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Transformer Failure Due to Circuit Breaker Induced Switching Transients

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michaeljmack

May 2011

Engineering
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TUTORIAL Transformer Failure Due to Circuit Breaker Induced Switching Transients 2011 I&CPS CONFERENCE 2011 NEWPORT BEACH, CA MAY 1 – 4, 2011 David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 daviddshipp@eaton.com Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 thomasjdionise@eaton.com
i TABLE OF CONTENTS 1.0 INTRODUCTION.....................................................................................1-1 2.0 CASE STUDY OVERVIEW.....................................................................2-1 3.0 EMTP MODELING & CASE STUDIES...................................................3-1 4.0 MOTOR STARTING AUTOTRANSFORMER CASE STUDY.................4-1 5.0 PT FAILURES.........................................................................................5-1 6.0 SNUBBER PERFORMANCE MEASUREMENTS ..................................6-1 7.0 SUMMARY..............................................................................................7-1 8.0 REFERENCES........................................................................................8-1
1-1 1.0 INTRODUCTION Switching transients associated with circuit breakers have been observed for many years. Recently this phenomenon has been attributed to a significant number of transformer failures involving primary circuit breaker switching. These transformer failures had common contributing factors such as 1) primary vacuum or SF-6 breaker, 2) short cable or bus connection to transformer, and 3) application involving dry-type or cast coil transformers and some liquid filled. This tutorial will review these recent transformer failures due to primary circuit breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromagnetic transients program (EMTP) will give case studies which illustrate how breaker characteristics of current chopping and re-strike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, especially the challenges of retrofitting a snubber to an existing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented including surge arresters, surge capacitors, snubbers and these in combination
2-1 2.0 CASE STUDY OVERVIEW
Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 1: Case Study Overview David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Case Study Overview: Data Center Transformer Failure Attributed to Circuit Breaker Induced Switching Transients System Configuration, Failure Analysis and RC Snubber Fix R. McFadden – JB&B – NY, NY D. Shipp – Eaton Electrical – Pgh, PA Presentation Purpose/Summary • Purpose #1 Safety: – Alert Design & User Community – Drive Solutions Through Market Interest • Unique Case History – Forensic Evidence – High Speed Power Quality Capture • Presentation Summary – Simplified System Overview – Forensic Review – Power Quality Snapshots – Installed RC Snubber Fix • Presentation Summary: David Shipp, Eaton – Phenomenon Detailed Explanation – Science of RC Snubber Design
Site Specifics • Utility Service: – 26kV – Double Ended Loop Through • Transformers – (6) Total – 26kV Primary, 480V Secondary – VPI – 3000kVA AA / 3990 kVA FA – 150 kVbil • Switching Device – Vacuum Circuit Breaker • Cable – 35kV, 133% EPR Insulation, 1/3 Concentric Ground M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” To Normal Side Of 480vac ATS (Typical) M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Simplified System Configuration Failure/Sequence of Events • All Transformers Fully Tested: • Pre-functional: Turns Ratio, Insulation Resistance, etc • Functional:UPS Full Load Tests, UPS Transient Tests, Data Center Room Validation Testing • Final Pull The Plug Test (PTP) • 4 Electricians “simultaneously” open (4) 26KV vacuum breakers to simulate a general utility outage. • All systems successfully transfer to standby generation but: • Loud Pop heard in Substation Room B • Relay for VCB feeding TB3 signaling trip • Decision made to shutdown generator test and investigate issue in “B” substation room • 2 Electricians “simultaneously” close (2) 26KV vacuum breakers to Substation Room “A” • Transformer TA3 fails catastrophically.
M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” 0.8MW M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Pre-PTP Condition 1.5MW 0.9MW 1.3MW M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Failure #1: De-Energization During PTP Audible Pop In Substation Room Relay Signaling Trip M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Failure #2: Energization During PTP Catastrophic Failure Relay Trips VCB
Coil to Coil Tap Burnt Off Failure #2 Transformer Failure On Energization Coil to Coil Conductor Burnt Off Suspected Area of Initial Flash Failure #2 Transformer Failure On Energization Coil to Coil Tap Burnt Off Failure #2 Transformer Failure On Energization
Coil to Coil Tap Burnt Off Upward Twist From Lower Blast Failure #2 Transformer Failure On Energization Flash/Burn Marks Failure #1 Transformer Failure On De-Energization Close up of Flash/Burn Marks Coil to Coil not Winding to Winding Failure #1 Transformer Failure On De-Energization This Transformer was Returned to the Factory and Passed all Standard IEEE Tests!
No Coil to Coil Cable Supports Some Burn Marks Match Point of Cable Contact 15KV Cable In 26KV Transformer Failure #1 Transformer Failure On De-Energization Some Burn Marks Match Vicinity of Cable Cable is Undamaged Failure #1 Transformer Failure On De-Energization Factory Installed Cable Support Unaffected Transformer
Current Transient: Failure #2 Transformer Energization Event Initiated A-B Phase 19,000 amp, 3 Phase 5 Cycle CB Clearing Time Second Transformer Inrush AØ BØ CØ AØ BØ CØ Current Transient: Failure #1 Transformer DeEnergization 8000amp A-B Phase Approx ¼ Cycle C Phase Shows 2nd Xfmr Load & Event Duration Breaker Induced Switching Transients
Case 1 Hydro Dam, MT 2005 • MV Vac Bkr Replacements Vendor “A” • 13.8 kV • 20 feet of cable • 50 kV BIL (W) ASL Dry Type Txmrs • Customer Energized before Vendor OK • Txmr Failed • No Surge Protection Applied CASE 2 Cleveland Hospital 3/06 • Vacuum Breakers – Vendor “A” and Vendor “C” • 13.8 kV • 95 kV BIL • Dry Type Txmr • 27 feet of Cable • Bkr Manufacturer Paid to Repair Failed 2500 KVA Txmr • Surge Protection Added Afterwards CASE 3 RAILROAD SUBSTATION 11/06 • Vacuum Breaker – Vendor “A” • 26.4 kV • 150 kV BIL • Generic Liquid Filled Rectifier Txmr • 37 feet of Cable
CASE 4 NJ DATA CENTER 12/06 • 26.4 kV – Vendor “B” • 4 Txmrs Switched Under Light Load • 2 Txmrs Failed-1 on Closing/1 on Opening • 40 Feet of Cable • 2 other Txmrs Did Not Fail - 80 Feet of Cable • Arresters Were Present CASE 5 OIL FIELD – AFRICA 6/07 • Vacuum Breaker – Vendor “D” • 33 kV • 7 Feet of Cable • Dry Type Txmr in 36 Pulse VSD • Arresters Were Applied • Txmr Failed Upon Energization CASE 9 Oil Drilling Ship – 6/2002 • Vendor “A” IEC Vac Bkrs in Vendor “D” Swgr • 11kV, 60 HZ • Cast Coil Dry Type IEC VSD Propulsion Txmr Failed (7500 kVA) – 75 kV BIL? • < 30 feet of Cable • Fed from Alternate Bus – Now 80 feet of Cable • No further Failures Reported
CASE 11 Hospital – Slide 1 • 13. 8 kV System • Vendor C Breakers/ Vendor F Txmrs • 2 Txmrs Failed During Commissioning • Differential Relay Tripped During motor Starting – under Highly Inductive Load CASE 11 Hospital – Slide 2 CASE 11 Hospital – Slide 3
CASE 12 Motor Starter Auto Txmr Failure • 4160V • 5000 HP • Reduced Voltage Auto Txmr Failed • Arresters on Wye Point • Internal Resonance • Layer Wound • Failed Layer to Layer CASE 12 Run Contactor Closes 3516 HZ CASE 12 - Sweep Frequency Test 4500 HZ (Admittance)
CASE 12 - Run Contactor Closes – 844 HZ With Snubber SWITCHING TRANSIENTS DUE TO VACUUM / SF6 BREAKERS • Opening -- Current Chop • Closing -- Prestrike/Re-ignition/Voltage Escalation • Vacuum Bkrs --Both Closing and Opening • SF6 -- Opening • Air -- Generally Acceptable Current Chop
Current Chop Current Chop in Vacuum is a Material Problem Current Chop • All Types of Interrupters Chop Current • This is not a Unique Feature of Vacuum Switching Inductive Circuits • Closing • Opening • Voltage Escalation • Surge Suppression
Switching Inductive Loads CLOSING Switching Inductive Loads OPENING Switching Inductive Loads VOLTAGE ESCALATION
SWITCHING TRANSIENT THEORY • “Thou Shalt Not Change Current Instantaneously in an Inductor” • Conservation of Energy – – You Cannot Create or destroy Energy – You Can Only Change Its Form ENERGY EQUATION 22 2 1 2 1 CVLI  OR C L IV  TOTAL VOLTAGE • Vt = Venergy + Vdc + Vosc • Venergy is from the Energy Equation • Vdc = DC Off-set that Sometimes is Present • Vosc = the Oscillatory Ring Wave
TRANSFORMER LIMITS • Magnitude – BIL Ratings • Dv/dt Limits • Both MUST Be Met • Dry Type Txmrs Particularly Susceptable • Liquid Filled Not Immune. • Consider “Hammer Effect” ANALYSIS • When Open Bkr, Txmr is Left Ungrounded • Ring Wave is a Function of its Natural Frequency LC NF 2 1  CASE 3 Waveforms Without Snubbers
CASE 3 Current – Without Snubbers Ichop CASE 3 Waveforms With Snubbers CASE 4 Data Center 12/06 • 26.4 kV • Vendor “B” Breakers • 4 Bkrs Switched at Once • 2 Dry Type Txmrs Failed (40 Ft of Cable) • 2 Txmrs Did not Fail (80 Ft of Cable) • Unfaulted Txmr Winding Failed BIL @162 kV ( Rated 150 kV)
CASE 4 Data Center, NJ 12/06 - W/O Snubbers 08-Jan-07 14.53.47 0 5 10 15 20 25 -200 -150 -100 -50 0 50 100 150 200 v [kV] t [ms](42) XFMA3C (48) XFMB3C Plot of the Transformer A3 and B3 primary voltage Phase C Open circuit breaker with chopped current of 9 Amperes -207 kV CASE 4 Data Center, NJ 12/06-With Snubbers 09-Jan-07 10.50.25 0 5 10 15 20 25 -40 -30 -20 -10 0 10 20 30 40 v [kV] t [ms](29) XFMA3B (35) XFMB3B Plot of the Transformer A3 and B3 primary voltage Phase B Open circuit breaker with chopped current of 9 Amperes Application of the R-C snubber circuit -39 kV CASE 6 Data Center 2, NJ 12/06 • 13.2 kV • Vendor “B” Breakers • 3 MVA Dry Type Txmrs • 60 ft Cable – Required Snubbers • 157 ft Cable – Required Snubbers • No Problem at Startup
CASE 6 13.2 kV SYSTEM – 119 kV @ 678 HZ CASE 6 WITH SNUBBERS – 33.6 kV @ 236 HZ CASE 7 Chemical Plant, NC 3/07 • 12.47 kV System • 20+ Year Old Oil Filled Txmrs • Vendor “A” Vacuum Bkrs retrofitted on Primary • 10 Feet of Cable • No Problem at Startup
CASE 7 425 kV - 12. 47 kV - 23 kHZ Ring Wave CASE 7 Added Snubbers – 12.47 kV CASE 8 DATA CENTER – Indiana 6/07 • 12.47 kV System • Vendor “A” Breakers • 270 Feet of Cable • No Additional Surge Protection Required
CASE 8 -55 kV at 800 HZ CASE 11 Hospital 186.9 kV / 2000 HZ CASE 10 – Snubber Reduced to 33.8 kV / 368 HZ
MITIGATING TECHNIQUES • Arresters • Surge Capacitors • Snubbers (RC) • Hybrid / Combinations • Liquid vs Dry Type • Electronic Zero Crossing Switching Snubber - 3 Phase Capacitor Generally Solidly Grounded System • Paper Mill - AL • 13.8 kV Snubbers – 2nd paper mill - beta site
Snubber – Single Phase Capacitors Low Resistance Grounded Systems • Paper Mill • Vendor “A” Swgr • 13.8 kV Snubber – Single Phase Capacitors • Silicon “Chip” Plant • Montana • Very Specialized Dry Type Txmrs • 13.8 kV • Cables < 100 feet • Primary Fused Switch AF Solution • Vendor “A” Vac Bkrs Capacitor Resistor Fuse Lightning Arrestor Blown Fuse Viewing Window RC Snubber Installed – Case 4
RC Snubber Installed – Case 4 Capacitor Resistor RC Snubber Installed – Case 4 Fuse Blown Fuse Indicator SWITCHING TRANSIENT STUDY • Quantifies Problem • Predict Exposure / Risk • Select Best / Most Cost Effective Solution • Do “What if” Cases • Verify Results
RECOMMENDATIONS • Factor into Design Up-front • Do Study – Results Are Bkr Manufacturer Specific • Use Protection Only When / Where Needed (if not there, cannot fail) • Fused or Unfused Snubbers?? • Loss of Fuse Detection?? • Fear Not! - Mitigating Techniques Have Been Proven. • Discrete Snubber Components?? Conclusion/Next Steps • This is a System Problem – Transformer, Cable, Switching Device, Proximity – Statistical Event , Possible Undetected Failures • Data Centers Fall into the Highest Risk Category – High Power Density – Close Proximities – Frequent Switching • Draft IEEE C57.142 – A Guide To Describe The Occurrence and Mitigation Of Switching Transients Induced By Transformer And Switching Device Interaction – Does not accurately warn users of all areas of concern – This case did not meet the areas of concern noted in Draft C57.142 – Need to push for formalization of the standard with new lessons learned • RC Snubbers – Transformer Manufacturer appears best positioned to implement the solution – Limited Cataloged Product (One Manufacturer) – Not embraced by all transformer manufacturers – Design Parameters of RC Snubber not clearly defined • Lives, Property and Uptime are all at risk QUESTIONS ?
3-1 3.0 EMTP MODELING & CASE STUDIES
1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 2: EMTP Modeling and Case Studies David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Transient Analysis Tools • Electromagnetic Transients Program (EMTP) • Developed by Hermann Dommel brought to BPA • Major contributors: Scott Meyer & Dr. Liu at BPA • Alternative Transients Program (ATP) • Alternative to commercialized EMTP • Free to all if agree not to commercialize it • EMTP-RV • PSCAD EMT/DC 3 3 ATP • ATP Draw interface • 3-phase modeling • Solution method - Trapezoidal integration • Robust for wide variety of modeling needs • Extensive library of models • Many options and features
2 4 4 Source Model • “3-phase wye cosine” • Resistance in ohms • Inductance in mili-Henries • Source Impedance • Z1 and X/R • Z0 and X/R SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT 5 5 Cable Model • “Pi Model” • Resistance in ohms • Inductance in mili-Henries • Cable charging in micro-Farads • ½ charging on sending end • ½ charning on receiving end • Multiple pi models in some cases CABLE 13.8KV C1 LCABLERCABLE C2 C/2C/2 6 6 Breaker Model • “Switch” • Time dependent switch • Open or close at a specific time • Opens at current zero unless specify otherwise • Current dependent switch • Needed for current chopping • Define current at which to open switch • Independent Poles (A, B and C independent) • Data request • Vacuum or SF6 breaker nameplate • Current chop (ask manufacturer) • Transient Recovery Voltage Ratings – T2, E2 and RRV VCB BKR
3 7 7 Transformer Model • “Three Phase Model” • Resistance in ohms • Inductance in mili-Henries • Magnetizing current • Winding capacitance • CH and CL for dry-types • CHL for oil-filled • More detailed modeling • Saturation • Hysteresis • Data Request • %Z and X/R, MVA • BIL TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 8 8 Complete System Model TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 CABLE 13.8KV C1 LCABLE RCABLE C2 C/2C/2 SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT VCB BKR 9 9 Time Step • Time Step – Integration Time Step • Choice depends on frequency of expected transient • Too large – miss high frequency effects • Too small – excessive simulation times • Nyquist Criteria • Minimum sample rate = 2 x frequency • Example 10-20KHz ring for VCB transient
4 10 10 Switching Transient Simulations • Switching transient simulations • EMTP and model requirements • Case studies • Data Center • Ferry Propulsion • Laddle Melt Furnace • Successful techniques/solutions 11 11 Ferry Propulsion Highlights • 60K passengers per day • 20M passengers per year • 4160V distribution • 3 x 2865kW diesel gens • 2 x 1865KW forward propulsion motors • 2 x 1865KW reverse propulsion 12 12 Problem • Forward propulsion drives ferry • Upon reaching dock, reverse propulsion stops ferry • July 1, 2009, reverse propulsion was lost entering dock • lost power and hit a pier at full speed • one serious injury and nine minor injuries • 750 to 800 passengers were aboard • impact did not send any passengers overboard • 1185KVA rectifier transformer failed on reverse propulsion motor • Evidence of insulation failure on first few turns of primary winding • Investigation into source of such failure VCB switching
5 13 13 Ferry - Oneline Vacuum Breaker Short Cable Dry-Type Transformer Forward Reverse 14 14 Ferry – Electrical Highlights • 3 x 2865kW diesel generators • 4160V, 3-bus system • Vacuum circuit breakers – 630A mains, ties, feeders • 2 x 1865kW motors (forward propulsion) • 2 x 1865kW motors (reverse propulsion) • 8 x 1185KVA dry-type transformers, 30kV BIL • 3-winding rectifier transformers • 12-pulse effective (6-pulse VFD per winding) • Feeder cable lengths of 50 feet each 15 15 Worst Case Scenarios • Feeder cable lengths of 50 feet • Each 1185KVA transformer has one 4160V feeder • Need to examine both feeders for each transformer • “Worst Case” Switching Transient Simulations • Close 4160V VCB to transf. with 50ft. cable (model validation) • Close 4160V VCB to transf. with 50ft. cable (re-ignition) • Open 4160V VCB to transf. with 50ft. cable (current chop) • Repeat each case with Snubber
6 16 16 Measurements Transient Overvoltages at Primary of Rectifier Transformer – VCB Closes 17 17 Study Cases • Case 1 – Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer with the distance of 50 feet. Simulated to match the Siemens measurements to ensure that the computer model is accurate. (Model Validation) • Case 2 – Same as Case 1, except with the RC snubber. • Case 3 - Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer, then the 4.16 kV feeder breaker opens during the inrush current. Shows the possibility of the vacuum breaker re-ignition. (Re-ignition) • Case 4 - Same as Case 3, except with the RC snubber. • Case 5 - Open the 4.16 kV feeder breaker feeding the three winding rectifier transformer under the light load condition. (Current Chop) • Case 6 – Same as Case 5, except with the RC snubber. 18 18 Case 1 – Model Validation Transformer primary voltage V max of 4.96kV < 30kV BIL Oscillation of 20,203Hz > 1000Hz
7 19 19 Case 2 – Valid Model with Snubber Transformer primary voltage V max of 3.982kV < 30kV BIL Oscillation of < 1000Hz 20 20 Case 3 – Re-ignition Transient Recovery Voltage (TRV) at VCB 21 21 Case 4 – Re-ignition with Snubber Transient Recovery Voltage (TRV) at VCB
8 22 22 Case 5 – Current Chop Transformer primary voltage V max of 31.9kV > 30kV BIL Oscillation of 958Hz ~ 1000Hz 23 23 Case 6 – Current Chop with Snubber Transformer primary voltage V max of 8.9kV < 30kV BIL Oscillation of 299Hz < 1000Hz 24 24 Summary of Current Chop Cases
9 25 25 Summary of Re-Ignition Cases 26 26 Recommendations • Install snubber at primary of each 1185KVA rectifier transformer • 40ohm resistor • Non-inductive • Peak voltage – 6 kVpeak • Peak energy – 2100 Joules • Average power – 190 Watt • 0.5uF capacitor • Rated voltage - 4.16kV 27 27 Data Center Highlights • Tier III • LEEDS Certified • 12.5MVA Capacity • 13.2KV Ring Bus
10 28 28 Data Center - Oneline Vacuum Breaker Short Cable Cast-Coil Transformer 29 29 Data Center – Electrical Highlights • 2 x 24.9 kV lines from Factory Shoals and Buzzard Roast • 2 x 12.5 MVA transformers • 13.2 kV “ring-bus” configuration • MSA and MSB and generator bus GPS • Vacuum circuit breakers – 600A mains & ties, 200A feeders • 3 x 2250 KW generators • 6 x 3750KVA cast coil transformers, 90kV BIL • 3 x DE Subs CSA, CSB and CSC • 3 x SE Subs MDSA, MDSB and MLB • Feeder cable lengths vary from 109 to 249 Feet 30 30 Worst Case Scenario • Feeder cable lengths vary from 109 to 249 Feet • Each 3750KVA transformer has two 13.2kV feeders • Need to examine both feeders for each transformer • Shortest of all 13.2kV cable runs to 3750KVA Transformers • MSB to Transformer CSC – 109 feet • MSA to Transformer CSC – 111 feet • “Worst Case” Switching Transient Simulation • Open 13.2 kV VCB at MSB to CSC with 109ft. cable • Open 13.2 kV VCB at MSA to CSC with 111ft. cable
11 31 31 Worst Case – Study Case 13 • Study Case 13 - Open the 13.2 kV feeder breaker at Bus MSB feeding the 3,750 kVA dry type transformer CSC with the shorter distance of 109 feet. The result for this case will represent the “worst-case” condition, the other feeder from Bus MSA has a longer feeder distance of 111 feet. • Study Case 14 – Same as Case 13, except with the application of the 0.25 μF surge capacitor. • Study Case 15 – Same as Case 13, except with the application of the snubber circuit with 30ohm and 0.25 μF surge capacitor. 32 32 Case 13 – no surge protection Load current at 13.2kV VCB Load current of 10A Chopped current of 6A 33 33 Case 13 – no surge protection Transformer CSC primary voltage VC max of 65.3kV > 95kV BIL VB max of 116kV > 95kV BIL VA max of 123kV > 95kV BIL Oscillation of 969Hz > 1000Hz
12 34 34 Case 14 – 0.25uF surge cap Load current at 13.2kV VCB Load current of 10A Chopped current of 6A 35 35 Case 14 – 0.25uF surge cap Transformer CSC primary voltage VA max of 29.4kV < 95kV BIL VB max of 19.1kV < 95kV BIL VC max of 15.7kV < 95kV BIL Oscillation of 215Hz < 1000Hz 36 36 Case 15 – snubber 30ohm and 0.25uF Load current at 13.2kV VCB Load current of 10A Chopped current of 6A
13 37 37 Case 15 – snubber 30ohm and 0.25uF Transformer CSC primary voltage VB max of 28.6kV < 95kV BIL VA max of 19.4kV < 95kV BIL VC max of 15.9kV < 95kV BIL Oscillation of 215Hz < 1000Hz 38 38 Case 15 – snubber 30ohm and 0.25uF Snubber Current – important for duty on resistor and capacitor IC peak of 7.7A IB peak of 8.0A IA peak of 6.8A 39 39 Comparison of Results No Snubber 0.25uF surge cap Snubber R=30ohm C=0.25uF
14 40 40 Measured Transients with Snubber 41 41 Recommendations • Install snubber at primary of each 3750KVA cast coil transformer 30ohm and 0.25uF • Install surge caps 0.25uF at each emergency generator • Install surge arresters at the following locations: • both incoming power transformers • every distribution dry type transformer • every generator • line side of both main circuit breakers • line side of the three generator circuit breakers • load side of every feeder breaker and every tie circuit breaker 42 42 Laddle Melt Furnace Highlights • Retiring 3 x 38MW EAFs • Adding 1 x 155MVA EAF • Adding 2 x 138kV lines to new EAF • Adding SVC at 34.5kV • Adding 1 x 20MW LMF
15 43 43 LMF & EAF - Oneline Vacuum Breaker Short Bus Oil-Filled Transformer Existing EAF New LMF 44 44 138KV UTILITY 4713MVA 3PH SC 9.26 X/R 50/66/83MVA 135.3/26.4KV 7.5%Z SF-6 BREAKER 2000A 1600A 27KV 13OHM AUTO LTC 56MVA 27-10KV 3.3%Z ALUMINUM IPS BUS 53FEET VACUUM BREAKER 1200A LMF XFMR 50/56MVA 25/.53KV 2.5%Z HEAVYDUTY COPPER PIPE 28FEET LMF 20MW LMF Vacuum Breaker Short Bus Oil-Filled Furnace Transformer New LMF SF6 Breaker Short Bus Oil-Filled Auto-Regulating Transformer 45 45 LMF Circuit – Electrical Highlights • 50MVA Power Transformer 135/26.4kV • 27kV system • SF6 circuit breaker – 2000A • Bus length of 53 feet • 56MVA autoregulating transformer, 200kV BIL • Vacuum circuit breaker – 1200A • Bus length of 28 feet • 50MVA oil-filled LMF transformer, 200kV BIL • 20MW LMF
16 46 46 Worst Case Scenarios • Examine switching transients at both transformers • “Worst Case” Switching Transients for Auto-Reg Transf • Open SF6 breaker to transf. with 53ft. bus (6A current chop) • 3 cycles after energizing Auto-Reg Tran, open SF6 bkr to transf. with 53ft. bus (re-ignition) highly inductive current • “Worst Case” Switching Transients for LMF Transf • Open VCB to transf. with 28ft. bus (6A current chop) • 3 cycles after energizing LMF Tran, open VCB to transf. with 28ft. bus (re-ignition) highly inductive current • Repeat each case with Snubber 47 47 Case 1 – Open VCB LMF Transformer primary voltage V max of 386kV > 200kV BIL Oscillation of 1217Hz > 1000Hz 48 48 Case 2 – Open VCB with Snubber LMF Transformer primary voltage V max of 56.4kV < 200kV BIL Oscillation of 200Hz < 1000Hz
17 49 49 Case 3 – Open SF6 Breaker Auto-regulating Transformer primary voltage V max of 23kV < 200kV BIL No oscillating frequency 50 50 Case 4 – Open SF6 Breaker with Snubber Auto-regulating Transformer primary voltage VB max of 54.7kV < 200kV BIL Oscillation of 197Hz < 1000Hz 51 51 Case 5 – VCB Re-ignition Transient Recovery Voltage for VCB
18 52 52 Case 5 – Highly Inductive Current Inrush current to LMF transformer 6000A Peak Inrush VCB Opens VCB Closes 53 53 Case 6 – VCB Re-ignition & Snubber Transient Recovery Voltage for VCB 54 54 Case 7 – SF6 Breaker Re-ignition Transient Recovery Voltage for Siemens SF6
19 55 55 Case 7 – Highly Inductive Current Inrush current to Auto-Regulating transformer 14000A Peak Inrush SF6 CB Opens SF6 CB Closes 56 56 Case 8 – SF6 Breaker Re-ignition & Snubber Transient Recovery Voltage for SF6 Breaker 57 57 Summary of Current Chop Cases
20 58 58 Summary of Re-Ignition Cases 59 59 Recommendations • Install snubber (100ohm and 0.15uF) at primary of each transformer • 100ohm resistor • Non-inductive • Peak voltage – 38 kVpeak • Peak energy – 17,500 Joules • Average power – 1000 Watt • 0.15uF, 34.5kV surge capacitor (not available) • 1-pole, 24kV, 0.13uF • 2-pole, 14.4kV, 0.5uF • Series combination gives 0.103uF
4-1 4.0 MOTOR STARTING AUTOTRANSFORMER CASE STUDY
1 Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – Larry Farr, Senior Member IEEE Principal Engineer Eaton Electrical Asheville NC Technical Advisor TC-17A IEC Chair of CANENA TC-17A SC-3 Arthur J. Smith III, Member IEEE Vice President Waldemar S. Nelson and Co. Inc. New Orleans, LA • Background Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – TODAY’S AGENDA • Investigation • Testing 1988 • Findings •Testing 2002-03 • Findings • Solution • Conclusions • Background Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained –
2 • For the past century the Auto- transformer or “Korndorfer” Starter has been a standard in the electrical industry. Background • In the late 1970s, unexplained “High Voltage Stress” failures started occurring. Background
3 Failures Were Occurring Worldwide • North Sea Platform 12kV starters failed multiple times. Problems solved with single phase autotransformers. Background • 20,000 hp-15kV starter in British Columbia failed four times until surge arresters from the Zero tap to ground were installed. • South America 2400 volt failed multiple times. • Gulf of Mexico – Multiple 5kV starter failures • IEC 60470 Clause 6.102.7 Change-over Ability Test • Off the West Coast of Africa – Multiple 5kV starter failures 3 Inches !! What Happened? Zero Tap Circuit to Ground Background High Voltage Flashover Telescoping Coils Layer to Layer Failure Modes Background
4 High Voltage Flashover Tap to Tap Over the Surface Background High Voltage Flashover Coil to Ground Background High Voltage Flashover Layer to Layer Failure Background
5 High Voltage Flashover Zero-tap Circuit to Ground Background Telescoping Coils Background • S and R Conducting Simultaneously • Flashover of S with R Closed Background Flashover of S with R Closing
6 Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 Testing Initial Testing Testing Initial Testing
7 Testing Initial Testing Testing Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 • Findings
8 Design Modifications • Changed Design to Three Coil Three Legged Autotransformer Findings • Changed Control Circuit to Include a Transition on Current Below 125% FLA • HOWEVER, Failures Continued at a Rate of Two to Three a Year ! The High Voltage Stress Failures Occurred When: • Transformers were on the 80% tap Findings • Transitions Were Forced at or Near Locked Rotor Current Remaining Failures Required a Closer Look and Additional Testing Findings
9 Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 Additional Testing • However, The Sample Rate Was only 50,000 Samples Per Seconds and I suspect some level of smoothing. • Confirmed the Test Results From 1988 on Three Coil Three Legged Transformers (February 2002 High Power Lab in Pennsylvania) Dielectric Withstand Two Layers of .005 Nomex 14kV rms for 60 Secs. Additional Testing
10 Partial Discharge Additional Testing November, December and January Belmont, North Carolina Additional Testing A Closer Look Additional Testing
11 February Arden, North Carolina Additional Testing Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 • Findings Issue Found ! Findings
12 Current Flat During Voltage Escalation Findings When Reignition Occurs Findings Each Burst 70,000 Volts/Micro Sec Each Burst 70,000 Volts/Micro Sec Findings
13 • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 • Findings • Solution Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – New Circuit for Autotransformer Starters Solution With Surge Arresters 12-13,000 Volts at 12-18 amps for 800 Micro Sec Solution
14 • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 • Findings • Solution • Conclusions Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – Conclusions • When controllers are configured for transition on current sensing below 125% FLA, high voltage potential is reduced but not eliminated. •When autotransformer starters are forced to transition before they reach near full speed, high voltages are generated on the 0% taps Conclusions • Vacuum contactor autotransformer starters and starter retrofits require surge arresters even if no problems were encountered with air break contactors • When 6 kV distribution surge arresters are installed on a 4,160 volt circuit from the 0% tap to ground, the voltage is clamped to 13 kV without the resultant high dv/dt across the transformer coil regardless of transition current
15 Conclusions • Care must be taken so the Metal Oxide arrestors do not conduct near normal operating voltages. • The 15kV Autotransformer Starter in British Columbia was set on the 55% tap. The starter used MV Circuit Breakers. Therefore all autotransformer starters need surge arrestors zero tap to ground or across the VI,s.
5-1 5.0 PT FAILURES
1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 4: Potential Transformer Failures David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Case Studies • Switching Transients – Opening • Switching Transients – Closing • Ferro- Resonance – Closing • Ferro-Resonance – Opening (20 HZ Saturation) • Internal Resonance 3 3 CASE 12 Midwest Data Center • 12.47 kV System / 120 MW Load • Bkr Pairs with Unloaded wye-wye PTs for Auto Transfer Sensing at Load End of Cables • Multiple Open and Closed Operations were Performed Preceding the Failure. • 1st failure – Smoke But fuses did not Blow – Cleared Manually.
2 4 4 CASE 12 Midwest Data Center • 2nd Failure – Identical Switching Events • Open Transitioned Back to Source “A” • A few Minutes Later A Load “Pop” Was heard. • More Smoke + B Phase Fuse Blew • Measurements Were Taken – Snuck Up on Problem without PT Loading – Risked Failure 5 5 CASE 12 Midwest Data Center 6 6 CASE 12 Closing From Generator Source – 300 kW Load
3 7 7 CASE 12 Opening From Generator Source – 300 kW Load 8 8 CASE 12 Primary Measurements - 300 kW Load Opening 9 9 CASE 12 Primary Measurements – Snubbers + 300 KW Load
4 10 10 CASE 12 - Closing Primary Measurements - 300 kW Load 11 11 CASE 12 – Closing - Snubber Primary Measurements - 300 kW Load 12 12 PT Failure – Example 13 • PT 1500VA, 14400/120V, open delta, in gen-tie swgr • System 13.8 kV wye-solidly grounded • Upstream VCB switching • Cable lengths of 250 to 3600 feet • Failures • #1 – catastrophic failure of PT in gen-tie swgr (2700ft cable) • #2 – a month later, catastrophic failure of PT in gen-tie swgr • No switching at time of failure/100s prior to failure • Possible failure modes • Ring frequency of transient overvoltage on closing • Ferroresonance (PT saturation) on opening
5 13 13 CASE 13 Switching Transient 1st Failure 14 14 PT transient on VCB closing (EMTP simulated) 15 15 PT - test without snubber (measured) • Transient overshoot to -17.5kVpeak • High frequency oscillation follows • Oscillation persists for more than ¼ cycle on phase-b Transient followed by high frequency ring Oscillation continues beyond ¼ cycle
6 16 16 PT - test without snubber (zoom view) 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 -18000 -16000 -14000 -12000 -10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 12000 Volts Recorded Volts/Amps/Hz Zoomed Detail: 04/27/2011 19:21:07.855.770.000 - 04/27/2011 19:21:07.857.283.100 Time 04/27/2011 19:21:07.854000 + us • Magnitude is not the only problem • High frequency as well as high magnitude • Overtime both will damage PT Magnitude effects – 17.5kV High frequency effects – 50kHz 11.27kVpeak nominal 17 17 PT - test with snubber • Overshoot to 12.25kVpeak • Lower frequency oscillation • Oscillation very well damped Transient just above normal crest Oscillation well damped 18 18 PT – test with snubber (zoom view) • Both magnitude and frequency are acceptable • PT is well protected by the snubber 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 -12000 -10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 12000 Volts Recorded Volts/Amps/Hz Zoomed Detail: 04/27/2011 20:09:12.582.800.000 - 04/27/2011 20:09:12.584.990.800 Time 04/27/2011 20:09:12.580000 + us Low frequency 2000Hz 11.27kVpeak nominalLow Magnitude 12.25kV
7 19 19 PT saturation on VCB opening simulated measured 20 20 PT saturation on VCB opening (EMTP) 21 21 PT with 100W loading resistors VCB opening
6-1 6.0 SNUBBER PERFORMANCE MEASUREMENTS
1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 5: Snubber Performance Measurements David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Snubber Performance Measurements • Data Center • Purpose of Measurements • Test procedures • Medium voltage measurements • Selecting a meter • Using voltage dividers • Safety concerns • Data Center challenges 3 3 Purpose of Performance Measurements • Ensure the proper operation of the snubbers • Measure the transient overvoltage waveforms at the transformer primary • produced during switching of the primary vacuum circuit breakers • verify they do not exhibit high frequency transients (magnitude, rate- of-rise and frequency). • Specify the meters and voltage dividers needed for the transient measurement • guide electrical contractor with their installation • electrical contractor to operate all equipment following the test procedure • After completion of the site monitoring during the snubber tests, analyze the measurement data and document the test results in an engineering report.
2 4 4 Data Center Highlights • Tier III • LEEDS Certified • 12.5MVA Capacity • 13.2KV Ring Bus 5 5 Data Center – Measurement Locations Vacuum Breaker Short Cable Cast-Coil Transformer 6 6 Data Center – Electrical Highlights • 2 x 24.9 kV lines from Factory Shoals and Buzzard Roast • 2 x 12.5 MVA transformers • 13.2 kV “ring-bus” configuration • MSA and MSB and generator bus GPS • Vacuum circuit breakers – 600A mains & ties, 200A feeders • 3 x 2250 KW generators • 6 x 3750KVA cast coil transformers, 90kV BIL • 3 x DE Subs CSA, CSB and CSC • 3 x SE Subs MDSA, MDSB and MLB • Feeder cable lengths vary from 109 to 249 Feet
3 7 7 RC Snubber at MVS - Typical 200 A Disconnect to transformerRC Snubber 13.8 kV 0.25 microFarad 30 ohm Cast Coil 3750KVA Transformer 8 8 RC Snubber at MVS - Typical Snubber Specs 13.8 kV 0.25 microFarad 30 ohm 9 9 Test Procedure • Test Procedure required by contractor • Prepared 2 weeks in advance of testing • developed the instructions • Included safety briefing each day • Site specifics supplied by the contractor • LO/TO instructions • Breaker operations • All meter connections made de-energized & LO/TO • No one in transformer room during tests • Signature of “Responsible Engineer” before test began • detailed test procedure form
4 10 10 Measurement Equipment • Measurement equipment included voltage dividers and a transient recording device • Voltage Dividers • Capacitive and resistive elements • 10MHz frequency response • Three-Phase Power Quality Recorder • transient voltage waveshape sampling • 200 nsec sample resolution • 5 Mhz sampling 11 11 Test Measurement Setup (Typical) 12 12 Voltage Divider Connections • Highly stranded No. 8, 15 kV insulated hookup wire • Insulated wire was a precaution – could have used bare conductor • maintained 8inch minimum separation for 15kV between phases and ground • Routed wires with gradual curve – no 90 degree bends • Connection at surge arrester at bus to transformer primary windings
5 13 13 Voltage Divider Connections 14 14 Voltage Divider Connections • Base of each divider grounded • Grounded to ground bus in primary disconnect • Used same highly stranded no. 8, 15kV insulated hookup wire • Connection to PQ Meter • Cables to match input impedance of meter to that of voltage divider • Voltage divider was calibrated prior to shipping to site 15 15 CT Connections • Primary interest was voltage at transformer • Secondary interest was snubber current • Connected CTs at ground side of snubbers
6 16 16 Initial view of events Energize (see zoom) De-energize (see zoom) • RMS trend of voltage • VCB energized and de-energized transformer 17 17 Zoom View of Energize • Slight overshoot above normal crest • Ring well damped 18 18 Zoom View of De-Energize • No transient on opening • Well damped
7 19 19 Other Tests – transformer natural frequencies • Sweep Frequency Response Analysis (SFRA) ImpedancePhaseAngle Frequency 20 20 Data Center Challenges • Contractor requested measurements • Advised contractor • First hookup/measurement – 1 day • Remaining 5 transformers – 2nd day • Prepare two voltage divider carts in advance • Contractor agreed to schedule but… • This was a construction site • Project manager had other objectives • Snubber testing viewed as an “inconvenience” • Voltage divider carts were not prepared • Be prepared to negotiate 21 21 Summary • Prepare test procedure – 2 weeks in advance • Request onelines and site photos to plan hookups • Ship all equipment in advance • For safety, two engineers perform measurements • Follow test procedure • Make all connections de-energized, LO/TO • No one in transformer room during test • Radio contact with contractor performing switching • Download data after each test • Analyze results before proceeding to next test • Begin building test report while on-site
7-1 7.0 SUMMARY
1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Summary David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 The Problem – In a Nutshell • Switching transients associated with circuit breakers observed for many years • Breaking opening/closing interacts with the circuit elements producing a transient • The severity of the transient is magnified by breaker characteristics • Current chopping on opening • Pre-strike or re-ignition on closing • In limited instances, the transient overvoltage exceeds transformer BIL resulting in failure • RC snubber in combination with surge arrester mitigates the transient 3 3 What to look for… “Rules of Thumb” to screen applications: • Generally, short distance between circuit breaker and transformer • about 200 feet or less • Dry-type or cast coil transformer • oil filled not immune and low BIL • Inductive load being switched • transformer, motor, etc. (light load or no load) • Circuit breaker switching characteristics: • chop (vacuum or SF6) or restrike (vacuum)
2 4 4 Predicting Transients – EMTP Simulations • For purposes of screening applications for damaging TOVs • Source, breaker, cable and transformer modeled • Breaker models for current chop and re-ignition TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 CABLE 13.8KV C1 LCABLE RCABLE C2 C/2C/2 SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT VCB BKR 5 5 Designing the Snubber • 15kV typical snubber & arrester • transformer protection • non-inductive ceramic resistor • 25 ohms to 50 ohms • surge capacitor • capacitor ratings 0.15 μF to 0.35 μF • 3-phase 13.8kV solidly ground • 1-phase 13.8kV LRG R C SA TX Surge Arrester Resistor Surge Cap 6 6 Performance Measurement Equipment • Test equipment includes voltage dividers and a transient recording device • Voltage Dividers • Capacitive and resistive elements • 10MHz frequency response • Three-Phase Power Quality Recorder • transient voltage waveshape sampling • 200 nsec sample resolution • 5 Mhz sampling
8-1 8.0 REFERENCES Switching Transients 1. Shipp, Dionise, Lorch and MacFarlane, “Transformer Failure Due to Circuit Breaker Induced Switching Transients”, IEEE Transactions on Industry Applications, April/May 2011. 2. Shipp, Dionise, Lorch and MacFarlane, “Vacuum Circuit Breaker Transients During Switching of an LMF Transformer”, IEEE Industry Applications Society Annual Meeting 2010, October 2010. 3. ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switching Transients Induced By Transformer And Switching Device Interaction, C57.142- Draft. 4. D. Shipp, R. Hoerauf, "Characteristics and Applications of Various Arc Interrupting Methods," IEEE Transactions Industry Applications, vol 27, pp 849- 861, Sep/Oct 1991. 5. ANSI/IEEE, Standard for AC High-Voltage Generator Circuit Breakers on a Symmetrical Current Basis, C37.013-1997. 6. ANSI/IEEE, Application Guide for Transient Recovery Voltage for AC High- Voltage Circuit Breakers, C37.011-2005. 7. D. Durocher, “Considerations in Unit Substation Design to Optimize Reliability and Electrical Workplace Safety”, ESW2010-3, 2010 IEEE IAS Electrical Safety Workshop, Memphis. Ferroresonance 1. IEEE Standard Dictionary of Electrical and Electronics Terms, ANSI/IEEE Std 100-1984. 2. Hopkinson, R.H., “Ferroresonant Overvoltages Due to Open Conductors,” General Electric, 1967, pp. 3 - 6. 3. Westinghouse Distribution Transformer Guide, Westinghouse Electric Corp., Distribution Transformer Division, Athens, GA, June 1979, revised April 1986, Chapter 4 Ferroresonance, pp. 36 - 40. 4. IEEE Guide for Application of Transformers, ANSI/IEEE C57.105-1978, Chapter 7 Ferroresonance, pp. 22 – 28. 5. Distribution Technical Guide, Ontario Hydro, Ontario, Canada, May 1999, original issue May 1978, pp. 72.1-1 – 72.1-10. 6. Greenwood, A., “ Electrical Transients in Power Systems”, Wiley & Sons, 1971, pp. 91-93. 7. Kojovic, L., Bonner, A., “Ferroresonance - Culprit and Scapegoat”, Cooper Power Systems, The Line, December 1998.
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 707 Transformer Failure Due to Circuit-Breaker-Induced Switching Transients David D. Shipp, Fellow, IEEE, Thomas J. Dionise, Senior Member, IEEE, Visuth Lorch, and Bill G. MacFarlane, Member, IEEE Abstract—Switching transients associated with circuit breakers have been observed for many years. Recently, this phenomenon has been attributed to a significant number of transformer failures involving primary circuit-breaker switching. These transformer failures had common contributing factors such as the following: 1) primary vacuum or SF-6 breaker; 2) short cable or bus con- nection to transformer; and 3) application involving dry-type or cast-coil transformers and some liquid-filled ones. This paper will review these recent transformer failures due to primary circuit- breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromag- netic transients program will give case studies which illustrate how breaker characteristics of current chopping and restrike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, particularly the challenges of retrofitting a snubber to an exist- ing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented, including surge arresters, surge capacitors, snubbers, and these in combination. Index Terms—Electromagnetic Transients Program (EMTP) simulations, RC snubbers, SF-6 breakers, surge arresters, switch- ing transients, vacuum breakers. I. INTRODUCTION TODAY, medium-voltage metal-clad and metal-enclosed switchgears that use vacuum circuit breakers are applied over a broad range of circuits. These are one of many types of equipment in the total distribution system. Whenever a switching device is opened or closed, certain interactions of the power system elements with the switching device can cause high-frequency voltage transients in the system. The voltage- transient severity is exacerbated when the circuit breaker op- erates abnormally, i.e., current chopping upon opening and prestrike or reignition voltage escalation upon closing. Such complex phenomena in combination with unique circuit char- acteristics can produce voltage transients involving energies Manuscript received June 4, 2010; accepted July 9, 2010. Date of publication December 23, 2010; date of current version March 18, 2011. Paper 2010- PPIC-188, presented at the 2010 IEEE Pulp and Paper Industry Conference, San Antonio, TX, June 21–23, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Pulp and Paper Indus- try Committee of the IEEE Industry Applications Society. The authors are with Eaton Electrical Group, Warrendale, PA 15086 USA (e-mail: daviddshipp@eaton.com; thomasjdionise@eaton.com; visuthlorch@ eaton.com; billgmacfarlane@eaton.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2010.2101996 Fig. 1. Simplified electrical distribution system for data center. which can fail distribution equipment such as transformers. Transformer failures due to circuit-breaker-induced switching transients are a major concern, which is receiving attention in a draft standard [1] and the focus of this paper. A. Forensic Evidence for a Unique Case Consider the case study of a new data center with a 26-kV double-ended loop-through feed to six dry-type transformers each rated at 3000 kVA AA/3390 kVA FA and 26/0.48 kV and delta–wye solidly grounded, as shown in Fig. 1. The trans- former primary winding is of 150-kV basic impulse insulation level (BIL). A vacuum breaker was used to switch the trans- former. The 4/0 cable between the breaker and the transformer was 33 kV and 133% ethylene propylene rubber. Primary arresters were installed. The transformers were fully tested, including turns ratio, insulation resistance, etc. Functional tests were completed, including uninterruptible power supply (UPS) full load, UPS transient, data center room validation, etc. In the final phase of commissioning, a “pull-the-plug” test was implemented with the following results. 1) De-Energization Failure #1. Four electricians “simultane- ously” opened four 26-kV vacuum breakers to simulate a general utility outage. All systems successfully trans- ferred to standby generation but a “loud pop” was heard in Substation Room B and the relay for the vacuum circuit breaker feeding transformer TB3 signaled a trip. 2) Energization Failure #2. Minutes later, two electri- cians “simultaneously” closed two 26-kV vacuum break- ers to substation Room A. Transformer TA3 failed catastrophically. 0093-9994/$26.00 © 2010 IEEE
708 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 2. Transformer failure #2 during energization. Fig. 3. Transformer failure #1 during de-energization. Failure #2 is shown in Fig. 2. Examination of the primary windings revealed that the coil-to-coil tap burnt off and the winding terminal showed an upward twist. The burn marks from the initial Flash indicated the transient concentrated on the first turns of the windings. Typically, closing the vacuum breaker to energize the transformer is the worst condition. Failure #1 is shown in Fig. 3. Examination of the primary windings revealed Flash and burn marks on the B-phase wind- ing at the bottom and middle. Those at the top indicate a coil- to-coil failure, not a winding-to-winding failure, and indicate a transient voltage with high dv/dt. Those in the middle were a result of the cable (used to make the delta connection) swinging free. Supports were only lacking for this jumper (oversight during manufacturing) which could not withstand the forces of the transient. This transformer passed the BIL test at 150-kV BIL but ultimately failed at 162-kV BIL. All six transformers and cables were identical, but only two failed during the vacuum-circuit-breaker switching. The significant difference was that the two failed units had 40 ft of feeder cable while the others had 80 or 100 ft of feeder cable. This short 40-ft cable, high-efficiency transformer, and vacuum circuit breaker proved to be the right combination to produce a damaging voltage transient on both energization and de-energization. B. History of Failures and Forensic Review The previous example is not an isolated case. Instead, it is representative of a growing number of transformer failures TABLE I HISTORY OF TRANSFORMER FAILURES RELATED TO PRIMARY VACUUM BREAKER SWITCHING due to primary switching of vacuum breakers. Table I details a history of transformers related to primary switching of vacuum breakers occurring within the past three years. In Case 1, in a hydro dam, the transformer was “value engineered” with a 13.8-kV primary-winding BIL of 50 kV. The BIL should have been 95 kV for the 13.8-kV class. The 1955 switchgear was replaced with modern vacuum breakers with only 20 ft of cable to the transformer. The user chose to energize the transformer before conducting a switching transient analysis and failed the transformer primary winding. The post mortem analysis revealed that no surge protection was applied. In Case 2, in a hospital, the vacuum breaker was close-coupled through 27 ft of cable to a 2500-kVA dry-type transformer with a 95-kV-BIL primary winding. The vacuum breakers were supplied with no surge protection because the particular vacuum breaker installed had a very low value of cur- rent chop. During vacuum breaker switching of the transformer, the transformer failed. The transformer was rewound, and surge protection/snubbers were installed. In Case 3, in a railroad substation, vacuum breakers applied at 26.4 kV were used to switch a liquid-filled rectifier transformer with 150-kV-BIL primary winding. The switching transient overvoltage (TOV) failed the middle of the primary winding. Forensic analysis determined a rectifier with dc link capacitors, and the transformer inductance formed an internal resonance that was excited by the switching. Such an LC series resonance typically fails the middle of the transformer primary winding. In Case 4, in a data center, vacuum breakers applied at 26.4 kV were used to switch six dry-type transformers with 150-kV-BIL primary windings under light load. Two transform- ers failed, one on breaker closing and the other on opening. The failed transformers were connected by 40 ft of cable to the vacuum breaker, while the other transformers had either 80 or 100 ft of cable. Arresters were in place at the time of failure, but there were no snubbers. In Case 5, in an oil field, a dry-type transformer for a variable speed drive (VSD) had multiple windings to achieve a 36-pulse effective “harmonic-free” VSD. A vacuum breaker at 33 kV was separated from the transformer by only 7 ft of cable.
SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 709 TABLE II CURRENT CHOP VERSUS CONTACT MATERIAL Arresters were applied on the primary winding. However, upon closing the breaker, the transformer failed. Finally, in Case 6, in an oil drilling ship, vacuum breakers designed to International Electrotechnical Commission (IEC) standards were applied at 11 kV and connected by 30 ft of cable to a dry-type cast-coil propulsion transformer rated at 7500 kVA. The transformer was also designed to IEC stan- dards, and the primary winding had a BIL of 75 kV. The IEC transformer BIL is much lower than the American National Standards Institute (ANSI) BIL for the same voltage class winding. The transformer failed upon opening the breaker. C. Common Parameters The severity of the voltage surge, i.e., high magnitude and high frequency, and the damage caused by the voltage surge are determined by the circuit characteristics. The following are some “rules-of-thumb” to screen applications for potentially damaging switching transient voltages. 1) generally, short distance between circuit breaker and transformer (about 200 ft or less); 2) dry-type transformer (oil filled and cast coil not immune) and low BIL; 3) inductive load being switched (transformer, motor, etc.); 4) circuit-breaker switching characteristics: chop (vacuum or SF-6) or restrike (vacuum). D. Underlying Concepts 3) Current Chop. When a vacuum breaker opens, an arc burns in the metal vapor from the contacts, which requires a high temperature at the arc roots [2]. Heat is supplied by the current flow, and as the current approaches zero, the metal vapor production decreases. When the metal vapor can no longer support the arc, the arc suddenly ceases or “chops out.” This “chop out” of the arc called “current chop” stores energy in the system. If the breaker opens at a normal current zero at 180◦ , then there is no stored energy in the system. If the breaker opens, chopping current at 170◦ , then energy is stored in the system. Current chop in vacuum circuit breakers is a ma- terial problem. Older vacuum interrupters (VIs) used copper–bismuth. Modern VIs use copper–chromium. Most copper–chromium VIs have a low current chop of Fig. 4. Voltage escalation due to successive reignitions. 3–5 A, offering excellent interruption performance and a moderate weld strength. Table II shows the average and maximum levels of current chop for copper–chromium, copper–bismuth, and other contact materials. It should be noted that both vacuum and SF-6 interrupters current chop. Current chop is not unique to vacuum breakers. 4) Reignition. Current chop, even though very small, coupled with the system capacitance and transformer inductance can impose a high-frequency transient recovery voltage (TRV) on the contacts. If this high-frequency TRV ex- ceeds the rated TRV of the breaker, reignition occurs. Repetitive reignitions can occur when the contacts part just before a current zero and the breaker interrupts at high-frequency zeros, as shown in Fig. 4. On each successive reignition, the voltage escalates. The voltage may build up and break down several times before inter- rupting. Although current-chop escalation with modern VIs is rare, a variation of this concept applies on closing called prestrike. 5) Switching inductive circuits. The transformer is a highly inductive load with an iron core. The effect of switching this inductive load and core must be considered. The current cannot change instantaneously in an inductor. Energy cannot be created or destroyed; only the form of energy is changed. The energy in the inductor is described by 1/2LI2 = 1/2CV 2 or V = I √ L/C. (1) From the energy equation, it can be seen that, for short cables, C is very small, which results in a very high surge impedance √ L/C. Energizing a cable produces a travel- ing wave which reflects when it meets the discontinuity in surge impedance between the cable and the transformer. The surge impedance of a cable may be under 50 Ω, while the surge impedance of the transformer is 300–3000 Ω. In theory, the reflection can be as high as 2 per unit.
710 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 5. Important circuit elements for EMTP modeling. Vacuum circuit breakers are prone to current chopping and voltage reignition while SF-6 circuit breakers are more prone to just current chopping. Air circuit breakers are not prone to either of any significant magnitude. Manufacturers design vacuum-circuit-breaker contacts to minimize the severity and occurrence of abnormal switching leading to severe voltage surges (the lowest current-chop characteristics are 3–5 A). Regardless of the circuit-breaker manufacturer, voltage surges do occur. E. Characteristics of the Voltage Transient and Transformer Limits The voltage transient that develops following the vacuum- circuit-breaker switching is influenced by three factors: stored energy, dc offset, and the oscillatory ring wave. The voltage component is due to the stored energy. The dc offset is de- termined by the X/R ratio of the cable and the transformer. The oscillatory ring wave is a result of the capacitance and inductance of the cable and the transformer. The magnitude of the voltage transient is compared with the transformer BIL. If the magnitude is excessive, then the transformer winding will likely fail line to ground. If the voltage transient has excessive rate of rise (dv/dt), then the transformer winding will likely fail turn to turn (natural frequency of ring wave). For the transformer to survive the transient, the insulation must be able to withstand both the magnitude and the dv/dt. Dry-type trans- formers are particularly susceptible to vacuum or SF-6 breaker switching transients. However, oil- or liquid-filled transformers are not immune. The oil has capacitance and acts like a surge capacitor to slow the rate of rise of the voltage transient. The trend in modern-day power systems is to install trans- formers with high-efficiency design. As a result, these high- efficiency transformers have a very small resistance, which offers little or no damping to the voltage transient. Also, the repetitive effect, i.e., small indentations in the insulation, can occur with each successive peak of the voltage transient. II. PREDICTING PERFORMANCE WITH SIMULATIONS A. Modeling the Circuit When a statistical approach is taken for switching transients, complex modeling requires a frequency-dependent transformer model and an arc model of the circuit breaker. For purposes of screening applications for potentially damaging switching transients, a simpler approach is suggested with the important Fig. 6. Simplified electrical system for ship propulsion drive. circuit elements modeled in electromagnetic transients program (EMTP) consisting of the source, breaker, cable, and trans- former, as shown in Fig. 5. The cable is represented by a Pi model consisting of the series impedance and half of the cable charging at each end. In some cases, multiple Pi models are used to represent the cable. The vacuum or SF-6 breaker is represented by a switch with different models for opening (current chop), restrike (excessive magnitude of TRV), reig- nition (excessive frequency of TRV), and closing (prestrike). The three-phase transformer model consists of the leakage im- pedance, magnetizing branch, and winding capacitances from high to ground and low to ground. For oil-filled transformers, the oil acts like a dielectric so the high-to-low capacitance is modeled. In cases requiring more detail, the transformer saturation and hysteresis effects are modeled. The choice of the integration time step will depend upon the anticipated frequency of the voltage transient. If it is too large, the time steps will “miss” the frequency effects. If it is too small, then this will lead to excessive simulation times. The Nyquist criteria call for a minimum sample rate of twice the anticipated frequency. In switching transients, the anticipated frequency is 3–25 kHz. When the circuit breaker opens, the transformer primary winding is ungrounded. Also, the ring wave is a function of the natural frequency of the circuit fnatural = 1/(2π √ LC). (2) The iron core of the transformer dominates the inductance of the circuit. The capacitance is very small for the dry-type transformer and short cable. Consequently, the circuit’s natural frequency is 3–25 kHz with relatively short cables.
SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 711 Fig. 7. Matching the simulation to field measurements. B. Mitigating the Switching Transient Various surge protection schemes exist to protect the transformer primary winding from vacuum-breaker-switching- induced transients. A surge arrester provides basic overvolt- age protection (magnitude only). The arrester limits the peak voltage of the transient voltage waveform. The surge arrester does not limit the rate of rise of the TOV. A surge capacitor in combination with the surge arrester slows down the rate of rise of the TOV in addition to limiting the peak voltage but does nothing for the reflection or dc offset. The number of arrester operations is greatly reduced because of the slower rate of rise. There is a possibility of virtual current chopping. Finally, adding a resistor to the surge capacitor and surge arrester provides damping, reduces the dc offset of the TOV waveform, and minimizes the potential for virtual current chopping. The resistor and surge capacitor are considered an RC snubber. Selecting the values of resistance and capacitance are best determined by a switching transient analysis study, simulating the circuit effects with and without the snubber. C. Matching the Model to Measurements The results obtained from simulation of switching transients in EMTP are only as good as the choice of model and data used. When available, field measurements taken during the switching transients enable verification of the EMTP model. The EMTP model can be adjusted as needed to match the actual field- measured conditions. To illustrate this approach, consider the ship propulsion electrical system in Fig. 6. The system consists of 3 × 2865-kW generators, a 4160-V three-phase bus, two 1865-kW drives/motors for forward Fig. 8. TRV leading to reignition during energization of drive transformer with and without snubber. TABLE III CURRENT CHOP AND REIGNITION CASES FOR DRIVE PROPULSION TRANSFORMER SWITCHING propulsion and identical drives for reverse propulsion, eight 1185-kVA dry-type transformers, and eight 630-A vacuum circuit breakers. The critical parameters are the vacuum circuit breaker, 50 ft of cable, and dry-type transformer of 30-kV BIL. Fig. 7 shows that the EMTP simulation results match the transients captured in the field with a high-speed power-quality meter (closing). The simulation shows 4.96 kVpeak, which is less than 30-kV BIL; however, the oscillation frequency of 20.2 kHz exceeds an acceptable limit of dv/dt. Having verified the model, a series of current-chop cases and reignition cases were run. Fig. 8 shows the TRV leading to reignition and the TRV with a snubber installed. Reignition occurs because the TRV peak, time to crest, and rate of rise of recovery voltage exceed IEEE ANSI C37.06 limits.
712 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 9. Simplified electrical distribution system for Tier III data center. The snubber reduces the TRV below the IEEE/ANSI limits for general-purpose vacuum breakers [3] and for generator breakers [4]. Table III summarizes the reignition cases and the current-chop cases. In all cases, the snubber is effective in reducing the transient voltage. D. Borderline Case It is important to note that not all applications involving primary switching of transformers using vacuum breakers re- quire snubbers. The large majority of applications do not re- quire snubbers. Switching transient studies are conducted to determine when snubbers are needed. In this paper, the cases were selected to show different situations requiring snubbers. For the system shown in Fig. 9, the results were border- line; therefore, a snubber was still applied for reliability pur- poses. The Fig. 9 system is a Tier III data center with two 24.9-kV incoming lines, two 12.5-MVA 25/13.2-kV transform- ers, a 13.2-kV ring bus, two 2250-KW generators, and six 3750-kV cast-coil transformers. Data centers fall into the highest risk categories because of their high load density, close proximities of circuit components, highly inductive transformers (high-efficiency designs), and frequent switching. The critical parameters for the Fig. 9 system are vacuum circuit breakers, 90-kV-BIL transformers, and cable lengths ranging from 109 to 249 ft. For the cable of 109 ft, the results of opening the vacuum breaker with current chopping of 8 A are shown in Fig. 10. The TOV is as high as 123 kVpeak on phase A which exceeds the transformer BIL of 95 kV. The TOV exhibits a significant dc offset because there is very little resistance in the highly inductive circuit. The oscillation frequency of 969 Hz is slightly less than the acceptable limit. A snubber is required to reduce the peak below 95-kV BIL. The results of adding a snubber are shown in Fig. 10. Note the significant reduction in the dc offset. The resistor in the snubber provides the reduction in dc offset as well as damping. The peak is reduced to 28.6 kV and an oscillation of 215 Hz, Fig. 10. TOV initiated by current chop during de-energization of cast-coil transformer with and without snubber. both within acceptable limits. Finally, field measurements were taken after the snubber was installed to ensure that the snubbers performed as designed. The field test setup for the snubber performance measurements is discussed in Section IV. The field measurements showed that the snubber limited the TOV within acceptable limits. E. Case of Switching a Highly Inductive Circuit Now, consider the vacuum breaker switching of a highly inductive circuit, such as the starting current of a large grinder
SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 713 Fig. 11. Simplified electrical distribution system for LMF. motor or an electric arc furnace. The vacuum-circuit-breaker switching of an electric arc furnace and ladle melt furnace transformers raises concern because of their high inductive cur- rents. High-frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple reignitions. As an example, the arc furnace circuit of Fig. 11 consists of a 50-MVA power transformer, 2000-A SF-6 breaker, 56-MVA autoregulating transformer, 1200-A vacuum breaker, and 50-MVA furnace transformer. The switching of the SF-6 and vacuum breaker was studied. The vacuum breaker, because of the 28-ft bus to the furnace transformer, was the worst case. The results opening the vacuum breaker with and without snubbers are show in Fig. 12. The TOV of 386 kVpeak exceeds the transformer BIL of 200 kV, and the oscillation of 1217 Hz exceeds the acceptable limit. Application of the snubber results in a TOV of 56.4 kVpeak that is below the transformer BIL, and the oscillation of 200 Hz is below the acceptable limit. The results for cases involving current chop and reignition are given in Table IV. F. Concerns for the Pulp and Paper Industry The previous examples illustrate that circuit-breaker-induced switching transients can fail transformers for specific com- binations of circuit parameters and breaker characteristics. Fig. 12. TOV during de-energization of LMF transformer with and without snubber protection. The examples show that the problem is not unique to one industry, application, vendor’s breaker, or transformer design. For the pulp and paper industry, there are many situations where circuit-breaker-induced switching transients are likely to damage transformers. The following examples are some of the more common scenarios encountered in the pulp and paper industry. 1) Vacuum breaker retrofit for primary load break switch in a unit substation. In the pulp and paper industry, there are numerous unit substation installations with primary load break fused switch and no secondary main breaker. This arrangement results in arc Flash issues on the low- voltage secondary. Limited space on the low-voltage side prevents installation of a secondary main breaker to mitigate the arc Flash issues. Retrofitting a vacuum circuit breaker in the primary of the unit substation, in place of the primary load break switch, and sensing on the secondary is a solution that provides both primary and secondary fault protection [5]. Unit substations may have oil-filled or dry-type transformers. The secondaries may be solidly grounded or resistance grounded. With the vacuum breaker closely coupled to the transformer, surge arresters and snubbers are most likely needed. 2) Vacuum breaker and rectifier (or isolation) transformer installation. Rectifier transformers are installed to serve dc drives such as those needed for feed water pumps to the boilers. Also, isolation transformers are installed to serve a large VSD or groups of smaller drives. Primary voltages may be 13.8 or 2.4 kV, and secondary voltages
714 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 may be 600 or 480 V. In both situations, vacuum breakers are installed in the primary and closely coupled to the transformer through a short run of bus or cable. Often, these transformers are inside and of dry-type design. 3) New unit substation with primary vacuum breaker. Recently, a paper mill installed a new metal-enclosed vacuum switchgear and a new 13.8/2.4-kV 7500-kVA transformer for a bag house for the generator boilers to meet Environmental Protection Agency requirements. The vacuum breaker was connected to the transformer through 5 ft of bus. While doing the coordination and arc Flash studies, the switching transient issue was identified. The equipment was installed and was awaiting startup and commissioning when the studies raised the concern. Before energizing the transformer, snubbers were quickly sized, obtained, designed, and installed. The screening criteria previously mentioned identify the aforementioned examples for potential damaging switching transient voltages due to vacuum breaker switching. The vacuum breaker, short distance to transformer, and dry-type transformer (or aged oil-filled transformer) are key variables to consider. With such short distance between breaker and transformer, most of these installations will require snubbers. One might conclude that standard snubbers could be applied. However, a switching transient study is still recommended to determine the unique characteristics of the circuit and custom design the snubber for the application. Given the limited space in each of these examples, it is unlikely that off-the-shelf standard snubbers would fit. A substantial part of the design effort includes determining how to best fit the snubbers into the new or existing unit substation or transformer enclosure. III. DESIGNING THE SNUBBER The preceding analysis has shown that, in some cases, switching transients can produce overvoltages that can result in equipment insulation failure. If the results of the switching transient study indicate a risk of overvoltage greater than the BIL of the equipment and/or if the dv/dt limits are exceeded, a surge arrester and snubber should be applied. The switch- ing transient study may also indicate that multiple locations require surge arresters and snubbers to protect the generator, transformer, or large motor. Additionally, the study specifies the necessary protective components and determines how close the protection must be placed to provide effective protection. A. Design Requirements At this point, custom engineering design determines how to best provide the protection needed for the equipment. The fol- lowing questions must be answered to ensure that the snubber design meets all criteria and specifications. 1) Is the switching transient protection cost effective? 2) What is the value of the equipment being protected? 3) What is the cost of lost production if the equipment fails from switching transients? 4) Can the protection be installed within existing equipment enclosures? Fig. 13. Typical snubber and arrester arrangement for transformer protection. Fig. 14. Example of standard snubber package for transformer. 5) Will equipment modification void equipment warranties? 6) Are there standard equipment packages that can accom- plish the switching transient protection? 7) Is the application an indoor or outdoor application? 8) Is there available space to mount the protective equipment in an external enclosure? 9) How can it be verified if the switching transient protection components are working effectively? 10) What alarms are necessary if the protective equipment components fail? B. Custom Engineering Design Fig. 13 shows the typical snubber arrangement for trans- former protection. A noninductive ceramic resistor and a surge capacitor are the basic components of a snubber design. Resistance values typically range from 25 to 50 Ω. Standard capacitor ratings that range from 0.15 to 0.35 μF are the basis of the design. Fig. 14 shows a standard 15-kV surge protection package. The arresters are mounted on the top of the enclosure. A three- phase surge capacitor is mounted on the bottom. Insulators and bus are located in the center. The cables can enter from top or bottom. A ground bus is located on the center right. If space heaters are required for outdoor locations, they are located on the lower left. Fig. 15 shows one phase of a custom snubber circuit. The custom design was required because there was not enough room in the transformer for the snubber components. The enclosure
SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 715 TABLE IV CURRENT CHOP AND REIGNITION CASES FOR LMF TRANSFORMER SWITCHING Fig. 15. 15-kV snubber mounted above the transformer. had to be mounted above the transformer. The cable connec- tions from the transformer were field installed and land on the copper bus. A 15-kV nonshielded jumper cable was used to make the connection. Each phase passed through an insulation bushing to the transformer below. Bus work was required to provide a solid support for the fragile resistors. Normally, only one resistor would be provided, but for this application, to achieve the delivery schedule, parallel resistors were designed to obtain the correct ohmic value (the correct single resistor value had long delivery). Fig. 16(a) and (b) shows a snubber assembly mounted in medium voltage switchgear. The photo on the left shows the single-phase surge capacitors mounted vertically. The black cylinders are ceramic resistors. A variety of options are avail- able to detect if the snubbers are functional. They range from nothing (oversized but treated like a lightning arrester) to very sophisticated loss of circuit detection. Glow tube indicators are shown at the top of Fig. 16(a), and a close-up is shown in Fig. 17(a). These glow tubes are visible through a window in the switchgear door and provide a visual indication of snubber Fig. 16. 15-kV snubber mounted in switchgear with glow tubes and current sensors. continuity. The purpose of the blue current sensors at the bottom of Fig. 16(a) is to monitor the continuity of the resistor and fuse (optional) and alarm on loss of continuity. A close-up of the current sensor is shown in Fig. 17(b). Some industries mandate fused protection. If there should be a broken resistor or a blown fuse, an alarm signal can be sent to the plant distributed control system or supervisory control and data acquisition system to alert the operating personnel that these snubber components have failed. Fig. 16(b) shows a continuation of the same snub- ber assembly. Three fuses are attached to the tops of the resistor. The fuses will isolate any fault that may occur in the snubber assembly and prevent loss of the breaker circuit. C. Special Design Considerations The nature of high-frequency switching transients requires special design considerations. The snubber designer should
716 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 17. 15-kV snubber visual indication (glow tubes) and continuity verifi- cation (current sensors). consider the location of the switching transient source when de- veloping the custom design layout of the protective equipment. Abrupt changes in the electrical path should be avoided. A low inductive reactance ground path should be designed, using non- inductive ceramic resistors and flat tin braided copper ground conductors. The minimum clearances of live parts must meet or exceed the phase-to-phase and phase-to-ground clearances of NEC Table 490.24. The enclosure should be designed to meet the requirements of IEEE Standard C37.20.2 1999. When the enclosure is mounted greater than 10 ft from the equipment to be protected, NEC tap rules may apply to the cable size required and additional circuit protective devices may be required. D. Custom Designs for the Pulp and Paper Industry As mentioned previously, given the limited space in each of the examples related to the pulp and paper industry, it is unlikely that off-the-shelf standard snubbers would fit. Instead, a substantial part of the design effort includes determining how to best fit the snubbers into the new or existing unit substation or transformer enclosure. Following are three examples of the custom design effort needed for snubber installation. 1) 13.8-kV Solidly Grounded System in Paper Mill. A vac- uum breaker was retrofitted into the enclosure for the primary load break switch of the unit substation with a dry-type transformer. The space for the snubber was extremely limited, as shown in Fig. 18 (left). Because the system was solidly grounded, the voltage on the surge capacitor was limited to 8 kV line to ground; therefore, it was possible to use a three-phase surge capacitor. The tight clearance required the use of glastic to insulate the components at line potential from ground. 2) 13.8-kV Resistance Grounded System in Paper Mill. An- other example of retrofitting a vacuum breaker for a primary load break switch with a 30-year-old oil-filled transformer. Because this snubber was needed immedi- ately, the only available resistors had to be paralleled to obtain the desired resistance, as shown in Fig. 18 (right). Again, glastic was used for insulation and to support the resistors. 3) 13.8-kV Low Resistance Grounded System. Snubbers were provided for a new metal-enclosed vacuum switchgear and a new 13.8/2.4-kV 7500-kVA transformer Fig. 18. Snubber with (left) three-phase and (right) single-phase surge capac- itors for 13.8-kV paper mill application. Fig. 19. Snubber for metal-enclosed 13.8-kV vacuum breaker. for a bag house. Single-phase surge capacitors were used. Single resistors of the right ohmic value were available. Adequate clearance did not require the use of glastic as shown in Fig. 19. IV. MEASUREMENTS TO VERIFY SNUBBER PERFORMANCE Following the installation of the snubbers, power quality measurements may be taken to ensure the proper operation of the snubbers. A high-speed scope or power quality disturbance analyzer should be used to measure the TOV waveforms at the transformer primary produced during switching of the primary vacuum circuit breaker. The measurements are used to verify that the waveforms do not exhibit excessive high-frequency transients (magnitude, rate of rise, and frequency). The test measurement setup generally consists of voltage dividers and a transient recording device. The voltage dividers should be made of capacitive and resistive components with a bandwidth of 10 MHz. The scope or power quality meter should be capable of transient voltage wave shape sampling,
SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 717 Fig. 20. Snubber performance measurement setup for 15-kV transformer using high bandwidth voltage dividers. Fig. 21. Voltage divider connections at 18-kV surge arrester on transformer primary bushing. 8000-Vpeak full scale, and 200-ns sample resolution (5-MHz sampling). An outage and lockout/tagout are needed to install the volt- age dividers, make connections to each of the three phases at transformer primary bushing, and make secondary connections to the transient recorder. Fig. 20 shows the test measurement setup for a typical cast-coil transformer. Voltage divider con- nections to the transformer primary were made, as shown in Fig. 21. Additionally, for tests requiring load at a transformer, a portable load bank may be connected to the transformer sec- ondary. A resistive load bank configurable to different load levels (300 or 100 kW) prevents destructive testing. V. CONCLUSION This paper has reviewed recent transformer failures due to primary circuit-breaker switching transients to show the sever- ity of damage caused by the voltage surge and discuss common contributing factors. Next, switching transient simulations in EMTP were presented to illustrate how breaker characteristics of current chopping and restrike combine with critical circuit characteristics to cause transformer failure in unique situations. In these limited instances, mitigation of the transients is accom- plished with snubbers custom designed to match the specific circuit characteristics. Design and installation considerations were addressed, particularly the challenges of retrofitting a snubber to an existing facility with limited space. Finally, the performance of the snubbers is verified with field measurements at the medium-voltage primary winding of the transformer. REFERENCES [1] ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switch- ing Transients Induced by Transformer and Switching Device Interaction. C57.142-Draft. [2] D. Shipp and R. Hoerauf, “Characteristics and applications of various arc interrupting methods,” IEEE Trans. Ind. Appl., vol. 27, no. 5, pp. 849–861, Sep./Oct. 1991. [3] Standard for AC High-Voltage Generator Circuit Breakers on a Symmetri- cal Current Basis, C37.013-1997, 1997. [4] Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers, C37.011-2005, 2005. [5] D. Durocher, “Considerations in unit substation design to optimize reliabil- ity and electrical workplace safety,” presented at the IEEE IAS Electrical Safety Workshop, Memphis, TN, 2010, Paper ESW2010-3. David D. Shipp (S’72–M’72–SM’92–F’02) re- ceived the B.S.E.E. degree from Oregon State Uni- versity, Corvallis, in 1972. He is a Principal Engineer with the Electrical Services and Systems Division, Eaton Corporation, Warrendale, PA. He is a distinguished scholar in power system analysis and has worked in a wide variety of industries. He has spent many years per- forming the engineering work associated with his present-day responsibilities, which include a wide range of services covering consulting, design, power quality, arc flash, and power systems analysis topics. Over the last few years, he has pioneered the design and application of arc-flash solutions, modifying power systems to greatly reduce incident energy exposure. He has written over 80 technical papers on power systems analysis topics. More than 12 technical papers have been published in IEEE Industry Applications Society (IAS) national publications and two in EC&M. He spent ten years as a professional instructor, teaching full time. He occasionally serves as a legal expert witness. Mr. Shipp is currently the Chair for the IEEE Industrial and Commercial Power Systems-sponsored Working Group on generator grounding. He has received an IEEE IAS Prize Paper Award for one of his papers and conference prize paper awards for six others. He is very active in IEEE at the national level and helps write the IEEE Color Book series standards. Thomas J. Dionise (S’79–M’82–SM’87) received the B.S.E.E. degree from The Pennsylvania State University, University Park, in 1978, and the M.S.E.E. degree with the Power Option from Carnegie Mellon University, Pittsburgh, PA, in 1984. He is currently a Senior Power Systems Engi- neer with the Power System Engineering Depart- ment, Eaton Corporation, Warrendale, PA. He has over 27 years of power system experience involving analytical studies and power quality investigations of industrial and commercial power systems. In the metal industry, he has specialized in power quality investigations, harmonic analysis and harmonic filter design for electric arc furnaces, rectifiers, and variable-frequency drive applications. Mr. Dionise is the Chair of the Metal Industry Committee and a member of the Generator Grounding Working Group. He has served in local IEEE positions and had an active role in the committee that planned the Industry Applications Society 2002 Annual Meeting in Pittsburgh, PA. He is a Licensed Professional Engineer in Pennsylvania.
718 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Visuth Lorch received the B.S.E.E. degree from Chulalongkorn University, Bangkok, Thailand, in 1973, and the M.S. degree in electric power engi- neering from Oregon State University, Corvallis, in 1976, where he was also a Ph.D. candidate and was inducted as a member of the Phi Kappa Phi Honor Society. During his Ph.D. studies, he developed the Short Circuit, Load Flow, and Two-Machine Tran- sient Stability programs. The Load Flow program has been used in the undergraduate power system analysis class. He also prepared a Ph.D. thesis on the Load Flow program using the third-order Taylor’s series iterative method. In 1981, he joined Westinghouse Electric Corporation, Pittsburgh, PA. He was responsible for conduction power system studies, including short circuit, protective device coordination, load flow, motor starting, harmonic analysis, switching transient, and transient stability studies. He also developed the Pro- tective Device Evaluation program on the main frame Control Data Corporation supercomputer. In 1984, he joined the Bangkok Oil Refinery, Thailand, where his primary responsibility was to design the plant electrical distribution system as well as the protection scheme for the steam turbine cogeneration facility. He was also responsible for designing the plant automation, including the digital control system for the plant control room. In 1986, he rejoined Westinghouse Electric Corporation. He performed power system studies and developed the Short Circuit and Protective Device Evaluation programs for the personal computer. In 1998, he joined the Electrical Services and Systems Division, Eaton Corporation, Warrendale, PA, where he is currently a Senior Power Systems Engineer in the Power Systems Engineering Department. He performs a variety of power system studies, including switching transient studies using the electromagnetic transients program for vacuum breaker/snubber circuit applications. He continues to develop Excel spreadsheets for quick calculation for short circuit, harmonic analysis, soft starting of motors, capacitor switching transients, dc fault calculation, etc. Bill G. MacFarlane (S’70–M’72) received the B.S.E.E. degree from The Pennsylvania State Uni- versity, University Park, in 1972. He began his career in 1972 with Dravo Corpo- ration, Pittsburgh, PA, as a Power System Design Engineer supporting this engineering/construction company with expertise in the design and construc- tion of material handling systems, chemical plants, pulp and paper mills, steel facilities, ore benefaction, mine design, and computer automation systems. He provided full electrical design services in heavy in- dustrial applications, primarily in the steelmaking and coal-preparation-related sectors. His responsibilities included client and vendor liaison and supervision of engineers, designers, and drafters. From 1978 to 2003, he was a principal Process Controls Systems Engineer with Bayer Corporation, Pittsburgh. He provided design and specification of instrumentation and control systems for chemical processes. He configured programmable logic controller and distrib- uted control systems. He was a Corporate Technical Consultant for power distribution systems. He had been the principal Electrical Engineer on many major projects with Bayer Corporation. He joined the Electrical Services and Systems Division, Eaton Corporation, Warrendale, PA, in 2004. At Eaton, he has been involved in a 230-kV substation design, ground grid design for substations, as well as in short circuit, protective device coordination, load flow, and arc-flash hazard analysis studies. He has also done power factor correction and harmonics studies to resolve power quality issues.
Vacuum Circuit Breaker Transients During Switching of an LMF Transformer David D. Shipp, PE Eaton Electrical Group Power Systems Engineering Warrendale, PA Fellow, IEEE Thomas J. Dionise, PE Eaton Electrical Group Power Systems Engineering Warrendale, PA Senior Member, IEEE Visuth Lorch Eaton Electrical Group Power Systems Engineering Warrendale, PA William MacFarlane, PE Eaton Electrical Group Power Systems Engineering Warrendale, PA Senior Member, IEEE Abstract— Switching transients associated with circuit breakers have been observed for many years. With the wide-spread application of vacuum breakers for transformer switching, recently this phenomenon has been attributed to a significant number of transformer failures. Vacuum circuit breaker switching of electric arc furnace and ladle melt furnace transformers raises concern because of their inductive currents. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re- ignitions. This paper will present a detailed case study of vacuum breaker switching of a new ladle melt furnace transformer involving current chopping and re-strike simulations using the electromagnetic transients program. A technique that involves a combination of surge arresters and snubbers will be applied to the ladle melt furnace to show the switching transients can be successfully mitigated. Additionally, some practical aspects of the physical design and installation of the snubber will be discussed. Keywords- Switching Transients, vacuum breakers, SF-6 breakers, LMF transformer, EMTP simulations, surge arresters, RC snubbers. I. INTRODUCTION Electric Arc Furnaces (EAF) are used widely in the steel industry in the production of carbon steel and specialty steels. The Ladle Melt Furnace (LMF) maintains the temperature of liquid steel after tapping the EAF and facilitates changes in the alloy composition through additives. In both cases, the furnace transformer is a critical component of the furnace circuit that is exposed to severe duty. The demands of the melt cycle may result in extensive damage to the furnace transformer due to electrical failures in the transformer. With advances in technology and metallurgy, the operation of arc furnaces today is significantly different. Heats of 4 to 5 hours with periods of moderate loading have been reduced to 3 to 4 hours with consistently high loading. Accompanying the shorter heats of sustained loading are many more switching operations. Combined, these factors impose thermal and electrical stresses on the transformer. Frequent switching operations have been enabled by the development of the vacuum switch. The vacuum switch has been designed for hundreds of operations in a day, for long life and low maintenance. With the advantages of the vacuum switch, also come the disadvantages of switching transient overvoltages. Depending on the characteristics of the vacuum switch and the power system parameters, these switching transient overvoltages can be of significant magnitude and frequency to cause transformer failure. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. According to statistics compiled by one insurance company [1], the application of vacuum switches has resulted in numerous failures of arc furnace transformers. These failures rates have been reduced by the application of surge arresters, surge capacitors and damping resistors [2]. The transients produced by the vacuum circuit breaker switching of an LMF transformer and their mitigation are the focus of this paper. A. The LMF Circuit of Interest Consider the new LMF circuit of Fig. 1 that consists of a 50 MVA, 135/26.4 kV power transformer, a 2000 A SF-6 breaker, a 56 MVA, 27/10 kV auto-regulating transformer, a 1200 A vacuum breaker and a 50 MVA, 25/0.53 kV furnace transformer. The SF-6 circuit breaker is separated by 53 feet from the auto-regulating transformer. The vacuum circuit breaker is separated by 28 feet from the LMF transformer. The normal configuration (1) consists of the new LMF and the existing 4EAF operating in parallel. One alternate configuration (2) of the LMF circuit consists of 4OCB and 4EAF out-of-service with 4LTC in standby-service. A second alternate configuration (3) of the LMF circuit consists of LMF LTC out-of-service with 4LTC switched on-line to source the LMF. Each of these three possible configurations of the LMF circuit was considered. Of the three, the normal configuration results in the shortest bus length between the vacuum breaker and the LMF transformer. The normal configuration also results in the shortest bus length between the SF-6 breaker and the LMF transformer.
Transformer Failure Due to Circuit Breaker Induced Switching Transients
Transformer Failure Due to Circuit Breaker Induced Switching Transients
Transformer Failure Due to Circuit Breaker Induced Switching Transients
Transformer Failure Due to Circuit Breaker Induced Switching Transients
Transformer Failure Due to Circuit Breaker Induced Switching Transients
Transformer Failure Due to Circuit Breaker Induced Switching Transients

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Transformer Failure Due to Circuit Breaker Induced Switching Transients

  • 1. TUTORIAL Transformer Failure Due to Circuit Breaker Induced Switching Transients 2011 I&CPS CONFERENCE 2011 NEWPORT BEACH, CA MAY 1 – 4, 2011 David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 daviddshipp@eaton.com Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 thomasjdionise@eaton.com
  • 2. i TABLE OF CONTENTS 1.0 INTRODUCTION.....................................................................................1-1 2.0 CASE STUDY OVERVIEW.....................................................................2-1 3.0 EMTP MODELING & CASE STUDIES...................................................3-1 4.0 MOTOR STARTING AUTOTRANSFORMER CASE STUDY.................4-1 5.0 PT FAILURES.........................................................................................5-1 6.0 SNUBBER PERFORMANCE MEASUREMENTS ..................................6-1 7.0 SUMMARY..............................................................................................7-1 8.0 REFERENCES........................................................................................8-1
  • 3. 1-1 1.0 INTRODUCTION Switching transients associated with circuit breakers have been observed for many years. Recently this phenomenon has been attributed to a significant number of transformer failures involving primary circuit breaker switching. These transformer failures had common contributing factors such as 1) primary vacuum or SF-6 breaker, 2) short cable or bus connection to transformer, and 3) application involving dry-type or cast coil transformers and some liquid filled. This tutorial will review these recent transformer failures due to primary circuit breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromagnetic transients program (EMTP) will give case studies which illustrate how breaker characteristics of current chopping and re-strike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, especially the challenges of retrofitting a snubber to an existing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented including surge arresters, surge capacitors, snubbers and these in combination
  • 5. Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 1: Case Study Overview David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Case Study Overview: Data Center Transformer Failure Attributed to Circuit Breaker Induced Switching Transients System Configuration, Failure Analysis and RC Snubber Fix R. McFadden – JB&B – NY, NY D. Shipp – Eaton Electrical – Pgh, PA Presentation Purpose/Summary • Purpose #1 Safety: – Alert Design & User Community – Drive Solutions Through Market Interest • Unique Case History – Forensic Evidence – High Speed Power Quality Capture • Presentation Summary – Simplified System Overview – Forensic Review – Power Quality Snapshots – Installed RC Snubber Fix • Presentation Summary: David Shipp, Eaton – Phenomenon Detailed Explanation – Science of RC Snubber Design
  • 6. Site Specifics • Utility Service: – 26kV – Double Ended Loop Through • Transformers – (6) Total – 26kV Primary, 480V Secondary – VPI – 3000kVA AA / 3990 kVA FA – 150 kVbil • Switching Device – Vacuum Circuit Breaker • Cable – 35kV, 133% EPR Insulation, 1/3 Concentric Ground M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” To Normal Side Of 480vac ATS (Typical) M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Simplified System Configuration Failure/Sequence of Events • All Transformers Fully Tested: • Pre-functional: Turns Ratio, Insulation Resistance, etc • Functional:UPS Full Load Tests, UPS Transient Tests, Data Center Room Validation Testing • Final Pull The Plug Test (PTP) • 4 Electricians “simultaneously” open (4) 26KV vacuum breakers to simulate a general utility outage. • All systems successfully transfer to standby generation but: • Loud Pop heard in Substation Room B • Relay for VCB feeding TB3 signaling trip • Decision made to shutdown generator test and investigate issue in “B” substation room • 2 Electricians “simultaneously” close (2) 26KV vacuum breakers to Substation Room “A” • Transformer TA3 fails catastrophically.
  • 7. M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” 0.8MW M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Pre-PTP Condition 1.5MW 0.9MW 1.3MW M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Failure #1: De-Energization During PTP Audible Pop In Substation Room Relay Signaling Trip M T-A3 CRACs Chillers T-A2 UPS T-A1 OTHER 40’ 80’ 100’+ 26kv Utility “A” M T-B3 CRACs Chillers T-B2 UPS T-B1 OTHER 40’ 80’ 100’+ 26kv Utility “B” Failure #2: Energization During PTP Catastrophic Failure Relay Trips VCB
  • 8. Coil to Coil Tap Burnt Off Failure #2 Transformer Failure On Energization Coil to Coil Conductor Burnt Off Suspected Area of Initial Flash Failure #2 Transformer Failure On Energization Coil to Coil Tap Burnt Off Failure #2 Transformer Failure On Energization
  • 9. Coil to Coil Tap Burnt Off Upward Twist From Lower Blast Failure #2 Transformer Failure On Energization Flash/Burn Marks Failure #1 Transformer Failure On De-Energization Close up of Flash/Burn Marks Coil to Coil not Winding to Winding Failure #1 Transformer Failure On De-Energization This Transformer was Returned to the Factory and Passed all Standard IEEE Tests!
  • 10. No Coil to Coil Cable Supports Some Burn Marks Match Point of Cable Contact 15KV Cable In 26KV Transformer Failure #1 Transformer Failure On De-Energization Some Burn Marks Match Vicinity of Cable Cable is Undamaged Failure #1 Transformer Failure On De-Energization Factory Installed Cable Support Unaffected Transformer
  • 11. Current Transient: Failure #2 Transformer Energization Event Initiated A-B Phase 19,000 amp, 3 Phase 5 Cycle CB Clearing Time Second Transformer Inrush AØ BØ CØ AØ BØ CØ Current Transient: Failure #1 Transformer DeEnergization 8000amp A-B Phase Approx ¼ Cycle C Phase Shows 2nd Xfmr Load & Event Duration Breaker Induced Switching Transients
  • 12. Case 1 Hydro Dam, MT 2005 • MV Vac Bkr Replacements Vendor “A” • 13.8 kV • 20 feet of cable • 50 kV BIL (W) ASL Dry Type Txmrs • Customer Energized before Vendor OK • Txmr Failed • No Surge Protection Applied CASE 2 Cleveland Hospital 3/06 • Vacuum Breakers – Vendor “A” and Vendor “C” • 13.8 kV • 95 kV BIL • Dry Type Txmr • 27 feet of Cable • Bkr Manufacturer Paid to Repair Failed 2500 KVA Txmr • Surge Protection Added Afterwards CASE 3 RAILROAD SUBSTATION 11/06 • Vacuum Breaker – Vendor “A” • 26.4 kV • 150 kV BIL • Generic Liquid Filled Rectifier Txmr • 37 feet of Cable
  • 13. CASE 4 NJ DATA CENTER 12/06 • 26.4 kV – Vendor “B” • 4 Txmrs Switched Under Light Load • 2 Txmrs Failed-1 on Closing/1 on Opening • 40 Feet of Cable • 2 other Txmrs Did Not Fail - 80 Feet of Cable • Arresters Were Present CASE 5 OIL FIELD – AFRICA 6/07 • Vacuum Breaker – Vendor “D” • 33 kV • 7 Feet of Cable • Dry Type Txmr in 36 Pulse VSD • Arresters Were Applied • Txmr Failed Upon Energization CASE 9 Oil Drilling Ship – 6/2002 • Vendor “A” IEC Vac Bkrs in Vendor “D” Swgr • 11kV, 60 HZ • Cast Coil Dry Type IEC VSD Propulsion Txmr Failed (7500 kVA) – 75 kV BIL? • < 30 feet of Cable • Fed from Alternate Bus – Now 80 feet of Cable • No further Failures Reported
  • 14. CASE 11 Hospital – Slide 1 • 13. 8 kV System • Vendor C Breakers/ Vendor F Txmrs • 2 Txmrs Failed During Commissioning • Differential Relay Tripped During motor Starting – under Highly Inductive Load CASE 11 Hospital – Slide 2 CASE 11 Hospital – Slide 3
  • 15. CASE 12 Motor Starter Auto Txmr Failure • 4160V • 5000 HP • Reduced Voltage Auto Txmr Failed • Arresters on Wye Point • Internal Resonance • Layer Wound • Failed Layer to Layer CASE 12 Run Contactor Closes 3516 HZ CASE 12 - Sweep Frequency Test 4500 HZ (Admittance)
  • 16. CASE 12 - Run Contactor Closes – 844 HZ With Snubber SWITCHING TRANSIENTS DUE TO VACUUM / SF6 BREAKERS • Opening -- Current Chop • Closing -- Prestrike/Re-ignition/Voltage Escalation • Vacuum Bkrs --Both Closing and Opening • SF6 -- Opening • Air -- Generally Acceptable Current Chop
  • 17. Current Chop Current Chop in Vacuum is a Material Problem Current Chop • All Types of Interrupters Chop Current • This is not a Unique Feature of Vacuum Switching Inductive Circuits • Closing • Opening • Voltage Escalation • Surge Suppression
  • 18. Switching Inductive Loads CLOSING Switching Inductive Loads OPENING Switching Inductive Loads VOLTAGE ESCALATION
  • 19. SWITCHING TRANSIENT THEORY • “Thou Shalt Not Change Current Instantaneously in an Inductor” • Conservation of Energy – – You Cannot Create or destroy Energy – You Can Only Change Its Form ENERGY EQUATION 22 2 1 2 1 CVLI  OR C L IV  TOTAL VOLTAGE • Vt = Venergy + Vdc + Vosc • Venergy is from the Energy Equation • Vdc = DC Off-set that Sometimes is Present • Vosc = the Oscillatory Ring Wave
  • 20. TRANSFORMER LIMITS • Magnitude – BIL Ratings • Dv/dt Limits • Both MUST Be Met • Dry Type Txmrs Particularly Susceptable • Liquid Filled Not Immune. • Consider “Hammer Effect” ANALYSIS • When Open Bkr, Txmr is Left Ungrounded • Ring Wave is a Function of its Natural Frequency LC NF 2 1  CASE 3 Waveforms Without Snubbers
  • 21. CASE 3 Current – Without Snubbers Ichop CASE 3 Waveforms With Snubbers CASE 4 Data Center 12/06 • 26.4 kV • Vendor “B” Breakers • 4 Bkrs Switched at Once • 2 Dry Type Txmrs Failed (40 Ft of Cable) • 2 Txmrs Did not Fail (80 Ft of Cable) • Unfaulted Txmr Winding Failed BIL @162 kV ( Rated 150 kV)
  • 22. CASE 4 Data Center, NJ 12/06 - W/O Snubbers 08-Jan-07 14.53.47 0 5 10 15 20 25 -200 -150 -100 -50 0 50 100 150 200 v [kV] t [ms](42) XFMA3C (48) XFMB3C Plot of the Transformer A3 and B3 primary voltage Phase C Open circuit breaker with chopped current of 9 Amperes -207 kV CASE 4 Data Center, NJ 12/06-With Snubbers 09-Jan-07 10.50.25 0 5 10 15 20 25 -40 -30 -20 -10 0 10 20 30 40 v [kV] t [ms](29) XFMA3B (35) XFMB3B Plot of the Transformer A3 and B3 primary voltage Phase B Open circuit breaker with chopped current of 9 Amperes Application of the R-C snubber circuit -39 kV CASE 6 Data Center 2, NJ 12/06 • 13.2 kV • Vendor “B” Breakers • 3 MVA Dry Type Txmrs • 60 ft Cable – Required Snubbers • 157 ft Cable – Required Snubbers • No Problem at Startup
  • 23. CASE 6 13.2 kV SYSTEM – 119 kV @ 678 HZ CASE 6 WITH SNUBBERS – 33.6 kV @ 236 HZ CASE 7 Chemical Plant, NC 3/07 • 12.47 kV System • 20+ Year Old Oil Filled Txmrs • Vendor “A” Vacuum Bkrs retrofitted on Primary • 10 Feet of Cable • No Problem at Startup
  • 24. CASE 7 425 kV - 12. 47 kV - 23 kHZ Ring Wave CASE 7 Added Snubbers – 12.47 kV CASE 8 DATA CENTER – Indiana 6/07 • 12.47 kV System • Vendor “A” Breakers • 270 Feet of Cable • No Additional Surge Protection Required
  • 25. CASE 8 -55 kV at 800 HZ CASE 11 Hospital 186.9 kV / 2000 HZ CASE 10 – Snubber Reduced to 33.8 kV / 368 HZ
  • 26. MITIGATING TECHNIQUES • Arresters • Surge Capacitors • Snubbers (RC) • Hybrid / Combinations • Liquid vs Dry Type • Electronic Zero Crossing Switching Snubber - 3 Phase Capacitor Generally Solidly Grounded System • Paper Mill - AL • 13.8 kV Snubbers – 2nd paper mill - beta site
  • 27. Snubber – Single Phase Capacitors Low Resistance Grounded Systems • Paper Mill • Vendor “A” Swgr • 13.8 kV Snubber – Single Phase Capacitors • Silicon “Chip” Plant • Montana • Very Specialized Dry Type Txmrs • 13.8 kV • Cables < 100 feet • Primary Fused Switch AF Solution • Vendor “A” Vac Bkrs Capacitor Resistor Fuse Lightning Arrestor Blown Fuse Viewing Window RC Snubber Installed – Case 4
  • 28. RC Snubber Installed – Case 4 Capacitor Resistor RC Snubber Installed – Case 4 Fuse Blown Fuse Indicator SWITCHING TRANSIENT STUDY • Quantifies Problem • Predict Exposure / Risk • Select Best / Most Cost Effective Solution • Do “What if” Cases • Verify Results
  • 29. RECOMMENDATIONS • Factor into Design Up-front • Do Study – Results Are Bkr Manufacturer Specific • Use Protection Only When / Where Needed (if not there, cannot fail) • Fused or Unfused Snubbers?? • Loss of Fuse Detection?? • Fear Not! - Mitigating Techniques Have Been Proven. • Discrete Snubber Components?? Conclusion/Next Steps • This is a System Problem – Transformer, Cable, Switching Device, Proximity – Statistical Event , Possible Undetected Failures • Data Centers Fall into the Highest Risk Category – High Power Density – Close Proximities – Frequent Switching • Draft IEEE C57.142 – A Guide To Describe The Occurrence and Mitigation Of Switching Transients Induced By Transformer And Switching Device Interaction – Does not accurately warn users of all areas of concern – This case did not meet the areas of concern noted in Draft C57.142 – Need to push for formalization of the standard with new lessons learned • RC Snubbers – Transformer Manufacturer appears best positioned to implement the solution – Limited Cataloged Product (One Manufacturer) – Not embraced by all transformer manufacturers – Design Parameters of RC Snubber not clearly defined • Lives, Property and Uptime are all at risk QUESTIONS ?
  • 30. 3-1 3.0 EMTP MODELING & CASE STUDIES
  • 31. 1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 2: EMTP Modeling and Case Studies David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Transient Analysis Tools • Electromagnetic Transients Program (EMTP) • Developed by Hermann Dommel brought to BPA • Major contributors: Scott Meyer & Dr. Liu at BPA • Alternative Transients Program (ATP) • Alternative to commercialized EMTP • Free to all if agree not to commercialize it • EMTP-RV • PSCAD EMT/DC 3 3 ATP • ATP Draw interface • 3-phase modeling • Solution method - Trapezoidal integration • Robust for wide variety of modeling needs • Extensive library of models • Many options and features
  • 32. 2 4 4 Source Model • “3-phase wye cosine” • Resistance in ohms • Inductance in mili-Henries • Source Impedance • Z1 and X/R • Z0 and X/R SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT 5 5 Cable Model • “Pi Model” • Resistance in ohms • Inductance in mili-Henries • Cable charging in micro-Farads • ½ charging on sending end • ½ charning on receiving end • Multiple pi models in some cases CABLE 13.8KV C1 LCABLERCABLE C2 C/2C/2 6 6 Breaker Model • “Switch” • Time dependent switch • Open or close at a specific time • Opens at current zero unless specify otherwise • Current dependent switch • Needed for current chopping • Define current at which to open switch • Independent Poles (A, B and C independent) • Data request • Vacuum or SF6 breaker nameplate • Current chop (ask manufacturer) • Transient Recovery Voltage Ratings – T2, E2 and RRV VCB BKR
  • 33. 3 7 7 Transformer Model • “Three Phase Model” • Resistance in ohms • Inductance in mili-Henries • Magnetizing current • Winding capacitance • CH and CL for dry-types • CHL for oil-filled • More detailed modeling • Saturation • Hysteresis • Data Request • %Z and X/R, MVA • BIL TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 8 8 Complete System Model TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 CABLE 13.8KV C1 LCABLE RCABLE C2 C/2C/2 SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT VCB BKR 9 9 Time Step • Time Step – Integration Time Step • Choice depends on frequency of expected transient • Too large – miss high frequency effects • Too small – excessive simulation times • Nyquist Criteria • Minimum sample rate = 2 x frequency • Example 10-20KHz ring for VCB transient
  • 34. 4 10 10 Switching Transient Simulations • Switching transient simulations • EMTP and model requirements • Case studies • Data Center • Ferry Propulsion • Laddle Melt Furnace • Successful techniques/solutions 11 11 Ferry Propulsion Highlights • 60K passengers per day • 20M passengers per year • 4160V distribution • 3 x 2865kW diesel gens • 2 x 1865KW forward propulsion motors • 2 x 1865KW reverse propulsion 12 12 Problem • Forward propulsion drives ferry • Upon reaching dock, reverse propulsion stops ferry • July 1, 2009, reverse propulsion was lost entering dock • lost power and hit a pier at full speed • one serious injury and nine minor injuries • 750 to 800 passengers were aboard • impact did not send any passengers overboard • 1185KVA rectifier transformer failed on reverse propulsion motor • Evidence of insulation failure on first few turns of primary winding • Investigation into source of such failure VCB switching
  • 35. 5 13 13 Ferry - Oneline Vacuum Breaker Short Cable Dry-Type Transformer Forward Reverse 14 14 Ferry – Electrical Highlights • 3 x 2865kW diesel generators • 4160V, 3-bus system • Vacuum circuit breakers – 630A mains, ties, feeders • 2 x 1865kW motors (forward propulsion) • 2 x 1865kW motors (reverse propulsion) • 8 x 1185KVA dry-type transformers, 30kV BIL • 3-winding rectifier transformers • 12-pulse effective (6-pulse VFD per winding) • Feeder cable lengths of 50 feet each 15 15 Worst Case Scenarios • Feeder cable lengths of 50 feet • Each 1185KVA transformer has one 4160V feeder • Need to examine both feeders for each transformer • “Worst Case” Switching Transient Simulations • Close 4160V VCB to transf. with 50ft. cable (model validation) • Close 4160V VCB to transf. with 50ft. cable (re-ignition) • Open 4160V VCB to transf. with 50ft. cable (current chop) • Repeat each case with Snubber
  • 36. 6 16 16 Measurements Transient Overvoltages at Primary of Rectifier Transformer – VCB Closes 17 17 Study Cases • Case 1 – Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer with the distance of 50 feet. Simulated to match the Siemens measurements to ensure that the computer model is accurate. (Model Validation) • Case 2 – Same as Case 1, except with the RC snubber. • Case 3 - Closing the 4.16 kV feeder breaker feeding the three winding rectifier transformer, then the 4.16 kV feeder breaker opens during the inrush current. Shows the possibility of the vacuum breaker re-ignition. (Re-ignition) • Case 4 - Same as Case 3, except with the RC snubber. • Case 5 - Open the 4.16 kV feeder breaker feeding the three winding rectifier transformer under the light load condition. (Current Chop) • Case 6 – Same as Case 5, except with the RC snubber. 18 18 Case 1 – Model Validation Transformer primary voltage V max of 4.96kV < 30kV BIL Oscillation of 20,203Hz > 1000Hz
  • 37. 7 19 19 Case 2 – Valid Model with Snubber Transformer primary voltage V max of 3.982kV < 30kV BIL Oscillation of < 1000Hz 20 20 Case 3 – Re-ignition Transient Recovery Voltage (TRV) at VCB 21 21 Case 4 – Re-ignition with Snubber Transient Recovery Voltage (TRV) at VCB
  • 38. 8 22 22 Case 5 – Current Chop Transformer primary voltage V max of 31.9kV > 30kV BIL Oscillation of 958Hz ~ 1000Hz 23 23 Case 6 – Current Chop with Snubber Transformer primary voltage V max of 8.9kV < 30kV BIL Oscillation of 299Hz < 1000Hz 24 24 Summary of Current Chop Cases
  • 39. 9 25 25 Summary of Re-Ignition Cases 26 26 Recommendations • Install snubber at primary of each 1185KVA rectifier transformer • 40ohm resistor • Non-inductive • Peak voltage – 6 kVpeak • Peak energy – 2100 Joules • Average power – 190 Watt • 0.5uF capacitor • Rated voltage - 4.16kV 27 27 Data Center Highlights • Tier III • LEEDS Certified • 12.5MVA Capacity • 13.2KV Ring Bus
  • 40. 10 28 28 Data Center - Oneline Vacuum Breaker Short Cable Cast-Coil Transformer 29 29 Data Center – Electrical Highlights • 2 x 24.9 kV lines from Factory Shoals and Buzzard Roast • 2 x 12.5 MVA transformers • 13.2 kV “ring-bus” configuration • MSA and MSB and generator bus GPS • Vacuum circuit breakers – 600A mains & ties, 200A feeders • 3 x 2250 KW generators • 6 x 3750KVA cast coil transformers, 90kV BIL • 3 x DE Subs CSA, CSB and CSC • 3 x SE Subs MDSA, MDSB and MLB • Feeder cable lengths vary from 109 to 249 Feet 30 30 Worst Case Scenario • Feeder cable lengths vary from 109 to 249 Feet • Each 3750KVA transformer has two 13.2kV feeders • Need to examine both feeders for each transformer • Shortest of all 13.2kV cable runs to 3750KVA Transformers • MSB to Transformer CSC – 109 feet • MSA to Transformer CSC – 111 feet • “Worst Case” Switching Transient Simulation • Open 13.2 kV VCB at MSB to CSC with 109ft. cable • Open 13.2 kV VCB at MSA to CSC with 111ft. cable
  • 41. 11 31 31 Worst Case – Study Case 13 • Study Case 13 - Open the 13.2 kV feeder breaker at Bus MSB feeding the 3,750 kVA dry type transformer CSC with the shorter distance of 109 feet. The result for this case will represent the “worst-case” condition, the other feeder from Bus MSA has a longer feeder distance of 111 feet. • Study Case 14 – Same as Case 13, except with the application of the 0.25 μF surge capacitor. • Study Case 15 – Same as Case 13, except with the application of the snubber circuit with 30ohm and 0.25 μF surge capacitor. 32 32 Case 13 – no surge protection Load current at 13.2kV VCB Load current of 10A Chopped current of 6A 33 33 Case 13 – no surge protection Transformer CSC primary voltage VC max of 65.3kV > 95kV BIL VB max of 116kV > 95kV BIL VA max of 123kV > 95kV BIL Oscillation of 969Hz > 1000Hz
  • 42. 12 34 34 Case 14 – 0.25uF surge cap Load current at 13.2kV VCB Load current of 10A Chopped current of 6A 35 35 Case 14 – 0.25uF surge cap Transformer CSC primary voltage VA max of 29.4kV < 95kV BIL VB max of 19.1kV < 95kV BIL VC max of 15.7kV < 95kV BIL Oscillation of 215Hz < 1000Hz 36 36 Case 15 – snubber 30ohm and 0.25uF Load current at 13.2kV VCB Load current of 10A Chopped current of 6A
  • 43. 13 37 37 Case 15 – snubber 30ohm and 0.25uF Transformer CSC primary voltage VB max of 28.6kV < 95kV BIL VA max of 19.4kV < 95kV BIL VC max of 15.9kV < 95kV BIL Oscillation of 215Hz < 1000Hz 38 38 Case 15 – snubber 30ohm and 0.25uF Snubber Current – important for duty on resistor and capacitor IC peak of 7.7A IB peak of 8.0A IA peak of 6.8A 39 39 Comparison of Results No Snubber 0.25uF surge cap Snubber R=30ohm C=0.25uF
  • 44. 14 40 40 Measured Transients with Snubber 41 41 Recommendations • Install snubber at primary of each 3750KVA cast coil transformer 30ohm and 0.25uF • Install surge caps 0.25uF at each emergency generator • Install surge arresters at the following locations: • both incoming power transformers • every distribution dry type transformer • every generator • line side of both main circuit breakers • line side of the three generator circuit breakers • load side of every feeder breaker and every tie circuit breaker 42 42 Laddle Melt Furnace Highlights • Retiring 3 x 38MW EAFs • Adding 1 x 155MVA EAF • Adding 2 x 138kV lines to new EAF • Adding SVC at 34.5kV • Adding 1 x 20MW LMF
  • 45. 15 43 43 LMF & EAF - Oneline Vacuum Breaker Short Bus Oil-Filled Transformer Existing EAF New LMF 44 44 138KV UTILITY 4713MVA 3PH SC 9.26 X/R 50/66/83MVA 135.3/26.4KV 7.5%Z SF-6 BREAKER 2000A 1600A 27KV 13OHM AUTO LTC 56MVA 27-10KV 3.3%Z ALUMINUM IPS BUS 53FEET VACUUM BREAKER 1200A LMF XFMR 50/56MVA 25/.53KV 2.5%Z HEAVYDUTY COPPER PIPE 28FEET LMF 20MW LMF Vacuum Breaker Short Bus Oil-Filled Furnace Transformer New LMF SF6 Breaker Short Bus Oil-Filled Auto-Regulating Transformer 45 45 LMF Circuit – Electrical Highlights • 50MVA Power Transformer 135/26.4kV • 27kV system • SF6 circuit breaker – 2000A • Bus length of 53 feet • 56MVA autoregulating transformer, 200kV BIL • Vacuum circuit breaker – 1200A • Bus length of 28 feet • 50MVA oil-filled LMF transformer, 200kV BIL • 20MW LMF
  • 46. 16 46 46 Worst Case Scenarios • Examine switching transients at both transformers • “Worst Case” Switching Transients for Auto-Reg Transf • Open SF6 breaker to transf. with 53ft. bus (6A current chop) • 3 cycles after energizing Auto-Reg Tran, open SF6 bkr to transf. with 53ft. bus (re-ignition) highly inductive current • “Worst Case” Switching Transients for LMF Transf • Open VCB to transf. with 28ft. bus (6A current chop) • 3 cycles after energizing LMF Tran, open VCB to transf. with 28ft. bus (re-ignition) highly inductive current • Repeat each case with Snubber 47 47 Case 1 – Open VCB LMF Transformer primary voltage V max of 386kV > 200kV BIL Oscillation of 1217Hz > 1000Hz 48 48 Case 2 – Open VCB with Snubber LMF Transformer primary voltage V max of 56.4kV < 200kV BIL Oscillation of 200Hz < 1000Hz
  • 47. 17 49 49 Case 3 – Open SF6 Breaker Auto-regulating Transformer primary voltage V max of 23kV < 200kV BIL No oscillating frequency 50 50 Case 4 – Open SF6 Breaker with Snubber Auto-regulating Transformer primary voltage VB max of 54.7kV < 200kV BIL Oscillation of 197Hz < 1000Hz 51 51 Case 5 – VCB Re-ignition Transient Recovery Voltage for VCB
  • 48. 18 52 52 Case 5 – Highly Inductive Current Inrush current to LMF transformer 6000A Peak Inrush VCB Opens VCB Closes 53 53 Case 6 – VCB Re-ignition & Snubber Transient Recovery Voltage for VCB 54 54 Case 7 – SF6 Breaker Re-ignition Transient Recovery Voltage for Siemens SF6
  • 49. 19 55 55 Case 7 – Highly Inductive Current Inrush current to Auto-Regulating transformer 14000A Peak Inrush SF6 CB Opens SF6 CB Closes 56 56 Case 8 – SF6 Breaker Re-ignition & Snubber Transient Recovery Voltage for SF6 Breaker 57 57 Summary of Current Chop Cases
  • 50. 20 58 58 Summary of Re-Ignition Cases 59 59 Recommendations • Install snubber (100ohm and 0.15uF) at primary of each transformer • 100ohm resistor • Non-inductive • Peak voltage – 38 kVpeak • Peak energy – 17,500 Joules • Average power – 1000 Watt • 0.15uF, 34.5kV surge capacitor (not available) • 1-pole, 24kV, 0.13uF • 2-pole, 14.4kV, 0.5uF • Series combination gives 0.103uF
  • 51. 4-1 4.0 MOTOR STARTING AUTOTRANSFORMER CASE STUDY
  • 52. 1 Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – Larry Farr, Senior Member IEEE Principal Engineer Eaton Electrical Asheville NC Technical Advisor TC-17A IEC Chair of CANENA TC-17A SC-3 Arthur J. Smith III, Member IEEE Vice President Waldemar S. Nelson and Co. Inc. New Orleans, LA • Background Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – TODAY’S AGENDA • Investigation • Testing 1988 • Findings •Testing 2002-03 • Findings • Solution • Conclusions • Background Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained –
  • 53. 2 • For the past century the Auto- transformer or “Korndorfer” Starter has been a standard in the electrical industry. Background • In the late 1970s, unexplained “High Voltage Stress” failures started occurring. Background
  • 54. 3 Failures Were Occurring Worldwide • North Sea Platform 12kV starters failed multiple times. Problems solved with single phase autotransformers. Background • 20,000 hp-15kV starter in British Columbia failed four times until surge arresters from the Zero tap to ground were installed. • South America 2400 volt failed multiple times. • Gulf of Mexico – Multiple 5kV starter failures • IEC 60470 Clause 6.102.7 Change-over Ability Test • Off the West Coast of Africa – Multiple 5kV starter failures 3 Inches !! What Happened? Zero Tap Circuit to Ground Background High Voltage Flashover Telescoping Coils Layer to Layer Failure Modes Background
  • 55. 4 High Voltage Flashover Tap to Tap Over the Surface Background High Voltage Flashover Coil to Ground Background High Voltage Flashover Layer to Layer Failure Background
  • 56. 5 High Voltage Flashover Zero-tap Circuit to Ground Background Telescoping Coils Background • S and R Conducting Simultaneously • Flashover of S with R Closed Background Flashover of S with R Closing
  • 57. 6 Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 Testing Initial Testing Testing Initial Testing
  • 58. 7 Testing Initial Testing Testing Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 • Findings
  • 59. 8 Design Modifications • Changed Design to Three Coil Three Legged Autotransformer Findings • Changed Control Circuit to Include a Transition on Current Below 125% FLA • HOWEVER, Failures Continued at a Rate of Two to Three a Year ! The High Voltage Stress Failures Occurred When: • Transformers were on the 80% tap Findings • Transitions Were Forced at or Near Locked Rotor Current Remaining Failures Required a Closer Look and Additional Testing Findings
  • 60. 9 Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 Additional Testing • However, The Sample Rate Was only 50,000 Samples Per Seconds and I suspect some level of smoothing. • Confirmed the Test Results From 1988 on Three Coil Three Legged Transformers (February 2002 High Power Lab in Pennsylvania) Dielectric Withstand Two Layers of .005 Nomex 14kV rms for 60 Secs. Additional Testing
  • 61. 10 Partial Discharge Additional Testing November, December and January Belmont, North Carolina Additional Testing A Closer Look Additional Testing
  • 62. 11 February Arden, North Carolina Additional Testing Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 • Findings Issue Found ! Findings
  • 63. 12 Current Flat During Voltage Escalation Findings When Reignition Occurs Findings Each Burst 70,000 Volts/Micro Sec Each Burst 70,000 Volts/Micro Sec Findings
  • 64. 13 • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 • Findings • Solution Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – New Circuit for Autotransformer Starters Solution With Surge Arresters 12-13,000 Volts at 12-18 amps for 800 Micro Sec Solution
  • 65. 14 • Background • Investigation • Testing 1988 • Findings • Testing 2002-03 • Findings • Solution • Conclusions Medium Voltage Reduced Voltage Autotransformer Starter Failures – Explaining The Unexplained – Conclusions • When controllers are configured for transition on current sensing below 125% FLA, high voltage potential is reduced but not eliminated. •When autotransformer starters are forced to transition before they reach near full speed, high voltages are generated on the 0% taps Conclusions • Vacuum contactor autotransformer starters and starter retrofits require surge arresters even if no problems were encountered with air break contactors • When 6 kV distribution surge arresters are installed on a 4,160 volt circuit from the 0% tap to ground, the voltage is clamped to 13 kV without the resultant high dv/dt across the transformer coil regardless of transition current
  • 66. 15 Conclusions • Care must be taken so the Metal Oxide arrestors do not conduct near normal operating voltages. • The 15kV Autotransformer Starter in British Columbia was set on the 55% tap. The starter used MV Circuit Breakers. Therefore all autotransformer starters need surge arrestors zero tap to ground or across the VI,s.
  • 68. 1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 4: Potential Transformer Failures David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Case Studies • Switching Transients – Opening • Switching Transients – Closing • Ferro- Resonance – Closing • Ferro-Resonance – Opening (20 HZ Saturation) • Internal Resonance 3 3 CASE 12 Midwest Data Center • 12.47 kV System / 120 MW Load • Bkr Pairs with Unloaded wye-wye PTs for Auto Transfer Sensing at Load End of Cables • Multiple Open and Closed Operations were Performed Preceding the Failure. • 1st failure – Smoke But fuses did not Blow – Cleared Manually.
  • 69. 2 4 4 CASE 12 Midwest Data Center • 2nd Failure – Identical Switching Events • Open Transitioned Back to Source “A” • A few Minutes Later A Load “Pop” Was heard. • More Smoke + B Phase Fuse Blew • Measurements Were Taken – Snuck Up on Problem without PT Loading – Risked Failure 5 5 CASE 12 Midwest Data Center 6 6 CASE 12 Closing From Generator Source – 300 kW Load
  • 70. 3 7 7 CASE 12 Opening From Generator Source – 300 kW Load 8 8 CASE 12 Primary Measurements - 300 kW Load Opening 9 9 CASE 12 Primary Measurements – Snubbers + 300 KW Load
  • 71. 4 10 10 CASE 12 - Closing Primary Measurements - 300 kW Load 11 11 CASE 12 – Closing - Snubber Primary Measurements - 300 kW Load 12 12 PT Failure – Example 13 • PT 1500VA, 14400/120V, open delta, in gen-tie swgr • System 13.8 kV wye-solidly grounded • Upstream VCB switching • Cable lengths of 250 to 3600 feet • Failures • #1 – catastrophic failure of PT in gen-tie swgr (2700ft cable) • #2 – a month later, catastrophic failure of PT in gen-tie swgr • No switching at time of failure/100s prior to failure • Possible failure modes • Ring frequency of transient overvoltage on closing • Ferroresonance (PT saturation) on opening
  • 72. 5 13 13 CASE 13 Switching Transient 1st Failure 14 14 PT transient on VCB closing (EMTP simulated) 15 15 PT - test without snubber (measured) • Transient overshoot to -17.5kVpeak • High frequency oscillation follows • Oscillation persists for more than ¼ cycle on phase-b Transient followed by high frequency ring Oscillation continues beyond ¼ cycle
  • 73. 6 16 16 PT - test without snubber (zoom view) 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 -18000 -16000 -14000 -12000 -10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 12000 Volts Recorded Volts/Amps/Hz Zoomed Detail: 04/27/2011 19:21:07.855.770.000 - 04/27/2011 19:21:07.857.283.100 Time 04/27/2011 19:21:07.854000 + us • Magnitude is not the only problem • High frequency as well as high magnitude • Overtime both will damage PT Magnitude effects – 17.5kV High frequency effects – 50kHz 11.27kVpeak nominal 17 17 PT - test with snubber • Overshoot to 12.25kVpeak • Lower frequency oscillation • Oscillation very well damped Transient just above normal crest Oscillation well damped 18 18 PT – test with snubber (zoom view) • Both magnitude and frequency are acceptable • PT is well protected by the snubber 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 -12000 -10000 -8000 -6000 -4000 -2000 0 2000 4000 6000 8000 10000 12000 Volts Recorded Volts/Amps/Hz Zoomed Detail: 04/27/2011 20:09:12.582.800.000 - 04/27/2011 20:09:12.584.990.800 Time 04/27/2011 20:09:12.580000 + us Low frequency 2000Hz 11.27kVpeak nominalLow Magnitude 12.25kV
  • 74. 7 19 19 PT saturation on VCB opening simulated measured 20 20 PT saturation on VCB opening (EMTP) 21 21 PT with 100W loading resistors VCB opening
  • 76. 1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Part 5: Snubber Performance Measurements David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 Snubber Performance Measurements • Data Center • Purpose of Measurements • Test procedures • Medium voltage measurements • Selecting a meter • Using voltage dividers • Safety concerns • Data Center challenges 3 3 Purpose of Performance Measurements • Ensure the proper operation of the snubbers • Measure the transient overvoltage waveforms at the transformer primary • produced during switching of the primary vacuum circuit breakers • verify they do not exhibit high frequency transients (magnitude, rate- of-rise and frequency). • Specify the meters and voltage dividers needed for the transient measurement • guide electrical contractor with their installation • electrical contractor to operate all equipment following the test procedure • After completion of the site monitoring during the snubber tests, analyze the measurement data and document the test results in an engineering report.
  • 77. 2 4 4 Data Center Highlights • Tier III • LEEDS Certified • 12.5MVA Capacity • 13.2KV Ring Bus 5 5 Data Center – Measurement Locations Vacuum Breaker Short Cable Cast-Coil Transformer 6 6 Data Center – Electrical Highlights • 2 x 24.9 kV lines from Factory Shoals and Buzzard Roast • 2 x 12.5 MVA transformers • 13.2 kV “ring-bus” configuration • MSA and MSB and generator bus GPS • Vacuum circuit breakers – 600A mains & ties, 200A feeders • 3 x 2250 KW generators • 6 x 3750KVA cast coil transformers, 90kV BIL • 3 x DE Subs CSA, CSB and CSC • 3 x SE Subs MDSA, MDSB and MLB • Feeder cable lengths vary from 109 to 249 Feet
  • 78. 3 7 7 RC Snubber at MVS - Typical 200 A Disconnect to transformerRC Snubber 13.8 kV 0.25 microFarad 30 ohm Cast Coil 3750KVA Transformer 8 8 RC Snubber at MVS - Typical Snubber Specs 13.8 kV 0.25 microFarad 30 ohm 9 9 Test Procedure • Test Procedure required by contractor • Prepared 2 weeks in advance of testing • developed the instructions • Included safety briefing each day • Site specifics supplied by the contractor • LO/TO instructions • Breaker operations • All meter connections made de-energized & LO/TO • No one in transformer room during tests • Signature of “Responsible Engineer” before test began • detailed test procedure form
  • 79. 4 10 10 Measurement Equipment • Measurement equipment included voltage dividers and a transient recording device • Voltage Dividers • Capacitive and resistive elements • 10MHz frequency response • Three-Phase Power Quality Recorder • transient voltage waveshape sampling • 200 nsec sample resolution • 5 Mhz sampling 11 11 Test Measurement Setup (Typical) 12 12 Voltage Divider Connections • Highly stranded No. 8, 15 kV insulated hookup wire • Insulated wire was a precaution – could have used bare conductor • maintained 8inch minimum separation for 15kV between phases and ground • Routed wires with gradual curve – no 90 degree bends • Connection at surge arrester at bus to transformer primary windings
  • 80. 5 13 13 Voltage Divider Connections 14 14 Voltage Divider Connections • Base of each divider grounded • Grounded to ground bus in primary disconnect • Used same highly stranded no. 8, 15kV insulated hookup wire • Connection to PQ Meter • Cables to match input impedance of meter to that of voltage divider • Voltage divider was calibrated prior to shipping to site 15 15 CT Connections • Primary interest was voltage at transformer • Secondary interest was snubber current • Connected CTs at ground side of snubbers
  • 81. 6 16 16 Initial view of events Energize (see zoom) De-energize (see zoom) • RMS trend of voltage • VCB energized and de-energized transformer 17 17 Zoom View of Energize • Slight overshoot above normal crest • Ring well damped 18 18 Zoom View of De-Energize • No transient on opening • Well damped
  • 82. 7 19 19 Other Tests – transformer natural frequencies • Sweep Frequency Response Analysis (SFRA) ImpedancePhaseAngle Frequency 20 20 Data Center Challenges • Contractor requested measurements • Advised contractor • First hookup/measurement – 1 day • Remaining 5 transformers – 2nd day • Prepare two voltage divider carts in advance • Contractor agreed to schedule but… • This was a construction site • Project manager had other objectives • Snubber testing viewed as an “inconvenience” • Voltage divider carts were not prepared • Be prepared to negotiate 21 21 Summary • Prepare test procedure – 2 weeks in advance • Request onelines and site photos to plan hookups • Ship all equipment in advance • For safety, two engineers perform measurements • Follow test procedure • Make all connections de-energized, LO/TO • No one in transformer room during test • Radio contact with contractor performing switching • Download data after each test • Analyze results before proceeding to next test • Begin building test report while on-site
  • 84. 1 Transformer Failure Due to Circuit Breaker Induced Switching Transients Summary David D. Shipp, PE Fellow, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 Thomas J. Dionise, PE Senior Member, IEEE Eaton Electrical Group 130 Commonwealth Dr. Warrendale, PA 15086 2 2 The Problem – In a Nutshell • Switching transients associated with circuit breakers observed for many years • Breaking opening/closing interacts with the circuit elements producing a transient • The severity of the transient is magnified by breaker characteristics • Current chopping on opening • Pre-strike or re-ignition on closing • In limited instances, the transient overvoltage exceeds transformer BIL resulting in failure • RC snubber in combination with surge arrester mitigates the transient 3 3 What to look for… “Rules of Thumb” to screen applications: • Generally, short distance between circuit breaker and transformer • about 200 feet or less • Dry-type or cast coil transformer • oil filled not immune and low BIL • Inductive load being switched • transformer, motor, etc. (light load or no load) • Circuit breaker switching characteristics: • chop (vacuum or SF6) or restrike (vacuum)
  • 85. 2 4 4 Predicting Transients – EMTP Simulations • For purposes of screening applications for damaging TOVs • Source, breaker, cable and transformer modeled • Breaker models for current chop and re-ignition TRANSFORMER RLRG LTRAN RTRANT1 T2 CH CL N:1 CABLE 13.8KV C1 LCABLE RCABLE C2 C/2C/2 SYSTEM SOURCE AT 13.8 KV XUTIL RUTIL VUTIL UN U UT VCB BKR 5 5 Designing the Snubber • 15kV typical snubber & arrester • transformer protection • non-inductive ceramic resistor • 25 ohms to 50 ohms • surge capacitor • capacitor ratings 0.15 μF to 0.35 μF • 3-phase 13.8kV solidly ground • 1-phase 13.8kV LRG R C SA TX Surge Arrester Resistor Surge Cap 6 6 Performance Measurement Equipment • Test equipment includes voltage dividers and a transient recording device • Voltage Dividers • Capacitive and resistive elements • 10MHz frequency response • Three-Phase Power Quality Recorder • transient voltage waveshape sampling • 200 nsec sample resolution • 5 Mhz sampling
  • 86. 8-1 8.0 REFERENCES Switching Transients 1. Shipp, Dionise, Lorch and MacFarlane, “Transformer Failure Due to Circuit Breaker Induced Switching Transients”, IEEE Transactions on Industry Applications, April/May 2011. 2. Shipp, Dionise, Lorch and MacFarlane, “Vacuum Circuit Breaker Transients During Switching of an LMF Transformer”, IEEE Industry Applications Society Annual Meeting 2010, October 2010. 3. ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switching Transients Induced By Transformer And Switching Device Interaction, C57.142- Draft. 4. D. Shipp, R. Hoerauf, "Characteristics and Applications of Various Arc Interrupting Methods," IEEE Transactions Industry Applications, vol 27, pp 849- 861, Sep/Oct 1991. 5. ANSI/IEEE, Standard for AC High-Voltage Generator Circuit Breakers on a Symmetrical Current Basis, C37.013-1997. 6. ANSI/IEEE, Application Guide for Transient Recovery Voltage for AC High- Voltage Circuit Breakers, C37.011-2005. 7. D. Durocher, “Considerations in Unit Substation Design to Optimize Reliability and Electrical Workplace Safety”, ESW2010-3, 2010 IEEE IAS Electrical Safety Workshop, Memphis. Ferroresonance 1. IEEE Standard Dictionary of Electrical and Electronics Terms, ANSI/IEEE Std 100-1984. 2. Hopkinson, R.H., “Ferroresonant Overvoltages Due to Open Conductors,” General Electric, 1967, pp. 3 - 6. 3. Westinghouse Distribution Transformer Guide, Westinghouse Electric Corp., Distribution Transformer Division, Athens, GA, June 1979, revised April 1986, Chapter 4 Ferroresonance, pp. 36 - 40. 4. IEEE Guide for Application of Transformers, ANSI/IEEE C57.105-1978, Chapter 7 Ferroresonance, pp. 22 – 28. 5. Distribution Technical Guide, Ontario Hydro, Ontario, Canada, May 1999, original issue May 1978, pp. 72.1-1 – 72.1-10. 6. Greenwood, A., “ Electrical Transients in Power Systems”, Wiley & Sons, 1971, pp. 91-93. 7. Kojovic, L., Bonner, A., “Ferroresonance - Culprit and Scapegoat”, Cooper Power Systems, The Line, December 1998.
  • 87. IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 707 Transformer Failure Due to Circuit-Breaker-Induced Switching Transients David D. Shipp, Fellow, IEEE, Thomas J. Dionise, Senior Member, IEEE, Visuth Lorch, and Bill G. MacFarlane, Member, IEEE Abstract—Switching transients associated with circuit breakers have been observed for many years. Recently, this phenomenon has been attributed to a significant number of transformer failures involving primary circuit-breaker switching. These transformer failures had common contributing factors such as the following: 1) primary vacuum or SF-6 breaker; 2) short cable or bus con- nection to transformer; and 3) application involving dry-type or cast-coil transformers and some liquid-filled ones. This paper will review these recent transformer failures due to primary circuit- breaker switching transients to show the severity of damage caused by the voltage surge and discuss the common contributing factors. Next, switching transient simulations in the electromag- netic transients program will give case studies which illustrate how breaker characteristics of current chopping and restrike combine with critical circuit characteristics to cause transformer failure. Design and installation considerations will be addressed, particularly the challenges of retrofitting a snubber to an exist- ing facility with limited space. Finally, several techniques and equipment that have proven to successfully mitigate the breaker switching transients will be presented, including surge arresters, surge capacitors, snubbers, and these in combination. Index Terms—Electromagnetic Transients Program (EMTP) simulations, RC snubbers, SF-6 breakers, surge arresters, switch- ing transients, vacuum breakers. I. INTRODUCTION TODAY, medium-voltage metal-clad and metal-enclosed switchgears that use vacuum circuit breakers are applied over a broad range of circuits. These are one of many types of equipment in the total distribution system. Whenever a switching device is opened or closed, certain interactions of the power system elements with the switching device can cause high-frequency voltage transients in the system. The voltage- transient severity is exacerbated when the circuit breaker op- erates abnormally, i.e., current chopping upon opening and prestrike or reignition voltage escalation upon closing. Such complex phenomena in combination with unique circuit char- acteristics can produce voltage transients involving energies Manuscript received June 4, 2010; accepted July 9, 2010. Date of publication December 23, 2010; date of current version March 18, 2011. Paper 2010- PPIC-188, presented at the 2010 IEEE Pulp and Paper Industry Conference, San Antonio, TX, June 21–23, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Pulp and Paper Indus- try Committee of the IEEE Industry Applications Society. The authors are with Eaton Electrical Group, Warrendale, PA 15086 USA (e-mail: daviddshipp@eaton.com; thomasjdionise@eaton.com; visuthlorch@ eaton.com; billgmacfarlane@eaton.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2010.2101996 Fig. 1. Simplified electrical distribution system for data center. which can fail distribution equipment such as transformers. Transformer failures due to circuit-breaker-induced switching transients are a major concern, which is receiving attention in a draft standard [1] and the focus of this paper. A. Forensic Evidence for a Unique Case Consider the case study of a new data center with a 26-kV double-ended loop-through feed to six dry-type transformers each rated at 3000 kVA AA/3390 kVA FA and 26/0.48 kV and delta–wye solidly grounded, as shown in Fig. 1. The trans- former primary winding is of 150-kV basic impulse insulation level (BIL). A vacuum breaker was used to switch the trans- former. The 4/0 cable between the breaker and the transformer was 33 kV and 133% ethylene propylene rubber. Primary arresters were installed. The transformers were fully tested, including turns ratio, insulation resistance, etc. Functional tests were completed, including uninterruptible power supply (UPS) full load, UPS transient, data center room validation, etc. In the final phase of commissioning, a “pull-the-plug” test was implemented with the following results. 1) De-Energization Failure #1. Four electricians “simultane- ously” opened four 26-kV vacuum breakers to simulate a general utility outage. All systems successfully trans- ferred to standby generation but a “loud pop” was heard in Substation Room B and the relay for the vacuum circuit breaker feeding transformer TB3 signaled a trip. 2) Energization Failure #2. Minutes later, two electri- cians “simultaneously” closed two 26-kV vacuum break- ers to substation Room A. Transformer TA3 failed catastrophically. 0093-9994/$26.00 © 2010 IEEE
  • 88. 708 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 2. Transformer failure #2 during energization. Fig. 3. Transformer failure #1 during de-energization. Failure #2 is shown in Fig. 2. Examination of the primary windings revealed that the coil-to-coil tap burnt off and the winding terminal showed an upward twist. The burn marks from the initial Flash indicated the transient concentrated on the first turns of the windings. Typically, closing the vacuum breaker to energize the transformer is the worst condition. Failure #1 is shown in Fig. 3. Examination of the primary windings revealed Flash and burn marks on the B-phase wind- ing at the bottom and middle. Those at the top indicate a coil- to-coil failure, not a winding-to-winding failure, and indicate a transient voltage with high dv/dt. Those in the middle were a result of the cable (used to make the delta connection) swinging free. Supports were only lacking for this jumper (oversight during manufacturing) which could not withstand the forces of the transient. This transformer passed the BIL test at 150-kV BIL but ultimately failed at 162-kV BIL. All six transformers and cables were identical, but only two failed during the vacuum-circuit-breaker switching. The significant difference was that the two failed units had 40 ft of feeder cable while the others had 80 or 100 ft of feeder cable. This short 40-ft cable, high-efficiency transformer, and vacuum circuit breaker proved to be the right combination to produce a damaging voltage transient on both energization and de-energization. B. History of Failures and Forensic Review The previous example is not an isolated case. Instead, it is representative of a growing number of transformer failures TABLE I HISTORY OF TRANSFORMER FAILURES RELATED TO PRIMARY VACUUM BREAKER SWITCHING due to primary switching of vacuum breakers. Table I details a history of transformers related to primary switching of vacuum breakers occurring within the past three years. In Case 1, in a hydro dam, the transformer was “value engineered” with a 13.8-kV primary-winding BIL of 50 kV. The BIL should have been 95 kV for the 13.8-kV class. The 1955 switchgear was replaced with modern vacuum breakers with only 20 ft of cable to the transformer. The user chose to energize the transformer before conducting a switching transient analysis and failed the transformer primary winding. The post mortem analysis revealed that no surge protection was applied. In Case 2, in a hospital, the vacuum breaker was close-coupled through 27 ft of cable to a 2500-kVA dry-type transformer with a 95-kV-BIL primary winding. The vacuum breakers were supplied with no surge protection because the particular vacuum breaker installed had a very low value of cur- rent chop. During vacuum breaker switching of the transformer, the transformer failed. The transformer was rewound, and surge protection/snubbers were installed. In Case 3, in a railroad substation, vacuum breakers applied at 26.4 kV were used to switch a liquid-filled rectifier transformer with 150-kV-BIL primary winding. The switching transient overvoltage (TOV) failed the middle of the primary winding. Forensic analysis determined a rectifier with dc link capacitors, and the transformer inductance formed an internal resonance that was excited by the switching. Such an LC series resonance typically fails the middle of the transformer primary winding. In Case 4, in a data center, vacuum breakers applied at 26.4 kV were used to switch six dry-type transformers with 150-kV-BIL primary windings under light load. Two transform- ers failed, one on breaker closing and the other on opening. The failed transformers were connected by 40 ft of cable to the vacuum breaker, while the other transformers had either 80 or 100 ft of cable. Arresters were in place at the time of failure, but there were no snubbers. In Case 5, in an oil field, a dry-type transformer for a variable speed drive (VSD) had multiple windings to achieve a 36-pulse effective “harmonic-free” VSD. A vacuum breaker at 33 kV was separated from the transformer by only 7 ft of cable.
  • 89. SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 709 TABLE II CURRENT CHOP VERSUS CONTACT MATERIAL Arresters were applied on the primary winding. However, upon closing the breaker, the transformer failed. Finally, in Case 6, in an oil drilling ship, vacuum breakers designed to International Electrotechnical Commission (IEC) standards were applied at 11 kV and connected by 30 ft of cable to a dry-type cast-coil propulsion transformer rated at 7500 kVA. The transformer was also designed to IEC stan- dards, and the primary winding had a BIL of 75 kV. The IEC transformer BIL is much lower than the American National Standards Institute (ANSI) BIL for the same voltage class winding. The transformer failed upon opening the breaker. C. Common Parameters The severity of the voltage surge, i.e., high magnitude and high frequency, and the damage caused by the voltage surge are determined by the circuit characteristics. The following are some “rules-of-thumb” to screen applications for potentially damaging switching transient voltages. 1) generally, short distance between circuit breaker and transformer (about 200 ft or less); 2) dry-type transformer (oil filled and cast coil not immune) and low BIL; 3) inductive load being switched (transformer, motor, etc.); 4) circuit-breaker switching characteristics: chop (vacuum or SF-6) or restrike (vacuum). D. Underlying Concepts 3) Current Chop. When a vacuum breaker opens, an arc burns in the metal vapor from the contacts, which requires a high temperature at the arc roots [2]. Heat is supplied by the current flow, and as the current approaches zero, the metal vapor production decreases. When the metal vapor can no longer support the arc, the arc suddenly ceases or “chops out.” This “chop out” of the arc called “current chop” stores energy in the system. If the breaker opens at a normal current zero at 180◦ , then there is no stored energy in the system. If the breaker opens, chopping current at 170◦ , then energy is stored in the system. Current chop in vacuum circuit breakers is a ma- terial problem. Older vacuum interrupters (VIs) used copper–bismuth. Modern VIs use copper–chromium. Most copper–chromium VIs have a low current chop of Fig. 4. Voltage escalation due to successive reignitions. 3–5 A, offering excellent interruption performance and a moderate weld strength. Table II shows the average and maximum levels of current chop for copper–chromium, copper–bismuth, and other contact materials. It should be noted that both vacuum and SF-6 interrupters current chop. Current chop is not unique to vacuum breakers. 4) Reignition. Current chop, even though very small, coupled with the system capacitance and transformer inductance can impose a high-frequency transient recovery voltage (TRV) on the contacts. If this high-frequency TRV ex- ceeds the rated TRV of the breaker, reignition occurs. Repetitive reignitions can occur when the contacts part just before a current zero and the breaker interrupts at high-frequency zeros, as shown in Fig. 4. On each successive reignition, the voltage escalates. The voltage may build up and break down several times before inter- rupting. Although current-chop escalation with modern VIs is rare, a variation of this concept applies on closing called prestrike. 5) Switching inductive circuits. The transformer is a highly inductive load with an iron core. The effect of switching this inductive load and core must be considered. The current cannot change instantaneously in an inductor. Energy cannot be created or destroyed; only the form of energy is changed. The energy in the inductor is described by 1/2LI2 = 1/2CV 2 or V = I √ L/C. (1) From the energy equation, it can be seen that, for short cables, C is very small, which results in a very high surge impedance √ L/C. Energizing a cable produces a travel- ing wave which reflects when it meets the discontinuity in surge impedance between the cable and the transformer. The surge impedance of a cable may be under 50 Ω, while the surge impedance of the transformer is 300–3000 Ω. In theory, the reflection can be as high as 2 per unit.
  • 90. 710 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 5. Important circuit elements for EMTP modeling. Vacuum circuit breakers are prone to current chopping and voltage reignition while SF-6 circuit breakers are more prone to just current chopping. Air circuit breakers are not prone to either of any significant magnitude. Manufacturers design vacuum-circuit-breaker contacts to minimize the severity and occurrence of abnormal switching leading to severe voltage surges (the lowest current-chop characteristics are 3–5 A). Regardless of the circuit-breaker manufacturer, voltage surges do occur. E. Characteristics of the Voltage Transient and Transformer Limits The voltage transient that develops following the vacuum- circuit-breaker switching is influenced by three factors: stored energy, dc offset, and the oscillatory ring wave. The voltage component is due to the stored energy. The dc offset is de- termined by the X/R ratio of the cable and the transformer. The oscillatory ring wave is a result of the capacitance and inductance of the cable and the transformer. The magnitude of the voltage transient is compared with the transformer BIL. If the magnitude is excessive, then the transformer winding will likely fail line to ground. If the voltage transient has excessive rate of rise (dv/dt), then the transformer winding will likely fail turn to turn (natural frequency of ring wave). For the transformer to survive the transient, the insulation must be able to withstand both the magnitude and the dv/dt. Dry-type trans- formers are particularly susceptible to vacuum or SF-6 breaker switching transients. However, oil- or liquid-filled transformers are not immune. The oil has capacitance and acts like a surge capacitor to slow the rate of rise of the voltage transient. The trend in modern-day power systems is to install trans- formers with high-efficiency design. As a result, these high- efficiency transformers have a very small resistance, which offers little or no damping to the voltage transient. Also, the repetitive effect, i.e., small indentations in the insulation, can occur with each successive peak of the voltage transient. II. PREDICTING PERFORMANCE WITH SIMULATIONS A. Modeling the Circuit When a statistical approach is taken for switching transients, complex modeling requires a frequency-dependent transformer model and an arc model of the circuit breaker. For purposes of screening applications for potentially damaging switching transients, a simpler approach is suggested with the important Fig. 6. Simplified electrical system for ship propulsion drive. circuit elements modeled in electromagnetic transients program (EMTP) consisting of the source, breaker, cable, and trans- former, as shown in Fig. 5. The cable is represented by a Pi model consisting of the series impedance and half of the cable charging at each end. In some cases, multiple Pi models are used to represent the cable. The vacuum or SF-6 breaker is represented by a switch with different models for opening (current chop), restrike (excessive magnitude of TRV), reig- nition (excessive frequency of TRV), and closing (prestrike). The three-phase transformer model consists of the leakage im- pedance, magnetizing branch, and winding capacitances from high to ground and low to ground. For oil-filled transformers, the oil acts like a dielectric so the high-to-low capacitance is modeled. In cases requiring more detail, the transformer saturation and hysteresis effects are modeled. The choice of the integration time step will depend upon the anticipated frequency of the voltage transient. If it is too large, the time steps will “miss” the frequency effects. If it is too small, then this will lead to excessive simulation times. The Nyquist criteria call for a minimum sample rate of twice the anticipated frequency. In switching transients, the anticipated frequency is 3–25 kHz. When the circuit breaker opens, the transformer primary winding is ungrounded. Also, the ring wave is a function of the natural frequency of the circuit fnatural = 1/(2π √ LC). (2) The iron core of the transformer dominates the inductance of the circuit. The capacitance is very small for the dry-type transformer and short cable. Consequently, the circuit’s natural frequency is 3–25 kHz with relatively short cables.
  • 91. SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 711 Fig. 7. Matching the simulation to field measurements. B. Mitigating the Switching Transient Various surge protection schemes exist to protect the transformer primary winding from vacuum-breaker-switching- induced transients. A surge arrester provides basic overvolt- age protection (magnitude only). The arrester limits the peak voltage of the transient voltage waveform. The surge arrester does not limit the rate of rise of the TOV. A surge capacitor in combination with the surge arrester slows down the rate of rise of the TOV in addition to limiting the peak voltage but does nothing for the reflection or dc offset. The number of arrester operations is greatly reduced because of the slower rate of rise. There is a possibility of virtual current chopping. Finally, adding a resistor to the surge capacitor and surge arrester provides damping, reduces the dc offset of the TOV waveform, and minimizes the potential for virtual current chopping. The resistor and surge capacitor are considered an RC snubber. Selecting the values of resistance and capacitance are best determined by a switching transient analysis study, simulating the circuit effects with and without the snubber. C. Matching the Model to Measurements The results obtained from simulation of switching transients in EMTP are only as good as the choice of model and data used. When available, field measurements taken during the switching transients enable verification of the EMTP model. The EMTP model can be adjusted as needed to match the actual field- measured conditions. To illustrate this approach, consider the ship propulsion electrical system in Fig. 6. The system consists of 3 × 2865-kW generators, a 4160-V three-phase bus, two 1865-kW drives/motors for forward Fig. 8. TRV leading to reignition during energization of drive transformer with and without snubber. TABLE III CURRENT CHOP AND REIGNITION CASES FOR DRIVE PROPULSION TRANSFORMER SWITCHING propulsion and identical drives for reverse propulsion, eight 1185-kVA dry-type transformers, and eight 630-A vacuum circuit breakers. The critical parameters are the vacuum circuit breaker, 50 ft of cable, and dry-type transformer of 30-kV BIL. Fig. 7 shows that the EMTP simulation results match the transients captured in the field with a high-speed power-quality meter (closing). The simulation shows 4.96 kVpeak, which is less than 30-kV BIL; however, the oscillation frequency of 20.2 kHz exceeds an acceptable limit of dv/dt. Having verified the model, a series of current-chop cases and reignition cases were run. Fig. 8 shows the TRV leading to reignition and the TRV with a snubber installed. Reignition occurs because the TRV peak, time to crest, and rate of rise of recovery voltage exceed IEEE ANSI C37.06 limits.
  • 92. 712 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 9. Simplified electrical distribution system for Tier III data center. The snubber reduces the TRV below the IEEE/ANSI limits for general-purpose vacuum breakers [3] and for generator breakers [4]. Table III summarizes the reignition cases and the current-chop cases. In all cases, the snubber is effective in reducing the transient voltage. D. Borderline Case It is important to note that not all applications involving primary switching of transformers using vacuum breakers re- quire snubbers. The large majority of applications do not re- quire snubbers. Switching transient studies are conducted to determine when snubbers are needed. In this paper, the cases were selected to show different situations requiring snubbers. For the system shown in Fig. 9, the results were border- line; therefore, a snubber was still applied for reliability pur- poses. The Fig. 9 system is a Tier III data center with two 24.9-kV incoming lines, two 12.5-MVA 25/13.2-kV transform- ers, a 13.2-kV ring bus, two 2250-KW generators, and six 3750-kV cast-coil transformers. Data centers fall into the highest risk categories because of their high load density, close proximities of circuit components, highly inductive transformers (high-efficiency designs), and frequent switching. The critical parameters for the Fig. 9 system are vacuum circuit breakers, 90-kV-BIL transformers, and cable lengths ranging from 109 to 249 ft. For the cable of 109 ft, the results of opening the vacuum breaker with current chopping of 8 A are shown in Fig. 10. The TOV is as high as 123 kVpeak on phase A which exceeds the transformer BIL of 95 kV. The TOV exhibits a significant dc offset because there is very little resistance in the highly inductive circuit. The oscillation frequency of 969 Hz is slightly less than the acceptable limit. A snubber is required to reduce the peak below 95-kV BIL. The results of adding a snubber are shown in Fig. 10. Note the significant reduction in the dc offset. The resistor in the snubber provides the reduction in dc offset as well as damping. The peak is reduced to 28.6 kV and an oscillation of 215 Hz, Fig. 10. TOV initiated by current chop during de-energization of cast-coil transformer with and without snubber. both within acceptable limits. Finally, field measurements were taken after the snubber was installed to ensure that the snubbers performed as designed. The field test setup for the snubber performance measurements is discussed in Section IV. The field measurements showed that the snubber limited the TOV within acceptable limits. E. Case of Switching a Highly Inductive Circuit Now, consider the vacuum breaker switching of a highly inductive circuit, such as the starting current of a large grinder
  • 93. SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 713 Fig. 11. Simplified electrical distribution system for LMF. motor or an electric arc furnace. The vacuum-circuit-breaker switching of an electric arc furnace and ladle melt furnace transformers raises concern because of their high inductive cur- rents. High-frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple reignitions. As an example, the arc furnace circuit of Fig. 11 consists of a 50-MVA power transformer, 2000-A SF-6 breaker, 56-MVA autoregulating transformer, 1200-A vacuum breaker, and 50-MVA furnace transformer. The switching of the SF-6 and vacuum breaker was studied. The vacuum breaker, because of the 28-ft bus to the furnace transformer, was the worst case. The results opening the vacuum breaker with and without snubbers are show in Fig. 12. The TOV of 386 kVpeak exceeds the transformer BIL of 200 kV, and the oscillation of 1217 Hz exceeds the acceptable limit. Application of the snubber results in a TOV of 56.4 kVpeak that is below the transformer BIL, and the oscillation of 200 Hz is below the acceptable limit. The results for cases involving current chop and reignition are given in Table IV. F. Concerns for the Pulp and Paper Industry The previous examples illustrate that circuit-breaker-induced switching transients can fail transformers for specific com- binations of circuit parameters and breaker characteristics. Fig. 12. TOV during de-energization of LMF transformer with and without snubber protection. The examples show that the problem is not unique to one industry, application, vendor’s breaker, or transformer design. For the pulp and paper industry, there are many situations where circuit-breaker-induced switching transients are likely to damage transformers. The following examples are some of the more common scenarios encountered in the pulp and paper industry. 1) Vacuum breaker retrofit for primary load break switch in a unit substation. In the pulp and paper industry, there are numerous unit substation installations with primary load break fused switch and no secondary main breaker. This arrangement results in arc Flash issues on the low- voltage secondary. Limited space on the low-voltage side prevents installation of a secondary main breaker to mitigate the arc Flash issues. Retrofitting a vacuum circuit breaker in the primary of the unit substation, in place of the primary load break switch, and sensing on the secondary is a solution that provides both primary and secondary fault protection [5]. Unit substations may have oil-filled or dry-type transformers. The secondaries may be solidly grounded or resistance grounded. With the vacuum breaker closely coupled to the transformer, surge arresters and snubbers are most likely needed. 2) Vacuum breaker and rectifier (or isolation) transformer installation. Rectifier transformers are installed to serve dc drives such as those needed for feed water pumps to the boilers. Also, isolation transformers are installed to serve a large VSD or groups of smaller drives. Primary voltages may be 13.8 or 2.4 kV, and secondary voltages
  • 94. 714 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 may be 600 or 480 V. In both situations, vacuum breakers are installed in the primary and closely coupled to the transformer through a short run of bus or cable. Often, these transformers are inside and of dry-type design. 3) New unit substation with primary vacuum breaker. Recently, a paper mill installed a new metal-enclosed vacuum switchgear and a new 13.8/2.4-kV 7500-kVA transformer for a bag house for the generator boilers to meet Environmental Protection Agency requirements. The vacuum breaker was connected to the transformer through 5 ft of bus. While doing the coordination and arc Flash studies, the switching transient issue was identified. The equipment was installed and was awaiting startup and commissioning when the studies raised the concern. Before energizing the transformer, snubbers were quickly sized, obtained, designed, and installed. The screening criteria previously mentioned identify the aforementioned examples for potential damaging switching transient voltages due to vacuum breaker switching. The vacuum breaker, short distance to transformer, and dry-type transformer (or aged oil-filled transformer) are key variables to consider. With such short distance between breaker and transformer, most of these installations will require snubbers. One might conclude that standard snubbers could be applied. However, a switching transient study is still recommended to determine the unique characteristics of the circuit and custom design the snubber for the application. Given the limited space in each of these examples, it is unlikely that off-the-shelf standard snubbers would fit. A substantial part of the design effort includes determining how to best fit the snubbers into the new or existing unit substation or transformer enclosure. III. DESIGNING THE SNUBBER The preceding analysis has shown that, in some cases, switching transients can produce overvoltages that can result in equipment insulation failure. If the results of the switching transient study indicate a risk of overvoltage greater than the BIL of the equipment and/or if the dv/dt limits are exceeded, a surge arrester and snubber should be applied. The switch- ing transient study may also indicate that multiple locations require surge arresters and snubbers to protect the generator, transformer, or large motor. Additionally, the study specifies the necessary protective components and determines how close the protection must be placed to provide effective protection. A. Design Requirements At this point, custom engineering design determines how to best provide the protection needed for the equipment. The fol- lowing questions must be answered to ensure that the snubber design meets all criteria and specifications. 1) Is the switching transient protection cost effective? 2) What is the value of the equipment being protected? 3) What is the cost of lost production if the equipment fails from switching transients? 4) Can the protection be installed within existing equipment enclosures? Fig. 13. Typical snubber and arrester arrangement for transformer protection. Fig. 14. Example of standard snubber package for transformer. 5) Will equipment modification void equipment warranties? 6) Are there standard equipment packages that can accom- plish the switching transient protection? 7) Is the application an indoor or outdoor application? 8) Is there available space to mount the protective equipment in an external enclosure? 9) How can it be verified if the switching transient protection components are working effectively? 10) What alarms are necessary if the protective equipment components fail? B. Custom Engineering Design Fig. 13 shows the typical snubber arrangement for trans- former protection. A noninductive ceramic resistor and a surge capacitor are the basic components of a snubber design. Resistance values typically range from 25 to 50 Ω. Standard capacitor ratings that range from 0.15 to 0.35 μF are the basis of the design. Fig. 14 shows a standard 15-kV surge protection package. The arresters are mounted on the top of the enclosure. A three- phase surge capacitor is mounted on the bottom. Insulators and bus are located in the center. The cables can enter from top or bottom. A ground bus is located on the center right. If space heaters are required for outdoor locations, they are located on the lower left. Fig. 15 shows one phase of a custom snubber circuit. The custom design was required because there was not enough room in the transformer for the snubber components. The enclosure
  • 95. SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 715 TABLE IV CURRENT CHOP AND REIGNITION CASES FOR LMF TRANSFORMER SWITCHING Fig. 15. 15-kV snubber mounted above the transformer. had to be mounted above the transformer. The cable connec- tions from the transformer were field installed and land on the copper bus. A 15-kV nonshielded jumper cable was used to make the connection. Each phase passed through an insulation bushing to the transformer below. Bus work was required to provide a solid support for the fragile resistors. Normally, only one resistor would be provided, but for this application, to achieve the delivery schedule, parallel resistors were designed to obtain the correct ohmic value (the correct single resistor value had long delivery). Fig. 16(a) and (b) shows a snubber assembly mounted in medium voltage switchgear. The photo on the left shows the single-phase surge capacitors mounted vertically. The black cylinders are ceramic resistors. A variety of options are avail- able to detect if the snubbers are functional. They range from nothing (oversized but treated like a lightning arrester) to very sophisticated loss of circuit detection. Glow tube indicators are shown at the top of Fig. 16(a), and a close-up is shown in Fig. 17(a). These glow tubes are visible through a window in the switchgear door and provide a visual indication of snubber Fig. 16. 15-kV snubber mounted in switchgear with glow tubes and current sensors. continuity. The purpose of the blue current sensors at the bottom of Fig. 16(a) is to monitor the continuity of the resistor and fuse (optional) and alarm on loss of continuity. A close-up of the current sensor is shown in Fig. 17(b). Some industries mandate fused protection. If there should be a broken resistor or a blown fuse, an alarm signal can be sent to the plant distributed control system or supervisory control and data acquisition system to alert the operating personnel that these snubber components have failed. Fig. 16(b) shows a continuation of the same snub- ber assembly. Three fuses are attached to the tops of the resistor. The fuses will isolate any fault that may occur in the snubber assembly and prevent loss of the breaker circuit. C. Special Design Considerations The nature of high-frequency switching transients requires special design considerations. The snubber designer should
  • 96. 716 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Fig. 17. 15-kV snubber visual indication (glow tubes) and continuity verifi- cation (current sensors). consider the location of the switching transient source when de- veloping the custom design layout of the protective equipment. Abrupt changes in the electrical path should be avoided. A low inductive reactance ground path should be designed, using non- inductive ceramic resistors and flat tin braided copper ground conductors. The minimum clearances of live parts must meet or exceed the phase-to-phase and phase-to-ground clearances of NEC Table 490.24. The enclosure should be designed to meet the requirements of IEEE Standard C37.20.2 1999. When the enclosure is mounted greater than 10 ft from the equipment to be protected, NEC tap rules may apply to the cable size required and additional circuit protective devices may be required. D. Custom Designs for the Pulp and Paper Industry As mentioned previously, given the limited space in each of the examples related to the pulp and paper industry, it is unlikely that off-the-shelf standard snubbers would fit. Instead, a substantial part of the design effort includes determining how to best fit the snubbers into the new or existing unit substation or transformer enclosure. Following are three examples of the custom design effort needed for snubber installation. 1) 13.8-kV Solidly Grounded System in Paper Mill. A vac- uum breaker was retrofitted into the enclosure for the primary load break switch of the unit substation with a dry-type transformer. The space for the snubber was extremely limited, as shown in Fig. 18 (left). Because the system was solidly grounded, the voltage on the surge capacitor was limited to 8 kV line to ground; therefore, it was possible to use a three-phase surge capacitor. The tight clearance required the use of glastic to insulate the components at line potential from ground. 2) 13.8-kV Resistance Grounded System in Paper Mill. An- other example of retrofitting a vacuum breaker for a primary load break switch with a 30-year-old oil-filled transformer. Because this snubber was needed immedi- ately, the only available resistors had to be paralleled to obtain the desired resistance, as shown in Fig. 18 (right). Again, glastic was used for insulation and to support the resistors. 3) 13.8-kV Low Resistance Grounded System. Snubbers were provided for a new metal-enclosed vacuum switchgear and a new 13.8/2.4-kV 7500-kVA transformer Fig. 18. Snubber with (left) three-phase and (right) single-phase surge capac- itors for 13.8-kV paper mill application. Fig. 19. Snubber for metal-enclosed 13.8-kV vacuum breaker. for a bag house. Single-phase surge capacitors were used. Single resistors of the right ohmic value were available. Adequate clearance did not require the use of glastic as shown in Fig. 19. IV. MEASUREMENTS TO VERIFY SNUBBER PERFORMANCE Following the installation of the snubbers, power quality measurements may be taken to ensure the proper operation of the snubbers. A high-speed scope or power quality disturbance analyzer should be used to measure the TOV waveforms at the transformer primary produced during switching of the primary vacuum circuit breaker. The measurements are used to verify that the waveforms do not exhibit excessive high-frequency transients (magnitude, rate of rise, and frequency). The test measurement setup generally consists of voltage dividers and a transient recording device. The voltage dividers should be made of capacitive and resistive components with a bandwidth of 10 MHz. The scope or power quality meter should be capable of transient voltage wave shape sampling,
  • 97. SHIPP et al.: TRANSFORMER FAILURE DUE TO CIRCUIT-BREAKER-INDUCED SWITCHING TRANSIENTS 717 Fig. 20. Snubber performance measurement setup for 15-kV transformer using high bandwidth voltage dividers. Fig. 21. Voltage divider connections at 18-kV surge arrester on transformer primary bushing. 8000-Vpeak full scale, and 200-ns sample resolution (5-MHz sampling). An outage and lockout/tagout are needed to install the volt- age dividers, make connections to each of the three phases at transformer primary bushing, and make secondary connections to the transient recorder. Fig. 20 shows the test measurement setup for a typical cast-coil transformer. Voltage divider con- nections to the transformer primary were made, as shown in Fig. 21. Additionally, for tests requiring load at a transformer, a portable load bank may be connected to the transformer sec- ondary. A resistive load bank configurable to different load levels (300 or 100 kW) prevents destructive testing. V. CONCLUSION This paper has reviewed recent transformer failures due to primary circuit-breaker switching transients to show the sever- ity of damage caused by the voltage surge and discuss common contributing factors. Next, switching transient simulations in EMTP were presented to illustrate how breaker characteristics of current chopping and restrike combine with critical circuit characteristics to cause transformer failure in unique situations. In these limited instances, mitigation of the transients is accom- plished with snubbers custom designed to match the specific circuit characteristics. Design and installation considerations were addressed, particularly the challenges of retrofitting a snubber to an existing facility with limited space. Finally, the performance of the snubbers is verified with field measurements at the medium-voltage primary winding of the transformer. REFERENCES [1] ANSI/IEEE, A Guide to Describe the Occurrence and Mitigation of Switch- ing Transients Induced by Transformer and Switching Device Interaction. C57.142-Draft. [2] D. Shipp and R. Hoerauf, “Characteristics and applications of various arc interrupting methods,” IEEE Trans. Ind. Appl., vol. 27, no. 5, pp. 849–861, Sep./Oct. 1991. [3] Standard for AC High-Voltage Generator Circuit Breakers on a Symmetri- cal Current Basis, C37.013-1997, 1997. [4] Application Guide for Transient Recovery Voltage for AC High-Voltage Circuit Breakers, C37.011-2005, 2005. [5] D. Durocher, “Considerations in unit substation design to optimize reliabil- ity and electrical workplace safety,” presented at the IEEE IAS Electrical Safety Workshop, Memphis, TN, 2010, Paper ESW2010-3. David D. Shipp (S’72–M’72–SM’92–F’02) re- ceived the B.S.E.E. degree from Oregon State Uni- versity, Corvallis, in 1972. He is a Principal Engineer with the Electrical Services and Systems Division, Eaton Corporation, Warrendale, PA. He is a distinguished scholar in power system analysis and has worked in a wide variety of industries. He has spent many years per- forming the engineering work associated with his present-day responsibilities, which include a wide range of services covering consulting, design, power quality, arc flash, and power systems analysis topics. Over the last few years, he has pioneered the design and application of arc-flash solutions, modifying power systems to greatly reduce incident energy exposure. He has written over 80 technical papers on power systems analysis topics. More than 12 technical papers have been published in IEEE Industry Applications Society (IAS) national publications and two in EC&M. He spent ten years as a professional instructor, teaching full time. He occasionally serves as a legal expert witness. Mr. Shipp is currently the Chair for the IEEE Industrial and Commercial Power Systems-sponsored Working Group on generator grounding. He has received an IEEE IAS Prize Paper Award for one of his papers and conference prize paper awards for six others. He is very active in IEEE at the national level and helps write the IEEE Color Book series standards. Thomas J. Dionise (S’79–M’82–SM’87) received the B.S.E.E. degree from The Pennsylvania State University, University Park, in 1978, and the M.S.E.E. degree with the Power Option from Carnegie Mellon University, Pittsburgh, PA, in 1984. He is currently a Senior Power Systems Engi- neer with the Power System Engineering Depart- ment, Eaton Corporation, Warrendale, PA. He has over 27 years of power system experience involving analytical studies and power quality investigations of industrial and commercial power systems. In the metal industry, he has specialized in power quality investigations, harmonic analysis and harmonic filter design for electric arc furnaces, rectifiers, and variable-frequency drive applications. Mr. Dionise is the Chair of the Metal Industry Committee and a member of the Generator Grounding Working Group. He has served in local IEEE positions and had an active role in the committee that planned the Industry Applications Society 2002 Annual Meeting in Pittsburgh, PA. He is a Licensed Professional Engineer in Pennsylvania.
  • 98. 718 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 47, NO. 2, MARCH/APRIL 2011 Visuth Lorch received the B.S.E.E. degree from Chulalongkorn University, Bangkok, Thailand, in 1973, and the M.S. degree in electric power engi- neering from Oregon State University, Corvallis, in 1976, where he was also a Ph.D. candidate and was inducted as a member of the Phi Kappa Phi Honor Society. During his Ph.D. studies, he developed the Short Circuit, Load Flow, and Two-Machine Tran- sient Stability programs. The Load Flow program has been used in the undergraduate power system analysis class. He also prepared a Ph.D. thesis on the Load Flow program using the third-order Taylor’s series iterative method. In 1981, he joined Westinghouse Electric Corporation, Pittsburgh, PA. He was responsible for conduction power system studies, including short circuit, protective device coordination, load flow, motor starting, harmonic analysis, switching transient, and transient stability studies. He also developed the Pro- tective Device Evaluation program on the main frame Control Data Corporation supercomputer. In 1984, he joined the Bangkok Oil Refinery, Thailand, where his primary responsibility was to design the plant electrical distribution system as well as the protection scheme for the steam turbine cogeneration facility. He was also responsible for designing the plant automation, including the digital control system for the plant control room. In 1986, he rejoined Westinghouse Electric Corporation. He performed power system studies and developed the Short Circuit and Protective Device Evaluation programs for the personal computer. In 1998, he joined the Electrical Services and Systems Division, Eaton Corporation, Warrendale, PA, where he is currently a Senior Power Systems Engineer in the Power Systems Engineering Department. He performs a variety of power system studies, including switching transient studies using the electromagnetic transients program for vacuum breaker/snubber circuit applications. He continues to develop Excel spreadsheets for quick calculation for short circuit, harmonic analysis, soft starting of motors, capacitor switching transients, dc fault calculation, etc. Bill G. MacFarlane (S’70–M’72) received the B.S.E.E. degree from The Pennsylvania State Uni- versity, University Park, in 1972. He began his career in 1972 with Dravo Corpo- ration, Pittsburgh, PA, as a Power System Design Engineer supporting this engineering/construction company with expertise in the design and construc- tion of material handling systems, chemical plants, pulp and paper mills, steel facilities, ore benefaction, mine design, and computer automation systems. He provided full electrical design services in heavy in- dustrial applications, primarily in the steelmaking and coal-preparation-related sectors. His responsibilities included client and vendor liaison and supervision of engineers, designers, and drafters. From 1978 to 2003, he was a principal Process Controls Systems Engineer with Bayer Corporation, Pittsburgh. He provided design and specification of instrumentation and control systems for chemical processes. He configured programmable logic controller and distrib- uted control systems. He was a Corporate Technical Consultant for power distribution systems. He had been the principal Electrical Engineer on many major projects with Bayer Corporation. He joined the Electrical Services and Systems Division, Eaton Corporation, Warrendale, PA, in 2004. At Eaton, he has been involved in a 230-kV substation design, ground grid design for substations, as well as in short circuit, protective device coordination, load flow, and arc-flash hazard analysis studies. He has also done power factor correction and harmonics studies to resolve power quality issues.
  • 99. Vacuum Circuit Breaker Transients During Switching of an LMF Transformer David D. Shipp, PE Eaton Electrical Group Power Systems Engineering Warrendale, PA Fellow, IEEE Thomas J. Dionise, PE Eaton Electrical Group Power Systems Engineering Warrendale, PA Senior Member, IEEE Visuth Lorch Eaton Electrical Group Power Systems Engineering Warrendale, PA William MacFarlane, PE Eaton Electrical Group Power Systems Engineering Warrendale, PA Senior Member, IEEE Abstract— Switching transients associated with circuit breakers have been observed for many years. With the wide-spread application of vacuum breakers for transformer switching, recently this phenomenon has been attributed to a significant number of transformer failures. Vacuum circuit breaker switching of electric arc furnace and ladle melt furnace transformers raises concern because of their inductive currents. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re- ignitions. This paper will present a detailed case study of vacuum breaker switching of a new ladle melt furnace transformer involving current chopping and re-strike simulations using the electromagnetic transients program. A technique that involves a combination of surge arresters and snubbers will be applied to the ladle melt furnace to show the switching transients can be successfully mitigated. Additionally, some practical aspects of the physical design and installation of the snubber will be discussed. Keywords- Switching Transients, vacuum breakers, SF-6 breakers, LMF transformer, EMTP simulations, surge arresters, RC snubbers. I. INTRODUCTION Electric Arc Furnaces (EAF) are used widely in the steel industry in the production of carbon steel and specialty steels. The Ladle Melt Furnace (LMF) maintains the temperature of liquid steel after tapping the EAF and facilitates changes in the alloy composition through additives. In both cases, the furnace transformer is a critical component of the furnace circuit that is exposed to severe duty. The demands of the melt cycle may result in extensive damage to the furnace transformer due to electrical failures in the transformer. With advances in technology and metallurgy, the operation of arc furnaces today is significantly different. Heats of 4 to 5 hours with periods of moderate loading have been reduced to 3 to 4 hours with consistently high loading. Accompanying the shorter heats of sustained loading are many more switching operations. Combined, these factors impose thermal and electrical stresses on the transformer. Frequent switching operations have been enabled by the development of the vacuum switch. The vacuum switch has been designed for hundreds of operations in a day, for long life and low maintenance. With the advantages of the vacuum switch, also come the disadvantages of switching transient overvoltages. Depending on the characteristics of the vacuum switch and the power system parameters, these switching transient overvoltages can be of significant magnitude and frequency to cause transformer failure. High frequency transients and overvoltages result when the vacuum breaker exhibits virtual current chop and multiple re-ignitions. According to statistics compiled by one insurance company [1], the application of vacuum switches has resulted in numerous failures of arc furnace transformers. These failures rates have been reduced by the application of surge arresters, surge capacitors and damping resistors [2]. The transients produced by the vacuum circuit breaker switching of an LMF transformer and their mitigation are the focus of this paper. A. The LMF Circuit of Interest Consider the new LMF circuit of Fig. 1 that consists of a 50 MVA, 135/26.4 kV power transformer, a 2000 A SF-6 breaker, a 56 MVA, 27/10 kV auto-regulating transformer, a 1200 A vacuum breaker and a 50 MVA, 25/0.53 kV furnace transformer. The SF-6 circuit breaker is separated by 53 feet from the auto-regulating transformer. The vacuum circuit breaker is separated by 28 feet from the LMF transformer. The normal configuration (1) consists of the new LMF and the existing 4EAF operating in parallel. One alternate configuration (2) of the LMF circuit consists of 4OCB and 4EAF out-of-service with 4LTC in standby-service. A second alternate configuration (3) of the LMF circuit consists of LMF LTC out-of-service with 4LTC switched on-line to source the LMF. Each of these three possible configurations of the LMF circuit was considered. Of the three, the normal configuration results in the shortest bus length between the vacuum breaker and the LMF transformer. The normal configuration also results in the shortest bus length between the SF-6 breaker and the LMF transformer.