Analysis of Transformer Tank RupturesElectric Power Research Institute Wayne Johnson email@example.com Nick Abi-Samra firstname.lastname@example.org
Survey On Transformer RuptureObjectives• Develop an understanding of the tank rupture process associated with internal faults.• Develop tools, and methods, for evaluating the influence of tank designs on rupture characteristics 2
Background• 42 transformer failures (tanks ruptured or deformed without rupture)• 22 utilities• 10-year period (1980-1990)• 7 transformer manufacturers• Different voltage levels• Different designs: (GSU’s, Auto, Phase Shifters, etc.) 3
Conclusions• The arc energy is the critical rupture parameter• Differences in transformer design and application are not major discriminators in the tank rupture• The fault energy capacity of a tank can be increased by increasing the tank rupture pressure limit and tank flexibility. – The pressure at which tanks rupture can be increased by local strengthening of weak points, while the tank flexibility can be increased by replacing large beams with a number of smaller beams (which permit greater deflection at a given stress level).• Venting to conservators or to auxiliary tanks was not found to be effective for heavy faults (those with arc power greater than 300 MW) 4
Conclusions• A long arcing time is not necessary for rupture: – About 75% of the cases occurred with arcing times less than 4.5 cycles.• Since arc energy is proportional to I2t, where the t is duration in seconds, the parameter which offers the most opportunity for control of the risk of rupture is the magnitude of the current, and Graph of Tank Deformation Rupture as a function of fault specifically, the peak crest current and fault duration. value of the first half cycle of the fault current (and the associated X/R of the circuit). 5
Power TransformerTank RuptureA Utilitys ExperienceMarc FoataIEEE Transformers CommitteeHouston, March 2010
PresentationAssessment of the risk Statistics Arc energyPrevention of tank rupture Venting ContainmentSpecification of a Tank pressure withstandrequirement 2
Assessing the risk - Statistics 3
Assessing the risk - Statistics 4
Assessing the risk - Statistics 5
Arc energy - 4 MJ 6
Arc Energy - 8 MJ 7
Arc Energy - 12 MJ 8
Arc Energy - 14 MJ 9
Arc energy vs Damage 10
Arc Energy - Calculation Earc = 0.9 V I tI – Arc current: Evaluated from short-circuit levelT – Fault clearing time: Depends mainly onprotectionV – Arc voltage: Very difficult to evaluate0.9 – Factor introduced for square waveform of V 11
Arc Energy - Recordings A 100 Unité Support et analyses - DESTT E BN TT MAIS MICOUA 80 E BN L-7019 MICOUA 60 E BN L-7011 MICOUA E BN L-7027 MICOUA 40 20 (kV) 0 -20 -40 -60 -80 205 210 215 220 225 230 235 240 245 250 255 (ms rel. à 05:34:32.2870) 2008-09-12 05:34:32.499 MICOUA 735-02 TENSION RÉSIDUELLE DÉFAUT TRANSFO T8-B 12 11 10 9 8 (MJoules) 7 6 5 ÉNTERGIE TOTALE (TT MAIS) 4 ÉNTERGIE TOTALE (TTC 7019) 3 ÉNTERGIE TOTALE (TTC 7011) 2 ÉNTERGIE TOTALE (TTC 7027) 1 0 -1 205 210 215 220 225 230 235 240 245 250 255 (ms rel. à 05:34:32.2870) 2008-09-12 05:34:32.499 MICOUA 735-02 ESTIMÉ DE LÉNERGIE TOTALE DANS LE DÉFAUT TRANSFO T8-B 20000 15000 10000 5000 (A) 0 -5000 -10000 COURANTS CÔTÉ 735 kV -15000 COURANTS APROX. CÔTÉ 315 kV -20000 190 200 210 220 230 240 250 260 270 (ms rel. à 05:34:32.2870) 2008-09-12 05:34:32.499 MICOUA 735-02 COURANTS DE DÉFAUT TRANSFO T8-B Lundi 15 Septembre, 2008 12 VECTEURDONNEESPARTAGE@AUTOMATISMESCOMPORTEMENTRAPPORTRAPP_2008TOSC_ANA20080912_053432499_ENERGIE_T8-B_MICOUA.TOS
Arc Energy - Evaluation of Arc Voltage60-100 V/cm range is often referredFor 40 kV, this means an arc length ofmore than 4 m !! 13
Arc Energy - Pressure Effect V = 55L PConstant 55 V/cmL is arc length (m)P is absolute pressure (atm)Pressure in the gas bubble at arc ignitioncan reach extremely high values 14
Prevention of tank rupture – Venting Simulations 15
Prevention of tank rupture – Venting Simulations 16
Prevention of tank rupture – Vented vs Non-vented Example 17
Prevention of tank rupture – Conclusions on ventingPressure reduction from a single 25 cmaperture is low and becomes negligiblewhen the arc is more than 1 meter away.An effective pressure venting strategywould require either a very large ventingduct or numerous small apertures in theclose vicinity of the arc. 18
Prevention of tank rupture - Containment 19
Prevention of tank rupture - ContainmentPresent design can contain up to 10 MJfor the largest tanks (735 kV)More resistant tank design can beachievedNeed to implement specifications withminimum energy requirement to meet.Energy requirement will be a compromisebetween the feasibility and the likelihood. 20
New Specification - PhilosophyPriority is given to the protection of theworkersWorst energy levels may not always becontainable by the tankFirst rupture point must be the coverRequired calculation tools must beaccessible to transformer designersMust take into account the highly dynamicphenomena involvedMust be easily verified 21
New specification - Formula ⎡ ⎤ Ps= F 1 kE ⎢100 + − 50⎥ ⎣ 4 100C ⎦Ps – Calculated tank pressure withstandF – Dynamic (time & location) amplificationE – Fault energy level to withstandK – Arc energy conversion factorC – Tank expansion coefficient 22
New specifications – Dynamic factor 3 2,5 2 F 1,5 1 0 20 40 60 80 100 120 140 160 180 200 C/V (x 10 -5 kPa-1)Time related dynamic factor (pressure anddeformation)Proximity related dynamic factorTakes into account tank volume 23
New specification : Hydro-QuébecsEnergy Containment Requirements Voltage Class Arc Energy 24
New specification - Implementation All transformer suppliers since 1992 have shown adequate tank withstand calculation capabilities. Since implementing energy level requirements (2006), manufacturers have been forced to improve their tank design. Detailed analysis by a number of manufacturers confirmed that all the specified energy requirements can be met. 25
Transformer Tanks- Some Factors Related to Rupture A Manufacturers Perspective IEEE Transformers Committee Houston, March 9, 2010 by Bill Darovny, P.Eng. Siemens Canada
Facts about Liquid Filled Transformer Tanks• C57.12.00 and C57.12.10 define the operating pressures for transformer tanks – full vacuum = -101.4 kPa (-14.7 psig) – pressure 25% above the normal operating pressure• Transformer tanks are not pressure vessels – are not required to be designed to the ASME Boiler and Pressure Vessel Code. • ASME code is mandatory when operating pressure exceeds 2 atmospheres (203 kPa).
Facts about Liquid Filled Transformer Tanks• C57.12.10 requires a pressure relief device to be mounted on the tank cover – Typically activate at 34.5 to 69 kPa (5 to 10 psig)• Most rectangular tanks will sustain internal pressures of 140 to 210 kPa (20 to 30 psig) before rupture• A tank on its own cannot be made strong enough to resist all magnitudes of internal pressure
Transformers have been saved fromrupture by common protection devices • Pressure Relief Devices • Gas Detector Relays • Rapid Pressure Rise Relays • Real time Gas MonitorsProvided the alarm signals are quickly recognized and the transformer is de-energized
Pressure Relief Device in Action
Not Every Tank Rupture Results in a Fire Unit gassing, GDR alarmed, long delay in response to alarm,ambient -30°C, reaction force to rupture shifted unit on the pad.
Some Units Burn After Tank Rupture- 750 MVA 500kV
Same Rating Different Supplier
The Location Where a Tank will Rupture is a Function of:1. the rate of change of the pressure increase Slow rate: • Pressure has time to distribute throughout the tank Fast rate: • Results in a pressure build-up at the source2. the co-ordinates of the pressure source inside the tank3. the closest weak spot to those co-ordinates
Slow Rate of Pressure Increase- Failed at 2 locations on the Cover- Tensile end reactions tore the welds at the ends of the stiffeners- Cover end angle rotated and the weld to tank flange cracked
Moderate Rate of Pressure Increase• Tank failed atthe cover joint• Some stiffenerson the tank wallwere permanentlydeformed• There were nocracks in the oilcontainmentwelds in the tankbody
Rapid Rate of Pressure Increase• Co-ordinates ofpressure source wasabout 1/3 tank height• Wall plate fracturestarted at the cornerwelds and ran almostfull height of the tank• Tank wall to bottomweld joint also failed
Rapid Rate of Pressure Increase• Cover weld did not fail• Tank failed at the highstress points in welds atthe tank corners andwall penetrations thenpropagated through thewall plates• Unit was returned tothe factory and rebuilt
Weakest Points of a Rectangular Tank• Main cover to tank wall flange - weld joint• Tank wall corners - weld joint• Tank wall to base plate - weld joint• High stress points at throats and large penetrations through the tank plates• High stress points at ends of stiffeners where the end reaction force is transmitted to the tank plate
Main Cover to Tank Wall Flange Mode of Weld Failure Typical Cover Welded Joint Upward and outward forces due to internal pressures Stress is concentrated at the weakest point - the root of the weld. Weld crack progresses outward from the root
Typical Rectangular Tank Corner Joints Stresses are on the face of the weld. The face of the weld is stronger than the root of the weld. Adding corner gussets will reinforce this joint
Cylindrical Tanks are Inherently Stronger than Rectangular Tanks• 110MVAR 735 kVshunt reactor• The tank wall isstressed in hooptension• Typically thecylinder walls cansustain pressures> 350 kPa (50 psig)• Weakest point isthe cover weld
Design to Help Reduce Tank Rupture• Design in service systems to detect faults early & de-energize quickly• Use detection & relief accessories – Real time gas monitors – Gas detector relays - ensure gas collection system / piping functions as intended – Rapid pressure relays – Pressure relief devices • To be effective, relief devices must be located close to the pressure source • Standard size relief devices may not prevent all tank ruptures
Design to Help Reduce Tank Rupture• The best location for a tank to fail is at the welded cover joint as this will minimize fluid loss• Strengthen the tank below the cover joint – Reinforce tank corners and wall to bottom joints with plates / gussets – Distribute stiffener end reaction forces with reinforcements or by connecting to stiffeners on adjacent walls – Reinforce around wall penetrations to reduce the high stress points
Transformer Tank Rupture and Mitigation Tutorial March 9, 2010Mitigation Research and Example Techniques Presented by Craig Swinderman Mitsubishi Electric Power Products, Inc.
Research on Transformer Tank Rupture Mitigation• Joint research performed in the mid-1980’s by three large Electric Utilities in Japan, Tokyo University, and several transformer manufacturers.• Goal was to study ways of reducing the risk of transformer tank explosion in urban substations/ underground substations.• Full scale model testing performed.• Arc energies calculated and gas generation rate observed.• Pressure rise models developed.• Tank construction countermeasures developed.
Summary of ResearchTank Explosion Study Process for Internal Fault VerificationProcess Tests Estimation of Fault Condition Internal Fault Arc Test in Oil Tank ① Arc Current ② Arc Voltage ③ Time Dissolved Gas Generation ④ Dissolved Gas Generation Volume Internal Pressure Pressure Rise Analysis Pressure Rise Test Increase Dynamic Analysis considering Oil Motion using Full Scale TxInternal Pressure Comparison between Internal Pressure and Tank Strength> Tank Strength Countermeasure Tank ① Tank Strength Improvement ② Protection Relay Improvement Explosion ③ System Improvement ④ Pressure Restrained Structure
Dynamics of Internal Fault (1) 10000 Arc Test in Oil Tank Arc Test in Oil Tank Arc Voltage (V) 1000 Electrode 100 Arc 10 100 1000 Arc Length (mm) Oil Gas Volume 1 Gas Volume (m3) Arc Current : 1.3 to 40.9 kA Time : 3Cycle (0.05sec) 0.1 Arc Energy : 0.11 to 2.64 MJ Gap Length : 100 to 300 mm*Reference – T. KAWAMURA, M. UEDA, K. ANDO, Y. MAEDA, Y. ABIRU, M. WATANABE, K.MORITSU “Prevention of Tank Rupture Due to Internal Fault of Oil Filled Transformers”, 0.01CIGRE, 12-02, 1988. 0.1 1 10 Arc Energy (MJ)
Dynamics of Internal Fault (2)Pressure Rise Test using Full Scale model for verifying pressure risePressure Rise Test using Full Scale model for verifying pressure rise- Pressure rise at internal fault can be simulated by powder combustion, considering nozzle area of container and powder amount.- Dead space (steel tank) was set for simulating internal parts(Core,Coil).in the tested tank. Measured Results Transformer : 275kV 300MVA Analytical Results Arc Energy : 142,000kW、 Time : 80msec Pressure (Single Line-Ground Fault at Upper Tank) Upper Tank Time Combustion Container Φ300,L570 Middle Tank Pressure Upper Tank Gas Outlet (Φ12*72) Time Lower Tank Pressure Time Middle Tank *Reference – T. KAWAMURA, M. UEDA, K. ANDO, Y. MAEDA, Y.Lower Tank Cartridge increment ABIRU, M. WATANABE, K. MORITSU “Prevention of Tank (Max.7500g) Rupture Due to Internal Fault of Oil Filled Transformers”, CIGRE, 12-02, 1988.
Results of Analysis• Decomposed gas generation calculated to be around 0.5L/kW sec for larger 275 kV class transformers at HV lead.*• Dynamic oscillation of fault pressure wave (kinetic energy) has a considerable influence on the pressure rise. (Dynamic Load Factor approx. 1.3 was recommended)*• Tank expansion characteristics and tank strength are important in determining the transformer’s capability to resist rupture.• Reinforcements can be made at the joining flange between the tank and cover (or flange between upper and lower tank for shell-form) to significantly improve the tank strength against rupture.*Reference – T. KAWAMURA, M. UEDA, K. ANDO, Y. MAEDA, Y. ABIRU, M. WATANABE, K. MORITSU “Prevention of Tank Rupture Due to Internal Fault of Oil Filled Transformers”, CIGRE, 12-02, 1988.
Example of Tank Strength Improvement Conventional Type Improved TypeConnecting Part Alleviation of Stress Improvement of Connecting Part Concentration Strength by Tie Reinforcement
Pressure Reducing Space• Diaphragm type Conservator Tanks can be used as effective pressure reducing space if the connection duct to the main tank is short, the cross sectional area of the duct is large (approx. 1.4 m dia.), and the air space in the conservator diaphragm is adequateDiaphragm (bladder) Conservator Tank Connection duct Transformer main tank •Tests on 300 MVA, 230 kV units have verified ability to withstand 15,000 MVA short circuit capacity without rupture of the tank. •Still requires operation of protective relays and circuit breakers to clear fault within approx. 60 - 80 ms.
Gas Insulated Power Transformers•Use SF6 Gas as the insulating and cooling mediuminstead of insulating oil.•First units produced in 1967.•Several thousand units of various sizes now inservice worldwide, several manufacturers.•Transformer applications: From Distribution classunits up to 400 MVA, 345 kV ratings.•Primarily used in substations located in urbanareas (including inside buildings, underground) dueto safety benefits.
Features of Gas Insulated Transformers• Use SF6 Gas as insulating and cooling medium, instead of oil.• Typically use special internal insulation materials such as plastics, special paper, and pressboard.• SF6 has excellent dielectric properties, but not as good for heat transfer.
Benefit of Gas Insulated Transformers SF6 Gas Insulation: non-flammable, compressible gas Pressure rise during an internal fault is slower than oil-immersed (non- compressible fluid), thus SF6 gas reduces the chances of tank explosionPressure Rise (%) 100 100 Tank Strength 80 80 60 Oil-Immersed 60 Transformer 40 40 Gas Insulated 20 20 Transformer 00 00 0. 22 0. 0. 44 0. 0. 66 0. 0. 88 0. 11 Fault Time (sec)
Application Example 15MVA, Three phase, 50Hz, Continuous Rating, Core-Form, Forced-Gas Natual Air, with On-Load Tap Changer GNAN/GFAN H.V. 64.5kV +10/-10% Star L.V. 6.6kV DeltaFor underground substation beneath office building
A VERY BRIEF HISTORY:PRESSURE RELIEF DEVICES(PRDs) AND THEIR USE ONPOWER AND DISTRIBUTIONTRANSFORMERS Transformer Tank Rupture and Mitigation - March 9, 2010 J.Herz, Qualitrol
BEFORE THERE WERE RE-SEALABLE PRDs…
Rupture Discs GOOSENECK CAN ADD TO BACK PRESSURE HAD TO BE REPLACED AFTER OPERATION - LEFT THE TRANSFORMER OPEN TO ATMOSPHERIC MOISTURE IN THE INTERIM TYPICALLY WITH NO ALARM More Recently THEY HAVE BEEN USED IN MULTIPLE SETS ALONG THE TOP AND BOTTOM OF A SINGLE TRANSFORMER TO MAXIMIZE THE PRESSURE RELIEF AREA AND TO BE LOCATED AS CLOSE AS POSSIBLE TO ANY POTENTIAL FAULT LOCATION. THEY ARE THE RELIEF MECHANISM FOR COMBINATION PRESSURE RELIEF/FIRE SUPPRESSION SYSTEMS
THERE WERE SEVERAL DIFFERENTDESIGN APPROACHES TO RE-SEALINGPRDs MADE BY VARIOUSMANUFACTURERSIN THE LATE 1950’s THE INDUSTRYSETTLED OVERWHELMINGLY ON ONEDESIGN.
THE DESIGN IS STILL IN FREQUENTUSE TODAY. IT OFFERS SIMPLICITYAND DURABILITY IN ADDITION TORE-SEALABILITY.REFINEMENTS SINCE HAVE TO DOWITH IMPROVING THE SEAL,SHIELDING AND PROTECTION,SWITCH CAPACITY, CORROSIONRESISTANCE, ETC
WITH OPERATING PRESSUREWHEN OPERATING PRESSURE ACTING ON THE LARGER AREAIS REACHED, TOP SEAL CIRCUMSCRIBED BY THE SIDEOPENS WHILE SIDE SEAL SEAL, THE SPRING IS RAPIDLYREMAINS BRIEFLY CLOSED COMPRESSED AND THE VALVE EXHAUSTS QUICKLY
8400 SCFM AT 50% OVERPRESSURE ON A 10 PSI PRD WASTYPICAL. NOW THERE ARE DEVICES WHICH GO TO 12,600 SCFM.
Most frequent question: Will it protect?Most frequent answer is: Depends. • location of fault • magnitude of fault • duration of fault
TEST 1: 1958 AT GE SCHENECTADY The test tank was approximately 6 ft in diameter and 4 ft deep, with the PRD mounted in the center of the 6 ft. diameter cover. The gas space was 10 inches below the cover which resulted in about 700 gallons of oil and 23 cu. ft. of gas (air). Ball nosed copper electrodes (2) were threaded with a small copper wire to trigger the arc, the highest of which was 25K amps and 20K volts. Oil, smoke, mist, spray blasted out of the relief device over a radius of about 40 feet.TEST 2: 1958 AT GE SCHENECTADY In tests performed by Jim Barr on a transformer with NO COVER , the fault was introduced near the bottom of the tank and the bottom of the tank BLEW OUT, at 10K amps and 10K volts.
There are thousands of events where rupturediscs and re-sealable PRDs have successfullyprotected transformers:“Internal arcing, breaker insulation breakdown, load tap changer problems, phaseangle regulator problems, and internalwinding problems” are some of the morecommon. Transformer Tank Rupture and Mitigation - March 9, 2010 J.Herz, Qualitrol
CONCLUSIONSTTR is a complex problem. The severity is a function of arc location, arc I and Tas well as oil volume and tank expansion characteristics.It’s possible to reduce the risk of TTR by performing modifications to the tank.PRDs help to protect the tank against low energy internal arcing faults.Fluids with high fire point will reduce the consequences of a tank rupture;however it is not yet proven if these fluids will prevent tank rupture.GITs will eliminate the risk of tank rupture.Improved electrical protection and electrical system design can also helpprevent TTR.The IEEE currently has no standards that provide guidance on TTR mitigation.
IEEE TRANSACTIONS ON POWER DELIVERYVolume: 24 Issue: 4 Date: Oct. 2009Page(s): 1959 - 1967 Power Transformer Tank Rupture and Mitigation - A Summary of Current State of Practice and Knowledgeby the Task Force of IEEE Power Transformer SubcommitteeNick Abi-Samra, Javier Arteaga, Bill Darovny, Marc Foata, Joshua Herz, Terence Lee, Van Nhi Nguyen, Guillaume Perigaud, Craig Swinderman, Robert Thompson, Ge (Jim) Zhang, and Peter D. Zhao
Action Next –Planning to Generate an IEEE Std You are all welcome to join