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Blackout Avoidance & Undervoltage Load Shedding
- 1. BLACKOUT AVOIDANCE & UNDERVOLTAGE LOAD SHEDDING BLACKOUTAVOIDANCE & UNDERVOLTAGE LOAD SHEDDING Chuck Mozina Consultant Beckwith Electric Chuck Mozina Consultant Beckwith Electric
- 2. BLACKOUTS How and whythey occur Why voltage rather than frequency is the leading edge indicator of system collapse How blackout conditions effect generators and generator protection Undervoltage load shedding Blackout Avoidance
- 3. RECENT BLACKOUTS 2003 -East Coast Blackout 2003 - Italian Blackout 2002 - Swedish Blackout 1997 - PJM Disturbance 1996 - West Coast Blackout 1995 - PECO Disturbance 1987 - City of Memphis Blackout Avoidance
- 4. Root Cause ofRecent Blackouts VOLTAGE COLLAPSE – WHY? Today, major generation sources are remote from load centers. This was NOT the case 35 years ago. This makes the power system very reliant on transmission system to transport power to load centers. Blackout Avoidance
- 5. VOLTAGE COLLAPSE –WHY? Purchase power from remote sources to save $$$. New generation built remotely from load centers. Little new transmission being built. Utility loads are increasingly made up of air conditioning motors susceptible to stall conditions due to transmission system faults. Root Cause of Recent Blackouts Blackout Avoidance
- 6. As lines tripbetween remote generation and load center, the reactance increases. This increases the reactive (VAR) losses-reducing the voltage at the load center. The voltage phase angle between the generators at the load center and remote generators also increase. REMOTE GENERATION LOCAL LOAD CENTER LINE 1 LINE 2 LINE 3 LINE 4 LINE 5 LINE 6 How Voltage Collapses Occur Blackout Avoidance
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- 8. Real Power (MW)Flow Example SYSTEM A SYSTEM B SYSTEM C Eg O Es O POWER FLOW Blackout Avoidance Pe = Eg Es Sin ( 0g- 0s ) X Where: Eg = Voltage at the Load Center Generation Es = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0g = Votage Angle and Local Generation 0s = Voltage Angle at Remote Generation POWER TRANSFER EQUATION Where: Es = Voltage at the Load Center Eg = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0s = Voltage Angle and Local Generation 0g = Voltage Angle at Remote Generation
- 9. Pe = EgEs Sin ( 0g- 0s ) X Where: Eg = Voltage at the Load Center Generation Es = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0g = Voltage Angle and Local Generation 0s = Voltage Angle at Remote Generation POWER TRANSFER EQUATION SYSTEM A Load = 5000 MW GEN. =5000 MW SYSTEM B Load = 5000 MW Gen. = 5000 MW SYSTEM C Load = 5000 MW Gen. = 5000 MW Eg O Es O POWER FLOW - 0 MW TRANSFER Real Power (MW) Flow Example Blackout Avoidance Where: Es = Voltage at the Load Center Eg = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0s = Voltage Angle and Local Generation 0g = Voltage Angle at Remote Generation
- 10. Pe = EgEs Sin ( 0g- 0s ) X Where: Eg = Voltage at the Load Center Generation Es = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0g = Voltage Angle and Local Generation 0s = Voltage Angle at Remote Generation POWER TRANSFER EQUATION Real Power (MW) Flow Example SYSTEM A Load = 5000 MW GEN. = 7000 MW SYSTEM B Load = 5000 MW Gen. = 5000 MW SYSTEM C Load = 5000 MW Gen. = 3000 MW Eg O Es O POWER FLOW - 2000 MW TRANSFER Blackout Avoidance Where: Es = Voltage at the Load Center Eg = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0s = Voltage Angle and Local Generation 0g = Voltage Angle at Remote Generation
- 11. SYSTEM A SYSTEM B SYSTEM C Eg O Es O REACTIVE POWER FLOWSMALL 1. To make reactive power flow you need to have a difference in voltage magnitude between Eg and Es. 2. Voltage on a power system can only be varied +/- 5% which is not enough difference to result in a significant VARS flow. 3. Thus VARS cannot be transmitted over long distances and must be generated locally near the load. Reactive Power (Mvars) Flow Blackout Avoidance
- 12. VAr Support mustbe provided at the load center. Two major sources of VAr support: Capacitor Banks – Double-Edge Sword. Vars go down with the square of voltage. Generators/ Synch. Condensers – A dynamic source of Vars. Can adjust VAr output rapidly during contingencies. Sources of Reactive (Var) Support Blackout Avoidance
- 13. LOAD RESPONSE TO LOW VOLTAGE LOADRESPONSE TO LOW VOLTAGE Blackout Avoidance
- 14. How Load Respondsto Low Voltage Basic Power System Remote Generation Transmission System Load Center Resistive Load Motor Load VAR Support Local Generation Ps PL QLVL Blackout Avoidance
- 15. Resistive load currentdecreases as voltage goes down Helping the system. Motor loads are constant KVA devices and increase their load current as voltage decreases- Hurts the system. During “Heat Storm” conditions, most load is motor load making blackout more likely. How Load Responds to Low Voltage Blackout Avoidance
- 16. Example of VoltageRecovery From a Transmission Fault- Rapid Voltage Collapse R e sid e n tia l V o lta g e R e co ve ry fo r P h o e n ix A re a In cid e n t o n Ju ly 2 9 , 1 9 9 5 Blackout Avoidance
- 17. RECENT BLACKOUTS REVISITED 2003- East Coast Blackout 2003 - Italian Blackout 2002 - Swedish Blackout 1997 - PJM Disturbance* 1996 - West Coast Blackout 1995 - PECO Disturbance* 1987 - City of Memphis* * Rapid Voltage Collapse Blackout Avoidance
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- 19. POWER SYSTEM INSTABILITIES FourTypes of Instability: Voltage* Steady State * Transient Dynamic *Involved in Recent Blackouts Blackout Avoidance
- 20. REMOTE GENERATION LOCAL LOAD CENTER LINE1 LINE 2 LINE 3 LINE 4 LINE 5 LINE 6 Eg O Es O POWER FLOW Voltage Collapse Scenario Blackout Avoidance
- 21. REMOTE GENERATION LOCAL LOAD CENTER LINE1 LINE 2 LINE 3 LINE 4 LINE 5 LINE 6 Eg O Es O POWER FLOW Voltage Collapse Scenario Blackout Avoidance
- 22. 0 1800 Max. Power Transfer All Linesin Service Line 1 Tripped Line 2 Tripped Pe Pmax = Eg Es X 0g - 0s 90o0 1800 Max. Power Transfer All Lines in Service Line 1 Tripped Line 2 Tripped Pe Pmax = Eg Es X 0g - 0s 90o Pe = Eg Es Sin ( 0g- 0s ) X POWER TRANSFER EQUATION REMOTE GENERATION LOCAL LOAD CENTER LINE 1 LINE 2 LINE 3 LINE 4 LINE 5 LINE 6 Eg Og Es Os POWER FLOW 0 1800 Max. Power Transfer All Lines in Service Line 1 Tripped Line 2 Tripped Pe Pmax = Eg Es X 0g - 0s 90o Steady State Instability Blackout Avoidance
- 23. Generator G GSU System Reactance V Xd XT XS V2 __1____ + 1 2 XT + XS Xd Per Unit MW Per Unit MVAR V2 ___1___ 1 2 XT + XS Xd a) MW - MVAR PER UNIT PLOT X R Xd + XT + XS 2 XT + XS Xd - XT + XS 2 b) R-X DIAGRAM PLOT Xd Steady State Instability Blackout Avoidance
- 24. GENERATOR RESPONDS TO BLACKOUT CONDITIONS GENERATORRESPONDS TO BLACKOUT CONDITIONS Blackout Avoidance
- 25. Gen. AVR Excitation Transformer Generator Step-up Transformer CT VT Generator Field Static Exciter GeneratorExcitation & AVR Control Blackout Avoidance
- 26. The Generator AVR(Automatic Voltage Regulator) Controls Field Current into the Rotor Which in Turn Controls Terminal Voltage and VAr Output/Input. G Reactive Power into System Reactive Power into Generator Real Power into System + MVAR Overexcited Underexcited 0 - MVAR G Under Excitation Limiter (URL) + MW MW MVARS Overexcitation Limiter (OEL) Rotor Winding Limited Stator Winding Limited Stator End Iron Limited MW MVAR System System Normal Overexcited Operation Underexcited Operation How Generators Provide Vars to the System Blackout Avoidance
- 27. DEPENDS ON THEMANUFACTURER: Some Limiters Change as the Square of the Voltage – 90% Voltage Results in 81% of Setting Some Proportional to Voltage – 90% Voltage result in 90% Setting Some Do Not Change with Voltage at all AVR Limiters Response During Low Voltage Blackout Avoidance
- 28. 0 1800 Max. Power Transfer All Linesin Service Breakers 1 and 2 Tripped PM = Pe Pmax = Eg Es X 0g - 0s 90o A2 0C A1 Transient Instability Blackout Avoidance
- 29. Typical Out-of-Step ImpedanceLOCI Blackout Avoidance
- 30. Occurs when fastacting AVR control amplifies rather than damps small MW oscillations. Occurs when generators are remote from load Solution is AVR Power System Stabilizers (PSS) – Low Freq. Filter Required by WECC in the Western USA for generators larger than 30 MVA Dynamic Instability Blackout Avoidance
- 31. GENERATOR PROTECTION RESPOND TOBLACKOUT CONDITIONS GENERATOR PROTECTION RESPOND TO BLACKOUT CONDITIONS Blackout Avoidance
- 32. Loss of Field(40) Overexcitation (24) Overexcitation (24) System Backup (21 & 51V) Under Frequency (81) Out of Step Protection (78) Key Generator Protection Functions Effected by Major System Disturbances Blackout Avoidance
- 33. Generator Protection Effectedby Major System Disturbances Loss of Field (40) – Must be Coordinated with AVR Control, Steady State Stability Limit and Secure Under Low Voltage. Overexcitation (24) - Coordinated with AVR Control. System Backup (21 & 51V) – Secure on Stable Power Swings and System Low Voltage. Must be Coordinated with Transmission Protection. Blackout Avoidance
- 34. Under Frequency (81)– Coordinated with System Load Shedding. Out of Step Protection (78) – Set to trip the Generator if it Losses Synchronism. Generator Protection Effected by Major System Disturbances Blackout Avoidance
- 35. +X -X +R-R - Xd’ 2 Xd Generator Capability Under Excitation Limiter(UEL) Heavy Load Light Load Impedance Locus During Loss of Field 1.0 pu Steady State Stability Limit Zone 1 Zone 2 Loss of Field (40) – Must be Coordinated with AVR Control, Steady State Stability Limit and Secure Under Low Voltage. Blackout Avoidance
- 36. Transformation From Mw-Mvarto R-X Plot Blackout Avoidance
- 37. G Reactive Power into System ReactivePower into Generator Real Power into System + MVAR Overexcited Underexcited 0 - MVAR G Under Excitation Limiter (URL) + MW MW MVARS Overexcitation Limiter (OEL) Rotor Winding Limited Stator Winding Limited Stator End Iron Limited Steady State Stability Limit MW MVAR System System Normal Overexcited Operation Underexcited Operation Overexcitation (24)-Coordinated with AVR Control Blackout Avoidance
- 38. Generator (IEEE/ANSI C-50.12&13) 1.05pu V/Hz on Gen. Base Transformers (IEEE/ANSI C-57.12 ) 1.05pu V/Hz loaded at output 1.10pu V/Hz unloaded Overexcitation Gen./Trans. Capability Blackout Avoidance
- 39. Figure #4C Overexcitation OperatingLimits Blackout Avoidance
- 40. Dual Set-point DefiniteTime 1.18pu V/Hz – 2-6 Sec. Delay 1.10 pu V/Hz – 45-60 Sec. Delay Inverse Time Curve 1.10pu V/Hz Pickup with Curve Selection to Match Gen./Trans V/Hz Capability Curve Typical V/Hz 24 Relay Settings Blackout Avoidance
- 41. J X R Z2 Z1 RPFA Max. Torque Angle Generator Capability Curve Z2 Reachat 50 to 67% of Generator Capability Curve Z2 Reach 120% of Longest Line but Must be Less than 80 to 90% of Capability Curve System Backup (21 & 51V) – Secure on Stable Power Swings and System Low Voltage. Must be Coordinated with Transmission Protection. Blackout Avoidance
- 42. J X R Z2 Z1 Max. Torque Angle Generator Capability Curve Z2 Reachat 50 to 67% of Generator Capability Curve Z2 Reach 120% of Longest Line but Must be Less than 80 to 90% of Capability Curve Z3 out of step blocking Load Encroachment Blocking RPFA Z3 Security Enhancements for Generator Distance Backup Protection Blackout Avoidance
- 43. Voltage Controlled OvercurrentRelays - Voltage control set below emergency system operating voltage. - Current pickup set at 30-40% of full load (Xd). - Time delay set to coordinate with transmission backup. Voltage Restrained Overcurrent Relays - Current pickup varies proportional to voltage and set 150% of gen. Rating at gen. Rated voltage. - Time delay set to coordinate with transmission backup. Generator Voltage Overcurrent (51V) Backup Blackout Avoidance
- 44. Coordinate Under FrequencyTripping of Generator With North American Electric Reliability Council (NERC) System Load Shedding Regions – WECC, ECAR, ERCOT, PJM, Others. Hydro Generators not affected by Under Frequency Gas Turbines Controls Run Back Mw Output When Frequency Drops Under Frequency (81) – Coordinated with System Load Shedding Blackout Avoidance
- 45. Out of StepProtection (78) – Set to trip the Generator if it Losses Synchronism Blackout Avoidance
- 46. Undervoltage Condition NotItself Harmful To Synchronous Generators – V/Hz is a Low Limit. Auxiliary System is Effected By Low Voltage – Auxiliary Motor Tripping Can Shut Down Gens. U.S. Nuclear Plants Have Second Level Voltage Separation Relays on Auxiliary System. Automatic Generator Control (AGC) can Cause Problems when the Power System Breaks-up into Islands. Undervoltage Power Plant Trippings Brought About By System Var Deficits Blackout Avoidance
- 47. SYSTEM UNDERVOLTAGE LOAD SHEDDING (UVLS) SYSTEMUNDERVOLTAGE LOAD SHEDDING (UVLS) Blackout Avoidance
- 48. Utility Undervoltage LoadShedding (UVLS) ATTEMPT TO BALANCE MVAR LOAD WITH MVAR SOURCES BY SHEDDING LOAD. TWO TYPES OF UVLS SCHEMES: Decentralized - Relays Measure Voltage at load to be shed. Centralized – Relays Measure Voltage at Key locations. Voltage transmitted to Central Location and combined with other System Information. Schemes Called SPS or Wide Area Protection. Blackout Avoidance
- 49. Status Of UtilityUndervoltage Load Shedding (UVLS) NERC - UVLS Not Mandatory - Recognized as a Cost-effective Method to Address Voltage Collapse. - Allowed Region to Establish Policy WECC - Most Aggressive in UVLS - Established UVLS Guidelines Blackout Avoidance
- 50. North American ElectricReliability Council (NERC) Regional Areas Blackout Avoidance
- 51. UVLS AT Utilities CentralizedDecentralized - BC Hydro - Puget Sound - Hydro Quebec - First Energy - Entergy - TXU - Public Service of New Mexico - PG&E Blackout Avoidance
- 52. Blackout Avoidance DESIGNING ASECURE UVLS SCHEME DESIGNING A SECURE UVLS SCHEME
- 53. Selection of VoltageRelays for UVLS Measure all Three Voltage or Positive Sequence Voltage. Use Low Voltage Cutoff. Consider Negative Sequence Blocking. Start Timer only if Voltage is Within Window. Use Relay with High Reset Ratio. Digital Voltage Relays are ideal for This Application. Blackout Avoidance
- 54. Three-Phase UVLS Logic 27 27B 47B Va≤ Vb Vc Setpoint #1 ≤ ≤ Setpoint #1 Setpoint #1 AND Adjustable Timer Undervoltage Trip AND SINGLE PHASE UNDERVOLTAGE Va ≤ Vb Vc Setpoint #2 ≤ ≤ Setpoint #2 Setpoint #2 OR xUNDERVOLTAGE BOCK V2 Setpoint #3≥ x NEGATIVE SEQUENCE OVERVOLTAGE BLOCK Blackout Avoidance
- 55. Positive Sequence UVLSLogic V1 = 1/3 ( Va + aVb +a2Vc ) Where: Va,Vb,Vc are line-to- neutral voltages a = 1l120o a2 = 1l240o Balanced Conditions: V1=Va=Vb=Vc. 27 27B 47B V1 ≤Setpoint #1 Adjustable Timer Undervoltage Trip AND POSITIVE SEQUENCE UNDERVOLTAGE Va ≤ Vb Vc Setpoint #2 ≤ ≤ Setpoint #2 Setpoint #2 OR xUNDERVOLTAGE BOCK V2 Setpoint #3≥ x NEGATIVE SEQUENCE OVERVOLTAGE BLOCK Blackout Avoidance
- 56. Point of VoltageMeasurement UTILITY TRANSMISSION SYSTEM 27 81 A C Trip Selected Circuits (A-D) Typical Distribution Substation Transformer with LTC B D 27 = Undervoltage Relay 81= Underfrequency Relay Blackout Avoidance
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- 58. UVLS Setting Considerations RelayEngineers Must Work Closely With System Planning Engineers to Design UVLS. Planning Engineers Have the Load Flow Data Required to Determine the Voltage Measurement Locations and Amount of Load to Shed. They also develop the P-V (Nose Curve) that will determine the Voltage Relay Pickup Setting. Time Delay for UVLS are Typically in the 2-10 Sec. Range – not in Cycles Range for UFLS. Blackout Avoidance
- 59. Undervoltage Relay Pickup MWLOAD VOLTAGE Vcollapse Setting Margin Relay and VT Accuracy Band V setting Allowable Operating Area Operating Margin Blackout Avoidance
- 60. Coordinating UVLS RelayPickup Blackout Avoidance
- 61. NEW M-3401 BECKWITH LOAD SHEDDINGRELAY NEW M-3401 BECKWITH LOAD SHEDDING RELAY Blackout Avoidance
- 62. NEW M-3401 RELAY BlackoutAvoidance
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- 64. M-3401 Protective Functions BlackoutAvoidance 4-Step Phase Undervoltage (27) Protection, single-phase and positive sequence 4-Step Phase Undervoltage, selectable as single phase or positive sequence responding, with Negative sequence overvoltage and single phase undervoltage supervision Phase Overvoltage (59) Protection Four-Step Over/Under Frequency (81) protection Rate of Change of Frequency (81R) protection IPSlogicTM takes the contact INPUT status and function status and generates OUTPUTS by employing (OR, AND, and NOT) boolean Logic and a timer
- 65. Three-Phase UVLS Logic 27 27B 47B Va≤ Vb Vc Setpoint #1 ≤ ≤ Setpoint #1 Setpoint #1 AND Adjustable Timer Undervoltage Trip AND SINGLE PHASE UNDERVOLTAGE Va ≤ Vb Vc Setpoint #2 ≤ ≤ Setpoint #2 Setpoint #2 OR xUNDERVOLTAGE BOCK V2 Setpoint #3≥ x NEGATIVE SEQUENCE OVERVOLTAGE BLOCK Blackout Avoidance
- 66. Positive Sequence UVLSLogic V1 = 1/3 ( Va + aVb +a2Vc ) Where: Va,Vb,Vc are line-to- neutral voltages a = 1l120o a2 = 1l240o Balanced Conditions: V1=Va=Vb=Vc. 27 27B 47B V1 ≤Setpoint #1 Adjustable Timer Undervoltage Trip AND POSITIVE SEQUENCE UNDERVOLTAGE Va ≤ Vb Vc Setpoint #2 ≤ ≤ Setpoint #2 Setpoint #2 OR xUNDERVOLTAGE BOCK V2 Setpoint #3≥ x NEGATIVE SEQUENCE OVERVOLTAGE BLOCK Blackout Avoidance
- 67. Blackout Avoidance 5 ProgrammableOutputs, 2 programmable inputs, and 1 self-test output Oscillographic Recording (COMTRADE file format) Time-Stamped Sequence of Events (SOE) recording for 32 events Metering of Voltage and Frequency Ports – one RS-232 port (COM1) on front and one RS-232 and 485 port (COM2) on rear Setting Software – M-3812 IPScom® For WindowsTM Communica- tions Software Modbus Protocol Relay Voltage Inputs Can Be Directly Connected (no VT required) for voltages < 480 V ac Continuous Self-Diagnostics M-3401 Standard Features
- 68. CONCLUSIONS Voltage Collapse isthe Major Cause of Blackouts in US Power Systems. UVLS is a Viable Method of Providing Protection to Avoid Slow Voltage Collapse Situations. UVLS May be too Slow to respond to Rapid Fault Induced Voltage Collapses. UVLS Requires close Cooperation Between Planners and Relay Engs. UVLS Schemes are More Difficult to Design and Set than UFLS. Blackout Avoidance
- 69. Generator Protection Needsto be Made More Secure During Low Voltage Conditions and be Coordinated with Generator Controls. Methods to do this are Scattered in Various Text Books and Manufactures Literature. My Paper Provides a Single Document with this Information. My Paper Highlights the Important Role the AVR Plays During Major Disturbances. CONCLUSIONS Blackout Avoidance
- 70. BLACKOUT AVOIDANCE & UNDERVOLTAGE LOADSHEDDING BLACKOUT AVOIDANCE & UNDERVOLTAGE LOAD SHEDDING THE END Questions ? THE END Questions ? Blackout Avoidance ©2008 Beckwith Electric Co., Inc.