Out of step
- 1. Copyright © SEL 2007 Out-Of-Step Protection Fundamentals Demetrios Tziouvaras Schweitzer Engineering Laboratories, Inc. IEEE PES San Francisco Chapter December 13, 2007
- 2. Introduction The aim of this presentation is to explain The fundamentals of out-of-step (OOS) protection Discuss which relays and relay systems are prone to operate during power swings Share experiences and lessons learnt from the past to avoid making the same mistakes
- 3. Introduction Interconnected systems experienced an increased number of large disturbances in the last 15 years Protective relay systems are often involved during major disturbances In many cases they prevent further propagation of the disturbance In some cases undesired relay operations have contributed to cascading blackouts
- 4. Outline Out of step (OOS) protection fundamentals Relay performance during OOS conditions Transmission lines Generators System design and protection improvements Conclusions
- 5. What Is a Power Swing? Variation of power flow which occurs when generator rotor angles are advancing or retarding relative to each other in response to: System faults Line switching Major load switching Loss of large generation Major system disturbances
- 6. Stable and Unstable Power Swings Definitions A power swing is considered stable if the generators do not slip poles and the system reaches a new state of equilibrium, i.e. an acceptable operating condition An unstable power swing results in a generator or group of generators experiencing pole slipping or loss-of- synchronism for which some corrective action must be taken. Out-of-step is the same as an unstable power swing.
- 7. Power Swings can Cause Undesired Protective Relay Operation Power swings can cause undesired relay operation that may lead to: Undesired tripping of power system elements at undesired network locations Weakening of the power system Possible cascading outages and shutdown of major portions of the power system Damage of circuit breakers due to uncontrolled tripping Loss of human life
- 8. Unstable Power Swings Damage System Integrity Pole slipping may damage generators and turbines Low voltage conditions experienced during unstable power swings may cause: Motor stalling Generator tripping Damage to voltage-sensitive loads Prolonged low voltages could cause instability of smaller areas within a utility’s system
- 9. Need for Out-of-Step Protection Generators operating asynchronously with the rest of the power system cannot regain stability as a result of any excitation or regulator action Asynchronous power system areas must be separated in a controlled fashion to avoid: Equipment damage Widespread outages in the power system
- 10. Philosophy of Power-Swing Protection Detect both stable and unstable power swings Block tripping of relay elements prone to operate during power swings Differentiate between stable and unstable power swings Separate the system into islands during out-of-step conditions
- 11. Philosophy of Power-Swing Protection Separate the system at locations that provide good balance of load/generation in the resulting system islands Trip only at pre-selected network locations and block tripping at all other locations Trip only under controlled transient recovery voltages or with low current
- 12. Power System Stability Brief Review
- 13. Power System Stability Definition The ability of the electric power system to regain a state of operating equilibrium after being subjected to disturbances such as faults, line switching, load rejection, loss-of-excitation, and loss of generation.
- 14. Power Flow Two-Machine System 1 2 3 4 Line 1 VS VR Line 2 X δ ⋅ = sin X VV P RSVS VR δ
- 15. Effect of Fault Type on Power Transfer δ P Three-Phase Fault Phase-Phase-Ground Fault Phase-Phase Fault Single-Line-Ground Fault Normal System 1 2 3 4 VS VR Line 1 Line 2
- 16. Transient Stability Concepts 1 2 3 4 Prefault state (Both lines in service) Fault state Fault state with breaker 3 open Post-fault state (Line 2 out) VS VR Line 1 Line 2
- 17. Equal-Area Criterion δ180° P0 P Fault (one breaker open) Fault Prefault Post-Fault 1 0 2 3 4 5 6 Area 2Area 1 1 2 3 4 VS VR Line 1 Line 2
- 18. Effect of Fault Clearing Time Unstable System Fault Post-Fault δ Prefault P
- 19. Stable and Unstable Power Swings Rotor Angle Stable System Unstable System t δ δ0 δ1
- 20. Angular Instability Distinguishing Features Large voltage variations Large power oscillations Loss of synchronism Zero voltage at the electrical center Frequency excursions
- 21. Angular Instability Large Voltage Variations 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -1 0 1 Voltage MagnitudePerunit Seconds 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0 0.5 1 1.5 PerUnit Seconds
- 22. Angular Instability Large Power Oscillations 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -1000 -500 0 500 Real and Reactive Power MW Seconds 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -400 -200 0 200 400 600 MVar Seconds
- 23. Angular Instability V1 and Angle of V1 / I1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.2 0.4 0.6 Positive-Sequence Voltage Magnitude PerUnit Seconds 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -200 0 200 400 Angle of (V1 / I1) Degrees Seconds
- 24. Relay Elements Prone to Operate During Power Swings
- 25. Stable and Unstable Power Swings Impedance Trajectories Stable Swing Unstable Swing R X
- 26. Relay Elements Prone to Operate During Power Swings Instantaneous phase overcurrent Directional and non-directional Undervoltage Short time or instantaneous Zone 1 distance Zone 2 distance used in POTT scheme
- 27. Line Relays Prone to Operate During Angular Instability Zone 1 distance or overreaching distance elements applied in DCB or POTT schemes Potential reasons are: Lack of PSB function or improper settings of the PSB function Lack of frequency tracking and long memory polarizing voltage Phasor measurement errors due to large excursions of system frequency during islanding
- 28. Relay Systems Unresponsive to Power Swings Phase comparison Line current differential Pilot-wire
- 29. Impedances Measured by Distance Relays 21 V I ZS ZL ZR VS VR ( ) SRLS RS S ZZZZ VV V IVZ −++ − ==
- 30. Impedance Locus for k = ES / ER = 1.0 δ ZL ZR X R δ increases δ decreases δ=180° ZS 0.5ZT Z R S S T Z 2 cotj1 2 Z Z −⎟ ⎠ ⎞ ⎜ ⎝ ⎛ δ −= R S E E k =
- 31. Two-Machine System Impedance Locii R X S k > 1 ES > ER Rk = 1 k < 1 ES < ER
- 33. Clarify the Terminology Unstable power swing Out of step (OOS) Out-of-step blocking (OSB) Power-swing blocking (PSB) Out-of-step tripping (OST) Pole-slip tripping
- 34. Power-Swing Protection Relay Functions Power-swing blocking (PSB) Detects both stable and unstable power swings Prevents operation of protection elements Out-of-step tripping (OST) Detects unstable power swings or OOS Separates system into islands with good generation / load balance
- 35. Conventional PSB Scheme Load Region X R A B Z1 Inner Z ElementDistance Element Outer Z Element
- 36. Conventional OST Scheme Load Region X R A B Z1 Inner Z ElementDistance Element Outer Z Element
- 37. Disadvantages of Conventional PSB Scheme Needs detailed system information Requires extensive system stability studies It is difficult to set for long lines with heavy loads May fail after severe disturbances on marginally stable systems May fail during swings with high slip frequency
- 38. Long Line With Heavy Load ZL => ZΣ A R ZR ZS Z2 X B Swing Locus Trajectory ZL
- 39. Short Line With Light Load ZL << ZΣ A R ZR ZS Z2 X B Swing Locus Trajectory ZL
- 40. Unstable Swing After Severe Disturbance A R ZR ZS Z2 X Swing Locus Trajectory B ZL
- 41. New Zero-Setting PSB Function Uses swing-center voltage (SCV) Has no user settings Does not need system parameters Does not require system stability studies Provides PSB during pole open Detects evolving faults during power swings
- 42. Swing-Center Voltage (SCV) ( ) ( ) ( ) ⎟ ⎠ ⎞ ⎜ ⎝ ⎛δ ⋅⎟ ⎠ ⎞ ⎜ ⎝ ⎛ δ +ω= 2 t cos 2 t tsinE2tSCV o' o o" Z1S•I Z1L•I VS ER δ SCV Z1R•I ES VR
- 43. SCV During System OOS Condition Swing-Center Voltage seconds voltage(pu) SCV Amplitude 0 0.05 0.1 0.15 0.2 -1 -0.5 0 0.5 1
- 44. Local Estimate of SCV: Vcosϕ ϕ⋅≈ cos|V|SCV S o'Z1S•I Z1R•Iθ VS ES ϕ SCV ER I Vcosϕ VR AnglepedanceImSystem:θ
- 45. Local Estimate of SCV: Vcosϕ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ δ ⋅= 2 cos1E1SCV ( ) dt d 2 sin 2 1E dt 1SCVd δ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ δ −=
- 46. Vcosϕ for 1-Rad/Sec OOS Condition 0 0 90 180 270 360 E1 E1/2 d (SCV1) / dt δ SCV1
- 47. Benefits of SCV for PSB Application Independent of system source and line impedance Bounded: Lower limit: zero Upper limit: close to one per unit Relates directly to angle difference of two sources, δ
- 48. Transmission Line Relay Performance During Out-of-Step Conditions
- 49. 500 kV System Station C Station D Station E Line 1 Line 3 Line 2 Unit 1 Unit 2 Lines 4, 5, and 6 Intertie System B System A
- 50. Angular Instability Z1 Trajectory – Line 2 at Station C -10 -5 0 5 10 -10 -8 -6 -4 -2 0 2 4 6 8 10 23 25 27 29 31 33 35 37 39 41 43 45 Positive-Sequence Impedance (Z1) Locus Im(Z1)ohm Re(Z1) ohm
- 51. EHV System – Northern California Station C Station D Station E Line 1 Line 3 Line 2 Unit 1 Unit 2 Lines 4, 5, and 6 Intertie System B System A
- 52. Zone 1 Operation During OOS PSB Function not Enabled
- 53. Zone 1 Distance Is Blocked First Slip Cycle
- 54. Zone 1 Distance Operates Second Slip Cycle
- 55. Zone 1 Distance Operates Second Slip Cycle Fast slip frequency change Setting fine-tuning could have prevented operation of Zone 1 Concentric zone settings Separation between concentric zones PSB timer All of these settings are difficult to make Long heavy loaded lines Require large number of stability studies
- 56. Proper Blocking of Distance Elements by Zero Setting PSB 0 5 10 15 20 -1 -0.5 0 0.5 1 SCV1 (Solid), dSCV1/dt (Dash) (pu),(pu/cyc) 0 5 10 15 20 Cycle S PSB DPSB 67QUB 3PF Reset PSB Reset SLD Set Start-Zn SSD Set
- 57. Generator Relay Performance During Disturbances
- 58. Units 1 and 2 Operations Station C Station D Station E Line 1 Line 3 Line 2 Unit 1 Unit 2 Lines 4, 5, and 6 Intertie System B System A
- 59. One Unit Trips by Undervoltage 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -1 -0.5 0 0.5 1 Station C Voltage Perunit Seconds
- 60. Unit 1: Three-Phase P and Q During OOS 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -2000 0 2000 Real Power MW Seconds 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 -1000 0 1000 Reactive Power MVar Seconds
- 61. Units 1 and 2 Trip Inst. Dir. Phase OC Relay -20 -10 0 10 20 -40 -30 -20 -10 0 10 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Positive-Sequence Impedance (Z1) Locus Im(Z1)ohm Re(Z1) ohm
- 62. Protection System and Other System Improvements to Preserve System Stability
- 63. Proper System and Protection Design Preserve System Stability Prevent the occurrence of out-of-step conditions Install sufficient transmission capacity Maintain adequate reactive reserves Apply high-speed relaying systems and high-speed reclosing Apply single-phase tripping and reclosing Apply wide-area stability controls
- 64. Wide-Area Stability Controls or SIPS Preserve System Stability Wide-area stability controls Generator dropping Direct load dropping Fast valving Insertion of breaking resistors Series and shunt capacitor insertion Use FACTS devices
- 65. Protection System and Other Improvements Improvements in transient stability High speed fault clearing Single phase tripping and reclosing Apply local breaker failure protection on all EHV and critical HV substations Special protection systems Controlled system separation UVLS and UFLS
- 66. Protection System Improvements Apply dual pilot protection relay systems on all EHV and critical HV systems with PSB capability, or with systems that are immune to stable or unstable power swings Replace secondary non-pilot line relay systems in non-critical HV lines with: A relay system that has similar functionality with the main pilot protection system Consider switching the communications channel to the secondary relay system when the Main 1 is out of service
- 67. Conclusions Utilities must take every action economically justifiable to preserve system stability Out-of-step tripping should be applied and operate only as a last resort to preserve system stability OST and PSB should be applied based on an inter-regional controlled system separation philosophy
- 68. Conclusions OOS tripping must separate the system at predetermined locations to minimize the effect of the disturbance OOS blocking compliments OOS tripping by blocking relay elements prone to operate and ensures a controlled system separation Controlled separation schemes provide a safety net to lessen the impacts of major disturbances
- 69. References Out-Of-Step Protection Fundamentals and Advancements http://www.selinc.com/techpprs/6163.pdf Zero-Setting Power-Swing Blocking Protection http://www.selinc.com/techpprs/6172_ZeroSetting_20050302.pdf Relay Performance During Major System Disturbances http://www.selinc.com/techpprs/6244_RelayPerformance_DT_20060914.pdf
- 70. Thank You