Protection primer

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Protection primer

  1. 1. Power System Protection FundamentalsWhat should we teach studentsabout power system protection? Copyright © SEL 2008
  2. 2. Agenda Why protection is needed Principles and elements of the protection system Basic protection schemes Digital relay advantages and enhancements Copyright © SEL 2008
  3. 3. Disturbances: Light or Severe The power system must maintain acceptable operation 24 hours a day  Voltage and frequency must stay within certain limits Small disturbances  The control system can handle these  Example: variation in transformer or generator load Severe disturbances require a protection system  They can jeopardize the entire power system  They cannot be overcome by a control system Copyright © SEL 2008
  4. 4. Power System ProtectionOperation during severe disturbances:  System element protection  System protection  Automatic reclosing  Automatic transfer to alternate power supplies  Automatic synchronization Copyright © SEL 2008
  5. 5. Electric Power System Exposure to External Agents Copyright © SEL 2008
  6. 6. Damage to Main Equipment Copyright © SEL 2008
  7. 7. Protection System A series of devices whose main purpose is to protect persons and primary electricpower equipment from the effects of faults The “Sentinels” Copyright © SEL 2008
  8. 8. Blackouts Characteristics Main Causes Loss of service in a  Overreaction of the large area or protection system population region  Bad design of the Hazard to human life protection system May result in enormous economic losses Copyright © SEL 2008
  9. 9. Short Circuits Produce High Currents Three-Phase Line a b c I Substation FaultThousands of Amps I Wire Copyright © SEL 2008
  10. 10. Electrical Equipment Thermal Damage t Damage Damage Curve Time I In Imd Short-Circuit Rated Value Current Copyright © SEL 2008
  11. 11. Mechanical Damage During Short Circuits Very destructive in busbars, isolators, supports, transformers, and machines Damage is instantaneous Mechanical Forces f1 f2 i1 i2Rigid Conductors f1(t) = k i1(t) i2(t) Copyright © SEL 2008
  12. 12. The Fuse Fuse Transformer Copyright © SEL 2008
  13. 13. Protection System Elements Protective relays Circuit breakers Current and voltage transducers Communications channels DC supply system Control cables Copyright © SEL 2008
  14. 14. Three-Phase Diagram of the Protection Team Copyright © SEL 2008
  15. 15. DC Tripping Circuit Copyright © SEL 2008
  16. 16. Circuit Breakers Copyright © SEL 2008
  17. 17. Current TransformersVery High Voltage CT Medium-Voltage CT Copyright © SEL 2008
  18. 18. Voltage Transformers Medium Voltage Note: Voltage transformers are also known as potentialHigh Voltage transformers Copyright © SEL 2008
  19. 19. Protective Relays Copyright © SEL 2008
  20. 20. Examples of Relay Panels Microprocessor- Based RelayOld Electromechanical Copyright © SEL 2008
  21. 21. How Do Relays Detect Faults? When a fault takes place, the current, voltage, frequency, and other electrical variables behave in a peculiar way. For example:  Current suddenly increases  Voltage suddenly decreases Relays can measure the currents and the voltages and detect that there is an overcurrent, or an undervoltage, or a combination of both Many other detection principles determine the design of protective relays Copyright © SEL 2008
  22. 22. Main Protection Requirements  Reliability  Dependability  Security  Selectivity  Speed  System stability  Equipment damage  Power quality  Sensitivity  High-impedance faults  Dispersed generation Copyright © SEL 2008
  23. 23. Primary Protection Copyright © SEL 2008
  24. 24. Primary Protection Zone Overlapping Protection Zone A 52 Protection Zone B To Zone A Relays To Zone B Relays Protection Zone A 52 Protection Zone B To Zone A Relays To Zone B Relays Copyright © SEL 2008
  25. 25. Backup Protection Breaker 5 Fails C DA E 1 2 5 6 11 12 TB F 3 4 7 8 9 10 Copyright © SEL 2008
  26. 26. Typical Short-Circuit Type DistributionSingle-Phase-Ground: 70–80%Phase-Phase-Ground: 17–10%Phase-Phase: 10–8%Three-Phase: 3–2% Copyright © SEL 2008
  27. 27. Balanced vs. Unbalanced Conditions IaIc Ic Ia Ib IbBalanced System Unbalanced System Copyright © SEL 2008
  28. 28. Decomposition of an Unbalanced System Copyright © SEL 2008
  29. 29. Power Line Protection Principles  Overcurrent (50, 51, 50N, 51N)  Directional Overcurrent (67, 67N)  Distance (21, 21N)  Differential (87) Copyright © SEL 2008
  30. 30. Application of Inverse-Type Relays Relay t Operation TimeI Radial Line Fault Load Copyright © SEL 2008
  31. 31. Inverse-Time Relay CoordinationI Distancet } ∆T } ∆T } ∆T Distance Copyright © SEL 2008
  32. 32. Addition of Instantaneous OC Element Relay t Operation Time I Radial Line Fault Load Copyright © SEL 2008
  33. 33. 50/51 Relay CoordinationI Distancet } ∆T } ∆T } ∆T Distance Copyright © SEL 2008
  34. 34. Directional Overcurrent Protection Basic Applications K L Copyright © SEL 2008
  35. 35. Directional Overcurrent Protection Basic Principle V I F2 F1 Relay Reverse Fault (F2) Forward Fault (F1) I V V I Copyright © SEL 2008
  36. 36. Overcurrent Relay Problem E I SETTING ≈ Z S1 + (0.8) Z L1 Relay operates when the following condition holds: I FAULT = I a > I SETTING As Z s1 changes, the relay’s “reach” will change, since setting is fixed E I FAULT ( LIMIT ) = ′ Z S1 + (0.8) Z L1 Copyright © SEL 2008
  37. 37. Distance Relay Principle L d I a , Ib , Ic Radial 21 Three-PhaseVa ,Vb ,Vc Line Solid FaultSuppose Relay Is Designed to OperateWhen: | Va |≤ (0.8) | Z L1 || I a | Copyright © SEL 2008
  38. 38. The Impedance Relay Characteristic R 2 + X 2 ≤ Z r21 X Plain Impedance Relay Operation Zone Z ≤ Z r1 Radius Zr1 Zr1 R Copyright © SEL 2008
  39. 39. Need for Directionality F2 F1 1 2 3 4 5 6 RELAY 3 X Operation Zone F1 F2 RNonselectiveRelay Operation Copyright © SEL 2008
  40. 40. Directionality Improvement F2 F1 1 2 3 4 5 6 RELAY 3 X Operation Zone Directional Impedance F1 Relay Characteristic F2 RThe Relay WillNot Operate forThis Fault Copyright © SEL 2008
  41. 41. Mho Element Characteristic(Directional Impedance Relay) Operates when: V ≤ I Z M cos( ϕ − ϕ MT ) Z ≤ Z M cos( ϕ − ϕ MT ) Copyright © SEL 2008
  42. 42. Three-Zone Distance ProtectionTime Zone 3 Zone 2 Zone 1 1 2 3 4 5 6 Time Zone 1 Is Instantaneous Copyright © SEL 2008
  43. 43. Line Protection With Mho Elements X C B A R D E Copyright © SEL 2008
  44. 44. Circular Distance Relay Characteristics X X PLAIN OFFSET IMPEDANCE MHO (2) R R X X LENS MHO (RESTRICTED MHO 1) R R X X OFFSET TOMATO MHO (1) (RESTRICTED MHO 2) R R Copyright © SEL 2008
  45. 45. Semi-Plane Type Characteristics X X DIRECTIONAL RESTRICTED DIRECTIONAL R R X X REACTANCE RESTRICTED REACTANCE R R X X OHM QUADRILATERAL R R Copyright © SEL 2008
  46. 46. Distance Protection Summary Current and voltage information Phase elements: more sensitive than 67 elements Ground elements: less sensitive than 67N elements Application: looped and parallel lines Copyright © SEL 2008
  47. 47. Directional ComparisonPilot Protection Systems Copyright © SEL 2008
  48. 48. Permissive Overreaching Transfer Trip Copyright © SEL 2008
  49. 49. Basic POTT Logic Key XMTRZone 2 Elements AND Trip RCVR Copyright © SEL 2008
  50. 50. Directional Comparison Blocking Scheme Copyright © SEL 2008
  51. 51. Basic DCB LogicZone 3 Key XMTR Carrier Coordination Time Delay CCZone 2 0 TripRCVR Copyright © SEL 2008
  52. 52. Differential Protection Principle Balanced CT Ratio CT CT Protected Equipment External Fault 50 IDIF = 0No Relay Operation if CTs Are Considered Ideal Copyright © SEL 2008
  53. 53. Differential Protection Principle CTR CTR Protected Equipment Internal Fault 50 IDIF > ISETTING Relay Operates Copyright © SEL 2008
  54. 54. Problem of Unequal CT Performance CT CT Protected Equipment External Fault 50 IDIF ¹ 0  False differential current can occur if a CT saturates during a through-fault  Use some measure of through-current to desensitize the relay when high currents are present Copyright © SEL 2008
  55. 55. Possible Scheme – PercentageDifferential Protection Principle ĪSP ĪRP CTR Protected CTR Equipment ĪS ĪR Relay (87) Compares: I OP = I S + I R | IS | + | IR | k ×I RT =k× 2 Copyright © SEL 2008
  56. 56. Differential Protection Applications  Bus protection  Transformer protection  Generator protection  Line protection  Large motor protection  Reactor protection  Capacitor bank protection  Compound equipment protection Copyright © SEL 2008
  57. 57. Differential Protection Summary The overcurrent differential scheme is simple and economical, but it does not respond well to unequal current transformer performance The percentage differential scheme responds better to CT saturation Percentage differential protection can be analyzed in the relay and the alpha plane Differential protection is the best alternative selectivity/speed with present technology Copyright © SEL 2008
  58. 58. Multiple Input Differential Schemes Examples Differential Protection Zone ĪSP ĪRP ĪT I1 I2 I3 I4 OPBus Differential: Several Inputs Three-Winding Transformer Differential: Three Inputs Copyright © SEL 2008
  59. 59. Advantages of Digital Relays Compatibility with Low maintenance Multifunctional digital integrated (self-supervision) systemsHighly sensitive, Highly reliable secure, and Adaptive (self-supervision) selectiveReduced burden Programmable on Low Cost Versatile CTs and VTs Copyright © SEL 2008
  60. 60. Synchrophasors Provide a“Snapshot” of the Power System Copyright © SEL 2008
  61. 61. The Future Improvements in computer-based protection Highly reliable and viable communication systems (satellite, optical fiber, etc.) Integration of control, command, protection, and communication Improvements to human-machine interface Much more Copyright © SEL 2008

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