Differential protection of power transformer

1. Thesis Defense By Advised By ANALYSIS OF TRADITIONAL AND IMPROVED TRANSFORMER DIFFERENTIAL PROTECTIVE RELAYS

2. Contents 1. Introduction 2. Background 3. Problem Statement 4. Design, Experiments & Modeling 5. Conclusion 6. References 9/9/20142

3. Transformer: Function principle and equivalent circuit 11/28/2015 3

4. Causes of Transformer Failures 11/28/2015 4  Transformer failures cause about 100 millions in England Only, and it’s happen for couple of kinds of Faults and failure:  INTERNAL FAULTS  – Incipient faults  • Overheating  • Over-fluxing  • Overpressure  – Active faults  • Short circuit in wye connected  windings  • Short circuits in delta windings  • Phase-to-phase faults  • Turn-to-turn faults  • Core faults  • Tank faults

5. Causes of Transformer Failures 11/28/2015 5  Winding failures 51%  Tap changer failures 19%  Bushings failures 9%  Terminal board failures 6%  Core failures 2%  Miscellaneous failures 13%  Differential protection can detect all of the types of  failures above

6. Power Transformer Differential Protection  Differential protection is one of the most reliable and popular techniques in power system protection.  In 1904, British engineers Charles H. Merz and Bernard Price developed the first approach for differential protection. 11/28/2015 6

7. Transformer Differential Protection special qualities Angle shifting N·30° due to vector group (0 ≤ N ≤ 11) for 3-phase transformers. Different current values of the CT- sets on the high voltage side (HV) and on the low voltage side (LV) Zero sequence current in case of external faultswill cause differential current Transformer-tap changer, magnetising current Transientcurrents: Inrush , CT-saturation

8. Relay Characteristics 11/28/20158 2.0 8.0 9.0 3.0 InOIRest InO IDiff 7.0 2.5 Slope 1 Slope 2 IDiff> 0 1.0 4.0 5.03.0 6.00 1.0 0.5 2.0 1.5 x Trip Block Total CT- error Tap- changer Magnet. current Example: Transformerwith Tap changer

9.  Current Mismatch Caused the Transformation Ratio and by Differing CT Ratios  Delta‐Wye Transformation of Currents  CT Saturation, CT Remanence, and CT Tolerance  Inrush Phenomena and Harmonic Content Availability  Over Excitation Phenomena  Switch Onto Fault concerns Challenges to Understanding Transformer Differential Protection 11/28/2015 9

10. Inrush Phenomena and Harmonic Content Availability 11/28/2015 10

11. Inrush Phenomena and Harmonic Content Availability 11/28/2015 11

12. Inrush Phenomena and Harmonic Content Availability 11/28/2015 12 residual flux – worst-case conditions result in the flux peak value attaining 280% of normal value point on wave switching number of banked transformers transformer design and rating system fault level System Impedance, and X/R ratio of the system

13. CT Saturation, CT Remanence, and CT Tolerance 11/28/2015 13

14. Background and History of Differential Protection of Power Transformer 11/28/2015 14 • The first solution to this problem was to introduce an intentional time delay in the differential relay by I. T. Monseth. • desensitize the relay for a given time, to override the inrush condition by E. Cordray. • Using all the harmonics to restrain the tripping signal. • Using 2nd and 5th harmonic for restraining or blocking

15. Background and History of Differential Protection of Power Transformer 11/28/2015 15

16. Problem Statement 11/28/2015 16  In order to provide a high security for differential protection in case of switching power transformer.  Inrush current still cause relay failures.  Trip signal can be initiated due to DC component with long time decay.  Continuous failures of relay to recognize inrush current will cause unwanted long duration interruptions.

17. Research Contribution 11/28/2015 17  suggested technique prevents the relay from tripping using DC component restraining combined with 2nd & 5th Harmonic blocking.  Suggest improvement in the existing setting for the relay installed in the grid to increase the security of those relays during switching of power transformer.

18. Design Analysis, Experiments & Modeling 11/28/2015 18 • Event recorded in 27/12/2012 at 11:34 AM: Primary Currents of Power Transformer

19. Design, Experiments & Modeling 9/9/201419 • Event recorded in 27/12/2012 at 11:34 AM: Binary Output of the Relay

20. Design, Experiments & Modeling 9/9/201420 • Event recorded in 27/12/2012 at 11:34 AM: Harmonics Contents at 0.0 time

21. Design, Experiments & Modeling 9/9/201421 • Event recorded in 27/12/2012 at 11:34 AM: Harmonics Contents at Tripping time

22. Design, Experiments & Modeling 9/9/201422 • Methodology DC Harmonic Restraining 2th harmonic Blocking

23. Design, Experiments & Modeling 9/9/201423 • Methodology IRT = K(Iw1 + Iw2) Iop > SLP*IRT + K5 I5 And Iop > K2 I2 • Where IRT is Restraining Current • Iop is Differential Current • SLP is Slope Characteristic of the Relay

24. Design, Experiments & Modeling 9/9/201424  Using Discrete Fourier Transformation • Discrete Fourier series representation of periodic sequence  The discrete Fourier series coefficients ,...,n,W)k(X ~ N ...,,n,e)k(X ~ N )]k(X ~ [IDFS)n(x~ N k nk N N k kn N j 10 1 10 1 1 0 1 0 2           ,...,k,W)n(x~ ...,,k,e)n(x~)]n(x~[DFS)k(X ~ N n nk N N n nk N j 10 10 1 0 1 0 2          

25. Relay Modeling 9/9/201425  Simulation of power system with Proposed Relay Methodology

26. Relay Modeling 9/9/201426  Simulation of power system with Proposed Relay Methodology

27. Relay Modeling 9/9/201427  DFT :  Using Full Wave Cycle Discrete Fourier Transform Method with eight samples per cycle Discrete Fourier Transform Block in Simulink

28. Relay Modeling 9/9/201428  Single Line Diagram of Simulated System

29. Design, Experiments & Modeling 9/9/201429  Simulation of power system with Proposed Relay Methodology  Case 1: Normal Switching ( at different angles)  Case 2: External Three & single phase Faults  Case 3: Single Line to Ground Fault  Case 4: Double Line Fault  Case 5: Double Line to Ground Fault

30. Design, Experiments & Modeling 11/28/201530  Simulation of power system with Proposed Relay Methodology  Case 1: Normal Switching ( at 0 angle) Primary Currents at Zero angle

31. Design, Experiments & Modeling 11/28/201531  Simulation of power system with Proposed Relay Methodology  Case 1: Normal Switching ( at 0 angle) Restraining Current in phase A

32. Design, Experiments & Modeling 11/28/201532  Simulation of power system with Proposed Relay Methodology  Case 1: Normal Switching ( at 0 angle) Differential, 2nd,DC Currents in phase A

33. Design, Experiments & Modeling 11/28/201533  Simulation of power system with Proposed Relay Methodology  Case 1: Normal Switching ( at 45 angle) Primary Currents at Zero angle in phase A

36. Design, Experiments & Modeling 11/28/201536  Simulation of power system with Proposed Relay Methodology  Case 1: Normal Switching ( at 90 angle) Primary Currents at 90 angle

39. Design, Experiments & Modeling 11/28/201539  Simulation of power system with Proposed Relay Methodology  Case 2: External Three phase Faults Primary Currents of Power Transformer

40. Design, Experiments & Modeling 11/28/201540  Simulation of power system with Proposed Relay Methodology  Case 2: External Three phase Faults Signal Trip

41. Design, Experiments & Modeling 11/28/201541  Simulation of power system with Proposed Relay Methodology  Case 4: Double Line Fault Primary Currents of Power Transformer

42. Design, Experiments & Modeling 11/28/201542  Simulation of power system with Proposed Relay Methodology  Case 4: Double Line Fault Signal Trip

43. Design, Experiments & Modeling 11/28/201543  Simulation of power system with Proposed Relay Methodology  Case 5: Double Line to Ground Fault Primary Currents of Power Transformer

44. Design, Experiments & Modeling 11/28/201544  Simulation of power system with Proposed Relay Methodology  Case 5: Double Line to Ground Fault Signal Trip

45. Design, Experiments & Modeling 11/28/201545  Summary of all tested cases Case Type Relay Response Trip signal release time (m sec) Loaded Unloaded Inrush Current Restrain No Trip Signal Single Line to Ground Trip 11.2 20 External Three phase Fault Restrain/Trip No Trip Signal No Trip Signal Double Line Fault Trip 7.5 6.2 Double Line to Ground Trip 20 21

46. Design, Experiments & Modeling 11/28/201546  Summary :  Fast in Operation and make no delay in case of faults.  security (no false trips).  distinguish between in inrush and other types of faults.  No need for system impedance Value and reduce measurement in the relay

47. 11/28/2015 47 Suggested Setting  Existing Setting: Function Value IDIFF Pickup 0.20 I/In0 1 Slope characteristic 0.25 I/In0 2 Slope characteristic 0.50 I/In0 2nd Harmonic Content 15% Cross Blocking for 2nd Harmonic 3 Cycle 5th Harmonic Content 30% Trip without Delay 7.5 I/In0

48. Relay Testing 11/28/2015 48 • Tests done by injection the recorded event again to relay . • Transplay the event by (OMOCRON 257 6output). • To analyze suggested setting through faults, Power System Model by ATP/EMTP environment.

49. Relay Testing Results 11/28/2015 49 No load Test of power transformer Current Transformer F/I curve

50. Relay Testing Results 11/28/2015 50 • Relay Setting Tested: Parameter s IDIFF Cross blocking 2nd content Case 1 0.25 3 Cycle 15 % Case 2 0.25 5 Cycle 15 % Case 3 0.25 15 Cycle 15 % Case 4 0.25 5 Cycle 10% Case 5 0.25 3 Cycle 12% Case 6 0.27 20 Cycle 15% Case 7 0.27 20 Cycle 20% Case 8 0.27 20 Cycle 25%

51. Relay Testing Results 11/28/201551 • Results for Each Suggested Setting : Parameter s Inrush SLG Case 1 Trip Trip at 20 ms Case 2 Trip Trip at 30 ms Case 3 Trip Trip at 20 ms Case 4 Trip Trip at 20 ms Case 5 Trip Trip at 19 ms Case 6 OFF Trip at 300 ms Case 7 OFF Trip at 400 ms Case 8 OFF Trip at 20 ms

52. Relay Testing Results 11/28/201552 • Conclusion • Inrush Current Events usually had a 2nd harmonic magnitude between 20-25 % in first 2 Cycles. • Cross Blocking function give suppress trip signal in case of inrush current with high DC components. • Most fault current has 2nd harmonic content lower than 19%. • DC component in inrush current could lead to relay misoperation.

53. References 1) Gerhard Ziegler “Numerical Distance protection” Second Edition, GmbH, GWA, 2) Sandro Gianny Aquiles Perez “Modeling Relays for Power System Protection Studies” A Thesis For the Degree of Doctor of Philosophy in the Department of Electrical Engineering University of Saskatchewan Saskatoon, Saskatchewan Canada by© Copyright Sandro G. Aquiles Perez, July 2006.E. Sortomme, S. 3) Arun G. Phadke, James S. Thorp “Computer Relying for Power Systems” Second Edition A John Wiley and Sons, Ltd., Publication. Copyright © 2009 4) MATLAB, the language of technical computing, Version 7.6.0.324(R2008a), 1984- 2008, The MathWork.INC. 5) H. Dommel, “EMTP Reference Manual,” Bonneville Power Administration 1986. E. A. Klingshirn, H. R. Moore, and E. C. Wentz, “Detection of faults in power transformers,” AIEE Transactions, pt. III, vol. 76, pp. 87–95, Apr. 1957. 6) I. T. Monseth and P. H. Robinson, Relay Systems: Theory and Applications. New York: McGraw Hill Co., 1935. 7) R. E. Cordray, “Percentage differential transformer protection,” Electrical Engineering, vol. 50, pp. 361363, May 1931 8) S. E. Zocholl, A. Guzmán, and D. Hou, “Transformer modeling as applied to differential protection,” in 22nd Annual Western Protective Relay Conference, Spokane, WA, Oct. 24–26, 1995. 11/28/201553

54. 11/28/201554

Differential protection of power transformer

Differential protection of power transformer

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