This thesis analyzes traditional and improved transformer differential protective relays. It proposes a technique using DC harmonic restraining combined with 2nd harmonic blocking to prevent relay tripping during transformer energization while maintaining security during faults. Simulation results show the method can distinguish between inrush and fault currents, with no unnecessary delays for faults. Testing of various parameter settings found blocking for 15-25% 2nd harmonic content over 3-20 cycles minimized misoperations without reducing security. The improved differential relay performance was validated through simulations and laboratory tests.

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
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3. Transformer: Function principle and equivalent circuit
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3

4. Causes of Transformer Failures
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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
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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.
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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
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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
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9

10. Inrush Phenomena and Harmonic Content
Availability
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10

11. Inrush Phenomena and Harmonic Content
Availability
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11

12. Inrush Phenomena and Harmonic Content
Availability
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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
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13

14. Background and History of Differential
Protection of Power Transformer
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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
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15

16. Problem Statement
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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
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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
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18
• Event recorded in 27/12/2012 at 11:34 AM:
Primary Currents of Power
Transformer

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

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

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

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

23. Design, Experiments & Modeling
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• 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
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 Simulation of power system with Proposed Relay
Methodology

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

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

28. Relay Modeling
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 Single Line Diagram of Simulated System

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

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

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

34. Design, Experiments & Modeling
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 Simulation of power system with Proposed Relay
Methodology
 Case 1: Normal Switching ( at 45 angle)
Restraining Current in phase A

35. Design, Experiments & Modeling
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 Simulation of power system with Proposed Relay
Methodology
 Case 1: Normal Switching ( at 45 angle)
Differential, 2nd,DC Currents in phase
A

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

37. Design, Experiments & Modeling
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 Simulation of power system with Proposed Relay
Methodology
 Case 1: Normal Switching ( at 90 angle)
Restraining Current in phase A

38. Design, Experiments & Modeling
11/28/201538
 Simulation of power system with Proposed Relay
Methodology
 Case 1: Normal Switching ( at 90 angle)
Differential, 2nd,DC Currents in phase A

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

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

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

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

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

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

45. Design, Experiments & Modeling
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 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
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 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
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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
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49 No load Test of power transformer
Current
Transformer
F/I curve

50. Relay Testing Results
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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
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• 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
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• 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.
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