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.
4. Causes of Transformer Failures
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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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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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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
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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12. Inrush Phenomena and Harmonic Content
Availability
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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
14. Background and History of Differential
Protection of Power Transformer
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• 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
16. Problem Statement
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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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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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• 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
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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
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
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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
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
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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
48. Relay Testing
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• 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.
50. Relay Testing Results
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• 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.
