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Turn-to-Turn Fault Detection in
Transformers Using Negative
Sequence Currents
Mariya Babiy1, Rama Gokaraju1, Juan Carlos Garcia2
1University of Saskatchewan, Saskatoon, Canada 2Manitoba HVDC Research Centre, Winnipeg, Canada
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Outline
Traditional Differential Relay
Proposed Differential Relay Using Negative Sequence
Currents
Results
Conclusions
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Drawback of the differential relay - low sensitivity
Figure 1 – Percentage Differential Relay of the Transformer
3
Traditional Differential Relay
Internal fault
External fault
I1
i1
I2
i2
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Difficult to detect low-level turn-to-turn faults.
Change in transformers terminal current is quite small.
IEEE C37.91-2000 indicates that ~10% of the transformer
winding has to be shorted to cause a detectable change in
terminal current.
Restraint characteristics usually set to about 20%.
Traditional Percentage Differential
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5 Fig.2 Traditional Differential Relay – Differential and restraining current for various
percentages of shorted turns.
10% of turns are shorted
3% of turns are shorted
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Other Differential Relaying Schemes
Sudden Pressure Relays are slow to operate (50-100ms).
Sachdev, Sidhu (1989) used electromagnetic equations during internal
faults (accurate for shorted turns > 5%).
Gajic et. al from ABB in (2005) presented negative sequence current
differential concept .
Crossley (2004) presented a technique based on increments of flux
linkages (accurate for shorted turns > 10%).
Wavelet transforms (Kunakorn, 2006) & ANN (Li, 2007). 6
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Negative Sequence Current Scheme
Compared to zero sequence currents, negative sequence current differential provides coverage for phase faults as well as for ground faults.
Two stages of comparison: Negative sequence magnitudes compared with a pre-set level of 1%. Directional Comparison: <[0-5 degrees] then it is an internal fault. If it
is 180 degrees then it is an external fault. For Δ-Y and Y-Δ the appropriate 30 degree phase shifts could be taken
into account.
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Primary currents Ia1, Ib1, Ic1
Secondary currents Ia2, Ib2, Ic2
Neg. seq. current INS_P
FFT
Neg. seq. current INS_S
FFT
Relay Logic
INS_P > Imin No
Yes INS_S > I min
No
Yes
If phase shift ≈ [0 – 50 ]
Block
Yes
No
Trip Figure 3. Negative sequence current based logic
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9 Figure 4. Direction of negative sequence currents during faults
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Simulation Results
Simulation model developed in PSCAD/EMTDC.
Three-phase transformer bank constructed using three
single phase transformer banks (each 33.3 MVA, 23/132
kV). Mutual coupling taken into account.
Number of turns on primary: 150. Number of turns on
secondary: 866.
Accurate values of leakage reactance were obtained for
different shorted turns.
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Figure 5. Negative sequence current magnitudes for various percentages of shorted turns on primary winding (Y-Y).
10% of turns are shorted
1% of turns are shorted
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Figure 7. Phase angle comparison between two negative sequence currents for various percentages of shorted turns on primary winding (Y-Y).
10% of turns are shorted
1% of turns are shorted
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Figure 8. Response of the proposed scheme during an external B-to-ground fault on the secondary side (Y-Y).
Negative sequence current magnitudes
Phase angle between two phasors of negative sequence currents
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Time, s
Mag
nitu
de,
A
INS_S INS_P Pre-set level
Figure 9. Negative sequence current magnitudes for 3% shorted turns on secondary (Δ-Y)
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Deg
rees
Time, s
INS_P_ph
INS_S_ph
300
Figure 10. Phase angle comparison between two negative sequence currents for 3% shorted turns on secondary (Δ-Y) with 30º phase shift
Deg
rees
INS_S_ph
INS_P_ph
Time, s
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Phase Load Amps
A 4.1759
B 4.5976
C 4.3906
Table 5.7 – Phase currents due to a small unbalance
Mag
nitu
de,
A
Time, s
INS_P and INS_S during the small unbalance
Pre-set level
Figure 12. Negative sequence current magnitudes due to a small unbalance in the power system
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Deg
rees
Time, s
INS_S_ph
INS_P_ph
Figure 13. Phase angle between two phasors of negative sequence currents during a small unbalance in the power system
Trip
Sig
nal
Time, s
Figure 14. Output signal from the proposed technique for a small unbalance in the power system
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Mag
nitu
de,
A
Time, s
INS_P
INS_S
Figure 15. Negative sequence currents magnitudes during inrush current
Deg
rees
Time, s
INS_S_ph
INS_S_ph
Figure 16. Phase angle between two phasors of negative sequence currents during inrush current
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• Simple to implement (uses standard sequence current logic).
• successfully detects turn-to-turn fault even when small number of turns are shorted ( 1-2%).
• accurately discriminates between internal and external faults.
• detection time approximately 12 ms. • Tested for various operating conditions, inrush
currents, unbalanced operation. • does not require any extra customization compared to
the traditional differential relay. 19
Conclusions