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Phasing Diagram
GCB
gen
Neut Grounding
Transf/Resistor
345kv Switchyard Recordings. Fault Clear in 4.5 cycles
Gen voltages (phase-neut) and current
400000
15
350000
300000
5
250000
-0.040
-0.023
-0.007
0.010
0.027
0.043
0.060
0.077
0.093
0.110
0.127
0.143
0.160
0.177
-5
200000
150000
-15
Van
100000
VBn
-25
50000
VCn
IA
0
IB
-35
IC
-45
-50000
-100000
T0
Fault
T1 – GCB phase A main contact
current interrupted.
Also Swyd bkr clears fault
(4.5 cycles)
T2 – GCB phases
B & C Main contact
Current interrupted
T3 – GCB phases
B & C interrupting contact
current interrupted
Generator Phase to Phase Voltages (computed)
30
20
10
0
-0.040
-0.023
-0.007
0.010
0.027
0.043
0.060
0.077
0.093
0.110
0.127
-10
-20
Vab
Vbc
Vca
-30
Vbc lower during time of fault.
Sharp jumps in voltage seen in prevoius slide phase-neutral are not evident in this slide phase-to-phase except
Vbc at moment of start of fault and moment of B, C main interrupting
0.143
0.160
SEL 300G Relay reported “Neutral Currents” and “ground currents”.
Ground currents are simple sum. Is this IN computed from more frequent samples, or just garbage?
IN
IG
2000
0
IG (right axis)
1000
IN (left axis) 0
2000
IG (right axis)
1000
0
-0.040
IN (left axis)
-0.023
-0.007
0
-0.040
-5
0.010
0.027
0.043
0.060
0.077
0.093
0.110
-5
0.127
0.143
0.160
0.177
-10
-0.023
-0.007
0.010
0.027
0.043
0.060
0.077
0.093
0.110
0.127
0.143
0.160
0.177
-10
-1000
-1000
-15
-15
-2000
-2000
-20
-3000
-20
-3000
-4000
-4000
-25
-25
SEL Relay neutral and ground, zoomed out. No significant change in In from before, during after…
suspect In is garbage data. What about IG… where does this dc come from?
2000
IN
IG
IG (right axis)
1000
IN (left axis)
0
-1.000
0
-0.500
0.000
0.500
1.000
1.500
-5
2.000
-10
-1000
-15
-2000
-20
-3000
-4000
-25
SEL Relay has provision to measure neutral current [terminals
Z07/Z08], but nothing connected here in our installation (see next
slide)
CT 2o relay-end wye point return current does not go to SEL relay.
[DCN 0402475]
Vector Diagram low side to explain why Ic > Ib between T0 and T1.
Assumptions
•Phasors rotating CCW. => rotate a phasor CW if want to make it more lagging
•CBA rotation
•A corresponds to X3, C corresponds to X1
•For simplicity, assume initial currents are in phase with voltage (primarily resistive)
•For simplicity, assume currents associated with high-side non-faulted leg are unchanged by the fault
•Assume curent associated with high side faulted leg increase in magnitude and rotate toward the
lagging direction (fault current is limited by primarily inductive impedances)
•In changing the vector Ibc from pre-fault to post-fault, we increase magnitude by factor of 2 and
rotate CW by approx 30 degrees (factor of 2 and 30 degrees chosen arbitrarily)
•Assign Reference directions for currents
•Inside the delta, currents flow in CW direction
•External to transformer (on low side), currents flow into the transformer
•Notation for currents: lower case inside delta, upper case outside transformer
•The result of these reference direction assignments using KCL is
•IA = Ica - Iab
•IB = Iab – Ibc
•IC = Ibc- Ica
IC = Ibc- Ica
B
IB = Iab - Ibc
Ibc
C
Iab
IA = Ica - Iab
Ica
A
Conclusion: Vector diagrams (next slide) show why we expect |IC| > |IB| under these assumptions.
Also explains why angle between IB and IC is > 120 while angle between IB and IA is <120 deg
Voltage diagram
Vector Diagrams (low side winding MT2A)
to explain why Ic > Ib if source impedance
is inductive (assumptions next slide)
B
VBC
C
VAB
VCA
A
Prefault
Current diagram
Inside the delta
B
Ibc
C
Prefault IA= Ica-Iab
-Iab
Prefault IB= Iab-Ibc
IA
IB
Iab
Iab
Ica
Ica
Prefault IC= Ibc-Ica
Ibc
-Ica
IC
-Ibc
A
Post-fault
Current diagram
Inside the delta
B*
Postfault IA= Ica-Iab
Postfault IB= Iab-Ibc
Postault IC= Ibc-Ica
Ibc
-Iab
IA
IB
-Ica
Iab
IC
C
Ibc
Ica
Iab
B*
Ica
-Ibc
A
*Unbalanced inside delta currents Iab Ibc Ica do not sum to 0 but external currents IA IB IC will (in absence of ground fault)
(IA+IB+IC = 0 based on expressions for IA IB IC above)