GE Power Management
735/737 Feeder Protection Relay
A-1
APPENDIX A
A.1 OVERCURRENT PROTECTION SAMPLE CALCULATIONS
A
APPENDIX A APPENDIXA.1 OVERCURRENT PROTECTION SAMPLE CALCULATIONS
A.1.1 CHARACTERISTICS
Overcurrent Protection for a 13.8 kV Feeder (from Short Circuit Study)
•
Feeder rating = 600 A
•
CT Ratio = 600:5
•
Maximum load on feeder = 546 A
•
Fault current for a 3 phase fault at the feeder end = 2800 A
•
Fault current for a ground fault at the feeder end = 1100 A
The relay pickup current should be approximately twice the maximum load and allow for coordination with ‘downstream’
devices. The relay must operate for the minimum fault current at the very end of the feeder.
A.1.2 PHASE TIMED O/C PICKUP
•
Primary current pickup level = 2 × 546 A = 1092 A
•
Secondary current pickup level = 1092 A
×
(5 / 600)= 9.1 A
•
Phase Pickup dial setting = (9.1 A / 5 A)
×
100 =182%
≈
180%
From coordination curves obtained from the coordination study, obtain a ‘coordinating time interval’ with the slowest down-
stream relay and/or fuse of approximately 0.3 seconds (normally between 0.2 and 0.5 seconds). Also obtain appropriate
curve shape and time multiplier from coordination curves. In this case, ‘very inverse’ and 2 times, respectively.
•
Phase Curve Shape dial setting = VERY INVERSE (HI); Phase Time Multiplier dial setting = 2
A.1.3 PHASE INSTANTANEOUS PICKUP
The setting of the phase instantaneous element for this feeder is chosen such that it will not operate for faults beyond the
first fused tap out from the feeder breaker.
•
Fault current at first fuse = 5200 A
•
Set phase instantaneous element at 6000 A
•
Secondary current pickup level = 6000 A
×
(5 / 600)= 50 A
•
Phase Instantaneous dial setting = 50 A / 5 A = 10
A.1.4 GROUND PICKUP
The minimum ‘operate current’ or pickup must be set above the maximum unbalance current that can be tolerated by the
system. This unbalance usually is the result of unequal loading of single phase taps between the three phases. Typical
maximum unbalance permitted is 20% of feeder rating, in this case 120 A (0.20 × 600). The relay must also operate for a
ground fault at the feeder end.
•
Primary ground pickup level = 240 A (120
<
pickup level
<
1100)
•
Secondary ground current pickup level = 240 A
×
(5 / 600)= 2 A
•
Ground Pickup dial setting = (2 A / 5 A)
×
100= 40%
From coordination study obtain curve shape and time multiplier.
•
Ground Curve Shape dial setting = VERY INVERSE (LO); Ground Time Multiplier dial setting = 3
A.1.5 GROUND INSTANTANEOUS
A setting for the ground instantaneous element for this feeder is chosen such that it will not operate for ground faults
beyond the first fused tap from the feeder.
•
Ground fault current at first fuse = 2100 A
•
Level for ground instantaneous element = 2400 A primary
•
Secondary ground instantaneous level = 2400 A
×
(5 / 600) = 20 A
•
Ground Instantaneous dial setting = 20 A / 5 A = 4
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