Powerengineer_84
Electrical
- Apr 4, 2018
- 5
This question pertains to application of 3000A outdoor metal clad 15kV class ANSI switch gear. The available three fault current is calculated to be 38kA with an X/R = 37. As this substation has a resistor grounded medium voltage system and the SLG fault current is 4kA with an X/R<1.
The substation has 4 transformers run in parallel and the bus ties CANNOT be run normally open to reduce fault current.
Given the amount of three phase fault current and the large offset, there is one school of thought that is proposing an ANSI 63kA gear installation the rationale being that for a bolted three phase feeder fault (all UG feeders from this substation) right outside the station (the worst case) the amount of fault current and offset suggests 63kA gear as appropriate.
There is another school of thought proposing an ANSI 40kA gear. The rationale for the 40kA proposal is that each transformer will contribute <10 kA to a fault and that SLG faults are the most prevalent.
I am not sure how the < 10 kA / transformer secondary rationalizes this; for a bolted three phase fault on a feeder just outside the station the 38kA fault current is going to go thru the feeder breaker and that is what the breaker will need to interrupt.
Thoughts?
The substation has 4 transformers run in parallel and the bus ties CANNOT be run normally open to reduce fault current.
Given the amount of three phase fault current and the large offset, there is one school of thought that is proposing an ANSI 63kA gear installation the rationale being that for a bolted three phase feeder fault (all UG feeders from this substation) right outside the station (the worst case) the amount of fault current and offset suggests 63kA gear as appropriate.
There is another school of thought proposing an ANSI 40kA gear. The rationale for the 40kA proposal is that each transformer will contribute <10 kA to a fault and that SLG faults are the most prevalent.
I am not sure how the < 10 kA / transformer secondary rationalizes this; for a bolted three phase fault on a feeder just outside the station the 38kA fault current is going to go thru the feeder breaker and that is what the breaker will need to interrupt.
Thoughts?