Delaying bus differential
Delaying bus differential
(OP)
Two quick questions:
1. In a high impedance differential protection system, is there any way to know or calculate the stress an 8 cycle delay can cause on the MOVs and resistor during an in-zone fault?
2. Are there any low impedance differential relays (ie sel487, B90) where a differential trip can be delayed?
1. In a high impedance differential protection system, is there any way to know or calculate the stress an 8 cycle delay can cause on the MOVs and resistor during an in-zone fault?
2. Are there any low impedance differential relays (ie sel487, B90) where a differential trip can be delayed?






RE: Delaying bus differential
For the SEL-487B the differential doesn't cause a trip unless it's in the trip equation. Instead of putting the differential elements in the trip equation just use them as inputs to timers and put the output of the timers in the trip equation.
What you'll lose if you do that is the high-speed determinations made before CTs can saturate. If you don't have CT saturation problems that won't be an issue.
RE: Delaying bus differential
RE: Delaying bus differential
No such limitation exists for the 487B, correct?
RE: Delaying bus differential
As long as the dropout time of the timer is set to zero, the timer stops timing as soon as the input condition is no longer present. With a PU time of 8 cycles, the condition could be there for 7.75 cycles, out for 0.25 cycles, back in for 7.75, etc. and never trip. Throw in a 0.5 cycle DO time and you'd get your trip. Both PU and DO are independent and could either be zero.
RE: Delaying bus differential
RE: Delaying bus differential
J. Horak, P-R Engineering, Colorado
RE: Delaying bus differential
In my case I am debating increasing the bus differential trip by several cycles, and if I do, an SEL587Z would be exposed to many more cycles of differential voltage before the lockout relay shorts the CTs. Even without a time delay, High impedance relays need have their CTs shorted because a failed circuit breaker might take 12 or more cycles before the bus becomes de-enrgized. That whole time the relay is being fed elevated voltage.