MV Protection Trip

[alingstone]

A 33kV feeder of ours connects to a remote substation B with a single 33/11kV Dyn11 transformer connected at B, and an 11kV distribution board at B. The relay on the 33kV feede CB at Substation A has Overcurrent & Earth Fault protection. There was a phase to phase fault and the 11kV CB at substation B opened on OC prot. About 2 ms AFTER the correct opening of the 11kV CB at sub B, the 33kV feeder at sub A tripped on 33kV earth fault prot, which we find strange. The 11kV feeder which faulted runs for about 2 miles on conjoint poles carying the incoming 33kV supply with the 11kV underbuilt bu about 2.5 metres. Any ideas as to why this may have happenned?

 

[odlanor]

davidbeach !!
I agree with you. I missed the fact that was distribution circuits underbuilt with transmission lines.Could be an evolutive fault to other line.

 

[stevenal]

Or maybe:

Short circuit forces from 11 kV fault current in the 11 and 33 kV conductors cause poles and conductors to swing. Swing path of one of the 33 kV conductors intersects a down guy or other grounded object.

I have heard of this scenario causing a line to line fault on the higher voltage circuit when the conductors slap together. Slight alteration for this situation.

 

[davidbeach]

I'll accept that possibility too. We've never had first hand evidence of what the transmission fault was, but distance to fault always puts it in the same area as the distribution fault, never through the transformer and back to the distribution fault location. But, there aren't that many locations on our system where even the most wildly swinging, but intact, conductor could flash over to an intact guy

 

[rbulsara]

davidbeach:
Thanks, just wanted to know your opinion. Your suspicion seems more probable of all scenarios.

I am still not clear what the breaker B operating 2ms AFTER A means. Did it pick up 2ms later or completed opening. Were they acting simultaneously or there is enough time difference to allow for hot gases to travel up 2.5m (8+) feet), cause flash over plus the relay time. Simultaneous operation would be more puzzling.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

2ms is 1/10 cycle at 50Hz (an assumption given 33kV, 11kV, and earth fault). For all intents and purposes things that happen less than 1/4 cycle apart happen simultaneously. I took it to mean that some reference, like the 52a contact opening, happened 2ms later on one breaker than on the other. Since both breakers had to have received a trip command 3-5 (or more) cycles earlier it was just luck of the draw that they happened in that sequence with that time difference.

 

[rbulsara]

That is the puzzling part, the secondary flash over 2.5 meter away plus operating time should have resulted in some lag. But again not sure we have the correct data.

Rafiq Bulsara

http://www.srengineersct.com