Power Systems Protection 1

[linkect]

I came across an MCSU relay(GEC) setting which puzzled me. The Core balance CT ratio is 0.5/0.0075 and the pick up was set at 1.5 with a plug setting of 1. What is the actual primary pick-up current in this case?

I thought it would be 1.5x CTR(66.67)= 100A, but the source of the feeder where the relay is installed has a secondary winding neutral earthing impedance that limit the EF current to 25A. I would not have thought that any protection engineer would have missed that out. Please help?

 

[Ohmly]

I think a plug setting of 1 means 1% of In, so what you have is 1.5×1% = 1.5% of In.

If In = 1 Amp, the pickup is 0.015 Amps secondary, which is 1 Amp primary with that CT Ratio.

 

[linkect]

Yes,you are right about the 1.5% of In. And truely, the In is stated as 1A on the ralay facia. I still am worried about this pick-up setting because the feeder being protected by the relay has a cable about 2.5km long and the charging current should be tripping this feeder constantly. There is bound to be capacitive leakage of some sort which can not be less than 150mA!!

I was forced to increase the pick up settings on cables feeders with shorter cables of the same size before. I am still amazed by the setting and I did not physically check if the relay is activated.