NGR and 50N/51N settings ? 1

[Mongao]

Hi Guys,

I have to protect a feeder with SEL-751, I was asked to do 50N and 51N,

On the source side, the feeder is connected to the NGR-grounded-Wye side of a Delta-Wye transformer; on the load side it is connected to the Delta side of a Delta-Wye grounded transformer, so there is no steady state 3I0 flowing under normal conditions due to any load unbalance; there is a 50/5 CT in the transformer neutral connected to the SEL-751.

For 50N, I have simply ran a L-G fault and set the relay slightly under the obtained result, but how may I set up 51N, if there is no say "allowed unbalance" at all ? What would be the pick-up setting ? Which curve to use ?

The NGR is rated 25A for 10 seconds,

Thanks a lot, any help will be very appreciated,

Best regards,

Mongao

 

[pwrengrds]

Set the 50N pickup at about 50% of the 25 amps, with some time delay, not more than 5-6 seconds. Set the 51N to about the same pickup and time it as a backup so that at rated current it will trip at 10 seconds.

 

[Mongao]

Thanks for helping !

Please, why should I add a 5-6 seconds delay ? There is no other device (say, there isn't a NGR monitoring device in this case), so, if a fault happens, why shouldn't I clear it immediately ?

Thanks again, your help is much appreciated !

 

[pwrengrds]

Not more than 5-6 seconds does not mean you can't go faster. The limit is due to not burning out the NGR. Timing is more of a coordination issue with other devices that might clear the fault first and the timing should be set as needed for coordination.

Take into consideration what the voltage will do on the other phases, if you have under and over voltage protection timing it should be considered. If you have lighting arresters rated only for phase to ground voltages beware, they should be rated for phase to phase voltage.

 

[Mongao]

Thanks a lot for helping, I really appreciate it !