Under Voltage Setting

M

Thread Starter

Meto

Hi,

I'm an electrician at a Fertilizers company in Egypt. I'm trying to find a rule of thumb setting for the busbar under-voltage relay time delay.

The reason is that the supply to the site was interrupted which caused equipment to stop. The result of this could have very serious consequences due to the nature of the process. The relay is set to trip the MV (6.6 KV motors) without tripping the incomer circuit breaker at (75% 0.2 S & 85% 10 S) of normal voltage. The time delay was set by MV switch gear vendor, but I'm wondering if the time was set too short, due to the effect a trip has on the safety of the plant? My understanding of under-voltage protection is to prevent motors etc from getting to hot because of increased current. I can't see why, if the time delay was say, 0.5 or 1 sec, the equipment wouldn't still be protected from prolonged "brownout's", but site power could be maintained if a very quick dip in power occurred. I'm not sure of the duration of the dip we had.

Now we are thinking to increase the time delay from 0.2 S to 1 S or 2 S.

Does this modification has a side effect? what is the standard recommended under voltage setting for MV motors?
 
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