SEF Relay

[StarDeltaPower]

I have a 1500kVA 11/0.415kV transformer, which has LV earth protection and upstream 11kV protection. The grading is at about 0.25s between the 2 curves under a fault on the LV side of the transformer.

I have been asked to add an SEF relay to the LV side protection. Normally I would grade this to trip a 2s more or less after the primary LV earthing protection. In this instance though, as the 11kV curve can not be altered and made to trip slower, should the SEF curve sit between the LV and 11kV curves? Or react slower than both the LV and 11kV protection as it's purpose is backup?

 

[davidbeach]

What winding configuration does the transformer have? Are any wye points solidly grounded or grounded through an impedance?

 

[RRaghunath]

'SEF' could mean sensitive earth fault protection. If true in your case also, then, SEF need not cordinate with the normal earth fault protection as the SEF current pick up setting will be very small and is expected to operate only for high resistance faults.
If 'SEF' mean Standby Earthfault Protection with current signal from transformer LV neutral CT, this has to coordinate with the incomer earthfault protection (coordinating margin - 0.25s min).
LV incomer earthfault protection trips incomer alone, whereas, SEF (SBEF) protection is wire to trip both HV & LV breakers.

 

[StarDeltaPower]

Thanks for the pointers.

It's actually an SBEF really. I'll ensure that it is set to trip around 0.25s after when the LV and HV relays are expected to trip, and in turn trip both relays.