Transformers: Restricted Earth Fault vs. Differential Protection

For transformers, normally one would go for a percentage biased differential protection, in which, a certain bias is set, say about 15% or 20%. In the case of an internal earth fault in the LV winding and if the HV winding is connected in DELTA, the HV side would not reflect the LV earth fault. In case the earth fault is in the HV winding (provided the HV side is connected to an earthed source) or even with an LV winding Earth fault and the HV winding also connected in STAR, the differential protection would operate only if the magnitude of the earth fault current is more than the bias set in the differential relay.

Whereas in an REF relay the setting can be as low as 5% or even 2% in some of today's numerical REF relays. So, an REF relay is more sensitive. If you ask, which one would be faster, in case the earth fault current is more than the bias set in the differential relay, both are instantaneous relays and any one might operate first.

If fault occur in ristricted zone of the transformer i.e for eg. if there is an winding to case(of transformer) fault , then the Differential protection relay(87T)will act quicker. If fault occurs outside the transformer then the Restricted earth fault relay(64 REF) will trip for which the NCT(neutral CT) is connected in neutral point of the secondary winding (Star connected).And the neutral terminal is connected to earth.

We have to take some precaution that the transformer will be mounted on the insulated base for such an arrangment.

If the fault is outside the transformer, then restricted earth fault protection will NOT operate for sure. Because, if the fault is outside the zone, then the current pumped into the REF relay by the NCT will be neutralised by current of the faulted phase CT. In fact, it is one of the very important property of the REF relay that it MUST NOT operate for external faults and this is called the STABILITY of the REF Relay.

As said earlier, REF will act for earth faults inside the zone and Differential will act for phase faults inside the zone. However, the differential relay may also act for earth faults inside the protected zone, provided the earth fault current is more than the bias set in the differential relay.

Thanks for more clarity.

Thanks.