Transforer REF Protection

 Transformer restricted earth fault relay

A transformer restricted earth fault (REF) relay is a protective relay used to detect earth faults in the winding of a transformer. It operates by measuring the current flowing into and out of the transformer's neutral point and comparing them. If the current difference exceeds a predetermined threshold, the relay will trip and isolate the transformer from the network to prevent damage.

The REF relay is designed to operate only on the earth fault within the protected zone of the transformer. The protected zone is defined as the area between the neutral point of the transformer and the point where the earth fault is most likely to occur. This is achieved by connecting the relay in a specific configuration, which is usually a star (wye) connection with a neutral CT connected to the relay.

Restricted earth fault protection is provided in electrical power transformer for sensing internal earth fault of the transformer. In this scheme, the CT secondary of each phase of an electrical power transformer are connected together as shown in the figure. Then common terminals are connected to the secondary of a Neutral Current Transformer or NCT.