Provides Earth fault Protection in Star Connected HV Winding (Internal Fault) of the Power Transformer.
It uses the Turrets CT’s and the Neutral CT’s.
The Secondary of each Phase of HV Turret CT’s (called as Phase CT’s) of the Transformer are connected together in parallel.
The common terminals of the Phase CT’s are connected to the secondary of a Neutral Current Transformer (NCT).
REF Relay is placed in parallel to the Phase CT’s and in Parallel to NCT.
Need/Importance of REF Protection:
Differential Relay may not be able to sense Earth Faults in the Winding as the fault magnitude may be smaller than the Set Differential Current, Id. In these case REF relay is more sensitive, as it uses the current flowing through the Neutral CT.
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