> how is the testing of an REF relay? what are the test REF circuit having?
Are you talking about Restricted earth fault relay...???
In a Restricted Earth fault scheme, single phase to ground fault occurring in the star winding of the transformers is detected. Restricted earth fault scheme are basically High impedance differential scheme with a stabilizing resistor and a normal sensitive over current relay. So for preliminary checks we can test it as a normal over current relay basically current and voltage pick up can be checked from CT secondary side. The most and provocative test which must be done in a REF scheme is the single phase to ground through fault stability test which provides a formidable assurance of the connections including CT polarities and a correct operation is assured during a through fault in the star side system. This test can be carried out by creating a single phase to ground fault in the transformer star side by connecting a phase side with neutral of the transformer (note: the connection in the phase side should be after the CT i.e towards feeder side) and excite the transformer from the secondary voltage side. During a single phase to ground fault in the system there should not be any REF relay operation which can be checked by measuring the current or voltage in or across the differential circuit connected in CT secondaries affirming that all the CT connections and ratio's are properly matched for transformer operations. If there is any current in the differential circuit noted during this test check for CT connection errors or ratio errors. It is required to be repeated on all phases of the transformer by shorting one phase to neutral one by one and exciting the transformer. It is equally important to check the REF relay operation during this test by reversing the secondary connections of one of the CT's by which there will be a current flow in the differential circuit.
The selection of the stabilizing resistor is basically done to avoid any false operation of the REF scheme during through fault condition due to CT accuracy errors during fault condition. The setting of the over current relay and the stabilizing resistor must be equally matched with the CT VA, Rct- CT secondary resistance, resistance of cables connected in secondary of the CT's.
