Neutral displacement relay:
Neutral Displacement relay is a short form of NDR. Neutral displacement relay is used to protect the transformer against earth fault in delta side winding. The ANSI code for Neutral displacement relay is 59N.
Neutral displacement relay operation:
The three number of potential transformers are connected in broken delta configuration. Here we are going to use the principle of broken delta configuration of the vector sum of the phase voltage is equal to zero. The output voltage of the open delta configuration will be given to the relay coil.
Under earth fault condition (consider L-G fault), due to the zero sequence current flow, the voltage in one phase get affected i.e in the remaining two phases is now equal to the phase-to-phase voltage with a displacement of 60 degrees. The voltage at the broken delta becomes 3Vo or three times the phase-to-ground voltage. Therefore, the vector sum of the voltage across the relay operating coil is not equal to zero. Hence the relay operates the circuit breaker.
A resistor is connected parallel to the relay coil to prevent Ferro resonance, a condition that occurs when the line capacitance and the inductance in the potential transformers reach a state of resonance.
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