Answer / madhu

the neutral grounding transformer is star connected on the
primary and has an open delta on it's secondary.Open Delta
has two terminals.A resistor is conected across these two
terminals. whenever a fault occurs, an emf will be induced
in the open delta and there will be a voltage drop in the
resistor connected. this voltage drop will be sensed by the
potential transformer connected across this resistor. at
the secondary of the potential transformer we have the
neutral displacement relay which will give a trip signal or
an alarm whenever its settings exceed a particular value.

Answer / m.babu

By giving these two wires to Neutral displacement relay.
in the event of earth fault condition some voltage appears
across the relay and it will trips the circuit breaker.
In healthy condition '0' volts across the relay.

Answer / gunasekaran

at balance condition voltage appear across the open delta
connection is zero at fault candition no longer zero.
so we r using this open delta connection for resudial over
voltage protection.

Answer / k.prakashchandra

This relay is used for stator earth fault protection for
generators.The vector sum of three phases in all corners of
delta connected PT secondaries is zero.One of the corners is
taken through a voltage relay.Operation is as above.

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