Answer Posted / kk
A reverse power relay is a directional power relay that is
used to monitor the power from a generator running in
parallel with another generator or the utility. The function
of the reverse power relay is to prevent a reverse power
condition in which power flows from the bus bar into the
generator. This condition can occur when there is a failure
in the prime mover such as an engine or a turbine which
drives the generator.
The failure can be caused to a starvation of fuel in the
prime mover, a problem with the speed controller or an other
breakdown. When the prime mover of a generator running in a
synchronized condition fails. There is a condition known as
motoring, where the generator draws power from the bus bar,
runs as a motor and drives the prime mover. This happens as
in a synchronized condition all the generators will have the
same frequency. Any drop in frequency in one generator will
cause the other power sources to pump power into the
generator. The flow of power in the reverse direction is
known as the reverse power relay.
Another cause of reverse power can occur during
synchronization. If the frequency of the machine to be
synchronized is slightly lesser than the bus bar frequency
and the breaker is closed, power will flow from the bus bar
to the machine. Hence, during synchronization(forward),
frequency of the incoming machine is kept slight higher than
that of the bus bar i.e. the synchroscope is made to rotate
in the "Too fast" direction. This ensures that the machine
takes on load as soon as the breaker is closed.
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