With reference to AC generators operating in parallel :
(a)
Explain why protection is necessary in the event of
power loss by the prime mover of one generator. (3m)
(b)
State how the protection required in (a) is achieved. (2m)
(c)
Describe the construction and operation of a design
suitable for protection against the situation arising in (a). (5m)
Answer :-
(a)
Protection must be incorporated when generators
intended to operate in parallel. In case one generator prime mover failure
occurred, the generator would act as synchronous motor. Thereby causing
overloading of the running generator and damage the LO system of the faulty
prime mover.
(b)
By installing a ‘reverse power relay’, motoring or
running as synchronous motor fault can be detected by this protection thus
tripping the generator circuit breaker.
(c) 1) It consists of a thin aluminium disc
which can rotate between two electromagnet.
2)
The permanent magnet acting on the disc provides
damping control.
3)
The upper electromagnet is energised from a voltage
transformer and the lower one from a current transformer.
4)
Hence two eddy currents are produced in the disc of the
generator supply is healthy.
5)
The deflection torque is produce due to the interaction
of these eddy current and the inducing flux. This torque is in the direction of
the power flow.
6)
An adjustable time delay is also incorporated. The disc
carries an insulated arm with a contact strip, which when the reverse power
relay operates, will bridge two contacts to complete a trip circuit thus
tripping the circuit breaker.
“””” to sketch a simple reverse power relay circuit / diagram. “”””’’’’’’
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