Question

State the necessary conditions required prior to the synchronizing of electrical alternators.

Describe the type of cumulative damage that may be caused when alternators are incorrectly Synchronized.

Explain how the damage referred to in (b) can be avoided / reduced.

For two alternators operating in parallel the consequences of:

Reduced torque from the prime mover of one machine.

Reduced excitation on one machine.

Answer.

The various important condition are.

The voltage of the incoming machine must be equal to that of bus bar.

The frequency of the incoming machine must be same as that of the bus bar.

The e.m.f of the incoming machine must be in phase with the bus bar voltage.

The phase sequence of eh incoming machine must be similar to that of the bus bar and switching arrangements must be such that, like phase of the machine and bus bar are connected when paralleling procedure is completed.

b) if synchronised when alternator running slow, the incoming machine would draw motoring current, which may operate its reverse power relay and trip the circuit breaker of the machine already on the bar due to overload.

Attempting to close the incoming breaker when generator not in synchronism at the instant of closing breaker, the voltage phase difference at the instance of closing breaker, the voltage phase difference causes a large circulating current between the machines.

This result in a large magnetic forces that pulls the generator in synchronism. This means rapid acceleration of one rotor and de-acceleration of the other rotor.

Mechanical damage could occur which may include

Deformation of stator windings.

Movement between stator and frame.

Failure of rotor diodes on brushless machine.

Twisted rotor shaft.

Localised cracking of shaft and keyway and broken coupling.

The large circulating current may also trip each generator breaker resulting in a black out.

c) To achieve proper synchronising and avoid, reduce damage caused due to improper synchronising, the operator should ensure that the incoming voltage is within 5% of the bus bar voltage, ideally the incoming machine frequency should be with in 0.2% of the bus bar frequency, which is indicated by 1 revolution of synchroscope every 10seconds.

It is usual to synchronise the incoming machine slightly fast so that the synchronizing surge does nit operate reverse power trip.

It is essential that the circuit breaker is closed at the instant, the incoming machine voltage is in phase with the bus bar voltage to limit synchronising torque exerted on the rotor.

This condition is indicated by 12 O’clock position on the sychroscope.

The circuit breaker is to be operated, slightly before this position [ usually 11 O’clock] to take into account the breaker closing time.

In case of synchronising lamp, correct synchronising occurs when the top lamp is dark and the bottom lamp are equally bright.

Check synchroniser are usually fitted to prevent faulty manual synchronising.

This unit prevent the incoming circuit breaker from closing unless the voltage frequency and phase angle differences are within preset limit.

Check synchroniser could be by passed by an override switch in case of emergency.

d) 1.

Voltage generated by the alternator is directly proportional to torque produced by the prime mover.

The magnetic torque exerted on the rotor due to Kw loading of generator causes the rotor to slow down

This reduction of speed is sensed by the governor.

But if the torque is reduced it directly affects the Kw load sharing, leading to total load shedding of the generator.

If the other generator is unable to take the load, preferential trip will occur.

Excitation will also be affected if mounted on same rotor.

Under frequency trip will occur.

2)

System voltage may fall slightly because the excitation of the other machine will increase and will affect any tendency for the voltage to fall.

Reactive current in form of large circulating current will flow between the faulty and other machine.

Current will cause damage to faulty machine and tripping of other machine.

Excitation loss detection system equipment should be fitted to trip the main breaker.