Question

With reference to the parallel operation of electrical generators:

(a) Explain how two alternators may be synchronised;

(b) State how two alternator may be synchronised if the synchroscope is defective;

(c) Explain how synchronisation may be performed without any switchboard instrumentation;

(d) Explain the likely consequences if the timing of synchronisation is ineptly executed.

The various important conditions 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 frequency.

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.

With the help of synchroscope and voltmeter, synchronisation is carried out. When synchroscope pointer rotates slowly in clockwise direction and comes to 1100 clock position close the breaker and switch off synchroscope.

(b) With 3 lamp method synchronisation of 2 generator can be carried out. When lamps rotate in clockwise direction, top lamp is dark and bottom 2 lamps are equally bright, close the breaker.

Synchronising with three lamps.

(c)

Connect a voltmeter as shown, across one pole of the open incoming generator circuit breaker. This procedure is more easily (and safely) performed at the synchroscope terminals behind the door of the synchronising panel at the front of the main switchboard. Check the circuit diagrams before such testing.

Adjust the generator speed until the voltmeter very slowly fluctuates from zero to maximum.

Close the breaker when the voltmeter indication passes through zero. Note, for this operation, an analogue (pointer and scale) meter is easier to follow than a digital type.

(d) At the instant of closing the breaker, the voltage phase difference causes a large circulating current between the machines which produces a large magnetic force to pull the generator voltages (and field poles) into synchronism. This means rapid acceleration of one rotor and deceleration of the other. The large forces may physically damage the generators and their prime-movers and the large circulating current may trip each generator breaker. Blackout, danger

Voltage phase difference.

Large circulating current.

Large magnetic force. – pull the generator voltage into synchronisim.

Rapid acceleration of one rotor and de-acceleration of other.

Large force may damage generator.

Large current may trip each generator.

Blackout.

Six damages that may be caused when machine are incorrectly synchronised

Damage to coupling.

Damage to generator windings.

Damage to prime mover.

Damage to electrical circuit due to high power surge.

Stator core damage.

Huge surge current could damage stator winding.