Answer MET Question 28

Question 134: A. State the necessary conditions required prior to the synchronizing of electrical alternators.

B. Describe the type of cumulative damage that may be caused when alternators are incorrectly synchronized;

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

D. For two alternators operating in parallel state the consequences of:

(i) Reduced torque from the prime mover of one machine.

(ii) Reduced excitation on one machine.

Answer: A. Necessary condtions:

1. The two generators must be of nearly equal ratings and phase sequence must be same;

2. The two generators must be of equal rpm, the incoming must be running at stable rated rpm.

3. The voltage of incomer must be equal to busbar.

4. Frequency must be equal to the busbar.

B.  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. It will result in Blackout, danger and embarrassment.

C. To avoid the above stated damage the synchronising operation to be carried out very carefully.
Parallel running is achieved in the two stages i.e Synchronising then Load Shaing. Both operations are, usually carried out automatically but manual control is still in common use and is generally provided as a back-up to the auto control. The generator already on-the-bars is called the running machine and the generator to be brought into service is the incoming machine.

To smoothly parallel the incoming generator, it must be synchronised with the live bus-bars. To achieve smooth manual synchronising, the incomer must be brought up to speed to obtain approximately the same frequency. The incoming generator voltage is set by its AVR or manually trimmed to be equal to the bus-bar voltage. Fine tuning of the speed can now be observed on the synchroscope or synchronising lamps. The incomer is adjusted so that the synchroscope indicator rotates slowly clockwise (fast direction) at about 4 seconds per indicator revolution. The circuit-breaker should be closed as the indicator approaches slowly to the 12 o'clock (in-phase) position.

D. For two alternators operating in parallel state the consequences of:
(i) Reduced torque from the prime mover of one machine.

When the torque of the prime mover of the alternator connected to the bus-bars is reduced, the alternator connected to it will lag in phase with respect to other alternator or alternators connected to the common bus-bars. It will cause a phase difference between V and E, the magnitude of E remaining unchanged. The resultant of these two voltages, $\displaystyle \small \mathrm{E_R}$  will cause a synchronising current $\displaystyle \small \mathrm{I_{sy}}$ . Thus the power delivered by the alternator, whose input power has been reduced will reduce by an amount equal to $\displaystyle \small \mathrm{EI_{sy} cos\phi_1}$.
Hence by decreasing the torque of the prime mover of one alternator, it will take reduced load and an equivalent load will increased on the other unit or units with which the machine is paralleled.
Additional points
Also in case of increased torque, it can be noted that if the output of the alternator, whose prime mover torque has been increased, become more than the total load being supplied, then the other machine or machines will operate as synchronous motor(s).
Also it is to be noted that active and reactive power loading of an alternator operating on infinite bus is controlled by controlling the input power to it and excitation respectively.

(ii) Reduced excitation on one machine.

When a total loss of excitation occurs in No. 2 machine; Generator No. 2 will run as an induction generator drawing its excitation kVAr from No. 1. Both generator currents will rise rapidly with No. 1 becoming more lagging while No. 2 runs with a leading p.f. A loss of excitation trip (if fitted) or the overcurrent relay should trip No. 2 generator probably causing an overload on No. 1. Alternatively, No. 1 trips on overcurrent which deprives No. 2 of excitation and its breaker trips out on undervoltage.