Selected Electrical Question And Answer for MEO Class 2 (6)

(a)                Briefly describe how a running AC generator would be taken off load and another generator put on load without a blackout.                                                     (3m)

(b)               State the danger arising from reducing the excitation of the outgoing generator before it is off loaded.                                                                                         (4m)

(c)                State what would be the effect on the current, voltage and frequency of the machine if an AC generator allowed down slightly during operation.                                (3m)

Answer :-

(a)                The incoming generator is synchronised with the running generator after having ensure that the frequency and voltage of both generators are identical, both having the same phase sequence and the phase angle difference between the voltage is zero. To minimise the possibility of blackout, both generator should be allow to run in parallel for a short time before taking the outgoing are off load.

(b)               When the excitation of the outgoing generator is reduced, the current taken by it is reduced. The current (load) of the other generator is thus increased and if exceed the rated current, the overload relay will trip the generator and a blackout would occur.

(c)                If one machine running alone:

1)      speed slow down, frequency decreases.  N a f/p.

2)      Voltage is not affected as it controlled by AVR.

3)      Impedance reduces (XL = 2PfL) with voltage being constant, current would therefore increase.

If two alternators in parallel:

If one generator slow down slightly, it would drop / shed part of its kW loading which would be taken up immediately by the other one. However, there will be no change in the frequency, voltage and current in the system as both are locked electrically.