With reference to AVRS: (a) explain their purpose;(4) (b) explain the effects that would be observed should an AVR give a low output when the generator is: (i) running on its own;(2) (ii) running in parallel with a second generator whose AVR has no fault.(4)
(a) Purpose of AVRs (4 marks)
The Automatic Voltage Regulator (AVR) controls the excitation current supplied to the generator’s field windings.
Its primary purpose is to maintain the generator terminal voltage constant under varying load conditions.
It ensures reactive power (kVAr) sharing when generators are running in parallel.
It helps maintain system stability and protects equipment against undervoltage/overvoltage.
(b)(i) Generator running on its own (2 marks)
If the AVR gives a low output:
The excitation is reduced, leading to low terminal voltage.
Consumers supplied by that generator will experience undervoltage, causing poor performance of motors and possible tripping of sensitive equipment.
(b)(ii) Generator running in parallel with a healthy generator (4 marks)
With a faulty AVR giving low output:
The affected generator’s excitation reduces, lowering its terminal voltage.
It will begin to absorb reactive power (kVAr) instead of supplying it.
The healthy generator will be forced to supply more of the reactive load, leading to possible overloading of its excitation system.
If unchecked, this can cause instability, overheating, and possible trip of the overloaded machine.
