MAJOR FAULTS IN GENERATOR
The major faults in case of generator can be classified as:
(a) Failure of prime movers:
Whenever there is a fault on prime mover side the conversion of mechanical power to electrical power stops. If this generator works in parallel with other generator sets it will start working as a synchronous motor running at synchronous speed and the prime mover will act as load on it. When generator starts running as a motor it is called as inverted operation. If the fault is cleared the generator will automatically pick up generation.
(b) Failure of excitation:
The turbo generator set under consideration works in parallel with other sets. If the field of the generator is interrupted it will continue to operate as an induction generator-delivering load at very low power factor. So the other generators will be overloaded, as they will have to supply the load of this faulty generator. This may lead to decrease the supply voltage and the stability of the system will be affected.
The fault generator can be switched off but for an automatic operation an under current relay in conjunction with time lagged tripping and time lagged reinforcing relay as shown in fig 1 can be used.
The under current relay is a moving coil type instrument and is fed through a shunt in the field circuit and its provided with a double trip circuit contact. When there is field circuit failure under current relay is no longer energized and the action moves up the operating armature up and in doing so it stops supply to time lagged reinforcing and trip relays, thus both of theses relays are operated simultaneously.
(a) Failure of insulation in the stator or interconnecting cable can be further divided into:
i. Interphase short circuits – These are accompanied by large fault currents, the values of which are dependent upon the capacity of the system to which the alternator is connected, the nature of inter connection to the system and the moment at which the short circuit occurs.
ii. Single phase to earth fault – This will lead top the burning of alternator winding when leakage current exceeds even 5 amps. This current is dependent upon the nature of neutral connection, i.e. whether its earthed or unearthed.
iii. Inter turn faults – In this type of fault short circuit exists between the turns of the same phase or between turns of parallel branches in the same phase. The magnitude of the fault current depends on the number of turns shorted. This is a dangerous type of fault
iv. Earth fault on the rotor – Shorting of the rotor at one point to earth in itself does not cause any damage. But the appearance of another fault to earth may sharply disturb the magnetic flux distribution, which will lead to unequal voltages being induced in the stator and increased vibration in the alternator.
Written by John on March 19th, 2009 with
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