Generator faults can be considered under the following heads: 1. Stator Winding Faults 2. Field Winding or Rotor Circuit Faults 3. Abnormal Operating Conditions.
1. Stator Winding Faults:
Such faults occur mainly due to the insulation failure of the stator coils.
The main types of stator winding faults are:
(i) Phase-to-earth faults
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(ii) Phase-to-phase faults and
(iii) Inter-turn faults involving turns of the same phase winding.
The stator winding faults are the most dangerous and are likely to cause considerable damage to the expensive machinery. So, automatic protection is absolutely necessary to clear such faults in the shortest possible time in order to minimise the extent of damage.
Phase-to-phase faults and phase inter-turn faults are less common, these usually develop into an earth fault. Inter-turn faults are more difficult to be detected.
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The effect of earth fault in the stator is two-fold:
(i) Arcing to core, which welds laminations together, causing eddy current hot spots on subsequent use. Repairs to this condition involve expenditure of considerable money and time.
(ii) Severe heating in the conductors damaging them and the insulation with possible fire breaks.
2. Field Winding or Rotor Circuit Faults:
Faults in the rotor circuit may be either earth faults (conductor-to-earth faults) or inter-turn faults, which are caused by severe mechanical and thermal stresses.
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The field system is normally not grounded (i.e., remains isolated from the earth) and, therefore, a single fault between field winding and rotor body due to insulation breakdown does not give rise to any fault current. However, a second earth fault will short circuit some part of the rotor winding and may thereby develop an unsymmetrical field system, giving unbalanced force on the rotor. This can cause severe vibration of the rotor with possible damage to the bearings. Thus a single earth fault can be tolerated for a while but it should not be allowed to continue. Rotor earth fault protection is provided in case of large generators.
Owing to a fault, there may be an unbalance in the three-phase stator currents. According to the theory of symmetrical components, unbalance three-phase currents have a negative sequence component, which rotates at synchronous speed in a direction opposite to the direction of rotation of rotor. So, double frequency currents are induced in the rotor.
This causes overheating of rotor and possible damage to the rotor. Unbalanced currents may also cause severe vibration, but the overheating problem is more acute. Rotor temperature indicators are used with large generators for detecting rotor overheating due to unbalanced loading of generator.
Rotor open-circuit faults, though rare, can cause arcing and thus result on serious conditions.
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Reduced excitation may occur due to open circuit or short circuit in the field or exciter circuits or a fault in automatic voltage regulator. When a generator loses its field excitation it speeds up slightly and continues to run as an induction generator deriving excitation from the system and supplying power at a leading power factor.
A fall in voltage will also occur due to loss to excitation which may result in loss of synchronism and system stability. There is also the possibility of overheating of the rotor due to induced currents in the rotor and damper windings. This can be avoided by using a tripping scheme which is so arranged that opening of field circuit breaker causes the tripping of generator unit breaker.
3. Abnormal Operating Conditions:
The abnormal operating conditions that are likely to occur in a generator are:
(i) Failure of prime mover (turbine) resulting in operation of the generator as a synchronous motor
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(ii) Failure of field
(iii) Unbalanced loading and subsequent heating of generator
(iv) Overloading
(v) Over-voltage at generator terminals
(vi) Over-speed
(vii) Ventilation failure and
(viii) Current leakage in the body of the generator.