The generating units, especially the larger ones, are relatively few in number and higher in individual cost than most other equipments. Therefore, it is desirable and necessary to provide protection to cover the wide range of faults which may occur in the modern generating plant.
Some of the important faults which may occur on an alternator are :
(i) failure of prime-mover
(ii) failure of field
(iii) overcurrent
(iv) overspeed
(v) overvoltage
(vi) unbalanced loading
(vii) stator winding faults
(i) Failure of prime-mover.
When input to the prime-mover fails, the alternator runs as asynchronous motor and draws some current from the supply system. This motoring condi-tions is known as “inverted running”.
(a) In case of turbo-alternator sets, failure of steam supply may cause inverted running. If
the steam supply is gradually restored, the alternator will pick up load without disturb-
ing the system. If the steam failure is likely to be prolonged, the machine can be safely
isolated by the control room attendant since this condition is relatively harmless. There-
fore, automatic protection is not required.
(b) In case of hydro-generator sets, protection against inverted running is achieved by pro-
viding mechanical devices on the water-wheel. When the water flow drops to an insuf-ficient rate to maintain the electrical output, the alternator is disconnected from the system. Therefore, in this case also electrical protection is not necessary.
(c) Diesel engine driven alternators, when running inverted, draw a considerable amount of power from the supply system and it is a usual practice to provide protection against
motoring in order to avoid damage due to possible mechanical seizure. This is achieved
by applying reverse power relays to the alternators which *isolate the latter during their
motoring action. It is essential that the reverse power relays have time-delay in opera-
tion in order to prevent inadvertent tripping during system disturbances caused by faulty
synchronising and phase swinging.
(ii) Failure of field.
The chances of field failure of alternators are undoubtedly very rare. Even if it does occur, no immediate damage will be caused by permitting the alternator to run without a field for a short-period. It is sufficient to rely on the control room attendant to disconnect the faulty alternator manually from the system bus-bars. Therefore, it is a uni- versal practice not to provide †automatic protection against this contingency.
(iii) Overcurrent
It occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system. Overcurrent protection for alternators is considered unnec- essary because of the following reasons :
(a) The modern tendency is to design alternators with very high values of internal imped-ance so that they will stand a complete short-circuit at their terminals for sufficient time without serious overheating. On the occurrence of an overload, the alternators can be disconnected manually.
(b) The disadvantage of using overload protection for alternators is that such a protection might disconnect the alternators from the power plant bus on account of some momen-tary troubles outside the plant and, therefore, interfere with the continuity of electric service.
(iv) Overspeed
The chief cause of overspeed is the sudden loss of all or the major part of load
on the alternator. Modern alternators are usually provided with mechanical centrifugal
devices mounted on their driving shafts to trip the main valve of the prime-mover when a
dangerous overspeed occurs.
(v) Over-voltage
The field excitation system of modern alternators is so designed that over-
voltage conditions at normal running speeds cannot occur. However, overvoltage in an
alternator occurs when speed of the prime-mover increases due to sudden loss of the alterna-tor load.
In case of steam-turbine driven alternators, the control governors are very sensitive to speed
variations. They exercise a continuous check on overspeed and thus prevent the occurrence of over- voltage on the generating unit. Therefore, over-voltage protection is not provided on turbo-alternator sets.
In case of hydro-generator, the control governors are much less sensitive and an appreciable time may elapse before the rise in speed due to loss of load is checked. The over-voltage during this time may reach a value which would over-stress the stator windings and insulation breakdown may occur.
It is, therefore, a usual practice to provide over-voltage protection on hydro-generator units. The over-voltage relays are operated from a voltage supply derived from the generator terminals. The relays are so arranged that when the generated voltage rises 20% above the normal value, they operate to
(a) trip the main circuit breaker to disconnect the faulty alternator from the system
(b) disconnect the alternator field circuit
(vi) Unbalanced loading. Unbalanced loading means that there are different phase currents in the alternator. Unbalanced loading arises from faults to earth or faults between phases on the circuit external to the alternator. The unbalanced currents, if allowed to persist, may
either severely burn the mechanical fixings of the rotor core or damage the field winding.
Fig. 22.1 shows the schematic arrangement for the protection of alternator against unbalanced
loading. The scheme comprises three line current transformers, one mounted in each phase, having their secondaries connected in parallel. A relay is connected in parallel across the transformer sec-ondaries. Under normal oper-ating conditions, equal currents
flow through the different phases of the alternator and their algebraic sum is zero.
Therefore, the sum of the currents flowing in the secondar-ies is also zero and no current
flows through the operating coil of the relay.
However, if unbalancing occurs, the currents
induced in the secondaries will be different and the resultant of these currents will flow through
the relay. The operation of the relay will trip the circuit breaker to disconnect the alternator from the system.
(vii) Stator winding faults.
These faults occur mainly due to the insulation failure of the stator windings. The main types of stator winding faults, in order of importance are :
(a) fault between phase and ground
(b) fault between phases
(c) inter-turn fault 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. Therefore, automatic protection is absolutely necessary to clear such faults in the quickest possible time in order to minimise the *extent of damage. For protection of alternators against such faults, differential method of protection (also knows as Merz-Price system) is most commonly employed due to its greater sensitivity and reliability. This system of protection is dis-
cussed in the following section