External faults:
During the external faults with large short circuit currents, severe mechanical stress will be imposed on the stator windings.
The temperature rise is, however, relatively slow, and a dangerous temperature level may be obtained after about 10 seconds.
The over-current and earth fault protection of the generator provides backup protection to external faults, while the primary protection is provided by the protective system of the respective equipment.
Overcurrent:
The overcurrent in the generator may occur due to an overload condition system or due to partial breakdown of winding insulation.
Overcurrent protection for generators is not taken seriously due to the following reasons.
Most of the time the overcurrent on the generator is a momentary condition, so it is not feasible to provide protection against the overcurrent otherwise such protection might disconnect the generators from the power plant bus which may cause the interruption of the electrical supply.
While designing the generators their impedance is kept high so that they can withstand a complete short circuit at their terminals for sufficient time without serious overheating.
Apart from the above two conditions generators can be disconnected manually on the occurrence of overcurrent or overload.
Failure of the prime mover:
When the input to the prime mover fails, the generator runs as a synchronous motor and draws some active power from the supply system.
If this is allowed to persist above 20 seconds, serious overheating of the steam turbine blades may occur, depending on the type of turbine and the design limits imposed by the manufacturer.
In the case of hydro generators sets, protection against this condition is achieved by providing mechanical devices on the water wheel.
When the water flow drops to an insufficient rate to maintain the electrical output, the generator is disconnected from the system. Therefore, in this case, electrical protection is not necessary.
Diesel engine-driven generators, when running as a synchronous motor, draws a considerable amount of power from the supply system and it is a usual practice to provide protection against motoring action in order to avoid damage due to possible mechanical seizure. This is achieved by applying reverse power relays to the generators.
Field winding faults:
Rotor faults include rotor inter-turn fault, rotor winding to earth faults, and open circuit in the rotor winding.
The chances of field failure of generators are very rare, but it may cause by mechanical and temperature stresses. No immediate damage will be caused by permitting the generator to run without a field (i.e. open-circuited field) for a short period.
The generator will work as an induction generator and draw reactive power from the supply. The attendant will disconnect the faulty generator manually.
The field system is not connected to the earth so that a single earth fault does not give rise to any fault current. A second earth fault will short circuit part of the winding and may thereby produce an asymmetrical field system giving unbalanced forces on the rotor. Such a force will cause excess pressure on bearing and shaft distortion, if not cleared quickly.
Over speed :
The main cause of the increase in speed is the sudden loss of all or the major part of the load on the generator. A mechanical centrifugal device is mounted on the driving shafts of the generators to protect the machines from over speed.
When a dangerous over-speed occurs this device trips the main valve of the prime mover.
Overvoltage :
There are basically two reasons by which an overvoltage occurs in a generator; these are
a) When the speed of the prime mover increases due to sudden loss of the generator load.
b) Direct lightning strokes on the system, switching surges, arcing grounds also causes the overvoltage on the generator.
Surge arresters and R-C surge suppressors were installed, for overvoltage protection. Also, the generator may assist in reducing some of the highest switching surges
Specially developed indoor type surge arresters are connected near generator terminals. These comprise a three-star connected unit plus another unit between star point and earth and thus provide over-voltage protection for all phases and between phases. Capacitors rated about 0.1 micro farade to earth are fitted to absorb the surge voltages.
Unbalanced loading :
When the current flowing through different phases of the generator is different then it is known as unbalanced loading. Unsymmetrical faults such as L-G, L - L, L - L - G, etc. in the system near the generating stations causes unbalanced loading to the generator.
Unbalanced loading on the generator mainly causes the rise to negative sequence currents which generates the negative sequence component of the magnetic field.
The magnetic field produced due to the negative sequence component of the unbalanced current rotates in the opposite direction of the main fields and induces e.m.f's of double frequency in the rotor winding. Thus the unbalanced loading causes rotor heating. A negative sequence protection scheme is used for unbalanced loading protection of generators.
Stator winding faults:
These faults occur mainly due to the insulation failure of the stator windings. The stator winding faults are most dangerous and are likely to cause considerable damage to the expensive machines. Therefore it should be cleared quickly by complete shut down of the generators. Only opening the circuit breaker does not help since the e. m.f. is induced in the stator winding itself. The field is opened and de-energized by field suppression. The stator faults include,
1) Phase to phase faults.
2) Phase to earth faults.
3) Stator inter-turn faults.
Automatic protection is absolutely necessary to clear such faults in the quickest time in order to minimize the damage to the machine. For protection against such faults, a different method of protection (Merz price protection) is most commonly employed.
Thermal overloading :
Continued overloading may increase the winding temperature to such an extent that the insulation will be damaged and its useful life will reduce.
Temperature rise can also be caused by the failure of the cooling system. In large machines, thermal elements (thermocouples or resistance thermometers) are embedded in the stator slots and cooling system.