Tuesday, July 5, 2011

Generator Protection

Generator Protection Part 1
1. Introduction
       The generators used in the power system are the alternators which produce very high a.c. voltages. The protection of generators is very much complex due to the following reasons,
1. The generators are very large machines producing very high voltages and are connected to busbars.
2. Various other equipments are always associated with the generators. Such equipments are prime movers, excitation systems, voltage regulators, cooling systems etc. Thus protection of generators must consider the presence of these other equipments also.
3. The generators are very costly, expensive and very important factor in a power system. The protection scheme must be such that it should not shut off the generators result in a power shortage.
       All these factors make the design of protection scheme for the generator, very much complex.
       Before studying the various protection schemes for the generators, let us discuss various faults which can occur associated with the generators.
2. General Faults
       The various faults which can occur associated with a generator can be classified as,
1. Stator faults : The faults associated with the stator of the generator.
2. Rotor faults : The faults associated with the rotor of the generator.
3. Abnormal running conditions : This includes number of abnormal conditions which may occur in practice, from which the generator must be protected.
       Let us discuss these faults in detail.
2.1 Stator Faults 
       The stator faults means faults associated with the three phase armature windings of the generator. These faults are mainly due to the insulation failure of the armature windings. The main types of stator faults are,
1. Phase to earth faults
2. Phase to phase faults
3. Inter-turn faults involving turns of same phase winding.
The most important and common fault is phase to earth fault. The other two are not very common while inter-turn fault is very difficult to detect.
2.1.1 Phase to Earth Fault
       These faults mainly occur in the armature slots. The faults are dangerous and can cause severe damage to the expensive machine. The fault currents less than 20 A cause negligible burning of core if machine is tripped quickly. But if the fault currents are high, severe burning of stator core can take place. This may lead to the requirement of replacing the laminations which is very costly and time consuming. So to avoid the damage due to phase to earth faults, a separate, sensitive earth fault protection is necessary for the generators alongwith the earthing resistance.
2.1.2 Phase to Phase Faults
The phase to phase faults means short circuit between two phase windings. Such faults are uncommon because the insulation used between the coils of different phases in a slot is large. But once phase to earth fault occurs, due to the over heating phase to phase fault also may occur. This fault is likely to occur at the end connections of the armature windings which are overheating parts outside the slots. Such a fault cause severe arcing with very high temperatures. This may lead to melting of copper and fire if the insulation is not fire resistant.
2.1.3 Stator inter-turn Faults
        The coils used in the alternators are generally multiturn coils. So short circuit between the turns of one coil may occur which is called an inter-turn fault. This fault occurs due to current surges with high value (L di/dt) voltage across the turns. But if the coils used are single turn then this fault can not occur. Hence for the large machines of the order of 50 MVA and more, it is a normal practice to use single turn coils. But in some countries, multiturn coils are very commonly used where protection against inter-turn faults is must.
2.2 Rotor Faults 
       The rotor of an alternator is generally a field winding as most of the alternators are of rotating field type. The field winding is made up of number of turns. So the conductors to earth faults and short circuit between the turns of the field winding, are the commonly occurring faults with respect to a rotor. These faults are caused due to the severe mechanical and thermal stresses, acting on the field winding insulation.
       The field winding is generally not grounded and hence single line to ground fault does not give any fault current. A second fault to earth will short circuit the part of the field winding and may thereby produce unsymmetrical field system. Such an unsymmetrical system gives rise to the unbalanced forces on the rotor and results in excess pressure on the bearings and the shaft distortion, if such a fault is not cleared very early. So it is very much necessary to know the existence of the first occurrence of the earth fault so that corrective measures can be taken before second fault occurs.
       The unbalanced loading on the generator is responsible to produce the negative sequence currents. These currents produce a rotating magnetic field which rotates in opposite direction to that of rotor magnetic field. Due to this field, there is induced e.m.f. in the rotor winding. This causes overheating of the rotor.
       Rotor earth fault protection and rotor temperature indicators are the essential and are provided to large rating generators.

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1 التعليقات:

  1. Hi there! great stuff, Thanks for sharing a very interesting and informative content, it helps me a lot, keep it up!
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