The modern electric power system consists of several elements e.g alternators, transformers, station bus bars, transmission lines and other equipment. It is desirable and necessary to protect each element from a variety of fault conditions which may occur sooner or later. As a matter of convenience, here we deals with protection of alternators transformers, bus bars, lines and protection against over voltages.
The most serious faults on alternators which requires immediate attention are the stator winding faults. The major faults on transformers occur due to short circuits in the transformer or their connections. The basic system used for protection against these faults in the differential relay scheme because differential nature of measurements makes this system much more sensitive than other protective systems.
Protection Of Alternators
The generating units, 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:
- failure of prime mover
- failure of field
- Unbalanced loading
- stator winding faults
Failure Of Prime Mover: When input to the prime-mover fails, the alternator runs as a synchronous motor and draws some current from the supply system. This motoring condition is known as “inverted running”
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 universal practice not to provide automatic protection against this contingency.
Over Current: It occurs mainly due to partial breakdown of winding insulation or due to overload on the supply system. Over current protection for alternators is considered unnecessary because of the following reasons:
- The modern tendency is to design alternators with very high values of internal impedance 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.
- 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 momentary troubles outside the plant and therefore, interfere with the continuity of electric service.
Over Speed: The chief cause of over speed 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.
Over Voltage: The field excitation system of modern alternators is so designed that over voltage conditions at normal running speeds cannot occur. However, over voltage in an alternator occurs when speed of the prime mover increases due to sudden loss of the alternator load.
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.
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:
- fault between phase and ground
- fault between phases
- 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 minimize the extent of damage. For protection of alternators against such faults, differential method of protection (also known as Merz-Price System) is most commonly employed due to its greater sensitivity and reliability.
Differential Protection Of Alternators
The most common system used for protection of stator winding faults employs circulating current principle. In this Scheme of protection, currents at the two ends of the protected section are compared. Under normal operating conditions, these currents are equal but may become unequal on the occurrence of a fault in the protected section. The difference of the currents under fault conditions is arranged to pass through the operating coil of the relay. The relay then closes its contacts to isolate protected section from the system. This form of protection is also known as Merz-Price circulating current scheme.
Balanced Earth Fault Protection
In small size alternators, the neutral ends of the three phase windings are often connected internally to a single terminal. Therefore, it is not possible to use Merz-Price circulating current principle described above because there are no facilities for accommodating the necessary current transformers in the neutral connection of each phase winding. Under these circumstances, it is considered sufficient to provide protection against earth faults only by the use of balanced earth fault protection scheme. This scheme provides no protection against phase to phase faults, unless and until they develop into earth-faults, as most of them will.
Stator Inter-Turn Protection
Merz-Price circulating current system protects against phase-to-ground and phase-to-phase faults. It does not protect against turn-to-turn fault on the same phase winding of the stator. It is because the current that this type of fault produces flows in a local circuit between the turns involved and does not create a difference between the currents entering and leaving the winding at its two ends current transformers are applied. However, it is usually considered unnecessary to provide protection for inter-turn faults because they invariably develop into earth-faults. In single turn generator, there is no necessity of protection against inter-turn faults. However, inter-turn protection is provided for multi-turn generators such as hydro-electric generators. These generators have double-winding armatures owing to the very heavy currents which they have to carry. Advantage may be taken of this necessity to protect inter-turn faults on the same winding. The figure shows the schematic arrangement of circulating-current and inter-turn protection of a 3 phase Double wound generator. The relays Rc provide protection against phase-to-ground and phase-to-phase faults whereas relays R1 provide protection against inter-turn faults.