28-01-2013, 11:55 AM
THE PROTECTION OF GENERATOR AND TRANSFORMER
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INTRODUCTION
The generators are the most expensive piece of equipment in a .c. power system and are subjected to most possible trouble than any other equipment. The aim of the project is to protect against all these abnormal conditions and yet to keep the protection simple and reliable has resulted in considerable divergence of opinion on the choice of the protection.
Transformers are used in the power system are also subjected to so many troubles these are used for step-up or step-down the voltage continuous power supply is required so we have to protect transformer.
The purpose of the power system is to generate and supply electrical energy to consumers. The system should be managed to deliver this energy to the utilization points with both reliability and economy. The power system represents a very large capital investment so it should be protected so as give the best service to the consumers.
PROTECTION EQUIPMENT
IMPORTANCE OF PROTECTION
A fail free power system is neither economically justifiable nor technically feasible. Faults can occur in any power system components generator , transformer , motors , buses and lines - though the transmission lines being exposed to environment are most vulnerable . Faults fall in to two general categories - short circuit faults open circuit faults. Short circuit fault are most ever kind , resulting in a very abnormal high currents. If followed to persist of time , short period of time , short circuit can lead extensive damage to equipment . Undesirable effects of short circuit faults are unmerited below. Arcing faults (most common) can vaporize equipment in vicinity leading to, possibly, fire and explosion e.g. in transformer and circuit breakers. Power system components carrying abnormal currents get over heated , with the consequent reduction in the life span of their insulation. Operating voltages can go above or below their acceptable values, leading to development of another fault or another fault or damage to utilization equipment. Consequent unbalanced system operation causes overheating of generator rotor s. Power flow is severely restricted, or even completely blocked, while the short circuit lasts. As a consequence of blockage of power flow, power system areas can lose synchronism. The longer the fault last, the more is possibility of loss of synchronism. Open circuit faults cause abnormal system operating and danger to personnel. Voltage tends to rise well beyond acceptable values in certain parts of the system with possibility of insulation failure and development of short circuit fault. While open circuit faults can be tolerated for a long period of time than short circuit fault, these cannot be allowed to persist, and must be removed. We shall devote our attention to most sever type of fault, i.e. the short circuit faults. There are also other abnormal operating conditions, which require remedying, but do not fall two categories of faults mentioned. Two unbalanced conditions are one is heavily unbalanced generator and the other is loss of excitation. Faults should be instantly detected and faulty section is isolated from the section in the shortest possible time. It is obviously not possible to do this manually, and its must, therefore, be accomplished automatically. Faults are detected automatically be means of relays, and faulty section isolated by C.B. connected to the boundaries of section. The combination of relays & C.B. is known as protective system.
OVER VIEW OF RELAYS
A protective relay is a device that detects the fault and initiates the operation of the circuit breaker is isolate the defective element from the rest of system. The relays detect the abnormal conditions in the electrical circuit s by constantly measuring the electrical quantities, which are different under normal and fault conditions. As the technology in the protective system is developed
CLASSIFICATION OF PROTECTION OF ALTERNATOR
Protection schemes of alternators are classified based on the rating of the machine and the purpose for which the machine employed. Basically protection schemes are categorized into:
CLASS-A PROTECTION SCHEMES
This scheme gives the protection against the faults within the unit and auxiliaries directly connected to the unit where in the entire unit with auxiliaries and prime movers to be shutdown instantly. The unit is stripped in this scheme via reverse power protection.
GENERATION STATOR 0-100% EARTH FAULT PROTECTION
The relay to be employed depends on the type of earthing of the generator neutral. By restricting the fault current the damage to the core can be limited so that the repair will be easy. For small size generators the earthing can be done through a resistor in this case an instantaneous over current relay is connected to the C.T. provided on the neutral load will give fast and discriminative protection if the generator is connected to the bus through a delta/star transformer if the generator is connected directly to the bus, one IDMT over current the relay has to be used so as to grade the relays on the downstream side. The absence of third harmonic voltage in the secondary of the earthing transformer indicates an earth fault near to the neutral point including the neutral point.
GENERATOR DIFFERENTIAL PROTECTION
Fig 3.2 shows the schematic diagram of percentage differential protection. It is used for the protection of generators above 1 MW. It protects against winding faults, i.e. phase to phase and phase to ground faults. This is also called biased differential protection. The polarity of the secondary voltage of C.T.s at a particular moment for an external fault has been shown in the fig. In the operating coil, the current sent by the lower C.T. and the relay do not operate cancels the current sent by the upper C.T. For an internal fault the polarity of the secondary voltage of the upper C.T. is reversed as shown in the fig 3.1. Now the operating coil carries the sum of the currents sent by the upper C.T. and the lower C.T. and it operates and trips the circuit breaker. The percentage differential protection does not respond to external faults and overloads. It provides complete protection against phase-to-phase faults. It provides protection against ground faults about 80-85% of the generator winding. It does not provide protection to 100% of the winding because it is influenced by the magnitude of the earth fault current which depends upon the method of neutral grounding. When the neutral is grounded through impedance, the differential protection is supplemented by sensitive earth fault relays.