Power System Protection
INTRODUCTION
In a power system protection, faults can occur in any components like generators, transformers, buses or transmission lines and so on. The apparatus or equipments and their associated auxiliaries employed for controlling, regulating or switching on or off the electrical circuits in the electrical power system is known as “switchgear”. Faults in power system generally fall into two categories “short-circuit faults” and “open circuit faults”. Whenever a short-circuit occurs, a heavy current flows through the equipment causing considerable damage to it and interruption of service. So, in order to protect the lines, generators, transformers and other electrical equipment from damage automatic protective device or switchgear is required. An automatic switch gear consists of the relays, circuit breakers, lightning arresters and fuse. When the fault occurs on any section of the system, protective relay of that section comes in operation and close the trip circuit of the breaker, which disconnects the faulty section. The healthy section continues to supply loads as usual and thus there is no damage to the equipment and no complete interruption of supply.
Out of the two faults namely short-circuit faults and open-circuit faults, short-circuit faults are the most severe kind, resulting in flow of abnormally high currents which can lead to extensive damage to the equipment. It can lead to fire, explosion, insulation failure, overheating of generator and sometimes even loss of synchronism. On the other hand, open-circuit faults causes abnormal system operation and danger to personnel. They can be tolerated for longer period of time than short-circuit faults. The faults are detected by relays (automatically) and the faulty section is isolated by circuit breakers. The combination of relays and circuit breakers is known as the “protective system”. For the complete protection of a power system equipment there must be a coordinated operation between relay and circuit breaker.
Protection is needed not only against short circuit and open circuit case but also against any other abnormal conditions which may arise on a power system. Examples of other abnormal conditions are overspeed of generators and motors, over-voltage, under-frequency, loss of excitation, overheating of stator and rotor of an alternator etc. Protective relays are also provided to detect such abnormal conditions and issue alarm signals to alert operators or trip circuit breaker.
