Saturday, 16 July 2016

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Power System Protection Apparatus

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Surge arresters, also popularly referred to as, surge protectors in the electrical engineering are a part of the family of electrical protection devices and equipment. These types of equipment are installed in an electric power system to make sure that the other complicated and expensive devices connected to the power system networks are not damaged. This damage can result from the conditions of overvoltage, over current, low voltage, voltage transients, voltage surges, etc. These conditions arise due to factors like, lightning strike, either, directly on or near the transmission networks, fault conditions most prominent of which are, single line to ground fault, double line to ground fault, line to line fault.

These abnormal conditions can cause a great deal of harm to the power system. The over current and over voltage surges can burn the insulation on the winding of the generator and transformers. This can to lead to breakdown of the power system for several hours. Moreover, the relays and circuit breaker assembly installed to provide early warning and control of the working conditions of the power system, can trip when exposed to these unstable conditions for a time longer than the standard time. This will result in circuit breaking operation, which will cause isolation of a healthy and working component of the power system. As a result a section of the power system will be cut off from the remainder and supply will be discontinued. This is again a major issue in designing power system protection equipment.

The power system protection devices are responsible to guarantee the safety of the power system equipment like generators, transformers, transmission cables, motors, etc. and thus are very essential for smooth functioning of the powers system.

The cause of surge in an electrical power system: Lightning is a natural phenomenon in which flow of charged particles takes place in between two points at different potentials, through air. The two points can be charge center of the cloud and ground, charge centers of two different clouds or two charged centers of the same cloud. When a lightning strikes directly on or even near a pole carrying charged conductors, high voltage surges are induced in the current carrying conductors that travel to the terminals. These voltage surges in turn give rise to their own over current surges.

This over current surge is the prominent cause of trouble. These over current surges can cause destruction of very important and very expensive terminal apparatus. To prevent this surge arresters are installed at the terminal of transmission line in a transmission and distribution network. 

Operating principle of surge arresters: To protect the terminal apparatus from the surges (both over current and over voltage) induced in the line, a surge reducing device like polymer surge arresters are installed in between the apparatus and the transmission line. The surge arrester employs special electrical devices called Varistors. The resistance of a varistor changes drastically with the change in the terminal voltage. As a result when the line voltage is very high, as in case of an over voltage surge, the resistance of the varistor connecting the line to the ground becomes very low. Thus the over current is set to the ground. In case of normal operating voltages, the resistance of the earthing varistor is close to infinity and the line is shorted to the terminal apparatus.


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