When the voltage in a system, raised beyond its rated voltage, then it is known as overvoltage. This overvoltage may be of transient or persistent nature. The main cause due to which overvoltage is produced in the power system may be conveniently grouped into two categories, namely, internal and external. Internal overvoltage has got their origin within the system itself, whereas external overvoltage is because of lightning on the lines.
This over voltage may cause damage to insulators and substation equipment. It is, therefore, necessary to provide a means to protect the insulators and other apparatus from the harmful effect of overvoltage. Some devices are available to reduce the amplitude and front steepness of surges. The following will be described here
- Rod gap
- Surge Diverter
- Overhead Earth Wire
Overhead Earth Wire
An overhead earth wire or ground wire is one of the most common devices used to protect the lines against lightning. It is the wire carried the line supports and runs over the phase conductors. The purpose of the earth wire is to block the direct lightning strokes, which would otherwise strike the phase conductors. The waves of lightning reach the adjoining towers which pass them to earth safely.
In case the resistance of electrical tower or ground is small, the lighting will be raised to very high voltage, which will cause a flash over from the tower to one or more phase conductors. Such a flashover is known as black flash over. Back flash over on the line can be minimized by reducing the tower footing resistance using driven rods and counterpoise where soil resistivity is high.
The rod gap is one of the most common frames of protective devices. It is an air gap between the ends of two rods. The gap setting should be such that it should break under all conditions before the equipment to be protected is affected. The chief merits of this device are simplicity, reliability and cheapness.
Rod gap has a some restrictions like they are unable to prevent the flow of power which flows in the gap after the breakdown. It is used in the places where continuity of supply is not of much importance. In such cases (where continuity is important) , automatic reclosing circuit breakers are used.
Surge-diverters or lightning arrester is a device used for distracting abnormal high voltage to ground without affecting the continuity of supply. Surge diverters are three types
- Expulsion-type surge diverter
- Valve-type surge diverter
- Metal-oxide surge diverter
The name surge diverters appear to be correct than lightning arrester.