Use of silicon carbide varistors in the prevention of overvoltages in electrical T&D protection systems

The non-linear behaviour of silicon carbide varistors can be used to prevent high voltages from damaging protection systems, used in electricity T&D networks. Such ´overvoltages´ may be created when high current transients are incident on high impedances within the protection systems. These may be due to the type of protection system used or created when current transformers are left in open circuited conditions. The reliability of protection systems is key to the sustainability of power transmission networks and becomes more critical as the networks are expanded. Use of overvoltage limiting components in these protection systems will then help to maintain their reliability. This paper firstly describes the electrical characteristics of silicon carbide varistors. Their role in current transformer protection units and high impedance differential relays is then discussed.