Silicon carbide varistors
Current transformers are used to monitor large currents within power grids and power stations, producing a current in the device’s secondary winding that is proportional to the current flowing in its primary winding.
byTom Galvin, Dominic Coll
- Introduction
1.1 Varistors and how they work
A varistor (varying resistor) is a semiconducting device with a voltage-dependent resistance. Sometimes known as a voltage dependant resistor (VDR) or a metal oxide varistor (MOV, which refers to a specific type of varistor), varistors play a critical role in ensuring the safe and reliable operation of electrical circuits by acting as control or compensation elements to ensure optimal operating conditions or to prevent excessive voltages from being reached.
1.2 Current transformers
Current transformers are installed worldwide in high voltage power grids and in power generation plants. A current transformer produces a stepped down alternating current in its secondary winding, proportional to the current in its primary winding. To do this, the current transformer has a large number of turns in its secondary winding, typically 400 to 800 turns but often greater for high current systems, and usually just one turn in its primary. This allows for safe monitoring and measurement of the primary current.
- Protecting current transformers using varistors
Varistors respond to the increasing voltages that occur during open-circuit conditions in a current transformer, preventing potentially dangerous overvoltage events, should the secondary be disconnected from the burden while the primary is still energised. A typical configuration for a High Impedance Differential Relay scheme is shown in Fig. 1.
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