Series reactors are used in a power system to limit the current in case of a fault. During fault conditions, extremely high currents flow through the system. If the busbars and circuit breakers are not able to withstand the fault currents, they can get damaged and the protection of the system can be seriously compromised. This is known as the fault level of a system.While the fault level of a power system is taken into account during the design process. Over the years, the fault level can change when new sources of power such as generators and transformers are added into the system. The increased fault level cannot be handled by the existing busbars and circuit breakers. Replacing these system components will be expensive and time-consuming and almost impossible.
In this scenario, reactors provide a simpler solution. During a fault condition, the maximum current which may flow is determined by the impedance in the path of the fault circuit. Reactors work by increasing the reactance, and consequently the impedance of the system they are fitted in. This leads to reduced fault currents.
Reactors are generally fitted between adjacent busbar sections or in series with feeders. While reactors are useful in containing the fault currents, the downside is that tend to introduce a mild drop in the voltage. Hence, regulation in the form of boosters or transformer tappings may be required.
There are different kinds of reactors such as air core reactors, oil immersed reactors, etc. Series reactors are used widely in transmission and distribution systems.