What Is a Shunt Reactor?

What Is a Shunt Reactor?

A shunt reactor is a piece of power line equipment that helps stabilize voltage during load changes. It also controls dynamic overvoltage and provides capacitive reactive power compensation in systems above 400 kV. They come in different sizes and designs, but they all perform the same function.

Shunt reactors are a critical component of any modern power system. They are used in long transmission lines and cable systems, high-voltage substations, and dense urban networks. Utilities must ensure that their systems can react quickly to changes in load and supply enough reactive power during a black start. Shunt reactors help to achieve this goal by allowing them to compensate for line parasitic capacitance, mitigate voltage transients caused by switching, and reduce the impact of short circuits on line conductors and switching apparatus.

The most common shunt reactor is a fixed-rate type, which has a constant rating and is connected to the line all the time. However, a new variable-rate shunt reactor (VSR) is now available that can be switched in and out of service to respond to changes in load. VSRs have a higher regulation range, up to 80% at some voltage levels, and they can be rated for much higher voltage levels than traditional fixed-rate shunt reactors.

VSRs use either an iron or air core to maintain a consistent impedance and avoid harmonic currents. These devices can be oil-immersed or dry, depending on the requirements of a specific application. They can be fitted with built-in protections, including Buchholz relay and oil pressure and temperature sensors for oil-immersed reactors and windings temperature sensors for dry shunt reactors. They can also be equipped with a differential evolution (DE) algorithm to design the core to minimize material costs and expected losses.

In addition to being able to react quickly to changes in load, shunt reactors are also designed to resist the effects of ferroresonance. This is a condition where the power transformer’s tertiary winding generates excessive capacitive current, which causes the receiving end of the line to have a voltage that is higher than the sending end. The excessive voltage will cause overfluxing of the power transformer and over stressing of the line insulator.

During a reignition transient, the very high rate-of-rise of the voltage across the shunt reactor causes severe voltage stress on its turn-to-turn insulation at the incoming end. This can damage the shunt reactor and interrupter, and may even exceed the maximum operating voltage of the device protecting it.

General-purpose interrupters are not designed to interrupt the small inductive currents of a shunt reactor. This can result in harmful high-energy reignitions that can damage the interruption device and the turn-to-turn insulation of the shunt reactor. To reduce this problem, the Southern States’ RLSwitcher was specifically developed to interrupt shunt reactors without any reignition problems. The RLSwitcher uses a unique contact gap to delay the first interruption until the contact gap is large enough to withstand the TRV presented by the shunt reactor. This avoids the high-energy reignitions that can damage many types of interrupters and can lead to premature failure of the shunt reactor’s switchgear.

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