What Is Shunt Reactor Switching?

Shunt reactor switching is the operation of connecting or disconnecting high-voltage inductive reactors from a power system bus or transmission line using circuit breakers or other switching devices. Because shunt reactors store energy in their magnetic fields during normal operation, interrupting or initiating the reactor current at high voltage produces transient overvoltages and oscillations that must be controlled to protect the reactor, the circuit breaker, and adjacent network equipment. Shunt reactor switching is a specialized discipline within high-voltage engineering that combines knowledge of electromagnetic transients, circuit breaker technology, and overvoltage protection.

The phenomenon is of particular importance on transmission systems rated 100 kV and above, where shunt reactors compensate for the leading reactive power generated by long underground cables and lightly loaded overhead lines. These reactors must be switched in and out of service frequently as load and generation conditions change.

Switching Overvoltages

When a circuit breaker opens to disconnect a shunt reactor, it must interrupt the inductive current near a current zero. If the breaker interrupts prematurely, before the natural current zero, or if it restrikes after interruption, the energy stored in the reactor's inductance drives a voltage transient whose peak can reach two to three times the normal system voltage. Even a clean interruption at the current zero leaves the breaker terminals at a high recovery voltage, since the reactor's terminal voltage lags the current by 90 degrees, meaning the source-side voltage is at its maximum precisely when the current passes through zero.

The magnitude of the transient overvoltage depends on the system voltage, the reactor rated power, the circuit breaker's dielectric recovery characteristics, and the presence of stray capacitance in the circuit. As detailed in the IEEE C37.109-2023 Guide for Protection of Shunt Reactors, these transients can cause insulation stress on the reactor windings, the circuit breaker interrupters, and any connected instrument transformers.

Controlled Switching and Surge Arresters

Two principal mitigation techniques are applied to limit shunt reactor switching overvoltages. Controlled switching, also called point-on-wave switching, synchronizes the circuit breaker's mechanical operation so that each pole closes or opens at a specific phase angle of the system voltage waveform, targeting the instant when the recovery voltage stress is minimized. Modern controlled switching controllers use a predictive model of the breaker's mechanical timing to issue the trip or close command far enough in advance that the breaker contacts arrive at the target phase angle within a few milliseconds.

Metal oxide surge arresters installed at the reactor terminals clamp overvoltages to a protective level determined by the arrester's energy absorption capability. The IEEE Xplore study on reactive power compensation using variable shunt reactors discusses how the combination of controlled switching and arrester protection provides redundant layers of overvoltage mitigation for variable reactor installations where switching frequency is high.

Circuit Breaker Requirements

Circuit breakers designated for shunt reactor switching duty must meet requirements beyond those for ordinary load or fault interruption. The breaker must be capable of interrupting small inductive currents, typically in the range of tens to a few hundred amperes, without producing severe chop-current transients. Current chopping occurs when the arc extinguishing process forces the current to zero before its natural zero, and the resulting magnetic energy collapse drives an overvoltage proportional to the chopping level and the system surge impedance. The IEEE EEP technical reference on shunt reactor connections, switching, and protection specifies the test quantities and type tests that breakers must pass to be certified for this application.

Applications

Shunt reactor switching has applications in a range of fields, including:

• High-voltage transmission substation reactive power management during light-load and night-time operation
• Long underground AC cable systems requiring frequent reactive compensation adjustment
• Offshore wind farm export cable overvoltage control
• Flexible AC transmission system (FACTS) installations incorporating thyristor-controlled reactors
• High-voltage direct current (HVDC) converter station reactive power balancing