Reactor Switching

When a load is de-energized, the contacts of the switching device begin to part, the gap is small, and the dielectric strength is low. It is not physically possible to fully separate the contacts instantaneously due to the inertia of those contacts, so the contacts must be accelerated.

As the gap widens, the distance increases, which increases the dielectric strength of the gap. Once the voltage waveform crosses zero, the voltage begins to increase in magnitude. The voltage across the opening contacts is referred to as recovery voltage. If it increases faster than the gap’s dielectric strength, the current will be reestablished through an arc across the contacts. The contacts continue to separate and the gap’s dielectric strength increases to the point, the arc is extinguished when the current waveform reaches the next zero crossing. Switching devices are designed to dissipate the energy of this arc and the small associated transient voltage disturbance.

Figure 1

Shunt reactor switching imposes a severe duty on the connected system and the switching device. Interrupting the relatively small inductive current, generally less than 300 A, is easy for most switching devices resulting in the interrupting device try to clear at a forced current zero (current chopping) or at the first current zero due to the low magnitude of the current. At this point, the device’s interrupter contacts are still very close to each other. The gap’s dielectric strength is typically not sufficient to prevent a reignition due to the very fast recovery voltage (see Figure 1). Since a shunt reactor is often switched daily, the repeated high magnitude reignitions have the same impact on reactor windings as repeated lightning strikes causing premature failure of reactors.  In addition, these re-ignitions damage the internal parts of the switching device causing their failure. Devices used for shunt reactor switching need the ability to mitigate reignitions and current chopping.