In field operations, one might occasionally hear a sharp "pop" when a vacuum circuit breaker interrupts a shunt reactor-often followed by a failure in the reactor or a cable termination. Investigations frequently point to "current-chopping overvoltage" as the culprit. How exactly does this overvoltage occur? It is actually quite straightforward.
Current Chopping: Interrupting the Current Before It Reaches Zero
Vacuum circuit breakers possess an exceptionally strong arc-extinguishing capability. When interrupting small inductive currents, the breaker may extinguish the arc forcibly-while the current is still at several amperes-rather than waiting for it to reach its natural zero-crossing point. This phenomenon is known as "current chopping." Since a reactor is an inductive load, the sudden drop of current to zero leaves the magnetic energy stored in the inductance with nowhere to go; it is forced into the stray capacitance present within the circuit.
Energy Transfer Drives Up the Voltage
Capacitances-such as the reactor's capacitance to ground or cable capacitance-combine with the inductance to form an LC oscillatory circuit. The stored magnetic energy flows into the capacitors, charges them to a peak, and then discharges back, creating an oscillation. At the moment the magnetic energy is fully converted into electric field energy, the voltage across the capacitors surges to a very high level-this is the current-chopping overvoltage. The oscillation frequency can reach several kilohertz, with an amplitude potentially several times higher than the system voltage.
Higher Chopping Values and Lower Capacitance Lead to More Severe Overvoltages
A larger chopping current value means more stored magnetic energy, resulting in a higher overvoltage. Similarly, a smaller circuit capacitance leads to a higher characteristic impedance, which further intensifies the overvoltage. While the chopping current for a vacuum circuit breaker is typically around 5A, specific circuit parameters can cause this overvoltage to reach levels that threaten the inter-turn insulation of the reactor or the integrity of cable terminations.
The Real Culprit Is Often Not the Current Chopping Itself
One critical detail is frequently overlooked: in field tests involving 35kV vacuum circuit breakers interrupting reactors, the overvoltage caused solely by current chopping is often not the most severe factor. Instead, it is the "reignition" in the first phase to clear-which can subsequently trigger simultaneous interruption across all three phases-that serves as the primary driver for these high-frequency, high-amplitude overvoltages. Current chopping acts as the trigger, while reignition escalates the situation.
Understanding this mechanism provides a basis for selecting appropriate RC snubbers or surge arresters, ensuring that switching overvoltages are kept within the limits of the insulation's withstand capability.
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