
Where i is the magnitude of the chopped current (0 to perhaps as high as 10 A or more), L is the reactor’s inductance, and C is the capacitance of the reactor (on the order of a few thousand picofarads). When C is small, especially likely with dry-type reactors often used on transformer tertiaries, the surge impedance term can be large, and hence the overvoltage can be excessive.
To mitigate the over-voltages, surge arresters are sometimes useful, but the application of a capacitor on the terminals of the reactor (or other equipment) have a capacitance on the order of 0.25–0.5 μF is very helpful. In the equation above, note that if C is increased from pF to μF, the surge impedance term is dramatically reduced, and hence the voltage is reduced.