New series capacitor protective device

A series capacitor is best protected from overvoltage during fault by a gap having low arc drop with no tendency to cut off. In this manner the capacitor is not subjected to repetitive voltage peaks during fault, and high current oscillations in the circuit formed by the capacitor and gap are minimized. A special design of graphite gap with self-centering arc characteristics is described. A new type of by-pass switch is also described for automatically by-passing the gap and series capacitor and removing the by-pass after the fault has cleared. The switch shown provides an inherent time delay on closing thus allowing time for the capacitor to discharge through the gap and avoid damage to the switch contacts. The switch also provides time delay on reopening thus eliminating the necessity for a holding means which has been a problem always with magnetic contactors in this application. The motive power for closing the switch is derived from a sealed bellows using high resistance metal and containing a volatile liquid. The passage of current through the bellows changes the vapor pressure and furnishes the forces required to operate the switch. A device of this type is not subject to damage by steep wave front surges associated with the discharge of the capacitor through the protective gap, nor is it damaged by intermittent line faults. The combination of this gap and switch has many advantages over a conventional gap and magnetic contactor and should permit a more general application of series capacitors to distribution circuits.