Voltage and current based control and triggering for isolator surge

Abstract

An isolator surge protector (ISP) is provided which blocks DC current and passes AC current and AC and DC fault currents. The ISP includes a main bypass path, including main bypass path switching devices, that is connected in parallel with a DC blocking capacitor. Primary triggering of the main bypass path switching devices occurs when the voltage across the ISP exceeds a primary voltage trigger level. Repeated triggering of the bypass path switching devices, and high current discharge of the high voltage DC blocking capacitor through the bypass path, under abnormal steady state voltage conditions, is prevented by initiating a secondary triggering period following primary triggering of the bypass path switching devices. During the extended secondary triggering period, triggering of the bypass path switching devices occurs at a low secondary voltage trigger level. Primary triggering of the bypass path switching devices preferably also occurs when the AC current in the DC blocking capacitor exceeds a selected primary current trigger level, thereby protecting the DC blocking capacitor from damage due to excessive currents. Primary triggering due to high AC current levels in the DC blocking capacitor is also followed by a secondary triggering period during which triggering of the bypass path SCRs occurs at the low secondary voltage trigger level. The ISP preferably also provides a warning indication when primary triggering of the bypass path switching devices occurs.