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.

Abstract: A high power DC blocking device is provided which blocks DC current, passes AC current, and limits the voltage across it under fault conditions. A DC blocking capacitor is used to block the flow of DC current, while allowing the passage of normal AC currents. A main bypass path includes switching devices which provide a low impedance path across the capacitor under fault conditions. Auxiliary switching devices are used to connect a storage capacitor across the device at the initiation of a fault, thereby charging the storage capacitor to a stored voltage level before the main switching devices are fired. After the fault passes, the auxiliary switching devices are fired once again to apply the stored voltage in the storage capacitor to the main switching devices to commutate them off. The storage capacitor and auxiliary switching devices also form part of a voltage clamp circuit which dissipates inductive energy stored in a system to which the DC blocking device is connected.

Abstract: An isolator surge protector for use with corrosion protection systems which are an inherent part of or coupled to an electrical power system, for power transformers, and for DC transmission systems, where it is necessary to present a high impedance to DC and a low impedance to AC up to a predetermined voltage level. Anti-paralleled thyristors in the isolator surge protector turn on during power surges. A bypass circuit insures turn off of the thyristors after a triggering event such as lightning or a 60 Hz surge has ended. The bypass circuit shunts DC bias currents, such as that which may be provided by a DC bias voltage source for cathodic protection, from the thyristors to ensure that the voltage across each thyristor falls below its holding voltage or current, and thus prevents the thyristor from being stuck in a continuous conduction mode. Power to operate the bypass circuit may be tapped from the surge which caused the thyristor to trigger so that the bypass circuit operates autonomously.