High power dc blocking device for ac and fault current grounding

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. Multiple backup firing circuits are provided which activate the low impedance bypass path in the event of failure of the main firing circuits or of the blocking device control system. The effectiveness of the DC blocking device is continuously monitored using a DC current sensing circuit which is capable of distinguishing small DC currents in the presence of large AC currents.