Method and apparatus for monitoring ground faults in isolated electrical systems

摘要

A device for monitoring ungrounded power distribution systems that indicates the total hazard current from the isolated power conductors to ground. A measurement signal having at least two frequency components different from the power system line frequency is applied across the combined fault impedances that occur between the power conductors and ground. Voltage responses to the measurement signal components are separated from the power system line voltage and from each other by frequency selective means. These distinct voltage responses are proportional to the fault admittance magnitude at the different measurement frequencies. The voltage responses are applied to the computational circuit whose output is proportional to the maximum fault admittance magnitude at the line frequency. A gain signal related to the imbalance of the fault impedances of the different power conductor is used to scale the previously described voltage responses. Thus, the output of the computation circuit is scaled to be proportional to the largest fault admittance on the distribution system which in turn is proportional to the total hazard current of the system.