Patent ID: 6785105
Filing Date: 2004-08-31
Classification: H02H

Abstract:
A ground fault detection system for use in a local protective relay for ungrounded power systems, comprising:a selected one of (a) a zero sequence voltage value from a local relay on a protected power line and (b) a zero sequence voltage value from a remote relay on the power line; total zero sequence current values from the local relay and from the remote relay; a circuit for calculating zero sequence impedance from the selected zero sequence voltage value and the total zero sequence current value; and a first tripping circuit for tripping a circuit breaker associated with the protected power line at a selected time interval following determination that the calculated zero sequence impedance value exceeds a selected threshold, the first tripping circuit being subject to a second tripping circuit for tripping the circuit breaker at a time interval greater than the selected time interval when the calculated zero sequence impedance exceeds a preselected value and when selected other circuit conditions exist.