Device to prevent malfunction of ground fault overcurrent relay

摘要

PROBLEM TO BE SOLVED: To prevent miss-trip due to unwanted supply of a DC current to a CT(current transformer) of the particular phase resulting by check, etc., in a ground fault protection circuit wherein PT(potential transformer) or GPT (grounding potential transformer) is not provided for reduction in size and low cost of GIS(glass-insulated switching device) and a trip output is provided for grounding over-current only responding to CT output. SOLUTION: Voltage change detectors dVD1, dVD2 which can detect a voltage changing rate are respectively provided for voltage detection electrodes K11, K21 for detecting a line voltage to invalidate trip output from grounding over current relays OCG1, OCG2 when a voltage change exceeding a predetermined changing rate setting value is not detected. Thereby, it is no longer required to connect the terminal of a measuring gate to the area extremely near the measuring object on the occasion of checking such as the measurement of contact resistance of switches LS11, LS21 and a degree of freedom of terminal connection can be improved to minimize movement of terminal, realize improvement of contact stability and simplify the measuring work.