Non-grounded circuit ground fault detection device, ground fault protection relay using the same, and ground fault detection method

摘要

PROBLEM TO BE SOLVED: To calculate a ground fault current caused to flow in an electric circuit of a non-grounded system electric path by a ground fault accident in premises of the electric circuit of the non-grounded system by simple arithmetic processing.SOLUTION: A ground fault detection apparatus 2 is connected to a potential transformer 7 which detects a zero-phase voltage Vo caused to appear in the electric circuit 1 of the non-grounded system by a ground fault accident in premises of the electric circuit 1 of the non-grounded system, and a zero-phase current transformer 8 which detects a zero-phase current Io caused to flow in the electric circuit 1 of the non-grounded system on the occurrence of the ground fault accident in the premises are connected. The apparatus includes a differential circuit 9 which differentiates the zero-phase voltage Vo obtained by the potential transformer 7 with respect to time, an arithmetic circuit 10 which sets ground static capacitance C of the premises of the electric circuit 1 of the non-grounded system, a multiplication circuit 11 which multiplies a differentiated value dVo/dt calculated by the differential circuit 9 by the ground static capacitance C calculated by the arithmetic circuit 10, and a subtraction circuit 12 which obtains a ground fault current Ig by subtracting a multiplication value C×(dVo/dt) calculated by multiplying the ground static capacitance C by the differentiated value dVo/dt from the zero-phase current Io.