MONITORING METHOD FOR DIELECTRIC DETERIORATION OF NONGROUNDED ELECTRIC CIRCUIT

摘要

PURPOSE:To accurately and economically monitor earth insulating resistance by comparing the magnitudes of the effective components of leakage currents flowing through the resistor between the neutral point of nongrounded electric circuits and the earth and the capacitor between one of the nongrounded electric circuits and the earth. CONSTITUTION:The neutral point of nongrounded cable ways 2, 4 is earthed through a resistor gammaand voltages of both terminals thereof are amplified by a high input impedance amplifier A to be applied to a synchronous detector M and the voltages of the electric circuits 2, 4 are applied to the other input terminal of the detector M through a voltage dropping transformer 5 and the output thereof is applied to the input of a subtractor DIF and also stored in a capacitor 3 through a switch SW3 and the output thereof is applied to the other terminal of the DIF. Switches SW 1-3 are turned OFF and the voltages of both terminals of the resistor gammaare amplified and detected to obtain the effective components of the same phase as the voltages of the electric circuits and the components are stored in a capacitor C. Next, the switch SW1 is turned ON and the effective component at this time is obtained and the outputs of both of them from the DIF are applied to a comparator CP to judge the positive and negative thereof. By this constitution, when the earth insulating resistor R1 of the electric circuit 2 is smaller than the resistor gamma, an alarm is emitted and dielectric deterioration can be monitored.