Insulation deterioration monitoring system for ungrounded power distribution systems

An insulation monitoring system is described for detecting ungrounded power distribution sections having an incipient intermittent ground caused by insulation deterioration. Test results are presented to demonstrate the effectiveness of the monitoring system. At first, the system concept is described. Next, the principle of the section detection method is given which is based on the phase relationship between the zero-phase sequence voltage (V₀) and zero-phase sequence current (I₀). Finally, test results, which demonstrate that the insulation monitoring system can detect a section having a ground current of several tens of mA, are presented