Measurement and automatic monitoring insulation resistance of AC/DC mixed unearthed networks

Where the danger of electric shock is high, for example underground mines or medical equipment directly connected to a patient, special ungrounded power systems (unearthed networks) may be used to minimize possible leakage current to the ground. In mixed unearthed networks (AC/DC-IT) comprising alternating and direct current circuits, AC part of system is connected with DC part through rectifyer valves. Commutation of the valves causes cyclic variation of configuration of the entire galvanically connected network. A distinct feature of AC/DC-IT systems is that voltages between all points of AC side and ground may have mean value different from zero. This characteristic parameter can be used as a signal to determine fault and position to ground or decrease in insulation resistance. However, it also affects the results of the network equivalent insulation resistance measurements when a DC source is used to check insulation. The paper presents a solution to eliminate this effect and proposes a device model to measure and monitor insulation resistance. The results of this study can be applied in the design and manufacture of automatic resistance measuring and monitoring devices for AC/DC mixed unearthed networks.