The method for measuring insulation resistance by superimposing the dc voltage has potential for detecting the deterioration of cable insulations. The deterioration of cable insulations can be detected by measuring current which flows through a grounded shielding wire. However, the earth current also flows from the cable sheath into the grounding wire as noise. Thus, to assess the deterioration properly, it is necessary to separate the earth current from the total current. On the other hand, to improve detection sensitivity, it is desirable to superimpose the highest possible dc voltage. In this case, provision must be made so that there are no adverse effects on the GPT.(2) Even if 5 0 4 dc is superimposed through the GPT, it is possible to prevent the magnetic saturation of GPT if the current injected by such superimposition is limited l(i.e., below max. 5 mA).(3) In lieu of a variable resistor, a variable dc voltage supply is employed in the bridge. The variable dc voltage supply requires no mechanically driven part, thus making it possible to realize a fully automatic monitoring system.
Recent preventive maintenance for equipment has grown to include diagnosis of the insulation of cables under hot‐line conduction. Much of the equipment constructed during the high‐growth period in the 1960s has become obsolete. Recent circumstances make it difficult to obtain a longer service interruption time.
Results of a study of the method that measures the live‐wire insulation resistance by superimposing dc 50 V to a high‐voltage busbar through GPT and forms a bridge with the resistance of a cable insulation are discussed. It is clarified that this live‐wire insulation diagnostic method is highly effective at detecting degradation of a cable insulation resistance and is, therefore, suitable for so‐called trend monitoring as well.
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