Development of the DC superimposition bridge method for the measurement of high insulation resistance of medium voltage XLPE cables in operations

Abstract

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.