Practical issues of partial discharge testing of low-voltage cables

Demand for low voltage cable diagnostics, arising from lifetime extension programs of power plants, grows recently. This paper reports on investigations of the authors with partial discharge testing of low voltage control, measurement and signal cables. The construction of low voltage cables differs in many ways from the middle voltage and high voltage cables, on which partial discharge diagnostics has been widely investigated and highly developed. The lack of semiconducting layers makes them less lossy on higher frequencies than MV and HV ones. These results in a different behavior of the propagation of PD induced impulses. Besides, without smoothing layers, these cables are not necessarily PD free at their test voltage. Due to the issues mentioned above, the bandwidth and impulse resolution of the measuring equipment were addressed as well as the discrimination between discharge impulses formed in the insulation bulk and on the conductor surfaces. Measurements were carried out on single and multiple conductor cables by equipments with different bandwidth. The results of the investigation are presented and discussed in this paper.