Determination of substation model for correct interpretation of on-line measured PD signals from MV cable systems

PD measurement is a proven diagnostic for MV cable system insulation. The PD charge is a relevant parameter for interpretation of test results. In case of off-line PD diagnostics, the measuring system is relatively easy to calibrate. For on-line measurements, the magnitude of signals from the cable under test is also dependent on the impedances in the connected substation. In order to obtain correct PD magnitudes for on-line measured signals, this impedance must be known. In this paper, a substation model is presented, as well a measuring method to obtain model parameter values. Experiments show good agreement with the model. The measuring method provides a solution to determine the substation impedance(s) on-line, so without any switching, disconnecting or making galvanic contact. This can be an important benefit to on-line PD measuring systems.