Investigation of dielectric properties of mixed PILC and XLPE cable insulation by the extended Voltage Response Method

Remarkable percentage of the currently operating medium voltage cable lines are still paper insulated lead covered (PILC) cables reaching or exceeding their lifetime expectancies. The old cables are replaced completely or partly to cross-linked polyethylene (XLPE) cables producing mixed PILC and XLPE insulated cable lines. By means of the Voltage Response Method the decay and return voltages are measured on a charged and shorted dielectric. From the initial slopes of these voltages the specific and the polarization conductivity can be determined, two properties that characterize the material. Therefore by following the change of these values the condition of the insulation can be estimated. Previous studies have revealed that in case of EVR measurements the XLPE sections can be replaced by their capacitance. Measurements were carried out on PILC and XLPE insulated cables separately and connecting them. The XLPE insulated cables had three different lengths enabling to examine mixed cables with different length ratio. By means of EVR measurement method the differences of polarization processes of different insulating materials can be investigated more precisely.