Broadband measurement of the conductivity and the permittivity of semiconducting materials in high voltage XLPE cables

Because of several technical and economical advantages XLPE (cross-linked polyethylene)-insulated power cable systems are increasingly used in the high voltage (HV) and extremely high voltage (EHV) range. High voltage XLPE-insulated cables consist of a copper or aluminium conductor (inner conductor), a semiconducting layer extruded over the inner conductor, an XLPE-insulation, an outer semiconducting layer, a metallic wire screen or aluminum sheath and an outer polyethylene sheath. The extruded semiconducting layers provide a well bonded and smooth interface between the conductors and the dielectric, avoiding electric field strength enhancements, which would lead to partial discharge and a premature breakdown. However, the XLPE-insulation is very sensitive to partial discharges (PD). To ensure the reliability of the whole cable system, sensitive laboratory PD tests before installation and on-site verification are required. One common method to detect PD is the measurement of the radiated fields of the PD with sensitive field sensors. These sensors couple PD signals through the outer semiconducting layer and therefore the properties of the semiconducting layer have a significant influence on the coupling mechanisms. Knowing the conductivity and permittivity and their dependence on frequency and temperature is important to optimise PD field sensors. Those sensors are usually placed above the outer semicon layer close to or within cable joints. This paper presents a measurement set-up to determine the frequency dependent conductivity and permittivity of small samples taken from the outer semicon layer of different medium and high voltage cables. The measurement set-up can handle extremely high, frequency dependent dielectric constants, which are typical for semiconducting materials (carbon black filled polymers). Besides, the temperature of the sample can be adjusted from 10/spl deg/C to 85/spl deg/C to study the temperature dependence of the properties of the semicon sample.