Influence of different test conditions on volume resistivity of polymeric insulated cables and polyethylene samples

Resistivity is one of the main quality and usefulness parameters of dielectric materials. For cable insulation the most significant is volume resistivity rather than surface resistivity. Measurement of resistivity is relatively difficult because of the influence of many different factors on its value. Exterior factors are temperature, humidity, surface pollution, time and type of applied voltage; interior factors are structure, impurities and additives. The measurement of resistivity is performed under DC voltage in most cases. Typical values of measured volume resistivity are 10¹⁵-10¹⁸ Ω-cm. After voltage is applied polarisation and current flow take place. Macroscopic polarisation influences a very long time of conduction current stabilization (up to a few hours and longer). The most important cable type in service is still the paper-insulated cable-twice the amount of medium voltage XLPE AC cables. But today´s newly installed cables in the voltage range up to 500 kV, have mostly XLPE (crosslinked polyethylene) insulation. Older designs of extruded cables had the PE (thermoplastic polyethylene) insulation. XLPE insulation is applied often in submarine AC cables too. The development of HVDC cables using polymeric insulation is delayed by the problem of space charge formation in a polymer which influences the dielectric response in the bulk and the effects at the electrodes