Electrical characterization of polymeric DC mini-cables by means of space charge and conduction current measurements

Abstract

The world’s first commercial High Voltage Direct Current (HVDC) transmission link was built in 1954 between the Swedish mainland and the island of Gotland. At that time, it was proved that HVDC transmission is technically feasible. Since then, HVDC cable systems have been used worldwide in electrical energy transportation. Most HVDC installations in use around the world today, use paper-insulated, oil-filled type cables. Extruded dielectric cables with cross-linked polyethylene (XLPE) has long been the preferred solution in HVAC applications due to a combination of low material and processing costs, reliability and appropriate mechanical and electrical properties. However, polymeric HVDC cables suffer greatly from space charge accumulation during dc voltage application and from ‘low’ depletion rate of accumulated space charge when the external field is removed. As a result, considerable modifications of the electric field distribution with respect to the geometric Laplacian field occur, especially in case of voltage polarity inversion. This may cause insulation degradation and premature breakdown. Manufacturers are trying to tackle the problems related to space charge phenomena by introducing additives to the insulation or semicon layers. The development of new polymeric materials with improved performance under dc electrical stress requires a thorough investigation of the properties governing charge injection, transport and trapping. Particularly mobility and trap depth distribution are very useful to describe and compare the behavior of different materials from the view point of charge dynamics and field modification. In this thesis, different polymeric mini-cables are examined under DC stresses with regard to their space charge dynamics. Two different types of XLPE insulation and four types of semi-conductive layers compose eight different combinations of mini-cables. The specimens are subjected to space charge measurements and conduction current measurements in order for their electric field thresholds to be determined. The threshold for space charge trapping is an important parameter for the design of insulation systems subjected to dc electrical fields. If the applied electric field exceeds the threshold, charge injected from the electrodes can accumulate in traps located at the interface with electrodes and in the insulation bulk. Furthermore, depolarization characteristics obtained at high electric fields and temperatures, are used in order to further investigate the performance of the mini-cables with respect to their apparent trap-controlled mobilities and trap depths, including the space charge distribution along the trap levels. The main goal of this thesis is to evaluate how the composition of insulation and semi-conductive layers affects the space charge dynamics in polymeric mini-cables.