Space charge accumulation in polymeric high voltage DC cable systems

Abstract

One of the intrinsic properties of the polymeric high voltage (HV) direct current (DC) cable insulation is the accumulation of electrostatic charges. Accumulated charges distort the initial Laplacian distribution of the electric field, leading to a local field enhancement that may cause insulation degradation and premature breakdown of the system. The general objective of the present study is to obtain a better understanding of the major factors that control the space charge processes in polymeric HVDC cable systems. In order to achieve this goal, two main factors of influence have been investigated. 1) Cable accessories, i.e. cable joints and cable terminations, are considered to be the weakest part of a cable system, because of the presence of a dielectric interface between the cable insulation and that of the accessory. This thesis aims at a better knowledge of the polarization phenomena occurring at dielectric interfaces. To that purpose, an accurate methodology has been developed for the experimental study of the space charge behavior at the dielectric interface. The pulsed electro-acoustic (PEA) method for the measurement of space charge has been reviewed in case of dielectric interfaces. Secondly, space charge accumulation has been experimentally observed at the dielectric interfaces of different test specimens. 2) Across the insulation of HVDC cable systems a temperature drop is present when the cable carries a current. This thesis aims to provide a better understanding about the mechanisms responsible for space charge accumulation when a temperature drop is present across the insulation of the cable system. To that purpose, a physical model has been developed for the prediction of space charge dynamics and electric field in loaded HVDC cable systems. The physical model has been validated by means of laboratory investigation.