Effect of thermal stress on the space charge distribution of 160 kV HVDC cable insulation material

Abstract

The injection and accumulation of space charge in the bulk of XLPE insulation will be intensified due to the ageing of HVDC cable insulation which will affect the normal operation of the HVDC cable. In this paper, the effects of thermal stress of different endurance time and temperature on the space charge distribution of HVDC cable were studied. The samples used in the tests were all slices of 160 kV DC cable insulation and the thermal stress temperatures were 90 and 135 °C, which represented the low temperature thermal stress and high temperature stress, respectively. The Pulsed Electro-Acoustic (PEA) method was used to measure the space charge accumulation and decay characteristics of the samples. And then, the electron trap properties of the test samples were analyzed based on the PEA space charge decay model. Meanwhile, in order to further understand the influence of thermal stress on the physicochemical property of the samples, Fourier Transform Infrared Spectrum (FTIR) and Differential Scanning Calorimetry (DSC) tests were conducted in this paper. The results showed that the 90 °C thermal treating could help to improve the crystalline morphologies of the XLPE at the early stage which led to the decrease of electro traps and space charge density. However, for a much longer thermal stress, the total amount of space charge would trend to increase with the gradually degradation of the XLPE caused by long term thermal stress. Besides, the high temperature thermal stress of 135 °C caused the rapid degradation of the XLPE molecular structure, reducing the crystallinity, creating more deep traps in the insulation and resulting in the accumulation of space charge. With more deep electron traps, the de-trapping process became much more difficult and slower.