Study On Initiation And Growth Characteristics Of Electrical Tree And Ageing Under Different Gas Pressure In XLPE Cable

Cross-linked polyethylene (XLPE) power cables are widely used in power systems because of its excellent electrical and mechanical properties. Electrical tree will occur in the insulation during long-term operation of the cable due to manufacturing processes and other factors, which results in local electric field distortion. What’s worse, water tree may lead to sudden breakdown of the cable and the security and stability of power system operation will be threated. In this paper, the influence low of initiation and growth of electrical tree under partial gas pressure are studied. Developed an experimental device which research the electrical tree under partial pressure cable electrical treeing test system to study the influence to the initiation and growth characteristics in glass state and high-elastic state of XLPE. The influence mechanism is analyzed and the ageing of insulation is investigated by moment features of partial discharge and dielectric spectroscopy. These methods are used to evaluate the growth state of electrical tree and has significance for security and stability of cable operation.Two models are used to study the partial discharge characteristics and the growth law when the gas pressure on the inner wall of the cable insulation or on the channel of the electrical tree. One of the models adopts short cable with a needle defect made in insulation and the other model was squares made of insulation material. The experiment show that the gas pressure on the inner wall has no effect on the initiation of the electrical tree and promote the growth of the tree at late aging period at low temperature. The pressure on the tip has no effect on the initiation of the tree, but cause the inhibition of the growth at low temperature. Gas pressure on the inner wall or on the tip promote the initiation and growth of the tree at high temperature. Analysts believe that the deformation of the large area of the high temperature softening region is the main factor that promote the growth of electrical tree and the inhibition of streamer discharge by the high gas pressure lead to reduced speed of the growth under low temperature. The reduction of mechanical moduli promote the initiation of the growth of the tree under high temperature. The enhancement of the streamer discharge and the tension force in molecular chain by high gas pressure cause the acceleration of the growth.Based on the partial discharge data, Hqmax(φ) spectra and Hn(φ) spectra have constructed and the moment eigenvalues of Hqmax(φ) spectra have calculated. With the skewness and kurtosis of Hqmax(φ) spectra and the discharge phase distribution of Hn(φ) spectra, the development stage of electrical tree is evaluated. The dielectric spectroscopy of XLPE squares are tested and the parameters of Debye relaxation model are fitted. At last the influence of gas pressure on the parameters is analyzed.