Influence Of HVDC Cable Insulation Ageing On The Space Charge Distribution Characteristics And The Study Of The DC Electrical Tree Properties

With the development of flexible DC transmission technology,the application of HVDC XLPE cable is more and more widely,and several HVDC cables have been in operation in China.The ageing of the DC cable insulation marterial will affect the space charge distribution characteristics in the bulk of insulation,affecting the safe operation of DC cables.Besides,with the increase of the operating voltage level of the DC cable and the outstanding of the harmonics problems,electrical trees under DC and AC_DC composite voltages become an nonnegligible problem.In this paper,the ageing characteristics of 160 k V DC cable insulation material under thermal stress,electrical stress and electro-thermal stress and their inflences on the space charge properties were studied.Besides,the init iation and growth properties of the DC electrical tree and the electrical trees under DC_AC composite voltages were also studied.The main research results are as followed.In order to investigate the influence of ageing on the space charge distribution of the HVDC cable insulation material,thermal stressed,electrical stress ed and electro-thermal stressed endurance tests were carried out on the XLPE peelings of 160 kV HVDC cable insulation.The average space charge density and electron trap properties were studied.The 90 ℃ 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.The high temperature thermal stress of 135 ℃ would cause the degradation of the XLPE molecular structures,resulting in the increase of the electron trap density,electron trap depth and the accumulation of space charge.The effects of the DC electriacl ageing were relatively samll,though the space charge density increased compared to the unaged one.With a combination of electrical and thermal stresses,the injection and migration of space charge in the electro-thermal stressed samples were more significantly.The depth and density of electron traps increased rapidly with the increase of ageing time.Meanwhile,in order to further understand the influence of ageing on the physicochemical properties of the samples,Fourier Transform Infrared Spectrum(FTIR)and Differential Scanning Calorimetry(DSC)tests were carried in this paper.At the early stage of the 90 ℃ thermal ageing,it helped to improve the crystalline morphologies of the XLPE due to the post-crosslinking and the FTIR properties did not change significantly during the period.As for the 135 ℃ thermal ageing,the crystalline morphologie decreased a lot compared to the unaged one and an obvious carbonyl absorption peak was observed due to the intense thermal oxidation.In contrast,the FTIR properties did not change much for the electrical and the electro-thermal stressed samples.The crystalline morphologie decreased a lot for the electro-thermal stressed samples.The effects of ageing on the DC breakdown strength of DC cable material were studied.It was found that the DC breakdown strength increased with the ageing time for the early stage of the 90 ℃ thermal ageing samples and it began to decrease as the ageing time increased further.The DC breakdown strength decreased a lot for the 135 ℃ ageing samples and electro-thermal samples.However,it did not change much for the electrical aged ones.The initiation and growth properties of the grounded DC tree were studied.The pre-stress time had little influence on the inception rate of the grounded DC tree,but the initiation properties had an obvious polarity effect.The electrical tree inception voltage of negative polarity was significantly lower than that of positive polarity and the average tree lengths of negative polarity were larger than those of positive polarity.In the case of repetitive grounded DC tree growth tests,the electrical tree growth rate under negative DC voltage was faster than the positive one at the first several shots.As the number of electrical tree branches under negative DC voltage increased,the growth rate greatly slowed down and it was overpassed by th e positive one.The growth rate of DC electrical tree was quite slow,and the growth mostly occurred at the voltage rising stage or in a period of time just after the rising stage.The growth rate of the positive DC electrical tree was much faster than the negative one.The results indicated that the injection and accumulation characteristics had an obvious effect on the initiation and growth properties of the electrical trees.The initiation and growth characteristics of electrical trees under AC_DC composite voltages(50Hz,1k Hz,2k Hz)were studied.The initial properties of the electrical tree were mainly affected by the AC components,and the AC components greatly reduced the DC tree inception voltages and made the growth rate increased significantly compared to that under pure DC voltage.The growth rate of the electrical tree increased with the increase of the positive DC bias voltage,but the effect of the negative bias voltages on the electrical tree growth rate was not significant.As the frequency of the AC component increased,the growth rates of the electrical trees decreased due to the increase number of the branches.