Study On Growth Characteristics Of Electrical Tree In DC Cable Insulation Under DC And Polarity Reversal Electric Field

With the development of power electronics technology in recent years,HVDC technology has become increasingly widespread.Polymer insulated HVDC cable is an important part of HVDC transmission system.And its insulation performance has been paid much attention to with the rapid development of HVDC transmission project.Electrical tree is an electrical aging phenomenon which occurs in polymer insulation material and endangers its insulation properties during long-term operation.At present,the researches on electrical tree in polymer insulation materials mostly focus on AC electric field,while the researches on the growth characteristics of electrical tree under DC electric field mainly focus on periodic DC grounding electric field,DC superimposed AC electric field and so on.In order to investigate the growth characteristics of induced electrical tree under constant DC electric field and the growth characteristics of electrical tree under polarity reversal electric field which caused by power flow reversal in HVDC transmission system,experiments of electrical tree have been designed and carried out with XLPE under DC electric field and polarity reversal electric field.Polarity reversal tree experiments were carried out at 30 ?,50 ?,70 ? and 90 ?.The length of polarity reversal period includes 150 s,300 s,600 s and 900 s.And the applied time proportion of positive and negative voltage includes the high proportion of negative voltage applied time in each cycle,the same proportion of positive and negative voltage applied time in each cycle and the high proportion of positive voltage applied time in each cycle.The experimental results show that temperature has a great influence on the length,width and average growth speed of electrical trees under polarity reversal electric field.And the proportion of positive and negative voltage applied time also has effects on the length,width and average growth speed of electrical trees under polarity reversal electric field.In a single polarity reversal period,when the proportion of positive voltage applied time is large,the length and width of polarity reversal electrical trees tend to be small.And electrical trees under polarity reversal electric field present the shape between the branch electrical trees and the jungle electrical trees.DC electrical tree experiments were carried out at 30 ?,50 ?,70 ? and 90 ? under DC electric field with positive and negative polarity and different DC voltages.And the DC voltages applied in the experiments were 25 kV,30 kV and 35 kV.The experimental results show that the average growth speed of electrical trees under positive polarity voltage have a tendency to increase with the rise of experimental temperature,while electrical trees under negative polarity voltage have obvious increases only when the experimental temperature reaches 90 ?.Under positive DC voltage,the average growth speed of electrical tree tends to increase with the rise of DC voltage.And the polarity of DC voltage has a great influence on the growth of electrical trees.Under the same conditions,the average growth speed of electrical trees under positive DC electric field is higher than that under negative DC electric field.It has a great significance to investigate the growth characteristics of induced electrical trees under constant DC electric field and the electrical trees under polarity reversal electric field for evaluating insulation life of DC cables,preventing electrical trees,prolonging insulation life of cables and maintaining safe and stable operation of DC transmission system.