| High-voltage cables and accessories are indispensable electrical equipments in the power grid,and their safe and stable operation plays an important role in country’s energy transmission and national economic development.However,during actual operation,some defects in the dielectric tend to initiate electrical trees,thereby reducing the insulation level of electrical equipment and causing accidents.Regarding the cause of trees inception,the current theory of charge injection-extraction and trapping are the main ones,both of which emphasize the role of space charge.At present,there is no connection between space charge and electrical tree based on experimental measurement.Therefore,this article uses the AC voltage and overvoltage in DC systems as the background to measure the space charge distribution during the tree initiation process in cross-linked polyethylene(XLPE)and its composite dielectric containing voltage stabilizers to explore the initiation and suppression mechanism of trees,aiming to provide support for the design of high-voltage cables and accessories.The main work and conclusions of this paper are as follows:(1)The minimum value of the initial voltage of the XLPE electrical tree under AC is obtained,and the phase interval of the space charge measurement experiment is determined through the partial discharge experiment.The electric field simulation is employed to calculate the maximum field intensity at the tip of the needle electrode.Based on this value,the voltage applied in the AC space charge measurement experiment is obtained.After analyzing the experimental data,the mechanism of XLPE tree initiation under AC stress is proposed.The results show that space charge injectionextraction occurs under AC voltage,and XLPE molecules may be ionized to produce heteropolar charges.Simultaneously,it is pointed out that the generation of high-energy electrons is a key factor in initiating trees.The dielectric molecular chain is gradually cracked through the electrons impact and Auger effect,which creates conditions for the initiation of electric treeing.(2)Similar to the AC situation,the maximum electric field strength of the needle tip is used to establish a connection between the tree and the space charge under the pulse voltage.Through the experiment of tree initiation and space charge measurement under unipolar pulse voltage,it is found that the pulse polarity and amplitude have an effect on tree inception.Compared with the negative pulse voltage,the positive pulse voltage is more likely to initiate trees,which is related to the discrepancy in the mobility of positive and negative charges.Comparing the space charge distribution results under different pulse field strengths,the mechanism of XLPE electrical tree initiation under pulse voltage is proposed.It is believed that the cumulative damage effect of the charges under pulse voltage is the main factor initiating electrical trees,and the theory is proved based on experimental results.(3)The electrical tree initiation characteristics of the composite dielectric under AC and pulse voltage are researched,and the mechanism of the voltage stabilizer to inhibit tree initiation is proposed based on the space charge measurement results.The experimental results show that the voltage stabilizer can trap the charges,hence,hindering the process of charge injection-extraction and transport under AC and pulse voltage respectively.By measuring the trap energy level and density of the composite dielectric,it is found that a certain concentration of voltage stabilizer will introduce deep traps in the XLPE,which makes it difficult for the charges to escape from the traps after being trapped.Therefore,the ability of XLPE to withstand electrical trees is improved. |
