Study On Space Charge Characteristics In Insulating Material For High Voltage DC Cables

The deviation of energy center and load center in China,the problem of power transmission is one of the major problems restricting the development of electric power in China.With the development of flexible DC transmission technology and the reduction of cost,the best way to make full use of renewable energy is to interconnect the clean energy bases of wind power and solar energy lamps on land with the main grid.Flexible DC transmission with high-voltage DC plastic cable is the main direction advocated by international power grid.Under the long-term effect of DC electric field,space charge will accumulate in the insulation material,which will cause electric field distortion,accelerate insulation aging and finally lead to insulation breakdown.Therefore,it is of great significance to study the space charge characteristics of DC cable insulation materials for the development of cable insulation materials and safe operation of power grid.In this paper,the space charge characteristics of XLPE insulation materials for DC cables are systematically studied by using the pulse electroacoustic space charge measurement system.The whole process of space charge from injection,accumulation,migration to dissipation in the insulation materials is revealed,in order to provide data and theoretical support for the development of DC cable insulation materials in China.In this paper,the basic electrical properties and space charge characteristics of two kinds of domestic and imported XLPE insulation materials for 500 k V cables are studied.The breakdown field strength of domestic 2# XLPE sample is the lowest,and that of Nordic chemical industry and domestic 1# XLPE sample is close,which is much higher than that of domestic 2# XLPE sample,mainly because the purity of domestic 2# XLPE sample is not enough,which contains impurities.The results show that the dielectric strength and the withstand voltage can be improved by adding appropriate amount of nano particles into the insulating materials.-Under 50 k V/mm polarization electric field,the space charge injection and accumulation of 1# XLPE insulating material made in China is the smallest,and the difference between 2# XLPE made in China and Nordic chemical industry is not significant.-The space charge accumulation of the three materials is more serious under the polarization electric field of 100 k V/mm.The space charge characteristics of the insulating materials made in China are similar to those of the insulating materials made in Nordic chemical industry.The space charge near the anode and the cathode are of the same polarity,while the space charge near the anode and the cathode of the sample made in China are of the different polarity.It is found that nanoparticles can effectively suppress space charge and electric field distortion at low field strength,but not at high field strength.In this paper,the space charge characteristics of 110 k V DC cable insulation chips with different service time are studied,and the experimental results are analyzed.With the increase of service time,the amount of space charge accumulation in the insulating chip is reduced from less to more,and the charge polarity is changed from homopolar to heteropolar.It is found that the deeper the trap depth is,the more the space charge is trapped,the larger the space charge accumulation is,the slower the charge dissipation and the smaller the carrier mobility are.The more the ratio of shallow traps,the greater the probability of charge dissipation and the greater the mobility of carriers.