Research Of Coupled Analysis Method For Electric-Thermal Field Of Power Cable Accessories Based On Equivalent Capacitance Method

With widely applications of power cables in the power system, the reliability of these devices is attracting more and more power providers and consumers'attention. Power cable accessories are used for connecting cables as well as connection between other electrical equipment and their operating status are directly related to the security of power grid, whereas the failure rate of the accessories in practice has always been high. The statistical analysis for failures of cable accessories showed that such failure caused by insulation aging accounted for more than 70%. Electric field and thermal field can affect the properties of insulation material and cause the aging of insulation. Aiming at improving design of insulation for cable accessory products and prolonging their service life, it is of great importance to study the coupling characteristics of electric field and thermal field inside cable accessories as well as analyze influences of the characteristics on the insulation.Based on the summary of failure types and causes of power cables and accessories, the influence mechanisms of electric field and thermal field on XLPE insulation was elaborated in this paper. According to the existing finite element theory for calculating electric field and thermal field of cable accessories, the formula which is on the basis of equivalent capacitance method was deduced from the dielectric loss basic formula. Applying this formula, the coupling relationship between the electric field and thermal field can be analyzed and the computing methods for inner heat resources of cable accessory were also obtained.Through the analysis of heat sources in the operating cable accessory, the coupling analytic model for electric-thermal field of high voltage cable accessories has been established using the ANSYS software. The simulation of the coupling characteristics for studying electric-thermal field of HOkV cable joint and terminal has been accomplished, and the different distribution characteristics of the fields have been obtained. In addition, the impact of various factors, on thermal field of cable accessory have been concerned, including the ambient temperature on operating temperature of cable joint, the working current on thermal field of cable joint and cable terminal as well as the dielectric loss angle tangent on thermal field of cable terminal. The practical fault about abnormal heating of 110kV cable terminal was simulated by field analysis method. The study showed that in some occasion aged insulation, ambient temperature and working current could raise the accessory temperature, which could lead the distribution characteristic of the inner electric field of the operating accessory deviate from that of design. Moreover, under some worse conditions, it can result in thermal or electric breakdown of products.