Analysis Of Insulation Characteristics And Simulation Of Electro-thermal Coupling Field For HVDC Cable Accessories

With the rapid development of HVDC transmission technology,the transmission voltage level and transmission power are gradually improved,and the insulation problem between cables and accessories in HVDC transmission engineering has become the focus of attention.In high-voltage power transmission engineering,cable accessories play an important role in transition and connection,etc,and belong to the fault prone position in the power transmission system.The research on high-voltage cable accessories involves multi-disciplines such as electrical,material,thermal and mechanics,etc,and each part influences and coupling each other,which is relatively complex.Therefore,it is of great significance to analyze the insulation characteristics of HVDC cable accessories and simulate the electric-thermal coupling field for ensuring the safe operation of HVDC transmission lines.In the research,the method of combining experimental rule exploration with finite element simulation is adopted.The physical and chemical properties,thermal properties,electrical insulation properties and mechanical properties of SIR and EPDM,the insulating materials used in HVDC cable accessories,were tested and analyzed.Based on the experimental test,the physical field simulation model of HVDC cable accessories is established,and the distribution of electric field inside HVDC cable and its accessories is calculated by finite element method.Then,based on Maxwell-Wagner polarization model,the characteristics and mechanism of charge accumulation at the interface of double-layer insulating medium were investigated.Firstly,the insulation material characteristics of HVDC cable accessories are studied.The physical and chemical properties of SIR and EPDM were characterized by SEM,FTIR and DSC.The results showed that the physical and chemical properties of SIR and EPDM were significantly different.The electrical properties of SIR and EPDM were characterized by dielectric,resistivity,thermal stimulation depolarization current,DC breakdown field strength and space charge.The test results show that the dielectric constant of the two materials decreases with the increase of temperature.At the same temperature,the dielectric constant of SIR is greater than that of EPDM.The conductivity of the two materials changed differently with temperature and electric field.When the temperature changed from 25℃ to 90℃,the conductivity of SIR increased by1 order of magnitude,and that of EPDM increased by about 3 orders of magnitude.The DC breakdown field intensities of SIR and EPDM are similar and both increase first and then decrease with the increase of temperature.Secondly,the electrical-thermal field simulation of HVDC cable accessories is carried out.Based on the insulation material characteristics of HVDC cable accessories,the electric field simulation model and thermal field simulation model of HVDC cable accessories are established to calculate the distribution of electric field and temperature field of cable accessories under different voltage amplitudes and different temperatures.The simulation results show that the electric field strength of the HVDC cable accessories increases with the increase of the applied voltage amplitude.The electric field concentration area and insulation weak links are near the main insulation,stress cone and HVDC shield tube of the cable.The electric field at the root of the stress cone is about 6.29 k V/mm,and the electric field at the cable insulation is about 14.36 k V/mm.The internal temperature field of HVDC cable accessories presents gradient distribution.At room temperature,the highest temperature of HVDC cable accessories is about 62℃,and the temperature difference of main insulation is about 8.5℃.Finally,the interface charge characteristics of HVDC cable accessories under the electro-thermal coupling were studied.Space charge is considered to be the key factor limiting the development of HVDC cable accessories.The mismatching of dielectric constant and conductivity between composite insulating medium will lead to the accumulation of interfacial charge.Based on the electric-thermal field simulation,the electric-thermal coupling field simulation model is established to calculate the influence of interface charge on the electric field distribution of HVDC cable accessories.The analysis of interface charge characteristics shows that the interface charge of cable accessories has a polarity transition temperature,which is about 36℃.When the temperature is lower than this,negative charge accumulates at the interface,and vice versa.The accumulation of negative charge will increase the electric field of cable insulation and weaken the electric field at the root of the stress cone,while the accumulation of positive charge will increase the local electric field distortion of cable insulation and stress cone.The simulation results show that the electric field of cable insulation increases by 36% and the electric field distortion of stress cone decreases by62% at room temperature due to negative charge.At 50℃,the positive charge causes the electric field of cable insulation to increase by about 8%,the electric field at the root of the stress cone is distorted by more than 50%,and the electric field inside the cable insulation is less than that outside,resulting in polarity reversal.