Electric Field Analysis Of Shielding System For VSC-HVDC Converter System Under AC And DC Voltage

Commutation technology is the core technology of HVDC,AC and DC conversion is achieved by the valve in the VSC-HVDC system.The insulator and the shield are important parts of the converter valve tower,Analyzing electric field of valve tower shield surface and insulator is important for the design of VSC-HVDC converter valve tower.For the electric field inside the insulator and the influence between the shield and the insulator under the condition of AC voltage,the boundary element and integral equation field domain element coupling method was proposed in this thesis.The method use the potential of the conductor surface,to obtain the electric field of conductor surface and the electric field inside the insulator.Using the direct coupling method to calculate the electric field and potential distribution of three-dimensional axisymmetric model,and the correctness of the method is verified by comparing with the two-dimensional finite element method calculation results of the axisymmetric model.According to the analogy of electric field distribution of dielectric and conductive medium,the direct coupling method is extended from the case in the dielectric to the conductive medium and the direct coupling method can be used in the case of DC voltage.The influence between the shield and the insulator was analyzed in this thesis.For one valve tower,insulator models with different layers of valve towers are established,the electric field inside insulators is calculated by the direct coupling method,the calculation results show that when the valve tower is modeled,the shield near the insulator will have great influence on the distribution of electric field inside the insulator,while the shield far away from the insulator will have little effect on the electric field inside the insulator.The valve tower shielding system models with or without insulators are established respectively,the surface electric field of the models is calculated by the direct coupling method,the analysis shows that ignoring the fittings and insulators will make the result smaller in the simulation calculation.The direct coupling method can consider the influence of fittings and insulators and improve the accuracy of calculation.According to the the voltage application mode of the system and connection mode of multiple valve towers in the actual project,the valve tower models of the typical position are established respectively,using the direct coupling method to calculate the surface electric field of the shielding system and the internal electric field of insulator.Compared with the AC and DC voltages alone,it can be concluded that the effect of AC and DC mixed voltages on the electric field of shielding surface is small,but the distribution of electric field inside insulators is greatly affected.