Research On Characteristics Of Polarity Reversal Field And Insulation Dielectric In Converter Transformer

In the primary HVDC transmission system,the converter transformer is one of the most important components,and it's reliable operation plays a vital role in the stable transmission of the HVDC transmission system.In the insulation system of the converter transformer,the main cause of the conduction insulation failure is partial discharge,so the safety of the oil-paper insulation system can guarantee the stable operation of the converter transformer.The conditions of oil-paper insulation materials determine the condition and life expectancy of many professional power equipment(including converter transformers).Based on the above analysis,it has become the key for the design of oil-paper insulation system to study the influence factors of the insulation dielectric and the distribution of the main insulation electric field of the converter transformer under the polarity reversal voltage.Firstly,in view of the fact that the conductivity of the insulating dielectric in the oil-paper insulation system changes nonlinearly under different frequency and temperature conditions in this thesis,the distribution of the polarity reversal electric field is studied.Firstly,the finite element software Matlab is used to numerically fit the non-linear change law of the conductivity of oil-paper insulation dielectric.Then the RC equivalent circuit is used for calculation and analysis,and ±500kV converter transformer end winding simulation model is established.ElecNet finite element analysis software is used to simulate the valve side winding end electric field.Finally,according to the simulation results,the influence of temperature and frequency on the electric field distribution of the main insulation is analyzed under the action of DC polarity reverse voltage.Secondly,in view of the fact that the dielectric constant of the oil-paper insulation material of the converter transformer is nonlinearly regularized under different initial water content and aging conditions.Firstly,the influence degree of the converter transformer insulation system is analyzed,and then the Matlab software is used to numerically fit the non-linear change law of the dielectric constant of the oil-paper insulation medium,and the frequency characteristics under polar reverse voltage are taken into account.The software simulates the electric field at the end of the valve side winding.Finally,based on the simulation results,the influence of the initial moisture content and the aging degree on the distribution of the main insulation electric field under the action of DC polarity reverse voltage is analyzed.Finally,the weak link of the insulation structure is found in the simulation results.In view of the influence of the structure size of the electric ring,the formed angle ring and the insulation structure size of the valve side winding end on the distribution of the main insulation electric field,a reasonable insulation structure is designed.An optimized scheme is proposed to ensure that there is sufficient Insulation margin.Finally,and the main insulation structure size of 500 kV converter transformer is determined.Through the research and analysis of this thesis,it will provide a certain theoretical guiding significance for the future research on the insulation system of converter transformers.