Research On Insulation Characteristics Of Converter Transformer Valve Side Winding Under Compound Electric Field

With the rapid development of high voltage direct current(HVDC)transmission,the converter transformer is an important equipment to connect the AC and DC lines.There is a great difference between the converter transformer and the power transformer.The voltage type of the converter is DC voltage,AC voltage,polarity reversal voltage,lightning impulse voltage and operating impulse voltage.Therefore,it is important to study the electric field distribution characteristics and the influence factors of the valve side winding of converter transformer under compound electric field.In this thesis,according to the insulation structure of the actual ±500kV transformer,the structure model of converter transformer is established,the calculation model of the electric field is determined,and the corresponding boundary conditions are given.Firstly,the electric field of converter transformer under DC voltage is calculated by using finite element analysis software,the electric field distribution,the maximum electric field intensity value and location are obtained,and then the factors which affect the electric field distribution are analyzed.Secondly,the distribution of the electric field under the AC voltage is calculated and the results are compared with that of the DC voltage.By the method of AC and DC linear superposition,the distribution of electric field is calculated and the distribution characteristics of electric field and influencing factors are gained under the superposition of AC voltage and DC voltage.The polarity reversal voltage of converter transformer is different from that of power transformer.Because of polarity reversal voltage,the dielectric is nonlinear.In this thesis,the influence of temperature on the dielectric nonlinearity is studied.By using finite element analysis software,the 2D model of transformer leakage magnetic field is established,the loss is calculated,and then the calculated loss is applied to the core and winding of the converter transformer by means of heat source loading.The 2D temperature distribution of converter transformer is obtained by simulation.This provides a basis for the electric field distribution of converter transformer under polarity reversal voltage when considering the influence of temperature gradient.The electric field of converter transformer under the polarity reversal voltage is calculated.The variation law of internal electric field and the influence factors of electric field distribution and the maximum electric field intensity are obtained.Due to the nonlinear effect of temperature on resistivity,the distribution of polarity reversal electric field and the variation trend of electric field intensity in different dielectric are compared and analyzed when considering the influence of temperature and without temperature.These conclusions and research methods of this thesis provide the basis and reference for the insulation analysis and structure design of converter transformer.