Study On Electric Field Characteristics At Ends Of Valve Side In Converter Transformer

In this paper, the purpose and significance of HVDC transmission was briefly discussed. The converter transformer is one of the key equipments, and the insulation and structure is the key technology and the main distinction between converter transformers and power transformers. Insulation fault is the highest while the products are tested and operated, and the fault of a converter transformer occurs in polarity reversal in most cases. HVDC insulation failure occurred mainly in the valve side winding, and its operation reliability and maintenance is very important to the normal operation of a converter transformer. So it is necessary for us to analyze the insulation problems of a converter transformer in the two aspects.On the basis of polarity reversal voltage theory analysis, in this paper, the distribution characteristics of end electrical field under different excitation and oil-paper compound insulation electric field of a converter transformer polarity reversal are analyzed. On the basis of which the electric field distribution characteristics of parallel-plate electric field and axisymmetrical electric field is discussed under polarity reversal voltage, a finite element method to calculate transient electric field is presented under polarity reversal voltage. Polarity reversal is a special phenomenon occurring during operation of a converter transformer. The windings are stressed by combined action of AC and DC electric fields, with the latter being a nonlinear, anisotropic one.This paper puts emphasis on the research of the electric field calculation of HVDC valve side winding in polarity reversal voltage. The transient process of polarity reversal is analyzed first and then a numerical method of computing its electric fields is presented. Considering the anisotropic property of oil-immersed pressboard, a new finite element model based on ANSYS method is presented and verified. The influence of medium resistivity and polarity reversal upon the internal insulation design is analyzed. The paper studies the relation of conductivity and field strength distribution and the relation of temperature and field strength distribution, and studies the influence of internal insulation structure and materials upon DC electric field distribution of external insulation. The method and model developed are applied to calculation of the electric field distribution in four typical winding end structures of the converter transformer. The results are analyzed and some conclusions of practical significance are obtained.