Research On Characteristics Of Complex Electrical Field And Analysis Of Insulation In Converter Transformer

Converter transformer is one of the most important equipment in high voltage direct current transmission system, which connects alternating current to direct current lines and transmission electric power from alternating current system to direct current lines, then from direct current lines to alternating current system. Different from other ordinary power transformers, Converter transformer is in complex environment. While operating, the valve side winding of converter transformer withstands not only AC high voltage and lighting impulse voltage, but also DC high voltage and AC superimposed DC voltage. Moreover, when switching off polarity reversal voltage, the valve side winding of converter transformer bears polarity reversal voltage. Its operation reliability is very important to the high voltage direct current transmission system. The insulation design is based on analyzing the internal electrical field distribution of converter transformer for withstanding the various voltages.In this thesis, the numerical calculation model of steady-state and transient electrical field are established, according to the product of 500kV converter transformer. The internal electrical field distribution characteristics of converter transformer under the action of AC high voltage, state DC voltage, AC superimposed DC voltage, polarity reversal voltage are calculated. The regularities of electrical field distribution are found out and the characteristics of insulation are analyzed. The influence factors of the electric field distribution under the action of steady-state DC voltage, AC high voltage and AC superimposed DC voltage are discussed. As for two influence factors of the electric field distribution—resistivity and permittivity, the internal electric field distributions are calculated and analyzed when changing three kinds of insulating material resistivity or permittivity ratio, and then the maximum electric field strength and the location of the maximum electric field strength are pointed out. According to the calculation results, some results are summarized and the placements of insulation failure are pointed out.It is another distinguishing feature that the converter transformer withstands the polarity reversal voltage. The internal electrical field distribution of converter transformer under the action of polarity reversal voltage is calculated and analyzed. During the polarity reversal, electric field distribution with time increasing and the maximum electric field strength in different insulating medium is calculated and analyzed. When changing three kinds of insulating material resistivity, the internal electric field distribution and the process of distribution of equal electric potential are calculated and analyzed. The reasons of the variation of the electric field distribution are analyzed and the method to improve the distribution of the electric field is found out. The conclusions and the methods discussed provide reference and theoretical basis for insulation design of converter transformer in a certain extent.