Study On The Breakdown Characteristic Of Oil-paper Insulation In Converter Transformor Under A Polarity Reversed Voltage

Converter transformer plays an important role in HVDC (high voltage direct current) transmission system, its reliability directly affect to the safe operation of the system. The operating converter transformer not only withstands AC voltage, lightning impulse and operative-over voltage but also DC voltage, superimposed AC-DC voltage and polarity reversal voltage. Under the influence of the voltage, there are considerable differences in the characteristics of inner electric field's distribution, breakdown characteristics of insulation material, space charge's accumulation and distribution, oil-flow electrification between a converter transformer and a normal power transformer. All of these may cause abnormal discharge and breakdown of the insulation system of a converter transformer.Firstly, to analyze the electrical field distribution on Valve Side Winding of converter transformer, the simulation model of oil-paper insulation is established. The electric field's distribution of the insulation under a polarity reversed voltage is simulated with FEM (Finite Element Method). The law of the electrical field distribution is summarized. The calculation results show that the characteristic of electric field distribution at valve side winding of converter transformer are both regular and complex. This is because there is space charge at the interface of dielectric. The distribution is complex.Secondly, taken a composite insulation system of oil and cardboard filled in two parallel tabulate electrodes as research object, the space charge accumulation and the electric field distribution in the above insulation system has been studied under the polarity reversed voltage. The work is carried out by means of establishing and solving a differential equation to describe the field distribution, and the main factors influencing the field distribution characteristics have also been studied. The research result shows that there is a long transition process of electric field redistribution on the interface of oil and cardboard caused by the space charge, which will lead to significant increase of the electric field in oil under the polarity reversed voltage.At last, to study the breakdown characteristic of oil-paper insulation, electrode system are designed,and then a correlative preprocessing system and measuring system are established. At room temperature, studying the breakdown characteristic of paperboard, oil and oil-paperboard under a polarity reversed voltage through test. At the same time, studying the temperature characteristic of these three kinds of insulation structure.The result show breakdown characteristic of oil and paperboard have obvious regularity, but breakdown characteristic of oil-paper is relatively complex.The study can be referred to the insulation design of converter transformer and the HVDC apparatus testing.