Study Of Partial Discharge Development Process Of Oil-paper Insulation Under Combined AC-DC Electric Field

Converter transformer is the key equipment in HVDC system, voltage type of valve side winding insulation on converter transformer is different from ordinary transformer during its operation, not only include ac voltage, lightning impulse and operating over-voltage, but dc voltage, dc superimposed ac and polarity reversal of dc voltage. Therefore, in such a complex electric field environment, partial discharge of oil-paper insulation of valve side winding is easily happened. However, at present, the research for partial discharge of oil-paper insulation under combined ac-dc voltage is less. Therefore it is necessary to study the whole process of partial discharge of oil-paper insulation under combined ac-dc voltage, from the start stage to the insulation breakdown, to fill in a certain research gap.Partial discharge experimental platform under combined ac-dc voltage was constructed in this paper, including needle plate electrode model and conventional impulsive current detection system, ultra high-frequency detection system, ultrasonic detection system, as well as gas chromatography analysis system in the oil. Partial discharge development process of oil-paper insulation under combined ac-dc voltage was studied by constant-voltage method. A comparative study of partial discharge development process of oil-paper insulation is made under combined ac-dc voltage and ac voltage by increased-voltage method.The results of the study indicate:in the study of partial discharge development process of oil-paper insulation under combined ac-dc voltage by constant-voltage method, the whole test process is divided into two phases, discharge development stage and discharge serious stage. Phenomena of two stages are intermittent discharge, discharge frequency low, the number of discharge fewer, big discharge energy. All kinds of gas content in oil rise significantly in discharge serious stage.In the comparative study of partial discharge development process of oil-paper insulation under combined ac-dc voltage and ac voltage by increased-voltage method, find discharge phenomenon obviously different. Inception voltage of partial discharge under combined ac-dc voltage depends on ac voltage. In partial discharge development process under combined ac-dc voltage, discharge frequency low, discharge is random. However, in partial discharge development process under ac voltage, discharge frequency higher and stable. Partial discharge development processes of two different voltages also have similarities, in the final breakdown stage, discharge become continuous.