Study On Influence Of The Breakdown Characteristics Of Oil-paper Insulation In Converter Transformer With Different Moisture Content

Converter transformer plays an important role in high-voltage direct current,and it is a bridge of connecting AC and DC power transmission grids. Its stabilitydirectly affects to the safe operation of the system. Compared with the ordinarytransformer, the operating converter transformer not only withstands AC voltage,lightning impulse and various operative-over voltages, but also subjects to DCvoltage, superimposed AC-DC voltage and polarity reversal voltage. Therefore, theelectric field distribution of insulation structure of the converter transformer andbreakdown characteristics of insulation material are more complicated. Based on theneeds of the transformer industry, this paper studies the breakdown characteristics oftypical oil-paper insulation with different moisture content under compound electricfield as follows:Firstly, the influence of different moisture contents and temperature on thedielectric parameters of oil and oil-impregnated pressboard was analyzed through theexperiment technique. Then the typical oil-pressboard compound insulating structurewas taken as the physical model and the corresponding equivalent circuit model wasgiven, and at the same time, the influence of different moisture content on theelectric field distribution of typical oil-pressboard insulation under compoundelectric field was analyzed on the basis of the experimental data and the moistureequilibrium formula in oil-paper system from ABB company.Secondly, combining with the present equipments in the laboratory, a completeexperimental system and pre-test and detection devices of oil-pressboard insulationbreakdown were built in order to study the breakdown characteristic of oil-pressboard insulation.At last, the influence of different moisture content under DC voltage, ACvoltage, AC superimposed on DC voltage and polarity reversal voltage on thebreakdown characteristic of oil-pressboard insulation at the room temperature wasstudied through experiment method. The results show that the trend of the breakdown voltage of oil-paper insulation has gradually declined with the increasingmoisture content.These researches provide the basis for designing converter transformerinsulation and testing of high-voltage DC facilities.