Surface Charge And Discharge Characteristics Of Oil-paper Insulation Under Electrothermal Complex Field

HVDC transmission technology will become the mainstream power transmission direction in the future,so as to solve the problem of long-distance and large-capacity transmission and rational absorption of new energy sources.As one of the most crucial equipment for connecting the DC system and the AC system,the reliability of the converter transformer affects the safety and stability of the entire power grid.Due to special operating conditions,the valve side bushing of the converter transformer connected to the converter valve is subject to complex voltage excitations,including DC voltage,AC/DC composite voltage,lightning or switching over-voltage.Under the complicated electric field,the oil-paper insulation of the valve side bushing is prone to electric field distortion due to charge accumulation,which causes a creeping discharge and seriously causes the insulation failure.At the same time,because of the loss generated during the operation of the transformer,the temperature inside the converter transformer rises,which has great influence on the insulating characteristics of oil-paper insulation.In this paper,the oil-paper insulation surface charge and creeping discharge characteristics of the converter transformer valve-side bushing under the electro-thermal composite field are studied.The main research contents and conclusions obtained in this paper are as follows:The surface charge characteristics of the oil-paper insulation were measured using surface potential decay measurement under DC voltage and temperature composite field,impulse voltage superimposed with a DC voltage(the superimposed voltage)and the superimposed voltage and temperature composite field,respectively.The results showed that the temperature has a significant influence on the surface charge characteristics.With the increasing temperature,the trap energy level of the oil-paper insulation become deeper and the number of shallow trap increased while the deep trap density decreased under the DC or superimposed voltage.Some charges will not be trapped and some more easily escaped from the trap,resulting in a decrease of the initial surface potential and an increase in surface charge dissipation.At the same temperature,when the DC voltage and the impulse voltage had the same polarity,the initial surface potential increases and the charge dissipation rate increased.When the DC voltage was opposite to the impulse voltage polarity,the initial surface potential and the charge dissipation speed increased first and then decreased as the impulse voltage increased.The surface discharge characteristics of the oil-paper insulation were measured using double finger electrodes under DC voltage and temperature composite field,the superimposed voltage and the compound field of temperature and the superimposed voltage.The results showed that as the temperature increased,more charges escaped from shallow trap to hit cellulose chain.The discharge channel is easily formed,thus,the creeping discharge was more likely to occur and the discharge voltage decreased.When a DC voltage and an impulse voltage ere superimposed,the composite voltage decreased as the DC voltage increased.