Motion Characteristics Of Solid Particles In Insulating Oil And Their Influence On Breakdown Voltage Of Insulating Oil

Converter transformer is the key equipment of High Voltage Direct Current Transmission System(HVDC),which safety is related to the stability of the HVDC system.Transformer oil is the main insulating materials of converter transformer,which insulating performance is related to safe and reliable operation of converter transformer.Transformer oil is a kind of liquid dielectric,which insulating performance is quite sensitive to solid particles originating from the process of operation of converter transformers.The particle will move around and gather at high electric field region by the effect of electric field force.This phenomena will reduce the insulating performance of insulating oil.The operation condition of converter transformer is different from AC transformer.The valve winding of converter transformer simultaneously withstands AC voltage and DC voltage,and its insulation defects is different from AC transformer.There are different particle motion between converter transformer and AC transformer because of the existence of DC voltage.Therefore,this paper investigate the process of Impurities Bridge under combined AC and DC voltage by using a dynamic analysis method.The mechanism of the breakdown of insulating oil contaminated with solid particle are obtained.The main contents are shown as follows:(1)The force condition of particle under electrical field are investigated,and the motion model of solid particle are obtained.This paper investigated the particle trajectory under different electrical field via the COMSOL simulation software.The relationship between particle sizes,voltage value,voltage type and particle velocity are obtained.(2)The experiment platform of shooting the particle trajectory in insulating oil is constructed.The effect of electric field distribution,voltage type,electric field value and particle content on the motion of particle are obtained.Experimental results showed that the particle will gather at high electric field area.When the content of particle are enough,there will be a particle bridge connecting the electrode.There is no particle bridge formed at AC voltage,and the particle bridge show up at DC voltage and combined AC and DC voltage.The research results shows that the voltage type and particle content are the key factors of the formation of Particle Bridge.(3)The breakdown voltage of insulating oil contaminated with solid particle under AC voltage,DC voltage and combined AC and DC voltage are implemented.The effect of voltage type on the breakdown voltage of insulating oil are investigate.The relationship between the breakdown voltages of insulating oil at different voltage type and particle content are obtained.Experimental results showed that the voltage type,particle type and particle content have significant influence on the breakdown voltage of insulating oil.The mechanism of the solid particles on the insulating oil breakdown process at different voltage type are analyzed by combined the simulation results of the solid particle trajectory and the formation of solid particles Bridges.