Effect Of Moisture Content Variation Of Oil/pressboard Insulation On Partial Discharge And Its Suppression Method In Valve Side Winding Of Converter Transformer

Oil-pressboard insulation is the main insulation of valve side winding of UHVDC converter transformer,and its discharge behavior threatens the safe operation of DC transmission system.At present,the partial discharge of converter transformer is mainly carried out under steady-state conditions,which cannot provide reliable theoretical basis for the insulation design of converter transformer.In this thesis,based on the actual operating conditions of converter transformer,the relationship among partial discharge,moisture content and electric field distribution of oil-pressboard insulation in valve side winding is established.The partial discharge mechanism of oil-pressboard insulation of converter transformer under operating conditions is revealed,and a partial discharge suppression method of oil-pressboard insulation modified by Zn O magnetron sputtering is proposed,which provides theoretical support and technical basis for safe and stable operation of converter transformer.The major accomplishments are as follows:(1)Based on the Fick’s second law of diffusion and water balance equation,the moisture migration model of oil-pressboard insulation was established and the simulation analysis was carried out.The variation of moisture content at oil-pressboard insulation interface under varying temperature was obtained.It is found that the moisture diffusion betweem oil and pressboard is the root cause of moisture accumulation or dissipation at oil-pressboard insulation interface.Based on Kerr electro-optic effect and AC electric field modulation technology,a test platform for electric field in insulating oil with calibrated Kerr coefficient of insulating oil is established.Combined with the electric field distribution model considering the moisture migration in oil-pressboard insulation,the variation of electric field strength of oil-pressboard insulation under variable temperature is obtained.It is found that moisture content,temperature changing rate,temperature variation amplitude and moisture migration coefficient are important factors affecting the occurence time and amplitude of the maximum field strength of oil-pressboard insulation.(2)The needle-plate discharge model was used to simulate the partial discharge behavior of oil-pressboard insulation under extremely uneven electric field,the AC and DC partial discharge characteristics of oil-pressboard insulation under variable temperature was obtained.The relationship between partial discharge,moisture content and needle-tip electric field is established.It is clarified that the interfacial moisture in oil is the main factor affecting the partial discharge characteristics in the heating stage,and the interfacial moisture in pressboard is the main factor affecting the partial discharge characteristics in the cooling stage.It is found that compared with the heating stage,the partial discharge in the cooling stage has the characteristics of high inception probability and strong burstiness,close attention should be paid to the insulation performance of oil-pressboard insulation when the oil temperature of converter transformer drops.(3)A method for improving the insulation performance of Zn O/cellulose composite insulating paperboard based on Zn O magnetron sputtering was proposed.The effects of Zn O sputtering on the composition of elements and functional groups,chemical bonds and crystal structure,surface and side elevation morphology and surface hydrophilicity of the composite insulation were obtained.The mechanism of Zn O sputtering on the moisture sorption,charge accumulation,partial discharge and breakdown process of oil-pressboard insulation was revealed,and the feasibility of surface discharge suppression of oil-pressboard insulation based on Zn O magnetron sputtering was proved.The influence of volume conductivity and space charge distribution on DC breakdown voltage is obtained.It is found that the improvement of DC breakdown voltage of oil-pressboard insulation needs to balance the internal space charge dissipation rate and the charge injection degree of the electrode.