Electric Field Calculation And Coil Surface Discharge Restraining Method Research Of Casting Dry Type Transformer

With the progress of the China's national economic development and urbanization advancement,the city's comprehensive buildings have sprung up while urban rail transport in large and medium-sized cities has enjoyed a rapid development across the country.Under this circumstance,epoxy cast dry-type transformers,characterized by non-toxicity,flame retardance,small volume,low noise,strong anti-short circuit ability,environmental fridendliness and other features,are widely used in city buildings,substations and tractive power supply system of rail transit.Nevertheless,with the widespread application of epoxy cast dry-type transformers,their operating environment is increasingly complex.Discharge phenomenon occurs frequently in insulation surface outside the coil due to some dry-type transformers' long-term work under damp,filthy and other severe conditions,which contributes to dendritic discharge marks on the insulation surface and leads to irreversible damage.If the impaired dry-type transformers run for a long time,they would be prone to insulation breakdown and trigger a blast.In response to these problems mentioned above,this topic studys the electric field calculation and surface discharge suppression measures on epoxy cast dry-type transformers with the main contents as follows:Firstly,the fundamental theory of electrostatic field calculation is elaborated and the finite element methods and implementation steps are analized;combining with the electromagnetic structural parameters of an actual dry-type transformer,this paper uses ANSYS software to build a two-dimensional finite element analysis model of electric field for the transformer with the prominence to the analysis for the electric field when the transfomer is under a variety of operating conditions,such as the clean and dry coil surface,damp surface,various thickness of insulation surface,depressed surface,polluted surface and lightning impulse;and the contrastive analysis of these findings is conducted with the result shows that the damp insulation surface is the most important reason for coil insulation surface discharge,providing a theoretical basis for the proposed surface discharge suppression measures.Secondly,taking the epoxy cast dry-type transformers' numerous models,different structures and varied wiring patterns into consideration,it is time-consuming and complicated to conduct modeling calculations respectively for the dry-type transformers.Therefore,in order to achieve fast electric calculations,based on the VB software,this paper develops a universal electric field calculation software on epoxy cast dry-type transformers,through which the analysis of the electric field can be easily gained after entering the structural parameters and electromagnetic parameters of dry-type transformers.Meanwhile,according to the existing lightning impulse calculation software,results can be imported to general calculation software,calculating the electric field distribution under lightning impulse.Finally,this paper discusses the mechanism of gas discharge along the solid surface,and comparatively analyzes the discharge reasons and characteristics along the solid surface both under highly uniform and severe non-uniform electric field.Considering the features of the electric field distribution of dry-type transformers under various operating conditions,this paper proposes the utility of hydrophobic coatings and semi-conductive coatings on the coil insulation surface,and the results after calculating the elctric field shows that the two surface treatment measures mentioned above can effectively uniform electric field distribution of insulation surface,which offering an important theoretical reference for the discharge suppression of coil insulation surface of the dry-type transformer.