Study Of Calculating Method Of Transient Electrical Field In Converter Transformer's Oil-Paper Insulation

Convert transformer is one of the most important equipments in the HVDC (high voltage DC) transmission system. The value side winding of the convert transformer is under DC and AC composite field, and also under the polarity reversal voltage during operation. So the internal electric field distribution, space charge accumulation and dissipation have big difference with the normal electrical transformer.In order to solve the transient electrical field in oil-paper insulation construction of converter transformer, a state equation was built based on the finite element method. The boundary conditions were combined into the equation by means of penalty method. And impulse function was introduced into the input part of the state equation, to simulate the step change of boundary potential when a polarity reversal occurred. The strength of the impulse presents the variation of the step change. Since the integral property of the impulse function, electric field under polarity reversal voltage can be solved easily.The oil immersed paper and board has anisotropic and nonlinear properties. This paper derived the FEM equation with the considering of anisotropic and nonlinear properties.Several commonly used numerical methods were compared via a simple model and its analytic solution. From the numerical results, we can see that state space method can get a high-precision numerical solution in the linear medium problem. But for the nonlinear medium problem, we need to use the so called Runge-Kutta method.Finally, the transient electric field was calculated of a practical converter transformer under polarity reversal test voltage, and the results showed the electric field of anisotropic nonlinear analysis is very different with that of isotropic linear analysis. Therefore, the anisotropic and nonlinear conductivity of oil-immersed paper should be taken into account for the accurate designing of insulation.