Research On Numerical Algorithms Of Electric Fields Under AC-DC Voltage And Polariy Reversal Voltage Of Converter Transformer

The valve siding of the converter transformer, which is one of the most important equipment in HVDC (High Voltage Direct Current) project, is imposed on periodic AC-DC hybrid electric fields under the operation condition, which contains higher harmonic voltage, and PR (polarity Reversal) electric field when the power flow reversing. To accurately compute the electric fields in oil-paper insulation structure under the above conditions is the foundation of insulation design and the further study on the distribution characteristics of the AC-DC hybrid electric field and PR electric field has great significance to converter transformer's design, manufacture and maintenance. This thesis is focus on the algorithms of calculating the AC-DC hybrid electric field and PR electric field in oil-paper insulation structure and the main content is as follow:(1) Study the algorithm of computing the AC-DC hybrid electric field by FEM in frequencyThe voltage imposed on the valve siding of converter transformer not only contains dc and ac component, but also has higher harmonic components. It is a common method for the designers to obtain the AC-DC hybrid electric field by directly superimposing the dc electric field and static electric field and neglecting the higher harmonic components. In this thesis, the FEM in frequency is adopted to analyze the hybrid electric fields in the linear dielectrics and the FEM in frequency combing the fixed point method and linearization method is proposed to get the periodic electric field distribution in the electric dependent dielectrics. The FEM in frequency is fully compared with transient method and the HBFEM (Harmonic Balance FEM) in analyzing the steady hybrid electric fields in linear and nonlinear dielectrics of a typical oil-paper model. The results show that FEM in frequency can not only reduce computational time compared to transient method, but also save computer memory compared to HBFEM and can be adopted to obtain the AC-DC hybrid electric field distribution of the actual converter transformer. The electric fields in linear, nonlinear and anisotropic nonlinear dielectrics are obtained and analyzed in this paper.(2) Study the influence on the electric field distribution at the end of each PR by the duration of the PR process in the electric dependent dielectricsBecause the current analysis of PR electric fields has not took the electric dependent nonlinearity and the duration of the PR process into consideration at the same time, this paper deeply study the influence on the electric distribution at the end of each PR by the short duration of PR process in the electric dependent nonlinear dielectrics. The computational results show that the duration time of the PR process has great influence on the electric field distributions in the electric dependent dielectrics, especially for the oil, which has low insulation margin. The maximum electric field of oil in the nonlinear dielectrics, which PR duration is2minutes, is about50percent less than the corresponding ones assuming the PR taking place without any delay, and the less electric strength is benefical for the insulation design.(3) The node charge density-potential finite element method for the PR's electric fieldIt is usually qualitatively pointed out that the charges and their induced electric fields could not change suddenly or have small variations and have influence on the PR electric field when analyzing the PR's transient electric field, and should be quantitatively study the charges and their charge induced electric fields. This paper propose an algorithm to obtain the charge and potential at the same time, which is called node charge (density)-potential finite element method, to study the transient electric field and charge. The method directly uses the node charge and potential as variables and has higher accuracy. The charge densities on the metal structure, such as static ring in converter transformer, are adopt to obtain the normal electric field accurately, which can provide guidelines for the structure design.(4) Compute the electric field, charge and charge induced electric field of a±500kV converter transformer under PR voltageThe transient electric fields and charge distributions of a±500kV converter transformer under PR voltage are obtained by the proposed node charge(density)-potential finite element method. The transient electric fields, charges and charge induced electric fields in isotropic linear dielectrics, isotropic nonlinear dielectrics, anisotropic linear dielectrics and anisotropic nonlinear dielectrics are compared and study the variety characteristics of the electric fields, charge distributions and the normal electric fields of the static ring.