Analysis of nonlinear electric field of converter transformer under polarity reversal voltage

The conductivity of insulation oil and paper of the converter transformer is influenced by electric field strength, temperature and pressure etc. In order to study the influence of electric dependent conductivity on electric field distributions, the nonlinear characteristics between electric field strength and conductivity of the oil is fitted by an exponential curve. Using the new nonlinear curve, the transient nonlinear electric field of a ±500 kV converter transformer under PR (polarity reversal) voltage is calculated by the C-N (Crank-Nicolson) method .The numerical results under linear and nonlinear conditions show that, when we consider the 1 minutes duration of the PR process, the linear maximum electric field strengths at the end of the ramp voltage of the PR are almost equal to the results under the step voltage, while the nonlinear maximum electric field strengths at the end of the ramp voltage of the PR are about 50 percent less than the results under the step voltage. The phenomena of the less value under the nonlinear condition are explained by the acceleration of the space charges under the electric dependent conductivity.