A typical model of converter transformer is adopted to study the linear and nonlinear electric fields of PR (Polarity Reversal) under the step voltage and ramp voltage, and the results show that the short duration of changing polarity should not be neglected when analyze the nonlinear electric fields of the actual converter transformer or bushing, on which conductivity is electrical dependent, otherwise, the numerical results of the step voltage would be higher than the ones under the ramp excitation and cause unnecessary difficulties to the insulation designs.