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Adjustment of wave front time and overshoot in lightning impulse test for transformer insulation

In the lightning impulse test applied to the transformers with large entrance capacitance, the wave front time Tf is easy to exceed the range specified in the standard. However adjusting the parameter of the generator to reduce Tf may cause the overshoot to exceed the range specified. The relationship between the wave front time and overshoot is analyzed in this paper. A method based on the time-domain analysis of zero-state response of the second order circuit is used to measure and calculate the entrance capacitance. The effect of RC circuit in a test applied to a 500 kV/334 MVA transformer is discussed with experiments and simulation. Since reducing the inductance is another approach to adjust the wave, the influence on the line inductance of line arrangement is studied by an experiment evaluating the protective effect of the SPD (Surge Protection Device) connected with the line. The results indicate that the protective effect of the SPD connected with twisted line is better than that connected with ring and parallel line, i.e. the inductance of twisted line is the least.