When the power transformer is subjected to the switching impulse test using inductive method

it is neces-sary to find the equivalent capacitance value of the transformer with voltage

for the purpose of predict whether thewaveform of switching impulse voltage meets the requirements and estimating the voltage efficiency. This paper rec-ommended the use of the converting method by square value of voltage-ratio to calculate the equivalent capacitance.According to the principle of equivalent impedance between the primary and secondary side of the transformer theo-ry

the capacitance of the winding to ground（or to other winding）is first converted to its top to ground

then the ca-pacitance of high voltage side will be equivalent to the low voltage side. For various connection modes of single phaseand three phase transformers

the calculation methods of equivalent capacitance and calculation results are given.Compared with other methods

the method introduced in this paper is simpler and more convenient