To investigate the effect of transformer windings under VFTO

a new transient modeling algorithm is proposed in this paper.Firstly it is assumed that no frequency-dependent impedance in transmission line

fuzzy neural network is used to compute distributed parameters of large transformer windings.Secondly in the case of easily destroyed former several windings

each is modeled as one unit

others are modeled with two windings as one unit

a transient modeling algorithm based on Finite Difference Time Domain（FDTD） method is put forward.When frequency-changing effect is obvious

Vector Fitting method is adopted to deal with frequency-dependent parameters by expanding time-domain form of internal impedance into a sum of finite exponential functions

which is easy for time-domain convolution

so a transient modeling algorithm considering frequency-dependent parameters is presented.Lastly through simulation illustrations validate effectiveness and feasibility of the presented algorithms.