The Very Fast Transient Simulation of transformer windings in parallel with MOV based on multi-conductor transmission lines theory

The directly invading of very fast transient overvoltage (VFTO) caused by the switching operation in GIS to transformer windings may severely damage their longitudinal insulation. In this paper, the protective effect of the MOV in parallel with transformer windings under VFTO is studied. The multi-conductor transmission lines (MTLs) model for transformer windings in parallel with MOV is set up. The finite- difference time-domain method (FDTD) is adopted to calculate the transient wave process in transformer windings with and without MOV respectively. In order to simplify the time domain convolution in using the FDTD ,Vector Fitting method is used to expend the frequency domain form of frequency-dependent impedance into a sum of finite exponential functions which are easy to be treated. A practical transformer winding is studied. The comparison between the calculating results with that of measuring not only indicates the validity of the model and computing method adopted, but also proves the feasibility of using MOV to protect transformer windings from VFTO.