The Research On The Dynamic Protection Characteristic Of MOV For Transformer Windings Under VFTO

Very Fast Transient Overvoltage (VFTO) is a kind of switching impulse overvoltage with very steep front caused by the switching operation in Gas Insulated Substation. Its directly invading to transformer windings may severely damage their longitudinal insulation. In this paper, we mainly study the protection characteristic of the MOV in parallel with transformer windings under VFTO. First, the multi-conductor transmission lines (MTLs) model for transformer windings was setup, the iterative computing equations based on the Finite-Difference Time-Domain method (FDTD) were deduced, and the Vector Fitting method was adopted to treat the frequency-dependent parameter. Second, the model of the transformer winding in parallel with MOV based on MTLs was developed, and the computing equations to solve the model based on FDTD were deduced. Finally, a practical transformer winding model was studied, the voltage distribution of the transformer windings under VFTO with and without MOV were measured and calculated respectively. 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.