Analysis And Calculation Of Very Fast Transient Over-voltage And Its Distribution In Transformer Windings

In gas insulated switch-gear (GIS), very fast transient over-voltage (VFTO) was caused by disconnector when it closing and switching off charging current circuit. VFTO has the characteristic of the steep front and high amplitude value. It has the wave head of several nanosecond to several tens nanosecond. VFTO has the characteristic of intercepting wave, and it is very dangerous to the different parts of GIS equipment, such as the insulation of bus support, casing pipe and switch self. It has the effect on the reliability of switch equipment, and endangers the insulating property of transformer which is connected to GIS.In this paper, a circuit of very fast transient over-voltage caused by disconnector of GIS when it closing and switching off charging current circuit. First of all, the dynamic circuit model and parameters of components were brought up, and the voltage equations were built up to calculate the transient over-voltage, and then numerically simulated VFTO. Secondly, the wave shapes of transient over-voltage to different parts were calculated, and the amplitude value and the occurred time of transient over-voltage to different components were presented. Finally, when the parameters of components changed, the effects on the amplitude value of VFTO were discussed. And then the characteristic of frequency and amplitude value of VFTO were analyzed so that the effects on the components insulation of GIS equipment were studied.In addition, VFTO threatens the insulation of electric power transformer, which is connected to GIS. In allusion to the characteristics, having utilized the transient circuit model of distributing parameters to transformer, the node voltage equations of calculating the transient over-voltage between turns and windings of electric power transformer, moreover, calculated the transient network and the transient over-voltage distributing between turns and windings of transformer. At the same time, the most over-voltage value and occurred place between turns and windings were presented.Multi-conductor transmission line (MTL) model of transformer windings is deduced under VFTO, in which the mathematical equations for calculating over-voltage in windings are given. In order to solve the over-voltage in transformer windings, the finite different time domain (FDTD) method is used. And then the comparison between MTL model method and transient circuit model of distributing parameters are given. The comparison results have shown that the MTL model can better reflect the transient voltage distribution to transformer. All the calculated results provide the academic support and technical reference to the insulation design of GIS and the connecting equipments.