Study Of Modeling And Simulation Of Transformer Winding Under The Very Fast Transient Over-voltage

More and more gas insulation substations have been put in use in extra voltage andlarge electrical power system for their advantage of less area occupancy, no atmosphericinfluence, steady run and maintained convenience. But due to the relatively slowmoving speed of disconnect contacts in GIS, when switched the capacitive current, anumber of pre-and re-strikes will occur, which results in steep front impulse waves.The very fast transient over-voltage is formed by the multiple refractions and reflectionsof the steep front impulse waves at all points when they encounter impedance changesin GIS. When the transformer is either connected directly to GIS or indirectly throughoverhead line or cables, the VFTO will be applied to the winding of transformer, whichmaybe damages the transformer insulation. It is indispensable to build the equivalentcircuit model of transformer winding to analyze the effects of transformer insulationunder the VFTO, and some equivalent circuit of winding models were obtained byforeign and domestic scholar.The internal structures of transformer are presented in the thesis, and the differentover voltages which determine transformer insulation are analyzed, through simulationthe simplified equivalent circuit of GIS, the very fast transient over voltages of the lineterminal of transformer are gained. According to the exact structure of the transformerwinding, the author presents a winding hybrid lumped parameter model whichcombines the branch model based on parallel cables in one turn with grouped-turnsmodel of two discs under quasi-stationary electromagnetic field for VFTO. By thebranch model based on parallel cable in one turn, the over voltage between turns can beexactly simulated during the steep front wave impulse, and combining with the twodiscs turns model, the time of analysis and calculation can be rapidly reduced. Networktopology and element size are derived from the geometry and properties of thetransformer, whose inductance and capacitance are assumed to be passive and to havelinear characteristics, however, which have an inherent feature of a medium, whichpropagates waves, The formulae of parameters are obtained. An eigenvalue andeigenvector method is mentioned to solve the network equations.Through setting up the netlist which can be identified by Saber to expresses theequivalent circuit of winding model, by using the Saber tool, the simulation analysis isperformed for the different equivalent circuit of winding models under the lightning fulland chopped wave impulse, and the voltage distribution along winding, voltage betweenturns and voltage of nodes to earth are found out. According to the simulation results,the design of the winding structure can be made.