Research On Transient Overvoltage Distribution In Large Transformer Windings Under VFTO

With the increasing of power system voltage level, the problem of very fast transient overvoltage (VFTO) generated by disconnector operation in gas insulated switchgear (GIS) which has attracted more and more attention. VFTO has the characteristic of the steep front and high frequency oscillation and it is not only very dangerous for the parts of GIS equipment, but also harmful for the power equipment which is connected to GIS. And the danger of power transformer is particularly serious.In this paper, VFTO at some nodes caused by disconnector operation in practical 500kV GIS is calculated and simulated by electromagnetic transient simulation software (ATP-EMTP), and 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. From these calculated results, some of VFTO disincentives are discussed.It is indispensable to build the equivalent circuit model of transformer winding to analyze the effects of transformer insulation under the VFTO, and some equivalent circuit of winding models were obtained by foreign and domestic scholar. For the characteristic of VFTO, multi-conductor transmission line (MTL) mode is used to analyze the transient response of power transformer windings under VFTO. And then the calculation method of the transformer winding distributed parameter is given. To discrete the time-domain telegraph equations of MTL, through combining the compact finite difference (CFD) theory and the backward differentiation formulas (BDF), which translate differential algebraic equations into algebraic equations, and then transient voltage distribution in transformer windings under VFTO is obtained by solving the algebraic equations. At last, typical parts of the transient voltage waveform are shown and the voltages distribution law is analyzed in transformer windings. Finally, compared electromagnetic transient simulation software ATP-EMTP results with the proposed method, and the simulation results has proved that the proposed method is feasible.