| More and more Gas Insulated Switchgear (GIS) has been put in use in electric power system for their advantage of less area occupancy, steady run and maintaining convenience. When disconnector switched on and switched off charging current circuit, Very Fast Transient Overvoltage (VFTO) is caused by slow contact sports, the interrupter switch their poor performance, the gap between contact will be renewed repeatedly occurred. 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.First of all, according to the actual structure of GIS, the various components of the equivalent reasonable approximation are brought up. The dynamic circuit model of focuing on components resistors, inductors, capacitors and lossless transmission line are confirmed, and the dynamic circuit model of transformer, bresker and disconnect switch which of arc and unburned arc as well. According to the formula of inductors and capacitors, parameters of components are also calculated.Secondly, in this paper, with regarding to a charged electric current circuit of practical 500kV GIS is presented, in which the disconnect switch is disconnecting and closing the bus. The dynamic circuit model of each component is confirmed, and the equivalence circuit model of VFTO is brought up. The node voltage equations of calculating the transient overvoltage are built up to calculate the transient overvoltage, and then numerically simulated VFTO. The wave shapes of transient overvoltage to different parts are calculated, and the amplitude value and the occurred time of transient overvoltage to different components are presented. VFTO poses a threat to the insulation of electric power transformer, which is connected to GIS. Finally, according to the VFTO waveform of the Discrete Fourier Transform, when the parameters of components changed, the effects on the amplitude value of VFTO are discussed. And then the characteristic of frequency and amplitude value of VFTO are analyzed so that the effects on the components insulation of GIS equipment are studied. The wave shape of VFTO along with time is also presented. Then when the parameter of transformer's entrance capacitance and bus's remain charge as well as support insulator are endowed with different value, the influence on the wave shape and the change rules of VFTO is discussed. In addition, selection of the larger arc of resistance values, appropriate to increase the length of cable and the use of zinc oxide surge arresters, also on the amplitude VFTO have some effect. With rapid development of the GIS UHP, we deal with the isolation switch operation by special fast transient attach great importance to the process. We will take the necessary measures to limit the overvoltage to enhance the reliability of GIS. |
PDF Full Text Request