Research On Statistical Law Of Very Fast Transient Overvoltage And Reference Waveform For Insulation Test

Very fast transient overvoltage(VFTO)generated during the switching of disconnector within gas insulated substation(GIS)put forward high demand for the insulation coordination of the electrical equipment in the extra and ultra high voltage substations.Currently,the insulation design of electrical equipment can only be considered in the most serious cases as the distribution characteristic of VFTO u nder different operation modes remains poorly understood.Meanwhile,since there is no recognized standard waveform for VFTO,no requirement has been made on VFTO tolerance in the insulation test of electrical equipment.Therefore,it is necessary to study the VFTO generated in GIS substation in different operation modes and conclude a reference waveform which can be used for the insulation test of related electrical equipment.In this paper,the electromagnetic transient calculation method is used to study the VFTO produced by the disconnector in different positions of the 1000 k V GIS substations.VFTO waveforms generated during a single breakdown are recorded.Four parameters,the maximum amplitude,the first peak gradient,the duration time and the main oscillation frequency,were selected as the characteristic parameters of VFTO to be studied in detail.Distributions of these parameters under different operating modes were obtained by the method of probability density estimation.Proposal of the position retrofitting VFTO damping device was given for the disconnectors with low insulation margin.Afterwards,according to the simulation results,a reference waveform for the VFTO insulation test was given by curve fitting method.The result shows that the maximum amplitude of VFTO in 1000 k V GIS substation is mainly distributed in 1.2p.u.~2.2p.u.,while the first peak gradient is mainly distributed between 3.0k V/ns to 8.0k V/ns.However,the maximum difference of the value of each parameter can reach up to 40% under different operation modes.The duration of a single breakdown VFTO is mainly between 3.0?s and 5.0?s,and the main oscillation frequency is mainly distributed in 0.5MHz ~1.0MHz.Based on the insulation level of 1000 k V electrical equipment,insulation margin of t he equipment with a short charged incoming bus is usually less than 10%,and should be protected by some damping devices.While,more than 15% insulation margin is left when most of the GIS bus are charged,which means the damping device can be omitted.With high accuracy and easy to produce,the double-exponential fitting method was used in defining the reference waveform of insulation test.The modified reference waveform for VFTO insulation test can be expressed as 0.4?s(±35%)/4.0?s(±25%).