Study On The Distribution Of Lightning Impulse And VFTO On Transformer Winding

In this paper,the distribution of lightning impulse voltage and the very fast transient voltage(VFTO)in transformer windings is mainly studied.Compared to the lightning impulse voltage wave,the very fast transient overvoltage wavefront time is shorter,and by the spectrum analysis we can see that very fast transient overvoltage includes high frequency,so the paper need to establish a different model for analysis.Transformers are play a important role in substations,after the breakdown occurs,insulation of the windings cannot recover on its own,so it will cause great economic losses.Now most of the transformer using zinc oxide arrester lightning protection,but the arrester production process,insulation aging and bad weather and other factors will affect the normal operation of the arrester.In addition,the actual lightning statistics,the transformer more affected by theimpact of lightning intrusion,and the arrester often malfunction,so the transformer lightning protection can not rely solely on lightning arrester,and inside the windings also should strengthen their own insulation protection.The wavefront time of the lightning impulse voltage wave is microsecond,its frequency components are mainly concentrated in the 0 ~ 50 kHz,so we need to take the line cake as a unit to establish the transformer model,this not only ensures the accuracy of the calculation,while simplifying the operation,speed up the speed of the solution;then the each component parameters of equivalent circuit model is given;Finally,the overvoltage distribution in transformer windings under various lightning impulses is simulated by Bergeron equivalent model method.Through the simulation calculation,the wavefront time and the tail time respectively mainly affect the longitudinal insulation and main insulation strength of the transformer winding;lightning impulse truncation can compensate for the shortcomings of wavefront extension;taking into account the power frequency voltage,when the superposition of the power frequency voltage and the standard lightning impulse voltage amplitude on the opposite polarity,the transformer winding over-voltage distribution is the most serious,at this time on the main and longitudinal insulation of the assessment than the pure standard lightning invasion more stringent.In addition,two kinds of measures to improve the transformer potential distribution are proposed,namely,ground capacitance current compensation and longitudinal capacitance compensation,and verified by examples.Very fast transient overvoltage(VFTO)is also one of the causes of transformer winding insulation damage.Comparing to the lightning impulse,very fast transient overvoltage's waveform steepness is large,that is,the waveform rise time is ns level,and the lightning impulse overvoltage is ?s level.The high frequency part contains frequencies up to 50 MHz and extremely high frequency parts up to 100 MHz,so we can not use centralized parameter model of the lightning impulse.For the characteristic of VFTO,we need to establish the high frequency model of the transformer windings,then the overvoltage distribution in the transformer winding was calculated by using the frequency domain method simulation.Through the simulation calculation,the line turns close to the VFTO invasion is easily to occur the insulation breakdown,it should strengthen the insulation protection here.