Simulation Research On Transformer Winding VFTO Based On MTL Model

Gas insulated switchgear(GIS)has the advantages of compact structure,land occupation and easy maintenance.It has been widely used in 110kV and above power grids,especially ultra-high voltage and ultra-high voltage AC transmission networks.When the isolating switch operates the no-load short bus in GIS,the contact moves at a slower speed and has no arc-extinguishing function.During the movement of the knife-gate,multiple breakdown and re-ignition will occur,forming a steeper traveling wave.Deflection and superposition occur at the uneven impedance of the transmission line,which is named Very Fast Transient Overvoltage(VFTO).The VFTO waveform has the characteristics of high amplitude,steep wave front,frequency bandwidth and multiple consecutive pulses.Therefore,when the VFTO wave is transmitted into the transformer,the winding insulation may have breakdown accidents due to resonance or voltage difference.Such as the transformer insulation breakdown accident occurred in Tianhuangping Power Station and Daya Bay Nuclear Power Plant,so it is necessary to study the distribution of VFTO in the transformer.At present,the main model of transformer to analyzing VFTO distribution is multi?conductor transmission line model(MTL)but still has some shortcomings:when using the multi-conductor transmission line frequency domain model to study the VFTO distribution inside the transformer,it is necessary to build a radial model of the winding in the electromagnetic software to solve the capacitance parameter,but a large number of transformer windings are given,simulation modeling brings a huge amount of work;in order to prevent the occurrence of turn-to-turn breakdown in transformer windings,transformer manufacturers often use a variety of winding methods to wind the transformer windings,which makes it diffi;cult to study the internal VFTO distribution of the transformer;when VFTO is known After the domain waveform and the frequency domain response of the transformer winding,there is a lack of a time-frequency conversion method with less error to obtain the time domain response of the transformer winding.In view of the above three shortcomings,this paper studies the automatic modeling of electromagnetic software,winding twining method and time-frequency conversion method.Based on the engineering practice,this paper starts from the frequency domain model of multi-conductor transmission line,realizes the function of automatic calculation of frequency domain and time domain response of transformer winding,and provides technical support for preventing insulation breakdown of transformer winding from time domain and frequency domain.Firstly,this paper studies the methods and steps of electromagnetic software(Maxwell ANSYS)modeling,points out the characteristics and difficulties in the modeling process,and proposes an automatic built-in winding model based on ActiveX technology to solve the capacitance parameter method:By inputting the structural parameters of the transformer winding,using MATLAB to control ANSYS The process of automatically establishing the winding model,obtaining the winding capacitance parameters and outputting the simulation results;then studying the winding twining mode commonly used in transformer manufacturers,and deducing its frequency domain transfer function,as the theoretical basis of the resonant frequency domain analysis of the transformer winding,In order to find the time domain response of the transformer winding,finally,based on the known VFTO waveform and the frequency domain transfer function of the winding,the common time-frequency conversion method is analyzed to calculate the calculation accuracy of the transformer winding response.The leaf series method solves the rest of the time-frequency conversion method using the Fourier transform method.Compared with the traditional manual modeling method,the transformer automatic modeling simulation method greatly reduces the workload of scientific researchers and ensures a lot of repetitive work while ensuring the same simulation accuracy.By solving the frequency domain transfer function of the common transformer winding twining method,as the theoretical basis of the resonant frequency domain analysis of the transformer winding,it also paves the time domain response of the transformer winding.Finally,the wave head part is solved by the Fourier series method.The rest of the time-frequency conversion scheme obtained by Fourier transform method takes into account the calculation accuracy and calculation time,and has good field practicality.Above results can be automatically obtained through the structural parameters of the transformer winding,which greatly simplifies the operation difficulty and operation steps.