Variation Of Transformer Residual Flux And Strategy For Reducing Inrush Current

Transformer is an important and expensive equipment in power system.Its inrush current has a negative impact on the transformer,relay protection,and power system.To eliminate the influence of inrush current,such as the method of identifying inrush current,and controlled switching strategy,have been the focus of domestic and foreign experts and scholars.Controlled switching method is one of the most promising methods for inrush current suppression.By controlling the closing angle,the prospective flux is equal to the residual flux,so as to avoid transformer cores being driven into saturation during energization.Inrush current can be suppressed theoretically.However,the residual flux of transformer cores before closing cannot be known,which is the difficulty and key to controlled switching method.This paper studies the change of the residual flux,puts forward one method to calculate stable residual flux,the controlled closing strategy and inrush current suppression scheme.Firstly,the influence of the overvoltage on the residual flux of the transformer cores is studied.In normal condition,the core flux changes periodically and lags the winding voltage by 90 degrees.When the transformer is opened,there is a transient process caught by chopping current,which is characterized by the overvoltage.A calculation method of the transformer overvoltage based on the equivalent circuit is proposed.The influence of the parameters and opening angle on the overvoltage is analyzed.It is proved by voltage integral that the calculated overvoltage will reduce the flux to zero.The influence of the real overvoltage on the residual flux of the transformer cores is analyzed by comparing the shape of the real overvoltage with the calculated.Secondly,the relationship between the stable residual flux and opening angle is studied.Through the detailed experiment of transformer opening,the relationship is explored.Based on the analysis of magnetization and reverse magnetization of ferromagnetic materials,a method for calculating stable residual flux of transformer cores is proposed for the first time,based on the assumption that the reversible magnetization is proportional to the external field strength.The method calculates the stable residual flux by using the initial flux and excitation current corresponding to the opening angle.The calculated results are inagreement with the experimental.The variation of the residual flux is revealed.Then,the controlled closing strategy is proposed according to the calculation method of stable residual flux.Based on the method,the relationship between the bias of flux and closing angle of single-phase and three-phase transformer is studied respectively,and the controlled closing strategy of single-phase and three-phase transformer is proposed.On this basis,a complete inrush suppression scheme is proposed,and a series of experiments are completed to verify the feasibility and effectiveness of the scheme.Finally,the main research work and innovation points of the paper are summarized,and the further research work is prospected.