Research On Controlled Switching Strategy Of Unload Transformer Based On Residual Flux Control

With the wide application of electric energy and the popularization of new energy,the number of substations and large-scale wind farms is gradually increasing.Transformer is one of the important components in wind farms and substations,and it plays the role of converting voltage in power systems.However,when the unloaded transformer is closed,the inrush current will be generated due to the saturation of the magnetic core flux of the transformer.The presence of the inrush current will cause the transformer differential protection to malfunction.Serious overcurrent will heat the transformer winding,reduce the transformer insulation performance,and affect the service life of the transformer.Although the traditional inrush current identification technology can identify the inrush current and prevent the transformer differential protection from malfunctioning during closing,it has always failed to suppress the inrush current and the damage to the transformer itself has always existed.With the application of new operating mechanism,the controlled switching technology is becoming more and more mature,and this technology can suppress the inrush current from the source.However,in the power grid,there are many traditional circuit breakers.This part of the circuit breaker has only one operating mechanism and cannot be operated in phases.This article studies this part of the ganged three-operated circuit breaker.First,the basic performance and structure of the transformer are analyzed.The basic principles of controlled switching strategy are analyzed.The influencing factors of the inrush current during the unloaded closing of the transformer are studied.The effects of residual flux level,the system impedance,and the closing phase on the inrush current are simulated and analyzed.The closing characteristics of the ganged three-operated circuit breaker are studied;the residual flux distribution in the transformer core when the circuit breakers with different phases during a period are analyzed.Based on the different residual flux distributions,proposed the unload transformer controlled switching strategy for ganged three-operated circuit breakers,that is,closing when the residual flux of the transformer core and its expected magnetic flux deviation is the smallest.A simulation model was built to verify the controlled switching strategy.The simulation results show that the proposed closing strategy can effectively suppress the inrush current generated by the closing of the unload transformer based on the ganged three-operated circuit breaker.Combining the proposed ganged three-operated circuit breaker controlled switching strategy with traditional controlled switching technology,and applying it to the closing of unload wind farm transformer.The inrush current and the sympathetic inrush generated by theclosing unload transformer of the wind farm are analyzed and studied,and a simulation model is built to analyze this.The simulation results show that the controlled switching can effectively suppress the inrush current and the sympathetic inrush generated by the unload closing of the wind farm transformer.Finally,a unload transformer closing test platform was built,and random closing was performed through the platform.The test showed that the transformer will generate a high amplitude inrush current during random closing,which can reach 6-8 times the rated current of the transformer.For the ganged three-operated circuit breaker,the proposed controlled switching strategy is used to energize,the inrush current in the transformer can be suppressed to 1.5 times the rated current.The test results show that the proposed strategy is effective and practical.