Research On The Startup Control Strategies Of Unified Power Flow Controller Based On MMC

Since entering the 21 st century,the structure and operation mode of the power grid have become more and more complex with the rapid development of China’s power grid and the extensive construction of smart grid,which sets a higher standard for regulation of the line power flow and grid stability.Therefore,FACTS has been more and more widely used in power system.The combination of modula r multi-level technology(MMC)technology and FACTS technology will result in stronger technical advantage.However,the start-up control brought by adopting the MMC structure is a prerequisite for the normal operation of the engineering application in the field.If the startup mode is improper,the system will have severe voltage and current impacts and may even cause system oscillation and affect the safety and stability of the AC system.Therefore,this paper studies the control strategy of the startup process of the unified power flow controller based on MMC structure.The main contents of the research are as follows :(1)Check the basic structure and working principle of UPFC in flexible AC transmission system.Analysis the modular multi-level(MMC)work and modulation methods and compare them by simulation.Establishe the mathematical model of the series-parallel converter structure of The MMC-UPFC respectively,and deduces the related control strategy.(2)The charging mechanism of MMC-UPFC is analyzed in detail.The start-up process is divided into four stages,namely uncontrolled rectifier charging stage,series side module voltage control stage,controllable rectifier charging stage and series side transformer zero voltage output stage.For each stage of the charging characteristics,the establishment of models,analysis and derivation of control strategies.For the voltage balancing problem of MMC sub-module during startup,a voltage-sharing control strategy of sub-module based on fast sequencing algorithm is designed to achieve low switching frequency under the same rectification effect.The correctness of the proposed strategy is verified by using PSCAD.(3)The control strategy for MMC-UPFC startup process is optimized.A control strategy based on variable structure controller for converter arm loop current suppression is proposed.First of all,a mathematical model to analyze the circulation of MMC bridge arm,a feedback linearization method and a sliding mode control theory are combined to design a controller to effectively suppress the circulation and improve the system robustness.(4)The parameter setting problem caused by the input of a large number of PI controllers in start-up control of MMC-UPFC is improved.By improving the firefly algorithm,the weighted index ITAE of voltage outer loop and current inner loop parameter in system control is taken as the objective function,The control parameters are optimized to improve the system output waveform.Using PSCA D model to simulate,the validity of the proposed optimization method is verified.