Study On Optimal Control Strategy Of MMC-HVDC System Under Asymmetric Condition

MMC-HVDC system has the characteristics of fast response,good controllability,flexible operation mode,small footprint of the same capacity and so on.It is the power transmission technology with the highest controllability and best adaptability in the world,and has been widely used in large-scale renewable energy grid connection,AC system interconnection,island power supply and other occasions.As the key technology of MMC,control strategy directly determines the actual performance of MMC-HVDC system.The research on MMC control strategy under normal working conditions has been relatively mature,while the research on MMC control strategy under asymmetric network pressure is relatively few and not in-depth enough.Although domestic and foreign scholars have carried out related studies and achieved certain results,there are still some shortcomings.On the basis of analyzing the basic operation principle of MMC,this paper studies the control strategy of MMC-HVDC system under the condition of asymmetric network voltage.Firstly,the topological structure and working principle of MMC are introduced in detail,and the design process of the overall control structure of MMC in the DQ axis coordinate system is given.The characteristics and advantages of two commonly used modulation strategies of MMC,namely carrier phase-shift modulation and approximation level modulation,are described respectively,which provide a theoretical basis for the subsequent research.Secondly,the internal and external operation characteristics of the MMC under asymmetric grid voltage conditions are analyzed,and an optimal control strategy for the MMC AC side current and bridge arm circulation under asymmetric conditions is proposed.In this strategy,a classical dual-sequence current controller is used to track the positive and negative sequence components of the DQ axis of the AC side current,and a phase-locked loop(Dual Second-order Generalized Integrator PLL)based on a dual-order generalized integrator is designed.DSOGI-PLL can accurately extract the phase and frequency information of voltage under asymmetric conditions,realize the accurate extraction of the positive and negative sequence DQ axis components of voltage and current,and improve the accuracy of AC side current control.An embedded Repetitive Controller(PI-RC)composed of serial PI controllers and Repetitive controllers is used to track the DC reference instructions of circulation without static difference.The double-frequence positive and negative zero-sequence components in the circulation are effectively suppressed.The detailed design process of PI-RC parameters is presented.Finally,from the perspective of energy flow of the MMC-HVDC system,an optimal control strategy of the MMC-HVDC system under the asymmetric AC side of the constant voltage converter station is proposed in combination with the control strategy mentioned in Chapter 3.Under the premise of controlling the three-phase current balance on the AC side,the relative balance of energy in the bridge arm of the constant-voltage converter station is realized by regulating each component in the circulation with the bridge arm energy controller.According to the dynamic information of the DC bus voltage,the output of the active power of the constant power converter station is appropriately reduced during the asymmetric working condition,which effectively prevents the sharp drop of the total energy of the bridge arm of the constant voltage converter station and the DC bus voltage,and improves the transient performance of the MMC-HVDC system.The validity of the proposed strategy is verified by the simulation model of MMC-HVDC system built by MATLAB/Simulink software.