Research And Fault Analysis Of Observer Based MMC System

Modular multilevel converter(MMC)has been widely used in HVDC and AC transmission because of its cascaded modular structure,which has the advantages of easy expansion,high fault tolerance and low output harmonic content.In order to ensure the normal operation of the system,it is necessary to keep the capacitor voltage balanced among the sub-modules of the same bridge arm.In the balancing strategy based on capacitor voltage sorting,the capacitor voltage of each module needs to be obtained in real time.When the number of modules is very large,the voltage measurement by voltage sensor not only increases the project cost,but also reduces the reliability of the system.In this paper,a reduced-order observer is proposed for this problem.It only needs to measure the voltage and current of six legs,and the capacitance voltages of all modules can be observed by combining the module switching function.The uniform asymptotic stability of the observer error model is proved by using the passive theory and the continuous sufficient excitation condition.Then,the MMC control system is designed based on the reduced order observer and operates under various conditions.When the system is in unbalanced condition of three-phase power grid,the control method under balanced condition will lead to abnormal operation of three-phase MMC.Therefore,a power / current coordination control strategy is designed to change the control effect of the system by adjusting the value of the coordination factor.In order to prevent the DC side current from being too large,the full bridge sub-module is used instead of the half bridge sub-module.The switching state,control strategy and observer design of the full bridge MMC and the half bridge MMC are analyzed.Finally,a three-phase MMC simulation model is built in Matlab / Simulink.The simulation results show that the observer-based MMC system can operate normally under different conditions.