【作者】 吴刚；

【导师】 夏超英；

【作者基本信息】 天津大学 ， 电气工程， 2018， 硕士

【摘要】 模块化多电平变流器(MMC)因其级联模块化结构,具有易扩展、容错性高、输出谐波含量低等优点,在高压直流输电、交流传动领域都取得了广泛应用,有很好的发展前景。为了保证系统正常运行,需要同桥臂各子模块之间电容电压保持均衡,而在基于电容电压排序的均衡策略中,需要实时得到各模块电容电压,当模块数很多时,采用电压传感器测量电压不仅增大了工程造价,还降低了系统可靠性。本文针对此问题提出一种降维观测器,仅需测量六个桥臂电压和电流,结合模块开关函数即可观测出所有模块电容电压。利用无源理论和持续充分激励条件证明了观测器误差模型的一致渐近稳定性,并分析了系统鲁棒性。然后,基于该降维观测器设计了MMC控制系统,并在多种工况下运行。当系统处于三相电网不平衡工况时,若使用平衡条件下的控制方法,将导致三相MMC出现不正常运行状况,为此,设计了功率/电流协调控制策略,通过调节协调因数的值来改变系统控制效果。当系统发生直流侧双极短路故障时,为防止直流侧电流过大,采用全桥子模块代替半桥子模块,分析了全桥型MMC和半桥型MMC的开关状态,控制策略及观测器设计等方面的异同。最后,在Matlab/Simulink中搭建了三相MMC仿真模型,仿真结果证实了基于观测器的MMC系统能够在不同工况下正常运行。更多还原

【Abstract】 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.更多还原