Control Strategy Research Of Modular Multilevel Converter Under Abnormal Working Conditions

Modular multilevel convert (MMC) attracts more and more attentions nowadays in high voltage and large power situation, because of its scalability. Researches on MMC applications such as high-voltage direct current (HVDC), medium voltage motor drive and reactive power compensation develop fast. Circle current control and capacitor voltage balancing are two central issues recent years, however control strategy of MMC under abnormal working conditions is meaningful in practical applications.In this paper three different abnormal working conditions are proposed and studied. Improved hierarchical control strategy based on direct arm current control is applied. Ac side voltages asymmetry problem is solved by additional power balance controller. Low frequency operation is achieved by injecting high frequency power. A hybrid MMC structure and its control strategy is proposed for abnormal working condition of dc side.Firstly, this paper summarize the researches of abnormal working conditions, topology and principle of typical MMC are investigated. Then a hierarchical control strategy under normal working conditions is proposed.Secondly, ac side voltages asymmetry working condition is taken into consideration. Simulations show the proposed control strategy is effective for inverter side MMC under abornal working condition, however the strategy is not applicable for rectifier side MMC. So two kinds of improved control strategies are put forward, and simulations in PSCAD verified the effectiveness.Thirdly, low frequency operation and corresponding control stratege are developed. The fluctuations of capacitor voltage is reduced by injecting high frequency power under low frequency operation. Furthermore an analog emulation model of induction motor is proposed to verified the control strategies of MMC and motor. Simulations in PSIM verified the effectiveness.Finally, the paper proposed a hybrid MMC structure which integrate ice-melting and static synchronous compensation (STATCOM) functions. A corresponding control strategy is developed to achieve the flexibility. This hybrid MMC can also reduce the overcurrent in dc side abnormal condition.