An Improved Strategy for Modular Multilevel Converter at Low Speed Operation

Abstract

Abstract.One of the main problems of the Modular Multilevel Converter (MMC) for ship electric propulsion applications is the significant magnitude of capacitor voltage ripple at low speed operation. The commonly used solution is harmonic injection. In this paper, the injection rules are explored through the dynamic relationship of MMC in a comprehensive way, and the harmonic characteristics of injection current are derived. Furthermore, the circulating current controller is modified according to the harmonics of injected current with changing frequency, which reduces the ripple of capacitor voltage effectively in a wide speed range. Simulation results are shown to demonstrate the effectiveness of the proposed method. IntroductionThe Modular Multilevel Converter (MMC) has been widely used in the field of high voltage direct current (HVDC) transmission [1,2], and has attracted increasing attention for the application of medium-voltage adjustable-speed motor drive [3,4,5]. The great potential of MMC for ship electric propulsion has been mentioned in [6]. The main challenge of the application of MMC-based drive system is the suppression of capacitor voltage ripple at low speed operation.In order to suppress the voltage ripples of submodule (SM) capacitor which increase significantly as the output frequency decreases, several schemes have been adopted. Harmonic injection is the most mature method at present. The values of injected components are usually derived from MMC's arm energy relations. Several components are neglected in the derivation process which make the derivation result lack of comprehensiveness.The injected circulating current is regulated by the circulating current controller which also controls the even harmonics caused by MMC's mechanism. The performance of the circulating current controller is related to the control accuracy of the injected current. Many control strategies have been used in the design of current controller, such as repetitive control [7], model predictive control [8], and multiple quasi-resonant control [9], but they are only designed to regulate the even harmonics at the rated frequency. Meanwhile some other literatures like [10], [11] put emphasis on the control of the injected current but neglect the even harmonics which needs to be regulated at the same time. In [12], the current controller contains six resonant controllers to regulate the second harmonic and injected harmonics at low speed operation, but it is not suitable with the changing frequency.In this paper, the injection rules are explored through the relations of mathematical model, and the proposed formula of harmonic injection is derived from the rules. The circulating current controller is