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This paper presents a new feed-forward strategy for suppressing low-order harmonic circulating currents in pulsewidth-modulated (PWM) based modular multilevel converters (MMCs). The approach is based on the new average model approach established in this paper, which identifies cross-coupling interactions between the system-level and submodule elements using decomposed dependent sources. As well as improving the damping of the system, the major advantage of this approach is that it maintains the natural DC bus balance property of a PWM modulated MMC by considering the upper and lower arms together (unlike existing feed-forward schemes which treat each MMC arm independently). Moreover, unlike existing direct control resonant strategies, which require careful gain tuning and are highly dependent on the ac system frequency, the proposed approach achieves wideband circulating current ripple suppression without requiring knowledge of the circulating current harmonic frequencies and needs only a simple proportional–integral (PI) controller to regulate the circulating current dc component for power balance. This makes it particularly suitable for use in multifrequency or variable-frequency ac systems. Extensive simulation and matching experimental results, including steady-state and transient responses compared against existing circulating current suppression approaches, validate the effectiveness and benefits of this new feed-forward compensation technique. [ABSTRACT FROM PUBLISHER]

Modular multilevel converters (MMCs) represent an emerging topology in medium-voltage (MV) and high-voltage applications. But for MV variable-speed motor drives, the MMC faces the challenge of large capacitor voltage ripple at low speeds. Delightfully, a hybrid MMC (HMMC) has proved to be potential in MV motor drives, since this topology has a lower capacitor voltage ripple and does not introduce any common voltage to the motor. This paper proposed a method to decrease the average capacitor voltage of the HMMC, which can effectively drive the motor at very low speeds without significantly increasing the submodule (SM) capacitance. Moreover, the selection of average capacitor voltage and the dimension of the SM capacitance are presented. Simulation and experimental results verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has several submodules in cascade. To control the submodule capacitors voltage, a new generalized dynamic voltage balancing algorithm along with a simple pulse width modulator structure is proposed in this paper. With proposed pulse width modulator structure, the dynamic voltage balancing algorithm can be easily implemented with any type of carrier based pulse width modulation (including level-shifted and phase-shifted pulse width modulation) scheme without any modifications. This method does not require the sorting technique for selection of submodule capacitors voltage. The performance of the proposed voltage balancing algorithm is presented for MMC with three-level flying capacitor submodules (MMC-3L-FC). The proposed method is also applicable to the conventional two-level half bridge submodules as well. The effectiveness of the proposed voltage balancing algorithm is verified with the phase-shifted carrier-based pulse with modulation scheme. The results obtained from the MATLAB/SIMULINK simulations on 6-kV/ 2.5-MW system and the dSPACE DS1103-based laboratory prototype of 208-V/3-kVA MMC-3L-FC system are in close relationship, and thus, the proposed methodology is validated. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is attractive for medium- and high-power applications because of its high modularity, availability, and power quality. In this paper, the current ripple on the dc link of the three-phase MMC derived from the phase-shifted carrier-based pulse-width modulation scheme is analyzed. A control strategy is proposed for the current ripple elimination. Through the regulation of the phase-shifted angles of the carrier waves in the three phases of the MMC, the current ripple on the dc link of the three-phase MMC can be effectively eliminated. Simulations and experimental studies of the MMC were conducted, and the results confirm the effectiveness of the proposed current ripple elimination control. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted. [ABSTRACT FROM AUTHOR]

This paper evaluates two different concepts for the control of the modular multilevel converter (MMC). Generally aiming at medium voltage applications, the considered MMC topology has a low number of submodules, and a pulse-width modulation (PWM) scheme is applied. A model predictive control (MPC) algorithm is developed and experimentally compared to a cascaded control scheme based on conventional PI controllers, which has been proposed by Hagiwara and Akagi. Steady-state and dynamic system performance as well as computational power requirements are discussed. The basis for the comparison is a scaled converter test bench operating from 560-V dc voltage. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) has become one of the most promising converter topologies for future high-power applications. A challenging issue of the MMC is the voltage balancing among arm capacitors. A good overall control system is also vital for the MMC, which should be based on sound mathematical model, readily adaptable for different applications, and capable of high performance. This paper presents a general control structure for MMC inverters, which is suitable for both voltage-based and energy-based control methods, and includes voltage balancing between the upper and lower arms. A new method for voltage balancing among arm capacitors, which is based on an improved pulse-width modulation, is also presented. The proposed method avoids some major disadvantages found in present voltage balancing methods, such as dependence on computation-intensive voltage sorting algorithms, extra switching actions, interference with output voltage, etc. Furthermore, all switching actions are evenly distributed among power devices. The proposed control system as a whole can serve as a promising solution for practical applications, especially when the number of submodules is fairly high. Simulation and experimental results verify the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]