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The modular multilevel converter (MMC) has become the most promising converter topology for medium- and high-power applications. In this paper, a sliding mode control (SMC) based method is proposed for the MMC. Based on analysis of the system dynamics under the proposed control method, relations among control parameters and their validity conditions are obtained, which provide a guidance for systematic controller design. Compared with the conventional proportional-integral based method, the proposed SMC-based method provides comparable steady state performance and faster dynamic responses, without compromising the computational effort or requirement for precise system model. Simulation studies in the PSCAD/EMTDC software environment are carried out to validate the performance and effectiveness of the proposed control method in the context of an HVdc transmission system. A laboratory prototype is also developed to experimentally validate the proposed control method. [ABSTRACT FROM AUTHOR]

This paper presents a general space vector modulation (SVM) method for the modular multilevel converter (MMC). Compared with earlier modulation methods, the proposed SVM method not only utilizes the maximum level number (i.e., $2n+ 1$, where $n$ is the number of submodules in the upper or lower arm of each phase) of output phase voltages, but also leads to an optimized control performance in terms of capacitor voltage balancing, circulating current suppression, and common-mode voltage reduction. The maximum level number is achieved by introducing a new equivalent circuit of the MMC, and the optimized control is obtained by selecting the optimal redundant switching states. Since the computational burden of the SVM scheme is independent of the voltage level number, the proposed method is well suited to the MMC with any number of submodules. Simulation and experimental results are presented to validate the proposed method. [ABSTRACT FROM PUBLISHER]

The three-level neutral point-clamped (NPC) converter has attracted much attention in high-power medium-voltage applications, but it is well-known that the NPC topology suffers from unequal power loss distribution among its semiconductor devices. The power loss unbalance problem becomes especially severe when the converter is operating at low fundamental frequencies. To overcome the loss unbalance problem of the NPC converter, the active NPC (ANPC) technique was introduced. However, it is demonstrated in this paper that in many cases, the conventional modulation method for the ANPC converter only marginally relieves the loss unbalance problem. This paper proposes a new modulation scheme (named the adaptive doubled frequency modulation) for the three-level ANPC converter, which significantly improves the power loss and thermal sharing among the semiconductor devices. The basic idea of the proposed scheme is adaptively adjusting the duty cycles of the switching states for every switching cycle, so as to optimize the power loss distribution. Simulation and experimental results verify this new method. [ABSTRACT FROM AUTHOR]

One of the main technical challenges associated with the modular multilevel converter (MMC) for any application is the optimal control of its states. Optimal control of the MMC requires determination of the following: 1) an accurate MMC model, 2) the MMC states to be controlled and their corresponding reference trajectories, and 3) an appropriate controller type/form and its parameters. The aforementioned tasks lead to the formulation of a multistate control optimization problem, which is tackled in this paper. This paper enhances the accuracy of the existing MMC state-space model by including the following: 1) a piecewise affine insulated-gate bipolar transistor/diode model and 2) the impact of dead time. This paper also proposes a simulation-based gradient-descent optimization algorithm (cosimulation) to optimize the controller gains, given the controller type/form and the reference state trajectory. The theoretical proofs of the gradients required in the optimization algorithm are also provided. The aforementioned optimization algorithm is applied to a variable-frequency MMC-based drive system. Experimental results on a scaled-down prototype and simulation results on a real-world application are provided to validate the accuracy of the proposed MMC model and to show the effectiveness of the proposed optimization algorithm. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is increasingly becoming popular for multi-MW drive systems. One of the main technical challenges associated with the operation of MMC for adjustable-speed drives is the large magnitude of submodule (SM) capacitor voltage ripple under constant-torque low-speed operation. This paper proposes two new control strategies to reduce the magnitude of the SM capacitor voltage ripple in the MMC-based adjustable-speed drive systems under constant-torque low-speed operation. The proposed control strategies are based on injecting a square-wave common-mode voltage at the ac-side and a circulating current within the phase-legs to attenuate the low-frequency components of the SM capacitor voltages. The frequency spectrum of the injected circulating current consists of components in the vicinity of either the common-mode frequency or the common-mode frequency and third harmonic of the common-mode frequency. This paper also provides: i) a theoretical comparison of the proposed control strategies with the existing ones; ii) a controller design methodology to systematically determine the controller gains of the proposed control strategies; and iii) a theoretical proof of stability of the proposed control strategies and their design methodology based on Lyapunov analysis of singularly perturbed nonlinear non-autonomous systems. A set of experimental results for various case studies on a laboratory-scale prototype are provided to support the theoretical proof of stability of the proposed control strategies and their design methodology, and to show the superior performance of the proposed strategies over the existing strategy. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) is the most promising converter topology for medium- and high-power applications. One of the main concerns in the operation of the MMC, particularly for high-power applications, is its efficiency, which should be maximized. Silicon Carbide (SiC)-based devices have the potential to provide significant efficiency improvement compared with silicon devices. However, the possibility and impact of using SiC-based devices instead of silicon devices for high-power conversion have not been thoroughly explored. This paper reports on the results obtained from a detailed study to evaluate the performance of MMCs based on medium-voltage SiC MOSFETs and diodes with hybrid MMCs that employ silicon IGBTs and SiC diodes. The results are based on detailed circuit simulations that use simple physics-based circuit models. The study suggests the potential for significant efficiency gain for MMCs based on SiC power devices. [ABSTRACT FROM AUTHOR]