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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]

The modular multilevel converter (MMC) has become the most attractive converter technology for medium/high-power applications, specifically for high-voltage dc (HVdc) transmission systems. One of the technical challenges associated with the operation and control of the MMC-based system is to precharge the submodule (SM) capacitors to their nominal voltage during the startup process. In this paper, considering various SM circuits, a generalized precharging strategy is proposed for the MMC-based systems, which can implement soft startup from the dc or ac side. The proposed precharging strategy can be applicable for various SM circuits and MMC configurations. The proposed startup strategy does not require extra measurements and/or auxiliary power supplies. The charging current is controlled by adjusting the changing rate of the number of blocked and bypassed SM capacitors. Based on the proposed startup strategy, the startup processes of MMC/MMC-HVdc systems with various SM circuits are analyzed and a generalized startup procedure for various MMC-HVdc systems is proposed. In addition, the uncontrollable steady-state SM capacitor voltages of various MMC-based systems are analyzed and the associated precharging time is also investigated. Performance of the proposed strategy for various MMC-HVdc systems is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment and experimental results are based on a scaled-down prototype. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) has become one of the most promising converter topologies for medium/high-power applications. Since the MMC is structured based upon stacking up a number of series-connected identical submodules (SMs), to improve its fault tolerance and reliability, SM failure detection and location is of significant importance. In this paper, the impact of open-circuit switch failures of the SMs on the operation of the MMC is analyzed. Based on the analysis under SM failure conditions, two SM failure detection and location methods are proposed, that is, a clustering algorithm (CA)-based method and a calculated capacitance (CC)-based method. In the proposed CA-based method, a pattern-recognition-based fault diagnosis approach, which employs the clustering algorithm to detect and locate the faulty SMs with open-switch failures through identifying the pattern of 2-D trajectories of the SM characteristic variables, is developed. The proposed CC-based method is based on the calculation and comparison of a physical component parameter, that is, the nominal SM capacitance, and is capable of failure detection, location, and classification within one stage. The performance of the proposed failure detection methods for an MMC system is evaluated based on time-domain simulation studies in the PSCAD/EMTDC software environment. The reported study results demonstrate the capabilities of the two proposed methods in detecting and locating any SM failure under various conditions accurately and efficiently. [ABSTRACT FROM PUBLISHER]

The modular multilevel converter (MMC) has attracted significant interest for medium-/high-power energy conversion applications due to its modularity, scalability, and excellent harmonic performance. One of the technical challenges associated with the operation of the MMC is the circulation of double-frequency harmonic currents within its phase legs. This paper proposes a circulating current control strategy in a double-frequency rotating reference frame, which, contrary to the existing solutions that are based on approximate/inaccurate models, relies on an experimentally identified nonparametric model of circulating currents to determine the coefficients of the controller. Minimizing the squared second norm of the error between the open-loop transfer function of the system and a desired one, the coefficients of the controller are determined. To guarantee the stability of the closed-loop system, the minimization problem is subjected to a few constraints. The validity and effectiveness of the proposed control strategy is confirmed, and its dynamic performance is compared with that of an existing solution by experimental results. [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]