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.

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.

The trend towards multimegawatt (multi-MW) wind turbines and the increasing interest in direct-drive variable-speed wind energy systems have made multilevel converters a promising candidate for large wind energy conversion systems. This paper presents a new hybrid modular multilevel converter for interfacing a full-scale, permanent magnet synchronous generator (PMSG)-based direct-drive variable-speed wind energy conversion system (WECS). The proposed hybrid converter, which is used on the grid side of the system, consists of a three-level modular multilevel converter (MMC) in series connection with three H-bridge modules. The generator-side converter is based on a conventional three-level neutral-point-clamped converter. The proposed hybrid converter, as opposed to the existing full-scale multilevel converter-based wind energy systems, provides structural modularity and a higher dc-bus voltage utilization. This paper formulates and analyzes the internal dynamics of the proposed hybrid converter including the circulating currents and the capacitor voltage ripples. The ac components of the circulating currents, if not properly reduced, increase the amplitude of the capacitor voltage ripples, rating values of the converter components, and losses. Based on the analysis, closed-loop circulating current and capacitor voltage ripple reduction techniques are developed. The reduction of capacitor voltage ripples help reduce the capacitor value. A mathematical model is also developed for the overall WECS. Performance of the overall WECS, under the proposed multilevel converter-based topology and controls, is evaluated based on time domain simulations in the PSCAD/EMTDC environment.

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.

The dc-dc modular multilevel converter (MMC), which originated from the ac-dc MMC, is an attractive converter topology for the interconnection of medium-/high-voltage dc grids. This paper presents design considerations for the dc-dc MMC to achieve high efficiency with reduced component sizes. A steady-state mathematical model of the dc-dc MMC in the phasor domain is developed. Based on the developed model, a design approach is proposed to size the components and to select the operating frequency of the converter to satisfy a set of design constraints while achieving high efficiency. The design approach includes sizing the arm inductor, submodule capacitor, and phase filtering inductor along with selection of the ac operating frequency of the converter. The accuracy of the developed model and the effectiveness of the design approach are validated based on the simulation studies in the PSCAD/EMTDC software environment. The analysis and developments of this paper can be used as a guideline for designing the dc-dc MMC.

Inclusion of power electronics allows increased controllability and stability in power systems. The simulation of such systems on a large-scale is challenging due to the presence of a large number of switches and nonlinear devices. This paper presents an advanced simulation algorithm to solve the aforementioned problem. The algorithm considers separation of differential algebraic equations (DAEs) on the basis of numerical stiffness and applies hybrid discretization algorithms to simulate the DAEs. The DAEs, in this paper, represent the nonlinear nonautonomous switched system dynamics of power systems. Stability analysis is performed on a general class of nonlinear nonautonomous switched systems to show the constraints under which the proposed algorithm is stable. To show the validity of the proposed algorithm, two case studies are considered: 1) single high-voltage direct current (HVdc) substation based on the modular multilevel converter (MMC); and 2) an example three-terminal MMC-HVdc system. Relaxation techniques are introduced to create a stable interface for the separated DAEs. The developed algorithms are also validated with PSCAD/EMTDC-detailed reference models.