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.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

Modular multilevel converter (MMC) is considered as a promising topology for voltage-source converter (VSC) high-voltage, direct current (HVDC) applications. This paper presents a new control strategy for MMC-HVDC under unbalanced grid conditions. First, a new inner loop current control strategy based on nonideal proportional resonant (PR) controllers in stationary αβ frame is designed, which is more concise compared to the existing dual sequence current control scheme. Second, an analytical expression for circulating current is obtained which shows that the circulating currents will be asymmetric under unbalanced grid conditions and can be decomposed into positive-, negative-, and zero-sequence component. In order to suppress all these components, a new circulating current suppressing strategy is analyzed and designed also based on nonideal PR controllers. Application of nonideal PR controllers makes the control system well adapt to the fluctuation of grid frequency. The effectiveness of the proposed control strategy is verified through a simulation case of a 251-level MMC-HVDC transmission system using real-time digital simulator.

Circulating Current Suppressing Strategy for MMC-HVDC Based on Nonideal Proportional Resonant Controllers Under Unbalanced Grid Conditions

This paper proposes a model predictive control (MPC) method for modular-multilevel-converter (MMC) high-voltage direct current (HVDC). To control the MMC-HVDC system properly, the ac current, circulating current, and submodule (SM) capacitor voltage are taken into consideration. The existing MPC methods for the MMC-HVDC system utilize weighting factors to configure the cost function in combinations of the SM capacitor voltage balancing algorithm, ac current control, and circulating current control. Because all combinations of the switch states are considered in order to minimize the cost function, their possible combinations increase geometrically according to the increase of the level of the MMC, which is a significant disadvantage. This paper proposes a new MPC method with a reduced number of states for ac current control, circulating current control, and the SM capacitor voltage-balancing algorithm. The proposed cost functions are divided into three types according to their control purposes. Each cost function determines the minimum number of states for controlling the ac current, circulating current, and SM capacitor voltage. The efficacy of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.

Model Predictive Control With a Reduced Number of Considered States in a Modular Multilevel Converter for HVDC System

This chapter covers the fundamental multilevel converter structures and modulation paradigms. The basic principles of the four main different multilevel converters have been discussed methodically: (1) cascaded H-bridges, (2) diode clamp, (3) flying capacitors, and (4) modular multilevel converter. Various modulation schemes for multilevel converters are also covered including fundamental frequency switching and harmonic elimination and several carrier-based and space vector-based pulse-width modulation (PWM) methods. The chapter also includes significant content on the operation and control of modular multilevel converter (MMCs) including techniques to balance capacitor voltage and reduce circulating current in converter arms.

Multilevel Power Converters

This paper proposes a control method for circulating currents in modular multilevel converters (MMCs) under unbalanced voltage conditions. Under balanced voltage conditions, only negative-sequence circulating currents exist. Consequently, the conventional method has considered only negative-sequence circulating currents in MMC. However, under unbalanced voltage conditions, there are positive-sequence circulating currents, zero-sequence circulating currents, and negative-sequence circulating currents in MMC. Thus, under unbalanced voltage conditions, a control method should consider all of these components. This study adopts a dual vector current controller as an ac-side current controller to reduce the ac-side active power ripple under unbalanced voltage. It analyzes the effect of the unbalanced voltage on circulating currents in MMC and then proposes a control method considering each component of circulating currents under the unbalanced voltage. The effectiveness of the proposed controlling method is verified through simulation results using PSCAD/EMTDC.

Circulating Current Control in MMC Under the Unbalanced Voltage

This paper proposes a control method for high voltage direct current(HVDC) with modular multilevel converter (MMC) under unbalanced voltage conditions considering the submodule(SM)`s current capacity and circulating current It is aimed to propose a control method in which the current peak value does not exceed the maximum value of HVDC-MMC by considering the current capacity of the SM under unbalance voltage conditions And it analyzes the effect of the unbalanced voltage on circulating currents in MMC and then proposes a control method considering each component of circulating currents under unbalanced voltages The effectiveness of the proposed controlling method is verified through simulation results using PSCAD/EMTDC

http://www.ndsl.kr/soc_img/society/kipe/JRJJC3/2013/v18n3/JRJJC3_2013_v18n3_270.pdf

Dea-Wook Kang

Papers

Circulating Current Control in MMC Under the Unbalanced Voltage

Control of HVDC-MMC Considering the Switching Device`s Current Capacity and Circulating Current under Unbalanced Voltage Conditions