Modular Multilevel Converter Research Articles

A Review on Modular Multi-level Converter Based HVDC Systems

Modular Multilevel Converters (MMCs) have emerged as a promising technology for High Voltage Direct Current (HVDC) transmission systems, offering several advantages such as reduced harmonic distortion, high voltage capability and modularity. This paper presents a comprehensive review of MMC-based HVDC systems, focusing on their applications, control strategies and challenges. The paper begins by discussing the basic principles and structure of MMCs, highlighting their modular design and operation. The paper delves into the control strategies employed in MMC-HVDC systems, emphasizing the importance of coordinated control between the AC and DC sides to ensure stable and efficient operation. Furthermore, the paper addresses the challenges associated with MMC-HVDC systems, such as fault ride-through capability, harmonic interactions and grid integration. It highlights recent advancements in control techniques and technologies that are being developed to address these challenges. The paper provides a valuable overview of MMC-based HVDC systems, highlighting their potential benefits and the ongoing research efforts to overcome their challenges. As the demand for renewable energy sources and long-distance power transmission increases, MMC-HVDC systems are poised to play a crucial role in shaping the future of the power grid

A lightweight MMC topology with recombined half‐bridge submodules for DC fault ride‐through

AbstractThe lightweight of modular multilevel converter (MMC) and the DC faults ride‐through ability are main challenges for MMC‐high voltage direct current (HVDC) transmission systems. By introducing the concept of time‐division multiplexing, an arm multiplexing MMC (AM‐MMC) topology with high utilization of submodules is presented to reduce the weight and volume of MMC. In order to block the DC side fault current, this paper proposes a novel submodule in AM‐MMC, instead of using full‐bridge submodules. The proposed recombined half‐bridge submodules of AM‐MMC (RHAM‐MMC) contains four half‐bridge submodules and an IGBT with reverse parallel diodes. The topology and operating principle of RHAM‐MMC are introduced in detail. The time‐division multiplexing of middle arms between upper and lower arms is achieved by introducing arm selection switches. Thus, a new type of arm switch and switching method is designed based on the switch state. The DC faults ride‐through strategy is carried out based on its DC fault characteristic analysis. In addition, the economy analysis is conducted on the switching loss and operating loss of RHAM‐MMC. Compared with the fault ride‐through capability of other sub‐modules (SMs), RHAM‐MMC performs better in terms of investment cost and device losses. The simulation results based on MATLAB/Simulink reveal that RHAM‐MMC can achieve the DC side fault ride‐through and show effectiveness of the DC fault ride‐through control strategy.

Thyristor Branch Loop-Based Hybrid Modular Multilevel Converters for DC Short-Circuit Faults

Thyristor Branch Loop-Based Hybrid Modular Multilevel Converters for DC Short-Circuit Faults

Research on Sub-Module Fluctuating Power Decoupling Strategy for Modular Multilevel Converter-Based Medium-Voltage Motor Drive System with Wide Speed Range

There is a problem of excessively large sub-module capacitance in modular multilevel converter (MMC) applications for medium-voltage motor drive systems, which is particularly noticeable during the low-speed operation of the motor. This paper proposes a low-capacitance MMC (LC-MMC) motor drive system based on lateral sub-module interconnection, achieving capacitance lightweighting across the wide speed range of the motor. The LC-MMC is based on the three-phase symmetrical fluctuating current input of the sub-modules in the interconnection channel, achieving their mutual cancellation, thus significantly reducing the sub-module capacitance size. This paper starts from the traditional MMC motor drive system and analyzes its specific requirements for sub-module capacitance. Then, it provides a detailed description of the LC-MMC scheme for achieving capacitance lightweighting across the wide speed range of the motor, and the two MMC motor drive systems are compared and evaluated. The evaluation results show that the volume of the LC-MMC is reduced by nearly 70% compared to a traditional MMC. Finally, the LC-MMC scheme is simulated and experimentally verified.

Comparative analysis of submodules and design of seventeen-level modular multilevel converter using a five-level submodule block

Abstract Multilevel inverters and various modulation techniques have been proven to be the best solutions to overcome all the limitations of conventional two- or three-level voltage source inverters. In recent years, among all topologies, modular multilevel converter (MMC) has advantages like low total harmonic distortion (THD), reduced filter requirement, fault-tolerant operation, scalability, modularity, transformerless operation, etc. The main application of MMC is found in high-voltage DC transmission (HVDC). This study describes and analyses the performance of two distinct MMC submodule (SM) topologies: the Half Bridge submodule (HBSM) and the Full Bridge submodule (FBSM). Switching pattern is simpler in HBSM topology but it has the disadvantage that it does not have DC fault current blocking capability. Whereas FBSM not only inherently can block the DC fault current but also has advantages like reduced volume of MMC and better performance. Further, having the capability to produce three voltage levels, it can be operated in boost mode. Also, by using a proper switching pattern, MMC can be designed to generate a various number of output levels by using the same five-level submodule block. Out of which 17 level MMC gives the best output. The MATLAB-Simulink model is used to simulate the circuit, and the findings are confirmed for a 17-level output voltage.

Design and Implementation of Modular Multi-Level Converter Based HVDC System for Grid Connection

This paper examines Modular Multilevel Converters (MMC) for harnessing power from offshore wind farms. MMCs use many simple Voltage Source Converter (VSC) submodules, enabling high-voltage and high-power applications. They offer faster response times and lower harmonic distortion than traditional two-level VSCs. The paper addresses modelling challenges due to numerous switching devices and discusses the development of Cascaded Two-Level (CTL) converters, which improve output voltage quality. Overall, the study highlights advancements in MMC technology that enhance HVDC transmission capabilities.

Advanced Venturini control for PMSM with modular multilevel matrix converter

This paper provides a comprehensive analysis of control strategies for modular multilevel matrix converters (MMMC) utilizing advanced second-order sliding mode control (ASOSMC) within the context of permanent magnet synchronous motor (PMSM) applications. The research introduces a novel control framework based on the Venturini method, which, when integrated with ASOSMC, achieves a unity input power factor while generating sinusoidal phase current waveforms with markedly reduced harmonic distortion. These advancements not only enhance overall system performance but also significantly improve power quality, addressing critical requirements in contemporary electrical engineering. Additionally, the study conducts a rigorous comparative analysis between classical Proportional Integral (PI) control and ASOSMC, elucidating the substantial limitations of PI control in nonlinear environments where dynamic adaptability is paramount. In contrast, ASOSMC demonstrates superior capability in managing disturbances and nonlinearities, thereby offering enhanced reliability and efficiency for MMMC systems. The findings illustrate that ASOSMC not only addresses the challenges posed by traditional control techniques but also contributes to the development of more robust control methodologies within power electronics. By establishing a clear superiority of ASOSMC over conventional methods, this research paves the way for future innovations in control strategies specifically tailored for high-performance applications that require resilience under varying operational conditions. Ultimately, this work underscores the necessity of integrating advanced control techniques to optimize the operational capabilities of modular multilevel converters, facilitating the creation of sophisticated solutions that meet the evolving demands of electrical systems and power quality enhancement in diverse applications. The implications of these findings are significant, offering foundational insights for both theoretical exploration and practical implementation in the field.

PV Based Single Stage Cascaded Seven Level Modular Multilevel Converter with ZSV Control for Grid Interfaced System

Objectives: Climatic perturbation and environmental factors affect solar power generation. When these solar panels are connected to the grid it causes power quality issues such as voltage harmonics, voltage distortion, etc. These problems are addressed in this paper using a distributed MPPT algorithm. Methods: Multilevel inverters have made remarkable contributions to renewable energy, high voltage, and other high-power applications. Hence, a single-stage Cascaded Seven-Level Modular Multilevel Converter (CSLMMC) is used to address the issues. Zero Sequence Voltage (ZSV) control is proposed for producing balanced power during varying irradiance conditions. The proposed method is simulated using MATLAB SIMULINK software. Partial shading is conquered by using distributed MPPT. Findings: The simulation of the proposed system is performed for a three-phase system. The three-phase Cascaded Modular Multilevel Converter is connected to the grid through two loads which are controlled by a Zero Sequence Voltage Controller. Power quality parameters such as current, harmonics, and power are analyzed for the proposed system. Novelty: THD analysis for a five-level cascaded H bridge converter and the proposed systems are compared and presented. Keywords: Photo voltaic, Cascaded seven-level modular multilevel converter, Zero sequence control, Maximum power point tracking, Voltage Source Converter

A voltage-balancing algorithm based on the gating signal rotation and Separated Circulating Current Controller applied on a Modular Multilevel Converter

The Modular Multilevel Converter (MMC) is being used for many medium/high-power energy conversion applications due to its superior advantages such as modularity, scalability, and low harmonic generation. However, the MMC control can be challenging. If the submodule (SM) capacitors are not properly balanced, a circulating current is induced due to the voltage mismatch between the capacitor submodules. This circulating current adds to the arm current, which increases its RMS and peak value, and thus increases system losses. Therefore, controlling the submodule capacitor voltage and the circulating current is an important step in any MMC control. In this paper, a Gating Pattern Rotation algorithm is used for SM capacitor voltage balancing, and an independent circulating current controller in the double reference frame is used to eliminate the circulating current. Furthermore, a voltage-oriented control is employed to eliminate harmonics and regulate the grid power factor. MATLAB Simulink simulates the control technique, and various simulation results are presented. With results showing their effectiveness in managing and improving MMC behavior.

Novel modular multilevel matrix converter topology for efficient high-voltage AC-AC power conversion

Introduction. This paper delves into the practical application of multilevel technology, particularly focusing on the capacitor-clamped converter as a promising solution for medium-to-high voltage power conversion, with specific emphasis on direct AC-AC switching conditions. Problem. The limitations of conventional single-cell matrix converters (MC) in efficiency and performance for medium-to-high voltage power conversion applications are well-recognized. Goal. The primary objective is to investigate the performance of the 3 phase modular multilevel matrix converter (3MC) with three flying capacitors (FCs) modeling. This investigation utilizes the Venturini method for gate pulse generation, aiming to compare the performance of the 3MC with standard converter designs. Methodology. To achieve the research goal, the Venturini method is adopted for generating gate pulses for the 3MC, representing a departure from conventional approaches. Detailed simulations employing MATLAB/Simulink are conducted to comprehensively evaluate the performance of the 3MC in comparison to conventional converter designs. Results. The simulation outcomes reveal a significant reduction of 73 % in total harmonic distortion (THD) achieved by the 3MC. This reduction in THD indicates improved robustness and suitability for medium-to-high voltage power conversion systems necessitating direct AC-AC conversion. These results highlight the efficacy of the 3MC in enhancing power conversion efficiency and overall performance. Originality. This paper contributes novel insights into the practical implementation of multilevel technology, particularly within the realm of capacitor-clamped converters. Furthermore, the utilization of the Venturini method for gate pulse generation in the 3MC represents an original approach to enhancing converter performance. Practical value. The research findings present significant advancements in multilevel transformer technology, offering valuable guidance for optimizing transformer design in various industrial and renewable energy applications. These contributions serve to enhance the development of reliable and efficient power systems, addressing critical needs in the energy sector. References 54, tables 3, figures 4.

Multi-Terminal DC Transformer for Renewable Energy Cluster Grid Connection

An AC (alternative current) power integration of distributed energies faces multi-fold challenges such as synchronization and weak grid-induced instability. In this study, a multi-terminal DC transformer is proposed for renewable energy clusters grid connection. The DC transformer provides multiple DC input ports for renewable energy collection, while the load port is connected to the medium-voltage DC grid via a modular multilevel converter (MMC). The multi-port topology enables flexible power transfer between multiple input sources to the load without additional components. The carrier layer modulation strategy is implemented to balance the MMC module voltage; bidirectional power transmission between multiple input sources is achieved through the phase shift modulation (PSM) method. First, we provided a detailed introduction of the proposed topology and working principle. A simulation model was built using the SIMULINK, and the simulation results verified the effectiveness of the proposed converter modulation strategy and phase shift modulation method. A corresponding hardware experimental platform was designed and built, and the modulation and voltage equalization functions of modular multilevel rectifiers were presented as well as the measured results of power transmission modulation of the converter under single-input and multi-input conditions. The results indicate that the proposed transformer can achieve multiple DC inputs and has multi-channel power transmission capabilities, making it suitable for renewable energy clusters grid connection.

Analysis of Optimal HVDC Back-to-Back Placement Based on Composite System Reliability

HVDC is a promising interconnection solution for connecting asynchronous systems and ensuring power control. In Indonesia, a remote industrial system in Sumatra is experiencing load growth and has the option to draw power from the Sumatra system. However, due to frequency differences, the use of HVDC is crucial. The Generation Expansion Planning has proposed six converters but not their interconnection points. This study will determine the most reliable interconnection locations. The chosen converters are modular multilevel converters (MMCs) with high modularity. The converter reliability modeling considers voltage levels, the number of modules, and redundancy strategies. This modeling is then implemented at the power system level to obtain the best placement at the available high-voltage (HV) substation options. Determining the best placement is based on the optimal reliability index. The optimal placement also includes the option to convert from HV to medium-voltage (MV) interconnection. MV interconnection offers higher flexibility but tends to be more expensive. The availability for HV converters is 99.69%, while for MV converters, it is slightly higher, at 99.81%. Additionally, converting from HV to MV reduces the SAIFI (system average interruption frequency index) from 0.2668 to 0.2284 occurrences per year, lowering the interruption cost from 7.804 million USD to 5.737 million USD per year. The sensitivity of interruption, investment, and maintenance costs shows that converting at least one HV converter to MV remains economical. In this case study, the optimal converter placement includes Area VI–2, recommended for conversion from HV to a more distributed MV configuration, improving reliability and economic efficiency.

A control strategy of DC transmission system for offshore wind power based on hybrid converters

Abstract As the DC transmission system of modular multilevel converters for offshore wind power suffers problems like large volume, heavy weight, difficulty in platform transportation, and high cost, a DC transmission scheme for offshore wind power based on diode rectifier units and modular multilevel converters is put forth in the present work; Meanwhile, the features of active-power coordination, the DC harmonic current, and the AC harmonic current of the hybrid converter in this proposed scheme are analyzed to propose control strategies for active power coordination control, DC harmonic current suppression, and AC harmonic current suppression of the hybrid converter, respectively. The performance and feasibility of the proposed strategies are verified by testing the hardware in the loop real-time simulation system for transmission of offshore wind power through hybrid converters based on RTDS. It is found that the proposed strategies could improve the stability, flexibility, and reliability of the system.

Theoretical and simulation analysis of DC bus current fluctuation control strategies of modular multilevel converter under AC side fault conditions

Modular Multilevel Converters (MMCs) play a crucial role in high-voltage DC (HVDC) systems due to their adaptable control and swift response capabilities. However, AC side faults could introduce asymmetric double-line frequency components into the MMC's circulating current, jeopardizing DC bus stability. Although strategies to mitigate DC side fluctuations are well-established, a detailed comparison of their operational impacts remains unexplored. The operational effects of MMCs under AC side faults are firstly examined in this study, including issues related to grid-connected current asymmetry and DC bus current fluctuations. Subsequently, two targeted control schemes are devised based on the goals of suppressing the double-line frequency components and only the zero-sequence components of circulating current. Through simulation, these approaches are contrasted, delineating their advantages and disadvantages in terms of circulating current suppression, submodules(SMs) capacitance voltage fluctuation, and other performance metrics, which could provide a foundational reference for designing and selecting MMC control strategies under AC side faults.

A hybrid pulse-width modulation technique for a modular multilevel converter with embedded energy sources

A hybrid pulse-width modulation technique for a modular multilevel converter with embedded energy sources

An inherent fault current blocking modular series multilevel converter using half-bridge sub-modules for grid integration

This paper proposes an inherent fault current blocking modular series multilevel converter (MSMC) using half-bridge (HB) sub-modules (SMs) for grid integration. The proposed converter uses passive filters to decouple the AC and DC side. Compared to conventional three-phase modular multilevel converters (MMCs) that use half-bridge sub-modules (HBSMs), the new MSMC design significantly reduces the number of switching devices required. Furthermore, one of the features of the proposed MSMC is its inherent ability to limit fault currents, which is achieved through its unique topological configuration whereas the conventional HB-MMC is susceptible to DC and AC faults. Despite these advancements, the new MSMC design retains all the desirable characteristics of conventional HB-MMCs, ensuring compatibility and ease of adoption. The paper thoroughly examines the operation of the MSMC under both steady-state and fault conditions. It also discusses the equivalent voltage decoupling circuit, energy balancing which is essential for the proper functioning of the converter. Additionally, the pre-charging process of the sub-module capacitors, a critical step for ensuring the safe start-up of the converter, is discussed in detail. Furthermore, Extended topologies can be derived from proposed topology for different applications.

A novel and easy‐to‐implement modulation scheme for reducing modulation complexity in modular multilevel converter

AbstractModulation strategy plays a key role in the operation of modular multilevel converters (MMC). Although many studies have been carried out on its optimization in MMC, the limitations of hardware implementation are seldom considered. This paper presents a novel and easy‐to‐implement modulation strategy that simplifies both hardware design and application. From the analysis in this paper, the widely‐used modulation methods have the problems of pulse width modulation (PWM) signal feedback and complex carrier configuration in hardware implementation. By setting up a flexible insertion and removal submodule and proposing a capacitor voltage balance algorithm based on duty ratio allocation, the proposed modulation method can allow for the separate design of the Algorithm Unit and the PWM Unit in the hardware, which solves the problem of PWM feedback and reduces the hardware requirement of the trigger system. Moreover, the proposed method can broaden the selection range of processor chips. Finally, a downscaled MMC prototype is built; the proposed method is verified by both simulation and experiment.

An Optimized Power-Angle and Excitation Dual Loop Virtual Power System Stabilizer for Enhanced MMC-VSG Control and Low-Frequency Oscillation Suppression

Modular Multilevel Converter Virtual Synchronous Generator (MMC-VSG) technology is gaining widespread attention for its ability to enhance the inertia and frequency stability of the power grid integrated with converter-interfaced renewable energy sources. However, the excitation voltage regulation in the MMC-VSG can generate equivalent negative damping torque and cause low-frequency oscillation problems similar to those in synchronous machines. This article aims to improve the system’s damping torque and minimize low-frequency oscillations by introducing a Virtual Power System Stabilizer (VPSS) into the power control loop. Building on the study of dynamic interactions between various control links of the MMC, this research establishes a reduced-order model (ROM) and a Phillips–Heffron state equation for the MMC-VSG single machine infinite bus system, using a hybrid modeling approach and a zero-pole truncation method. It also analyzes the mechanism of low-frequency oscillations in the MMC-VSG system through the damping torque method. The analysis reveals that the negative damping torque produced during the excitation voltage regulation process causes changes in the virtual power angle, which in turn increases the risk of low-frequency oscillation in the MMC-VSG. To address this issue, the article proposes an optimized control method for the MMC-VSG dual power loop architecture (power-angle/excitation) VPSS. This strategy compensates for the inadequate damping torque of a single loop VPSS and effectively suppresses low-frequency oscillations in the system.

Comprehensive assessment of voltage and current source PV‐based modular multilevel converters

AbstractRecently, modular multilevel converters (MMCs) gained popularity for the grid integration of photovoltaic (PV), due to their many advantages, including low total harmonic distortion, high control flexibility, and distributed maximum power point (MPP) tracking capability. Two distinguished families of MMCs exist: voltage source and current source. Voltage source MMCs are mostly studied, but current source MMCs offer advantages under certain operating conditions. This article compares voltage and current source MMCs, for PV integration. To this aim, several key indicators are identified: number of components, energy stored in passive elements, semiconductor power rating, and the number of MPP trackers. The results of the analysis, performed in MATLAB©, show that for a fixed number of output voltage levels, power rating, and switching frequency, voltage source MMCs have simpler control and higher number of MPP trackers. In contrast, current source MMCs minimize the semiconductor power rating, the number of components and the energy stored in passive elements. Regarding efficiency, in the analyzed case study, voltage source MMCs perform better under both homogeneous and non‐homogenous irradiance conditions. This article provides a tool to select the optimal solution based on the required target (e.g. efficiency, energy storage etc.), given the specific characteristics of the application.

A Novel Modular Multilevel Converter Topology with High- and Low-Frequency Modules and Its Modulation Strategy

To resolve the issue of the difficultly in effectively balancing the output performance improvement, cost reduction, and efficiency improvement of a medium-voltage modular multilevel converter (MMC), a novel MMC (NMMC) topology based on high- and low-frequency hybrid modulation is proposed in this study. Each arm of the NMMC contains a high-frequency sub-module composed of a heterogeneous cross-connect module (HCCM) and N − 1 low-frequency sub-modules composed of half-bridge converters. The high-frequency bridge arm of the HCCM in this study adopts SiC MOSFET devices, while the commutation bridge arm and low-frequency sub-module of the HCCM adopt Si IGBT devices. For the NMMC topology, this study adopts a high/low-frequency hybrid modulation strategy, which gives full play to the advantages of low switching loss in SiC MOSFET devices and low on-state loss in Si IGBT devices. In addition, a specific capacitor voltage balance strategy is proposed for the HCCM, and the working state of the HCCM is analyzed in detail. Furthermore, the feasibility and effectiveness of the proposed topology, modulation strategy, and voltage balancing strategy are verified by experiments. Finally, the proposed topology is compared with the existing MMC topology in terms of device cost and operating loss, which proves that the proposed topology can better balance the cost and efficiency indicators of the device.