Modular Multilevel Converter Topology Research Articles

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.

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.

Design of main parameter of modular multilevel converter applied in power grid simulator

Abstract In the context of application in a power grid simulator, we investigated the parameter design issues of several crucial components of the Modular Multilevel Converter (MMC) topology applied to the power grid simulator system. These components include the bridge arm inductance and submodule capacitance. Based on the fundamental circuit of the MMC applied to the power grid simulator system, we introduced an equal discharge time constant H. We analyzed the capacitor voltage fluctuation rate and the equivalent capacitance discharge time constant “H”, discovering that they still exhibit an inversely proportional relationship across different engineering scenarios. Consequently, a general method for determining submodule capacitance parameters was proposed, and in conjunction with previous engineering parameters, recommended values for an equal discharge time constant of 40 ms were provided. Furthermore, we introduced the concepts of double-frequency circulating resonant angular frequency and phase resonant angular frequency, outlining the principles for determining the bridge arm reactance values. Through the analysis of corresponding parameters in previous engineering studies, universally applicable recommended values for the bridge arm reactance parameters were proposed, considering the unit resonant angular frequency as the power grid frequency angular frequency 1.0ω.

Optimal number of supercapacitors per submodule in the energy storage system based on a modular multilevel converter with embedded balance control

As renewable energy sources interconnected to the electric grid increase, deploying distributed energy storage systems along the grid is a viable solution to sustain grid stability. One of the critical aspects of any energy storage system is its efficiency in transferring power to the grid. Another important aspect is managing high power levels in a short time and being easily expandable in terms of energy capacity and power handling. For this reason, supercapacitors are suitable energy storage devices to fulfill these requisites. This article focuses on analyzing the losses and improving the efficiency of a supercapacitor energy storage system based on a modular multilevel converter, which accomplishes all the abovementioned functions and capabilities. The analyzed energy storage system is based on submodules, including the power electronic interface and the supercapacitors. Hence, the system can be easily expanded because the submodule provides the functions of balancing the energy storage devices, the DC/AC, and the DC/DC conversions by using the modular multilevel converter topology approach. Therefore, any number of submodules can be connected, and the operator can make an array of the desired power, energy, and voltage ratings. This study details a probabilistic loss analysis for the proposed supercapacitor energy storage system and presents a case study using 3000F supercapacitor cells for a 6.6 kW single-phase system. The improved submodule version is validated through simulations and experimentally with a lab-scale prototype. The results show how the proposed methodology increases the system's overall efficiency with a 95 % confidence level.

Hardware-in-the-loop setup for enhanced modular multi-level converter with reduced circulating currents

Owing to its essential features, such as modularity and exceptional power quality, the modular multilevel converter (MMC) emerges as the optimal converter topology for high-voltage direct current (HVDC) applications. Traditionally, MMCs are controlled through a method called nearest level modulation (NLM), which generates N+1 AC output voltages, where N represents the number of sub modules (SMs) per arm. In this paper, we introduce a modified NLM technique designed to yield 2N+1 and 4N+1 levels, with a focus on efficiently controlling internal dynamics. The proposed MMC is evaluated using a hardware-in-the-loop (HIL) environment to obtain real-time simulation outcomes. This MMC topology demonstrates a reduction in circulating currents and capacitor voltage ripple.

Investigation of a 25 kV–50 Hz Railway-Substation Power Supply Based on a Back-to-Back Modular Multilevel Converter Topology

This paper presents a preliminary study of a 25 kV–50 Hz railway substation power supply system. The control of a back-to-back converter based on modular multilevel converter (MMC) technology was investigated to fit with the power quality requirements of the application. One of the main challenges is the presence of constraining load conditions, under which the train circulation variability, low-frequency harmonics and critical power transients can notably decrease the power quality and lead to instability. In order to address this, cascaded controllers based on resonant controllers are proposed to ensure the desired performance. Furthermore, balancing voltage algorithms are added to avoid stress phenomena and additional losses in the studied power conversion interface. The paper presents the design of the control stages and demonstrates the robust performance of the system using a realistic loading condition of a railway substation.

An Embedded DC Power Flow Controller Based on Full-Bridge Modular Multilevel Converter

Meshed dc grids help improve the current network's configuration, flexible power management, and reliable power supply. However, the existing power converters implemented in dc grids do not have enough controllability in adjusting each feeder's power, leading to transmission congestion, overloads, and high-power loss issues. The DC power flow controller (DCPFC) is prominent in addressing problems with better system performance. This paper proposes an embedded DCPFC (eDCPFC) with full-bridge modular multilevel converter (FB-MMC) topology. The eDCPFC is directly integrated into a host MMC. The device's energy is balanced through the coordinated control of the eDCPFC and the host MMC without using isolation transformers or external power supplies. The working principle and the corresponding control strategies of the FB-MMC type eDCPFC and its host MMC are elaborated. Experimental results conducted in a scaled-down prototype validate the feasibility and effectiveness of the proposed topology and control schemes under typical working conditions.

A modified modular multilevel converter to reduce the second order ripples in the submodule capacitor voltage: Design and analysis

SummaryThe modular‐multilevel converter (MMC) faces the problem of second order ripples in the sub‐module DC capacitor voltage during the low frequency operation at the load side. To sort out this problem, this paper demonstrates a three‐phase modified MMC that includes a novel circuit placed between upper and lower arms of each phase, where load is connected through proposed circuit. The aim of the proposed circuit is to reduce the magnitude of the ripples in each sub‐module's DC capacitor voltage; moreover, it also uses a less number of components compared with the conventional circuits. In order to keep the capacitor voltage at its rated value and to prevent the development of common mode voltage, the proposed circuit uses the high frequency‐based power transfer control channel for balancing power between the upper or top and lower or bottom arms. Furthermore, the mathematical analysis, operating principle, and proposed control is explained in detail for the conventional MMC and modified MMC. The simulation results have been obtained for the modified MMC with resistive load by using MATLAB and Simulink. An experimental test results have been verified with simulation results for the modified MMC topology.

An Efficient Half-Bridge MMC Model for EMTP-Type Simulation Based on Hybrid Numerical Integration

Electromagnetic transient (EMT) simulation is critical and fundamental in the design and operation of the modular multilevel converter (MMC). This paper proposes a highly efficient EMT simulation model for MMC based on hybrid numerical integration. The topology and operational principle of MMC are elaborated first. Based on this, an efficient simulation model for MMC is proposed. Each arm of the MMC is reduced to a two-node Norton equivalent circuit in the main network simulation. The computation of the capacitor dynamic equations is decoupled from that of the arm inductor dynamic equation. They are computed in a leapfrog manner. This makes the equivalent conductance of the MMC arm constant and greatly improves the simulation efficiency. Moreover, the dynamic equations of the capacitors are also decoupled from each other. Various operational conditions of the MMC are also considered, and accuracy and adaptability are thus ensured. Extensive studies on power systems with different scales verify the accuracy and improved efficiency of the proposed EMT simulation model for MMC.

Sizing method of a novel hybrid energy storage considering adaptive inertia control

This paper introduces a novel hybrid energy storage system (HESS) with a focus on adaptive inertia control and its sizing methodology. The HESS is built upon the modular multilevel converter (MMC) topology, incorporating hybrid submodules, batteries, and supercapacitors (SC). The sizing method for the hybrid energy storage system in the proposed HESS is discussed, with a specific emphasis on its application in frequency response scenarios. The center of inertia frequency (COIF) is selected to represent the overall system's frequency characteristic. By examining COIF deviation and the Rate of Change of Frequency (ROCOF) during contingency events, the inertia and damping parameters of the HESS are designed. Additionally, an adaptive inertia approach is incorporated to further reduce the SC capacity. The analysis of COIF dynamics under contingency with adaptive inertia is performed, and an analytical expression of COIF dynamics is derived. Subsequently, a sizing method for the SC and battery in the HESS, considering adaptive inertia, is proposed. The effectiveness of the proposed sizing method is verified through simulation results, demonstrating its suitability for optimizing the design of the hybrid energy storage system in the novel HESS configuration.

Interconnecting industrial multi-microgrids using bidirectional hybrid energy links

Sharing and exchange energy among nearby industrial microgrids are crucial, especially with high energy requirements for their production targets and costly energy storage systems that may be oversized for their operations. Facilitating energy exchange can provide an economic advantage for industrial production by utilizing cheaper energy sources and reducing production costs. This manuscript presents an efficient approach for transferring large energy packets with minimal energy losses using high-voltage direct current (HVDC) energy transmission. The manuscript methodology focuses on implementing an industrial multi-microgrid using a modular multilevel converter. This converter utilizes two power link channels: a three-phase AC and an HVDC link, creating a hybrid energy transmission between microgrids. When a substantial amount of energy to transfer, the HVDC method enhances overall efficiency by reducing copper losses and mitigating issues associated with the AC link, such as harmonics and skin effects. The modular multilevel converter topology offers high flexibility and the use of fewer converters. Additionally, the HVDC link eliminates distance restrictions for energy transfer between industrial microgrids. A case study illustrates the functionality of this topology, demonstrating optimized power transfer and decreased energy losses. This methodology allows industrial microgrids to enhance energy efficiency and productivity while minimizing operational costs.

Low-frequency operation control method for medium-voltage high-capacity FC-MMC type frequency converter

The application of modular multilevel converters (MMCs) to large drive systems is subject to severe low-frequency operation restrictions. The fluctuation of capacitor voltage and the high amplitude of common mode voltage in sub-modules is a thorny problem. In this paper, a novel fly-across capacitor modular multilevel converter topology is adopted to eliminate low-frequency voltage ripples by using the fly-across capacitor as a power transfer channel between the upper and lower bridge arms of the MMC. A novel finite compensation method is proposed—instead of the traditional full compensation method—that introduces a real-time variable limiting factor to change the amplitude of the mixed injected high-frequency differential-mode voltage and high-frequency differential-mode current and reduce the amplitude of the common-mode voltage on the AC side while lowering the current stress of the power devices. Finally, a complete system simulation model is constructed, and the topology with the proposed control strategy are verified to have good output characteristics under different operating conditions; good results are achieved in suppressing the sub-module fluctuation and common-mode voltage.

Loss distribution analysis and accurate calculation method for bulk-power MMC

Accurate evaluation of power losses in a modular multilevel converter (MMC) is very important for circuit component selection, cooling system design, and reliability analysis of power transmission systems. However, the existing converter valve loss calculation methods using the nearest level modulation (NLM) method and the traditional sorting-based capacitor voltage balancing strategy are inaccurate since the submodule (SM) switching logics in the MMC arms are uncertain. To solve this problem, the switching principle of the SMs in the sorting-based voltage balancing strategy is analyzed. An accurate MMC power loss calculation method based on the analysis of loss distribution of various SM topologies, including half-bridge submodule (HBSM), full-bridge submodule (FBSM) and clamp double submodule (CDSM), is proposed in this paper. The method can accurately calculate the losses caused by the extra switching actions during the capacitor voltage balancing process, thus greatly increasing the calculation accuracy of switching losses compared with existing methods. Simulation results based on a practical ± 350 kV/1000 MW MMC-HVDC system with variety of MMC topologies with different voltage balancing strategies demonstrate the effectiveness of the proposed method.

A hybrid DC-DC modular multilevel converter with capacitors parallel connectivity for arm energy balancing

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms’ capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach.

A Lightweight Hybrid Modular Multilevel Converter Topology for DC Fault Blocking

A Lightweight Hybrid Modular Multilevel Converter Topology for DC Fault Blocking

A Modular Multilevel Converter Station With Multiple MVac Ports

Medium voltage (MV) ac/dc hybrid distribution network attracts much interest due to its high reliability, high power quality, and high compatibility with distributed generation and dc loads. As the key enablers to interconnect ac and dc grids, the converter stations usually adopt the modular multilevel converter (MMC) topology due to its superior modularity and scalability. Unlike the conventional stations that are implemented with multiple MMCs to accommodate multiple ac feeders, a novel multi-ac-port MMC station (MACP-MMC), which evolves from the MMC topology, is proposed in this paper to interconnect different ac feeders and facilitate flexible power exchange in the MV hybrid distribution network. In addition to the benefits inherited from the MMC, the proposed MACP-MMC also features easy multiport extension, bidirectional power flow adjustment, high power density and low cost. The operation principle, energy balance mechanism, and control strategies are elaborated. Finally, its effectiveness is verified by both simulation results in a 3-MVA case and experimental results obtained from a 3-kVA scaled-down lab prototype.

A Modular Multilevel Converter-Based Pulsed Electric Field Generator Design for Electroporation Applications

After a short historical background and mentioning common application areas and different clinical modalities of pulsed electric fields and the phenomena such as electro permeabilization and electroporation are introduced based on the concept of the basic mechanism. Subsequently, pulse generation (PG) and the adapted modular multilevel converter (MMC) topology that is to be covered are introduced including the operating principles and very basic analysis. Finally, performance of the proposed MMC with four half bridge submodules is tested on a prototype built in laboratory. A bidirectional voltage is produced with 80 V amplitude pulsating at 100 kHz operation frequency.

Control method of an AC-AC MMC for open-end stator winding wind turbine generation system

With the increasing of power rating and voltage level of wind turbine, the traditaional low voltage converter is being replaced by multilevel converter. This paper proposes a new ac-ac modular multi-level converter (MMC) connecting the open-end stator winding permanent magnet synchronous generator (OW-PMSG) to AC power grid. The working principle of the proposed topology is analyzed. Under different grid conditions, a unified capacitor voltage balance control method and a current control method are proposed. Aiming at the low grid voltage ride through problem, a distributed braking chopper circuit and the corresponding control strategy is given. Simulation is conducted for verification. Compared with the back to back (BTB) MMC for OW-PMSG, the number of capacitors and arm inductors of the proposed topology are reduced by 43% and 33%. Moreover, the number of the state variables of the average capacitor voltages and currents of the proposed topology are reduced by 33% and 47%, reducing the control complexity. The proposed topology also has higher efficiency than the BTB MMC for OW-PMSG. Compared with the other ac-ac MMC topologies for OW-PMSG, the combined power rating of the power devices of the proposed topology is much lower and thus the proposed topology has higher power density.

A simplified dual-stage model predictive controller for modular multilevel converters

The Modular Multi-Level Converter (MMC) topology is suitable for use in medium and high voltage applications. To cater to the multi-objective control requirements of the MMC, several Model-Predictive Control techniques have been developed. However, most of the available MPCs do not sufficiently address the simplification of the apparently complex switching logic of the MMC in regard to its practical implementation. To fill this gap, a modulated dual stage MPC is proposed here that controls the ac output current using a few simple steps, while maintaining sufficient control over the circulating current. The two MPC controller stages are used in a cascaded manner. The first stage calculates the voltage reference for grid current control and the second stage MPC selects the appropriate switching state to ensure circulating current control. The proposed technique uses carrier-based PD-PWM to retain its superior harmonic performance and ensures 2N+1 output pole voltage levels. While avoiding use of any cost function, a generalized fixed step approach is also presented that is independent of N value. The dual stage MPC presented in this paper is executed once in every switching time period to maintain a fixed switching frequency operation. Simulation and experimental studies show satisfactory performance of 25-level and 7-level MMC respectively, in a grid connected mode. Transient case study is also shown to demonstrate fast dynamic performance of the proposed controller.

An ingenious MMC topology appropriate for motor drives across their entire frequency spectrum

Modular multilevel converter (MMC) modules have popped up as among the best choices for medium and high-powered uses. This paper proposes a control scheme for the entire frequency range of operation for the MMC, focusing on supplying a three-phase machine. The machine is required to be controlled in the outer as well as the inner loop. Standard field oriented control (FOC) manages the three-phase machine in the outer closed loop while the inner control has to come up against the problem of energy balancing. That is unevenly distributed and stored in the capacitance of the upper and lower arms of the converter. There are two operating methods used in the inner control loop: a low-frequency method is used for start-up and low-speed operation, and a high-frequency method is for higher speed. In low-frequency mode (LF-mode), a special control strategy has to be implemented to minimize the energy oscillation in the capacitances of the converter arms. It makes utilization of the 3-phase machine's common mode voltage (Vc) as well as internal circulatory currents to verify a symmetrical energy distribution inside this MMC arms and also to avert whatever AC currents inside the DC source.