Multilevel Converter Topology Research Articles

Comparative Study of Multilevel Converters using DQ Current Control

This paper presents a comprehensive comparative analysis of different multilevel converter topologies, including 2-level, Neutral Point-Clamped (NPC), Cascaded H-Bridge (CHB), and T-Type converters using the DQ current control strategy. The study aims to evaluate the performance of these topologies concerning current Total Harmonic Distortion (THD) and ripple in the direct and quadrature components of the current. A mathematical model of the grid is presented to facilitate the simulation study. The DQ current control scheme is implemented to regulate the current in both the direct and quadrature axes. The simulations are conducted under various operating conditions to capture the converters’ performance across different load scenarios and modulation techniques. Results indicate significant variations in the performance metrics among the different converter topologies. The 2-level converter exhibits higher THD and ripple compared to the more complex multilevel topologies.

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.

Non-Integrated and Integrated On-Board Battery Chargers (iOBCs) for Electric Vehicles (EVs): A Critical Review

The rising Greenhouse Gas (GHG) emissions stemming from the extensive use of automobiles across the globe represent a critical environmental challenge, contributing significantly to phenomena such as global warming and the deterioration of air quality. To address these challenges, there is a critical need for research and development in electric vehicles (EVs) and their associated charging infrastructure, including off-board and on-board chargers (OBCs). This paper aims to bridge the gaps in existing review literature by offering a comprehensive review of both integrated and non-integrated OBCs for EVs, based on the authors’ knowledge at the time of writing. The paper begins by outlining trends in the EV market, including voltage levels, power ratings, and relevant standards. It then provides a detailed analysis of two-level and multi-level power converter topologies, covering AC-DC power factor correction (PFC) and isolated DC-DC topologies. Subsequently, it discusses single-stage and two-stage non-integrated OBC solutions. Additionally, various categories of integrated OBCs (iOBCs) are explored, accompanied by relevant examples. The paper also includes comparison tables containing technical specifications and key characteristics for reference and analysis.

Review paper on multilevel converters topology for VSC-based HVDC transmission system connected offshore wind power plant

This review paper offers a comprehensive analysis of multilevel converters (MLCs) within Voltage Source Converter (VSC)-based High Voltage Direct Current (HVDC) transmission systems, focusing on their integration with offshore wind power plants. Through an exhaustive exploration, the paper investigates various MLC configurations and control strategies, assessing their potential to enhance the performance, efficiency, and reliability of offshore wind energy integration into the power grid. By examining the intricate interplay between MLCs, VSC-based HVDC systems, and offshore wind power plants, this review aims to provide valuable insights into the evolving landscape of renewable energy integration technologies. Key topics covered include an overview of offshore wind power generation, the significance of HVDC transmission for offshore wind farms, and the role of MLCs in overcoming integration barriers. Furthermore, the paper delves into control strategies, performance evaluation metrics, and future perspectives and challenges. Through its in-depth analysis, this review contributes to a deeper understanding of the potential of MLCs in advancing the efficiency and reliability of VSC-based HVDC systems, ultimately facilitating the seamless integration of offshore wind energy into the broader power grid.

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 Comprehensive Review of Compensation Control Techniques Suitable for Cascaded H-Bridge Multilevel Inverter Operation with Unequal DC Sources or Faulty Cells

A cascaded H-bridge multilevel converter topology is the ultimate solution for energy conversion in various industrial applications due to its exceptional features, such as high modularity and fault-tolerant capability. However, two circumstances can lead to unbalanced operation of the inverter, potentially causing a decrease in its reliability and survivability: unequal DC voltage sources and faulty cells. In recent decades, scientists and engineers have conducted intensive research and meaningful studies to propose control solutions capable of maintaining the stable and continuous operation of the inverter under these operational concerns. Typically, each challenge is addressed separately using a distinct compensation control scheme in the existing literature. The paper aims to offer a comprehensive review of the existing compensation control schemes appropriate for CHBMIs operating under unbalanced conditions. It overviews the most popular control schemes and summarizes their usefulness in such scenarios. The theoretical foundations of each control scheme are presented and discussed, including their operating principles, implementation schemes, advantages, and disadvantages. The paper concludes with suggested future trends that require further research for CHBMIs’ continued growth and adoption in various industrial applications.

Method to Enable Reduced Sensor Capacitor Voltage Estimation in Modular Multilevel Converters

Bulk power applications such as shipping increasingly consider multilevel converter topologies such as modular multilevel converters (MMC), which offers the advantages of scalability, good power quality, and reconfigurability. The internal functioning of MMC requires complete knowledge of the capacitor voltages that make up their submodules meaning a large number of sensors are needed and thus a high number of potential points of failure exist. To increase reliability and reduce investment costs, state estimation techniques such as KALMAN filters have been employed to replace the physical sensors. Analytical techniques based on the knowledge of arm current, arm voltage, and submodule states have also been developed. These techniques exploit the fact that at an insertion index of 1, the arm voltage equals the capacitor voltage on the submodule which permits the estimation algorithm to refresh periodically with measured data thereby increasing the accuracy. This method requires a long refresher time, especially when many submodules are used per arm. In this study, we propose an improved analytical estimation by not only using unity insertion indices, but also exploiting transitions between two successive insertion indices. The study was carried out on a 4 submodule per arm MMC system. The estimated capacitor voltages were then compared with sensor-based voltage measurements confirming the validity of the proposed method. It was then integrated into a complete MMC controller including the inner controls such as circulating current and capacitor voltage balancing.

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.

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.

Open circuit fault diagnosis for a five-level neutral point clamped inverter in a grid-connected photovoltaic system with hybrid energy storage system

Introduction. Recently, the number of high and medium voltage applications has increased dramatically. The connection between these different applications requires series-parallel combinations of power semiconductors. Multilevel converter topologies provide major advantages to these applications. In this paper, a grid-connected photovoltaic system with a hybrid energy storage system using a five-level neutral point clamped inverter is studied. Although the multilevel inverter has many advantages over the two-level inverter, it has a high probability of experiencing an open circuit fault. In this context, the five-level inverter has 24 controllable switches, one of which may experience an open circuit fault at any time. Therefore, it plays an important part in the reliability and robustness of the whole system. The novelty of this paper presents an approach to accurately detect the open circuit fault in all insulated gate bipolar transistors of a five-level neutral point clamped inverter in a photovoltaic power generation application with a hybrid energy storage system. Purpose. Before using fault-tolerant control to ensure service continuity, fault diagnosis techniques must first be used, which are the crucial phase of reliability. Methods. A detection method based on the maximum and minimum error values is proposed. These errors are calculated using the expected and measured line-to-line pole voltages. Results. The open circuit fault detection method is implemented using MATLAB/Simulink. Simulation results showed the accuracy of detecting the open circuit fault in all insulated gate bipolar transistors in a short time. Moreover, this method is adaptable to several applications and is also robust to transient regimes imposed by solar irradiation and load variations.

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.

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.

Design and analysis of a new multi-level inverter topology with a reduced number of switches and controlled by PDPWM technique

With their many advantages, including low power dissipation in power switches, low harmonic content, and reduced electromagnetic interference (EMI) from the inverter, multilevel converter (MLI) topologies are becoming more and more in demand in high and medium power applications. This paper introduces a novel multi-level symmetric inverter topology with adopted control. The objectives of this article are to architecturally define the positions of the various switches, to choose the right switches and to propose an inverter control strategy that will eliminate harmonics while producing the ideal output voltage/current. By using fewer switching elements, fewer voltage sources, and switches with a total harmonic content (THD) which reduces losses and a drop in minimum voltage (Vstrssj), the proposed topology is more efficient than conventional inverters with the same number of levels. The new topology will be demonstrated using a seven-level single-phase inverter. For various modulation indices, MATLAB-SIMULINK is used to study and validate the topology.

A new flying capacitor multilevel converter topology with reduction of power electronic components

High power capacity and reliability are characteristics of multilevel inverters. The using a collection of DC sources can produce a terminal voltage that is very close to sinusoidal. The power quality can be improved by adding more levels, but this makes the control system more complicated and expensive. The number of power components in a multilevel inverter has been studied for decades. So, research needs to be done on multilevel inverter configurations to find ways to add levels with fewer power switches than with traditional topologies and those that have already been proposed. In this research, a new power-efficient arrangement of a flying-capacitor inverter is introduced. In order to illustrate the suggested topology, a seven-level multilevel inverter is constructed and demonstrated in a simplified form. Fewer power components, including power switches, capacitors, and gate driver circuits, are required in this topology than in other topologies described in the recent literature, which is one of its main advantages. The improvement mentioned above can be seen in the way this topology works, which is shown by the characteristics of the circuit. MATLAB/Simulink R2021a is used to simulate and verify the circuit to ensure the proposed topology is correct.