Multilevel Power Converter Research Articles

Observability and Observed-Based Control for a Class of Output Modular Systems With Application to Modular Multilevel Power Converters

Observability and Observed-Based Control for a Class of Output Modular Systems With Application to Modular Multilevel Power Converters

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.

Thermal analysis of an integrated motor drive with a switching cell array power converter

In modern electric propulsion systems the electric motor is tightly integrated with the power converter and control unit. These so called Integrated Motor Drives can allow for vast improvements in efficiency and volume reduction, due to the elimination of redundant electric components, and the shared case and cooling system between the motor and the power converter. The cooling systems used in these applications must be carefully designed as the motor and power electronics components may feature greatly different temperature limits. This work discusses the development of a thermal model for an integrated motor drive composed by a six phase permanent magnet motor with hairpin windings and a switching-cell-array-based multilevel power converter. The system is modelled by means of a lumped parameter thermal network which includes an accurate model of the motor stator and rotor, as well as the arrangement of the power switches on the converter boards. The conductive resistances are simply obtained from geometric data and material properties. Instead, the convective resistances are evaluated by means of empirical models available in the literature and a CFD model for the cooling jacket. The developed model is employed to assess the feasibility of the presented arrangement of motor and power electronics components from the thermal point of view.

A novel power conversion structure for grid-connected photovoltaic applications based on MLI and LeBlanc transformer using IRSA technique

This article proposes a new energy conversion structure by employing a hybrid approach for grid-tied photovoltaic (PV) applications. This structure depends on the LeBlanc transformer and multilevel inverter (MLI). The proposed hybrid system combines the honey badger algorithm (HBA) and the reptile search algorithm (RSA). Crocodiles hunting behavior is enhanced by the HBA technique, also known as the IRSA technique. Voltage source inverters (VSI) are used in the proposed multilevel power converter. The MLI output is attached to the LeBlanc transformer. Multi-string technology is essential to the PV system's configuration. This innovative power converter's structural layout allows for an output voltage at the MLI's output. The proposed IRSA approach is utilized to regulate this power converter. This control system permits a fast and robust response from the MLI. This is also ensured by using the IRSA technique. The performance of the proposed hybrid method is run in MATLAB, and the performance is compared with various existing methods. From the simulation, the proposed approach-based efficiency is higher than the existing one. The proposed method shows a high efficiency of 99% compared with other existing methods, such as the salp swarm algorithm (SSA), bee colony optimization (BCO), and grasshopper optimization algorithm (GOA).

VSG Control for Cascaded Three-Phase Bridge Based Battery Inverter

With the increasing number of new energy sources connected to the grid, the unbalanced output of three-phase grid-connected inverters and the lack of no inertia and damping characteristics in the traditional microgrid control system will seriously affect the stability of voltage, frequency, and power angle for microgrids. This paper proposes a novel cascaded three-phase bridge inverter topology for the battery system used for the electric vehicle. Compared with traditional cascaded H-bridge inverters, the proposed multilevel inverter can achieve self-adaptive balance for three phases. The mathematical model of a cascaded three-phase bridge inverter is established in this paper. Based on the voltage and current equations of a multilevel inverter, a new modulation strategy named carrier phase-shifted-distributed pulse width modulation (CPSD-PWM) was developed, which is more suitable for cascaded three-phase bridge inverters. The harmonic analytic equations of carrier phase-shifted pulse width modulation (CPS-PWM) and CPSD-PWM are constructed by the double Fourier analysis method. Compared with the traditional PWM modulation strategy, the CPSD-PWM can reduce the output harmonics and improve the balance of the three-phase output, which can realize the three-phase adaptive balance in the cascaded three-phase bridge inverter. This paper develops a cascaded three-phase bridge multilevel power converter system based on the virtual synchronous generator (VSG) control strategy. The voltage and frequency of inverter output can be accurately controlled in both island mode and grid-connected mode through active power-frequency regulation and reactive power–voltage regulation, and the stability of primary frequency regulation for the multilevel microgrid inverter can be improved by collaborative optimization of virtual inertia and virtual damping. The CPSD-PWM modulation strategy and VSG control strategy are verified by the simulation results and experimental data for the cascaded three-phase bridge inverter.

A Sensor-Based System for Fault Detection and Prediction for EV Multi-Level Converters.

Power electronic converters and alternating current motors are the actual driving solution applied to electric vehicles (EVs). Multilevel inverters with high performance are modern and the basis for powering and driving EVs. Fault component detection in multilevel power converters requires the use of a smart sensor-based strategy and an optimal fault analysis and prediction method. An innovative method for the detection and prediction of defects in multilevel inverters for EVs is proposed in this article. This method is based on an algorithm able to determine in a fast and efficient way the faults in a multilevel inverter in different possible topologies. Moreover, the fault detection is achieved not only for a single component, but even for several components, if these faults occur simultaneously. The detection mechanism is based on the analysis of the output current and voltage from the inverter, with the possibility of distinguishing between single and multiple faults of the power electronic components. High-performance simulation programs are used to define and verify the method model. Additionally, with this model, harmonic analysis can be performed to check the correctness of the system's operation, and different fault scenarios can be simulated. Thus, significant results were obtained by simulation on various topologies of multilevel converters. Further, a test bench was developed in order to verify some failure situations on a three-level inverter.

A Predictive Current Control Strategy for a Medium-Voltage Open-End Winding Machine Drive

This paper presents a medium-voltage drive based on an open-end winding induction machine supplied by a multilevel power converter topology. The power converter consists of cascaded two-level three-phase voltage source inverters (VSI) connected to each side of the machine windings and each VSI is fed by an isolated DC supply. The topology has been previously reported in the literature as a sinusoidal pulse-width modulation operating in an open loop. In this work, a closed-loop model predictive control (MPC) strategy is proposed. MPC offers a much simpler method to control the power switches of the inverter compared to complex modulation strategies that are typically used in multilevel converters. Moreover, the advantage of reducing the common-mode voltage offered by the open-end winding configuration is fully exploited in this work. Simulation results are presented to validate the performance of the proposed topology and control method.

Fully Integrated Multilevel Power Converter for SRM Drive With Charging Capabilities (G2V) for Electric Vehicle Application

This article presents a fully integrated multilevel power converter topology (IML-PCT) with integrated battery charging capability for a switched reluctance motor (SRM) drive. The battery charger is integrated within the IML-PCT, which eliminates the requirement of any additional module for charging purposes. The proposed IML-PCT applies higher energization and de-energization voltages during the driving mode, which leads to an enhanced constant torque region for SRM drive. For the battery charging mode, the proposed IML-PCT exhibits the characteristics of an onboard charger (OBC) capable of charging the battery energy storage system directly via a standard ac outlet. The OBC functionality is realized by reconfiguring the proposed IML-PCT into an interleaved bridgeless boost power factor correction circuit (IBB-PFCC). The charging inductors for IBB-PFCC are realized by reutilizing/reconfiguring all the windings of the four-phase SRM. Also, with the proposed IBB-PFCC configuration, the charging current in the windings leads to an average zero torque production. Thus, the rotor is kept stationary during battery charging mode without any external braking mechanism. Detailed simulation and experimental studies on a 1.1-kW laboratory prototype four-phase SRM demonstrate ac level-1 (120 V/240 V) charging performance of the proposed IML-PCT.

Control of Cascaded Multilevel Converter for Wave Energy Applications

This paper proposes a control scheme for a wave energy conversion system based on a linear generator and a cascaded multilevel converter. The mechanical conversion system is composed of a buoy connected directly to a linear generator. The windings of the generator are individually controlled by a cascaded multilevel power converter using independent maximum power point tracking to improve energy harvesting. The output of the cascaded converter is controlled to keep the DC capacitors balanced and generate a multilevel voltage at the output terminals which reduces the losses in the underwater transmission line. Experimental results show the performance of the proposed control scheme maximizing the power generation by imposing a current with the same waveform of the induced voltage and balancing the DC capacitors.

A New MPC Formulation Based on Suboptimal Voltage Vectors for Multilevel Inverters

Different new model predictive control (MPC) formulations have been recently proposed to reduce the real-time computation load, which makes the MPC algorithm promising for multilevel power converters. Unlike the conventional MPC formulation, which searches for the optimal switching state at each sampling time, the existing computationally efficient MPC formulations are to search for the optimal output voltage in the first stage. In the second stage, only the switching state redundancy under this optimal output voltage established in the first stage will be employed to achieve multiobjective. These computationally efficient MPC formulations based on searching an optimal voltage vector are usually validated on the power converter topologies with abundant switching redundancy or without floating capacitors. However, for the emerging topologies with less switching redundancy and floating capacitors, such as the five-level (5L) T-type nested neutral point clamped (T-NNPC) converters topology, the existing computationally efficient formulations based on optimal output voltage vector can lead to potential capacitor control failure due to the sacrificed multiobjective control performance. To address this issue, this article presents a novel MPC formulation based on suboptimal output voltage vectors considering both the system’s multiobjective control performance and computation burden reduction. With the determined suboptimal voltage vectors, a small group of the switching state candidate can be established to improve the system’s multiobjective control performance and efficiency. The proposed MPC formulation is finally validated on a 5L T-NNPC topology.

Novel discontinuous PWM for dc-link voltage balancing of multilevel quasi-nested topologies

ABSTRACT The use of multilevel power converters for its application in high demanding industrial processes and grid connected renewable energy conversion systems has nowadays become the most suitable solution. The main advantages of these converter configurations are: reduction in the blocking voltage stress, lower harmonic ratings, smaller filters for output and input stages. The proposed quasi-nested topology merges the advantages of the classical multilevel topologies, such as the NPC and ANPC configurations, addressing less active switching devices, lower blocking voltage stress. A novel discontinuous modulation technique, with controlled discontinuous gain, is implemented in order to deal with the dc-link voltage unbalance. Simulation and experimental validation results in a scaled-down power 1,5 kW three-phase converter prototype under different operational conditions using different modulation index are presented. The proposed four-level converter topology allows generating an additional output voltage level with the same semiconductor count as conventional three-level inverters such as NPC and ANPC with superior waveform quality, with a THDv reduction of 34.54% in comparison to the clamped inverters. Experimental tests carried out in a 1.5 kW setup verify the feasibility of the proposed topology and the developed discontinuous modulation scheme, to achieve dc-link voltage compensation.

A novel step‐up topology for multilevel power conversion

AbstractRenewable energy sources employ a power conditioning unit for injecting power into the grid. Such resources of small capacity are characterized by low generated voltage and thus require a high voltage boost at the inversion stage. This paper proposes a novel transformer‐less, single‐phase grid‐connected, 13‐level inverter to yield the voltage boost. The presented topology is realized through 12 unidirectional switches, 2 diodes, and 3 capacitors. A single‐stage circuit synthesizes 13 levels with a single input source and yielding an overall voltage gain of six. The adapted modulation technique is robust enough to keep the capacitors self‐balanced for all values of the modulation index. The proposed topology is modular and cost‐effective in comparison to other similar typologies due to reduced device count and space requirement. The presented topology is validated experimentally with satisfactory operation in yielding input voltage boost by a factor of six.

Reducing Submodule Capacitance for Modular Multilevel Converter-Based Medium-Voltage Wind Power Converter

This study presents a strategy to reduce the capacitance of the submodule (SM) capacitor of a modular multilevel converter-based medium-voltage wind power converter. The design of the SM capacitor of the modular multilevel converter (MMC) should consider the actual operating conditions, especially the influence of wind speed, because wind speed will affect the voltage ripple and voltage amplitude of the SM capacitor. In the traditional method, the capacitance design of the SM capacitor will be based on relatively high wind speed and leave a certain safety margin. However, in most cases, the system operates at the highest-frequency wind speed (HFWS). For the MMC-based wind energy conversion system, a constant capacitor voltage ripple (CCVR) control method is proposed. Using this method, the SM capacitor voltage ripple can be significantly reduced by injecting the second harmonic component of circulating current. In this way, the SM capacitor with smaller capacitance can be used. Finally, the effectiveness of the proposed method is validated in RT-Lab Platform.

Power Electronics Converters for the Internet of Energy: A Review

This paper presents a comprehensive review of multi-port power electronics converters used for application in AC, DC, or hybrid distribution systems in an Internet of Energy scenario. In particular, multi-port solid-state transformer (SST) topologies have been addressed and classified according to their isolation capabilities and their conversion stages configurations. Non-conventional configurations have been considered. A comparison of the most relevant features and design specifications between popular topologies has been provided through a comprehensive and effective table. Potential benefits of SSTs in distribution applications have been highlighted even with reference to a network active nodes usage. This review also highlights standards and technical regulations in force for connecting SSTs to the electrical distribution system. Finally, two case studies of multi-port topologies have been presented and discussed. The first one is an isolated multi-port bidirectional dual active bridge DC-DC converter useful in fast-charging applications. The second case of study deals with a three-port AC-AC multi-level power converter in H-Bridge configuration able to replicate a network active node and capable of routing and controlling energy under different operating conditions.

Multiplexed Optical Link for Gating Pulse Transmission in Multilevel Converters

Multilevel power converters play an increasingly important role in energy transmission, high-power electric drives and renewable energy systems. The transmission of control signals for the semiconductor switches in power converters is critical for the operation of the system, and therefore optical communications links are employed to minimize electromagnetic interference. However, conventional parallel communication topologies have the drawback of requiring a high number of independent optical fibers and transmitter-receiver pairs as the number of transmitted signals increase, which limits the flexibility and scalability of the link. In this paper we propose the design of an optical link with wavelength division multiplexing (WDM) based on a digital micromirror device (DMD) and a linear detector array as receiver. The main advantages of the proposed system are the high flexibility and scalability of the communications link. Simulation results are obtained from a mathematical modeling of the transmitter, the link and the receiver. The communication is established by means of an asynchronous protocol in order to ensure the correct decoding of the transmitted gating signals. A feasibility analysis of the optical link is carried out in terms of the waveform quality of the multilevel inverter output voltage, measured by its total harmonic distortion (THD). The simulation results obtained in MATLAB/Simulink environment validate the methodology to enable a future implementation of the proposed optical communication link.

Embedded Real-Time Simulator for Sensorless Control of Modular Multi-Level Converters

This paper suggests the application of an embedded real-time simulator (eRTS) in the context of voltage–sensorless control of a modular multilevel power converter (MMC). This eRTS acts as an observer and ensures digital redundancy in the case of any fault occurring among the capacitor voltage sensors of the MMC submodules. Hence, in such a faulty situation, the MMC controller switches from the measured voltages to their estimated counterparts. As for the digital implementation, to ensure a high level of integration of the overall control system, the Xilinx Zynq-7020 system-on-chip field programmable gate array (SoC-FPGA) device was used. The controller was implemented in the hardwired ARM Cortex-A9 processor, with a 100 µs time step. Regarding the time-sensitive blocks (PWM, eRTS and measurements filtering), a full hardware implementation was privileged, using the FPGA fabric. The execution time of these blocks was 710 ns with a 100 MHz system clock, and the synchronization with the analog to digital acquisition chain was made with a 5 µs time resolution. The whole proof-of-concept system was experimentally tested, including the time/area evaluation of the implemented designs and the experimental validation of the eRTS estimations in both healthy and faulty scenarios.

An Improved Sliding Mode Control with Integral Surface for a Modular Multilevel Power Converter

This paper presents a novel method for current control for a modular multilevel converter (MMC). The proposed current control methodology is based on a modified sliding mode control (SMC) with proportional and integral (PI) sliding surface which allows fast transient responses and improves the robustness of the MMC control performance. As the proposed method is derived via Lyapunov direct method, the closed-loop stability is ensured and results in globally asymptotically stable. Furthermore, the reaching time is also guaranteed by the proposed method, leading to fast transient responses. The proposed method is validated by comparing with some existing methods, which are proportional integral controller and conventional SMC, via offline and hardware-in-loop (HIL) simulations where a 10 MW, medium-voltage MMC system is tested. According to these results, the proposed method is able to provide fast transient responses, zero overshoot, and robustness to the weak grid and short-circuit conditions.

Control and Optimization of Lattice Converters

Multilevel converters continue their upward trend in renewable generation, electric vehicles, and power quality conditioning applications. Despite having satisfactory voltage capabilities, mainstream multilevel converters suffer from poor current sharing performances, thereby leading to the development of lattice converters, i.e., a strong and versatile type of future multilevel power converters. This article addresses two problems faced by lattice converters. First, we propose and detail how to optimize the efficiency of a given lattice converter by controlling the on/off states of H-bridge submodules. Second, we introduce the method that determines the voltage at each node of the converter in order to satisfy output voltage and current requirements. Design and analysis of lattice converters need a different mathematical toolbox than routinely exercised in power electronics. By use of graph theory, this article provides control methods of 3 × 3 and 4 × 4 lattice converters, satisfying various control objectives such as input/output terminals and output voltages. We further validate the methods with simulation results. The methodologies, algorithms, and special cases described in the article will aid further design and refinement of more efficient and easy-to-control lattice converters.

A New Decentralized Space Vector PWM Method for Multilevel Single-Phase Full Bridge Converters

This paper proposes a decentralized control structure and method for a multilevel single-phase power converter using space vector pulse width modulation (SVPWM). The focus of this paper is on the decentralized control structure for the power converter cells that will exchange information with neighboring cells in order to adjust the switching vector and switching time for each cell. In this study, the switching vectors and the corresponding switching times of each cell will be self-determined based on the phase angle of two neighboring cells. Normally, SVPWM applied to the multilevel power converters need complete information about the total cells and cell’s position to build a control algorithm. Meanwhile, a decentralized space vector pulse width modulation (DSVPWM) method is proposed that can be applied to power converters with any number of cells and can be considered as a multilevel SVPWM method. In addition, the decentralized multilevel single-phase power converter has high flexibility with which it is possible to easily adjust the number of active cells, so that the output voltage can be adjusted quickly; this provides the ability to dynamically reconfigure without interrupting the power energy supply process. The proposed control structure and method are effectively verified based on simulation and experimental results. Experimental results are evaluated based on a 9-level single-phase power converter, which has an RL load with rated parameters 220 V/500 W.

Common-Mode Voltage Elimination in Multilevel Power Inverter-Based Motor Drive Applications

The industry and academia are focusing their efforts on finding more efficient and reliable electrical machines and motor drives. However, many of the motors driven by pulse-width modulated converters face the recurring problem of common-mode voltage (CMV). In fact, this voltage leads to other problems such as bearing breakdown, deterioration of the stator winding insulation and electromagnetic interferences (EMI) that can affect the lifespan and correct operation of the motors. In this sense, multilevel converters have proven to be a useful tool for solving these problems and mitigating CMV over the past few decades. Among other reasons, because they provide additional degrees of freedom when comparing with two-level converters. However, although there are several proposals in the scientific literature on this topic, no complete information has been reviewed about the CMV issues and the different multilevel alternatives that can be used to solve it. In this context, the objective of this work is to determine how multilevel power converters provide additional degrees of freedom to make the reduction of the CMV possible by using specific modulation techniques, making it easier for engineers and scientists in this field to find solutions to this problem. This document consists of a descriptive study that collects the strengths and weaknesses of most important multilevel power converters, with special emphasis on how CMV affects each of them. In addition, the differences of modulation techniques aimed to the CMV reduction are explained in terms of output voltage, operating linear range, and generated CMV. Considering this last, it is recommended to use those modulation techniques that allow the generation of CMV levels of 0 V in order to be able to completely eliminate said voltage.