Multilevel Converter Topology Research Articles

Benchmarking of Modular Multilevel Converter Topologies for ES-STATCOM Realization

In recent years, the integration of the high-power static synchronous compensator (STATCOM) and energy storage in the same device has gained interest. Such a system is referred to as ES-STATCOM. Modular multilevel converter (MMC) topologies constitute a promising converter family for ES-STATCOM realization, providing a modular and scalable solution with a high efficiency that handles high-power and high-voltage ratings in grid applications. There is a gap in technical literature discussing the design and the comparison of MMC-based ES-STATCOMs while utilizing batteries to find the most suitable MMC topology for ES-STATCOMs. Therefore, this paper benchmarks MMC family members for ES-STATCOM realization. Both centralized and distributed energy storage approaches are investigated. The proposed design flowcharts can be employed for comparison and optimization purposes. In total, seven topologies are compared in terms of number of cells, required silicon area and total battery volume. Different semiconductor devices and battery types are analyzed. The result indicates that centralized energy storage systems are the most suitable due to their design flexibility, low volume and small silicon area. Moreover, the possibility of using over-modulation in MMC using bridge cells has an important role in the optimization of ES-STATCOM. The results for the adopted case study shows that the decentralized approach can lead to 55% higher silicon area and 30% higher volume than the centralized approach. The double-star bridge cell MMC with centralized energy storage is determined as the most suitable solution for ES-STATCOM systems.

A Hybrid-Arm Modular Multilevel Converters Topology With DC Low Voltage Operation and Fault Ride-Through Capability for Unidirectional HVDC Bulk Power Transmission

In this paper, a novel hybrid-arm modular multilevel converters (HA-MMCs) topology with DC low voltage operation and fault ride-through capability is proposed for unidirectional HVDC bulk power transmission. The HA-MMCs topology is a combination of two types of converter arms: the half-bridge sub-module based arm (HBSM-arm) and the diagonal bridge sub-module based arm (DBSM-arm). Replacing the traditional HBSM-MMC, the HA-MMCs are operated in current and reactive power control modes during normal conditions and can work as the rectifier or the inverter. Since the DBSM can provide bipolar voltages with unipolar currents, the HA-MMCs need not change its control strategy under DC low voltage conditions caused by the high-impedance faults. Under DC pole-to-pole or pole-to-ground faults, the HA-MMCs can both clear the DC fault current and work as the static synchronous compensator (STATCOM) to provide reactive power to support the AC grid. Compared with other MMC topologies with DC fault ride-through capability, the HA-MMCs topology is more economical for significantly decreasing the number of power devices. The operation states, control strategy and DC low voltage operation capability of the HA-MMCs are presented and simulation results of the LCC-HA-MMC hybrid system using PSCAD/EMTDC validate the characteristics and advantages of the HA-MMCs.

Dtc of 3-Level Npc Inverter and Mmc Fed 3-Phase Induction Motor

Direct torque control is an utmost control technique to attain high performance of AC drives. This paper presents controlling of 3-phase Induction Motor (IM) fed from a 3-level Modular Multilevel Converter (MMC) topology which results in minimization of torque and flux ripples when compared to a 3-level Neutral Point Clamping (NPC) inverter based on Direct Torque Control(DTC). As well this in turn can reduce the Total Harmonic Distortion (THD) of the output current and voltage. In recent, modular multilevel converters are becomes popular due to their excellent scalability, redundancy and lesser harmonics for high power applications. The objective of this paper is to improve the steady state and dynamic performances of 3-phase induction motor fed from MMC with DTC using MATLAB/Simulation.

Unified Power Quality Conditioner using PV Fed SVPWM based Multilevel Converters for Power Quality Improvement

This paper presents a solar photovoltaic based unified power quality conditioner (PV-UPQC). The proposed system consists of a shunt compensator (ShC), series compensator (SeC) and PV array connected in parallel to a common dc-link. A three-level diode clamped multilevel inverter configuration is used as shunt and series converters. The SVPWM technique provides reduction in the common mode voltage and the balancing of the dc link voltage. The shunt compensation converter extracts power from PV array and provides compensation for load harmonic currents. The series compensation converter compensates for the power quality (PQ) problems such as grid voltage sags/swells and source harmonic voltages. The proposed system utilizes space vector pulse width modulation (SV-PWM) in multilevel converter topology for the mitigating the power quality problems using PV-UPQC. The proposed system improves PQ and also utilize RES, it also simultaneously improves both current and voltage disturbances. The performance of the system is analyzed in MATLAB/Simulink for nonlinear load conditions.

Comparative Analysis of 5 level, 7 level and 9 level Half-Bridge Modular Multilevel Converter

This paper uses the topology of Modular Multilevel Converter (MMC) to demonstrate Direct Current (DC) / Alternating current (AC) conversion implementation. For drive applications, a theoretical background is presented. The 5-stage MMC switching system is explained. The simulation was performed using cascaded interconnection of half-bridge submodules for single legs of three phase 5 level, 7 level and 9 level modular multilevel inverters. This paper is intended to examine Total Harmonic Distortion (THD) for different voltage stages.

A Modular Multilevel Converter With Active Power Filter for Submodule Capacitor Voltage Ripples and Power Losses Reduction

Modular multilevel converter (MMC) topology has been widely used in the medium-/high-voltage applications. However, large capacitance is needed to suppress submodule (SM) capacitor voltage ripples. This article presents a modified MMC with the active power filter (APF-MMC) paralleled between the N th SM of the upper arm and the first SM of the lower arm in each phase without changing the original topology. It is intended to decrease the capacitor voltage fluctuations and power losses of SMs by diverting the phase low-frequency ripple powers into the storage capacitor of the additional active power filter circuit. Both the detailed SM capacitor voltage fluctuations, which can be reduced up to 50% of the original value in the traditional MMC topology, and the SM average currents, which are proportional to SM switching losses and can be reduced up to 87.83% of the original value, are analyzed. The arm rms currents are also derived, which are related to SM conduction losses. Moreover, the operating principle, modulation method, and control strategy are presented in detail. Finally, the APF-MMC and the traditional MMC topology simulation and experimental platforms were built. The simulation and experimental results demonstrate the validity and performance of the APF-MMC topology and control strategy by comparing with the traditional MMC topology.

An Extremely High Power Density Asymmetrical Back-to-Back Converter for Aerospace Motor Drive Applications

Higher-voltage-standard and higher-power-rating aerospace power systems are being investigated intensively in the aerospace industry to address challenges in terms of improving emissions, fuel economy, and also cost. Multilevel converter topologies become attractive because of their higher efficiency under high-voltage and high-switching-frequency conditions. In this paper, an asymmetrical-voltage-level back-to-back multilevel converter is proposed, which consists of a five-level (5L) rectifier stage and a three-level (3L) inverter stage. Based on the comparison, such an asymmetrical back-to-back structure can achieve high efficiency and minimize the converter weight on both rectifier and inverter sides. A compact triple-surface-mounted heatsink structure is designed to realize high density and manufacturable thermal management. This topology and structure are evaluated with a full-rating prototype. According to the evaluation, the achieved power density is 2.61 kVA/kg, which is 30% higher than that of traditional solutions. The efficiency at the rated power of the back-to-back system is 95.8%.

Novel Uninterruptible Phase-Separation Passing and Power Quality Compensation Scheme Based on Modular Multilevel Converter for Double-Track Electrified Railway

Over-voltage and over-current problems of locomotives when passing phase separation and negative sequence current penetration seriously influence the safety of double-track electrified railway and public power systems. In order to solve these problems, this paper proposes a novel uninterruptible power supply phase separation passing and power quality compensation (UPSP-PQC) scheme for double-track electrified railway. Three working modes of UPSP-PQC are put forward, including uninterruptible phase separation passing mode, power quality compensation mode and uninterruptible phase separation passing priority, and power quality compensation optimum mode. A three-leg modular multilevel converter (MMC) topology of UPSP-PQC is proposed and the corresponding control strategy has been studied. PSCAD/EMTDC simulation is performed to verify the correctness and effectiveness of the proposed scheme and its control method.

A Comparative Study of Different Multilevel Converter Topologies for High Power Photovoltaic Applications

A Comparative Study of Different Multilevel Converter Topologies for High Power Photovoltaic Applications - written by Priyanka A. Bettawar , Sumit R. Punam published on 2020/01/10 download full article with reference data and citations

A New Multilevel AC/DC Topology Based H-Bridge Alternate Arm Converter

The current popular modular multilevel converter topology (MMC) needs a large number of cascaded sub modules and switches, a high voltage level of DC bus and a large loss. Based on these problems, a new multilevel converter topology with H-bridge alternate arm is proposed. This topology combines two structures of H-bridge sub module cascade and H-bridge switch series arm. In high-voltage and high-power applications, on the premise of outputting the same AC voltage level, this topology reduces the DC bus voltage level, system loss and cost, and the number of components, simplifies the control method, and improves the system reliability compared with the MMC structure. In this paper, the working mechanism of the topology is described in detail. On this basis, the control strategy of sub module capacitor voltage balancing is proposed. Finally, the PSIM simulation of the topology is carried out, and a single-phase prototype is made in the laboratory. The simulation and experimental waveforms are obtained, which verify the correctness and feasibility of the topology structure.

Wielopoziomowe przekształtniki energoelektroniczne – topologie, zasada działania, metody modulacji

In this paper selected issues concerning topological structure, operation and control as well as modulation methods used in multilevel converters are presented. There is reviewed general topologies of multilevel converters, which include Diode-Clamped Converters, Capacitor-Clamped Converters, Cascade Converters and Hybrid Converters. The general structure of a multilevel converter consisting of several stages depending on its application and topology is described. In the structure of operation, the stages of Outer Control Loop, Inner Control Loop, DC voltage stabilization and modulation are specified. Next, the division of modulation methods used in multilevel converters is presented and a detailed description of selected modulation methods is made. In the further part of the article the modulation methods were described in detail, i.e. the Level-Shifted Carrier Modulation Technique and Phase-Shifted Carrier Modulation Technique were used to present the analysis of power losses in multilevel converters.

A V2G Integrated Battery Charger Based on an Open End Winding Multilevel Configuration

A new approach to obtain an integrated battery charger is described in this paper, based on the Asymmetrical Hybrid Multilevel Converter topology. Such a particular open-end winding motor configuration, which has proved to be more efficient than conventional inverter topologies in EV motor drive applications, can be turned in an on-board battery charger only by acting on the control system. Thus, no circuit reconfiguration through electro-mechanical switches is required. Moreover, by introducing a single extra power switch, a bilateral power flow can be managed enabling vehicle to grid operations. The obtained integrated battery charger can be supplied either by a standard ac single-phase grid, either by a dc power source for direct connection to domestic energy resources. The proposed approach enables a new remarkable function to the asymmetrical hybrid multilevel converter at a marginal extra cost, thus mitigating the larger complexity and cost of such an inverter if compared with conventional topologies.

7-Level Semi Cross Switched Multilevel Inverter Fed Induction Motor Drive

Multilevel inverters have more prominent features than 2-level inverter due to various advantages like voltage quality, low EMI etc. The semi cross switched multilevel converter topology need less number of semiconductor switches compared to cascaded H-bridge multilevel inverter, and can be implemented to any number of voltage levels. The operating modes of 7-level semi cross switched multi level inverter are discussed. Three phase seven level inverter fed induction motor is implemented in MATLAB/SIMULINK.

An Improved Control Strategy for the Bus Interlinking Converter Based on Three-level Topology in AC-DC Hybrid Microgrid

Considering the increasing requirements of the AC-DC hybrid microgrid (HMG) for the transmission power capacity and voltage level, the several bus interface converters (BICs) with the two-level PWM converter topology, need to be connected between AC bus and DC bus in parallel, which could generate circulating current and cycle power. The cost of the modular multilevel converter topology based BIC is too high. Hence, this paper proposes to apply the BIC with three-level topology to the HMG, and a droop control strategy with the midpoint potential hysteresis regulation link is designed, which can effectively suppress the fluctuation of midpoint potential without affecting the normal operation of the BIC in the grid connected or off mode. In addition, the principle of neutral point fluctuation and the process of hysteresis control are introduced, and the detailed design of dual-mode control strategy based on droop curve are also given. Finally, the effectiveness of the improved control strategy is verified by simulation.

A Qualitative Assessment of a Modified Multilevel Converter Topology M2LeC for Lightweight Low-Cost Electric Propulsion

A Cascade H Bridge (CHB) is evaluated for both electric vehicle motor traction control and off-vehicle charging against the Power ElectronicsUK Automotive Challenge for cost and mass for the year 2035. By combining the power electronics with batteries using low-voltage MOSFET transistors in a series cascade arrangement the cost and mass targets could be met 12 years earlier (in 2023 and 20 times lighter if an application specific integrated circuit (ASIC) is used. A 200 kW peak reference car was used to evaluate cost and mass benefits using four different topologies of power electronics. Vehicle installation is shown to be simplified as only passive cooling is required removing the need for liquid cooling systems and the arrangement is inherently safe; no high voltages are present when the vehicle is stationary. The inherently higher efficiency of CHB increases vehicle range. The converter with integrated batteries can also behave as an integrated on-board battery charger delivering additional off-vehicle benefits by removing the need for costly external chargers.

ZPUC: A New Configuration of Single DC Source for Modular Multilevel Converter Applications

Z packed U-cell (ZPUC) converter topology is presented in this paper as a new type of multilevel converter topology that can be operated in a single phase as well as in three-phase configurations while using a single DC source. Since each U-cell includes two switches and one capacitor, in this topology, three U-cells are needed to generate 5 or 7 voltage levels. Moreover, the configuration proposed for the ZPUC is more appropriate for high-power application modular multilevel converters (MMCs) to increase the voltage levels compared to other topologies. Accurate voltage balancing on small-sized auxiliary capacitors is due to integrated modulation strategy without using additional controllers; additionally, the reduction of total harmonic distortion (THD) in AC currents for higher voltage levels is an advantage of this configuration. A full topology sequence of operation and performance analysis of ZPUC based on the 5-L inverter is investigated in Matlab-Simulink and experimentally validated on a 3 kVA prototype. The obtained results illustrate the good dynamic performance of the proposed topology and the implemented integrated switching pattern voltage balancing.

Analysis of Cable Overvoltages in Symmetrical Monopolar and Rigid Bipolar HVDC Configuration

The symmetrical monopolar configuration is the prevailing scheme configuration for high voltage direct current (HVDC) interconnectors utilizing the modular multilevel converter (MMC) topology. However, the rigid bipolar configuration is gaining significance, as first projects are currently in planning stage. The purpose of this paper is to analyse both HVDC scheme configurations with regard to cable voltage stresses originating from short-circuit faults. The study focuses on HVDC cables with extruded insulation connected to MMCs equipped with half-bridge (HB) submodules. Basic design aspects in rigid bipolar configuration such as the station grounding concept are discussed. Moreover, various station internal as well as dc faults are examined by means of electromagnetic transient (EMT) simulations. Occurring cable voltage stresses are analyzed with respect to voltage polarity, peak value and wave front-parameters and compared in symmetrical monopolar and rigid bipolar configuration. It is demonstrated that cable overvoltages with same polarity as the dc operating voltage are significantly reduced in schemes operating in rigid bipolar configuration. Voltage reversals caused by the discharge process during a cable fault are nearly independent of the selected scheme configuration. Obtained results are relevant with regard to upcoming projects in rigid bipolar configuration and provide insights to further refine insulation co-ordination aspects related to dc cable systems.

Model Predictive Control of a Nine-Level Internal Parallel Multilevel Converter With Phase-Shifted Pulsewidth Modulation

Among various multilevel converter topologies, the nine-level internal parallel multilevel converter (9L-IPMC) stands out due to its capability to incorporate the advantages of both parallel modularity and active neutral point clamped converter. However, suppressing the circulating current and balancing the neutral-point voltage and six flying capacitor voltages while providing fast output current control is always a challenging task in 9L-IPMC. This is particularly true when the flying capacitor is small. This article proposes a model predictive control (MPC) scheme for the 9L-IPMC with phase-shifted pulsewidth-modulation (PS-PWM) to achieve these goals. By considering the high-frequency cell in 9L-IPMC as a whole, PS-PWM can be integrated into MPC scheme, and no parameter tuning process is required for the output current control. The internal circulating current suppression and flying capacitor voltage balancing are achieved by the flexibility of the inherent redundancy in 9L-IPMC. A thorough experimental evaluation of the proposed method has been conducted to validate the effectiveness of the proposed MPC scheme.

A New Generalized Multilevel Converter Topology Based on Cascaded Connection of Basic Units

In this paper, a new single-phase multilevel converter (MLC) topology based on the cascade connection of novel basic units is presented. The proposed basic unit generates 17-level output voltage waveform and can be extended for higher voltage levels by using a simple cascade connection. Both the proposed basic unit and cascaded topologies are compared with other state-of-the-art MLC topologies. From the comparison results, it will be shown that the proposed topology has several advantages such as the reduced number of power electronic components, lesser number of dc sources, and blocking voltage. Moreover, the proposed MLC has reduced power losses and improved efficiency. The operability and feasibility of the proposed converter are validated through extensive simulation and experiments. Finally, the corresponding results affirming the predominance of the proposed topologies are presented.

Systematic Synthesis and Derivation of Multilevel Converters Using Common Topological Structures With Unified Matrix Models

Multilevel converters (MLCs) have been increasingly adopted in both low-power low-voltage systems and high-power high-voltage applications. In recent years, many new topologies have been proposed, and a few of them have been successfully implemented in industry. While searching for new topologies, synthesis and derivation principles of multilevel topologies are critical for converter design. In this article, a generalized synthesizing approach of multilevel topologies is proposed and analyzed with the considerations of the voltage source, current source, and matrix-type MLCs. Based on the proposed method, the topological relationship among these three types MLCs is revealed through the stage-based common circuit structure. In addition to the graphic-based approach, a mathematical approach is proposed for representing the MLC topologies in this work. The matrix-based model is utilized to unify and verify the derivation and simplification process in a systematic way through many MLC examples in this article. Finally, demonstration examples are presented to show how the proposed principles can be used to derive new topologies covering five-level to nine-level converters. With the ever-increasing research efforts on MLCs, it is hoped that this article can provide a new approach that inspires the development of more interesting and practical MLC topologies for various applications.