Nearest Level Modulation Research Articles

Circuit Configuration of a Reduced Count Modular Multi-Level Inverter

ABSTRACT A novel topology of the modular multilevel inverter with the ability to boost the input voltage is proposed in this work. The proposed single-source structure requires neither any external controller nor transformers to balance the voltages of the capacitors. In addition, the output voltage polarity of the inverter is generated inherently with a low switching frequency, making it suitable for both residential and industrial applications. The Nearest Level Modulation (NLM) is applied to control the operation of the inverter. The detailed comparative analysis of the proposed inverter with other inverters indicates that the proposed structure can produce similar voltage levels with a fewer number of elements and less voltage stresses. Moreover, to verify the performance of the proposed converter, a single-phase 13-level prototype was implemented in the laboratory environment and the test results were compared to the simulation ones. The advantages of low harmonic contents with Total Harmonic Distortion (THD) of 6.5% and 92.69% efficiency make the proposed inverter appropriate for applications with a limited number of low voltage sources.

Mission Profile Emulator for Individual Submodule in Modular Multilevel Converter With Nearest Level Control

The mission profile emulator (MPE) proves to be an effective and cost-efficient tool to validate the design and reliability of the submodule (SM) in a modular multilevel converter (MMC). However, the low-frequency and random switching behaviors of SMs in the MMC with nearest level modulation (NLM) undermine the testing accuracy of MPE in many aspects, including the large ripples in the loading current and the lack of flexibility in control dynamics. To tackle these problems, a MPE with special control structure is introduced in this article to test the individual SM modulated with NLM. The proposed testing method is realized with a common single-phase configuration and the current ripples are suppressed by separating the control functions for two arms in a full-bridge circuit. What's more, the proposed approach gets rid of the need for historical gating signals of the SM under test, and offers a virtual control method to autonomously generate the SM gating signals that can realize the self-balancing of capacitor voltage and the emulation of NLM behaviors in MMC system. Simulations and experimental measurements are also given to validate the proposed method.

Power quality investigation of CHB nine‐level converter based large‐scale solar PV system with different modulation schemes

Multilevel converters (MLCs) are extensively used in solar photovoltaic (SPV) applications owing to their advantages such as low total harmonic distortion (THD) in the converter voltage, reduction in device stress, and switching losses. A suitable modulation technique is important for the efficient closed-loop control of megawatt (MW)-scale solar photovoltaic plants. This work utilises different modulation techniques, such as phase-shifted (PS) multicarrier pulse width modulation (PWM), selected harmonic elimination (SHE), and nearest level modulation (NLM), for switching of cascaded H-bridge (CHB) converter-based large-scale SPV systems. The investigation on improving power quality is presented with a suitable fast Fourier transform (FFT) analysis and comparative graphs. The presented control and modulation enhance the power quality of the output current being fed to the grid in the dynamic solar profile. Moreover, the low switching frequency employed in this photovoltaic converter at a high power rating increases the system efficiency. Graphical illustrations of losses with fundamental and PWM switching were analysed for the MW-rated system. The obtained results show that SHE-PWM provides the best performance for large-scale solar power plants. Furthermore, the IEEE-519 standard was met for both grid voltages and currents. The system was modelled and simulated in MATLAB/Simulink and validated in a real-time environment.

Self-balanced switched-capacitor multilevel inverter with asymmetric double-wing structure

In order to solve the problems of complex structure, high power losses and voltage stress of switches of existing 13-level inverters, this paper presents a new multilevel inverter (MLI) to convert a single DC source’s voltage to an AC voltage with 13 levels. It consists of a switched-capacitor (SC) unit, a T-type unit and an inverting half-bridge. The SC unit can be placed either on the right side of the DC source to halve the input voltage and supply it to the end-side T-type unit, or on the left side of the DC source to double the input voltage and supply it to the end-side inverting half-bridge. Of course, multiple SC units with the same circuit configuration can be placed on both right and left sides of the DC source, simultaneously, to generate more output levels. In addition to flexible structure, the proposed MLI has the advantages of low standing voltage of switches, high boosting capability and power conversion efficiency as well as self-balanced capacitor voltages, which is very suitable for renewable energy sources applications such as photovoltaic and fuel cells. The circuit configuration, working principle, performance analysis under the nearest level modulation and comparative study are presented in detail. And experimental results are provided to evaluate steady state and dynamic characteristics of the proposed MLI.

Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy

The research paper presents the control strategy to reduce THD (Total Harmonic Distortions) losses by the implementation of the Nearest Level Modulation control technique in a Modular Multilevel Converter. Modular Multilevel Converter is found one of the leading technologies in Power Electronics & Control, its applications are very common in HVDC systems, FACTS (Flexible Alternating-current Transmission system), Variable frequency drives and Electric vehicles as well. The power quality of MMC is better and has lesser THD in comparison to conventional converters like 2-level converters with carrier-based modulation techniques. The MMC has been designed with high scalability and has high voltage and power capacity. Sub-module is an integral part of MMC which is built up as an identical and controllable part of it. This converter is also called a controllable voltage source (VSC). Researchers aim to come up with a detailed review of control methods and necessary operations applied to MMC-based systems for HVDC, particularly focusing to control the total harmonic distortions. Power converters use many modulation techniques, but the existing techniques contribute to a great part in switching losses. MMC up to 49 levels, by implementing the Nearest Level Modulation (NLM) technique, is robust and has less complexity for the systems like MMC-HVDC, and the levels control the total harmonic distortions. In this research paper, the reduction of THD by increasing the voltage levels in MMC is comprehensively evaluated. The simulation results in MATLAB/Simulink are used to examine and confirm the proposed control strategy for stable operation of MMC for HVDC application.

A generalized diode containing bidirectional topology for multilevel inverter with reduced switches and power loss

AbstractMultilevel inverters (MLIs) have made great strides in utilizing renewable energy and industrial applications compared to conventional two‐level inverters in terms of output voltage quality, smaller size of the output filter, and higher efficiency. This paper describes a new diode containing bidirectional topology for single‐phase MLI. This structure is presented to reduce the count of switches, isolated voltage sources, and drivers that can increase the number of output levels modularly. This converter, despite the use of diodes, can feed bidirectionally the low power factor inductive–resistive loads. The proposed structure has been designed to achieve a higher output voltage levels relative to the number of switches, drivers, and direct current (DC) voltage sources. Three different algorithms have been proposed to determine the values of DC voltage sources. To confirm the superiority of the proposed structure, a detailed comparison has been carried out with other MLI topologies. Then the proposed topology is simulated in Matlab/Simulink environment using the nearest level modulation technique. Finally, experimental results are presented to confirm the performance of the designed structure in symmetric and asymmetric topologies.

Power Quality Improvement in Modular Multilevel Inverter Using for Different Multicarrier PWM

Pulse width modulation (PWM) is a powerful technique employed in analog circuit convert with a microprocessor based digital output. Besides, Pseudo Random Multi Carrier (PRMC) involves in two random PWM strategies to minimize the harmonic order for 9- level cascaded multilevel H-bridge (CHB) inverter and 9-level Modular Multilevel inverter are introduced. The design mainly focuses on the (Pulse Width Modulation) PWM method, in which two nearest voltage levels are approached in estimated output voltage prediction based on the Partial swarm optimization (PSO) algorithm, and it conveys a random variation in the pulse position of output by Pseudo Random Multi Carrier- Pulse Width Modulation (PRMC-PWM). The CHB and the Modular inverters generate low distortion output by using PMRC. The simulation and prototype circuit are developed for the nine level output using sixteen switches and ten with Resistive-Inductive (R-L) load variation condition. The power quality is improved in CHB and Modular inverter (MoI) with minimized harmonics in various modulation index (MI) as varied from 0.1 up to 0.8. The circuit is designed by using a Field Programmable Gate Array (FPGA), Implementing a PSO algorithm for both CHB, and MoI are proposed. The comparisons of results are verified with lower order harmonics and find the best switching angle across the MLI switches. Modular inverter furthermore investigates with PRMC, Random Nearest level (RNL) modulation scheme are presented, and the proposed circuit is along with the respective degree of the output voltage were synthesized in non-linear load by the development of reactive power across a motor load.

An Optimized Hybrid PWM Strategy for Five Level NPC VSI With Unequal DC-Links in a PV System

This article presents an optimized hybrid pulsewidth modulation (PWM) strategy for five-level neutral point clamped (5L-NPC) inverter with unequal dc-link capacitor voltages fed by differential power processing (DPP)-based photovoltaic (PV) system. The generation of balanced output from the 5L-NPC inverter with the unequal dc-link capacitor voltages becomes necessary in a PV system. The unequal dc-link capacitor voltages cause low order harmonics in inverter output voltage. A hybrid PWM strategy combining the advantages of space-vector PWM (SVPWM) and the nearest level modulation (NLM) methods is proposed to address the issues associated with unequal dc-link capacitor voltages in a five-level NPC inverter. The proposed hybrid PWM is optimized to achieve balanced output voltage with less total harmonic distortion at fundamental frequency. In-depth theoretical analysis of sector identification algorithm and dwell time computation for the proposed optimized hybrid PWM method are described. Simulation and experimental results are illustrated for several combinations of unequal dc-link capacitor voltages to validate the functionality and performance of the proposed optimized hybrid PWM method.

A Study on Common Mode Voltage Reduction Strategies According to Modulation Methods in Modular Multilevel Converter

Low level modular multilevel converter (MMC) is a promising candidate for medium voltage applications such as MVDC (medium voltage DC current) transmission and megawatt machine drives. Unlike high-level MMC using nearest level modulation (NLM), the low-level MMC using the pulse width modulation (PWM) or NLM + PWM is affected by a common mode voltage (CMV) due to a frequent change of a switching state. This CMV causes electromagnetic interference (EMI) noise, common mode current (CMC) and bearing current leading to a reduction in the efficiency and durability of the motor drive system. Therefore, this paper provides a mathematical analysis on how the switching state affects the CMV and proposes three software based CMV reduction algorithms for the low level MMC system. To reflect the characteristic of MMC modulation strategy for upper and lower reference voltage independently, two separate space vectors are used. Based on the analysis, three different CMV reduction algorithms (complete CMV reduction (CCR), DPWM CMV reduction (DCR) and partial CMV reduction (PCR)) are proposed using NLC + PWM modulation strategy. The performance of the proposed CMV reduction algorithms was verified by both simulation and experimental result.

Investigation and Reliability Analysis of Fifteen-level Asymmetrical Multilevel Inverter Topology using Nearest Level Control Technique with low Component Count

Background:: In multilevel inverters (MLI) as the number of level increases, there is a proportionate increase in the count of the semiconductor devices that are employed. Methods:: An asymmetrical multilevel inverter topology using a bidirectional switch is presented which employs lesser number of power electronic devices to produce fifteen levels at the output voltage. Nearest Level modulation (NLM) technique is used to generate the switching pulses and reliability analysis is performed using Markov reliability methodology. The operating principle of the proposed MLI and its performance abilities is verified through MATLAB/Simulink and a prototype is developed to provide the experimental results. Results:: Total Harmonic Distortion (THD) is computed for proposed MLI for different types of loads in simulation environment as well as in the developed hardware prototype. The fifteen level is achieved by using only 9 switches and 3 DC sources in comparison to the 28 switches and 6 DC sources required by the traditional cascaded H-bridge inverter. Conclusion:: The simulation and hardware results confirm the suitability of the proposed fifteen level MLI as the total component count and the requirement of DC sources reduces considerably.

High Quality NLM Method for Medium Voltage Hybrid Bridge MMC

Medium voltage hybrid bridge multilevel converter (MMC) usually has a lower number of links. Therefore, the traditional Nearest Level Modulation (NLM) method has the problems of low equivalent switching frequency and poor output quality. And the traditional Carrier Phase-shifted Pulse Width Modulation (CPS-PWM) modulation has problems such as difficulty in hybrid bridge control, difficulty in redundant configuration, and large amount of calculation. In response to the above problems, this paper proposes a new high-quality NLM method. Based on the traditional NLM modulation method, PWM modulation output and module voltage equalization are realized through the time difference between one module input and one module exit in each control cycle. It achieves the purpose of improving the equivalent switching frequency and output power quality, and at the same time has the advantages of the traditional NLM modulation method with small calculation amount, redundant configuration and hybrid bridge modulation. This article describes the three modulation methods and compares their effects. Finally, the theoretical analysis is verified by PSCAD simulation.

Modified Nearest Level Modulation for Full-Bridge Based HVDC MMC in Real-Time Hardware-in-Loop Setup

Modular Multilevel Converter (MMC) is an emerging converter topology for medium and high voltage applications. Nearest level Modulation (NLM) is the conventional control topology used to control the MMC that produces the N+1 AC output waveform. In previous research work, the Modified NLM has been already proposed, producing a 2N+1 and 4N+1 output waveform while utilizing a half-bridge (HB) submodule (SM) topology. However, half-bridge-based MMC has a similar behavior as two-level Voltage Source Converter (VSC) and cannot block DC fault current in case of DC-side short circuit fault. So, in recent years, full-bridge-based MMC topology is preferably used by manufacturers as it has DC fault blocking capabilities. This paper presents the Modified NLM for Full bridge (FB) SM topology to take the critical benefits of FB SM topology and improve power quality. The proposed method is simpler to implement and produces a 4N+1 AC output waveform. The THD of the output voltage and current reduces to half compared to the conventional NLM method. The proposed method is verified using LabVIEW Multisim co-simulation and as well as real-time simulation.

An Improved Low Switching Frequency Modulation Scheme for a Symmetric CCS-Multilevel Inverter

In this paper, a low switching frequency modulation scheme is proposed for the symmetrical configuration of cross-connected sources-based multilevel inverter (CCS-MLI). The proposed scheme, named modified nearest level modulation (mNLM), is an improvement over the conventional nearest level modulation (NLM) scheme for MLI. The implementation of mNLM, compared to the NLM, has shown a considerable reduction in total harmonic distortion (THD) at the ac output of MLI. The mNLM works through variations in the switching angles of the MLI. By suitably varying the switching angles, the THD content is brought to the minimum. An algorithm for the same is also proposed which enables the mNLM to be implemented on other MLI topologies. Simulation is carried out in MATLAB/Simulink environment and it is also extended to a hardware prototype. The effectiveness of the proposed modulation scheme is also seen in terms of improved inverter parameters such as rms voltage, rms current and generated power.

An Analytical Method to Determine the Optimal Switching of Modular Multilevel Converter in HVDC System

The lifetime of the submodules (SMs) in a modular multilevel converter (MMC) is significantly impacted by its switching frequency. In this work, the determination of the switching frequency, to be applied to the nearest level modulation (NLM) method used in the SMs, is formulated as a linear integer optimization problem (LIOP). It is shown that there is an ideal minimum switching frequency under which all voltage constraints on the SMs are satisfied. As the loading level on the converter increases, additional switching events are generated and the solution of the LIOP is to yield the gating signals to ensure the voltage constraints are still met and the switching loss is reduced. The LIOP is approximated as a series of Lagrangian relaxation linear programming sub-problems. Each of the sub-problems is then shown to have integrality property and can be solved using the subgradient technique. The solution time is a polynomial function of the number of SMs and the number of voltage constraints in each arm of the converter. The quality of the solution is $\varepsilon $ -optimal and the corresponding gating signals for each SM are generated by a proposed two-level coordinated control scheme. The feasibility of the proposed optimization method is demonstrated through simulation study performed on a practical 201-level ±200kV/400MW MMC-HVDC system model constructed in a real-time digital simulator (RTDS) platform. Results of the simulation indicate that the tolerance on the voltage difference between any pair of SMs has to be adjusted when the MMC operates at the determined optimum switching.

Feed-forward Modulation Technique for more Accurate Operation of Modular Multilevel Converters

Modular multilevel converters have become the prominent topology for medium- and high-voltage applications. The performance of these converters highly depends on the accuracy of the used modulation approach, for which the capacitor voltage of submodules (SM) are usually assumed to be equal. This article exhibits that ignoring the capacitor voltage differences among SMs adversely affects the system performance. This becomes more obvious the larger the capacitor voltage differences are. Hence, this article proposes a more accurate feedforward modulation approach that takes into account either the instantaneous capacitor voltage value and the real output voltage in the modulation stage. As a result, in applications where larger SM voltage differences are expected, the current distortion and control performance are improved. Particularly, switching–saving approaches benefit from this method as it enables their operation with reduced switching losses without the downsides of increased distortion due to capacitor voltage differences. The proposed approach is analyzed and compared with the nearest-level modulation and with the level-shift PWM. Simulations and experimental validation are presented to confirm the effectiveness of the proposed technique.

Implementation of a Novel Variable Structure Nearest Level Modulation on Cascaded H-Bridge Multilevel Inverter

The output voltage of a Multilevel Inverter (MLI) can be controlled using various low and high frequency modulation strategy. Apart from meeting the load voltage demand, the modulation strategy also tries to reduce the harmonics present in the output voltage waveform. In this paper, a novel modulation strategy, a Variable Structure Nearest Level Modulation (VSNLM) has been proposed with the objective to reduce the Total Harmonic Distortion (THD) thus giving better output voltage quality and smoother control. The development of the mathematical model and subsequent analysis has been carried out. Moreover, the proposed modulation scheme has been implemented on 7, 9, 11, and 13 level Cascaded H Bridge inverter and a generalized mathematical model of the proposed scheme has been developed. A comparison of the proposed strategy with the conventional NLC is presented in the paper. The results confirm the superior performance of proposed strategy in terms of THD. Experimental results have also been presented for validation.

New Asymmetrical Modular Multilevel Inverter Topology With Reduced Number of Switches

In this article, a new single-phase multilevel inverter is introduced with a reduced number of power switches and reduced voltage stress on power switches. The proposed topology consists of four input dc sources and nine semiconductor switches (eight unidirectional and one bidirectional switch). The topology can be used for asymmetrical voltage source configuration to generate seventeen voltage levels. The extended topology is constructed by a series connection of the topology circuit to produce higher voltage levels with less voltage stress on the switches without modifying the existing structure. Comparison is made with traditional and recently introduced topologies based on the number of power switches, dc sources, total blocking voltage of switches, and gate driver circuits, to prove the proposed topology’s superiority. A simple nearest level modulation has been deployed as the switching scheme. Validation on the viability of the proposed topology has been carried out through simulation and hardware experimental setup.

Binary-Quintuple Progression Based 12-Switch 25-Level Converter With Nearest Level Modulation Technique for Grid-Tied and Standalone Applications

This article introduces a cascaded packed U cell (CPUC) multilevel converter (MLC) to achieve a higher-level count in converter voltage with a minimum number of switches. Here, two five-level packed U cell topologies are connected in a cascaded manner to obtain 25 levels in its output converter voltage. The switch count in CPUC is reduced to 12, as compared to the number of semiconductor devices used for obtaining 25 levels in converter output. A binary-quintuple progression is used for selection of voltage ratios between dc voltage sources and capacitors. CPUC is operated at low-frequency switching, using the nearest level modulation technique. The fundamental switching frequency ensures reduced switching losses as compared to pulsewidth modulation schemes. The converter performance is analyzed for grid-tied and standalone applications. The performance parameters such as total harmonic distortion (THD) of converter voltage and THD of grid/load current are examined. The CPUC configuration is modeled and simulated in MATLAB/Simulink, and test results are taken using OPAL-RT test bench. The acquired simulation and test results confirm viability, practicability, acceptability, and cost-effectiveness of CPUC-MLI converter over existing MLC topologies for efficient power conversion.

Single-Source Three-Phase Multilevel Inverter Assembled by Three-Phase Two-Level Inverter and Two Single-Phase Cascaded H-Bridge Inverters

Multilevel inverters (MLIs) with reduced number of switching devices and dc sources have compact size, reduced cost, and higher efficiency. This article proposes a transformer-based single-source MLI with reduced number of components and small size transformers. The proposed topology is a hybrid three-phase MLI assembled by a conventional three-phase two-level inverter (TTI) and two single-phase cascaded H-bridge inverters (CHBIs). The proposed MLI has a low ratio of number of active switches to number of output voltage levels. In order to use only one dc source, the CHBIs are cascaded with the TTI through transformers. An individual nearest level modulation (NLM) has been developed to control voltage of this inverter. Moreover, power distribution between its modules is analyzed. The total power of the transformers is 28% of the inverter rated power. Simulation and experimental results of a 13-kVA prototype verify the efficiency of the proposed arrangement.

A Generic Scheduling Algorithm for Low-Frequency Switching in Modular Multilevel Converters With Parallel Functionality

Since the introduction of modular multilevel converters (MMC), various additional features have been introduced for MMCs, among them parallel inter-module connectivity. However, such a parallel mode requires appropriate modulation and switching strategies to exploit its full potential. Low-frequency switching modulation is widely used with half-bridge modules and a large body of research focusses on the optimum control and scheduling of these methods. However, so far, a suitable adaptation for MMCs with parallel mode is missing, and existing methods perform suboptimum. This article proposes a generic scheduling algorithm for simple and low-cost integration with low-frequency switching modulation. The proposed method reduces control complexity, minimizes power loss, and can be combined with most modulation techniques, including nearest-level modulation (NLM) and selective harmonic elimination. The text in this article provides a general algorithm for integrating this method with other control levels and analyzes its effect on MMCs with parallel functionality. Moreover, it examines the performance of the system with NLM combined with the proposed scheduler and compares it to other alternatives. Results show up to a 50% reduction in power loss with a similar modulation technique as well as significantly lower THD and power loss compared to optimized phase-shifted carrier modulation as another sensorless alternative method.