Multilevel Power Converter Research Articles

A novel 7-Level symmetric inverter module with less circuit components

Multilevel Power Converters (MPCs) have vast potential in power electronics research and development of DC-AC electrical energy conversion system. MLIs have become an inescapable power electronics device in various power conversion applications due to the incompatibility of traditional two-level inverters. MLIs are extremely popular for medium power and high AC voltage applications, and thus predicted to gather incessant attention in future. This article suggests a novel single-phase symmetric 7-level inverter suitable for a renewable energy application with an effort to lower the count of switching components and conducting devices in the path for the flow of current. The proposed symmetric type inverter circuit could produce 7-level output voltage by using three DC sources and seven power electronic switches. The suggested inverter structure can be used to generate any number of voltage levels, and it can produce all output voltage levels (i.e.,+ve,-ve and zero), which significantly reduces the inverter's total standing voltage. The performance of the suggested 7-level inverter topology was confirmed by simulation results obtained in MATLAB/Simulink.

Finite‐control‐set model predictive control with a constant switching frequency for single‐phase grid‐connected photovoltaic inverter

Taking the advantage of easy implementation in the discrete digital control system, the finite-control-set model predictive control (FCS-MPC) is widely used in multi-phase or multi-level power converters. As the smallest independent operation unit in photovoltaic (PV) distributed power generation systems, the PV inverter has the most direct influence on the power quality. However, the lack of switching states and output voltage levels restricts the development of FCS-MPC in single-phase PV inverters. In order to improve the quality of the PV inverter output current, a constant switching frequency FCS-MPC (CFS-FCS-MPC) method is proposed for single-phase grid-connected PV inverter in this paper, which can thus reduce output current ripples and lower harmonics. Compared with the conventional FCS-MPC method, the proposed CFS-FCS-MPC method synthesizes the switching state with a nonzero and a zero switching state in one sampling period. In addition, an optimum allocation of switching states and execution time for the synthesized switching state is presented. Experimental results have verified the effectiveness of the proposed CSF-FCS-MPC method for the output current, achieving lower current ripples and higher current quality with a constant switching frequency.

EVALUATING THE POSSIBILITY OF APPLYING CONTROLLED COMPENSATION SYSTEMS TO IMPROVE EFFICIENCY IN ELECTRIC GRIDS

The paper considers issues of increasing efficiency of multilevel power converter based on IGCT and IGBT modules to improve energy performance by compensating reactive power in electric networks. By the example of a local power grid using SEMIS SimulationTool design tool the results of calculations of variants of multilevel converters with IGCT and IGBT are presented. The study of possibilities to increase the efficiency of multilevel power converters (MPE) for reactive power compensation (RPC) in local electrical networks (LES) on the basis of plenipotentiary controlled semiconductor cells is of particular interest, because the efficiency of network operation is insufficient, especially in changing load modes with large electric power losses (PEM). And from these positions, one of the most effective tools are fast controlled (PFC) systems. For this purpose, research has been carried out to obtain the best results. Used in calculations tool and methodology are certified software products and algorithms of ABB-SEMIS Simulation Tool computer design.

Improved 25-level inverter topology with reduced part count for PV grid-tie applications

<span lang="EN-US">A new bidirectional multilevel inverter topology with a high number of voltage levels with a very reduced number of power components is proposed in this paper. Only TEN power switches and four asymmetric DC voltage sources are used to generate 25 voltage levels in this new topology. The proposed multilevel converter is more suitable for e-mobility and photovoltaic applications where the overall energy source can be composed of a few units/associations of several basic source modules. Several benefits are provided by this new topology: Highly sinusoidal current and voltage waveforms, low Total Harmonic Distortion, very low switching losses, and minimum cost and size of the device. For optimum control of this 25-level voltage inverter, a special Modified Hybrid Modulation technique is performed. The proposed 25-level inverter is compared to various topologies published recently in terms of cost, the number of active power switches, clamped diodes, flying capacitors, DC floating capacitors, and the number of DC voltage sources. This comparison clearly shows that the proposed topology is cost-effective, compact, and very efficient. The effectiveness and the good performance of the proposed multilevel power converter (with and without PWM control) are verified and checked by computational simulations.</span>

Three-Level Inverter Control Techniques: Design, Analysis, and Comparisons

This work addresses the analysis and design of various Proportional-Integral-Derivative (PID) control techniques for a three-level inverter. Multilevel power converters are modern and basic elements of high-voltage electric drive and power supply systems. By using simulations and specific computer-aided design tools, the overall functional characteristics of multilevel converters, as well as the electrical demands of the components, can be accurately assessed to obtain an appropriate control solution. An innovative and detailed software model of a three-level inverter is developed and then used for the implementation of control techniques. Several tuning methods are used to tune PID controllers for two specific cases: the multilevel inverter with a linear load and with an asynchronous motor load, respectively. A detailed analysis and comparisons of the quality criteria and control performance are achieved. This analysis shows that the choice of controller type depends on the inverter load. For the linear load, proper results are obtained with a PI Nichols-tuned controller, and for the asynchronous load, with a PI controller tuned via a modified Hokushin method. The computer-aided design tools can be further used for the simulation of the equipment in various operating conditions, normal and fault, following all functional parameters.

A nine-switch nine-level converter new topology with optimal modulation control

<span lang="EN-US">Multilevel power converters are becoming increasingly used in several sectors: energy, grid-tie renewable energy systems, High voltage direct current (HVDC) power transmission, and a multitude of industrial applications. However, the multilevel converters consist of several drives and a high number of power switches, which leads to a considerable cost and an increased size of the device. Thus, a novel topology of a multilevel bidirectional inverter using a reduced number of semiconductor power components is proposed in this paper. Without any diode clamped or flying capacitor, only nine switches are used to generate nine voltage levels in this new topology. The proposed multilevel converter is compared with the conventional structures in terms of cost, the number of active power switches, clamped diodes, flying capacitors, DC floating capacitors, and the number of DC voltage sources. This comparative analysis shows that the proposed topology is suitable for many applications. For optimum control of this multilevel voltage inverter and to reduce switching losses in power semiconductors, a hybrid modulation technique based on fundamental frequency modulation and multi-carrier-based sinusoidal pulse-width modulation schemes is performed. The effectiveness of the proposed multilevel power converter is verified by simulation results.</span>

MF_CSA AI based Controlling Technique for Single Phase Converter Output Enhancement at Different Loading Conditions

Renewable and sustainable sources such as SPV systems and WT energy conversion systems are now seen as a promising and growing alternative energy source in view of global emissions and deterioration of the FP. the main objective of the study designing of a solar PV system with varying irradiation in MATLAB/SIMULINK and enhance its output capacity before its integration with the grid. Also, the system has to be made to drive an unbalanced load and dynamic load so that it is capable of handling the change in the power demand of the system. And to design a suitable controller for the multilevel power converter such that it produces better output results than the traditional converter. AI technique has to be incorporated in the converter designing. This work provides a comprehensive design and implementation of power regulatory per phase inverter with proposed MF-CSA (Multi-Function Crow Search Algorithm) optimization controller. The Inverter has been provided with a proposed optimization technique while integrating it with the grid. The voltage output of the system from the modeled solar system with varying irradiation and temperature control is being fed to the inverter for DC to AC conversion

Investigation of fault-tolerant capabilities of some recent multilevel inverter topologies

ABSTRACT In this paper, some recently proposed multilevel power converters are investigated for their fault-tolerant capabilities and improved reliability. These reported topologies are generally reduced device multilevel inverter (RD-MLIs) configurations that cannot tolerate switch failures due to absence of redundant states. Present work therefore aims to incorporate open-circuit fault-tolerant capability by adding few redundant states in switching sequences of existing topologies. The work in this paper objectively presents two existing topologies of five-level and seven-level Multi-Level Inverters (MLIs) modified for fault-tolerant (FT) capability against single switch open-circuit (OC) faults. The modification for fault-tolerant feature (FT-MLI) has intrinsic self-voltage balancing capability, low cost and control simplicity. The modified FT-MLI uses multi-carrier sinusoidal pulse-width modulation (SPWM) technique to generate control switching pulses as per desired switching sequences. Simulation and experimental results of these modified FT-MLI topologies under healthy, faulty and post-fault mode are presented and discussed. Experimental results verify the feasibility and effectiveness of these modified FT-MLI topologies which are suitable for safety-critical applications.

A Method of Reconfiguring Flying Capacitor Multilevel Input Stage for Wide Input Voltage Range Converter Design

Wide input or output voltage range converters are required in various applications, including railway, industrial, military, aerospace, and many more. However, it has been challenging to achieve and maintain high efficiency under a wide range of input or output voltages of operation because of shifting operating conditions and compromised component selections required to cover a wide voltage range of operation. In this article, a method of reconfiguring the flying capacitor multilevel power conversion input stage is revealed for a wide voltage range of operation. A pulse voltage is produced by the input stage. With proper configuration under different input voltage levels, the voltage across key components, namely power switches, inductors, and capacitors, can be maintained relatively constant. In this way, components can be selected and optimized for very confined operating conditions. Also, the amplitude and frequency of the pulse voltage produced by the input stage are also maintained relatively constant. Therefore, a followed filter stage, a resonant tank, or an isolation + rectifier stage can be designed and operated with optimization. High and maintained overall efficiency are achieved over a wide change of the input voltage.

Design, Operation, and Loss Characterization of a 1-kW GaN-Based Three-Level Converter at Cryogenic Temperatures

This article investigates the potential for high power density, high-efficiency power conversion using GaN-based flying capacitor multilevel power converters at low temperature. Specific focus is given to the application of hybrid aircraft powered by liquefied natural gas. Building off of prior characterization of GaN switches over temperature, this article outlines the impact of near-cryogenic operation on the losses of a complete 1-kW inverter. Specific focus has been given to the characterization of the losses in ceramic capacitors and inductors at near-cryogenic operation. A 1-kW GaN-based three-level power converter was designed and successfully tested from room temperature down to $-$ 140 $^{\circ }$ C. Along with the first demonstration of a flying capacitor multilevel converter and associated components at such low temperature, a breakdown of losses over temperature is included to facilitate a better understanding of the opportunities and challenges introduced by this article.

Simulations and experimental validation of one cycle controlled nine-level inverter using FPGA

This paper presents an extensive simulation-based and experimental validation of One-Cycle Control (OCC) over a nine-level power inverter using Cyclone III FPGA from Intel. OCC is a technique that exploits the nonlinear nature of multilevel power converters to overcome issues such as fast dynamic response, robustness and perturbations on either the source or the load side. Simulations performed in Modelsim demonstrate the elimination of dynamic and steady-state error between the switched output and reference signals on application of OCC. Simulink-based development and testing of the multilevel inverter model is also carried out under varying load conditions to record the reference signal, current, power factor and total harmonic distortion. The applied control scheme exhibits negligible current distortion and instability concerns when the converter is lightly loaded. Altera Quarturs II is further used for the VHDL-based implementation and experimental testing of the developed algorithm thus, validating the robustness of the control strategy.

Novel Predictive Control for the IPMSM Fed by the 3L-SNPC Inverter for EVAs: Modified Lyapunov Function, Computational Efficiency, and Delay Compensation

This paper proposes a novel predictive strategy based on a model predictive control (MPC) for the interior permanent magnet synchronous motors (IPMSMs) driven by a three-level simplified neutral-point clamped inverter (3L-SNPC) for electric vehicle applications (EVAs). Based on the prediction of the future behavior of the controlled variables, a predefined multiobjective cost function incorporates the control objectives which are evaluated for every sampling period to generate the optimal switching state applied directly to the inverter without the modulation stage. The control objectives in this paper are tracking current capacity, neutral-point voltage balancing, common-mode voltage control, and switching frequency reduction. The principal concepts of the novel scheme are summarized as follows: first, the delay compensation based on the long horizon of prediction is adopted by a multilevel power converter structure. Second, based on the modified Lyapunov candidate function, both stability and recursive feasibility are ensured of the proposed predictive scheme. Third, the practicability of the real-time implementation is improved by the proposed “static voltage vector” (SVV) and “single state variation” (SSV) principles. Finally, the proposed concepts are implemented in the novel predictive control formulation as additional constraints without compromising the complexity and the good performances of the predictive controller. Therefore, only the switching states that guarantee the stability and the reduction of calculation burden criteria are considered in the evaluation of cost function. The proposed predictive scheme based on the “SVV” principle has demonstrated superior performance in simulation compared with the proposed scheme with the “SSV” principle. The computational burden and switching frequency rates are reduced by 35% and 56.22%, respectively.

Power Loss and Thermal Impedance Modeling of Multilevel Power Converter With Discontinuous Modulation

The hard switching operation of the insulated gate bipolar transistor (IGBT)-based multilevel converters (MLCs) in the transformer less direct integration of renewable power plants produces a considerable amount of switching and conduction power loss. This eventually causes higher junction temperature and lower thermal stability. The symmetrically designed MLC using third harmonic sixty-degree bus-clamped pulse width modulation (BCPWM) technique demonstrates reduced switching losses and better harmonic spectra. However, the development of analytical power loss model is a complex and challenging issue due to the discontinuous switching mode of IGBTs for the BCPWM in MLC. In this regard, an analytical model for calculating the average and the rms currents for the hard switching operation of IGBTs and antiparallel diodes (APDs) with BCPWM is proposed. Furthermore, to calculate the device turn-on and turn-off losses, a discrete form of equation is presented to evaluate the transition times of switching more precisely. The numerical power loss values of IGBTs and APDs found from the derived analytical equations for MLC are compared with the simulation and experimental results. The numerical results agree well with the simulated and the experimental outcomes, which validate the proposed analytical loss model. Moreover, an advanced three dimensional thermal impedance model with chip-wise interdependent multilayer thermal coupling effect is also considered in this paper to realize the critical thermal loading condition of the switching devices.

A Multi-Level Power Converter Interface for a Battery Energy Storage System

— This study proposes a multi-level power converter for a battery energy storage system (BESS) with bidirectional power flow. The multi-level power converter includes a DC-DC power converter, a selection switch set and a full-bridge inverter. The DC-DC power converter performs bidirectional power conversion and provides a DC voltage that is greater than the peak voltage of the utility. Integrating the selection switch set and the full-bridge inverter produces a five-level AC voltage that controls the real power flow and the reactive power flow between the battery set, the DC-DC power converter and the grid. Because partial power is directly processed using only the selection switch set and the full-bridge inverter, the DC-DC power converter is operated only when the grid voltage is greater than the voltage of the battery set. Therefore, the power conversion efficiency for the proposed BESS is improved. The proposed BESS uses only eight power electronic switches, so the power circuit is simplified. A hardware prototype with a digital signal processor controller is used to verify the performance of the proposed BESS. The experimental results are as expected.

Design of a robust current controller for grid‐connected photovoltaic systems based on cascaded multilevel inverter

This work proposes a design method for robust state feedback H ∞ grid current controller applied to single-phase grid-connected photovoltaic (PV) systems. The proposed method achieved high system reliability even dealing with grid distorted voltages and other power quality issues. The robustness of the H ∞ controller was verified in the face of parametric uncertainties at the first-order output filter, and also a satisfactory disturbance rejection performance was achieved. The proposed design is based on linear matrix inequalities associated with constraints involving H ∞ optimisation and a pole placement analysis on the continuous-time domain. A discrete-time analysis is also presented in order to indicate the simplicity of its implementation by a microcontroller or similar devices. Based on the presented results and in order to validate the proposed controller, its robust performance was compared with two linear and well-known control methods: a classical PI controller, in terms of robustness; and a robust H ∞ resonant controller, regarding the power quality at the connection point with the grid-utility. Although the conclusions were listed regarding a low voltage PV grid-interactive multilevel power converter, the proposed H ∞ control design (and its concepts) can be applied to a great variety of renewable energy resources systems.

Direct power control with neutral-point voltage balance of grid-connected three-level NPC inverter

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected renewable energy sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. This paper presents a three-level direct power control (DPC) scheme for a grid-connected NPC inverter with neutral point voltage (NPV) balance. The developed DPC scheme provides the active (P) and reactive (Q) powers control and commonly it preserves the voltages of the two DC-link capacitors at one half of the full DC bus voltage. At each sampling period, the adequate space vector is selected from a predefined switching table designed regarding the effect of the inverter's space vectors on both, P and Q powers, and their ability to achieve the NPV balance. Numerical simulations are carried out, and confirm the effectiveness of the proposed DPC approach in controlling the P and Q powers and minimising the fluctuations of the DC link capacitors voltages.

Direct power control with neutral-point voltage balance of grid-connected three-level NPC inverter

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected renewable energy sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. This paper presents a three-level direct power control (DPC) scheme for a grid-connected NPC inverter with neutral point voltage (NPV) balance. The developed DPC scheme provides the active (P) and reactive (Q) powers control and commonly it preserves the voltages of the two DC-link capacitors at one half of the full DC bus voltage. At each sampling period, the adequate space vector is selected from a predefined switching table designed regarding the effect of the inverter's space vectors on both, P and Q powers, and their ability to achieve the NPV balance. Numerical simulations are carried out, and confirm the effectiveness of the proposed DPC approach in controlling the P and Q powers and minimising the fluctuations of the DC link capacitors voltages.

Open-circuit fault-tolerance in multilevel inverters with reduced component count

High component count and subsequent effects on volume and reliability have been the major concerns for practical applications of multilevel inverters (MIs). Recent emergence of the so-called reduced component count MIs (RCC-MIs) has been driven by the attempts to reduce the number of power switches for multilevel power conversion. In many of such topologies, the aspect of fault tolerance has not been given full consideration. In this work, some of the recently proposed RCC-MI topologies have been considered and analyzed in the light of imparting fault tolerance capability in the case of open-switch failure of any one of the power switches. In an RCC-MI, the occurrence of an open-circuit fault in a power switch would often lead to shut down due to the lack of redundant switching states. To overcome these occurrences, an optimal addition of power switches is described in this work so that the desired redundant states can be synthesized. The modified RCC-MI topologies so obtained have been analyzed under the normal and faulty conditions and computer simulations have been carried out using MATLAB/Simulink, and corresponding results have been presented. The results so obtained are experimentally validated to prove the feasibility of the proposed fault-tolerant topologies.

Modelling approach for multi‐carrier‐based pulse‐width modulation techniques utilised in asymmetrical cascaded H‐bridge inverters

The increasing use of medium-voltage drives and high-power equipment requires a detailed study on the switching method and topology of multi-level inverters. Asymmetric cascaded H-bridge topologies that have unequal input dc voltages with different devices in various parts of the cascaded H-bridge inverter (CHB) are representative of significant improvements in medium-voltage industrial drives. Various modulation strategies are used in multi-level power conversion applications. In the multi-carrier switching method for asymmetric CHB, the number of switches is less than carriers and is usually used in the off-line switching method. These methods of switching are not applicable to online systems. In this study, equations for combining different pulse-width modulation (PWM) to use the controllability advantage of the multi-carrier method are introduced in asymmetric topologies. Then, the dual Fourier series equations are applied to model each inverter switching relation of the asymmetric cascaded blocks. To model each inverter switching relation of the asymmetric blocks using sinusoidal PWM, the dual Fourier series equations were applied. The presented analytical modelling switching method was validated using simulation and experimental case studies.

Circuit-Field Coupling Methodology for Incorporation of Power Electronic Devices: A Piecewise-Linear Approach

A new circuit-field coupling methodology for a single-phase transformer connected to a half-wave rectifier is presented. The rectifier is composed of switched electronic devices (EDs) which can be modeled through a piecewise-linear approximation technique. The modified nodal analysis (MNA) is used to solve the circuit where the EDs are represented with stamps that are systematically incorporated into the system of differential equations. The transformer is modeled using a 2-D finite-element (FE) approach, leading to a coupled circuit-field system, which is solved using proprietary software. The system is also implemented experimentally using an xPC-Target data acquisition system, showing close correspondence with simulation results. Thus, this paper provides the basis for implementing multi-level power converters connected to transformers.