Conventional Two-level Converter Research Articles

Concert Exploration of DFIG Grid-integrated MMC using Artificial Intelligence

The performance of grid-connected wind turbine systems that use a Doubly Fed Induction Generator (DFIG) is explored in this work. In the suggested configuration, two conversion stages are present. In the first step, Alternating Current (AC) voltage is changed into Direct Current (DC) voltage. At this point, the conventional two-level converter is implemented. The second stage involves the conversion of DC to AC using a five-level Modular Multilevel Converter (MMC). Voltage fluctuation is observed in the DC link. It is anticipated that the Fuzzy Logic Control (FLC) approach will be able to adjust the DC link voltage at the MMC of the second stage of conversion. For the purpose of the Pulse Width Modulation (PWM) system, the current controller is accountable for regulating the current signals and the reference signals. As a result, the PI Controller (PIC) is being evaluated for use in this function. Carrier-based Phase Disposition (PD) PWM is employed for MMC switch control. The proposed controllers’ performance is validated in Matlab/Simulink.

Reliability Assessment of Power Semiconductor Devices for a 13-Level Boost Inverter Topology

Since the last few decades, research on multilevel inverters (MLIs) has been growing in the power electronics field. The MLI is preferred over conventional two-level converters due to its improved performance. In recent years, different types of MLI architectures have been proposed, each employing a unique combination of power semiconductor devices to produce a multilevel output voltage waveform. The reliability of a multilevel inverter is a crucial performance indicator that must be taken into account at every stage from design to commercialization. MLI is vulnerable to a variety of failure modes and processes due to a large number of interdependent components. In this paper, a 13-level boost inverter topology has been analyzed and test results are presented and discussed. The analysis has been carried out in terms of open-circuit fault (OCF), short-circuit fault (SCF), power loss, and thermal analysis. The results have been discussed and recommendations have been made for the better design of the inverter topology to improve reliability.

Capacitor Voltage Balancing Control for a Novel 5-Level Dual Active Bridge Converter

Multilevel technology is an attractive solution to increase the cost effectiveness and power density of dc solid state transformers. In this article, a novel five-level (5L) dual active bridge (DAB) converter topology is proposed. By stacking two neutral-point-clamped (NPC) converters at the primary side, the proposed topology could quadruple the input dc-bus voltage of a conventional two-level (2L) DAB converter. Besides, by analyzing the working principle of the 5L-DAB converter, a voltage balancing control method is carried out. With the aid of the voltage balancing method, voltages of the four series-connected dc-bus capacitors in the 5L-DAB converter can be actively balanced, guaranteeing safe operation voltage for all the semiconductor switches. After that, the operation range of the proposed voltage balancing method is derived by theoretical analysis. Finally, the performance of the proposed converter topology and voltage balancing method have been verified by experimental results.

Potentials and Comparison of Inverter Topologies for Future All-Electric Aircraft Propulsion

Decarbonization of the air transport sector is a major challenge for the upcoming years. To achieve this goal, electrification of the propulsion for small to medium civil aircraft is a key enabler for various concepts, ranging from hybrid over fuel cell to full battery powered aircraft. While small electric aircraft for lower altitudes already exist, it is necessary to extend the power range to several megawatts for passenger aircraft flying at medium to high altitudes. Here, the question of the most promising inverter topology and dc supply voltage level is yet to be answered. Challenges include lightweight, high efficiency, and reliability due to cosmic-ray-induced failures. To address this, a broad range of topologies, covering conventional two-level, three-level inverters, and modular multilevel converters (MMCs), are compared in this article. For this purpose, an inverter design methodology is introduced, including chip area optimization, inverter loss, and weight calculation. As a prerequisite, a loss model for high-voltage silicon carbide (SiC) MOSFETs is derived and used to estimate the characteristics for voltage levels not readily available today. Uncertainty is addressed by covering several scenarios.

Single-Phase to Single-Phase Three-Wire Power Converters Based on Two-Level and Three-Level Legs

This article presents two single-phase to single-phase three-wire converters based on two-level and three-level neutral-point-clamped legs. The converters present a shared-leg between the grid and load. A space-vector pulsewidth modulation technique considering the harmonic distortion and the semiconductor losses is presented, besides the dc-link voltage-balance technique to balance the neutral-point voltage. The converters can supply the loads with constant amplitude and frequency under grid voltage disturbances. These characteristics make the proposed converters suitable for applications as uninterrupted power supply. The proposed converters are compared to a conventional two-level converter from simulated results for evaluating the semiconductor losses and harmonic distortion. The experimental results are provided to illustrate and validate the operation of the proposed systems.

A multi-rotor medium-voltage wind turbine system and its control strategy

With the continuous increase in wind turbine power ratings, the growing turbine size makes the turbine manufacture, transport and installation more challenging. At MWs power level, the conventional two-level converters with a 690V voltage have to handle very high current, which causes substantial losses and high cost of power cables, switchgears, etc. To solve this issue, a multi-rotor medium-voltage modular wind turbine configuration is proposed in this paper. Each cell is composed of a turbine rotor, a permanent magnet synchronous generator (PMSG) and a converter. Each PMSG is driven directly by each rotor. The input and output power stages of each converter consist of a three-phase active rectifier and an H-bridge inverter. H-bridge inverters are connected in series to achieve medium-voltage grid connection. This wind turbine configuration has advantages such as more wind power capture, higher reliability, lower transport and installation cost, easy extension to higher power and grid voltage ratings and fault-tolerant operation capability. A control strategy of the wind turbine with maximum wind power capture is proposed. An imbalance compensation control scheme is also proposed to keep three phase output current balanced. Simulation results and experimental tests have validated the proposed wind turbine topology and control strategy.

A simplified SVPWM method for cascaded multilevel inverters

A highly popular alternative in medium voltage and high-power applications is multilevel converters because of their superior performance over conventional two-level converters. The most commonly used control methods in the case of multilevel inverters are sine pulse width modulation (SPWM) and space vector pulse width modulation (SVPWM) methods. Among these two control strategies, SVPWM has superior performance over SPWM in terms of DC bus voltage utilization along with a reduction in total harmonic distortion (THD) of line voltages. The classical SVPWM method has various drawbacks such as computational complexity for identifying the location of reference voltage vector, sector identification, region identification, memory requirement to store lookup tables for switching vectors. The novel simplified SVPWM technique is presented for cascaded H-Bridge multilevel inverter (CHBMLI) in this paper. This simplified SVPWM method has overcome the drawbacks of the classical SVPWM method. This new technique has been implemented into a five-level CHBMLI to evaluate performance and also to compare with the SPWM method. The simulation has been performed in MATLAB software.

Qualitative and quantitative analysis of the reliability of NPC and ANPC power converters for aeronautical applications

In more electrical aircraft, most flight control wing surface actuators are either electro-hydrostatic or electro-mechanical, whether in manual or autopilot mode. Their reliability is essential for flight safety; failure of even one actuator can have catastrophic consequences. According to ARP4761 guidelines for the development and certification of critical aircraft systems, the probability of failure of flight-control actuator components must be less than 10-9 per hour of flight. In this paper, we compare the probability of failure per hour (PFH) of conventional two-level power converters and three-level NPC and ANPC converter topologies designed in our laboratory. In accordance with ARP procedures, PFH was calculated using an approach that includes qualitative analysis based on failure mode and effect analysis (FMEA) methodology as well as quantitative analysis based on truth tables. ANPC converter met all the requirements of the functional hazard assessment in all cases of defects. Finally, we discuss factors that determine the feasibility of implementing power converters in flight control systems.

Model Predictive Current Control with Asymmetric Stacked Multilevel Inverter and LCL-Filter Based STATCOM

The work presented in this paper deals with a proposal of a new topology of a multilevel inverter to act as a Static synchronous Compensator (STATCOM). The proposed inverter is the five-level Asymmetric Stacked Multi-Level Inverter (ASMLI). One of the essential features of this inverter that distinguishes it from the conventional types is that it achieves the required voltage levels with fewer switching devices, leading to simplifying the control process. Moreover, the work includes using a Finite Control Set Model Predictive Current Control (FCS-MPCC) to control the proposed structure. The FCS-MPCC control strategy performs the finite optimization process at the current sampling instant to provide the optimum switching states to the inverter at the next sampling instant. Therefore, this control strategy allows injecting harmonic current and reactive power compensation to reduce source current distortion and improve the voltage profile and power factor. The optimization mechanism reduces the cost function, which is a function of measuring the network current's deviation from the reference value and how the capacitor voltage deviates from the required values. LCL-filter was used to connect this setup to the grid, and its resonance was actively damped using the multivariable capabilities of the FCS-MPCC. The proposed control framework was simulated using MATLAB/Simulink 9.1 environment and tested in a distorted and healthy network compared to a conventional two-level converter with RL-filter. The STATCOM was used to inject reactive power to raise the source power factor to unity and reduce source current harmonics by injecting harmonic current. The proposed prototype could absorb 70% of source current harmonics, which is nearly 25% better than a conventional inverter, inject an appropriate amount of reactive power, and raise the source power factor to unity in two case scenarios. The performance achieved was promising at steady-state operation and speedy response during transients with balanced capacitors voltages.

Analysis and experimental verification of improved two-level converter behaviors under unbalanced AC conditions

An improved two-level converter (I2LC) represents a practical design trade-off between the conventional two-level converter (C2LC) and modular multilevel converter (MMC), in which the MMC like AC and DC fault responses and relative simplicity of C2LC power circuit and control are prioritized. Therefore, this paper presents theoretical analysis, which helps to develop broader understandings of the I2LC, and identification of the needed countermeasures to ensure correct operation during balanced and unbalanced grid conditions. The correctness of the findings and observations drawn from the presented analysis has been validated with the aid of scaled-down simulations and experimentations of the I2LC. Unlike the C2LC, it has been revealed that the I2LC can offer two concurrent operational objectives at system level during unbalanced AC grid conditions, such as balanced output currents and ripple-free DC link current. Further investigation conducted on the performance of the I2LC based two-terminal DC link during asymmetrical AC faults corroborates the theoretical analysis and scaled-down simulation and experimental studies.

A Real-Time Fault Diagnosis for Neutral-Point-Clamped Inverters Based on Failure-Mode Algorithm

The fault diagnosis in multilevel converters is of great importance since they are comprised of more solid-state switches, compared with the conventional two-level converters. In this article, we propose a fast and efficient real-time fault detection and faulty component identification for an open-switch fault diagnosis based on the failure-mode algorithm for a single-phase five-level neutral-point-clamped inverter. Component modeling, complicated calculations, and load parameters are not required for realizing the proposed approach. Furthermore, the proposed strategy does not require any extra sensors. The signals already available in the control system, such as the applied switching pattern and terminal voltage, fulfill the fault diagnosis. Thanks to the proposed fault detection strategy, any misdiagnosis arising from noise existence, response time of the sensors, turn on and turn off delay times of the power switches, and measurement errors are avoided. The simulations and experiments are carried out to confirm the performance and effectiveness of the presented strategy.

A New Multilevel Inverter Topology With Reduced Power Components for Domestic Solar PV Applications

Power electronic converters are used to nullify the input fluctuations from a solar photovoltaic unit because of intermittent solar irradiance and to make the terminal voltage grid compatible with the desired frequency. The conventional two-level converters suffer from low power quality and high voltage stress. In this article, a new multilevel inverter topology called Dual Source Multilevel Inverter (DS-MLI) with fewer power switches is proposed for solar PV power conversion systems. It can operate in symmetric and asymmetric operating modes with no cascading. This reduces the switching components required to produce several levels in the staircase voltage waveform. A closed-loop control algorithm is designed using the state-space averaging technique, and we assess the dynamic behaviour of the system under step change. We carry the simulation out in MATLAB environment. The experimental prototype of DS-MLI rated 1 kW is fabricated using FGA25N120-ANTD IGBTs, and an eco-sense made solar PV emulator is used for analysing the performance of DS-MLI while interfacing with solar PV unit. We compare the suggested scheme with its conventional counterpart in the aspects of components required, cost and efficiency, and the results are presented.

Application of common-mode-free control topology to a two-level inverter

The use of asynchronous motors in electric drive systems has become widespread and convenient. Also, since the frequency converters became wide-spread and popular devices, induction motors became the most popular machine type in both constant- and variable-speed applications. However, the conventional two-level frequency converter’s output three-phase waveform is not fully repeating ideal sine waves provided from the grid. The so-called common-mode voltage appears because of specifics of the converter operation and works as the source of harmful parasitic phenomena of bearing currents. Flowing through the motor bearings, these currents providing a list of undesirable effects, finally causing the bearing properties degradation and early system failure. Current paper deals with the novel control method described earlier and show the method implementation by the means of the two-level frequency converter.

Modeling, Control, and Experimental Verification of a DFIG With a Series-Grid-Side Converter With Voltage Sag, Unbalance, and Distortion Compensation Capabilities

This article presents the modeling and experimental validation of a recently presented DFIG wind power system driven by a two-level three-phase converter in which the grid-side converter is connected in series with the grid. The series-grid-side converter is able to compensate for stator-voltage sags, unbalance and harmonic distortion resulting in a high-power-quality system with considerable low-voltage-ride-through capability. The proposed control technique makes it feasible to obtain such high-power-quality system using a conventional two-level three-phase converter composed of 12 IGBTs, with no need of extra switches, while maintaining the usual high efficiency and low cost inherent to the DFIG configuration. Moreover, using the proposed linear model, one can design the dc-link-voltage controller's parameters according to the desired dynamic behavior. Experimental results are presented using a 15 kW test bench, which validate the proposed control technique.

Application of Multi-Level Converter for Fast Current Control in Small-Scale DC Power Network

DC microgrids have been emerging as next-generation small-scale electric power networks, where the line impedance is very low. This phenomenon causes large currents in the microgrids, even for a slight change in voltage; therefore, it is critical for a power flow controller to have faster transient response and precise power flow control. In this study, multi-level converters are applied as the power flow controllers to realize high-speed and high-precision power flow control in a dc microgrid. The output filter can be small, as a multi-level converter is used. This paper also presents the design of the output LC filter of a multi-level converter to satisfy a requirement of current ripple. We experimentally verified that a multi-level converter with a smaller filter can realize high-speed and high-precision power flow control for low line impedance conditions compared with the conventional two-level converters.

A Generalized Switching Strategy and Capacitor Sizing Algorithm for Granular Multilevel Converters

The granular multilevel converter (GMC) is a multilevel converter built by hybridizing a conventional two-level converter with an asymmetrical cascaded H-bridge converter. Addition of each additional H-bridge submodule doubles the number of the GMC output voltage steps. Thus, with a small number of power electronic switches and associated gate drive signals and circuits, a GMC can offer a high-quality ac voltage to meet the stringent requirements of high-performance drives and grid-tied converters in low and medium voltages. This paper proposes: 1) a generalized switching strategy to control the output voltage as well as capacitor voltages of H-bridge submodules for a GMC with an arbitrary number of submodules $n$ (this proposed strategy is scalable, which allows for easy modification of the voltage rating or the number of the voltage steps of the GMC) and 2) a systematic analysis for optimal sizing of the submodule capacitors to minimize the required total energy of the installed capacitors. Simulation and experimental case studies evaluate the performance of the proposed switching and capacitor sizing algorithms.

An Analytical Method to Evaluate and Design Hybrid Switched-Capacitor and Multilevel Converters

This paper investigates the use of multilevel conversion in dc–dc applications that require a large voltage conversion ratio. A quantitative method that can serve as a guide to compare and design multilevel topologies for large conversion ratio applications is presented. The proposed method keeps the conduction loss and switching loss constant across the different converters and employs the passive component volume as the single performance metric. As examples, flying capacitor multilevel converters and hybrid switched-capacitor (SC) converters are compared to conventional two-level buck converters, and are shown analytically to have significantly reduced passive component size. Three converter prototypes are implemented, based on the presented methodology to experimentally validate the method as well as demonstrate the advantages of multilevel and hybrid SC converters.

An Asymmetric Three-Level Neutral Point Diode Clamped Converter for Switched Reluctance Motor Drives

An asymmetric three-level neutral point diode clamped converter for switched reluctance motor drives is proposed in this paper. The modulation method, the dc-link voltage balancing algorithm, and the current control scheme of this converter are presented. The proposed three-level converter is compared with the conventional two-level asymmetric half-bridge converter in terms of cost, current ripple, noise, and power losses. Compared to the conventional two-level converter, the proposed three-level converter has much lower current ripple, lower noise, and higher efficiency. The effectiveness of the proposed converter is verified by both simulation and experimental results.

High-Efficiency mosfet-Based MMC Design for LVDC Distribution Systems

Low-voltage dc (LVdc) distribution networks have the potential to release larger capacity without having to upgrade the existing cables. One of the main challenges of LVdc networks is the extra customer-end dc–ac conversion stage. This paper proposes and evaluates a five-level Si mosfet -based modular multilevel converter (MMC) as a promising alternative to the conventional two-level insulated gate bipolar transistor-based converter. This is due to the comparatively higher efficiency, power quality and reliability, and reduced electromagnetic (EM) emissions. A comprehensive analysis of a Si mosfet five-level MMC converter design is performed to investigate the suitability of the topology for LVdc applications. Detailed theoretical analysis of the five-level MMC is presented, with simulated and experimental results to demonstrate circuit performance. To suppress the ac circulating current, especially the dominant second harmonics, this paper presents a double line-frequency proportional integral (PI) with orthogonal imaginary axis control method. Comparison of simulation and experimental results with those for double line-frequency proportional resonant control shows that the proposed PI controller has a better performance. In addition, it is simpler to implement and more immune to sampling/discretization errors.

Improved Two-Level Voltage Source Converter for High-Voltage Direct Current Transmission Systems

This paper presents an improved two-level voltage source converter for dc transmission systems with relatively low rated power and dc operating voltage. Unlike conventional two-level converter, the presented converter employs two distributed cell capacitors per three phase; thus, do not contribute any current when converter is blocked during dc short-circuit fault as in modular multilevel converter case. The use of three-phase cells is proven to be beneficial because the arm currents do not contain second-order harmonic currents, and cell capacitors tend to be small as they only experience high-order harmonic current associated with the switching frequency. For the same rated dc-link voltage and switching devices, the rated power of the improved two-level converter will be twice than that of the conventional two-level converter. Averaged, switching function, and electromagnetic transient simulation models of the improved two-level converter are discussed and validated against detailed switch model. The viability of the improved two-level converter for high-voltage dc applications is examined, considering dc and ac short-circuit faults. Besides, reduced complexity of the control and power circuit of the improved two-level converter, it has been found that its transient responses to ac and dc faults are similar to that of the modular multilevel converter.