Multilevel Topology Research Articles

A Fault-Tolerant Hybrid Cascaded H-Bridge Multilevel Inverter

The cascaded H-bridge (CHB) inverter is one of the most attractive multilevel topologies for renewable energy applications. Due to the fact that CHB inverters employ a large number of components, they suffer from a higher probability of fault, which reduces the system reliability. A fault-tolerant operation for a CHB inverter is described in this article. New features ensure reliable and robust operation of the converter in the event of a fault. The proposed strategy uses an additional cross-coupled CHB (X-CHB) unit in companion with the existing CHB units to support the output voltage and ensure continuity of operation in the event of an open/short-circuit fault. The operation of the proposed X-CHB inverter is described in detail. Simulation and experimental verification of the proposed concept are demonstrated using a seven-level CHB. Both simulation and experimental results confirm the fault-tolerant operation of the X-CHB for a battery energy storage system in case of switch faults.

SiC-Based Bidirectional Multilevel High-Voltage Gain Switched-Capacitor Resonant Converter with Improved Efficiency

This paper presents the research results of the bidirectional multilevel resonant switched capacitor converter (MRSCC). The converter can achieve a high voltage ratio in multilevel topology, which limits the voltage stress on switches and is able to operate with high power efficiency. The converter can be applied as an interconnector between DC voltage systems used for various applications. This paper presents a method that significantly improves the efficiency of the MSRCC through topology modification. Furthermore, the feasibility of the converter was demonstrated with the use of SiC and Si MOSFET switches, together with suitable passive components. It was demonstrated that the proposed modification of the topology makes the converter very efficient in SiC-based ones and can significantly improve the efficiency of Si MOSFET converters. The series of test results of the SiC-based converter is a novel aspect presented in this paper and shows promising achievements of efficiency. The results were obtained from the laboratory setup of 5 kW and 0.5/2 kV MRSCC. To demonstrate the bidirectional operation of the converter, a back-to-back setup (0.5/2/0.5 kV) was used. It also demonstrates that such a high-voltage gain converter can be accurately tested with the use of laboratory equipment with a typical voltage range.

Mathematical model of distributed registry architecture based on cloud and fog technologies

The formalized statement of the problem of the distribution of computing and information resources for storing and processing data, as well as the provision of services at the levels of the cloud and fog systems within the framework of optimizing the multi-level topology of the computing network of the distributed registry system from cloud and fog computing paradigm, is presented. The possibility of using a multi-level representation of the topological structure to determine the optimal network architecture of a distributed registry system based on cloud and fog computing using the genetic algorithm for a fractal model of information traffic is shown, in particular, the results of an experiment to minimize response time from the number of generations for Poisson and fractal models are presented traffic.

Determination of efficiency of a five-level multilevel topology utilizing a single bidirectional-blocking-bidirectional-conducting switch

Abstract Numerical simulation for calculation of losses of a new cascaded multilevel inverter (MLI) topology depending upon a bi-directional-blocking-bidirectional–conducting-switch is summarized in this paper. The numerical circuit simulation is performed in PSIM software by utilizing thermal models of power electronic switches. Calculation of conduction and switching losses of a 73 W inverter by relying on data sheet values has also been made. A comparison of efficiency of 73 W inverter using numerical simulation as well as analytical calculation has also been made. The new proposed inverter is a compound of both unidirectional as well as bidirectional power electronic switches challenging the attempt to reduce device count of the cascaded MLI.

Influence of Current Ripples in Cascaded Multilevel Topologies on the Aging of Lithium Batteries

Previous studies have proposed to use cascaded multilevel topologies in stationary battery energy storage systems (BESSs) or in BESSs of electric vehicles, due to the balancing capability or the high efficiency of these topologies. Because of the elimination of the direct current (dc) bus, the batteries in these topologies are expected to experience large current ripples. However, whether these current ripples can accelerate the aging of the batteries has not yet been dedicatedly investigated, although it is a determining factor for the feasibility of this category of topologies. Therefore, this article first summarizes the existing studies regarding the influence of current ripples on the aging of lithium batteries, which proves the necessity of an experimental investigation. Then, a long-term aging experiment on battery cells is conducted, in order to examine the influence of the current ripples in cascaded multilevel topologies. According to the experimental results and the conclusions in previous studies, the ripples in cascaded multilevel topologies generally have a negligible influence on the aging of batteries, except in certain scenarios. These scenarios can be identified by the three preconditions, first, current ripples must contain micro charge and discharge cycles; second, the microcycles are below 10 Hz; third, low frequency microcycles must contribute a large amount of charge throughput.

Simplified Predictive Duty Cycle Control of Multilevel Converters With Internal Identical Structure

Classical finite control set model predictive control of multilevel converters faces two major limitations: heavy computation burden and low steady-state performance. On the other hand, the internal paralleled or cascaded identical structure is extensively employed in multilevel converters as the voltage level increases. Taking advantage of the internal identical structure in multilevel converters, a simplified predictive duty cycle control method for multilevel converters is proposed in this article. First, a series of switching pairs with the identical structure are modeled as a virtual switching state, and the number of the corresponding voltage vectors can be significantly reduced. Then, a least-square optimization problem is formulated to calculate the duty cycles for optimal output current tracking using these virtual voltage vectors. The output current quality is enhanced due to the employment of the interleaved output voltage stage with phase-shifted pulsewidth modulation. As the number of voltage vectors is significantly reduced, the computation burden is drastically reduced. The proposed method can be applied in various multilevel topologies with internal identical structures. Simulation and experimental results on the multilevel converters with internal cascaded/paralleled identical cells are presented to verify the good performance and application value of the proposed method.

Sampling-Time Harmonic Control for Cascaded H-Bridge Converters With Thermal Control

Cascaded H-bridge converter (CHB) is a multilevel topology that is a well-suited solution for multiple applications such as flexible ac transmission systems or motor drives. This paper is focused on a CHB where the cells present an aging mismatch. This can be caused by the maintenance operation which forces the replacement of some damaged cells of the converter with new or repaired ones. In this paper, a new improved approach of the active thermal control (ATC) of the CHB using discontinuous pulsewidth modulation (PWM) (D-PWM) is presented. The D-PWM technique is used to reduce the power losses of one cell reducing its average temperature in order to increase its remaining lifetime. However, the combination of D-PWM with traditional phase-shifted PWM (PS-PWM) introduces high harmonic distortion in the output voltage of the CHB converter at twice the carrier frequency. A detailed harmonic distortion analysis of the CHB output voltage when the D-PWM based ATC is active is presented. From this analysis, a modification of the traditional PS-PWM is derived to eliminate the harmonic distortion at twice the carrier frequency. Experimental results show how the ATC using D-PWM is achieved whereas the harmonic distortion around twice the carrier frequency is eliminated.

Cascaded Multilevel PV Inverter With Improved Harmonic Performance During Power Imbalance Between Power Cells

The difference in power cell irradiances in cascaded multilevel converters results in different duty cycles among those cells when maintaining the maximum power point tracking (MPPT). However, the difference in cell duty cycles is undesired since it is proportional to the output voltage and current distortions. To this regard, a multilevel topology for photovoltaic (PV) applications is proposed, where an H6 bridge power cell is used instead of an H-bridge one. In case of solar irradiance mismatch among the power cells, the proposed converter injects power with lower voltage from the shaded cells without altering the PV voltage; hence maintaining the MPPT operation. This modification allows us to retain an equal duty cycle in all the power cells whatever the meteorological conditions are present; consequently, maintaining good output voltage and current waveform qualities. To test the effectiveness of the proposed solution, a detailed simulation model as well as an experimental prototype is built. The obtained results show that the proposed topology provides significantly improved output voltage and current qualities compared to the cascaded H-bridge one. The performance of the proposed topology compared to one offering improved harmonics performance, according to the European efficiency, has been also compared, where an enhancement of 2.64% has been registered.

A Novel Four Switch Infinite Level Inverter

A novel Four Switch Infinite Level Inverter (FSILI) is proposed in this paper. In conventional multilevel inverters, as the number of levels increases the output voltage becomes more sinusoidal. Unlike conventional multilevel topologies, the output voltage level in the proposed topology depends upon the switching frequency. Since the switching frequency is very high, the output voltage level approaches infinity, thus the name Infinite Level Inverter. Proposed topology requires only one inductor and capacitor reducing the size, weight and thus cost of the overall system. Inherent buck operation is happening in the proposed topology with a sine varying duty ratio PWM control. Steady-state analysis and design of the inverter are carried out. The proposed topology is simulated using Matlab/Simulink to evaluate the theoretical analysis and operation. A hardware prototype is also developed to validate the operation of proposed FSILI.

Model analysis, simplified control and sensitivity verification of modular multilevel DC–DC converter with parallel branches

In view of the modular multilevel topology with its uses in medium-voltage DC vessel integrated power systems, this paper deals with typical modular multilevel DC–DC converter (MMDC) topologies and AC control methods. In terms of DC control, a brief introduction is first made about a MMDC topology with parallel branches and its static characteristics. Then, an analysis is focused on the modeling of the MMDC and its model simplification. An optimal control strategy is proposed for the simplified model. Finally, the sensitivity of this system to model and parameter uncertainties is verified both theoretically and experimentally. The obtained results show that the proposed MMDC consisting of a heterogeneous full-bridge submodule and a parallel-branch structure can make the submodule voltage self-balancing and the range of output voltage wide. The use of an optimal control strategy based on a simplified model can lead the system to achieve good static and dynamic performance and robustness.

Neighborhood optimization of intelligent wireless mobile network based on big data technology

With the rapid construction and deployment of LTE, the multi-standard, multi-element and multi-level network topology framework of mobile wireless network has been formed, and the fine configuration of neighborhood has become the primary guarantee of the wireless network. In view of the shortcomings of traditional network performance, this paper proposes a time series prediction algorithm based on large data by using a large number of real network existing data and combining the characteristics of big data. Based on the time series feature extraction process of large data method, this paper introduces the concept of burst component and uses different methods to extract component feature to mine the massive data in depth, so as to optimize the intelligent neighborhood of mobile communication network. Then, anomaly detection of the core performance indicators of the wireless mobile network is proposed. Finally, data analysis is carried out for each type of cell, and data prediction is used to obtain the operation state, the proposed optimization algorithm improves the success rate of the wireless network. Experiments verify the reliability and superiority of the algorithm proposed in this paper, especially, the accuracy of the proposed method improves by 5%.

A 3D digital modelling method for railway station subgrade based on generalized hexahedron

ABSTRACT This study aims to propose an efficient modelling method for railway station subgrade based on cross-sectional data. A modelling principle is firstly established to determine the connection relationships between the adjacent cross sections with large differences. And the connected areas between adjacent cross sections are constructed into a solid model based on the method of ‘Generalized Hexahedron’. In addition, the dissection algorithm for the solid model with multi-level topology is studied, realizing the fast cross-section extraction at an arbitrary position. The proposed method is adopted to build a 3D digital subgrade model in a real case study of Ganquan North Station. The results show that the proposed method can realize the efficient modelling and analysis of the digital subgrade model.

Asymmetrical Triangular Current Mode (ATCM) for Bidirectional High Step Ratio Modular Multilevel Dc–Dc Converter

Direct current (Dc) networks have proven advantages in high voltage direct current (HVDC) transmission systems, and now they are expanding to medium- and low-voltage distribution networks. One of the major challenges is to develop reliable dc-dc voltage transformation achieving high efficiency and performance, especially at high voltage and high step ratio. New resonant modular multilevel topologies have arisen as an alternative, mainly because of advantages such as optional use of transformers, natural voltage balance, simple control, and soft-switching capability. However, this type of operation generates a high peak current, does not allow control of power flow in all power range, and has a limited range of voltage variation. This article proposes an asymmetrical triangular current mode applied to high step ratio modular multilevel dc-dc converters. The proposed modulation increases the efficiency and achieves bidirectional control of the power, soft-switching, and a natural balance of the voltage in the cell capacitors. The experimental results show the bidirectional operation and the capacitor voltage balance of the converter under different operating conditions with higher efficiency (97.72%) and lower peak current compared to previous reports of this topology using resonant operation.

A control scheme for grid-tied hybrid modular multilevel converter under grid voltage unbalance

This paper presents a finite-control-set predictive current control (FCS-PCC) scheme for the recently proposed hybrid modular multilevel converter (HMMC)-based grid connected system. The HMMC has several advantages over the conventional modular multilevel converter (MMC) as well as other multilevel topologies. This topology has reduced number of switch counts compared to the conventional MMC, eliminates the problem of circulating current and having higher efficiency. The required number of sub-modules (SM) is half of the conventional MMC, along with an H bridge circuit per phase. This paper analyses the real and reactive power control through FCS-PCC and addresses the AC side grid unbalance by third harmonics injection method. During grid side voltage unbalance, there is a problem of the unequal amount of power exchange between per-phase converter and grid, which needs to be taken care of with an appropriate control method. This study proposed a method to add or subtract an amount of third harmonic component with the reference current signal given to the predictive controller to maintain the grid current at the desired level. The whole concept has been presented along with a simulation study in MATLAB/Simulink environment and the results are presented in support of the described concept.

New Figure-of-Merit Combining Semiconductor and Multi-Level Converter Properties

Figures-of-Merit (FOMs) are widely-used to compare power semiconductor materials and devices and to motivate research and development of new technology nodes. These material- and device-specific FOMs, however, fail to directly translate into quantifiable performance in a specific power electronics application. Here, we combine device performance with specific bridge-leg topologies to propose the extended FOM, or X-FOM, a Figure-of-Merit that quantifies bridge-leg performance in multi-level (ML) topologies and supports the quantitative comparison and optimization of topologies and power devices. To arrive at the proposed X-FOM, we revisit the fundamental scaling laws of the on-state resistance and output capacitance of power semiconductors to first propose a revised device-level semiconductor Figure-of-Merit (D-FOM). The D-FOM is then generalized to a multi-level topology with an arbitrary number of levels, output power, and input voltage, resulting in the X-FOM that quantitatively compares hard-switched semiconductor stage losses and filter stage requirements across different bridge-leg structures and numbers of levels, identifies the maximum achievable efficiency of the semiconductor stage, and determines the loss-optimal combination of semiconductor die area and switching frequency. To validate the new X-FOM and showcase its utility, we perform a case study on candidate bridge-leg structures for a three-phase 10 kW photovoltaic (PV) inverter, with the X-FOM showing that (a) the minimum hard-switching losses are an accurate approximation to predict the theoretically maximum achievable efficiency and relative performance between bridge-legs and (b) the 3-level bridge-leg outperforms the 2-level configuration, despite utilizing a SiC MOSFET with a lower D-FOM than in the 2-level case.

A control scheme for grid-tied hybrid modular multilevel converter under grid voltage unbalance

This paper presents a finite-control-set predictive current control (FCS-PCC) scheme for the recently proposed hybrid modular multilevel converter (HMMC)-based grid connected system. The HMMC has several advantages over the conventional modular multilevel converter (MMC) as well as other multilevel topologies. This topology has reduced number of switch counts compared to the conventional MMC, eliminates the problem of circulating current and having higher efficiency. The required number of sub-modules (SM) is half of the conventional MMC, along with an H bridge circuit per phase. This paper analyses the real and reactive power control through FCS-PCC and addresses the AC side grid unbalance by third harmonics injection method. During grid side voltage unbalance, there is a problem of the unequal amount of power exchange between per-phase converter and grid, which needs to be taken care of with an appropriate control method. This study proposed a method to add or subtract an amount of third harmonic component with the reference current signal given to the predictive controller to maintain the grid current at the desired level. The whole concept has been presented along with a simulation study in MATLAB/Simulink environment and the results are presented in support of the described concept.

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 New Dynamic Voltage Restorer with Multi-level Open-end Winding Topology

In this paper, a variety of main circuit structures of DVR based on open-end winding are presented. Two inverters are connected in series to realize the multi-level structure and simplify the hardware cost. According to the mathematical model of open-end DVR, a three loop control strategy based on load voltage feedback is implemented to suppress the resonance of LCL filter. Aiming at the common mode voltage problem of open-end winding DVR, a common mode voltage suppression strategy based SVPWM is proposed to completely suppress the common mode voltage of high frequency and low frequency of the system. Finally, an open-end DVR experimental platform is built to verify the effectiveness of the control strategy and modulation strategy.

Medium Voltage Large-Scale Grid-Connected Photovoltaic Systems Using Cascaded H-Bridge and Modular Multilevel Converters: A Review

Medium-voltage (MV) multilevel converters are considered a promising solution for large scale photovoltaic (PV) systems to meet the rapid energy demand. This article focuses on reviewing the different structures and the technical challenges of modular multilevel topologies and their submodule circuit design for PV applications. The unique structure of the converter’s submodule provides modularity, independent control of maximum power point tracking (MPPT), galvanic isolation, etc. Different submodule circuits and MPPT methods to efficiently extract the PV power are reviewed. The integration of the multilevel converters to PV systems suffers unbalanced power generation during partial PV shading conditions. Several balancing strategies to solve this problem are presented and compared to give a better understanding of the balancing ranges and capabilities of each strategy. In addition, the paper discusses recent research advancements, and possible future directions of MV converters-based large-scale PV systems for grid integration.