Multilevel Converters: An Enabling Technology for High-Power Applications

| Multilevel converters are considered today as the state-of-the-art power-conversion systems for high-power and power-quality demanding applications. This paper presents a tutorial on this technology, covering the operating principle and the different power circuit topologies, modulation methods, technical issues and industry applications. Special attention is given to established technology already found in industry with more in-depth and self-contained information, while recent advances and state-of-the-art contributions are addressed with useful references. This paper serves as an introduction to the subject for the not-familiarized reader, as well as an update or reference for academics and practicing engineers working in the field of industrial and power electronics.Ministerio de Ciencia y Tecnología DPI2001-3089Ministerio de Eduación y Ciencia d TEC2006-0386

Grid-Connected Single-Star Bridge-Cells Modular Multilevel Cascaded Converter with Selective Harmonic Elimination Techniques

Nowadays, Renewable Energy Sources (RESs) are receiving enormous attention due to the noticeable exhaustion of fossil fuel and its emission of greenhouse gases. DC-AC converters have attracted the attention of the researchers, as they are entailed to integrate RESs to the grid to comply with the grid frequency and voltage requirements. Due to the high penetration of RESs, especially with elevated power levels, high-power converters are needed, which necessitates higher voltage and current ratings of the semiconductor devices. The unavailability of high voltage semiconductor devices has directed the attention to the use of either series connection of semiconductor devices or Multilevel Inverters (MLIs). MLIs allow using several low rated semiconductors to hold the high output power of the inverter. The MLI output waveform is close to sinusoidal in nature, therefore it may require a small filter to enhance the output power quality. There are many types of MLIs, where the most common MLIs are Flying Capacitor, Diode Clamped, and Modular Multilevel Cascaded Converter (MMCC). The MMCC can be classified into three main formations, the Single-Star Bridge-Cells MMCC (SSBC-MMCC), the Double-Star Bridge-Cells MMCC (DSBC-MMCC), and the Double-Star Chopper-Cells MMCC (DSCC-MMCC). The main advantage of the MMCC is the modularity and scalability. In addition, the MMCC does not require any clamping diodes or flying capacitors for clamping the voltage across the switches. In this thesis, the MMCC will be used to integrate high-power RESs to Grid. Nevertheless, the high-power applications necessitate low switching frequency operations. One of the most common controlling techniques of MLI with low frequency operation is the Selective Harmonic Elimination (SHE). SHE insures also the output current Total Harmonic Distortion (THD) to be minimized. One disadvantage of the SHE method is that the complexity of the algorithm along with the equations used is increased by the increase of the MMCC number of levels. Therefore, other alternatives of SHE techniques will be studied in this work to overcome this complexity. This thesis focuses typically on MMCC, particularly the SSBC-MMCC. In this work, a high-power grid-connected SSBC-MMCC is controlled with three different SHE techniques, complying with low switching frequency operation limitation in high-power applications. In addition to the Conventional SHE (C-SHE) technique, Quasi-SHE (Q-SHE) and Asymmetrical-SHE (A-SHE) approaches are proposed and assessed. Q-SHE and A-SHE approaches are based on eliminating selected low order harmonics (for instance, eliminating the fifth and seventh order harmonics), irrelevant to the number of employed levels provided that the number of levels allows for the required harmonic elimination. Compared with the C-SHE approach, the Q-SHE and A-SHE require less computational burden in solving the required equation groups, especially when a high number of levels and/or multiple switching angles for each voltage level are needed, while maintaining the same dv/dt of the output voltage. A 5MW, 17-level, grid-connected SSBC-MMCC, controlled in the synchronous rotating reference frame, is employed for assessing the addressed SHE techniques. The assessment is validated through simulation results using Matlab/Simulink platform

Design and Application of Hybrid Multilevel Inverter for Voltage Boost

Today many efforts are made to research and use new energy sources because the potential for an energy crisis is increasing. Multilevel converters have gained much attention in the area of energy distribution and control due to its advantages in high power applications with low harmonics. They not only achieve high power ratings, but also enable the use of renewable energy sources. The general function of the multilevel converter is to synthesize a desired high voltage from several levels of dc voltages that can be batteries, fuel cells, etc. This dissertation presents a new hybrid multilevel inverter for voltage boost. The inverter consists of a standard 3-leg inverter (one leg for each phase) and H-bridge in series with each inverter leg. It can use only a single DC power source to supply a standard 3-leg inverter along with three full H-bridges supplied by capacitors or batteries. The proposed inverter could be applied in hybrid electric vehicles (HEVs) and fuel cell based hybrid electric vehicles (FCVs). It is of voltage boosting capability and eliminates the magnetics. This feature makes it suitable for the motor running from low to high power mode. In addition to hybrid electric vehicle applications, this paper also presents an application where the hybrid multilevel inverter acts as a renewable energy utility interface. In this dissertation, the structure, operation principle, and modulation control schemes of the proposed hybrid multilevel inverter are introduced. Simulation models and results are described and analyzed. An experimental 5 kW prototype inverter is built and tested

Grid integration of renewable power generation

This thesis considers the use of three-phase voltage and current source inverters as interfacing units for renewable power, specifically photovoltaic (PV) into the ac grid. This thesis presented two modulation strategies that offer the possibility of operating PV inverters in grid and islanding modes, with reduced switching losses. The first modulation strategy is for the voltage source inverter (VSI), and exploits 3rd harmonic injection with selective harmonic elimination (SHE) to improve performance at low and high modulation indices, where the traditional SHE implementation experiences difficulties due to pulse dropping. The simulations and experimentation presented show that the proposed SHE allows grid PV inverters to be operated with less than a 1kHz effective switching frequency per device. This is vital in power generation, especially in medium and high power applications. Pulse dropping is avoided as the proposed modified SHE spreads the switching angles over 90°, in addition increasing the modulation index. The second proposed modulation strategy, called direct regular sampled pulse width modulation (DRSPWM), is for the current source inverter (CSI). It exploits a combination of forced and natural commutation imposed by the co-existence of an insulated gate bipolar transistor in series with a diode in a three phase current source inverter, to determine device dwell times and switching sequence selection. The DRSPWM strategy reduces switching frequency per device in a CSI by suspending each phase for 60°, similar to VSI dead-band, thus low switching losses are expected. Other benefits include simple digital platform implementation and more flexible switching sequence selection and pulse placement than with space vector modulation. The validity of the DRSPWM is confirmed using simulations and experimentation. This thesis also presents a new dc current offset compensation technique used to facilitate islanding or grid operation of inverter based distributed generation, with a reduced number of interfacing transformers. The proposed technique will enable transformerless operation of all inverters within the solar farm, and uses only one power transformer at the point of common coupling. The validity of the presented modulation strategies and dc current offset compensation technique are substantiated using simulations and experimentation.This thesis considers the use of three-phase voltage and current source inverters as interfacing units for renewable power, specifically photovoltaic (PV) into the ac grid. This thesis presented two modulation strategies that offer the possibility of operating PV inverters in grid and islanding modes, with reduced switching losses. The first modulation strategy is for the voltage source inverter (VSI), and exploits 3rd harmonic injection with selective harmonic elimination (SHE) to improve performance at low and high modulation indices, where the traditional SHE implementation experiences difficulties due to pulse dropping. The simulations and experimentation presented show that the proposed SHE allows grid PV inverters to be operated with less than a 1kHz effective switching frequency per device. This is vital in power generation, especially in medium and high power applications. Pulse dropping is avoided as the proposed modified SHE spreads the switching angles over 90°, in addition increasing the modulation index. The second proposed modulation strategy, called direct regular sampled pulse width modulation (DRSPWM), is for the current source inverter (CSI). It exploits a combination of forced and natural commutation imposed by the co-existence of an insulated gate bipolar transistor in series with a diode in a three phase current source inverter, to determine device dwell times and switching sequence selection. The DRSPWM strategy reduces switching frequency per device in a CSI by suspending each phase for 60°, similar to VSI dead-band, thus low switching losses are expected. Other benefits include simple digital platform implementation and more flexible switching sequence selection and pulse placement than with space vector modulation. The validity of the DRSPWM is confirmed using simulations and experimentation. This thesis also presents a new dc current offset compensation technique used to facilitate islanding or grid operation of inverter based distributed generation, with a reduced number of interfacing transformers. The proposed technique will enable transformerless operation of all inverters within the solar farm, and uses only one power transformer at the point of common coupling. The validity of the presented modulation strategies and dc current offset compensation technique are substantiated using simulations and experimentation

Interleaved Buck Converter Based Shunt Active Power Filter with Shoot-through Elimination for Power Quality Improvement

The “shoot-through” phenomenon defined as the rush of current that occurs while both the devices are ON at the same time of a particular limb is one of the most perilous failure modes encountered in conventional inverter circuits of active power filter (APF). Shoot-through phenomenon has few distinct disadvantages like; it introduces typical ringing, increases temperature rise in power switches, causes higher Electromagnetic Interference (EMI) and reduces the efficiency of the circuit. To avert the “shoot-through”, dead time control could be added, but it deteriorates the harmonic compensation level. This dissertation presents active power filters (APFs) based on interleaved buck (IB) converter. Compared to traditional shunt active power filters, the presented IB APFs have enhanced reliability with no shoot-through phenomenon. The instantaneous active and reactive power (p-q) scheme and instantaneous active and reactive current component (id-iq) control scheme has been implemented to mitigate the source current harmonics. Type-1 and Type-2 fuzzy logic controller with different membership functions (MFs) viz. Triangular, Trapezoidal and Gaussian have been implemented for the optimal harmonic compensation by controlling the dc-link voltage and minimizing the undesirable losses occurred inside the APF. Additionally, the adaptive hysteresis band current controller (AHBCC) is being implemented to get the nearly constant switching frequency. The performance of the control strategies and controllers for the presented IB APF topologies has been evaluated in terms of harmonic mitigation and dc-link voltage regulation under sinusoidal, unbalanced sinusoidal and non-sinusoidal voltage source condition. This dissertation is concerned with the different topologies of 3-phase 4-wire IB APFs viz. split capacitor (2C) topology, 4-leg (4L) topology, transformer based full-bridge IB APF or single capacitor based FB IB APF (1C 3 FB IB APF) and full-bridge IB APF (FB IB APF) for low to medium power application. Moreover, APF topology is now being expanded to multilevel VSIs for high power application. Thanks to flexible modular design, transformerless connection, extended voltage and power output, less maintenance and higher fault tolerance, the cascade inverters are good candidates for active power filters with the utility of high power application. The cascaded FB IB APF is modelled with no shoot-through phenomenon by using multicarrier phase shifted PWM scheme. Extensive simulations have been carried out in the MATLAB / Simulink environment and also verified in the OPAL-RT LAB using OP5142-Spartan 3 FPGA to support the feasibility of presented IB APF topologies, control strategies and controllers during steady and dynamic condition. The performance shows that IB-APF topologies bring the THD of the source current well below 5% adhering to IEEE-519 standard. A comparison has also been made, based on SDP (switch device power) between the IB-APF topologies

Transformer-Less Cascaded Voltage Source Converter Based STATCOM

In this work, a transformer-less voltage source converter (VSC) based STATCOM is proposed with a combination of cascaded conventional three-phase voltage source inverters. This modular structure provides multilevel operation with reduced switch count and independent DC-link capacitors. The actual contribution of this paper is the transformer-less configuration of a conventional cascaded voltage source converter which provides reduced cost and volume as compared to other transformer-less converter configurations. The system provides reactive power compensation with better power quality when connected to the nonlinear power electronics load also. A simple control system is provided for balancing the Dc link capacitor voltage and reactive power compensation. The validation of the proposed model is analyzed with simulation using MATLAB/SIMULINK software and the results are obtained with different linear and nonlinear load configurations

Application of the cascaded multilevel inverter as a shunt active power filter

Abstract unavailable please refer to PD

Enhanced decoupling current scheme with selective harmonic elimination pulse width modulation for cascaded multilevel inverter based static synchronous compensator

This dissertation is dedicated to a comprehensive study and performance analysis of the transformer-less Multilevel Cascaded H-bridge Inverter (MCHI) based STATic synchronous COMpensator (STATCOM). Among the shunt-connected Flexible AC Transmission System (FACTS) controllers, STATCOM has shown extensive feasibility and effectiveness in solving a wide range of power quality problems. By referring to the literature reviews, MCHI with separated DC capacitors is certainly the most versatile power inverter topology for STATCOM applications. However, due to the ill-defined transfer functions, complex control schemes and formulations were emerged to achieve a low-switching frequency high-bandwidth power control. As a result, adequate controller parameters were generally obtained by using trial and error method, which were practically ineffective and time-consuming. In this dissertation, the STATCOM is controlled to provide reactive power (VAR) compensation at the Point of Common Coupling (PCC) under different loading conditions. The goal of this work is to enhance the performance of the STATCOM with the associated proposed control scheme in achieving high dynamic response, improving transient performance, and producing high-quality output voltage waveform. To evaluate the superiority of the proposed control scheme, intensive simulation studies and numerous experiments are conducted accordingly, where a very good match between the simulation results and the experimental results is achieved in all cases and documented in this dissertation

High-performance motor drives

This article reviews the present state and trends in the development of key parts of controlled induction motor drive systems: converter topologies, modulation methods, as well as control and estimation techniques. Two- and multilevel voltage-source converters, current-source converters, and direct converters are described. The main part of all the produced electric energy is used to feed electric motors, and the conversion of electrical power into mechanical power involves motors ranges from less than 1 W up to several dozen megawatts

電力系統に接続されたモジュラー・カスケードHブリッジ多段インバータのディジタル制御

九州工業大学博士学位論文 学位記番号：生工博甲第310号 学位授与年月日：平成30年3月23日1. Introduction|2. Cascaded H-bridge Multilevel Inverter And FPGA Hardware Co-simulation|3. LCL filter interfaced DSTATCOM|4. Multiband Hysteresis Current Controlled CHMLI|5. Multiband HCC for Cascaded H-bridge inverter based DSTATCOM|6. Conclusion九州工業大学平成29年