The Essential Role and the Continuous Evolution of Modulation Techniques for Voltage-Source Inverters in the Past, Present, and Future Power Electronics

The cost reduction of power-electronic devices, the increase in their reliability, efficiency, and power capability, and lower development times, together with more demanding application requirements, has driven the development of several new inverter topologies recently introduced in the industry, particularly medium-voltage converters. New more complex inverter topologies and new application fields come along with additional control challenges, such as voltage imbalances, power-quality issues, higher efficiency needs, and fault-tolerant operation, which necessarily requires the parallel development of modulation schemes. Therefore, recently, there have been significant advances in the field of modulation of dc/ac converters, which conceptually has been dominated during the last several decades almost exclusively by classic pulse-width modulation (PWM) methods. This paper aims to concentrate and discuss the latest developments on this exciting technology, to provide insight on where the state-of-the-art stands today, and analyze the trends and challenges driving its future

A hybrid multilevel converter for medium and high voltage applications

This paper investigates the suitability of the hybrid multilevel converter for medium and high voltage application. The converter operation, modulation, and capacitor voltage balancing method are described in detail. The ability of the hybrid multilevel converter to operate with different modulation indices and load power factors is investigated. It has been established that the hybrid multilevel converter is capable of operating independent of load power factor. Operation with variable modulation index increases voltage stresses on the converter switches and does not alter the fundamental voltage magnitude as in all known voltage source converter topologies. The viability of the hybrid multilevel converter for medium and high voltage applications is confirmed by simulations

Analysis of Modulation and Voltage Balancing Strategies for Modular Multilevel Converters

Modular multilevel converters are an emerging voltage source converter topology suitable for many applications. The increased utilization of HVDC power transmission solutions has resulted in modular multilevel converters becoming a more common converter type. Other applications include interfacing renewable energy power sources to the grid and motor drives. Modular multilevel converters are beneficial for medium voltage motor drives because the properties of this converter topology, such as low distortion, allow for an efficient motor drive design. Modular multilevel converters are designed where instead of using 6 IGBTs as in a conventional voltage source converter, numerous low-rated IGBTs are used to produce the desired voltage. The converter is made up of a series of IGBT half-bridge circuits with a capacitor across both devices. Benefits of this converter include reduced semiconductor device costs due to the ability to use more commercially available low-rated IGBTs and reduced or potentially the elimination of filter components. The number of voltage levels which corresponds to the number of submodules is what causes the harmonic reduction allowing for this omission. Other benefits include lower operating switching frequencies which also results in reduced converter losses. A drawback to using modular multilevel converters is an increase in the complexity of the control schemes and data processing requiring many more sensors and because of this a thorough understanding of the benefits and limitations of all the control strategies is desired. One area of control flexibility is in the pulse width modulation and voltage balancing algorithms applicable to modular multilevel converters. The pulse width modulation options are multicarrier solutions that focus on two categories: phase-shifted PWM which utilizes multiple carrier waveforms with the same frequency and amplitude but a different phase shift and level-shifted techniques which utilize multiple carrier waveforms with identical frequency and amplitude but a different DC bias. An important aspect of modular multilevel converters is that the capacitor voltages need to be as closely balanced to the desired DC voltage as possible with a typical acceptable voltage ripple of 10%. In order to achieve this, various voltage balancing algorithms have been developed for modular multilevel converters with this work focusing on two common algorithms. This work focuses on analyzing both modulation techniques and voltage balancing algorithms using a range of metrics to better understand the most applicable strategies based on the specific application of the converter. A MATLAB/Simulink model using SimPowerSystems of a 21-level three phase modular multilevel converter has been built in order to implement and analyze the various methodologies. The result will be a comprehensive analysis of the optimal approach based on capacitor voltage ripple, converter power loss, and converter voltage THD

Double-Carrier Phase-Disposition Pulse Width Modulation Method for Modular Multilevel Converters

Modular multilevel converters (MMCs) have become one of the most attractive topologies for high-voltage and high-power applications. A double-carrier phase disposition pulse width modulation (DCPDPWM) method for MMCs is proposed in this paper. Only double triangular carriers with displacement angle are needed for DCPDPWM, one carrier for the upper arm and the other for the lower arm. Then, the theoretical analysis of DCPDPWM for MMCs is presented by using a double Fourier integral analysis method, and the Fourier series expression of phase voltage, line-to-line voltage and circulating current are deduced. Moreover, the impact of carrier displacement angle between the upper and lower arm on harmonic characteristics is revealed, and further the optimum displacement angles are specified for the circulating current harmonics cancellation scheme and output voltage harmonics minimization scheme. Finally, the proposed method and theoretical analysis are verified by simulation and experimental results

Performance Evaluation of Pulse Width Modulation Techniques for Losses Reduction in Modular Multilevel Flying Capacitor Converter

This paper presents the analysis and evaluation of power losses in a modular multilevel flying capacitor converter (MMFCC) controlled by three different pulse width modulation techniques. A new unipolar hybrid PWM scheme, which combines phase disposed PWM (PD-PWM) and phase shifted PWM (PS-PWM), is proposed. Detailed electrical and thermal models of the single star configured FC cells are implemented and simulated using MATLAB/SIMULINK and PLECS. The conduction and switching losses of semiconductor devices and power losses of floating capacitors in the simulated MMCC are evaluated. The results show that the proposed PWM scheme gives the lowest overall power losses and hence the highest efficiency of the three methods under different modulation index variations. Furthermore, the quality of the voltage waveform of the proposed method is as good as that obtained by using PS-PWM

New Topologies and Advanced Control of Power Electronic Converters for Renewable Energy based Microgrids

Solar energy-based microgrids are increasingly promising due to their many features, such as being environmentally friendly and having low operating costs. Power electronic converters, filters, and transformers are the key components to integrate the solar photovoltaic (PV) systems with the microgrids. The power electronic converters play an important role to reduce the size of the filter circuit and eliminate the use of the bulky and heavy traditional power frequency step-up transformer. These power converters also play a vital role to integrate the energy storage systems such as batteries and the superconducting magnetic energy storage (SMES) unit in a solar PV power-based microgrid. However, the performance of these power converters depends upon the switching technique and the power converter configuration. The switching techniques can improve the power quality, i.e. lower total harmonic distortion at the converter output waveform, reduce the converter power loss, and can effectively utilize the dc bus voltage, which helps to improve the power conversion efficiency of the power electronic converter. The power converter configuration can reduce the size of the power converter and make the power conversion system more efficient. In addition to the advanced switching technique, a supervisory control can also be integrated with these power converters to ensure the optimal power flow within the microgrid. First, this thesis reviews different existing power converter topologies with their switching techniques and control strategies for the grid integration of solar PV systems. To eliminate the use of the bulky and heavy line frequency step-up transformer to integrate solar PV systems to medium voltage grids, the high frequency magnetic linkbased medium voltage power converter topologies are discussed and compared based on their performance parameters. Moreover, switching and conduction losses are calculated to compare the performance of the switching techniques for the magnetic-linked power converter topologies. In this thesis, a new pulse width modulation technique has been proposed to integrate the SMES system with the solar PV system-based microgrid. The pulse width modulation technique is designed to provide reactive power into the network in an effective way. The modulation technique ensures lower total harmonic distortion (THD), lower switching loss, and better utilization of dc-bus voltage. The simulation and experimental results show the effectiveness of the proposed pulse width modulation technique. In this thesis, an improved version of the previously proposed switching technique has been designed for a transformer-less PV inverter. The improved switching technique can ensure effective active power flow into the network. A new switching scheme has been proposed for reactive power control to avoid unnecessary switching faced by the traditional switching technique in a transformer-less PV inverter. The proposed switching technique is based on the peak point value of the grid current and ensures lower switching loss compared to other switching techniques. In this thesis, a new magnetic-linked multilevel inverter has been designed to overcome the issues faced by the two-level inverters and traditional multilevel inverters. The proposed multilevel inverter utilizes the same number of electronic switches but fewer capacitors compared to the traditional multilevel inverters. The proposed multilevel inverter solves the capacitor voltage balancing and utilizes 25% more of the dc bus voltage compared to the traditional multilevel inverter, which reduces the power rating of the dc power source components and also extends the input voltage operating range of the inverter. An improved version magnetic-linked multilevel inverter is proposed in this thesis with a model predictive control technique. This multilevel inverter reduces both the number of switches and capacitors compared to the traditional multilevel inverter. This multilevel inverter also solves the capacitor voltage balancing issue and utilizes 50% more of the dc bus voltage compared to the traditional multilevel inverter. Finally, an energy management system has been designed for the developed power converter and control to achieve energy resiliency and minimum operating cost of the microgrid. The model predictive control-based energy management system utilizes the predicted load data, PV insolation data from web service, electricity price data, and battery state of charge data to select the battery charging and discharging pattern over the day. This model predictive control-based supervisory control with the advanced power electronic converter and control makes the PV energy-based microgrid more efficient and reliable

Cascaded Converters For Integration And Management Of Grid Level Energy Storage Systems

ABSTRACT CASCADED CONVERTERS FOR INTEGRATION AND MANAGEMENT OF GRID-LEVEL ENERGY STORAGE SYSTEMS by ZUHAIR ALAAS December 2017 Advisor: Dr. Caisheng Wang Major: ELECTRICAL ENGINEERING Degree: Doctor of Philosophy This research work proposes two cascaded multilevel inverter structures for BESS. The gating and switching control of switching devices in both inverter typologies are done by using a phase-shifted PWM scheme. The first proposed isolated multilevel inverter is made up of three-phase six-switch inverter blocks with a reduced number of power components compared with traditional isolated CHB. The suggested isolated converter has only one battery string for three-phase system that can be used for high voltage and high power applications such as grid connected BESS and alternative energy systems. The isolated inverter enables dq frame based simple control and eliminates the issues of single-phase pulsating power, which can cause detrimental impacts on certain dc sources. Simulation studies have been carried out to compare the proposed isolated multi-level inverter with an H-bridge cascaded transformer inverter. The simulation results verified the performance of the isolated inverter. The second proposed topology is a Hierarchal Cascaded Multilevel Converter (HCMC) with phase to phase SOC balancing capability which also for high voltage and high power battery energy storage systems. The HCMC has a hybrid structure of half-bridge converters and H-bridge inverters and the voltage can be hierarchically cascaded to reach the desired value at the half-bridge and the H-bridge levels. The uniform SOC battery management is achieved by controlling the half-bridge converters that are connected to individual battery modules/cells. Simulation studies and experimental results have been carried on a large scale battery system under different operating conditions to verify the effectiveness of the proposed inverters. Moreover, this dissertation presents a new three-phase SOC equalizing circuit, called six-switch energy-level balancing circuit (SSBC), which can be used to realize uniform SOC operation for full utilization of the battery capacity in proposed HCMC or any CMI inverter while keeping balanced three-phase operation. A sinusoidal PWM modulation technique is used to control power transferring between phases. Simulation results have been carried out to verify the performance of the proposed SSBC circuit of uniform three-phase SOC balancing

Carrier-based sinusoidal pulse-width modulation techniques for flying capacitor modular multi-level cascaded converter

Carrier-based sinusoidal pulse-width modulation (PWM) techniques, such as phase disposoed PWM(PD-PWM) and phase shifted PWM (PS-PWM), are widely applied to control the modular multilevel cascaded converters (MMCC) having full H-bridge as sub-modules. This paper evaluates these PWM techniques when controlling a variant of the H-bridge MMCC, i.e. the MMCC five-level flying capacitor converter as sub-modules. This MMCC poses an extra challenge to PWM schemes; namely maintaining two inner floating capacitor voltage balancing. Two novel PWM techniques known as the swapped carrier PWM techniques are introduced for the control of this converter. The paper compares them with the two conventional ones using a performance metrics composed of voltage waveform performance, capability in natural flying capacitor voltage balancing, converter power loss, and switch utilisation. The results show that the proposed new PWM schemes outperform both conventional methods in both switching and conduction power losses and achieve similar performance like the PS-PWM under the three other metrics

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

Enhanced control strategy of full-bridge modular multilevel converter

This paper describes a control approach that allows the cell capacitor voltages of the full-bridge modular multilevel converter (FB-MMC) to be controlled independent of the input dc link voltage. Moreover, this control approach offers the possibility of operating FB-MMC from bi-polar dc link voltages; thus, creating new possibilities for building generic hybrid dc grids with reversible dc link voltage, where the conventional line commutated current source converters can operate alongside voltage source converters. Furthermore, the presented control approach improves the dc fault ride-through of the FB-MMC compared to existing approaches. This could be achieved by an active control of the arm currents and cell capacitor voltages, and full exploitation of the FB-MMC redundant switch states. Operation of the FB-MMC with reversible DC link voltage and decoupled control of the cell capacitor voltages from the dc link voltage are demonstrated using simulations. The major findings and implications of this work are highlighted