Model Predictive Control Technique of Multilevel Inverter for PV Applications

Renewable energy sources, such as solar, wind, hydro, and biofuels, continue to gain popularity as alternatives to the conventional generation system. The main unit in the renewable energy system is the power conditioning system (PCS). It is highly desirable to obtain higher efficiency, lower component cost, and high reliability for the PCS to decrease the levelized cost of energy. This suggests a need for new inverter configurations and controls optimization, which can achieve the aforementioned needs. To achieve these goals, this dissertation presents a modified multilevel inverter topology for grid-tied photovoltaic (PV) system to achieve a lower cost and higher efficiency comparing with the existing system. In addition, this dissertation will also focus on model predictive control (MPC) which controls the modified multilevel topology to regulate the injected power to the grid. A major requirement for the PCS is harvesting the maximum power from the PV. By incorporating MPC, the performance of the maximum power point tracking (MPPT) algorithm to accurately extract the maximum power is improved for multilevel DC-DC converter. Finally, this control technique is developed for the quasi-z-source inverter (qZSI) to accurately control the DC link voltage, input current, and produce a high quality grid injected current waveform compared with the conventional techniques. This dissertation presents a modified symmetrical and asymmetrical multilevel DC-link inverter (MLDCLI) topology with less power switches and gate drivers. In addition, the MPC technique is used to drive the modified and grid connected MLDCLI. The performance of the proposed topology with finite control set model predictive control (FCS-MPC) is verified by simulation and experimentally. Moreover, this dissertation introduces predictive control to achieve maximum power point for grid-tied PV system to quicken the response by predicting the error before the switching signal is applied to the converter. Using the modified technique ensures the iii system operates at maximum power point which is more economical. Thus, the proposed MPPT technique can extract more energy compared to the conventional MPPT techniques from the same amount of installed solar panel. In further detail, this dissertation proposes the FCS-MPC technique for the qZSI in PV system. In order to further improve the performance of the system, FCS-MPC with one step horizon prediction has been implemented and compared with the classical PI controller. The presented work shows the proposed control techniques outperform the ones of the conventional linear controllers for the same application. Finally, a new method of the parallel processing is presented to reduce the time processing for the MPC

A multilevel medium-voltage inverter for step-up-transformer-less grid connection of photovoltaic power plants

Recently, medium (0.1-5 MW) and large (>5 MW) scale photovoltaic (PV) power plants have attracted great attention, where medium-voltage grid connection (typically 6-36 kV) is essential for efficient power transmission and distribution. A power frequency transformer operated at 50 or 60 Hz is generally used to step up the traditional inverter's low output voltage (usually ≤400 V) to the medium-voltage level. Because of the heavy weight and large size of the power frequency transformer, the PV inverter system can be expensive and complex for installation and maintenance. As an alternative approach to achieve a compact and lightweight direct grid connection, this paper proposes a three-phase medium-voltage PV inverter system. The 11-kV and 33-kV PV inverter systems are designed. A scaled down three-phase 1.2-kV test rig has been constructed to validate the proposed PV inverter. The experimental results are analyzed and discussed, taking into account the switching schemes and filter circuits. The experimental results demonstrate the excellent feature of the proposed PV inverter system. © 2011-2012 IEEE

Isolated Single-stage Power Electronic Building Blocks Using Medium Voltage Series-stacked Wide-bandgap Switches

The demand for efficient power conversion systems that can process the energy at high power and voltage levels is increasing every day. These systems are to be used in microgrid applications. Wide-bandgap semiconductor devices (i.e. Silicon Carbide (SiC) and Gallium Nitride (GaN) devices) are very promising candidates due to their lower conduction and switching losses compared to the state-of-the-art Silicon (Si) devices. The main challenge for these devices is that their breakdown voltages are relatively lower compared to their Si counterpart. In addition, the high frequency operation of the wide-bandgap devices are impeded in many cases by the magnetic core losses of the magnetic coupling components (i.e. coupled inductors and/or high frequency transformers) utilized in the power converter circuit. Six new dc-dc converter topologies are propose. The converters have reduced voltage stresses on the switches. Three of them are unidirectional step-up converters with universal input voltage which make them excellent candidates for photovoltaic and fuel cell applications. The other three converters are bidirectional dc-dc converters with wide voltage conversion ratios. These converters are very good candidates for the applications that require bidirectional power flow capability. In addition, the wide voltage conversion ratios of these converters can be utilized for applications such as energy storage systems with wide voltage swings

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

Grid Integration of DC Buildings: Standards, Requirements and Power Converter Topologies

Residential dc microgrids and nanogrids are the emerging technology that is aimed to promote the transition to energy-efficient buildings and provide simple, highly flexible integration of renewables, storages, and loads. At the same time, the mass acceptance of dc buildings is slowed down by the relative immaturity of the dc technology, lack of standardization and general awareness about its potential. Additional efforts from multiple directions are necessary to promote this technology and increase its market attractiveness. In the near-term, it is highly likely that the dc buildings will be connected to the conventional ac distribution grid by a front-end ac-dc converter that provides all the necessary protection and desired functionality. At the same time, the corresponding requirements for this converter have not been yet consolidated. To address this, present paper focuses on various aspects of the integration of dc buildings and includes analysis of related standards, directives, operational and compatibility requirements as well as classification of voltage levels. In addition, power converter configurations and modulation methods are analyzed and compared. A classification of topologies that can provide the required functionality for the application is proposed. Finally, future trends and remaining challenges pointed out to motivate new contributions to this topic

Z Source Inverter Topologies-A Survey

Need for alternative energy sources to satisfy the rising demand in energy consumption elicited the research in the area of power converters/inverters. An increasing interest of using Z source inverter/converter in power generation involving renewable energy sources like wind and solar energy for both off grid and grid tied schemes were originated from 2003. This paper surveys the literature of Z source inverters/converter topologies that were developed over the years

Quasi impedance source based high power medium voltage converter for grid integration of distributed energy sources

The next generation of Power Electronics systems would need to be able to work at higher power levels, higher switching frequencies, compact size, and higher ambient temperatures, as well as should have improved energy efficiency than existing Silicon (Si) devices. As a result, new wide bandgap semiconductor technologies must be introduced to address Si's physical limitations. Silicon Carbide (SiC) devices are becoming popular because of their outstanding properties that address all the requirements of the next generation Power Electronics system. On the other hand, the converter topology still plays a major role in deciding the overall system performance. Hence the major objective of this dissertation is to devise new multilevel quasi impedance source (qZS) based converter topologies using SiC devices to achieve a compact, highly efficient, and modular solution for grid integration of Solar PV Energy Source to the utility grid. Other objectives include modification in the PWM methods to address the problem of unequal power-sharing in Solar PV multilevel converters. By using qZS as the front-end power converter several different power converter topologies have been developed and presented in this dissertation. The detailed design, modulation, loss analysis, and control have been developed for multi module cascaded structure. Level-shifted PWM technique is developed at first for two cascaded modules which are similar to the standard Phase opposed disposed Pulse width modulation (PODPWM). However, this control method cannot be directly applied to a higher number of modules. For more than two cascaded modules a unified combined hybrid PWM technique is developed and presented. During normal balanced operation, the power among the modules is unequal. To address the unequal power sharing problem, further modification in the PWM technique is done called the Carrier rotation technique. For providing the isolation between the low voltage PV panels and the high voltage AC grid, a modified Inverter topology, and a new modulation technique is developed. The presented technique, however, is limited to a single module, and more research is needed to implement for cascaded structure. Front-end qZS based single-stage DC-AC-DC converter is developed as an alternative of one of the most popular conventional dual active bridge (DAB) converter. The proposed converter offers reduced component count while maintaining the continuous input current. The detailed operation, modulation technique, simulation, and experimental result are presented to show the superiority of the developed qZS Cascaded Multilevel Converter. The developed power converter has strong commercialization potentia