Addressing control and capacitor voltage regulation challenges in multilevel power electronic converters

Multilevel power electronic converters are the current industry solutions for applications that demand medium voltage, reasonable efficiency, and high power quality. The proper operation of these types of power converters requires special control, modulation methods, and capacitor voltage regulation techniques. Both developing capacitor voltage regulation methods and addressing their associated issues with such fall within the primary focus of this dissertation. In this dissertation an investigation was conducted on the capacitor voltage regulation constraints in cascaded H-bridge multilevel converters with a staircase output voltage waveform. In the proposed method, the harmonic elimination technique is used to determine the switching angles. A constraint was then derived to identify modulation those indices that lead to voltage regulation of the capacitor. This constraint can be used in optimization problems for harmonic minimization to guarantee capacitor voltage regulation in these types of converters. Furthermore, a capacitor voltage regulation method was developed using redundant state selection for a flying capacitor active rectifier. This method reduces the number of switching instances by using both online and offline state selection procedure. Additionally, a start-up procedure is proposed that pre-charges the all of capacitors in the rectifier to both avoid overstressing the switches and obtain a smoother start-up. Finally, a flexible capacitor voltage regulation method is proposed that provides the ability to control the voltage of the capacitors in both cascaded H-bridge and hybrid multilevel converters. In this method, the capacitor voltage in each individual H-bridge cell is independently regulated by controlling the active power of each cell

Investigation on Cascade Multilevel inverter for Medium and High-Power Applications

It is hard to connect a single power semiconductor switch directly to medium voltage grids (2.3, 3.3, 4.16, or 6.9 kV). For these reasons, a new family of multilevel inverters has emerged as the solution for working with higher voltage levels. Multilevel inverters have received more attention in industrial application, such as motor drives, static VAR compensators and renewable energy systems, etc. Primarily multilevel inverters are known to have output voltages with more than two levels. As a result, the inverter output voltages have reduced harmonic distortions and high quality of waveforms. Additionally, the devices are confined to fraction of dc-link voltage. These characteristics make multilevel inverter to adopt for high-power and high-voltage applications. A good number of multilevel inverter topologies have been proposed during the last two decades. Contemporary research has engaged novel converter topologies and unique modulation schemes. Moreover, four major multilevel inverter structures have been reported in the literature these are as follows: cascaded H-bridges inverter (CHB) with separate dc sources, diode clamped (neutral-clamped), and flying capacitors (capacitor clamped), P2 Multilevel inverters. Although different multilevel inverter exists, Cascade Multilevel Inverter (CMI) is one of the productive topology from multilevel family. In reality, on comparing with other multilevel based topologies, CMI feature a high modularity degree because each inverter can be seen as a module with similar circuit topology, control structure, and modulation. Therefore, in the case of a fault in one of these modules, it is possible to replace it quickly and easily. Moreover, with an appropriated control strategy, it is possible to bypass the faulty module without stopping the load, bringing an almost continuous overall availability. All this features make CMI an outstanding power converter. However, one of the greatest limitations of CMI is utilization of separate DC source for each H-Bridge cell. This not only increases cost but also affects the reliability of the system. This is the key motivation for this dissertation. In the present work, we have investigated different CMI based topologies with separate and single DC sources and finally proposed a new CMI based configuration with single dc source by using three-phase transformers. The proposed CMI based inverter presented in this thesis is well defined with logical and mathematical approach. Additionally to illustrate the merits, it is compared with traditional multilevel inverters. The feasibility of proposed inverter is demonstrated with different illustrations and confirmed by experimental results. The proposed CMI is well suited for grid / photovoltaic and FACTS systems. To elevate the application of proposed CMI a shunt active power filter (APF) design is demonstrated. In this case, the goal is to inject, in parallel with the load, compensation current to get a sinusoidal source current. The proposed APF is verified through Matlabsimulation. Finally, Opal-RT verifications are performed to verify the final design

Selective harmonic elimination methods for a cascaded H-bridge converter

In recent years there has been an increased demand for integration of renewable energy into the electricity grid. This has increased research into power converter solutions required to integrate renewable technology into the electricity supply. One such converter is a Cascaded H-Bridge (CHB) Multilevel Converter. Operation of such a topology requires strict control of power flow to ensure that energy is distributed equally across the converters energy storage components. For operation at high power levels, advanced modulation methods may be required to ensure that losses due to non-ideal semiconductor switching are minimised, whilst not compromising the quality of the voltage waveform being produced by the converter. This thesis presents several low switching frequency modulation methods based on Selective Harmonic Elimination (SHE) in order to address these two operational issues. The methods presented involve manipulating the H-Bridge cell voltages of the CHB converter to control power flow. Simulated results are supported by experimental verification from a seven level, single phase CHB converter

Selective harmonic elimination methods for a cascaded H-bridge converter

In recent years there has been an increased demand for integration of renewable energy into the electricity grid. This has increased research into power converter solutions required to integrate renewable technology into the electricity supply. One such converter is a Cascaded H-Bridge (CHB) Multilevel Converter. Operation of such a topology requires strict control of power flow to ensure that energy is distributed equally across the converters energy storage components. For operation at high power levels, advanced modulation methods may be required to ensure that losses due to non-ideal semiconductor switching are minimised, whilst not compromising the quality of the voltage waveform being produced by the converter. This thesis presents several low switching frequency modulation methods based on Selective Harmonic Elimination (SHE) in order to address these two operational issues. The methods presented involve manipulating the H-Bridge cell voltages of the CHB converter to control power flow. Simulated results are supported by experimental verification from a seven level, single phase CHB converter

Comparative Analysis of Multilevel Converters for Medium-Voltage Applications

The electric energy demand has been steadily growing during the last century, and all forecasts indicate that it will keep growing in the following years. Within this frame, and due to all the problems that this demand increase generate in the environment, it is necessary improving the current techniques of electric energy conversion and transmission in order to increase the whole system efficiency. On the other hand, it is also necessary increasing the renewable energy resources exploitation through more efficient generation systems. According to these lines, the power electronics systems that have been installed in the last decades allowed to obtain better efficiency from the renewable natural resources like the wind or the solar power. These systems have also notably improved the quality of the power supplied, reducing the losses through what are known as power quality applications. Power converters are currently essential in any power electronics system. Within them, the multilevel converters specially suppose a breakthrough compared with the classical two level converters, as they allow obtaining voltage and current signals with lower harmonic content, what means fewer losses in high power medium voltage applications. In this Thesis a comparative study of some multilevel converter topologies normally used in high power medium voltage applications is done. The objective is analyzing in detail each topology and comparing it with the rest following different criteria, with the aim to know the advantages and drawbacks of each one and to realize which one is more suitable for each application

Modulation techniques for the cascaded H-bridge multi-level converter

This thesis investigates space-vector modulation and one-dimensional modulation applied to the cascaded H-bridge multi-level converter as a model for one port of the UNIFLEX-PM power converter system. The UNIFLEX-PM converter is a modular system including galvanic isolation at medium frequency intended to replace transformers in future distribution and transmission systems. Power converters in this application must produce good quality voltage waveforms with low power loss. In this work, modulation methods are developed using theoretical analyses and simulation studies, before being verified experimentally using a low voltage, laboratory-based power converter operating at the low switching frequencies applicable to high-power applications. Using space-vector modulation, the relationship between the phase of the sampling process and the distortion of the line voltages is used to reduce the harmonic distortion of the output voltages. Different loads are attached to the cells of the cascaded H-bridge converter and limits are derived determining the range of loads for which it is possible to equalize the capacitor voltages. An algorithm which uses redundant states to balance the capacitor voltages without increasing the switching frequency is applied to space-vector modulation and one-dimensional modulation and its performance is compared to the derived limits. The geometrical effect of capacitor voltage ripple on the space-vector diagram is used to derive the influence on the spectrum of the line-voltages. It is identified that second and fourth harmonics of the capacitor voltages contribute to fifth and seventh harmonics of the line voltages. A feed-forward scheme to compensate for the ripple of the capacitor voltage is derived and is shown to reduce the magnitude of un-wanted harmonics. All the methods developed in this thesis can be applied to converters with any number of cells

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters

An 'active' passive-filter topology for low power DC/AC inverters

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This thesis presents a new output passive filter for voltage source inverter applications which is based on a shunt connected single tuned filter topology. The proposed circuit has the advantage of tracing harmonic components wherever its location in the frequency spectrum. The change in the harmonic location might be as a result of a change in the inverter operating frequency. Also, the proposed filter achieves harmonic reduction close to the traditional single tuned passive filter. In order to show the superiority of the proposed model, a comparison is introduced with other self tuning harmonic filters showing merits and drawbacks of each technique. The proposed circuit (when integrated in square wave inverter) has also shown a tremendous reduction in the switching losses in comparison with high frequency Pulse Width Modulation inverter. Mathematical analyses showing the design of the proposed filter together with extensive simulation results to verify the design are also introduced. The practical implementation of the system is presented and the results show excellent agreement with the theory and simulation. In order to appreciate the proposed filter a new method for classifying passive power filters is introduced. The review includes a comparison of these configurations showing their merit and drawbacks

Modulation techniques for the cascaded H-bridge multi-level converter

This thesis investigates space-vector modulation and one-dimensional modulation applied to the cascaded H-bridge multi-level converter as a model for one port of the UNIFLEX-PM power converter system. The UNIFLEX-PM converter is a modular system including galvanic isolation at medium frequency intended to replace transformers in future distribution and transmission systems. Power converters in this application must produce good quality voltage waveforms with low power loss. In this work, modulation methods are developed using theoretical analyses and simulation studies, before being verified experimentally using a low voltage, laboratory-based power converter operating at the low switching frequencies applicable to high-power applications. Using space-vector modulation, the relationship between the phase of the sampling process and the distortion of the line voltages is used to reduce the harmonic distortion of the output voltages. Different loads are attached to the cells of the cascaded H-bridge converter and limits are derived determining the range of loads for which it is possible to equalize the capacitor voltages. An algorithm which uses redundant states to balance the capacitor voltages without increasing the switching frequency is applied to space-vector modulation and one-dimensional modulation and its performance is compared to the derived limits. The geometrical effect of capacitor voltage ripple on the space-vector diagram is used to derive the influence on the spectrum of the line-voltages. It is identified that second and fourth harmonics of the capacitor voltages contribute to fifth and seventh harmonics of the line voltages. A feed-forward scheme to compensate for the ripple of the capacitor voltage is derived and is shown to reduce the magnitude of un-wanted harmonics. All the methods developed in this thesis can be applied to converters with any number of cells

Contributions on spectral control for the asymmetrical full bridge multilevel inverter

Las topologías de circuitos inversores multinivel pueden trabajar a tensiones y potencias mayores que las alcanzadas por convertidores convencionales de dos niveles. Además, la conversión multinivel reduce la distorsión armónica de las variables de salida y en algunos casos, a pesar del aumento de elementos de conmutación, también reduce las pérdidas de conversión al incrementarse el número de niveles. La reducción de distorsión alcanzada por el número de niveles puede aprovecharse para reducir las pérdidas de conmutación disminuyendo la frecuencia de las señales portadoras. Para reducir aún más esta frecuencia sin degradar el espectro, nosotros controlamos las pendientes de las portadoras triangulares. Primero se han desarrollado dos modelos analíticos para predecir el espectro del voltage de salida, dependiendo de: el índice de modulación MA, la razón de distribución de voltaje K de las fuentes de alimentación , y las cuatro pendientes de las portadoras{r1, r2, r3, r4}. El primer modelo considera el Muestreo Natural y se basa en Series Dobles de Fourier (SDF) mientras que el segundo modelo, utiliza la Serie Sencilla de Fourier (SSF) introduciendo el concepto de Muestreo Pseudo-Natural, una aproximación digital de la modulación natural. Ambos modelos son programados en Matlab, verificados con Pspice y validados con un prototipo experimental que contiene un modulador digital implementado con DSP.La concordancia entre las modulaciones natural y pseudo-natural, asi como entre sus respectivos modelos, es aprovechada por un algorítmo genético (AG) donde la THD es la función costo a reducir. Después de varios ensayos y de sintonizar el AG, se genera una matriz que contiene conjuntos de portadoras optimizadas dentro un rango específico de las variables {MA,K} y es probada con un segundo prototipo en lazo cerrado. Un lazo lento digital modifica las portadoras creadas por un dsPIC en modulaciones PWM; estas son demoduladas y sus amplitudes corregidas por un lazo de acción anticipada. Estas portadoras se comparan con una referencia sinusoidal que a su vez es modificada por variables de estado, generando finalmente la modulación multinivel en lazo cerrado. Los resultados finales demuestran la fiabilidad de la reducción de armónicos usando la programación de las pendientes de las portadoras. Palabras claves: inversor multinivel, PWM, distorsión armónica, modelo espectral, pendiente de portadora, conjunto de portadoras, distribución de niveles, Serie Doble de Fourier, Serie Simple de Fourier, muestreo natural, muestreo regular, muestreo pseudo-natural , Algoritmos Genéticos.Multilevel inverter (MI) topologies can work at higher voltage and higher power than conventional two-level converters. In addition, multilevel conversion reduces the output variables harmonic distortion and, sometimes, in spite of the devices-count increment, the conversion losses can also decrease by increasing the number of levels. The harmonic distortion reduction achieved by increasing the number of levels, can be used to further reducing the switching losses by decreasing the inverter carrier frequencies. To reduce even more the switching frequency without degrading output spectrum, we control the triangular carrier waveforms slopes. First, to achieve this target, two analytical models have been created in order to predict the inverter output voltage spectrum, depending on diverse parameters: the amplitude modulation index MA, the voltage distribution K of the inverter input sources, and the four carrier slopes {r1, r2, r3, r4}. The first model considers Natural Sampling and is based on Double Fourier Series (DFS) whereas the second model based on Simple Fourier Series (SFS), introduces the concept of Pseudo-Natural Sampling, as a digital approximation of the natural modulation. Both models are programmed in Matlab, verified with Pspice simulations and validated with a first experimental prototype with a DSP digital modulator.The good agreement between natural and pseudo-natural modulations, as well as their respective DFS and SFS models, is exploited by a Genetic Algorithm (GA) application where THD is the cost function to minimize. After testing and properly tuning the GA, a framework matrix containing the optimized carriers set for a specific range of variables {MA,K} is generated and then, tested with a second, closed-loop prototype. A slow digital loop modifies the carrier slopes created by dsPIC microcontroller as PWM modulations, whose amplitude, once demodulated, are affected by a feed-forward loop. These carriers, compared with a sinusoidal reference, state-feedback modified, generate finally the closed-loop multilevel modulation. The final results demonstrates the feasibility of harmonic reduction by means of carrier slopes programming. Keywords: multilevel inverter, PWM, harmonic distortion, spectral modeling, carrier slope, carriers set, level distribution, Double Fourier Series, Simple Fourier Series, natural sampling, regular sampling, pseudo-natural sampling, Genetic Algorithms