Common mode performance is important for photovoltaic applications where the common mode voltage can become hazardous to people near the solar installation and can cause reliability concerns in inverters. The proposed dual-buck inverter uses hybrid unipolar modulation and a topology that is modified from the standard full-bridge dual-buck inverter to address the common mode voltage concerns. In the proposed design, the fast-switching side of the inverter is identical to a half-bridge dual-buck inverter, while the side that switches at line frequency uses a half-bridge of the standard H-bridge inverter topology. The motivation of this design is to realize the benefits of unipolar modulation and the dual-buck topology, while improving the poor common-mode voltage performance associated with unipolar modulation by utilizing hybrid switching. Unipolar switching has benefits which carry over to the hybrid switching scheme, such as reduced current ripple allowing use of smaller inductors. Additionally, the dual-buck topology enables the effective use of faster switches due to the elimination of dead time and reverse recovery concerns by using devices such as wide-bandgap GaN HEMTS and SiC Schottky diodes. The proposed inverter topology also realizes the benefits of the dual-buck topology while using half of the number of diodes and inductors compared to a standard full-bridge dual-buck inverter. The use of this modified dual-buck topology and hybrid unipolar modulation results in an inverter which has favorable common mode voltage characteristics. These characteristics indicate that this inverter would be useful in applications sensitive to common mode voltage concerns, such as photovoltaic applications. The performance of this topology using hybrid unipolar modulation is investigated using simulations and by creating and testing a 300-watt prototype inverter. / Master of Science / The popularity of photovoltaic panels has been increasing rapidly in recent years due to popular desire to reduce reliance on nonrenewable energy sources and steady reductions in the cost of solar power installations. The DC power provided by photovoltaic panels requires an inverter to create AC power to interface with the grid. However, in some scenarios the common-mode voltage can induce leakage current in the system, which can be hazardous to nearby people. Leakage current is larger for systems with high parasitic capacitance and for inverters that create high frequency components in their common mode voltage. Photovoltaic panels tend to have high parasitic capacitance, causing leakage current concerns. Additionally, advancements in wide bandgap devices enable inverters to operate at increasingly higher switching frequencies, and this is typically advantageous because it allows size reduction of expensive and heavy components used in inverter output filters. However, this can exacerbate leakage current concerns by introducing high frequency components to the common mode voltage. These developments create an incentive to investigate inverter designs that can mitigate leakage current concerns by creating favorable common mode voltage waveforms. Many existing solutions require circuit topologies with additional switches or use additional components like an isolation transformer or an additional common mode filter. These solutions add cost and complexity to inverter design. This thesis investigates a circuit topology based on a dual-buck inverter using hybrid unipolar switching, which will effectively utilize wide bandgap devices operating at high frequencies. The use of hybrid unipolar switching produces favorable common mode voltage characteristics that mitigates leakage current concerns while maintaining the quality of the output waveform, and the topology uses fewer diodes and inductors than a traditional dual-buck inverter. The design is evaluated through simulation and by creating and testing a 300-watt prototype to determine if it is suitable for photovoltaic applications and other applications where common mode voltage and leakage current are major concerns.

The electrification of transportation is increasingly of interest to governments around the world as a means of contributing to the achievement of climate change goals. Transportation is a significant source of greenhouse gas emissions, but it is also the backbone of the global economy and local mobility. Electrification is widely seen as a promising pathway to reducing greenhouse gas emissions from transportation while continuing to support economic growth. Multiphase machines have distinctive features that draw attention in the transportation electrification domain due to their features. Recently, powertrains based on the current-source inverter (CSI) are getting more attention to be a more reliable structure for Electric Vehicles (EVs) by replacing the dc-link capacitor with a choke inductor. This thesis combines these two technologies to develop a more reliable, compact powertrain for heavy-duty electric vehicles. First, a survey covers the recent advances in several aspects such as topology, control, and performance to evaluate the possibility and the future of exploiting them more in EV applications. The six-phase drives are extensively covered here because of their inherent structure as a dual three-phase system which eases the production process. The survey presents the different topologies used in dual three-phase drives, the modulation techniques used to operate them, the status of using multiphase drives in traction applications industrially, and the upcoming trends toward promoting this technology. New powertrain configurations for heavy-duty electric vehicles (HDEV) are proposed based on current-source inverters (CSI) and asymmetrical six-phase electric machines. Since the six-phase CSI comprises two three-phase CSIs, multiple configurations can arise based on the connection between the two CSIs. In this context, the proposed powertrain configurations are based on parallel, cascaded, and standalone six-phase CSIs. The standalone topology is based on separating the two three-phase converters by supplying each converter with a dedicated dc-dc converter. A new and straightforward method is proposed to extend the six-phase standalone CSI. The proposed technique employs the vector space decomposition (VSD) to mitigate the inverter current harmonics and extend the linear modulation region by about 8%. For motor drive applications, increasing the fundamental output component can reflect higher torque production capability for the same drive size, given that thermal limits are not exceeded. Moreover, to increase the drive's reliability, space vector modulation (SVM) techniques are developed to operate the six-phase CSI while reducing the common-mode voltage (CMV) content associated with the switching of semiconductors. The SVM techniques select the switching states associated with the minimum CMV value offline to eliminate the need for measurements. Experimental validation of the proposed algorithms is presented to operate a scaled-down six-phase PMSM fed by the proposed powertrain configuration. These proposed techniques make the CSI- based powertrain a promising solution for future HDEVs in terms of cost, performance, and reliability. / Thesis / Candidate in Philosophy

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Distributed energy is an attractive alternative to typical centralized energy sources specifically for remote locations not accessible to the electricity grid. With the continued advancement into new renewable technologies like solar, wind, fuel cell etc., off-grid standalone systems are becoming more attractive and even compeating on a cost basis for rural locations. Along with the environmental and sustainable movement, these technologies are only going to get more and more popular as time goes on. Power electronic converters are also advancing which will help the shift in electricity options. Creating innovative power electronic systems will be important when moving toward smaller, more e cient and higher power density solutions. As such, this thesis will aim to design and create an integrated power electronic system for an o -grid standalone solar application designed for remote rural locations with no access to electricity, or in locations which could bene t from such a system. It is designed for a DC input source from 24V-40V, such as a solar panel, and can operate four di erent loads; one 12V-24V 100 W DC load, charge a 48V battery, run three 5V cell phone charger outputs and run one 230V, 50Hz, 1 kW AC load. A boost converter, buck converter, phase shifted full bridge isolated DC-DC converter and a single phase inverter are implimented in the integrated system to achieve these outputs. A comparison of similar products on the market are presented and compared with the proposed design by showing the product speci cations, advantages and disadvantages of each. A discussion of each converter in the system is presented and will include operation, design and component selection. An in-depth design process for the inductor within the boost converter is presented and will cover core, winding design and an optimization algorithm using the Genetic Algorithm (GA) is used to compare di erent ferrite based C-C shaped inductors. More speci cally, the core material selected is Ferroxcube 3C97 and the inductor comparions are between di erent Litz bundled windings from New England Wire Tecnologies and a customized rectangular winding. The GA optimizes around the lowest volume by comparing the di erent inductor designs using the di erent Litz winding constructions and the custom rectangular winding constrictuion. The rectangular winding achieves the lowest volume and will be compared with a three phase interleaved boost design implimenting a CoilCraft inductor. The buck converter is the simplest converter and is designed using the traditional methods in literature. An in-depth design process for the phase shifted full bridge converter is also done wherein the zero voltage switching (ZVS) is achieved. The DC-AC inverter is the last converter designed within the integrated system and covers input capacitor sizing, and output lter design. There are speci c distributed energy standards that must be followed when connecting loads to the system and so the purpose of the lter is to lter out the voltage harmonics. The control techniques for each converter is also discussed and shown to operate in both simulation and in experimentally. The losses within the system are discussed and the required equations are de ned / Thesis / Master of Applied Science (MASc)

The past few decades have seen a rapid sales increase and technological development of electric vehicles (EVs). As the key part of the electrical powertrain systems, the traction machine drive systems in modern EVs are composed of voltage source inverters (VSI) and electric machines. In this thesis, multiphase VSIs are studied and designed to achieve volume reductions when compared with existing 3-phase benchmark VSIs. Different existing switching strategies for arbitrary phase number multiphase VSIs are investigated resulting in an understanding of best practice and a newly proposed switching strategy. Thus, the first contribution of this thesis is switching strategies that support subsequent investigations and experimental validation. DC-link capacitor and heat sink are two bulkiest components in VSIs and hence it is more efficient to decrease their volumes to achieve the compactness improvement. The investigation methodology and procedure for arbitrary phase number VSI DC-link capacitor requirements, i.e. capacitance and RMS current ratings, are firstly developed. Increased phase number decreases the DC-link capacitor requirements and hence the VSI volume significantly. Throughout this analysis, the connected multiphase machine is considered appropriately, though no electric machine design is described in the thesis. While other authors have studied DC-link current ripple, this thesis qualifies and quantifies the system benefits. This is the second contribution. Multiphase VSIs thermal models are built and their respective thermal performances studied and evaluated against a reference 3-phase benchmark VSI. The power loss deviation among different semiconductor dies is lower or even eliminated in the multiphase VSIs. Furthermore, the multiphase integrated design VSIs have a significant heat sink volume reduction when compared to the 3-phase benchmark VSI. This study and concluding benefits are the third contribution. Finally, comparative test validations are made on an experimental set-up designed to illustrate the benefits of a 9-phase against a reference 3-phase system. Here, the test hardware and implementation are carefully designed to representatively illustrate performance benefits. / Thesis / Doctor of Philosophy (PhD)

The work in this dissertation presents the first attempt in the literature to propose the concept of "soft switch". The goal of "soft switch" is to develop a standard PWM switch cell with built-in adaptive soft switching capabilities. Just like a regular switch, only one PWM signal is needed to drive the soft switch under soft switching condition. The core technique in soft switch development is a built-in adaptive soft switching circuit with minimized circulation energy. The necessity of minimizing circulation energy is first analyzed. The design and implementation of a universal controller for implementation of variable timing control to minimize circulation energy is presented. The controller has been tested successfully with three different soft switching inverters for electric vehicles application in the Partnership for a New Generation Vehicles (PNGV) project. To simplify the control, several methods to achieve soft switching with fixed timing control are proposed by analyzing a family of zero-voltage switching converters. The driver based soft switch concept was originated from development of a base driver circuit for current driven bipolar junction transistor (BJT). A new insulated-gate-bipolar-transistor (IGBT) and power metal-oxide-semiconductor field-effect-transistor (MOSFET) gated transistor (IMGT) base drive structure was initially proposed for a high power SiC BJT. The proposed base drive method drives SiC BJTs in a way similar to a Darlington transistor. With some modification, a new base driver structure can adaptively achieve zero voltage turn-on for BJT at all load current range with one single gate. The proposed gate driver based soft switching method is verified by experimental test with both Si and SiC BJT. The idea is then broadened for "soft switch" implementation. The whole soft switched BJT (SSBJT) structure behaves like a voltage-driven soft switch. The new structure has potentially inherent soft transition property with reduced stress and switching loss. The basic concept of the current driven soft switch is then extended to a voltage-driven device such as IGBT and MOSFET. The key feature and requirement of the soft switch is outlined. A new coupled inductor based soft switching cell is proposed. The proposed zero-voltage-transition (ZVT) cell serves as a good candidate for the development of soft switch. The "Equivalent Inductor" and state plane based analysis method are used to simply the analysis of coupled inductor based zero-voltage switching scheme. With the proposed analysis method, the operational property of the ZVT cell can be identified without solving complicated differential equations. Detailed analysis and design is proposed for a 3kW boost converter example. With the proposed soft switch design, the boost converter can achieve up to 98.9% efficiency over a wide operation range with a single gate drive. A high power inverter with coupled inductor scheme is also designed with simple control compared to the earlier implementation. A family of soft-switching converters using the proposed "soft switch" cell can be developed by replacing the conventional PWM switch with the proposed soft switch. / Ph. D.

A single-phase grid connected transformerless photovoltaic (PV) inverter for residential application is presented. The inverter is derived from a boost cascaded with buck converter along with a line frequency unfolding circuit. Due to its novel operating modes, high efficiency can be achieved because there is only one switch operating at high frequency at a time, and the converter allows the use of power MOSFET and ultra-fast reverse recovery diode. This dissertation begins with theoretical analysis and modeling of this boost-buck converter based inverter. And the model indicates small boost inductance will leads to increase the resonant pole frequency and decrease the peak of Q, which help the system be controlled easier and more stable. Thus, interleaved multiple phases structure is proposed to have small equivalent inductance, meanwhile the ripple can be decreased, and the inductor size can be reduced as well. A two-phase interleaved inverter is then designed accordingly. The double-carrier modulation method is proposed based on the inverter's operation mode. The duty cycle for buck switch is always one if the inverter is running in boost mode. And the duty cycle for boost switches are always zero if the inverter is running in buck mode. Because of this, the carrier for boost mode is stacked on the top of the carrier for buck mode, as a result, there is no need to compare the input and output voltage to decide which mode the inverter should operate in. And the inverter operates smoothly between these two modes. Based on similar concept, three advanced modulation methods are proposed. One of them can help further improve the efficiency, and one of them can help increase the bandwidth and gain, and the last one takes the advantage of both. Based on similar concept, another three dual-mode double-carrier based SPWM inverters are proposed. With both step-up and step-down functions, this type of inverter can achieve high efficiency in a wide range because only one switch operates at the PWM frequency at a time. Finally, the simulation and experiment results are shown to verify the concept and the tested CEC (California Energy Commission) efficiency is 97.4%. It performs up to 2% more efficiently better than the conventional solution. / Ph. D.

In recent decades there has been a rapidly growing demand for high quality, uninterrupted power. In light of this fact, this study has addressed some of the causes of poor power quality and control strategies to ensure a high performance level in inverter-fed power systems. In particular, specific loading conditions present interesting challenges to inverter-fed, high power systems. No-load, unbalanced loading, and non-linear loading each have unique characteristics that negatively influence the performance of the Voltage Source Inverter (VSI). Ideal, infinitely stiff power systems are uninfluenced by loading conditions; however, realistic systems, with finite output impedances, encounter stability issues, unbalanced phase voltage, and harmonic distortion. Each of the loading conditions is presented in detail with a proposed control strategy in order to ensure superior inverter performance. Simulation results are presented for a 90 kVA, 400 Hz VSI under challenging loading conditions to demonstrate the merits of the proposed control strategies. Unloaded or lightly loaded conditions can cause instabilities in inverter-fed power systems, because of the lightly damped characteristic of the output filter. An inner current loop is demonstrated to damp the filter poles at light load and therefore enable an increase in the control bandwidth by an order of magnitude. Unbalanced loading causes unequal phase currents, and consequently negative sequence and zero sequence (in four-wire systems) distortion. A proposed control strategy based on synchronous and stationary frame controllers is shown to reduce the phase voltage unbalance from 4.2% to 0.23% for a 100%-100%-85% load imbalance over fundamental positive sequence control alone. Non-linear loads draw harmonic currents, and likewise cause harmonic distortion in power systems. A proposed harmonic control scheme is demonstrated to achieve near steady state errors for the low order harmonics due to non-linear loads. In particular, the THD is reduced from 22.3% to 5.2% for full three-phase diode rectifier loading, and from 11.3% to 1.5% for full balanced single-phase diode rectifier loading, over fundamental control alone. / Master of Science