Silicon carbide (SiC) power switching devices promise to be a major breakthrough for new generation ac three-phase power converters, offering increased junction temperature, low specific on-resistance, fast switching, and low switching loss. These characteristics are desirable for increasing power density, providing faster system dynamics, and improving power quality. At present, the normally-on SiC JFET prototypes available from SiCED are the first SiC power switches close to commercialization. The objective of this work is to characterize the switching behavior of the prototype SiC JFET devices, as well as demonstrate the feasibility of achieving high switching frequency for a 2 kVA three-phase converter. The switching characterization of the 1200 V SiC JFET prototypes is shown for a wide range of operating conditions such as switched voltage, switched current, and junction temperature. The SiC JFET is shown to be a fast-switching, low-loss device offering performance benefits compared to traditional silicon (Si) power devices of similar ratings. Utilizing the SiC JFET, a three-phase ac buck rectifier is then demonstrated with a 150 kHz switching frequency and a rated power of 2 kVA. Additionally, improvements are made to the charge control scheme for the buck rectifier allowing power factor compensation and reduction of input current transients. / Master of Science

This thesis proposes one comparison methodology for single and three-phase high power high frequency transformers in power conversion systems. The objective is to compare the volume of the transformers. And single and three-phase Dual Active Bridge Converter (DAB1 and DAB3) topologies with single and three-phase isolating transformers are selected for the transformer comparison. Design optimization of power transformer has been studied and simplified models have been built for the single and three-phase transformer design optimization in this work, including assumptions for core shapes, materials, winding structures and thermal model. Two design methods have been proposed according to different design constraints, named T – B Method and J – B Method separately. T – B Method is based on feature of the core, which has the major limits of maximum flux density and temperature rise. The flux density should not reach the saturation value of the core, and temperature rise should meet specifications in different applications to assure the performance of the core (permeability, saturation flux density, and core loss) and the insulation of the wire. And J – B Method starts from the comparison of area product in conventional design method. The relationship between area product of transformer cores and the flux and current of the transformer in design is analyzed. There is specified relationship between area product of single and three-phase transformers if flux and current densities are specified for both. Thus J – B Method is proposed with the design constraints of specified current and flux density. Both design methods include both single and three-phase transformer design. One example case for single and three-phase transformer comparison is selected as high power high frequency DAB conversion system. Operation principles are studied for both DAB1 and DAB3 based on previous work. And transformer design based on the T – B and J – B Methods are carried out and transformer volumes are compared. And results show that three-phase transformer has little benefit in volume or thermal than single-phase transformer, when they are utilized in single-phase DAB and three-phase DAB converters separately. Scaled-down single and three-phase DAB systems have been built and volume and thermal tests have been carried out. / Master of Science

Three-phase rectifiers/inverters are widely used in grid-tied applications. Take the electric vehicle (EV) charging systems as an example. Within a certain space designated for the chargers, quick charging yet high efficiency are demanded. According to the current industry practice, with a power rating between 10 and 30 kW, the power density are limited by silicon (Si) power semiconductor devices, which make the systems operate at only up to around 30 kHz. The emerging wide bandgap (WBG) power semiconductor devices are considered as game changing devices to exceed the limits brought by their Si counterparts. Much higher switching frequency, higher power density and higher system efficiency are expected to be achieved with WBG power semiconductor devices. Among different types of WBG power semiconductor devices, Silicon Carbide Metal-Oxide-Semiconductor Field-Effect Transistors (SiC MOSFETs) are more popular in current research conducted for tens of kW power converter applications. However, the commonly adopted hard switching operation in this application still leads to significant switching loss at high frequency operation even for SiC-based systems. With the unique feature that the turn-off energy is almost negligible compared with the turn-on energy, critical conduction mode (CRM) based zero voltage soft switching turn-on operation is preferred for the SiC MOSFETs to eliminate the turn-on loss with small penalty on the conduction loss and on the turn-off loss. With this soft switching operation, switching frequency of SiC-based systems is able to be pushed to more than ten times higher than Si-based systems, and therefore higher power density yet even higher system efficiency can be achieved. The CRM-based soft switching is applied to three-phase rectifiers/inverters under the unity power factor operating condition first. Decoupled CRM-based control is enabled, and the inherent drawback of wide switching frequency variation range at CRM-based operation is overcome by the proposed novel modulation technique. It is the first time that CRM-based soft switching modulation is demonstrated in the most conventional three-phase H-bridge ac–dc converter, and more than three-time size reduction compared with current industry practice yet 99.0% peak efficiency are achieved at above 300 kHz switching frequency operation. Then this proposed soft switching modulation technique is extended to non-unity power factor operating conditions especially for grid-tied inverter system applications. With several improvements on the modulation, a generalized CRM-based soft switching modulation technique is proposed, which is applicable to both the unity and non-unity power factor conditions. With the power factor down to 0.8 lagging or leading according to commercial products, above 98.0% peak efficiency is achieved with the generalized soft switching modulation technique at above 300 kHz switching frequency operation. Furthermore from the aspect of electromagnetic interference (EMI), compared with the traditional Si-based design, CRM operation brings higher differential-mode (DM) EMI noise, and higher dv/dt with SiC MOSFETs brings higher common-mode (CM) EMI noise. What's more, hundreds of kHz switching frequency operation makes the main components of the system EMI spectrum located within the frequency range related to the EMI standard (150 kHz – 30 MHz). Therefore, several methods are adopted for the reduction of EMI noise. The total inductor current ripple is reduced with multi-channel interleaving control in order to reduce DM EMI noise. The balance technique is applied in order to reduce CM EMI noise. With PCB winding coupled inductors, the well-controlled parasitic parameters make the balance technique able to be effective for a uniform reduction of CM EMI noise from 150 kHz to above 20 MHz. In addition, PCB winding based magnetic designs are beneficial to achieving manufacture automation and reducing the labor cost. / Doctor of Philosophy / Power electronics and power conversion are crucial to many applications related to electricity, such as consumer electronics, domestic and commercial appliances, automobiles, data centers, utilities and infrastructure. In today's market, quality and reliability are usually considered as a given; high efficiency (low loss), high power density (small size and weight) and low cost are the main focuses in the design of power electronics products. In the past several decades, significant achievements in power electronics have been witnessed thanks to the silicon (Si) semiconductor technology, especially the Si power semiconductor devices. Nowadays, the development of Si power semiconductor devices is already close to the theoretical limits of the material itself. Therefore, in order to meet the increasing demands from customers in different applications, wide bandgap (WBG) based power semiconductor devices, namely Gallium Nitride (GaN) and Silicon Carbide (SiC), are becoming attractive because of its great potential compared with their Si counterparts. In literature, great contributions have already been made to understanding the WBG based power semiconductor devices. It is exciting and encouraging that some of the GaN-based power electronics products featuring high efficiency, high power density and low cost have been commercialized in consumer electronics applications. However, when pursuing these objectives, previous literature has not shown any applications of high frequency soft switching technology into the high power ac–dc conversion (usually three-phase ac–dc) in a simple way as the low power ac–dc conversion (usually single-phase ac–dc) in consumer electronics products. The key to achieving high efficiency, high power density and low cost is the high frequency soft switching operation. For single-phase ac–dc systems, the research on the realization of soft switching by control strategies instead of additional physical complexity has been intensively conducted, and this technology has also been adopted in the current industry practice. Therefore, the major achievement of this work is the development of a generalized soft switching control strategy for three-phase ac–dc systems, without adding any physical complexity, which is applicable to the simplest and most conventional three-phase ac-dc circuit topology. The proposed soft switching control strategy features bidirectional (rectifiers/inverters) power conversion, active/reactive power transfer, grid-tied/stand-alone modes, and scalability to multi-channel interleaved operation. Furthermore, with high frequency, the integration of magnetic components with embedded windings in the printed circuit board (PCB) becomes feasible, which is also beneficial to achieving electromagnetic compatibility (EMC) and manufacture automation. Based on the proposed control strategy and design methodology, a SiC-based 25-kW three-phase high frequency soft switching rectifier/inverter is developed for various applications such as electric vehicle (EV) charging stations, uninterruptible power supplies (UPS) and renewable energy based utilities.

This dissertation presents work conducted at the Center for Power Electronics Systems (CPES) at Virginia Polytechnic Institute and State University. Chapter 1 introduces the concept of impedance measurement, and discusses previous work on this topic. This chapter also addresses issues associated with impedance measurement. Chapter 2 introduces the analyzer architecture and the proposed algorithm. The algorithm involves locking on to the voltage vector at the point of common coupling between the analyzer and the system via a PLL to establish a D-Q frame. A series of sweeps are performed, injecting at least two independent angles in the D-Q plane, acquiring D- and Q-axis voltages and currents for each axis of injection at the point of interest. Chapter 3 discusses the analyzer hardware and the criteria for selection. The hardware built ranges from large-scale power level hardware to communication hardware implementing a universal serial bus. An eight-layer PCB was constructed implementing analog signal conditioning and conversion to and from digital signals with high resolution. The PCB interfaces with the existing Universal Controller hardware. Chapter 4 discusses the analyzer software. Software was written in C++, VHDL, and Matlab to implement the measurement process. This chapter also provides a description of the software architecture and individual components. Chapter 5 discusses the application of the analyzer to various examples. A dynamic model of the analyzer is constructed, considering all components of the measurement system. Congruence with predicted results is demonstrated for three-phase balanced linear impedance networks, which can be directly derived based on stationary impedance measurements. Other impedances measured include a voltage source inverter, Vienna rectifier, six-pulse rectifier and an autotransformer-rectifier unit. / Ph. D.

This dissertation is devoted to a unified and comprehensive study of zero-current-transition (ZCT) soft-switching techniques for high-power three-phase PWM inverter applications. Major efforts in this study are as follows: 1) Conception of one new ZCT scheme and one new ZCT topology; 2) Systematic comparison of a family of ZCT inverters; 3) Design, implementation and experimental evaluation of two 55-kW prototype inverters for electric vehicle (EV) motor drives that are developed based on the proposed ZCT concepts; and 4) Investigation of the ZCT concepts in megawatts high-frequency power conversions. The proposed ZCT techniques are also applicable to three-phase power-factor-correction (PFC) rectifiers. In order to minimize switching losses, this work first proposes a new control scheme for an existing three-phase ZCT inverter circuit that uses six auxiliary switches. The proposed scheme, called the six-switch ZV/ZCT, enables all main switches, diodes and auxiliary switches to be turned off under zero-current conditions, and in the meantime provides an opportunity to achieve zero-voltage turn-on for the main switches. Meanwhile, it requires no modification to normal PWM algorithms. Compared with existing ZCT schemes, the diode reverse-recovery current is reduced significantly, the switching turn-on loss is reduced by 50%, the resonant capacitor voltage stress is reduced by 30%, and the current and thermal stresses in the auxiliary switches are evenly distributed. However, a big drawback of the six-switch ZV/ZCT topology, as well as of other types of soft-switching topologies using six auxiliary switches, is the high cost and large space associated with the auxiliary switches. To overcome this drawback, this work further proposes a new three-phase ZCT inverter topology that uses only three auxiliary switches-- the three-switch ZCT. The significance of the proposed three-switch ZCT topology is that, among three-phase soft-switching inverters developed so far, this is the only one that uses fewer than six auxiliary switches and still has the following three features: 1) soft commutation for all main switches, diodes and auxiliary switches in all operation modes; 2) no modification to normal PWM algorithms; and 3) in practical implementations, no need for extra resonant current sensing, saturable cores, or snubbers to protect the auxiliary switches. The proposed six-switch ZV/ZCT and three-switch ZCT inverters, together with existing ZCT inverters, constitute a family of three-phase ZCT inverters. To explore the fundamental properties of these inverters, a systematic comparative study is conducted. A simplified equivalent circuit is developed to unify common traits of ZCT commutations. With the visual aid of state planes, the evolution of the family of ZCT inverters is examined, and their differences and connections are identified. Behaviors of individual inverters, including switching conditions, circulating energy, and device/component stresses, are compared. Based on the proposed six-switch ZV/ZCT and three-switch ZCT techniques, two 55-kW prototype inverters for EV traction motor drives have been built and tested to the full-power level with a closed-loop controlled induction motor dynamometer. The desired ZCT soft-switching features are realized together with motor drive functions. A research effort is carried out to develop a systematic and practical design methodology for the ZCT inverters, and an experimental evaluation of the ZCT techniques in the EV motor drive application is conducted. The design approach integrates system optimization with characterizations of the main IGBT device under the ZCT conditions, selection, testing and characterization of the auxiliary devices, design and selection of the resonant inductors and capacitors, inverter loss modeling and numerical analysis, system-level operation aspects, and layout and parasitic considerations. Different design aspects between these two ZCT inverters are compared and elaborated. The complexity of the 55-kW prototype implementations is compared as well. Efficiencies are measured and compared under a group of torque/speed points for typical EV drive cycles. Megawatts high-frequency power conversion is another potential application of the ZCT techniques. The integrated gate commutated thyristor (IGCT) device is tested and characterized under the proposed six-switch ZV/ZCT condition, and the test shows promising results in reducing switching losses and stresses. Improvements in the IGCT switching frequency and simplification of the cooling requirements under ZCT operations are discussed. In addition, a generalized ZCT cell concept is developed based on the proposed three-switch ZCT topology. This concept leads to the discovery of a family of simplified multilevel soft-switching inverters that reduce the number of auxiliary switches by half, and still maintain desirable features. / Ph. D.

Front-end converters with power factor correction (PFC) capability are widely used in distributed power systems (DPSs). Most of the front-end converters are implemented using a two-stage approach, which consists of a PFC stage followed by a DC/DC converter. The purpose of the front-end converter is to regulate the DC output voltage, supply all the load converters connected to the distributed bus, guarantee current sharing, and charge a bank of batteries to provide backup energy when the power grid breaks down. One of the main concerns of the power supply industry is to obtain a front-end converter with a low-cost PFC stage, while still complying with required harmonic standards, especially for high-power three-phase applications. Having this statement in mind, the main objective of this dissertation is to study front-end converters for DPS applications with PFC to meet harmonic standards, while still maintaining low cost and performance indices. To realize the many aforementioned objectives, this dissertation is divided into two main parts: (1) two-stage front-end converters suitable for telecom applications, and (2) single-stage low-cost AC/DC converters suitable for mainframe computers and server applications. The use of discontinuous conduction mode (DCM) boost rectifiers is extensively explored to achieve simplicity, while reducing the cost for DPS applications. Interleaving of DCM boost rectifiers is also explored as an alternative approach to further reduce the system cost by reducing the filtering requirements. All the solutions discussed are implemented for 3kW applications, while 6kW is obtained by interleaving two converters. / Ph. D.

Over the last two decades there has been an increased use of three-phase AC power systems that may not be connected to the main power grid, such as the power systems on more-electric airplane and all-electric ships. Power-electronic converters are usually a significant part of these systems, which provide excellent performance. But their negative incremental impedance nature increases the possibility of system instability. A small-signal analysis that uses interface impedances defined in the synchronous frame is developed by Belkhayat at Purdue in the mid-90s to access the system stability. The system impedance varies with the operating point. Thus the impedance has to be obtained online at the desired operating point, on even in situ. Literature investigates its use with system models, but the lack of equipment to measure such impedance prevents its use in practical systems. Measurement of impedances of each component enables the prediction of system stability before building the real system. The impedance data can also be used to investigate the instability in the system after it is built. The capability of impedance measurement can save the cost and time of system integrators. After reviewing the state-of-the-art development of impedance measurement systems, the dissertation analyzes several systematical error sources in the system, which includes the signal processing and sampling circuits, the phase estimation for coordinate transformation and the injection device connection, and proposes the solution to reduce their influence. Improved algorithm and system architecture are proposed to increase the measurement speed and accuracy. Chirp signal is used as an excitation signal to extract impedances at a group of frequencies at one time. The use of both shunt current injection and series voltage injection improves the SNR of measured signal. Oversampling, cross-correlation and frequency domain averaging technique are used to further reduce the influence of noise. An instrument is built based on the proposed solution. A voltage source inverter is used to generate the perturbation. A PXI computer is used for real-time signal processing. A PC is used for data post processing and measurement process control. Software is developed to fully automate the measurement. The designed unit is tested with various linear and nonlinear load. The test result shows the validity of the proposed solution. / Ph. D.

Phasor Measurement Units (PMUs) are widely acknowledged as one of the most significant developments in the field of real-time monitoring of power system. By aligning time stamps of voltage and current phasor measurements, which are consistent with Coordinated Universal Time (UTC), a coherent picture of the power system state can be achieved through either direct measurements or simple linear calculations. With the growing number of PMUs installed or planned to be installed in the near future, both utilities and research institutions are looking for novel applications of synchrophasor measurements from these widely installed PMUs. In this dissertation, the author proposes two new PMUs measurements applications: three-phase instrument transformer calibration, and three-phase line parameter calculation with instrument transformers. First application is to calibrate instrument transformers. Instrument transformers are the main sensors used in power systems. They provide isolation between high voltage level of primary side and metering level of the secondary side. All the monitoring and measuring systems obtain input signals from the secondary side of instrument transformers. That means when instrument transformers are not accurate, all the measurements used in power system are inaccurate. The most important job of this dissertation is to explore a method to automatically calibrate all the instrument transformers in the power system based on real-time synchrophasor measurements. The regular instrument transformer calibration method requires the instrument transformer to be out of service (offline) and calibrated by technicians manually. However, the error of instrument transformer changes when environment changes, and connected burden. Therefore, utilities are supposed to periodically calibrate instrument transformers at least once a year. The high labor and economic costs make traditional instrument transformer calibration method become one of the urgent problems in power industry. In this dissertation we introduce a novel, low cost and easy method to calibrate three-phase instrument transformers. This method only requires one three-phase voltage transformer at one bus calibrated in advance. All other instrument transformers can be calibrated by this method as often as twice a day, based on the synchrophasor measurements under different load scenarios. Second application is to calculate line parameters during calibrating instrument transformers. The line parameters, line impedance and line shunt admittance, as needed by utilities are generated by the computer method. The computer method is based on parameters, such as the diameter, length, material characteristics, the distance among transmission line, the distance to ground and so on. The formulas to calculate line parameters have been improved and re-modeled from time to time in order to increase the accuracy. However, in this case, the line parameters are still inaccurate due to various reasons. The line parameters errors do affect the instrument transformers calibration results (with 5% to 10% error). To solve this problem, we present a new method to calculate line parameters and instrument transformers in the same processing step. This method to calibrate line parameter and instrument transformers at the same time only needs one pre-calibrated voltage transformer and one pre-calibrated current transformer in power system. With the pre-calibrated instrument transformers, the line parameter as well as the ratio correction factors of all the other instrument transformers can be solved automatically. Simulation results showed the errors between calculated line parameters and the real line parameter, the errors between calibrated ratio correction factors and the real ratio correction factors are of the order of 10e-10 per unit. Therefore, high accuracy line parameters as well as perfectly calibrated instrument transformers can be obtained by this new method. This method can run automatically every day. High accuracy and dynamic line parameters will significantly improve power system models. It will also increase the reliability and speed of the relay system, enhance the accuracy of power system analysis, and benefit all other researches using line parameters. New methods of calculating line parameter and the instrument transformer calibrations will influence the whole power industry significantly. / Ph. D.

This thesis applies the concept of a virtual-synchronous-machine- (VSM-) based control to a conventional 250-kW utility-scale photovoltaic (PV) inverter. VSM is a recently-developed control scheme which offers an alternative grid-synchronization method to the conventional grid-tracking control scheme, which is based on the dq phase-locked-loop- (PLL-) oriented vector control. Synchronous machines inherently synchronize to the grid and largely partake in the stabilization of the grid frequency during power system dynamics. The purpose of this thesis is primarily to present the design of a grid-forming control scheme based on the VSM and the derivation of the terminal dq-frame ac impedance of the small-signal model of the inverter and control scheme. This design is also compared to the design of the conventional grid-tracking control structure, both from a loop design and terminal dq-frame ac impedance standpoint. Due to the inherent lax power-balance synchronization, the grid-forming control scheme results in 1 to 2 decades' lower frequency range of negative incremental input impedance in the diagonal elements, which is a favorable condition for stability. Additionally, the stability of the grid-forming control scheme is compared to the conventional grid-tracking control using the generalized Nyquist criterion (GNC) for stability under three modes of operation of active and reactive power injection. It is found that the connection is stable for both control schemes under unity power factor and fixed reactive power modes; however, the grid-forming control is able to inject twice the amount of active power under the voltage regulation mode when compared to the grid-tracking control. / Master of Science / Concerns about the current and future state of the environment has prompted government and non-profit agencies to enact regulatory legislation on fossil fuel emissions. In 2017, electricity generation comprised 28% of total U.S. greenhouse gas emissions with 68% of this generation being due to coal combustion sources. As a result, utilities have retired a number of coal power plants and have employed alternative means of power generation, specifically renewable energy sources (RES). Most RES operate as variable-frequency ac sources (wind) or dc sources (solar) and are interfaced with the power grid through ac-dc-ac or dc-ac converters, respectively, which are power-electronic devices used to control the injection of power to the grid. Conventional converters synchronize with the grid by tracking the phase of the voltage at the point of common coupling (PCC) through a phase-locked loop (PLL). While power system dynamics significantly affect the performance of a PLL, and, subsequently, inverters' operation, the initial frequency regulation during grid events is attributed to the system's inherent inertia due to the multitude of synchronous machines (SM). However, with the steady increase of RES penetration, even while retaining the number of SM units, the net inertia in the system will decrease, thus resulting in prolonged responses in frequency regulation to the aforementioned dynamics. This thesis investigates the control of variable-frequency sources as conventional synchronous machines and provides a detailed design procedure of this control structure for photovoltaic (PV) inverter applications. Additionally, the stability of the connection of the inverter to the grid is analyzed using innovative stability analysis techniques which treat the inverter and control as a black box. In this manner, the inner-workings of the inverter need not be known, especially since it is proprietary information of the manufacturer, and the operator can measure the output response of the device to some input signal. In this work, it is found that the connection between the inverter and grid is stable with this new control scheme and comparable to conventional control structures. Additionally, the control based on synchronous machine characteristics shows improved stability for voltage and frequency regulation, which is key to maintaining a stable grid.

This thesis describes new methods of evaluating the efficiency of three-phase induction machines using synthetic loading. Synthetic loading causes the induction machine to draw full-load current without the need to connect a mechanical load to the machine's drive shaft. The synthetic loading methods cause the machine to periodically accelerate and decelerate, producing an alternating motor-generator action. This action causes the machine, on average over each synthetic loading cycle, to operate at rated rms current, rated rms voltage and full-load speed, thereby producing rated copper losses, iron loss and friction and windage loss. The excitation voltages are supplied from a PWM inverter with a large capacity DC bus capable of supplying rated rms voltage. The synthetic loading methods of efficiency evaluation are verified in terms of the individual losses in the machine by using a new dynamic model that accounts for iron loss and all parameter variations. The losses are compared with the steady-state loss distribution determined using very accurate induction machine parameters. The parameters were identified using a run-up-to-speed test at rated voltage and the locked rotor and synchronous speed tests conducted with a variable voltage supply. The latter tests were used to synthesise the variations in stator leakage reactance, magnetising reactance and the equivalent iron loss resistance over the induction machine's speed range. The run-up-to-speed test was used to determine the rotor resistance and leakage reactance variations over the same speed range. The test method results showed for the first time that the rotor leakage reactance varied in the same manner as the stator leakage and magnetising reactances with respect to current. When all parameter variations are taken into account there is good agreement between theoretical and measured results for the synthetic loading methods. The synthetic loading methods are applied to three-phase induction machines with both single- and double-cage rotors to assess the effect of rotor parameter variations in the method. Various excitation waveforms for each method were used and the measured and modelled efficiencies compared to conventional efficiency test results. The results verify that it is possible to accurately evaluate the efficiency of three-phase induction machines using synthetic loading.