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The use of single-phase line-interfaced power converters in electrical power systems is rapidly growing due to the changing nature and power quality requirements of electrical loads. Most applications require these single-phase line-interfaced power converters to be compact and efficient, and depending on application meet additional performance, cost, and reliability targets. This thesis presents innovative system architectures, circuit topologies, design methodologies, and control strategies for highly compact and efficient single-phase ac-dc and ac-ac line-interfaced power converters. First, a comprehensive design methodology for step-down isolated two-stage ac-dc converters is presented which compares various designs and operating modes and selects the optimal design based on overall volume and efficiency. Additionally, a new control strategy is presented for a compact front-end soft-switched power-factor correction (PFC) stage to ensure compliance with strict electromagnetic interference (EMI) regulations. A 1-kW universal-input to 28V-output isolated ac-dc prototype converter is built to showcase performance benefits of proposed design and control strategies. This prototype achieves a high-power-density of 84W/in^3 and maintains greater than 93% efficiency across a wide output power range. Next, the functionality of the proposed ac-dc converter is further enhanced by incorporating a new droop control strategy for parallel operation of multiple similar ac-dc converter modules. The proposed control strategy uses the input current of the secondary dc-dc stage of two-stage ac-dc converters in conjunction with variable droop resistance to achieve near-perfect parallel operation. A multi-module ac-dc conversion system is built to validate the proposed droop control strategy. The parallel modules achieve a current distribution error of less than 2% near their maximum output power. Multiple ac-ac conversion applications are also addressed in this thesis. For highly cost-sensitive applications, two compact and efficient single-stage ac-ac converters are presented which utilize a comprehensive design methodology centered around minimizing the total cost of components. Moreover, innovative control strategies are presented for both ac-ac converters to enable output voltage regulation under input voltage and output load fluctuations. Both single-stage ac-ac prototype converters, utilizing the proposed design and control strategies, are built and tested. The 600-W 480Vrms-to-264Vrms prototypes achieve power densities exceeding 40W/in^3 while maintaining conversion efficiencies of greater than 96% across majority of the output load. Finally, a much more feature-rich ac-dc-ac converter is also proposed for advanced ac-ac conversion applications, such as data center online uninterruptible power supplies (UPS). The proposed transformer-less two-stage ac-ac converter is based on a new circuit topology which can operate at high switching frequencies (up to several MHz) and utilize 50% lower dc-bus capacitance than conventional split-dc-bus topologies. A 1-kVA 120Vrms prototype ac-dc-ac converter is built and extensively tested to showcase performance improvements. This prototype achieves high peak conversion efficiency of greater than 95% and high power density of 26.4W/in^3 while utilizing long-life but relatively bulky film dc-bus capacitors.

Microgrids can provide significant operational benefits to the connected grid when operating in the grid-connected mode, and energy security to the end users when operating in islanded mode. This research has first developed microgrid dynamic models which consist of multiple facility-scale microgrids with different types of distributed energy resources and are interconnected through the medium-voltage distribution system network. The dynamic models are represented by a set of ordinary differential equations to support analytical studies. The models also consider both conventional and renewable generation resources such as synchronous generators and photovoltaics, as well as battery energy storage systems. For converter interfaced systems, both grid-feeding type and grid-forming type converters are modeled in order to support the microgridislanded mode of operation.The stability of the microgrid systems in islanded mode is studied through small signal stability and sensitivity analysis. The participation factor study shows that the stability issues are associated with the system impedance loads parameters. Then this thesis presents a novel primary control strategy based on both the linear and nonlinear output regulation theory for voltage and frequency regulations in microgrid systems with fast-response converters. The proposed control strategy can accurately track voltage and frequency set points while mitigating system transients in the presence of disturbance events. Therefore, it overcomes the main shortcomings of the common droop-basedcontrol methods such as large steady-state voltage and frequency deviations and poor transient performance. Different control schemes are designed while considering trade-offs between communication requirement and system dynamic performance. Their effectiveness is validated through Matlab SIMULINK simulation studies involving a medium-voltage microgrid with both synchronous generation resources and converters. Although the proposed control schemes are centralized, practical implementation is possible with available communication links in microgrids and embedded hardware technologies.

This dissertation proposes general methodologies for designing hierarchical control schemes for dc microgrids loaded by constant power loads (CPLs). CPLs form a major proportion of the system loads in many microgrids. Without proper control, CPLs present destabilizing effect at the dc microgrid. In addition to stable operation of microgrid, proper current sharing among paralleled sources is essential. The proposed hierarchical control strategy consists of two control levels. The lower level consists of droop-based primary controllers which enables current-sharing among paralleled sources and also damps limit cycle oscillations due to CPLs. The higher level consists of secondary controller which compensates for voltage deviations due to primary controller. This higher level is implemented either as autonomous controllers or as a centralized controller. In the case of autonomous secondary controllers, they operate alongside of primary controllers in each of the paralleled converters. In the case of centralized secondary controller, a remote secondary controller uses a high speed communication link to communicate to local controllers. Interfacing sources with different characteristics and voltage ranges necessitates the use of complex converter topologies. As an initial step towards implementing hierarchical control scheme for such microgrids with CPLs, a linear controller is proposed for dc microgrids with standalone SEPIC, Cuk and Zeta converters. During the first stage of the two stage controller, limit cycle oscillations are damped by inserting a virtual resistance in series with the converter input inductor. During the second stage, an integral controller is added to the first stage to compensate for voltage deviations. For microgrids containing different converter topologies, stability of equilibrium points is examined and stability conditions are derived and explained. Experiments performed on a prototype microgrid are used to verify the proposed control laws. Expanding study on stability of microgrids, the maximum real power load in a dc microgrid bus is traced geometrically. The generalized circle diagram approach used in a conventional power system is modified for this purpose. The different types of buses present in a dc microgrid are described and the locus of operating points is obtained. The proposed method is verified by simulations on an example dc microgrid.

This thesis provides insight into quantitative accuracy assessment of the modeling and simulation of power electronic converters. Verification, Validation, and Uncertainty quantification (VVandUQ) provides a means to quantify the disagreement between computational simulation results and experimental results in order to have quantitative comparisons instead of qualitative comparisons. Due to the broad applications of modeling and simulation in power electronics, VVandUQ is used to evaluate the credibility of modeling and simulation results. The topic of VVandUQ needs to be studied exclusively for power electronic converters. To carry out this work, the formal procedure for VVandUQ of power electronic converters is presented. The definition of the fundamental words in the proposed framework is also provided. The accuracy of the switching model of a three-phase Voltage Source Inverter (VSI) is quantitatively assessed following the proposed procedure. Accordingly, this thesis describes the hardware design and development of the switching model of the three-phase VSI.

The use of power electronic converters is increasing rapidly in application to clean energy power generation systems such as solar/wind power generation and electric vehicles. Electro Magnetic Interference (EMI) is a major concern in power electronic based energy conversion systems. The EMI issues in the PWM inverter system arises from the high dv/dt and di/dtrates, which are due to the fast switching of the power semiconductor devices. Dithering the switching frequency of an inverter over a particular range can reduce the dv/dt stresses of the power semiconductor devices, and consequently the EMI noise. In this research, a dithering algorithm to reduce the dv/dt of the switches is proposed and the relationship between the parameters used in the dithering of the switching frequency is developed. A relationship is developed among the various parameters involved in the dithering concept. Two important parameters involved in dithering concept are the dithering frequency range (f_min to f_max), and the sweeping frequency (f_sweep) for a given fundamental frequency. An analysis is done for different dithering ranges by sweeping frequencies for minimization of the EMI. A relationship between the important parameters is developed by doing the FFT analysis of the switch voltage of the two level and three level inverter system for various combinations of dithering ranges and