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Battery energy storage system (BESS) is promising to be implemented in residential applications for supporting PV integration, load shifting, and backup power purposes. For this application, 48V second-life battery draws more and more attentions for their cost-effectiveness, safe voltage level, reliability, and potential large market. This thesis proposes the comprehensive control and design of an isolated battery-driven grid interface (IBDGI) with the dual-active-bridge (DAB) converter for residential applications with 48V battery pack. The three-phase DAB converter is a promising candidate as the front-end DC/DC converter in the two-stage IBDGI due to its high efficiency, high power density, and low capacitance requirement. An effective design strategy for the three-phase DAB converter is proposed based on the zero-voltage-switching (ZVS) zone and back-ow power to achieve high efficiency for a wide operating voltage range and different load conditions. Based on the power loss model, an easily-implemented variable switching frequency operating method is proposed to further increase the efficiency at light load conditions. The dead-time effect is observed in the three-phase DAB converter. To avoid the dead-time effect and better understand the phenomena, a comprehensive analysis is proposed. All the cases of the dead-time effect in the three-phase DAB converter are analyzed in terms of the buck, boost, and matching states. The expressions of the transmission power, constraint conditions, and key time of the dead-time effect are derived for each state. The operation waveforms of the dead-time effect are also presented. The hybrid capacitor bank composed by the LC resonant lter with electrolytic capacitor and lm capacitor is utilized for the DC bus of the IBGDI. The electrolytic capacitors work as passive decoupling purpose while the lm capacitor is responsible for high switching harmonic ltering. Moreover, a current sharing method between the hybrid capacitor bank is proposed to extend the electrolytic capacitor's life. The LCL single-phase inverter is applied for the downstream of the IBDGI. A step-by-step design procedure of the LCL lter with passive damping is proposed for the 120V/240V dual grid-tied and standalone modes. The PR controllers are also designed for the LCL inverter for standalone and grid-tied modes. At the system level, a novel second harmonic current (SHC) reduction strategy is proposed for the IBDGI with the three-phase DAB converter by adding a load current feedforward (LCFF) path to the DAB voltage closed-loop controller. This method will suppress the SHC without modi cations of the original controller's bandwidth, which make it easy to be implemented. The small-signal model of the three-phase DAB converter is provided and veri ed by the step response. The parameter sensitivity analysis for the LCFF method is proposed to show that the SHC is well suppressed within ±20% parameter error. The proposed converter and control methods are veri ed by simulation and experimental results. / Thesis / Doctor of Philosophy (PhD)

Lithium-ion based battery energy storage system has become one of the most popular forms of energy storage system for its high charge and discharge efficiency and high energy density. This dissertation proposes a high-efficiency grid-tie lithium-ion battery based energy storage system, which consists of a LiFePO4 battery based energy storage and associated battery management system (BMS), a high-efficiency bidirectional ac-dc converter and the central control unit which controls the operation mode and grid interface of the energy storage system. The BMS estimates the state of charge (SOC) and state of health (SOH) of each battery cell in the pack and applies active charge equalization to balance the charge of all the cells in the pack. The bidirectional ac-dc converter works as the interface between the battery pack and the ac grid, which needs to meet the requirements of bidirectional power flow capability and to ensure high power factor and low THD as well as to regulate the dc side power regulation. A highly efficient dual-buck converter based bidirectional ac-dc converter is proposed. The implemented converter efficiency peaks at 97.8% at 50-kHz switching frequency for both rectifier and inverter modes. To better utilize the dc bus voltage and eliminate the two dc bus bulk capacitors in the conventional dual-buck converter, a novel bidirectional ac-dc converter is proposed by replacing the capacitor leg of the dual-buck converter based single-phase bidirectional ac-dc converter with a half-bridge switch leg. Based on the single-phase bidirectional ac-dc converter topology, three novel three-phase bidirectional ac-dc converter topologies are proposed. In order to control the bidirectional power flow and at the same time stabilize the system in mode transition, an admittance compensator along with a quasi-proportional-resonant (QPR) controller is adopted to allow smooth startup and elimination of the steady-state error over the entire load range. The proposed QPR controller is designed and implemented with a digital controller. The entire system has been simulated in both PSIM and Simulink and verified with hardware experiments. Small transient currents are observed with the power transferred from rectifier mode to inverter mode at peak current point and also from inverter mode to rectifier mode at peak current point. The designed BMS monitors and reports all battery cells parameters in the pack and estimates the SOC of each battery cell by using the Coulomb counting plus an accurate open-circuit voltage model. The SOC information is then used to control the isolated bidirectional dc-dc converter based active cell balancing circuits to mitigate the mismatch among the series connected cells. Using the proposed SOC balancing technique, the entire battery storage system has demonstrated more capacity than the system without SOC balancing. / Ph. D.

This dissertation aims to present a detailed analysis of the grid voltage disturbance in frequency domain for the current control design in the grid-tie inverter applications and to propose current control techniques in order to minimize its impact and maximize feasibility of the power conditioning system in distributed generations. Because the grid voltage is constantly changing, the inverter must be able to response to it. If the inverter is unable to respond properly, then the grid voltage power comes back to the system and damages the fuel cell power conditioning systems. A closed-loop dynamic model for the current control loop of the grid-tie inverter has been developed. The model explains the structure of the inverter admittance terms. The disturbance of the grid voltages has been analyzed in frequency domain. The admittance compensator has been proposed to prevent the grid voltage effect. The proposed lead-lag current control with admittance compensator transfers current properly without system failure. In order to get rid of the steady-state error of the feedback current, a proportional-resonant controller (PR) has been adopted. A PR control with admittance compensation provides great performance from zero power to full power operation. In addition, active and reactive power flow controller has been proposed based on the PR controller with admittance compensation. The proposed active and reactive power flow control scheme shows a wide range power flow control from pure leading power to pure lagging power. Finally, the proposed controller scheme has been verified its feasibility in three phase grid-tie inverter applications. First of all, a half-bridge grid-tie inverter has been designed with PR controller and admittance compensation. Then three individual grid-tie inverters has been combined and produced three phase current to the three phase grid in either balanced condition or unbalanced condition. The proposed control scheme can be applied not only single phase grid-tie inverter application, but also three phase grid-tie inverter application. This research can be applicable to the photovoltaic PCS as well. This technology makes renewable energy source more plausible for distributed generations. / Ph. D.

The generic concept of quasi-single-stage (QSS) power conversion topology for ac-dc rectification and dc-ac inversion is proposed. The topology is reached by direct cascading and synchronized switching of two variety of buck or two variety of boost switching networks. The family of QSS power converters feature single-stage power processing without a dc-link low-pass filter, a unidirectional pulsating dc-link voltage, soft-switching capability with minimal extra commutation circuitry, simple PWM control, and high efficiency and reliability. A new soft-switched single-phase QSS bi-directional inverter/rectifier (charger) topology is derived based on the QSS power conversion concept. A simple active voltage clamp branch is used to clamp the otherwise high transient voltage on the current-fed ac side, and at the same time, to achieve zero-voltage-switching (ZVS) for the switches in the output side bridge. Seamless four-quadrant operation in the inverter mode, and rectifier operation with unity power factor in the charger (rectifier) mode are realized with the proposed uni-polar center-aligned PWM scheme. Single-stage power conversion, standard half-bridge connection of devices, soft-switching for all the power devices, low conduction loss, simple center-aligned PWM control, and high reliability and efficiency are among its salient features. Experimental results on a 3 kVA bi-directional inverter/rectifier prototype validate the reliable operation of the circuit. Other single-phase and three-phase QSS bi-directional inverters/rectifiers can be easily derived as topological extensions of the basic QSS bi-directional inverter/rectifier. A new QSS isolated three-phase zero-voltage/zero-current-switching (ZVZCS) buck PWM rectifier for high-power off-line applications is also proposed. It consists of a three-phase buck bridge switching under zero current and a phase-shift-controlled full-bridge with ZVZCS, while no intermediate dc-link is involved. Input power and displacement factor control, input current shaping, tight output voltage regulation, high-frequency transformer isolation, and soft-switching for all the power devices are realized in a unified single stage. Because of ZVZCS and single-stage power conversion, it can operate at high switching frequency while maintaining reliable operation and achieving higher efficiency than standard two-stage approaches. A family of isolated ZVZCS buck rectifiers are obtained by incorporating various ZVZCS schemes for full-bridge dc-dc converters into the basic QSS isolated buck rectifier topology. Experimental and simulation results substantiate the reliable operation and high efficiency of selected topologies. The concept of charge control (or instantaneous average current control) of three-phase buck PWM rectifiers is introduced. It controls precisely the average input phase currents to track the input phase voltages by sensing and integrating only the dc rail current, realizes six-step PWM, and features simple implementation, fast dynamic response, excellent noise immunity, and is easy to realize with analog circuitry or to integrate. One particular merit of the scheme is its capability to correct any duty-cycle distortion incurred on only one of the two active duty-cycles which often happens in the soft-switched buck rectifiers, another merit is the smooth transition of the input currents between the 60o sectors. Simulation and preliminary experimental results show that smooth operations and high quality sinusoidal input currents in the full line cycle are achieved with the control scheme. / Ph. D.

State-of-the-art silicon-based power supplies have reached a point of maturity in performance. Efficiency, power density, and cost are major trade-offs involved in further improvements. Most products are custom designed with significant non-recurrent engineering and manufacturing processes that are labor intensive. In particular, conventional magnetic components, including transformers and inductors, have largely remained the same for the past five decades. Those large and bulky magnetic components are major roadblocks toward an automated manufacturing process. In addition, there is no specific approach to reduce electromagnetic interference (EMI) in conventional practices. In certain cases, EMI filter design even requires a trial-and-error process. With recent advances in wide-bandgap (WBG) power semiconductor devices, namely, SiC and GaN, we have witnessed significant improvements in efficiency and power density, compared to their silicon counterparts. In a power factor correction (PFC) rectifier/inverter, the totem-pole configuration with critical conduction mode (CRM) operation to realize zero-voltage switching (ZVS) is deemed most desirable for a switching frequency 10 times higher than current practice. With a significantly higher operating frequency, the integration of inductors with embedded windings in the printed circuit board (PCB) is feasible. However, a PCB winding-based inductor has a fundamental limitation in terms of its power handling capability. The winding loss is proportional to the magnetomotive force (MMF), which is Ni. That is to say, with the number of layers (turns) and currents increased, winding loss is increased nonlinearly. Furthermore, for a large-size planar inductor, flux distribution is usually non-uniform, resulting in dramatically increased hysteresis loss and eddy loss. Thus, current designs are challenged by the capability to increase their power range. To address those issues, a modular building block approach is proposed in this dissertation. A planar PCB inductor is formed by an array of pillars that are integrated into one magnetic core, where each pillar handles roughly 750 W of power. The winding loss is reduced by limiting the number of turns for each pillar. The core loss is minimized with a proposed planar magnetic structure where rather uniformly distributed fluxes were observed in the plates. The proposed approach has a similar loss to a conventional litz wire-based design but features a higher power density and can be easily assembled in automation. A 3 kW high frequency PFC converter with 99% efficiency is demonstrated as an example. Furthermore, PCB-based designs up to 6 kW are provided. Another challenge in a WBG-based PFC/inverter is the high common-mode (CM) noises associated with the high dv/dt of the WBG devices. Symmetry and cancellation techniques are often employed to suppress CM noises in switching power converters. Meanwhile, shielding technique has been demonstrated to effectively suppress CM noises in an isolated converter with PCB-based transformer design. However, for non-isolated converters, such as PFC circuits, none of the techniques mentioned above are deemed applicable or justifiable. Recently, the balance technique has been demonstrated to effectively suppress CM noises up to a point where the parasitic ringing between the inductor and its winding capacitor is observed. This dissertation presents an improved balance technique in a PCB-based coupled inductor design that compensates for the detrimental effect of the interwinding capacitors. A CM noise model is established to simplify the convoluted couplings into a decoupled representation so as to illustrate the necessary conditions for realizing a balanced network. In the given 1 kW PFC example, CM noise suppression is effective in the frequency range of interest up to 30 MHz. The parasitic oscillation of inductors, known to be detrimental for CM noise reduction, is circumvented with the improved magnetic structure. By applying the balance technique to a PFC converter and the shielding technique to an LLC DC/DC converter, significant noise reductions were realized. This provides the opportunity to use a simple one-stage EMI filter to achieve the required EMI noise attenuation for a server power supply. This dissertation further offers an in-depth study on reducing the unwanted near-field couplings between the CM/DM inductors and DM filter capacitors, as well as unwanted self-parasitics such as the ESL of the DM capacitors. An exhaustive finite element analysis (FEA) and near field measurements are conducted to better understand the effect of frequency on the polarization of the near field due to the displacement current. The knowledge gained in this study enables one to minimize unwanted mutual coupling effects by means of physical placement of these filter components. Thus, for the first time, a single-stage EMI filter is demonstrated to meet the EMI standard in an off-line 1 kW, 12 V server power supply. With the academic contributions in this dissertation, a PCB winding-based inductor can be successfully applied to a high-frequency PFC/inverter to achieve high efficiency, high power density, automation in manufacturing, lower EMI, and lower cost. Suffice it to say, the proposed approach enables a paradigm shift in the designing and manufacturing of a PFC/inverter for the next generation of power supplies. / Doctor of Philosophy / State-of-the-art silicon device-based switching power supplies have reached a point of maturity in performance. Efficiency, power density, and cost are major trade-offs involved in performance improvements. Most products are custom designed, requiring significant non-recurrent engineering and labor-intensive manufacturing processes. In particular, conventional magnetic components, including transformers and inductors, have largely remained the same for the past five decades. Those large and bulky magnetic components are major roadblocks toward an automated manufacturing process. In addition, there is no specific approach to reduce electromagnetic interference (EMI) in conventional practices. In consequence, a large multi-stage EMI filter is usually adopted between the power converter and the grid to reduce the EMI noise. It generally occupies 1/4-1/3 of the total converter volume. In certain cases, EMI filter design even requires a trial-and-error process. Suffice it to say, EMI is still regarded as both science and art. With recent advances in wide-bandgap (WBG) power semiconductor devices, namely, SiC and GaN, we have witnessed significant improvements in efficiency and power density, compared to their silicon counterparts. With GaN devices, the switching frequency of a PFC converter is able to be increased by 10 times compared to the state-of-the-art design without compromising efficiency. With a significantly higher operating frequency, the integration of inductors with embedded windings in the printed circuit board (PCB) is feasible. However, the state-of-the-art PCB winding-based inductor has a fundamental limitation in power range. Its winding loss and core loss increase dramatically in high powers. To address this issue, a modular building block approach is proposed in this dissertation. A planar PCB inductor is formed by an array of pillars that are integrated into one magnetic core, where each pillar handles roughly 750 W of power. The winding loss is reduced by limiting the number of turns for each pillar. The core loss is minimized with a proposed planar magnetic structure where rather uniformly distributed fluxes have been observed in the magnetic core plates. A 3 kW high-frequency PFC converter with a 99% peak efficiency is demonstrated as an example. Furthermore, PCB-based designs up to 6 kW are provided. Another challenge in a WBG-based PFC/inverter is the high common-mode (CM) noises caused by the high switching speed of the WBG devices. Symmetry and cancellation techniques are often employed to suppress CM noises in switching power converters. Meanwhile, shielding technique has been demonstrated to effectively suppress CM noises in an isolated converter with PCB-based transformer. However, for non-isolated converters, such as PFC circuits, none of the techniques mentioned above are deemed applicable or justifiable. Recently, the balance technique has been demonstrated to effectively suppress CM noises up to several MHz. However, the CM noise reduction is not effective beyond that. This dissertation presents an improved balance technique in a PCB-based coupled inductor to circumvent the limits. In the given 1 kW PFC example, CM noise suppression is effective in the frequency range of interest up to 30 MHz. By applying the balance technique to a PFC converter and the shielding technique to an LLC DC/DC converter, significant noise reductions were realized. This provides the opportunity to use a simple one-stage EMI filter to achieve the required EMI noise attenuation for a server power supply. It features a smaller volume compared to a conventional multi-stage filter. To further enhance the filter's performance at high frequencies, an exhaustive finite element analysis and near field measurements are conducted to better understand the effect of frequency on the polarization of the near field due to the displacement current. The knowledge gained in this study enables one to minimize unwanted mutual coupling effects through physical placement of these filter components. Several approaches for improving the filter performance at high frequency are conducted. With these approaches applied, a single-stage filter is demonstrated in an off-line 1 kW, 12 V server power supply. Thus, for the first time, a single-stage EMI filter can be contemplated to meet the EMI standard in server power supplies. With the academic contributions in this dissertation, a PCB-winding based inductor can be successfully applied to a high-frequency PFC/inverter to achieve high efficiency, high power density, automation in manufacturing, lower EMI, and lower cost. Suffice it to say, the proposed approach in this work enables a paradigm shift in the designing and manufacturing of a PFC/inverter for the next generation of power supplies.

This dissertation presents control, analysis, and design of silicon carbide (SiC)-based critical conduction mode (CRM) high-frequency soft-switching three-phase ac-dc converters (inverter and rectifier). The soft-switching technique with SiC devices grounded in CRM makes the operation of the ac-dc converter at hundreds of kHz possible while maintaining high efficiency with high power density. This is beneficial for rapidly growing fields such as electric vehicle charging, photovoltaic (PV) systems, and uninterruptable power supplies, etc. However, for the soft-switching technique to be practically adopted to real products in the markets, there are a lot of challenges to overcome. In this dissertation, four types of the challenges are carefully studied and discussed to address them. First, the grid-tied inverters used for distributed energy resources, such as PV systems, must continue operating to deliver power to the grid, when it faces flawed grid conditions such as voltage drop and voltage rise. During abnormal grid conditions, delivering constant active power from the inverter to the grid is essential to avoid large voltage ripples on the dc side because it could trigger over-voltage protection or harm the circuitries, eventually shutting down the inverter. Hence, in such cases, unbalanced ac currents need to be injected into the grid. When the grid voltages and the ac currents are not balanced, there is a chance for the CRM soft-switching inverter to lose its soft-switching capability. Continuous conduction mode operation emerges, causing hard-switching where discontinuous conduction mode (DCM) operation is expected. This leads to huge turn-on loss and high dv/dt noise at the active switch's turn-on moment. To eradicate the hard-switching problem, two improved modulation schemes are developed; one with off-time extension in the CRM phase, the other by skipping switching pulses in the DCM phase. The DCM pulse skipping is applied for a variety of grid imbalance cases, and it is proven that it can be a generalized solution for any kinds of unbalanced grid conditions. Second, the CRM soft-switching scheme with 2-channel interleaving achieves high efficiency at heavy load. Nevertheless, the efficiency plunges as the output load is reduced. This is not suitable for PV inverters, which take account of light load efficiency in terms of "weighted efficiency". Small inductor currents at light load cause the switching frequency to soar because of its CRM-based operation characteristic, causing large switching loss. To increase the inductor current dealt with by the first channel, a phase shedding control is proposed. Gate signals for the second channel are not excited, increasing the first channel's inductor current, thus cutting down the first channel's switching frequency. To prevent the unwanted circulating current formed by shared zero-sequence voltage in the paralleled structure, only two phases in the second channel working in high frequency are shed. The proposed phase shedding control achieves a 0.5 to 3.9 % efficiency improvement with light loads. Third, due to the usage of SiC devices, high dv/dt generated at switching nodes over the system parasitic capacitance causes substantial common mode (CM) noise compared to that with Si devices. In this case, a balance technique with PCB winding inductors can effectively reduce the CM noise. First, winding interleaving structure is selected to minimize the eddy current loss in the windings. But the interwinding capacitance caused by the winding interleaving structure aggravates the CM noise. Impact of the interwinding capacitance on the CM noise is analyzed with a new inductor model containing the interwinding capacitance. Then, finally, a novel inductor structure is proposed to remove the interwinding capacitance and to improve the CM noise reduction performance. The soft-switching ac-dc converter built with the final PCB magnetics features almost similar efficiency compared to that with litz-wire inductor and 14 to 18 dB CM noise reduction up to 15 MHz. Lastly, the soft-switching technique is extended to inverters in standalone mode. To meet tight ac voltage total harmonic distortion requirements, a current control in dq-frame is introduced. As for the ac voltage regulation at no-load, on top of the improved phase shedding control, a frequency limiting with fixed frequency DCM method is applied to prevent excessive increase in the switching frequency. Then, how to deal with short-circuit at the output load is investigated. Since the soft-switching modulation violates inductor voltage-second balance during the short-circuit, the modulation method is switched to a conventional sinusoidal PWM at fixed frequency. It is concluded that all the additional requirements for the standalone inverters can be satisfied by the introduced control strategies. / Doctor of Philosophy / The world is facing an unprecedented weather crisis. Global warming is getting more severe because of excessive amount of carbon emission. In an effort to overcome this crisis, paradigm of energy and lifestyle of people have changed. Penetration of distributed energy resources (DERs) such as wind turbines, and photovoltaic systems has been dramatically increased. Instead of internal combustion engine vehicles (EVs), electric vehicles hit the mainstream. In these changes, power electronics plays a critical role as the key element of the systems. Especially, three-phase inverter/rectifiers are essential parts in such applications. Most important aspects of the three-phase inverter/rectifier are efficiency and power density. In the past decades, Silicon (Si) power devices were mostly used for the systems and the technology based on Si has almost reached to its physical limits. The switching frequency of Si-based inverter/rectifier is limited below 20 – 30 kHz to reduce switching loss. This impedes high power density due to bulky passive components such as inductors and capacitors. Nowadays, the advent of wideband gap such as Silicon Carbide (SiC) and Gallium Nitride (GaN) power devices gives us a great opportunity to improve the efficiency and the power density with its high switching speed capability, low switching energy and low on-resistance. The SiC power devices are more suitable for DERs and EVs due to higher voltage rating. Using SiC power devices allows to increase inverter/rectifier' switching frequency about five times to have similar efficiency with those based on Si power devices, making the power density high. However, there is still room to push the switching frequency even higher to hundreds of kHz with soft-switching. In this sense, studies on soft-switching techniques for three-phase inverter/rectifier have been intensively conducted. Particularly, soft-switching techniques based on critical conduction mode (CRM) are regarded as the most promising solutions because it does not have any additional circuits to achieve the soft-switching, keeping the system as straightforward as possible. However, most of the studies for the CRM-based soft-switching three-phase inverter/rectifier mainly focus on limited occasions such as ideal operation conditions. For this technique to be widely used and adopted in industry, more practical cases for the systems need to be studied. In this dissertation, the soft-switching three-phase inverter/rectifier under diverse situations are investigated in depth. First, behavior of the soft-switching inverter/rectifier under unbalanced grid conditions are analyzed and control methods are developed to maintain its soft-switching capability. Second, how to improve light load efficiency is explored. Circulating current issue for the light load efficiency improvement is analyzed and a control method is proposed to eliminate the circulating current. Third, a design methodology and considerations of inductors based on PCB magnetics are discussed to reduce electromagnetic noise and improve system efficiency. Lastly, the soft-switching technique is extended to standalone mode applications dealing with strict voltage regulation, no-load operation, and output short-circuit.

This thesis proposes a novel approach to power electronics system design that is based on the open-architecture distributed digital controller and modular power electronics building blocks (PEBBs). The proposed distributed digital controller partitions the controller in three levels of control authority. The power stage controller, designated as hardware manager, is responsible for low-level hardware oriented tasks; the high level controller, designated as applications manager, performs higher-level application-oriented tasks; and the system level controller handles system control and monitoring functions. Communications between the hardware-oriented controller and the higher-level controller are implemented with the previously proposed 125 Mbits/sec daisy-chained fiber optic communication protocol. Real-time control and status data are communicated by means of communication protocol. The distributed controller on the power converter level makes the system open, flexible and simple to use. Furthermore, this work gives an overview and comparison of current state-of-the-art communication protocols for real-time control applications with emphasis on industrial automation and motion control. All of the studied protocols have been considered as local area networks (LAN) for system-level control in power converter systems. The most promising solution has been chosen for the system level communication protocol. This thesis also provides the details of design and implementation of the distributed controller. The design of both the hardware and software components are explained. A 100 kVA three-phase voltage source inverter (VSI) prototype was built and tested using the distributed controller approach to demonstrate the feasibility of the proposed concept. / Master of Science

Within the past decade, talk has arisen of shifting the utility grid from centralized, radial sources to a distributed network of sources, also known as distributed generation (DG); in the wake of deregulation, the California energy crisis, and northeastern blackouts. Existing control techniques for DG systems are designed to operate a system either in the connected or disconnected (islanding) mode to the utility; thus not allowing for both modes to be implemented and transitioned between. Existing detection and re-closure algorithms can also be improved upon. Dependent upon the method implemented, detection algorithms can either cause distortions in the output or completely miss a disturbance. The present re-closure process to reconnect to the utility is to completely shutdown and wait five minutes. The proposed methods of this study improve upon existing methods, via simulation and hardware experimentation, for DG systems that can intentionally islanding themselves. The proposed, "switched-mode", control allows for continuous operation of the system during disturbances by transitioning the mode of control to reflect the change in the system mode (grid-connected or islanding). This allows for zero downtimes without detrimental transients. The proposed detection method can sense disturbances that other methods cannot; and within 25 ms (approximately 1.5 line-cycles at 60 Hz). This method is an improvement over other methods because it eliminates the need to purposely distort the outputs to sense a disturbance. The proposed re-closure method is an improvement over the existing method due to the fact that it does not require the system to de-energize before re-synchronizing and reconnecting to the utility. This allows for DGs to continuously supply power to the system without having to shut down. Results show that the system is generally ready to reconnect after 2 to 5 line cycles. / Master of Science

Current source is used in auxiliary power supplies, battery chargers, and LED drivers. The battery chargers are required to provide constant current within a wide output voltage range, similar to LED drivers. The load-independent (LI) Class-E inverter is a promising topology for such applications since it can realize zero-voltage switching (ZVS) within a wide load range. Class-E current source can be achieved by converting constant voltage (CV) Class-E inverter to current source with a trans-susceptance network or using parallel resonant topology. The design and analysis of LI Class-E inverters usually assume a high-Q resonant load tank so that the load current/voltage is sinusoidal. While this is the case in RF applications, it's not required in DC-DC power conversion. Besides, high-Q design leads to high inductance and increased voltage/current stress on the resonant components, increasing converter volume, loss, and cost. This work aims to provide a design guideline for the CC Class-E inverter when significant harmonics are present by reflecting the trade-off between load range and voltage stress, with the help of a modified frequency domain analysis method to eliminate the iteration existing in the time domain analysis. Output current variation and voltage stress can be automatically quantified when circuit parameters vary. Generalized load range contours are obtained to guide the circuit design. With the help of the analysis, a 10-W dual-output Class-E gate power supply is designed with optimized magnetics and reduced isolation capacitance. Compared with CC Class-E based on trans-susceptance network, the parallel resonant CC Class-E inverter has smaller part counts due to its low-order resonant network. However, the current topology suffers from limited maximum output power. In this work, a coupled-inductor based parallel resonant CC Class-E inverter is proposed with more than 2 times maximum power without increasing part counts. / Doctor of Philosophy / Current source is used in auxiliary power supplies, battery chargers, and LED drivers. The battery chargers are required to provide constant current within a wide output voltage range, similar to LED drivers. The load-independent (LI) Class-E inverter is a promising topology for such applications since it can realize zero-voltage switching (ZVS) within a wide load range. This work aims to provide a new design guideline for the CC Class-E inverter when significant harmonics are present by reflecting the trade-off between load range and voltage stress, with the help of a modified frequency domain analysis method to eliminate the iteration existing in the time domain analysis. Output current variation and voltage stress can be automatically quantified when circuit parameters vary. Generalized load range contours are obtained to guide the circuit design. With the help of the analysis, a 10-W dual-output Class-E gate power supply is designed with optimized magnetics and reduced isolation capacitance. Compared with CC Class-E based on trans-susceptance network, the parallel resonant CC Class-E inverter has smaller part counts due to its low-order resonant network. However, the current topology suffers from limited maximum output power. In this work, a coupled-inductor based parallel resonant CC Class-E inverter is proposed with more than 2 times maximum power without increasing part counts.