Renewable energy and distributed generation are getting more and more popular, including photovoltaic modules (PV), wind turbines, and fuel cells. The renewable energy sources need the power electronics interface to the utility grid because of different characteristics between the sources and the grid. No matter what renewable energy source is utilized, inverters are essential in the microgrid system. Thanks to flexible modular design, transformerless connection, extended voltage and power output, less maintenance and higher fault tolerance, the cascade inverters are good candidates for utility interface of various renewable energy sources. This dissertation proposes a new type of cascade inverters based on dual-buck topology and phase-shift control scheme. Compared to traditional cascade inverters, they have enhanced system reliability thanks to no shoot-through problems and lower switching loss with the help of using power MOSFETs. With phase-shift control, it theoretically eliminates the inherent current zero-crossing distortion of the single-unit dual-buck type inverter. In addition, phase-shift control can greatly reduce the ripple current or cut down the size of passive components by increasing the equivalent switching frequency. An asymmetrical half-cycle unipolar (AHCU) PWM technique is proposed for dual-buck full-bridge inverter. The proposed approach is to cut down the switching loss of power MOSFETs by half. At the same time, this AHCU PWM leads to current ripple reduction, and thus reducing ripple-related loss in filter components. Therefore, the proposed PWM strategy results in significant efficiency improvement. Additionally, the AHCU PWM also compensates for the zero-crossing distortion problem of dual-buck full-bridge inverter. Several PWM techniques are analyzed and compared, including bipolar PWM, unipolar PWM and phase-shifted PWM, when applied to the proposed cascade dual-buck full-bridge inverter. It has been found out that a PWM combination technique with the use of two out of the three PWMs leads to better performance in terms of less output current ripple and harmonics, no zero-crossing distortion, and higher efficiency. A grid-tie control system is proposed for cascade dual-buck inverter with both active and reactive power flow capability in a wide range under two types of renewable energy and distributed generation sources. Fuel cell power conditioning system (PCS) is Type I system with active power command generated by balance of plant (BOP) of each unit; and photovoltaic or wind PCS is Type II system with active power harvested by each front-end unit through maximum power point tracking (MPPT). Reactive power command is generated by distributed generation (DG) control site for both systems. Selective harmonic proportional resonant (PR) controller and admittance compensation controller are first introduced to cascade inverter grid-tie control to achieve better steady-state and dynamic performances. / Ph. D.

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.