High-fidelity power injection from a fuel cell onto a variable-frequency AC power bus

Variable-frequency alternating current (AC) power increasingly exists in grid systems, for example on commercial aircraft, and DC power sources feeding these variable-frequency AC grids require novel synchronization approaches to deal with the system dynamics. In order to further the knowledge of DC sources in a variable-frequency power system, the two main foci of this dissertation are the (1) control and synchronization of the inverter to the variable-frequency main distribution bus and (2) power sharing between the fuel cell system and main turbine generator on an aircraft.;The application to aircraft systems is of particular interest, since engine driven generators are no longer mechanically compensated to run at a constant frequency. In the past, on aircraft, auxiliary power units have fed the power system through the direct current (DC) or 400 Hz AC bus; however, new power system configurations may require a fuel cell to provide extra power to a variable-frequency bus. Variable-frequency power systems may also be applicable on ships and isolated microgrids where they may have efficiency advantages over fixed-frequency systems [1]. Power systems with low-inertia may also be variable frequency. For example, according to the EN 50160 standard for electricity supplied by public distributions systems, frequency is allowed to vary as much as +/-15% in isolated system [3].;A fuel cell or other DC source providing power to a variable-frequency AC power bus, specifically on aircraft, is a feasible option for electrical power transfer; however, aspects of the power conversion process can be improved upon to advance the integration of such systems. The control and synchronization of the inverter to the variable-frequency bus presents a myriad of challenges that have not yet been explored in the context of traditional power systems; therefore, the first objective is to develop a methodology for connecting a variable frequency inverter to a weak variable-frequency power system; specifically the work focuses on the problem of deriving an adequate phase reference and comparing alternative controller architectures for the inverter. Additionally, power sharing strategies for fuel cells, energy storage systems, and generators have been studied extensively for microgrid applications [4]; however, the uncontrollable (from the electrical standpoint) variable-frequency due to the generator attached to a jet engine makes this power sharing problem unique.;The broader implications of this work are in facilitating the use of fuel cells, or any DC-power supplied inverter, in variable-frequency autonomous power systems where frequency sensing is important for the control and synchronization of power electronics. An example of such a microgrid having variable frequency would be in the direct combination of a wind-turbine and solar array system through a power converter. Research also suggests that variable-speed micro-turbines are more efficient, and because of lower inertia at lower speeds they need a back-up source to prevent stall during large increases in load [6]. Combine heat and power plants such as high-speed micro-turbines and high temperature fuel cells as demonstrated in [7] may also be an application for a variable frequency bus.