Date of Award

1-1-2009

Document Type

Campus Access Dissertation

Department

Electrical Engineering

First Advisor

Roger A. Dougal

Abstract

Synchronization of DC-source fed inverters to AC power systems is essential to the integration of DC sources such as fuel cells, batteries, and photovoltaics. In many cases, a phase detector determines the angle of the system which is used in the inverter control for the synchronization; however, there may be errors in the phase detection as a result of a trade-off between harmonic filtering and response time of the detector. These errors may propagate through the control to the output of the inverter and increase harmonic content or degrade the inverter's dynamic response. Such trade-offs are especially significant in highly variable-frequency systems where the phase-detector response time may be critical. Variable-frequency power systems exist in isolated power systems with low inertia such as microgrids and ships, and on aircraft due to the main generators being directly coupled to the aircraft jet turbines. While the frequency in most isolated power systems may vary +/-15% from the nominal 50 or 60Hz, aircraft power systems may vary as much as 380-800Hz in only a few seconds.

In this work, a new method of phase detection is proposed to reduce phase errors in variable-frequency systems through a feed-forward technique. The proposed method is compared with existing phase detectors both in simulation and through hardware evaluation and is verified to be superior in its dynamic response. Additionally, it is demonstrated that phase-detection errors may be reduced by inverter control structure configurations. Finally, the variable-frequency aircraft power system under study is combined with a fuel-cell fed DC to AC inverter in simulation and a small-signal stability study using Lyapunov's indirect method is performed. The results demonstrate that the fuel cell, or any other DC-source, impedance and the inverter controller has a direct effect on the system small-signal stability.

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