New Multi-Phase Diode Rectifier Average Models for AC and DC Power System Studies
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More power semiconductors are applying to the aircraft power system to make the system smaller, lighter and more reliable. Average models provide a good solution to system simulation and can also serve as the basis to derive the small signal model for system-level study using linear control theory. A new average modeling approach for three-phase and nine-phase diode rectifiers with improved ac and dc dynamics is proposed in this dissertation. The key assumption is to model the load current using its first-order Taylor Series expansion throughout the entire averaging time span. A thorough comparison in the time domain is given of this model and two additional average models that were developed based on different load current assumptions, using the detailed switching models as the benchmark. The proposed average model is further verified by experimental results. In the frequency domain, the output impedance of a nine-phase diode rectifier is derived, and the sampling effect in the average model is investigated by Fourier analysis. The feeder's impedance before the rectifier is modeled differently in the output impedance in contrast in the equivalent commutation inductance. The average model is applied to the resonance study in a system composed of a synchronous generator, a nine-phase diode rectifier and a motor drive. The Thevenin's and Norton's equivalent circuits are derived to construct a linearized system. The equivalent impedance are derived from the average models, and the source are obtained from the switching circuit by short-circuit or open-circuit. Transfer functions are derived from the harmonic sources to the bus capacitor voltage for resonance study. The relationship between the stability and the resonance is analyzed, and the effect of controllers on the resonance is investigated. Optimization is another system-level application of the average model. A half-bridge circuit with piezoelectric actuator as its load is optimized using genetic algorithm. The optimization provides the possibility to design the actuator and its driving circuit automatically.