Power Control Systems for PM Synchronous Flywheel Alternators

Abstract: Flywheel systems are attractive in hybrid and electric vehicles, being able to handle the large power when accelerating and for regenerative braking. The combination of a flywheel device with a battery source has several advantages, such as higher peak power capacity, higher energy density and a decrease of the number of charging/discharging cycles in the battery. The Flywheel Energy Storage System here investigated has its novelty in the use of a double wound flywheel motor/generator to divide the system in two different voltage/power levels. High-Voltage (HV) side connects the flywheel machine to the wheel motor and Low-Voltage (LV) side connects the flywheel machine to the battery. This thesis deals with the power control systems involved in the connection between different components of the system. Bidirectional DC/DC and DC/AC converters are used to connect the flywheel motor/generator to the battery in the LV side. The DC/AC converter design and construction is the main focus of the present licentiate thesis. Two different scaled prototypes have been tested. A speed motor drive was designed and tested with the first scaled prototype, in which different filter and motor load connections were investigated. Results have shown that low harmonic distortion can be obtained in the inverter output voltage and current. The second scaled prototype is a Two Voltage Levels Machine. The bidirectional DC/AC converter has been designed so the LV side of the system could work in a fast and efficient way. Simulations have shown the dynamics of the system, which has also been tested experimentally under a drive cycle. The flywheel has shown to be able of delivering smooth power to the battery side, despite the variations in the load side. A scaling factor has been applied in order to extrapolate the results to a full-scaled system. Investigations have also been made about the battery recharging process. The main challenge is the control of the power flow despite the voltage variations of the flywheel machine as dependent on its charge state, i.e. rotational speed. A DC/DC buck/boost converter under PI control has been designed and simulated, being able to keep either the current or the voltage constant on the battery side when the machine speed is decreasing.

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