Implementation and Control of an AC/DC/AC Converter for Double Wound Flywheel Application

An all-electric driveline based on a double wound flywheel, connected in series between main energy storage and a wheel motor, is presented. The flywheel works as a power buffer, allowing the battery to deliver optimized power. It also separates electrically the system in two sides, with the battery connected to the low voltage side and the wheel motor connected to the high voltage side. This paper presents the implementation and control of the AC/DC/AC converter, used to connect the flywheel high voltage windings to the wheel motor. The converter general operation and the adopted control strategy are discussed. The implementation of the AC/DC/AC converter has been described from a practical perspective. Results from experimental tests performed in the full-system prototype are presented. The prototype system is running with satisfactory stability during acceleration mode. Good efficiency and unity power factor could be achieved, based on vector control and space vector modulation. 1. Introduction Extensive research has been recently done on Electric Vehicles (EVs) [1, 2]. The development of an efficient and robust propulsion system is essential to the feasibility of EVs [3]. But, even though sophisticated engines and advanced electrical power trains exist, the main issue, the long-term energy storage, has not been resolved. All-electric drivelines based on battery, supercapacitor, flywheel, and combinations of these are being widely discussed and tested, attempting to lower the requirement on power density from the batteries [4, 5]. The propulsion system in development at Uppsala University is based upon a double wound flywheel energy storage device [6]. The flywheel under study is physically divided into two voltage levels through the stator winding. The high voltage side of the flywheel is connected to the wheel motor, whereas the low voltage side is connected to the battery, as shown in Figure 1. The system is bidirectional and the power can either flow from the battery to the wheel motor (acceleration mode) or from the wheel-motor to the battery (regenerative braking) [7]. Figure 1: All-electric driveline schematics: The power can flow in both directions. In acceleration mode, the flywheel function is basically to provide the variant power requested by the wheel motor, so that the battery delivers a smoother output power. In braking mode, the wheel motor acts as a generator, and the flywheel is responsible for storing the regenerated energy. The system needs a considerable number of power electronics converters and electronic controllers in order