Flywheel energy storage systems are focused as uninterruptible power supplies (UPS) from the viewpoint of environmental friendliness and high durability performance. Using a low-speed, heavy, simple flywheel, and a low-cost squirrel-cage induction motor/generator, this paper proposes two applications as follows; 1) 11-kW voltage sag compensator using a capacitor self-excited induction generator without semiconductor converters; 2) UPS composed of the flywheel system and an engine generator. From some experimental results, an ideal voltage sag compensator and UPS are realized by the low-technology flywheel system.
This paper presents two applications about power quality improvement using a quite simple flywheel induction motor. In our first work, a new flywheel voltage sag compansator without using semiconductor converters is developed. Second, a hybrid uninterruptible power supply (UPS) composed of the flywheel system and an engine generator is tested. Figure 1 shows an induction motor just only attached with a flywheel. A voltage sag compensator is a very simple configuration as shown in Fig. 2; only an 11 kW squirrel-cage induction motor, a heavy flywheel and a self-excited capacitor are needed. When voltage sag occurs, an interconnection switch SW1 is immediately turned off. At the same time, the induction motor becomes a capacitor self-excited induction generator. Thus, the flywheel system can provide electrical energy to the load at constant speed because of the heavy flywheel. Figure 3 is one of the experimental voltage sag test results using 3.7 kW load. It shows that this system can compensate the voltage sag without any influence on the load voltage waveforms. In addition, higher harmonics can be reduced because no semiconductor converters are used.Next, a hybrid UPS system with the flywheel system and a 5 kVA
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