Development of long life three phase uninterruptible power supply using flywheel energy storage unit

According to the development of computer applications, uninterruptible power supplies (UPS) are indispensable to the industrial field. But the cost of maintaining a conventional UPS is very high, because frequent replacement of parts which have short lifetime is necessary. This paper describes the research and development of a new UPS which has long life parts for maintenance free operation. To lengthen the lifetime, the following techniques are introduced: (1) a flywheel energy storage unit having more than 20 years lifetime; and (2) an electrolytic capacitorless inverter and power converter. By using these techniques, a three-phase UPS rated 5 kVA, 200 V is developed, and excellent performance is obtained: input power factor is over 99.7%; output voltage distortion is under 1.6%; the transformerless UPS produces a lightweight system; the UPSs have the function of automatic output voltage balance using auxiliary diode rectifier; and input current harmonic distortion is less than 1.2%, even if a single-phase load is connected