Transfer Time Suppressor (TTS) for line-interactive Uninterruptible Power Supplies

Abstract

Passive standby and line-interactive uninterruptible power supplies (UPSs) have lower cost and size than double conversion UPS and their efficiency is higher. On the other hand, their voltage regulation is poor (or non-existent) during the normal mode of operation. Moreover, both UPSs have a transfer time (also defined as switching time) from normal mode to stored energy mode when the utility power fails. During this switching time (a few milliseconds), the load is still connected to the grid and, therefore, it is affected by the abnormal line conditions. In previous papers, authors proposed different methods for filling voltage notches in line-interactive UPSs. In this paper, a method which eliminates not only notches, but also voltage surges, is proposed. It is based on two capacitors charged one with positive and the other with negative voltage. If any abnormal line condition takes place in the, for example, positive period, the positive capacitor is connected to the load. This connection is then modulated in order to obtain a sinusoidal waveform. This method can make line-interactive UPS more competitive than the double conversion UPS in many situations and cases.