Elimination of transfer time effects in line-interactive and passive standby UPSs by means of a small-size inverter

The main drawback of line-interactive and passive standby uninterruptible power supplies (UPS) is the transfer time when changing from normal mode of operation to battery mode of operation. During this time, the critical loads protected by the UPS are under the influence of the grid disturbance which forced the mode change. On the other hand, equipment protected by a double conversion UPS is never affected by any disturbance in the mains. Nevertheless, the efficiency of this topology is lower and its size and cost are considerably higher. Hence, the elimination of the transfer process between modes in line-interactive and passive standby UPSs would make them more competitive than double conversion topology in many situations. In this paper, a method for eliminating the transfer time is proposed. It is based on a half-bridge inverter with a hysteretic controller whose output voltage is only applied to the load during the transfer time. Considering the random nature of grid failures and the short duration of the transfer time, the design and selection of the components is nothing but typical. This also leads to a non-standard hysteretic controller that needs to be fully analyzed. Experimental results are provided for a 750-W line-interactive UPS with a transfer time of 4 ms.