One of the drawbacks of the straightforward off-line UPS design is that the load must be transferred to the inverter as soon as the bypass supply voltage reaches voltage limits acceptable to the load. This means that the UPS might transfer between bypass and inverter quite frequently if it is set up to operate with a critical load having a tight voltage tolerance. Apart from the power break each time this occurs, this method of operation incurs frequent battery usage which reduces battery life and might perhaps result in a battery that is inadequately charged when it is called upon to support a prolonged mains blackout.
A buck-boost system transformer connected in the bypass line helps overcome this problem. The transformer has tapped secondary windings, which are selected by relays to either step-up or step-down the bypass voltage as appropriate to maintain the UPS output voltage within the required output voltage limits. This means of controlling the output voltage permits a wider variation of bypass voltage to exist before the output voltage reaches its limits and initiates a load transfer to inverter.
A typical UPS in this category will sustain the load voltage over a bypass voltage range of +20% to -30%. Note that although the output voltage is maintained within its preferred window using this method, buck/boost switching unavoidably leads to a degree of step voltage changes as tap changes take place.
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