How Does a UPS Work?

There are several categories of static UPS systems available. Broadly speaking, UPS Modules fall within one of three operational design architectures, namely off-line, line interactive and on-line.

However, irrespective of their individual design criteria certain features are common to all forms of static UPS systems. For example, they all contain batteries which store energy when the mains supply is available. Converting the battery charge into an alternating current (ac) supply in times of mains failure. All systems must therefore include a battery charger and a power inverter circuit.

As described above, the battery provides a power source for the inverter when the mains supply fails, whereupon it discharges at a rate determined by the critical load connected to the UPS output. The inverter automatically shuts down when its dc supply falls below a certain voltage. How does a UPS workTherefore the duration for which the critical load can be supported in times of mains failure depends upon the battery capacity and the percentage applied load.

A typical Uninterruptible Power Supply (UPS) system will contain sufficient battery capacity to support its fully rated output load for 5 to 15 minutes. Extend this by adding further battery cabinets, or selecting batteries of a higher capacity. The battery backup time is autonomy time.

Virtually all systems contain a ‘bypass’ system which, in conjunction with some form of output switching circuit, provides a means of connecting the critical load directly to the mains supply. The output switching circuit is implemented using solid-state switching devices. The rules governing the static switch control depend on the UPS operating mode.

Line-Interactive System

This type of UPS covers a range of hybrid devices that attempt to offer a higher level of performance than conventional off-line designs by adding voltage regulation features in the bypass line. The two most popular types of system in this category employ either a buck/boost transformer or a ferroresonant transformer.

Like off-line models, line-interactive UPS normally supply the critical load through the bypass line and transfer it to the inverter in the event of a bypass supply failure.
The battery, charger and inverter power blocks are utilised in the same manner as in an off-line system. Due to the added ‘regulation’ circuits in the bypass line, the load is transferred to the battery-fed inverter supply less often.
This makes this type of system slightly more efficient in terms of running costs. Also better ‘wear’ compared with an off-line system.

Buck/Boost Transformer Design


One of the drawbacks of the straightforward off-line design is

– The load must transferred to the inverter.
This must happen 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. When 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. Resulting in a inadequately charged battery! Unable to support a prolonged mains blackout.

A buck-boost transformer connected in the bypass line helps overcome this problem. The transformer has tapped secondary windings, selected by relays to either step-up or step-down the bypass voltage. Likewise maintaining 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, initiating 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%.

Use this method to maintain output voltage within its preferred window.
Buck/boost switching unavoidably leads to a degree of step voltage changes as tap changes take place.

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