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 i.e. they all contain batteries which store energy when the mains supply is available, and a means of 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.

ups1As 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, therefore 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. However, in most cases this can be extended by adding further battery cabinets or selecting batteries of a higher capacity. The battery backup time is often referred to as the 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. In most cases 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 but due to the added ‘regulation’ circuits in the bypass line the load is transferred to the battery-fed inverter supply less often, making this type of system slightly more efficient in terms of running costs and better ‘wear’ compared with an off-line system.

ups2Buck/Boost Transformer Design
One of the drawbacks of the straightforward off-line 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 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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