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The challenge of choosing a UPS system

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With power outages commonplace this winter, South African companies are looking at various technologies to ensure a continuity of supply. In addition to the ubiquitous generator, an uninterruptible power supply (UPS) system is key to a well-rounded solution. Philip Hampton, chief technology officer at Powermode provides some pointers to the correct selection of a UPS.

Unlike the limited choice of 'standby' or 'on-line' UPS systems of only a few years ago, today there are more options to choose from. In this article we'll look at four of the most popular: 'standby', 'ferroresonant standby', 'line interactive', and 'on-line double conversion'.
Each one is suited to a specific 'UPS topology' or UPS design. However, within the parameters of each design there may be many different performance characteristics.
As a result, choosing a UPS system to meet your company's needs might be significantly more complex than first thought.

Standby UPS
The standby UPS is the simplest and least expensive UPS design. Its primary power source is electrical supply – from Eskom or your local utility company – and its secondary power source is a battery pack, which is kept charged by the UPS's built-in charger.
When the AC (alternating current) power fails, a 'transfer switch' changes to the battery pack which uses an 'inverter' to change the current from DC (direct current) to AC. When line power is restored, the UPS switches back.
Standby UPS systems are generally used as backup for desktop PCs. They are not used for more critical functions because, when the power fails, the switch to battery power is NOT instantaneous. This delay can cause severe damage to critical computer components.
When buying one of these units, compare the 'switch time' or 'transfer time' between different brands and relate your findings to the 'hold time' of your computer system(s). The hold time is the length of time your computer can handle having its power input cut before being restored with out resulting damage.
Obviously, the transfer time needs to be (significantly) less than the hold time, in order for the UPS to work for you.
If you buy a standby UPS, make sure it incorporates surge suppression and filtering features for the times when it is running off standard power.

Ferroresonant Standby
Next step up the ladder is the 'ferroresonant' standby UPS. The big difference is that the standby UPS's transfer switch (that selects between power sources) has been replaced by a ferroresonant transformer.
The most important effect of this transformer is that it tends to smooth out power problems in the line – spikes, brownouts, etc – and not pass them on. This makes this class of UPS well-suited for line filtering.
More significantly, the core of the ferroresonant transformer stores energy in its magnetic field and acts a buffer in the switch over from line current to battery power.
For just a fraction of a second, the output will function off the energy in the transformer itself thus reducing the chances of the computer equipment being affected during the transfer.
Ferroresonant standby UPSs are usually available in sizes of between 3 – 15 kVA, making them suitable for even the largest of servers.

Line Interactive

The line interactive UPS should not be confused with an on-line system, although it uses a totally different design philosophy compared to the standby UPS.
AC line power is still the primary power source, and the battery is the secondary. However, the battery charger, inverter and source selection switch have all been replaced by a combination inverter/converter, which both charges the battery and converts its energy to AC output as required.
When the line power is operating, the inverter/converter charges the battery; when the power fails, it operates in reverse.
The main advantage of this design is that the inverter/converter unit is always connected to the output, powering the equipment. This allows for faster response times in the event of a power failure.
The inverter/converter is also fitted with circuitry to filter out 'noise' and spikes, and to regulate the power output, providing additional power during brownouts and filtering output during surges.
The line-interactive UPS is commonly used in units sized from 0.5 to 5 kVA. It is superior to the standby UPS, being generally more efficient and reliable.
Unfortunately, it still has the handicap of a transfer time, and thus does not provide the level of protection of an on-line UPS system.

On-Line Double Conversion
This is the most common – and the most popular – type of UPS for applications above 10kVA. From a design perspective, the On-line Double Conversion UPS system is very similar to the standby UPS, except that the primary power path is the inverter instead of the AC mains supply.
As such, failure of the input AC does not cause activation of the transfer switch, because the input AC is charging the backup battery source which provides power to the output inverter.
During an input AC power failure, the unit's on-line operation results in no transfer time whatsoever, providing almost ideal electrical output performance for mission critical computer systems.