Art of supply

Art of supplyIn the words of Shakespeare, power corrupts – absolute power corrupts absolutely…. but then he never had uninterruptible power supplies. Steve Bush reports Protect your system Get it now. “Don’t dither. It’s like burglar alarms,” said Peter Bently of independent UPS solution provider Uninterruptible Power Supplies, “You know you need one, but only go and get one after you have been burgled, or in this case, had an expensive power failure. Think of it as insurance.” Make sure the UPS is reliable. “Supplier pedigree is very important, seek references,” said Bently. Maintain your UPS. “It is important to have a maintenance programme in place to ensure the UPS is always available. A maintenance contract should have guaranteed call-out and one or two preventative maintenance visits each year,” said Bently. Some larger UPSs now have automatic telephone links to maintainers and do not rely on the end user spotting trouble.  
 
 
The millennium bug is quite rightly scaring the hell out of big companies, and it should be frightening you too. The developed world is increasingly reliant on computers and the data they store. When computers go down, people are inconvenienced at the minimum, and bankrupted, irradiated or killed at the other end of the scale.
But the millennium bug is not the only thing that causes computers to crash, in fact it is the new kid on the block.
The big established threat to stable computer operation, which has been happily crashing important data processing and control computers for years, is power failure. If power disappears without warning, computers have no time to shut down in an orderly fashion and data and, or, control is lost.
Fortunately, the power problem has been around for long enough for people to have got to grips with it. The answer is uninterruptible power supplies, or UPSs.
These sit between computers and the mains and take over when the mains voltage drops. Further to this, some types also ‘condition’ the mains, removing spikes, noise and voltage fluctuations.
UPSs vary in size. The smallest are rated at 250VA and protect a single PC, the largest types operate at several MVA and can protect a whole building. Very high power types often include a diesel generator to prolong back-up.
Although computer users are the largest customers for UPSs, they are not the only market and point-of-sale terminals in shops, whole shops and industrial concerns are among other users.
There are three basic types of uninterruptible power supply: off-line, line-interactive and on-line. Each type has its place in the market, although there is significant overlap.
In all types, a battery supplies the load through a voltage inverter if the mains supply disappears. This battery is charged when the mains is present.
Off-line UPSs are the simplest, cheapest, type and provide basic protection against the absence of the mains supply. They tend to be small, between 250VA and 500VA and capable of supplying a PC or small workstation and its monitor, or one sales terminal in a shop. The European UPS Market According to Frost and Sullivan, in Europe revenues for UPS manufacturers are forecast to grow from just over $1.4bn, representing almost three quarters of a million units in 1998, to almost $2bn and 2.9 million units in 2004. This represents a compound annual growth rate of six per cent by value.
Currently about 80% of UPS applications are within the computer industry and this sector will continue to dominate the market as other industries have fewer critical applications, and knowledge of UPS technology in many industries remains poor. Conversely, many new UPS applications have emerged as the computer industry converges with other industries such as telecommunications to provide complete ‘information solutions’.
In 1998, Germany was the largest European market for UPS with an estimated 20 per cent share, followed by France (17 per cent), the United Kingdom (14 per cent), Italy (nine per cent), Benelux (seven per cent) and Spain and Portugal (six per cent).
The three leading companies in the European UPS market are Liebert, APC and MGE (formerly part of Merlin-Gerin). APC and MGE each have a market share of 12 per cent whilst Liebert holds 10 per cent. Other key players include Chloride, Exide, Silicon, IMV, Siemens, Best and AEG.
In an off-line UPS, the load is supplied direct from the mains when the mains is present and, because of this, the load is not protected against mains-borne transients, noise or voltage variations.
When the mains fails, it takes around 5ms for an off-line UPS to re-establish supply to the load. “This delay is rarely a problem for loads, most modern PCs cope with 100 to 200ms gaps”said Jorma Mannerkoski, director of product marketing at UPS maker Exide Electronics.
Off-line UPSs tend to have simple inverters and supply the load with a square-waves or stepped waveforms (sometimes called trapezoidal waves) rather than sine waves. They are typically brick-sized or slightly larger, depending on the length of time the battery can supply the load. Ten minutes is the normal maximum.
Status indication to the user is generally limited to a couple of LEDs, one to say the unit is connected to the mains and one to say that the inverter is operating. Occasionally there is another to warn that the battery is about to run flat.
These units tend to be built down to a price and the battery’s life is likely to be under five years before it must be changed. Built-in battery health monitoring is severely limited.
Line-interactive types are similar to off-line types in that their inverters only operate when the mains stops. But when the load is operating from the mains, they add power conditioning. This takes the form of transient removal, interference filtering and voltage regulation – normally through the use of a ta pped transformer. These additional capabilities add a cost as well as a slight size penalty when compared with off-line designs. However, their mains-cleaning capability removes more of the risk of load-upset.
In addition, line-interactive types, because they are not cut to the bone to save cost like off-line types, are more likely to have sine-wave output inverters which are less likely to cause problems with sensitive loads.
The diagram opposite shows a line-interactive UPS with a bi-directional ‘converter’ rather than the traditional battery charger – inverter combination.
Power capacities vary between around 700VA to several kW. Expected loads include small servers, large workstations and multiple PCs. UPS batteries
Regardless of type, at the heart of every UPS is its battery. And despite all the advances in other technologies, lead-acid types still prevail. “Probably 95 per cent of UPS have lead-acid batteries,” said Sean Fitzgerald, product manager at UPS maker Liebert, “some use NiCds, but these tend to be expensive.”
As electronics have shrunk physically, the battery is starting to dominate the size of UPS, occupying 50 per cent of the volume in smaller types – and this in not likely to change. “Lead-acid technology is moving ahead incrementally,”said Fitzgerald. WIthout a breakthrough, and no one sees anything on the horizon, the battery will continue to set the size of UPSs and UPS installations.
Battery life before capacity has reduced to unacceptable levels is generally five years, but 10 year life batteries are available and are used in some large installations. The price difference between five and 10 year batteries is diminishing, making the better quality batteries more attractive.  
 
On-line UPSs do not rely on a direct connection between mains and load. Instead, a front-end rectifier-regulator supplies an intermediate voltage which both charges the battery and feeds the inverter, which in turn supplies the load.
This topology allows all output characteristics, voltage, wave shape and frequency to be controlled and all noise and spikes to be rejected. In addition, changeover from mains to battery and back is transparent to the load, with no change-over gap.
The extra electronics make on-line UPSs physically bigger and the continuous double conversion of power makes them waste some power all the time. On the other hand, they are built to handle this continuous dissipation so their battery operating duration can be extended by fitting a larger battery. In off-line and line interactive types, heat sinking tends to be sized for the back-up time available from the battery fitted. A larger battery can cause these units to overheat after a while.
Power ratings in on-line UPSs range upwards from 500VA to several MVA.
Battery monitoring, including automatically running on battery back-up for a short period to assess battery health, can be included in both line-interactive and on-line UPSs.
More comprehensive types also connect directly to suitable equipped loads to warn them of impending battery depletion. “We supply serial links and software – for 23 operating systems – to shut down computers properly before the battery runs flat,” said Jorma Mannerkoski of Exide Electronics.
Large high-end UPSs can often connect directly to the telephone system and call for maintenance action from specialist without end-user action.
Unlike the Millennium bug, there is a solution to protect computers from power failures. Uninterruptible power supplies are available from single PC versions up to those which can protect a building. Types differ and should be chosen to match the security level required and the sensitivity of the load concerned.


Leave a Reply

Your email address will not be published. Required fields are marked *

*