Uninterrupted power is now a universal requirement. To cater for this need, there are hundreds of UPS suppliers to choose from. Not surprisingly, price has become a major factor in the selection of equipment, which is often viewed more as a commodity than as an important part of computers and data communication networks. The choice of a UPS is too important, however, to be decided purely on price. Phil Kreveld investigates.
The single most important issue in deciding on a UPS is the choice between "line-interactive" and "online" types. The former charges its standby batteries while the power from the main network is connected to the computer or other equipment. When a power failure occurs, it switches over to the battery. The problem with this type of system is that there is a small - but sometimes critical - time gap in the power to the computer, etc during the switching over from main power network to the battery.
The latter, the so-called "online" type, does not switch over. It is always in service. When power fails, the computing equipment attached to the output does not even suffer the most minute interruption of power. For obvious reasons, the online UPS is by far the preferred choice.
What you DO need to know about power suppliesThe power supplies that are part of computers and other office and communication equipment are designed to be connected to the main power network. Generally the alternating voltage supplied by the network to the input to these power supplies, is close to an ideal sine wave. The current drawn by a typical power supply is anything but a sine wave, generally speaking.
Why is it so, and is it a problem? Electronic power supplies convert alternating voltage to direct (DC) voltage. The DC output has energy storage capacity built into it. The current that is required to replenish lost energy (ie, that is supplied to the computer or other equipment), can only flow for very short time spans in each alternating voltage cycle. The shorter the time the charging current can flow, the more intense it is. So-called switch-mode power supplies are a good example - and these days, they're everywhere!
But where is the problem? There is usually no problem while the equipment is directly connected to the main power network. The problems can start, though, when the connection is made via a UPS. The ability of many UPS units to deliver short-time, high-current pulses is very limited, and is described in their specifications (if mentioned at all) under the title of "crest factor". The inability to deliver sufficiently "peaky" current can produce problems in a computer's power supply and therefore also in the computer itself.
Crest factor - it's important
A UPS is used to provide assurance for the correct functioning of important data handling equipment during power failure. The best assurance that can be given is by way of an "online" UPS. These units consist of three parts: a converter which turns the main power network AC voltage to DC; a DC link with battery charging function; and an inverter which turns DC back into AC voltage. For these online systems, the crest factor specification is very important, because the online UPS units are the sole source of AC power to the connected equipment - and not just during periods of power failure.
For the technically inclined, crest factor is defined as the ratio of the peak current that flows for a half cycle divided by the rms (root mean square) current flowing over that period. Irrespective of the precise definition, it is important, when purchasing a UPS, to ask for the output crest factor specification.
Put it this way: if you purchase a UPS with a low crest factor (we think, less then 5), it may well be necessary to spend more dollars on a higher output-power rated unit than that indicated by the connected load. It works approximately as the "square" of the ratio of crest factors. A 5kVA (5000 volt-amp) unit with a crest factor of 5 can be equivalent to a 14kVA unit with form factor 3 - it depends on the type of equipment that is attached to the output of the UPS!
Input voltage range
The input voltage range of a UPS is a very important parameter. The reason for this is not hard to explain. Between a complete power failure and normal, 100 per cent power, there are all kinds of "intermediate" states. Would you consider 80 per cent of standard voltage a power failure? Of course not! But what, if the UPS had a different opinion? If the voltage dropped below the lower operating voltage limit, the UPS would be discharging its standby batteries! Therefore, if an 80 per cent voltage, or generally speaking low voltage condition, happened often, then, come the power failure, you might find that your batteries have insufficient charge to keep your important equipment alive.
The wider the tolerance of input voltage to the UPS, the less the batteries are called on for service!
You wouldn't think of pulling a cart along a road by pulling UP on its tow-rope. By pulling on the rope, as close as possible, parallel to the road surface, the force needed to move the cart is minimised. The equivalent applies in electrical power. Going back to the mechanical example, when the rope is parallel with the cart and road, the power factor is ONE (unity)! If the rope were at an angle of 45 degrees, 41 per cent more force would be needed, and the power factor would no longer be unity but 0.7.
There are many UPS units, computers and other electronic equipment which have poor (significantly less than 1.00) power factors. Instead of drawing minimum current, they require more current. Taking the above example, a power supply or UPS with an input power factor of 0.7 requires 40 per cent more current than the same unit operating at a power factor of 1.00.
Not only can you save on your electricity bill when you select equipment with good power factor, there is also reduced disturbance to other equipment connected to the main powernet. If a UPS with high power factor is chosen then equipment with poor power factor, attached to the UPS output, is insulated from the main power network.
Pick the size to suit the load
Simplicity rules! For a centralised computing or other data handling facility (eg, a local network in a building), the best bet usually is a single UPS appropriately rated for the combined electrical load. It is a more economical solution for simple reasons. There are no connectors, protective devices, wiring, cabinets, etc (all part of each UPS) to replicate. But is it more secure or at least as secure when compared to a raft of separate UPS units, each with a lower rating to suit its individual load? In short, -Yes!
The above may seem surprising, but comes about because of very practical considerations. If a UPS fails for some reason and needs to be serviced, it must either be bypassed so that its load can remain under power, or there has to be a spare UPS available. Connecting more load to another UPS under load is only an option if that UPS is capable of handling the necessary additional power output. Buying UPS units with such margins of power reserve isn't economical at all!
The above argument also goes for so-called large UPS units which, inside their cabinetry, house a number of small UPS units, paralleled together. This is a UPS Lodging House! Either the residents are overrated (costing collectively more than a single UPS capable of handling the aggregate load) or, when one fails, the others are put under strain and in due course, also fail through being overloaded!
What you need when the patient is under the knifeSo we're back to bypassing. What is needed in a UPS so as to provide for times when service may be needed, is a Manual Bypass. It is switched on by the service technician while work is being done on the UPS. Its connected load notices absolutely no interruption. For reasons of stripping cost out of small UPS units, these manual bypasses are not incorporated! One note of warning: many if not most UPS units have internal bypass arrangements, ie, as part of the electronics. These latter bypasses come into action when, perhaps for a short period, the connected load draws too much current for the rating of the UPS. When the UPS fails, so usually does the internal bypass, after a short time!
So far we've concentrated on the most important points. They are basics and, they do have an influence on price. One should not just focus on bells and whistles - but consider the fact that a UPS must function reliably AT ALL TIMES, as well as during power failures!
For many users, the choice of a UPS should be considered as a mission-critical decision. The basic things that should be looked at have been discussed here. They should always be weighed against price!
The uninterrupted power supply industry is experiencing a boom due to the ever-increasing amount of data found on company servers due to an expansion in small server capacity and Internet and intranet use.
The mission-critical nature of this internally generated and stored data is sending executives in droves to developers and resellers who can offer protection against the inevitable failure of power, noise interruptions and other environmental hazards.
American Power Conversion
Symmetra Power Array. The Symmetra Power Array is an uninterrupted power supply system based on RAID type architecture and aimed at the server and business-critical application market.
The system includes four main modules: the power modules, which are 4kVA each, the Battery module, the Main Intelligence module and the Redundant Intelligence module. These all run in unison and can be "hotswapped while the load is up, running and fully protected", according to APC.
The Symmetra Power Array provides a redundant real-time power path that allows the system some reserve power capacity, which in turn supports load-sharing capacity.
The module design allows for easy scalability as more modules can be added and the kVA increased.
The management applications also make APC's UPS an attractive option as the suite allows administrators to shut down multiple file servers and reboot individual locked devices.
Maintenance can be an internal matter as the modular nature of the system allows for simple support. The UPS remains operational and protected during any service.
The Symmetra makes it easier by automatically giving warning about any problems and self-diagnosing these dilemmas.
Chloride Power Electronics
The Synthesis Unit. Intended for use to protect mission-critical applications in the telecommunications, finance, transport and government sectors, the Synthesis has a power range of 4 to 30kVA.
It has the standard UPS features of modularity, bypass switches and easy scalability and load sharing capabilities as the Synthesis model can be paralleled with other units.
Chloride feels that the exceptional quality of its maintenance program will set the Synthesis apart from other UPSs as it will enable optimum use of technology.
According to Chloride sources maintenance is integral to ensuring that early warning systems are operational and relevant to particular organisations and that extended battery life adds value to the purchase of a Synthesis UPS.
Powerware Profile Online UPS. The Powerware Profile Online system aims to protect workstations, light industrial applications, centralised multiserve sites and telecommunications equipment from power interruptions.
The Profile range of UPSs has a power range between 8kVA and 12.5kVA which allows 20 minutes of backup time.
It has the contemporary standard modular design and "hot swappable" battery configuration. BTR prides itself on its Advanced Battery Management, which incorporates Exide Electronics' technology.
BTR claims that this will double a battery's service life, optimise recharge time and provide early warning of battery failure.
Also included is FailSafe III and LanSafe III software that ensures data integrity.
This system allows multiple battery packs to be added that are easily connected.
The UPS is easily upgraded and serviced without having to power down a critical load due to a bypass switch.
'Smart' Series UPS. Built along similar lines to other UPSs in the 3.5kVA to 30kVA range, the Smart series offers backup time of 10 to 18 minutes. It also has modular architecture allowing for easy upgrades and servicing whilst the system is still up and running.
Power Tech has included a global communications facility that enables different servers and Lans connectivity. To support this feature a network management solution is part of the package.
SOLA 610 range. This range of UPS solutions covers power outputs from 7.5kVA to 20kVA and claims to protect midrange computers, application and database servers and networking equipment.
Similar to other brand models it has battery addition options and includes an automatic internal bypass facility for ease of service or upgrades.
The 610 range has a management feature, The BestLink Adapter, which assists with the organisation of the unit via connecting to the network or Internet.
With a no-frills pack the backup time in this series runs from seven to 34 minutes on full load.
The systems all have a full two-year warranty with the option to extend it further. vContactsAPCTel (02) 9955 9366BTRTel (02) 4276 4764Chloride Power ElectronicsTel (02) 9980 7788Power TechTel 1800 635 733SOLATel (02) 9949 6000