There are 3 main UPS technologies – Offline, Line Interactive and Online Double Conversion – and two different types of inverter – square wave (also called pseudo-sine wave or modified sine wave) and sine-wave.

An Offline UPS provides basic levels of power protection. There is usually a degree of surge suppression incorporated and when the input mains voltage goes out of tolerance (that is too high/low or fails) the UPS inverter starts up and provides power to your equipment. There is a break in the mains supply when this occurs in the region of 10-20 thousands of a second which will generally go unnoticed by the majority of equipment. The inverter in offline UPS is nearly almost always a square wave.

A line interactive UPS is similar to an offline UPS but has the added benefit of voltage regulation. This means that it will reduce the mains voltage if it goes too high (called “buck”) or raise the mains voltage when it goes too low (called “boost”). It will do this without reverting to battery and hence conserve battery life. Since a line interactive UPS has an inline transformer, it also provides in-built filtering and hence a higher degree of power protection. Like the offline types, there is also a break during the transition from mains power to battery power. In higher quality line interactive units, this break may be as low as only 2 to 4 thousandths of a second. Line Interactive UPS Systems come with square wave as well as sine wave inverters.

The online double conversion UPS System is considered by many to provide the highest levels of power protection. The inverter is always on, and provides power to the load. This means that there is no deviation in output voltage and no break if the UPS reverts to battery power. The output waveform in an online UPS System will almost always be a sine-wave, generally of very high quality and can enhance the operation of certain equipment. An online double conversion UPS also has a bypass circuit, which allows power to be continually fed to your load even if the UPS develops a fault or is overloaded.

In choosing your technology you need to consider:

1. The power consumption of your load
2. The type of load you have
3. How critical your load is
4. The environment in which the UPS will sit
5. The required runtime
6. Your budget

Your normal UK socket outlet is rated at 13Amps which means the most power you can draw from a UK outlet is 3KVA, or 3KW. (Power Measurement will be covered in a separate paper). Above this level (for specialist equipment, or hardwired installations) most UPS will be online double conversion technology as the economies of scale start making other technologies non cost effective. Below this level, however all technologies are available, although above around 2KVA, line interactive systems start to become heavy and unwieldy due to the size of transformer that is required.

If your load is made up of computer type power supplies, then both square wave and sine wave products will power this equipment adequately. If your load contains motors, transformers, pumps or other inductive components (on the input power supply) then square wave systems are not suitable and you must opt for a sine-wave system.

Any load that is susceptible to mains disturbances such as in analytical equipment or audio applications should also choose a sine-wave system. Where mains distortions affect equipment performance then you need to opt for online double conversion where a pure sine-wave is always present.

If your load is critical for operation then the increased benefit of online double conversion technology should be used. This provides additional security against all power disturbances as well as the comfort of a fall-back bypass in case a fault develops with the UPS. You may wish to parallel together UPS and again, this can only be achieved with online double conversion UPS. (We’ll cover parallel systems and redundancy in a separate paper). If you have a PC where you are looking for simple battery backup to stop annoyance rebooting or tripping then an offline or line interactive unit would suffice.

Offline and line interactive UPS Systems are generally quiet in operation and do not utilise cooling fans in normal operation (usually). This means they are suitable to be placed in an office or home environment. Online Double Conversion UPS systems require forced cooling and can be quite noisy and therefore tend not to be suitable for use in an office environment.

If you are running your UPS in conjunction with an emergency backup generator then the benefits of online double conversion technology come to the fore, as the UPS will cover for any transition to generator operation and also provide a constant unchanging power source during generator start up and any shifts that may occur in frequency and or voltage during load switching.

Offline UPS tend to be the smallest of all technologies so can be useful to provide emergency ride through for areas where space is at a premium.

Long runtimes on UPS are better handled with online UPS Systems. This is because they are designed to operate continually from the inverter. In which case it does not matter if the input power is from the rectifier or the battery – the operation is the same. Offline and Line Interactive units tend not to be designed for this continuous operation. (Battery selection and runtime will be covered in a separate paper.) It is also unwise to power any equipment with a square wave inverter for any prolonged period of time as you could cause degradation to electronic components. Square wave systems are designed to allow basic computer systems to ride through brief power outages or time to shut down a system in the event of a prolonged power cut.

You may have already made up your mind which technology you need, but let’s take a look at relative costs. Offline UPS systems used to be by far the cheapest. However nowadays an offline UPS and a line interactive square wave unit are typically similar prices. Online Double Conversion used to be far more expensive than other technologies, but now is comparable to a high quality sine-wave line interactive UPS.

With regard to running costs, the offline UPS is the most efficient as in normal operation the input power goes straight through to the load, so the only power loss is to keep the battery float charged and power the UPS logic. Line Interactive units are similar in efficiency but experience more losses due to the transformer required for the buck and boost facility. Online Double Conversion however, has the drawback of being the least efficient of the technologies.

In summary, for simple low cost battery backup then the offline or line interactive square wave systems are suitable. For high grade protection in a quiet environment, or where your load type demands it, opt for a sine-wave line interactive unit. However, if you want the best power solution, then we recommend online double conversion technology as it is compatible with all load types and provides the highest degrees of power protection.

We’ve got the IP-PRO-Evolution at under £45 delivered. This will provide you with quality surge protection as well as battery backup allowing you to continue to trade even if the lights go off for a while.

If you’re short of space then what about the OPTI VS575C for £50 delivered? This is an ideal system for a EPOS terminal.

When money’s tight it’s easy to skimp on the power protection and not spend the money, but at these low prices it’s crazy not to.

They bring up a valid point about overvoltage leading to damage of equipment. Your normal mains supply is designed to operate at 230V±10%, which means a maximum voltage entering your building of 253V. However, the substation providing this voltage has to be able to do so during full power loading. Let’s say you’re on an industrial park and the substation is providing power to all the buildings – the IT infrastructure, the telecom systems, the lighting, the air conditioning, elevators, escalators etc. The load can be quite substantial, but let us take a figure of say, 1000Amps (equivalent to 10 houses). If the impedance on that line was half of one tenth of an Ohm – 0.05Ω the voltage drop across the cable using good old Ohm’s law would be 50V. This means that the substation needs to set its output voltage to around 280V so that when the power reaches your building it is 230V and within limits. However, if the load is suddenly removed – all the air conditioners are switched off, the buildings are empty and nobody is at home, all of a sudden you are hit with 280V, as the lower current causes less voltage to be dropped across the supply cables.

Some people call this a surge and think that surge suppression devices will protect them against it. In fact, this is not a surge but rather a voltage swell or overvoltage condition. (A surge is an overvoltage condition too, but of short duration -usually µseconds), and in order to safeguard your equipment you need to have some form of overvoltage protection. The only way to achieve this is by the use of either voltage regulators or by the Uninterruptible Power Supply (UPS).

A voltage regulator is a (usually mechanical) device that incorporates a tap changing, or continually variable transformer to keep the output voltage to a tight tolerance.

The Uninterruptible Power Supply, however will also provide overvoltage protection by keeping the voltage within limits. How well it does this depends upon the type of technology used:

• The Offline Uninterruptible Power Supply will provide overvoltage protection by dropping to battery as soon as the mains voltage is out of limits. This will protect your equipment but if this happens regularly or for prolonged periods, the UPS battery will drain and you will lose power.
• The Line Interactive Uninterruptible Power Supply will provide overvoltage protection by incorporating some voltage regulation. When the mains goes to high, the UPS System will “buck” the voltage downward by changing taps on a transformer. This has the benefit over the Offline UPS System in that there is no dropping to battery for marginal overvoltage conditions.
• The Online Uninterruptible Power Supply, (aka Online Double Conversion Uninterruptible Power Supply) provides the best possible overvoltage protection. It has a very wide input voltage window, which means it can take very high voltages (as well as very low voltages) without reverting to battery. What’s more the voltage supplied to your system is constant and unchanging regardless of what is happening to the input voltage.

It’s another string to the Uninterruptible Power Supply bow, as not all power problems are as obvious as the power cut. Give your equipment overvoltage protection with a Uninterruptible Power Supply from UPSMart.

So depending upon what level of problem you are experiencing, or what your risk analysis tells you what you should protect against determines the choice of product used:

Since most UPS systems will contain a surge supressor and filter, a UPS is the ideal power protection solution, however consider a power conditioner if you do not require any form of battery backup. The choice of system will be made according to the following table:

Note that 3, 6 and 9 are grey areas, VFD and VI may provide protection against these, dependent upon type of UPS and level of protection employed.

Other Considerations
Once you have decided on the minimum level of technology needed you will need to look at the options you require, and if there are any special requirements. For example, you may need the unit to be situated in a rack server, or the UPS may need to be used in a patient vicinity environment and therefore have special certification requirements. You may have long run time requirements, and need to choose a UPS that can deal with this.

©2008 UPSMart

For more information see OPTI Surge Buster