All Uninterruptible Power Supply or UPS Systems consist of a rectifier which converts your normal mains AC power into DC power, an inverter that converts DC power into AC power, and an energy storage medium which is usually lead acid batteries.
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:
- The power consumption of your load
- The type of load you have
- How critical your load is
- The environment in which the UPS will sit
- The required runtime
- 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.