Back up business

UPS applications have grown substantially over the past decade. Most major industries are now using ups to protect their core business against mains failures, mains supply fluctuations, power surges, and neighbouring disturbances in the electrical supply.

Although the functionality of the ups remains the same, the choice of selecting a rotary or static ups for industrial applications has become contentious.

In recent years, static ups have commanded a major share of the ups market, although rotary ups still maintain popularity in industrial environments where ratings are in excess of 800 kVA. The problem is that rotary technology is up to 30% more expensive than the alternative static offerings.

Major components in a static ups

The harmonic filter on the input side of the ups is normally either an LC passive filter or one that employs the latest technology using active harmonic conditioning. These manage the re-injected harmonics or electrical pollutive components that can increase as the load diminishes. So why is it that for industrial environments, many engineers design ups systems at 100% load, when in real terms the application runs at approximately 60%? This oversizing can often mean that the true potential harmonic pollution has not been correctly calculated at the design stage.

Typically, where a six pulse rectifier with LC filter is running at full load, the total harmonic distortion current (thdi) represents approximately 5%, whereas the same device will show thdi of about 20% at half load. This is where the benefits of an active harmonic conditioner are prevalent as it can easily address and compensate for the changing levels of harmonic distortion.

The increased power factor at partial loads when utilising an LC passive filter should also be considered, as the increased power factor can make the automatic voltage regulator oscillate, but this can easily be avoided by using an active harmonic conditioner.

Since loads supported by ups are critical, it is important that the designer selects a true on-line, dual conversion type as the ups system. Whereas, with a single conversion type where the inverter and rectifier are combined, there are distinct weaknesses to consider. If the switching doesn't work, then the ups will fail. Alternatively, in cases where standby generators are used for longer back-up periods, the ups spends more time on battery due to the generator frequency shifts that cannot be corrected by off-line single conversion ups.

The rectifier or charger on the input side of the ups is generally a six pulse, 12 pulse or 24 pulse devices. By design, six pulse devices are simple and considered highly reliable, although some would suggest that the higher the pulse, the lower the harmonic pollution. However, this is not necessarily the case as you sacrifice reliability when moving to a higher pulse number due to the increased component count.

Take an example. A six pulse rectifier and harmonic filter can limit thdi to around 5%. Although the same can be said for a 12 pulse rectifier and filter, the downside is that the greatly increased component count of two six pulse rectifiers and transformer, significantly reduces the mean time between failure (mtbf). Cost, weight, component count and running costs should be considered and they should consider the inrush current demanded by 12 and 24 pulse systems due to the inherent design characteristics that include transformers.

It is the battery bank that holds the mains stored energy for every ups system and that provides the back-up power from anything between five minutes and several hours, depending upon the application. The battery bank is connected via a circuit breaker to the dc link and is available in the event of a mains disturbance.

The types of batteries utilised are of the sealed lead acid (SLA) variety and normally have either a 5, 7·5 or 10 year design life, but with an operational life of between 2-3 years less. With the battery being the weakest link in a ups system, the design criteria for industrial applications should always be for a minimum two string battery with end of life autonomy.

Batteries are very sensitive to surrounding temperatures and environments. If exposed for long periods to high temperatures (35&C and above), their operational life can be reduced by up to 50%. In order to reach optimum life, batteries should not be exposed to temperatures greater than 25&C or lower than 15&C.

Inverters have changed dramatically in recent years with thyristor technology being replaced with IGBT technology. This new technology has helped to improve the efficiency and handling of non-linear loads (computers and SMP type loads) with extremely low voltage distortion. With thyristor technology, peaks of the voltage waveform were frequently flattened. This does not occur with the more recent IGBT technology. A further advantage of IGBT is that it reduces the size of the inverter stacks and allows repairs to be made more easily.

The final component of any industrial ups system is the static switch. This utilises thyristor switches to provide a bypass path for maintenance, or in the event of internal ups malfunction on the load side, and transfer the load to the mains without a break.

Reliability

Reliability is the key issue of any ups system. It is essential that the ups components are selected and designed on the basis of their ability to work continuously with minimum problems and disruptions.

Although industrial applications require a ups system with a high mtbf, equally important is the mean time to repair (mttr) period which should be as short as possible. This is a problem with rotary ups. The mttr can span several hours for large systems, seriously affecting the operation of critical applications. It should be common practice that large users select a ups system with high flexibility in respect of maintenance. This can normally be achieved by the use of a redundant ups configuration, typically N+1.

Total harmonic distortion current

Designers should consider that the thdi on a redundant system or any ups utilising phase-shift transformers for containment of harmonic pollution will be subject to change. The thdi at the common busbar will vary dramatically if and when the redundant ups or any of the operational units are off-line.

Although phase-shift transformers are used to minimise costs, they are also adopted to control harmonic pollution to provide a more economic solution. However, one should be aware that if a phase-shift transformer fails or is taken down for maintenance, then the thdi at the common busbar will show significant harmonic pollution that may cause problems, such as tripping circuit breakers.

Fault clearing capacity

It is essential to consider the fcc implications when designing systems. You need to make sure that any downstream fault can be cleared by using the inverter capability rather than depending on the mains bypass circuit.

There is misunderstanding in the market when comparing fault clearing capabilities between static and rotary ups. Static ups are able to provide fault clearing of 1·75 x In as compared to 2 x In provided by rotary systems both operating at 415 V. Although it has been stated that rotary ups can deliver fault clearing current at the order of 10 x In, this is at zero volts and so means that computers are no longer supported as they cannot operate at zero volts.

Monitoring and control

Due to digitally controlled ups design and growth in the IT arena, it is now easy to control and monitor ups remotely.

UPS can also automatically dial up to four telephone numbers in the event of an imminent failure or malfunction. These telephone numbers can be programmed to dial in any sequence as specified by the user in order to reach key personnel.

Conclusion

UPS physical sizes have reduced dramatically over the last 15 years due to the introduction of IGBT technology which, in some cases, has helped to eliminate the output transformer from the ups system. However, particular care needs to be taken at the design stage of any system incorporating ups that are backed up by standby generators.

It is important to consider the implications in a situation where there is long term or extended mains failure. There are two main issues to consider: the size of the alternator which should be capable of supporting the load faced by the ups and its harmonic culture. Of course, this does not apply if there is an active harmonic conditioner interposed between the load and the generator.

In the event that the standby generator is required to support the load, one needs to be confident that the fcc of the generator is adequate when compared with the mains building transformer.

Wherever there are critical applications demanding continuous clean electrical power, ups will be the answer.