Today, office and industrial equipment invariably needs electrical power to operate. Although the consequences of any power failure depend on the nature of the equipment and its application, they are increasingly likely to be serious or even catastrophic.
Computer systems, for example, typically cannot tolerate a power break of even a few milliseconds without failing. The potential threat of such events to business security and possibly human safety stretches far beyond the immediate risk to the computer hardware. Accordingly, facilities managers must appreciate mains power issues, what power protection is and how to apply it, as organisations become inexorably more dependent on sensitive electronic equipment.
Although power failure events, or blackouts, are obvious threats to on-site equipment, other conditions can also cause problems. Brownouts occur when the mains supply cannot cope with its overall load and the voltage levels reduce, in extreme cases for periods measured in hours.
Mains power can also sag, or drop in voltage level for a few cycles, usually after a large load such as air conditioning or rotating machinery is switched on. Conversely, switch-off of such loads can cause voltage surges, where a voltage increase above normal is sustained for more than one cycle.
Spikes are short duration rapid voltage transitions superimposed on the mains waveform by external events such as lightning strikes or switching of high electrical currents. Non-linear loads such as computers, photocopiers, laser printers and variable speed drives impose harmonics on the mains supply. These can cause a disproportionate rise in current and temperatures, leading to equipment overheating and component failure.
Not all on-site equipment is susceptible to such conditions, but a significant proportion is likely to be. Such equipment is often known as the critical load, partly to reflect its requirement for power free of availability or control problems, and partly because its continuous operation is essential to its organisation’s viability.
Critical loads include computer and communications systems, industrial process control and medical equipment, point of sale terminals and online transaction processing hardware. Within these loads, brownouts or sags can cause equipment malfunction or rebooting where computers believe they are being re-started. Spikes can damage load equipment, while surges can degrade switched-mode power supplies and cause premature equipment failures. Equally serious is the potential for data loss and corruption.
Power protection possibilities
Critical loads can however be protected from these power problems, with UPS offering the most comprehensive protection currently available. UPSs are solid-state assemblies that connect to the incoming mains supply and in turn feed power to the site’s critical load. UPS systems contain batteries which store electrical energy when the mains supply is available, then feed it to the critical load whenever the mains is compromised.
Therefore, a UPS must also have a rectifier, battery charger and an inverter to convert the DC battery power into an AC mains supply level to suit the critical load. All modern UPSs also include a bypass system and a switch allowing direct connection of the critical load to the incoming mains supply when required.
These components are typically arranged in dual conversion configuration in which, during normal operation, the incoming mains supply is rectified and used for float-charging the battery, then inverted back to the critical load AC level. During mains failure, the charger shuts down and the battery takes over the mains supply by discharging through the inverter.
This topology offers the best possible protection for the critical load for two reasons. Firstly, whenever the mains fails or falls outside acceptable limits transfer to the battery is invisible to the load, which suffers no power interruption or disturbance.
Secondly, the inverter and rectifier act as a barrier to mains borne noise and transient voltage excursions as well as providing a well-regulated output voltage. The load enjoys protected power at all times, irrespective of whether it is supplied from the mains or the UPS battery.
UPS factors
When the case for UPS protection becomes clear, suppliers can advise on suitable products matching them to site requirements. However site managers can start by considering the key factors relating to UPS deployment and maintenance as an efficient and effective power protection facility. These include taking advantage of the latest technology, maximising UPS availability, choosing suitable battery autonomy, and making adequate provision for repairs and maintenance.
Trends in UPS development over recent years have had a significant impact on their size, and consequently on their resilience to failure, availability and maintainability. Originally, UPS designs typically included a transformer to boost the inverter output to a level compatible with the critical load requirement.
Since the 1990s, however, advances in semiconductor technology have eliminated the output transformer, yielding a significant reduction in UPS size and weight, and improving energy efficiency. This in turn has allowed the more recent concept of modular design, where the smaller, lighter implementation means that a UPS can comprise a number of independent modules in a rack rather than one large standalone installation.
These modules can be connected in a parallel redundant configuration to achieve very high resilience to failure. For example, a 120 kVA load could be shared by four 40 kVA modules. This includes redundant capacity, so if one module fails the others can continue to fully support the entire load.
Repair of a failed module can be achieved simply by a ‘hot-swap’ replacement – an operation that can be completed in about half an hour, compared with the six hours usually needed for in-situ repair of a standalone system. The UPS’s availability, which is a comparison between the equipment’s Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR) is also significantly improved, with up to 99.9999% being possible.
Battery autonomy refers to the time for which a fully charged UPS battery could support the critical load during a mains failure. It depends on the battery capacity compared with the critical load size. Statistically 95% of all mains disturbances last either for less than five minutes or for several hours.
A battery autonomy time of 10 – 30 minutes, depending on site requirements, is therefore standard. Some loads must remain on line even if a mains failure or problem lasts for several hours – a requirement not realistically supported by battery autonomy times. A typical solution to this is to use a generator, in which case the UPS’s role is to provide sufficient battery autonomy for generator to start-up and supply power.
Maintenance of modular systems becomes easier as repairs require less skill as well as less time, and stockholding is simplified. However a preventive maintenance program is always recommended, especially for batteries which have a finite life.
Regular inspection of batteries and other key components can ensure that problems are resolved before they cause failure. Reputable UPS suppliers can advise on and offer service plans for planned maintenance together with call-out support appropriate to the needs of each specific site load.
Against this background, the need for UPS protection is no longer in doubt. Therefore, the focus has rightly shifted to choosing the best UPS solution for the application. This evaluation is supported by a recent Uninterruptible Power Supplies Limited (UPSL) industry survey, which sought to discover ICT equipment users’ views and experiences relating to these issues.
In the survey, over 75% of the respondents experienced a power outage within the last 12 months that required their UPS to protect their critical equipment and users do not expect this situation to improve – with 78% believing that power reliability is going to become a major concern within the next ten years.
Given the critical requirement to provide protection on demand, it is hardly surprising that 86% of the survey’s respondents regard reliability as the UPS’s most important attribute. However most also recognise that guaranteed power protection takes more than just the UPS hardware.
Over 72% ranked quality of service as highly important or important, while over 90% consider maintenance and emergency callout support to be as important to their business as the hardware itself.
