Pioneering solutions for total power protection

In today's 24/7 on demand world, mission-critical business systems must be available 100% of the time. Downtime cannot be tolerated, which is where an uninterrupted power supply can assist. At Uninterruptible Power Supplies Ltd, we pride ourselves on delivering industry-leading power protection solutions combined with service excellence to ensure systems are 'Always ON'.

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  • When budgeting for uninterruptible power supply systems, it's essential to consider the total cost of ownership rather than just the original purchase price. If operating costs and upgrades are taken into account, an apparently lower-cost initial purchase can often be ultimately more expensive for the purchaser than a better-technology solution with a slightly higher original price tag.
    In this article Kenny Green, Technical Support Manager at Uninterruptible Power Supplies Ltd, a Kohler company, explains how better UPS topology can reduce the total cost of ownership and improve reliability while doing so.
    Most modern businesses’ reputation and survival depend ultimately on the quality and availability of their electrical power supply. Even brief power transgressions can take down any ICT equipment they reach, immediately disrupting any banking, purchasing or other transactions currently in progress. The Bank of England’s 2014 Q3 Bulletin notes that ‘Over 98% of sterling payments by value are made electronically….an (IT) infrastructure failure could greatly inhibit — or remove entirely — the ability of individuals and firms to make their payments. This would have severe consequences for economic activity’. The Bulletin cites power failures as a potential cause of such scenarios.
    Any operator faced with this possibility wants the best UPS protection technology available, while being aware that any funding request will be closely scrutinised in today’s economic climate. Fortunately, however, there is an answer to this dilemma; consider the UPS’s total cost of ownership rather than simply its catalogue purchase price. The best-performing UPS technology will cost a little more to buy initially than older products, but within a year the higher cost of the modular system is recovered.
    Although there are several reasons for this, they all relate to how UPS technology and topology have evolved. Accordingly we can look at this evolution and see how it generates cost-saving benefits alongside improved technical performance.

    Improved efficiency cuts costs

    Legacy systems used transformers, which made them large and heavy by today’s standards. For example a data centre with a 120 kVA load could theoretically have been supplied by a single, floor-standing 120 kVA UPS unit. In reality however, redundancy would probably be required to ensure better availability; this would be implemented with two 120 kVA units sharing the load in a 1+1 redundant configuration. As a result, neither unit could ever be more than 50% loaded, which for a transformer-based system meant a significant reduction in efficiency. It also meant investing in substantially more UPS capacity than actually necessary.
    The advent of transformerless technology has allowed much smaller and lighter UPS implementations – in fact to UPS modules that can be incrementally added to a racking frame to achieve an application’s required power capacity and redundancy. Free-standing individual units are no longer necessary. The 120 kVA load of the above example could therefore be satisfied by a single rack containing four 40 kVA ‘hot swap’ plug-in modules. The load remains fully supported with n+1 redundancy, while the total UPS capacity has been reduced from 240 kVA to 160 kVA.
    Although the purchase price per kVA for modular UPSs will be slightly higher than for legacy types, this difference will be partly offset by the reduction in purchased capacity and in floor space required for installation. However important savings in operating costs will also be made as the modular solution is more efficient than a transformerless implementation – especially one that cannot operate at more than 50% loading. Table 1 below shows that for our 120 kVA example, these savings will amount to over £38,000 for a five year operating period when the associated reduction in cooling costs is taken into account.
    Table 1: Legacy vs. modular transformerless UPS: Comparative operating costs over five-year period
    Although these savings alone will more than offset any difference in original purchase price, modular UPSs’ inherent efficiency can cut costs even further. Some UPSs are efficient enough to qualify for the Energy Technology List, or ETL. As part of the Enhanced Capital Allowance tax scheme for businesses, this government-managed list of energy-efficient plant and machinery includes UPSs as well as items such as boilers, electric motors and air-conditioning systems.  A business that pays income or corporation tax can claim 100% first year capital allowance on any ETL-listed product purchased. The ETL’s accompanying Energy Technology Criteria List (ETCL) has a complete section on UPSs. This defines and describes UPS technology, including the criteria for ETL eligibility.
    These ETCL criteria include UPS efficiency thresholds for eligibility at different percentages of full load for static systems of 10 kVA or more.Transportation costs themselves can also be halved, and installation costs can be considerably reduced, by using the smaller, lighter modular technology.
    During UPS operation, modular topology can also reduce ongoing costs due to spare part stock and logistics issues. Because traditional systems demand in-situ component-level repairs, maintaining a suitable set of spare parts is difficult and costly – especially as users tend to buy the most extensive and costly kit for maximum security. Even then, there is no guarantee that the kit obtained will be effective or contain the parts required for every possible failure that could arise.
    Modular systems, however eliminate these complications. Instead a single spare plug-in module will suffice; even when modules of different power ratings are being used, simply holding a module of the highest kVA rating installed will cover all eventualities, Training is simplified and less time-consuming, and trained technicians can swap modules within 15 minutes. Repairing by module-swapping in this way can save up to 50% on logistics and stock management costs.
    Modular system upgrading is also far simpler, faster and cheaper as extra capacity can be added simply by plugging in additional modules without even interrupting power to the critical load. The building work, increase in footprint and interruption to supply associated with extending traditional systems is eliminated.

    Hot swap modules and their impact on UPS availability

    So we can see that over time the modular solution becomes more attractive in cost terms – but does it compromise power availability by doing so? Not at all; in fact the reverse is true. A UPS’s availability is increased if its mean time to repair (MTTR) is reduced, and a key feature of a modular system is its minimal MTTR figure. If a hot swappable module does fail, it can be withdrawn from the UPS frame without even interrupting power to the load. A replacement module can be plugged into the rack immediately, reducing UPS repair time to as little as half an hour.
    By contrast, if a legacy system fails, it must be shut down, isolated from its mains supply and repaired in situ; a process that typically takes 6 hours to complete. This factor means that whereas a transformer-based system can offer ‘five nines’ (99.999%) availability, a modular UPS provides ‘six nines’ (99.9999%) availability. ‘Five nines’ equates to 2.5 minutes’ downtime per year while ‘six nines’ equates to 32 seconds.


    In today’s online commercial environment, power failures that bring down ICT systems are unacceptable, yet UPS systems, as significant capital equipment items, must be carefully cost-justified. Fortunately the best technology currently available need not be the most expensive, provided the total cost of its ownership is considered. In this article we have seen the technical, tax and logistics reasons why modular UPS technology, although initially slightly more expensive to purchase, becomes the most economical solution – while increasing UPS power availability - once all costs are taken into account.
  • Since the financial crisis, capital for new projects has become much harder to obtain. Investing speculatively in projected future demand for IT capacity has become impossible in many circumstances. Businesses are instead starting to use a modular, scalable approach to their data centre requirements; starting from a minimal size and growing incrementally in pace with the demand for capacity.
    To achieve this agility, support services such as the UPS system must be as scalable as the IT equipment it serves. In this article, Kenny Green, Technical Support Manager at Uninterruptible Power Supplies Ltd, a Kohler company, looks at modern UPS topology and how it can provide the modularity and scalability essential to today’s data centre requirements. 

    The concept of scalable data centres

    The worst of the financial crisis may be over, although its effects on our way of thinking persist today and probably into the foreseeable future. When capital is now released for new projects, it is usually only after stringent analyses of the ROI possibilities and risks. Data centres, with all their ICT processing equipment and infrastructure capital costs, are no exception to these investment inhibitions and scrutiny.
    These circumstances have allowed the concept of modular, or scalable data centres to gain a hold. A modular approach allows an enterprise to buy in only enough data processing resource for its immediate needs. Extra capacity can be added incrementally, with only incremental costs, at short notice and with little disruption to service. These possibilities exist irrespective of whether the data centre is standalone or part of a larger DC campus.
    While significantly reducing upfront capital costs, easy scalability offers other advantages too. As ICT technology is evolving rapidly, data centres can rapidly exploit new opportunities they create to boost leading-edge performance and efficiency. Users benefit from appropriately scaled solutions, and from the most efficient technology currently available.
    To reap these benefits fully, the modular and scalable approach must be extended to the data centre’s infrastructure as well as its ICT equipment.  Cooling equipment and uninterruptible power supplies (UPSs) are major components of this infrastructure. Both offer opportunities to improve scalability and efficiency through new technology and fresh approaches. Improved cooling efficiency for example, is sometimes achieved by using containerised data centres with sealed racks, or individual computer room air conditioning (CRAC) systems within IT equipment rows for more targeted cooling.

    The benefits of modular UPS topology

    The scalable data centre benefits significantly from the right modular UPS system topology because it boosts electrical efficiency while extending the scalability concept. The last few decades have seen enterprises in commerce, industry and other sectors become increasingly dependent on UPS systems to ensure protected, uninterrupted power for their data resource. Earlier systems were implemented as relatively large, free-standing monolithic devices that were inflexible. By today’s standards older systems are inefficient. If demand increased beyond the original system’s capacity, a newer, larger system could be added or substituted.  This however was after considerable upheaval and interruption to UPS service as the building work and cabling installation tasks were completed. Due to these considerations, many users were tempted to install capacity far in excess of their immediate needs, which led to tying up capital on underutilised equipment. Since the loading on each UPS system decreased, efficiency was reduced.
    Adding redundancy was also costly and wasteful. If a data centre was being supported by a standalone 50 kVA capacity UPS, redundancy could only be achieved at the cost of another entire 50 kVA system. Loading on each system could never exceed 50%, further reducing efficiency.
    However, the advances we have seen in semiconductor technology, and the resulting move to transformerless solutions, have allowed major reductions in UPS size and weight. These are particularly significant because they make modular UPS topology possible. Instead of a single fixed, monolithic installation, UPSs become cost- and energy-efficient systems based on aggregations of smaller modules working together in parallel. These can easily be incrementally scaled to users’ exact requirements – and just as easily scaled again whenever those requirements change.


    An example of modular UPS topology

    The concept is clearly illustrated by Uninterruptible Power Supplies Ltd.’s PowerWAVE 9500DPA. This is a modular three-phase UPS system, as shown in Fig.1. Its capacity can be scaled from 100 kW all the way to 3 MW if required. The system is based on 19” frames with industry-standard dimensions; these accept individual 100 kW ‘plug-in’ modules, which users can simply add – or remove – as required. Each PowerWAVE 9500DPA frame accommodates from one to five modules, accordingly providing a capacity up to 500 kW. This flexibility is known as the UPS’s vertical scalability. Additionally, up to six frames can be paralleled together to provide a total UPS capacity of 3 MW; a property referred to as horizontal scalability.
    With its scalability coupled to an efficiency of up to 96.1%, this PowerWAVE design fits comfortably with the overall concept of data centre scalability. Once the frames are installed, modules only require plugging in, so no further building work or installation of cables or switchgear is necessary. There are no UPS reconfiguration constraints to compromise the flexibility of a modular data centre.
    In fact, the modules have ‘hot swap’ capabilities, so the UPS does not even have to be taken off line while modules are added or removed; the entire operation is invisible to the critical load. In this context, hot swappability also boosts another feature of modular UPS systems that remains critical to all UPS users: high availability. As we shall see, modular systems achieve this by efficiently building redundancy into their configuration.
    Fig.1: Uninterruptible Power Supplies PowerWAVE 9500 modular three-phase UPS system
    The modular design of our UPS example is implemented as a truly distributed parallel architecture. This means that there are no central static switches or any other single points of failure to compromise system reliability. Each module comprises a fully-functional UPS, with no dependence on external UPS components to keep it on line. A number of modules can be assembled into an N+1 redundant system to maximise availability. A 300 kW load, for example, could be supported by four 100 kW modules; if any one module fails, the remaining three can fully support the load until the faulty module is replaced.
    How high is this availability? Very high indeed. In the PowerWAVE 9500DPA system, ‘six nines’ or 99.9999% availability becomes possible. This is because one of the key factors controlling a system’s availability is its mean time to repair (MTTR) – and for a hot swap modular system, this reduces to half an hour, compared with six hours for a standalone UPS system.
    The flexibility, space advantages and reduced redundant capacity requirements of a modular system compared with standalone implementations are highlighted by different levels of redundancy protection as shown in Figs. 2 and 3 below. Fig. 2 represents a variation of the N+1 arrangement mentioned above; here, a modular UPS system within a single rack supports the 200 kW load. N+1 redundancy is achieved with just 100 kW redundant capacity, and there is room within the frame to plug in further modules should power demand increase. There is no need to find space or carry out building work to add the extra capacity. By contrast, the standalone system requires two complete units and, inevitably, 200 kW of redundant capacity. If the critical load then increases, floor space will have to be found for a third unit, together with the associated UPS installation work, cabling and disruption.
    These contrasts are further magnified in the situation represented by Fig.3. Here, a particularly critical load is supplied by two entirely independent power feeds; each feed is protected by a UPS with N+1 redundancy. The advantages of the modular UPS frame over the standalone units are the same as for the simpler N+1 example, except that the standalone UPS solution now includes 600 kW of redundant capacity, compared with 200 kW for the modular frames. Also, the standalone UPS implementation now requires four separate units, which must be increased by a further two if any future expansion becomes necessary.

    Fig.2: UPS N+1 system – Standalone vs modular topology
    Fig.3: UPS 2(N+1) system – Standalone vs modular topology


    Overall, we have seen how choosing a UPS system with the right modular topology supports the vision of enterprises seeking the benefits of a modular data centre approach. The scalability of the UPS system can easily match that of the entire data centre. UPS capacity can be quickly and incrementally added to match growing loads, without interruption to the power supply or disruption within the data centre building. All of this can be achieved whole enjoying high levels of UPS efficiency and minimal need for cooling. At the same time, hot swap modularity allows up to 99.9999% availability – a consideration never far from any data centre operator’s mind.
  • Power Usage Effectiveness, or PUE, has become a popular metric for measuring the overall electrical energy efficiency of a data centre. It compares ‘useful’ power used for data processing with total power taken from the grid.
    In this article Alan Luscombe, Director at Uninterruptible Power Supplies Ltd, a Kohler company, looks at the significance of PUE and how modern UPS topology can help to improve it.

    What is PUE?

    Driven by continuously-growing demand for secure data processing capacity, dedicated data centres have become truly enormous. Colocation provider Switch’s SuperNAP data centre campus in Las Vegas, for example, has a mission-critical power capacity of up to 200MW and can house up to 20,000 cabinets. 
    As power demand has climbed to these levels, energy efficiency has become a critical issue for both commercial and political reasons. In recognition of this, the Green Grid – an industry group focused on data centre efficiency – created the Power Usage Effectiveness or PUE metric to determine a data centre’s efficiency.  PUE is defined as the ratio between the amount of power entering a data centre and the amount usefully consumed by the data-processing load within it. As a data centre’s efficiency improves, its PUE drops; an ‘ideal’, perfectly-efficient data centre would have a PUE of 1.
    According to the Uptime Institute’s Data Center Industry Survey 2013, the typical data centre has an average PUE of 1.65, so for every 1.65W taken from the utility, only 1W is used directly for IT activity. The ‘useful’ power is consumed by data processing hardware including servers, storage and telecommunications equipment. The ‘overhead’ is due to chillers and other cooling equipment, switchgear and UPSs. As cooling equipment has become more efficient, attention has turned to UPS systems as they offer the major remaining PUE improvement opportunity.
    PUE values can vary continuously over a 24 hour period as data centre loads change, on both the overhead and IT equipment sides. Outside ambient temperature changes can also affect cooling equipment and its contribution to the PUE value. Nevertheless, PUE provides a useful comparative indicator that can reveal improvements and changes within the data centre. One slightly counterintuitive result occurs if a data centre succeeds in bettering its IT hardware’s efficiency – the overall PUE deteriorates. However this effect can be countered, and PUE improved, if UPS energy efficiency is improved. In fact, the benefits are two-fold; in addition to direct energy savings, raised UPS efficiency cuts energy use by reducing cooling requirements. Accordingly, we now look at two ways of improving UPS efficiency.

    UPS technology and efficiency

    Modern UPS topology has evolved through the availability of improved power semiconductor technology. This has allowed the earlier phase-controlled rectifier design to be replaced by a fixed rectifier and DC boost converter as shown in Fig.1. In this configuration the DC converter’s output voltage remains constant for a fairly wide range of voltages on the unregulated DC busbar, so its output to the regulated busbar is unaffected by AC mains aberrations. It is also permits the UPS inverter to generate an output AC voltage at a level compatible with the incoming utility mains. The battery charger reduces the regulated busbar’s high voltage to a level suitable for battery float charging.
    The configuration shown, like its earlier counterpart, is known as a double-conversion online type. During normal operation the rectifier and inverter protect the critical load from mains-borne noise and transient voltage excursions, while providing a well-regulated output voltage. If the mains fails, or exceeds preset voltage limits – typically +10% to -20% - the battery takes over until mains is restored. Transfers between battery and mains are invisible to the load, while regulation and protection also continue without degradation.

    Fig. 1: Transformerless UPS block diagram
    Transformerless technology offers several advantages, of which two are key: Improved efficiency, and significant savings in size and weight. Fig.2 compares transformer-based and transformerless efficiencies against load. Transformerless efficiency is higher across the entire load range, with an overall improvement of around 5%. Cooling costs as well as direct energy losses are substantially reduced.
    Fig.2: UPS efficiency curve – transformerless vs. transformer-based solutions
    The phase-controlled rectifier within a transformer-based UPS presents a lagging power factor to the mains supply, which moves further from unity as the load reduces. By contrast, the transformerless UPS’s free-running rectifier presents a power factor much closer to unity and far less load-dependent. This reduces the magnitude of the input currents, therefore reducing the size of the power cabling and switchgear. In some instances, electricity running costs are reduced.
    As previously mentioned, size and weight are substantially reduced. For example a 120kVA transformer-based UPS would have a 1.32m2 footprint while weighing 1,200Kg. By contrast, a transformerless type occupies 0.64m2 and weighs 310Kg. These reductions in physical dimensions have had an enormous impact on UPS topology, because they have enabled the concept of modular UPS systems. Instead of one large, inflexible unit, a system can comprise a number of rack-mountable modules – each an independent UPS in its own right – paralleled together to provide the overall capacity and redundancy required. Scalability is easy, and if the modules are ‘hot swap’ types, they can be added or removed without having to take the system off line. As modules with capacities from 10KVA are available, UPS capacity can be incrementally and closely matched to the exact load requirement. 
    This means that transformer-based UPS systems use more energy than their transformerless alternatives for two reasons. Firstly, as Fig. 2 shows, the transformer approach is inherently less efficient. Secondly, as their capacity cannot be incrementally adjusted to their load, transformer types inevitably find themselves operating well below their rated capacity, and as Fig. 2 also shows, this further reduces their efficiency.
    As an example, consider a 120KVA load that requires UPS support with N+1 redundancy. The maximum loading per unit of a transformer-based UPS solution would be 50%, if two 120KVA units are used. Efficiency would be 91%. By contrast a transformerless modular UPS solution could achieve N+1 redundancy using four 40KVA modules, with 75% loading and 94.5% efficiency. Based on 9.0p/KWh, the total cost of cooling and energy losses for a year’s operation of the transformer-based UPS would be £13,264; the transformerless figure would be £5,637. Therefore, over five years, £38,135 will be saved by using modular transformerless UPS technology.

    Eco-mode operation

    A modern modular UPS system can achieve 96.1% efficiency while operating on-line. For many data centres, this is the default mode. This can be increased to 99% efficiency, or possibly more, by switching to eco-mode. Fig.3 shows the eco-mode principle; in normal operation, the critical load is fed directly from the raw mains. The UPS components and battery back-up only come into use during a mains failure.
    The attraction of eco-mode is that bypassing the UPS components during normal operation allows very high efficiencies. The downside is that the critical load is constantly exposed to any noise, spikes or other mains-borne problems. Every data centre manager has to consider the historical power quality of his mains supply and the sensitivity of his on-site equipment, and weigh the exposure risk against the energy-saving benefits of eco-mode.

    Fig. 3: UPS system in Eco-mode


    In this article we have seen that PUE is a popular metric for data centre power supply efficiency. As data processing equipment efficiency improves, overall PUE can deteriorate. Raising the efficiency of essential support systems such as the UPSs is vital to maintain or improve PUE values. This can be done primarily by using modern UPS hardware based on transformerless technology which offers about a 5% efficiency improvement over traditional transformer-based designs. Efficiency can be taken to 99% or better by operating in eco-mode, however this benefit must be balanced against the risk of exposing the critical load to raw mains during normal operatio