What do today’s data centres expect of their UPS?

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UPSs that effectively protect their critical load from mains power failures and disturbances are essential to modern data centres. To successfully fulfil this role in today’s business conditions, they must do so while offering near-perfect availability, high efficiency and easy scalability. In this article, Kenny Green, technical support manager at Uninterruptible Power Supplies Ltd (UPS Ltd), a Kohler company, looks at how available UPS topology allows data centre operators to meet these exacting requirements

Today, no data centre operates without an uninterruptible power supply (UPS) in place to protect the load from mains-related disturbances and power failures. If the load is unprotected, such events have the potential to cause irrevocable damage to IT hardware. Significant as this damage could be, it is unlikely to be as serious as the impact on business and reputation resulting from loss of data, or IT availability in a 24/7 online service environment.

With this in mind, data centre operators will judge a UPS by its level of availability, combined with the quality of protection it provides from mains failures and transients while on-line. However, current business, economic and even political conditions impose other pressures. The greatest of these is the need to improve energy efficiency. Although this is largely to minimise the steadily increasing cost of energy, cutting carbon emissions and ‘Going Green’ is also increasingly important.

Another factor imposed by modern conditions is that data centres’ processing loads can change rapidly as demand for IT resource grows. To remain effective, UPSs must be readily scalable to keep pace with these rapid changes.

Meeting the requirements of today’s data centre users

Today’s UPSs allow data centre operators to overcome these issues. To see how they do so, we can look more closely at their technology, and at how they utilise this technology to fulfil their role.

Fig. 1 shows the major UPS components. Incoming raw mains is fed to a rectifier/charger for conversion to a DC output. This output supplies the inverter input and charges the UPS battery. When the incoming mains supply is available, the rectifier/charger keeps the battery fully charged, while the inverter also uses its DC level to develop an AC output for the critical load. If the AC mains supply fails, the inverter draws DC from the battery.

Because the battery is part of the DC bus, switchover between battery and rectifier, and back again, is seamless. The mains failure is entirely invisible to the critical load, provided it lasts less than the battery’s autonomy. The critical load is protected from incoming power aberrations as well as failures.The UPS rectifier and inverter provide a barrier to mains-borne noise and transient voltage excursions in addition to providing a well-regulated AC output.

Fig.1: Typical UPS block diagram

Given that the UPS’s mains failure protection capability is subject to its battery autonomy, operators must have a strategy for handling power outages that exceed this. This strategy depends on whether or not the load must continue running throughout the mains failure. If this is not essential, a battery runtime of about 10 minutes will be sufficient to ensure that the ICT equipment has a safe, well-ordered shutdown. If the application must continue running throughout a power outage, the UPS must be provided with extra batteries or, preferably deployed with a back-up generator.

To summarise, we have seen that the UPS’s job is to be always available, providing a level of protection from mains supply failures and events compatible with the nature of the critical load. However, we have also mentioned that in today’s conditions it must achieve this with the best possible energy efficiency and easily-implemented scalability.

Modular UPS topology

The solution lies in both the technology and the topology available in the latest developments in UPSs. Whereas, earlier-design UPSs used a transformer to step up their inverter’s output to the required AC voltage level, advances in power semiconductor technology, particularly the Insulated Gate Bipolar Transistor (IGBT) have allowed the transformer to be eliminated. This has had a number of profound effects on modern UPS design.

Firstly, transformerless UPSs are about 5% more efficient than transformer-based products. Fig.2 shows this, while revealing that efficiency is improved over the entire load spectrum from 100% down to 25%. As a result, substantial reductions in electricity running costs and heating losses are achieved. Additionally, the power factor is improved, while total input current harmonic distortion (THDi) is reduced, bringing further cost savings and improved reliability.

Fig.2: Transformer-based and transformerless UPS efficiency curves

While transformerless technology is extremely important for its energy savings, its reductions in size and weight also have far-reaching effects. These reductions result from eliminating both the transformer and the phase controlled rectifier. A transformer-based 120 kVA UPS, for example, weighs 1200 kg and has a footprint of 1.32 m2. By contrast, a transformerless 120 kVA UPS weighs just 310 kg, with a footprint of 0.64 m2.

The significance of this is that it allows UPSs, even in high power installations, to be configured as sets of independent rack-mounted modules. For example, with the PowerWAVE 9500DPA, up five modules, each of 100 kW, can be accommodated within a single UPS frame. A UPS can be scaled to a 100 kW load with a single module, then incremented in 100 kW steps to 500 kW, matching the load as it grows. This flexibility in populating the frame is known as vertical scalability. For loads beyond 500 kW, up to five additional frames can be added, providing horizontal scalability up to 3 MW.

Alternatively, a <400 kW load can be supported by five 100 kW modules. This means that if one module fails, the other four can continue to fully support the load, as they still have 400 kW capacity between them. As one module is redundant, this is known as N+1 redundancy. Modules can be ‘hot-swapped’; a process where a faulty module can be removed, simply by sliding it out of the UPS frame, and replaced with another without interrupting power to the critical load. This also has a positive effect on Mean Time to Repair (MTTR).

Minimised MTTR contributes to increased availability, with modular UPS systems offering availabilities to 99.9999%. These UPSs are well-equipped to fulfil the critical power protection role expected of them by today’s data centres – and they can do so with a true online efficiency exceeding 96%.

The last 30 years of UPS development has without question had a significant effect on IT power security and today, R&D is still responsible for driving sales growth – meaning you can count on further step changes in efficiency in the coming years. So, as the way data centres are used and managed develops over the next decade, UPS manufacturers will undoubtedly continue to invest and innovate, using the latest technological advances to ensure your load is as protected as it ever can be.

For more information about Uninterruptible Power Supplies Ltd and the UPS Systems they offer please visit www.upspower.co.uk

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