UPS systems - How ‘FAT’ is your UPS system?
Peter Bentley, sales director at Uninterruptible Power Supplies, explains how ensuring UPS systems are lean and fit for purpose helps to keep data centres running
The health of a data centre is highly reliant on the health of its uninterruptible power supply. A regular assessment of the ‘FAT' factors - flexibility, availability and total cost of ownership - helps optimise UPS systems to increase efficiency, save running costs and reduce CO2 emissions. Rising energy costs and pressure to shrink carbon footprints make these achievements all the more desirable.
Flexibility
There are two distinct aspects of flexibility of UPS systems: The first relates to an individual UPS system's flexibility to efficiently protect its critical load in the data centre, even when the load changes over time, as it inevitably does. There is huge potential to reduce the electricity consumption of data centres, and to alleviate the burden on their stretched cooling systems, by continually matching the capacity of UPS systems to their respective critical loads. State-of-the-art modular 'transformerless' UPS systems are much more flexible than their traditional counterparts at matching load requirements and delivering optimum efficiency.
The second and equally important aspect of flexibility relates to a UPS system's ability to increase the level of protection it provides to its critical load. The level of protection is a matter of strategic choice by the data centre manager and is a trade-off between availability (not only of the UPS system but also the critical load) and initial capital costs to achieve that level of availability. However, capital expenditure should not be considered on its own; installation, operation and upgrade costs are important factors in how investments in power protection will be paid back (see total cost of ownership').
Figure 1 illustrates how the ability to ‘right-size' modular UPS systems promotes efficiency. The limited flexibility of a free-standing UPS would require the initial installed power to exceed the data centre's anticipated capacity requirements, resulting in a wasteful gap due to oversizing. However, the flexibility and scalability of modular UPS enables power to be added as the data centre requirements grow (without increasing footprint).
Inserting and removing ‘hot-swappable' UPS modules facilitates right-sizing a UPS system to its anticipated critical load. The trend for the size of critical loads, especially those within data centres, is to increase. However, right-sizing for decreasing loads is equally easy.
Availability
Since power problems are the largest single cause of computer downtime, increasing power availability is the most effective way for IT managers to increase their overall systems availability. The single most important issue in increasing power availability is to decrease the mean time to repair (MTTR) of the power protection system.
Referring to Figure 2, the availability of a traditional UPS system and an advanced modular UPS system are compared. The UPS system on the left comprises two 120kVA free-standing UPSs in 1+1 parallel-redundant configuration, and the one on the right comprises four 40kVA ‘hot-swappable' UPS modules in 3+1 parallel-redundant configuration.
Their MTBFs are 600,000 and 400,000 hours, and their MTTRs are 6 hours and 0.5 hours respectively. However, the availability of the free standing solution is 0.99999 (five nines) while the modular solution provides an availability of 0.999999 (six nines). This higher availability increases overall system availability by a factor of 10 compared to free-standing (non-modular) UPS systems that are not hot swappable.
Figure 3 categorises power protection systems in quadrants according to how well they meet the requirements of high power availability, redundancy and hot-swappability. As more components in a system become hot-swappable, the system moves from the bottom to the top of the graph; and as more components become redundant, it moves from left to right. The modern, modular UPS provides the highest power availability and the highest level of protection for IT managers' critical loads.
Parallel UPS systems comprise either centralised parallel architecture (CPA) or de-centralised parallel architecture (DPA). While CPA systems offer a cost benefit by sharing common components, the drawback is that this centralised configuration introduces a number of ‘single points of failure' into the system, which adversely affect its availability. DPA systems, with effectively no single-point-of-failure, offer very high availability. The additional capital expenditure on a DPA system will therefore be recouped by providing enhanced protection against revenue losses caused by system failures.
Total Cost of Ownership (TCO)
The lower purchase price of traditional UPS technology must also be offset against significantly higher operating costs in comparison with a modular system. In fact, reductions in energy loss costs mean that the potential higher outlay for a modular system can be recovered within the first year of operation. There are also a number of longer term benefits that contribute to significant cost savings with modular technology - more than £25,000 over five years.
TCO should also take account of the weight, volume and footprint of UPS systems, since these factors can increase transport and installation costs by 50% or more and impinge on valuable square footage. Compared with an advanced modular UPS, the traditional UPS system, based on two units, needs two to three times the amount of floor space and, since many utilize transformer based technology, weigh up to two or three times more.
Further significant reductions of TCO can accrue from rationalising installation, training and maintenance services, and by re-cycling reusable UPS modules. The scalability of modular systems also contributes major savings. Upgrading a traditional UPS requires extra space, costly cabling and taking the UPS off line. With a modular UPS, the upgrade is performed by simply inserting the additional power modules into the rack, provided the system's distribution and frame have been specified for the maximum foreseeable requirement. Such upgrades can be safely performed without any interruption to the load, without increasing the footprint, and with no additional work on site. This flexibility makes upgrading a system very easy, and with very little additional cost.
Modern hot-swap-modular, double-conversion, true on-line UPS systems optimise all the elements of FAT. However, many older UPSs employ out-of-date technology and are often sized incorrectly for today's needs. Such inefficiencies mean that companies could be burning excess electricity and creating needless heat emissions, compromising efforts to reduce their carbon footprint. In all, thousands of pounds can be saved on uninterruptible power supply expenditure, simply by giving systems a FAT health check.
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