SUPPLEMENT FEATURE UPS & STANDBY POWER
REAPING THE BENEFITS D
Kenny Green, technical support manager at Uninterruptible Power Supplies, discusses parallel redundant system optimisation and the benefits of modular UPS design
epending on its load capacity, a UPS module may comprise a rack
mountable unit or a larger floor standing installation. Either way many UPS solutions comprise two or more modules connected in parallel. This can be done to increase either capacity or, more commonly, UPS availability. UPS systems that are paralleled to
increase availability are known as redundancy systems. For full redundancy, decentralised parallel architecture should be used - that is, each module should be capable of operating independently, without reliance on any centralised or common components. If a major organisation suffers a power
glitch, it doesn’t take long for the effects to be felt - not only by large numbers of frustrated customers, but also in terms of adverse publicity. Faced with such scenarios,
organisations are highly motivated to invest in the best available uninterruptible power supply (UPS) solutions and to deploy them in a way which maximises their power availability. In this article, we look at why parallel, modular UPS configurations offer the best, most truly effective redundancy solutions.
PARALLEL REDUNDANT SYSTEMS A system has redundant capacity that translates into fault resilience if it has more capacity than the load actually requires. However, achieving redundancy while efficiently using UPS capacity needs a modular approach. Consider, for example, a load of 120kVA. This could be supported with a traditional, single unit 120kVA UPS system. For redundancy though, two such units would need to be paralleled to supply the load, creating a large system with a total capacity of 240kVA. This means that under normal conditions the UPS is only half loaded and has reduced efficiency, while capital expenditure has been wasted on capacity that isn’t needed. Now consider the same 120kVA load
being supplied by smaller UPS modules rated at 40kVA each. Three modules can support the load, but redundancy is achieved by adding a fourth to create a total capacity of 160kVA. Each UPS module now has a 75% load and operates at a higher efficiency level,
S8 SEPTEMBER 2014 | UPS & STANDBY POWER
while excess capacity has been reduced from 120 to 40kVA. Generically, these parallel redundant systems are said to offer N+1 redundancy, where N is 3 in the above example. If extra security is required, more than one redundant module can be added to create an N+x redundant system. ‘x’ is referred to as the coefficient of redundancy for the system.
CENTRALISED PARALLEL SYSTEMS These N+x modular solutions, now widely used in data centres and other critical load environments, can be implemented as either centralised or de-centralised parallel architectures. The first approach offers some financial economies, while the second provides the ultimate in power availability. A ‘centralised’ architecture is so called because its implementation includes a centralised static switch (CSS) as a shared component. This saves some money as it eliminates the need for each UPS module to have its own static switch. However, it impairs the overall system’s resilience to failure because it introduces a ‘single point of failure’ into the power path. During normal operation all the UPS
modules are online while being synchronised to one another and to the standby mains supply. They share the load current equally. If a module fails, it automatically isolates itself from the system by rapidly opening its output static switch. The healthy modules continue to support the critical load if the remaining system capacity allows, otherwise the load is transferred to the standby mains by the CSS.
DECENTRALISED PARALLEL SYSTEMS A de-centralised system comprises a set of parallel modules feeding the load directly. Each module is entirely autonomous and in fact is usually capable of working in ‘stand alone’ mode - no shared components are used, so no single point of failure exists. One module acts as ‘master’ while the others are ‘sub-master’. If the master fails, or is taken offline for maintenance, the next UPS in line will take over, changing from sub-master to master while doing so. Given that de-centralised parallel systems offer optimal redundancy and
Right and below: when a major bank experienced a power failure early one Saturday afternoon, they claimed to have fixed it within 20 minutes – yet in that period, customers could not use their debit or credit cards for making purchases, withdrawing cash from ATM machines or accessing their online accounts. The incident made the BBC News at the time, negatively impacting the company financially and in credibility
power availability, it’s worth remembering the modular topography that makes these systems so economical in both cost and size. Modern UPS modules’ compact, light design allows several to be assembled into a single rack to achieve the required N+1 redundant configuration. This design is enabled by transformerless technology, which has significantly reduced the size and weight associated with earlier transformer- based designs.
The appearance of parallel rack
mounting modules has also facilitated ‘hot swapping’, which makes a further important contribution to a redundant UPS’s power availability. Hot swapping allows operators to replace a faulty module with a healthy one without interruption of conditioned power to the critical load. MTTR is minimised, significantly improving availability.
CONCLUSION In this article we have seen how modern UPS design encompasses parallel redundancy, fully de-centralised architecture, rack-mounting UPS modules and hot swapping to achieve optimum power availability. Guaranteeing this availability remains the UPS’s primary function, and makes a UPS system essential to the viability of any sizeable enterprise today.
Uninterruptible Power Supplies (UPSL)
www.upspower.co.uk T: 0800 731 3269
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