Feature UPS & Standby Power
Considerations for availability A
UPS availability is mission critical in activities involving ICT equipment. But what does the term UPS availability actually mean? Mike Elms, technical support manager for UPSL, provides the answers
vailability is the primary con- cern for data centre operators because it is a measure of how much time per year their ICT resource is operational and available. It is formally defined as:
Availability =
Mean Time Between Failures (MTBF) MTBF + Mean Time To Repair (MTTR)
This equation shows that we can increase availability by reducing MTTR as well as by increasing MTBF, and the best results come from employing both of these strategies. A UPS system’s reliability is the probability that it can perform its designed function of supplying unin- terrupted, clean power over a given time period. This reliability is driven by the quality of the components used, and improves with better quality, more expensive devices. However, as cost is increased, it reaches a plateau where further spending is no longer rewarded by further reliability. At this point we need another tool to drive further increase in MTBF.
Fault tolerance and availability The answer is to build a fault tolerant system. Imagine for example, a 120kVA load served by two free stand- ing UPS units, each of 120kVA capac- ity. Either unit can continue to fully support the load if the other fails and through such fault tolerance, the MTBF of the total UPS installation is significantly better than that of a single unit entirely dependent on the relia- bility of its own components. While a single UPS unit might achieve an MTBF of 50,000 hours to 200,000 hours, a fault tolerant redundant system could achieve 1,250,000 hours, depending on its configuration. This effect is shown in Figure 1 (below).
Such configurations are generically
known as N+n redundant systems, where ‘N’ (Need) is the number of UPS units essential to support the critical load, and ‘n’ is the number of redun- dant units. Accordingly, this example comprises a 1+1 redundant configura- tion. Although, as we have shown, this improves MTBF and therefore avail- ability, it’s not the best possible solu- tion in terms of efficiency and cost. Data centre managers are con- stantly under pressure to extract the best possible power availability from the least possible budget and floor space, and with the UPS technology now available, more steps can be taken to help them. Firstly, consider the 1+1 redundant configuration - by definition it can never be more than 50% loaded. This is highly inefficient in both capital cost and operating cost terms. A better solution is to configure a 4+1 system which can run at up to 80% loading. Increasing the load like this can improve the UPS’s efficiency and reduce running costs, while capital expenditure is reduced as less excess capacity is being purchased. For the 120kVA example, we could achieve a 4+1 configuration using five free standing 30kVA units, any four of which could deliver 120kVA if one
Right: Table 1 - effects of
redundancy and modularity on availability
MTBF Hrs MTTR Hrs Availability
Below: Figure 1 - effect of component quality and redundancy on UPS MTBF
Free standing system, no redundancy
Up to 200,000 6
99.997%
formers. The extent of weight and space this saves is so significant that a transformerless 30kVA UPS unit can be implemented as a slide-in rack module rather than as a free standing floor unit. We can now build our 120kVA 4+1 redundant configuration vertically as five modules within a single 19” frame, occupying minimal floor space. However, this rack mounted modu- lar approach also offers more advan- tages because the modules can be hot swapped, or removed and replaced without taking the system offline. This reduces the MTTR to around half an hour, compared with the six hours typically needed for free stand- ing unit repair. This has an important impact on availability which, as shown earlier, can be improved by reducing MTTR as well as by increas- ing MTBF. Table 1 (below) illustrates the effect of these factors (‘1+1’ vs ‘4+1’ redundancy, and hot swap modularity), on UPS availability. The availability figures have been obtained by using the equation as described below, and expressing the results as a percentage.
Availability =
1+1 redundant free standing system
1,250,000 6
99.999%
unit fails. In this scenario, the 4+1 con- figuration does have one disadvantage compared with its 1+1 alternative - as it has more components, its MTBF is reduced from 1,250,000 to 500,000 hours. We have therefore improved efficiency at the cost of reduced MTBF and availability, although this effect can be addressed by reducing MTTR.
Hot swappability/reduced MTTR In addition, by turning to modern, modular UPS technology, efficiency levels can be optimised, availability can be improved and floor space requirements can be reduced. By using solid state IGBT devices, UPSs can dispense with output trans-
Electrical Engineering DECEMBER/JANUARY 2013 MTBF MTBF+MTTR
4+1 redundant free standing system
500,000 6
99.9988%
4+1 redundant rack mount modular system
500,000 0.5
99.9999%
From Table 1 we can see that a 4+1 redundant system has less availabil- ity than a 1+1 configuration, but as we have shown, it is more energy efficient. However, managers of mis- sion critical ICT installations can obtain the best possible power pro- tection by choosing a hot swap rack mount configuration, such as the 4+1 example in the table. It benefits sig- nificantly from its reduced MTTR, and offers the best efficiency from the least floor space, and at 99.9999%, the best availability.
UPSL
www.upspower.co.uk T: 0800 171 2320
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