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
Availability is the primary concern 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 uninterrupted, 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 standing UPS units, each of 120kVA capacity. 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 reliability 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 (right).
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 redundant units. Accordingly, this example comprises a 1+1 redundant configuration. Although, as we have shown, this improves MTBF and therefore availability, it’s not the best possible solution in terms of efficiency and cost.
Data centre managers are constantly 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 unit fails. In this scenario, the 4+1 configuration 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 transformers. 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 modular approach also offers more advantages 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 standing unit repair. This has an important impact on availability which, as shown earlier, can be improved by reducing MTTR as well as by increasing 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 = MTBF
MTBF+MTTR
From Table 1 we can see that a 4+1 redundant system has less availability than a 1+1 configuration, but as we have shown, it is more energy efficient. However, managers of mission critical ICT installations can obtain the best possible power protection by choosing a hot swap rack mount configuration, such as the 4+1 example in the table. It benefits significantly from its reduced MTTR, and offers the best efficiency from the least floor space, and at 99.9999%, the best availability.
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