Achieving near-perfect availability
Wed 13 Apr 2016
Who heals the healer? In power protection terms, this conundrum is about recognising that while UPSs are essential for protecting critical loads from power problems, their success depends on their own fitness for purpose. With the continuously increasing ubiquity of online transaction processing, IT system availability is more than ever of primary concern for operators, and power availability is its largest single component.
Power availability is a measure of how much time per year an IT system has acceptable power. It is also the ratio between Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR). Its value can be improved both by maximising MTBF and by minimising MTTR times – modern UPS systems use techniques that exploit both of these.
Earlier UPSs tended to use transformer-based designs, resulting in large, floor-standing units. The advent of transformerless technology allowed manufacturers to develop smaller, more modular designs, which offered several advantages. Users could specify the UPS capacity they needed far more accurately. Modular design also supports future growth with equal efficiency, as modules can be added according to changing needs.
Another important advantage of modular design is its efficient support of redundant configurations. A UPS comprising five 10 kVA modules supporting a 50 kVA load is referred to as a capacity system. It operates without any problems, provided all five modules remain healthy and deliver full power. However, if one should fail, the other four will have to supply 12.5 kVA each and would be overloaded. The load may be transferred to bypass, exposing it to raw mains power and all its potential aberrations. To prevent this, a redundancy system can be set up, comprising at least one more module to fully support the load. This makes the system resilient to the failure of a single module. Redundant systems offer significantly improved availability compared with capacity implementations.
We have assumed that the UPS topology is ‘decentralised’ – each module operates as a complete, independent unit with no components shared by all the modules. However, some systems are available with a more centralised architecture, in which all modules connect to the load through a shared centralised static switch (CSS). The advantage is reduced costs, but the problem is that the CSS represents a single point of failure, which adversely affects the system’s availability.
Modular rackmounting systems
The highest possible availability is achieved by using a modern, modular rackmounting system. Firstly, these provide true Decentralised Parallel Architecture (DPA) as each module is fully self-contained. Additionally, many such systems offer a ‘hot swap’ capability, which also improves availability. If a module in a hot swap system fails, it can be removed and replaced with a working unit without having to put the system into maintenance bypass, or interrupting power to the load. Eliminating these requirements or having to effect repairs to equipment while within the UPS means that MTTR times can be reduced to minutes, compared with the repair time of hours, typically needed for monolithic systems.
This article first appeared in Data Centre Management, Spring 2016