In the below video Mike Elms, Managing Director of Centiel UK answer the question “what is availability?” and explains “how true modular technology can improve it”.
Availability is defined as readiness of a system to provide a corrective service. Steady-state availability is the most commonly used metric for availability quantification and is defined as the fraction of time a system is operational during its expected lifetime.
Availability equals Mean Time to Failure (MTTF) divided by MTTF plus Mean Time to Repair (MTTR).
MTTF is a mean time between two consecutive failures, whereas Mean Time to Repair is a mean time needed for the repair. In other words, MTTF is the average time during which the system is up after it has been repaired and before it fails again.
To make the comparison between architectures straight forward, steady-state availability is usually expressed with a number of nines. For example “Five nines” availability means that steady-state availability is 0.99999 or 99.999%.
Availability of a system example of 160kVA output therefore, has a reduced availability of 0.999993 equivalent to 3 minutes and 40 seconds per year.
To boost dependability, redundant UPS units are introduced, known as n+1 configuration. Here, the system may continue operation with the remaining n modules. The MTTR of a modular architecture decreases to only 0.5h. Such systems may tolerate failure of a single UPS unit, however, again failure of any of the parallel buses makes the entire system fail.
Centiel’s N+1 modular architecture introduces numerous changes to improve the overall dependability. Specifically the control logic allows the communication between UPS units to be maintained even when one of the parallel buses has failed.
In addition the MTTR of a single UPS is additionally decreased by placing the Static Bypass Switch fuse on each module at the frame level, out of the module. In this way, the fuse may be replaced without the need of pulling-out or opening the entire module. We have assumed the MTTR of the fuse is five minutes and the fuse causes 5% of the overall module failures.
In Centiel’s architecture, there are changes that increase system’s MTTF and decrease MTTR. In this white paper we only consider two: the improved parallel bus configuration and isolating the Static Bypass Switch module fuse. In this way, Centiel’s architecture significantly outperforms the one with typical N+1 architecture, as it increases from six and a half to even ten and a half nines. Annual downtime is decreased from 12 seconds to 0.0006 Seconds
However, the 1+1 configuration also has a limited output of 40kVA. As the output increases, availability of both architectures slightly decreases, but the level of availability improvement with the Centiel architecture (when compared to a typical one with the same level of redundancy) gets even higher. For example, when the output is 160kVA (4+1 configuration), a typical architecture has availability of six nines (annual downtime is 32 s), whereas availability with the Centiel architecture is nine nines or 0.03 seconds downtime per year.
Read our availability white paper to find out more.
