Worked Examples
UPS Systems
Before proceeding, which is the more reliable system, Option A or Option B?
Introduction
This example compares the reliability of two UPS systems, Option A and Option B. The components in Option A are identical to those in Option B, the difference between the two systems is the way the UPS modules are arranged. Option A is a set of parallel redundant UPS modules with a common static bypass supply (simulated by co2). In Option B the two UPS modules are arranged such that only one UPS module is on-line at any time in a master-slave arrangement. When the master fails it statically bypasses (co1) to the slave UPS. When the slave UPS fails it statically bypasses (co2) to the mains supply.
View UPS option A: UPSOptionA
View UPS option B: UPSOptionB
Working Out
A reliability analysis was performed on Option A and Option B using the Analyst software..
| OPTION A | OPTION B | |
| MTBF | 19 yrs | 12 yrs |
| MTTR | 0.39 hrs | 0.42 hrs |
| Availability | 99.999767 | 99.999599 |
| Unavailability | 1.22 min/yr | 2.11 min/yr |
Simulation results suggest that option A will be more reliable. The question is, why?
Explanation
In addition to reliability analysis, Analyst has the capability to perform analog simulations even with systems containing lead-acid batteries. The mains supply to each UPS system was failed in turn. In each case, Analyst was able to visually demonstrate the subsequent system response to this change.
In Option A, when the mains was failed (simply by clicking on it), Analyst showed the two UPS batteries discharging together. The batteries lasted 1 hour 30 minutes.
When Option B mains was failed, Analyst again showed the actual operation of the system. First the master UPS battery came on-line, when it was discharged, Analyst's programmable logic controller (plc1), sensed the failure and automatically transferred the load to the slave UPS. The load continued to be powered until the slave UPS battery was also discharged, a total time of 1 hour and 4 minutes.
The difference in the two systems is now evident. Option A has longer battery reserve time! Analyst was able to model the subtle differences in the way the two UPS batteries were being used and also correctly predicted the effect on reliability of the two systems!
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for electrical engineers working with critical ac and dc power systems.
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Powertechnic. Test-drive the demo available from the download area.
Footnote:
When two batteries A and B simultaneously power a load, the resultant
capacity will be higher than that of a system in which the same two batteries take it in
turn to power the same load.