Worked Examples
UPS Systems
Before proceeding, can you guess which is the more reliable system and why?
Introduction
This example compares the reliability of two UPS systems Option A, and Option B. The components in Option A are all identical to those in Option B, the difference being the way the two UPS modules are arranged in each case. 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.
To view UPS option A: UPSOptionA
To view UPS option B: UPSOptionB
Working Out
The two schematics shown above were drawn with and taken directly from Analyst. Each schematic took approximately 5 minutes to draw and test.
Using Analyst, a reliability analysis was performed on the two options, the computation taking less than a minute in each case with the following results:
| 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 |
The simulation results suggest that option A will be more reliable. Congratulations if your guess was correct! The question is, why?
Explanation
In addition to reliability analysis, Analyst also has the ability to do 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 show the subsequent response of the system 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 system in actual operation. First the master UPS battery came on-line, then 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 reason for the difference in the two systems is now evident. Option A has a longer battery reserve time! Analyst was not only able to model the subtle differences in the way the two UPS batteries were being used, but it also correctly predicted the effect this would have on the reliability of the two systems!
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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.