Advantages and Dis-advantages of a Parallel Modular UPS System

Note: The conclusions drawn in this article are the personal views of the author. The website or author is not responsible for any unforeseen damages 070-551-VB caused by implementing these techniques.

Modular UPS systems are a smart conception. They are small, light, compact, hot swappable, and low cost modules which can be added when required and removed when there is no further use for it. Parallel connected UPS systems are more distinctive in their own way. There are two sides to a coin, with many advantages come dis-advantages too, read on.

Advantages:

A parallel modular UPS system beyond doubt is very advantageous to a data center. Here are a few:

Higher Availability:

 

When analysing a UPS system it is obvious that availability is a major criteria when considering a purchase. Availability of a UPS is defined as follows:
920-121

AV= MTBF/ (MTBF+MTTR) = 1/ (1+ MTTR /MTBF).

Where, AV – Availability, MTBF – Mean Time Between Failure, MTTR – Mean Time To Repair.

The value of mean time between a failure depends on, the number of parallel units and level of redundancy. Swapping of failed modules due to hot swapping feature will reduce the mean time to repair.

Least Floor Space:

 

Most standard stand-alone systems come in the horizontal stacking form factor. This drastically increases floor space as more systems are added. Modular UPSs systems are designed to be stacked vertically reducing space by almost 25%!

Reduced Maintenance Failure:

 

Popular reviews show that 30% of UPS failures are due to errors caused by maintenance staff during repair. Hot swappable modules can be replaced and repaired at a later time. Hence, the failed UPSs can be sent to a service station for repairs, greatly minimizing maintenance failures.

Extreme Efficiency:

 

Efficiency of a UPS reaches its peak when the load is at maximum rating. Modular UPSs allow power modules to be added, this keeps the ratio high and efficiency higher than normal.

Isolation:

 

 The greatest advantage of the parallel operated UPSs is the isolation. The load and supply always remains balanced even though there may be a break due to unforeseen events. 

Dis-advantages:

The parallel operated modular UPSs have a few dis-advantages that are usually neglected. It is best to know these just in case some fine tuning has to be made to the backup power system in general.

Not too many in parallel:

 

Generally a low mean time to repair a parallel modular UPS system is more advantageous than a low mean time between failures. In a system of lower rating, multiple modules may be placed on a single board, hence multiple modules in parallel which means smaller mean time between repairs, and hence this acts as a major drawback. The same principle can be applied to batteries in parallel on these systems.

Relatively Higher Maintenance:

 

Increasing the number of UPS modules further reduces system reliability due to increase in number of failures at a given time and the general availability due to the low availability between failures.

Failure of Individual Components:

 

In a parallel modular UPS, the entire rack may serve as a single UPS. A single failure would amount to a common failure in all parallel modules which may be a common control unit or even a battery bank. This may cause complete output breakdown. Hence it is advisable for the data center to go for an N+1 or N+ 2 modules where N is the number of UPS needed by the data center for normal backup practice. This leads to added costs.

The advantages of a parallel modular UPS system clearly outweigh the dis-advantages. A good parallel connected modular UPS may be the key to an efficient data center power backup.