Modular UPS: Pros and Cons of Modular Uninterrupted Power Supply ( UPS) Systems

UPSonNet

site search by freefind

Spread Your Word On the Globe

Y our Products on this Site

Newsletter Sign-In

UPSonNet NewsLetter
February 2011

UPSonNet

site search by freefind

Spread Your Word On the Globe

Y our Products on this Site

Editorial Topic by Meir Portnoy: Pros and Cons of Modular UPS Systems
Certain links in this Newsletter require UPSonNet member subscription; Sign UP for full site access

Modular Three Phase UPS systems are lately gaining momentum, due to the attractive concept
based on small, light, compact, hot swappable, low cost modules which can be added when
needed. Unfortunately, with the merits come also certain flaws; this editorial considers both.

Our September 2010 Newsletter titled "The Right UPS Protection Strategy for Now and the
Future" examined different scenarios related to Three phase Scalable UPS systems, indicating the
need to form a UPS protection strategy, and how such strategy, which takes in account the
needed level of protection, future prospects and optimum cost can be implemented with the aid of
a presented Three Phase Scalable UPS Guide.

Most current Three Phase UPS systems have scaling ability, which enables to add parallel UPS
units whenever additional power or redundancy is required. A unique scalable solution is presented
by a Modular UPS approach. These systems are generally based on a rack type enclosure that
includes battery cabinet at its bottom, and hot swappable modules located one above the other,
which can be added whenever more power is needed.

The modular UPS approach has the following indisputable advantages:

1. It provides lowest floor space, compared to other scalable solutions.

Unlike conventional stand alone cabinet approach, which increases horizontally the required floor
space, when parallel systems are added, the modular approach is based on vertical utilization of the
existing rack. A review of systems in the mentioned above guide, indicates that a modular
approach takes only about 25% of floor space that should be allocated for a scalable cabinet type
solution, with the same total power capacity as the modular rack when fully loaded.

2. It enables achieving higher Availability compared to same number of stand alone
parallel UPS Systems.

UPS systems are generally intended to work continuously. Availability is defined as the ratio of
time in which the UPS is operating correctly, expressed as MTBF (Mean Time Between Failures),
to the total time, which includes also the time it takes to restore a failed UPS system to normal
operation, expressed as MTTR (Mean Time To Repair); in equation format: AV= MTBF
/(MTBF+MTTR) = 1/(1+ MTTR /MTBF).

Availability is the most important parameter to ascertain proper site protection. When Availability of
UPS systems with equal number of parallel units is compared, the modular approach has a distinct
advantage. Whereas MTBF figure depends mainly on the number of parallel units and level of
redundancy, hot swappable modules enable fast swapping by local personnel of failed modules,
decreasing practically the MTTR figure to the time it takes to deliver a new replacement module to
the facility.

3. It enables to achieve higher efficiencies in installation with scalable load.

UPS efficiency reaches its maximum level when the load is at its maximum rating. The ability to
increment total UPS power by adding small modules when needed, while maintaining high ratio of
total load to total UPS capacity, renders the most efficient solution.

4. It reduces dramatically the probability of UPS failure during maintenance.

Reviews indicate that 30% of UPS failures are caused by technicians' errors during UPS repair or
maintenance. Hot swappable modules, which need to be repaired, are isolated from the UPS and
sent to the service center, diminishing the probability of UPS failure during service.

Reduced reliability is the main problem with most Modular UPS systems, due to the
following reasons:

1. Low MTBF of Electronic UPS modules.

A modular approach calls for relatively high quantity of small parallel power modules. Most UPS
systems are based on 10kVA or 20kVA modules, located in a 50 kVA or 100kVA Rack. For
additional power, manufacturers recommend to add additional Racks in parallel.

From manufacturing view point, 10kVA, and even 20 KVA UPS systems can be build on a single
printed circuit mother board, thus enabling low cost production, outsourced from PCB assemble
manufacturers. The user gets more flexibility, higher efficiency, and lightweight swappable units,
which can be handled by a single person, thus reducing cost and MTTR.

On the other side, system MTBF is inversely proportional to the number of parallel UPS units. The
relatively small capacity of a basic module increases the number of parallel modules needed to
achieve the required total power. MTBF figure of ten parallel modules is ten times lower than two
parallel modules.

The associated reduction in Reliability and Availability, the nuisance and high maintenance costs
will increase when number of modules, and associated number of UPS failures goes up, and may
make the modular approach unpractical for systems exceeding 4 to 5 modules in parallel.

2. Low MTBF of UPS Batteries

Similar logic applies to UPS batteries; Modular approach increases the number of batteries,
inversely decreasing the MTBF figure. Most UPS systems or modules include a battery bank
comprising about 16 to 20 batteries connected in series for a UPS with 208V AC output, up to 32
to 40 batteries for a UPS with 400V AC output. Failure of a single battery disables the total battery
bank. Assuming that each UPS has its own battery bank, ten modules in parallel will have 160 to
400 batteries. Certain UPS manufacturers use 64 batteries with dual center tap configuration, thus
reaching 640 batteries in a 10 module 400V UPS system.

The problem becomes more complicated due to the incremental nature of adding modules
whenever power demand grows. The ability to implement a periodic practice of battery
replacement, aimed to increase battery MTBF, recommended in our April 2010 Newsletter, titled
Impact of backup batteries on UPS Reliability, becomes very hard to accomplish, leading to a
dispersed battery failure behavior, where stated lifetime of the battery dictates it's average MTBF.
We might therefore expect MTBF figure, due to battery failure to go down to 219Hours
[24x365x5/(20x10)] for a UPS system with 10 parallel modules, each with 20 batteries rated for 5
year operation, going down to 110 Hours for a similar 400VAC configuration.

3. Limited Redundancy

Increasing the number of parallel modules reduces both system reliability by increasing the number
of failures in a given time, and systems availability by decreasing the time between failures.

As outlined in our UPS Strategy Guide, increasing the number of parallel modules may require
upgrading the N+1 redundancy level to N+ 2, by addition of a redundant module to maintain the
required Availability level.

Unfortunately, many modular UPS systems are limited to N+1 redundancy, designed for only one
redundant module in the UPS rack. A simple addition of an excess module doesn't solve the
problem, because the system lacks the means to reveal required overload and other needed
conditions for N+ 2 redundancy, and will not alarm whenever the new redundancy isn't met.

4. High Single Failure Probability.

Unlike conventional stand alone UPS systems, each equipped with all the needed UPS function
blocks, certain common components in most modular UPS systems may serve all parallel modules
in the UPS Rack. A single failure in following functional components, if common to all modules in
the rack, may override redundancy, affecting UPS operation or causing total output failure:

Common Transfer Switch
Modules of certain UPS manufacturers include a Rectifier/Charger and DC to AC Inverter which is
connected at its output to a common three phase AC buss; whereas a single Transfer Switch
designed to supply full rack capacity is located separately in the UPS rack. Malfunction of the
transfer switch may affect load transfer to alternate source, when needed, discontinuing output
power in severe cases.

Common Control Unit
Certain modular systems have a common supervising synchronizing and communicating unit,
where a single failure may inhibit alarms and even bring down all parallel UPS modules.

Common Battery Bank
Many manufacturer's use a common battery bank, aimed to serve all modules in the UPS rack.
This configuration enables to decrease the amount of battery strings and increase reliability, by
using batteries with higher capacity.

Whereas the single dedicated battery for each module has the necessary means to monitor, test and
control the health of its battery bank, the common battery approach may pose a problem if no
provisions are made to monitor and control each string in a common battery cabinet. Lack of such
measures and inability to isolate a failed battery string may cause a single point failure, which can
harm healthy batteries and trip the common battery protection.

Conclusion

In conclusion, Modular UPS system architecture isn't always the best solution to all scalable needs,
but it should be thought over as a part of a general UPS strategy, as discussed in our UPS Strategy
Guide.

Implementation without prior consideration of a "Go as You Grow" approach, by means of a
modular UPS system, starting with two 10kVA modules in parallel redundant cinfiguration, may
end up with a 100kVA system, which includes ten modules, and has the reliability, and failure rate
of ten UPS systems connected together with hundreds of batteries.

In addition, selection of a modular approach requires thorough knowledge of the particular UPS
design including detailed information about common items, in order to see the entire picture. Most
UPS manufacturers however provide very vague information. Generally, no design details, and no
block diagram is presented on published product literature, and additional digging is required to
reach a comprehensive decision.

I welcome remarks. Please send to: meirp@upsonnet.com

UPSonNet Website News

We have informed in our last Newsletter the launching of North American Professional UPS
Suppliers Guide. Since then some additional companies were added. For more information please
see UPSonNet Website News in January 2011 Newsletter.

Recently published UPSonNet News Release titled: New UPS Suppliers Guide Helps Realizing
Green UPS Solutions, explains the need and reason behind the new guide.

UPS Industry News

News products from various fields related to UPS systems are briefed on February UPS-News,
including UPS systems, batteries, test equipment, semiconductors, fuel cells, super capacitors and
more.

Two items desire special attention:

Minutemen cites results of Frost and Sulivan's IT managers survey, which indicates a clear trend
of longer UPS backup times requirement, in excess of 30 minutes.

Energy Star informs phase 2 of UPS testing According to Final Draft of ENERGY STAR Test
Method and summarizes interesting phase 1 results of 236 tests from 5 UPS manufacturers
performed according to IEC standard 62040-3, which can be accessed by means of the link in the
relevant news brief.

We welcome remarks on any Newsletter issue. Please e-mail to: denis@upsonnet.com
__________________________________________________________________

To receive this newsletter in your e-mail, please subscribe.

Please forward this e-mail to a friend that might be of interest