Page 248 - Uninterruptible Power Supplies

Some System Failures: The Light of Experience!
246 Chapter Ten

continuously energized and do not experience a cold ambient tempera-
ture but standby generator transformers are idle for most of their life
and are only occasionally energized.
This failure indicates the need for the designer to be aware of the
unexpected! This incident was followed by a change to the specification
for generator transformers to ensure that a visible and adequate oil
level is maintained down to 10°C.

Inadequate Protection against Driving Rain
This incident was an extreme inconvenience and a potential failure.
The site was on the west coast of Scotland and the generator had a rat-
ing somewhat above 1 MW.
The ventilation air inlet had the usual weatherproof louver which
faced seaward and toward the prevailing wind. In normal circum-
stances the louver may have been adequate, but at this location the
generator room was the on the ground floor of a two- or three-story
building. Above the inlet louver there was a high blank wall which
caught all the driving rain, which then ran down the wall toward the
louver. The louver had not been designed to cope with the large quan-
tity of water and much of it was drawn into the generator room.
The building should have incorporated some feature which diverted
the rainwater from above before it reached the intake louver, a form of
guttering for instance. In addition, it would have been expedient to
build a wall or other construction to prevent driving rain from reaching
the louver. This failure demonstrates the need to be aware, at the plan-
ning stage, of potential problems which may not be obvious. This
installation would undoubtedly have passed its usual commissioning
tests, but it would not have been raining heavily at the time!

Unconventional Use of a Harmonic Filter
The installation provided power for an office block serving a financial
institution. It comprised a static, uninterruptible power supply rated
at 120 kVA and a much larger standby set rated at 1 MW. The load was
the usual mix of communication, computing, and display equipment.
The uninterruptible power supply was unusual in that it included a
harmonic filter connected to the UPS output, the purpose being to
reduce the harmonic content of the load to comply with Engineering
Recommendation G.5/3 when using the bypass supply. A contactor
enabled the use of the filter to be controlled. The UPS connections were
as indicated in Fig. 10.1.
The filter comprised, for each phase, three series resonant circuits
tuned for the third, fifth, and seventh harmonics, and connected across

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