Page 238 - Uninterruptible Power Supplies

Notes on Systems Installation
236 Chapter Nine

ignored and then blamed when called upon to supply power during an
emergency and failing. The problems here are as follows:

A tendency for operational staff to ignore them as a passive object
seemingly not contributing anything to the task they are faced with
Such units are often hidden from view and the tell-tale warning
lamps are either ignored or not even seen!
Small units suffer from a lack of maintenance and indeed it has to be
understood that maintenance costs for such systems may be exception-
ally high, for example, replacement of a battery may be uneconomical.

Clearly, solutions to these problems are fairly straightforward.
Advise operators of the qualities of such units, place units in a position
where they can monitored, and regularly check them. It should be
noted that many such sets may be interrogated remotely electronically.
An unusual application for UPS systems is railway signaling.
Clearly, such loads are vitally important. Any loss of power on busy
routes may cause not only dangerous conditions but it may take days
to reorganize traffic flow. The availability of a suitable power source
trackside is not always immediately to hand. Some areas requiring
power can be remote and so the answer has been to use the raw power
available to the electric traction units. Such power sources are very
often 25 or 33 kV with some wide variations in regulation: 30 percent
has to be considered and surges have been reported up to 60 kV. In
some instances, recordings have indicated a twice crossover of 0 volts
in one-half cycle. Clearly a power supply fraught with problems!
To work under such conditions, the UPS has to be protected against
wide voltage variations and surges, this is achieved by well-designed
isolating transformers and surge diverters. The wave trace shown in
Fig. 9.1 illustrates input, and the output voltage (upper trace) which is
approximately 650 V single phase.
Designs vary depending to a great extent on variations to be expected
in the input voltage range. One school of thought is to use standard
UPS units well protected against the vagaries of the high-voltage sup-
ply and illustrated in a photo of a system using two Silcon/APCC 40-
kVA modules with isolating transformers and controls (see Fig. 9.2).
The other approach is to use a specially designed UPS unit again with
full isolating transformers and controls. Where such wide variations in
supply are encountered a normal bypass circuit is clearly unfeasible,
and one alternative is to use a a hot standby UPS (i.e., a UPS in work-
ing mode but off load) switchover via a static switch. Figure 9.3 is of a
specially designed 60-kVA UPS to operate from a 25-kV power source
and with IP54 construction.

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