Page 55 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 53

■ The use of two sets, each rated for one half of the essential load,
allows part of the installation to be supplied with standby power
after the failure of one set. This assumes that the installation is such
that it can continue to operate on a reduced scale. On loss of supply
both sets are started and either paralleled before the essential load
is applied, or the first available set supplies essential load 1, the sec-
ond set is then paralleled and essential load 2 is connected.
■ If a diesel engine is run on light load for a sustained period of time,
products of combustion are deposited on the cylinder bores and lead
to “glazing” and lubrication problems. In addition combustion may be
incomplete, leading to carbon deposits within the combustion cham-
bers. Where the electrical load is likely to vary and the lower limit
approaches one third of the upper limit, the use of two sets should be
considered, each rated at say 50 percent of the essential loading. On
light load one set can then be shut down, but note that the second set
has to be restored, either manually or automatically, before the full
essential load can be supplied. If redundancy is required three sets
would be used.
■ The use of two independently running sets makes it possible to sep-
arate industrial-type loads from loads requiring a clean supply. The
two sets may be of different sizes and to allow for the failure of either
set, manual switching can be provided to rearrange the loading on
the healthy set.
The use of two sets doubles the probability of a failure of one set but
the probability of a total failure of the standby supply is very much
reduced.

Modes of Operation
Where multiple sets are required to be paralleled before supplying the
essential load, time will be required for the paralleling procedure and
the time between the loss of normal supply and the availability of the
standby supply will be greater than that which occurs for a single set
of similar rating. The maximum time for paralleling one set to the bus-
bars will probably be on the order of a minute. Most critical loads nowa-
days are supplied through an uninterruptible power supply with a long
period of autonomy, and battery-maintained emergency lighting is nor-
mal in buildings. In such installations the duration of the “dead” period
cannot be claimed to be important. It probably makes little difference
whether the break in supply is 15 seconds, 30 seconds, or a minute.
If, however, in a two set installation it is required to keep the “dead”
period to a minimum, an alternative mode of loading can be used. On

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