Page 60 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
58 Chapter Two

Operating regimes such as those described in the last two para-
graphs should not be used indiscriminately, they require skilled and
competent operating personnel who have the necessary experience and
confidence to operate switchgear under the somewhat stressful condi-
tions that always apply after a failure of the normal supply. For the
benefit of operating personnel a wall chart should display relevant
information such as that given above.

The Generator Voltage
Most standby generating sets are of a rating well below 1 MW and will be
connected to systems using the standard final distribution voltage which,
in the United Kingdom, is 400/230 volts. However generating sets having
a rating approaching 1 MW may be associated with a high-voltage distri-
bution system. In the United Kingdom, the predominant high-voltage
distribution voltage is 11 kV, but 20kV is used in some rural areas.
Each generator manufacturer will have determined a rating at
which the transition from low voltage to high voltage occurs on grounds
of costs and practicability. A generator designed for say, 100 kW at high
voltage would comprise much insulation and very little copper; like-
wise a generator designed for say 5 MVA at low voltage would comprise
much copper and very little insulation. For every rating there is an
optimum voltage but choice is usually limited to the standard distribu-
tion voltages which in the United Kingdom are 400 V, 11 kV, and 20 kV.
The precise rating at which the transition occurs depends on details of
manufacturing techniques but it is likely to be of the order of 1250 kVA.
The current rating of a 1250-kVA generator at 400 V is 1804 amperes
and cables will be of large cross section and intractable and may be dif-
ficult to install. If during the planning of an installation such difficul-
ties are foreseen, it may be worth considering generating at low voltage
and feeding directly into a step-up generator transformer. However,
unless high voltage is already in use within the installation, its intro-
duction is likely to cause problems with the competence of personnel
and operating and safety procedures.

The Electricity Supply Regulations 1988
The Electricity Supply Regulations are currently applicable, within the
United Kingdom, to suppliers of electricity, but they are soon to be
replaced by the Electricity Safety, Quality and Continuity Regulations.
These notes are included here as they indicate the underlying philoso-
phy which seems unlikely to be changed by the new document.
The Regulations set out the requirements for the distribution of elec-
tricity, from its origin (a generating station or a substation) to the var-

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