Page 75 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 73

break, and for test runs it is similarly possible to transfer the essential
load to the standby supply without a break, while leaving the nonessen-
tial load connected to the normal supply. It should be noted that this
advantage can only be gained if the normal supply circuit breaker con-
nects to the section of busbar that supplies the nonessential load. (See
Figs. 2.2 and 2.3.) These procedures require paralleling for short peri-
ods only and the agreement with the network operator usually limits
the duration to 5 min.
The technical requirements are set out in Engineering Recommenda-
tion G59/1 and its supporting document Engineering Technical Report
113, both published by the Electricity Association.

Features Required on the Generating Set
The generating set should have been constructed with parallel running
in mind. The coupling between the engine and alternator should be suf-
ficiently robust to withstand an attempt to synchronize in phase oppo-
sition without suffering damage; a shear pin is sometimes provided
between the alternator and the coupling. The alternator poles should
be provided with pole face damper windings to prevent phase swinging.
As the set will be feeding into an infinite system the engine governor
will have no control over the speed and the voltage regulator will have
no control over the voltage. Instead the governor is used to control
power (kW e ) and voltage regulator to control power factor (cos ). For
synchronizing purposes the engine governor and the voltage regulator
will require remote adjusting facilities.

The Supply Authority’s Requirements
For reasons connected with the safety of their operating and mainte-
nance personnel the network operator will require electrical protection
to ensure that their distribution network is not energized from the
standby supply. For long-term continuous paralleling the protection
required is complex but for the short-term occasional paralleling
applicable to standby installations the protection required will usually
consist of under- and overfrequency, under- and overvoltage, overcur-
rent, earth leakage, and a 5-min timer, in addition the network opera-
tor may require protection against neutral voltage displacement. The
settings of the relays required for paralleling will be determined by the
network operator.
The purpose of the under- and overfrequency and voltage protection
is to detect a loss of normal mains and to prevent the standby genera-
tor back-feeding a part of the supply authority’s distribution system
(islanding). The purpose of the neutral voltage displacement protection

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