Page 14 - Uninterruptible Power Supplies

Standby Power Generating Sets
12 Chapter One

of excitation power and to total collapse of the generator output. In
such circumstances the fault may not be cleared by the protection
equipment and the standby supply may be out of use until the fault is
removed manually from the system. The probability of this happening
is affected by whether one, two, or three generator phases are used to
supply the field power, and by which phases are affected by the fault.
To avoid a collapse of the generator output, a current transformer
may be added to each phase of the generator output. The current trans-
former secondary windings are connected to the excitation system and
maintain the field power until the fault is cleared by the protective
equipment. There is an alternative system, less widely encountered,
which is applicable only to salient pole machines and uses a separate
stator winding specially to back up the field supply during faults. This
winding is distributed in the stator to take advantage of the third har-
monic voltages which appear when heavy currents flow from a salient
pole generator. The third harmonic voltages provide the field power
during faults in the same way as the previously described fault current
maintenance current transformers.

Pole Face Damper Windings
Salient pole generators should be fitted with pole face damper windings
having interconnections between the poles. These windings improve the
waveform by attenuating harmonic fluxes in the machine air gap; where
machines are to be run in parallel they are essential to prevent phase
swinging, the phenomena in which the machine rotors oscillate in angu-
lar rotation (and position) either side of the desired position. In this con-
dition the damper windings are in motion relative to the stator flux,
voltages are induced and the resultant current flow has a damping effect.

Generator Transformers
A generator transformer is used where the voltage of the generator dif-
fers from that of the system which it is intended to supply. The trans-
former primary windings are permanently connected to the generator
output terminals, without any switchgear or protective equipment
between them and for practical purposes the transformer is regarded
as part of the generator. The transformer windings may be delta/star
or star/interstar and the neutrals of the generator and of the secondary
windings are earthed in the usual way.
When stepping down to feed a low voltage system a delta/star con-
figuration will usually be used. An interstar secondary winding has a
high impedance to zero sequence currents and is not therefore suitable
to feed a low voltage distribution system. If stepping up to feed a high
voltage system a delta/star or star/interstar configuration may be used.

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