Page 13 - Uninterruptible Power Supplies

Standby Power Generating Sets
Standby Power Generating Sets 11

ROTATING PERMANENT
PILOT MAIN ALTERNATOR AC
EXCITER MAGNET FIELD R
AC EXCITER
DC AC DC

N Y
S

B
ROTATING

VOLTAGE SENSING
Figure 1.4 Excitation system using a pilot exciter.

MAIN ALTERNATOR AC
R
EXCITER
DC AC DC
ROTATING
RECTIFIER
Y

ROTATING B
N
AVR
FAULT CURRENT
MAINTENANCE

VOLTAGE SENSING
Figure 1.5 Excitation system taking a supply from the generator output.

The permanent magnet pilot exciter has the advantage that it pro-
vides a supply to the main exciter field that is independent of the gen-
erator output. If the generator output suffers a short circuit the field
remains excited and the generator is able to deliver its short circuit
current until protective equipment operates to clear the fault.
Where there is no pilot exciter, power for the exciter field is taken
from the generator output terminals. With this arrangement, unless
precautions are taken, a fault on the distribution system will lead to a
loss of voltage on one or more phases, which in turn can lead to a loss

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