Page 73 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 71

decides which machine it is; it will usually be the first generator on
line. The resistor should be rated to limit any earth current to the
current rating of one machine. Figure 2.15 illustrates the electrical
connections.
■ Each generator is earthed through its own earthing resistor. Circulating
currents will be limited to an acceptable level by the resistors and the
complications of contactors and a logic system are avoided. The cir-
culating currents are unlikely to be high but the rating of the resis-
tors should be continuously rated to dissipate the resulting losses.
Figure 2.16 illustrates the electrical connections.
■ The supply is earthed through an earthing transformer, which is sim-
ilar to the low-voltage static balancer but is not rated to carry any
neutral current. The generator star point terminals are not used and
the star point of the earthing transformer is connected to the earth-
ing resistor.

Only one contactor is to
be closed at any time

Resistor

R Y B E
Figure 2.15 Neutral earthing of high-voltage multiple sets using con-
tactors and a common resistor.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.