Page 72 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
70 Chapter Two

Alternator No 1

Alternator No 2

Static balancer

R Y B N E
Figure 2.14 Connecting the neutral for low-voltage multiple sets using a static balancer.

■ With the generator maker’s approval the generator neutrals are par-
alleled. Unless the generators are identical and are working under
identical conditions of excitation and loading there will be some third
harmonic currents circulating in the neutrals.

High-Voltage Installations
The supply should be earthed through an earthing resistor as described
for the single set installation but, for the reasons described in a forego-
ing paragraph, it may not be advisable to connect together the star
points of a multiple set installation. There are several methods of over-
coming the difficulty, the matter should be agreed with the generator
manufacturer:

■ Each machine is provided with an earthing contactor connected to a
common earthing resistor but only one contactor is closed at any
time. A logic system ensures that one machine is earthed and

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