Page 69 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 67

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Figure 2.11 Triplen harmonic currents flowing in the windings
of paralleled generators due to differing machine waveforms
become additive in the neutral.

interconnected there can be no triplen harmonic circulating currents
and the problem will not arise.
If the machines are identical and are running under identical exci-
tation and loading conditions their neutral terminals may be safely
connected together. However machines which are thought to be identi-
cal may differ and the excitation and loading conditions will be subject
to the normal tolerances of the voltage regulators and load sharing sys-
tems. The failure of a voltage regulator, a speed governor, a load shar-
ing system, or a kVAr sharing system may cause unexpectedly heavy
circulating currents to flow in the neutral connections. For these rea-
sons, the generator manufacturer should be consulted before the neu-
tral terminals of paralleled generators are connected together.

Low-Voltage Installations
The supply should be solidly earthed as described for the single set
installation but, for the reasons described in the preceding paragraphs,
it may not be advisable to connect together the machine neutrals of a
multiple set installation. There are several methods of overcoming the
difficulty, the matter should be agreed with the generator manufacturer:

■ Each machine is provided with a neutral contactor but only one con-
tactor is closed at any time. A logic system ensures that the neutral

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