Page 68 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
66 Chapter Two

Standby Electrically interlocked Supply Authority's
generator circuit breakers circuit breaker
Standby supply Normal supply
R

Y Normal
supply
B

E
(if provided)
Earthing
resistor

Essential Non-essential
load load

Figure 2.10 Typical connections for alternative supplies at high voltage.

Neutral Connections for Multiple Sets
Not Intended to Run in Parallel with the
Normal Supply
Running Generators in Parallel- and
Triplen-Current Flow
Triplen currents have not been previously mentioned and it is appro-
priate to introduce them here. The harmonic orders 3, 6, 9, 12, etc. are
known as triplen harmonics or triplens. They have zero-phase sequence
and triplen currents flowing in the phase conductors become additive in
the neutral, hence the concern expressed in the following text.
The voltage generated by a loaded salient pole generator will include
a third harmonic and other triplen components the magnitude of which
will be dependent upon the machine design, its excitation, and its load-
ing. If the machines are identical and are equally loaded and excited
the triplen harmonic voltages in each of the machines will be equal, in
phase and balanced and there will be no resulting current flow. If the
machines are not identical or are not equally loaded or excited, triplen
harmonic currents will circulate between the machines as shown in
Fig. 2.11. Any triplen load currents taken from them will similarly be
additive in the neutral busbar return path as shown in Fig. 2.12.
It is likely that both the above conditions will exist at the same
time and for the triplen currents in the neutral connections to be
unexpectedly large. When salient pole generators are to be run in
parallel, consideration should be given to the possibility of undesir-
able third harmonic currents circulating in the neutral connections
of the machines. If the neutrals of paralleled generators are not

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