Page 77 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 75

■ It is not possible to use the standby supply independently unless the
neutral contactor is closed.

An alternative procedure is to install a reactor connected between the
generator star point and the supply neutral. The reactor is to have suf-
ficient reactance to limit the triplen harmonic current flow to within the
limits set by ER G.5/4. This arrangement is discussed in ER G.59/1 and
in ETR 113 but it is more complex and expensive than the preceding
procedure and would seem to have limited application for single sets.

Earthing Low-Voltage Supplies
When the normal and standby supplies are running in parallel the neu-
tral connection will usually be taken from the normal supply and,
depending on the earthing arrangements and the agreement with the
network operator, there may be a requirement for an earthing contac-
tor or for an additional pole on the neutral contactor mentioned in an
earlier paragraph. For multiple earthed systems, including PME, the
neutral and earth connections are combined and a single pole neutral
contactor is used. Figure 2.17 illustrates the electrical connections. For
single-point earthed systems such as TN-S or TT an additional pole is
required on the neutral contactor to avoid a second earth connection to
the normal supply neutral. Figure 2.18 illustrates the electrical con-
nections. If a separate earthing contactor is used, electrical interlocks,
as required for the neutral contactor, must be provided.
As was the case for the diagrams related to alternative connections,
Figs. 2.17, 2.18, and 2.19 have been derived from diagrams appearing
in ETR 113. Figure 2.17 provides a parallel path for the neutral cur-
rent, the undesirable consequences of such an additional path have
already been discussed at the end of the section titled “Neutral
Connections for Single Sets Not Intended to Run in Parallel with the
Normal Supply” in connection with Fig. 2.7.

Earthing High-Voltage Supplies
For high-voltage systems the network operator may use solid, resistor,
reactor, or arc suppression coil earthing methods and a second earth-
ing point will not normally be allowed. When a high-voltage standby
supply is operating independently its star point must be connected to
earth, usually through an earthing resistor, but when it is operating in
parallel with the normal supply, the earth connection must be opened,
for which purpose an earthing contactor is used. Figure 2.19 illustrates
the electrical connections. With this arrangement the standby supply
star point is not used and the system relies upon the normal supply for
earthing.

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