Page 71 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 69

Only one contactor is to
be closed at any time

R Y B N E
Figure 2.13 Connecting the neutral for low-voltage multiple sets using
contactors.

The static balancer is a vital component connected to the main bus-
bars and thought should be given to the electrical protection that
should be provided. If it is tripped from the busbars due to a fault the
standby supply has to be shut down due to the lack of a neutral con-
nection. It follows that any protection such as overcurrent or earth
fault should have current settings and time delays which avoid the
probability of spurious tripping.
■ Each machine is provided with a reactor connected between its star
point and the neutral busbar. These will attenuate the third and
higher harmonic currents without offering significant impedance at
fundamental frequency. The reactors have the effect of increasing the
generator’s zero sequence impedance and data will normally be
required from the generator manufacturer before any calculations
can be undertaken. If there is fundamental current flow in the neu-
tral due to load imbalance between the phases the reactors will
increase the voltage unbalance—a compromise has to be reached
between unbalanced phase voltages and triplen current flow.

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