Page 57 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
Interconnecting the Standby and Normal Supplies 55

voltage regulators to introduce a small drop in terminal voltage as the
kilovar load increases as indicated by Fig. 2.5. The difference between
Figs. 2.4 and 2.5 is that in the former the axes represent speed and kW
loading, and in the latter they represent voltage and kVAr loading. As
the paralleled sets will be running at the same voltage, if they have the
same droop settings, load sharing will be achieved automatically. One
method of achieving the droop is for each regulator to be provided with
a signal from a quadrature drop current transformer connected in one
phase of its generator output. The quadrature drop signal is added to
the terminal voltage sampled by the voltage regulator and thereby
introduces a voltage droop.
If the voltage droop is not acceptable, an alternative system of shar-
ing the kilovar loading is used, this requires interconnections between
the voltage regulators and maintains constant voltage. If one machine
is taking too great a share of kilovars the sharing signal will cause its
regulator to reduce its excitation.
Where frequency and voltage droop are used to share load and kVAr
the droop will be, say, 3 or 4 percent and it is usual to arrange for the
nominal frequency and voltage to be achieved at normal running load.

Interconnections with the Normal Supply
The main power interconnections between the normal supply, the
standby supply, and the essential and nonessential loads depend on the
number of sets in use and whether the power changeover is performed
with two circuit breakers or an arrangement of contactors. Where the
normal and standby supplies are required to be paralleled, the follow-
ing comments are applicable but the reader should in addition refer to
the section titled “Paralleling the Standby and Normal Supplies.”

Figure 2.5 Kilovar load sharing between two sets using a voltage regulator with a quad-
rature droop characteristic.

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