Page 56 - Uninterruptible Power Supplies

Interconnecting the Standby and Normal Supplies
54 Chapter Two

loss of supply both sets are started and the first available set supplies
essential load 1, the second set is then paralleled and essential load 2
is connected.

Load Sharing
When more than one set supplies the load, they will usually be of the
same rating in which case they should share the kilowatt and kilovar
loads equally. If they are of different ratings they should share the
loads in proportion to their ratings.
The simplest system of kilowatt load sharing is for the engine speed
governors to allow a small drop in speed as the load increases as indi-
cated by Fig. 2.4. The paralleled sets must of necessity run at the same
speed and if each set has a similar load/speed characteristic, load shar-
ing will automatically be achieved. If at some time the load sharing
between two sets becomes unequal, both sets will be running at speeds
which are not compatible with their load/speed characteristics and
their governors will react to correct the errors.
If the paralleled sets are of the different ratings, provided that the
droop settings of each of the sets are the same, the load will be shared
proportionally to their ratings. It can be seen from Fig. 2.4 that the load
taken by any set can be increased or decreased by reducing or increas-
ing the droop setting (the slope of the characteristic) or by altering the
set speed (the vertical position of the characteristic).
If the frequency droop is not acceptable, an alternative system of shar-
ing the kilowatt loading is for the engine speed governors to be arranged
for isochronous (constant speed) running. This requires electrical inter-
connections between the governors to maintain load sharing.
Kilovar load sharing between generators running in parallel is depen-
dent upon the rotor excitation which in turn is determined by the volt-
age regulators. As with kW load sharing the simplest system is for the

Figure 2.4 Kilowatt load sharing between two sets using a speed droop governor
characteristic.

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