Page 105 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 103

and many other harmonic currents producing an rms total harmonic
distortion of 110 percent. In three-phase systems triplen harmonic cur-
rents have zero phase sequence and become additive in the neutral
return path. In a distribution system supplying a large computer
installation the neutral current can be large and can exceed the phase
current; the system should be designed to allow for this. This statement
applies whether the load is being supplied from the standby supply or
from the normal supply.
Small computers may be connected directly to the supply, and while
most computers of importance will be supplied from an uninterruptible
power supply, they may on occasions be connected directly to the nor-
mal or the standby supply when the uninterruptible power supply is
operating in bypass mode. Where standby power generation is at low
voltage, the triplen harmonic current components will appear in the
generator neutral and can cause overheating. If the total neutral cur-
rent is likely to exceed say 15 percent of rated current, the generating
set manufacturer should be advised.

Fluorescent and Discharge Lighting
These are nonlinear loads taking a current rich in odd harmonics. BS
EN/IEC 61000-3-2 allows a third harmonic of up to 30 percent. The
same problems arise with neutral currents as are described for com-
puter loads in the preceding subsection.

Cyclically Varying Loads
A load such as a rotating radar aerial will vary cyclically at a regular
rate due to the aerial’s rotation and the more or less static direction of
the wind. If the regular rate of variation is comparable to the natural
frequency of response of the engine governor, continuous hunting about
the mean speed may occur. Continuous motion of the governor parts
and linkages can lead to premature wear and to lost motion in the fuel
control mechanism. Increasing the speed of response of the governor
may lead to instability, and reducing the speed of response may result
in the frequency not remaining within specification. The generator set
manufacturer should be aware of a significant load of this nature at an
early stage of the contract.

Motors
Any large motors in the load will affect the supply voltage during start-
ing. An induction motor will take a current of six or seven times its
rated current at a power factor of say 0.15 when started direct on line.
A squirrel cage motor taking a starting current equal to the generator

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