desktop PC's, uninterruptible power supplies,
variable frequency drives, radar power supplies, and
solid state ballasts in florescent light fixtures.
They cause distortion of the source voltage and
current waveforms that can have harmful effects on
many types of electrical equipment and electronics,
including generators. Non-linear loads are similar
to short circuits in that they provide momentary,
sub-cycle-duration, short-circuiting of two phases.
Switching power supplies consist of
SCR/thyristors-controlled rectifier bridges which
act as three single-phase loads, each connected
across two phases of the power system. When the
SCR/thyristors are switched on and off a notch in
the voltage waveform will occur as a result of an
instantaneous phase-phase short-circuit during the
commutation of current. A low generator
subtransient reactance minimizes the voltage
waveform distortion in the presence of such loads.
For this reason, when the non-linear loads comprise
25% or more of the loads served, the generator
subtransient reactance should be limited to more
than 0.12.
Generators are particularly vulnerable to control
problems and instability, excessive winding heating,
neutral overheating, reduced efficiency, reduced
torque, shaft fatigue, accelerated aging, and
induced mechanical oscillations when non-linear
loads are applied without careful consideration of
the generator's capability to supply them. Measures
which can be used to mitigate the effects of
non-linear loads on generators include: procurement
of low impedance generators with special windlings
to compensate for the additional heating;
self-excited generators; use of 2/3 pitch factor
(rather than 5/6 pitch) generator windlings; and
generator derating with oversized neutrals.
For large non-linear loads, filter traps which are
tuned to the dominant harmonic frequencies of the
non-linear loads should be procured/provided with
the load component. This approach is normally less
costly than procurement of specially designed or
derated generators.
For combinations of linear and non-linear loads
where the percentage of non-linear loads is small
relative to the capacity rating of the generator
(25% or less), standard generator configurations are
normally acceptable.
Provide a list of the non-linear loads in the
parameter schedule either on the drawings (and
denoted on the single-line diagram) or in tabular
form in the specification section. The list should
contain a description of the load including
equipment type, whether the rectifier is 6-pulse or
SECTION 16264A
Page 17
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