Non-linear Loads: Non-linear loads are addressed in
IEEE Std 519. They are loads that draw a
non-sinusoidal current waveform when supplied by a
sinusoidal voltage source. Typical non-linear loads
include solid state switching power supplies,
computer power supplies (including those found in
desktop PC's, uninterruptible power supplies,
variable frequency drives, radar power supplies, and
solid state ballasts in fluorescent 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/thyristor-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 no more
than 0.12.
Delete Subtransient Reactance from the
Engine-Generator Parameter Schedule where the genset
manufacturer is responsible for sizing the generator
breaker and where the non-linear loads served are
less than 25%.
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 windings to
compensate for the additional heating; installation
generators; use of 2/3 pitch factor (rather than 5/6
pitch) generator windings; 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.
SECTION 16263A
Page 21
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