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Figure 2: In general, unbalanced currents can be resolved into
a-b-c (positive sequence), a-c-b (negative sequence) and a0-
b0-c0 zero sequence components.
BY
PHIL
KREVELD
I
a
I
c
I
b
I
a-
I
b-
I
c-
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a+
i
b+
i
c+
Thyristor-based converters in variable-speed drives for
fans, chillers, compressors, etc, have lower power factors than
insulated gate bipolar transistor (IGBT) and gate turn-off (GTO)
thyristor switched converters. The latter two have a power
factor very close to 1.00.
Phase-fired thyristor equipment – used in heating, gluing,
heat treatment, etc – has a poor displacement power factor.
Phase firing at 30° (1.7 milliseconds from the zero-crossover)
produces a displacement power factor of 0.78. The harmonics
content for the current is close to 50%.
Harmonics further reduce the total power factor but have
no bearing on displacement power factor.
HARMONIC FILTERING
It’s often assumed that harmonic filtering improves power
factor, but this is true only for total power factor.
Many active harmonic filters provide limited displacement
power factor correction. However, their total kVAr capacity
invariably has to be apportioned between harmonics
mitigation and displacement power factor correction, thus
limiting one or the other capacity range.
In some installations, capacitor-based correction equipment
is combined with active filtering. In such cases, care must be
taken to place the capacitor bank at the incomer, and not on
the bus feeding the harmonics-generating load.
THE STATCOM
The equipment type referred to here is properly called a
static synchronous compensator.
It is often referred to as a STATCOM, and a simplified
schematic is shown in Figure 1.
The circuit looks a lot like those employed in active harmonic
filters, but in this case the control scheme is different in
important ways.
i
20
i
60
i
co
PQSine
™
Active Harmonic Filter for
Ideal Power Quality
Product highlights
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For sales and inquiries from Australia
and New Zealand please contact:
PQSine@ramelec.com.auor phone 1300 309 303
www.epcos.com/pqsineWall-mounted design Floor-mounted design