Application Note

2 Fluke Corporation How adjustable speed drives affect power distribution
For the SCR converter, there
are three main issues that affect
line-side PQ:
•
Commutation notches. SCR
switching or commutation is
such that there are brief
moments when two phases
will both be “ON.” This causes
what is in effect a momentary
short circuit that tends to col-
lapse the line voltage. This
shows up as “notches” on the
voltage waveform. These
notches cause both high V-
THD and transients. The
solution is to place a reactor
coil or isolation transformer in
series with the drive’s front
end to clean up both problems.
•
Displacement Power Factor
declines as drive speed
decreases. This is not as seri-
ous a problem as it sounds,
because the power require-
ment of the drive-motor-load
decreases even more.
•
Harmonic currents, typically
the 5th and 7th, are generated
by VSI drives.
Diode convertor with
Pulse Width Modulation
(PWM) drives
The other and more common
c
onverter design uses diodes and
is used in the PWM drive. The
diodes require no switching con-
trol circuitry. One of the main
trends in the industry has been
the proliferation of PWM drives,
mainly due to the continued
development of fast-switching,
efficient IGBTs (Insulated Gate
Bipolar Transistors) used in the
inverter section of the drive
(inverters turn dc to ac). For all
practical purposes, PWM drives
are the industry standard.
For the diode converter, the
main PQ issue is harmonics. The
actual harmonic orders being
generated depend on the numb
er
of diodes in the front end. For
three-phase conversion, a mini-
mum set of six diodes is required
.
This “six-pulse” converter will
generate 5th and 7th harmonics.
If a 12-pulse converter were
used, the 11th and 13th harmon-
ics will be generated instead of
the 5th and 6th — and, very
importantly, for the same load,
the amplitude of the 11th and
1
3th would be considerably less
than the 5th and 6th. Therefore,
the THD would be less. The vast
majority of drives, however, are
six-pulse PWM style, which is
one reason we see so much 5th
harmonic on the system.
Harmonics solutions
There are a number of solutions
to mitigating drive-generated
harmonics:
Harmonic trap filters (Figure 5)
These are typically LC networks
connected in parallel at the
source of the harmonics (in other
words, at the drive input). They
are tuned to just below the 5th
harmonic (typically 280 Hz) and
will tend to sink both 5th and
much of the 7th harmonic.
Obviously, they must be sized to
the harmonic-generating load.
Phase-shift transformers
This can be as simple as a delta-
wye transformer feeding one
drive(s) and a delta-delta feeding
another drive(s). There is a 30
degree phase shift effect
between these two configura-
tions, which effectively results in
cancellation of harmonics at the
closest upstream P
CC (Point of
Common Coupling). The cancella-
tion effect is optimal when both
loads are more or less equal
.
Figure 2. Voltage notching.
Figure 3. Typical ASD harmonic waveform.
Figure 4. Pulse Width Modulated (PWM) ASD.