Application Note

26 Fluke Corporation Power Quality Troubleshooting
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 mo-
ments when two phases will
both be “ON.” This causes
what is in effect a momen-
tary short circuit that tends
to collapse 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 re-
actor coil or isolation trans-
former in series with the
drive’s front end to clean up
both problems.
•
Displacement Power Factor
declines as drive speed de-
creases. This is not as serious
a problem as it sounds, be-
cause the power requirement
of the drive-motor-load de-
creases even more.
•
Harmonic currents, typically
the 5th and 7th, are gener-
ated by VSI drives.
Diode Convertor with
Pulse Width Modulation
(PWM) Drives
The other and more common
converter design uses diodes
and is used in the PWM drive.
The diodes require no switching
control circuitry. One of the
main trends in the industry has
been the proliferation of PWM
drives, mainly due to the con-
tinued development of fast-
switching, efficient IGBTs
(Insulated Gate Bipolar Transis-
tors) used in the inverter section
of the drive (inverters turn dc to
ac). For all practical purposes,
PWM drives are the industry
standard.
For an in-depth discussion of
the efffects of PWM-IGBT drives
on motors, see “Measurement of
ASDs with Fluke Meters,” docu-
ment number G0416UEN.
For the diode converter, the
main PQ issue is harmonics. The
actual harmonic orders being
generated depend on the num-
ber of diodes in the front end.
For three-phase conversion, a
minimum set of six diodes is re-
quired. This “six-pulse” con-
verter will generate 5th and 7th
harmonics. If a 12-pulse con-
verter were used, the 11th and
13th harmonics will be gener-
ated instead of the 5th and
6th—and, very importantly, for
the same load, the amplitude of
the 11th and 13th 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 (Fig. 7.5)
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 har-
monic. Obviously, they must be
sized to the harmonic-generat-
ing 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 PCC (Point
of Common Coupling). The can-
cellation effect is optimal when
both loads are more or less
equal.
Figure 7.2 Voltage Notching.
Figure 7.3 Typical ASD Harmonic Spectrum
Figure 7.4. Pulse Width Modulated (PWM) ASD.