Application Note

Power Quality Troubleshooting Fluke Corporation 25
AC ASDs can be both a source
and a victim of poor PQ (see
“Measurement of Adjustable
Speed Drives with Fluke
Meters,” document number
G0416UEN, for more information
on drive troubleshooting).
ASDs as Victim Loads
Although ASDs are usually
depicted as the culprit in the
PQ scenario, there are ways
in which they can be a victim
load as well.
Capacitor switching transients
High-energy (relatively low-
frequency) transients that are
characteristic of utility capacitor
switching can pass through the
service transformer, feeders,
and converter front-end of the
drive directly to the dc link bus,
where it will often cause a dc
link overvoltage trip. Input di-
odes could also be blown out
by these transients.
Voltage distortion
If high-voltage distortion shows
up as excessive flat-topping, it
will prevent dc link capacitors
from charging fully and will
diminish the ride-through
capability of the drive. Thus a
voltage sag which would not
normally affect a drive will
cause the drive to trip on
undervoltage.
Improper grounding will affect
the internal control circuits of
the drive, with unpredictable
results.
ASDs as Culprit Loads
A drive can definitely be a “cul-
prit load” and have a major im-
pact on system PQ. But before
we talk of problems, let’s put in
a good word for the positive ef-
fects of drives on PQ. First of all,
they offer built-in soft-start ca-
pabilities. This means there will
Section 7
PQ Troubleshooting of Adjustable Speed Drives
be no inrush current and no
voltage sag effect on the rest of
the system. Secondly, if the
drive is of the PWM type, with a
diode converter front-end, the
Displacement Power Factor is
high (commonly >95% at rated
load) and more or less constant
throughout the range. This
means that drives can reduce
energy usage and correct for
DPF at the same time. It’s a
good thing too, because drives
and PF correction capacitors
don’t mix (see “Power System
Resonance,” page 28). Caps are
vulnerable to the higher fre-
quency harmonic currents gen-
erated by drives, since their
impedance decreases as fre-
quency increases.
The type of drive has a major
impact on the PQ symptoms, be-
cause of the different converter
designs (converters or rectifiers
turn ac to dc and are the first
stage of the drive). There are
two major types of converter
design.
SCR Convertor with Voltage
Source Inverter/Variable
Voltage Inverter (VSI/VVI)
Drives
Commonly called six-step
drives, they use SCRs (Silicon -
Controlled Rectifiers) in their
converter front-ends (the fol-
lowing discussion applies to
CSI, Current Source Inverter
drives, which also use SCRs).
VSI and CSI drive designs
tended to be applied on larger
drives (>100HP). SCR converters
control the dc link voltage by
switching on (or “gating”) cur-
rent flow for a portion of the ap-
plied sine wave and switching
off at the zero-crossing points.
Unlike diodes, SCRs require
control circuits for gate firing.
Table 7.1 Line-side measurements on ASDs.
Measurement Look for Instrument
Voltage waveform
•
Voltage notching (SCR converters) 43
•
Flat-topping
Harmonic spectrum Harmonic orders and amplitudes, before and 43, 41B
after filter application
Displacement PF For PWM drives, DPF should remain high even 43, 41B
at low speeds (it will typically decrease slightly)
Voltage unbalance Less than drive manufacturer specs, or current 43, 41B
overload trips can result. The drive may have a
higher limit for unbalance than the motor.
Figure 7.1 Voltage Source Invertor (VSI) ASD
Induction
Motor
M
Line
Reactors