Specifications

Catalog WSC/WDC-4 43
VFD Line Harmonics
Care must be taken when applying VFDs due to the effect of line harmonics on the electric system.
VFDs cause distortion of the AC line because they are nonlinear loads; that is, they don't draw
sinusoidal current from the line. They draw their current from only the peaks of the AC line, thereby
flattening the top of the voltage waveform. Some other nonlinear loads are electronic ballasts and
uninterruptible power supplies.
Line harmonics and their associated distortion may be critical to AC drive users for three reasons:
1. Current harmonics can cause additional heating to transformers, conductors and switchgear.
2. Voltage harmonics upset the smooth voltage sinusoidal waveform.
3. High-frequency components of voltage distortion can interfere with signals transmitted on the AC
line for some control systems.
The harmonics of concern are the 5
th
, 7
th
, 11
th
, and 13
th
. Even harmonics, harmonics divisible by
three, and high magnitude harmonics are usually not a problem.
Current Harmonics
An increase in reactive impedance in front of the VFD helps reduce the harmonic currents. Reactive
impedance can be added in the following ways:
1. Mount the drive far from the source transformer.
2. Add line reactors.
3. Use an isolation transformer.
Voltage Harmonics
Voltage distortion is caused by the flow of harmonic currents through a source impedance. A
reduction in source impedance to the point of common coupling (PCC) will result in a reduction in
voltage harmonics. This may be done in the following ways:
1. Keep the PCC as far from the drives (close to the power source) as possible.
2. Increase the size (decrease the impedance) of the source transformer.
3. Increase the capacity (decrease the impedance) of the busway or cables from the source to the
PCC.
4. Make sure that added reactance is downstream (closer to the VFD than the source) from the PCC.
The IEEE 519-1991 Standard
The Institute of Electrical and Electronics Engineers (IEEE) has developed a standard that defines
acceptable limits of system current and voltage distortion. A simple form is available from McQuay
that allows McQuay to determine compliance with IEEE 519-1991.
Line reactors, isolation transformers, or phase-shifting transformers may be required on some
installations.