Eaton PowerXL DG1 - Total Harmonic Distortion (THD); Reactive Power Compensation Devices

17
Chapter 2—Engineering considerations
POWERXL SERIES VFD MN040002EN—February 2018 www.eaton.com
Total harmonic distortion (THD)
Non-linear consumers (loads) in an AC supply system
produce harmonic voltages that again result in harmonic
currents. These harmonic currents at the inductive and
capacitive reactances of a mains supply system produce
additional voltage drops with different values that are
then overlaid on the sinusoidal mains voltage and result
in distortions. In supply systems, this form of “noise” can
give rise to problems in an installation if the sum of the
harmonics exceeds certain limit values.
Non-linear consumers (harmonics producers) include
forexample:
•
Induction and arc furnaces, welding devices
•
Current converters, rectifiers and inverters, soft starters,
variable frequency drives
•
Switched-mode power supply units (computers, monitors,
lighting), uninterrupted power supply (UPS)
The THD value (THD = Total Harmonic Distortion) is
defined in standard IEC/EN 61800-3 as the ratio of the
rms value of all harmonic components to the rms value
of the fundamental frequency. It is given in percent of the
totalvalue.
The THD value of the harmonic distortion is stated in
relation to the rms value of the total signal as a percentage.
On a variable frequency drive, the total harmonic distortion
is around 28–36%.
To assist in the calculation of system harmonics, a
HarmonicEstimation Calculator Tool is available at
www.eaton.com/drives.
Reactive power compensation devices
Special compensation measures on the power supply side
is not required for PowerXL Series drives, which take on
very little reactive power of the fundamental harmonics
from the AC power supply network (cosØ ~0.98).
In the AC power networks with non-choked reactive current
compensation devices, current deviations can enable parallel
resonance and undefinable circumstances.
In the project planning for the connection of frequency
inverters to AC power networks with undefined
circumstances, please consider using AC chokes.