Chapter 2—Engineering Considerations
DG1 Series VFD MN040002EN—March 2014 www.eaton.com 11
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 120%.
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 DG1 Series drives, which take on very little
reactive power of the fundamental harmonics from the AC
power supply network (cosw ~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.
fundamental component
;
U
U
UU
U
U
U
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3
100%
+
—
—
++
THD
n
n
order harmonic component
n
th
34
2
1
1
222
2
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