2 Engineering
2.2 Electrical power network
30 DC1 variable frequency drive 12/13 MN04020003Z-EN www.eaton.com
2.2.4 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 which 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.
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.
2.2.5 Idle power compensation devices
Compensation on the power supply side is not required for the variable
frequency drives of the DC1 series. From the AC power supply network
they only take on very little reactive power of the fundamental harmonics
(cos ~ 0.98).
￫
In the project planning for the connection of three-phase
supplied variable frequency drives (DC1-3…), consider only AC
supply systems that handle permitted asymmetric divergences
in the mains voltage +3 %.
If this condition is not met or if the unbalance at the connection
location is not known, it is recommended to use a recom-
mended mains choke (please refer to the annex, "Mains chokes"
section, page 214).
￫
In order to reduce THD values (up to 30%), it is recommended
to use a recommended mains choke (please refer to the annex,
"Mains chokes" section, page 214).
￫
In the AC supply systems with non-choked reactive current
compensation devices, current deviations can enable parallel
resonance and undefinable circumstances.
In the project planning for the connection of variable frequency
drives to AC supply systems with undefined circumstances,
consider using main chokes.
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