Mains Supply Interference/Harmonics; Residual Current Device - Danfoss VLT AutomationDrive FC 300 Design Manual

FC 300 Design Guide

Mains Supply Interference/Harmonics

"
A frequency converter takes up a non-sinusoidal
current from mains, which increases the
input current I . A non-sinusoidal current is
RMS
transformed by means of a Fourier analysis and
split up into sine wave currents with different
frequencies, i.e. different harmonic currents I
with 50 Hz as the basic frequency:
N
The harmonics do not affect the power consumption
directly but increase the heat losses in the
installation (transformer, cables). Consequently,
in plants with a high percentage of rectifier
load, maintain harmonic currents at a low
level to avoid overload of the transformer and
high temperature in the cables.
NB!:
Some of the harmonic currents might disturb communication equipment connected to the same
transformer or cause resonance in connection with power-factor correction batteries.
Harmonic currents compared to the RMS
input current:
To ensure low harmonic currents, the frequency converter is equipped with intermediate circuit
coils as standard. This normally reduces the input current I
The voltage distortion on the mains supply depends
on the size of the harmonic currents multiplied
by the mains impedance for the frequency in
question. The total voltage distortion THD is
calculated on the basis of the individual voltage
harmonics using this formula:

Residual Current Device

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You can use RCD relays, multiple protective earthing or earthing as extra protection,
provided that local safety regulations are complied with.
If an earth fault appears, a DC content may develop in the faulty current.
If RCD relays are used, you must observe local regulations. Relays must be suitable for
protection of 3-phase equipment with a bridge rectifier and for a brief discharge on power-up
see section Earth Leakage Current for further information.
How to Install
MG.33.B4.02 - VLT is a registered Danfoss trademark
Harmonic currents I
1
Hz 50 Hz
I
RMS
I
1
I
5
I
7
I
11-49
by 40%.
RMS
I I
5 7
250 Hz 350 Hz
Input current
1.0
0.9
0.4
0.2
< 0.1
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