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 trans-
RMS
formed by means of a Fourier analysis and split up
into sine-wave currents with different frequen-
cies, i.e. different harmonic currents I
as the basic frequency:
The harmonics do not affect the power consump-
tion 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 de-
pends on the size of the harmonic currents multi-
plied by the mains impedance for the frequency in
question. The total voltage distortion THD is cal-
culated on the basis of the individual voltage har-
monics using this formula:

Residual Current Device

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.
108
How to Install
Harmonic currents
Hz
with 50 Hz
N
I
RMS
I
1
I
5
I
7
I
11-49
by 40%.
RMS
THD
%
MG.33.B8.02 - VLT ® is a registered Danfoss trademark
I
1
50 Hz 250 Hz
Input current
2 2
U U
= + + ... +
5 7
I I
5 7
350 Hz
1.0
0.9
0.4
0.2
< 0.1
( )
2
U U of U
%
N
N