Residual Current Device - Danfoss FC 300 Design Manual

FC 300 Design Guide
8.10.1 Mains Supply Interference/Harmonics
A frequency converter takes up a non-sinusoidal current from mains,
which increases the input current I
RMS
formed 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:
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 cur-
rents 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 by 40%.
RMS
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:

8.11.1 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
. A non-sinusoidal current is trans-
N
Earth Leakage Current
for further information.
MG.33.BC.02 - VLT ® is a registered Danfoss trademark
Harmonic currents I
Hz 50 Hz
I
RMS
I
1
I
5
I
7
I
11-49
2 + 2 + ... +
THD % = U U U N
5 7
(U % of U)
N
8 Electrical Installation
I I
1 5 7
250 Hz 350 Hz
Input current
1.0
0.9
0.4
0.2
< 0.1
2
8
193