Mains Supply Interference/Harmonics; Residual Current Device - Danfoss VLT Series Design Manual

5. How to Install

5.8. Mains supply interference/Harmonics

5.8.1. Mains Supply Interference/Harmonics
A frequency converter takes up a non-sinus-
oidal current from mains, which increases the
input current I
transformed by means of a Fourier analysis
and split up into sine wave currents with dif-
ferent frequencies, i.e. different harmonic cur-
rents I
5
The harmonics do not affect the power con-
sumption directly but increase the heat losses
in the installation (transformer, cables). Con-
sequently, 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.
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 fre-
quency in question. The total voltage distor-
tion THD is calculated on the basis of the
individual voltage harmonics using this formu-
la:

5.9.1. 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
Earth Leakage Current
110
. A non-sinusoidal current is
RMS
with 50 Hz as the basic frequency:
N
NB!
Some of the harmonic currents might disturb communication equipment connected
to the same transformer or cause resonance in connection with power-factor cor-
rection batteries.
for further information.
MG.11.B2.02 - VLT
Harmonic currents I
Hz 50 Hz
I
RMS
I
1
I
5
I
7
I
11-49
by 40%.
RMS
2
THD U
% = +
5
®
is a registered Danfoss trademark
®
VLT HVAC Drive Design Guide
I I
1 5 7
250 Hz 350 Hz
Input current
1.0
0.9
0.4
0.2
< 0.1
(
2 2
U U U
+ ... +
N
N
7
)
of U
%