Residual Current Device - GE AF-650 GP Design Manual

7.10.1 Mains Supply Interference/Harmonics
A frequency converter takes up a non-sinusoidal current from mains, which
increases the input current I
RMS
means of a Fourier analysis and split up into sine-wave currents with different
frequencies, i.e. different harmonic currents I
quency:
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.
7
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
40%.
The voltage distortion on the mains supply depends on the size of the har-
monic 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:

7.11.1 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 Earth Leakage Current for further information.
156
. A non-sinusoidal current is transformed by
with 50 Hz as the basic fre-
N
Harmonic currents I
1
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
AF-650 GP Design Guide
I I
5 7
250 Hz 350 Hz
Input current
1.0
0.9
0.4
0.2
< 0.1
by
RMS
2