Residual Current Device - Danfoss VLT AQUA Drive Design Manual

5. How to Install
The harmonics do not affect the power con-
sumption directly, but they do increase the
heat losses in the installation (transformer,
cables). Consequently, in plants with a high
percentage of rectifier load, maintain harmon-
ic currents at a low level to prevent an over-
load of the transformer and high temperature
in the cables.
5
Harmonic currents compared to the RMS input
current:
To ensure low harmonic currents, the adjustable frequency drive is standard-equipped with in-
termediate circuit coils. This normally reduces the input current I
The voltage distortion in the line supply depends on the size of the harmonic currents multiplied
by the line power impedance for the frequency in question. The total voltage distortion THD is
calculated on the basis of the individual voltage harmonics using this formula:
THD

5.10.1. Residual Current Device

You can use RCD relays, multiple protective grounding or grounding as extra protection, provided
that local safety regulations are complied with.
If a ground 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
Ground Leakage Current
112
NOTE
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.
2 2
U U
% = + + ... +
5 7
for further information.
MG.20.N2.22 - VLT
I
RMS
I
1
I
5
I
7
I
11-49
2
(U % of U)
N
U
N
® is a registered Danfoss trademark.
VLT ® AQUA Drive Design Guide
Input current
1.0
0.9
0.4
0.2
< 0.1
by 40%.
RMS