Danfoss VLT AutomationDrive FC 301, Руководство по конструкции, Страница: 55 / 200

2e Make sure that the impedance from
frequency converter to building ground
is lower that the grounding impedance
of the machine. This can be difficult for
pumps.
2f Make a direct ground connection
between the motor and load motor.
3. Lower the IGBT switching frequency.
4. Modify the inverter waveform, 60° AVM vs.
SFAVM.
5. Install a shaft grounding system or use an
isolating coupling.
6. Apply conductive lubrication.
7. Use minimum speed settings if possible.
8. Try to ensure the line voltage is balanced to
ground. This can be difficult for IT, TT, TN-CS or
Grounded leg systems.
9. Use a dU/dt or sinus filter.
5.3 Mains Supply Interference/Harmonics
A frequency converter takes up a non-sinusoidal current
from mains, which increases the input current I
RMS
. A non-
sinusoidal current is transformed by means of a Fourier
analysis and split up into sine-wave currents with different
frequencies, i.e. different harmonic currents I
N
with 50 Hz
as the basic frequency.
Harmonic currents I
1
I
5
I
7
Hz 50 250 350
Table 5.7 Transformed Non-sinusoidal Current
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.
175HA034.10
Illustration 5.4 Intermediate Circuit Coils
NOTICE
Some of the harmonic currents might disturb communi-
cation equipment connected to the same transformer or
cause resonance in connection with power-factor
correction units.
Input current
I
RMS
1.0
I
1
0.9
I
5
0.4
I
7
0.2
I
11-49
< 0.1
Table 5.8 Harmonic Currents Compared to the RMS Input
Current
To ensure low harmonic currents, the frequency converter
is equipped with intermediate circuit coils as standard. DC-
coils reduce the total harmonic distortion (THD) to 40%.
5.3.1 The Effect of Harmonics in a Power
Distribution System
In Illustration 5.5 a transformer is connected on the primary
side to a point of common coupling PCC1, on the medium
voltage supply. The transformer has an impedance Z
xfr
and
feeds a number of loads. The point of common coupling
where all loads are connected together is PCC2. Each load
is connected through cables that have an impedance Z
1
,
Z
2
, Z
3
.
Illustration 5.5 Small Distribution System
Harmonic currents drawn by non-linear loads cause
distortion of the voltage because of the voltage drop on
the impedances of the distribution system. Higher
impedances result in higher levels of voltage distortion.
Current distortion relates to apparatus performance and it
relates to the individual load. Voltage distortion relates to
system performance. It is not possible to determine the
voltage distortion in the PCC knowing only the load’s
harmonic performance. To predict the distortion in the
System Integration
VLT
®
AutomationDrive FC 301/FC 302 Design Guide, 0.25-75 kW
MG33BF02 - Rev. 2013-12-20 53
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