3-20
Engineering Guide CDA3000
3 Selection of inverter module
Additional comments on the IT system
For devices, machines and plant which need to be run in an IT system, an
isolating transformer must be fitted. The secondary side of the power
transformer must be configured as a TN system (TN-S with isolated
grounding lead).
Radio interference suppression filters must not be inserted/opera-
ted in the IT system. The IEC1800-3 standard stipulates that the fil-
ters cannot be used in this type of system because correct opera-
tion of the insulation monitoring cannot then be guaranteed.
3.2.2 Loading on the
supply system
All inverter systems draw a non-sinusoidal current from the system. This
is because of the 1/3-phase input rectifier in the inverter input. This non-
sinusoidal current consumption results in voltage distortions (THD=Total
Harmonic Distortion) in the system.
Depending on local conditions, line chokes may need to be inserted to
reduce the voltage distortions. A line choke reduces the voltage distortion
in the system by approx. 67%.
System load
Without line choke With line choke Change
4 kW inverter, line
impedance
0.6 mH
4 kW inverter, line
impedance
0.6 mH
Without line
choke to
with line
choke
Voltage distortion (THD) 99 % 33 % -67 %
Mains current amplitude 18.9 A 9.7 A -48 %
Mains current effective 8.5 A 6,23 A -27 %
Commutation notches
referred to the mains volt-
age
28 V 8 V -70 %
Life of the DC-link capaci-
tors
Nominal life
2 to 3 times nomi-
nal life
+200 to 300 %
Table 3.15 Change in system load resulting from insertion of a line choke
with 4 % short-circuit voltage based on the example of a 4 kW
inverter CDA34.010
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