Operating Principle Of Line Filters; Magnitude Of Leakage Or Interference Currents - Siemens SINAMICS G130 Engineering Manual

Operating principle of line filters

High-frequency interference in the variable-speed drive system is caused by the IGBTs (Insulated Gate Bipolar
Transistors) switching at high speed in the motor-side inverter of the converter unit. These switching operations
produce very high rates of voltage rise dv/dt. For further information about this phenomenon, please refer to the
section "Effects of using fast-switching power components".
The high rates of voltage rise at the inverter output generate large, high-frequency leakage currents which flow to
ground across the capacitance of the motor cable and motor winding. These must return via a suitable path to their
source, i.e. the inverter. When shielded motor cables are used, the high-frequency leakage or interference currents
I pass via the shield to the PE busbar or the EMC shield busbar in the cabinet.
Leak
If the cabinet or the chassis unit would not contain filters of any type which could offer this high-frequency
interference current a low-resistance return path to the inverter, then all the interference current would flow via the
line-side PE connection of the cabinet to the transformer neutral (I
(rectifier) via the three phases of the three-phase supply. If this were the case, the interference current would
superimpose high-frequency interference voltages on the line voltage and thus influence or even destroy other loads
connected to the same point of common coupling as the cabinet itself. The interference at the connection point would
match the level defined by category C4.
Due to the line filter which is a standard feature of SINAMICS Chassis the high-frequency interference current gets a
low-resistance return path to its source so that a high percentage of the interference current I
inside the chassis unit. As a result, the supply system is loaded with lower interference currents I
interference level at the point of common coupling drops to the level of category C3.
If the optional line filter is installed in the cabinet in addition to the standard line filter fitted to SINAMICS Chassis,
almost all the interference current I
supply is reduced still further (I
I
without Line filter (category C4)
PE
High-frequency interference current at the line-side PE connection as a function of line filters

Magnitude of leakage or interference currents

The magnitude of the high-frequency leakage currents depends on a large number of drive parameters. The most
important influencing factors are:
•
Level of the line voltage V
•
Rate of voltage rise dv/dt produced by fast-switching IGBTs in the inverter,
•
Pulse frequency f
•
Converter output with or without motor reactor or motor filter,
•
Impedance Z
W
•
Inductance of grounding system and all ground and shield connections.
The inductance values of the grounding system and the exact grounding conditions are normally not known so it is very
difficult in practice to precisely calculate the leakage currents that are likely to occur. It is however possible to work out
the theoretical maximum values of the leakage current I
grounding system inductance is negligible and the line filter action is ideal. In this case, the peak value of leakage
current î can be calculated as follows from the DC link voltage V
Leak
Fundamental Principles and System Description
is diverted before it can exit the drive system and the load on the power
Leak
<< I ), i.e. the interference level drops to the value defined for category C2.
PE Leak
I
with Line filter (category C3)
PE
or the DC link voltage V
Line
of the inverter,
P
or capacitance C of motor cable,
V
=
DCLink
î
Leak
Z
W
= I ) and from there back to the converter
PE Leak
of the converter
DCLink
carried by the motor cable shield if we assume that the
Leak
and the impedance Z
DCLink
.
SINAMICS Engineering Manual - May 2008
Engineering Information
can flow via the filter
Leak
< I
PE Leak
I
with Line filter (category C2)
PE
,
of the motor cable:
W
57/396
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and the