5
vacon • 31
BASIC VARIANTS
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The filter consists of three separate chokes, one for each leg (Figure 31). It is not possible to use a
traditional 3-phase choke because the current is DC (common mode) and the magnetic flux does
not have a natural return path in the core structure. There would be only a very small inductance
generated by stray flux and this kind of situation can lead to a burnt filter. That is why a 3 x 1-phase
structure is necessary. In addition, the stray capacitance should be small. Foil winding with many
overlapping turns is not recommended. One way to minimize the stray capacitance is to use wire
winding in one layer.
Figure 31. Simple filter consist of three separate chokes.
The target is that the filtering could be done with chokes only. That is a simple solution, and one
benefit of not having capacitors is that switching the battery/supercapacitor is possible without any
current spikes. When IGBTs are disabled, the connection requires only that the DC-link is higher or
equal. If the output current ripple requirement is low and the required inductance would lead to an
impractical design, one possibility is to use an optional filtering capacitor (Figure 32). The best case
would be to add one more choke to form an LCL-filter structure (Figure 33). Without interleaving,
an LCL-filter is recommended, otherwise ripple will be big (nine times bigger).
Figure 32. Optional capacitor (C) connected to filter output.
DC+
DC-
Battery
C
DC
L
1
DC+
DC-
Battery
C
DC
L
1
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