BASIC VARIANTS
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5
Figure 33. LCL-filter structure is best possible solution if output ripple is critical.
The optional capacitor forms an LCL-circuit with the cables and the battery. It is important to know
the cable/battery impedance/inductance in order to evaluate possible resonances and calculate a
safe capacitance value. The impedance of the battery seems to depend on many things so this is not
an easy task. The resonance frequency for the LCL-circuit can be calculated as:
Note that the inductance of the converter side choke is one third because of the parallel connection.
In addition, there can be resonance can happen between L1 and C or L2 and C:
All these resonance frequencies must be well below the switching frequency. In standard inverter
applications, the LCL-circuit resonance frequency is usually one third of the switching frequency.
Because equivalent switching frequency at output is three times bigger, in theory it would be
possible to use higher resonance frequencies. But as explained earlier, the switching frequency is
not totally eliminated at output because of non-idealities in the control and chokes.
Because battery properties are usually not well known, it is recommended to dimension the
optional capacitor as follows:
DC+
DC-
Battery
C
C
DC
L
1
L
2
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