Design and optimization of LCL filters for grid-connected converters

This paper describes a method for designing LCL filters to be used in distributed generation systems. It is especially suitable for converters with low switching frequency and/or converters with modulation schemes generating harmonics in the entire frequency range. This property distinguishes the method from the well-known design processes. As an example, a 5 MW medium-voltage NPC converter for wind energy plants is regarded more detailed. The proposed method is based on the fact that the converter´s spectrum is known, for which a calculation approach is presented. The discussed design process enables a filter design meeting all requirements from the grid code concerning the voltage quality for all relevant operating ranges. In general, it is suitable for all converter topologies and modulation schemes generating periodical output voltages.