Design of LCL filters in consideration of parameter variations for grid-connected converters

In this paper an efficient design process of LCL filters for grid-connected converters is described. It is especially developed to design filters for converters with low switching frequency generating harmonics in the entire frequency range. In these special applications, some small parameter variations can cause the given limits, like the allowed voltage harmonics, to be exceeded. In the worst case, the resonance frequency of the filter can be excited to an unacceptable extent. To avoid this, the tolerances of the filter parameters, the grid voltage and the grid frequency are taken into account when designing the filter. This essential consideration of the parameter variations distinguishes this new method from the well-known design processes. As an example, a filter design for a 5 MW wind energy generator with full-scale three-level medium-voltage converter is regarded more detailed. Measurements on a scaled low-voltage laboratory setup validate the theoretical calculations and complete the examination. The discussed design process enables a filter design meeting all grid code requirements concerning the voltage quality for all relevant operating ranges in consideration of various parameter variations. The method is suitable for all converter topologies and modulation schemes generating periodical output voltages.