Control Design of Grid Synchronization Systems for Grid-Tied Power Converters Using Symmetrical Optimum Method: A Comprehensive Reference

Control design of advanced grid synchronization systems is complex as they are often adaptive nonlinear feedback systems with one or more filtering networks and feedback loops in their structure. This article intends to contribute to addressing this issue. It is shown that the phase or frequency estimation loop of many advanced grid synchronization systems, including $3\phi$ and $1\phi$ phase-locked loops and frequency-locked loops equipped with in-loop filters and prefilters, can be well-Approximated by a standard type-2 control system in the low-frequency range and an extension of the symmetrical optimum (SO) approach could be used to efficiently tune their control parameters. To better understand all this, an overview of the SO method and its extended version is provided and several case studies are presented. The limitations of this design approach and possible solutions are also discussed.