High Performance Stationary Frame Filters for Symmetrical Sequences or Harmonics Separation Under a Variety of Grid Conditions

This paper proposes a group of high performance filters for fundamental positive / negative sequences and harmonics detection under varied grid conditions based on a basic filter cell. The filter cell is demonstrated to be equivalent to a band-pass filter in the stationary frame, and is easily implemented using a multi-state-variable structure. To achieve high performance in different grid conditions, cascaded filters are developed for distorted and unbalanced grids. This paper also analyzes the robustness of the filter for small frequency variations, and improves its frequency-adaptive ability for large frequency changes. Furthermore, it is proved that this filter can also be applied for the synchronization in a single-phase system. Considering the digital implementation of the filter, four discretization methods and the resulting limitations are investigated. The effectiveness of the presented filters is verified by experiments.