A Fixed-Frame Positive-Sequence Extractor: an Alternative to the MAF-Based Park-Filter.

Park-filters based on moving average filters (MAF) are a popular technique to extract the fundamental positive sequence component with zero-phase-shift from electrical three-phase signals, which is used to build references for power converters' controllers, as well as to help in grid synchronization tasks. Their implementation is, however, computationally burdensome, as it requires mapping the signals from the alpha beta-fixed reference frame coordinates to the synchronous frame coordinates, where they are processed using the moving average filter, and the result is finally transformed back to alpha beta-fixed reference frame coordinates. To overcome this issue, the present letter proposes and experimentally tests an alternative filter that directly operates on the alpha beta-fixed reference frame, thus avoiding the Park transformations. This considerably reduces the computational burden to almost one fifth. The proposed filter comprises a comb-filter cascaded with a second order harmonic oscillator. The proposed filter, referred to as CF-SOHO, guarantees the extraction of the fundamental positive sequence component with unitary gain and without phase-shift from unbalanced and harmonic-distorted signals, i.e., all harmonics (including DC components) contained in the incoming signals are fully suppressed. It can also cope with typical electrical stationary and dynamic disturbances.