This paper proposes a new three-phase positive sequence detector. The scheme is based on a stationaryreference frame and a Moving Average Filter (MAF) that guarantees the complete cancellation of harmonicsand grid imbalances. The performance of the MAF is mathematically analyzed and a proper selectionof the optimal filter’s window width is realized. The proposed detector operates in open loop andthere is no PI controller to be tuned. Thus, the dynamic response and simplicity is improved compared toother solutions. Performance of the proposed detector is verified through simulation and experiment. Itshows very good performance under extreme grid voltage conditions, allowing fast detection of the gridvoltage positive sequence (within one grid voltage cycle).