A novel method for detecting voltage sags is proposed for three-phase three-wire power systems. The detecting method which uses the 90°-leading-phase-shift operator is derived from instantaneous symmetrical components theory and is constructed on the stationary and orthogonalαβ reference frame. Detection of positive and negative components is decoupled. Sage-Husa Kalman filtering is used to generate in-quadrature signals and serves as a filter for fundamental component detection. Euclidean vector norm is adopted to locate voltage sag and reset Kalman filtering parameters. Simulation results show that the proposed method has advantages of fast response and high steady accuracy and verify the correctness and feasibility of the proposed method.