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　orthogonal　αβ　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.　©　2012　State　Grid　Electric　Power　Research　Institute　Press.