Aiming　at　the　problem　of　detecting　three-phase　voltage　sag　quickly　and　accurately,　a　novel　decoupled　detection　method　with　steady-state　and　compensation　components　was　presented.　Firstly,　a　decoupled　model　for　fundamental　voltage　positive　and　negative　sequence　components　was　built　on　the　stationary　and　orthogonal　αβ　reference　frame　based　on　instantaneous　symmetrical　components;　according　to　the　presented　model,　the　Kalman　filter　with　steady-state　and　compensation　components　was　constructed.　Afterwards,　mathematical　morphological　filter　and　grille　fractal　were　adopted　to　detect　voltage　step　change,　and　then,　reset　the　parameters　of　Kalman　filter　to　accelerate　transient　response.　Finally,　the　proposed　method　was　used　for　different　faults　detection　and　DVR　compensation.　Simulation　results　show　that　the　response　time　is　just　2~3　sample　times,　ten　times　faster　than　dq　method,　and　verify　that　the　proposed　method　has　advantages　of　detection　response　time　and　accuracy.　©　2012　Chin.　Soc.　for　Elec.　Eng.