Accurate　and　real-time　detection　of　compensated　components　plays　an　important　role　in　compensation　results　of　active　power　filter.　Under　the　complicated　condition　of　unsymmetrical　and　distorted　three-phase　system　voltage　with　frequency　fluctuation,　a　novel　fundamental　positive-sequence　active　current　detection　method　based　on　series　second　order　generalized　integrator　(S-SOGI)　is　proposed.　Moreover　the　S-SOGI　has　three　functions　of　phase-shift,　filtering　and　assisted　frequency　locking.　Compared　with　normal　dq　transformation　detection　method,　the　novel　method　is　exposed　on　αβ　reference　frame　without　PLL　and　LPF,　and　the　fundamental　positive-sequence　active　current　components　can　be　obtained　exactly　and　quickly.　At　last　the　simulation　results　show　the　efficiency　and　accuracy　of　the　improved　method　under　Matlab/Simulink.