The　existence　of　interference　signals,　such　as　decaying　direct　current　(DDC)　components　and　high-order　harmonics,　makes　certain　errors　in　detecting　the　amplitudes　and　phases　of　fundamental　frequency　component　in　the　distortion　signals　of　the　power　grid.　The　time　constant　of　the　DDC　component　usually　exceeds　45　ms,　and　its　duration　is　long.　To　solve　this　problem,　this　paper　proposes　a　half-cycle　four-point　sampling　detection　algorithm　for　DDC　components　in　distorted　signals.　The　proposed　algorithm　can　greatly　shorten　the　detection　response　time　for　the　DDC　component.　For　the　distorted　signals　that　contain　both　DDC　components　and　high-order　harmonics,　the　above　DDC　component　detection　algorithm　is　combined　with　the　high-order　harmonic　detection　algorithm　in　the　dq　coordinate　system　after　the　half-cycle　integration　of　the　distorted　signals.　The　influence　of　interference　signal　can　be　filtered　out　at　the　same　time　in　a　half-cycle　of　the　power　frequency,　and　the　fundamental-frequency　component　in　the　distorted　signal　can　be　detected　accurately.　Finally,　a　semi-physical　real-time　simulation　model　in　MATLAB/Simulink　is　built　to　verify　the　effectiveness　of　the　proposed　algorithm　in　terms　of　detection　accuracy　and　response　time.　©　2021　Automation　of　Electric　Power　Systems　Press.