A fast synchronous reference frame (SRF)-based sag/swell detection method for static transfer switches (STSs) is proposed. In the proposed method, the positive sequence d-q components are passed through a differentiator The resulting values are then crisscrossed with the positive sequence d-q components and calculated to extract the original positive sequence components. The obtained original positive sequence d component is used as an ideal error signal to generate the transfer signal. This improves the response time of sag/swell detection with respect to conventional methods. Simulation studies using the PSCAD/EMTDC software program confirm the improvement.