A medium voltage Solid-State Transfer Switch (SSTS) has been designed in this paper. A dq transformation based method with both timeliness and reliability is deployed to detect voltage sag, by subtracting the AC components from the dq values of voltages, the RMS value of the remaining part is compared with a pre-set voltage threshold to generate transfer command signal. BBM and MBB switching strategies are compared and analyzed, and MBB switching strategy, which triggers thyristor according to the current direction on primary feeder and voltage drop direction between preferred and alternate sources, is utilized to shorten the transfer time, so sensitive loads can be transferred to alternate source very fast. A 10kV SSTS simulation model is established using PSCAD and simulation results verify the instantaneity and reliability of dq voltage detection method and the effectiveness and feasibility of MBB switching strategy