With the increasing penetration of grid-connected photovoltaic power stations, the fault currents of distribution grids are altered greatly. As a result, there is a risk of incorrect operation for traditional differential current protection that is widely used for dedicated power distribution lines connected to photovoltaic power stations. Aimed at the problems, a novel protection strategy based on the active injection method is proposed. A typical characteristic harmonic current is added into the transient control loop of grid-connected inverter applied for photovoltaic power station after the grid faults. Based on the consideration of the inverter's control stability, both the harmonic current amplitude and its frequency are selected so as to always exists a characteristic harmonic current that is always equal to 0.1 times the rated current of photovoltaic power stations. At the same time, the initial phase angle of harmonic voltage is chosen in order to ensure the correct actions of proposed protection strategy. On this basis, a novel protection criterion is proposed based on the amplitude ratio of harmonic currents and the cosine of the phase angle difference between the fundamental current and the aforementioned harmonic current. The proposed coordinated control and protection strategy is verified for the active distribution grids connected with photovoltaic power stations based on PSCAD / EMTDC.