Control strategy under unbalanced grid voltage conditions is one of the most important issues for grid integration of high penetration photovoltaic (PV) systems. The output power flow of PV systems is analyzed and the reference instruction of grid-connected current is deduced. Module value detection method is developed, which is a new method for detecting the positive and negative sequence components. The method realizes the detection of the positive and negative sequence components in αβ coordination based on mathematical relationship between the module value of the space vector and that of the positive sequence vector and negative sequence vector. It has the advantages of clear geometrical concept and low calculation cost. Finally, the simulation is carried out under the unbalanced grid voltage conditions. The simulation results verify the effectiveness of the proposed method.