According to the control requirements of the low voltage ride-through for photovoltaic power generation, it is very important to study the fast and accurate detecting method of the voltage under the unbalanced grid voltage. This paper analyzes the different working principles of frequency locked loop technology. Meanwhile, simulation models of dual second order generalized integrator frequency locked loop (DSOGI-FLL), cascaded second order generalized integrator frequency locked loop (Cascaded-SOGI-FLL)and reduced order resonant regulator frequency locked loop (ROR-FLL) are established by using Matlab/Simulink platform. By simulating the imbalanced grid voltage situation, this paper compares the simulation results of three kinds of frequency locked loop technology. The results show that the DSOGI-FLL and ROR-FLL methods have fast response speed and can adapt to the situation of the grid voltage drop and frequency change, the Cascaded-SOGI-FLL has relative slow response speed and can adapt to the situation of the grid voltage existing harmonic. This work is supported by National Natural Science Foundation of China (No. 61203224).