PLL algorithm for power generation systems robust to grid voltage faults

In the case of unbalanced grid faults, one of the main problems for the Distributed Power Generation Systems (DPGSs) controller is the second harmonic ripple in the control, consequence of the phase unbalance caused by the fault. The second harmonic propagates into different sections of the controller and can have a negative influence on the controller even leading to trip out of the system. This paper proposes a novel Phase-Looked Loop (PLL) algorithm which is able to filter out the negative sequence and to provide a clean synchronization signal. Along with the conventional PI controller in the PLL structure, the proposed algorithm employs a repetitive controller to deal with the second harmonic. The purpose of the repetitive controller is to amplify the second harmonic in the reference of the PI controller, so that this can be better rejected. Simulation and experimental results are used to validate the robustness of the proposed system when running on grid faults.