Design of Single-Phase PLL with DC Offset Rejection

To control grid-connected inverters, grid synchronization is essential, and a Phase-Locked Loop (PLL) technique is commonly applied for this purpose. Among various PLL techniques, the Second Order Generalized Integrator (SOGI)-PLL is widely used due to its robustness against external noise. However, when the grid voltage contains a DC offset, the SOGI-PLL exhibits steady-state error, which is a limitation. To address this issue, this paper applies the Cascade SOGI (CSOGI)-PLL, which has a robust characteristic against DC offsets. Applying the CSOGI-PLL requires designing the parameters of a fourth-order transfer function and tracking the grid frequency by adjusting the resonant frequency. In this paper, a simplified design method using a second-order system for the damping coefficient of the CSOGI with a complex transfer function is proposed. A separate PLL is employed to update the resonant frequency of the CSOGI in response to rapid frequency variations, and frequency limitation and anti-windup control are applied to ensure stability during transients. To validate the effectiveness of the proposed method, simulations and experiments were conducted.