PLL-Induced Modal Resonance of Grid-Connected PMSGs With the Power System Electromechanical Oscillation Modes

This study investigates the impact of a grid-connected permanent magnet synchronous generator (PMSG) for wind power generation on the power system electromechanical oscillation modes (EOMs) caused by a phase-locked loop (PLL). A closed-loop interconnected model of a power system with the PMSG is established, where the PLL and rest of the power system are modeled as two separate open-loop interconnected subsystems. Analysis based on the established model indicates that, when an open-loop PLL oscillation mode (POM) is close to an open-loop EOM of concern on the complex plane, the dynamic interactions between the PMSG and the power system may become strong. It is very likely that the modal resonance due to the closeness of the open-loop POM and EOM may degrade the small-signal stability of the power system. In this paper, case studies of an example power system with PMSGs are presented. It is demonstrated that open-loop modal resonance occurs when power system operating conditions change and the PLL parameters are tuned. Strong dynamic interactions between the PMSGs and power system are caused by the open-loop modal resonance, resulting in considerable damping degradation of the low-frequency electromechanical oscillations.