Properties and physical interpretation of the dynamic interactions between voltage source converters and grid: electrical oscillation and its stability control

Voltage source converters (VSCs) play an important role in the power conversion of renewable power generation (RPG) systems. Conventional power systems are considerably affected by power electronic devices in systems with a high percentage of RPG. Various abnormal interactions in the form of oscillations between VSCs and grid have been reported, whereas the mechanism at the core is still lacking understanding. In this study, the properties of interactions between the VSC and grid are investigated, and through an analytical model the mechanisms of electrical oscillations are revealed. First, a simple resistance-inductance-capacitance (RLC) equivalent to the small-signal model of VSC-grid system was derived based on the knowledge of virtual passive element effects of VSC. Then, an intrinsic oscillatory point in the RLC circuit was identified, the damping characteristics of the current controller and phase-locked-loop (PLL) at this point were analysed. Subsequently, a critical stability criterion for the determination of PLL bandwidth that may trigger oscillations was established. For the improvement of the overall damping, a VSC stabiliser was proposed. Finally, the mechanism analysis and analytical criteria were verified by time-domain simulations in PSCAD/EMTDC.