Modeling and analysis of grid-synchronizing stability of a Type-IV wind turbine under grid faults

Abstract

Transient behavior and response of wind turbines have been extensively studied in the context of the low-voltage-ride-through (LVRT) for many years, in which the identified transients are mostly associated with circuit responses, whereas underlying stability issues relevant to converter control yet remain concealed. Recently, small-signal analyses of grid-tied converters have shown that the phase-locked-loop (PLL) may interact with other converter controls when synchronized to nonideal grids, if not properly treated, it may lead to oscillations. Enlightened by this, this paper will explore the PLL effects on the stability of a Type-IV wind turbine, however, from a large-signal analysis viewpoint. To achieve this, a nonlinear reduced-order model focusing on the transient interaction of PLL and the faulty grid is developed rigorously from the model assumption to verification. Based on this, the grid-synchronizing stability (GSS) provoked by grid faults is identified and the mechanism of which is revealed through the equal-area-principle (EAP). Then, impacts of system parameters (e.g. PLL bandwidth) on GSS margin are quantitatively evaluated by calculating the critical-clearing-time (CCT), and the acquired knowledge could be useful guidelines for PLL parameter design. Finally, all the analyses are verified by a switching model of the Type-IV wind turbine system in PSCAD/EMTDC.