Qihao Shen, Meng Zhan
• This study built a nonlinear model with transient control, capturing PV grid-tied dynamics in the full LVRT. • The 3D columnar attraction basin allows direct transient stability study on the PLL phase plane. • Transient stability parameter regions are wider; the during-fault equilibrium point is not a necessary condition. • At the initial state of Stage 3, the transient stability boundary of the system is determined. Large-scale photovoltaic power generation makes the transient characteristics of power grid under severe faults particularly complicated. Different from the traditional synchronous generator, the transient stability condition and evolution law of the PV grid-tied systems remain to be studied. In this article, by considering the whole low voltage ride-through process including pre-fault, during-fault, early fault recovery, and late fault recovery four stages, the corresponding nonlinear simplified models are established and the transient stability evolution law is uncovered. It is shown that the stability of during-fault stage does not solely determine the system transient stability, the system can recover its stability as long as the fault is removed in time, and the transient stable parameter region is wider than what researchers previously thought. In addition, different climbing speeds of the early fault recovery stage do not influence the system stability. Therefore, the transient stability criterion is given by whether the initial state of the early fault recovery stage is located within the attraction basin of the early fault recovery stage. All these provide in-depth physical insight for the transient synchronization stability evolution law of the PV grid-tied systems and establish a theoretical basis for stability control strategies in engineering as well.