Transient Stability Evaluation and Decoupled Control for Grid-Following Converters Considering Nonideal Alternating Current Control

In most previous studies about transient synchronization stabilities of grid-following converters (GFLC), alternating current control (ACC) dynamics are often neglected. However, the bandwidth of the ACC cannot be selected too high in some low-switching cases, such as large wind turbines. The adverse effects of ACC must be considered to avoid over-optimistic evaluation of transient stability. There are seldom quantitative and analytic large signal analyses that take the ACC dynamics into account. To fill this gap, an iteration-based accurate transient stability evaluation method is proposed in this article. First, the model of the ACC and line dynamics are deduced. The time-domain analytic solution of current dynamics is obtained. Furthermore, the multivariate implicit function equation set concerning current-frequency-power angle's mapping relation under the critical stable condition is constructed and the transient stability boundary is solved based on the proposed iterative algorithm. The interaction mechanism between the phase-locked-loop (PLL) and ACC is accurately quantified. A simplified transient stability criterion is deduced to preliminarily estimate the adverse effects of ACC on GFLC's transient stability. In addition, a stability-enhanced decoupled PLL strategy is proposed to enable the setting of PLL bandwidth unconstrained by the interaction from ACC, which significantly improves the dynamic response of the GFLC. Simulation and experiments verify the effectiveness and superiority of the proposed stability evaluation method and decoupled PLL strategy.