Analytical Current Injection Feasible Region and Transient Stability Enhancement Control for PLL Synchronized VSC During Low Voltage Grid Fault

How to analysis the phase-locked loop synchronous (PLL-Syn) transient stability issue in weak grid connected VSC system (WG-VSCs), and further guide the transient stability enhancement control design is of significance to the WG-VSCs, which is also the main motivation of this paper. First, from the perspective of ensuring PLL-Syn transient stability, the concept of the current injection feasible region (CIFR) is proposed for the first time. Moreover, a modified equal area criterion is utilized to obtain the analytical boundary of the feasible region, which can be significantly affected by the indefinite damping of PLL. To expand the CIFR and improve the PLL-Syn transient stability, a simple enhancement control is proposed. Eventually, the PSCAD/EMTDC simulation is performed to validate the theoretical analysis.