Transient Stability Enhancement with High Shares of Grid-Following Converters in a 100% Converter Grid

With increasing shares of power electronics-based generation in power grids, grid-following converters may become unstable during faults, resulting from a loss of phase-locked loop (PLL) synchronism. Even when current grid code low voltage ride through (LVRT) requirements are met, PLLs may still become unstable under high shares of grid-following converters, due to much reduced reactive current support from (online) synchronous generators or grid-forming converters. Consequently, a readily implementable transient stability enhancement approach is developed for grid-following converters using a reactive current priority current limiting strategy for faults on a transmission network. The proportional gain for the reactive current injection is determined by formulating an optimization-based transient stability problem, which ensures a valid PLL equilibrium point, maximizes active power output, fully exploits converter current capacity, and, at least, satisfies existing grid code LVRT requirements. An additional PLL frequency-feedback PI term is used to enhance PLL dynamic stability, in recognition of parameter estimation errors and imperfect control. A case study (100% converter-based grid) verifies that the proposed solution enables grid robustness against faults and the permissible share of grid-following converters to be increased (especially in weak grids).