Composite Power-Frequency Synchronization Loop for Enhanced Frequency Response Considering Current and Power Limits of Grid-Forming Converters

Grid-forming (GFM) control is a promising solution to provide damping and frequency support services in low-inertia power systems with an increasing share of inverter-based resources. This article investigates the available frequency response services from GFM converters during frequency disturbances, considering the impact of physical limitations of the converter and underlying power source (i.e., current and power ratings). Specifically, current limiters that are used for protection of power semiconductors restrict the achievable output power range of GFM converters and make them susceptible to loss of grid synchronization during large frequency disturbances. The inclusion of power limiters guarantees a stable equilibrium point without exceeding the current limit during disturbances, but negatively impacts frequency stability by reducing the effective damping factor. To address these problems, a composite power-frequency synchronization loop (CPFSL) is proposed as an alternative to the widely adopted power-synchronization loop (PSL). The proposed CPFSL can coordinate with current and power limiters, and enhances the damping factor of GFM converters by incorporating a phase-locked loop-based droop term into the conventional PSL. GFM converters with the CPFSL are capable of providing adaptive frequency response, which fully utilizes the available power headroom of dc-side energy sources, while maintaining grid synchronization during extreme grid events. A comparison with other GFM control methods demonstrates the improved synchronization stability of the CPFSL during severe frequency events. Furthermore, the response of CPFSL-based GFM converters during frequency disturbances, voltage disturbances, and under weak grid and islanded conditions is verified experimentally using a hardware setup of a grid-connected GFM converter.