Frequency Response Analysis of Grid-Forming Converters Considering Current and Power Limitations

Modern power systems are more frequently faced with frequency events due to an increasing share of inverter-based resources (IBRs) and the phase-out of synchronous generators. Grid-forming (GFM) control is adopted as a promising solution to provide damping and frequency response services in a low-inertia power system. This paper discusses the available frequency support capability of GFM converters by taking its physical constrains into consideration, i.e. current and power ratings. An analysis is conducted on the influence of current and power limiters, used for protecting power semiconductors and batteries, on the frequency response of GFM converters. Specifically, the use of current limiters makes GFM converters prone to losing grid synchronization during large grid frequency changes. The inclusion of power limiters guarantees the existence of a stable equilibrium point during the fault, but it affects frequency stability by reducing the effective damping factor of GFM converters. The effective damping factor is mutually determined by power set-point, power limit and the severity of frequency disturbance. A hardware setup of a grid-connected GFM converter system is developed and experimental results are provided to validate the analysis on frequency response during severe frequency disturbances.