Comparative assessment of typical control realizations of grid forming converters based on their voltage source behaviour

The generation of power system is transitioning from conventional synchronous generators with voltage source behaviour to power converter interfaced renewables. Power converters typically excel a current source behaviour, which fundamentally changes the dynamics of the power systems and resulting in stability challenges. To address these, a new type of control that can enable the converters to operate in a voltage source behaviour, referring to as grid forming control (GFC), is drawing significant interest from industry and academia. However, the reported control loops of the GFCs do not have a unified structure. Different control structures of GFC would lead to different pros and cons in different operational conditions based on their underlining control realization principles, the amount of parameters, and their setting rooms. Therefore, the stability augmenting voltage source behaviour cannot be considered as equal as for all the reported GFC realizations. This paper provides a critical review and discussion on the impact of inner control loop realizations of the GFC's reported in the literature on their stability during steady-state and large disturbances. Three typical GFC structures by inner loop controls based on, (1) cascaded voltage and current control, (2) inner current control, (3) no inner loop, are chosen for in-depth investigation. The analysis revealed that inner loops could negatively impact the voltage source behaviour of a GFC due to the complex control structure and the associated challenges of parameter tuning. The MW-level GFC with inner loops could potentially become unstable in a weak power system. Additionally, it is also revealed that GFC with cascaded control can operate stable for a narrow range of network impedance than other two types of controls. Furthermore, it is also shown that slow response behaviour based on cascaded inner loop can negatively impact dynamic reactive and active power-sharing and the fast-acting current limiting capabilities.