Beyond Virtual Synchronous Machine Control

Doktorsavhandling, 2025

Grid-forming converter control is considered one of the most important contributions to handle the challenges of converter-dominated power grids. This is due to its advantages over classic grid-following control structures, such as more robust operation in weak grids and the ability to intrinsically provide grid-supporting services like fast fault-current contribution and inertial support. Nevertheless, grid-following converters show superior properties in some areas such as dynamic performance in strong grids and effective converter current limitation, in particular in comparison to those grid-forming control structures that are limited to the emulation of synchronous machines, i.e. virtual synchronous machines.

The aim of this thesis is to investigate the possibilities to combine the advantages of both grid-forming and grid-following converter control through the development of a grid-forming control approach that combines robustness and intrinsic grid support with the dynamic performance and fault-ride through capability of grid-following control. Through a study of existing control designs and grid-connection requirements for converter-interfaced generation as well as power electronic applications within transmission systems such as high voltage direct current (HVDC) transmission systems or flexible alternating current transmission systems (FACTS), gaps in the existing strategies are identified.

A voltage-based current-limitation strategy that maintains grid-forming behaviour even during current limitation is presented. In concurrence, the transient stability of grid-forming converters during current limitation due to voltage dips and frequency disturbances is studied and improved through the development of an inertia emulation loop. The focus is then moved to the behaviour of the converter in the frequency domain to ensure sufficient damping provision as well as prevent adverse control interactions. This is addressed by developing a decoupled grid-forming control structure, in which individual control parameters influence different parts of the frequency response, enabling an effective shaping of the converter's behaviour. Finally, a tuning approach that is based on performance requirements is presented for the virtual admittance control parameters.

Through these results, this thesis contributes to the development of converter control strategies that intrinsically provide grid support and enable the operation of sustainable, converter-dominated power grids.