Grid-Forming Control of Voltage Source Converters Based on the Virtual-Flux Orientation

The increasing penetration of renewable generation in power systems is causing growing concern about system stability. Grid-forming (GFM) control has been appointed as the technology required for achieving a high penetration of renewables in the grid, as it successfully contributes to the power system stability. This paper proposes a novel grid-forming control scheme for voltage source converters (VSC) achieving the operational characteristics of a virtual synchronous machine (VSM). The proposed scheme is based on the orientation of a defined virtual flux to a reference axis obtained from the emulation of the synchronous generator swing equation. The virtual flux is obtained by the integration of the VSC internal voltage and it is oriented to the reference axis by means of a flux controller that also controls the flux magnitude. In this way, the VSC synchronism is maintained, operating as a voltage source connected to the grid. Moreover, the flux orientation in turn allows to control the VSC active power, while the flux magnitude control allows to regulate the VSC reactive power or terminal voltage in isolated mode. The paper also demonstrates that the stability of GFM converters is negatively affected when emulating high inertia values. So, a stabilizer is also proposed for damping these modes. Finally, due to fact that GFM converters behave as voltage sources, their low voltage ride through capability becomes an issue, because they naturally respond to faults with high short-circuit currents, that cannot be withstand by power electronics converters. Therefore, a method for limiting fault currents is proposed as well in the paper. The proposed grid-forming control scheme has been implemented in a real-time control platform and validated using comprehensive simulation models in a real-time simulator for assessing its grid-forming capability