As sustainability has increasingly gained ground in the global energy landscape, renewable energy sources such as wind and solar play an essential role in electricity generation. In contrast to conventional power plants, which use synchronous generators to provide critical grid supporting functions, renewable energy sources do not inherently offer these stabilizing services. This paradigm shift constitutes a fundamental challenge - maintaining a stable and reliable grid without conventional generation.

Grid-forming (GFM) controls have emerged as a promising solution for this challenge. Inverters operating in grid-forming mode can control voltage and frequency, emulating the behavior of conventional synchronous generators. This feature enhances grid stability, especially in systems with a high share of renewable generation.

This paper focuses on control aspects that are especially relevant for GFM control structures without inner current controllers, referred to as open-loop or direct voltage control in the literature. While this control category offers the possibility of faster control dynamics, as one control cascade can be omitted, it also causes other control challenges. To handle the inverter currents in case of faults, due to the absence of current controllers, virtual impedances are usually applied for current limitation, where a state feedback of the inverter current is used to calculate the voltage drop across the virtual impedance. A time derivative of the measured current must be computed to mimic the impedance's behavior accurately, constituting a general issue for measured signals (noise amplification). Different approaches can be found that try to avoid an ideal time derivative element, like a differential lag component (washout filter) for the measured currents or even a quasi-stationary implementation of the impedance that does not require a time derivative at all. While these approaches work well during symmetrical fault cases, they fail to provide the correct negative sequence current components during asymmetrical faults. To solve this issue, an observer-based structure is presented that allows computation of the time derivative of the measured current, although no derivative element is used in the control.

PSCAD simulation studies highlight and compare the behavior of different virtual impedance implementations for the GFM open-loop voltage control during fault cases. A model of an offshore wind turbine generator developed within the research project OffWiPP (Offshore Wind Farms as Power Plants) is used for the investigations. However, the insights and statements of this work are not limited to offshore wind power systems but apply to grid-forming controls, using the open-loop or direct voltage control, in general.