The grid integration of renewable energy sources interfaced through power electronic converters is undergoing a significant acceleration to meet environmental and political targets. The rapid deployment of converters brings new challenges in ensuring robustness, transient stability, among others. In order to enhance transient stability, transmission system operators established network grid code requirements for converter-based generators to support the primary control task during faults. A critical factor in terms of implementing grid codes is the control strategy of the grid-side converters. Grid-forming converters are a promising solution which could perform properly in a weak-grid condition as well as in an islanded operation. In order to ensure grid code compliance, a wide range of transient stability studies is required. Time-domain simulations are common practice for that purpose. However, performing traditional monolithic time domain simulations (single solver, single domain) on a converter-dominated power system is a very complex and computationally intensive task. In this paper, a co-simulation approach using the MOSAIK framework is applied on a power system with grid-forming converters. A validation workflow is proposed to verify the co-simulation framework. The results of comprehensive simulation studies show a proof of concept for the applicability of this co-simulation approach to evaluate the transient stability of a dominant grid-forming converter-based power system.

The massive integration of wind power plants into the transmission system calls for a careful scrutinization of their dynamic behaviour and the interactions with the power system in the transient stability time-frame of interest. This paper presents a sensitivity analysis based approach to investigate the impact of full converter wind turbine generators on the transient stability of the interconnected power system. A combination of different fault ride through and voltage support mechanisms such as 1) low voltage ride through, 2) voltage-dependent reactive power injection, 3) current limitation strategy during grid faults, and 4) the application of inertia emulation capability are considered for the sensitivity analysis. Critical clearing times are used as the transient stability indicator. The implementation of the wind turbine models are according to IEC 61400-27-1 and the assessment is done through time domain simulations using DIgSILENT PowerFactory. The results on a modified version of the IEEE 9-bus system show that by more effective use of the existing controllers of wind turbines, the share of wind power plants can be increased substantially.