## 6 Conclusions and Recommendations  
Time-domain simulation is an important instrument for ensuring transient stability of a converter-dominated power system in the planning and development phase of wind parks and other renewable energy generation plants. This paper explored the efficacy of a mosaik-based co-simulation framework for transient stability evaluation of a grid-forming converter-based RES. mosaik enables a successful coupling of different FMU-based simulators of power system in Powerfactory and converter controller in Simulink. The efficacy has been shown by triggering different types of dynamic events, control parameters, and systemic boundaries. The co-simulation results were compared to the results of the monolithic simulation. The impact of the transmission events, such as voltage sags caused by faults on the RES controller operation, was demonstrated. The results show a wide consistency between co-simulation and the monolithic simulation in almost all of the study cases. The exception was during a severe voltage dip close to the converter, which induced spikes in the converter current, on its turn generating reactive power at fault ignition and clearing. It has been investigated and it appears that the spike is due to the inevitable delay in transferring the information between the FMUs and the high sensitivity of the voltage-source converter modelled in PowerFactory to this delay. The master algorithm of mosaik is based on a discrete time scheduler that introduces a delay into the data exchange between controller and the power system FMU. Reducing the synchronisation time-step size is a solution to reduce the delay. However, it will decrease the overall (speed) performance of the co-simulation. Another solution is to look for interaction protocols or interfacing techniques that are dedicated to the solver of PowerFactory or to the properties of the wind turbine model. Despite this minor discrepancy, the co-simulation framework is considered widely applicable for transient stability assessment, which was also reflected in the critical clearing times and the root squared differences compared to the reference simulation in PowerFactory. Additionally, the performance of the co-simulation in different grid conditions was investigated. Two cases, weak-grid conditions and islanded operation, were studied. The low-frequency oscillations observed under these conditions were eliminated by applying a power system stabilizer in the converter controller. This did not impair the efficacy of the mosaik-based co-simulation approach. The coordination between the FMUs has been investigated in two ways: via dedicated Python code that implements a rudimentary scheduler, and via mosaik, which boasts a production grade implementation of a discrete event scheduler and offers high usability and flexibility in terms of scenario creation. This will be addressed in future research in \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Case Study** & mosaik & \
\begin{tabular}{l} **FMI-Based** \\ **Co-Simulation** \\ \end{tabular} &
\begin{tabular}{l} **Monolithic** \\ **Simulation** \\ \end{tabular} \\ \hline
5.1 Step in the voltage & 225.4 & 246.6 & 1.5440 \\ set-point & & & \\ \hline
5.2 Controller parameter & 231.1 & 237.2 & 1.5744 \\ modification (for \(k_{\text{drop}}\) = 0.02) & & & \\ \hline
5.3 Response during & 246.4 & 252.8 & 1.6788 \\ faults (for FCT = 150 ms) & & & \\ \hline
5.4 Weak-grids & 240.3 & 244.7 & 1.5126 \\ (with PSS) & & & \\ \hline
5.5 Islanded operation & 226.2 & 232.6 & 1.4769 \\ (with PSS) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Execution times of the simulations [s] for the case studies of Sections 5.1–5.5. (Intel Xeon W-2123, 3.6 GHz, 8 GB RAM). which this platform will be applied for assessing transient stability of future power grids using the design of experiment techniques. The proposed co-simulation approach revolves around the assessment of systemic impacts of wind turbine generators.