# Overload Mitigation of Inertial Grid-Forming Inverters Under Frequency Excursions
## 7 Conclusions and Future Work  
This study analyzes and compares three strategies for overload mitigation in inertial GFM inverters under frequency excursions. The findings confirm that while all strategies can reduce the active power overload by adjusting inertial contribution and primary frequency regulation, each strategy presents specific limitations. The parallel PI strategy is simple but has slower recovery, making it less effective in fast grid transients. The AL strategy offers faster recovery but is sensitive to system uncertainties. Finally, the EFS strategy has greater robustness to uncertainties at the cost of reduced damping during normal operation.  
Several technical challenges remain to be addressed to enhance the performance of GFM inverters under grid frequency excursions. One of the main challenges is managing the inertia, as GFM inverters must balance reducing inertial contribution for overload mitigation while ensuring sufficient inertia to maintain grid stability. Determining the minimum required inertia to avoid instability while optimizing power output poses a significant challenge. Damping performance is another critical issue, especially in cascaded control structures, where interactions between controllers can reduce damping and affect inverter stability. The sensitivity to weak grids when relying on auxiliary PLLs also needs further research.  
Future research lines should focus on evaluating the interoperability of multiple GFM inverters with different overload mitigation strategies within complex power systems. This includes assessing their performance in large-scale grids with dynamic operating conditions. Another relevant topic is the coordination of overload mitigation strategies and fast protection algorithms, such as those designed for LVRT events, to avoid adverse interactions and ensure reliable operation during faults. Additionally, further experimental validation under realistic grid conditions is needed to assess the effectiveness of the proposed strategies, especially under changing grid conditions.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Virtual** & **Parallel PI** & **Angle Limiter** & **External Frequency Support** \\ \hline Freq, regulation power elimination & Yes & Yes & Yes & Yes \\ Inertial power reduction & No & Yes & Yes & Yes \\ Inner controller requirements & No & No & Voltage drop between inverter and FCC & Voltage drop between inverter and FCC \\ PLL required & No & No & Yes & Yes. Not for synchronization \\ Anti-windup required & No & No & Yes & No \\ Sensitive to grid impedance & No & No & Reduced damping in weak grids (\(L_{g}/L_{c}\)) & No \\ Sensitive to delays and uncertainties & No & No & Yes & No \\ Overload recovery & Good & Poor & Good & Best \\ Impact on normal operation & No & No & No & Reduced damping \\ \hline \hline \end{tabular}
\end{table}
Table 3: Summary of the characteristics of different overloading schemes.  
**Author Contributions:** Conceptualization, A.O.; methodology, A.O., A.S.-R. and M.Z.; software, A.O. and J.R.-G.; validation, A.O. and J.R.-G.; formal analysis, A.O., A.S.-R., M.Z. and F.J.A.; investigation, A.O., A.S.-R., M.Z. and F.J.A.; resources, A.O. and F.J.A.; data curation, A.O.; writing--original draft preparation, A.O., A.S.-R., M.Z. and F.J.A.; writing--review and editing, A.O., A.S.-R., M.Z., F.J.A. and J.R.-G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Basque Government (GISEL research group IT1522-22).  
Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.