# Relationship between Fault Level and System Strength in Future Renewable-Rich Power Grids
## 6 Conclusions  
In this paper, the relationship between the fault level and the system strength in future grid scenarios of high penetration of PE-based RESs is examined. Our assessment has included comparison of the measurement-based methods proposed by the up-to-date literature for estimation of the Thevenin impedance at different nodes with the IEC 60909 methods and dynamic simulations using the adjusted IEEE nine-bus test system modelled in DIgSILENT PowerFactory. The measurement-based methods have been utilized for calculating the fault level, hence scanning the system strength. This has been achieved by creating a perturbation caused by load change and capacitor switching for generating measurement sets that are required for estimating the Thevenin impedance at different nodes in the system. The fault level was calculated using steady-state simulations based on IEC 60909 standards which employ the Thevenin equivalent and account for the fault current contributions from PE-based RESs. The fault level calculated using load change-based Thevenin impedance (LCTI), capacitor switching-based Thevenin impedance (CSTI), and IEC 60909 methods have been compared with the actual fault level results obtained from the dynamic simulations considering three different scenarios of RES penetrations (\(p=0\%\), \(p=22\%\) and \(p=56\%\)). This comparison has aimed to investigate the extent to which these methods can represent the Thevenin impedance correctly, hence a valid system strength estimation in future scenarios with high penetration of RESs. The results have shown that the system strength, either indicated by the steady-state Thevenin impedance or the fault level in faulty conditions, has been affected significantly by the increased penetration of RESs and the displacement of SGs. It has also been shown that the concept of fault level and system strength can still be interchangeably used to represent each other in scenarios of low penetration of RESs. However, this might not be valid when increasing the penetration of RESs and displacing large central SGs. In other words, although the PE-based RESs might contribute to the system strength at steady-state conditions, their different behaviour during faulty conditions might not reflect this contribution properly. Hence, the fault level in scenarios of high penetration of RESs might not be an accurate measure of the system strength and vice versa. Furthermore, the results obtained have shown that the fault level calculated based on the measurement-based methods (i.e., LCTI and CSTI) cannot lead to the same conclusion about the system strength. Regardless of the difference between the results of LCTI and CSTI methods in representing the system strength, they have shown very low values of fault level, hence system strength, when compared with dynamic simulations at high penetration scenarios of RESs. It can be concluded that the steady-state system strength, if indicated by the actual fault level obtained from the dynamic simulations, is overestimated, which necessitates the need to differentiate between these two concepts in future scenarios. In terms of the fault level calculated using the steady-state simulations based on the IEC 60909 standards, it can be concluded that the accuracy of the results might not be satisfactory when considering scenarios of high penetration of RESs, but still can provide a much better insight about the fault level in faulty conditions. Conceptualization, R.A. and Q.S. ; Methodology, R.A., H.M., M.A.-O. and S.H. ; Software, R.A., S.A. and S.H. ; Validation, R.A., M.K. and H.M.; Formal analysis, R.A. and S.H. ; Investigation, R.A., M.K., H.M., S.A., M.A.-O. and Q.S. ; Resources, M.K. ; Data curation, R.A. and M.A.-O. ; Writing--original draft, R.A. and H.M.; Writing--review & editing, M.K., H.M., S.A., Q.S. and S.H. ; Visualization, R.A. and M.K. ; Supervision, M.K. ; Project administration, M.K. ; Funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript. This research received no external funding.