# Relationship between Fault Level and System Strength in Future Renewable-Rich Power Grids
However, this might not be captured by using the fault level scanning tool. This necessitates the need to better understand the correlation between the fault level and the system strength represented by the system impedance. Traditionally, using the Thevenin impedance has been employed for steady-state fault calculations by most standards, such as IEEE and IEC standards [10; 11]. This is not only due to its simplicity, but also due to the accuracy achieved in estimating the fault currents. This is because of the direct and linear (inversely) relationship between the impedance and the fault current flowing at the point of study. Such relationship is attributed to the physical behaviour of the classical SGs during faulty conditions. These SGs provide a time-decaying fault current which is inversely related to their impedance seen during the fault. SGs' fault currents are usually of very high values which go multiple times above its rating (e.g., up to six p.u.). Nevertheless, the Thevenin equivalent method may not provide the same accuracy when the fault current comes to the systems from RESs with different characteristics and limited fault current contributions when compared with SGs (e.g., up to 1.4 p.u.) [9]. Hence, these Thevenin-based methods attempted to modify their approach to better include the impact of such RESs in fault calculation methods. For instance, IEC 60909 standards suggested that the PE-based RESs should be modelled as a current source with infinite impedance. Aside from fault calculation standards, many studies have attempted to map the system strength and the fault level in power systems. These studies can be categorized into two main classes:  
1. Those that studied the application of measurement-based methods for Thevenin impedance and fault level estimation [12; 13; 14; 15; 16]. Although such studies provided a valuable insight, they were mostly analysing the system impedance and the fault level in classical/existing power systems without considering the impact of very high RES penetration. 2. Those focused on employing of the fault level and the short circuit ratio (SCR) for evaluating the system strength for PE-based RES integration purposes [17; 18; 19; 20; 21; 22]. Such studies assess the strength of the AC/DC connection when integrating RESs. Some of these studies have suggested modified SCR-based metrics for better scanning of the system strength. However, they were restricted to the point of common coupling (PCC) and did not consider the fault current contribution from RESs. Moreover, they have not clearly pointed to the impact of the fault contribution from PE-based REs on those suggested metrics. In recent literature, researchers have started to raise questions on the accuracy of employing the fault level for reflecting the traditional concept of Thevenin impedance and the system strength in future renewable-rich scenarios [23; 24]. For instance, researchers in [23] have concluded that the classical definitions of grid strength based on short-circuit power and Thevenin impedance are inadequate as they may fail to describe the system behaviour. The work in [24] argues that a fault level may not be a valid measure of the equivalent system impedance, i.e., the system strength in systems with large-scale integration of PE-based REs. In this paper, we aim to examine the effectiveness of using a fault level-based approach as a system strength scanning tool that reflects the Thevenin impedance in future grid scenarios with high penetration of REs. (i.e., PE-based generators). Our analysis focuses on achieving a better understanding of the correlation between the fault level and the system strength in such scenarios. The work in this paper is divided into two steps: first, estimation of the fault level in renewable-rich grids by utilizing the measure-based Thevenin impedance, and second, comparison between these estimated fault levels with those obtained from stead-state and dynamic simulations.