# Review of Power System Resilience Concept, Assessment, and Enhancement Measures
## 1 Introduction  
Extreme events have an impact on the operation of distribution and transmission grids, thereby affecting a large number of customers [1]. As our society becomes increasingly reliant on electricity for various infrastructure and services, providing a continuous power supply is crucial. In order to make the power grid more flexible and intelligent and to better withstand natural disasters, the concept of resilience has recently been applied [2, 3]. Extreme events have the potential to cause significant power outages, making it crucial and urgent to enhance the resilience of the power system [4]. The average number of billion-dollar disaster events in the United States from 2020 to 2023 exceeds twice the average from 1980 to 2023, as shown in Figure 1. As of 8 August 2023, a total of 15 weather/climate disaster events had occurred that year, including 13 severe storm events, which had a significant economic impact on the affected areas [5]. [PERSON] et al. [6] argue that storms have compromised the integrity of the power system in Minnesota. Compared to typical outages or failures, natural disasters often result in more extensive damage, requiring a longer recovery time for the system. Despite the implementation of disaster preparedness plans and the strengthening of grid infrastructure, power outages have become increasingly severe in recent years, with the majority caused by extreme events. Traditional power systems are generally designed based on reliability requirements, namely the \"N-1\" or \"N-2\" standard. Moreover, several reliability metrics such as SAIDI, SAIFI, and others have been widely used in different power systems. These metrics provide information on the duration or frequency of occurrences of power interruptions. However, extreme weather events have gradually increased; thus, these low-probability but high-impact extreme events should be of concern and urge the design of new metrics to evaluate power system resilience. A resilient system can help reduce the impacts of extreme events on it and recover the power system rapidly after these events. Therefore, the issue concerning the assessment and enhancement of power system resilience is crucial. In recent years, the topic of power system resilience has emerged, and extensive research works have been conducted. In terms of resilience assessment and metrics, [PERSON] et al. [7] categorized existing resilience assessment metrics into pre-event metrics, post-event metrics, and other commonly used metrics, and explained their characteristics and definitions. [PERSON] et al. [8] discussed resilience assessment metrics from the perspectives of steady-state and transient operations. [PERSON] et al. [9] proposed a classification scheme for quantitative power system resilience metrics. To enhance power system resilience, many works have proposed various methods; for instance, [PERSON] et al. [2] discussed numerous methods, technologies, and future trends to improve power system resilience from the aspects of resilience-based planning, response, and restoration. [PERSON] et al. [10] proposed a method to improve system resilience by connecting tie switches to recovery paths in a radial grid topology. [PERSON] [11] explored various strategies for enhancing proactive resilience, including proactive strategies, machine learning, distributed energy resources, and renewable energy integration. In terms of the impacts of extreme events, [PERSON] et al. [12] investigated the types of natural disasters that occurred in Taiwan and the factors causing power system outages or damages. [PERSON] et al. [13] analyzed the impact of extreme events on the power system from the perspectives of generation, grid, and load. However, these articles provided less description of the meaning of resilience, which is due to the lack of a standard definition for resilience. This paper reviewed the definitions of resilience by different organizations and summarized the main characteristics of power system resilience.