# Review of Power System Resilience Concept, Assessment, and Enhancement Measures
## 3 Understanding Power System Resilience
### Differentiate Resilience from Similar Concepts  
The concepts of reliability and robustness are generally confused with resilience. Reliability refers to the ability of a power system to continuously deliver power to users with acceptable quality and the required quantity, even in the event of failures, thus \begin{table}
\begin{tabular}{c c} \hline
**Organizations** & **Characteristics** \\ \hline UKERC [59] & Withstanding, Absorption, Adaptation, Recovery \\ DOE [60] & Withstanding, Recovery \\ NIAC [61] & Anticipation, Resistance, Absorption, Adaptation, Recovery \\ PPD-21 [62] & Withstanding, Adaptation, Recovery \\ ASIS [63] & Resistance, Withstanding, Adaptation, Recovery \\ UN/ISDR [64] & Resistance, Withstanding, Adaptation, Recovery, Learning \\ \hline \end{tabular}
\end{table} 
Table 2: The characteristics in the definition of resilience by various organizations. \begin{table}
\begin{tabular}{c c} \hline
**Duration** & **Characteristics** & **Intention** \\ \hline \multirow{3}{*}{Before the disruption} & \multicolumn{3}{c}{Predicting the occurrence of events} \\  & \multicolumn{3}{c}{Assess the potential damage from events} \\  & \multicolumn{3}{c}{Pre-event planning} \\ \hline \multirow{3}{*}{During the disruption} & Resistance & Minimize the impact of the disruption \\  & Withstanding & Maintain basic functionality \\  & Absorption & Absorb the impacts of the disruption \\  & Adaptation & Modify or reorganize system configurations \\ \hline \multirow{3}{*}{After the disruption} & Recovery & Restore system elements quickly \\  & \multicolumn{3}{c}{The system returns to a steady state} \\  & Learning & Learn experiences from events \\ \cline{1-1}  & \multicolumn{3}{c}{The system establishes a learning mechanism} \\ \hline \end{tabular}
\end{table} 
Table 3: The main characteristics of power system resilience. Figure 2: The conceptual construction of power system resilience. maintaining the functionality of the grid [56]. For example, to prevent cascading failures, the power grid isolates certain circuits when a failure occurs, resulting in power outages for some customers. However, the power grid continues to operate reliably [6]. Compared to resilience, reliability is often seen as \"keeping the lights on\", while resilience is seen as \"quick recovery when the lights go out\" [61]. In other words, discussions on reliability focus on serving all loads, while discussions on resilience focus on the survivability of the system. Robustness refers to the ability of a system to maintain its functionality in the presence of a set of disruptions [69]. Sometimes, systems are designed to be sufficiently robust to withstand anticipated shocks, while in other cases, alternative or redundant systems need to be designed so that the system can withstand interruptions in the event of accidents [61]. High levels of robustness can make a system brittle, leading to large-scale failures. For comparison, a grid with robustness, when subjected to impact, may break like an oak tree in a storm, while a resilient grid can bend like a need in a storm. In other words, robustness means resistance to external changes, which is related to strength, whereas resilience refers to flexibility and survivability when faced with unexpected disruptions [56]. Typical power outages are usually quickly repaired or restored with a low impact on the power grid but a high probability of occurrence and are termed LIHP events. However, power outages caused by extreme events often result in significant damage to the power system, requiring more time for repairs and recovery. Although the probability of such events is low, their impact on the power grid is substantial, and they are termed HILP events. When comparing reliability, robustness, and resilience, reliability primarily focuses on handling LIHP events, while resilient systems possess the flexibility to withstand HILP events. Robustness, on the other hand, does not primarily focus on withstanding HILP events but can also be applied to LIHF events.