## 1 Introduction
### Review of Curtailment Methods  
A notable example of curtailment can be found in the United States and Germany [16]. In the United States, curtailment is largely implemented through market-based mechanisms and the Pro-Rata method, which prioritize economic efficiency by curtailing the lowest-cost generators--typically renewable energy plants--first. For example, in the California ISO (CAISO), curtailment predominantly occurs during midday when solar power generation peaks, resulting in approximately 3% of renewable energy being curtailed in 2022 [17]. To mitigate this issue, ESS and demand response programs are actively utilized [18; 19]. Currently, South Korea lacks a comprehensive, market-based mechanism for managing curtailment. Instead, practices are largely non-market-based and include methods, such as Last In, First Out (LIFO) and Pro-Rata [20]. The LIFO method is a curtailment strategy commonly employed in power grids to manage DERs during periods of overgeneration or grid congestion. This approach prioritizes curtailing the output of DERs based on their connection order to the grid, where the most recently connected resources are curtailed first [21]. The underlying principle of LIFO is to maintain fairness by protecting the operational continuity of earlier-connected DERs, which are often considered to have higher priority due to their longer-standing grid participation. Under the LIFO method, the curtailment sequence is determined by the chronological order of the grid connection. For example, if DER-1, DER-2, and DER-3 are connected to the grid in that order, DER-3, being the most recently connected, will be curtailed first. Onlywhen the required curtailment exceeds DER-3's available capacity will curtailment extend to DER-2 and so on. Figure 1 illustrates examples of the LIFO method. While LIFO ensures a degree of fairness by preserving the operational stability of earlier-connected DERs, it may lead to inefficiencies in certain cases. For instance, the method does not account for the real-time operational characteristics of DERs, such as their current output or contribution to grid stability. As a result, LIFO might not always optimize the overall efficiency of the grid or maximize the utilization of renewable energy resources. On the other side, the Pro-Rata method is one of the approaches used to fairly curtail the output of multiple DERs operating simultaneously within a power grid. This method is characterized by allocating curtailment amounts proportional to the output of each DER. There are two main variants of this approach: one based on the current active output of each DER and the other based on their maximum active output capacity [21]. Figure 2 illustrates examples of the Current Active Output-based Pro-Rata and Maximum Active Output-based Pro-Rata methods. In the current active output-based approach, the curtailment amount for each DER is determined proportionally to its current contribution to the power grid. In this case, the proportionality coefficient is calculated by distributing the total curtailment requirement among DERs based on their current output levels. This method enables fair and real-time curtailment by reflecting the actual operational contributions of each DER to the grid. For instance, as illustrated in Figure 2a, if there are three DERs--DER-1, DER-2, and  
Figure 1: LIFO example. Figure 2: (**a**) Current Active Output-based Pro-Rata and (**b**) Maximum Active Output-based Pro-Rata. DER-3--With varying current active output levels, DER-1, which has the highest current active output, will be required to reduce its output by a larger proportion, whereas DER-3, which has a relatively lower output, will be curtailed by a smaller proportion. Conversely, the maximum active output-based approach allocates curtailment amounts based on the maximum active output capacity of each DER. In this case, the proportionality coefficient is calculated by dividing the total curtailment requirement among DERs according to their maximum active output.