## 1 Introduction  
Since the adoption of the Paris Agreement in 2015, global CO\({}_{2}\) emissions have risen by approximately 4% [1]. In the future, energy transition and proactive efforts such as revising the nationally determined contributions in each country are needed to reduce emissions. The Korean government has been struggling with the enactment and reformation of policies and systems related to climate change and energy transition. The Energy Transition Roadmap and the Renewable Energy 3020 Implementation Plan were established in 2017 [2, 3], and many efforts have been made since then to reduce coal power plants and expand new and renewable energy resources. In the Ninth Basic Electric Power Supply and Demand Plan established in 2020, the ratio of renewable energy generation to the total electricity generation in 2040 is targeted at 26.3% (for reference, its value in 2020 was 7.5%) [4]. To enhance the realizability of this plan and revitalize the related industrial ecosystems under these plans, the Fifth Renewable Energy Basic Plan was established in 2020 [5]. Along with these policy and institutional efforts, one of the essentials in introducing massive renewable energy resources (RERs) is to enhance the flexibility of power systems, although comprehensive strategies considering the energy sector coupled with using technologies such as power to gas (P2G) and power to heat (P2H) equally count [6]. The large penetration of RER into the power system is likely to increase the volatility and uncertainty of the net load, i.e., the load minus the output of the RER [7]. A growing amount of variable generation, such as wind and photovoltaic (PV) power plants, requires more flexibility, where flexibility is defined as the ability of a power system to respond to the change in net load [8]. The flexibility issue indicates a situation associated with a lack of flexibility [9; 10]. System operators have begun to confront this problem and make various efforts to enhance adequate flexibility. It is related to not only improving the related infrastructure and system, such as the development of market mechanisms for flexibly supplying resources and preemptive network reinforcement, but also advancing the RER output forecasting technique and the flexibility-considered operation [11; 12]. The uncertainty of the RER may largely affect the flexibility of the power system, as it relates to the shortage risk of flexibility. The uncertainty of the RER in the power system indicates the forecast error of the RER output, which may cause an unserved net load. Irrespective of how well the RER output is predicted, its forecast error is inevitable. Thus, the system operator should consider the forecast error when determining an adequate level of flexibility [13]. The RER forecast error has been modeled using various types of probability distributions, such as normal [14; 15; 16], Weibull [17], Rayleigh [18], and beta [19] distributions. In most studies on flexibility evaluation, the RER forecast error has often been assumed to follow a normal distribution, which has a limit in reflecting the characteristics of actual error distributions [20; 21]. Skewness and kurtosis are representative characteristics of error distribution [22]. If these characteristics are neglected in the flexibility evaluation, the system operator may underestimate or overestimate the shortage risk of flexibility and accordingly may not be able to respond to the net load variation. Although normal distribution [23; 24] and conditional distributions based on historical data [25] have been used in existing research on flexibility evaluation, the effect of the characteristics of the forecast error on flexibility is yet to be explicitly analyzed. This study investigates the effect of changes in the uncertainty characteristics of the RER on the flexibility of a power system. An index called the ramping capability shortage probability (RSP) is used to assess flexibility [26].