Reactive power reserves are important for grid stability and have a variety patterns depending on fault location as power systems become larger and complex. This paper presents the clustered effective reactive reserve (CEQR), an indicator that is suitable for identifying the risk of dynamic voltage stabitlity in terms of power system operation. This indicator provides the following features: i) It calculate the realistic reactive reserve to recognize dynamically changing system conditions; ii)it derive clustered areas to identify valid regions for system control. The proposed method uses the sensitivity between dynamic reactive resources or load buses and a particular bus to obtain the correlation. And since this indicator is calculated using only the system topology, current and maximum outputs of the generators, there is less computational burden. This study verified the features of clustered effective reactive reserve(CEQR) by analyzing the practicable dynamic voltage collapse scenarios in the Korean electric power(KEPCO) system. In addition, as a countermeasure against the dynamic voltage collapse, the control performance of blocking tap changer scheme using the proposed indicator is described.

Renewable energy curtailment often occurs to accommodate large amounts of renewable energy sources in power systems while maintaining system stability and reliability. Widely known methods, such as new transmission line construction, the introduction of demand-side resources, and the reduction of conventional generator output, can minimize the occurrence of curtailment; however, there are difficulties in introducing them because of social and economic problems. For these problems, the Jeju power system adopted a battery energy storage system (BESS) resource to mitigate the curtailment and secure frequency stability with the high penetration of renewable energy. The small-size Jeju island power system is operated with reliability must-run (RMR) units and high-voltage direct current (HVDC) lines connected to the mainland. Since the number of RMR units contributes to frequency stability by providing inertia, reducing the number of operating units for curtailment mitigation is difficult. Therefore, in this paper, based on the current “Carbon-Free island” policy and operation plan of the Jeju power system, we proposed a BESS for reducing the number of RMR units, observe the effect of reducing curtailment using the BESS, and suggest a practical operation plan to reduce the number of RMR units under conditions that secure frequency stability.

With the increase in inverter-based resources, maintaining system stability has made frequency maintenance criteria an important factor. In South Korea, there are two frequency maintenance criteria for N-1 and N-2 contingency faults, and consistency between these criteria is necessary to ensure the rational use of limited frequency response resources. This article proposes a method to verify the consistency of the criteria based on the minimum inertia in the system. The proposed methodology has the following salient features: i) developing a methodology to verify consistency between criteria based on minimum system inertia, ii) identifying the inconsistency in South Korea's existing frequency maintenance criteria and the need for their revision, iii) evaluating the impact of inconsistent frequency maintenance criteria on non-synchronous generation and system non-synchronous penetration. The methodology was validated on the IEEE 39 bus test system and applied to 7040 hourly operational data from the energy management system of the South Korean to verify the differences in criteria. The main result is that the two frequency maintenance criteria are inconsistent, require more frequency response resources in the N-1 criterion. The findings provide practical implications for system planners and operators, highlighting the need for frequency maintenance criteria revision in South Korea.

In low-inertia systems with a high penetration of renewable energy, the rotational kinetic energy and inertia constant are significant factors in determining frequency stability. The energy released owing to the frequency decrease during contingency represents a portion of the inertia that a synchronous machine possesses in the normal state. However, when securing inertia or planning additional resources to secure frequency stability, inertia in the normal state is analyzed as the standard rather than the amount of energy released during a fault. Therefore, in this paper, we define the actual energy emitted from a synchronous machine as Effective inertia. In order to evaluate Effective inertia in various operating conditions, we conducted a comprehensive review on approximately 24,627 cases from the years 2019, 2020, and 2021. As a result, in systems with low rotational kinetic energy, both low- and high-frequency nadirs were observed, indicating high uncertainty. However, Effective inertia presented a consistent trend regarding the energy release aligned with the minimum frequency. For instance, the rotational kinetic energy required to satisfy the frequency standard was 23 GWs, while the required Effective inertia was 858 MWs. We emphasize that securing inertia based on rotational kinetic energy includes additional imaginary energy that does not contribute to frequency, resulting in an energy requirement greater than that needed for Effective inertia. Therefore, in order to secure the frequency stability of the future system, the actual required energy amount based on Effective inertia will be presented and utilized in the inertia market and FFR (Fast Frequency Response) resource design.