The responses of an inverter-based distributed generator (IBDG) to abnormal voltage and frequency are different from those of a conventional generator because of the inverter's operating modes. In particular, although the momentary cessation (MC) mode is essential to protecting the distribution system, the MC mode can affect the power system transient stability by temporarily ceasing significant amounts of IBDG generation. To mitigate these negative effects, this study analyzed the impacts of MC capability on the power system transient stability. First, the transient stability was analyzed in relation to MC capability using the single-machine equivalent (SIME) method. A critical MC operating point (CMCOP) was then proposed to assess the severity of a transient impact and to secure the transient stability under the contingency conditions. A case study based on the Korean power system validated the feasibility and effectiveness of the proposed method.

The responses of an inverter-based distributed generator (IBDG) to abnormal voltage and frequency are different from those of a conventional generator owing to the difference in the operating modes. In particular, the momentary cessation (MC) mode deteriorates the transient stability of normal power systems by ceasing to provide active and reactive power to the grid. However, in a high-generation area, where a significant amount of generation is concentrated and where transient instability exists under a contingency, MC operation is conducive to the transient stability because the electrical output of critical generators increases to cover the local loads under this condition. This effect can cause frequency instability if a sizeable portion of the IBDG output is lost owing to the operating modes. To ensure transient and frequency stability, this study analyzed the effects of operating modes and generator tripping on the high-generation area. A method for determining the capacity limit of the IBDGs in the high-generation area was then developed to ensure power system stability. The effectiveness and feasibility of the proposed method were verified by conducting a case study on the Korean power system.

Efforts to reduce greenhouse gas emissions constitute a worldwide trend. According to this trend, there are many plans in place for the replacement of conventional electric power plants operating using fossil fuels with renewable energy sources (RESs). Owing to current needs to expand the RES penetration in accordance to a new National power system plan, the importance of RESs is increasing. The RES penetration imposes various impacts on the power system, including transient stability. Furthermore, the fact that they are distributed at multiple locations in the power system is also a factor which makes the transient impact analysis of RESs difficult. In this study, the transient impacts attributed to the penetration of RESs are analyzed and compared with the conventional Korean electric power system. To confirm the impact of the penetration of RESs on transient stability, the effect was analyzed based on a single machine equivalent (SIME) configuration. Simulations were conducted in accordance to the Korean power system by considering the anticipated RES penetration in 2030. The impact of RES on transient stability was provided by a change in CCT by increasing of the RES penetration.

An energy paradigm shift has rapidly occurred around the globe. One change has been an increase in the penetration of sustainable energy. However, this can affect the reliability of power systems by increasing variability and uncertainty from the use of renewable resources. To improve the reliability of an energy supply, a power system must have a sufficient amount of flexible resources to prevent a flexibility deficit. This paper proposes a countermeasure for protecting nonnegative flexibility under high-level penetration of renewable energy with robust optimization. The proposed method is divided into three steps: (i) constructing an uncertainty set with the capacity factor of renewable energy, (ii) searching for the initial point of a flexibility deficit, and (iii) calculating the capacity of the energy storage system to avoid such a deficit. In this study, robust optimization is applied to consider the uncertainty of renewable energy, and the results are compared between deterministic and robust approaches. The proposed method is demonstrated on a power system in the Republic of Korea.