Critical inertia can be defined as the minimum kinetic energy stored in generators that should be kept for maintaining the frequency stability of the power system. As the frequency control performance of the power system is maintained according to its control criterion, during the inertia response time frame, the expected energy imbalance can be calculated by accumulating the imbalanced power caused by the credible contingency and calculating the available inertia response by considering the allowable operating limit of the frequency. Since the frequency control criterion can be met when the available inertia response becomes larger than the expected energy imbalance, the critical inertia can be calculated by the kinetic energy of the generators, of which the available inertia response is equal to the expected energy imbalance due to the contingency. With this in mind, this paper derives the energy balance condition for the frequency stability in the inertia response time frame based on the frequency control criterion and calculates the minimum inertia, which should be kept in the generators from the energy balance condition for meeting the criterion. In addition, the effectiveness of the proposed method is verified through various case studies employing Korean power systems.

In this paper, considering the electric power generation characteristics of variable renewable sources(VRS) due to the dependence on the weather conditions the factors that would limit the increasing penetration level of VRS in the power system are analyzed, and then the method for evaluating the steady-state penetration limit of VRS is presented for each factor. In addition, through study cases employing the planning data developed from the 9th Basic Plan for Long-term Electricity Supply and Demand and historical operation data of Korean power system, the effectiveness of the proposed method is verified.

Due to environmental policies in the electric power industry, the proportion of Variable Renewable Source(VRS) in the power system is increasing. However, since the power outputs of VRS are very variable and its capability for providing the inertia response is limited, the high penetration of VRS may lead to violate the operating standards of the power system and result in deteriorating the power system stability. One of the solutions to alleviate these problems is power flow and frequency control using High Voltage Direct Current(HVDC) interconnecting the power systems between countries, and through this, the penetration limit of VRS can be increased. For this, this paper proposes a method to evaluate the penetration limit of VRS considering the HVDC interconnection of power system between countries. In the case study, the effectiveness of the propsoed method is verified by evaluating the VRS penetration limit of Korean power system.