As the penetration rate of renewable enery resources (RES) in the power system increases, uncertainty and variability in system operation increase. The application of energy storage systems (ESS) in the power system has been increased to compensate for the characteristics of renewable energy resources. Since ESS is a controllable and highly responsive power resource, primary frequency response and inertia response are possible in case of system contingency, so it can be utilized for frequency regulation (FR) purposes. In frequency regulation, reduction of the Rate of Change of Frequency (RoCoF) and increase the frequency nadir by improving the response characteristics are important factors to secure frequency stability. Therefore, it is important to control ESS with proper parameters according to changing system situation. In this paper, we propose a method to calculate and apply a frequency droop, which is basically required according to the power system condition based on swing equation and effective inertia assessment. In addition, a method to estimate RoCoF droop according to the correlation with frequency by estimating the systematic inertia in the current situation is proposed. The case study for verification of the proposed method was performed through dynamic simulation using actual Korean power system data. The results show that the proposed method is more effective than the governor-free of the conventional thermal generator and conventional droop control-based FR-ESS.

To date, power system analysis has been performed separately for transmission and distribution systems. Due to the small influence of distribution systems on transmission systems, separate analyses have had no accuracy problems in existing power systems. However, as the amount of distributed generation (DG) in distribution systems increases, neighboring distribution systems and even transmission systems can be affected by the distributed generation. Therefore, a power system operator needs a new system to analyze the power system, one that considers the mutual interactions between the transmission and distribution systems. This paper presents with applications and case studies a transmission and distribution integrated monitoring and analysis system for high DG penetration. The integrated system analyzes the mutual interaction between the transmission and distribution systems due to DG. The preliminary evaluation of the DG connections is automated in this system, using real time online data. Case studies with practical data show the need and effectiveness of transmission and distribution integrated monitoring and analysis for real power systems with high DG penetration.

The application of the direct current (DC) transmission is increasing through the interconnection between grids or the renewable energy resource integration. Various types of DC transmission topology are researched, and the hybrid multi-terminal high voltage DC (HVDC), called the “MTDC”, is one of the research subjects. The hybrid multi-terminal HVDC is the MTDC system that is composed with the Line Commutated Converter (LCC) and Voltage Source Converter (VSC). Most hybrid MTDC research has been focused on the connection of the renewable energy generation sources, especially offshore wind farms. However, the DC grid built with a hybrid MTDC was recently proposed due to the development of the converter technology. Therefore, the DC grid is expected to be able to substitute some parts of the transmission grid instead of the alternating current (AC) system, and the operation strategies of the DC grid are still being researched. The DC grid has the advantage of being able to control the power flow, which can even improve the stability of the connected AC system. The dynamic model is required to analyze the improvement of the AC system by the operation strategy of the hybrid MTDC, however, there is no generic model for the system. In this paper, an operation strategy of the hybrid MTDC is proposed to improve the stability of the AC power system by increasing the utilization of parallel AC transmission lines under the contingency condition. Furthermore, studies on the modeling method for a hybrid MTDC analysis were performed. The proposed modeling method and operation strategy were verified in simulations for which a modified IEEE 39 bus test system was used. The improvement of transient stability by the proposed hybrid MTDC system was shown in the simulation results.

Many of the recent projects for new transmission line have considered the high-voltage direct current (HVDC) system, owing to the many advantages of the direct current (DC) system. The most noteworthy advantage is that a cable can serve as a substitute for the overhead transmission line in residential areas; therefore, the HVDC system application is increasing, and as the number of DC systems in the power system increases, the interaction assessment regarding the HVDC system gains importance. An index named multi-infeed interaction factor (MIIF) is commonly used to estimate the interaction between power converters; however, the HVDC system is composed of two converters and a transmission line. The MIIF represents the interaction between the rectifiers and inverters, but not for the whole system. In this work, a method to assess the interaction of the whole system was therefore studied. To decide on the location of the new HVDC transmission system at the planning stage, in consideration of the interaction of the existing DC system, the line flow change distribution factor, according to the HVDC-transmission capacity change, was examined. Also, a power system transient -stability analysis was performed with different HVDC system locations, depending on the distribution factor. The simulation results indicate that when the factor is higher, two HVDC systems have a stronger interaction and are less stable in the transient state.

This paper proposes two novel power control strategies to improve the angle stability of generators using a Back-to-Back (BTB) system-based voltage source converter (VSC). The proposed power control strategies have two communication systems: a bus angle monitoring system and a special protection system (SPS), respectively. The first power control strategy can emulate the behaviour of the ac transmission to improve the angle stability while supporting the ac voltage at the primary level of the control structure. The second power control scheme uses an SPS signal to contribute stability to the power system under severe contingencies involving the other generators. The results for the proposed control scheme were validated using the PSS/E software package with a sub-module written in the Python language, and the simple assistant power control with two communication systems is shown to improve the angle stability. In conclusion, BTB VSCs can contribute their power control strategies to ac grid in addition to offering several existing advantages, which makes them applicable for use in the commensurate protection of large ac grid.

The Korean government is targeting the development of a renewable energy penetration of 30–35% by 2040 of the total generation. These conditions will decrease network stability due to a lack of inertia, especially in isolated systems, such as Jeju Island. HVDC systems with several balancing arrangements of reserve operation are used to maintain variability and uncontrollability of RES penetration. This paper presents the fast frequency reserves of HVDC control systems for frequency stability enhancement in the isolated power system by using a combined frequency containment process with grid stability standards. A new MIDC reserve operation control with angle and voltage deviation at the bus converter was developed to provide a faster and more appropriate balancing arrangement compared to the other concept. In addition, two layers of energy and transfer-capacity flexibility were considered to prevent the need to hunt for that balancing arrangement, as well as low nadir frequency, unavailability of the reserve and other constraints caused by each region having a different network size, HVDC interconnection capacity, and type. The proposed control schemes were verified by simulations on the Korean power system model implemented in PSS/E for different sizes of disturbance.