Korean Power System Research Articles

지자기 폭풍에 의한 국내 전력계통 최대 유도 전계량 분석

지자기 폭풍에 의한 국내 전력계통 최대 유도 전계량 분석

BESS를 이용한 대규모 발전단지 제약완화 효과에 관한 연구

In case of 765kV transmission line tripping, there is a Special Protection Sheme(SPS) in Korea power system. To avoid generator over-speed, preselected generators can be tripped by SPS. However, the system frequency can be under the Under Frequency Relay(UFR) 1st pickup level 59.92Hz when large scale generators are tripped. Accordingly, generation constraints can arise until new transmission lines are installed. In this paper, we proposed BESS for mitigating generation constraints of large scale generators, and we applied to the Korean power system in 2022 to analyze the effects of BESS application.

A Study on Optimal Power System Reinforcement Measures Following Renewable Energy Expansion

Renewable energy generation capacity in Korea is expected to reach about 63.8 GW by 2030 based on calculations using values from a power plan survey (Korea’s renewable energy power generation project plan implemented in September 2017) and the “3020” implementation plan prescribed in the 8th Basic Plan for Long-Term Electricity Supply and Demand that was announced in 2017. In order for the electrical grid to accommodate this capacity, an appropriate power system reinforcement plan is critical. In this paper, a variety of scenarios are constructed involving renewable energy capacity, interconnection measures and reinforcement measures. Based on these scenarios, the impacts of large-scale renewable energy connections on the future power systems are analyzed and a reinforcement plan is proposed based on the system assessment results. First, the scenarios are categorized according to their renewable energy interconnection capacity and electricity supply and demand, from which a database is established. A dynamic model based on inverter-based resources is applied to the scenarios here. The transmission lines, high-voltage direct current and flexible alternating current transmission systems are reinforced to increase the stability and capabilities of the power systems considered here. Reinforcement measures are derived for each stage of renewable penetration based on static and dynamic analysis processes. As a result, when large-scale renewable energy has penetrated some areas in the future in Korean power systems, the most stable systems could be optimally configured by applying interconnection measure two and reinforcement measure two as described here. To verify the performance of the proposed methodology, in this paper, comprehensive tests are performed based on predicted large-scale power systems in 2026 and 2031. Database creation and simulation are performed semi-automatically here using Power System Simulator for Engineering (PSS/E) and Python.

Dynamic-Model-Based AGC Frequency Control Simulation Method for Korean Power System

To fulfill the need of operating power systems more effectively through diverse resources, frequency control conditions for maintaining a balance between generators and loads need to be provided accurately. As frequency control is generally achieved via the governor responses from local generators and the automatic generation control (AGC) frequency control of the central energy management system, it is important to coordinate these two mechanisms of frequency control efficiently. This paper proposes a dynamic-model-based AGC frequency control simulation method that can be designed and analyzed using the governor responses of generators, which are represented through dynamic models in the planning stage. In the proposed simulation model, the mechanism of the AGC frequency control is implemented based on the dynamic models of the power system, including governors and generators; hence, frequency responses from the governors and AGC can be sequentially simulated to coordinate and operate these two mechanisms efficiently. The effectiveness of the proposed model is verified by simulating the AGC frequency control of the Korean power system and analyzing the coordination effect of the frequency responses from the governors and AGC.

Stochastic Modeling Method of Plug-in Electric Vehicle Charging Demand for Korean Transmission System Planning

The number of plug-in electric vehicles (PEVs) has rapidly increased owing to the government’s active promotion policy worldwide. Consequently, in the near future, their charging demand is expected to grow enough for consideration in the planning process of the transmission system. This study proposes a stochastic method for modeling the PEV charging demand, of which the time and amount are uncertain. In the proposed method, the distribution of PEVs is estimated by the substations based on the number of electricity customers, PEV expansion target, and statistics of existing vehicles. An individual PEV charging profile is modeled using the statistics of internal combustion engine (ICE) vehicles driving and by aggregating the PEV charging profiles per 154 kV substation, the charging demand of PEVs is determined for consideration as part of the total electricity demand in the planning process of transmission systems. The effectiveness of the proposed method is verified through case studies in the Korean power system. It was found that the PEV charging demand has considerable potential as the additional peak demand in the transmission system planning because the charging time could be concentrated in the evening peak time.

Hardware-in-the-Loop Simulation Using Real-Time Hybrid-Simulator for Dynamic Performance Test of Power Electronics Equipment in Large Power System

This paper presents the hardware-in-the-loop simulation for dynamic performance test (HILS-DPT) of power electronic equipment replicas using a real-time hybrid simulator (RTHS). The authors developed the procedure of HILS-DPT, and as an actual case example, the results of HILS-DPT of Static VAR Compensator (SVC) replica using RTHS is presented. RTHS is a co-simulation tool that synthesizes real-time simulator (RTS) with transient stability program to perform real-time dynamic simulation of a large power system. As power electronics applications have been increasing, the electric utilities have performed HILS-DPT of the power electronics equipment to validate the performance and investigate interactions. Because inspection tests are limited in their ability to validate its impact on the power system during various contingencies, all power electronics equipment newly installed in the Korean power system should take HILS-DPT using large-scale RTS with replicas since 2018. Although large-scaled RTS offers an accuracy improvement, it requires lots of hardware resources, time, and effort to model and simulate the equipment and power systems. Therefore, the authors performed SVC HILS-DPT using RTHS, and the result of the first practical application of RTHS present feasibility comparing the result of HILS-DPT using large-scale RTS. The authors will discuss the test results and share lessons learned from the industrial experience of HILS-DPT using RTHS.

Enhancement of linearity and constancy of PMU‐based voltage stability index: application to a Korean wide‐area monitoring system

This study presents a new wide-area voltage stability monitoring method using synchronised phasor measurements. The salient features of the proposed index are the following: (i) It ensures constancy, which is the ability to maintain the same scale even if the system changes. (ii) It is pseudo-linear with respect to the power system stability level, which enhances intuitiveness for real-time operation. Accordingly, stability can be uniformly compared in various situations, and thus, the index can be used as a solid criterion or threshold point for system operation. A verification study of the proposed index with regard to constancy is carried out using Korean power system operation data. Furthermore, an implementation of the proposed method for the Korean wide-area monitoring system is presented, and it is demonstrated that the proposed index detects event more clearly than previous stability index through an actual event measured using phasor measurement units.

Application of the First Replica Controller in Korean Power Systems

The purpose of this paper is to introduce, examine, and evaluate the industrial experiences and effectiveness of a Thyristor Controlled Series Compensator (TCSC) replica controller installed in Korea in 2019 through a review of its configuration, test platform, and practical application, and further to propose operational guidelines for replica controllers. Four representative practical cases were conducted: a Dynamic Performance Test (DPT) under a sufficiently large-scale power system prior to the Site Acceptance Test (SAT), pre-verification for on-site controller modification during operation stage, parameter tuning to mitigate the control interaction, and time domain simulation for Sub-Synchronous Torsional Interaction (SSTI). None of these four cases can be performed in a Factory Acceptance Test (FAT) or on-site. Therefore, TCSC control performance was accurately verified under the entire Korean power system based on a large-scale real-time simulator, which demonstrated its effectiveness as a powerful tool for operations including multiple power electronics devices. Our review herein of these four practical cases is expected to show the usefulness of replica controllers, to demonstrate their strength to deal with practical field events, and to contribute to the further expansion of the application area from a perspective of electric utility.

확장된 토폴로지를 갖는 배전변전소의 지능형 고장복구 알고리즘의 연구

This paper propose an intelligent restoration algorithm for newly extended distribution substations equipped with double bus double brakers structure, which are widely used in Korean power systems. The complete hierarchical restoration process are proposed in detail including distribution buses which have extended topology. As the dramatic intelligent automation of power systems is in progress so far, it is anticipated that the results would be applied to IEC protocols in the future as well as the state estimation function for distribution substations.

Detection and Analysis of Electromechanical Oscillation in Power Systems with Low-Sampled Data Using Modal Analysis Methods

Electromechanical oscillations between interconnected generators are considered a major threat to the secure operation of power systems. Therefore, oscillation monitoring systems in real-time are of critical importance to detect the danger of poorly damped oscillations. For the detection and analysis of the oscillations, high-temporal-resolution measurements are required according to the Nyquist theorem. This paper proposes a novel algorithm for the identification of electromechanical oscillations using low-sampled data such as supervisory control and data acquisition (SCADA) measurements. The lack of temporal resolution of the data is compensated by using low-sampled data sets at multiple different locations. At a target location, a high-sampled data-signal can be reconstructed using mode shape information obtained from model-based modal analysis. The variable projection method is then used to detect oscillations and estimate oscillation components including frequency and damping ratio. Case studies based on practical Korean power systems are presented to evaluate the performance of the proposed method. Simulation results show that the proposed method can detect and identify electromechanical oscillations with low-sampled data.

Impact Analysis of Transmission Congestion on Power System Flexibility in Korea

This study proposes a method to evaluate the impact of transmission congestion on the flexibility of a power system, based on the ramping capability shortage expectation (RSE). Here, flexibility refers to the ability to retain a power balance in response to changes in the net load. The flexibility issue arises due to the extensive integration of renewable energy resources; specifically, the higher the degree of integration, and the greater the variability and uncertainty in the power system. Flexibility is further limited by the net transfer capacity (NTC) of transmission lines. Here, we propose a method capable of capturing the extent to which transmission congestion affects the power system, to identify transmission reinforcement options for improved flexibility. In Korea, transmission congestion occurs frequently in regions to the north and southeast. A case study for a Korean power system in 2030 was conducted. Simulation results showed that the impact of transmission reinforcement in flexibility tends to be proportional to the NTC and is greater when the penetration level is low.

Determining the Capacity Limit of Inverter-Based Distributed Generators in High-Generation Areas Considering Transient and Frequency Stability

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.

Recurrent Neural-Network-Based Maximum Frequency Deviation Prediction Using Probability Power Flow Dynamic Tool

This paper proposes a recurrent neural network (RNN)-based maximum frequency deviation forecasting model for power systems with high photovoltaic power (PV) penetration. The proposed RNN model extracts the nonlinear features and invariant structures exhibited in regional PV power output data and time-variable frequency data in case of contingency. To capture the regularity and random characteristics of PV power output, a probability power flow-dynamic tool (PPDT) for uncertain power system modeling has been developed. This tool considers all possible combinations of PV power generation patterns, even those with low probability, such as those caused by passing clouds. The results are verified by a comparison of various artificial intelligence methods using case studies from the South Korean power system. An online dispatch algorithm that considers the frequency constraints for a designated contingency can be implemented by using the proposed model.

Oscillation Recognition Using a Geometric Feature Extraction Process Based on Periodic Time-Series Approximation

Oscillations may cause both economic and technical problems such as a reduction in overall system reliability. Therefore, detecting and preventing oscillatory behavior that affects power systems is important. This paper proposes an oscillation recognition method that includes monitoring and extracting features in a recursive and sequential manner in a time-series measurement in power systems. We propose a geometric feature extraction process for recognizing oscillations by constructing an average system and Poincare map for time-series measurement. The proposed process provides the features of a system’s damping and frequency of oscillation, and the developed monitoring systems are based on nonlinear dynamics. The circulating oscillatory behavior is represented on a finite-integer-delay embedded time-series plane, extracted by a Poincare map construction, and examined directly along the trajectory to monitor the features of the oscillation according to damping and frequency. Oscillatory behavior recognition is tested on IEEE’s second benchmark system for subsynchronous resonance to verify the fast extraction of oscillation components. In addition, a case study for Korean power systems with a high penetration of renewable energy and application on actual measurement data is carried out to demonstrate the practical application of the process.

육지계통 HVDC 확대와 DC Tap 설정에 따른 융통조류 여유량 산정에 관한 연구

The load continues to be concentrated in the metropolitan area, and the power generation is reduced in the metropolitan area, and the power generation in the non-metropolitan area is increased, so that power transmission is inevitably made through interface lines. In this environment, it is necessary to secure additional transmission lines because existing interface lines have reached the limit to transmit large-scale power. KEPCO has installed a number of HVDC alternatives, which will affect the determination of the limit of the interface power flow that is the basis for operating the system. In this paper, how to calculate the interface flow limit in this environment is described. According to DC tap setting in HVDC facilities, the interface power flow margin changes are monitored. It also discusses how to do with FV(Flow-Voltage) analysis in the future. The simulations were performed using the 8th planned Korean power systems data.

Assessment of Primary Control Reserve Requirement from Generator Considering a Battery Energy Storage System

In Korean power system, 376 MW of battery energy storage system (BESS) functions as primary control reserve. The current total requirement of primary control reserve is 750 MW, which is procured by 376 MW of BESS and 374 MW of conventional generators. The BESS installed in the Korean power system can supply its energy for 10 min after loss of generators. If all the energy of the BESS is supplied during the system frequency restoration, this can cause a shortage in primary control reserve. Therefore, a new approach for reserve procurement is required to cope with the shortage in primary control reserve in case the second contingency might happen. Pumped storage hydro generators are good candidates for the additional reserve sources. In this paper, we propose a methodology for procuring additional primary control reserve considering their availability varying in real time and droop characteristics. In the case study, the effect of the proposed methodology is verified through a dynamic simulation.

Evaluation Method for Penetration Limit of Renewable Energy Sources in Korean Power System

As Korea aims to increase the extent to which renewable energy sources (RES) account for up to 20% of the total power generated in the country by 2030, the feasibility of this target is a major concern. This concern largely results from the Korean power system possessing unique characteristics, such as its electrical isolation and high density. To achieve the RES target, the reliable operation of the power system must coexist with an increased share of RES power generation. This study proposes a method to evaluate the penetration limit of RES in the Korean power system considering the existing plans for the long-term electricity supply and demand, as well as its operational requirements. The Korea electric power corporation (KEPCO) planning database of the Korean power system for the next 15 years was employed to determine the penetration limit of RES considering the reliability criteria, including the minimum power generation of conventional sources, primary frequency control requirement, 10-min reserve requirement, and frequency stability.

Control Strategy of BESS for Providing Both Virtual Inertia and Primary Frequency Response in the Korean Power System

As the battery energy storage system (BESS) has been considered to be a solution to the diminished performance of frequency response in the Korean power system, in which renewable energy resources (RESs) are expected to increase rapidly, this paper proposes a control strategy for providing both the virtual inertia and primary frequency response considering the MW-scale BESS installed by the Korea Electricity Power Corporation (KEPCO). The benefit of such a fast and flexible BESS can be maximized by the proposed control strategy for making it provide both the inertia and primary frequency response, which would be deficit with the increased RES. In the proposed control strategy, the state of charge (SOC) is maintained in the specific range in which the life cycle is maximized, the interference of SOC recovery by frequency control is minimized, the responding capacity for providing the virtual inertia response is maximized during the transient period, and the performance requirements for frequency response are satisfied. The effectiveness of the proposed strategy is verified by both Korean power system model-based simulation and on-site operations.

Utilization of Energy Storage System for Frequency Regulation in Large-Scale Transmission System

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.

과도안정도와 미소신호안정도 관점에서 국내 전력계통의 임계관성계수 평가

The increase of inverter-based renewable energy sources reduces inertia constant of power systems and the loss of the system inertia may cause power system stability problems. To ensure a secure energy delivery system, these problems should be analyzed thoroughly from various perspectives about power system stability. This paper presents the estimation of the critical inertia of Korean power systems, which is the minimum amount of inertia required to maintain power system dynamic securities. The estimation has been made by gradually reducing the system inertia of Korean power systems and then by analyzing the change of dynamic securities in terms of transient and small signal stabilities. In the transient stability aspect, the decrease of critical clearing time(CCT) has been analyzed with four different reduced inertial models of Korean power systems. With the small signal stability perspective, the change of the system damping ratio has been studied. Based on the results from the stability studies, the critical inertia has been determined and it can be considered as the indirect limit of renewable energy shares in Korean power systems.