Autonomous state estimation and its application to the autonomous operation of the distribution system with distributed generations

The objective of this thesis is to propose guidelines for advanced operation, control, and protection of the restructured distribution system by designing the architecture and functionality for autonomous operation of the distribution system with DGs. The proposed architecture consists of (1) autonomous state estimation and (2) applications that enable autonomous operation; in particular, three applications are discussed: setting-less component protection, instant-by-instant management, and short-term operational planning. Key elements of the proposed approach have been verified: (1) the proposed autonomous state estimation has been experimentally tested using laboratory test systems and (2) the feasibility of the setting-less component protection has been tested with numerical simulations.Ph.D

Optimal Scheduling of Battery Energy Storage Systems and Demand Response for Distribution Systems with High Penetration of Renewable Energy Sources

The penetration of renewable energy sources (RESs) is increasing in modern power systems. However, the uncertainties of RESs pose challenges to distribution system operations, such as RES curtailment. Demand response (DR) and battery energy storage systems (BESSs) are flexible countermeasures for distribution-system operators. In this context, this study proposes an optimization model that considers DR and BESSs and develops a simulation analysis platform representing a medium-sized distribution system with high penetration of RESs. First, BESSs and DR were employed to minimize the total expenses of the distribution system operation, where the BESS model excluding binary state variables was adopted. Second, a simulation platform based on a modified IEEE 123 bus system was developed via MATLAB/Simulink for day-ahead scheduling analysis of the distribution system with a high penetration of RESs. The simulation results indicate the positive effects of DR implementation, BESS deployment, and permission for electricity sales to the upper utility on decreasing RES curtailment and distribution system operation costs. Noticeably, the RES curtailments became zero with the permission of bidirectional power flow. In addition, the adopted BESS model excluding binary variables was also validated. Finally, the effectiveness of the developed simulation analysis platform for day-ahead scheduling was demonstrated

Impact of Transformer Topology on Short-Circuit Analysis in Distribution Systems with Inverter-Based Distributed Generations

Distributed energy resources (DERs), recently introduced into distribution systems, are mainly inverter-based distributed generations (IBDGs), which have different short-circuit behaviors from conventional synchronous-based distributed generations (SBDGs). Hence, this study presents a comprehensive analysis of the short-circuit behaviors of distribution systems with IBDGs, based on sequence networks and superposition, from the perspectives of interconnected transformers, and observes the flow of zero-sequence fault currents with different transformer topologies. Moreover, two- and three-winding transformers with various bank connection types and groundings are investigated. It was concluded that the transformer topology and its grounding influence the fault current contribution in zero-sequence networks, and the high penetration of IBDGs alters the fault current magnitude and phase angles

Probabilistic Stability Evaluation Based on Confidence Interval in Distribution Systems with Inverter-Based Distributed Generations

This study proposed a probabilistic methodology based on a confidence interval with the aim of overcoming the limitations of deterministic methods. A stability evaluation technique was required because the output variability of renewable energy can lead to instability of the distribution system. The proposed method can predict the possibility of violating stability in the future. It can also provide a theoretical basis for securing distribution system stability and improving operational efficiency by assessing the in-stability risk and worst-case scenarios. Because of steady-state analysis in the distribution system to which solar power is connected, the probability of violating the standard voltage during the daytime when PV fluctuations are severe was the highest. Moreover, as a result of a simulation of a three-phase short-circuit in the distribution system that is connected to the PV and WT, it was observed that it could violate the allowable capacity of the CB owing to the effects of the power demand pattern and output variability

Control Strategy for Line Overload and Short Circuit Current of Networked Distribution Systems

The expected increase in renewable energy sources (RESs) and electric vehicles (EVs) connected to distribution systems will result in many technical constraints. A meshed network is a promising solution; however, some remarkable challenges must be overcome. Among these, this paper mainly focuses on the line overload and short circuit current of a networked distribution system (NDS) in Korea, an advanced form of meshed network. An NDS refers to a system in which there exists permanent linkages between four feeders and N × N communication-based protection. We propose a method, which employs the tap changing control algorithm of the series reactor to control line overload and short circuit current. MATLAB/Simulink was used to evaluate the proposed method. Three different types of distribution system were employed. First, the utilization rate and feeder imbalance were analyzed in steady-state condition. Subsequently, the short circuit current was analyzed in short circuit condition. The results revealed that the proposed method can effectively prevent line overload in up to 82.7% of cases, enhance the utilization rate by up to 79.9%, and relieve the short circuit current; that is, it can contribute to system stability and the economic operation of an NDS

Cloud Cover Forecast Based on Correlation Analysis on Satellite Images for Short-Term Photovoltaic Power Forecasting

Photovoltaic power generation must be predicted to counter the system instability caused by an increasing number of photovoltaic power-plant connections. In this study, a method for predicting the cloud volume and power generation using satellite images is proposed. Generally, solar irradiance and cloud cover have a high correlation. However, because the predicted solar irradiance is not provided by the Meteorological Administration or a weather site, cloud cover can be used instead of the predicted solar radiation. A lot of information, such as the direction and speed of movement of the cloud is contained in the satellite image. Therefore, the spatio-temporal correlation of the cloud is obtained from satellite images, and this correlation is presented pictorially. When the learning is complete, the current satellite image can be entered at the current time and the cloud value for the desired time can be obtained. In the case of the predictive model, the artificial neural network (ANN) model with the identical hyperparameters or setting values is used for data performance evaluation. Four cases of forecasting models are tested: cloud cover, visible image, infrared image, and a combination of the three variables. According to the result, the multivariable case showed the best performance for all test periods. Among single variable models, cloud cover presented a fair performance for short-term forecasting, and visible image presented a good performance for ultra-short-term forecasting

Mitigating Subsynchronous Torsional Interaction Using Geometric Feature Extraction Method

This paper proposes a method to mitigate subsynchronous torsional interaction detected during power system operation. This innovative method employs the delay reconstruction of the damping controller of a thyristor-controlled series compensator. This addresses the need to detect and manage stability and electromagnetic transients in power systems caused by the increasing use of fast-response power electronics. Previously, severe oscillation conditions could be avoided via analysis of the subsynchronous torsional interaction scenarios during the planning stage, enabling the suppression of oscillations. However, planning, modeling, and analysis for various scenarios becomes more difficult as the complexity of the power system increases, owing to the use of renewable energy and the incorporation of topology changes. Therefore, interest in measurement data-based real-time oscillation analysis has increased. The first step of the mitigation strategy proposed herein reconstructs nonlinear time-series data to detect subsynchronous torsional interaction in real time and generate alert signals. The second step of the strategy is that the controller mitigates oscillations by controlling the firing angle using the geometric feature extraction method. In this paper, the relaxation of the frequency oscillation in the subsynchronous region of about 22 Hz and about 18 Hz was verified through two simulation cases

Mitigating Subsynchronous Torsional Interaction Using Geometric Feature Extraction Method

This paper proposes a method to mitigate subsynchronous torsional interaction detected during power system operation. This innovative method employs the delay reconstruction of the damping controller of a thyristor-controlled series compensator. This addresses the need to detect and manage stability and electromagnetic transients in power systems caused by the increasing use of fast-response power electronics. Previously, severe oscillation conditions could be avoided via analysis of the subsynchronous torsional interaction scenarios during the planning stage, enabling the suppression of oscillations. However, planning, modeling, and analysis for various scenarios becomes more difficult as the complexity of the power system increases, owing to the use of renewable energy and the incorporation of topology changes. Therefore, interest in measurement data-based real-time oscillation analysis has increased. The first step of the mitigation strategy proposed herein reconstructs nonlinear time-series data to detect subsynchronous torsional interaction in real time and generate alert signals. The second step of the strategy is that the controller mitigates oscillations by controlling the firing angle using the geometric feature extraction method. In this paper, the relaxation of the frequency oscillation in the subsynchronous region of about 22 Hz and about 18 Hz was verified through two simulation cases

Strategy for Optimal Grid Planning and System Evaluation of Networked Distribution Systems

The meshed network may become a standard for future distribution systems owing to its various benefits regarding voltage profile, reliability, losses, and the distributed generation (DG). Therefore, in Korea, there is a plan to introduce an advanced form of meshed network called a networked distribution system (NDS). This refers to a system with permanent linkages between four distribution lines (DLs) and N×N communication-based protection. To properly introduce NDS to an actual grid, this study proposes a strategy for optimal grid planning and system evaluation. Four different topologies and four practical indicators are explained. First, load imbalance is used to find the optimal grid that maximizes the load capacity. Second, line overload, fault current, and temporary overvoltage (TOV) were used to evaluate the necessity of load transfer, availability of circuit breakers, relay settings, and system stability. PSCAD/EMTDC were employed for the simulation. This study establishes the construction and evaluation guidelines of NDS for distribution system operators (DSOs)