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This paper proposes a new approach to estimate the state of charge of Li-ion batteries, the hybrid-type intelligent state of charge observer. The proposed observer has the combination structure of a sliding mode observer and a fuzzy PI observer and adopts the expert system based on Takagi–Sugeno fuzzy reasoning to switch those two nonlinear observers. This scheme embodies the advantages; the fast dynamic response by the sliding-mode observer and the chattering attenuation in steady state by the fuzzy PI observer. Some comparative simulation verifies and demonstrates the performance and the superiority of the proposed observer.

This paper presents strategies for a voltage controller in which droop control and var reserve control (an auxiliary controller) are included in delta-connected H-bridge STATCOM. To supply the linear property and load sharing effect, the droop control has been applied to a voltage controller. In addition, the var reserve control has been chosen to enlarge the output margin depending on the dead band range. According to the installation environment, the two auxiliary controllers are chosen. However, the combination control of two auxiliary inputs has not been determined. In this paper, three different control strategies (combination methods) are proposed, and the trajectory of equilibrium point is identified. The three combination methods are implemented from HILS tests.

Most power systems at islands in South Korea are stand-alone microgrids, which are disconnected from main grid, and generally have small power demand. Although the diesel generator has many advantages as main power sources for small power system, it has problems of very high operational and maintenance cost. To solve these problems, the South Korean government has replaced conventional diesel generators with renewable energies based on converter-based generators. In this study, the power system in Dukjuk Island in South Korea is modeled in detail based on practical data. Then, the voltage and frequency responses of the stand-alone microgrid are analysed corresponding to the penetration ratio of total diesel generation.

This study proposes an energy management system (EMS) for stable operation of isolated microgrid which is composed of diesel generators, wind turbines, photovoltaic generation systems and battery energy storage systems (BESS). In the EMS, a power dispatch control is proposed in order to minimise diesel operation. There is also a state-of-charge (SOC) control to allow continuous use of the BESS. By dispatching the scheduled power of the diesel generators, the SOC can be regulated within its limits. A simulation model for isolated microgrid was developed using DIgSILENT Power Factory® software. To verify the performance of the proposed EMS, case studies are performed using practical data of Dukjuk Island in Korea.

With the increase of renewable energy penetration and various types of load demand, the 70 kV transmission lines are needed in power system of South Korea. In order to apply the new voltage level to the power system, its stability and reliability must be preferentially verified. The limitation on total capacity of all distributed generations (DGs) in the new voltage level can guarantee the stability of power system. This paper proposes an estimation method for the allowable capacity of DGs in the 70 kV transmission system and its sub-distribution system. First, reasonable technical requirements are established based on the protective coordination, short circuit capacity, and voltage stability. Then, the allowable capacities of DGs are estimated for new 70 kV transmission system and its sub-distribution system. As a result, the hosting capacities acquired by investigation of the protection coordination and the short-circuit current are 160 MVA for the test system with a single type of DGs. On the other hand, the various hosting capacities are derived by studying the voltage stability of test system.

This paper describes the operation characteristics of using superconducting cables in dc transmission systems, focusing on dc current and cable parameters. A superconducting power transmission system need not operate at high voltage levels to reduce I2R losses. This eliminates the need for high voltage insulation and large transformers. Voltage source converter (VSC) transmission has come into use in the last decade. The VSC transmission is often used in cases where an ac system is not practical, generally due to long underground or undersea cables, and excessive losses. The combination of VSC and low-voltage dc (LVDC) superconducting transmission cables could be a good solution to minimize the losses. Issues related to the impact of superconducting cables on VSC LVDC transmission system will be presented along with computer simulation results.

The paper describes implementation and application of voltage management system (VMS) to a physical power system for bringing stable and robust operation. The VMS has a role of balancing reactive power generation of the generators, which are controlled by the VMS. The VMS consists of continuous voltage controller (CVC) and discrete voltage controller (DVC). Those are used to maintain the system voltage within stable range. Especially, the CVC induces a supplementary signal to automatic voltage regulator (AVR) to balance reactive power generation of all involved generators. In the paper, the physical application of the VMS is described and the important considerations are introduced. Also, the paper shows the results of the stable application of the VMS.

This paper presents the modeling of insulated-gate bipolar transistor (IGBT) in electromagnetic transients program (EMTP) simulation for the reliable calculation of switching and conduction losses. The conventional approach considering the physical property of switching devices requires many attribute parameters and large computation efforts. In contrast, the proposed method uses the curve fitting and interpolation techniques based on typical switching waveforms and a userdefined component with variable resistances to capture the dynamic characteristics of IGBTs. Therefore, the simulation time can be efficiently reduced without losing the accuracy while avoiding the extremely small time step, which is required in simulation by the conventional method. The EMTP based simulation includes turn-on and turn-off transients of IGBT, saturation state, forward voltage of free-wheeling diode, and reverse recovery characteristics, etc. The effectiveness of proposed modeling for the EMTP simulation is verified by the comparison with experimental results obtained from practical implementation in hardware.

In the last decade, the installation of distributed generations (DGs) by many independent power producers (IPPs) has been being increased in distribution systems of South Korea by the active policy of government to expand power generations based on renewable energy sources. However, it is difficult to unlimitedly admit the penetration of DGs on a practical distribution system due to its stability and reliability problems. Therefore, it is required to reasonably regulate the allowable capacity of DGs in distribution system. This paper newly proposes a methodology and procedure to estimate the allowable capacity of DGs connected to a physical distribution system. All technical requirements derived for the estimation are established by considering protective coordination, short circuit current, and voltage stability. Then, they are evaluated by applying them to a practical distribution system in South Korea.

This paper presents the results of field tests on Voltage Management System (VMS) using hybrid voltage control, which utilizes coordinated controls of various reactive power resources such as generators, FACTS and switched shunt devices to regulate the pilot bus voltage in a voltage control area. It also includes the results of performance test on RTDS-based test bed in order to validate the VMS before installing it in Jeju power system. The main purpose of the system is adequately to regulate the reactive power reserve of key generators in a normal condition with coordination of discrete shunt devices such as condensers and reactors so that the reserves can avoid voltage collapse in emergency state in Jeju system. Field tests in the automatic mode of VMS operation are included in steady-states and transient states. Finally, by the successful operation of VMS in Jeju power system, the VMS is proved to effectively control system voltage profiles in steady-state condition, increase system MVAR reserves and improve system reliability for pre- and post-contingency.

The paper describes a method to damp out low-frequency oscillations by design and tuning of both power system stabilizer (PSS) and controllable series capacitive reactance compensator (SCRC) controllers in power system. The PSS is generally used to mitigate the low-frequency oscillation by injecting supplementary signal to automatic voltage regulator (AVR) and the SCRC is used to enhance the transfer capacity for the transmission line in power system. Especially, the design of the external controller of the SCRC can provide effective damping for the low-frequency oscillation. In the paper, the effect of the design of both controllers on the system stability is analyzed. Also, the parameter tuning method is introduced and the performances of the proposed method are carried out by time-domain simulation with single-machine connected to infinite bus.

The systematic optimal tuning of power system stabilizers (PSSs) using the dynamic embedded optimization (DEO) technique is described in this paper. A hybrid system model which has the differential-algebraic-impulsive-switched (DAIS) structure is used as a tool for the DEO of PSSs. Two numerical optimization methods, which are the steepest descent and Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithms, are investigated to implement the DEO using the hybrid system model. As well as the gain and time constant of phase lead compensator, the output limits of PSSs with nonsmooth nonlinearities are considered as the parameters to be optimized by the DEO. The simulation results show the effectiveness and robustness of the PSSs tuned by the proposed DEO technique on the IEEE 39 bus New England system to mitigate system damping.

The high penetration of distributed energy resources (DERs) in the distribution system brings new challenges related to voltage variations and reverse power flow. The sudden changes in electric power generation from the DERs due to climate conditions or their internal malfunction might cause high fluctuations in system voltage. To enhance the voltage stability when a disturbance occurs, this paper proposes a new reactive power management method to control each DER based on voltage sensitivity analysis. The voltage variation has the different features in both steady-state and transient conditions. In particular, its transient behavior depends on the type of DERs. Therefore, the particular optimal control for improving the dynamic response of voltage is difficult to apply for all types of DERs. In contrast, the voltage variation in steady-state can be controlled with the reactive power management by the sensitivity analysis between reactive power generation and system voltage. Even though this paper focuses on the reduction of voltage variation in steady-state, the relationship between the voltage variations in steady-state and transient conditions is also analyzed. The effectiveness of the proposed method is verified with several case studies on the practical distribution system in South Korea by using the time-domain simulation based on the PSCAD/EMTDC® software. The results show that the proposed reactive power management method can improve the dynamic voltage responses in both steady-state and transient conditions when the distribution system has the high penetration of renewables.

This paper presents the design of the advanced voltage management system (AVMS) including manual operation mode in order to control reactive power generation in a practical power system. The existing secondary voltage controller (SVC) has a demerit occurring as a large transient phenomenon shortly after the SVC begins to work. This malfunction results from the difference of the operation statuses before and after the SVC operates. For solving the problem and implementing the voltage control safely, the AVMS including the manual operation mode is developed by Korea Electric Power Corporation. Aside from the fact that the AVMS overcomes the drawback, it can also execute the coordination control of both continuous voltage controller (CVC) and discrete voltage controller (DVC). The CVC controls reactive power generation sources such as generator and flexible ac transmission system devices which can generate reactive power continuously and linearly. On the other hand, the DVC controls the discrete reactive power sources such as switched shunt capacitors and reactors. The performances of the proposed AVMS are carried out by time-domain simulation in the Jeju Island power system with real-time digital simulator.

The paper describes an application of voltage management system (VMS) to a physical power system. The VMS has a role of balancing reactive power generation of the generators, which are controlled by the VMS. The VMS consists of continuous voltage controller (CVC) and discrete voltage controller (DVC). Those are used to maintain the system voltage within stable range. Especially, the CVC induces a supplementary signal to automatic voltage regulator (AVR) to balance reactive power generation of all involved generators. In the paper, the physical application of the VMS is described and the important considerations are introduced. Also, the paper shows the results of the stable application of the VMS.

This paper analyzes the effect of a superconducting dc cable on the VSC LVDC system in determining the size of the quenching resistance of the cable. The HTS dc cable has zero impedance at steady state operation. However, the HTS has resistance value when it loses its characteristics. The resistance value affects on circuit breakers operation which prevents the dc current flow through superconductor when quenching occurs. The simulation results showed that the larger quenching resistance value of superconductor could be good solution of thermodynamic recovery problem.

In this paper, the robustness in practical control problems is studied and verified. For such study and verification, the position control of a twin rotor MIMO system is considered in terms of both the simulation and the experiment. With the coupled model of the twin rotor MIMO system, the conventional input-output feedback linearization based nonlinear controller is designed first without the consideration of model uncertainties and its performance is analyzed through both the simulation and the experiment. Based on the analysis results, a robust controller, the sliding mode controller for the twin rotor MIMO system is designed. Accordingly, it is concluded that the importance of the robustness consideration in practice and practical issue for designing a robust controller are verified.

This paper describes a method to design an external controller for secondary voltage controller (SVC) and tune both controllers in order to improve transient stability with real-time simulation. Recently, Korea Electric Power Corporation (KEPCO) has developed a novel voltage management system (VMS), a kind of SVCs, and installed in Jeju Island. It has been operated successfully to balance reactive power reserve of the controlled generators and enhance the voltage stability. However, the VMS has an inherent demerit in short-term recovery after a disturbance such as load change and tripping transmission line occurs. In this paper, after having a discussion related to the demerit in detail, an external controller is proposed. Also, a method to tune the external controller for reducing the bad reaction of the VMS is implemented with real-time digital simulator (RTDS) which is a fully digital power system simulator in real-time. The performance is evaluated by time-domain simulation.

This paper deals with the impact analysis of the impedance change of 345/154[kV] power transformers on the KEPCO system's overall performance. Through the steady-state and dynamic analysis of power system, the maximum available impedance of power transformers were determined. Checking violation of short-circuit current ratings and transformer overload, parallel operation of power transformers, calculation of voltage variation ratio according to the impedance changes of power transformers are included in the steady-state analysis. In addition, transient and voltage stability analysis are also performed in the study. Available magnitudes to be able to change the impedance of the transformers in KEPCO system are finally determined in the paper.

【This paper, as the second part of the paper, dealt with the field test and test results to validate PSS(Power System Stabilizer) parameters which are previously tuned in Part 1 paper. In Part 1 of the paper, the selection of parameters such as lead-lag time constants for phase compensation and system gain was optimized by using linear & eigenvalue analyses and they were verified through the time-domain transient stability analysis. In part 2, the performance of PSS was finally verified by the generator's on-line field test. Through the comparisons of simulation results and measured data before and after tuning of the PSS, the models of generator and its controllers including AVR, Governor and PSS used in the simulation are verified and confirmed.】

This paper deals with parameter tuning of the Power System Stabilizer (PSS) for 612 MVA thermal power plants in the KEPCO system and its validation in a field test. In this paper, the selection of parameters, such as lead-lag time constants for phase compensation and system gain, is optimized using linear and eigenvalue analyses. This is then verified through the time-domain transient stability analysis. In the next step, the performance of PSS is finally verified by the generator's on-line field test. After the field test, measured and simulated data are also compared to prove the effectiveness of the models used in the simulations.

This paper presents a study to estimate the composition of an electric load, i.e. to determine the amount of each load class by the direct measurements of the total electric current waveform from instrument reading. Kalman filter algorithm is applied to estimate the electric load composition on a consumer side of a distributed power system. The electric load supplied from the different voltage level by using a non-ideal delta-wye transformer is also studied with consideration of the practical environment for a distributed power system.

The paper describes an optimal parameter selection method of automatic voltage regulator (AVR) for a large power plant in order to improve power system stability. The AVR is generally used to control the output voltage of the generator by regulating the excitation voltage in a generator rotor. In order to improve transient stability in power system, the paper proposes the method to select optimal parameters of the AVR based on exact power system modeling. The performances of the proposed method are carried out by timedomain simulation by applying fault to a power system.

This paper reviews the various operation techniques of wind turbines within the stand-alone microgrid. First, the paper focuses on the feature and operation of the stand-alone microgrid when it includes wind turbines. Then, various methods to operate and control wind turbines within the stand-alone microgrid are introduced. In addition, main considerations are explained for the operation of a wind turbine generator (WTG) in a stand-alone microgrid. Finally, the technical challenges for this issue are discussed.

The paper describes the coordination control of PSS (power system stabilizer) and FACTS (flexible AC transmission system) devices in order to improve low-frequency oscillation followed by disturbances. The PSS is generally used to mitigate the low-frequency oscillation and the FACTS is used to enhance the transfer capacity for the transmission line in power system. In order to improve small signal and transient stability in power system, the paper proposes the combination of the PSS and FACTS. The performances of the proposed method are carried out by time-domain simulation with single- machine connected to infinite bus system.

This paper presents the design of the advanced voltage management system (AVMS) including manual operation mode in order to control reactive power generations in practical power system. The existing secondary voltage controller (SVC) has a demerit occurring large transient phenomenon shortly after the SVC begins to work. This malfunction results from the difference of the operation status before and after the SVC operates. For solving the problem and implementing the voltage control safely, the AVMS including the manual operation mode is developed by Korea Electric Power Corporation (KEPCO). As well as the AVMS overcomes the drawback, it can execute the coordination control of both continuous voltage controller (CVC) and discrete voltage controller (DVC). The CVC controls reactive power generation sources such as generator and FACTS devices which can generate reactive power continuously and linearly. Otherwise, the DVC controls the discrete reactive power sources such as switched shunt capacitors and reactors. The performances of the proposed AVMS are carried out by time domain simulation in the Jeju Island power system with real-time digital simulator (RTDS).

This paper describes the design of a nonlinear controller in a power system by using the reduced multivariate polynomial (RMP) optimization algorithm with the one-shot training property. The RMP model is applied to estimate its Hessian matrix in addition to identifying the trajectory sensitivities obtained from hybrid system modeling for the power system. In this paper, the saturation limiter of the power system stabilizer (PSS), which is an important nonlinear controller to improve low-frequency oscillation damping performance, is tuned optimally by using Hessian matrix estimated by the RMP model. The performance of the optimal output limits determined by the proposed method is evaluated by applying the large disturbance such as a three-phase short circuit to a power system.

This paper describes the nonlinear parameter optimization of power system stabilizer (PSS) by using the reduced multivariate polynomial (RMP) algorithm with the one-shot property. The RMP model estimates the second-order partial derivatives of the Hessian matrix after identifying the trajectory sensitivities, which can be computed from the hybrid system modeling with a set of differential-algebraic-impulsive-switched (DAIS) structure for a power system. Then, any nonlinear controller in the power system can be optimized by achieving a desired performance measure, mathematically represented by an objective function (OF). In this paper, the output saturation limiter of the PSS, which is used to improve low-frequency oscillation damping performance during a large disturbance, is optimally tuned exploiting the Hessian estimated by the RMP model. Its performances are evaluated with several case studies on both single-machine infinite bus (SMIB) and multi-machine power system (MMPS) by time-domain simulation. In particular, all nonlinear parameters of multiple PSSs on IEEE benchmark two-area four-machine power system are optimized to be robust against various disturbances by using the weighted sum of the OFs.

This paper describes the optimal tuning for the output limits of the power system stabilizer (PSS), which can improve the system damping performance immediately following a large disturbance. The non-smooth nonlinear parameters such as the saturation limits of the PSS cannot be tuned by the conventional methods based on linear approaches. To implement the systematic optimal tuning for the output limits of the PSS, a feedforward neural network (FFNN) is applied to the hybrid system model based on the differential-algebraic-impulsive-switched (DAIS) structure. The FFNN is firstly designed to identify the trajectory sensitivities obtained from the DAIS structure. Thereafter, it estimates the second-order derivatives of an objective function J, which is used during iterations of optimization process. The performance of the optimal output limits tuned by the proposed method is evaluated by applying a large disturbance to a power system.

The output limits of the power system stabilizer (PSS) can improve the system damping performance immediately following a large disturbance. Due to non-smooth nonlinearities from the saturation limits, these values cannot be determined by the conventional tuning methods based on linear analysis. Only ad hoc tuning procedures can been used. A feedforward neural network (FFNN) (with a structure of multilayer perceptron neural network) is applied to identify the dynamics of an objective function formed by the states, and thereafter to compute the gradients required in the nonlinear parameter optimization. Moreover, its derivative information is used to replace that obtained from the trajectory sensitivities based on the hybrid system model with the differential-algebraic-impulsive-switched (DAIS) structure. The optimal output limits of the PSS tuned by the proposed method are evaluated by time-domain simulation in both a single machine infinite bus system (SMIB) and a multi-machine power system (MMPS).