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This paper describes a method for implementing a gradient-based nonlinear optimization technique for optimal tuning of nonlinear power system controllers such as a flexible ac transmission system (FACTS) via a combination of hardware and software in the real-time digital simulator (RTDS). The RTDS offers fundamental advantages over other commercial software tools because it is a fully digital power system simulator that operates in real time. Therefore, nonlinear optimal parameters can be effectively determined under similar environments to actual physical systems by this real-time implementation. In this paper, the Broyden–Fletcher–Goldfarb–Shanno optimization algorithm, which has a quadratic convergence property, is implemented in order to design the nonlinear controllers in power systems. Its effectiveness is demonstrated with two design examples intended to improve low-frequency oscillation damping in power systems. After illustrating an example to optimize the nonlinear parameters of the power system stabilizer, this paper introduces the optimal design of an external controller for the series capacitive reactance compensator, which is one of the FACTS devices, in real-time simulation. [ABSTRACT FROM AUTHOR]

The selection of placement and size for multiple distributed generators based on renewable energies becomes an important issue in operating modern power systems safely and reliably. This paper extends the previous study in our earlier paper by treating the problems for their optimal locations and sizes in more realistic way based on the power loss sensitivities and Kalman filter algorithm, respectively. In particular, the proposed method is implemented by the simulation on a part of practical distribution network of 60 MVA scale, which is located at the Do-gok area in Seoul, Korea. The new factor, called as the optimal locator index, is developed to determine the optimal locations in a systematic and effective manner by the power loss sensitivities. Also, the Kalman filter algorithm is applied to determine the optimal sizes by choosing the step size of 10 KW, which is a much smaller increment than the 10-MW step used in our earlier paper, without requiring high computational efforts. This selected step size of 10 KW is realistic and suitable to apply in practical test systems. [ABSTRACT FROM AUTHOR]

The controllable capacitive reactance can be used as the input variable for the external controller of a series capacitive reactance compensator (SCRC) to improve the damping of low, frequency oscillations of the rotor angle and active power in a power system. Conventional linear PI controllers are tuned for best performance at one specific operating point of the nonlinear power system. At other operating point its performance degrades. Nonlinear optimal neuro-controllers are able to overcome this degradation. In this paper, the dual heuristic dynamic programming (PUP) optimization algorithm is applied to design an external nonlinear optimal neuro-controller for the SCRC. Simulation studies using PSCAD/EMTDC® software are presented. [ABSTRACT FROM AUTHOR]

As renewable energy penetrates the power system, system operators are required to curtail output power from generation units to balance the power supply and demand. However, large curtailment from wind power plants (WPPs) may instantly cause excessive output power decrement, causing system frequency to drop significantly before reaching its nominal value. In order to solve this problem, this paper proposes a cooperative control framework to determine the operation of WPPs in two control methods, which are the stepwise inertial control (SIC) method and the curtailed control method. The proposed framework first determines the WPPs to operate in the curtailed control method to provide the required power curtailment. Next, it determines the WPPs to operate in the SIC method considering their releasable kinetic energy to provide an effective inertial response and compensate for the sudden excessive output power decrement caused by other WPPs operated in the curtailed control method. Therefore, each WPP is operated in one of two control methods to provide required power curtailment while reducing the sudden excessive output power decrement. To verify the effectiveness of the proposed cooperative control framework, several case studies are carried out on the practical South Korea electric power system. [ABSTRACT FROM AUTHOR]

This paper proposes a computationally effective method for optimal operation of multiple distributed generations (DGs) in dc distribution systems to improve their efficiency. There have been many research works to select the optimal sizes of multiple DGs. However, these optimal sizes are valid only in the particular operating point. For optimal operations of power systems, the DG sizes must be determined all the time according to the various load conditions. The proposed method can maintain the distribution system operating in its optimal condition with only single optimization process, without regard to load variations. Therefore, it can moderate the serious problems, derived from inaccurate load forecasting. To verify the performance of the proposed method, the dc system is first implemented with the real distribution system of Do-gok area in Seoul, Korea. Then, the real hourly-load data, acquired from SCADA system in Korea, are used to simulate the practical load variations. [ABSTRACT FROM PUBLISHER]

This paper describes a study on the optimal location of a resistive superconducting fault current limiter (SFCL) applied to an electric power grid. The resistive SFCL, which is designed to provide the quick system protection in the event of a fault, can have different effects on the planning and operation of a power system depending on its location. To select the optimal location of the SFCL, the sensitivity analysis of power changes and/or power losses in the system with respect to its resistive value occurred in series with a transmission line during a fault is introduced. Moreover, the optimal location determined by the proposed method is coordinated with the corresponding optimal resistive value of the SFCL to improve low-frequency oscillation damping performance of the system. The IEEE benchmarked four-machine, two-area test system is used to evaluate the effectiveness of the proposed method with the case studies based on time-domain simulation. [ABSTRACT FROM AUTHOR]

In South Korea, the existing diesel generators are being replaced with renewable energy sources (RESs) in several stand-alone microgrids. However, their reliability and stability are not still guaranteed by fluctuations of the system caused by RESs, arising from unpredictable changes in environmental factors. Moreover, since the inertia that is originally provided by the diesel synchronous generators is reduced, the overall system becomes highly sensitive to frequency variations. As a result, the installation of RESs must be well planned while considering the upper limit of their installed capacity. In this paper, a new method for evaluating the maximum penetration capacity of photovoltaic (PV) generators to ensure the stable operation of a microgrid is proposed. For the analytical approach, two frequency stability indexes, which are the minimum instantaneous frequency and the quasi-steady-state (QSS) frequency after a disturbance, are used. The capacity of PV to be installed is limited by considering the characteristics of other generators. In addition, the efficiency of the energy storage system (ESS) is also analyzed to determine the optimal capacity of both PV and ESS. The effectiveness of the proposed method is firstly proven through the mathematical analysis. Then, case studies on a practical stand-alone microgrid in South Korea are carried out using a time-domain simulation based on the DIgSILENT PowerFactory® software. [ABSTRACT FROM AUTHOR]

Increase in power demand has required providing infrastructure, such as huge power plants and transmission lines. However, the recent trends in power system planning and operation are being toward maximum utilization of existing electricity infrastructure with tight operating margins because of new constraints placed by economic, political, and environmental issues. This might cause serious stability problems when a severe fault or a sudden outage of critical lines occurs. Therefore, the special protection scheme (SPS) has been used by generator tripping and successive load shedding to prevent a large-scale blackout by the cascading effect. If the number of tripping generators in the SPS operation is reduced, the enormous cost of power outage can be saved. This paper analyzes the enhancement of SPS via the transient stability improvement by the static synchronous compensator (STATCOM) applied to a generation side of Korea electric power system. First, its characteristic related to the SPS is described. Then, the proper capacity of STATCOM is determined with the equal area criterion to reduce the number of tripping generators. Also, the new control method for STATCOM is proposed for its full utilization. Finally, its effectiveness is verified by the case study with the practical data of Korea electric power system. [ABSTRACT FROM AUTHOR]

This paper proposes a computationally efficient technique for estimating the composite load model parameters based on analytical similarity of parameter sensitivity. When the model parameters are updated in the optimization procedure to best fit the actual load dynamics, i.e., measurements, parameters of similar sensitivity representation in the given mathematical model structure are updated in the same manner at every iterative step. This research allows for practically reducing the number of load model parameters to be identified in the estimation process and the overall computational cost while preserving the desired complexity and accuracy of the original model. This approach consequently facilitates the parameter estimation in the optimization process and helps manage increased number of parameters often criticized for adopting the dynamic composite load model via measurement-based approach. Case studies for the real power system demonstrate the computational efficiency and intact accuracy of the proposed method with reference to the existing methods of estimating all the parameters of the given composite load model independently. [ABSTRACT FROM AUTHOR]

The high penetration of wind power decreases the system inertia and primary frequency reserve while replacing the conventional synchronous generators (SGs). Therefore, if the system operator does not take appropriate action on the remaining generation units (GUs) operation, high penetration of wind power will aggravate the frequency stability. To solve this problem, wind power plants (WPPs) may provide the inertial response and primary frequency response (PFR) to support the frequency stability. However, due to the variability of renewable energy, WPPs may not provide adequate frequency response whenever it is required. This paper proposes an algorithm to determine the operation of GUs to provide appropriate PFR for a power system with high penetration of wind power. Through the proposed algorithm, it calculates the required PFR to restore the decreased frequency stability caused by the high penetration of wind power. Then, while considering the available PFR from WPPs, it redetermines the droop coefficient of SGs governor to provide the sufficient PFR to recover the frequency stability. Finally, the effectiveness of the proposed algorithm is verified on the practical Korean electric power system. [ABSTRACT FROM AUTHOR]

This paper proposes a method to determine the optimal size of superconducting magnetic energy storage (SMES) to improve the stability of distribution power system with photovoltaic (PV) generation. The output power of PV system fluctuates according to changing weather conditions. Then, the system is subject to be unstable. In order to improve its stability, the SMES is applied. In general, the SMES is being considered as a strong candidate among energy storage devices in industry due to its high efficiency, fast response, and high energy density. [ABSTRACT FROM PUBLISHER]

This paper describes a study to determine the optimal resistance of a superconducting fault current limiter (SFCL) connected to a wind-turbine generation system (WTGS) in series considering its protective coordination. The connection of the WTGS to the electric power grid may have serious effects on the stability and reliability of the overall system during a contingency due to the increase in fault currents. Moreover, it causes malfunction in the protective devices such as overcurrent relays (OCRs). To deal with this problem, the SFCL is applied to reduce the level of fault current increased by the connection of the WTGS. Then, to determine the optimal parameter of the SFCL, protective coordination between the existing multiple OCRs is considered based on the computation of fault current by the equivalent voltage source method. [ABSTRACT FROM AUTHOR]