Search

This paper deals with the power integration of large urban railway systems to improve regenerative energy utilization. Current regenerative energy utilization is low because there is no special plan for using this energy. If the railway system is integrated with other railway systems, the efficiency is expected to be improved. Through the case studies, to find the realistic effect of integrated operation, real system for the Seoul Metro lines, especially line 5 and 7, had been applied.

This paper proposes frequency-shift acceleration control for anti-islanding of an inverter-based distributed generator. The proposed control is based on frequency positive feedback in the synchronous dq frame. Overall, the scheme of the proposed control is described. The design methodology of the frequency-shift acceleration gain is presented based on small-signal stability and step input response. The proposed control is intended for zero nondetection zone, noncompromised power quality, and easy implementation without additional devices required. Simulation and experimental results verify that the proposed anti-islanding scheme and the gain design approach are effective and useful in real applications.

Recently, many signal-processing techniques, such as fast Fourier transform, short-time Fourier transform, wavelet transform (WT), and wavelet packet transform (WPT), have been applied to detect, identify, and classify power-quality (PQ) disturbances. For research on PQ analysis, it is critical to apply the appropriate signal-processing techniques to solve PQ problems. In this paper, a new time-frequency analysis method, namely, the Gabor-Wigner transform (GWT), is introduced and applied to detect and identify PQ disturbances. Since GWT is an operational combination of the Gabor transform (GT) and the Wigner distribution function (WDF), it can overcome the disadvantages of both. GWT has two advantages which are that it has fewer cross-term problems than the WDF and higher clarity than the GT. Studies are presented which verify that the merits of GWT make it adequate for PQ analysis. In the case studies considered here, the various PQ disturbances, including voltage swell, voltage sag, harmonics, interharmonics, transients, voltage changes with multiple frequencies and voltage fluctuation, or flicker, will be thoroughly investigated by using this new time-frequency analysis method.

A standard of International Electrotechnical Commission (IEC) has defined the flicker measuring specification in an analog manner. But the analog specs are inefficient when implementing a flickermeter in the real world according to the standard. Therefore, it is necessary to convert its original analog filters to the corresponding digital forms as well as applying other digital processing techniques such as data sampling, bit resolution, and filtering theory. Even though a digital flickermeter may meet the requirements of the IEC standard, its outputs will vary according to the analog-to-digital filtering conversion methods used such as bilinear transformation, impulse invariance transformation, and covariance invariant transformation. This paper presents the characteristics of various filter conversion methods and the influences on the digital implementation of a flickermeter, which are simulated by using MATLAB®. Finally, this paper concludes with which methods are the most adequate to implement an IEC digital flickermeter, helping ultimately to avoid ambiguities from analog prototypes described in original standard.

Power quality simulation plays an important role in many practical cases, for example, when deciding the capacity of the related mitigation devices, assessing the influence of installing a nonlinear load in the distribution part, dissolving the quality issues between utilities and customers, and so on. For these purposes, many dedicated tools have been used in order to assess the level of quality distortions by various kinds of PQ indices. However, there are few modules that can accurately simulate the flicker phenomenon, that is, P st and the nonlinear and chaotic behavior of the electrical arc furnace, which is one of the representative nonlinear loads. This paper deals with the useful and simple modules for the voltage flicker simulation in the distribution and transmission level under the environment of Matlab/Simulink. With these modules, the various conditions of distribution systems and the capacities of arc furnaces with the chaotic characteristic can be easily taken into account.

This paper presents power-control strategies of a grid-connected hybrid generation system with versatile power transfer. The hybrid system is the combination of photovoltaic (PV) array, wind turbine, and battery storage via a common dc bus. Versatile power transfer was defined as multimodes of operation, including normal operation without use of battery, power dispatching, and power averaging, which enables grid- or user-friendly operation. A supervisory control regulates power generation of the individual components so as to enable the hybrid system to operate in the proposed modes of operation. The concept and principle of the hybrid system and its control were described. A simple technique using a low-pass filter was introduced for power averaging. A modified hysteresis-control strategy was applied in the battery converter. Modeling and simulations were based on an electromagnetic-transient-analysis program. A 30-kW hybrid inverter and its control system were developed. The simulation and experimental results were presented to evaluate the dynamic performance of the hybrid system under the proposed modes of operation.

A voltage drop in the electrical circuit must be unavoidable. The voltage drop in the electrical circuit means a loss of heat. The heat lost would change the characteristics of the insulator and thus, the insulating performance would be towered resulting in electric leakage, electric shock, power failure, fire and other accidents. Hence, an optimized design against the voltage drop in the electrical circuit must be an important factor determining safety and economy of electrical facilities. This study analyzed the effects of voltage drop on the electrical circuit for such low-voltage electrical facilities requiring the public safety foremost and subject to multi-distributed random loads as street lamps, buildings and subway stations, and thereupon, developed an optimized voltage drop computation program to enhance safety and economy of those electrical facilities.

This paper addresses improving the voltage stability limit of interface flow between two different regions in an electric power system using the Static Synchronous Series Compensator (SSSC). The paper presents a power flow analysis model of a SSSC, which is obtained from the injection model of a series voltage source inverter by adding the condition that the SSSC injection voltage is in quadrature with the current of the SSSC-installed transmission line. This model is implemented into the modified continuation power flow (MCPF) to investigate the effect of SSSCs on the interface flow. A methodology for determining the interface flow margin is simply briefed. As a case study, a 771-bus actual system is used to verify that SSSCs enhance the voltage stability limit of interface flow.

This paper proposes a probabilistic evaluation for the transient stability of electrical power systems incorporating FACTS devices. The uncertainties of the fault location and relay operation time play important keys in power system instability evaluation. The TCSC and SVC are employed for the reduction of system instability probability. This method is demonstrated by the WSCC test system and the results are compared with and without FACTS by means of Monte Carlo simulation.

This paper presents a harmonic analysis model for the Korean high-speed railway. Since the harmonic current flows through the catenary system, the catenary system must be modeled exactly to analyze harmonic current amplifications and the total harmonic distortion. In the present work, the line constants of five conductors in the catenary system were calculated by the reduction method, and each subsystem, which consisted of the Korean high-speed railway system, was modeled by the eight-port representation. Simulation results using the proposed model were compared to field test data of the Korean high-speed railway system, and the accuracy of the harmonic analysis model was verified. Also, the harmonic characteristics of the Korean high-speed railway system are presented.

This paper presents the fault analysis of Korean AC electric railway system. The AC electric railway system is modeled by PSCAD/EMTDC tool. This system model is composed of the Scott-transformer, the autotransformer, the running rails, the protection wires, the feeders, messenger wires and contact wires, etc. After obtaining the models of the fundamental elements describing Korean AC electric railway system, studies for the steady state and fault conditions are carried out. Simulation result and hand calculation for the fault analysis are compared with field data. Finally, we acquire the specific characteristic of Korean AC electric railway system. The proposed model would be a powerful tool for designing and planning Korean AC electric railway system.

This paper presents a new concept of reactive reserve-based contingency constrained optimal power flow (RCCOPF) for voltage stability enhancement. This concept is based on the fact that increase in reactive reserves is effective for enhancement of voltage stability margins of post-contingent states. In this paper, the proposed algorithm is applied to voltage stability margin of interface flow. Interface flow limit, in the open access environment, can be a main drawback. RCCOPF for enhancement of interface flow margin is composed of two modules, modified continuation power flow (MCPF) and optimal power flow (OPF). These modules are recursively performed until satisfying the required margin of interface flow in the given voltage stability criteria.

This paper reports a study on determining the steady-state voltage stability limit of interface flow on a set of interface lines between the metropolitan region and the neighboring regions in the Korea Electric Power Corporation (KEPCO). The interface flow limit considering severe contingencies is determined with a concept of interface flow margin, which is the active power flow margin of the key transmission lines between one region and others under a fixed load demand condition. The paper proposes a procedure to determine secure limit of active power flow on a set of specified interface lines. The procedure uses the modified continuation power flow (MCPF) tracing a path of power flow solutions with respect to generation shift to vary interface flow. In simulation, voltage stability limits of the metropolitan interface flow of the 1998 KEPCO system are determined.

For real-time power system analysis, there exists a limitation of the scale of the system to be simulated; hence, it is necessary for real-time simulation to develop reduced equivalent systems that preserve the desired properties of the original system. In this paper, a procedure for developing equivalent systems whose scale is within the hardware capacity of the Korea Electric Power Corporation (KEPCO) real-time simulator is proposed. In addition, the KEPCO equivalent systems are developed by applying the proposed procedure. The comparison between the original system and equivalent systems illustrates the capabilities of the proposed procedure.

As the importance of the improvement of energy efficiency is constantly increasing, a study has been done on the implementation of an energy storage system, to improve load efficiency, on the railway which has relatively large regenerative energy. The pre-existing regenerative energy not only has a low efficiency but also increases the catenary, which has a possibility to create problems in the electrical device voltage, during the regeneration process. In this paper, the improvement of the efficiency and stabilization of the catenary voltage by the implementation of an energy storage device is being discussed. The composition of the interface, control strategy and reduced railway model for simulation are being proposed.

Voltage flicker is defined as "the impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time" according to IEC standard. And it is also known as voltage fluctuation, light flicker, and flicker. In Korea, the equivalent 10 Hz voltage flicker index has been used as the standard index. But now we are faced with the change into the new standard for flicker because U.S.A and many advanced European countries adopted the IEC 61000 standard as a criterion of voltage flicker. This paper introduces the digital flicker measuring algorithm based on IEC 61000-4-15 in detail and new approach for our unique distribution system.

This paper deals with the AC electric railway system modeling using EMTDC program. This system model is composed of Scott-transformers, auto-transformers, running rails, protection wires, feeders, catenary and contact wires, and so on. After obtaining the fundamental frequency model of the AC electric railway system and analyzing its behavior, we have analyzed and tested Korean high-speed railway system focused on the amplification o f harmonic current to verify the proposed model. The simulation results from the proposed approach and the measurement data from the test are described in the paper.

In this paper, a fast contingency screening algorithm using SIME for the transient stability monitoring of the KEPCO system is implemented. This method evaluates the first-swing stability of a large number of contingencies in a short time with reliable accuracy. The salient feature of the proposed screening algorithm is as follows. First, critical cluster of generators is identified by a new index in the early stage of the time domain simulation. Thus, the computational time required for finding the equivalent one machine infinite bus system can be reduced significantly. Second, clustering critical machines can perform even in very stable cases. It allows us to avoid extra calculation of the time trajectory that is needed in the SIME for classifying the stable cases. Third, the screening capability is improved using the power-angle trajectory and subdividing the contingency classification.

A fast contingency screening algorithm using SIME for the transient stability monitoring of the KEPCO system is implemented. This method evaluates the first-swing stability of a large number of contingencies in a short time with reliable accuracy. The salient features of the proposed screening algorithm are as follows. First, a critical cluster of generators is identified by a new index in the early stage of the time domain simulation. Thus, the computational time required for finding the equivalent one machine infinite bus system can be reduced significantly. Second, clustering critical machines can perform even in very stable cases. This allows us to avoid the extra calculation of the time trajectory that is needed in the SIME for classifying the stable cases. Third, the screening capability is improved using the power-angle trajectory and subdividing the contingency classification. The test results on the KEPCO system show that the proposed method can be adopted as a contingency screening function and the transient stability assessment for the transient stability monitoring of the KEPCO system.

For any one control area, the rest of the interconnection is considered as one equivalent area. System time error and inadvertent interchange energy of an area are considered as a vector with two coordinates. This vector can be decomposed into two vector components: a component caused by the area itself, and a component caused by the rest of the interconnection.

Nathan Cohn's newly proposed technique of coordinated, system-wide correction of time error and inadvertent interchange is examined and validated by computer simulations. Area and system performances are investigated for a sudden change in load or generation in a three-area power system. The following effects are examined: changing the time of correction; simultaneous versus individual correction of time error and inadvertent interchange; the type of disturbance; the governor delay time; and the presence of sustained errors.

This paper introduces a new concept of interface flow margin in terms of steady-state voltage stability. The interface flow margin is the active power flow margin of the key transmission lines between one region and others under a fixed load demand condition. In addition, the paper proposes a technique to trace interface flow-voltage (f-V) curves by applying a continuation method. From the f-V curves, interface flow margin can be easily obtained, which is useful to determine secure flow limit of the specified interface. In the case study, illustrative examples with the New England 39-bus system and a practical 771-bus system are shown.