The effect of voltage sags becomes important increasingly in the recent high technology industries. This article presents the recent survey results of current status on voltages sags, mitigation methods and future trends.
The planar solid oxide fuel cell (SOFC) with the Y2O3 stabilized ZrO2 electrolyte is expected to be a candidate for the distributed power sources in the next generation due to its high efficiency of power generation. In this study, we have analyzed the system performance of the SOFC and gas turbine combined cycle of about 500 kW output, using a two-dimensional simulation program for planar SOFC with the internal reforming. The effects of cell temperature, SOFC pressure, recirculation ratios of fuel and air, utilization ratios of fuel and air, and average current density at SOFC on the system efficiency were analyzed under typical operating condition taking account of realistic efficiencies and heat losses. The addition of Cheng cycle to the SOFC and gas turbine combined cycle improves the system efficiency by 1—3%. The 500 kW class SOFC, gas turbine and Cheng combined cycle gives the high efficiency of 61.2% (HHV) under the typical condition when the SOFC pressure is set to about 2.0 MPa.
This paper presents an arbitrary diagonal wire on an rectangular surface composing a cubic cell in an electromagnetic analysis based on the orthogonal FDTD (Finite-Difference Time-Domain) Algorithm. One of the numerical electromagnetic analyzing algorithms is the FDTD method based on Maxwell’s equation. The basic FDTD method divides the analyzed space into cubic cells and directly calculates the electrical and magnetic fields of the cells by discretizing the Maxwell’s equation of electromagnetic fields, where the derivatives with respect to time and space are replaced by a numerical difference. The development of computer performance brings about an actual execution of the FDTD method on a usual personal computer recently. In dealing with a diagonal and curved wire, the boundaries of which do not coincide with the finite-difference grid lines, the staircase approximation has been commonly used. However, the approximation causes the large error in a resonant frequency and a propagation time of a system including the diagonal or curved wire. The proposed method can express a diagonal and curved wires on a rectangular surface composing a cubic cell by transforming the general integral form of Maxwell’s equation to the different integral form around the wires. This proposed method is also useful to calculate surge propagation on an arbitrary three-dimensional skeleton structure including a diagonal or curved grid such as a tower model and so on.
A new method of detecting loss of synchronism between groups of generators is proposed. Suppose a group of generators swinging coherently losses synchronism against the remaining generators that also swing coherently. The loss of synchronism can be detected by using the relative phase angle α between current and voltage. The current is measured on a transmission line connecting two sub-networks to which these two groups of generators are belonged respectively and the voltage on a bus connected to the transmission line. Also, the magnitude of current flowing through the transmission line and that of voltage are measured and used for detection. When loss of synchronism takes place between two groups, the relative phase angle α exceeds 90 degrees. Also at the instant when α exceeds 90 degrees, following two conditions hold: the magnitude of voltage is near to the minimum value and that of current is near to the maximum value. These characteristics are used for the method. Theoretical background is explained first and the validity of the proposed method is shown by numerical examples using the model network EAST10.
This paper investigates transfer limit surface as a key to voltage stability of electric power system. Transfer limit surface is defined in nodal loading parameter space as a set of critical points. It is concluded that transfer limit surface of general power system with inequality constraints is considered to be continuous, non-smooth, and convex under some realistic assumptions. It should be noted that the non-smoothness is not so serious. Above properties of transfer limit surface is confirmed numerically through examples of two small-scale power systems with inequality constraint(s). Numerical methods to compute normal vector of transfer limit surface in the context of continuation methods are also shown.
It is well known that grounding resistance has a current-dependent characteristic, whose phenomena and models have been reported and proposed by many investigators as well as Liew and Darveniza. As far as lightning analysis, especially in Japan, the grounding resistance has been simulated as a simple constant lump resistance. The goal of the authors’ investigation is to utilize those characteristics to lightning protection and make a certain methodology to save construction cost for UHV transmission tower on very high resistivity soil. We, therefore, focus on the characteristic of grounding resistance of tower footing for 500 kV transmission line. In this paper, we discuss measurement on high-current injected experiment using an actual tower footing base and its analysis, where we employ dynamic characteristic of the grounding resistance proposed by Liew and Darveniza. Comparison between the observed result and the calculated one is also discussed.
A finite-difference time-domain (FDTD) method has become popular in analyzing surge phenomena as well as transient electromagnetic fields because of its high flexibility and straightforwardness. One of the representative limitations of the FDTD method in Cartesian coordinate system is the use of staircase approximation to deal with curved surfaces and slanting thin wires, which are tilted with respect to the coordinate axes. In analyzing a conductor system including curved surfaces, the accuracy may be maintained if the conductor system and its surrounding space are divided into very small cells. It, however, requires long computation time and large memories. The staircase approximation of a slanting wire results in an artificially slowed propagation speed and a lowered resonance frequency. This flaw is inherent in the staircase approximation. In the present paper, surges on a slanting wire and on a cylindrical conductor have been analyzed by the FDTD method with non-rectangular cells, the shapes of which are suitable to fit the curved surface and the slanting conductor. An absorbing boundary condition for the FDTD method using non-rectangular cells has been discussed and the numerical stability has been tested. Also, the accuracy has been investigated in comparison with an analysis based on the method of moments.
Recently it is necessary to reduce ELF (Extremely Low Frequency) magnetic field, which arises from the electric power line in the building. The magnetic shielding with ferromagnetic material is effective in the reduction of ELF magnetic field from the three-phase electric power line. Then, the current of the electric power line fluctuates in proportion to use load, season and time. Therefore, we experimentally measured the magnetic shielding effectiveness with current fluctuation. Also, we numerically calculated them using the finite element method reflected the nonlinearity of the magnetization characteristics. The magnetic material selected thin plate of the permalloy. Characteristics of single-layer shielding and multi-layer shielding were examined. It was confirmed that the shielding effectiveness increased with the increase in the current and that it decreases from the threshold. As a result, it was proven that the shielding property was able to estimate by the finite element method analysis considering magnetization curve.
In this paper, the method of deriving the practical-use equivalent circuit of 3 winding transformer for calculating magnetizing inrush current of a transformer is shown. On the basis of this equivalent circuit, the simulation circuit for general-purpose programs, such as EMTP, can be derived. Reference is made also about the influence to a magnetizing inrush current calculation result by equivalent circuit constitution.
In this paper, the view of the magnetic linkage between other leg windings of a three-phase three-leg core-type transformer core is clarified. In a three-phase three-leg core-type transformer, even if the geomagnetically induced current of a quasi- direct current by the magnetic storm flows in, the asymmetrical DC magnetization of the core is not carried out at all. In a zigzag-Y connection transformer, it can restrict to a three-phase three-leg core transformer, and single-phase load can be taken between the terminal of a secondary Y connection, and a neutral point.
This paper describes characteristics of current imbalance in AC-operated superconducting coil and trial for its suppression with iron core method. Measurements of strand current in the frequency less than —1 Hz revealed that the current imbalance significantly occurred to the frequency increase, and that the critical frequency almost corresponds to a frequency where the coil leakage inductance is comparable to the coil resistive. Such a current imbalance was not seriously affected by ferromagnetic materials set in the coil. The iron core method was effective for suppression of the current imbalance, especially in the low frequency region, and inductance measurements suggested that the suppression effect was attributed to inductance compensation as well as reduction of effective electro-magnetic coupling between coil strands.
Energizing of transformers generates high amplitude inrush currents that reduce power quality and can potentially damage the transformers. Controlled closing for transformers taking account of the residual flux is studied to eliminate the inrush current. Since the residual flux must be precisely acquired before controlled closing to determine the optimum instant for energizing, a new method of estimating the residual flux by using the measured transformer voltage is proposed. The optimum closing target taking account of the dielectric and mechanical characteristics of a gas circuit breaker is also studied. The verification using a single phase transformer showed that the maximum error of the estimated residual flux was less than 10%, and the controlled closing using the proposed method eliminated the inrush current to less than 15% of random closing.
Three-dimensional electric fields analysis was applied to the tip of water trees in XLPE cable. First, pre-breakdown detection was curried out on “needle-shaped” water trees. The results were analyzed by the three-dimensional electric field F.E.M. The water tree was simplified by a spheroid in the analysis. The position of the trees, length, tip radius were read from the microphotographs. The analysis was done on all 11 examples. The test results could be explained well when the conductivity of the water tree region was assumed to be 5×10-7S/m. Next, the electric fields of tip of three kinds of water tree (“blue”, “needle-shaped” and “white” water tree) were analyzed. Three kinds of water tree were expressed by changing the conductivity of water tree region. And, a distance from water trees to inside half conductor was made to change in three kinds. These were analyzed about the cable (insulation thickness: 3, 6, 9 mm) of 6.6, 22, 66 kV respectively. It was found out that “blue” and needle-shaped tree in the 66 kV cable and “blue” tree in the 22 kV cable may cause a breakdown under the operation voltage. As for other cases, the tree may propagate without making breakdown until it bridges the insulation. And, the possibility that the growth of “white” water tree declined rapidly in 66, the 22 kV cable when it touches inner semi-conducting layer so that the tip electric fields of the tree are the same as the average electric fields of the cable was suggested.
Numerical simulations of the closed-cycle disk MHD generation experiment with TIT (Tokyo Institute of Technology) Fuji--1 blow-down facility are performed. In the calculations, the r--z two-dimensional time-dependent simulation code developed by the authors that can take the effect of water contamination into account is used, and the experimental conditions of Run A4109 operated by Disk--F4 generator are selected as the numerical conditions. When the water contamination is the lowest level realized in the experiments, the simulation results coincide with the experimental results reasonably well, though there exist some discrepancies caused by inaccuracy of used basic plasma parameters, limitations of the two-dimensional approximation, etc. The voltage-current curve is almost linear, indicating that the MHD interaction is relatively weak and the flow field is mainly determined by the back pressure. The increase of the water contamination level results in the decrease of the seed ionization rate at the generator channel inlet, leading to the steep deterioration of the generator performance.
Tokyo Electric Power Company (TEPCO) introduced the Lightning Position and Tracking System (LPATS) Version III in 1990. The system uses the time-of-arrival technique for lightning location. TEPCO had studied the operating characteristics of the system, and had confirmed its effectiveness. However, further improvement of the functions and performance was necessary from the viewpoint of utilities. TEPCO, therefore, began development of the LPATS-T in cooperation with the Global Atmospherics, Inc. and formally put the system into operation in 1996. Performance of LPATS-T at TEPCO was evaluated based on comparison with actual lightning current measurement data at transmission towers. Sixty-one lightning locations at transmission towers were detected by LPATS-T from 1996 to 2000. Location error was estimated to be about 650 m on average. TEPCO corrected the lightning locations by the geographical propaga-tion path. As a result, location error was effectively improved when the lightning wavelength was set at 2, 000 m. The precision of lightning location by LPATS-T reduced the average error from 650 m to 330 m. Also, the standard deviation was 180 m from 390 m.