Research and development is underway in electric power storage technology for efficient utilization of power at night and flexible response to peak demand during the daytime. Recently, the stage of verification using actual equipment has been reached for applications to deal with momentary voltage sags, and for the application of multi-use applications such as the function of absorbing fluctuations in natural energy. We discuss recent trends in electric power storage technology mainly for the sodium-sulfur battery, which at present is the most practical battery for storing electricity.

View full abstract

This paper investigates a new regulation in competitive power systems so that the dispersed generators can contribute to the reduction in CO₂ emissions, while cooperating with the existing large-scale central station generation of electric utility Two kinds of scenario concerning installation of DG are considered. First scenario is similar to the regulation of the present power systems, in which the utility has a responsibility of plan and operation of the whole power systems and DG are installed to reduce the cost of electricity by customers. In this case, the utility offers the time of use rate to customers to control installation and operation of DG indirectly. The other scenario is a future competitive environment in which DG can sell electricity to customers under the same condition as the utility In this case, this paper assumes that every customer is restricted the use of electricity to satisfy constraints of the CO₂ emission imposed to every customer.

View full abstract

Researches on the unit commitment with transmission network have been reported recently. But, in most of these researches, they discussed mainly on the security constrained unit commitment, while the relationship between unit commitment and transmission losses was not considered. However, from a standpoint of operating reserve for ensuring power supply reliability, an unit commitment considering transmission losses is required. Further, under the deregulation and liberalization of electric power industry, not only line's security but also transmission losses are expected to play an important role in calculating the network access charge, and unit commitment taking into account transmission losses is also desired from this viewpoint.
In this paper, an unit commitment approach with both transmission losses and line flow constraint is presented. Based on a heuristic iterative optimization method, first, an initial schedule is created by using a successively decommitting unit approach that is proposed in this paper. Then, we find out a constraints included unit commitment schedule by a heuristic iterative optimization approach, in which an algorithm being able to get rid of line overload by DC optimal power flow is developed. Through numerical simulations on two test power systems, the effectiveness of the proposed method is shown.

View full abstract

Accurate and equitable evaluation of power system operation with the consideration of transmission network is increasingly demanded by power industry due to the recent deregulation. Conventionally, the computation for the indices of reliability and economics is generally performed by Monte Carlo method, which however is time-consuming simulation especially for large systems with the consideration of a large number of contingencies. This paper aims to develop a fast Monte Carlo method for power system reliability evaluation by exploiting the properties of power system operation. As shown in this paper, in contrast to the conventional Monte Carlo simulation, the proposed method not only considerably improves the computation efficiency in terms of CPU time, but also has high accuracy from the statistical viewpoint.

View full abstract

In a transmission network open access environment, an efficient comprehensive generation scheduling taking into account the transmission network will be required. This paper proposes a practical hydro-thermal unit commitment scheduling considering the transmission network.
The proposed technique consists of a multi-stage screening algorithm for processing the operation constraints and a feasible solution is obtained using a heuristic multi-stage approach. At first stage, we decide an initial schedule by relaxing various operation constrains of the thermal units and reservoir storage constrains of hydro power stations. Second stage processes the thermal unit operation constrains of the spinning reserve and minimum up and down time taking into account the hydro system operation. At final stage, we screen the violation of reservoir storage of hydraulically coupled power stations. In each stage, calculation of transmission losses using an optimal DC power flow is carried out and it is expected to realize a more detailed and realistic schedule than the conventional one in which transmission losses are ignored. The application results for a 53 buses model power system including 12 thermal units and 24 hydro power stations of 7 rivers show the effectiveness of the proposed method.

View full abstract

Research has been done on distributing textbooks and lecturing in classrooms by using WWW. Using WWW for education allows a number of network users to easily deliver new teaching materials using pictures and different kinds of animation. It is important for computer-aided education to combine textbooks which cover fundamental and applied contents with easily understandable pictures and simulation programs. Exercises are important to allow users to master textbook contents as well as the textbook described above. However, differences in user ability make it difficult to master the contents: it is painfully difficult for users to solve exercises which are not suitable for their ability. This situation makes users lose their study volition. In the psychology of learning, the advantages of programmed learning for the above problem are well known and documented. Studies have reported on stand-alone computer-aided education support systems that use programmed learning. A network-base education support system must be equipped with a function to instruct different mastery level users as well as stand alone systems. However, past network-base education support systems are lacking in this.
In this paper, the authors propose a new education support system for power engineering. The proposed system has the following characteristics. The system is equipped with a function to select exercises automatically according to the mastery level of each user. Similar exercises are created to modify exercises, which are previously registered in the proposed system. Not only simulation programs but also pictures and different kinds of animation are used to explain the text. This paper also illustrates an example of the proposed system.

View full abstract

Combined cycle power plant which combines gas turbine and steam turbine can achieve high energy efficiency. Many combined cycle plants have been installed in the world. However, a large-scale blackout due to combined cycle chain trip-out occured in Malaysia. The cause of this chain trip is considered to be large drop of system frequency. From these backgrounds, there is a need to investigate stability of the combined cycle power plant for the frequency drop. Several dynamic models of combined cycle plant have been proposed. In our analysis, we use some of them and build a model for a single-shaft combined cycle plant. We execute numerical simulations to see how the combined cycle plant behaves when system frequency drops. The cause and the countermeasure of the combined cycle plant trip-out are explained.

View full abstract

To cope with restructuring of electricity market, application of 22kV distribution system to areas with high load density, is promising. In this paper, we have studied the neutral grounding method considering low voltage rise and induced voltage to communication line in case of ground fault. We have performed EMTP(Electro-Magnetic Transients Program) simulation on several distribution system models with variation of line length and assembling representing actual distribution systems. We have also compared the simulation result with measurement performed on line imitating actual 22kV distribution line to confirm the accuracy of simulation result. Consequently, we have clarified the permissible range of neutral grounding resistance.

View full abstract

Thyristor-controlled series capacitor (TCSC) compensates transmission line reactance, and substantially improves transmission capability. It also does not cause subsynchronous resonance (SSR) accompanying conventional series capacitors. With an appropriate angle of thyristor firing, electrical damping becomes almost zero, which is called SSR neutral. This nature comes from TCSC itself but not from firing angle control. However, negative damping increases for firing angle 170_??_180° where little current flows thyristors. In this paper, we investigate improvement of damping by control of firing angle. First, we oscillate the firing angle at a frequency, and analytically calculate variation of the damping. Then, we show that the damping improves at all frequency if the oscillation is in phase with that of rotor angle. Synchronizing torque decreases however, so a limit must be put on the control gain. On the other hand, if their phases are 90° apart, the damping may improve in one frequency range, but it deteriorates in another range at the same time. This case happens when we feedback rotor speed deviation to control the firing angle. Lastly, we execute numerical simulation of SSR, and verify our investigation.

View full abstract

Overvoltage generated by lightning is a threat to indoor electrical equipments in information-oriented and computerized society. Countermeasures for preventing the overvoltage on an indoor wiring are demanded. It is well known that low-voltage arresters and common grounding wires are effective for lightning protection. However, the effects of the position of the grounding point and the layout of the common grounding wire for controlling the overvoltage have never been investigated yet.
In order to propose an effective countermeasure using a common grounding wire, we have investigated the effects of the position of the grounding point and the layout of the common grounding wire on reduction of the overvoltage, using a full-scale test distribution line with a customer's facility and the EMTP analysis. Main results are as follows:
(1) A common grounding wire installed along the indoor wiring can reduce the overvoltage between the indoor wiring and the grounding more than that installed under the floor, when low-voltage arresters are installed at the panel board.
(2) One-point grounding of the common grounding wire at the panel board is more effective than multi-point grounding for preventing failure of surge protective devices.

View full abstract

It has been pointed out that a very strong electromagnetic field is induced occasionally along aerial power lines located near a medium wave (MW) broadcasting station, and the construction and the maintenance of power lines are sometimes endangered. We have presented a method to predict a dangerous span before the construction and the maintenance, and a use of a loading of appropriate inductor to decrease the induced field the case where the resonant loop is composed of three power lines, i.e., a top conductor, a middle conductor and a bottom conductor and part of towers.
In this paper, the effect of loading inductor for the case that the resonant loop includes a ground wire is presented. Since the ground wire is connected directly to the tower, the inductor can not be loaded into the ground wire. It is shown that the field strength in the vicinity of three power lines can be suppressed by loading the inductor to three power cables each. However, the field in the vicinity of the ground wire can not be suppressed significantly. Measured values of the field strength in the vicinity of the bottom conductor is presented to confirm the suppression effect.

View full abstract

This paper describes flashover voltage characteristics of CO₂/N₂/SF₆ gas mixtures under uniform field, focusing on whether or not the flashover voltage can be increased by adding CO₂ into N₂/SF₆ gas mixture as found under non-uniform field experiments. First, uniform field experiments are carried out by applying standard lightning impulse voltages to a parallel plane electrode while changing the gas pressure from 0.1 to 0.6MPa. It was found that flashover voltage of CO₂/N₂/SF₆ gas mixture increases as much as a few to 10% as comparison with N₂/SF₆ gas mixture. Additionally, it was expected that dielectric strength of 60%CO₂/30%N₂/10%SF₆ gas mixture is equal to that of SF₆ at 0.5MPa when the gas pressure increases up to about 0.9 MPa. In order to confirm this expectation, flashover voltages of the gas mixture at 0.9MPa are investigated using practical size coaxial electrodes, and then flashover voltages among SF₆, 90%N₂/10%SF₆ and 60%CO₂/30%N₂/10%SF₆ gas mixtures are experimentally compared. As a result, positive V_FO of 60%CO₂/30%N₂/10%SF₆ gas mixture at 0.9MPa is almost same as negative V_FO of pure SF₆ at 0.5MPa. This result suggests that CO₂/N₂/SF₆ gas mixture is superior to N₂/SF₆ gas mixture on withstand impulse voltage under uniform field as well as non-uniform field formed by a free particle or defects in GIS.

View full abstract

The construction of the UHV designed transmission lines had been finished in 1999, and UHV designed transmission lines have been in operation at 500kV. On UHV designed transmission lines, the transmission towers are high, and a distance between the phase conductor and the ground wire is long. From these viewpoints, it is important to evaluate the characteristics of direct lightning strokes to the phase conductors. However, the characteristics of direct lightning strokes have hardly been known yet. So, we have been observing direct lightning strokes to study their characteristics.
This paper presents results of the observation and analysis of direct lightning strokes to the phase conductors on UHV designed transmission lines in three years from 1998 until 2000. We report mainly about the peak current, the front duration and the tail duration of the current waveform, which are most fundamental parameters as the characteristics of the direct lightning strokes. In this paper, at first an example of the observation and the analytic process which EMTP was used for is shown, next, the statistics distributions of the direct lightning strokes parameters are reported. This paper will be useful for the rationalization in insulation design of the electric power equipment.

View full abstract

For developing a rational lightning protection design of power distribution lines, we have investigated on sparkover characteristics when direct lightning hits on power distribution lines. In the previous study, lightning impulse voltages with negative polarity were applied to a rod electrode under the condition that an open wire of overhead distribution line was placed together with a tree. Then the rate of sparkover to an open wire and or to an open wire via a tree was examined.
In order to understand the phenomena, we made simultaneous observation of discharge development using an image converter camera and pre-discharge current. From the experimental result, the shielding effect of the tree near distribution lines is evaluated.

View full abstract

Results of numerical study on performance of closed loop in the CCMHD single power generation system are presented. The small closed loop with thermal input of 2.3MW was designed. The fluid-dynamical and thermal performance of the closed loop was calculated for the first time by one-dimensional numerical simulation. The result indicates that pressure loss and heat loss are large in the designed small closed loop. The large pressure loss was due to the wall friction or the pebble bed and it occurred at the supersonic nozzle, the MHD channel and the regenerative heat exchanger used for argon cooling. The enthalpy extraction remained low (=17.6%) owing to the large pressure loss and as a result, the thermal efficiency of the closed loop remained only 21.6%. The enthalpy extraction and the thermal efficiency became the maximum at the optimal inlet total pressure of 0.60MPa. At lower inlettotal pressure than the optimal one, the electrical conductivity, the flow velocity and the electrical efficiency became higher. However, the total pressure ratio (=exit/inlet) in the MHD channel became larger particularly at low inlet total pressure because of the increase in the pressure loss due to pebble bed and wall friction. These led to the existence of the optimal inlet total pressure.

View full abstract

For rationalization of insulation design, it is necessary to grasp the characteristics of the over voltage precisely. To clarify the level of over voltage which occur in electric network, EMTP is frequently used as the calculation tool. Nevertheless there are few reports in which the calculation precision of EMTP in distribution lines was examined. So, we had made an experiment to measure the characteristics of over voltages which occur in cable line, and to confirm the calculation precision of EMTP concerned with transient over voltage in distribution line, we compared the calculated wave form with the measured wave form. In this paper, we present the characteristics of over voltages that we measured, and the result of examination of calculation precision.

View full abstract