bluuebunny/crossref_metadata_2025 huggingface dataset:
Water mist formation in vapor‐mist dielectrics and its effect
Authors: Nagaki Yasunari , Itabashi Satoru , Mitsui Hideo , Sone Mototaka
Published: July 2007

It has been proposed that vapor‐mist dielectrics in which mixing liquid droplets such as tetrachloroethlene in a gas have the ability of increasing flashover, impulse voltage, and of suppressing partial discharge. Vapor‐mist methods have been performed on other materials and the effect of the mist can also be obtained on benzene, hexane, and even water as general materials. Although these studies have been carried out, neither the discharge mechanism on vapor‐mist nor the reason for the increase of the discharge voltage is understood fully because the mist is of various sizes. On the other hand, agglomeration of water molecules can be studied with the FR‐IR method, and are classified into monomer, dimer, trimer (including polymer). Water gives mist stats which are significant in discussing effects.

This paper describes states of water mist and their effect under nonuniform field with positive and negative polarities. Water mist in vapor exists as dimer and trimer, and their amount decreases with the reduction of temperature. These mists have no effect on discharge suppression. However, when the mists are formed by space charges, the inception voltage is increased by these mists in the negative polarity.
Residential solar‐powered air conditioners
Authors: Tanaka Kunio , Makino Yasuhiro , Sakoguchi Etsuo , Takeoka Akio , Kuwano Yukinori
Published: July 2007

The power demand in Japan increases yearly especially in summer, and peaks in the daytime. The main cause is the drastically increased load of air conditioners used widely in general residences. The load pattern of air conditioners is very similar to the output pattern of a photovoltaic (PV) array.

Therefore, Sanyo has been developing two types of residential solar‐powered air conditioners. One is a unidirectional system by using a dc/dc converter which controls the voltage of a PV array at its optimal operation point and boosts the voltage up to the dc voltage of an inverter‐type air conditioner. The other is a bi‐directional system by using a bi‐directional current‐control converter, which regenerates surplus power into a commercial system from a PV array, and suppresses the input current distortion of an air conditioner.

In this paper, the configurations and experimental results are described and discussed.
An evaluation of the optimal generation mix for controlling carbon dioxide emissions
Authors: Tanabe Ryuya , Yasuda Keiichiro , Yokoyama Ryuichi
Published: July 2007

This paper presents an efficient computational algorithm for selecting the optimal generation mix considering CO2 emissions. To demonstrate the effectiveness and feasibility of the proposed method, a fundamental study of the evaluation of the optimal generation mix for controlling CO2 emissions is indicated. Furthermore, by using a parametric analysis which considers load characteristics as parameters, a general trend for the optimal generation mix which is affected by controlling CO2 can be derived.

The proposed method is based on an optimization method known as simulated annealing. In the method, solutions in a generation mix problem are equivalent to state of a physical system, and the cost of a solution is equivalent to the energy of a state. The proposed method can easily accommodate not only CO2 emissions but also many practical constraints of generation expansion planning, such as integer solutions of unit capacities, condition of existing units, and so on.

Case studies with various annual load patterns (combinations of annual load factors and the shapes of annual load duration curve) are presented and discussed. Consequently, a general trend for selecting generation technologies that should be added to a power system is derived, i.e., a useful guideline for studying generation expansion planning under controlling CO2 emissions can be provided.
Configuration optimization for a coupled 200‐MWe MHD channel and magnet coil system
Authors: Nishimura Ryo , Aoki Yoshiaki , Kayukawa Naoyuki
Published: July 2007

This paper reports on the results of the optimization of the cross‐sectional shape of a magnet coil system including an MHD channel. This optimization was carried out for a 200‐MWe supersonic Faraday‐type MHD generator. It was shown that the channel length can be shortened more than 30 percent, without the decrease of the enthalpy extraction by the optimization of the coil shape from the crescent shape coil producing a uniform magnetic field. It was estimated also that the capital cost for a commercial MHD/steam combined plant can be decreased more than 6 percent by this coil shape optimization.
Investigation of impedance and line parameters of a finite length multiconductor system
Authors: Ametani Akihiro , Ishihara Akiko
Published: July 2007

The earth‐return impedance and capacitance formulas of a finite parallel multiconductor with arbitrary positions have been proposed. A comparison with measured results has proved that the accuracy of the impedance formula is satisfactory. By applying the formulas, the characteristics of the finite line parameters such as a propagation constant and a characteristic impedance are investigated in comparison with those given by the conventional assumption of an infinite line. The impedance of the infinite line is greater and the capacitance is smaller than those of the finite line. The difference increases as the line length and source frequency decrease, and the height, radius and separation distance increase. According to the foregoing, the attenuation constant and the characteristic impedance of the infinite line are greater and the propagation velocity is smaller than those of the finite line.
Development of unbalanced load flow program for large power systems
Authors: Suzuki Mamoru , Ishii Takahiro , Kamio Mitsukazu , Suzuki Hiroshi , Iba Kenji
Published: July 2007

The idea of unbalanced load flow calculation was proposed many years ago. At that time, however, the needs for such techniques was not urgent. Modern power system networks are comprised of long untransposed transmission lines. Therefore, for some kinds of analysis, it is now almost impossible to treat a system as though it were a symmetrical network. The aims of most previous studies were oriented to solve voltage/current imbalance in local or small systems, as local imbalance was a serious concern. This is still an important issue, but more recently needs have become concentrated on practical bulk power systems, since principal EHV lines are entirely untransposed.

In this paper, the development of practical unbalanced load program and practical experience with it are reported. This program was developed for steady‐state analysis of large‐scale practical networks under many possible unbalanced conditions. The Newton‐Raphson method in polar coordinates is used, since fast and moderate convergent characteristics are suitable for solving solutions. The program was applied to practical case studies. The models contain all of the 500‐kV and most of the 275‐kV overhead transmission lines in TEPCO. Since the density of the Jacobian matrix is higher than in single‐phase load flows, the number of nonzero elements is extremely large. Convergent characteristics are excellent in all cases. The fear of numerical instability has been allayed.
Active‐passive power circuit
Authors: Funato Hirohito , Kawamura Atsuo
Published: July 2007

The recent development of a semiconductor device technology realizes small‐size, lightweight, high performance power switching devices. However, reactive elements such as an inductor or a PWM inverter achieves high‐performance current control, which is applied to the active filter, and so on [2, 4]. If a compact and high efficient reactive component is developed, the power converter technology may drastically change the application field.

In this paper, the concept of variable active‐passive reactance (VAPAR) which consists of an inverter and passive elements is proposed. The VAPAR behaves like a two‐terminal reactive element and the produced active reactance component works as a passive reactive component, including in transient state.

The principle of VAPAR and two possible control schemes to realize VAPAR are realized. Through simulations and experiments, it is verified that VAPAR shows reactive characteristics not only in steady state, but also in transient state, as proposed, and basic characteristics of both control schemes are clarified.
Degradation diagnosis and estimation of residual life of insulating material using pattern recognition of phase‐angle resolved partial‐discharge pulse‐occurrence distribution
Authors: Komori Fumitaka , Nishiguchi Noriaki , Hikita Masayuki , Mizutani Teruyoshi
Published: July 2007

A prototype of partial discharge (PD) pulse measurement and degradation diagnosis expert system (PDM/ES‐I) is constructed using a personal computer for insulation degradation diagnosis of materials experiencing PDs. For 0.1‐mm‐thick low‐density polyethylene (LDPE) with CIGRE Method II electrode system, temporal change of phase‐angle‐resolved PD pulse occurrence distribution was measured with PDM/ES‐I.

The so‐called “pattern recognition” of the PD pulse distribution is applied to diagnosis of insulation degradation and assessment of residual life. Experimental results revealed that in spite of large scattering of time to breakdown, i.e., life, from 5 to 43 hours, all the tested samples exhibited similar evolution of the profile of PD pulse occurrence compared with five standard normalized PD patterns which had been registered in advance as five representative degradation stages from the initial degradation category to the final one.

The degradation stage determined by the pattern recognition (PR) method proved to vary from the initial degradation category to the final one in a correct turn. Consequently, the PR method permitted an exact determination of the degree of degradation at a given aging time. In other words, the PR method was capable of predicting the residual life of LDPE with a CM‐II electrode system.
Electrical resistance of electroconductive plastic composite with carbon fiber filler
Authors: Katsumata Makoto , Endo Morinobu , Yamanashi Hidenori , Ushijima Hitoshi
Published: July 2007

To obtain a new electroconductive composite, the vapor‐grown carbon fiber (VGCF) has been applied to the thermoplastic resin composite as a conductive filler. The stability of electrical conductivity of the composite is investigated.

The resistivity of VGCF composite is 1.7 × 10−1 Ω·cm at room temperature which is one order smaller than that of commercial grade composites with carbon filler. The stability of electrical conductivity, under temperature change (−35 to +60°C) and for exposure tests (at room temperature and at 60°C), the VGCF composites have high degree of stability. The stability of electrical conductivity of the present composites are superior to those of conventional carbon fibers such as PAN‐based, under tensile stresses and bending stresses. It is suggested that the fine conductive networks in the present composites give rise to these desirable performances.
Quenching current level coordination in superconducting power transmission systems
Authors: Tang Yue‐Jin , Yokomizu Yasunobu , Hayakawa Naoki , Goto Yasuyuki , Matsumura Toshiro , Okubo Hitoshi , Kito Yukio
Published: July 2007

In the present power transmission system, electric power devices are equipped under the concept of insulation coordination and are protected from lightning overvoltage and switching surge by the arrester. However, the future superconducting power transmission system is characterized by the heavy current transmission capability, the lower transmission loss and lightning‐surge‐free circumstances. Thus, in the superconducting power transmission system, superconducting devices should be protected rather from quenching by the overcurrent of a short‐circuit fault than from the breakdown by over‐voltage.

In this paper, a new concept of the coordination for the superconducting transmission system is proposed as a “quenching current level coordination” instead of the insulation coordination. Furthermore, it is pointed out that the quenching current level coordination has the duality relationship with the present insulation coordination. A superconducting fault current limiter should play an important part of the quenching current level coordination to prevent the other superconducting devices from quenching.
Controllability of arc jet from arc horns with slits
Authors: Sunabe Kinya , Inabe Tsuginori , Fukagawa Hiromasa , Kito Yukio
Published: July 2007

Ultrahigh‐voltage transmission lines must be protected from lightning surges and the large‐current fault arcs which follow. Horns which are installed at the jumpers on the strain towers have a role in protecting the insulators from the fault arcs by coordinating insulation strength and holding large‐current fault arcs between the horns. Although a flat surface of the electrode tip with a small diameter effectively meets these requirements, it results in the deterioration of corona discharge depressing capabilities in a steady state on power transmission lines. To improve corona discharge depressing capabilities and to reduce deflection of arc jets, finally two types of hemispherical‐shaped arc horn electrodes with slits are developed. The arc jet control performance of those horns was disclosed experimentally as follows:

Two types of arc horn electrodes with our or six slits show remarkable arc jet control performance in the current of larger than 5 kA.

The addition of a rod‐type central electrode at the center of the hemispherical‐shaped electrodes results in the considerable extension of arc jet controllability in the wider current zone by containing the arc spot within the tip area of the central electrodes.

Based on the foregoing results, arc horn electrodes with slits have been designed and utilized for practical application.
High efficiency a‐Si//poly‐Si tandem solar cell
Authors: Horiuchi Toshikazu , Ma Wen , Lim C. C. , Yoshimi Masashi , Hattori Kiminori , Sada Chitose , Okamoto Hiroaki , Hamakawa Yoshihiro
Published: July 2007

A series of experimental trials has been carried out to realize the high efficiency a‐Si//Poly‐Si tandem solar cell. Employing p‐type μc‐SiC as a wide‐gap heterojunction window, a‐SiC as an interface buffer layer and n‐type μc‐Si as a back ohmic contact layer, 17.2 percent conversion efficiency has been achieved with the structure of ITO/p μc‐SiC/n Poly‐Si/n μc‐Si heterojunction.

Utilizing an optically transparent a‐Si p‐i‐n cell as a top cell and inserting an optical coupler between the top and the Poly‐Si bottom cell, a high total efficiency of 21.0 percent has been obtained so far on the four‐terminal tandem cell. This conversion efficiency value represents a world record for a‐Si basis tandem solar cells.
Current distribution in parallel‐connected YBCO high‐temperature superconducting elements with different critical currents
Authors: Yokomizu Yasunobu , Ito Koji , Matsumura Toshiro , Okubo Hitoshi , Kito Yukio
Published: July 2007

A high‐temperature superconductor is investigated for application to a fault‐current limiter. However, the current‐carrying capacity of a single element is too low for practical use. This parallel connection of superconducting elements is required to increase the current capacity. This paper discusses an ac current distribution between two YBa2Cu3O7‐x superconducting elements connected in parallel that differ in electrical aspects such as a critical current level and voltage‐current characteristic.

As the instantaneous value of the current rises from zero to peak, the following processes are observed: (1) the transition from superconducting to normal conducting state is initiated in the element with the smaller critical current; (2) the current value in the element is kept constant until the other element with the larger critical current level becomes normal conducting state; and (3) only when the instantaneous value of the ac current exceeds the summation of the individual critical current values of each element does the voltage across parallel‐connected elements appear. These results are caused by the quick recovery characteristic of YBCO superconductors from the normal conducting state to the superconducting state.
Active vibration control of a motor system by using H∞ control theory
Authors: Ouchi Shigeto , Mita Tsutomu , Yano Koji
Published: July 2007

This paper considers the vibration control of a motor system which has a motor and a load connected with a flexible shaft. However, this system often generates a shaft torsional vibration. Traditional methods of treating this problem to adjust the PID controller so that the closed‐loop frequency response is slower than that of the vibration mode. On the other hand, one method has already been proposed in which the vibration is suppressed by a disturbance observer.

This paper proposes a new approach based on H∞ control theory. For comparison, a PI control system based on classical control theory also is constructed.

The results of several experiments show that compared with the PI control system, the H∞ control system is effective in suppressing the vibration. Further, the H∞ controller obtained in the study consists of a PI controller and a series compensator that functions as an active vibration controller.
Lightweight type linear induction motor and its characteristics
Authors: Osawa Satoru , Yoshimuro Makoto , Karita Mitsuji , Ebihara Daiki , Yokoi Toshiaki
Published: July 2007

At the Institute for Posts and Telecommunications Policy, a postal transportation system has been studied. This system would make use of linear induction motor (LIM)‐driven vehicles for transporting mail between post offices in the Tokyo Metropolitan area. The system would connect the main post offices in the Tokyo area with tunnels circularly constructed underground and would transport mail with linear induction motor‐driven vehicles.

In this study, it is found that if vehicle weight is reduced, climbing capability could be considerably improved. Accordingly, the potential for reducing the weight of vehicle‐mounted LIMs has been explored, and a prototype lightweight LIM has been manufactured. This paper reports on the lightweight LIM and its characteristics.
Impedance frequency characteristics of MAGLEV power feed circuits
Authors: Ema Satoshi , Ajiki Koji
Published: July 2007

In Japan, the MAGLEV (magnetically levitated train) is now well along in development testing. The MAGLEV power source must supply a variable voltage and variable frequency source, and contains many higher harmonics which cause many problems such as inductive interference. Thus it is necessary to clarify the impedance frequency characteristics of the MAGLEV power feed circuit.

In view of this situation, experiments and investigations concerning the impedance frequency characteristics of the coils for propulsion and the power feeding circuit at the Miyazaki Test Track have been performed but no theoretical analysis has been done.

Thus a theoretical analysis was performed in this paper by applying the inverted L equivalent circuit to the coil for propulsion, from which the characteristics of the coils for propulsion were analyzed.

In addition, the characteristics of the power feeding circuit could be clarified by assuming the equivalent circuit of the power feeder line.

As a result, the measured values and calculated values were almost equal in both the characteristics of the coils for propulsion and those of the power feeding circuit.
Effect of lightning overvoltages on low‐voltage power distribution lines due to direct lightning hits to overhead ground wire
Authors: Imai Yasutomo , Fujiwara Nobuyuki , Yokoyama Hiroshi , Shimomura Tetsuro , Yamaoka Koichi , Ishibe Shinji
Published: July 2007

Overhead ground wires and surge arresters have been installed to protect high‐voltage power distribution lines and apparatus from overvoltages induced by nearby lightning strokes. The effects of surge arresters for protection of high‐voltage distribution lines against direct lightning strokes have already been investigated using the digital simulation program EMTP (Electromagnetic Transients Program). With regard to the protection of low‐voltage distribution lines from overvoltages induced by lightning strokes, experimental analyses using a scale model line have been reported.

This paper reports on the comparison between the experimental analyses and EMTP simulation of power distribution lines, including low‐voltage lines, and the validity of EMTP simulation. Furthermore, this paper analyzes the overvoltages on low‐voltage power distribution lines against direct lightning strokes to overhead ground wire using the digital simulation.
Hybrid‐type stability control method based on combination of conventional PSS and fuzzy control
Authors: Iskandar Marzan Aziz , Satoh Masashi , Mizutani Yoshibumi , Suzuki Akio , Ishizeki Mitsuo
Published: July 2007

This paper presents a hybrid‐type stability control method based on fuzzy control to improve power system stability. The proposed method consists of a speed deviation input fuzzy controller (F1), an electrical power deviation input analog‐type PSS (ΔP‐type PSS), and a fuzzy judgment mechanism (F2). By using the membership function of the magnitude of speed deviation/acceleration of the generator, the following control schemes are considered. If the magnitude is large, then the supplementary stabilizing signal computed by F1 is used to damp oscillations as fast as possible. Otherwise, the output signal of ΔP‐type PSS is used to avoid undesirable noise effect due to noisy input signal of the speed deviation of F1. The proposed control method increases the power system stability greatly, and it is simple to implement. The effectiveness of the method is demonstrated by one‐machine infinite‐bus and multi‐machine model systems.
Optimal planning of cogeneration systems under time‐of‐use rates
Authors: Asano Hiroshi , Imamura Eiichi , Sogai Shigeio
Published: July 2007

For commercial customers with a cogeneration system (CGS), a mixed integer programming model is used to examine effects of time‐of‐use (TOU) rate structure on the optimal operation of the system as well as its optimal capacity. Three case studies are performed for a hotel, a hospital, and an office building (20,000 m2 in total floor space, respectively). The optimal capacity of the generator under the flat rate is 50 to 70 percent of peak power demand. Effects of the on‐peak/off‐peak energy charge ratio are saturated at a ratio of 5. It is not until an energy charge ratio of between 2 and 5 that the purchase of off‐peak power increases in summer when price reduction is applied to city gas. As the energy charge ratio is raised, cheaper off‐peak power is substituted for city gas and the sum of electricity energy charge and gas energy charge decreases. The simulation results indicate that properly designed TOU rates may provide commercial customers an incentive to operate a cogeneration system with an appropriate mix of the self‐generation and commercial power.
Use of multifield induction machine to determine the characteristics of self‐excited generator systems at variable speed, constant voltage and constant frequency
Authors: Taka Koji , Shibata Fukuo
Published: July 2007

This paper considers a generator system using variable energy such as wind power. The variable energy influences the voltage and frequency of a generator system. However, the load requires constant voltage and frequency. Therefore, this paper proposed that a generator system have the following good points:

This generator is a brushless system which makes use of the multifield induction machine;

Frequency converter of small capacity which is used for constant voltage and frequency in load at variable speed;

The PWM inverter can supply reactive power to the self‐excited generator; and

Voltage wave of load is sine wave byLCfilter of the frequency converter.

The multifield induction machines of this generator system have 4 poles and 8 poles similar to stator winding. The rotor of the multifield machine is especially cage type. This rotor can transform among the 4‐pole field and 8‐pole field on the stator.

In this paper, an experimental study of the brushless, self‐excited, constant voltage, constant frequency, variable speed and sine wave generator system by multifield induction machine is described. The component of this generator system, operation of multifield induction machine, characteristics of generator system, and the effect ofLCfilter in frequency converter are described.
Improvement of partial discharge (PD) resistance of epoxy resin by using sulfur‐containing additives
Authors: Kako Yasuhiko , Watanabe Shigetaka , Higashimura Yutaka , Matsunobu Kenji
Published: July 2007

In the field of high‐voltage engineering, it has been desired for a long time to improve the internal PD resistance of epoxy resin without lowering other properties. Recently, the authors discovered that some sulfur compound as curing agents or simple additives improved the PD resistance of rubber markedly, and here an attempt has been made to apply this fact to the case of epoxy resin.

As was expected, some sulfur compounds containing SS‐bond suppress the deterioration of epoxy resin due to internal partial discharges, and the larger the molecular weight of organic group combining the SS‐bond, the longer the effective life of additives. Moreover, to make full use of these additives, it is very important to restrict moisture in the insulation systems.

From this study, the authors have proposed the chemical formulation of the suppressive effect of additives for deterioration of epoxy resin due to internal partial discharges.
Fuzzy inference application to planning of distribution network switching
Authors: Imamura Yuzuru , Kanoi Minoru , Kawakami Junzo
Published: July 2007

Optimal switching pattern planning is the main subject of distribution network reconfiguration to recover from a blackout or to avoid a blackout due to maintenance and construction. This is a combinatorial optimization problem, and an enumerative check of every pattern is necessary to obtain the optimal solution.

In practice, successive heuristic search methods are adopted to obtain the quasi‐optimal solution within a limited computation time. However, a very long computing time is still required when the problem size becomes slightly larger. The step‐by‐step nature of the methods results in many iterations, although it ensures satisfaction of the constraints. Reduction of the iterations is considered to be necessary to enhance processing speed.

In this paper, “supply adequacy” is introduced as a new criteria index to guess the solution, which is obtained by fuzzy inference for each possible pair of source and load zone. The reconfiguration pattern is obtained at once or in a few processing steps by assigning to each zone the source with the highest adequacy. Numerical examples show that the proposed method can obtain the solution faster than the ordinary method, more remarkably with the increase of problem size.
Characteristics of discharge induced by laser‐generated plasmas in atmospheric air
Authors: Honda Chikahisa , Muraoka Katsunori , Takuma Tadasu , Akazaki Masanori , Kinoshita Fumihiro , Katahira Osamu
Published: July 2007

The possibility of laser‐induced lightning has been investigated for the future application in the protection of electric power systems from lightning strokes. This technique induces lightning strokes artificially along a laser‐produced breakdown plasma channel and conducts them to a harmless place on the ground.

To clarify the plasma formation mechanism and the discharge (sparkover) induction process, sparkover induction experiments are performed for two types of long‐gap electrode configurations, plate‐to‐rod and plate‐to‐plate, with a 50‐J TEA CO2 laser.

In the present paper, the sparkover voltage characteristics and interferometric measurement on laser‐produced plasma mainly are described, together with sparkover induction for a series of metal beads arranged equidistantly which simulate a plasma channel.
Power system dynamic equivalence based on a new model reduction technique
Authors: Takimoto Akira
Published: July 2007

Power system dynamic equivalents play an important role in the effort to reduce the computational burden of the transient stability analysis of power systems. This paper proposes a new efficient practical algorithm for forming linear dynamic equivalents for power systems based on modal analysis. The main feature of the proposed method is that the accuracy of the equivalent model is greatly improved by converting a coordinate system representing the subsystem from the R‐J coordinate system of the main system to the reference generator D‐Q coordinate system of the subsystem. The order reduction is based on aggregation of similar modes and elimination of the inferior modes without significant effect on the frequency response of the subsystem. Thus the equivalent obtained by the proposed method can be used for both steady‐state and transient stability analysis studies.

To verify the dynamic equivalent performance stability simulation results for a model power system composed of 16 generators also are shown.
More intelligent DC railway electrical power systems with traction power control
Authors: Takagi Ryo , Sone Satoru
Published: July 2007

The load of a dc railway has very large peaks while the average power is low. This makes the capacity of wayside power equipment larger. Recently, the ratio of peak power to average power has become even higher because of the introduction of a large number of electric multiple units with a regenerative brake system. This has also made the contact system unreceptiveness of the regenerated energy a significant problem. To solve these problems, researchers are attempting to add intelligence to the power system.

This paper describes a novel method of intelligently controlling the traction power of trains. In this system the trains reduce their accelerating power to cut current peaks in wayside substations. With the proposed method the load peaks can be reduced by up to 31 percent, which means that the characteristics of the railway load have been changed drastically. The delays of the trains involved are negligible. Also, the unreceptiveness of the regenerated power can almost be eliminated.

In the first part of this paper, the idea of traction power control is described. Then a description is given of how the idea is applied to wayside substation load peak suppression or receptiveness improvement, with some simulation results to prove the method. Finally, the possibility is discussed of further improvement of the system by adding communication capability between trains and wayside equipment.
Reluctance control magnetic suspension system—suspension system with permanent magnet and linear actuator
Authors: Higuchi Toshiro , Oka Koichi
Published: July 2007

A new type of magnetic suspension system is proposed which uses a permanent magnet. In this system, the attractive force is adjusted by controlling the reluctance of the magnetic circuit. Since no electromagnet is necessary, this system is effective for saving energy and avoiding heat generation. The composition of this paper is as follows.

First, the principle of this suspension system is explained and various types of magnetic circuits are introduced for reluctance control. Second, the possibility of one of them is examined from the viewpoint of linear control theory. Third, an experimental device is developed based on the proposed suspension method. The reluctance of this device is controlled by the air gap which is between the magnet and the suspended object, and which is adjusted by driving a permanent magnet by piezoelectric actuator. Experimental results support the theoretical predictions well.
Development of XLPE insulated DC cable
Authors: Maekawa Yuichi , Yamaguchi Akio , Hara Makoto , Sekii Yasuo
Published: July 2007

This paper describes the development of dc XLPE cable. Through a series of material investigations, an XLPE compound containing highly purified special filler was developed.

To check the dc insulation performance of the cable insulated with this new material, a prototype cable with 9 mm insulation thickness was manufactured. It was confirmed that the performance of the prototype cable was excellent. Based on the study of the prototype cable, a 250‐kV dc cable with 20 mm insulation thickness was designed and manufactured. Through a series of voltage tests, excellent dc insulation performance of the developed cable was verified.
Preventive control based on complex‐valued load‐flow technique to avoid voltage instability in power systems
Authors: Kameoka Naoki , Kita Hiroyuki , Tanaka Eiichi , Hasegawa Jun , Nishiya Ken‐Ichi
Published: July 2007

This paper presents a method for obtaining preventive control strategies based on nonlinear optimization for power systems which will incur voltage instability when load demand increases. An algorithm is proposed to determine an appropriate and effective control action taking into account the operating constraints to widen the margin between the present operating point and a voltage collapse point, thereby improving the system state.

The method proposed here is based on a complex‐valued load‐flow technique using the Newton‐Raphson method which has been developed already by the authors. Consequently, the preventive control algorithm also can evaluate the voltage instability in the event the present operating point becomes closer to the critical point and the system state becomes unfeasible at increased loading point. Further, in optimization, two‐types of objective functions are introduced so that voltage stabilization and dissolution of constraint violation can be attained simultaneously. In addition, this paper discusses the extension of the proposed preventive control to the successive control method which carries out the control action to avoid voltage collapse while the load demand is increasing. Numerical examples for various model systems demonstrate the effectiveness of the proposed method.
Vector‐controlled induction motors using neural network
Authors: Shimane Kazuo , Tanaka Shigeru , Tadakuma Susumu
Published: July 2007

Slip‐frequency vector control generally is used for variable speed induction motor drives due to its excellent response characteristics. However, it is sensitive to the variation of motor parameters since the flux current and slip‐frequency commands are computed using the motor parameters. As a result, the system performance will be degraded if the controller parameters do not match the motor parameters.

The authors propose a new approach of vector control in which quick responses and system robustness are obtained concurrently by the learning capabilities of neural network. In the newly developed method, controller parameters are learned in real time at medium and high speeds. Using these learned parameters, the low‐speed performance of the controller can also be improved greatly.

This paper describes the theoretical analysis, simulation and experimental results of the proposed method.
A parameter tuning for dynamic simulation of power plants using genetic algorithms
Authors: Miyamoto Yuichi , Miyatake Tatsuya , Kurosaka Soh , Mori Yoshinobu
Published: July 2007

This paper reports a method of dynamic simulation parameter tuning for a coal‐fired power plant using genetic algorithms (GA). GA is a search algorithm based on the mechanics of natural selection and natural genetics. GA also is one of the effective methods for optimization problems and it requires a formalization of problems and a fitness function definition. Because the dynamic simulation can be repeated and the error between real data and simulation results is defined by a fitness function, it is reasonable to consider genetic operations.

To satisfy environmental laws and regulations, the control of NOx emission is important for the operation of power plants. Therefore, an NOx analytical model is designed and used for the dynamic simulation at the change of loads, the kind of coal, and so on. In this paper, the optimal parameters for the simulation are determined by GA.
Reduction of higher harmonic sequences in terminal voltage of an AC exciting synchronous generator using magnetically anisotropic slot wedges
Authors: Ide Kazumasa , Watanabe Masatoshi , Sato Masaki , Takahashi Miyoshi , Takahash Noriyoshi
Published: July 2007

This paper describes relationships between characteristics of a magnetically anisotropic slot wedge and voltage higher harmonics in an ac exciting synchronous generator terminal due to permeance ripple caused by an open‐slot‐type rotor. From the relationships obtained, practical guidelines were formulated for machine designs applying the magnetically anisotropic slot wedge. The results are as follows.

(1) Equivalent permeability of the magnetically anisotropic slot wedge became isotropic wedge permeability for air‐gap flux ripple.

(2) Two ways were identified to reduce voltage‐higher harmonics by using the slot wedges.

(3) The level of leakage flux passing through the anisotropic slot wedge was determined only by its permeability in the width direction.

(4) Effectiveness of the magnetic anisotropic slot wedges inserted into the stator slots was verified from experimental results using a model machine of the open‐ slot‐type rotor.
A novel islanding protection system for photovoltaic inverters
Authors: Okado Chihiro , Kimoto Kenichi , Shinohara Hirofumi , Ichinose Takashi
Published: July 2007

It is very difficult to detect an islanding condition of a power distribution line with conventional voltage or frequency relays while the output power and the load power of utility interactive PV inverter units are in nearly balanced state in both active power and reactive power. This is because a sufficiently large voltage or frequency change cannot be expected at such a balanced state.

Many studies have been reported so far to complement dead bands in combination of active and passive methods to prevent the islanding, but none of them has been successful.

This paper presents the principle of a new active method called slip mode frequency shift (SMS). With this method the reactive power between the inverter and the load is made unbalanced intentionally to cause the frequency to shift as if it slips down a slide. The performance when the dead bands are eliminated effectively in the range of practical use has been examined through simulation and experiments.
A zero‐current‐switching based three‐phase pulse width modulation inverter
Authors: Fujita Hideaki , Akagi Hirofumi
Published: July 2007

With the remarkable progress of switching devices, the switching frequency of a voltage‐source PWM inverter has become higher and higher. A high‐frequency PWM inverter gives great benefits in the reduction of current ripples and acoustic noises. However, high‐frequency hard switching causes the increase of switching losses and EMI to be solved.

The authors propose a zero‐current‐switching based three‐phase PWM inverter which has small resonant circuits on the ac side. The current flowing in a switching device is a sum of the resonant current and the load current. Since the switching device is turned on and off at zero current, the switching losses and electromagnetic noises are greatly reduced. This paper described the principle of the zero current switching operation, the design of the resonant circuits and the control scheme for the new soft switching inverter. Moreover, it shows interesting experimental results obtained by the zero‐current‐switching PWM inverter which drives an induction motor of 2.2 kW.
Development of automatic fault point locating and sectional isolating system for power distribution line
Authors: Kakimoto Hitoshi , Hayashi Takehiro , Handa Hirosi , Yukihira Kenzi , Okamura Yukio , Odaka Naotaka
Published: July 2007

This paper describes the development of the Automatic Fault Point Locating and Sectional Isolating System. This system locates a single‐line‐to‐ground fault point at each pole by using ground‐current sensors, and isolates the faulted section by controlling pole switches with no outage of unfaulted sections.

The ground‐current sensor is connected to the line between allied ground terminals of pole devices and grounded conductor. It detects a ground fault current of more than 0.64 amperes that is determined so as to avoid an unexpected operation by stray current, but to detect the fault current accurately, and sends out an emergency signal with time delay of 0.1 s.

Sectional isolation starts when both the emergency signal from the sensor and relay operation signals from a substation are received and ends within a feeder circuit‐breaker tripping time of 1.5 s.
Modeling by petri net with place invariants for sequential control systems
Authors: Cai Yigang , Nishii Ikuya , Sekiguchi Takashi
Published: July 2007

It is more difficult to model a discrete system than a continuous system. Recently, a great number of applications of Petri nets to the design and the analysis of discrete systems has been reported. However, a common problem in its applications is that the required computer memories and the computation times increase explosively in accordance with the increase in the number of systems components. Some methods to divide or to reduce Petri nets have been proposed to solve this problem. Although the liveness and boundedness of Petri nets are held in the divided or reduced Petri nets, the reachability problem cannot be solved by these methods.

This paper proposes another method to model discrete systems by Petri nets with place invariants. A sequential control system will be described as a typical kind of discrete system, and its structural characteristics will be used in modeling. Each component of a sequential control system will be modeled by a sub‐Petri net with place invariants. There are many components in one sequential control system, but not each one is necessarily complicated. Most sub‐Petri nets do not have so many places or transitions. It is also well known that a Petri net with place invariants is bounded and can be live by placing sufficient tokens into its initial marking. Besides, the reachability problem is not so difficult to solve in a small sub‐Petri net. Further, an activating relation to combine two or more sub‐Petri nets is defined, and some rules to reduce conflicts among enabled transitions will be described for simulations.
Improvement of dielectric strength on transformer winding using new aramid paper
Authors: Endoo Kaoru , Uwano Yoshito , Hiraishi Kiyoto , Oonuma Toshiyuki , Uemura Ryuuji
Published: July 2007

Large power transformers using perfluorocarbon liquid have advantages of high dielectric strength, excellent cooling ability, and nonflammability.

To make the transformer more compact, it is very important to improve the insulation material.

Recently, the authors succeeded in developing a new type of aramid insulation paper which has no fluff on its surface to avoid field concentration.

In this study, an outline of the developed paper and the experimental results of the insulation strength on turn‐to‐turn and intercoil models are described.

It was found that the new aramid paper increases the insulation strength 1.2 (turn‐to‐case) to 1.7 (intercoil case) times that of the existing insulating paper.
Magnetic‐shield type superconducting fault current limiter with high Tc superconductors
Authors: Kajikawa Kazuhiro , Kaiho Katsuyuki , Tamada Noriharu , Onishi Toshitada
Published: July 2007

The continuous development of electric power utilities has led to the increase in problems concerning fault currents. The use of fault current limiters, which suppress fault currents below a certain level, has been examined widely. The authors have studied a superconducting fault current limiter that is based on the magnetic shielding characteristics of superconductors.

In this paper, three kinds of fault current limiters with high Tc superconductors and their results tested at liquid nitrogen temperature are presented. The experimental results show that the impedances of limiters in the fault condition became higher than those in the normal condition but were lower than the anticipated ones.

Next, a numerical calculation model to analyze the experimental results is presented. The calculation results with this model agreed with the aforementioned experimental ones very well.

From these studies it is concluded that it is necessary for the sudden transition from the superconducting state to the fully resistive state to be caused by suitable means.
A new self‐excitation scheme for three‐phase synchronous generators
Authors: Fukami Tadashi , Miyamoto Toshio , Shibata Fukuo , Taka Koji
Published: July 2007

A new self‐excitation scheme has been developed for three‐phase synchronous generators. The generator that uses this scheme behaves as a flat‐compound machine without automatic voltage regulators. The stator of the generator is provided with an armature winding that generates the fundamental and 2nd‐harmonic mmfs. The rotor is equipped with a field winding and a harmonic winding. The harmonic winding is magnetically coupled to the 2nd‐harmonic mmf produced by the armature winding. The current flows in the stator armature winding by way of a three‐phase set of capacitors connected across its three‐phase terminals and the ac voltage is induced in the rotor harmonic winding due to the armature 2nd‐harmonic mmf. By connecting the harmonic winding to the field winding through a rectifier circuit, self‐excitation of the generator is obtained. This excitation scheme is both self‐excited and brushless, and no exciter is required. Additionally, voltage regulation due to load variation can be improved by connecting suitable capacitors.

In this paper the operating principle and circuit configuration of the generator are explained and the experimental results using a 1‐kVA prototype machine are shown. The generator characteristics are theoretically clarified by approximate analysis, and appropriateness of the theory and the usefulness of the proposed scheme are experimentally confirmed.
An experimental investigation of the electrical breakdown phenomena of interface insulation in prefabricated‐type joints
Authors: Takahashi Yoshihisa , Hara Makoto
Published: July 2007

To establish the reliability of the insulating performance of prefabricated‐type joints (PJs), dielectric breakdown characteristics of interface insulation were studied.

Using two types of electrodes, which simulate the interface configuration in an actual 275‐kV PJ, ac and impulse BDSs, together with long‐term performance of the interface, were evaluated under various conditions. Among the many factors which would influence the insulating characteristics of the interface, the effect of the silicone oil layer, as well as the applied external pressure, were examined. It was determined that the BDSs were influenced greatly by silicone oil application, by the applied interface pressure, and by the pretreatment of the surface of XLPE insulation. On the basis of the result of experiments, the functions of the silicone oil layer in the insulation interface of the PJ were examined.

The authors concluded that the thinner silicone oil layer in the insulation interface is favorable for the establishment of the insulation interface of the PJ.