摘要:

AbstractLow‐density polyethylene (LDPE) is used widely as an electric insulating material in electric power cables. The generation of electrical treeing due to aging of LDPE may lose the reliability of its insulating capability. Therefore it is important to suppress its treeing and to improve its electrical properties so that the reliability of cables can be improved. For this purpose, the authors prepared LDPE to which various inorganic materials were added.This investigation focuses particularly on lithium carbonate and quartz which were effective in suppressing the treeing when added to LDPE. Lithium carbonate‐added (0.25 phr) LDPE in two hours of discharge showed the tree length 150 μm shorter than that (560 μm) of nonadded LDPE, exhibiting an obvious suppressing effect on treeing generation. It was found that this effect was more significant than quartz‐added LDPE and even at elevated temperatures lithium carbonate‐added LDPE exhibited suppression of treeing. The thermal decomposition temperature at which lithium carbonate‐added (1 phr) LDPE loses 50 percent of its weight rose 100°C more than nonadded LDPE and 60°C more than that of quartz‐added LDPE, which showed improvement in heat res

ISSN:0424-7760    

DOI:10.1002/eej.4391140101

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractMelt‐spun ribbons of Nd‐Fe‐Co‐B‐Cu‐Nb system alloys were prepared by the single roller rapid‐quenching method. The effects of composition, wheel velocity and heat‐treatment on the magnetic properties were investigated. The magnetic properties of bonded magnets prepared by compression molding of optimally annealed ribbons were also measured. A maximum energy product of 152.1 kJ/m3was obtained for the Nd11Fe72Co8B7.5Cu0.5Nb1.0alloy ribbon prepared at a wheel velocity of 17.1 m/s.From the TEM observation on the above Nd11Fe72Co8B7.5Cu0.5Nb1.0ribbon, the particle size of these ribbons was determined to be between 10 and 40 nm. It is conjectured that the high remanence is observed by the magnetic interaction of these fine particles, and it was found that these fine particles were achieved by Nd‐Fe‐Co‐B compound in addition with Nb and Cu at the same time.The amorphous Nd11Fe72Co8B7.5Cu0.5Nb1.0ribbons prepared at a wheel velocity of 20.7 m/s were crystallized by heat treatment, and the optimum annealing condition was found to be at 650°C for 15 min. Its corresponding value of (BH)maxis 144.5 kJ/m3. A maximum energy product of 88.1 kJ/m3was achieved with Nd11Fe72Co8B7.5Cu0.5Nb1.0bonded magnets made from the opti

ISSN:0424-7760    

DOI:10.1002/eej.4391140102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractNew technologies such as synchronized switching and power electronics will make it possible to change the configuration of a transmission network in future electric power systems. It is called VIPS (Variable Impedance Power System) by the authors who have been studying the effects of those technologies on power system stabilities.This paper considers the improvement of transient stability by means of switching‐over control of transmission lines. It is clear that in some power systems, the simultaneous energization of the line de‐energized in the normal state with the fault clearing better improves first‐swing stability than the pre‐ and post‐fault energization of the line. The energy function‐based index to select such a transmission line is derived. In addition, this paper shows that multiple switching of a transmission line improves damping of post‐fault conditions, where minimization of time derivative of energy functions is used. The switching‐over control is applied to multimachine power systems and its effectiveness

ISSN:0424-7760    

DOI:10.1002/eej.4391140103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractLoad‐flow calculation on electric power systems is one of the most fundamental analyses in system operation and planning. There are many reasons to calculate load‐flow solutions, and the contingency evaluation is a representative one.In contingency evaluation, it is necessary to repeat load‐flow calculation for tremendous patterns depending on branch or generation outages. Then, computing efforts are generally very heavy. Thus various techniques have been proposed to reduce computing time, especially by adopting equivalencing for external network. On the other hand, the network equivalents techniques also become important at present from the viewpoint of the necessity of decentralized monitor and control on bulk power systems.This paper describes a newly developed method of load‐flow equivalents based on the Ward equivalent for both direct (dc) solution and (ac) solution. Numerical results also are shown to clarify the accuracies and practicality of the proposed method through the comparison of the representative REI a

ISSN:0424-7760    

DOI:10.1002/eej.4391140104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractA superconducting multistranded cable is used to realize high current capacity for ac use. The critical current value of the cable is reported to be less than the simple summation of the individual critical current value of each strand. The causes for such a degradation of the critical current value have not been revealed.This paper investigates the current distribution in multistrands before and after their quenching by using seven‐strand superconducting cable and 7x7 cable. The following experimental results are derived: (1) the quenching is initiated at one strand in the cable; (2) the current in the quenched strand is transferred into the other strands; (3) an avalanche of quenching is induced among the strands; and (4) the central strand is quenched finally among the strands. The critical current values of the 7‐ and 7 × 7‐stranded cables also are measured. These values are in good agreement with the predicted values based on the mutual inductance among the strands. It is concluded that the unbalance of the current distribution in the superconducting multistrands can be one of the promising causes for the degradation of the critical current

ISSN:0424-7760    

DOI:10.1002/eej.4391140105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractWith the increase in the size and capacity of electric power systems and the growth of widespread interconnections, the problem of power oscillations due to the reduced system damping has become increasingly serious. Since a Superconducting Magnetic Energy Storage (SMES) unit with a self‐commutated converter is capable of controlling both the active (P) and reactive (Q) power simultaneously and quickly, increasing attention has been focused recently on power system stabilization by SMES control.This paper describes the effects of SMES control on the damping of power oscillations. By examining the case of a single generator connected to an infinite bus through both theoretical analyses and experimental tests (performed with a SMES unit with maximum stored energy of 16 kJ and an artificial model system), the difference in the effects betweenPandQcontrol of SMES is clarified as follows:1In the case ofPcontrol, as the SMES unit is placed closer to the terminal of the generator, the power oscillations will decay more rapidly.2In the case ofQcontrol, it is most effective to install the SMES unit near the midpoint of the system.3By comparing thePcontrol withQcontrol, the former is more effective than the latter based on the conditions that the SMES unit location and the control gain are the sam

ISSN:0424-7760    

DOI:10.1002/eej.4391140106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractRecently, PWM rectifiers which realize high‐power factor and almost sinusoidal ac input current have been proposed and some of them have already been put into practice.In many cases the PWM rectifiers are of current type, that is, they have a smoothing reactor on the dc side. In such rectifiers, anLCfilter has to be inserted on the ac side to reduce current harmonics due to the PWM operation. However, theLCfilter may cause waveform distortion and transient oscillation of the ac side current.As a solution to this problem, a new control method is proposed introducing ac side current control by means of state feedback of theLCfilter. In this case, both the dc output current and the ac input current controls should be achieved by the PWM operation of the rectifier. Furthermore, to obtain sufficient controllability regardless of the operating condition, the dc output and ac input current controls should be done independently. To meet these requirements, existing control methods for PWM rectifiers are not directly applicable.In this paper, a novel control circuit configuration and a method of generating a PWM pulse pattern suitable for the proposed control method are described in detail. From the viewpoint of stability, feedback coefficients and circuit parameters are investigated on the basis of root locus analysis.Some experimental results also are shown to confirm the effectiveness of the proposed control method and the validity of the analytical results. Improvement of the ac side current waveform and the stability of the transient response are demonstrated by these experimental result

ISSN:0424-7760    

DOI:10.1002/eej.4391140107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractOne of the representative vector‐controlled induction motor drive systems without a speed sensor is composed of a rotor flux estimator, a PI torque current controller and a conventional vector‐controlled current source. However, the stability analysis of this system has not been reported.In this paper, the flux estimator is interpreted as a flux observer. Then a linear model of the system is proposed by deriving the equations of the flux estimator in a synchronously rotating reference frame. By computing the trajectories of the poles and zeros and the transient responses, the following results are obtained.(1) If the rotor flux is estimated using only the stator equations (voltage model), the system becomes unstable when the actuator stator resistance is smaller than the value used by the controller.(2) By choosing a small time constant of the first‐order lag element in the flux estimator (reciprocal number of observer gain), the poles and zeros on the imaginary axis move to the stable region. However, another pair of poles has reverse performance.(3) From the viewpoint of stability, the gain of the PI torque current controller should be chosen as large as possible and the stator and rotor resistances in the controller should be set to values smaller than their actual v

ISSN:0424-7760    

DOI:10.1002/eej.4391140108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractA superconducting generator (SCG) consists of a nonmagnetic cryogenic rotor, which contains a super‐conducting field winding cooled by liquid helium, and an airgap armature winding, which makes it possible to obtain a high magnetic field in the airgap. Since the SCG construction differs from that of the conventional generator, practical application requires that its electrical and mechanical design methods be established.This paper describes electrical characteristics equations of the SCG, electromagnetic shield characteristics of double dampers and proof test results of a 50‐MVA experimental SCG. Equations for the generator parameters (voltages, output capacity, reactances, time constants, flux densities), which are useful for the practical design of the SCG, are derived from two‐dimensional electromagnetic analysis considering winding thickness and the edge effect. From comparison with measured and calculated values of no‐load, sudden 1–3 phase short‐circuit and slip extended tests, these equations were confirm

ISSN:0424-7760    

DOI:10.1002/eej.4391140109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY

摘要:

AbstractIt is necessary for a linear induction motor (LIM) to have large thrust force in applications for the vertical movement. If it does not have sufficient thrust force, it cannot lift even itself. A ratio (thrust force/weight of motor), therefore, is the most important factor for the design of this motor.In this paper, assuming the LIM has no slot‐effect and infinite length, a simple analytical model is introduced for steady‐state operation. By comparison with the results of detailed numerical analysis, it is verified to be applicable for basic evaluation of the motor. The design of the motor for vertical movement is carried out and the ratio is maximized. The basic dimensions of a feasible motor are given for such applicat

ISSN:0424-7760    

DOI:10.1002/eej.4391140110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1994

数据来源: WILEY