摘要:

AbstractThis paper describes the influence of space charge on the growth of an electrical tree which is an important factor of the dielectric breakdown in polyethylene‐insulated power cables. Polyethylene and ethylene vinyl‐acetate copolymer with a semi‐conducting needle were used as the treeing specimens. The statistical analysis showed that the growth rate and the unbalance of positive and negative partial discharges increased as the vinyl‐acetate content increased, and that the growth per cycle decreased with the increase in frequency. A model of the tree growth accompanied by the space charge behavior was proposed considering the above experimental results. The tree growth is considered to be caused by the dielectric breakdown due to the field enhancement at the tree tip during the positive half cycle of the applied voltage, the field enhancement being due to residual negative space charge injected during the previous negative half cycle. The negative space charge in polyethylene would be mainly supplied through the partial discharge inside t

ISSN:0424-7760    

DOI:10.1002/eej.4391120801

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

年代:1992

数据来源: WILEY

摘要:

AbstractTo take full advantage of combined effects of metal and plastic materials, a composite tape combining lead foil which lacks mechanical strength with plastic film which imparts strength has been developed. This composite tape is designed to serve the purpose of a water‐impervious layer in an XLPE cable. Evaluation of its properties such as corrosion resistance, fatigue, expansion and shrinkage was made. The results of tests indicate that the combination of lead foil with plastic film is more effective in several ways than either alone. As one of its applications, the lead laminate tape was used as a water‐impervious layer in an XLPE cable. This composite material may be used not only in XLPE cable applications but also in many other various fields of indus

ISSN:0424-7760    

DOI:10.1002/eej.4391120802

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

年代:1992

数据来源: WILEY

摘要:

AbstractAC superconducting wire is being developed for such electrical equipment as superconducting transformers and superconducting generators. AC loss reduction is of primary concern in the development of such high‐efficiency equipment. In reducing ac losses, it is necessary to develop assessment methods for ac loss in test samples which have shapes similar to the end‐product equipment.This paper described a least squares calculation of ac losses of superconducting wire as a function of frequency and magnetic field strength measured in test coils.Two sample solenoid coils were made to test the influence of different capacities and winding methods on ac losses in ac superconducting coils with rated capacities of 500 kVA and 20 kVA, and impregnated (epoxy resin) and nonimpregnated windings.The ac losses in the superconducting coils were measured by a calorimetric method using the evaporating rates of liquid helium. Estimated ac losses in the superconducting wire of the two coils were compared with Joule losses of copper conductors at ambient temperature. As a result of this comparison, a low‐loss ac superconducting wire winding can be made for electrical equipment rather than employing conventional copper winding when used under low magnetic fields less than

ISSN:0424-7760    

DOI:10.1002/eej.4391120803

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

年代:1992

数据来源: WILEY

摘要:

AbstractCloud‐to‐ground lightning flashes in the Niigata area in Japan have been observed for a few years by a magnetic direction‐finder network. Data obtained so far are analyzed carefully taking into account the operating characteristics of the system. As a result, frequency distributions of return‐stroke current for each polarity are produced, and their seasonal variations and the occurrence probabilities of high current exceeding 200 kA are clarified.Based on the estimated current distributions, back‐flashover rates of 500‐kV 2 cct transmission lines are calculated. It is known that the back‐flashover caused by downward flashes can account for most of the line outages in the summer for both single‐ and multiple‐line faults. However, this mechanism cannot explain the high outage rate in the winter experienced in the winter thunderstorm area in the late 1970s. In spite of the low lightning flash density in the winter, the multiple‐line fault rate in the winter caused by back‐flashovers is predicted to be comparable to that in the summer, because of the seasonal variation of the frequency distributions of

ISSN:0424-7760    

DOI:10.1002/eej.4391120804

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

年代:1992

数据来源: WILEY

摘要:

AbstractTo maintain power system security, the authors are developing an integrated security monitoring and control (ISMAC) system which consists of the three main functions: security monitoring, preventive control and emergency control. This paper focuses on the dynamic preventive control which deals with the transient stability immediately after the contingency has occurred. The proposed method is based on the transient stability assessment using the pattern recognition with two‐dimensional feature space. Therefore, a preventive control strategy can be obtained rapidly. An index which represents the severity of the contingency quantitatively (security index) is defined by the distance from a linear decision surface which divides a feature plane into a stable and an unstable region. Further, this method has also the advantage that it is possible to consider the effect of the control devices or damping to some extent and specify the operator demand for stabilization flexibly.The effectiveness of the proposed method is ascertained through numerical examples for model power system

ISSN:0424-7760    

DOI:10.1002/eej.4391120805

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

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper is a comprehensive analysis of the characteristics and economics of systems which treat municipal refuse and utilize its contained energy as the energy source to generate not only electric energy but also the heat energy for district heating and cooling. The following two systems are investigated: (1) SystemAwhich is composed of a plant to incinerate municipal refuse and a cogeneration system with a bleeder/condensate turbine power generation unit; and (2) SystemBwhich is the system proposed here and is composed of a pyrolysis furnace for treating municipal refuse, a two‐stage‐cleaning process for refining generated pyrolysis gas, and a cogeneration system constructed by a combined cycle power generation unit using cleaned pyrolysis gas as its fuel gas. It was estimated that the maximum net power‐generating efficiency of SystemBis 24.8 percent, and is higher by 63 percent compared to that of SystemA.Economics of the systems have also been investigated, and it was shown that SystemAif the extra electric energy of the systems can be supplied directly to various facilities, which is different from the case where it should be sold only to electric companies. Effects of changes in major social and technological factors which affect the economics of the two systems are also analyzed, and it was shown that the possibility is considered to be high that the economics of SystemBwill become more advantageous than that of SystemAin the f

ISSN:0424-7760    

DOI:10.1002/eej.4391120806

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

年代:1992

数据来源: WILEY

摘要:

AbstractThis paper investigates the contamination of the surface of various insulators suspended in the air at a certain altitude and connected in series. It is shown that salt‐proof dishes attached to pin‐type insulators are effective against contamination.Exposure tests of insulators on Okinawa have been made since 1985 and the results are as follows:1The higher the placement of the insulator, the heavier the contamination.2The amount of contamination on the downward surface of the suspended insulators at the lowest position was considerably smaller than that of the insulators at the other positions.3The extent of contamination of the pin‐type insulators with the salt‐proof dishes was far less in comparison to that of pin‐type insulators without salt‐proof dishes.4The maximum equivalent salt‐deposit density was related closely to the small amount of precipitation during the observation period and the occurrence of high wind speed, which was highe

ISSN:0424-7760    

DOI:10.1002/eej.4391120807

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

年代:1992

数据来源: WILEY

摘要:

AbstractAn on‐line fault diagnosis scheme for adaptive controlled systems designed by a self‐tuning approach is proposed. A physical parameter change in the controlled system can effectively be detected by using Kullback Discrimination Information (KDI) as an index for model discrimination. In the adaptive controlled system, parameter changes may occur under the normal operation, as well as under a failed situation. In order to decide whether the detected system parameter change really means a fault occurrence or not, a fuzzy inference approach to fault diagnosis is considered. Some appropriate membership functions which describe fuzzy events of the fault are constructed to implement the fuzzy inference. In this way, useful knowledge about fault modes obtained from, e.g., experts, can be introduced into the model‐based diagnosis technique. Simulation studies on a second order damped oscillator have been carried out to demonstrate the effectiveness of the m

ISSN:0424-7760    

DOI:10.1002/eej.4391120808

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe future demand for electricity is uncertain and difficult to forecast, but it is necessary to do so in order to construct power plants. In that sense, a large‐scale power plant is disadvantageous because it takes a long time to construct. On the other hand, although it takes less time to construct, a small‐scale power plant is more expensive. Therefore large‐ and small‐scale power plants could complement the disadvantages of each other. This paper develops a stochastic dynamic programming model to investigate the possibility. The model determines the capacities of large‐ and small‐scale power plants in the face of uncertain demand to minimize the expected value of the total cost. Computed results have shown that small‐scale plants can play a significant role in this case even if they are more expensive than large

ISSN:0424-7760    

DOI:10.1002/eej.4391120809

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

年代:1992

数据来源: WILEY

摘要:

AbstractThe conventional sliding mode control or variable structure control (VSC) of a nonlinear uncertain system is well known for its robust property and simplicity of control law. However, the use of either is permitted only on the assumption that the upper bound of parameter variation or nonlinearity is known and that the complete information about the state is available. Although the former has recently been solved with adaptive robust control theory, the latter does not appear to be solved.This paper attempted to solve this problem using the technique of VSS adaptive robust control theory. That is, a VSS adaptive observer and a sliding mode control incorporated with this observer was proposed. The robust stability of the closed system can be proved applying Lyapunov's second method.

ISSN:0424-7760    

DOI:10.1002/eej.4391120810

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

年代:1992

数据来源: WILEY