摘要:

Computations of mode-scattering coefficients arising from ionospheric perturbations at sunrise and sunset lines are reported in the paper. The computations are based on an analysis that exploits the concept that radio waves at low frequencies propagate in a nonuniform, spherical, ‘Earth-ionosphere waveguide’ with finite, surface-impedance boundaries. The computed data are compared with available experimental data at 18kHz for both day-night and night-day propagation paths. The computed data for v.l.f. radio frequencies indicate that, for an ionospheric effective-height change of 20km, the magnitudes of the first-second and second-first mode-conversion coefficients pass through minima about 16kHz provided that the length of the transition region is less than 800km.

DOI:10.1049/piee.1970.0142

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The mechanism of subconductor oscillation has defeated analytical formulation for a considerable period of time. Oscillation ‘inhibitors’ have been designed without a true understanding of the underlying destabilising mechanism and, in consequence, none has really proved successful. In the paper, it is shown that subconductor oscillation may be ascribed to classical flutter and that, for lines with conventional spacing (e.g. CEGB bundled lines with two or four parallel conductors spaced at 10.7 or 16 diameters), such oscillations are more likely when the vertical, natural, structural frequency exceeds its horizontal counterpart than when these frequencies are equal. The theoretical predictions of flutter boundaries are vindicated, with remarkable consistency, by dynamical model tests performed in an open-jet wind tunnel. Attention is also drawn to the possibility of static instability (divergence) or a leeward subconductor when the wind speed is sufficiently high. This possibility, again, has seemingly been overlooked in the past and warrants careful attention. It is emphasised that the use of line spacers, whose primary dynamic function is to detune horizontal and vertical natural frequencies, may, in fact, foment instability at certain bundle orientations.

DOI:10.1049/piee.1970.0144

出版商:IEE    

年代:1970

数据来源: IET

摘要:

Considerable effort has been expended in attempting to explain subconductor oscillation. This oscillation has proved to be a major nuisance ever since increased power requirements necessitated the use of multiconductor bundles on overhead power-transmission lines. In recent publications, the author has pointed to a classical flutter mechanism which is highly sensitive to small changes in the basic mechanical system and which always requires a difference in still-air natural frequencies (e.g. in the horizontal and vertical senses) to admit the possibility of energy extraction from the airstream. The present paper summarises the published results, giving sources of reference and outlining relevance to full-scale problems.

DOI:10.1049/piee.1970.0145

出版商:IEE    

年代:1970

数据来源: IET

摘要:

With increased voltage for power transmission, bundle conductors have become widely used in recent years. The paper suggests a numerical method of obtaining the electromagnetic characteristics of bundle conductors by a digital computer. The accuracy of the method used is assessed by applying it to isolated conductors, which involve analytic solutions. The method of solution used is successive approximations to the integral equations, and it is found to be highly convergent in all practical cases.

DOI:10.1049/piee.1970.0146

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The basic transmission characteristics of power cables, such as attenuation and surge impedance as functions of frequency, are required as data for transient analysis and for the interpretation of h.v.-surge recordings. Measurements of capacitance, inductance and associated dielectric andI2Rlosses for sample lengths of 132 kV power cable have been carried out over a wide frequency range from 50 Hz to 800 kHz. The results concerning the main cable insulation at ambient temperature are shown in graphs of tan δ and the percentage variation in capacitance against the logarithm of frequency. The influences of skin effect are shown in graphs of inductance and series resistance against frequency for a short-circuited cable; they are, at least for a range of ultra-audio frequencies, broadly as might be predicted on the basis of conventional ‘skin-depth’ theory. From the measured quantities, characteristic impedances and attenuation coefficients have been calculated and presented graphically; a step-voltage test yielded a surge impedance in agreement with that calculated for a frequency of 1 MHz. The cable samples were wrapped with metal tape, and the capacitance and tan δ measurements were carried out between the cable (outer) screen and the metal tape. Results concerning the neoprene and hessian serving (required for idealised studies concerning propagation in the sheath-earth mode) are presented in graphs similar to those for the main insulation. The variation with temperature of the effective permittivity and tan δ of the impregnated-paper insulation, obtained by heating the core of the cable to simulate the cable loss at full rating, is also given with the relevant thermal measurements.

DOI:10.1049/piee.1970.0147

出版商:IEE    

年代:1970

数据来源: IET

摘要:

When double-circuit transmission lines are operated with one of the circuits isolated and earthed at both ends, relaying conditions can arise that influence the choice of the forward-reach settings of distance protection. Earth-fault conditions during single-circuit working in this way give rise to apparent-impedance values presented to the protection that are generally less, and to errors in impedance measurements that are correspondingly greater, than for the same fault conditions during normal operation, when both circuits are in service together. The paper reports computer studies which seek to compare the discriminative performance of distance protection under earth-fault conditions during double-circuit and single-circuit operation and which, in particular, provide an account of the relaying conditions likely to be encountered in single-circuit operation. Apparent impedance values and associated relaying errors are summarised in tabular form for fault and prefault conditions relevant to a typical 400 kV interconnection, on the basis of which an assessment of the discriminative performance of distance protection in single-circuit working is made.

DOI:10.1049/piee.1970.0148

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The paper describes the results of a series of short-line fault tests on the 275 kV CEGB system at Thorpe Marsh switching station by a collaborative committee. These tests were planned with two main objectives: cross-checking calculated system characteristics with measured values and the demonstration of circuit-breaker performance, both with as high a fault current as possible. Since the short-line fault has recently assumed greater significance owing to the increase of short-circuit currents, the tests were made for this condition. Details of the test arrangements and instrumentation, which included a special Faraday cage, are given. A fault current of up to 20 kA was obtained. Overhead-line transient and 50 Hz parameters are derived from the test results and compared with calculations. Similarly, the source-side transient recovery-voltage waveshapes are presented and compared with calculated waveshapes. Despite the complexity of the system, good agreement was obtained for these and for calculations of fault current. The calculation methods are described and references are given. Although the fault current represented only about 75% of the circuit-breaker rating under this condition, the tests gave a useful demonstration of the capability of circuit breakers developed by the unit-test principle.

DOI:10.1049/piee.1970.0149

出版商:IEE    

年代:1970

数据来源: IET

摘要:

A fault-detection scheme intended to be capable of providing the necessary information for a direct digital control of a.c.-d.c. convertors is described. The proposed scheme provides fast, safe, unique and unambiguous discrimination between the fault condition and its origin through the initial stages of the fault. It is shown that these requirements can only be achieved by sequential logic. Backup detection, based on a completely different principle, is also provided for the automatic location of possible circuit faults within the main detection scheme.

DOI:10.1049/piee.1970.0152

出版商:IEE    

年代:1970

数据来源: IET

摘要:

The load scheduling problem of hydroelectric/thermal systems can be split into long-range and short-range problems. The short-range problem is to allocate the hydroelectric energy available at each station over the period of day, so as to meet the load demand and minimise the overall cost of thermal generation. If the head variations in reservoirs are neglected and if the period is discretised, the problem can be posed as a nonlinear-programming problem of a special kind in terms of the busbar-voltage variables. An algorithm which splits the problem into smaller-dimension problems is described. It is particularly suitable for large systems, in view of the storage requirements of the digital computer. An example, consisting of two hydroelectric stations, three thermal stations and two load stations is solved, and the results are presented.

DOI:10.1049/piee.1970.0153

出版商:IEE    

年代:1970

数据来源: IET

DOI:10.1049/piee.1970.0154

出版商:IEE    

年代:1970

数据来源: IET