摘要:

Transistorised amplifiers for video transmission over coaxial cables incorporate power units with d.c.-d.c. convertors that produce an aggregate effect similar to that of band-limited random noise at low frequency. The subjective effect of such noise, centred at 3.2 and at 7.5kHz, has been investigated. Tests were also made with sinewave noise at the frequencies giving the worst effect in the regions of 1 and 3.2kHz and 1 and 3.2MHz. Results are summarised in the form of signal/noise ratios for a series of ‘imp’ mark points that has recently been introduced.

DOI:10.1049/piee.1968.0069

出版商:IEE    

年代:1968

数据来源: IET

摘要:

In a communication net in which the maximum attainable flows are determined by the vertex capacities, there is, in general, a high degree of redundancy among the paths available for flow assignments in the net. A technique is given by which the set of all nonredundant paths, called proper flow paths, is uniquely determined. The method employs a direct and simple means of eliminating the redundancies in a pathfinding routine proposed previously by Paz, involving symbolic noncommutative multiplications among the entries of a modified vertex-adjacency matrix. A useful byproduct ensues with the immediate identification of vertexes which are not in any vertex cut set. This simplifies the problem of identifying all vertex cut sets in the net. A presentation of proper-flow-path theory in communication nets is included in the paper.

DOI:10.1049/piee.1968.0070

出版商:IEE    

年代:1968

数据来源: IET

摘要:

The approach adopted is to write down a wave equation which applies to the medium as a whole. The physical parameters of the medium appear in the coefficients in this equation. It is shown that meaningful solutions can be obtained when the physical parameters possess finite discontinuities. A similar approach was suggested by Brillouin, but was not successfully completed. Some examples are given in which solutions may be obtained by other methods; e.g. using boundary conditions at surfaces of discontinuity.

DOI:10.1049/piee.1968.0071

出版商:IEE    

年代:1968

数据来源: IET

摘要:

At the present time the Area Boards are spending between £8 million and £20 million on capital work, mainly connected with the expansion of the supply network. The cost of equipment to transform from high to medium voltage, together with associated substation equipment and the connections to the high- and medium-voltage networks, represents in the Merseyside and North Wales Electricity Board (Manweb) area somewhere of the order of 30% of the total capital costs. Consequently, a small percentage saving in the number of substations required for the network can represent a considerable sum of money. This saving could result from a decrease in the number of transformers installed if an increase in the mean loading of the existing and future transformers could be considered acceptable. As this mean or average loading is increased, so also is the load on the most heavily loaded transformers, and this has to be limited, owing to the possibility of a certain number of these transformers exceeding the loading at which deterioration is excessive. The load on these transformers has to be considered under conditions of inclement weather and/or outage conditions, which could occur at any time. From a statistical survey of the network conditions in the Merseyside and North Wales Electricity Board area, it can be shown that, within a given probability, a higher mean loading can be considered acceptable, provided that the variation from this mean, i.e. the standard deviation of the individual loadings to the mean, is controlled. The form of this control is considered.

DOI:10.1049/piee.1968.0072

出版商:IEE    

年代:1968

数据来源: IET

摘要:

The interaction between the controllers of h.v. d.c. convertors connected to the same d.c. system raises problems in the design and analysis of the dynamic behaviour of the convertors under transient conditions. Studies of controller behaviour can be made on d.c.-system models, but the modelling procedure is usually costly and time-consuming. For the initial investigation of suitable controller parameters for a multiterminal d.c. system, a simplified method is proposed in the paper for which a digital program has been written. The program enables the d.c.-system response to a step change of a.c. or d.c. conditions to be studied for about 200ms after the disturbance, a time period comparable with the response of most d.c.-system controllers. The a.c. system is not represented in any detail, but the d.c. system is simulated by a distributed-constant line to enable the effect of surges to be taken into account. Examples are given of the behaviour of h.v. d.c.-convertor controllers incorporating gain linearisation, staleness factors and control changeover after a voltage disturbance on the a.c. side. The paper concludes with an estimation of the accuracy of the method for initial-design purposes and the computer-storage and time requirements.

DOI:10.1049/piee.1968.0073

出版商:IEE    

年代:1968

数据来源: IET

摘要:

Resistance-network analogues have been used experimentally for longitudinal step-by-step solution of the thermal problem in separate-water-pipe cooling of underground e.h.v. cables. In parallel with this technique, mathematical models have been programmed for digital and analogue computers. The feature of the digital solution is the use of two programs; the first need only be run once, giving results which are applicable as boundary conditions to various cable and pipe geometries in the second program, in which both radial and square meshes are used. The analogue program in which the digital results are used is simple, versatile and suitable for general application to most cable geometries. The derivations of the thermal equations and the development of the computer programs are given in detail, and results for typical cable systems are presented.

DOI:10.1049/piee.1968.0075

出版商:IEE    

年代:1968

数据来源: IET

摘要:

A sampling technique has been developed which allows a comparison of instantaneous values derived at different instants of time, thereby dispensing with the need to phase-shift or mix signals derived from the primary line quantities. Both amplitude and phase comparisons are used to derive a variety of impedance characteristics. The relays combine high sensitivity with a high resistance to interference. Detailed results are presented for a current-polarised relay with a characteristic surrounding the probable fault area.

DOI:10.1049/piee.1968.0076

出版商:IEE    

年代:1968

数据来源: IET

摘要:

The paper deals with the transformation by numerical techniques of fields having polygonal boundaries. Modern direct-search techniques for the minimisation of multivariable functions are employed, together with a method of integration in which the step length is continuously and automatically varied. The complete solution procedure is highly efficient; it converges smoothly for all boundary shapes studied and has the important feature for general application that it is virtually independent of experience on the part of the analyst. A discussion of typical functional variations and a transformation simplifying the integration of functions with complex limits are included. An example is considered to show the application of the method, and guidance is given on the performance to be expected in the general application of the method.

DOI:10.1049/piee.1968.0079

出版商:IEE    

年代:1968

数据来源: IET

摘要:

Measurements were made of the permittivity and loss tangent of polyethylene, poly(4-methyl pentene-1), polyethylene terephthalate, polyvinyl chloride (containing 33% tritolyl phosphate), poly-ω-undecanolactum, cellulose triacetate and insulating paper for temperatures between 4.2 and 295°K over a frequency range from 47Hz to 89.5kHz. In polyethylene and poly(4-methyl pentene-1), a single small absorption region (maximum value, 1−2×10−4was observed in the temperature range 150–200°K. The loss tangent of polyethylene decreased to a constant value (∼2×10−5) near absolute-zero temperature, whereas, in the case of poly(4-methyl pentene-1), below 70°K it increased with increasing frequency and decreasing temperature. With the exception of polyvinyl chloride, which did not exhibit any distinct absorption regions, the remaining materials investigated possessed one or more loss regions with a maximum loss tangent between 4×10−3and 3×10−4. The peaks occurred in the temperature range 150–300°K. The loss tangent of these materials decreased to 1−3×10−4as the temperature was reduced to 4.2°K, with the exception of poly-ω-undecanolactum, for which it was 3×10−5

DOI:10.1049/piee.1968.0080

出版商:IEE    

年代:1968

数据来源: IET

DOI:10.1049/piee.1968.0081

出版商:IEE    

年代:1968

数据来源: IET