GB2059670A

GB2059670A - A power supply system for three-phase current of medium frequency and high voltage cable for conducting a three-phase current of medium frequency

Info

Publication number: GB2059670A
Application number: GB8030584A
Authority: GB
Original assignee: Piller Anton GmbH and Co KG
Current assignee: Piller Anton GmbH and Co KG
Priority date: 1979-09-26
Filing date: 1980-09-23
Publication date: 1981-04-23
Also published as: DE2938864C2; FR2466086B1; GB2059670B; DE2938864A1; FR2466086A1

Abstract

A high voltage cable, for a three-phase power supply system, comprising six phase conductors, 4, 6, 8, 10, 12 and 14, each of the same cross-sectional area arranged symmetrically around a central null or protective conductor 2, the phase conductors being connected together in oppositely-situated pairs at their ends. <IMAGE>

Description

SPECIFICATION A power supply system for three-phase current of medium frequency and a high voltage cable for conducting a three-phase current of medium frequency The invention relates to a power supply system for three-phase current, of medium frequency, in particular a system for a frequency of 400 Hz. or more, and to a high voltage cable for conducting three-phase current of medium frequency.

Three-phase current of medium frequency, 400 Hz. in particular, is used, for example, in aeronautical, military and computer technology. In such cases it is sometimes necessary to transmit high powers. A series of phenomena appear in the transmission of high powers at a frequency of 400 Hz. or more which are practically without significance in 50/60 Hz.

supply mains and therefore do not usually need to be taken into account.

In medium frequency power supplies there appears, in particular: 1. increased transmission losses; 2. increased drop in the reactive voltage; 3. voltage assymetry at the ends of the sup ply cable, and 4. induced currents in protective conductor loops.

With a higher working frequency, a considerable increase in the resistance of a conductor is produced by current displacement effects due to both skin effect and short range effect.

The skin effect causes a current displacement in the conductor produced by the field of the conductor and the short range effect leads to current displacement by the field of neighbouring conductor. At 400 Hz. they cause considerable losses. Owing to the increased resistance connected therewith a considerable increase in the ohmic voltage drop is produced. As the relative increase in resistance increases rapidly with increasing cross-sectional area, large cross-sections have been avoided as far as possible up to the present time and instead of these a change has been made to a plurality of cables connected in parallel.

The voltage drop in a conductor is made-up of the effective voltage drop, I x R, and the reactive voltage drop, I x cos f. These two voltage drops should be added geometrically.

Owing to the frequency dependence of the reactive voltage drop, this comes into marked prominence at a medium frequency of 400 Hz. in contrast to a 50 Hz. system where it is practically without significance.

The inductance of a cable depends on the ratio of the cable diameter to the distance between the conductors and is thus almost independent of the conductor cross-sectional area. Thus the reactive resistance of a cable of a given construction cannot be changed by an increase in cross-sectional area. In this case again the parallel connection of a plurality of cables has provided the only remedy up to the present time.

Maintenance of the symmetry of the voltage at the ends of a cable, which is necessary within narrow tolerances for most usages, is only possible when the effective voltage drop and the reactive voltage drops are equally great in all three phase conductors of a cable.

This is true for a three-conductor cable when the distances between the conductors are of equal magnitude. In known cables, however, the introduction of a fourth conductor, which must be provided as a protective of null conductor, leads to difficulties. If such a four conductor cable of the usual constructional type is used, differences of about 30% between the working inductances arise for the various conductors. These lead to marked assymmetry of the voltages at the ends of the cable. By the use of the fourth conductor as a centre point conductor, because of the differing mutual inductances with the phase conductors a considerable voltage is induced in the centre point conductor, even with symmetrical load, and this increases the asymmetry still further. An ordinary four conductor cable is therefore unsuitable for use in medium frequency power supply systems.Symmetrical voltage relationships can be achieved with cables in which the three phase conductors are at equal distances from each other and the null conductor is mounted as a concentric external conductor. However, such a cable has high transmission losses owing to the eddy currents which appear in the concentric external conductor.

If, in an asymmetric cable construction, the fourth conductor is used as a protective or null conductor, a voltage is induced within it because of differing mutual inductances. If protective conductor loops are formed in extensive cable systems, as often occurs unintentionally owing to multiple earthing, currents are produced in these loops which may reach the same order of magnitude as the working current. If cables of different crosssectional areas are used in mains systems, dangerous overloading of the protective conductor may result from this effect. Careful planning of the protective conductor system, taking into account all protective circuits, is thus indispensable. In a NYCY conductor, protective conductor loops cause no problems because of the symmetrical position of the protective conductor.

It is an object of the invention to devise a power supply system for three-phase current of medium frequency and a high voltage cable for conducting three-phase current of medium frequency, by means of which it is possible to transmit high powers with substantially lower losses so that, in most cases, it is possible to operate with a single cable and to eliminate parallel circuits and the known method of compensation of the reactive voltage drop by means of series capacities.

This object is solved according to the invention by means of a power supply system having a supply cable comprising six phase conductors of equal cross-sectional area arranged symmetrically around a central null or protective conductor, each pair of oppositelysituated phase conductors being connected together at their ends to form the conductor for one phase of the supply.

The central null or protective conductor preferably has the same cross-sectional area as each of the phase conductors.

A high voltage cable for conducting a threephase current of medium frequency, according to the invention, comprises six phase conductors each of the same cross-sectional area arranged symmetrically around a central null or protective conductor, the phase conductors being connected together in oppositely-situated pairs at their ends.

The central null or protection conductor preferably has the same cross-sectional area as each of the phase conductors.

By way of example, a high voltage cable in accordance with the invention is illustrated schematically in the accompanying drawing, which is a transverse cross-section through the cable.

The cable illustrated comprises six phase conductors 4, 6, 8, 10, 12 and 14 positioned around a central conductor 2. The six phase conductors are arranged symmetrically round the central conductor, which is used as a null or protective conductor. Each of the six phase conductors consists of twisted single wires and each is separately insulated. The phase conductors are preferably wound around the central conductor. The conductor assembly is provided externally with a non-conducting sheath 1 6 which gives protection against damp and mechanical damage.

The cross-sectional area of each of the phase conductors is between 25 and 1 25 mm2 according to the power to be transmitted. Larger cross-sectional areas are, however, possible. With conductor cross-sectional areas greater than 50 mm2, current displacement effects occur, even with the cable in accordance with the invention, at medium frequencies and these lead to losses. The losses are greatly reduced if wire insulated with a coating of lacquer, i.e., enamelled wire, is used for a plurality of wires which are twisted to form each phase conductor of the cable.

When phase conductors consisting of twisted enamelled wire are used, the current displacement effect is so greatly reduced that even cables with a conductor cross-sectional area up to 250 mm2 may still be used economically.

When the high voltage cable in accordance with the invention is used in a power supply system of medium frequency, two diametrically-opposite phase conductors, that is the pairs of conductors 4 and 10; 6 and 1 2 and 8 and 14 are connected in parallel, so that each of the phases R, S and T of the cable consists of two symmetrically-opposite individual phase conductors. The conductors of each pair of phase conductors are connected together at their ends.

In comparison with normal cables and also with the parallel connection of a plurality of cables, the composite cable in accordance with the invention, having the same crosssectional area of copper, is distinguished by: 1. smaller losses; 2. higher load capacity because of the greater surface area; 3. smaller stray field; 4. lower inductance; 5. a symmetrical construction including the null conductor.

By the use of a single composite cable having large copper cross-sectional area instead of the parallel connection of a plurality of cables, necessary up to the present time, it is possible to make the installation of the composite cable substantially easier than a plurality of cables connected in parallel.

Claims

1. A power supply system for three-phase current of medium frequency, in which a supply cable comprises six phase conductors of equal cross-sectional area arranged symmetrically around a central null or protective conductor, each pair of oppositely-situated phase conductors being connected together at their ends to form the conductor for one phase of the supply.

2. A power supply system according to Claim 1, in which the central null or protective conductor of the supply cable has the same cross-sectional area as each of the phase conductors.

3. A high voltage cable, for conducting a three-phase current of medium frequency comprising six phase conductors each of the same cross-sectional area arranged symmetrically around a central null or protective conductor, the phase conductors being connected together in oppositely-situated pairs at their ends.

4. A high voltage cable according to Claim 3, in which the central null or protective conductor has the same cross-sectional area as each of the phase conductors.

5. A high voltage cable according to Claim 3 or 4, in which the six phase conductors are wound around the central null or protective conductor.

6. A high voltage cable according to any of Claims 3 to 5, in which each of the phase conductors is formed from a plurality of twisted insulated wires.

7. A high voltage cable according to any one of Claims 3 to 6 in which the assembly of the six phase conductors and the central null or protective conductor are enclosed within a non-conducting sheath.

8. A high voltage cable according to Claim 3 constructed and arranged substantially as described herein and shown in the accompanying drawing.