THREE PHASES CONDUCTOR ARRANGEMENT

Abstract: Method for generating low magnetic fields from a three-phase-conductor arrangement, as well as such an arrangement. Four phase conductors are used, one phase (R) being split into two phase conductors (R/2, R/2) which are arranged one on each side of the phase conductors (S, T) of the unsplit phases in a planar phase arrangement. The two outer phase conductors (R/2, R/2) are interconnected, thereby forming a loop. Capacitive elements (53) can be arranged in the loop, preferably in loop-forming cross connections (51).

Description: THREE PHASES CONDUCTOR ARRANGEMENT Technical Field The present invention generally relates to three- phase-conductor arrangements, and specifically to methods and arrangements using phase splitting and configuration 5 of phase conductors for the primary purpose of obtaining a reduction of the magnetic field generated by a three- phase-conductor arrangement. Technical Background 0 Today it is common knowledge that it is desirable to reduce the magnetic field in the vicinity of three-phase- conductor arrangements, both in consideration of any effects on the health and in consideration of disturbing effects on electronic equipment. Typical three-phase-con- 5 ductor arrangements are in this context, for example, overhead and underground power lines, bus bar systems in substation switch gears and generator bus bars in power plants and the like. Regarding overhead lines, attempts have been made to 0 reduce the generated magnetic field by different kinds of symmetrical phase conductor arrangements comprising split phases having five or six phase conductors, implying that two and, respectively, three phases have been split into two phase conductors each. This results in costly and 5 extensive arrangements. Particular problems arise in connection with exist¬ ing three-phase-conductor arrangements having three phase conductors, which in many cases cannot be rebuilt in a reasonable manner for reducing the generated magnetic 0 field. Objects of the Invention The main object of the present invention is to pro¬ vide a method for reducing in a simple, inexpensive and CORRECTED effective manner the magnetic field from a three-phase- conductor arrangement. Another object of the present invention is to pro¬ vide a method for magnetic field reduction, which does not require extensive symmetrical phase conductor confi¬ gurations. A further object of the present invention is to pro¬ vide a method for magnetic field reduction, which is very well adapted for use in existing three-phase arrangements having three phase conductors in planar configuration. One more object of the invention is to provide a method which renders it possible, in addition to magnetic field reduction, to also alternatively easily use phase conductors in various conventional arrangements having three phase conductors. One more object of the invention is to provide a method which makes it possible, in a three-phase arrange¬ ment for transmission over long distances, to simulta¬ neously increase the transmission capacity and reduce the generated magnetic field as compared with a conventional arrangement having three phase conductors. Summary of the Invention The above-mentioned objects are achieved by a method, an arrangement and a use having the features stated in the accompanying claims. The invention thus is based on the knowledge that an unsymmetrical arrangement of four phase conductors, of which two are connected to one and the same phase and form a loop, surprisingly yields an essential magnetic field reduction as compared with a conventional arrange¬ ment of three phase conductors. The two "split" phase conductors are arranged each on one side of and outside the two phase conductors in the middle, which belong to the non-split phases. The loop is formed by connecting the two outer phase conductors with each other at least at the beginning and, respectively, at the end of the phase splitting. Connections can be carried out in a con¬ ventional manner. In overhead lines, they may consist of, for example, suspension cross-connecting conductors (slacks) in connection with suspension from used pole constructions. In bus bars, they consist naturally of the connecting bars of each cell, extending to the two split phase conductors. In the inventive arrangement, the four phase conduc¬ tors, seen transversely of the extent of the phase con- ductors, are thus arranged in an extended configuration, preferably at least essentially planar or straight. The phase conductors are advantageously equidistantly spaced, the space between them being kept as small as possible, while considering the necessary requirements for insula- tion etc. The phase conductor arrangement can be essen¬ tially horizontal, vertical or inclined in general. According to a first aspect of the invention, a sub¬ stantial part of the magnetic field reduction is obtained by the currents which will circulate in an unscrewed loop formed of the outer phase conductors. The generating of these circulating currents can be promoted by connecting capacitive elements to the loop. Such capacitive ele¬ ments, for example capacitors, are advantageously con¬ nected to the loop-forming connections, preferably sym- metrically, i.e. in both connections forming a loop. In overhead lines, the capacitive elements may, for example, be suspended from cross-connecting conductors (slacks) . As compared with a planar phase arrangement having three phase conductors, use of the present invention implies that, in case of an unscrewed loop, the magnetic field can be reduced, in any case to about one third of the normal. If optimum compensation (about 60%) of the inductance of the loop by means of capacitive elements connected to the loop is used, a further halving of the magnetic field can be achieved. Totally seen, the magne¬ tic field then decreases to about one sixth. The circu¬ lating current can then typically be about 20% of the phase current, as compared with about 10% in the uncom- pensated case. In bar arrangements, the circulating current can be stimulated by giving the bars of the split phase large dimensions. It has been found that the field then decreases by approximately a factor eight, even without compensation. It will be appreciated that the present invention can advantageously be used to reduce in a controlled man- ner the magnetic field from a special distance of a power line having a conventional planar arrangement of three phase conductors by rebuilding the distance involved according to the present method. In conventional overhead lines supported by portal- type line poles, it has in many cases proved to be pos¬ sible to arrange the two unsplit phase conductors between the two pole legs of the respective line pole and the two phase conductors obtained by splitting, one on each out¬ side of the two pole legs, while maintaining the requi- site insulation between the phases and to the poles, respectively. This results in a halving of the phase dis¬ tance, which yields an extra halving of the magnetic field, which totally decreases to about one twelfth, if maximum compensation is also utilised. In buried lines, it may in many cases be relatively easy to arrange an additional phase conductor by the side of three conventionally arranged, spaced-apart parallel phase conductors and at a distance from the closest of said three phase conductors, which preferably equals approximately the mutual distance between said three phase conductors. The additional phase conductor is con¬ nected to the remotest phase conductor of said three fur¬ ther phase conductors so as to form the necessary loop. As a rule, the connections required do not constitute any problem, since cable-type phase conductors are involved, i.e. insulated phase conductors that may intersect each other without any problems. Also in conjunction with bar-type phase conductors there are, as a rule, no difficulties in arranging an additional bar in parallel with three bars which consti¬ tute a conventional bar arrangement. It will be appreciated quite easily that the inven¬ tion may be used both for new constructions and for reconstructions. Especially for long lines, it is conve¬ nient to use screwing for the purpose of reducing the disequilibrium between the impedances of the different phases. Such screwing may be easily effected by splitting the three phases one after the other according to the invention along preferably equally long line portions. As compared with a conventional arrangement with three phase conductors in planar configuration, a screwed arrangement according to the invention yields about 20% lower induc¬ tive reactance without compensation and about 30% lower inductive reactance with optimum compensation. This results in an increase of the transmission capacity by about 25% without compensation and by about 50% with optimum compensation. The losses at the same total con¬ ductor cross-section will be insignificantly greater if each of the outer phase conductors has a cross-section which is at least half of the cross-section of the other two phase conductors. According to a second aspect of the invention, a magnetic field reduction can be achieved, while avoiding losses, which may be especially advantageous in conjunc¬ tion with line arrangements * for transmission over long distances. Then use is made of screwed loops together with line screwing of the type mentioned above. Screwed loops mean that circulating currents in the loops are avoided, at the same time as it has surprisingly been found that the inductive reactance does not seem to be affected to any considerable extent. The magnetic field reduction obtained is a halving compared with a conven¬ tional three-conductor arrangement, and the transmission capacity increases by about 25%. If loop compensation is also utilised, which is possible, the transmission capa¬ city increases still more. A further favourable effect of the four-conductor arrangement according to the invention is that the short- circuit forces acting on the conductors will become low. Compared with a common, planar three-conductor arrange¬ ment, it appears that the forces in a three-phase short- circuit decrease by about 40% in the power line case and by about 50% in the bar case even without compensation. With compensation, the decrease will be considerably above 50% in both cases. The use of four phase conductors according to the invention, however, also implies redundancy conditions, which may be used to provide, if necessary, various alternative configurations of arrangements with only three phase conductors. This can render continued ope¬ ration possible if a phase conductor has to be tempo¬ rarily taken out of operation, for example owing to a fault, service or the like. The necessary phase switching and breaking/switching of transverse connections must, of course, be carried out. The preparations for this can be made in a switch gear or in some other suitable place. When using the inventive connecting possibility, it may in some cases be convenient to give the two outer phase conductors at least the same cross-section as the two other phase conductors, for an overall optimising purpose. The use of four phase conductors according to the invention may, in conjunction with overhead lines, in some cases result in simplified wiring, since the conduc¬ tors can be suspended in pairs from two separate poles, one conductor on either side of each pole, which yields great flexibility. A conductor arrangement designed according to the invention also has the advantage of being convertible into direct-current transmission, the four conductors being used for two dipoles with two conductors each. The invention will now be described in more detail by means of some embodiments, reference being made to the accompanying drawing. Brief Description of the Drawings Fig. 1 schematically illustrates a basic embodiment of the invention. Fig. 2 schematically illustrates a further embodi¬ ment of the invention, with a compensated loop. Fig. 3 schematically illustrates a third embodiment of the invention, which uses screwing. Fig. 4 schematically illustrates a fourth embodiment of the invention, in which loop screwing is used. Fig. 5 schematically illustrates an example of a reconstruction of a conventional three-conductor arrange¬ ment into a four-conductor arrangement according to one embodiment of the invention. Fig. 6 is a diagram illustrating the magnetic field reduction obtained in a reconstruction according to Fig. 5. Fig. 7 is a schematic perspective view of an embodi¬ ment of a bus bar arrangement according to the invention. Description of Preferred Embodiments Fig. 1 illustrates how the present invention is used over a line portion L for reduction of the magnetic field generated by the conductor arrangement which comprises three phases R, S, T. Three input phase conductors 1, 2, 3 extend in parallel in a planar phase arrangement. At the beginning of the line portion L, the R phase conduc¬ tor 1 is split into two phase conductors 4, 5 extending in parallel with the S and T phase conductors 2, 3 and on either side thereof, while maintaining the planar phase arrangement and with unchanged, equal mutual distance between the phase conductors 2, 3, 4, 5. At the end of the line portion L, the phase conductors 4, 5 are joined such that the R phase conductor 1 is resurrected. In the embodiment shown, the phase conductor 4 is a direct extension of the input phase conductor 1, and at the end of the line portion , the output phase conductor 1 is a direct extension of the phase conductor 4. The phase conductor 5 is connected to the beginning and, respectively, to the end of the phase conductor 4 by cross connections 6 and 7, respectively. In conjunction with overhead lines, the cross connections 6, 7 are, as will be immediately appreciated, suitably designed as hanging slacks arranged in connection with a line-sup¬ porting pole construction. It will be appreciated that the output phase conduc¬ tor 1 could alternatively be a direct extension of the phase conductor 5. Fig. 2 illustrates how it is possible, in an embodi¬ ment of the general type as shown in Fig. 1, to compen¬ sate for the inductance in the loop that is formed by the conductors 4, 5, 6 and 7. The compensation is provided by capacitive elements 8, 9 being connected to the cross connections 6 and 7, respectively. These elements may consist of, for instance, capacitors suspended in slacks, as illustrated in Fig. 5. In the embodiment of Fig. 2, the output R phase con¬ ductor 1 starts from the opposite side compared with the embodiment of Fig. 1. In other words, it is a direct extension of the phase conductor 5. As is directly appa¬ rent, this results in an improved symmetry, while the two capacitive elements 8, 9 are arranged each in one loop half. Fig. 3 illustrates an embodiment, showing how screw¬ ing can be applied when using the present invention, for the purpose of reducing impedance disequilibrium. In the case illustrated, a basic arrangement as shown in Fig. 1 is assumed. The R, S and T phases are in turn split each along a section L/3. The R phase is split as shown in Fig. 1, although the cross connection 7 has two parts 7' and 7". The S phase 2 is, correspondingly, split into two phase conductors 11, 12 by means of cross connections 13' 13" and 14', 14", respectively, while the T phase 3 is split into two phase conductors 15, 16 by means of cross connections 17', 17" and 18, respectively. Fig. 4 schematically illustrates an example of using the invention in conjunction with a screwed loop. Here, the R phase is split into two phase conductors 21, 22 which form a screwed loop together with end cross con¬ nections 23, 24 of the same type as in Fig. 1. In other words, the loop has the general configuration of an eight, the transverse central loop portions 21", 22" which intersect each other at 25, however, not being in contact with each other. The cross connections 21", 22" are positioned in the middle of the line portion L and may be designed in a manner similar to the cross con¬ nections 23, 24. This loop design does not produce any circulating currents and, consequently, no loop losses, at the same time as the inductive reactance does not increase significantly. Fig. 5 illustrates how a 220 kV power line having three phase conductors can be rebuilt according to the present invention so as to produce lower magnetic fields. Here, the conductors are suspended from a portal-type line pole comprising two poles 31, 32 and one crossbeam 33. Dashed lines indicate the conventional suspension of the three phase conductors R, S and T from insulators 35, 36 and 37. In rebuilding to an embodiment according to Fig. 2, it has been found to be possible to reduce the phase distance by half, while maintaining the necessary insulation. This means that two phase conductors S and T can be suspended symmetrically between the poles 31, 32, suitably in angular insulator pairs 41, 42 and 43, 44, respectively. The third phase R is split and the two resulting phase conductors R/2 are suspended on either side outside the poles 31, 32, also suitably in angular insulator pairs 45, 46 and 47, 48 respectively. The mutual distance between the phase conductors S and T equals the distance between these phase conductors and the respective exterior phase conductors R/2 and T/2. The two outer phase conductors R/2 and T/2 are interconnected by a cross connection 51 in the form of a slack conductor, in whose lowest part a capacitor 53 is connected. Fig. 6 shows graphs of the strength of the generated magnetic field as a function of the distance laterally from a line of symmetry of the conductor arrangement, measured at a distance from the ground of 1.5 m. The graph 61 illustrates the magnetic field from the con¬ ventional three-conductor arrangement. The graph 62 illustrates the magnetic field from the four-conductor arrangement obtained after the reconstruction, however without compensation, i.e. without capacitors 53. The graph 63 illustrates the corresponding magnetic field with compensation, i.e. the capacitors 53 being connect¬ ed. As is directly apparent, an extremely good reduction of the magnetic field is obtained. In the embodiment shown, the effected halving of the phase distance also contributes, viz. a halving of the magnetic field. Fig. 7 illustrates a section of a bus bar arrange¬ ment according to the present invention. The bar arrange¬ ment comprises two centrally positioned conventional bars 71, 72 which are connected to the R phase and the T phase, respectively, via upright connecting bars 73, 74, respectively. The S phase is split into two outer bars 75, 76 which are connected to an upright connecting bar 77 via bar portions 78, 79 extending in opposite direc- tions from the bar 77. The bars 77, 78 and 79 are together of essentially Y configuration. Simultaneously, the bars 78, 79 constitute a mutual connection between the outer bars 75, 76. The bars 75, 76 have an angular configuration with a vertical portion 81 which (in vertical direction) is wider than the bars 73, 74, and a horizontal portion 82 extending inwardly from the upper edge of the vertical portion 81. The portions 82 are interconnected by cross connections 83, suitably in any case in positions corre¬ sponding to the cross connection 78, 79. As will be immediately appreciated, this bar arrangement is very stable, at the same time as the two outer bars have advantageously great dimensions.

Claims: CLAIMS 1. A method for providing low magnetic fields from three-phase electric conductor arrangements, comprising the use of phase-splitting and configuration of phase conductors for reducing the generated magnetic field, c h a r a c t e r i s e d by splitting a phase into two phase conductors and arranging said two phase conductors one on each side of the phase conductors of the unsplit phases in an extended phase arrangement seen in cross- section, the first-mentioned two phase conductors being interconnected so as to form a loop. 2. The method as claimed in claim 1, c h a r a c - t e r i s e d in that the four phase conductors, seen in cross-section, are arranged in an at least essentially planar or straight phase arrangement. 3. The method as claimed in claim 1 or 2, c h a r ¬ a c t e r i s e d by arranging the four phase conductors essentially equidistantly spaced from each other seen in cross-section. 4. The method as claimed in any one of the preceding claims, c h a r a c t e r i s e d by arranging capacitive elements in the loop, preferably in one or more of the loop-forming connections, thereby promoting the genera¬ tion of magnetic-field-reducing circulating currents. 5. The method as claimed in any one of claims 1-3, c h a r a c t e r i s e d by screwing the loop so as to at least essentially eliminate circulating currents in the loop, thereby avoiding loop losses. 6. The method as claimed in any one of the preceding claims, c h a r a c t e r i s e d by splitting in turn the three phases over preferably equally long line por¬ tions, thereby obtaining screwing which reduces disequli- brium between impedances of the phases. 7. The method as claimed in any one of the preceding claims in conjunction with overhead lines supported by portal-type line poles, c h a r a c t e r i s e d by placing the two unsplit phase conductors between the two pole legs of each line pole and the two phase conductors obtained by splitting one on each outside of the two pole legs. 8. The method as claimed in any one of claims 1-6, in conjunction with underground lines, c h a r a c ¬ t e r i s e d by placing an extra phase conductor by the side of three conventionally and separately arranged, parallel phase conductors and at a distance from the closest of said three phase conductors approximately equalling the mutual distance between said three phase conductors, and connecting the extra phase conductor with the remotest phase conductor of said three phase conductors so as to form a loop. 9. The method as claimed in any one of the preceding claims, c h a r a c t e r i s e d by arranging the phase conductors such that, when required, one split phase con¬ ductor, or both of them, together with the two other phase conductors, or one of them, can constitute a con¬ ventional three-phase-conductor arrangement composed of three phase conductors. 10. The method as claimed in claim 9, c h a r a c ¬ t e r i s e by using four phase conductors having at least essentially the same conductor cross-section. 11. An arrangement for three-phase transmission of electric current, comprising essentially parallel and separately arranged phase conductors, c h a r a c t e r ¬ i s e d in that it consists of four phase conductors, of which two central, separated phase conductors are allo¬ cated one phase each, the other two phase conductors being separately arranged one on each side of the two central phase conductors and being interconnected so as to form a loop and being allocated the third phase. 12. The arrangement as claimed in claim 11, c h a r a c t e r i s e d in that the phase conductors, seen transversely of the extent of the line, are arranged at least essentially in alignment with each other. 13. The arrangement as claimed in claim 11 or 12, c h a r a c t e r i s e d in that capacitive elements are arranged in the loop for promoting the generation of mag¬ netic-field-reducing circulating currents, said capaci¬ tive elements preferably being arranged in loop-forming connections between said other two phase conductors. 14. Use of four at least essentially parallel phase conductors in a three-phase electric conductor arrange¬ ment, two separate, centrally arranged phase conductors being allocated one phase each and the other two phase conductors being allocated the third phase and being separately arranged one on each side of the two centrally arranged phase conductors and being interconnected so as to form a loop, such that the magnetic field generated by the conductor arrangement is reduced along the extent of the conductor arrangement, which is enclosed by the loop, the inductance of said loop preferably being compensated by arranging capacitive elements in said loop for promot¬ ing the generation of circulating currents therein. 15. Use of four at least essentially parallel phase conductors in a three-phase electric conductor arrange¬ ment, two separate, centrally arranged phase conductors being allocated one phase each and the other two phase conductors being allocated the third phase and being separately arranged one on each side of the two centrally arranged phase conductors and being interconnected so as to form a loop, the three phases in turn being split over preferably equally long line portions, such that screw¬ ing, which reduces disequilibrium between the impedances of the phases, is obtained, and each loop being screwed, thereby at least essentially eliminating circulating cur¬ rents in the loop, whereby the generated magnetic field is reduced, loop losses are avoided and the transmission capacity is increased relative to a conventional conduc¬ tor arrangement having three phase conductors.

Inventors: PETTERSSON PER, PETTERSSON, PER
Applicants: PETTERSSON PER, VATTENFALL AB, PETTERSSON, PER
Application year: 1995
Main classification: H02G7
IPC codes: H02G7