Abstract:
A method of designing a single- or a multi-phase electric cable for conducting current through insulated conductors and creating a weak external magnetic field, so as to obtain a cable wherein at least one of the above-mentioned conductors is assembled from two or more insulated sub-conductors connected in parallel, and wherein the sum of cross-sectional areas of the sub-conductors is equal to a design cross-sectional area of the conductor. The arrangement in the cable is such that each of the sub-conductors is adjacent to a conductor or a sub-conductor associated with either a different phase or a different current direction, and the sum of magnetic moments of magnetic dipoles formed from all currents passing through the cable is zero.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method of designing multi-conductor electric cables (both single-phase and multi-phase) which create a very weak external magnetic field, and to the structure of such cables per se. 
     BACKGROUND OF THE INVENTION 
     Scientific research and investigation of influence of continuous exposure to existing environmental alternating electromagnetic field, which have been completed to date, have arrived at very significant conclusions. For example, it has been acknowledged that epidemiological evidence points to human health hazards in exposure to ambient alternating electromagnetic field environments exceeding 0.2 μT. A dose-dependence of childhood leukemia is suggested for power frequency fields in the range 0.2-0.4 μT. Assessment of the ambient magnetic environment in these studies at sites near power transmission and distribution lines has generally not taken account of much higher but more focal fields in the immediate vicinity of operating devices in the home and workplace. Resulting risk estimates may thus underestimate the true exposure levels from all sources. Although largely neglected in the emphasis on magnetic field bio-effects, there is also a body of laboratory evidence relating biologically significant effects, particularly in cerebral tissue calcium binding, to electric field exposures in the range 10-100 V/m. It must be emphasized that epidemiological studies do not rule out effects of electromagnetic fields on cancer risk, even large ones. This is because of limitations in exposure assessment and undoubted misclassification of exposure, as well as the absence of truly unexposed subjects. 
     The Regulations of the Israeli Electricity Law stipulate in Section 31 that, at medical sites where bio-potential measurements are provided (such as emergency departments in hospitals, ECG or EMG laboratories or the like) electric cables are to be screened to avoid or diminish interference caused by electrical equipment. Section 31 of the Regulations states that in such locations the maximal allowed value of magnetic induction be 2 *10 −7  to 4*10 −7  T (i.e. 2 to 4 mGs). 
     Screens which are utilized for protection from the excessive magnetic field are usually capable of reducing external magnetic field intensity by about 10-30%. Such a screen is effective for protecting the cable from ambient magnetic fields, but it decreases only slightly the external magnetic field created by the electric cable itself. 
     It should be mentioned that an electromagnetic field created in the vicinity of a current conducting cable may have a harmful effect on precise electronic instruments, computers and communication devices. In the prior art, attempts have been made to improve transmission capability of a multi-wired electrical conductor by reducing its coefficient of additional losses. 
     RU 2025014 describes a three-phase current cable for supply of electric energy users in three-phase circuits with frequency up to 10 kHz. The cable contains phase current conductors (for example, A,B,C), wherein each of the current conductors is in the form of a pair of parallel connected wires. The pairs of wires A,B,C are placed opposite to each other relative to the center of the conductor. 
     SU 1836766 discloses an electricity supply system, having three-phase current conductors with phases made in the form of parallel connected wires set symmetrically relative to a central wire. As explained in the patent specification, when a three-phase current passes through the current conductors, two equal oppositely directed magnetic fluxes are formed and smaller counter EMF&#39;s (electromotive forces) are induced in the cable; the inductive resistance is thereby reduced together with the coefficient of additional losses. The system is declared to have an increased transmission capability. 
     It should be emphasized, that both of the above-mentioned technical solutions are focussed on achieving a minimal internal magnetic interaction in the multi-phase conductor for improving its transmission capability, and the goal is gained by providing a symmetrical structure of parallel connected wires of different phases. 
     U.S. Pat. No. 3,675,042 (Merriam) entitled “Apparatus for power transmission utilizing superconductive elements”, discloses a long electrical power transmission line utilizing superconductivity in which each conductor includes a superconductive portion and a normally conductive portion having high thermal conductivity with the two portions being in electrical and thermal contact along substantially their entire lengths. The conductors are in the shape of thin wires to provide a low internal magnetic field and permit high current densities. The conductors are connected in pairs into a plurality of direct current circuits which in turn are connected to one another in parallel and are arranged in a plurality of circular clusters to further minimize the internal magnetic field and which may be selectively connected between a power source and one or more loads. 
     In such an arrangement, the conductor material operates at or near zero degrees absolute. As the conductor radius increases, for constant current density, the magnetic flux density increases linearly in accordance with the equation:        B   =       I     2      π                 r       ·   μ                            
     where: 
     B=magnetic flux density 
     r=radius of conductor 
     μ=magnetic permeability 
     I=conductor current 
     j=current density 
     There thus exists a critical magnetic flux density for superconductive material, beyond which the material ceases to superconduct. For this reason, as described at col. 6, lines 70ff and recited in the claims, the diameter of the conductor must be limited to no more than about 2 mm. 
     Each superconductive core is surrounded by copper cladding to quench fire if core loses superconductivity and gives rise to heating of the core. Thus, the actual distance of adjacent cores is significantly increased by the diameter of the copper cladding, leading to an increased external magnetic field. 
     U.S. Pat. No. 3,675,042 is thus directed to minimizing the internal magnetic flux density without regard to the external magnetic field. In contrast thereto, the invention is directed to minimizing the external magnetic field. 
     British Patent Publication No. 2 059 670 describes a high voltage cable for a three-phase power supply system, comprising six phase conductors each of the same cross-sectional area arranged symmetrically around a central null or protective conductor. The phase conductors are connected together in oppositely-situated pairs at their ends. GB 2 059 670 has as an objective the requirement to obtain voltage symmetry at the end of the cable and to limit the losses in a 3-phase line at higher frequencies. No suggestion is made to reduce the external magnetic field. 
     SUMMARY OF THE INVENTION 
     It is the two-fold object of the present invention to provide a method for designing a single-phase or a multi-phase electric cable having a very weak external magnetic field and, correspondingly, to provide a novel structure of such cables. 
     According to one aspect of the invention, the above object can be achieved by a method of designing a single- or multi-phase electric cable capable of conducting current through insulated conductors and creating a weak external magnetic field, the method comprising the following steps: 
     (a) assembling at least one of said conductors from two or more insulated sub-conductors to be connected in parallel, wherein the sum of cross-sectional areas of the sub-conductors is equal to a design cross-sectional area of said conductor, and wherein the sum of currents to pass through the sub-conductors is equal to a given current to pass through said conductor; 
     (b) arranging said conductors in the cable in such a manner that each of said sub-conductors is adjacent to a conductor or a sub-conductor associated with either a different phase or a different current direction; and 
     (c) ensuring a predetermined minimal strength of the external magnetic field by checking the following condition for the sum of magnetic moments:            ∑     i   =   1     N                         M   .     _     i       =   0                          
     where N is a total number of the magnetic dipoles, and i is a number of a particular dipole. 
     The insulated sub-conductors may have cross-sections of a circular, rectangular or any other shape. 
     Needless to say, that according to the Kirchhoff&#39;s Law the sum of all currents passing through all conductors and sub-conductors of the cable must be equal to zero. It is understood to those skilled in the art that the cable should be designed according to its operational conditions. 
     Preferably, the sub-conductors and conductors assembled and arranged according to the above definition should be placed in the cable as close as possible to one another. It is readily understood that technical limitations will be imposed by the design voltage, by quality and thickness of the electrical insulation, as well as by the cross-section of the wire. 
     As an option, the method may also comprise a step of twisting the arranged conductors in the cable. 
     A specific calculation step can be applied to the above method for ensuring that the above-described configuration provides a predetermined minimal strength of an external magnetic field. 
     For single-phase cables the calculation step includes arranging a number of so-called magnetic dipoles from currents passing via the mentioned sub-conductors and conductors, dividing the dipoles into groups, determining a value and direction of magnetic moment of each of said groups, and adjusting the arrangement of said conductors and sub-conductors in such a manner that the sum of magnetic moments of component dipoles in each of said groups is substantially zero. 
     In order to better understand the principle of the proposed approach, the following terms will be acknowledged herein below: 
     A dipole is a pair of currents having equal values and opposite directions and passing via a pair of adjacent wires (whether being a non-divided conductor and a sub-conductor, or two sub-conductors) in the cable. 
     A magnetic moment M of a dipole is a vector which can be defined as follows: 
     
       
           {dot over ({overscore (M)})}=μ   0   *{dot over (I)}*D*l   0   *{overscore (n)}   0 , 
       
     
     where: 
     μ 0  is the magnetic permeability of vacuum, 
     I is the value of one of the equal and opposite currents in the dipole, 
     {dot over (I)} indicates the phase of alternating current 
     D is the distance between the parallel wires in the dipole, 
     l 0  is an elementary length of the wire being one unit of length, 
     {overscore (n 0 )} is an elementary vector being perpendicular to the surface where the elementary lengths of two wires of the dipole are located; the vector n 0  can be carried in parallel to itself, its direction thus defined according to the right gimlet rule. 
     Using the above definitions, the following condition of designing a single-phase cable with a plurality of sub-conductors can be written down:            ∑     i   =   1     N                         M   .     _     i       =   0                          
     (where N is a total number of the dipoles, and i is a number of a particular dipole). 
     It has been found by the inventor, that even a pair of dipoles arranged in the cable according to the above described rules enables to achieve a significant reduction of the external magnetic field. The greater the total number of sub-conductors (or the greater the number of the formed pairs of dipoles in the formed multidipole), the weaker will be the external magnetic field created at any predetermined distance from the cable. It has been found that the larger the number of parallel wires and the smaller the distance therebetween, the higher is the degree of attenuation of the external magnetic field. In a case of an infinite number of wires spatially mixed in the cable, no external magnetic field would be created, like in the case of a coaxial electric cable. 
     Moreover, the longer the distance from the center of the cable, the sharper will be the character of attenuation of the external magnetic field. 
     In other words, the method comprises the step of adjusting the degree of attenuation of the external magnetic field by selecting a number of sub-conductors for assembling said conductors of the cable. 
     For example, by applying the above mentioned method to a single-phase cable, one may obtain a degree of attenuation of the magnetic field of many hundreds or thousands, or even more at an exemplary distance of 50 cm from the center of the cable (!), (compared to a very weak attenuation of about 10 to 30% which can be reached by screening of equivalent conventional cables). 
     For multiphase cables the additional calculation step may include calculating a resulting magnetic field created by all conductors and sub-conductors in the cable and adjusting a number, cross-section and configuration of the sub-conductors in the cable to obtain maximal decrease of an external magnetic field in the vicinity of the cable. In such a case, the magnetic flux density in the center of the cable is usually essentially equal to zero, which factor might be helpful for correct designing of the inventive cable. 
     In terms of magnetic moments, the calculation step may comprise determining magnetic dipoles formed in each phase conductor of the multiphase cable and a neutral wire, and arranging the dipoles to satisfy a system of equations wherein each of said equations is built for a specific phase conductor of the cable:                    ∑     n   =   1     N                         M   .     _     Rn       =   0     ;                     ∑     P   =   1     P                         M   .     _     sp       =   0     ,             ⋯                   ∑     q   =   1     Q                         M   .     _     Tq       =   0     ,     where   :                                  
     RS, . . . T—symbolize conductors of different phases of a multiphase cable; 
     N, P, . . . Q—symbolize total numbers of sub-conductors in each of the phase conductors R, S, . . . T, respectively; 
     n, p, . . . q—each symbolize a specific number of a sub-conductor in the phase conductors R, S, . . . T respectively; 
     {dot over ({overscore (M)})} Tq —symbolizes a particular magnetic moment created by a current passing in a sub-conductor q of the phase conductor T and a corresponding part of the current in the neutral conductor, when present, or in another phase when no neutral conductor is present. 
     The preferred version of the method for designing the multiphase cable includes the step of assembling each of m single-phase conductors of the cable from n equal sub-conductors, and the step of arranging said sub-conductors in a circle so that an angle α between each two sub-conductors is about 360°/m·n, and an angle β between each two sub-conductors belonging to the same phase is about 360°/n. 
     The degree of attenuation of the magnetic field which can be achieved by applying the method to multi-phase cables depends on the construction of a specific cable (number of sub-conductors, their arrangement, etc.); if required, the degree may reach hundreds or thousands. It is understood, however, that complexity of the cable&#39;s construction will put a certain limitation to the maximal decrease of the external magnetic field. 
     It is known to those skilled in the art that the stronger the external electromagnetic field created by a cable, the easier the penetration of any ambient electromagnetic field into the cable, thereby creating electric disturbances therein, jamming in transmission lines, etc. It can now be seen, that if the magnetic field around the inventive cable can be minimized, the cable will be less sensitive to any external magnetic field. Such a property is of special importance for sensitive electronic devices (such as precise measurement instruments, computers, TV-sets, etc) and for all high frequency electronic devices. A value of mutual inductance between a reference cable and a cable of interest may serve as a measure of sensitivity of the cable of interest to external magnetic field disturbances. 
     Calculations and measurements which have been undertaken by the inventors, has proven that the mutual inductance of any modification of the inventive cable is much smaller than that of an appropriate conventional cable, and that the inventive cables are significantly less sensitive to external magnetic fields. The inventive cables also have smaller self-inductance than the conventional cables, thus the voltage drop along a transmission line formed by the inventive cables will be decreased. 
     According to a second aspect of the invention, there is provided a single-phase or a multi-phase electric cable for conducting current through insulated conductors and creating a weak external magnetic field, wherein: 
     at least one of said conductors is assembled from two or more insulated sub-conductors to be connected in parallel, wherein the sum of cross-sectional areas of the sub-conductors is equal to a design cross-sectional area of said conductor, and wherein the sum of currents to pass through the sub-conductors is equal to a given current to pass through said conductor; 
     the arrangement being such that each of said sub-conductors is adjacent to a conductor or a sub-conductor associated with either a different phase or a different current direction. 
     It is understood that the arrangement wherein the conductor(s) and sub-conductors (or the sub-conductors only) are placed in the cable as close as possible to one another, is preferable. 
     It should be emphasized, that the present invention does not impose on the sub-conductors any requirements of symmetry, equal cross-sections or specifically stated distances therebetween. 
     With respect to multi-phase cables, the invention also allows that at least one phase conductor in the cable is not assembled from sub-conductors. 
     According to one embodiment of a single-phase cable without a neutral wire, one conductor thereof is assembled from two sub-conductors which are symmetrically placed near the other (non-split) conductor from its two diametrically opposite sides. 
     In accordance with an alternative embodiment of the single-phase cable, each of the two conductors thereof comprises two or more sub-conductors to be connected in parallel to each other. 
     In the preferred embodiment of a multiphase cable, each of its m phase conductors may be assembled from n equal sub-conductors, and the sub-conductors are arranged in a circle so that an angle α between each two adjacent sub-conductors is 360°/m*n, and an angle β between each two sub-conductors belonging to the same phase is 360°/n. 
     The multi-phase cable, according to one specific embodiment of the invention, may comprise a number of phase conductors each being assembled from two or more insulated sub-conductors being equal or non-equal in cross-section; said sub-conductors being mixed in the cable in a manner providing for a minimal external magnetic field. 
     It is known, that in the case of a symmetric load of a multiphase cable, current in the zero-wire (neutral wire) is absent. In this case, position of the zero-wire in the cable is not important. However, for cases where a multiphase cable is loaded non-symmetrically, it is preferable that the zero-wire is positioned in the center of the cable. It has been noticed that the inventive construction of the cable is especially helpful for reducing external magnetic fields created around such non-symmetrically loaded multiphase cables. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawings in which: 
     FIG. 1 a  (prior art) is a schematic cross-sectional view of a conventional single-phase cable without a neutral wire. 
     FIG. 1 b  is a schematic diagram of superposition of magnetic field intensity vectors created by currents flowing in the cable shown in FIG. 1 a.    
     FIG. 2 a  is a schematic cross-sectional view of one embodiment of the inventive single phase cable. 
     FIG. 2 b  is a schematic diagram of superposition of magnetic field intensity vectors created by the cable shown in FIG. 2 a.    
     FIG. 3 a  is a schematic cross-sectional view of another embodiment of the single phase cable according to the invention. 
     FIG. 3 b  schematically shows how the inventive cable can be represented as a system of dipoles. 
     FIG. 4 is a schematic cross-sectional view of yet another embodiment of the single-phase cable according to the invention. 
     FIG. 5 is a schematic cross-sectional view of still a further embodiment of the single-phase cable according to the invention. 
     FIG. 6 is an embodiment of the inventive single-phase cable with a neutral conductor, shown in cross-section with n=3. 
     FIG. 7 is a modification of the embodiment shown in FIG. 6, wherein (n=4), without a neutral conductor. 
     FIG. 8 (prior art) is a schematic cross-sectional view of a conventional three phase cable. 
     FIG. 9 is a schematic cross-sectional view of one embodiment of a three phase cable according to the invention, where n=3. 
     FIG. 10 is another embodiment of the inventive three phase cable, shown in cross-section, where n=4. 
     FIG. 11 is yet another embodiment of a three phase cable having split and non-split phase conductors. 
     FIG. 12 is a modified embodiment of FIG.  11 . 
     FIG. 13 is an embodiment of a three phase cable having conductors and sub-conductors having square-like cross-sections. 
     FIG. 14 is a modification of the embodiment shown in FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In order to explain the mechanism of attenuation of the external magnetic field in the vicinity of the invented cable, a simple numerical calculation will be performed for a conventional single phase cable with two wires (FIGS. 1 a  and  1   b ) and for an equivalent cable with four wires (FIGS. 2 a  and  2   b ). The magnetic field will be computed at two points: one being remote by 50 cm, and the other by 2 m from the center of the cable. 
     FIG. 1 a  refers to the Prior Art and illustrates a conventional single-phase cable  10  in its cross-section. The cable is comprised of two parallel insulated main conductors  11  and  12  each having a cross-section S main , via which a single-phase current I passes in mutually opposite directions, as indicated in the drawing. 
     FIG. 1 b  illustrates a schematic vector diagram explaining a way to calculate intensity H of the magnetic field created by the cable  10  shown in FIG. 1 a  at the point A located at the distance r=50 cm from the center of the cable. The intensity H total  is calculated according to the known method of superposition of the magnetic fields created by the two conductors  11  and  12  at the predetermined point. To this end, the following data and considerations will be utilized for the exemplary calculation: 
     radius of one conductor (including insulation) is 2 mm, so the distance between the centers of the conductors is 4 mm which corresponds to a single phase cable carrying current of about 30 A; 
     distance between each of the conductors and the point A is equal to: 
     
       
         
           r 
           11 
           =r 
           12 
           ={square root over (50 2   +0.2   2 )} (cm) 
         
       
     
     each conductor creates a vector of intensity H i  (where i is 11 or 12), which can be divided into two component orthogonal vectors as follows: 
     
       
           H   iy   H   i  sin α and  H   ix   =H   i  cos α 
       
     
     intensity H i  of the magnetic field created by one of the conductors at the point A (where distances r 11 =r 12 ) can be calculated as follows:            H   i     =       I     2      π                   r   i                         (     A        /        cm     )         ;                          
     the two component vectors along axis x will compensate each other, and the total intensity of the magnetic field at the point A will be: 
     
       
           H   total =2 H   y   
       
     
     After suitable calculations according to the above formulae and for the current I=30 A we obtain: 
     at a distance of 50 cm: H total =2,5462*I*10 −5  (A/cm);  B total =0.96 mG; 
     at a distance of 2 m:  B total =6.0*10 −2  mG; 
     FIG. 2 a  illustrates a schematic cross-section of one embodiment of the inventive single-phase cable  20 , wherein each of two main conductors are assembled from a pair of sub-conductors having equal cross-sections, and any one of the sub-conductors is adjacent to those of the other main conductor. Sub-conductors  21  and  24  together carry the same total current I=30A in one direction; since these sub-conductors have equal cross-sections S main /2, each of them carries a current I/2. The total cross-section of the main conductor formed by the sub-conductors  21  and  24  is equal to the cross-section of the conductor  11  (or  12 ) of the cable  10 . The same applies to the sub-conductors  22  and  23  carrying the total current I in the opposite direction. From the functional point of view the cable  20  is completely equivalent to the cable  10  of FIG. 1 a.    
     In order to check which external magnetic field will be created, for example, at point A located at a distance 50 cm from the center of the cable  20  (i.e. the same distance as in the example of FIG. 1 b ), the attention is drawn to FIG. 2 b.    
     In order to be on the safe side, the same distance 4 mm between the wires (sub-conductors) will be maintained in the calculation, despite the fact that it could be decreased since the cross-section area of the wires became two times smaller. For the decreased distance between the wires, even a higher degree of attenuation could be obtained. 
     FIG. 2 b  illustrates a schematic vector diagram which shows superposition of magnetic fields created by the four sub-conductors shown in FIG. 2 a.  As in the former example, the sum of the x-components of the magnetic field created by a pair of wires  21 ,  22  and a pair of wires  23 ,  24  will be equal to zero. Therefore, only the y-components of the magnetic field remain, which can be written down as (H 21 +H 22 ) Y  and (H 23 +H 24 ) Y . These components are almost equal and since they are oppositely directed, their sum is very small: 
     
       
           H   total   =|H   21   +H   22   |   Y   +|H   23   +H   24 | Y   
       
     
     Upon calculation of each of the four components of the above formula (the method of calculation is analogous to that effected for the conductors of conventional cable  10  above, but just for conductors with currents I/2 and radiuses being equal to the square root of 2), the following result has been obtained: 
     at a distance of 50 cm and for I/2=15A: 
     
       
           H   total =4.075* I/ 2*10 −7  (A/cm),  B   total =7.7*10 −3  mG. 
       
     
     The degree of the magnetic field attenuation with respect to the corresponding B total  created by the conventional cable of FIG. 1 is equal to 125. 
     In an analogous way, at a distance of 2 m: 
     
       
           B   total =1.2*10 −4  mG. 
       
     
     The degree of the magnetic field attenuation with respect to the corresponding B total  created by the conventional cable of FIG. 1 is equal to 500. 
     It is readily seen that the magnetic field created by the inventive cable  20  is significantly weaker than that created by the conventional cable  10 . Quantitatively, the degree of the magnetic field attenuation achieved by the embodiment of FIG. 2 a  is equal to 125 at a distance of 50 cm, which result is much better than the most effective screens known in the prior art. For longer distances the attenuation is even stronger. 
     FIG. 3 a  illustrates another simple modification of the inventive single-phase cable  30 , which comprises one main conductor  31  similar to the conductor  12  in FIG. 1 a,  i.e. carries current I in one direction, and two sub-conductors  32  and  33  (placed at two diametrically opposite sides of the conductor  31 ) forming a second main conductor and carrying current I in the opposite direction. Each of the sub-conductors has a cross-section (S main /2), where S main  is the conductor&#39;s  31  cross-section, and carries a current equal to I/2. The attenuation degree of this embodiment will be about 126 at a distance of 50 cm, and 490 at a distance of 2 m. 
     FIG. 3 b  illustrates how the embodiment of FIG. 3 a  can be schematically represented as two dipoles according to the definition given in the Summary of the Invention (a so-called quadrupole configuration) which are shown by the dotted contours  34  and  35 . Each one of the dipoles includes two equal and oppositely directed currents; the current I passing through the main conductor  31  is represented as a pair of two unidirectional currents I/2. It is understood, that the two dipoles  34  and  35  have magnetic moments 
     (M=μ 0 *{overscore (I)}/2*D*l 0 *{overscore (n)} 0 ) which are equal and oppositely directed. According to the invention, such a construction satisfies the so-called condition of “a number of dipoles” described in the Summary of the Invention, and therefore enables to reduce significantly the external magnetic field. It should be mentioned, that the embodiment of FIG. 2 b  also satisfies the condition, since two pairs of dipoles can be formed from the currents passing via the four sub-conductors. 
     FIG. 4 schematically illustrates yet another embodiment  40  of a single phase cable, having five sub-conductors of one type ( 41 ,  43 ,  45 ,  47 , and  49 ) and four sub-conductors of another type ( 42 ,  44 ,  46 ,  48 ), mixed in the cable. The sub-conductor  45  carries a current I/2 and has cross-section S main /2, while the sub-conductors  41 , 43 , 47  and  49  have cross-sections S main /8 and transmit currents I/8, respectively. All four conductors  42 ,  44 ,  46  and  48  have cross-sections equal S main /4 and carry currents I/4, respectively. Three pairs of dipoles are formed in this modification (shown by dotted lines). It has been proven that the degree of the magnetic field attenuation, achieved by this embodiment of the inventive cable at a distance of 50 cm from the center of the cable (with reference to the conventional cable of FIG. 1 a ), is about 1.054*10 7 , i.e. more than ten million (!). At a distance of 2 m the degree of attenuation is even more, i.e. 6.67*10 8 . 
     FIG. 5 demonstrates another arrangement  50  of a single-phase cable, wherein each of sub-conductors  53  and  54  has the cross-section S main /2 and transmits current I/2, while sub-conductors  51 ,  52 ,  55 ,  56  have the cross-section S main /4 and carry currents I/4, respectively. Two pairs of dipoles are formed in this embodiment and the degree of the magnetic field attenuation at a distance of 50 cm appears to be equal 42,000, i.e. it is lower than that of the embodiment in FIG. 4 (three pairs of dipoles), though it is much higher than that of FIG. 2 a  (one pair of dipoles). At a distance of 2 m the attenuation degree will be about 667,000. 
     FIG. 6 illustrates an arrangement 60 of sub-conductors in a single-phase cable with a zero wire. Each main conductor is assembled from three sub-conductors, and the sub-conductors of two types alternate with one another, surrounding the zero wire. It has been calculated that the attenuation degree of this embodiment at a distance of 50 cm is about 63,400. At a distance of 2 m the degree is about 985,000. The cable  60  can also be used as a three-phase cable with two parallel sub-conductors for each phase ( 61  and  64  for one phase,  62 ,  65  for the second phase, and  63 ,  66  for the third phase). The attenuation degree of such a three-phase cable will be about 65. See also FIGS. 9 and 10 below. 
     FIG. 7 shows an embodiment 70 similar to that in FIG.  6  and having the attenuation degree of about 65*10 6  at a distance of 50 cm, and 41.67*10 8  at a distance of 2 m. 
     It should be mentioned that all the described single-phase cables were designed for the same current of 30 A and for a constant current density. 
     FIG. 8 illustrates a cross-section of a conventional three phase cable  80  which can be referred to as Prior Art. The cable includes three main single-phase conductors S, R and T. An optional neutral wire (zero-wire)  0  is shown with a dotted line. In this three phase cable the diameter of each insulated phase conductor is 20 mm, so the distance between the conductors is also about 20 mm. Such a three phase cable is suitable for a phase current of about 240 A. The magnetic induction created by the conventional cable  80  at a distance of 50 cm from its center is B=55 mG. At a distance of 2 m from the center of the cable the induction is of about B=3,4 mG. 
     FIG. 9 illustrates an embodiment  90  of a multi-phase inventive cable, where each of the phase conductors is assembled from three sub-conductors (i.e. n=3) marked with one and the same letter. The sub-conductors of three different phases alternate with each other and surround the zero-wire. The angle α between each two sub-conductors is about 360°/(3*3)=40, and an angle β between each two sub-conductors belonging to the same phase is about 360°/3=120. The attenuation degree of this embodiment at the distance 50 cm is about 209, and at a distance of 2 m it will be about 448. 
     FIG. 10 illustrates another embodiment  100 , being a modification of that shown in FIG.  9 . In the embodiment  100  each single-phase conductor is divided into four sub-conductors (n=4). The attenuation degree at a distance of 50 cm is about 4,600, and at a distance of 2 m it is about 71,600. 
     FIG. 11 illustrates a further embodiment of a three phase cable with a coaxial zero wire  0  and three phase conductors. One phase conductor R conducts current I=240 A. Two conductors S and T are divided into pairs of sub-conductors S 1 , S 2  and T 1 , T 2  respectively, and each of the sub-conductors carries current being I/2. The sub-conductors are symmetrically arranged around the phase conductor R placed in the center of the cable. The attenuation degree of this cable with respect to that shown in FIG. 8 is about 26 at a distance of 50 cm, and about 102 at a distance of 2 m. 
     FIG. 12 shows a slightly modified embodiment, where each of the phase conductors S and T is divided into three equal sub-conductors uniformly distributed around the non-divided conductor R. The attenuation degree provided by this embodiment is about 830 at a distance of 50 cm, and 13,200 at a distance of 2 m. 
     FIG. 13 shows an embodiment of a three phase cable having conductors and sub-conductors non-circular in their cross-section, the number thereof being similar to the arrangement in FIG.  11 . The attenuation degree for the cable shown in this drawing is slightly smaller than that calculated for the cable of FIG. 11, i.e. it is about 25 at a distance of 50 cm and 90 at a distance of 2 m. 
     FIG. 14 illustrates a more complex embodiment of the three phase cable assembled from non-circular wires (the total number of sub-conductors is 15). At 50 cm from the center of the cable the embodiment provides a degree of attenuation of the magnetic field equal to 1209, and at 2 m from the center of the cable,—a degree of about 19,200. 
     It should be emphasized that though in FIGS. 11 to  13  the non-divided conductor is marked to belong to phase R, it might belong as well to either phase S or phase T. 
     All values of the attenuation degree are given in comparison with: 
     a conventional single phase cable described with reference to FIGS. 1 a  and  1   b  (for embodiments of the inventive single phase cable shown in FIGS. 2 to  7 ); and/or 
     a conventional three phase cable described with reference to FIG. 8 (for inventive embodiments of FIGS. 9 to  14 ). 
     Similarly to the case of single phase cables, all the above-described three phase cables were designed for the same phase current of 240 A and for a constant current density value. 
     As has already been mentioned, the inventive cables have lower self-inductance, and lower mutual inductance than the respective conventional single phase and three phase cables. 
     For single phase cables the following results have been obtained. Self-inductance of a single phase cable L phase,Fig.No  has been computed for each of the cables shown in the following drawings: 
     L 11 —is self-inductance of a single-phase cable shown in FIG. 1 a  (the conventional single phase cable); 
     L 12 —is self-inductance of a new single-phase cable shown in FIG. 2 a;    
     L 16 —is self-inductance of a new single-phase cable shown in FIG. 6; 
     L 17 —is self-inductance of a new single-phase cable shown in FIG.  7 . 
     Then, three ratios were calculated, each one showing how the self-inductance of one of the inventive single phase cables is reduced in comparison with that of the conventional single phase cable: 
     
       
           L   12   /L   11 =0.316;  L   16   /L   11 =0.261;  L   17   /L   11 =0.202. 
       
     
     Also, mutual inductance M Fig.No.(first cable):FigNo.(second cable)  was computed for the following four pairs of cables at a constant distance from the inventive (new) cable: 
     M 11 —is mutual inductance between two conventional single-phase cables shown in FIG. 1 a;    
     M 12 —is mutual inductance between the new single phase cable shown in FIG. 2 a  and the conventional single-phase cable shown in FIG. 1 a.    
     M 16 —is mutual inductance between the new single-phase cable shown in FIG.  6  and the conventional single-phase cable shown in FIG. 1 a.    
     M 17 —is mutual inductance between the new single-phase cable shown in FIG.  7  and the conventional single-phase cable shown in FIG. 1 a.    
     Further, the following ratios were obtained to show how the mutual inductance, created in arrangements where the new cables participate, decreased in comparison with that created between the conventional cables: 
     
       
           M   12   /M   11 =0.0261;  M   16   /M   11 =0.00864;  M   17 /M 11 =0.00540. 
       
     
     For the case of three phase cables, the self-inductance and the mutual inductance were calculated and written down in a similar way: 
     L 3,8 —is self-inductance of the conventional three phase cable shown in FIG.  8 . 
     L 3,6 —is self-inductance of a new three phase cable according to the arrangement shown in FIG. 6; 
     L 3,9 —is self-inductance of a new three phase cable shown in FIG. 9; 
     L 3,10 —is self-inductance of a new three phase cable shown in FIG.  10 . 
     The ratios between the self-inductances are as follows: 
     
       
           L   3,6   /L   3,8 =0.490;  L   3,9   /L   3,8 =0.341;  L   3,10   /L   3,8 =0.257. 
       
     
     Mutual inductances M 3,6 , M 3,8 , M 3,9 , M 3,10  for each of the above-mentioned three phase cables were calculated similarly to the case of single phase cables, where the first (reference) cable in each respective pair is a single phase conventional cable shown in FIG. 1 a.  In other words, the mutual inductance for each of the pairs including the above three phase cable was computed for an external magnetic field induced by a single phase cable, and at a constant distance from the investigated cables. 
     The ratios between the obtained mutual inductances show that three phase cables having more sub-conductors create lower mutual inductance with external magnetic field. Since the mutual inductance for a three phase cable is different for its different phases, the ratios were computed for average values: 
     
       
           M   3,6   /M   3.8 =0.260;  M   3,9   /M   3,8 =0.0766;  M   3,10   /M   3,8 =0.0402. 
       
     
     The obtained results indicate a clear advantage of the inventive cables over the conventionally used cables.