Electricity: conductors and insulators – Anti-inductive structures
Reexamination Certificate
2001-11-05
2003-01-14
Nguyen, Chau N. (Department: 2831)
Electricity: conductors and insulators
Anti-inductive structures
C174S034000
Reexamination Certificate
active
06506971
ABSTRACT:
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 &mgr;T. A dose-dependence of childhood leukemia is suggested for power frequency fields in the range 0.2-0.4 &mgr;T. 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'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
&mgr;=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 foll
Erlitzki Michael
Grach Yosef
Browdy and Neimark
Nguyen Chau N.
The Israel Electric Corporation Ltd.
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