JPH0566003B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a zero-phase current transformer. The current value of the sum or difference of the currents flowing through the three phases of the first, second, and third phases, that is, the zero-sequence current value, is the one-round integral of H→・d→ along the path surrounding all of them. It is well known that this can be obtained if the medium surrounding the wire is homogeneous and the relative magnetic permeability of the medium is constant. Among the above-mentioned media, air, vacuum, and the like are industrially easy to use. The above-mentioned letter symbol H→ is the strength of the magnetic field, and d→ is the minute length. For the purpose of finding the one-round integral value of H→・d→, under the condition that the current is alternating, we can use an alternating number of n-turn coils of equal cross-sectional area and equal pitch arranged along the same path as described above. Measure the output voltage. The output voltage across the coil is equal to the time-differentiated value of B→·ns→, where s→ is the cross-sectional area of the coil, and B→ is the product of transmittance μ and H→. If the current to be measured is a commercial frequency current, this output voltage is so small that subsequent signal processing becomes difficult. For this reason, it is well known that a coil with a magnetic core is used when the purpose is to measure the zero-sequence current of a commercial frequency circuit. The magnetization characteristics of magnetic materials used as magnetic cores are such that the relationship between magnetic field strength and magnetic flux density, that is, the relationship between H and B, is nonlinear for all materials, and of course, the relationship between H and μ The same applies to the relationship between them. Therefore, in the case of a zero-phase current transformer in which the outside of the electric wire is a non-homogeneous medium consisting of the magnetic core and air, depending on the magnitude of the current flowing through the three phases, and where the coil passes through the magnetic core. A device that detects an imaginary zero-sequence current is often created by combining the arrangement and shape of electric wires. In fact, in the disclosures of technologies in this field, there are very few disclosures that the zero-sequence characteristics do not change due to changes in the load current. Fig. 1 shows a conventional zero-phase current transformer. In the figure, 10 is the magnetic core, and capital letters A, B, and C are the primary phases of the first, second, and third phases, respectively, outside the penetration area of the magnetic core. Conductor configuration, lowercase a indicates the split configuration of the first phase conductor at the penetration site, and for the second and third phases, the lowercase letters b and c respectively
FIG. FIG. 2 also shows a conventional zero-phase current transformer. In the figure, 10 is a magnetic core (zero-phase iron core), and 5 is a zero-phase secondary winding.
1 is the primary conductor of the first phase, 2a, 2b, 2c, 2d
are the primary conductors of the second phase, 3a, 3b, 3c,
3d is the third phase primary conductor. The drawback of these basic concepts for conventional zero-phase current transformers is that if a two-phase short circuit occurs between the first and second phases or between the first and third phases, the magnetic field to the zero-phase core The problem lies in the common misconceptions that the distribution will be symmetrical or that the force received by each conductor will be a symmetrical component of force. If a zero-sequence current can be detected, the currents in the three phases are unbalanced, and even if the conductors of the three phases are geometrically balanced, the flowing currents are unbalanced. Therefore, as mentioned above, it is impossible for the magnetic field distribution to become symmetrical or for each conductor to have symmetrical component forces. In fact, the results of these re-experiments have revealed that the zero-sequence characteristics vary greatly depending on the load current. Taking this point into consideration, a conventional zero-phase current transformer has a first phase conductor that is a cylinder, a second phase conductor, and a third phase conductor that are circular tubes, and these three conductors are arranged concentrically. In addition, a zero-phase current transformer with a coaxial solid conductor as the through-hole primary conductor was prototyped, and its characteristics were investigated. As a result, the possibility of realizing a zero-phase current transformer that is not affected by the magnitude of load current was demonstrated, but it had the drawbacks of being extremely difficult to manufacture and expensive. Taking all these results into consideration, the invention was made to use coaxial cables as feedthrough primary conductors to compensate for the drawbacks of coaxial conductors with solid conductors. The technology for making coaxial cables that carry large currents has made great progress as the budget for nuclear fusion research increases, and there is no need to worry about using coaxial cables for low-voltage circuits. However, for high voltage applications, there are concerns from the viewpoint of electrical insulation. The present invention examines electromagnetic field theory from the basics, utilizes the results, and is completely unaffected by the magnitude of load current.
This invention makes it possible to realize a zero-phase current transformer suitable for high-voltage, large-current circuits at a low cost. The invention of the present application will be explained in detail below. In Figure 3, at point A, a current J flows from the back of the page to the front, and at point B, which is a distance l away from point A.
Consider the case where the current at point J flows in the same direction.
The direction of the magnetic field generated at point P by the current flowing at points A and B is H〓 ₁ is perpendicular to the straight line AP, and H〓 ₂ is perpendicular to the straight line BP, so their combined value is It is possible to display in a cylindrical coordinate system with the origin moved to the midpoint of straight line AB. The length of straight line AP is R ₁ and the length of OP is R, respectively.
If the length of BP is R ₂ and the angle POB is θ, then H _R = H〓 ₁ sin (θ − θ ₁ ) − H〓 ₂ sin (θ _{2 −} θ) = −J / 4π l ² Rsin2θ / (R ² +l ² /4) ² −R ² l ² co
sθ(1), and H _R can be expressed as a function of R and θ. Also H〓
is H〓=H〓 ₁ cos(θ−θ ₁ )+H〓 ₂ cos(θ ₂ −θ)=J
/2π
R {2(R ² + l ₂ /4) − l ² cos ² θ} / (R ² + l ² /4) ²
−R ² l ² cos ² θ(2) In a cylindrical coordinate system, each line element is dR
and Rdθ, the formula for the lines of magnetic force is −dR/l ² sin2θ=Rdθ/2{2(R ² +l ² /4)−l ² cos ²
θ}(3) can be obtained. 100 in FIG. 3 is a virtual current that greatly contributed to completing the invention of this application. Virtual current refers to what would happen if a large number of conductors with equal central angles were arranged on a circular arc centered at the origin, and all of the currents flowed through a cylinder arranged at the origin. It is an electric current for thinking. The virtual current in Figure 3 is 2 as shown in the figure.
It is J. Letting the virtual current be I, the magnetic field outside the conductor is only the θ component, which can be expressed by the following equation. Hθ=I/2πR (4) In the process of completing the explanation of the present application, the formula for expressing H〓 shown in formula (2) was expanded. These equations were compared with equation (4), and it was examined in what kind of through-through primary conductor the ratio of the two approaches 1 at the position of smaller R. The formula to display the H _R component as shown in formula (1) is:
It is not useful for anything other than drawing lines of magnetic force. Expression (2) is expressed as H〓 by a conductor of logarithm n with equal central angles.
It is relatively easy to extend the formula to find .
Expanding equation (2), H〓=J/2π _o-1 〓 〓 ^k=0 R{2(R ² +l ² /4)−l ² cos ² (θ−kπ/n)}
/(R ² +l ² /4) ² −R ² l ² cos ² (θ−kπ/n) (5). Equation (5) is on the arc of distance R from the origin and at any central angle position away from the conductor.
Give H〓. Although only examples in which the number of conductors is an even number are used in the explanation, this is only because it is easy to explain using mathematical formulas. Even if the numbers are odd, the purpose of this application can be achieved if the central angles of all the conductors are equal, and the degree to which the aforementioned ratio approaches 1 is determined by the value corresponding to the two even numbers that sandwich the odd number. You can guess. The letter symbol k in equation (5) is a number attached to the conductor on the arc through which the in-phase current flows, and k = 0 is the central angle of 0, k = 1 is π/n, and k = 2 is the central angle.
2π/n... From equation (5), if we consider the change in Hθ on the circumference of a constant R, Hθ becomes the maximum value at θ = 0, π = n, 2π/n..., and θ = π/2n, 3π/2n. , 5π=2n...
It becomes the minimum value at .... Figure 4 shows H〓max on the circumference of R=l.
This shows how and H〓min change alternately. By normalizing these H〓max and H〓min using virtual currents, that is, by dividing both the left and right sides of equation (5) by both the left and right sides of equation (4), it can be used dimensionlessly and in a wide range of ways. Technical data is available. Table 1 shows that the logarithm is 2 or more, R = 0.6l, 0.75l,
H〓max and H〓 for combinations such as l, 2l, etc.
Indicates the standard value of min. FIG. 5 is a schematic diagram showing the concept of how the lines of magnetic force generated by a group of conductors with a logarithm of 4 gradually approach a circle as they move away from the origin 0 in the figure. 15 in the diagram
are lines of magnetic force, 16 is an arc of radius R, 17 and 18
are both on the arc, and H〓max and H〓
min is a vector representing the concept of the strength of the magnetic field that changes in response to the central angle of the conductor; 42 is the third phase conductor; in the example shown, the logarithm of the conductor is 4; Furthermore, R ₀ is the radius at which the lines of magnetic force become a circle that cannot be seen or identified if it is this far from the origin. In the invention of the present application, the required H〓max and
First, know H〓min, then find out the specific dimensions.
It is characterized by knowing R ₀ and arranging a magnetic core with a secondary coil wound around it. The method that led to the invention of the present application selects a space for consideration in which the medium surrounding the conductor through which the current flows is equal to the permeability of vacuum and is homogeneous, and when determining the composite value of the magnetic field strength, the superposition principle is used. be applicable. Next, a circular magnetic core with a secondary coil wound around it is placed in the circular magnetic field. Considering the former, by freely using the principle of superposition, we can calculate how far the space is from the circumference by in-phase currents divided and arranged on the same circumference.
It is possible to consider whether the lines of magnetic force generated at that position are circular. In the previous explanations, we have described the results of mathematical analysis and numerical calculations regarding this point. the result,
The maximum and minimum values of the H component of the magnetic field generated in a space on the circumference of the same radius outside the coaxial conductor are normalized by the magnetic field caused by the virtual current flowing through the center of the coaxial conductor. It becomes easy to know the spatial position of the radius, which is reduced to a difference of less than 1/10,000 times compared to . In the space outside of it,
It also becomes easier to arrange the ring-shaped magnetic body, ie, the magnetic core, around which the secondary coil is wound. The shape of the magnetic lines of force generated at the position where the magnetic core is arranged is circular, and the strength of the generated magnetic field is a weak magnetic field strength due to the composite value of each phase current,
Moreover, the circular shape of the lines of magnetic force avoids the extremely difficult problem of refraction of lines of magnetic force at the boundary between a magnetic material and air. The weak magnetic field also allows the zero-phase current transformer to operate in a region where the magnetic material has high permeability. To explain these descriptions more clearly, in the present application, in the middle of the ring of magnetic material that makes up the magnetic core, extra lines of magnetic force that cause measurement errors enter the ring or exit from the ring. It has the characteristic that it does not cause any damage. FIG. 6 is a conceptual diagram of an embodiment of the present invention. 5 in the diagram
0 is a first phase conductor in which a large number of conductive bare wires are twisted together to form a core wire with a circular cross section without insulating the wires between each other. On top of that, a semiconducting insulator is applied to increase the dielectric strength.

[Table] Covering 31 is applied to provide insulation coating for the purpose of maintaining interphase insulation. In the figure, 21 is an interphase insulation coating between the first phase and the second phase. The finish of this insulation must be axially symmetrical. Conductors having a much smaller cross-sectional area than the first phase conductor, which has been insulated in advance, are arranged so that the insulation coatings of these thin wires are in close contact with each other, and are arranged concentrically. Since these conductor groups are used as second phase conductors, it is natural that the sum of the cross-sectional areas of these thin wires is equal to the cross-sectional area of the first phase conductors. For this reason, the concentric divided arrangement may become a one-layer arrangement, or it may become necessary to form a multi-layer arrangement of two or three layers. Examples of multilayer arrangements are shown in FIGS. 7 and 8.
The symbol 42 is used for the conductor in the figure in consideration of the third phase conductor. FIG. 7 shows a state in which there is no deviation in center angle between the first and second layer conductors. In the schematic diagram shown in the figure, the logarithm of the conductor is n=12. In FIG. 8, there is a center angle deviation between the first, second, and third phase conductors. Therefore, in this schematic diagram, the logarithm n=18. The explanation returns to FIG. 6. The second phase conductor is symbol 41
Indicated by A semiconductive coating 32 is applied thereon, and an insulating coating 22 for the purpose of maintaining interphase insulation between the second and third phases is applied, and a third phase conductor is applied on top of it in exactly the same manner as the second phase conductor. The phase conductors 42 are divided and arranged concentrically. As can be seen from the foregoing description, in order to achieve the objectives of the present application, the third phase conductor 42 is
It is important that the wire be a thin wire with a smaller cross-sectional area than the phase conductor 41, and such considerations make it possible to increase the value of the logarithm n. After arranging the third phase conductors, a coaxial portion of the penetrating primary conductor, which has a semiconductive insulating coating 33 and an interphase insulating coating 23 applied thereon, passes through the magnetic core 10. Further, in the figure, 5 is a secondary coil wound around a ring-shaped magnetic body (magnetic core), 70 and 80 are both output terminals of the secondary coil, and 9
0 is the electrostatic shield applied to the secondary coil. This primary through conductor is constructed by leaving the part that includes the through part, dismantling the part that is outside the through part, and removing the insulation covering from the required length of the conductor for each phase. Each of the three-phase conductors is collectively used as a terminal of a through-hole primary conductor. As explained above with reference to FIGS. 4, 5, 6, 7, and 8, the normalization ( normalized) H〓max and H〓
Using a calculation formula, find the distance R ₀ at which each value of min differs by 1/10,000 or less from 1, and place a ring-shaped magnetic core around which a secondary coil is wound outside. The distance R ₀ is determined by the 8th distance from the conductor arrangement shown in Figure 7.
It is clear from the above explanation that the conductor arrangement shown in the figure is smaller. It is also clear from the above description that the distance R until the magnetic lines of force due to the conductor current disposed at the outermost side converge into a circular shape is the largest. In the conceptual diagram of the embodiment shown in Fig. 6, if the configuration of the third phase conductor is removed, it becomes a through primary conductor of only the first and second phases, and the conductor configuration becomes the leakage detector itself. Of course. Figure 9 embodies the invention of the present application, setting the load current flowing through the through primary conductor to 100 amperes, and after confirming that the output voltage of the secondary coil is only 0.00 mV or less depending on the load current, Prepare a separate current circuit that is completely independent of the current circuit that flows through the conductor, and run an extra current that corresponds to the zero-sequence current through a wire that is attached to the primary conductor. Since this corresponds to electric current, it is a diagram in which the value of extra current (zero-sequence current value) is scaled on the horizontal axis, and the output voltage of the secondary coil is scaled on the vertical axis. This characteristic is the same as the magnetization characteristic of the magnetic core, which does not change at all even if the load current flowing through the primary conductor changes from several amperes to 100 amperes. The figure also shows that even when the load current is 100 amperes, it is possible to detect zero-sequence currents of about 0.2 milliamperes. The experiment was conducted by numerically calculating a space in which both the standardized H〓max and H〓min values differed by less than 1/10,000 of 1, and placing the magnetic core outside the space. Ta. The magnetic core has an outer diameter of 51.3 mm, an inner diameter of 31.1 mm, and a width of 13.2 mm.
The number of turns of the secondary coil is 210.
It is times. In order to examine the machining margin that is a problem in embodying the present application, the central axis of the coaxial conductor and the central axis of the magnetic core are shifted with a current of 100 amperes flowing through the primary conductor. So, no matter how far apart the distance between the two axes is, the output voltage of the secondary coil is 0.00.
It was confirmed that the machining margin below mmV was 3 mm. A primary conductor consisting of a first phase and a second phase was prototyped and used as a through primary conductor. It was clear that using a thinner wire as the second phase conductor was less likely to generate an imaginary zero-sequence voltage in the secondary coil, even if it was made offset from the axis of the magnetic core. . As explained above with reference to the theory underlying the invention and the drawings, the invention has an economical effect in that a zero-phase current transformer with excellent characteristics not previously available can be obtained at a low price.

[Brief explanation of drawings]

FIGS. 1 and 2 are schematic diagrams showing conventional zero-phase current transformers. 3, 4, and 5 are diagrams explaining the theory underlying the present invention. FIG. 6 is a conceptual diagram of an embodiment according to the present invention. 7 and 8 are explanatory diagrams of conductor pairs according to the present invention. FIG. 9 is a diagram showing zero-phase characteristics using concrete proportionality according to the present invention.

Claims (1)

[Claims] 1 A core wire with a circular cross section, which is used as the first phase conductor, and after applying an axially symmetrical insulation coating on it, the core wire becomes the second phase conductor on the outer circumference. Conductor wires with a circular cross section that is much thinner than the cross-sectional area of The conductor wire with a circular cross section, which will become the third phase conductor, is divided and arranged in layers in the same way as the second phase conductor, and the outermost part is coated with an axially symmetrical insulation coating. A magnetic field due to a virtual current flowing through the coaxial center of the H〓 component of a magnetic field that forms a coaxial conductor in which only in-phase conductors are arranged and is generated in a space on the circumference of the same radius outside the coaxial conductor. A ring-shaped magnetic material in which a secondary coil is wound coaxially with the coaxial conductor in the space outside the radius where the difference between the maximum and minimum values and 1 is reduced to 1/10,000 or less, standardized by A zero-phase current transformer characterized in that a coaxial conductor (magnetic core) is arranged and the coaxial conductor passes through the center of the magnetic core.