Coil winding method for expanding detection bandwidth, coil structure and its design method

Abstract

The present invention discloses a coil winding method, a coil structure and a design method thereof for expanding the detection bandwidth. The coil structure includes a cylindrical outer skeleton, three inner skeletons with coincident axes, and several sections of two-layer windings. The first layer of each section of two-layer winding is wound with the outer skeleton as the outer layer and the first inner skeleton and the third inner skeleton alternately wound as the inner layer; the second layer of winding is wound with a section on the outer skeleton that is the same as the outer layer of the first layer of winding as the outer layer and the second inner skeleton as the inner layer; the winding directions of the first layer of winding and the second layer of winding are opposite, but the current directions of the turns are the same. Starting from the bandwidth of the abnormal high-frequency transient signal generated from the discharge of the transmission line to the fault, the present invention uses an equal-arc multi-row double-layer winding to design the coil, ensuring that the coil has a relatively wide measurement bandwidth without sacrificing the detection sensitivity and anti-interference performance.

Description

Technical Field

[0001] The present invention relates to the technical field of abnormal high-frequency transient signal detection for overhead transmission lines, and particularly relates to a coil structure for expanding the detection bandwidth and a design method thereof. Background Art

[0002] Weak discharge signals will be generated in overhead transmission lines due to tree obstacles, insulator fouling, etc. Moreover, during this process, as time goes by, the degree of discharge will intensify until a fault state is formed. Therefore, such high-frequency transient current signals will fluctuate within a large frequency range.

[0003] Currently, two types of sensors are mainly used to monitor abnormal high-frequency transient current signals in overhead transmission lines. One type is a high-frequency small current detection sensor with an iron core. This type of sensor can exhibit good response performance for detecting high-frequency small currents, but the number of turns of the wire and the magnetic material have a great impact on the bandwidth. At the same time, it is very easy to reach the magnetic saturation state during the monitoring process of the transmission line, and it is not suitable for the monitoring process from discharge to fault.

[0004] The second is to utilize the advantage of the Rogowski coil without magnetic saturation, which is applicable to measuring any high-frequency current. However, due to uneven wire turns and inaccurate design of the effective area of the return wire in the Rogowski coil, the anti-interference performance is not good, and the frequency band measurement range is limited. Many existing studies have started from the measurement bandwidth of the Rogowski coil and have also achieved good results. For example, a more complex PCB Rogowski coil has been designed to improve the measurement bandwidth and anti-interference performance, but the detection sensitivity cannot be guaranteed, and even a part of the sensitivity will be consumed to improve the bandwidth. Summary of the Invention

[0005] The purpose of the present invention is to provide a coil structure for expanding the detection bandwidth and a design method thereof, which can solve the problem of sacrificing detection sensitivity to provide measurement bandwidth and anti-interference performance in the prior art.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] In a first aspect, the present invention provides a coil structure for expanding the detection bandwidth, including a cylindrical outer skeleton, three inner skeletons with coincident axes, and several two-layer windings. The first layer of each two-layer winding is wound with the outer skeleton as the outer layer and the first inner skeleton and the third inner skeleton alternately as the inner layer; the second layer of winding is wound with a section on the outer skeleton that is the same as the outer layer of the first layer of winding as the outer layer and the second inner skeleton as the inner layer; the winding directions of the first layer of winding and the second layer of winding are opposite, but the current directions of the wire turns are the same.

[0008] Further, the number of turns of each two-layer winding is the same.

[0009] Further, the spacing between each two-layer winding is the same.

[0010] In a second aspect, the present invention provides a coil winding method for expanding the detection bandwidth, which is wound using the above coil structure:

[0011] The wire routing sequence of the first layer of winding is: starting from a point D1 at the top of the outer skeleton 0 as the starting point, passing through A0 at the top of the first inner skeleton, winding along the inner surface of the first inner skeleton to the bottom of the first inner skeleton, and then winding to D2 at the bottom of the outer skeleton 0 , and then winding along the outer surface of the outer skeleton to D1 at the top of the outer skeleton 1 , and then winding to the top C0 of the third inner skeleton, winding along the inner surface of the third inner skeleton to the bottom of the third inner skeleton, and then winding to D2 at the bottom of the outer skeleton 1 , and so on until the end point D4 of the winding on the top of the outer skeleton n ;

[0012] The wire routing sequence of the second layer of winding is: using the end point D4 of the first layer of winding n as the starting point, in the opposite direction to the first layer of winding to the top B of the second inner skeleton n , winding along the inner surface of the second inner skeleton to the bottom of the second inner skeleton, and then continuing to wind to D3 at the bottom of the outer skeleton n , winding along the outer surface of the outer skeleton to D4 at the top of the outer skeleton n-1 , and then continuing to wind to the top B of the second inner skeleton n-1 , and so on to complete the winding of the second layer of winding.

[0013] Furthermore, the included angle α formed by the wire routing points of two adjacent turns of winding on each inner skeleton and the center of the skeleton is equal; the included angle β formed by the wire routing points of two adjacent turns of winding between the first inner skeleton and the third inner skeleton and the center of the skeleton is equal; the distance between two adjacent turns of winding on the outer skeleton is equal.

[0014] A design method for the above coil structure includes the following steps:

[0015] The dimension design of the outer skeleton and all inner skeletons meets the following formula requirements:

[0016] R i =(2 1 / 2 -1)R d ;

[0017] R c -R a =2(R b -R a )=C, where C is a constant;

[0018] where: R i represents the cross-sectional radius of the second inner skeleton; R drepresents the cross-sectional radius of the outer skeleton; R c represents the cross-sectional radius of the third inner skeleton, R a represents the cross-sectional radius of the first inner skeleton;

[0019] such that the internal capacitance of each section of double-layer winding is equal;

[0020] For each inner skeleton, the angle α formed by the wire-walking points of two adjacent turns of winding and the center of the skeleton is equal; for the first inner skeleton and the third inner skeleton, the angle β formed by the wire-walking points of two adjacent turns of winding and the center of the skeleton is equal; the distance between two adjacent turns of winding on the outer skeleton is equal.

[0021] A coil structure for expanding the detection bandwidth and its design method according to the present invention start from the bandwidth of abnormal high-frequency transient signals from the discharge of the transmission line to the occurrence of a fault, and use an equal-arc multi-row double-layer winding to design the coil, ensuring that the coil has a relatively wide measurement bandwidth without sacrificing the detection sensitivity and anti-interference performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic diagram of the skeleton of the coil structure of the present invention;

[0023] Figure 2 is a schematic diagram of the corresponding relationship between the winding points of the inner and outer skeletons of the present invention;

[0024] Figure 3 is a winding diagram observed from the top or bottom after the winding of the present invention is completed;

[0025] Figure 4 is half of the plane top view of the coil structure of the present invention;

[0026] Figure 5 is the equivalent circuit diagram of the coil when the current frequency is relatively high;

[0027] Figure 6 is a schematic diagram of the equal-spacing multi-section coil design of the present invention;

[0028] Figure 7 is a schematic diagram of the decomposition of the first and second layer turn units of the present invention;

[0029] Figure 8 is a schematic diagram of the arrangement of the first and second layer turn segments of the side winding of the present invention;

[0030] Figure 9 is a schematic diagram of the area not included in the projection of the turn into the xoy plane. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] The following describes the embodiments of the present disclosure in detail with reference to the drawings.

[0032] The following describes the embodiments of the present disclosure through specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the content disclosed in this specification. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all the embodiments. The present disclosure can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present disclosure. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present disclosure without creative efforts belong to the scope of protection of the present disclosure.

[0033] The coil structure for expanding the detection bandwidth of the present invention, as Figure 1 shown, includes a cylindrical outer skeleton D with coincident axes, three inner skeletons, and several two-layer windings. The first layer of each two-layer winding is wound with the outer skeleton D as the outer layer and the first inner skeleton and the third inner skeleton alternately as the inner layer; the second layer of winding is wound with a section on the outer skeleton D that is the same as the outer layer of the first layer of winding as the outer layer and the second inner skeleton as the inner layer. The number of turns of each winding is the same, and the spacing between each winding is the same.

[0034] It should be noted that: the first inner skeleton, the second inner skeleton, and the third inner skeleton of the present invention have the following several combination methods, which will be described in combination with Figure 1 as follows:

[0035] The first combination method: the first inner skeleton is A, the second inner skeleton is B, and the third inner skeleton is C;

[0036] The second combination method: the first inner skeleton is A, the second inner skeleton is C, and the third inner skeleton is B;

[0037] The third combination method: the first inner skeleton is B, the second inner skeleton is A, and the third inner skeleton is C.

[0038] Among the above three combination methods, the first inner skeleton and the third inner skeleton can be interchanged. Equivalent to having 6 combination methods.

[0039] Each combination method can achieve the beneficial effect of the present invention of ensuring that the coil has a relatively wide measurement bandwidth without sacrificing the detection sensitivity and anti-interference performance. The working principle, design method, and winding method of each combination method are similar. The following will take the first combination method as an example for detailed description. Since the working principles of other combination methods are similar, they will not be elaborated here.

[0040] In combination with Figure 2 and Figure 3 the winding method of the above two-layer winding will be described:

[0041] The first layer of winding is wound with the outer skeleton D as the outer layer and the inner skeletons A (equivalent to the first inner skeleton) and C (equivalent to the third inner skeleton) alternately wound as the inner layer; the second layer of winding is wound with the outer skeleton D as the outer circle and the inner layer winding skeleton B (equivalent to the second inner skeleton) as the inner layer.

[0042] Each layer of winding includes an inner layer winding and an outer layer winding. The winding wound on the surface of the outer skeleton is the outer layer winding, and the winding wound on the surface of the inner skeleton is the inner layer winding.

[0043] Figure 2 and Figure 3 In, the dotted line is the back winding and the solid line is the front winding. For easy observation, the arc of the outer layer winding is intentionally enlarged. The wiring sequence of the first layer of winding is: starting from D1 at the top of the outer skeleton 0 as the starting point, passing through A0 at the top of the first inner skeleton A, winding along the inner surface of the first inner skeleton A to the bottom of the first inner skeleton A, and then winding to D2 at the bottom of the outer skeleton D 0 , and then winding along the outer surface of the outer skeleton D to D1 at the top of the outer skeleton D 1 , then winding to C0 at the top of the inner skeleton C, winding along the inner surface of the inner skeleton C to the bottom of the inner skeleton C, and then winding to D2 at the bottom of the outer skeleton D 1 , and so on until D4 at the end of this section of winding at the top of the outer skeleton n , completing the winding of the first layer of winding for this section of winding. The sequence of the path that the first layer of winding takes on the inner skeleton is A0, C0, A2, C2,..., A n-1 , C n-1 , and the path on the outer skeleton is D1 0 , D2 0 , D1 1 , D2 1 ,..., D1 n-1 , D2 n-1 , D1 n , D2 n .

[0044] Start winding the second layer of winding in the opposite direction, but the direction of the wound turns remains the same, so as to ensure that the current direction inside the turns remains unchanged. If the first layer of winding is wound clockwise, the second layer is wound counterclockwise. The meaning of the direction of the turns remaining the same is: if the outer layer winding of the first layer of winding on the outer skeleton is wound from the top to the bottom and the current direction is from top to bottom, then the outer layer of the second layer of winding on the outer skeleton is also wound from the top to the bottom and the current direction is also from top to bottom. The second layer of winding is wound with the outer surface of the outer skeleton and the inner surface of the second inner skeleton as the carriers. As Figure 2As shown, its wire routing direction is opposite to that of the first-layer winding. Taking one section of the wire routing as an example: The first-layer winding winds to the top D4 of the outer skeleton n and then to the top B of the inner skeleton B n , winds along the inner surface of the inner skeleton B to the bottom of the inner skeleton B, and then continues to wind to the bottom D3 of the outer skeleton D n , winds along the outer surface of the outer skeleton D to the top D4 of the outer skeleton D n-1 , and then continues to wind to the top B of the inner skeleton B n-1 , and so on to complete the winding of the second-layer winding.

[0045] The path of the second-layer winding on the inner skeleton is B n 、B n-1 、...、B1、B0, and the path on the outer skeleton is D4 n 、D3 n 、D4 n-1 、D3 n-1 、...、D4 1 、D3 1 、D4 0 、D3 0 . The difference between the second layer and the first layer is that the wire turn direction changes from right to left to from left to right, while the winding direction of the wire turns remains unchanged, that is, the current direction inside the wire turns remains unchanged. Complete this section of winding. After a certain distance, start winding the second section of winding according to the above method until the surface of the outer skeleton is covered.

[0046] The present invention also provides a design method for the above coil structure, including the following steps:

[0047] Step S1, the size design of the outer skeleton and all inner skeletons meets the following formula requirements:

[0048] R i =(2 1 / 2 -1)R d ; (1)

[0049] R c -R a =2(R b -R a )=C, where C is a constant; (2)

[0050] Where: R i represents the cross-sectional radius of the second inner skeleton; R d represents the cross-sectional radius of the outer skeleton; R c represents the cross-sectional radius of the third inner skeleton, and R a represents the cross-sectional radius of the first inner skeleton.

[0051] The process of the size design of the outer skeleton and all inner skeletons is:

[0052] Theoretically, the larger the width of the coil cross-section, the higher the sensitivity. However, the size of the sensor installed on the overhead line is limited. Figure 4 Figure 5 is a top view of a half-turn of the skeleton of the coil that has not started winding in the present invention. From the inside to the outside, the inner layer has three rows A, B, and C, and the outermost layer is row D. The radii of each row from the center point O are successively corresponding to R a , R b , R c , R d .

[0053]

[0054] In formula (3), N, μ0, and h are respectively the total number of turns of the wire wound on the coil, the magnetic permeability of vacuum, and the height of the skeleton. R d，i (i = a, b, c) is the distance from the center of the turns surrounded by each inner row and the outermost row to the center point O. R d and R i are respectively the distances from the outermost row and each inner row to the current-carrying conductor, that is, the axis of the skeleton. δ (δ = 2πR i / N) is the shortest distance between two adjacent turns. It can be determined from formula (3) that when R i =(2 1 / 2 -1)R d , the induced voltage u takes the maximum value. Combining with the design of the present invention, taking row B (i.e., R b ) satisfies the condition for the maximum value of u, that is, i = b, while rows A and C are designed according to formula (2).

[0055] Step S2: Make the internal capacitance of each winding section equal.

[0056] When the current frequency is relatively high, the capacitance and inductance inside the coil have a great influence on the output voltage. The equivalent circuit diagram is as Figure 5 shown.

[0057]

[0058] In formula (4), R is the internal resistance of the coil, L is the internal inductance of the coil, R S is the external damping resistance of the coil, and C is the internal capacitance of the coil. It can be explained from the above formula that the bandwidth is: when the current frequency is relatively high, the smaller the internal parameters R, L, and C of the coil, the wider the bandwidth.

[0059] In the present invention, to effectively expand the bandwidth while maintaining the coil sensitivity unchanged and having strong anti-interference performance, the equal-arc multi-segment double-layer winding method is used to divide the coil into equal multi-segments with the same number of turns. Therefore, based on formula (4), the bandwidth expansion formula of the present invention is analyzed according to formula (5).

[0060]

[0061] In formula (5), the sensitivity of the coil itself remains unchanged, i.e., M j N s remains unchanged. To increase the measurement bandwidth of the coil, i.e., the internal resistance R j and internal inductance L j and internal capacitance C j of the j-th segment are small enough and satisfy formula (6). Since the edge effect has a greater impact on the resistance and inductance in the high-frequency case, the present invention mainly considers the internal capacitance C j .

[0062] R j = R j-1 L j = L j-1 C j = C j-1 (6)

[0063] Step S3: The angles α formed by the wire routing points of two adjacent turns of wire on each inner skeleton and the center of the skeleton are equal; the angles β formed by the wire routing points of two adjacent turns of wire between the first inner skeleton and the third inner skeleton and the center of the skeleton are equal; the distances between two adjacent turns of wire on the outer skeleton are equal.

[0064] It is illustrated as follows Figure 6 :

[0065] Figure 6 The dotted lines in

[0066] are auxiliary lines drawn and not actually shown. The multi-rows refer to the three inner rows A, B, and C of the coil. The structural relationship of the three inner rows satisfies formula (2). The multi-row design is used to alternately arrange the first and second layer of turns during winding and add return wires at the same time. n- 1OA n = α1, α2 is the angle formed by two adjacent wire routing points on the inner skeleton B and the center of the coil, i.e., ∠B0OB1 = ∠B1OB2 =... = ∠B n-1 OB n = α2, α3 is the angle formed by two adjacent wire routing points on the inner row C and the center of the coil, i.e., ∠C0OC1 = ∠C1OC2 =... = ∠C n-1 OC n= α3. β1 and β2 are the angles formed by the wiring points of two adjacent turns of wire between the first inner skeleton and the third inner skeleton and the center of the skeleton, that is, ∠C0OA0 = ∠C1OA1 =... = ∠C n OA n = β1 = β2. α and β satisfy the relationship of formula (7).

[0067] α - β = ∠A k OC k-1 = ∠C k-1 OB k-1 (k = 1, 2,..., n) (7)

[0068] Point D1 0 , D2 0 , D3 0 , D4 0 , D1 1 , D2 1 ,..., D1 n , D2 n , D3 n , D4 n are arranged on the extension lines (dashed lines in the figure) of the coil center and each point in the inner three rows, and starting from the starting point, every 4 points form a cycle until the nth cycle, and n satisfies the relationship of formula (8).

[0069] n = [2π / α] (8)

[0070] Since the coil is a double-layer winding with high density, combined with formula (7), according to the geometric relationship, there is a relationship of formula (9) for the distance between two adjacent points on the outer skeleton.

[0071] D1 0 D2 0 = D2 0 D3 0 = D3 0 D4 0 =... = D1 n D2 n = D2 n D3 n = D3 n D4 n (9)

[0072] The design of equal angles is the design of equal radians, which is used to evenly divide the wire turns into multiple segments, and make the number of wire turns and the arrangement of each segment of the coil equal as much as possible to reduce the influence of the internal capacitance of the coil.

[0073] Coil internal parameters:

[0074] (1) Coil mutual inductance:

[0075] Any turn unit of the first layer of winding can be regarded as composed of five turn segments as shown in Figure 7 , namely A k-1 A k-1 ’, A k-1 ’D2 k-1 , D2 k-1 D1 k , D1 k C k-1 . Then the mutual inductance between the i-th turn unit of the first layer of winding and the current-carrying conductor is equal to the sum of the mutual inductances of each turn segment, that is:

[0076] M i = M i,1 + M i,2 + M i,3 + M i,4 + M i,5 (i = 1, 2, 3,..., n) (10)

[0077] Similarly, any turn unit of the second layer of winding also satisfies formula (10).

[0078] Since the turns of the first and second layers are wound alternately in the same direction, for any segmented structure, it satisfies M in formula (5) j , that is:

[0079]

[0080] (2) Capacitance to ground inside the coil:

[0081] The capacitance to ground inside the coil is mainly the internal capacitance generated between the winding conductor and the shielding layer. For the two-layer winding method designed in the present invention, according to Gauss's theorem, the magnetic field intensity generated by the winding conductor per unit length, based on the vector sum, cancels out a part of each other, and relatively reduces the magnitude of the charge q carried by the winding conductor per unit length, as shown in Figure 8 The arrangement of the turn segments of the first and second layers on the side, and the capacitance between the side turn segments and the shielding layer is calculated by formula (12).

[0082]

[0083] In formula (12), u is the voltage difference between the winding conductor per unit length and the coil shielding layer, H is the height of the shielding layer, R w is the cross-sectional diameter of the winding conductor, ε is the permittivity, and θ is the angle between the side turn segment and the Z-axis. Among them, θ of the side turn segment is calculated by formula (13).

[0084]

[0085] Since β satisfies the relationship in formula (7), θ is definitely existent. Similarly, the capacitances between the front and back of the coil and the shielding layer can be calculated according to formulas (12) and (13).

[0086] The coil of the present invention based on the equal-arc multi-row design has a smaller internal capacitance to ground compared to the Rogowski coil with a single-layer winding. Also, since the first and second layer turns are wound in the same direction alternately, the capacitance to ground for any segmented structure satisfies formula (6).

[0087] (3) Anti-interference performance:

[0088] According to the design methods of formulas (7) and (9), the winding uniformity of the turns can be obtained. Whether between turns or between segmented structures, the coil can achieve good winding uniformity.

[0089] By using the winding method of the second layer, the large turn area surrounded by the return wire and the large turn area surrounded by the outgoing wire are strictly symmetric and equal. Figure 9 is the area that the turn projects into the xoy plane and is not included in the coil plane. Combining formulas (2) and (9), there is the relationship of formula (14).

[0090] 2S b =S c +S a (14)

[0091] Therefore, the area surrounded by the outgoing wire (the first layer) turns is equal to the area surrounded by the return wire (the second layer) turns, and the magnetic field perpendicular to the coil plane can be completely cancelled out, having strong anti-interference performance.

[0092] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "middle", "length", "upper", "lower", "front", "rear", "vertical", "horizontal", "inner", "outer", "radial", "circumferential", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation to the present invention.

[0093] In the present invention, unless otherwise clearly specified and limited, the first feature being "on" the second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. The meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically limited.

[0094] In the present invention, unless otherwise clearly specified or defined, terms such as "installed", "connected", "joined", "fixed", etc. shall be construed in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection, an electrical connection, or capable of communicating with each other; it may be directly connected, or indirectly connected through an intermediate medium, and it may be the internal communication of two components or the interaction relationship between two components, unless otherwise clearly defined. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0095] The above are only illustrative of the embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, any modifications, equivalent replacements, improvements, etc. made without creative efforts within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A coil structure for expanding the detection bandwidth, characterized in that, It includes a cylindrical outer skeleton, three inner skeletons, and several sections of two-layer windings with the same axis. The first layer of each section of the two-layer winding is wound with the outer skeleton as the outer layer and the first inner skeleton and the third inner skeleton alternately wound as the inner layer; the second layer of the winding is wound with a section on the outer skeleton that is the same as the outer layer of the first layer of the winding as the outer layer and the second inner skeleton as the inner layer; the winding directions of the first layer of the winding and the second layer of the winding are opposite, but the current directions of the turns are the same; the turns of each section of the two-layer winding are the same; the spacing between each section of the two-layer winding is the same; the angle formed by the wire-walking points of two adjacent turns of the winding on each inner skeleton and the center of the skeleton α are all equal; the angle formed by the wire-walking points of two adjacent turns of the winding between the first inner skeleton and the third inner skeleton and the center of the skeleton β are all equal; the distance between two adjacent turns of the winding on the outer skeleton is equal.

2. A coil winding method for expanding the detection bandwidth, characterized in that Wind using the coil structure described in claim 1: The wiring sequence of the first layer of winding is as follows: starting from a point D1 at the top of the outer skeleton 0 as the starting point, passing through A0 at the top of the first inner skeleton, winding along the inner surface of the first inner skeleton to the bottom of the first inner skeleton, and then winding to D2 at the bottom of the outer skeleton 0 , and then winding along the outer surface of the outer skeleton to D1 at the top of the outer skeleton 1 , and then winding to C0 at the top of the third inner skeleton, winding along the inner surface of the third inner skeleton to the bottom of the third inner skeleton, and then winding to D2 at the bottom of the outer skeleton 1 , and so on until the end point D4 of this section of winding at the top of the outer skeleton n ; The wiring sequence of the second-layer winding is as follows: starting from the end point D4 of the first-layer winding n and going in the opposite direction of the first-layer winding to the top B of the second inner skeleton n , winding along the inner surface of the second inner skeleton to the bottom end of the second inner skeleton, and then continuing to wind to the bottom end D3 of the outer skeleton n , winding along the outer surface of the outer skeleton to the top end D4 of the outer skeleton n-1 , and then continuing to wind to the top B of the second inner skeleton n-1 , and so on to complete the winding of the second-layer winding.

3. A design method for a coil structure for expanding the detection bandwidth as described in claim 1, characterized in that, It includes the following steps: Design the dimensions of the outer skeleton and all inner skeletons to meet the requirements of the following formula: R i = （2 ½ -1） R d ； , where C is a constant; Wherein: R i represents the cross-sectional radius of the second inner skeleton; R d represents the cross-sectional radius of the outer skeleton; R c represents the cross-sectional radius of the third inner skeleton, R a represents the cross-sectional radius of the first inner skeleton; Make the internal capacitance of each double-layer winding equal; The included angle formed by the wire routing points of two adjacent turns of wire on each inner skeleton and the center of the skeleton α are all equal; the included angle formed by the wire routing points of two adjacent turns of wire between the first inner skeleton and the third inner skeleton β are all equal; the distance between two adjacent turns of wire on the outer skeleton is equal.

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