A zero-sequence current transformer and a manufacturing method thereof

Abstract

The present invention relates to a zero-sequence current transformer, comprising a coil core and a shell, wherein the coil core is arranged in the shell; the coil core is a combined core, comprising a first core and a second core, wherein the first core is arranged in the second core; the first core is a silicon steel sheet core divided into at least two parts in the diameter direction, and the second core is a closed core made of a high magnetic permeability material. The present invention also includes a method for manufacturing the zero-sequence current transformer. The present invention adopts a hybrid core composed of cut silicon steel and Bomalloy, which can enable the zero-sequence current transformer to maintain a high accuracy at very small currents, and is not prone to magnetic saturation, and maintains a high accuracy within a wide range; the second core adopts a Bomalloy core, which can enable the zero-sequence current transformer to maintain a high accuracy at very small currents, and the second core also reduces the leakage magnetic flux from the cross section and the tight slit, and can also prevent electromagnetic induction from interfering with the outside.

Description

Technical Field

[0001] The invention belongs to the technical field of current transformers, and in particular relates to a zero-sequence current transformer and a manufacturing method thereof. Background Art

[0002] Zero-sequence current transformer (hereinafter referred to as: zero-sequence CT) is widely used to detect single-phase grounding faults. The zero-sequence current transformer is a three-phase primary conductor passing through the inside of the ring core, and the current vector sum of the primary conductor is transformed and output from the secondary coil. In order to make the zero-sequence current transformer applicable to different usage scenarios, the zero-sequence current transformer is required to have high accuracy in a wide range from very small currents (about 0.2A when used in a neutral point non-grounded system or a neutral point grounded system through an arc suppression coil) to larger currents (about 600A when used in a neutral point grounded system through a small resistance).

[0003] Existing zero-sequence current transformers generally use closed iron cores. Silicon steel cores have high saturation magnetic induction intensity, but low effective magnetic permeability, while Bomal alloy has high initial magnetic permeability and low saturation magnetic induction intensity. In order to make the zero-sequence current transformer have high precision under low current conditions, it is necessary to use materials with high magnetic permeability; and in order to be unsaturated under high current conditions, the iron core must have strong anti-saturation ability. There is a certain contradiction between the two, and it is difficult to meet the wide range and high precision requirements of zero-sequence current transformers.

[0004] At the same time, when a single-phase fault occurs in the power system (especially in a neutral point grounding system with a small resistance), the fault current often contains a transient DC component. When the transient DC component is superimposed on the AC, DC polarization occurs, and the core of the zero-sequence current transformer is prone to magnetic saturation, causing the secondary output to be distorted. Among the current algorithms for single-phase grounding faults, some use the transient signal of the zero-sequence current transformer as a basis. The distortion of the secondary output of the zero-sequence current transformer will affect the detection of the fault. Therefore, it is necessary to improve the existing zero-sequence current transformer. Summary of the invention

[0005] The present invention provides a zero-sequence current transformer and a manufacturing method thereof to solve the technical problems existing in the known technology. The zero-sequence current transformer proposed by the present invention is unlikely to cause magnetic saturation and effectively prevents the increase of the phase error between the primary current and the secondary output.

[0006] The present invention includes the following technical solutions:

[0007] A zero-sequence current transformer comprises a coil core and a shell, wherein the coil core is arranged in the shell; the coil core is a combined core, comprising a first core and a second core, wherein the first core is arranged in the second core; the first core is a silicon steel sheet core divided into at least two parts in a diameter direction, and the second core is a closed core made of a high magnetic permeability material.

[0008] Furthermore, the zero-sequence current of the zero-sequence current transformer is the vector sum of the phase currents flowing through the three-phase primary conductors (i.e. ), and after transformation by the secondary winding, it is output from the two secondary terminals at the bottom of the casing.

[0009] Furthermore, the second iron core is made of Permalloy and is divided into an inner iron core and an outer iron core, and the first iron core is arranged between the inner iron core and the outer iron core.

[0010] Furthermore, after the first iron core is assembled into a ring, the surface is wrapped with a first insulating tape, and the split surface of the first iron core is tightly fitted and has an air gap. After the air gap is inserted into the winding iron core, the magnetic saturation phenomenon under large AC and DC polarization conditions can be avoided. At the same time, the magnetic resistance of the air gap part is hundreds of times that of the closed iron core, and the magnetic resistance of the slit part is dominant. The excitation impedance is reduced to one percent, the excitation current increases accordingly, and the error of the secondary output relative to the primary input current of the zero-sequence current transformer increases. The open air gap of the iron core can also reduce the residual magnetism of the iron core when it works in an asymmetric magnetic field state, and the larger the air gap, the smaller the residual magnetism of the iron core when the coil current drops to zero, so that the iron core of the same volume can output greater power.

[0011] Furthermore, the two ends of the first insulating tape are respectively connected to the inner ring iron core and the outer ring iron core; the second insulating tape is wrapped around the surface of the annular intermediate product composed of the first insulating tape, the first iron core and the second iron core, and the secondary coil is wrapped around the outside of the second insulating tape.

[0012] Furthermore, the second insulating tape and the secondary coil are arranged in the housing and are covered by pouring of thermosetting resin.

[0013] Furthermore, the thermosetting resin may be epoxy resin, phenolic resin or the like.

[0014] A method for manufacturing a zero-sequence current transformer comprises the following steps:

[0015] S1. Make the first iron core; S2. Split the first iron core and wrap the first insulating tape on its surface to obtain intermediate product No. 1; S3. Set the inner iron core and the outer iron core at the inner circle and outer circle of intermediate product No. 1 respectively to obtain intermediate product No. 2; S4. Wrap the second insulating tape on the surface of intermediate product No. 2 to obtain intermediate product No. 3; S5. After winding the secondary coil on the surface of intermediate product No. 3, put it into an outer shell, evacuate it, inject thermosetting resin into the shell and heat it to cure it to obtain a zero-sequence current transformer.

[0016] Furthermore, the S1 includes S1-1 winding the silicon steel sheet on a fixture and fixing it into an annular closed magnetic core by welding; S1-2 cutting the annular closed magnetic core and dividing it into a plurality of equal arc-shaped magnetic cores.

[0017] Furthermore, the inner ring iron core in S2 is connected to the inner circle of the first iron core through a first insulating tape, and the outer ring iron core is connected to the outer circle of the first iron core.

[0018] Furthermore, the inner ring core and the outer ring core in S3 are both formed by winding electrical steel strips with tooling, the starting parts of the winding of the two are fixed on the first intermediate product and the thickness of the inner ring core is less than the thickness of the outer ring core.

[0019] The present invention has the advantages and positive effects:

[0020] 1. The present invention adopts a hybrid core composed of cut silicon steel and Permalloy, which can enable the zero-sequence current transformer to maintain a high accuracy at very small currents, and is not prone to magnetic saturation, maintaining a high accuracy within a wide range; the second core adopts a Permalloy core, which has a high magnetic permeability and can enable the zero-sequence current transformer to maintain a high accuracy at very small currents.

[0021] 2. When the present invention is used, the divided silicon steel sheet cores are combined and brought into close contact with each other, and a tiny air gap is formed between the divided surfaces, thereby improving the anti-saturation ability of the core, ensuring that the zero-sequence current transformer is not saturated when detecting a larger current, and at the same time suppressing the extreme drop in the magnetic permeability of the core that affects the accuracy of the zero-sequence current transformer.

[0022] 3. The second core of the present invention also reduces the leakage magnetic flux from the cross section and the tight slits, and also plays a role in preventing electromagnetic induction from interfering with the outside.

[0023] 4. In the manufacturing process of the zero-sequence current transformer of the present invention, the secondary coil is firstly wound on the cut iron core and then the insulating resin is poured. Various stresses are applied during these processes, resulting in deviations in the bonding between the cut surfaces in the first iron core. The thermosetting resin easily enters the interlayer gaps of the silicon steel sheets of the first iron core, causing serious degradation of the magnetic properties of the first iron core, thereby affecting the output characteristics of the zero-sequence current transformer. In the present invention, the penetration of the thermosetting resin during the pouring process is suppressed by winding the insulating tape, so that the first iron core is firmly fixed, and the degradation of the magnetic properties during the manufacturing process of the zero-sequence CT is suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a three-dimensional diagram of the appearance of the zero-sequence current transformer;

[0025] Figure 2 It is an axonometric diagram of the zero-sequence current transformer with three-phase primary conductors passing through it;

[0026] Figure 3 is a cross-sectional view of a zero-sequence current transformer;

[0027] Figure 4 It is the schematic diagram of the zero-sequence current transformer;

[0028] Figure 5 It is the equivalent circuit of the zero-sequence current transformer;

[0029] Figure 6 yes Figure 5 The current vector diagram;

[0030] Figure 7 yes Figure 5 The distribution diagram of current in each part when DC polarization occurs;

[0031] Figure 8 It is a graph showing the change in the core magnetic flux density when the transient DC component is superimposed on the AC;

[0032] Fig. 9 This is the waveform diagram when magnetic saturation is caused by DC polarization;

[0033] Fig.10 This is an example diagram of the distorted waveform of the load current when the core is magnetically saturated;

[0034] Fig.11 It is a schematic diagram of the magnetic characteristic curves of different iron cores;

[0035] Fig.12 It is a schematic diagram of the coil excitation magnetic impedance curve for different iron cores;

[0036] Fig.13 is a plan view of the first core;

[0037] Fig.14is a three-dimensional perspective view of the first core;

[0038] Fig.15 15-(a) is a hysteresis loop of a closed core, 15-(b) is a hysteresis loop of a closed core with a plurality of air gaps, and 15-(c) is a hysteresis loop of the first core of the present invention;

[0039] Fig.16 It is a front view schematic diagram of intermediate product No. 1;

[0040] Fig.17 yes Fig.16 AA section diagram in;

[0041] Fig.18 is a schematic diagram of the process of winding the first insulating tape;

[0042] Fig.19 This is a front view of the intermediate product No. 2;

[0043] Fig. 20 yes Fig.19 BB cross-section diagram in FIG.

[0044] Fig.21 This is a front view of the intermediate product No. 3;

[0045] Fig. 22 yes Fig.21 Schematic diagram of CC section in;

[0046] Fig.23 is a schematic diagram of the process of winding the second insulating tape;

[0047] Fig.24 This is a schematic cross-sectional view of the No. 3 intermediate product after being wound by the secondary coil;

[0048] In the figure, 1-zero-sequence current transformer; 2-secondary output terminal No. 1; 3-secondary output terminal No. 2; 4-housing; 1-1 is intermediate product No. 1, 1-2 is intermediate product No. 2, and 1-3 is intermediate product No. 3;

[0049] 10-first iron core; 11-first arc-shaped magnetic core; 12-second arc-shaped magnetic core; 20-first insulating tape; 30-second iron core; 31-inner iron core; 32-outer iron core; 40-second insulating tape; 50-secondary coil; 60-thermosetting resin. DETAILED DESCRIPTION

[0050] In order to further disclose the content, features and effects of the present invention, the following examples are given and described in detail with reference to the accompanying drawings.

[0051] Example 1: See attached Figure 1-15A zero-sequence current transformer includes a coil core and a housing 4, wherein the coil core is disposed in the housing 4; the coil core is a combined core, comprising a first core 10 and a second core 30, wherein the first core 10 is disposed in the second core 30; the first core 10 is a silicon steel core divided into two parts in the diameter direction, and the second core 30 is a closed core made of a high magnetic permeability material. The second core 30 is made of a Permalloy alloy and is divided into an inner core 31 and an outer core 32, wherein the first core 10 is disposed between the inner core 31 and the outer core 32.

[0052] like Figure 2 As shown, the zero-sequence current of the zero-sequence current transformer 1 is the vector sum of the phase currents flowing through the three-phase primary conductors (i.e. ), after transformation by the secondary winding, output is obtained from the No. 1 secondary terminal output 2 and the No. 2 secondary terminal output 3 at the bottom of the housing 4.

[0053] like Figure 4-6 As shown, I0 is the vector sum of the three-phase currents, Z b is the load of zero-sequence current transformer 1, the turns ratio of the primary winding and the secondary winding is 1:n; I0 / n is the secondary current of zero-sequence current transformer 1, I L is the excitation current, the secondary load side current is Iz, r′1 is the impedance of the primary winding converted to the secondary, r2 is the secondary winding impedance, Z b It can be seen that when the volume of the winding core remains unchanged, if the magnetic permeability of the core is low, the excitation impedance is small, the excitation current increases, and the error of the zero-sequence current transformer 1 is large when the current is small.

[0054] When a single-phase grounding fault occurs in the power system, the fault current often contains a transient DC component. First, the mechanism of DC polarization caused by the transient DC component is described. The current change curves of each part are as follows: Figure 7 shown. Figure 5 When the "DC polarization phenomenon" occurs in the equivalent circuit in the circuit, the transient DC component that decays exponentially is input into the equivalent circuit, and the excitation current (I L ) in the magnetizing inductance (X L ) flows in a "mountain-shaped" waveform. The magnetic flux generated by this "mountain-shaped" current increases in one direction. Figure 8 As shown in FIG. 1 , when the AC magnetic flux 101 is superimposed on the DC magnetic flux 102, it becomes a curve 103, which easily produces magnetic saturation. Figure 9-10 As shown in the figure, when the AC magnetic flux 111 is superimposed on the DC magnetic flux 112 generated by the transient DC component and becomes curve 113. Due to the magnetic saturation of the iron core, the load side current I z The waveform 114 is distorted from a sine wave state to a Fig.10 The load current in the waveform 116 is distorted. In the worst case, when there is residual flux (Br) in the core, the magnetic flux increases from the residual flux (Br), which is more likely to cause magnetic saturation. In this way, the system current cannot be faithfully reproduced, which may cause misjudgment when an accident occurs.

[0055] like Fig.11 As shown, the magnetic characteristic curve 121 of the oriented silicon steel core and the magnetic characteristic curve 122 of the Permalloy core, in the area with small magnetic field strength H, the magnetic characteristics of the oriented silicon steel core can be corrected by the characteristics of the Permalloy core, while the magnetic characteristics of the oriented silicon steel core dominate in the area with large magnetizing force H, thereby obtaining the magnetic characteristic curve 123 of the hybrid core, which can increase the magnetic permeability within a small current range.

[0056] like Fig.12 As shown, relative to the excitation magnetic impedance curve 124 of the oriented silicon steel core, the excitation magnetic impedance curve 125 of the BoMo alloy core is corrected in the region of small input current and the two are combined to obtain the excitation magnetic impedance curve 126 of the hybrid core. The zero-sequence current transformer 1 using the hybrid core can ensure accuracy in a small current range.

[0057] like Figure 3 , Fig.13 , Fig.14 As shown, the first core 10 of the present invention is cut into a first arc-shaped magnetic core 11 and a second arc-shaped magnetic core 12 of the same shape in the diameter direction. The first cutting surface 11a and the second cutting surface 12a of the two semi-annular magnetic cores are in close contact. After the first core 10 is assembled into a ring, the surface is wrapped with a first insulating tape 20, and the split surface of the first core 10 is tightly fitted and has an air gap. After the air gap is inserted into the winding core, the magnetic saturation phenomenon under large AC and DC polarization conditions can be avoided. At the same time, the magnetic resistance of the air gap part is hundreds of times that of the closed core, and the magnetic resistance of the slit part is dominant. The excitation impedance is reduced to one percent, the excitation current increases accordingly, and the error of the secondary output relative to the primary input current of the zero-sequence current transformer increases. The open air gap of the core can also reduce the residual magnetism of the core when it works in an asymmetric magnetic field state, and the larger the air gap, the smaller the residual magnetism of the core when the coil current drops to zero, so that the core of the same volume can output greater power.

[0058] like Fig.15As shown, the slope of the curve 15-(a) is large, Br / Bm is also large, there is residual magnetism at the position (Br), and Bm is quickly reached at this position during magnetization, thereby producing magnetic saturation; the slope of the hysteresis loop 15-(b) becomes smaller, Br decreases, but because Br / Bm is also small, it is difficult to reach the maximum magnetic flux density Bm, magnetic saturation is difficult, but the magnetic resistance will become larger, the excitation impedance will become very small, and the excitation current will become larger. The present invention achieves the setting of the air gap by providing a slit in the first iron core 10 and then making the slit portion in close contact. Compared with an iron core simply having a slit inserted therein, the increase in magnetic resistance can be suppressed, the extreme drop in excitation impedance can be suppressed, and Br / Bm can be reduced, thereby achieving Fig.15 -(a) and 15-(b). The first cut surface 11a and the second cut surface 12a can also be polished to make the combination tighter, suppress the increase of magnetic resistance, and then realize the hysteresis loop shown in 15-(c). In this way, the magnetic resistance of the core is greater than the magnetic resistance of the closed core, and the magnetic resistance can be suppressed to be lower than the core simply provided with slits. Therefore, even if a transient DC component is superimposed, it is not easy to cause magnetic saturation, and the accuracy is suppressed.

[0059] The two ends of the first insulating tape 20 are connected to the inner iron core 31 and the outer iron core 32 respectively; the surface of the annular second intermediate product 1-2 composed of the first insulating tape 20, the first iron core 10, and the second iron core 30 is wrapped with the second insulating tape 40, and the second insulating tape 40 is wrapped with the secondary coil 50. The second insulating tape 40 and the secondary coil 50 are arranged in the housing 4 and are cast and covered by the thermosetting resin 60. The thermosetting resin 60 can be epoxy resin, phenolic resin, etc.

[0060] See attached Figure 3 , Attachment Figure 16-24 , a method for manufacturing a zero-sequence current transformer, comprising the following steps: S1, making a first iron core 10; the S1 includes S1-1 winding a silicon steel sheet on a fixture and fixing it into a ring-shaped closed magnetic core by welding; S1-2 cutting the ring-shaped closed magnetic core and dividing it into two equal arc-shaped magnetic cores (i.e., a first arc-shaped magnetic core 11 and a second arc-shaped magnetic core 12).

[0061] like Figure 16-18 As shown, S2, after the first iron core 10 is assembled, the first insulating adhesive 20 is wrapped on its surface to obtain an intermediate product No. 1-1; in the S2, the inner ring iron core 31 is connected to the inner circle of the first iron core 10 through the first insulating adhesive 20, and the outer ring iron core 32 is connected to the outer circle of the first iron core 10.

[0062] Since the first core 10 is formed by laminating electrical steel strips, the thermosetting resin 60 can easily enter the gaps between the layers of the electrical steel strips, and stress will be generated when the thermosetting resin 60 is cured, which will deteriorate the magnetic properties of the first core 10. Unlike the closed core, the first core 10 may peel off due to stress due to the presence of a cut portion, which will have a greater impact on the degradation of the characteristics. Therefore, by wrapping the first insulating tape 20 around the first core 10, the two semi-ring-shaped magnetic cores are fixed to increase the adhesion between the first cut surface 11a and the second cut surface 12a, which can inhibit the side of the first core 10 or the first cut surface 11a and the second cut surface 12a from entering the thermosetting resin 60, and improve the residual current characteristics. The first insulating tape 20 is wrapped over and over again, which can further inhibit the thermosetting resin 60 from penetrating from the gaps between the layers on the side of the first core 10 during casting.

[0063] like Figure 19-20 As shown, S3, the inner ring iron core 31 and the outer ring iron core 32 are respectively arranged at the inner circle and the outer circle of the No. 1 intermediate product 1-1 to obtain the No. 2 intermediate product 1-2; the inner ring iron core 31 and the outer ring iron core 32 in S3 are formed by winding electrical steel strips with tooling, and the starting parts of the winding of the two are fixed on the No. 1 intermediate product 1-1 and the thickness of the inner ring iron core 31 is less than the thickness of the outer ring iron core 32.

[0064] Since the shrinkage of the outer ring of the first core 10 is greater than that of the inner ring during the curing process of the thermosetting resin 60, it is desirable that the thickness of the inner ring core 31 is smaller than the thickness e2 of the outer ring core 32. For example, the thickness of the inner ring core 31 is set to 2.5 mm, and the thickness of the outer ring core 32 is set to 3.5 mm. In this way, when pouring the thermosetting resin 60, the deformation of the first core 10 caused by stress during the curing shrinkage process can be more accurately suppressed, the adhesion between the first cutting surface 11a and the second cutting surface 12a is suppressed, and a stable residual current characteristic can be obtained.

[0065] like Figure 21-23 As shown, S4, a second insulating tape 40 is wound around the surface of the second intermediate product 1-2 to obtain a third intermediate product 1-3; the second insulating tape 40 further inhibits the thermosetting resin 60 from penetrating from the gap between the side surface layers of the first iron core 10 during the casting process. The second insulating tape 40 can protect the coating of the wire constituting the secondary coil 50, and at the same time inhibit the position deviation of the inner iron core 31 and the outer iron core 32 caused by external force during the subsequent winding process of the secondary coil 50.

[0066] like Fig.24As shown, S5, after the secondary coil 50 is wound on the surface of the third intermediate product 1-3, it is placed in the housing 4 to evacuate and the thermosetting resin 60 is injected into the housing 4 to heat and cure, thereby obtaining the zero-sequence current transformer 1. When the secondary coil 50 is directly wound on the third intermediate product 1-3, the first iron core 10 will be deformed due to the stress generated by the winding tension, but the first iron core 10 is covered by the first insulating tape 20, the inner iron core 31, the outer iron core 32 and the second insulating tape 40, which can suppress the stress applied to the first iron core 10 and the deterioration of the residual current characteristics caused by the stress.

[0067] Embodiment 2: A zero-sequence current transformer, the first core 10 is divided into three equal parts in the diameter direction, and the three arc-shaped magnetic cores can be made of different materials, but they still need to be assembled into a ring shape and the cut surfaces must fit tightly. The second core 30 is made of ferrite material or iron-based amorphous alloy material. The other structures are basically the same as those of Embodiment 1.

[0068] Although the preferred embodiments of the present invention are described above, the present invention is not limited to the above specific embodiments, which are merely illustrative and not restrictive. Under the guidance of the present invention, a person skilled in the art can make many forms without departing from the scope of the present invention and the claims. All of these belong to the protection scope of the present invention.

Claims

1. A zero-sequence current transformer, comprising a coil core and a housing (4), wherein the coil core is arranged in the housing (4), characterized in that: The coil core is a combined core, comprising a first core (10) and a second core (30), wherein the first core (10) is arranged inside the second core (30); the first core (10) is a silicon steel sheet core divided into at least two parts in a diameter direction, and the second core (30) is a closed core made of a high magnetic permeability material.

2. The zero-sequence current transformer according to claim 1, characterized in that: The zero-sequence current of the zero-sequence current transformer (1) is the vector sum of the phase currents flowing through the three-phase primary conductors, and is output from two secondary terminals at the bottom of the housing (4) after being transformed by the secondary winding.

3. The zero-sequence current transformer according to claim 2, characterized in that: The second iron core (30) is made of Permalloy and is divided into an inner iron core (31) and an outer iron core (32), and the first iron core (10) is arranged between the inner iron core (31) and the outer iron core (32).

4. The zero-sequence current transformer according to claim 3, characterized in that: After the first iron core (10) is assembled into a ring, a first insulating tape (20) is wound around the surface, and the divided surfaces of the first iron core (10) are in contact with each other and have air gaps.

5. The zero-sequence current transformer according to claim 4, characterized in that: The two ends of the first insulating tape (20) are respectively connected to the inner iron core (31) and the outer iron core (32); the surface of the annular intermediate product composed of the first insulating tape (20), the first iron core (10) and the second iron core (30) is wrapped with the second insulating tape (40), and the secondary coil (50) is wrapped around the outside of the second insulating tape (40).

6. The zero-sequence current transformer according to claim 5, characterized in that: The second insulating tape (40) and the secondary coil (50) are arranged in the housing (4) and are covered by pouring of thermosetting resin (60).

7. A method for manufacturing a zero-sequence current transformer, characterized in that: The following steps are involved: S1. Make a first iron core (10); S2. After assembling the first iron core (10), wrap a first insulating tape (20) on its surface to obtain a first intermediate product (1-1); S3. Arrange an inner iron core (31) and an outer iron core (32) on the inner circle and outer circle of the first intermediate product (1-1) respectively to obtain a second intermediate product (1-2); S4. Wrap a second insulating tape (40) on the surface of the second intermediate product (1-2) to obtain a third intermediate product (1-3); S5. After winding a secondary coil (50) on the surface of the third intermediate product (1-3), place it in a housing (4), evacuate it, and inject a thermosetting resin (60) into the housing (4) to heat and cure it, to obtain a zero-sequence current transformer (1) as described in claim 6.

8. The method for manufacturing a zero-sequence current transformer according to claim 7, characterized in that: The S1 includes S1-1, winding the silicon steel sheet on the fixture and fixing it into a ring-shaped closed magnetic core by welding; S1-2, cutting the ring-shaped closed magnetic core and dividing it into a plurality of equal arc-shaped magnetic cores.

9. The method for manufacturing a zero-sequence current transformer according to claim 7, characterized in that: The inner iron core (31) in S2 is connected to the inner circle of the first iron core (10) through a first insulating tape (20), and the outer iron core (32) is connected to the outer circle of the first iron core (10).

10. The method for manufacturing a zero-sequence current transformer according to claim 9, characterized in that: The inner iron core (31) and the outer iron core (32) in S3 are both formed by tooling winding electrical steel strips, the starting parts of the winding of the two are fixed on the first intermediate product (1-1), and the thickness of the inner iron core (31) is smaller than the thickness of the outer iron core (32).

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