Zero sequence current transformer

Abstract

The invention relates to a zero sequence current transformer applied in the electrical field, which comprises an iron core made of a ferromagnetic material with high initial permeability, a secondary winding is wound on the iron core, the outer side of the secondary winding is coated with a magnetic shielding layer, and the initial permeability of the ferromagnetic material in the magnetic shielding layer is higher than that of the iron core; the secondary winding is equally divided into a plurality of parts and then connected in series end to end, the output end of the secondary winding is connected with a plurality of divider resistors in series, each divider resistor is provided with a plurality of taps, and voltage output by each tap is subjected to data acquisition according to needs. And a voltage signal output by the tap with the highest amplitude but not exceeding a limit value is used as a standard basis. According to the invention, main magnetic flux and leakage magnetic flux generated by primary current and secondary current of each phase are the same as much as possible, and unbalanced output is minimized.

Description

Technical Field

[0001] The invention relates to a zero-sequence current transformer, in particular to a racetrack type zero-sequence current transformer. Background Art

[0002] At present, the known zero-sequence current transformer features are as follows:

[0003] ① Use a ring-shaped ultra-microcrystalline iron core (ordinary silicon steel sheets can also be used if there are no special requirements), wind the secondary winding, and put it on all primary wires after insulation and protection to detect zero-sequence current. Since the initial magnetic permeability of ultra-microcrystalline materials is relatively high, smaller zero-sequence currents can also be detected;

[0004] ② The zero-sequence current transformer in the cable loop can generally be made into a ring shape. The three-phase conductors of the cable are generally arranged in an equilateral triangle. The conductors of each phase have good symmetry and balance characteristics relative to the iron core. If it is placed on a three-phase conductor arranged in a straight line, the zero-sequence current transformer must be made into a runway type to reduce the volume and facilitate installation;

[0005] ③ The racetrack zero-sequence current transformer, because each phase conductor is not spatially symmetrical relative to the core, will cause the following disadvantages:

[0006] a. The three-phase primary current is symmetrical in time, but is very asymmetrical in space relative to the core. Each phase current generates different leakage flux outside the core and different main flux inside the core. The main flux and leakage flux after the three-phase flux vector synthesis will not be zero, and an unbalanced current (also called residual current) will be output. This residual current will exist in the secondary circuit as long as there is load current.

[0007] b. When the three-phase load current increases or the three-phase current becomes more asymmetric, the residual current output will also increase;

[0008] c. When a ground fault occurs, a true zero-sequence current will be generated, which will be superimposed with the residual current vector and output to the protection device. Since the residual current amplitude is even larger than the true zero-sequence current signal, and the phase also changes randomly, it will be superimposed with the true zero-sequence current and input to the protection device as the output current. However, the phase and amplitude of the superimposed output current may be completely different from those of the zero-sequence current. The protection device cannot distinguish the residual current component and can only judge the input as the true zero-sequence current, which will lead to misjudgment and refusal or malfunction.

[0009] ④ For different power grids and different grounding transition resistances, the zero-sequence current varies widely, possibly from 10mA to 50A, while the transformation ratio of the zero-sequence current transformer is constant and can only be selected according to the maximum current. If a very small zero-sequence current passes through the primary side, the excitation flux density is not in the linear range of the magnetization curve of the iron core, and the output current will be too small or even undetectable, which will seriously affect the normal protection judgment;

[0010] In order to solve the problem of too large current variation range, the secondary coil can use a tap to change the number of turns of the secondary coil according to the size of the primary current, but this will bring the following problems:

[0011] a. Only one tap can be used during operation. Two or more taps cannot be used at the same time to find the most accurate data reading, because once the second or more taps are connected to the load, part of the excitation current will be diverted, the measurement error will increase sharply, and it cannot be used;

[0012] b. During operation, the taps can be replaced with power, but the load must be connected between the taps to be used first, and then the load connected between the original taps must be disconnected. This operation is not only unsafe, but also impossible to implement in some occasions, and will also cause the measurement function to be lost for a short period of time, affecting the safety of operation;

[0013] ⑤ According to foreign literature, there is a method to reduce the residual current value: split each phase conductor into two or more strands passing through the iron core, and arrange each conductor in each phase symmetrically with the center point of the iron core as the reference, so that the magnetic motive force generated by each phase current in the main magnetic circuit and the leakage magnetic circuit is as equal as possible. This solution can be used in low-voltage leakage protection switches, but it is difficult to use in high-voltage switches or high-voltage circuits, because high-voltage electricity requires high insulation strength between phases, relative to the ground, relative to the iron core, and relative to the secondary coil. The symmetrical arrangement of each phase conductor will take up a lot of space and is actually impossible to achieve.

[0014] SUMMARY OF THE INVENTION

[0015] In view of the above-mentioned defects and problems existing in the prior art, the present invention designs a non-closed magnetic shielding layer (4) with a higher initial magnetic permeability (compared with the iron core), so that most of the leakage magnetic flux is gathered in the magnetic shielding layer, the magnetic resistance is reduced, and the leakage magnetic impedance generated by the three-phase current is as similar as possible. Since the main magnetic circuit is the same magnetic circuit for the three-phase current loop, the excitation impedance of each phase is slightly different only due to the different relative positions. However, the spatial positions of the leakage magnetic circuits of each phase are significantly different, and the leakage reactance has a large difference, which also generates residual current output. The smaller the difference in the three-phase leakage reactance, the smaller the residual current output. The present invention also provides a plurality of resistor voltage divider output taps, each of which can carry a measurement circuit load at the same time. When reading data, only the data measured by the tap with the highest resolution is read, and according to the needs of the digital circuit, the current signal is directly converted into a voltage signal, which is convenient for A / D conversion.

[0016] In order to achieve the above object, the present invention provides the following technical solutions:

[0017] A zero-sequence current transformer comprises an iron core 6 made of a ferromagnetic material with a high initial magnetic permeability, a secondary winding 5 wound on the iron core 6, a magnetic shielding layer 4 coated on the outer side of the secondary winding 5, and the initial magnetic permeability of the ferromagnetic material in the magnetic shielding layer 4 is higher than that of the iron core 6; the secondary winding 5 is equally divided into a plurality of parts and then connected in series head to tail, a plurality of voltage-dividing resistors are connected in series at the output end of the secondary winding 5, the voltage-dividing resistors have a plurality of taps, the voltage outputted by each tap is subjected to data acquisition as required, and the voltage signal outputted by the tap with the highest amplitude but not exceeding the limit value is used as the standard basis.

[0018] Furthermore, the magnetic shielding layer 4 includes two shielding layers made of different high initial magnetic permeability materials, wherein the initial magnetic permeability of one layer is higher than that of the other layer, but the magnetic saturation inflection point is lower than that of the other layer.

[0019] Furthermore, the conductive loop of the magnetic shielding layer 4 formed by the magnetic conductive material on the plane orthogonal to the main magnetic flux is separated by the insulating pads (4-5) and no longer constitutes another short-circuited secondary winding. The circuit formed by the magnetic shielding layer 4 can only be an open circuit.

[0020] Furthermore, the middle of the iron core 6 is provided with an A-phase conductor 1, a B-phase conductor 2, and a C-phase conductor 3 passing side by side, and the secondary winding 5 is divided into four sections according to the number of turns, two of which are respectively wound around the upper and lower straight portions of the iron core 6 corresponding to the C-phase conductor 3 and the B-phase conductor 2, and the other two sections are respectively wound around the upper and lower straight portions of the iron core 6 corresponding to the B-phase conductor 2 and the A-phase conductor 1.

[0021] Furthermore, the multiple voltage-dividing resistors connected in series to the output end of the secondary winding 5 convert the zero-sequence current signal into a voltage output, and the multiple taps and a common end derived from the voltage-dividing resistors are respectively connected to the grounding protection device, and the taps are automatically selected as needed to detect the zero-sequence current.

[0022] Beneficial effects:

[0023] ① When the three-phase load current is relatively large, or the three-phase current asymmetry is large (in the most serious case, one-phase circuit is broken), the secondary winding will output a large residual current. When a single-phase grounding fault occurs in the power grid, this residual current will be superimposed on the zero-sequence current, changing the amplitude and phase of the output zero-sequence current. The grounding protection device mainly uses the phase of the zero-sequence current as the main judgment basis, which will seriously affect the judgment of the grounding fault direction. After adding the magnetic shielding layer, the residual current value is greatly reduced, avoiding the influence of the residual current on the normal zero-sequence current signal, especially when there is a high-resistance grounding fault, the zero-sequence current is extremely small, but the load current is large and one phase is broken (the load current will often increase when the phase is broken). The residual current may greatly affect the phase of the output current (for the grounding protection device, it is the zero-sequence current);

[0024] ② The resistor voltage divider output is adopted to improve the resolution of weak zero-sequence current (high resistance grounding);

[0025] ③Simple structure and low cost.

[0026] Description of the drawings

[0027] Figure 1 This is a forward schematic diagram of a zero-sequence current transformer;

[0028] Figure 2 This is an axial schematic diagram of a zero-sequence current transformer;

[0029] Figure 3 This is a cross-sectional view of the zero-sequence current transformer along the AA direction;

[0030] Figure 4 This is an axial view of the zero-sequence current transformer after disassembly;

[0031] Figure 5 This is the equivalent circuit diagram of a zero-sequence current transformer.

[0032] In the figure: 1-A phase primary conductor; 2-B phase primary conductor; 3-C phase primary conductor; 4-1-upper outer shielding layer; 4-2-lower outer shielding layer; 4-3-lower inner shielding layer; 4-4-upper inner shielding layer; 4-5-insulating pads; 5-secondary winding; 6-iron core. DETAILED DESCRIPTION

[0033] The zero-sequence current transformer comprises: a secondary winding 5 is wound on the zero-sequence current transformer core 6, the secondary winding 5 is connected to a voltage-dividing resistor R1-R3, the voltage-dividing resistor has multiple taps, the voltage output from each tap can be collected simultaneously, and the voltage signal output from the tap with the highest amplitude but not exceeding the limit value is used as the standard basis. At the same time, the secondary winding 5 is equally divided into four parts 5-1, 5-2, 5-3, 5-4, 5-1, 5-2 are respectively wound on the upper and lower straight parts of the core 6 between the C-phase conductor 3 and the B-phase conductor 2, and 5-3, 5-4 are respectively wound on the upper and lower straight parts of the core 6 between the B-phase conductor 2 and the A-phase conductor 1 (see attached Figure 4 ), and finally connect the head and tail in series, which can also reduce the residual current output.

[0034] The zero-sequence current transformer also includes: a magnetic shielding layer 4 wrapped around the iron core 6 and the outer side of the secondary winding 5, the magnetic shielding layer 4 is separated by insulating pads 4-5, and forms a non-closed magnetic circuit in the orthogonal direction of the main magnetic circuit of the iron core, but is a closed magnetic circuit along the direction of the main magnetic circuit of the iron core (see attached Figure 3 ).

[0035] (1) A-phase primary conductor 1-constitutes a three-phase primary circuit;

[0036] (2) Phase B primary conductor 2 - forms a three-phase primary circuit;

[0037] (3) C-phase primary conductor 3- constitutes a three-phase primary circuit;

[0038] (4) Magnetic shielding layer

[0039] (4-1) upper outer shielding layer 4-1-consists of a magnetic shielding layer (a material with high initial magnetic permeability such as nickel-iron alloy);

[0040] (4-2) lower outer shielding layer 4-2-consists of a magnetic shielding layer (a material with high initial magnetic permeability such as nickel-iron alloy);

[0041] (4-3) lower inner shielding layer 4-3-consists of a magnetic shielding layer (a material with high initial magnetic permeability such as nickel-iron alloy);

[0042] (4~4) upper inner shielding layer 4~4-consists of a magnetic shielding layer (a material with high initial magnetic permeability such as nickel-iron alloy);

[0043] (4-5) Insulation pads 4-5-block the closed conductive circuit formed by the magnetic shielding layer in the plane orthogonal to the main magnetic flux;

[0044] (5) Secondary winding 5-outputs zero-sequence current and is connected to voltage divider resistors R1 to R3;

[0045] (5~1) Secondary winding first part 5~1-used to reduce residual current;

[0046] (5~2) The second part of the secondary winding 5~2-is used to reduce the residual current;

[0047] (5~3) The third part of the secondary winding 5~3-is used to reduce the residual current;

[0048] (5~4) The fourth part of the secondary winding 5~4-is used to reduce the residual current.

[0049] (6) Iron core 6 constitutes the main magnetic circuit of zero-sequence magnetic flux, and uses ferromagnetic material with a lower initial magnetic permeability than the magnetic shielding layer but higher initial magnetic permeability than ordinary silicon steel sheet materials, generally using ultra-fine crystal magnetic permeability material.

[0050] The following is attached to the instruction manual Figure 1 ~Instructions attached Figure 5 And specific implementation methods further illustrate the present invention:

[0051] Electromagnetic working principle of zero-sequence current transformer

[0052] When the power grid is operating normally, the conductors of each phase of the three-phase circuit are insulated from the ground, but there is distributed capacitance to the ground (because the line is long). During normal operation, due to the symmetry of the three-phase voltage to the ground, the current to the ground generated by each phase is also symmetric, and the sum of their vectors is also zero, and the zero-sequence current is also zero. When a single-phase grounding fault occurs, the grounding phase voltage decreases (the reduction value is related to the grounding excess resistance value and the line capacitance to the ground value), the voltage of the two non-grounded phases increases, the current to the ground generated by each phase is no longer symmetrical, and the sum of their vectors is no longer zero, so there is a zero-sequence current output. The zero-sequence current transformer core is mounted on the three-phase primary circuit, and the secondary side also senses 3 times the zero-sequence current value.

[0053] Since the positions of the three-phase conductors relative to the core are not symmetrical (especially the runway core), the current induced in the core by each phase current cannot have the same amplitude and a phase difference of 120°, and the vector sum is not zero, forming a residual current output. This residual current may be much larger than the zero-sequence current (the larger the load current, the larger the residual current, and the larger the grounding resistance, the smaller the zero-sequence current). When a single-phase grounding occurs, the residual current is superimposed on the true zero-sequence current (vector sum), which will make the detected current signal not a true zero-sequence current signal (the most serious is that the phase changes). The grounding protection device just needs the phase and amplitude of the zero-sequence current to determine the line and direction of the grounding fault. It can be seen that the residual current has a very serious impact on the grounding protection.

[0054] The equivalent principle of zero-sequence current transformer is shown in the attached Figure 5 This figure draws each phase equivalent current loop separately and then superimposes them together for analysis. In the figure: X Aσ , X Bσ、 X Cσare the equivalent leakage reactance of the primary circuits of the three phases A, B and C, respectively, and r A1 、r B1 、r C1 are the equivalent resistance of each phase primary circuit, r Am、、 r Bm、 , r Cm are the equivalent resistance of each phase excitation circuit (although the three phases share a magnetic circuit, the equivalent resistance will be slightly different), X Am、 , X Bm、 , X Cm are the equivalent inductive reactance of each phase excitation circuit (the three phases share one magnetic circuit, but the equivalent inductive reactance may also be slightly different), r A2 、r B2 、r C2 are the equivalent resistance of the secondary circuit of each phase, X A2 , X B2、 X C2 They are the equivalent leakage reactances of the three-phase secondary circuits A, B, and C respectively, and R1~R3 ​​are the resistors for voltage sampling and voltage division of the secondary output circuit respectively.

[0055] Depend on Figure 5 It can be seen that r A1 、r B1 、r C1 Each phase can be basically equal (because the primary circuit resistance is very small), because each phase shares a secondary winding r A2 、r B2 、r C2 are also completely equal, r Am、、 r Bm、 , r Cm Since they share the same core, the difference is not big. Aσ , X Bσ、 X Cσ The differences between the phases may be large, X A2 , X B2、 X C2 The differences between the phases will also be very large, X Am、 , X Bm、 , X Cm The phases are quite different (because the primary conductor is asymmetrical relative to the core). Therefore, even if the three-phase current is completely symmetrical, an unbalanced current (residual current) I will be generated. L If X Aσ , X Bσ、 X Cσ Equal, let X A2 , X B2、 X C2 Equal, let X Am、 , X Bm、 , X Cmare also equal, that is, the leakage reactance of the primary circuit of each phase is equal, the leakage reactance of the secondary circuit is equal, and the magnetizing reactance is equal, so I L is zero (the difference in effective resistance of each phase can be ignored). The ultimate goal of the present invention is to make the residual current I L As small as possible to reduce the impact on zero-sequence current.

[0056] Key points of the structure of a zero-sequence current transformer:

[0057] ① On the basis of the original zero-sequence current transformer, a magnetic shielding layer 4 of ferromagnetic material is coated on the outside to make the leakage flux generated by the primary current and secondary current of each phase as similar as possible, so as to minimize the unbalanced output;

[0058] ② In order to prevent the magnetic shielding layer from magnetic saturation, two layers of shielding (inner and outer) are used. One layer has a higher magnetic saturation inflection point, but the starting magnetic permeability is also slightly higher. The other layer has a lower magnetic saturation inflection point, but the starting magnetic permeability is required to be high. This not only avoids magnetic saturation but also meets the requirement of high starting magnetic permeability.

[0059] ③ Since the material of the magnetic shielding layer is also a conductor, it is coated on the outside of the iron core 6 and the secondary winding 5, forming a new secondary circuit and being connected in parallel with the original secondary winding 5, which is equivalent to the original secondary circuit being short-circuited and unable to output normal zero-sequence current. The magnetic shielding layer 4 is padded with insulating pads 4-5 in the direction of the orthogonal plane of the main magnetic flux to cut off the conductive closed circuit, and no longer forms another short-circuited secondary winding, and can only operate in an open circuit;

[0060] ④ Divide the secondary winding into 4 equal sections, and wind them around the upper and lower straight parts of the core 6 in the middle of phases A and B, and in the middle of phases B and C respectively, to reduce the degree of asymmetry and effectively reduce the residual current output.

[0061] ⑤Insert three resistors in series in the secondary circuit to convert the zero-sequence current signal into voltage output, then lead out three taps and a common terminal, which are respectively connected to the grounding protection device. The taps are automatically selected as needed to detect the zero-sequence current to ensure sufficient zero-sequence current resolution.

[0062] Working principle of zero-sequence current transformer:

[0063] ①See Appendix Figure 3 , the leakage flux generated by the phase currents that did not pass through the main magnetic circuit will not pass through the same magnetic circuit in space, which will cause the leakage reactance of the primary circuit and the secondary circuit to be inconsistent. Figure 5 , even if the three-phase symmetry There will also be a residual current I LOutput. When the overall magnetic shielding layer 4 is formed by the upper outer shielding layer 4-1, the lower outer shielding layer 4-2, the lower inner shielding layer 4-3, and the upper inner shielding layer 4-4, since the initial magnetic permeability of the magnetic shielding layer 4 is more than 100 times that of non-magnetic conductors such as air, even if there is very weak leakage flux, it will be gathered into the magnetic shielding layer 4. Most of the leakage flux generated by the three-phase current will pass through the magnetic shielding layer 4 to form a closed magnetic circuit, and increase the magnetic induction intensity by nearly 100 times. Since the three phases share a magnetic shielding magnetic circuit, the magnetic resistance is the same, and the leakage reactance (X Aσ , X Bσ、 X Cσ ), each phase secondary circuit leakage reactance (X A2 , X B2、 X C2 ) are also basically the same. Since the three phases share a core 6, the excitation reactance of each phase is X Am、 , X Bm、 , X Cm is also very close, which eventually makes the residual current output I L It will be very small, and after superposition with the zero-sequence current, it will not cause serious distortion of the output zero-sequence current waveform, thus meeting the requirements of the grounding protection device.

[0064] ②See Appendix Figure 3 The insulating pads 4-5 block the closed conductive loop formed by the magnetic shielding layer 4 in the plane direction orthogonal to the main magnetic flux, so that the second secondary winding formed by the magnetic shielding layer can only be open-circuited but not short-circuited, and no longer affects the output of the secondary winding 5.

[0065] ③ The secondary winding 5 is divided into 4 equal parts according to the number of turns, which can also reduce the residual current value, especially when phase A or phase C is broken and accompanied by a ground fault. Since the secondary winding 5 is closer to the primary conductor and is symmetrically distributed, the residual current output can be effectively reduced.

[0066] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A zero-sequence current transformer, characterized in that: The invention comprises an iron core (6) made of a ferromagnetic material with a high initial magnetic permeability, a secondary winding (5) wound on the iron core (6), a magnetic shielding layer (4) coated on the outer side of the secondary winding (5), and the initial magnetic permeability of the ferromagnetic material in the magnetic shielding layer (4) being higher than that of the iron core (6); the secondary winding (5) is equally divided into a plurality of parts and then connected in series head to tail, a plurality of voltage-dividing resistors are connected in series at the output end of the secondary winding (5), the voltage-dividing resistors have a plurality of taps, the voltage outputted by each tap is subjected to data acquisition as required, and the voltage signal outputted by the tap with the highest amplitude but not exceeding the limit value is used as the standard basis.

2. A zero-sequence current transformer according to claim 1, characterized in that: The magnetic shielding layer (4) comprises two shielding layers made of different high initial magnetic permeability materials, wherein the initial magnetic permeability of one layer is higher than that of the other layer, but the magnetic saturation inflection point is lower than that of the other layer.

3. A zero-sequence current transformer according to claim 1, characterized in that: The conductive loop of the magnetic shielding layer (4) formed by the magnetic conductive material on the plane orthogonal to the main magnetic flux is separated by the insulating pads (4-5) and no longer forms another short-circuited secondary winding. The circuit formed by the magnetic shielding layer (4) can only be an open circuit.

4. A zero-sequence current transformer according to claim 1, characterized in that: The middle of the iron core (6) is provided with an A-phase conductor (1), a B-phase conductor (2), and a C-phase conductor (3) passing through side by side. The secondary winding (5) is divided into four sections according to the number of turns, two of which are respectively wound on the upper and lower straight sections of the iron core (6) corresponding to the C-phase conductor (3) and the B-phase conductor (2), and the other two sections are respectively wound on the upper and lower straight sections of the iron core (6) corresponding to the B-phase conductor (2) and the A-phase conductor (1).

5. A zero-sequence current transformer according to claim 1, characterized in that: The multiple voltage-dividing resistors connected in series at the output end of the secondary winding (5) convert the zero-sequence current signal into a voltage and output it. The multiple taps and a common end derived from the voltage-dividing resistors are respectively connected to a grounding protection device, and the taps are automatically selected as needed to detect the zero-sequence current.

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