Zero sequence current transformer

Abstract

The utility model relates to a zero sequence current transformer applied in the electrical field, which comprises an iron core made of 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 utility model, 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] This utility model relates to a zero-sequence current transformer, particularly a racetrack-type zero-sequence current transformer. Background Technology

[0002] Currently, the known characteristics of zero-sequence current transformers are as follows:

[0003] ① A toroidal microcrystalline iron core (or ordinary silicon steel sheet for non-special requirements) is wound with secondary windings, insulated and protected, and then looped around all primary conductors to detect zero-sequence current. Because microcrystalline materials have high initial permeability, even small zero-sequence currents can be detected.

[0004] ② Zero-sequence current transformers used in cable circuits can generally be made into rings. The three-phase conductors of a cable are generally arranged in an equilateral triangle, and the symmetry and balance characteristics of each phase conductor relative to the iron core are good. If it is used on a straight-line arrangement of three-phase conductors, the zero-sequence current transformer must be made into a racetrack shape to reduce its size and facilitate installation.

[0005] ③ Racetrack-type zero-sequence current transformers, due to the spatial imperfections of each phase conductor relative to the iron core, will cause the following disadvantages: a. The three-phase primary currents are symmetrical in time, but spatially asymmetrical relative to the iron core. The leakage flux generated by each phase current outside the iron core is different, and the main flux generated inside the iron core is also different. The main flux and leakage flux after the vector synthesis of the three-phase fluxes will not be zero, resulting in an unbalanced current (also called residual current). This residual current will exist in the secondary circuit as long as there is load current; b. When the three-phase load current increases or the three-phase currents become more asymmetrical, this will also increase the residual current output; c. When a ground fault occurs, a true zero-sequence current will be generated, which will be vector-superimposed with the residual current and output to the protection device. Since the residual current amplitude is even larger than the true zero-sequence current signal and the phase changes randomly, it is superimposed with the true zero-sequence current and is used as the output current input to the protection device. The phase and amplitude of this superimposed output current may be completely different from the phase and amplitude 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 failure to operate or false operation.

[0006] ④ Due to different power grids and varying grounding resistances, the zero-sequence current varies greatly, potentially ranging from 10mA to 50A. However, the transformation ratio of the zero-sequence current transformer remains constant, requiring selection based on the maximum current. If a very small zero-sequence current passes through the primary side, the excitation flux density will not fall within the linear range of the core's magnetization curve, resulting in an excessively small or undetectable output current, severely impacting normal protection judgment. To address the issue of excessive current variation, taps can be used on the secondary coil to adjust the number of turns according to the primary current. However, this presents the following problems: a. Only one tap can be used during operation; two or more taps cannot be used simultaneously to find the most accurate data reading. Once a second or more taps are connected to a load, the excitation current will be diverted, drastically increasing the measurement error and rendering the device unusable. b. Taps can be changed while the circuit is energized, but the load must be connected between the taps to be used before disconnecting the original load connections. This operation is not only unsafe but also impractical in some situations, and may cause temporary loss of measurement function, affecting operational safety.

[0007] ⑤ Foreign literature reports a method to reduce residual current: split each phase conductor into two or more strands and pass them through the iron core. Each conductor in each phase is arranged symmetrically with respect to the center point of the iron core, so that the magnetomotive force generated by the current in each phase in the main magnetic circuit and the leakage magnetic circuit is as equal as possible. This scheme 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 very high insulation strength between phases, between phases and ground, between phases and the iron core, and between phases and the secondary coil. The symmetrical arrangement of split conductors in each phase would occupy a lot of space, making it practically impossible to implement. Utility Model Content

[0008] To address the aforementioned deficiencies and problems in existing technologies, this invention designs a non-closed magnetic shielding layer 4 with higher initial permeability (compared to the iron core), concentrating most of the leakage flux within the shielding layer, reducing magnetic reluctance, and ensuring that the leakage magnetic impedance generated by the three-phase current is as similar as possible. Since the main magnetic circuit is the same for the three-phase current loop, the excitation impedance of each phase differs slightly only due to their relative positions. However, the spatial positions of the leakage magnetic circuits of each phase differ significantly, resulting in a large difference in leakage reactance and thus generating residual current output. The smaller the difference in leakage reactance among the three phases, the smaller the residual current output. This invention also provides multiple resistive voltage divider output taps, each of which can simultaneously carry a measurement circuit load. When reading data, only the data measured by the tap with the highest resolution is read. Furthermore, according to the needs of digital circuits, the current signal is directly converted into a voltage signal, facilitating A / D conversion.

[0009] To achieve the above objectives, this utility model provides the following technical solution: a zero-sequence current transformer, comprising an iron core 6 made of a ferromagnetic material with high initial permeability, a secondary winding 5 wound on the iron core 6, and a magnetic shielding layer 4 covering the outer side of the secondary winding 5, wherein the initial 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 divided into several equal parts and then connected in series end to end; multiple voltage divider resistors are connected in series at the output end of the secondary winding 5; the voltage divider resistors have multiple taps; the voltage output of each tap is collected as needed, and the voltage signal output by the tap with the highest amplitude but not exceeding the limit is used as the standard basis.

[0010] Furthermore, the magnetic shielding layer 4 comprises two shielding layers made of different high initial permeability materials, one of which has a higher initial permeability than the other, but a lower magnetic saturation inflection point.

[0011] Furthermore, the conductive circuit formed by the magnetic material in the magnetic shielding layer 4 on the orthogonal plane of the main magnetic flux is interrupted by the insulating pad (45), and no longer constitutes another short-circuited secondary winding. The circuit formed by the magnetic shielding layer 4 can only be an open circuit.

[0012] Furthermore, the iron core 6 has A-phase conductor 1, B-phase conductor 2, and C-phase conductor 3 arranged side by side in the middle. The secondary winding 5 is divided into 4 segments according to the number of turns. Two segments are wound on the upper and lower straight sections of the iron core 6 between the C-phase conductor 3 and the B-phase conductor 2, respectively. The other two segments are wound on the upper and lower straight sections of the iron core 6 between the B-phase conductor 2 and the A-phase conductor 1, respectively.

[0013] Furthermore, the multiple voltage divider resistors connected in series at the output terminal of the secondary winding 5 convert the zero-sequence current signal into a voltage output. The multiple taps and a common terminal of the voltage divider resistors are respectively connected to the grounding protection device, and the taps are automatically selected as needed to detect the zero-sequence current.

[0014] Beneficial effects:

[0015] ① When the three-phase load current is relatively large, or the three-phase current asymmetry is significant (in the most severe case, a single-phase circuit is broken), the secondary winding will output a large residual current. When a single-phase ground 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. Since grounding protection devices mainly rely on the phase of the zero-sequence current for judgment, this will seriously affect the judgment of the direction of the ground fault. Adding a magnetic shielding layer greatly reduces the residual current value and avoids the influence of the residual current on the normal zero-sequence current signal. Especially when there is a high-resistance ground fault, the zero-sequence current is extremely small, but the load current is very large and one phase is broken (the load current often increases when a phase is broken). The residual current may greatly affect the phase of the output current (which is the zero-sequence current for the grounding protection device).

[0016] ② The use of resistor voltage divider output improves the resolution of weak zero-sequence current (high-resistance grounding);

[0017] ③ It has a simple structure and low cost. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a zero-sequence current transformer in the forward direction;

[0019] Figure 2 This is an axial schematic diagram of a zero-sequence current transformer.

[0020] Figure 3 This is a cross-sectional view of a zero-sequence current transformer along axis AA.

[0021] Figure 4 This is an axial view of a zero-sequence current transformer after disassembly.

[0022] Figure 5 This is the equivalent circuit diagram of a zero-sequence current transformer.

[0023] In the diagram: 1-Phase A primary conductor; 2-Phase B primary conductor; 3-Phase C primary conductor; 41-Upper outer shielding layer; 42-Lower outer shielding layer; 43-Lower inner shielding layer; 44-Upper inner shielding layer; 45-Insulating pad; 5-Secondary winding; 6-Iron core. Detailed Implementation

[0024] This zero-sequence current transformer includes: a secondary winding 5 wound on the core 6 of the zero-sequence current transformer; the secondary winding 5 is connected to voltage dividing resistors R1 to R3; the voltage dividing resistors have multiple taps, and the voltage output from each tap can be simultaneously acquired, with the voltage signal output from the tap with the highest amplitude but not exceeding the limit being used as the standard. Simultaneously, the secondary winding 5 is equally divided into four parts 51, 52, 53, and 54. 51 and 52 are wound on the upper and lower straight sections of the core 6 between the C-phase conductor 3 and the B-phase conductor 2, respectively; 53 and 54 are wound on the upper and lower straight sections of the core 6 between the B-phase conductor 2 and the A-phase conductor 1, respectively (see appendix). Figure 4 Finally, the two ends are connected in series, which can also reduce the residual current output.

[0025] This zero-sequence current transformer also includes: a magnetic shielding layer 4 covering the outside of the iron core 6 and the secondary winding 5. The magnetic shielding layer 4 is separated by an insulating pad 45, forming a non-closed magnetic circuit in the orthogonal direction of the main magnetic circuit of the iron core, but a closed magnetic circuit along the direction of the main magnetic circuit of the iron core (see appendix). Figure 3 ):

[0026] (1) Phase A primary conductor 1- constitutes a three-phase primary circuit;

[0027] (2) Phase B primary conductor 2- constitutes a three-phase primary circuit;

[0028] (3) Phase C primary conductor 3- constitutes a three-phase primary circuit;

[0029] (4) Magnetic shielding layer;

[0030] (41) Upper outer shielding layer 41 - constitutes a magnetic shielding layer (high initial magnetic permeability material such as nickel-iron alloy).

[0031] (42) Lower outer shielding layer 42 - constitutes a magnetic shielding layer (high initial magnetic permeability material such as nickel-iron alloy);

[0032] (43) Lower inner shielding layer 43 - constitutes a magnetic shielding layer (high initial magnetic permeability material such as nickel-iron alloy).

[0033] (44) Upper inner shielding layer 44 - constitutes a magnetic shielding layer (high initial magnetic permeability material such as nickel-iron alloy).

[0034] (45) Insulating pad 45 - Blocking magnetic shielding layer forms a closed conductive circuit in the plane orthogonal to the main magnetic flux;

[0035] (5) Secondary winding 5 - output zero-sequence current, connected to voltage divider resistors R1 to R3;

[0036] (51) Secondary winding, first part 51 - for reducing residual current;

[0037] (52) Second part of the secondary winding 52 - for reducing residual current;

[0038] (53) Secondary winding, third part 53 - for reducing residual current;

[0039] (54) Secondary winding, fourth part 54 - for reducing residual current;

[0040] (6) The iron core 6- constitutes the zero-sequence magnetic flux main magnetic circuit. It uses a ferromagnetic material with a lower initial permeability than the magnetic shielding layer but a higher initial permeability than ordinary silicon steel sheet material. Generally, it uses a microcrystalline permeability material.

[0041] The following is in conjunction with the instruction manual appendix. Figure 1 ~Instruction manual attached Figure 5 The present invention will be further explained in detail with reference to specific embodiments: the electromagnetic working principle of the zero-sequence current transformer.

[0042] During normal operation of the power grid, the conductors of each phase in the three-phase circuit are insulated from the ground, but there is distributed capacitance to the ground (due to the long length of the line). Under normal operation, because the three-phase voltages to ground are symmetrical, the ground currents generated by each phase are also symmetrical, and their vector sum is zero, so the zero-sequence current is also zero. When a single-phase ground fault occurs, the voltage of the grounded phase decreases (the decrease is related to the grounding resistance and the line capacitance to ground), and the voltages of the two ungrounded phases increase. The ground currents generated by each phase are no longer symmetrical, and their vector sum is no longer zero, resulting in a zero-sequence current output. The core of the zero-sequence current transformer is wrapped around the three-phase primary circuit, and the secondary side induces a zero-sequence current value of 3 times.

[0043] Because the positions of the three-phase conductors relative to the iron core are not symmetrical (especially in racetrack-type iron cores), the current induced in the iron core by each phase cannot have the same amplitude and a 120° phase difference, resulting in a non-zero vector sum and the formation of 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; the larger the grounding resistance, the smaller the zero-sequence current). When a single-phase ground fault occurs, this residual current is superimposed (vector sum) with the true zero-sequence current, causing the detected current signal to be not the true zero-sequence current signal (most seriously, the phase changes). Grounding protection devices precisely need the phase and amplitude of the zero-sequence current to determine the line and direction of the ground fault. Therefore, it is evident that residual current has a very serious impact on grounding protection.

[0044] The equivalent principle of a zero-sequence current transformer is shown in the appendix. Figure 5 This diagram shows the equivalent current loops of each phase separately and then superimposes them together for analysis. In the diagram: X Aσ X Bσ、 X Cσ These are the equivalent leakage reactances of the three-phase primary circuits A, B, and C, respectively, r A1 r B1 rC1 These are the equivalent resistances of the primary circuits of each phase, r. Am、 r Bm、 r Cm These are the equivalent resistances of the excitation circuits for each phase (although the three phases share a single magnetic circuit, their equivalent resistances will differ slightly), X Am、 X Bm、 X Cm These are the equivalent inductive reactances of each phase's excitation circuit (the three phases share a single magnetic circuit, but the equivalent inductive reactances will vary slightly), r A2 r B2 r C2 These are the equivalent resistances of the secondary circuits of each phase, X. A2 X B2、 X C2 These are the equivalent leakage reactances of the three-phase secondary circuits A, B, and C, respectively, and R1 to R3 are the voltage sampling and voltage dividing resistors of the secondary output circuit, respectively.

[0045] Depend on Figure 5 It can be seen that r A1 r B1 r C1 Each phase can be essentially equal (because the primary circuit resistance is extremely small), since each phase shares a single secondary winding r. A2 r B2 r C2 They are also completely equal, r Am、 r Bm、 r Cm Since they share the same iron core, the difference is not significant. However, X Aσ X Bσ、 X Cσ The differences between the phases can be quite large, X A2 X B2、 X C2 The differences between the phases can also be significant, X Am、 X Bm、 X Cm The phases differ significantly (due to the asymmetry of the primary conductors relative to the iron core). Therefore, it can be seen that even if the three-phase currents are perfectly symmetrical, an unbalanced current (residual current) will still 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 Cm They are also equal, meaning the leakage reactance of the primary circuit is equal, the leakage reactance of the secondary circuit is equal, and the magnetizing reactance is equal, so that I... LThe residual current I is zero (the differences in effective resistance between phases are negligible). The ultimate goal of this invention is to ensure that the residual current I is zero regardless of the load current I. L Keep it as small as possible to minimize its impact on the zero-sequence current.

[0046] Key structural features of a zero-sequence current transformer:

[0047] ① On the basis of the original zero-sequence current transformer, a layer of ferromagnetic material magnetic shielding layer 4 is wrapped 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.

[0048] ② To prevent magnetic saturation of the magnetic shielding layer, a double-layer shielding (inner and outer layers) is used. One layer has a higher magnetic saturation inflection point, but allows for a slightly higher initial permeability. The other layer has a lower magnetic saturation inflection point, but requires a high initial permeability. This avoids magnetic saturation while meeting the requirement of a high initial permeability.

[0049] ③ Since the material used for the magnetic shielding layer is also a conductor, it covers the outside of the iron core 6 and the secondary winding 5, forming a new secondary circuit that is essentially connected in parallel with the original secondary winding 5. This is equivalent to the original secondary circuit being short-circuited and unable to output normal zero-sequence current. By placing an insulating pad 45 on the magnetic shielding layer 4 in the direction of the orthogonal plane of the main magnetic flux to isolate the conductive closed circuit, it no longer forms another short-circuited secondary winding and can only operate in an open circuit.

[0050] ④ Divide the secondary winding into 4 equal parts and wind them into the upper and lower straight sections of the iron core 6 in the middle of phases A and B and the middle of phases B and C respectively. This reduces the degree of asymmetry and can also effectively reduce the residual current output.

[0051] ⑤ Insert three resistors in series in the secondary circuit to convert the zero-sequence current signal into a voltage output. Then, lead out three taps and a common terminal and connect them to the grounding protection device. The taps are automatically selected as needed to detect the zero-sequence current, ensuring sufficient zero-sequence current resolution.

[0052] Working principle of zero-sequence current transformer:

[0053] ①See appendix Figure 3 The leakage flux generated by the phase currents that did not pass through the main magnetic circuit will not flow through the same magnetic circuit in space, thus causing inconsistencies in the leakage reactance between the primary and secondary circuits. See Appendix. Figure 5 Even if the three sides are symmetrical There will also be residual current I LOutput. With the overall magnetic shielding layer 4 consisting of the upper outer shielding layer 41, lower outer shielding layer 42, lower inner shielding layer 43, and upper inner shielding layer 44, since the initial permeability of the magnetic shielding layer 4 is more than 100 times that of non-magnetic materials such as air, even very weak leakage flux will be concentrated within the magnetic shielding layer 4. The vast majority of the leakage flux generated by the three-phase current will form a closed magnetic circuit through the magnetic shielding layer 4, increasing the magnetic induction intensity by nearly 100 times. Because the three phases share a single magnetic shielding circuit with the same magnetic reluctance, the leakage reactance (X) of each phase's primary circuit is... Aσ X Bσ、 X Cσ ), leakage reactance of each phase secondary circuit (X) A2 X B2、 X C2 The results are basically the same, since the three phases share a single iron core 6, and the excitation reactance of each phase is X. Am、 X Bm、 X Cm It is also very close, ultimately resulting in the residual current output I. L The voltage will be very small, and even when superimposed 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.

[0054] ② See Appendix Figure 3 The insulating pad 45 blocks the closed conductive loop formed by the magnetic shielding layer 4 in the direction of the main magnetic flux orthogonal plane, so that the second secondary winding formed by the magnetic shielding layer can only be open-circuited and not short-circuited, and no longer affects the output of the secondary winding 5.

[0055] 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 disconnected and accompanied by a ground fault, the secondary winding 5 is closer to the primary conductor and is symmetrically distributed, which can effectively reduce the residual current output.

[0056] The above description is only a specific embodiment of this utility model, but the protection scope of this utility model is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this utility model should be included within the protection scope of this utility model.

Claims

1. A zero-sequence current transformer, characterized in that, The system includes an iron core (6) made of ferromagnetic material with high initial permeability, a secondary winding (5) wound on the iron core (6), and a magnetic shielding layer (4) covering the outer side of the secondary winding (5). The initial 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 divided into several equal parts and then connected in series. Multiple voltage divider resistors are connected in series at the output end of the secondary winding (5). The voltage divider resistors have multiple taps. The voltage output of each tap is collected as needed, and the voltage signal output by the tap with the highest amplitude but not exceeding the limit is used as the standard.

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 permeability materials, one of which has a higher initial permeability than the other, but a lower magnetic saturation inflection point.

3. A zero-sequence current transformer according to claim 1, characterized in that, The conductive circuit formed by the magnetic material in the magnetic shielding layer (4) on the orthogonal plane of the main magnetic flux is isolated by the insulating pad (45) and no longer constitutes another short-circuited secondary winding (5). 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 iron core (6) has A-phase conductor (1), B-phase conductor (2) and C-phase conductor (3) running side by side in the middle. The secondary winding (5) is divided into 4 sections according to the number of turns. Two sections are wound on the upper and lower straight parts of the iron core (6) between the C-phase conductor (3) and the B-phase conductor (2), respectively. The other two sections are wound on the upper and lower straight parts of the iron core (6) between the B-phase conductor (2) and the A-phase conductor (1), respectively.

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

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