Design method of current transformer

Abstract

The invention discloses a design method of a current transformer. The current transformer comprises an iron core, a secondary winding and a framework, the design method comprises the following steps: acquiring the induced potential of a secondary winding, the effective sectional area of an iron core, the frequency of the current transformer and the first number of turns of the secondary winding; calculating the magnetic flux density of the current transformer according to the induced potential, the effective sectional area, the frequency and the first number of turns; when the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the framework so as to control the magnetic flux density to be within the first preset range; wherein the first preset size comprises a first inner diameter, a first outer diameter and a first height of the framework; and when the error of the current transformer is not within the second preset range, the exciting current is changed by adjusting the first preset size of the framework so as to control the error to be within the second preset range. By the adoption of the technical scheme, the design requirement can be met only by adjusting the size of the framework, the scheme is simple, and cost is low.

Description

Technical Field

[0001] The present invention relates to the technical field of current transformers, and in particular to a design method of a current transformer. Background Art

[0002] The measuring current transformer converts the large current of the power system into a small current of 5A or 1A, and together with the electric energy meter, it forms the current measurement circuit of the electric energy metering device. Its performance is directly related to the accuracy of electric energy metering and affects the fairness of electric energy trade settlement. The higher the accuracy level of the measuring current transformer, the better, and the smaller the instrument safety factor, the better. In my country's ultra-high voltage power system, the standard stipulates that the accuracy of the measuring-level current transformer is 0.2S and the instrument safety factor is FS5. In the same current transformer, to improve the accuracy level, the magnetic flux density must be reduced, but the reduction of magnetic flux density will increase the saturation multiple of the core, and the instrument safety factor will also increase accordingly.

[0003] The existing current transformers that can simultaneously meet the requirements of high-precision level and instrument safety factor have complex designs and high costs. Summary of the invention

[0004] The invention provides a design method of a current transformer to solve the problem that the existing current transformer which can simultaneously meet the requirements of high precision level and instrument safety factor has a complex design scheme and high cost.

[0005] According to one aspect of the present invention, a design method of a current transformer is provided, wherein the current transformer comprises: a secondary winding and a frame; the secondary winding comprises an iron core;

[0006] The design method comprises:

[0007] Obtaining the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer and the first number of turns of the secondary winding;

[0008] Calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns;

[0009] When the magnetic flux density is not within a first preset range, the induced potential is changed by adjusting a first preset size of the skeleton to control the magnetic flux density to be within the first preset range; wherein the first preset size includes a first inner diameter, a first outer diameter, and a first height of the skeleton;

[0010] When the error of the current transformer is not within a second preset range, the excitation current of the current transformer is changed by adjusting the first preset size of the skeleton to control the error to be within the second preset range.

[0011] Optionally, obtaining the effective cross-sectional area of ​​the core includes:

[0012] Calculate the first cross-sectional area of ​​the core according to the second preset size of the core; wherein the second preset size includes the second inner diameter, the second outer diameter and the second height of the core;

[0013] The effective cross-sectional area is determined according to the first cross-sectional area and a filling factor.

[0014] Optionally, obtaining the induced potential of the secondary winding includes:

[0015] Obtaining the total impedance of the secondary circuit and the rated secondary current of the current transformer;

[0016] The induced potential of the secondary winding is determined according to the total impedance and the rated secondary current.

[0017] Optionally, the current transformer further comprises: a compensation winding; there is no electromagnetic induction between the compensation winding and the secondary winding;

[0018] Obtaining the total impedance of the secondary circuit includes:

[0019] Obtaining a first resistance of the secondary winding and a second resistance of the compensation winding;

[0020] determining a total resistance of the secondary circuit according to the first resistance and the second resistance;

[0021] Obtaining the active component of the secondary load, the reactive component of the secondary load and the leakage reactance of the secondary circuit;

[0022] The total impedance of the secondary circuit is calculated according to the total resistance, the active component, the reactive component and the leakage reactance.

[0023] Optionally, the secondary winding comprises a secondary winding wire;

[0024] Obtaining a first resistance of the secondary winding includes:

[0025] Calculating a second cross-sectional area of ​​the secondary winding conductor according to the rated secondary current and the current density of the secondary winding;

[0026] Determining a wire diameter of the secondary winding wire according to the second cross-sectional area;

[0027] Determining a third cross-sectional area of ​​the secondary winding wire according to the wire diameter;

[0028] Calculating a first turn length of the secondary winding according to a second preset size of the core; the second preset size includes a second inner diameter, a second outer diameter and a second height of the core;

[0029] determining a first total turn length of the secondary winding according to a product of the first turn length and a first number of turns of the secondary winding;

[0030] The first resistance is calculated according to the third cross-sectional area and the first total turn length.

[0031] Optionally, the compensation winding comprises a compensation winding conductor;

[0032] Obtaining the second resistance of the compensation winding includes:

[0033] Calculating a second turn length of the compensation winding according to a first preset size of the frame;

[0034] Calculating a second total turn length of the compensation winding according to the second turn length and a second number of turns of the compensation winding;

[0035] Obtaining a fourth cross-sectional area of ​​the compensation winding conductor;

[0036] A second resistor is determined according to the fourth cross-sectional area and the second total turn length.

[0037] Optionally, when the error of the current transformer is not within a second preset range, changing the excitation current of the current transformer by adjusting the first preset size of the skeleton to control the error to be within the second preset range includes:

[0038] When the error of the current transformer is not within the second preset range, adjusting the first preset size of the frame to change the excitation current of the current transformer to adjust the error of the current transformer;

[0039] When the error of the current transformer is still not within the second preset range, the second preset size of the iron core is adjusted to change the excitation current of the current transformer to control the error to be within the second preset range.

[0040] Optionally, when the error of the current transformer is not within a second preset range, changing the excitation current of the current transformer by adjusting the first preset size of the skeleton to control the error to be within the second preset range includes:

[0041] When at least one of the ratio difference and the phase difference of the current transformer is not within the second preset range, the first preset size of the skeleton is adjusted to change the excitation current of the current transformer to control the error to be within the second preset range.

[0042] Optionally, calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns includes:

[0043] The magnetic flux density is calculated according to the following magnetic flux density formula;

[0044]

[0045] Among them, B represents the magnetic flux density of the current transformer, E represents the induced potential of the secondary winding, f represents the frequency of the current transformer, N represents the first number of turns of the secondary winding, and A represents the effective cross-sectional area of ​​the iron core.

[0046] Optionally, calculating the total impedance of the secondary circuit according to the total resistance, the active component, the reactive component and the leakage reactance includes:

[0047] Calculate the total impedance of the secondary circuit according to the following total impedance formula of the secondary circuit;

[0048]

[0049] Where Z2 represents the total impedance of the secondary circuit, R f represents the active component of the secondary load, X f represents the reactive component of the secondary load, r represents the total resistance of the secondary circuit, and X represents the leakage reactance of the secondary circuit.

[0050] In an embodiment of the present invention, the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer and the first number of turns of the secondary winding are first obtained, and the magnetic flux density of the current transformer is calculated according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns. According to the magnitude of the magnetic flux density, a suitable first preset size of the skeleton is selected so that the designed current transformer can meet the safety factor requirements while ensuring the measurement accuracy. By adopting the above technical solution, it is only necessary to adjust the size of the skeleton to meet the design requirements, and the solution is simple and low-cost.

[0051] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0053] Figure 1 is a schematic diagram of the structure of a first current transformer provided according to an embodiment of the present invention;

[0054] Figure 2 is a flow chart of a first current transformer design method provided according to an embodiment of the present invention;

[0055] Figure 3 is a flow chart of a second current transformer design method provided according to an embodiment of the present invention;

[0056] Figure 4 is a flow chart of a third current transformer design method provided according to an embodiment of the present invention;

[0057] Figure 5 is a flow chart of a fourth current transformer design method provided according to an embodiment of the present invention;

[0058] Figure 6 is a flow chart of a fifth current transformer design method provided according to an embodiment of the present invention;

[0059] Figure 7 is a flow chart of a sixth current transformer design method provided according to an embodiment of the present invention;

[0060] Figure 8 4 is a schematic diagram of the structure of a second current transformer provided according to an embodiment of the present invention. DETAILED DESCRIPTION

[0061] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0062] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, any variation of the terms "including" and "having" is intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0063] Figure 1 It is a schematic diagram of the structure of a first current transformer provided according to an embodiment of the present invention. Figure 21 is a flow chart of a first current transformer design method according to an embodiment of the present invention. This embodiment is applicable to designing a current transformer that meets both high-precision grade and instrument safety factor requirements. Figure 1 As shown, the current transformer includes: a secondary winding 2 and a frame 3; the secondary winding 2 includes an iron core 1.

[0064] like Figure 2 As shown, the design method based on the above structure includes:

[0065] S110, obtaining the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer, and the first number of turns of the secondary winding.

[0066] Specifically, Figure 1 As shown, the current transformer is a special transformer that converts the primary current into a secondary current in proportion to the turns ratio of the primary winding (not shown in the figure) and the secondary winding 2 according to the principle of electromagnetic induction. In the power system, the current transformer often has a large overcurrent flowing through the primary winding. In order to prevent the instrument of the secondary circuit of the current transformer from being impacted by the large current, it is hoped that the current of the secondary winding 2 of the measuring current transformer will no longer increase strictly in proportion under the overcurrent condition, and the instrument safety factor (FS) requirement is proposed. Based on this, the design of the current transformer needs to meet the requirements of the instrument safety factor while ensuring the measurement accuracy. It should be noted that the instrument safety factor of the current transformer refers to the ratio of the instrument safety current to the rated primary current, and the instrument safety current refers to the minimum primary current of the measuring current transformer under the rated secondary load, whose composite error is not less than 10%. To improve the accuracy level in the same current transformer, the magnetic flux density must be reduced, but the reduction in magnetic flux density will increase the saturation multiple of the core 1, and the instrument safety factor will also increase accordingly, which may cause the instrument safety factor to be greater than the recommended value (such as 5 or 10), thereby reducing the safety of the instrument. Therefore, when designing a current transformer, it is necessary to reasonably control the value of the magnetic flux density. Further, to calculate the magnetic flux density of the current transformer, first obtain the induced potential of the secondary winding 2, the effective cross-sectional area of ​​the core 1, the frequency of the current transformer, and the first number of turns of the secondary winding 2, where the induced potential refers to the magnetic coupling of the primary winding and the secondary winding 2 through the core 1. When an AC voltage is applied to the primary winding, according to the principle of electromagnetic induction, an alternating magnetic field will be generated in the core 1, and the magnetic field will pass through the secondary winding 2, thereby generating an induced potential in the secondary winding 2. The frequency of the current transformer is usually 50Hz or 60Hz, which is the frequency of the AC power grid in the area. The first number of turns of the secondary winding 2 refers to the number of turns of the coil used to convert the current size in the current transformer.

[0067] S120. Calculate the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency, and the first number of turns.

[0068] After obtaining the induced potential, effective cross-sectional area, frequency and the first number of turns, the magnetic flux density formula is entered as follows:

[0069]

[0070] The magnetic flux density of the current transformer can be calculated.

[0071] Among them, B represents the magnetic flux density of the current transformer, E represents the induced potential of the secondary winding 2, f represents the frequency of the current transformer, N represents the first number of turns of the secondary winding 2, and A represents the effective cross-sectional area of ​​the core 1.

[0072] S130. When the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the skeleton to control the magnetic flux density to be within the first preset range.

[0073] The first preset size includes the first inner diameter, the first outer diameter and the first height of the skeleton 3; the first preset size is the size estimated by the designer based on experience. In addition, the skeleton 3 selects a non-ferromagnetic material commonly used epoxy laminated glass cloth board. The advantage of such a setting is that it will not cause electromagnetic interference to the secondary winding 2, ensuring that the current transformer can work accurately and reliably.

[0074] Specifically, Figure 1 As shown, the first preset range can be the range that the magnetic flux density needs to meet when the safety factor of the current transformer is met. Exemplarily, when the material of the core 1 of the current transformer is a nanocrystalline alloy, the saturation magnetic flux density of this material is 1.2T. If the instrument safety factor Fs is required to be 5, that is, the current transformer can reach saturation at 5 times the rated primary current, then the first preset range can be 0.24T~0.6T, that is, the magnetic flux density of the current transformer under the rated primary current needs to be controlled between 0.24T~0.6T.

[0075] According to the magnetic flux density formula, the induced potential is positively correlated with the magnetic flux density. Adjusting the size of the skeleton 3 can change the total resistance of the secondary circuit, and then change the total impedance of the secondary circuit. The total impedance is positively correlated with the induced potential. Therefore, adjusting the size of the skeleton 3 is equivalent to changing the induced potential, thereby controlling the size of the magnetic flux density.

[0076] Exemplarily, when the magnetic flux density is lower than 0.24 T, the magnetic flux density can be controlled to be between 0.24 T and 0.6 T by increasing the first preset size of the skeleton 3, such as reducing the first inner diameter, increasing the first outer diameter, or increasing the first height. When the magnetic flux density is higher than 0.6 T, the magnetic flux density can be controlled to be between 0.24 T and 0.6 T by reducing the first preset size of the skeleton 3, such as increasing the first inner diameter, reducing the first outer diameter, or reducing the first height.

[0077] S140. When the error of the current transformer is not within the second preset range, the excitation current is changed by adjusting the first preset size of the skeleton to control the error to be within the second preset range.

[0078] Specifically, the accuracy level of the current transformer is an important indicator to measure its measurement accuracy, which reflects the measurement error of the current transformer for the primary current in actual operation. The accuracy level is usually expressed in percentage. The smaller the value, the smaller the measurement error of the current transformer and the higher the accuracy. The excitation current is the current used to generate the magnetic field in the core 1 in the current transformer. When the magnetic flux density is small, the magnetic field strength in the core 1 is weak, and the required excitation current is also small. The existence of the excitation current will cause the error of the current transformer. The excitation current will generate an additional magnetic field in the core 1, and the additional magnetic field will interact with the magnetic field in the core 1 to produce errors. Therefore, a smaller excitation current will have a smaller error and higher accuracy. The second preset range refers to the standard error range, which is defined according to the accuracy level of the current transformer. Different accuracy levels correspond to different standard error ranges. When the error of the current transformer exceeds the second preset range, this usually means that the error of the current transformer is large and the accuracy is low. At this time, it is necessary to reduce the magnetic flux density, thereby reducing the excitation current and improving the error. The error can be reduced and the accuracy can be improved by reducing the first preset size of the skeleton 3, reducing the magnetic flux density, and lowering the exciting current.

[0079] Exemplarily, when the magnetic flux density of the current transformer is 0.5T, although it is within the first preset range, the error of the current transformer may not be within the second preset range. When it is within the first preset range, but the error is not within the second preset range, reduce the first preset size of the skeleton 3, reduce the magnetic flux density and then reduce the excitation current to reduce the error and increase the accuracy. In fact, in order to meet the accuracy level requirements, the smaller the magnetic flux density, the better, but because it also needs to meet the instrument safety factor requirements. Therefore, when adjusting the first preset size and reducing the magnetic flux density, the magnetic flux density cannot be lower than 0.24T.

[0080] In an embodiment of the present invention, the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer and the first number of turns of the secondary winding are first obtained, and the magnetic flux density of the current transformer is calculated according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns. According to the magnitude of the magnetic flux density, a suitable first preset size of the skeleton is selected so that the designed current transformer can meet the safety factor requirements while ensuring the measurement accuracy. By adopting the above technical solution, it is only necessary to adjust the size of the skeleton to meet the design requirements, and the solution is simple and low-cost.

[0081] Figure 3FIG. 1 is a flow chart of a second current transformer design method provided according to an embodiment of the present invention. This embodiment further explains how to obtain the effective cross-sectional area of ​​the core. Figure 3 As shown, the method includes:

[0082] S210, calculating a first cross-sectional area of ​​the core according to a second preset size of the core.

[0083] The second preset size includes the second inner diameter, the second outer diameter and the second height of the core 1;

[0084] Specifically, the designer estimates the second inner diameter, the second outer diameter and the second height of the core based on experience, and calculates the first cross-sectional area of ​​the core 1 , ie, the apparent cross-sectional area, based on the second inner diameter, the second outer diameter and the second height.

[0085] S220. Determine an effective cross-sectional area according to the first cross-sectional area and the filling factor.

[0086] Specifically, the filling factor is a correction factor that takes into account factors such as internal gaps, defects, and uneven material density of the core 1. The filling factor is usually less than 1, which reflects the compactness and uniformity of the actual material of the core 1. By multiplying the apparent cross-sectional area by the filling factor, an effective cross-sectional area that is closer to the actual situation can be obtained. For example, the core 1 can be a nanocrystalline material, in which case the filling factor is 0.75.

[0087] S230: Obtain the induced potential of the secondary winding, the frequency of the current transformer, and the first number of turns of the secondary winding.

[0088] S240, calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns.

[0089] S250. When the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the skeleton to control the magnetic flux density to be within the first preset range.

[0090] S260. When the error of the current transformer is not within the second preset range, the excitation current is changed by adjusting the first preset size of the skeleton to control the error to be within the second preset range.

[0091] In an embodiment of the present invention, the first cross-sectional area is first obtained according to the second size of the iron core, and then, considering the influence of factors such as internal gaps, defects and uneven material density of the iron core, the effective cross-sectional area is calculated according to the first cross-sectional area and the filling factor, thereby improving the accuracy of subsequent calculation of the magnetic flux density and further ensuring the rationality and feasibility of the current transformer design.

[0092] Figure 44 is a flow chart of a third current transformer design method provided according to an embodiment of the present invention. The embodiment of the present invention further describes how to obtain the induced potential of the secondary winding.

[0093] S310. Obtain the total impedance of the secondary circuit and the rated secondary current of the current transformer.

[0094] Specifically, the total impedance of the secondary circuit refers to the impedance presented by the secondary side (i.e., the output side) of the current transformer. The rated secondary current of the current transformer refers to the maximum current value that the secondary side can stably output under the specified working conditions (such as rated frequency, rated voltage, etc.). The rated secondary current can be 5A or 1A.

[0095] S320. Determine the induced potential of the secondary winding according to the total impedance and the rated secondary current.

[0096] Specifically, the total impedance and the rated secondary current are substituted into the induced potential formula: E=Z2I2 to calculate the induced potential of the secondary winding 2. Among them, E represents the induced potential of the secondary winding 2, Z2 represents the total impedance of the secondary circuit, and I2 represents the rated secondary current.

[0097] S330, obtaining an effective cross-sectional area of ​​the iron core, a frequency of the current transformer, and a first number of turns of the secondary winding.

[0098] S340, calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns.

[0099] S350. When the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the skeleton to control the magnetic flux density to be within the first preset range.

[0100] S360. When the error of the current transformer is not within the second preset range, the excitation current is changed by adjusting the first preset size of the skeleton to control the error to be within the second preset range.

[0101] The embodiment of the present invention first obtains the total impedance of the secondary circuit and the rated secondary current of the current transformer, and then determines the induced potential of the secondary winding according to the total impedance and the rated secondary current, providing a basis for subsequent calculation of the magnetic flux density, and further ensuring the rationality and feasibility of the current transformer design.

[0102] Figure 5 FIG. 1 is a flow chart of a fourth current transformer design method provided according to an embodiment of the present invention. Based on the above embodiment, this embodiment further explains how to obtain the total impedance of the secondary circuit. Figure 1As shown, the current transformer further includes a compensation winding 4 . Since the frame 3 is made of non-ferromagnetic material, there is no electromagnetic induction phenomenon between the compensation winding 4 and the secondary winding 2 .

[0103] like Figure 5 As shown, based on the above structure, the design method includes:

[0104] S400: Obtain a first resistance of the secondary winding and a second resistance of the compensation winding.

[0105] Specifically, Figure 1 As shown, a compensating winding 4 is added to the current transformer, and the compensating winding 4 is wound on the frame 3. In addition, when the secondary winding 2 is a single transformation ratio, the secondary winding 2 and the compensating winding 4 are arranged in accordance with: Figure 1 The second end S2 of the secondary winding 2 is connected to the first end N1 of the compensation winding 4. The first end S1 of the secondary winding 2 and the second end N2 of the compensation winding 4 are used as lead wires. Based on this structure, the total resistance of the secondary loop can be the sum of the first resistance and the second resistance. Therefore, before calculating the total resistance of the secondary loop, it is necessary to first obtain the first resistance of the secondary winding 2 and the second resistance of the compensation winding 4.

[0106] Further, the secondary winding 2 includes a secondary winding wire; obtaining the first resistance of the secondary winding includes:

[0107] The second cross-sectional area of ​​the secondary winding wire is calculated according to the rated secondary current and the current density of the secondary winding 2, the wire diameter of the secondary winding wire is determined according to the second cross-sectional area, the third cross-sectional area of ​​the secondary winding wire is determined according to the wire diameter, the first turn length of the secondary winding 2 is calculated according to the second preset size of the core 1, the second preset size includes the second inner diameter, the second outer diameter and the second height of the core 1, the first total turn length of the secondary winding 2 is determined according to the product of the first turn length and the first number of turns of the secondary winding 2, and the first resistance is calculated according to the third cross-sectional area and the first total turn length.

[0108] For example, the material of the secondary winding wire can be copper, and the current density of copper is 2A / mm 2 , the current density of the secondary winding 2 can be the copper current density 2A / mm 2 , the rated secondary current is 5A. At this time, the theoretical cross-sectional area of ​​the secondary winding wire can be calculated by the ratio of the rated secondary current to the current density, which is 2.5mm 2 That is, the second cross-sectional area of ​​the secondary winding wire. In order to ensure that the secondary winding 2 can safely carry the expected maximum secondary current, the actual cross-sectional area of ​​the secondary winding wire, that is, the third cross-sectional area, usually needs to be larger than the theoretical cross-sectional area. Therefore, the appropriate wire diameter of the secondary winding wire can be determined based on the second cross-sectional area. For example, the wire diameter can be 1.8mm, and the calculated third cross-sectional area is 2.54mm 2 Greater than 2.5mm2 .

[0109] At the beginning of the design of the current transformer, the designer will estimate the size of the core based on experience, that is, the second preset size. The second preset size includes the second inner diameter, the second outer diameter and the second height of the core 1. According to the second inner diameter, the second outer diameter and the second height of the core 1, the length of each turn of the secondary winding 2, that is, the first turn length, can be calculated. The rated transformation ratio refers to the current ratio between the primary winding and the secondary winding 2. In order to achieve the required rated transformation ratio, the designer needs to select a suitable first number of turns of the secondary winding 2. For example, when the primary current is 800A and the secondary current is 5A, the number of turns of the primary winding is 1, and the first number of turns of the secondary winding 2 is 160. The first total turn length of the secondary winding 2 can be determined by multiplying the first turn length by the first number of turns of the secondary winding 2, and then the third cross-sectional area and the first total turn length are substituted into the first resistance calculation formula:

[0110]

[0111] The first resistance can be calculated, wherein R1 represents the first resistance of the secondary winding 2, ρ represents the resistivity of the secondary winding wire material, L1 represents the first total turn length of the secondary winding 2, A3 represents the third cross-sectional area of ​​the secondary winding 2, and K represents the winding coefficient.

[0112] Further, the compensation winding 4 includes a compensation winding wire; and obtaining the second resistance of the compensation winding includes:

[0113] The second turn length of the compensation winding 4 is calculated according to the first preset size of the skeleton 3, the second total turn length of the compensation winding is calculated according to the second turn length and the second number of turns of the compensation winding, the fourth cross-sectional area of ​​the compensation winding wire is obtained, and the second resistance is determined according to the fourth cross-sectional area and the second total turn length.

[0114] Specifically, the designer obtains the length of each turn of the compensation winding 4, i.e., the second turn length, based on the estimated second inner diameter, second outer diameter, and second height. The material of the compensation winding 4 may be copper. The second number of turns of the compensation winding 4 may be set arbitrarily according to actual conditions. The total turn length of the compensation winding 4 is calculated based on the product of the second turn length and the second number of turns. The fourth cross-sectional area of ​​the compensation winding conductor may be set arbitrarily according to actual conditions. For example, the fourth cross-sectional area of ​​the compensation winding conductor may be set equal to the third cross-sectional area of ​​the secondary winding conductor. Substitute the fourth cross-sectional area and the second total turn length into the second resistance calculation formula:

[0115]

[0116] The second resistance can be calculated, wherein R2 represents the second resistance of the compensation winding 4, ρ represents the resistivity of the compensation winding wire material, L2 represents the first total turn length of the compensation winding 4, A4 represents the fourth cross-sectional area of ​​the compensation winding 4, and K represents the winding coefficient.

[0117] It can be understood that when the final calculated magnetic flux density cannot meet the accuracy level requirements or the instrument safety factor requirements, the first preset size can be adjusted to change the second turn length of the compensation winding and the second resistance; the magnetic flux density can also be adjusted by adjusting the cross-sectional area of ​​the compensation winding conductor or the second number of turns of the compensation winding to change the second resistance.

[0118] S410 , determining a total resistance of the secondary circuit according to the first resistance and the second resistance.

[0119] Specifically, the total resistance of the secondary circuit is determined according to the sum of the first resistance and the second resistance.

[0120] S420, obtaining the active component of the secondary load, the reactive component of the secondary load and the leakage reactance of the secondary circuit.

[0121] Specifically, before designing the current transformer, the designer will obtain some rated parameters such as rated capacity and rated secondary current, and substitute the rated capacity and rated secondary current into the secondary load formula:

[0122]

[0123] Among them, Z represents the secondary load, S represents the rated capacity, and I represents the rated secondary current.

[0124] The secondary load can be determined. For example, when the rated capacity is 15VA and the rated secondary current is 5A, it can be calculated that the secondary load is 0.6Ω.

[0125] The active and reactive components are determined based on the secondary load and the phase difference.

[0126] Specifically, the secondary load and phase difference are substituted into the active component formula:

[0127]

[0128] Among them, R f represents the active component, Z represents the secondary load, Indicates the power factor of the active component. In addition, It is a constant usually equal to 0.8.

[0129] For example, when the secondary load is 0.6Ω, the active component can be calculated to be 0.48W.

[0130] Substituting the secondary load and phase difference into the reactive component formula:

[0131]

[0132] Among them, X frepresents the reactive component, Z represents the secondary load, Indicates the power factor of the reactive component. In addition, It is a constant usually equal to 0.6.

[0133] For example, when the secondary load is 0.6Ω, the active component can be calculated to be 0.36W

[0134] It should be noted that the leakage reactance of the secondary circuit can be determined according to the number of turns of the secondary current. The leakage reactance increases by 0.05Ω for every 200 turns. For example, when the number of turns is 400, the leakage reactance of the secondary circuit can be 0.1Ω, and when the number of turns is 600, the leakage reactance of the secondary circuit can be 0.15Ω. When the number of turns is 160 turns but less than 200 turns, the leakage reactance is generally 0.05Ω based on 200 turns.

[0135] S430, calculating the total impedance of the secondary circuit according to the total resistance, active component, reactive component and leakage reactance.

[0136] Specifically, the total resistance, active component, reactive component and leakage reactance are substituted into the following secondary circuit total impedance formula to calculate the secondary circuit total impedance;

[0137]

[0138] Where Z2 represents the total impedance of the secondary circuit, R f represents the active component of the secondary load, X f represents the reactive component of the secondary load, r represents the total resistance of the secondary circuit, and X represents the leakage reactance of the secondary circuit.

[0139] S440. Obtain the rated secondary current of the current transformer.

[0140] S450. Determine the induced potential of the secondary winding according to the total impedance and the rated secondary current.

[0141] S460, obtaining an effective cross-sectional area of ​​the iron core, a frequency of the current transformer, and a first number of turns of the secondary winding.

[0142] S470, calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns.

[0143] S480. When the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the skeleton to control the magnetic flux density to be within the first preset range.

[0144] S490. When the error of the current transformer is not within the second preset range, the excitation current is changed by adjusting the first preset size of the skeleton to control the error to be within the second preset range.

[0145] In summary, in the embodiment of the present invention, when the magnetic flux density is not within the first preset range, adjusting the first preset size of the skeleton can change the second resistance of the compensation winding, and the secondary loop impedance will also change accordingly, thereby changing the induced potential of the secondary winding, and further adjusting the magnetic flux density of the current transformer. That is to say, when designing a current transformer, when the current transformer cannot simultaneously meet the instrument safety factor and accuracy level requirements, it is only necessary to adjust the size of the skeleton to meet the design requirements. The solution is simple and low cost.

[0146] Figure 6 1 is a flow chart of a fifth current transformer design method provided according to an embodiment of the present invention. Based on the above embodiment, this embodiment further describes how to change the excitation current by adjusting the first preset size of the skeleton to control the error within the second preset range when the error of the current transformer is not within the second preset range. Figure 6 As shown, the design method of the current transformer includes the following steps:

[0147] S510, obtaining the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer, and the first number of turns of the secondary winding.

[0148] S520, calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns.

[0149] S530. When the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the skeleton to control the magnetic flux density to be within the first preset range.

[0150] S540, when the error of the current transformer is not within the second preset range, adjusting the first preset size of the skeleton to change the excitation current to adjust the error of the current transformer;

[0151] S550: When the error of the current transformer is still not within the second preset range, adjust the second preset size of the core to change the excitation current to control the error to be within the second preset range.

[0152] Specifically, the current transformer needs to be placed in a shielding tube. Due to the limitation of the shielding tube, the size of the skeleton 3 cannot be infinitely enlarged. In order to meet the error requirements while setting a reasonable size of the skeleton 3, when the size of the skeleton 3 is limited by the shielding tube, the second size of the core 1 can be fine-tuned to meet the accuracy level requirements of the current transformer.

[0153] In the embodiment of the present invention, when the frame size is limited, the second preset size of the core is adjusted to change the excitation current to control the error within the second preset range, thereby designing a current transformer with a more reasonable size.

[0154] Figure 71 is a flow chart of a sixth current transformer design method provided according to an embodiment of the present invention. Based on the above embodiment, this embodiment further describes how to adjust the first preset size of the skeleton to change the excitation current to control the error within the second preset range when the error of the current transformer is not within the second preset range. Figure 7 As shown, the design method of the current transformer includes the following steps:

[0155] S610, obtaining the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer, and the first number of turns of the secondary winding.

[0156] S620, calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns.

[0157] S630. When the magnetic flux density is not within the first preset range, the induced potential is changed by adjusting the first preset size of the skeleton to control the magnetic flux density to be within the first preset range.

[0158] S640. When at least one of the ratio difference and the phase difference of the current transformer is not within the second preset range, adjust the first preset size of the skeleton to change the excitation current to control the error to be within the second preset range.

[0159] Specifically, the error of the current transformer includes the ratio difference and the phase difference. The accuracy level of the current transformer is defined by the ratio difference and the phase difference. Therefore, it is necessary to satisfy that the ratio difference and the phase difference are within the second preset range at the same time. For example, when the accuracy level of the current transformer is 0.2S, when the primary current reaches 1% of the primary rated current, the ratio difference should not exceed ±0.75%; when the primary current reaches 5% of the primary rated current, the ratio difference should not exceed ±0.35%; when the primary current reaches 20% of the primary rated current, the ratio difference should not exceed ±0.2%; when the primary current reaches 100% of the primary rated current, the ratio difference should not exceed ±0.2%; when the primary current reaches 120% of the primary rated current, the ratio difference should not exceed ±0.2%. When the accuracy level of the current transformer is 0.2S, when the primary current reaches 1% of the primary rated current, the phase difference should not exceed ±30 minutes; when the primary current reaches 5% of the primary rated current, the phase difference should not exceed ±15 minutes; when the primary current reaches 20% of the primary rated current, the phase difference should not exceed ±10 minutes; when the primary current reaches 100% of the primary rated current, the phase difference should not exceed ±10 minutes; when the primary current reaches 120% of the primary rated current, the phase difference should not exceed ±10 minutes. Only when the current transformer meets all the above conditions can the current transformer meet the accuracy level requirements. The following is an example of when the primary current reaches 100% of the primary rated current. When the primary current reaches 100% of the primary rated current, the ratio difference exceeds ±0.2%, indicating that the error is large and the current transformer does not meet the 0.2s accuracy level. Therefore, it is necessary to adjust the first preset size to change the excitation current to control the ratio difference to not exceed ±0.2%; or when the primary current reaches 100% of the primary rated current, the angle difference exceeds ±10 minutes, indicating that the error is large. At this time, it is also necessary to adjust the first preset size to change the excitation current to control the angle difference to not exceed ±10 minutes; or when the primary current reaches 100% of the primary rated current, the ratio difference exceeds ±0.2% and the angle difference exceeds ±10 minutes, indicating that the error is large. At this time, it is also necessary to adjust the first preset size to change the excitation current to control the ratio difference to not exceed ±0.2% and the angle difference does not exceed ±10 minutes.

[0160] In an embodiment of the present invention, whether the current transformer meets the accuracy level requirement is judged based on whether the ratio difference and the phase difference are both within a second preset range. When at least one of the ratio difference or the phase difference is not within the second preset range, the first preset size is adjusted to change the excitation current to control the error to be within the second preset range, thereby ensuring that the current transformer can meet the accuracy level requirement.

[0161] Furthermore, Figure 8 1 is a schematic diagram of the structure of a second current transformer provided according to an embodiment of the present invention. The embodiment of the present invention describes the connection method between the secondary winding of the current transformer and the compensation winding when the secondary winding has a multi-ratio.

[0162] like Figure 8 As shown, when the secondary winding 2 is a multi-ratio, the secondary winding 2 has n gears, and n compensation windings 4 need to be wound accordingly. Take the secondary winding 2 with 3 gears as an example. When the secondary winding 2 has 3 gears, the first end of the secondary winding 2 is S1, the second end is S2, the third end is S3, and the fourth end is S4; the first end of the compensation winding 4 is N1, the second end is N2, the third end is N3, the fourth end is N4, the fifth end is N5, and the sixth end is N6. Among them, the second end S2 of the secondary winding 2 is connected to the first end N1 of the compensation winding 4, the third end S3 of the secondary winding 2 is connected to the third end N3 of the compensation winding 4, and the fourth end S4 of the secondary winding 2 is connected to the fifth end N5 of the compensation winding 4. The first end S1 of the secondary winding 2, the second end N2, the fourth end N4 and the sixth end N6 of the compensation winding are used as lead wires. This design method is easy to operate and convenient to wire. The overall structure is compact and simple, which is convenient for product installation and transportation.

[0163] It should be understood that the various forms of processes shown above can be used to reorder, add or delete steps. For example, the steps described in the present invention can be executed in parallel, sequentially or in different orders, as long as the desired results of the technical solution of the present invention can be achieved, and this document does not limit this.

[0164] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A design method for a current transformer, characterized in that: The current transformer comprises: a secondary winding and a frame; the secondary winding comprises an iron core; The design method comprises: Obtaining the induced potential of the secondary winding, the effective cross-sectional area of ​​the iron core, the frequency of the current transformer and the first number of turns of the secondary winding; Calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns; When the magnetic flux density is not within a first preset range, the induced potential is changed by adjusting a first preset size of the skeleton to control the magnetic flux density to be within the first preset range; wherein the first preset size includes a first inner diameter, a first outer diameter, and a first height of the skeleton; When the error of the current transformer is not within a second preset range, the excitation current of the current transformer is changed by adjusting the first preset size of the skeleton to control the error to be within the second preset range.

2. The design method according to claim 1, characterized in that: Obtaining the effective cross-sectional area of ​​the core includes: Calculate the first cross-sectional area of ​​the core according to the second preset size of the core; wherein the second preset size includes the second inner diameter, the second outer diameter and the second height of the core; The effective cross-sectional area is determined according to the first cross-sectional area and a filling factor.

3. The design method according to claim 1, characterized in that: Obtaining the induced potential of the secondary winding includes: Obtaining the total impedance of the secondary circuit and the rated secondary current of the current transformer; The induced potential of the secondary winding is determined according to the total impedance and the rated secondary current.

4. The design method according to claim 3, characterized in that: The current transformer further comprises: a compensation winding; there is no electromagnetic induction between the compensation winding and the secondary winding; Obtaining the total impedance of the secondary circuit includes: Obtaining a first resistance of the secondary winding and a second resistance of the compensation winding; determining a total resistance of the secondary circuit according to the first resistance and the second resistance; Obtaining the active component of the secondary load, the reactive component of the secondary load and the leakage reactance of the secondary circuit; The total impedance of the secondary circuit is calculated according to the total resistance, the active component, the reactive component and the leakage reactance.

5. The design method according to claim 4, characterized in that: The secondary winding includes a secondary winding conductor; Obtaining a first resistance of the secondary winding includes: Calculating a second cross-sectional area of ​​the secondary winding conductor according to the rated secondary current and the current density of the secondary winding; Determining a wire diameter of the secondary winding wire according to the second cross-sectional area; Determining a third cross-sectional area of ​​the secondary winding wire according to the wire diameter; Calculating a first turn length of the secondary winding according to a second preset size of the core; the second preset size includes a second inner diameter, a second outer diameter and a second height of the core; determining a first total turn length of the secondary winding according to a product of the first turn length and a first number of turns of the secondary winding; The first resistance is calculated according to the third cross-sectional area and the first total turn length.

6. The design method according to claim 4, characterized in that: The compensating winding comprises a compensating winding conductor; Obtaining the second resistance of the compensation winding includes: Calculating a second turn length of the compensation winding according to a first preset size of the frame; Calculating a second total turn length of the compensation winding according to the second turn length and a second number of turns of the compensation winding; Obtaining a fourth cross-sectional area of ​​the compensation winding conductor; A second resistor is determined according to the fourth cross-sectional area and the second total turn length.

7. The design method according to claim 1, characterized in that: When the error of the current transformer is not within a second preset range, changing the excitation current of the current transformer by adjusting the first preset size of the skeleton to control the error to be within the second preset range includes: When the error of the current transformer is not within the second preset range, adjusting the first preset size of the frame to change the excitation current of the current transformer to adjust the error of the current transformer; When the error of the current transformer is still not within the second preset range, the second preset size of the iron core is adjusted to change the excitation current of the current transformer to control the error to be within the second preset range.

8. The design method according to claim 1, characterized in that: When the error of the current transformer is not within a second preset range, changing the excitation current of the current transformer by adjusting the first preset size of the skeleton to control the error to be within the second preset range includes: When at least one of the ratio difference and the phase difference of the current transformer is not within the second preset range, the first preset size of the skeleton is adjusted to change the excitation current of the current transformer to control the error to be within the second preset range.

9. The design method according to claim 1, characterized in that: Calculating the magnetic flux density of the current transformer according to the induced potential, the effective cross-sectional area, the frequency and the first number of turns includes: The magnetic flux density is calculated according to the following magnetic flux density formula; Among them, B represents the magnetic flux density of the current transformer, E represents the induced potential of the secondary winding, f represents the frequency of the current transformer, N represents the first number of turns of the secondary winding, and A represents the effective cross-sectional area of ​​the iron core.

10. The design method according to claim 4, characterized in that: Calculating the total impedance of the secondary circuit according to the total resistance, the active component, the reactive component and the leakage reactance comprises: Calculate the total impedance of the secondary circuit according to the following total impedance formula of the secondary circuit; Where Z2 represents the total impedance of the secondary circuit, R f represents the active component of the secondary load, X f represents the reactive component of the secondary load, r represents the total resistance of the secondary circuit, and X represents the leakage reactance of the secondary circuit.

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