A power taking current transformer and a design method thereof

Abstract

This invention discloses an energy-harvesting current transformer and its design method, relating to the field of current transformer technology. The energy-harvesting current transformer portion can employ either a closed-core or open-core structure. The secondary winding has multiple secondary leads with varying numbers of turns. When the primary current is small, a capacitor can be connected in parallel on the secondary side to increase the secondary output power. When the primary current is large, a resistor can be connected in parallel on the secondary side to shunt the current and prevent core saturation.

Description

Technical Field

[0001] This invention relates to the field of current transformer technology, specifically to an energy-harvesting current transformer and its design method. Background Technology

[0002] With the continuous development of power grid intelligence and automation, real-time monitoring applications in power systems are becoming increasingly widespread, such as transmission line insulation monitoring, distribution network fault location, and conductor icing monitoring. However, transmission lines have high voltage levels, and the monitoring equipment is in a complex environment with high voltage and strong magnetic fields, requiring a highly stable power supply.

[0003] Existing current transformers are the main power supply method, but they have the following problems: 1. When the primary current is small, the power output is insufficient and the power supply dead zone is large; 2. When the primary current is large, the iron core overheats severely, the secondary voltage is too high, endangering the safety of personnel and equipment, and the power input power is too high, damaging the downstream power supply circuit; 3. Unlike conventional current transformers, the secondary load is uncertain, and the maximum output power needs to be calculated. When an open structure is used, the calculation is very complicated. Simulation analysis is usually performed using simulation technology, which has a high technical threshold and is time-consuming.

[0004] In view of this, it is necessary to study an energy harvesting current transformer and its design method, so as to improve the safety of personnel and equipment and avoid insufficient power. Summary of the Invention

[0005] The purpose of this invention is to provide an energy-harvesting current transformer and its design method to solve the technical problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides an energy-harvesting current transformer, which can be a closed-core structure or an open-core structure. The open-core structure includes an iron core and fasteners. An inner insulation layer is wound around the outer side of the iron core, and secondary conductors are wound around the outer side of the inner insulation layer. The secondary conductors have multiple leads, each led out by a secondary lead wire. An outer insulation layer is wound around the outer side of the secondary conductors, and epoxy resin is poured onto the outer side of the outer insulation layer. The current transformer is cut into two parts, and in use, the two parts are aligned and fixed together with fasteners. The closed-core structure eliminates the need to cut the current transformer and fix it with fasteners. Parallel resistors and parallel capacitors can be connected in parallel on the secondary side of the energy-harvesting current transformer.

[0007] The secondary winding of the current transformer has multiple secondary leads with different numbers of turns. When the primary current is low, a secondary lead with fewer turns is used; when the primary current is high, a secondary lead with more turns is used. When the primary current is low, a capacitor can be connected in parallel on the secondary side to increase the secondary output power. When the primary current is high, a resistor can be connected in parallel on the secondary side to shunt the current and prevent core saturation.

[0008] Preferably, in the open-core current transformer, the fastener is fixed to the outside of the current transformer, and the fastener is used to fasten the two aligned parts of the current transformer into one piece.

[0009] Preferably, the core is made of silicon steel, microcrystalline alloy, amorphous alloy, permalloy, or other similar materials.

[0010] Preferably, the number of turns of the multiple leads of the secondary conductor is different, so that different leads can be used for different primary currents.

[0011] Preferably, the parallel resistor is a shunt resistor, which is used in parallel when the primary current is too large to serve as a shunt resistor.

[0012] Preferably, the parallel capacitor is a compensation capacitor, which is used in parallel when the primary current is small to compensate for the inductive current.

[0013] A second aspect of the present invention provides a design method for designing an energy-harvesting current transformer as shown above, the method comprising the following steps: Step S1: Determine the structure of the energy harvesting current transformer, whether it is an open core structure or a closed core structure, and initially select the core magnetic flux density, core size, number of turns of the secondary winding, load resistance value, parallel resistance value, and parallel capacitance value. Step S2: Calculate the secondary induced electromotive force under different primary current conditions; Step S3: Calculate the actual working magnetic flux density of the core based on the secondary induced electromotive force. If the actual working magnetic flux density exceeds the preset saturation magnetic flux density threshold, adjust the load resistance value, parallel capacitance value, and parallel resistance value, and return to step S2 for recalculation. If the error between the actual working magnetic flux density and the preset core magnetic flux density in step two exceeds the allowable range, update the preset core magnetic flux density with the actual working magnetic flux density, and return to step S2 for iterative calculation until the magnetic flux density value converges. Step S4: Based on the converged parameters, calculate the actual output power on the load; if the actual output power does not meet the design requirements, adjust the number of turns of the secondary winding, the load resistance value and the parallel capacitor value, return to step S2 to recalculate, the actual output power has a maximum value, until this maximum output power is calculated; Step S5: If the maximum output power still does not meet the design requirements, increase the effective cross-sectional area of ​​the core 1, change the number of turns of the secondary winding, change the core material, etc., and repeat steps S1 to S5 until the output power under each primary current condition meets the requirements, thereby completing the design of the energy harvesting current transformer.

[0014] Preferably, when designing an open-core structure current transformer, multiple core samples are first made. After the core is cut open, the cuts are aligned and tightened. Test turns are wound around the core, and the BH curve and loss angle curve of the open core are tested. After statistical analysis, the BH curve and loss angle curve of the open core are summarized and used for the calculation of the open-core structure current transformer.

[0015] Compared with the prior art, the beneficial effects of the present invention are: The current transformer in this invention features multiple leads with varying numbers of turns on the secondary conductor, which are then led out via secondary lead wires. A parallel resistor on the secondary side is used for current shunting, which can be connected in parallel when the primary current is too high, thus shunting the current, reducing the secondary voltage, and ensuring the safety of personnel and equipment. A parallel capacitor on the secondary side also compensates for inductive current; when the primary current is low, it is connected in parallel to compensate for the inductive current, improving the secondary output voltage and power.

[0016] This invention provides a design method suitable for energy-harvesting current transformers, solving the problems of insufficient power when the primary system current is small and core saturation voltage waveform distortion when the current is large. It can accurately calculate the secondary output power, maximum secondary output power, capacitance of the parallel capacitor, and resistance of the parallel resistor under different primary currents and secondary loads. Simultaneously, it reduces design complexity and improves design efficiency. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the open-core structure energy harvesting current transformer of the present invention; Figure 2 This is a cross-sectional view of the energy harvesting current transformer of the present invention.

[0018] In the diagram: 1. Iron core; 2. Inner insulation layer; 3. Secondary conductor; 4. Outer insulation layer; 5. Epoxy resin; 6. Fastener; 7. Secondary lead wire; 8. Parallel resistor; 9. Parallel capacitor. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "front end," "rear end," "both ends," "one end," and "the other end," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "equipped with," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0022] Example 1: Please refer to Figure 1 and Figure 2 An energy harvesting current transformer and its design method; The current transformer adopts an open core structure, including: a core 1 and fasteners 6. An inner insulation layer 2 is wound around the outside of the core 1, and secondary conductors 3 are wound around the outside of the inner insulation layer 2. The secondary conductors 3 have multiple leads, which are led out by secondary lead wires 7. An outer insulation layer 4 is wound around the outside of the secondary conductors 3, and epoxy resin 5 is poured on the outside of the outer insulation layer 4. Parallel resistors 8 and parallel capacitors 9 are installed in parallel in the secondary lead wires 7. Fasteners 6 are fixed to the outside of the current transformer and are used to fasten the two cut and aligned parts of the current transformer into one piece. The core 1 is made of silicon steel. The number of turns of the multiple leads of the secondary conductors 3 are different, so different leads can be used for different primary currents. The parallel resistor 8 is a shunt resistor, which is used in parallel when the primary current is too large to shunt the current. The parallel capacitor 9 is a compensation capacitor, which is used in parallel when the primary current is small to compensate for the inductive current.

[0023] The secondary winding of the current transformer has multiple secondary leads with different numbers of turns. When the primary current is low, a secondary lead with fewer turns is used; when the primary current is high, a secondary lead with more turns is used. When the primary current is low, a capacitor can be connected in parallel on the secondary side to increase the secondary output power. When the primary current is high, a resistor can be connected in parallel on the secondary side to shunt the current and prevent core saturation.

[0024] Example 2: The energy harvesting current transformer adopts an open structure and uses a silicon steel core. Several prototypes are first fabricated, cut in half lengthwise, and the cuts are aligned and tightened. The magnetization curve of the silicon steel core after the cut is then tested by winding a measuring winding around it. B - H curves and B - i Curve. Initially select the core magnetic flux density, estimate the core cross-sectional area based on the minimum current required for power output, determine the range of secondary winding turns, set the number of turns at each exit point of the secondary winding, and determine the load impedance. Z 2n Within the initial range, determine the load resistance and parallel capacitance values ​​at the minimum current required for power output, and calculate the load impedance. Z 2n Value and power factor angle f 2n Calculate the DC resistance of the secondary winding and the secondary leakage reactance, and calculate the total impedance of the secondary circuit. Z 2 and the total power factor angle of the secondary circuit f 2. Calculate the secondary induced electromotive force. E 2. Recalculate the magnetic flux density of the iron core B C When comparing the calculated magnetic flux density with the initial value, if the difference is significant, the recalculated core magnetic flux density is used. B C The value is used to replace the initial core magnetic flux density, and the calculation is repeated. E 2 values, then recalculate the core magnetic flux density. B C This process is repeated until the magnetic flux density of the iron core is reduced. B C The error value meets the requirements. Calculate the secondary terminal voltage. U 2n and load R L Output power P 2n If the output power P 2nIf the output power is less than the required value, change the load resistance and parallel capacitor values, and recalculate until the maximum output power is found. If the maximum output power still does not meet the requirements, increase the core cross-sectional area and recalculate. Repeat this process until the maximum output power meets the requirements. Then, recalculate the output power and core magnetic flux density when the current is high. If the core magnetic flux density is saturated, reduce the load resistance value, use a secondary output with more turns, and add a parallel shunt resistor to prevent the core from saturating even with a large primary current. The calculation formulas can be entered into spreadsheet cells for quick parameter adjustments and manual data adjustments for iterative calculations. This method of designing energy-harvesting current transformers ensures that performance requirements are met and improves design efficiency.

[0025] Specifically, in order for the secondary load to obtain greater power, the secondary DC resistance should be less than the load resistance.

[0026] ; ; ; In the formula: R 2—Secondary winding DC resistance; R L —Load resistance, typically 5Ω to 500Ω; N 2—Number of turns in the secondary winding; r —Resistivity of the secondary conductor; L 2—Average turn length of secondary conductors; S 2—Cross-sectional area of ​​the secondary conductor.

[0027] During the energy extraction process, the energy extraction voltage must also meet the startup voltage of the hardware circuit; otherwise, the circuit cannot operate and cannot supply power to the load. Therefore, the number of turns in the secondary winding must meet the following requirements: ; In the formula: N 2—Number of turns in the secondary winding; U 3—Hardware circuit startup voltage; f —Grid frequency; B C —The magnetic flux density of the iron core shall not exceed the saturation magnetic flux density; S C —Effective cross-sectional area of ​​the iron core.

[0028] Because the primary current varies over a wide range, it is necessary to consider both the insufficient power draw when the current is low and the distortion of the output voltage waveform caused by core saturation when the current is high. The secondary winding can be configured with multiple leads with different numbers of turns; leads with fewer turns are used when the current is low, and leads with more turns are used when the current is high.

[0029] Estimate the cross-sectional area of ​​the iron core: Based on the number of turns in a primary winding N 1. Typically, it is 1 turn. The number of conductors, cross-sectional area and arrangement of the primary winding, the thickness of the secondary winding, the thickness of the insulation layer, the assembly space, the thickness of the metal shield shell (if any), the thickness of the shield winding (if any), etc. are used to estimate the shape of the energy harvesting current transformer, the size of the internal window and the size of the internal window of the iron core.

[0030] Determine the core material, such as silicon steel core, microcrystalline alloy core, permalloy core, etc.

[0031] When estimating the cross-sectional area of ​​the iron core, assuming the iron core is unsaturated and neglecting core losses and secondary DC resistance losses, we have: ; ; ; Then the cross-sectional area of ​​the iron core S C for: ; In the formula P 2 represents the required secondary output power value. B C The initial magnetic flux density value, not exceeding the saturation magnetic flux density, is the primary current. I 1. The required core cross-sectional area varies within a certain range. The maximum core cross-sectional area should be selected. Usually, the core cross-sectional area is the largest when the primary current is small.

[0032] Load calculation: When the primary current varies within a certain range, the load R L Changes in load will affect power consumption. R L Increasing the voltage will lead to an increase in the secondary voltage, and the output power will first increase and then decrease, with a maximum output power value.

[0033] Calculate the load impedance for different primary currents Z 2n and power factor angle f 2n .

[0034] When the primary current is small, a capacitor can be connected in parallel. In this case, the load impedance after the capacitor in parallel should be calculated. Z 2n Value and power factor angle.

[0035] When the primary current is very large, a resistor can be connected in parallel to shunt the current. In this case, the load impedance and power factor angle after the parallel shunt resistor should be calculated.

[0036] Load impedance Z 2n The following formula should be satisfied: ; Initial load resistance R L The values ​​of the parallel capacitor, the parallel resistor, and the load impedance. Z 2n The above formula should be satisfied.

[0037] Once the core dimensions and the number of turns in the secondary winding are determined, the DC resistance and leakage reactance of the secondary winding can be calculated, followed by the total impedance of the secondary circuit. Z 2 and the total power factor angle of the secondary circuit f 2.

[0038] Maximum power calculation: Calculate the secondary induced electromotive force under different primary currents E 2 values: ; In the formula: i - Core loss angle; Z 0 - Magnetizing impedance; Z 2-Secondary total impedance.

[0039] In the formula L C The average magnetic path length of the iron core, m The permeability of the iron core at the corresponding magnetic flux density. Because the magnetization curve of a typical iron core B Take the peak value, and H To take the valid value, calculate... m It is necessary to magnetize the iron core curve at this time. B value divided by work out B Valid value.

[0040] Using the calculated E 2-value recalculated core magnetic flux density B C If it exceeds the saturation magnetic flux density of the iron core, the initial load resistance should be reduced. RL If the primary current is large, the parallel resistance value can be changed and the calculation recalculated until it is less than the core saturation magnetic flux density. Then, compare this recalculated magnetic flux density with the initial value. If the difference is significant, adjust the recalculated core magnetic flux density. B C Value replaces initial magnetic flux density B C Value, recalculate E 2 values, then recalculate the core magnetic flux density. B C This process is repeated until the magnetic flux density of the iron core is reduced. B C Continue until the value error meets the requirements (e.g., less than 1%).

[0041] Secondary terminal voltage U 2n for: ; load R L Output power P 2n for: ; The secondary output power has a maximum value. To find this maximum value, different values ​​can be changed. Z 2n Solve for each value separately, and then find the maximum value. If the calculated value at this point... P 2n Value less than required P 2n The value should be adjusted by increasing the core cross-sectional area and recalculating until the result is achieved. P 2n The value meets the requirements.

[0042] Typically, the primary winding of a current transformer has only one turn and cannot be adjusted. To meet the secondary output power requirements when the primary system current is relatively low, the effective cross-sectional area of ​​the iron core can be increased. S C 2. Appropriately reduce the number of turns in the secondary winding N 2 (when) N 2. When reduced too much, P 2) It may not increase but decrease; choose magnetic permeability. m High-quality core materials (such as low-loss silicon steel cores, microcrystalline alloy cores, permalloy cores, etc.) and appropriately reducing the average magnetic path length of the core. L (Often limited by structural constraints), methods include changing the capacitance of the parallel capacitor, changing the diameter of the secondary conductor, etc. When the primary system current is very large, the secondary output power should be reduced, which can be achieved by using a secondary output with more turns, disconnecting the parallel capacitor, or connecting a parallel resistor.

[0043] By incorporating the above calculation formulas into the cells of a spreadsheet, parameters can be quickly adjusted, and data can be manually adjusted to achieve cyclic calculations, thereby improving design efficiency.

[0044] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A current transformer for power extraction, characterized in that include: The iron core (1) and fasteners (6) are provided with an inner insulation layer (2) wound around the outside of the iron core (1). A secondary conductor (3) is wound around the outside of the inner insulation layer (2). The secondary conductor (3) is provided with multiple outlets. The outlets of the secondary conductor (3) are led out by secondary lead wires (7). An outer insulation layer (4) is wound around the outside of the secondary conductor (3). Epoxy resin (5) is poured on the outside of the outer insulation layer (4). The secondary lead wire (7) can be connected in parallel with a parallel resistor (8) and a parallel capacitor (9).

2. A power taking current transformer according to claim 1, characterized in that: The energy harvesting current transformer includes an open core structure and a closed core structure.

3. The energy harvesting current transformer according to claim 1, characterized in that: When an open core structure is used, the fastener (6) is fixed on the outside of the current transformer. The fastener (6) is used to fasten the two aligned parts of the current transformer into one piece.

4. The energy harvesting current transformer according to claim 1, characterized in that: The core (1) is made of silicon steel, microcrystalline alloy, amorphous alloy, or permalloy.

5. The energy harvesting current transformer according to claim 1, characterized in that: The secondary conductor (3) has multiple leads with different numbers of turns, allowing different leads to be used for different primary currents.

6. The energy harvesting current transformer according to claim 1, characterized in that: The parallel resistor (8) is a shunt resistor, which is used in parallel when the primary current is too large to serve as a shunt resistor.

7. The energy harvesting current transformer according to claim 1, characterized in that: The parallel capacitor (9) is a compensation capacitor, which is used in parallel when the primary current is small to compensate for the inductive current.

8. A design method for an energy-harvesting current transformer, characterized in that, Designing an energy-harvesting current transformer as described in claims 1-7 includes the following steps: Step S1: Determine the structure of the energy harvesting current transformer, whether it is an open core structure or a closed core structure, and initially select the core magnetic flux density, core size, number of turns of the secondary winding, load resistance value, parallel resistance value, and parallel capacitance value. Step S2: Calculate the secondary induced electromotive force under different primary current conditions; Step S3: Calculate the actual working magnetic flux density of the core based on the secondary induced electromotive force. If the actual working magnetic flux density exceeds the preset saturation magnetic flux density threshold, adjust the load resistance value, parallel capacitance value, and parallel resistance value, and return to step S2 for recalculation. If the error between the actual working magnetic flux density and the preset core magnetic flux density in step two exceeds the allowable range, update the preset core magnetic flux density with the actual working magnetic flux density, and return to step S2 for iterative calculation until the magnetic flux density value converges. Step S4: Based on the converged parameters, calculate the actual output power on the load; if the actual output power does not meet the design requirements, adjust the number of turns of the secondary winding, the load resistance value and the parallel capacitor value, return to step S2 to recalculate, the actual output power has a maximum value, until this maximum output power is calculated; Step S5: If the maximum output power still does not meet the design requirements, increase the effective cross-sectional area of ​​the iron core (1), change the number of turns of the secondary winding, change the iron core material, etc., and repeat steps S1 to S5 until the output power under each primary current condition meets the requirements, thereby completing the design of the energy harvesting current transformer.

9. The design method of an energy-harvesting current transformer according to claim 8, characterized in that: When designing an open-core current transformer, multiple core samples are first made. After cutting the core, the cuts are aligned and tightened. Test turns are wound around the core, and the BH curve and loss angle curve of the open core are tested. Through statistical analysis, the BH curve and loss angle curve of the open core are summarized and used for the calculation of the open-core current transformer.

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