Design method of rapid saturation energy-taking current transformer

Abstract

The invention discloses a rapid saturation energy-taking current transformer design method, which comprises the following steps of: obtaining theoretical values of main design parameters of an iron core sectional area and a number of turns of a secondary winding through low-current saturation design in combination with a secondary working voltage requirement; and meanwhile, a certain design margin and a market actual iron core product model are considered, so that actual design parameters can be determined. Meanwhile, aiming at the voltage overrun problem of the energy-taking current transformer designed by the theoretical method in a wide-range current scene, two preferred design schemes are provided. The problem of efficient design of the energy-taking current transformer is solved, the universality is high, the design method is simple and efficient, compared with a common current transformer, a corresponding energy-taking current transformer product overcomes the defects that application of the common current transformer in a wide-range current scene is limited, energy is wasted, and heat dissipation exists, and the energy-taking current transformer product is high in practicability. And meanwhile, the problems of large size, high cost, large-current overload and the like also exist.

Description

Technical Field

[0001] The present invention relates to the technical field of current transformer design, and particularly relates to a design method for a fast-saturation energy-taking current transformer. Background Art

[0002] In emerging fields such as new energy and smart grids, the demand for intelligent monitoring of power lines is increasing day by day. However, how to provide stable electrical energy for these monitoring devices, especially in remote areas or scenarios where external power supply cannot be provided, has become a difficult problem to be solved urgently. The energy-taking method using current transformers has become a mainstream solution because it utilizes the principle of electromagnetic induction and has advantages such as non-invasiveness and no need for external power supply.

[0003] Although ordinary current transformers perform well under small-range current conditions, their application in wide-range current scenarios is limited, with problems such as energy waste and heat dissipation. At the same time, there are also problems such as large volume, high cost, and overloading of large currents.

[0004] In summary, the application of ordinary current transformers for self-powered energy acquisition has many defects. While there are numerous scenarios for mutual inductance and energy acquisition applications, the traditional one-to-one design method based on requirements does not have universality, is difficult to be widely promoted and applied on a large scale, and has low efficiency. For mutual inductance and energy acquisition applications in different scenarios, a general design method for energy-taking transformers is needed; for wide-range current scenarios, an efficient and general design method for energy-taking transformers is even more needed. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects existing in the prior art and provide a design of an energy-taking transformer based on the principle of rapid saturation of the iron core, aiming to solve the problems of volume, cost, and large-current overload of ordinary current transformers, and provide guidance for engineering practice.

[0006] To achieve the above purpose, the present invention designs a design method for a fast-saturation energy-taking current transformer. By designing an appropriate cross-sectional area of the iron core and the number of turns of the secondary winding, the transformer can be quickly saturated at low current. After saturation, the current output by the secondary does not increase linearly with the primary current. The design method (which the present invention calls the low-current saturation design method) includes the following steps: S1. Calculate the theoretical cross-sectional area of the iron core according to the power formula of the fast-saturation transformer , and the power formula includes: Wherein, is the saturation power; is the primary current; is the saturation magnetic density of the iron core; is the theoretical cross-sectional area of the iron core; According to the pre-determined target measured current range, take the minimum current value as , the power on the load side is , so that the theoretical cross-sectional area of the iron core can be calculated ; S2. According to the cross-sectional area of the iron core, calculate the theoretical number of secondary winding turns of the winding. When the current transformer is saturated, the theoretical number of secondary winding turns can be calculated according to the secondary output voltage formula , and the secondary output voltage formula includes: Among them, is the secondary output voltage, in volts V, is the operating frequency, both of which can be determined in advance according to user requirements, is the number of secondary winding turns; , , , are all known quantities. For example, the working magnetic density of silicon steel core is about 2T when saturated; Furthermore, select the actual cross-sectional area of the iron core product to be more than 5% larger than the theoretical cross-sectional area to ensure a certain margin for the output power.

[0007] Furthermore, when designing the actual number of secondary winding turns, it should be considered that the actual output voltage is slightly greater than 5% to 20% of the working voltage required by the load for the energy-taking current transformer, so as to ensure a certain margin for the output voltage.

[0008] Furthermore, aiming at the problem of too high upper limit voltage in the low-current saturation design method, the iron core includes a main magnetic path and a shunt magnetic path, that is, the magnetic shunt technology is adopted in the structural design to enhance the saturation characteristics of the current transformer under large current.

[0009] Furthermore, for another solution to the problem of too high upper limit voltage in the low-current saturation design method, the secondary voltage output of the current transformer is connected to a voltage stabilizing circuit or a voltage stabilizing chip, and a stable output voltage is achieved through circuit adjustment, and both the voltage stabilizing circuit and the voltage stabilizing chip belong to mature technologies.

[0010] The advantages and beneficial effects of the present invention are as follows: The present invention provides a design method for a fast-saturation energy-taking current transformer. Through low-current saturation design and in combination with the secondary working voltage requirements, the theoretical values of the core cross-sectional area and the number of secondary winding turns, which are the main design parameters, can be obtained. At the same time, considering a certain design margin and the actual core product models in the market, the actual design parameters can be determined. The present invention also provides two preferred design solutions for the voltage over-limit problem existing in the energy-taking current transformer designed according to the above theoretical method in a wide-range current scenario. The present invention solves the problem of efficient design of the energy-taking current transformer, has strong versatility, a simple and efficient design method. Compared with ordinary current transformers, the corresponding energy-taking transformer products overcome the limitations in the application of ordinary current transformers in a wide-range current scenario, have problems of energy waste and heat dissipation, and also have problems such as large volume, high cost, and large-current overload. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is the test schematic diagram of the current transformer designed by the present invention; Figure 2 is the structural diagram of a magnetic shunt energy-taking current transformer.

[0012] Markings in the figure: CT, current transformer; 1, main magnetic circuit; 2, shunt magnetic circuit; 3, coil. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0013] The following combines the drawings and embodiments to further describe the specific embodiments of the present invention. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention.

[0014] First, analyze the design principle of the energy-taking device using an ordinary current transformer.

[0015] The ordinary current transformer is based on the following calculation formulas: Primary current and secondary current: ; Secondary output electrical energy: P2 = E2 2 / R2, .

[0016] Among them, is the primary current, is the secondary current, is the load resistance. By adjusting the number of turns of the secondary winding, the requirements of different load powers can be met. In the case of a relatively narrow primary current range, by precisely designing the number of turns With the core size, a stable power output can be obtained. However, when the primary current range is wide, especially in scenarios from dozens of amperes to hundreds of amperes, traditional current transformer designs face the following challenges: Linear constant current source characteristic: When the primary current increases, the secondary side output power increases significantly, resulting in excessive energy waste.

[0017] Core volume and cost: To obtain sufficient power output at low currents, the core and winding designs must be increased, significantly raising the cost and volume of the current transformer.

[0018] Heat dissipation and energy release problems: At high currents, the power is too high, requiring additional voltage division, energy dissipation, and heat dissipation designs, further increasing complexity.

[0019] Assume that the primary current range of the power line is 50A to 500A, the diameter of the primary cable is 50mm, the power requirement of the monitoring system is 0.5W, and the voltage requirement is 5V. According to the calculation: Required current = 0.5 / 5 = 0.1A; Number of turns of the secondary winding = 50 / 0.1 = 500T.

[0020] To ensure that the minimum current can have sufficient power output, a current transformer with an input of 50A, an output of 0.1A, and a load of 0.5W is considered in the design. If 50A meets the power requirements, then 500A will surely meet the power extraction requirements of the monitoring system. The specific parameter design of the current transformer will not be elaborated here. For relevant design literature by the author, the relevant calculation results are shown in the following table. It can be seen that the energy extraction current transformer does not require high accuracy, and the exciting current of the current transformer has little impact on energy extraction. Therefore, it is not necessary to use ultra-microcrystalline or permalloy with a high magnetic permeability for production. Instead, a silicon steel core with a larger magnetic induction intensity can be directly used for production. The energy output of the energy extraction current transformer produced in this way is also larger. Of course, if the primary current is small, such as in the milliampere range, then first, it is necessary to consider that the core can sense the primary current. At this time, ultra-microcrystalline or permalloy is preferably used for production. For the 50A energy extraction current transformer discussed in this invention, with a relatively large current, a silicon steel core of 60 * 75 * 10 can be completely used for production, and its winding parameter is to wind 500T with a wire diameter of 0.46mm.

[0021] However, the defects brought by this design are very obvious, such as the overload problem: when the primary current increases to 500A, the output voltage and power of the current transformer increase exponentially. The current transformer can output 0.1A and 0.5W of electrical energy at 50A. At the rated 50A, its magnetic flux density is about 0.76T, which is also the optimal magnetic induction intensity working range of the current transformer. However, 500A is 10 times that of 50A, and the saturation magnetic flux density of silicon steel is 2T. 2T / 0.75T = 2.67, close to 3 times the margin. Considering the non-linear growth after saturation, there should be a growth of about 5 times. That is, when the secondary current reaches 500A, the secondary current is about 0.5 * 5 = 2.5A, and its voltage has to increase from 5V to about 25V, resulting in an increased demand for voltage division, serious heat dissipation problems, and complex structural design.

[0022] Cost issue: After calculation, the mass of the core of this energy-taking current transformer reaches 124g, and the mass of the enameled wire is 32g, resulting in a relatively high manufacturing cost.

[0023] Embodiment 1: The present invention proposes a design method for a fast-saturating energy-taking current transformer. By designing an appropriate core cross-sectional area and the number of secondary windings, the current transformer can be quickly saturated at low current. After saturation, the current output by its secondary does not increase linearly with the primary current. The design method (which the present invention calls the low-current saturation design method) includes the following steps: S1. Calculate the theoretical cross-sectional area of the core according to the power formula of the fast-saturating current transformer , and the power formula includes: Among them, is the saturation power, which is taken as the unit W in this embodiment; is the primary current, with the unit of ampere A; is the saturation magnetic flux density of the core, which is related to the material, and for silicon steel it is about 2T; is the theoretical cross-sectional area of the core, which is taken as the unit of in this embodiment; According to the pre-determined target measured current range, take the minimum current value as , and the power on the load side is , so as to calculate the theoretical cross-sectional area of the core ; The above formula does not contain the load, indicating that the output power of the current transformer at the initial saturation only relates to the primary-side current , the maximum magnetic flux density of the core and the cross-sectional area of the core . Of course, the load and the number of secondary windings will affect the saturation degree of the core. Therefore, we can calculate the cross-sectional area of the core on the premise of already having the load power.

[0024] S2. Calculate the theoretical number of secondary winding turns of the winding according to the cross-sectional area of the iron core. When the current transformer is saturated, the theoretical number of secondary winding turns can be calculated according to the secondary output voltage formula. The secondary output voltage formula includes: Among them, is the secondary output voltage, with the unit of volt V. is the operating frequency, both of which can be determined in advance according to user requirements. is the number of secondary winding turns; , , , are all known quantities. For example, when the silicon steel core is saturated, the working magnetic density is about 2T. It can be seen that the secondary output voltage is related to the cross-sectional area and the number of turns of the iron core. If the cross-sectional area of the iron core is large, the number of secondary turns of the current transformer can be correspondingly less, and vice versa. Then how to determine the relationship between the two? Here, the power needs to be considered. The greater the power, the greater the current required for the output of the current transformer, and the fewer the number of secondary turns of the current transformer. At the same time, in order to make the current transformer saturated as soon as possible, the number of secondary turns should not be too large.

[0025] The design principle of the present invention is to utilize the principle that the iron core of the current transformer is saturated when working at low current. Once the iron core of the current transformer is saturated, the current output by the secondary will no longer increase linearly. Then when the current transformer is saturated, its voltage value will not increase to a certain extent, that is, in the saturated state of the current transformer, we change its original "constant current source" state to a "constant voltage source" state, so as to stably supply power to the secondary circuit and reduce costs.

[0026] Preferably, the actual cross-sectional area of the iron core product is selected to be more than 5% larger than the theoretical cross-sectional area to ensure a certain margin for the output power.

[0027] Preferably, when designing the actual number of secondary winding turns, it should be considered that the actual output voltage is slightly greater than 5% and less than 20% of the working voltage required by the load for the energy-taking current transformer, so as to ensure a certain margin for the output voltage.

[0028] For the cross-sectional area design, assuming that the requirements of the power line are the same as those of the ordinary current transformer, that is, =0.5W, =50A, =2T as an example, according to the power formula, the theoretical cross-sectional area of the iron core can be calculated as S = 0.210 The iron core includes a main magnetic path.

[0029] Based on the inner hole of the primary cable being 50 mm and the existing silicon steel core product models, a silicon steel wound core of 60*65*10 can be selected, and its actual cross-section is S = 0.245 (16.7%), which also ensures that there is a certain margin for power.

[0030] For the winding design, a voltage of about 5V needs to be output, with a margin, and it is designed to be 5.5V (10%). According to the output voltage formula, = 50Hz, = 2T, then the designed number of winding turns is N2 = 505, and at the same time, it is wound with 0.565 mm enameled wire (it can be calculated and selected with reference to the case where the current density is 4 in the case of an overload of 10 times the current). To achieve the same current measurement, the fast saturation energy-taking current transformer (also known as the fast saturation energy-taking current transformer) designed by the present invention has the following differences compared with the ordinary current transformer: Specification Ordinary current transformer Fast saturation energy-taking transformer Cost The iron core weighs 124g, with a relatively high cost The iron core weighs 38.6g, with a low cost Output characteristic Biased towards constant current source Biased towards constant voltage source Application scenario Current measurement and protection equipment Power supply for low-power devices As can be seen from the above table, there are significant differences in the output characteristics between the fast saturation energy-taking current transformer and the ordinary current transformer, and the core differences can be analyzed from the working principles of the two, which tend to be a constant voltage source and a constant current source respectively. The following is a detailed comparison of the similarities and differences between the two: 1. Fast saturation energy-taking current transformer: Tending to a constant voltage source, the core design of the fast saturation energy-taking current transformer enables it to quickly reach the saturation state, restricting the further change of the magnetic flux density. When the change of the core magnetic flux tends to be stable, the secondary induced voltage is close to constant, showing the characteristics similar to a constant voltage source. The output voltage is mainly determined by design parameters (such as turns ratio, saturation magnetic flux density), and is less affected by the load impedance.

[0031] 2. Ordinary current transformer: Tending to a constant current source, the core of the ordinary current transformer works in the linear region, and its magnetic flux density has a linear relationship with the primary current. The secondary induced current is jointly determined by the turns ratio and the load impedance. The output current is directly proportional to the primary current, and has a weak relationship with the change of the secondary load impedance or voltage, showing the characteristics similar to a constant current source.

[0032] Although there are obvious advantages, the design method of this embodiment still has certain defects in the wide current range.

[0033] Figure 1The following is the principle block diagram of the product test for this embodiment. The measured results show that under the conditions of a primary current of 50 A and a load of 10 ohms, the output voltage is 5.5 V, meeting the design requirements. However, when the primary current is 500 A, the output voltage measured by a multimeter is 8.3 V. This is mainly because the iron core does not actually reach the fully saturated state at low currents. As the primary current increases, although the secondary output voltage is relatively stable, it still rises slowly. At the same time, due to the deep saturation of the current transformer, although the voltage measured by the multimeter is the effective value of 8.3 V, the actual current transformer has been deeply saturated, resulting in its peak value far from being times the relationship of the effective value. The measured peak value is found to be around 24 V. For this growth situation, when we increase the primary current to 3500 A and test it with an oscilloscope, the CT output effective value is 24.5 V, and the peak voltage is around 112 V. That is to say, in a wide current range, when the upper limit current is saturated, attention should be paid to the test of the peak current. If it exceeds the voltage range of the secondary saturation device, corresponding measures need to be taken for control. Generally, there are the following solutions.

[0034] Example 2: The difference from Example 1 is that taking the output power requirement = 12 W, = 50 A, = 2 T as an example, according to the power formula, the theoretical cross-sectional area of the iron core can be calculated as S = 5.05 , and a silicon steel core wound with 120 * 180 * 20 is selected, and its actual cross-section is S = 5.88 (design margin 16.4%); the output nominal voltage is 24 V, and it is designed to be 25.2 V with a 5% margin. Therefore, N2 = 96 is designed.

[0035] Example 3: The difference from Example 1 is that in view of the problem of too high upper limit voltage in the low-current saturation design method, the iron core in this embodiment includes a main magnetic path and a shunt magnetic path, that is, the magnetic shunt technology is adopted in the structural design to enhance the saturation characteristics of the current transformer under large currents. Its principle is to use the shunt magnetic path to share the magnetic flux: at low currents, the magnetic flux is mainly concentrated in the main iron core with a higher magnetic permeability; when the current increases and the main iron core is quickly saturated, the excess magnetic flux is shunted through the high magnetic density secondary iron core with an air gap, so as to achieve a stable voltage output. However, the magnetic shunt structure is relatively complex and the manufacturing cost is relatively high, which is not conducive to wide promotion.

[0036] Specifically, as Figure 2As shown in the figure, a magnetic shunt current transformer includes a main magnetic circuit 1, a shunt magnetic circuit 2, and a coil 3 disposed on the main magnetic circuit 1; the main magnetic circuit 1 is a closed annular magnetic circuit, and the shunt magnetic circuit 2 includes two separately arranged U-shaped magnetic circuits; the main magnetic circuit 1 is formed by stacking a plurality of main magnetic circuit laminations, and the shunt magnetic circuit 2 is formed by stacking a plurality of main magnetic circuit laminations and a plurality of shunt magnetic circuit laminations.

[0037] Embodiment 4: The difference from Embodiment 1 is that, aiming at the problem of too high upper limit voltage existing in the low-current saturation design method, the secondary voltage output of the transformer in this embodiment is connected to a voltage stabilizing circuit or a voltage stabilizing chip, and a stable output voltage is achieved through circuit adjustment, and both the voltage stabilizing circuit and the voltage stabilizing chip belong to mature technologies.

[0038] Generally, the AC voltage output by the transformer is first rectified and filtered, and then enters the DC / DC conversion current to obtain the corresponding voltage and power. This solution is simple and feasible, the output voltage is stable, the cost is low, and it can effectively solve the problem that the voltage exceeds the design range, and has good practicability and popularization value.

[0039] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and refinements can be made, such as designing the main magnetic circuit, the shunt magnetic circuit, and the circuit for outputting the secondary voltage after voltage stabilization at the same time, as well as the selection method of the iron core and the winding method of the winding, etc., but the design cannot be separated from the two core parameters of the cross-sectional area and the number of turns. Therefore, these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A design method for a fast-saturation energy-taking current transformer, characterized in that By designing an appropriate cross-sectional area of the iron core and the number of turns of the secondary winding, the mutual inductor is quickly saturated at low current. After saturation, the current output by the secondary does not increase linearly with the primary current. The design method includes the following steps: S1. Calculate the theoretical cross-sectional area of the iron core according to the power formula of the fast saturation transformer , and the power formula includes: Among them, is the saturation power; is the primary current; is the core saturation magnetic flux density; is the theoretical cross-sectional area of the core; According to the pre-determined target measured current range, take the minimum current value as , the power on the load side is , calculate the theoretical cross-sectional area of the iron core ; S2. Calculate the theoretical number of secondary winding turns of the winding according to the cross-sectional area. When the mutual inductor is saturated, calculate the theoretical number of secondary winding turns according to the secondary output voltage formula , and the secondary output voltage formula includes: Among them, is the secondary output voltage, is the operating frequency, is the number of turns of the secondary winding.

2. A design method for a fast-saturation energy-taking current transformer according to claim 1, characterized in that, Select an iron core product with an actual cross-sectional area more than 5% larger than the theoretical cross-sectional area.

3. A design method for a fast-saturation energy-taking current transformer according to claim 1, characterized in that When designing the actual number of turns of the secondary winding, it should be considered that the actual output voltage is more than 5% and less than 20% of the working voltage required by the energy-taking mutual inductor for the load demand.

4. A design method for a fast-saturation energy-taking current transformer according to claim 1, characterized in that, The iron core includes a main magnetic path and a shunt magnetic path.

5. A design method for a fast-saturation energy-taking current transformer according to claim 1, characterized in that The secondary voltage output of the mutual inductor is connected to a voltage stabilizing circuit or a voltage stabilizing chip.

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