Design method of zero sequence current transformer

Abstract

The invention relates to a design method of a zero sequence current transformer, and belongs to the technical field of transformers. According to the technical scheme, the method comprises the steps that the number of turns N2 of a secondary winding, load impedance Z2n, the total impedance Z2 of a secondary loop, a power factor angle phi 2 of the secondary loop, excitation impedance Z0 and an iron core loss angle theta are calculated, then known primary current I1 and the number of turns N1 of the primary winding are substituted into a formula, the secondary output voltage U2 can be calculated, and if U2 is not within the error range of the rated secondary output voltage U2n, the load impedance Z2n and the total impedance Z2 of the secondary loop are calculated. And all parameters need to be adjusted for recalculation, so that the error requirements are finally met. According to the design method, design errors possibly occurring in conventional current transformer error calculation are avoided. The cost can be reduced, and the performance requirement can be met when the structure is limited.

Description

Technical Field

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

[0002] Zero-sequence current transformer is a current transformer specially used to transform zero-sequence current. In the power system, zero-sequence current transformer is usually matched with relay protection device. When the line generates zero-sequence grounding current (or grounding fault), the device components are selectively tripped or sent out signals to achieve the function of protecting or detecting the normal operation of the line. Zero-sequence current transformer sometimes requires the secondary output to be not a current signal, but a voltage signal. In addition to resistance, the secondary load sometimes also connects capacitors in parallel to increase the secondary output voltage. At this time, the traditional design calculation method for calculating the error of the current transformer can no longer be simply applied. Even if the secondary output voltage is converted into secondary current, and the secondary resistance and capacitance are converted into secondary load (VA) and power factor (cosφ), it is possible that the core excitation current is greater than the primary current during the error calculation process. Because the core excitation current cannot be greater than the primary current, calculation errors will occur and the design task cannot be completed. Summary of the invention

[0003] The purpose of the present invention is to provide a design method for a zero-sequence current transformer, which can avoid design errors that may occur in conventional current transformer error design calculations, reduce costs, still meet performance requirements when the structure is limited, and solve the above-mentioned technical problems existing in the background technology.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A design method for zero-sequence current transformer. First, calculate the secondary winding turns N2 and load impedance Z 2n , the total impedance of the secondary circuit Z2, the power factor angle of the secondary circuit φ2, the excitation impedance Z0 and the core loss angle θ, and then substitute the known primary current I1 and the number of primary winding turns N1 into the following formula:

[0006]

[0007] The secondary output voltage U2 can be calculated. If U2 is not within the rated secondary output voltage U 2n Within the error range, it is necessary to adjust the parameters and recalculate them so that they finally meet the error requirements.

[0008] When the zero-sequence current transformer is limited by structure and cannot make U2 meet the requirements by adjusting various parameters, or when you want to reduce costs, increase the magnetic permeability μ of the core at the rated magnetic flux density by K times (generally not more than 2) to make U2 meet the requirements; at this time, the excitation current value at the rated magnetic flux density in the core control data should be divided by K.

[0009] Usually, the primary winding turns of the zero-sequence current transformer are 1 turn and cannot be adjusted. If you want to increase the secondary output voltage U2, you can increase the effective cross-sectional area S of the core, appropriately reduce the secondary winding turns N2 (when N2 is reduced too much, U2 may not increase but decrease), select core materials with high magnetic permeability μ (such as low-loss silicon steel core, ultra-fine crystal alloy core, Permalloy core, etc.), appropriately reduce the average magnetic path length L of the core (often due to structural limitations and not much change), change the capacitance of the parallel capacitor, change the diameter of the secondary wire, etc.

[0010] When the zero-sequence current transformer is limited by the structure and cannot meet the requirements of U2 through the above methods, or when you want to reduce costs, you can improve the performance requirements of the core instead of increasing the cross-sectional area of ​​the core, that is, improve the requirements of the core magnetic permeability μ, so that U2 meets the requirements. Because in order to improve the qualified rate of the core, the magnetization curve of the core is set conservatively. Usually, the actual performance of the core exceeds the performance calculated by the magnetization curve. For example, the magnetic permeability μ of the core at the rated magnetic flux density is increased by K times. K cannot be too large (generally not more than 2), otherwise it may cause the qualified rate of the core to be too low, so that U2 meets the requirements. At this time, the excitation current value at the rated magnetic flux density in the core control data should be divided by K.

[0011] Compared with the prior art, the invention has the following beneficial effects: providing a design calculation method suitable for zero-sequence current transformers, avoiding design errors that may occur in conventional current transformer error calculations, reducing costs, and meeting performance requirements when the structure is limited. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 This is a phasor diagram of a zero-sequence current transformer according to an embodiment of the present invention. DETAILED DESCRIPTION

[0013] The technical solutions in the embodiments of the present invention are described clearly and completely below. 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 are within the scope of protection of the present invention.

[0014] A design method for zero-sequence current transformer. First, calculate the secondary winding turns N2 and load impedance Z 2n, the total impedance of the secondary circuit Z2, the power factor angle of the secondary circuit φ2, the excitation impedance Z0 and the core loss angle θ, and then substitute the known primary current I1 and the number of primary winding turns N1 into the following formula:

[0015]

[0016] The secondary output voltage U2 can be calculated. If U2 is not within the rated secondary output voltage U 2n Within the error range, it is necessary to adjust the parameters and recalculate them so that they finally meet the error requirements.

[0017] When the zero-sequence current transformer is limited by structure and cannot make U2 meet the requirements by adjusting various parameters, or when you want to reduce costs, increase the magnetic permeability μ of the core at the rated magnetic flux density by K times (generally not more than 2) to make U2 meet the requirements; at this time, the excitation current value at the rated magnetic flux density in the core control data should be divided by K.

[0018] Zero-sequence current transformers generally require that when the number of turns of the primary winding is N1 (usually 1 turn) and the rated primary zero-sequence current is I 1n (A), the rated secondary output voltage should be U 2n (V), the secondary is connected to a load resistor R (Ω), and a capacitor C (F) may also be connected in parallel.

[0019] Calculate the load impedance Z 2n and power factor angle φ 2n :

[0020] The admittance Y of the load resistance R R =1 / R,S,

[0021] The admittance Y of the parallel capacitor C C =jωC,S, where ω=2πf, f is the rated frequency, Hz,

[0022] Load admittance Y after load resistance and capacitance are connected in parallel 2n =Y R +Y C , S.

[0023] The load impedance Z after the load resistance and capacitor are connected in parallel 2n =1 / Y 2n ,Ω, its actual part is the resistance R 2n , the imaginary part is the reactance X 2n .

[0024] The load power factor angle φ after the resistor and capacitor are connected in parallel 2n =tg -1 (X 2n / R 2n ),°.

[0025] Rated secondary current I2n =U 2n / Z 2n ,

[0026] Generally, the current density of the secondary conductor shall not exceed 2A / mm 2 Determine the diameter of the copper wire, taking into account the mechanical strength and winding process requirements of the copper wire. Generally, the diameter of the copper wire should not be less than 0.2mm.

[0027] Estimate the number of turns of the secondary winding N2 = I 1n N1 / I 2n ,

[0028] According to the primary winding turns N1, the number of primary winding conductors, cross-sectional area and arrangement, secondary winding thickness, insulation layer thickness, assembly space, metal shielding shell thickness (if any), shielding winding thickness (if any) and other data, first estimate the shape of the zero-sequence current transformer, the size of the internal window and the size of the internal window of the iron core.

[0029] Determine the core material, such as silicon steel core, ultrafine crystal alloy core, Permalloy core, etc. Ultrafine crystal alloy is preferred. Usually the magnetic flux density of the zero-sequence current transformer is very low, and the core cross-sectional area S can be estimated as 0.03T:

[0030]

[0031] Where B m It is the initially selected rated magnetic flux density value, generally 0.03T.

[0032] After the core cross-sectional area is determined, the core size can be further determined. After the core size is determined, the arrangement of the secondary winding and the DC resistance R of the secondary winding can be determined. CT :

[0033]

[0034] Where: ρ—conductor resistivity, the resistivity of copper conductor at 55℃ is 0.02Ω·mm 2 / m;

[0035] L0—length of each turn of the secondary winding, mm;

[0036] N2—number of turns of secondary winding;

[0037] L y —Secondary winding lead length, m;

[0038] S0—Secondary conductor cross-sectional area, mm 2 .

[0039] Total resistance of secondary circuit R2=R CT +R 2n ,Ω, where RCT is the DC resistance of the secondary winding, R 2n is the resistance of the load resistor and the capacitor in parallel.

[0040] Total reactance of secondary circuit X2=X CT +X 2n ,Ω, where X CT is the secondary winding leakage reactance, X 2n It is the reactance of the load resistance and capacitance in parallel.

[0041] Total impedance of secondary circuit

[0042] Secondary circuit power factor angle φ2 = tg -1 (X2 / R2),°.

[0043] The relationship between the core excitation current I0 and the magnetic flux Φ is:

[0044] I0N1=ΦR0

[0045] Where: R0—magnetic resistance of zero-sequence current transformer, L is the average magnetic path length of the core, μ is the magnetic permeability of the core, and S is the effective cross-sectional area of ​​the core.

[0046] The relationship between the magnetic flux Φ and the secondary induced electromotive force E2 is:

[0047]

[0048] The peak value of magnetic flux density B is:

[0049]

[0050] get

[0051]

[0052] Because the excitation impedance It turns out that:

[0053]

[0054] because Conclusion

[0055]

[0056] The zero-sequence current transformer satisfies the magnetomotive force balance equation

[0057]

[0058] Where: —primary current, A;

[0059] —Secondary current, A;

[0060] —excitation current, A;

[0061] N1—number of turns of primary winding;

[0062] N2—Number of turns of secondary winding.

[0063] The phasor diagram of the zero-sequence current transformer is shown in Figure 1 .

[0064] In the figure, θ is the core loss angle.

[0065] From the phasor diagram we can get:

[0066] (I1N1) 2 =(I0N1 cosθ+I2N2 sinφ2) 2 +(I0N1 sinθ+I2N2 cosφ2) 2

[0067] Will and Substituting into the above formula and rearranging it, we get:

[0068]

[0069] Also because It turns out that:

[0070]

[0071] in μ is the magnetic permeability of the core corresponding to the magnetic flux density, Because the B value in the magnetization curve of the core is usually the peak value, and H is the effective value, when calculating μ, the B value in the magnetization curve of the core needs to be divided by Calculate the effective value of B.

[0072] Use I 1n Substituting I1, the rated primary current I can be calculated 1n The secondary output voltage U2 at this time, if U2 is not within the rated secondary output voltage U 2n Within the error range, it is necessary to adjust the parameters and recalculate them so that they finally meet the error requirements.

[0073] Usually, the primary winding turns of the zero-sequence current transformer are 1 turn and cannot be adjusted. If you want to increase the secondary output voltage U2, you can increase the effective cross-sectional area S of the core, appropriately reduce the secondary winding turns N2 (when N2 is reduced too much, U2 may not increase but decrease), select core materials with high magnetic permeability μ (such as low-loss silicon steel core, ultra-fine crystal alloy core, Permalloy core, etc.), appropriately reduce the average magnetic path length L of the core (often due to structural limitations and not much change), change the capacitance of the parallel capacitor, change the diameter of the secondary wire, etc.

[0074] When the zero-sequence current transformer is limited by the structure and cannot meet the requirements of U2 through the above methods, or when you want to reduce costs, you can improve the performance requirements of the core instead of increasing the cross-sectional area of ​​the core, that is, improve the requirements of the core magnetic permeability μ, so that U2 meets the requirements. Because in order to improve the qualified rate of the core, the magnetization curve of the core is set conservatively. Usually, the actual performance of the core exceeds the performance calculated by the magnetization curve. For example, the magnetic permeability μ of the core at the rated magnetic flux density is increased by K times. K cannot be too large (generally not more than 2), otherwise it may cause the qualified rate of the core to be too low, so that U2 meets the requirements. At this time, the excitation current value at the rated magnetic flux density in the core control data should be divided by K.

[0075] In the embodiment, the performance requirements of the zero-sequence current transformer are: rated primary current 100mA, rated primary turns 1 turn, rated frequency 50Hz, rated secondary output voltage 144mV±3%, secondary load resistance 1020Ω, secondary parallel capacitance 1.2μF, and a racetrack shape is required.

[0076] According to the design calculation method, it is calculated that:

[0077] Secondary turns N2 = 661 turns,

[0078] The core is made of ultra-microcrystalline alloy core, with an inner diameter of 68mm, an outer diameter of 80mm, a height of 15mm, a straight length of 200mm, and an effective cross-sectional area of ​​0.675cm 2 The average magnetic circuit length is 63.25cm. The secondary conductor uses φ0.21 enameled round copper wire. The conductor direct resistance R CT =23.43Ω, secondary leakage reactance X CT =1Ω.

[0079] Secondary load impedance Z 2n =952.04Ω, total impedance of secondary circuit Z2 =973.59Ω, power factor angle of secondary circuit φ2 =-20.48°.

[0080] The rated magnetic flux density B = 0.149T. The magnetic permeability μ = 0.0663H / m at 0.149T is calculated from the magnetization curve. The core loss angle θ = 5.42° and the excitation impedance Z0 = 0.00222Ω are obtained from the magnetization curve.

[0081] The calculated secondary output voltage U2 = 118.21 mV, which does not meet the requirement of 144 mV ± 3%.

[0082] At this time, the magnetic permeability can be increased by K = 1.75 times, and the secondary output voltage U2 = 142.02mV, meeting the performance requirements. At this time, the original core control data (excitation turns 1 turn, control turns 1 turn): the induced potential is not less than 0.223mV when the excitation current is 0.1A, is changed to the induced potential is not less than 0.223mV when the excitation current is 0.1 / 1.75 = 0.057A.

[0083] This design and calculation method for the zero-sequence current transformer can ensure that it meets performance requirements and can also reduce costs.

Claims

1. A design method for a zero-sequence current transformer, characterized in that: First calculate the number of turns of the secondary winding N 2. Load impedance Z 2n , total impedance of secondary circuit Z 2. Secondary circuit power factor angle f 2. Excitation impedance Z 0 and core loss angle i , and then the known primary current I 1 and primary winding turns N Substitute 1 into the following formula: The secondary output voltage can be calculated U 2. If U 2 Not at rated secondary output voltage U 2n Within the error range, it is necessary to adjust the parameters and recalculate them so that they finally meet the error requirements.

2. The design method of a zero-sequence current transformer according to claim 1, characterized in that: When the zero-sequence current transformer is limited by the structure and cannot be adjusted by adjusting the parameters U 2 To meet the requirements or reduce costs, the magnetic permeability of the core at the rated flux density m improve K times, U 2 meets the requirements; at this time, the excitation current value under the rated magnetic flux density in the core control data should be divided by K .

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