Method for measuring and calculating tripping current of instantaneous tripping device of low-voltage circuit breaker

Abstract

The invention relates to a tripping current measuring and calculating method for an instantaneous tripping device of a low-voltage circuit breaker. The method comprises the following steps: S1, calculating magnetic attraction force; s2, calculating an instantaneous tripping current; s3, calculating an instantaneous tripping current multiple; and S4, carrying out analogy calculation on the instantaneous tripping current multiple. According to the method, the tripping current value, meeting the standard requirement, of the instantaneous tripping device is completed through one-time design in a calculation and test combined mode, the one-time success rate and the qualification rate of instantaneous tripping detection are improved, the design time is greatly shortened, and the design cost is greatly reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of low-voltage circuit breakers, and in particular relates to a tripping current calculation method of an instantaneous tripping device of a low-voltage circuit breaker. Background Art

[0002] With the improvement of social automation and industrialization, the power consumption of factories has gradually increased, and at the same time, the demand for power transmission and distribution equipment in the power grid has increased. In the power distribution level, circuit breakers are an important protection link, and the stability of their performance is crucial. When a short circuit or overload fault occurs in the line, the circuit breaker can operate reliably to achieve effective protection of the power grid and equipment. In particular, thermal magnetic circuit breakers have been widely used due to their stable and reliable mechanical structure and low price.

[0003] When a short circuit occurs in the line, the low-voltage circuit breaker uses an instantaneous trip device to provide short-circuit protection for the line and equipment. The tripping characteristics of the instantaneous trip device of the low-voltage circuit breaker must meet the requirements of GB14048.2 or GB10963.1. In the actual design and production process, in order to realize the measurement of the tripping current of the instantaneous trip device, the common method is to repeatedly make prototypes and conduct continuous test verification to finally determine the instantaneous tripping current value, which consumes a lot of time and cost. For R&D projects, time targets and cost targets are often not adjustable. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for calculating the tripping current of an instantaneous tripping device of a low-voltage circuit breaker. By combining calculation and experiment, a tripping current value of an instantaneous tripping device that meets the standard requirements is designed once, thereby improving the one-time success rate and the qualified rate of instantaneous tripping detection, greatly shortening the design time and reducing the design cost.

[0005] The present invention solves the technical problem by the following technical solutions:

[0006] A method for calculating the tripping current of an instantaneous tripping device of a low-voltage circuit breaker, the method comprising the following steps:

[0007] S1. Calculation of magnetic attraction

[0008] When the current in the line flows through the low-voltage circuit breaker, a magnetic field is formed in the coil of the instantaneous tripping device. The moving and static iron cores of the instantaneous tripping device generate a magnetic attraction F in the magnetic field formed by the coil. The calculation formula of the magnetic attraction F is:

[0009]

[0010] Where: B is the magnetic induction intensity, S is the cross-sectional area of ​​the magnetic pole, μ o is the magnetic permeability;

[0011] Assume the cross-sectional area of ​​the magnetic pole is S = A,

[0012] Right now

[0013]

[0014] Without considering the leakage magnetic factor, the calculation formula of magnetic induction intensity B is:

[0015] B=μ o *H (3)

[0016] Where: H is the magnetic field strength, and the calculation formula is:

[0017]

[0018] Where: s is the length of the magnetic circuit, i.e. the air gap; i is the current flowing through the coil;

[0019] Substituting 4) into 3) we get

[0020]

[0021] Substituting 5) into 2), we get:

[0022]

[0023] Where: n is the number of coil turns; i is the current; μ o is the vacuum magnetic permeability, which is 4π*10 -7 N / A 2 ;

[0024] S2, instantaneous tripping current calculation

[0025] When a short-circuit current occurs in the line, the value of i at the moment when the instantaneous tripping device operates is the instantaneous tripping current value. The calculation formula for the instantaneous tripping current is:

[0026]

[0027] S3, instantaneous tripping current multiple calculation

[0028] Assuming that the nominal rated current of the low-voltage circuit breaker is In, when a short-circuit fault occurs in the distribution circuit, the fault current flowing through the coil is i. Assuming i is x times the rated current, that is, i=x In, then substitute i=x In into 7), the instantaneous tripping current multiple x is

[0029]

[0030] S4. Analogy calculation of instantaneous tripping current multiples

[0031] For circuit breakers of the same frame rating, assuming that the nominal rated current of the circuit breaker is In1, the current i1 flowing through the coil is a times the rated current, that is, i1 = a*In1; assuming that the nominal rated current of the circuit breaker is In2, the current i2 flowing through the coil is b times the rated current, that is, i2 = b*In2; then equation 6) is:

[0032]

[0033]

[0034] Divide formula 9) by formula 10) to get

[0035]

[0036] Formula 11) is rearranged to obtain:

[0037]

[0038] The instantaneous tripping current multiple b is related to the instantaneous tripping current multiple a, currents In1 and In2, coil turns n1 and n2, and reaction spring force values ​​F1 and F2. Therefore, the instantaneous tripping current is converted into the instantaneous tripping current multiple for analysis and calculation.

[0039] The positive effects that the present invention can produce are:

[0040] 1. After the space of the instantaneous tripping device of the present invention is determined, the core size and the air gap size are fixed values, and there is no adjustment space. The predetermined instantaneous tripping current is the design median value determined by the standard, and there is basically no adjustable space. During the measurement process, only the number of coil turns and the spring force value can be adjusted to achieve the predetermined instantaneous tripping current value. From the perspective of manufacturing technology, the spring force value is more adjustable and easier to implement, so it is preferred to adjust the spring reaction force value.

[0041] 2. In the actual production process of the present invention, most of the designs of instantaneous tripping devices are derived from the original model. In this case, the analogy method of the present invention can be directly used for theoretical calculation, thereby eliminating the influence of magnetic leakage and the precision of parts manufacturing in the manufacturing process on the test results. From the comparison of the calculation and test data of the present invention, it can be seen that the analogy calculation method obtains the design result at one time.

[0042] 3. The purpose of the present invention through theoretical calculation is to accurately and quickly achieve the established design goals, avoid the waste of time and cost caused by repeated prototype trial production and testing, greatly improve the first-time success rate and the qualified rate of instantaneous tripping detection, shorten the design time and reduce the design cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 It is a schematic diagram of the structure of the instantaneous tripping device of the present invention;

[0044] Figure 2 This is a curve diagram of the instantaneous tripping current multiple of the prototype when the spring force value F=0.68N of the present invention;

[0045] Figure 3 This is a curve diagram of the instantaneous tripping current multiple of the prototype when the spring force value F=0.96N of the present invention. DETAILED DESCRIPTION

[0046] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0047] A method for calculating the tripping current of an instantaneous tripping device of a low-voltage circuit breaker, the innovation of which lies in:

[0048] The instantaneous tripping device is composed of a static iron core 1, a moving iron core 4, a coil 3 and a reaction spring 2. Figure 1 As shown, the dynamic and static iron cores generate magnetic field forces in the magnetic field formed by the energized coil; when a short circuit occurs in the line and the current increases to a sufficient value, the magnetic field forces generated by the dynamic and static iron cores in the magnetic field overcome the reaction force of the spring, and the dynamic and static iron cores are attracted, thereby driving the low-voltage circuit breaker mechanism to trip the mechanism, thereby realizing short-circuit protection of the line, and the current value when it reaches a sufficient value is the instantaneous tripping current of the circuit breaker.

[0049] In different application scenarios, the instantaneous tripping current value has different tripping form requirements, that is, different instantaneous tripping current ranges. When designing, we cannot blindly pursue the lower limit instantaneous tripping current value required by the circuit breaker tripping form, but should also consider the pass rate of the instantaneous tripping characteristics and the stability of the instantaneous tripping characteristics during its manufacturing process. Therefore, in the actual research and development process, more designs use the median of the tripping current range in the instantaneous tripping characteristics as the median of the design.

[0050] The steps of the method are:

[0051] 1. Calculation of magnetic attraction

[0052] The current in the line flows through the circuit breaker, forming a magnetic field in the coil of the instantaneous tripping device. The moving and static iron cores generate magnetic attraction in the magnetic field formed by the coil.

[0053] Maxwell's electromagnetic force formula:

[0054]

[0055] Where F: electromagnetic attraction, B: magnetic induction intensity, S: magnetic pole cross-sectional area, μ o : Magnetic permeability.

[0056] For the convenience of subsequent analysis, let the cross-sectional area of ​​the magnetic pole S = A, that is

[0057]

[0058] If factors such as magnetic leakage are not considered, the magnetic induction intensity

[0059] B=μ o *H (3)

[0060] Where H: magnetic field strength.

[0061] Magnetic field strength

[0062]

[0063] Where s is the length of the magnetic path, that is, the air gap; i is the current flowing through the coil.

[0064] Substituting 4) into 3) we get

[0065]

[0066] Substituting 5) into 2), we get:

[0067]

[0068] in:

[0069] F: magnetic attraction; A: effective cross-sectional area of ​​magnetic pole; s: air gap; n: number of coil turns; i: current; μ o :For instantaneous tripping device, it is vacuum magnetic permeability 4π*10 -7 N / A 2 .

[0070] When a short circuit current occurs, the value of i at the moment when the instantaneous tripping device is actuated is the instantaneous tripping current value. From formula 6), it can be seen that the spring reaction force value F is proportional to the square of the number of coil turns n and is proportional to the square of the current i flowing through the coil.

[0071] 2. Calculation of instantaneous tripping current

[0072] The instantaneous tripping current is obtained from formula 6):

[0073]

[0074] For circuit breakers of the same structure, the magnetic tripping device is composed of the same moving iron core and static iron core, and the effective cross-sectional area A of the magnetic pole and the air gap s are constant. The products of the same frame grade but different rated currents vary in the number of coil turns n and the value of the reaction spring force F. From formula 7), it can be seen that the instantaneous tripping current i is proportional to the square root of the spring force F and inversely proportional to the number of coil turns n.

[0075] 3. Calculation of instantaneous tripping current multiples

[0076] Assume that the nominal rated current of the circuit breaker is In. When a short circuit occurs in the power distribution circuit, the fault current flowing through the coil is i. Let i be x times the rated current, that is, i=x In. Substituting i=x In into 7), the instantaneous tripping current multiple x is

[0077]

[0078] 4. Analogy calculation of instantaneous tripping current multiples

[0079] From formula 7), it can be seen that the instantaneous tripping current is related to the spring reaction force value F, the effective cross-sectional area A, the air gap s, and the vacuum magnetic permeability μ. o It is related to the number of coil turns n. o is a fixed value, while the reaction spring force F, effective cross-sectional area A, air gap s, and coil turns n are all related to the manufacturing process accuracy during the parts manufacturing process, and all have dimensional tolerances. It can be seen that the accuracy of parts manufacturing will directly affect the size of the instantaneous tripping current.

[0080] In addition, during the actual circuit breaker power-on process, the instantaneous tripping device has leakage magnetic phenomena. The distribution of leakage magnetic factors in practice is complex, and the theoretical calculation is also complex. Manual calculation is common in design, so in order to simplify the design calculation process, leakage magnetic problems are often not calculated. However, its impact cannot be ignored.

[0081] Based on the above two issues, the analog calculation method is used to design and study the instantaneous tripping current.

[0082] For circuit breakers of the same frame rating, assuming that the nominal rated current of the circuit breaker is In1, the current i1 flowing through the coil is a times the rated current, that is, i1 = a*In1. For circuit breakers of the same frame rating, assuming that the nominal rated current of the circuit breaker is In2, the current i2 flowing through the coil is b times the rated current, that is, i2 = b*In2.

[0083] Then formula 6) can be listed as:

[0084]

[0085]

[0086] Divide formula 9) by formula 10) to get

[0087]

[0088] Formula 11) is rearranged to obtain:

[0089]

[0090] It can be seen from formula 12) that the instantaneous tripping current multiple b is related to the instantaneous tripping current multiple a, currents In1 and In2, coil turns n1 and n2, and reaction spring force values ​​F1 and F2. Formula 12) no longer includes parameters such as reaction spring force value F, effective cross-sectional area A, and air gap s.

[0091] Through the above analysis and the comparison of formulas 7) and 12), it can be seen that the instantaneous tripping current multiple b in formula 12) is only related to the spring reaction force value, rated current, number of coil turns and instantaneous tripping current multiple a. The above data are all measurable and quantifiable. The influence of problems such as part accuracy and magnetic leakage on the calculation results has been eliminated in formula 12). Therefore, formula 12) is used to convert the instantaneous tripping current into the instantaneous tripping current multiple for analysis and calculation.

[0092] The correctness and accuracy of the method of the present invention are verified by testing the prototype data

[0093] 1. Theoretical calculation of instantaneous tripping current and setting of spring reaction force value

[0094] The analysis and verification is carried out by taking the miniature circuit breaker standard GB10963.1 as an example. The provisions of the standard GB 10963.1 "Electrical accessories - Overcurrent protection circuit breakers for household and similar places - Part 1: Circuit breakers for alternating current" are shown in Table 1 below.

[0095] Table 1

[0096]

[0097] The instantaneous tripping current standard for C-characteristic circuit breakers requires 5 to 10 times the rated current, and the median value of the instantaneous tripping current group is 7.5 times the rated current. In actual use scenarios, when an instantaneous short-circuit current occurs, the earlier the instantaneous tripping device trips, the more conducive it is to protecting the line and equipment. Therefore, when designing, the instantaneous tripping current value of the C-characteristic circuit breaker is usually 7 times the rated current.

[0098] In the circuit breakers of the same frame rating, the C characteristic 32A product is taken as an example to analyze and calculate the tripping current and multiple of the instantaneous tripping device.

[0099] Fixed value parameter Pole cross-sectional area A = π*(5.5 / 2) 2 mm 2 , air gap s = 3mm, vacuum permeability μ o =4π*10 - 7 N / A 2 .

[0100] Rated current In=32A, coil turns n=3, spring force F=0.68N.

[0101] The theoretical instantaneous tripping current is obtained from formula 7:

[0102] That is, i=213.4A.

[0103] The instantaneous tripping current i=aIn, and the instantaneous tripping current multiple a=6.6 is calculated. The instantaneous tripping current 6.6In is between the instantaneous tripping current (5-10)In required by the standard, and is near the designed instantaneous tripping current median 7In.

[0104] 2. Verification of instantaneous tripping current multiples

[0105] The test verification of instantaneous tripping characteristics in GB10963.1 is defined in clause 9.10.3:

[0106] For a C-characteristic circuit breaker, starting from the cold state, a current equal to 5In is passed through all poles in series. The disconnection time should be no less than 0.1s. Then, starting from the cold state again, a current equal to 10In is passed through each pole. The circuit breaker should trip in less than 0.1s.

[0107] In the process of designing and studying the instantaneous tripping current, it is necessary to detect its specific instantaneous tripping current value and disconnection time. During the test process, an oscilloscope can be used to detect its specific instantaneous tripping current value and disconnection time.

[0108] According to the parameter values ​​of A, s, n, F, etc. determined in Section 3.1, 10 prototypes with a rated current of In = 32A were made and the instantaneous tripping current was tested. In order to facilitate analysis and comparison, the instantaneous tripping current value obtained by the test was converted into the instantaneous tripping current multiple for analysis. The test results are shown in Table 2.

[0109] Table 2

[0110]

[0111] The distribution diagram of the measured values ​​is shown in Figure 2 The measured instantaneous tripping current range is (5.55~6.01)*In, and the measured instantaneous tripping current multiple is much lower than the predetermined C characteristic design instantaneous tripping current multiple median of 7 times. It does not meet the design requirements and needs to be adjusted.

[0112] 3. Verification of instantaneous tripping current multiple adjustment

[0113] In the design process of the circuit breaker, once the product shape is determined, the space of the instantaneous tripping device is also determined, and then the cross-sectional area A of the core and the air gap s are fixed values. It can be concluded from formula 7 that the purpose of adjusting the instantaneous tripping current i can be achieved by adjusting the number of coil turns n and the spring force value F. From the perspective of manufacturing technology, the spring force value is more adjustable and easier to achieve, so the adjustment of the spring reaction force value F is given priority.

[0114] The sample In analyzed in this patent is In=32A, so In1=In2=32A, n1=n2=3 in formula 11), and formula 11 can be simplified to:

[0115]

[0116] Then formula (13) is only related to the spring force value F and the instantaneous tripping current multiple a. a and F1 are known quantities. If b is to be increased, the F2 value needs to be increased. That is, the instantaneous tripping current is increased by increasing the spring force value F, and the spring force value F is increased to F2 = 0.96N.

[0117] Assume that the minimum value of the measured value detected in Table 2 is i1'=5.55In, the maximum value is i1"=6.01In, and F1=0.68N. Then take a'=5.55, a"=6.01 in formula 13). Substituting a', F1, and F2 into formula 13), b'=6.59 is obtained; substituting a", F1, and F2 into formula 13), b"=7.14 is obtained; that is, when the reaction spring is changed to F2=0.96N, the theoretical instantaneous tripping current calculated based on the measured value obtained by F1=0.68N is 6.59In~7.14In.

[0118] The theoretical calculation results are obtained after adjusting the reaction spring force F according to the measured values, and the instantaneous tripping current data of the new prototype is tested. The test results are shown in Table 3.

[0119] Table 3

[0120]

[0121] The measured instantaneous tripping current multiple data range is (6.77~7.17)*In. The measured tripping current multiple is about 7 times the median of the designed instantaneous tripping current multiple, and the consistency is very good. The distribution diagram of the measured values ​​is shown in Figure 3 .

[0122] 4. Comparative analysis of instantaneous tripping current multiple theoretical data and test data

[0123] The theoretical calculation data and measured data of this patent are summarized in Table 4.

[0124] Table 4

[0125]

[0126] When the spring force value F = 0.68N, there is a certain difference between the theoretical instantaneous tripping current and the instantaneous tripping current actually measured during the trial production of the prototype. This difference is normal in the actual production process.

[0127] The present invention simplifies the leakage magnetic field and other factors in the design process and the accuracy problem in the parts manufacturing process. The actual measured instantaneous tripping current value (5.55-6.01) In of the trial prototype should be lower than the theoretical calculated value of 6.6 In.

[0128] The theoretical calculation value before making the prototype of F = 0.96N is based on the data measured when the prototype was made when F = 0.68N. The data obtained by the theoretical calculation of F = 0.96N is based on formula 13). Formula 13) no longer contains the reaction spring force value F, cross-sectional area A, air gap s and other data, eliminating the influence of parts accuracy on the calculation results, and using analog calculation to eliminate the influence of factors such as magnetic leakage ignored in the theoretical calculation when F = 0.68N. Therefore, the instantaneous tripping current data measured by the prototype when F = 0.96N is very close to the theoretical instantaneous tripping current data, and the result is very accurate. The theoretical calculation data when F = 0.96N is The measured data is The agreement between the theoretical calculated value of instantaneous tripping current multiple and the measured value reached 97.3%.

[0129] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for calculating the tripping current of an instantaneous tripping device of a low-voltage circuit breaker, characterized in that: The steps of the method are: S1. Calculation of magnetic attraction When the current in the line flows through the low-voltage circuit breaker, a magnetic field is formed in the coil of the instantaneous tripping device. The moving and static iron cores of the instantaneous tripping device generate a magnetic attraction F in the magnetic field formed by the coil. The calculation formula of the magnetic attraction F is: Where: B is the magnetic induction intensity, S is the cross-sectional area of ​​the magnetic pole, μo is the magnetic permeability; Assume the cross-sectional area of ​​the magnetic pole is S = A, Right now Without considering the leakage magnetic factor, the calculation formula of magnetic induction intensity B is: B=μo*H(3) Where: H is the magnetic field strength, and the calculation formula is: Where: s is the length of the magnetic circuit, i.e. the air gap; i is the current flowing through the coil; Substituting 4) into 3) we get Substituting 5) into 2), we get: Where: n is the number of coil turns; i is the current; μo is the vacuum magnetic permeability, which is 4π*10- 7 N / A 2 ; S2, instantaneous tripping current calculation When a short-circuit current occurs in the line, the value of i at the moment when the instantaneous tripping device operates is the instantaneous tripping current value. The calculation formula for the instantaneous tripping current is: S3, instantaneous tripping current multiple calculation Assuming that the nominal rated current of the low-voltage circuit breaker is In, when a short-circuit fault occurs in the distribution circuit, the fault current flowing through the coil is i. Assuming i is x times the rated current, that is, i=x In, then substitute i=x In into 7), the instantaneous tripping current multiple x is S4. Analogy calculation of instantaneous tripping current multiples For circuit breakers of the same frame rating, assuming that the nominal rated current of the circuit breaker is In1, the current i1 flowing through the coil is a times the rated current, that is, i1 = a*In1; assuming that the nominal rated current of the circuit breaker is In2, the current i2 flowing through the coil is b times the rated current, that is, i2 = b*In2; then equation 6) is: Divide formula 9) by formula 10) to get Formula 11) is rearranged to obtain: The instantaneous tripping current multiple b is related to the instantaneous tripping current multiple a, currents In1 and In2, coil turns n1 and n2, and reaction spring force values ​​F1 and F2. Therefore, the instantaneous tripping current is converted into the instantaneous tripping current multiple for analysis and calculation.

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