A closing final voltage calculation method for a direct current circuit breaker contact system

Abstract

The application discloses a method for calculating closing final voltage of a DC circuit breaker contact system, and simultaneously takes the electric force less than the closing final voltage and the highest temperature lower than the melting point of the contact material as the qualified criterion of the closing final voltage; the closing final voltage calculation considering the radius change of the conductive bridge is used; when the transient thermal simulation is performed on the complex model, the steady-state thermal simulation result of the small current is used as the boundary condition of the transient thermal simulation, and whether the transient thermal simulation result is lower than the melting point of the material is used as the convergence criterion of the iterative calculation of the closing final voltage; and the method provided by the application can realize the forward calculation of the closing final voltage of the DC circuit breaker with the delay protection requirement, solves the problem that the closing final voltage of the DC circuit breaker is difficult to be calculated forwardly, shortens the research and development cycle of the circuit breaker, and saves the test cost.

Description

Technical Field

[0001] This invention belongs to the technical field of Class B DC circuit breaker design, and particularly relates to a method for calculating the final closing pressure of a DC circuit breaker contact system. Background Technology

[0002] With the continuous development of DC power supply and distribution technology, Class B DC circuit breakers with time-delay protection functions have been widely used in rail transit, new types of ships, and other fields. Short-time withstand performance is a key technical indicator of Class B circuit breakers, and the final closing pressure of the contact system is the most important factor affecting short-time withstand performance. Currently, the final closing pressure of circuit breakers is basically still in the verification stage, and there is a lack of methods for forward calculation of the final closing pressure.

[0003] With the continuous increase in capacitance, the short-time withstand performance requirements of circuit breakers have risen to over 100kA. In this situation, if the required final closing pressure cannot be accurately calculated, failure of the short-time withstand test will lead to damage to the circuit breaker sample. The traditional R&D path of test-improvement-retest is no longer suitable for the rapid iteration and updates required in current product development. Therefore, from a practical engineering perspective, researching methods for calculating the final closing pressure of circuit breakers is of great significance.

[0004] The short-time withstand test of circuit breakers is divided into rated short-time withstand test and peak short-time withstand test, which examine the electrodynamic stability and thermal stability of the circuit breaker respectively. The two tests are conducted separately. Currently, the verification or calculation of the closing final pressure focuses on the electrodynamic effect and thermal effect of the current respectively. It is believed that in the rated short-time withstand test, the closing final pressure should ensure that the highest temperature of the contacts when the current passes through is lower than the melting point of the material, and in the peak withstand test, it should ensure that the electrodynamic force generated when the current passes through is less than the closing final pressure.

[0005] However, existing engineering test data shows that, whether it is the rated short-time withstand test or the peak withstand test, the electrodynamic effect and thermal effect of the current are the direct causes of test failure. Therefore, considering the two separately is inconsistent with the actual test results.

[0006] For the reasons mentioned above, there is an urgent need for a method to calculate the final closing pressure of the contact system that comprehensively considers the electrodynamic effect and thermal effect of the current, so as to provide a theoretical basis for the forward design of the contact system of Class B DC circuit breakers. Summary of the Invention

[0007] The purpose of this invention is to provide a method for calculating the final closing pressure of a DC circuit breaker contact system, which solves the problem of inaccurate calculations caused by considering only electrodynamic forces and not thermal effects in the calculation of the final closing pressure of Class B circuit breakers, and also solves the problem that the conductive bridge model widely used in the existing simulation calculations of electrodynamic forces and contact resistance of DC circuit breaker contact systems does not take into account the iterative change of the conductive bridge radius with electrodynamic forces.

[0008] To achieve the above objectives, the technical solution of the present invention is: a method for calculating the final closing pressure of a DC circuit breaker contact system, used in a contact system including contact fingers and moving and stationary contacts, implemented according to the following steps:

[0009] Step S1, let i=0, use formula (1) to calculate the initial conductive bridge radius r when the iterative change of the conductive bridge radius reaches the convergence condition considering only the electrodynamic force. i The initial Holm force F between the moving and stationary contacts at this time is calculated according to formula (2). H (i) The Lorentz force F between the moving and stationary contacts was calculated using simulation tools. L (I) Using formula (3), calculate the initial closing final pressure F that is still greater than the electromotive force between the moving and stationary contacts when the radius of the conductive bridge reaches the convergence condition. N (i), where μ0 is the permeability of the medium, I is the short-time withstand current, Δr is the convergence condition for the change in the radius of the conductive bridge, ξ is the contact coefficient of the contact surface, and H B Let A be the Brinell hardness of the contact material, and A be the apparent contact area of ​​the contact; where formula (1) is... Formula (2) is Formula (3) is The Lorentz force F when the current is I L (I) Obtained through simulation;

[0010] Step S2, measure the final closing pressure F. N (i) The contact resistance R0 of the circuit breaker contact system (i.e., R) i (i=0), the contact resistance in subsequent iterations is calculated using formula (4); where formula (4) is ;

[0011] Step S3, use formulas (5) and (6) to calculate the resistivity σ(i) and thermal conductivity λ(i) of the conductive bridge section at this time; where formula (5) is... Formula (6) is , where h is the height of the conductive bridge, r is the radius of the conductive bridge, T is the absolute temperature, and L is the Lorentz coefficient;

[0012] Step S4: Use Ansys to perform transient thermal analysis of the simulation model, build a thermal simulation model of the contact system, and establish a conductive bridge model in the simulation model according to the above calculation results to perform transient thermal analysis and calculate the highest temperature of the contact system at this time.

[0013] Step S5: Determine whether the highest temperature of the contact system exceeds the melting point of the contact material. If the condition is met, determine that the calculation is complete and terminate the calculation, outputting the final closing pressure of the contact system; otherwise, proceed to step S6.

[0014] Step S6: Verify whether the design value of the closing final pressure is greater than the closing final pressure of the electromotive force between the moving and stationary contacts. If so, record the convergence radius of the conductive bridge at this time: denote i=i+1, and increase the closing final pressure by one iteration step ΔF to F. N (i+1)= F N (i)+ΔF, calculate the contact resistance R at this time using formula (4). i The radius r of the conductive bridge at this time is calculated using formula (7). i Repeat step S3 for iterative calculation; where formula (7) is .

[0015] Furthermore, in step S1, the change in electrodynamic force caused by the change in the radius r of the conductive bridge being less than Δr is set as a convergence condition, i.e., r i -r i+1 =Δr, if electrodynamic force is considered, the radius of the conductive bridge is calculated using the formula... calculate.

[0016] Furthermore, in step S2, the final closing pressure F calculated in step S1 is measured using an instrument. N (0) is the contact resistance, denoted as the initial value R0 as a reference. The contact resistance R of the contact system under the final closing pressure is then calculated.

[0017] Furthermore, in step S3, the final closing pressure calculated in step S1 is first set to F. N0 The contact resistance measured in step S2 is set as R0, and thereafter the contact resistance changes when the closing final pressure changes. Meanwhile, the contact resistance generally consists of two parts: the film resistance and the absorption resistance, i.e., R = R m +R s The film resistance is represented by the bulk resistance of the conductive bridge, i.e. The shrinkage resistance is generally calculated using the following formula: resistivity of the conductive bridge section The thermal conductivity of the conductive bridge section is λ = T × L × σ, where the value of the reference coefficient m is related to the contact form of the moving and stationary contacts. For point contact, m is 0.5; for surface contact, m is 1; and for line contact, m is 0.7.

[0018] Furthermore, step S4 involves importing the resistivity σ(0) and thermal conductivity λ(0) of the conductive bridge portion calculated in step S3 into the Ansys electromagnetic module and thermal analysis module. To reduce the difficulty of setting boundary conditions in the transient thermal analysis of complex simulation models, a steady-state thermal simulation is first performed with a small current, i.e., a current of several amperes. The results of the steady-state thermal simulation are then used as boundary conditions in the transient thermal analysis. The current in the steady-state thermal simulation can be set to no more than 50 amperes. In the transient thermal analysis, the model is meshed with a hexahedron, which helps to improve the simulation accuracy.

[0019] Furthermore, the radius r of the conductive bridge needs to be recalculated before repeating step S3. i+1 resistivity σ i+1 With thermal conductivity λ i+1 Formula (7) is .

[0020] Furthermore, if the termination calculation condition is not met, the closing final pressure is increased by one iteration step. The specific steps include: using whether the highest temperature of the contact system exceeds the melting point of the contact material as a pass / fail criterion; if it exceeds the melting point, the closing final pressure is updated using the following formula: F N =F N0 +ΔF, where ΔF is the manually set iteration step, which can be set to F. N0 20-30%.

[0021] Compared with the prior art, the present invention has the following beneficial effects: by comprehensively considering the electrodynamic effect and thermal effect of the current during short-term withstand process, a transient thermal simulation model is established to express the contact resistance by setting relevant parameters of the conductive bridge, taking into account the change in the radius of the conductive bridge caused by the current flowing through.

[0022] This invention provides a convenient and rapid calculation method for the forward design of the closing final pressure of Class B DC circuit breakers. It can simultaneously meet the requirements of electrodynamic stability and thermal stability during short-term withstand processes. The calculation is simple, easy to apply in engineering, and can shorten the circuit breaker development cycle and save testing costs. Attached Figure Description

[0023] Figure 1 This is a flowchart of the method for calculating the final closing pressure of the present invention;

[0024] Figure 2 This is a simplified model of a circuit breaker contact system in one embodiment of the present invention;

[0025] Figure 3 This is a schematic diagram illustrating the change process of the conductive bridge radius and electrodynamic force in one embodiment of the present invention;

[0026] Figure 4 This is a schematic diagram illustrating the change of conductive force with the final closing pressure in one embodiment of the present invention.

[0027] Figure 5 This is a schematic diagram of a transient thermal simulation process in one embodiment of the present invention;

[0028] Figure 6 This is a schematic diagram illustrating the change in the highest temperature of the contact system with the final closing pressure in one embodiment of the present invention. Detailed Implementation

[0029] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0030] The closing final pressure calculation method of this invention is applicable to contact systems containing contact fingers and moving and stationary contacts, namely, frame circuit breakers, molded case circuit breakers, and other Class B circuit breakers with short-time withstand requirements (DC circuit breakers with time delay protection requirements, i.e., those with short-time withstand performance requirements), or multi-contact finger structure contact systems, such as frame circuit breakers and molded case circuit breakers, to solve the problem of inaccurate circuit breaker closing calculation.

[0031] The electrodynamic force of the contact system consists of two parts: the Lorentz force and the Holm force. The Holm force is a repulsive force, while the Lorentz force, depending on the contact system structure, can be either a repulsive force or a compensating force. When the sum of the two is a repulsive force, the electrodynamic force is repulsive; when the sum is a compensating force, the electrodynamic force is compensating. Establishing a conductive bridge for electrodynamic simulation is a common method. Addressing the issue that current calculations of the final closing pressure do not consider changes in the conductive bridge radius, this invention proposes a method for calculating the final closing pressure of a DC circuit breaker contact system, as follows... Figure 1 As shown, it includes the following steps.

[0032] (1) Calculate the closing final pressure F that is still greater than the electric repulsion force of the moving and stationary contacts when the radius of the conductive bridge reaches the convergence condition. N .

[0033] The closing final pressure that ensures the radius of the conductive bridge reaches the convergence condition is still greater than the electromotive force between the moving and stationary contacts is F L (I) represents the Lorentz force when the current is I. It depends only on the magnitude of the current and can be obtained through simulation. If it is a compensating force, its value is negative; if it is a repulsive force, its value is positive. F H (i) is the Holm force between the moving and stationary contacts, which is affected by the radius of the conducting bridge and the magnitude of the current, and is calculated as follows: Where A is the apparent contact area of ​​the contact, the radius of the conductive bridge is calculated using the following formula. , where μ0 is the permeability of the medium.

[0034] According to the formula The calculated closing pressure is still greater than the closing electric force between the moving and stationary contacts when the radius of the conductive bridge reaches the convergence condition.

[0035] At this time, the changes in the radius of the conductive bridge and the electrodynamic force are as follows: Figure 3 As shown.

[0036] The specific process is as follows: Set the change in electrodynamic force caused by a change in the radius of the conductive bridge being less than Δr as a negligible convergence condition, i.e., r i -r i+1 =Δr, if electrodynamic force is considered, the radius of the conductive bridge is calculated using the formula... Calculate, where F N F is the final closing pressure of the contact system. L For Lorentz force, F H H is the Holm force, ξ is the contact coefficient of the contact surface, and H B The Brinell hardness of the material.

[0037] The contact system in this embodiment is as follows: Figure 2 As shown in the table below. The required input parameters include: dielectric permeability, convergence condition of the conductive bridge radius change, contact coefficient of the contact surface, Brinell hardness of the contact material, short-time withstand current, and apparent contact area of ​​the contact. The relevant parameters of the contact system are shown in the table below.

[0038] .

[0039] Based on the data given in the table above, according to the formula... Calculate the radius of the conductive bridge when the convergence condition is met, and solve for r. i =0.0754mm.

[0040] According to the formula and Calculate the Holm force between the moving and stationary contacts on a single contact finger. Simulation results show that the Lorentz force on the contact system under a 100kA current is 357.044N, which is a repulsive force.

[0041] This step uses formula (1) to calculate the initial conductive bridge radius that converges during the iterative change of the conductive bridge radius when only considering the electrodynamic force. ; Calculate the Holm force between the moving and stationary contacts at this time according to formula (2). ; Calculate the initial closing final pressure using formula (3) .

[0042] (2) Use the instrument to measure the initial value of the contact resistance R0 as a reference, and use the formula to calculate the contact resistance R of the contact system under the final closing pressure at this time.

[0043] When the final closing pressure is 804N, the contact resistance is 9.84μΩ. Therefore, the contact resistance on a single contact finger is 9.84μΩ. When the final closing pressure changes, the resistance is determined by the formula... Calculate the contact resistance.

[0044] However, it should be noted that during the current-carrying process, the electrodynamic force acts on the contact pressure, which is not equal to the final closing pressure. The electrodynamic force changes with the final closing pressure as follows: Figure 4 As shown.

[0045] When the final closing pressure is 804N, the electrodynamic force is 753.8979N, and the actual contact pressure is 50.1021N. At this time, the contact resistance is... .

[0046] (3) Calculate the resistivity ρ and thermal conductivity λ of the conductive bridge section at this time.

[0047] Set the final closing pressure calculated in step (1) as F. N0 The contact resistance measured in step (2) is set as R0. The contact resistance when the closing final pressure changes thereafter is... The reference coefficient m is related to the contact form of the moving and stationary contacts; if it is a line contact, then m = 0.7.

[0048] The contact resistance R is generally composed of two parts: the film resistance and the absorption resistance, i.e., R = R m +R s , where R m R is the membrane resistance. s This is the shrinkage resistance. The film resistance is represented by the bulk resistance of the conductive bridge, i.e. Where ρ is the resistivity of the conductive bridge portion; h is the height of the conductive bridge; and r is the radius of the conductive bridge, obtained through the formula in step (1). The shrinkage resistance portion is generally calculated using the following formula: .

[0049] When using a conductive bridge to express contact resistance, the resistivity and thermal conductivity of the conductive bridge portion can be calculated using the following formula: .

[0050] The thermal conductivity of the conductive bridge section is calculated using the following formula: λ = T × L × σ, where λ is the thermal conductivity, L is the Lorentz coefficient, and T is the absolute temperature.

[0051] This step requires inputting the following parameters: conductive bridge height, conductive bridge radius, absolute temperature, and Lorentz coefficient; the relevant parameters of the conductive bridge are shown in the table below.

[0052] .

[0053] According to the table above, resistivity .

[0054] The thermal conductivity of the conductive bridge section is calculated using the following formula: λ = 290.1 ​​× 2.44 × 10⁻⁶ -8 ×2.11×107 =149.59w / (m·K).

[0055] This step uses formulas (5) and (6) to calculate the resistivity of the conductive bridge section at this point. With thermal conductivity .

[0056] (4) Using the above calculation results as parameters, use Ansys finite element analysis to perform transient thermal analysis of the simulation model and calculate the highest temperature of the contact system.

[0057] Import the resistivity ρ and thermal conductivity λ of the conductive bridge part calculated in step (3) into the Ansys electromagnetic module and thermal analysis module; in order to reduce the difficulty of setting boundary conditions in the transient thermal analysis of complex models, first perform steady-state thermal simulation at a current of no more than 50 amperes, and use the results of steady-state thermal simulation as boundary conditions in transient thermal simulation.

[0058] In steady-state thermal simulation, the current can be set to no more than 50 amperes; in transient thermal analysis, the model is divided into hexahedral meshes, which helps to improve simulation accuracy.

[0059] This step requires input parameters including: the melting point of the contact material, the highest temperature in the transient thermal analysis, and the iteration step size of the final closing pressure. The relevant parameters for thermal simulation are shown in the table below.

[0060] .

[0061] Simulation interface as Figure 5 As shown, the highest temperature at this time was 16937.3℃, which far exceeded the material's melting point of 1300℃.

[0062] (5) Determine whether the highest temperature of the contact system is higher than the melting point of the contact material. If it is lower, the calculation is completed and the calculation is terminated. If it is higher, proceed to step (6).

[0063] (6) If the termination calculation condition is not met, the closing final pressure is increased by one iteration step to verify whether the closing final pressure design value is greater than the closing final pressure of the electric force between the moving and stationary contacts. If the requirement is met, the convergence radius of the conductive bridge at this time is recorded.

[0064] After entering the iterative step, the change in the highest temperature of the contact system with the final closing pressure is as follows: Figure 6 As shown, when the closing final pressure is 2204N, the highest temperature is 1273.3℃, ​​which is lower than the material melting point. The calculation ends, and the output closing final pressure is 2204N.

[0065] If the highest temperature in the transient thermal analysis is lower than the melting point of the contact material, the calculation stops, and the closing final pressure at that time is output. If the highest temperature in the transient thermal analysis is higher than the melting point of the contact material, the closing final pressure is increased by one iteration step. Whether the highest temperature of the contact system exceeds the melting point of the contact material is used as the pass / fail criterion. If it exceeds the melting point, the closing final pressure is updated using the following formula: F N =F N0 +ΔF, where ΔF is the manually set iteration step, which can be set to F. N0 20-30%.

[0066] After each iteration, the electromotive force changes (increases), causing the corresponding conductive bridge radius to decrease. Therefore, the conductive bridge radius, resistivity, and thermal conductivity need to be recalculated. Thus, the conductive bridge radius r needs to be recalculated before proceeding to step S3. i Resistivity ρ and thermal conductivity λ: , , , λ i =T×L×σ i .

[0067] The above are preferred embodiments of the present invention. Any changes made to the technical solution of the present invention that do not exceed the scope of the technical solution of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for calculating the final closing pressure of a DC circuit breaker contact system, used for a contact system including contact fingers and moving and stationary contacts, characterized in that: The steps are as follows: Step S1, let i=0, calculate the initial conductive bridge radius when the iterative change of the conductive bridge radius reaches the convergence condition considering only the electrodynamic force. Calculate the initial Holm force between the moving and stationary contacts at this time. Calculate the Lorentz force F between the moving and stationary contacts. L (I) Calculate the initial closing pressure that ensures the radius of the conductive bridge is still greater than the electromotive force between the moving and stationary contacts when the radius of the conductive bridge reaches the convergence condition. Where μ0 is the permeability of the medium, I is the short-time withstand current, Δr is the convergence condition for the change in the radius of the conductive bridge, ξ is the contact coefficient of the contact surface, and H B A is the Brinell hardness of the contact material, and A is the apparent contact area of ​​the contact. Step S2, measure the final closing pressure F. N (i) The contact resistance R0 of the circuit breaker contact system; Step S3: Calculate the resistivity of the conducting bridge. The thermal conductivity λ = T × L × σ, where h is the height of the conductive bridge, r is the radius of the conductive bridge, T is the absolute temperature, and L is the Lorentz coefficient; Step S4: Use Ansys to build a thermal simulation model of the contact system, establish a conductive bridge model based on the above calculation results, perform transient thermal analysis, and calculate the highest temperature of the contact system. Step S5: Determine whether the highest temperature of the contact system exceeds the melting point of the contact material; otherwise, terminate the calculation; otherwise, proceed to step S6. Step S6, let i = i + 1, increase the final closing pressure by one iteration step ΔF to F. N (i+1)= F N (i)+ΔF, calculate the contact resistance R at this time. i Calculate the radius r of the conductive bridge at this time. i Repeat step S3.

2. The method for calculating the final closing pressure of a DC circuit breaker contact system according to claim 1, characterized in that, The convergence condition in step S1 is set such that the change in electrodynamic force caused by the change in the radius r of the conductive bridge being less than Δr can be ignored: r i -r i+1 =Δr, the formula for calculating the radius of the conductive bridge considering electrodynamic force is: .

3. The method for calculating the final closing pressure of a DC circuit breaker contact system according to claim 1, characterized in that, In step S2, the closing final pressure F calculated in step S1 is measured using an instrument. N (0) is the contact resistance, denoted as the initial value R0 as a reference. The contact resistance R of the contact system under the final closing pressure is then calculated.

4. A method for calculating the final closing pressure of a DC circuit breaker contact system according to claim 1, 2, or 3, characterized in that, Step S3 first depends on the closing final pressure F. N0 Calculate the contact resistance R0 when the final closing pressure changes. At the same time, the contact resistance R = R m +R s membrane resistance Shrinkage resistance resistivity of the conductive bridge section The value of the reference coefficient m is related to the contact form of the moving and stationary contacts.

5. The method for calculating the final closing pressure of a DC circuit breaker contact system according to claim 4, characterized in that, Step S4 involves importing the resistivity σ and thermal conductivity λ of the conductive bridge into the Ansys electromagnetic module and thermal analysis module. A steady-state thermal simulation is first performed with a current of several amperes, and the results of the steady-state thermal simulation are used as boundary conditions in the transient thermal analysis.

6. The method for calculating the final closing pressure of a DC circuit breaker contact system according to claim 5, characterized in that, Before repeating step S3, the radius r of the conductive bridge needs to be recalculated. i+1 resistivity σ i+1 With thermal conductivity λ i+1 The formula is .

7. The method for calculating the final closing pressure of a DC circuit breaker contact system according to claim 5, characterized in that, In step S6, whether the highest temperature of the contact system exceeds the melting point of the contact material is used as the qualification criterion. If it exceeds the melting point of the material, formula F is used. N =F N0 +ΔF updates the final closing pressure, where ΔF is set to F. N0 20-30%.

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