Real-time measurement method for contact resistance and load capacity of switch cabinet in live operation

Abstract

The application provides a real-time calculation method for contact resistance and load capacity of a switch cabinet in a live operation, comprising: establishing a 3D model of the switch cabinet and performing heat source and temperature distribution analysis; based on the heat source and temperature distribution analysis results, a qualitative thermal circuit model is constructed for the purpose of solving the steady-state temperature of the switch cabinet contact; based on the qualitative thermal circuit model, a qualitative expression with undetermined coefficients of the contact temperature rise, contact resistance and load current of the switch cabinet is obtained; based on the qualitative expression, a function expression of the relationship between the contact resistance of the switch cabinet contact, the actual load capacity of the switch cabinet and the current measured steady-state temperature rise and load current is obtained; a temperature fluid field model of the switch cabinet is established to quantitatively calculate the numerical value of the contact temperature rise; the calculation result of the temperature fluid field is used to solve the undetermined coefficient to obtain the corresponding quantitative expression. The application solves the problem that the contact resistance of the disconnecting switch of the switch cabinet cannot be measured live, and even when the power is off, it is also difficult to measure independently.

Description

Technical Field

[0001] This invention relates to the field of switchgear, specifically a method for real-time calculation of the contact resistance and load capacity of switchgear during energized operation. Background Technology

[0002] Overheating defects in switchgear typically originate from localized overheating at the contacts of handcart-type disconnectors. The main causes are increased contact resistance due to loose contact springs, bolts, or inadequate contact fingers, resulting in an actual load capacity lower than the rated value. Switchgear with these contact defects operating beyond its actual load capacity for extended periods can lead to contact overheating, oxidation and melting, and deterioration of insulation performance, potentially causing short circuits, large-scale fires, and other accidents, severely impacting the reliability of the power grid.

[0003] The load capacity of switchgear is directly related to the contact resistance of the disconnector contacts and other contact points. Currently, contact resistance can only be measured during switchgear de-energization tests; there is no method for directly measuring contact resistance while the switchgear is energized. Furthermore, due to the limitations of sealed vacuum circuit breakers, it is impossible to insert a circuit resistance tester between the circuit breaker and the disconnector contacts. Therefore, even during de-energization tests, only the total circuit resistance of the switchgear can be measured, and the contact resistance of individual disconnector contacts cannot be accurately measured. During peak summer and winter power demand periods, switchgear frequently operates under heavy loads, and it is difficult to request power outages for maintenance. Therefore, real-time monitoring of the actual maximum load capacity of the entire switchgear station, and the corresponding rational load allocation, is crucial for the safe and stable operation of the switchgear.

[0004] Currently, the national standard GB / T 11022-2020 "Common Technical Requirements for High Voltage Switchgear and Control Equipment Standards" and the State Grid Corporation's guideline "Modular Construction of Substations Version 2.0" (〔2021〕31) clearly stipulate the need to install contact temperature sensors, especially in high-current switchgear, and establish contact temperature rise limits. However, existing switchgear contact temperature monitoring methods can only measure contact temperature, have weak data mining capabilities, and lack load capacity verification functions. When the switchgear is not under heavy load, the contact temperature is within the temperature rise limit range when the contact resistance is normal, moderately deteriorated, or severely deteriorated, and the temperature measuring device will not issue an early warning. However, when the load increases to near the rated value, the contact temperature of the defective contact will rapidly exceed the standard temperature range, resulting in overheating defects.

[0005] In summary, real-time acquisition of contact resistance values ​​and load capacity calculations for operating switchgear under energized conditions is of great significance for switchgear operation and maintenance and dispatching. It also represents a deep-dive application of existing switchgear contact temperature and load current data. The former enables the formulation of differentiated operation and maintenance strategies for switchgear, allowing for the reasonable and quantitative reduction of the operating load on switchgear with general contact defects and the timely shutdown of switchgear with severe contact defects, thereby preventing switchgear overheating and assisting in the rational and timely formulation of maintenance plans, achieving the goal of preventing problems before they occur. The latter can assist substation dispatchers in rationally formulating dispatching plans and allocating the load of each switchgear according to the actual load capacity.

[0006] Currently, numerous patents exist for methods to detect and alarm in real-time at various contact points inside switchgear, based on direct measurement by internal temperature sensors or estimation using external temperature measurements. However, existing contact temperature monitoring methods only measure the current contact temperature and lack load capacity verification functionality. When the switchgear load is low, the temperatures of both normal and defective contacts remain within the rated range, and the temperature measuring device does not issue an alarm. However, when the load increases to near the rated value, the temperature of defective contacts rapidly exceeds the standard temperature range, leading to overheating.

[0007] The key reason why load capacity verification cannot be achieved is that the contact resistance value cannot be measured while the switchgear is energized. This is because the contact resistance of the disconnecting switches in switchgear changes due to incomplete closing, long-term vibration, and deformation during operation. Compared to transformers, lines, and cables, the resistance value exhibits randomness and cannot be directly measured during operation, making load capacity verification more difficult than for other equipment. Furthermore, the complex structure, multiple heat sources, and various heat exchange methods of switchgear also increase the difficulty of constructing load capacity prediction models. Typically, only thermal circuit models with clear physical meaning but only qualitative analysis can be constructed, or temperature-fluid field models that can be quantitatively calculated but cannot directly characterize the relationship between contact temperature and other variables. Summary of the Invention

[0008] The purpose of this invention is to provide a method for real-time calculation of the contact resistance and load capacity of a switchgear in operation. By measuring the load current and contact temperature of the switchgear in operation, the load resistance of the switchgear is calculated based on the thermal circuit theory model and regression method. Based on this, the load capacity of the switchgear is further calculated. This overcomes the shortcomings of existing switchgear contact resistance measurement schemes, which require non-energized operation, and the deficiencies of existing switchgear contact temperature measurement methods, which have weak data mining capabilities and lack load capacity verification functions.

[0009] To achieve the above objectives, the present invention provides a method for real-time calculation of the contact resistance and load capacity of a switchgear in operation, comprising the following steps:

[0010] (1) Establish a 3D model of the switch cabinet and perform heat source and temperature distribution analysis based on the 3D model of the switch cabinet;

[0011] (2) Based on the analysis results of heat source and temperature distribution, a qualitative thermal circuit model is constructed for the purpose of solving the steady-state temperature of switch cabinet contacts, and the expression of the steady-state temperature T1 of the contacts is solved.

[0012] (3) Based on the qualitative thermal circuit model and the expression of the steady-state temperature T1 of the contact, a qualitative expression with undetermined coefficients is derived for the temperature rise of the switch cabinet contacts, contact resistance, and load current.

[0013] (4) Based on the qualitative expression with undetermined coefficients of the switch cabinet contact temperature rise, contact resistance, and load current, transform the expression to obtain the functional expression of the relationship between the contact resistance of the switch cabinet contacts, the actual load capacity of the switch cabinet, and the currently measured steady-state temperature rise and load current.

[0014] (5) Establish a temperature fluid field model for the switchgear and quantitatively calculate the contact temperature rise under multiple load currents and contact resistances;

[0015] (6) Using the calculation results of the temperature fluid field, based on the binary nonlinear regression method, all the undetermined coefficients in the qualitative thermal circuit model and the contact resistance of the switch cabinet contacts and the actual load capacity function expression of the switch cabinet are obtained, and a quantitative expression of the relationship between the contact resistance of the switch cabinet contacts, the actual load capacity of the switch cabinet and the currently measured steady-state temperature rise and load current is obtained.

[0016] Furthermore, step (1) involves analyzing the heat source and temperature distribution based on the 3D model of the switchgear, including:

[0017] Heat source distribution analysis: 1) The ohmic loss Q1 of the contact resistance r1 of the contact is the main source of thermal defects; 2) The ohmic loss Q2 of the copper busbar near the contact of this phase; 3) The eddy current loss Q3 of the galvanized steel plate shell and the partition; 4) The ohmic loss Q4 of the copper busbar of the other two phases.

[0018] Temperature distribution analysis: 1) Temperature T1 at the contact of a certain phase disconnector; 2) Average temperature T2 of the copper busbar near the contact of this phase; 3) Average temperature T3 of the outer casing and partition near the contact; 4) Average temperature T4 of the copper busbar of the other two phases near the contact. There are equivalent thermal resistances R1, R2, R3, and R4 between each temperature point due to heat conduction, convection, and radiation heat transfer. There is an equivalent thermal resistance R0 between the partition and the outer casing and the ambient temperature T0.

[0019] The thermal resistance expressions for heat conduction, heat convection, and heat radiation properties are shown in equations (1), (2), and (3):

[0020]

[0021]

[0022]

[0023] Where δ is the thickness of the heat-conducting layer, λ is the thermal conductivity of the material, A is the heat-conducting area, h is the convective heat transfer coefficient, A is the radiative area, σ is the Boltzmann constant, and ε is the emissivity of the material surface.

[0024] Furthermore, in step (2), the expression for the steady-state temperature T1 of the contact is obtained, including: listing the thermal circuit equations (4) based on the qualitative thermal circuit model, and obtaining the expression for the steady-state temperature T1 of the contact (5):

[0025]

[0026]

[0027] Furthermore, step (3) derives qualitative expressions with undetermined coefficients for the switchgear contact temperature rise, contact resistance, and load current based on the qualitative thermal circuit model and the expression for the steady-state contact temperature T1, including:

[0028] (1) Thermal resistance analysis

[0029] R1 is the thermal resistance with thermal conductivity, and its expression is shown in equation (1); R2, R3, R4, and R0 have thermal convection properties without considering radiation, and their expressions are shown in equation (2).

[0030] (2) Heat source analysis

[0031] Under the above assumptions, the thermal resistance values ​​in equation (5) are approximately considered constants, and each heat source can be considered to be proportional to the square of the load current, i.e.:

[0032] Q i =I 2 r i (i = 1, 2, 4)

[0033] Q3=(K e I) 2 r3 (6)

[0034] Where r1, r2, r3, and r4 are the contact resistance of the contact, the resistance of the copper busbar of this phase, the resistance of the casing and the partition, and the resistance of the copper busbar of other phases, respectively, and K e The ratio of average eddy current value to load current value on the simplified housing and partition;

[0035] (3) Derivation of the expression relating switchgear contact temperature rise to contact resistance and load current

[0036] The contact temperature in equation (5) can be simplified as follows:

[0037] T = T0 + I 2 (ar c +b) (7)

[0038] The qualitative expression for the temperature rise of switchgear contacts relative to contact resistance and load current, with undetermined coefficients, can be written in the form of temperature rise relative to ambient temperature as follows:

[0039] ΔT=T-T0=I 2 (ar c +b) (8)

[0040] Where a and b are undetermined coefficients related to the thermal resistance of each part of the cabinet and the busbar resistance, as shown in equation (9):

[0041]

[0042]

[0043] Furthermore, step (4) transforms the qualitative expression with undetermined coefficients regarding the temperature rise of the switchgear contacts, contact resistance, and load current to derive a functional expression relating the contact resistance of the switchgear contacts, the actual load capacity of the switchgear, and the currently measured steady-state temperature rise and load current, including:

[0044] Based on equation (8), the contact resistance r of the switch cabinet contacts is derived. c The function to be determined is a calculation function with the measured steady-state temperature rise ΔT and load current I as input quantities:

[0045]

[0046] Set the temperature rise ΔT in equation (8) to the temperature rise limit ΔT in GB / T 11022-2020. max =75K, thus obtaining the maximum load current I max The expression:

[0047]

[0048] Substituting the contact resistance calculation function (10) into equation (11), we obtain the result with the maximum load current I. max The function to be determined is a calculation function with the measured steady-state temperature rise ΔT and load current I as input quantities:

[0049]

[0050] Using equations (11) and (12) under the operating condition of the switchgear, the contact resistance r, which was originally impossible to measure under energized conditions, is calculated by using the steady-state temperature rise ΔT and load current I, which are easily measurable in actual operation. c And estimate the maximum allowable load current I of the switchgear.max .

[0051] The present invention has the following beneficial effects:

[0052] (1) A method has been realized that in a switch cabinet under energized operation, the contact resistance of the contact can be calculated under energized conditions based solely on the steady-state load current and the contact temperature of the handcart-type disconnector. This solves the problem that the contact resistance of the disconnector in the switch cabinet cannot be measured under energized conditions, and it is even difficult to measure independently when the power is off. This allows substation maintenance personnel to monitor the contact defects in the switch cabinet at any time.

[0053] (2) A method has been developed to calculate the actual maximum load capacity of switchgear in operation based solely on steady-state load current and handcart disconnector contact temperature measurement. This solves the problem that the actual maximum load capacity of switchgear in operation cannot be obtained at present. The online monitoring data of switchgear has been thoroughly analyzed, which can be used by substation dispatchers to reasonably allocate load plans based on the actual maximum load capacity of switchgear. Attached Figure Description

[0054] Figure 1 This is a flowchart of the method for real-time calculation of switchgear contact resistance and load capacity during energized operation according to the present invention;

[0055] Figure 2 This is a 3D model diagram of a switch cabinet according to an embodiment of the present invention, wherein (a) is a physical model and (b) is a simplified model;

[0056] Figure 3 It is an equivalent thermal circuit model of the switchgear;

[0057] Figure 4 It is a regression analysis curve of contact temperature with respect to load current and contact resistance;

[0058] Figure 5 It is an equipotential graph for measuring contact resistance under different load currents and contact temperature rises;

[0059] Figure 6 It is an equipotential graph for calculating the maximum load current under different load currents and contact temperature rise conditions. Detailed Implementation

[0060] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0061] Please see Figure 1 This invention provides a method for real-time calculation of the contact resistance and load capacity of a switchgear during energized operation, comprising the following steps:

[0062] Step 1: Establish Figure 2 The 3D model of the switch cabinet shown is used to analyze the heat source and temperature distribution near the contact of a certain phase (such as the contact defect phase). Figure 2 The components corresponding to each number are as follows: 1-Cable compartment, 2-Busbar compartment, 3-Circuit breaker compartment, 4-Instrument compartment, 5-Cable joint, 6-Current transformer, 7-Busbar, 8-Circuit breaker, 9-Handcart.

[0063] Analysis of heat source distribution and temperature distribution patterns based on a 3D model of the switchgear:

[0064] (1) Heat source distribution analysis

[0065] 1) The ohmic loss Q1 of the contact resistance r1 of the contact is the main source of thermal defects; 2) The ohmic loss Q2 of the copper busbar near the contact of this phase; 3) The eddy current loss Q3 of the galvanized steel shell and the partition; 4) The ohmic loss Q4 of the copper busbar of the other two phases.

[0066] (2) Temperature distribution analysis

[0067] 1) Temperature T1 at the contact of a phase disconnector; 2) Average temperature T2 of the copper busbar near the contact of that phase; 3) Average temperature T3 of the casing and partition near the contact; 4) Average temperature T4 of the copper busbars of the other two phases near the contact. There are equivalent thermal resistances R1, R2, R3, and R4 between each temperature point due to heat conduction, convection, and radiation. There is also an equivalent thermal resistance R0 between the partition and casing and the ambient temperature T0 (to avoid ambiguity, R represents thermal resistance and r represents electrical resistance in this application).

[0068] The thermal resistance expressions for heat conduction, heat convection, and heat radiation properties are shown in equations (1), (2), and (3).

[0069]

[0070]

[0071]

[0072] Where δ is the thickness of the heat-conducting layer, λ is the thermal conductivity of the material, A is the heat-conducting area, and h is the convective heat transfer coefficient, which is typically 20 W / (m²) for natural air convection inside the switch cabinet. 2 ·K), where A is the radiating area, σ is the Boltzmann constant, σ=5.67×10 -8 W / (m 2 ·K 4ε is the surface emissivity of the material; the ε of the copper busbar is 0.03, and the ε of the galvanized steel plate is 0.25.

[0073] Step 2, based on the Figure 2 Based on the heat source and temperature analysis results of the 3D model of the switchgear shown in (a), a qualitative thermal circuit model is constructed with the aim of solving the steady-state temperature of the switchgear contacts, and the expression for the contact temperature T1 is solved.

[0074] Analogous to a circuit system, an equivalent thermal diagram of the area near the switchgear contacts can be obtained with appropriate simplifications, such as... Figure 3 As shown.

[0075] based on Figure 3 The qualitative thermal circuit model shown can be used to derive the thermal circuit equation set (4) and obtain the expression for the steady-state temperature T1 of the contact (5).

[0076]

[0077]

[0078] The following simplified assumptions are made to rationalize the thermal circuit model:

[0079] (1) The changes in resistance and thermal conductivity of each metal material within the allowable temperature rise range are not considered.

[0080] (2) Neglecting radiative heat transfer. Because according to equations (2) and (3), at an ambient temperature T2 = 25℃ (298.15K) and a contact temperature of T1 = 100℃ (373.15K), which is the maximum limit specified in GB / T 11022-2020, the ratio of radiative heat transfer resistance to convective heat transfer resistance is R. σ / R h =76.77, therefore R can be ignored when the two are connected in parallel. σ Similarly, the R on the surface of the galvanized steel partition... σ / R h =9.21, in reality the temperature rise of the partition surface is much smaller than the temperature rise of the contact, so this ratio is greater than the above estimated value.

[0081] (3) Considering that defects often occur in single phases, the average temperature of the non-defective phase contacts and conductors is consistent, and the temperature distribution of the shell and partition is relatively uniform.

[0082] Step 3, based on Figure 3 Based on the qualitative thermal circuit model shown and the expression for the steady-state temperature T1 of the contact, qualitative expressions with undetermined coefficients are derived for the temperature rise of the switchgear contacts, contact resistance, and load current.

[0083] (1) Thermal resistance analysis

[0084] Figure 3 In the equation, R1 is the thermal resistance with thermal conductivity, and its expression is shown in equation (1); R2, R3, R4, and R0 have thermal convection properties without considering radiation, and their expressions are shown in equation (2).

[0085] (2) Heat source analysis

[0086] Under the above assumptions, the thermal resistance values ​​in equation (5) can be approximated as constants, and the heat sources can be considered to be proportional to the square of the load current, i.e.:

[0087] Q i =I 2 r i (i = 1, 2, 4)

[0088] Q3=(K e I) 2 r3 (6)

[0089] Where r1, r2, r3, and r4 are the contact resistance of the contact, the resistance of the copper busbar of this phase, the resistance of the casing and the partition, and the resistance of the copper busbar of other phases, respectively, and K e The ratio of average eddy current value to load current value on the simplified housing and partition.

[0090] (3) Derivation of the expression relating switchgear contact temperature rise to contact resistance and load current

[0091] The contact temperature in equation (5) can be simplified to:

[0092] T = T0 + I 2 (ar c +b) (7)

[0093] The form of temperature rise relative to ambient temperature during writing is:

[0094] ΔT=T-T0=I 2 (ar c +b) (8)

[0095] Where a and b are undetermined coefficients related to the thermal resistance of each part of the cabinet and the busbar resistance, as shown in equation (9):

[0096]

[0097]

[0098] Step 4: Based on the qualitative expressions with undetermined coefficients for switchgear contact temperature rise, contact resistance, and load current, transform them to derive the calculation functions for switchgear contact resistance and actual load capacity with steady-state temperature rise ΔT and load current I as input quantities.

[0099] According to equation (8), the contact resistance r of the switch cabinet contacts can be derived. c The function to be determined is a calculation function with the measured steady-state temperature rise ΔT and load current I as input quantities:

[0100]

[0101] In addition, the temperature rise ΔT in equation (8) is set to the temperature rise limit ΔT in GB / T 11022-2020. max =75K, and the maximum load current I can be obtained. max The expression:

[0102]

[0103] Substituting the contact resistance calculation function (10) into equation (11), we can obtain the result with the maximum load current I. max The function to be determined is a calculation function with the measured steady-state temperature rise ΔT and load current I as input quantities:

[0104]

[0105] Using equations (11) and (12), the contact resistance r, which was originally impossible to measure under energized conditions, can be calculated in the operating state of the switchgear by using the steady-state temperature rise ΔT and load current I, which are easily measurable in actual operation. c And estimate the maximum allowable load current I of the switchgear. max .

[0106] Step 5: Establish a temperature fluid field model for the switchgear and perform contact temperature rise calculations under different contact resistances and load currents.

[0107] The aforementioned qualitative thermal circuit model significantly simplifies the internal temperature distribution, material properties, and heat transfer paths and methods within the switchgear. Therefore, the accuracy of the contact resistance and load current calculation functions needs to be verified through simulation or experiments. Furthermore, the contact resistance r... c The calculation function (11) contains undetermined coefficients a and b related to multiple thermal resistances and heat sources. However, the internal structure of the switch cabinet is complex in practice, and it is impossible to directly use equations (1) and (2) to calculate the thermal resistance at various points by giving material properties and geometric relationships of components. In particular, the convective heat transfer coefficient h in equation (2) varies at different locations with different airflow velocities inside the cabinet. In addition, the eddy currents in the baffles are also difficult to calculate by means of thermal circuits.

[0108] Therefore, it is necessary to go through such Figure 2 The switchgear temperature and fluid field model shown in Figure (b) is used to calculate the contact resistance r within multiple given ranges using magnetic field and temperature fluid field calculation software equipped with finite element or finite volume methods. cThe temperature rise ΔT of the contact under load current I.

[0109] Step 6: Using the calculation results of the temperature fluid field, use bivariate nonlinear regression to obtain the undetermined coefficients in the qualitative thermal circuit model, and obtain the quantitative expression of the functional relationship between the contact resistance of the switchgear contacts, the actual load capacity of the switchgear and the currently measured steady-state temperature rise and load current.

[0110] Using software toolkits with binary nonlinear regression capabilities, such as MATLAB's cftool toolkit, the switchgear contact temperature values ​​under various load currents and contact resistances are used as data sources. Regression analysis is performed on the undetermined coefficients a and b in equation (8), thereby deriving the quantitative forms of equations (8), (10), and (12). The above quantitative expressions (10) and (12) are the calculation formulas for the switchgear contact resistance and actual maximum load capacity that can be used for substation operation and maintenance scheduling.

[0111] Step 7: Implement field applications.

[0112] Based on the final calculation functions (10) and (12) for switchgear contact resistance and maximum load capacity, tables or equipotential graphs of switchgear contact resistance and maximum load capacity under different load currents and contact temperatures are made for use by on-site maintenance personnel, or corresponding calculation software modules are developed in the substation data center to realize real-time calculation and early warning.

[0113] Specific calculation example:

[0114] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the operation steps of the method of the present invention are described below with an example.

[0115] 1. Introduction to the example model

[0116] This example uses a KYN28A-12 type 630A self-cooled switchgear as an example to illustrate the entire process and specific steps of the method. The simulation physical model of the switchgear is as follows: Figure 2 As shown in a, the simplified model is as follows: Figure 2 As shown in (b).

[0117] Figure 2 The cabinet dimensions of the switchgear are 1800mm (depth) × 1000mm (width) × 2300mm (height), and the busbar specifications are 10 × 125 × 3mm. 2 The simplifications made in the model include

[0118] 1) Retain components that affect the distribution of the fluid field inside the cabinet, including non-current-carrying components such as the main circuit, contact box, post insulator, and handcart; 2) Ignore parts that have little impact on the heating and cooling of the current-carrying circuit, such as the instrument room and connecting parts; 3) Simplify details, such as filling bolt holes, and simplify complex mechanisms such as plum blossom contacts or overlapping joints into cylindrical or square plate structures with the same total resistance.

[0119] 2. Material settings for the simulation model

[0120] Regarding the parameter settings of the example model, the conductive circuits of the overhead incoming busbar, upper branch busbar, and lower branch busbar of the switchgear are made of copper, while the supporting insulators, circuit breaker housings, and contact boxes are made of epoxy resin; the handcart, partitions, and cabinet doors are made of galvanized steel plates, and their physical parameters are shown in Table 1.

[0121] Table 1 Material physical parameters (20℃)

[0122]

[0123]

[0124] To address the deterioration of contact resistance at the contact points of disconnecting switches, this invention employs a virtual material method for contact resistance modeling. The overall resistance value is controlled by adjusting the resistivity of the virtual material at the contact and overlap, which is equivalent to an equal-resistivity cylindrical or square plate structure. The overall resistance of the contact and overlap ranges from 15 μΩ under normal conditions to 20 times the maximum resistance under defective conditions, i.e., 300 μΩ.

[0125] 3. Calculation results of temperature and fluid field

[0126] Temperature and fluid field calculations were performed on the above simulation model under multiple temperature load current and contact resistance conditions using temperature and fluid field simulation software based on the finite element and finite volume methods. Taking phase A as an example, the calculation results are shown in Table 2.

[0127] Table 2 Simulation Calculation Results of Temperature Rise of Switchgear Contacts (K)

[0128]

[0129] 4. Regression analysis of undetermined coefficients in the qualitative thermal circuit model

[0130] Based on the qualitative expression (8) of the contact temperature with undetermined coefficients in the thermal circuit analysis, a bivariate nonlinear regression analysis was performed on the simulated temperature fluid field data in the table above, and the results were plotted as follows: Figure 4 The regression surface of the contact temperature with respect to the load current and contact resistance is shown, and the undetermined coefficient a in equation (8) is obtained as 9.729 × 10. -7 b = 1.052 × 10 -4 .

[0131] The root mean square error (RMSE) obtained from the regression calculation is 4.35, meaning the average error of the contact temperature is 4.35 K. With a limit of 75 K for ΔT, the relative error is 5.80%, indicating that the regression equation can well reflect the relationship between contact temperature, load current, and contact resistance. The regression error mainly stems from the assumptions in the thermal circuit model regarding the average equivalent of the copper busbars and surface temperatures near the switchgear contacts, and the assumption that conductor resistance does not change with temperature.

[0132] Substituting the undetermined coefficients a and b obtained from the regression analysis into the contact resistance calculation function (10), and under the given load current I and steady-state temperature rise ΔT, the values ​​of the contact resistance calculation function (10) and the maximum load current calculation function (12) are calculated, and the equipotential lines of the two calculation functions can be plotted, such as... Figure 5 and Figure 6 As shown, the table can be used by operation and maintenance personnel to look up the measured values ​​of contact resistance and maximum load capacity based on the relatively stable load current and contact temperature measured over a longer period of time.

[0133] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for real-time measurement of contact resistance and load capacity of an on-line switchgear, characterized in that, Comprising the following steps: (1) Establishing a switch cabinet 3D model, and performing heat source and temperature distribution analysis based on the switch cabinet 3D model; (2) Based on the heat source and temperature distribution analysis results, constructing a qualitative thermal circuit model for solving the switch cabinet contact steady-state temperature, and solving the expression of the contact steady-state temperature T1; (3) Based on the qualitative thermal circuit model and the expression of the contact steady-state temperature T1, obtaining a qualitative expression of the switch cabinet contact temperature rise with the contact resistance and the load current with undetermined coefficients; (4) Based on the qualitative expression of the switch cabinet contact temperature rise with the contact resistance and the load current with undetermined coefficients, transforming to obtain a functional expression of the relationship between the contact resistance of the switch cabinet contact, the actual load capacity of the switch cabinet, and the current measured steady-state temperature rise and load current; (5) Establishing a switch cabinet temperature fluid field model, and quantitatively calculating the contact temperature rise under multiple groups of load currents and contact resistances; (6) Using the calculation results of the temperature fluid field, based on the binary nonlinear regression method, solving all the undetermined coefficients in the qualitative thermal circuit model and the functional expression of the contact resistance of the switch cabinet contact and the actual load capacity of the switch cabinet, and obtaining a quantitative expression of the relationship between the contact resistance of the switch cabinet contact, the actual load capacity of the switch cabinet, and the current measured steady-state temperature rise and load current.

2. The method of claim 1, wherein the method is characterized by: In step (1), the heat source and temperature distribution analysis based on the switch cabinet 3D model includes: Heat source distribution analysis: 1) The ohmic loss Q1 of the contact resistance r1 of the contact is the main source of heat defect; 2) The ohmic loss Q2 of the copper bar near the contact; 3) The eddy current loss Q3 of the galvanized steel plate shell and the partition plate; 4) The ohmic loss Q4 of the copper bars of the other two phases; Temperature distribution analysis: 1) The temperature T1 at the contact of the disconnector of a certain phase; 2) The average temperature T2 of the copper bar near the contact of the phase; 3) The average temperature T3 of the shell and the partition plate near the contact; 4) The average temperature T4 of the copper bars of the other two phases near the contact, and there are equivalent thermal resistances R1, R2, R3, R4 between each temperature point due to heat conduction, convection and radiation heat exchange, and there is an equivalent thermal resistance R0 between the partition plate and the shell and the environment temperature T0; The thermal resistance expressions of the thermal conduction, thermal convection and thermal radiation properties are shown in equations (1), (2) and (3): Where δ is the thickness of the thermal conduction layer, λ is the thermal conductivity of the material, A is the thermal conduction area, h is the convective heat transfer coefficient, A is the radiation area, σ is the Boltzmann constant, and ε is the material surface emissivity.

3. The method of claim 2, wherein the method is characterized by: In step (2), the expression of the contact steady-state temperature T1 is solved, including: based on the qualitative thermal circuit model, the thermal circuit equation group (4) is listed, and the expression (5) of the contact steady-state temperature T1 is obtained:

4. The method of claim 3, wherein the method is characterized by: In step (3), based on the qualitative thermal circuit model and the expression of the contact steady-state temperature T1, a qualitative expression of the switch cabinet contact temperature rise with the contact resistance and the load current with undetermined coefficients is obtained, including: (1) Thermal resistance analysis R1 is the thermal resistance of thermal conduction property, the expression is shown in equation (1); R2, R3, R4, R0 are of thermal convection property without considering radiation, the expression is shown in equation (2); (2) Heat source analysis Under the above assumptions, each thermal resistance value in equation (5) is approximately considered as a constant, and each heat source can be considered as being proportional to the square of the load current, i.e.: Q i =I 2 r i (i = 1, 2, 4) Q3 = (K e I) 2 r3(6) Wherein, r1, r2, r3, r4 are respectively contact resistance, self-phase copper bar resistance, shell and partition resistance, other-phase copper bar resistance, K e is the ratio of the average eddy current value on the shell and the partition to the load current value; (3) Switchgear contact temperature rise and contact resistance, load current relationship expression derivation The contact temperature in equation (5) is simplified as: T = To + I 2 (ar c +b) (7) The qualitative expression of switchgear contact temperature rise and contact resistance, load current with undetermined coefficients is written as: ΔT = T - T0= I 2 (ar c + b)(8) Where a, b are undetermined coefficients related to the thermal resistance of each part of the cabinet and the busbar resistance, as shown in equation (9):

5. The method of claim 4, wherein the method is characterized by: Step (4) is based on the qualitative expression of switchgear contact temperature rise and contact resistance, load current with undetermined coefficients, and the function expression of the relationship between the contact resistance of the switchgear, the actual load capacity of the switchgear and the current measured steady-state temperature rise, load current is obtained, including: According to equation (8), the switchgear contact resistance r c The calculation function for the function to be sought, with the measured values of the steady-state temperature rise ΔT and the load current I as input quantities: Set the temperature rise ΔT in formula (8) to the temperature rise limit ΔT in GB / T 11022-2020 max = 75 K, the maximum load current I max Expression: Substituting the contact resistance measurement function (10) into (11) gives the maximum load current I max The measurement function for the function to be solved, which takes the measured values of the steady-state temperature rise ΔT and the load current I as input quantities: Using formula (11) and formula (12) in the running state of the switch cabinet, the contact resistance r which cannot be measured under the condition of live measurement is calculated by the steady-state temperature rise ΔT and the load current I which are easy to measure in actual operation c , and the maximum load current I allowed by the switch cabinet is estimated max .

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