A design method for DC bus capacitor ripple current parameters in electric vehicles

Abstract

This invention relates to a method for designing DC bus capacitor ripple current parameters for electric vehicles. The method first requires collecting the operating condition information of the entire electric vehicle and converting this information into a finite number of equivalent operating points. The weight of each operating point in the entire operating cycle is calculated. Then, these finite number of vehicle operating points are mapped to the inverter load. This mapped operating point is applied to the electrical performance simulation of the inverter system to calculate the relationship between the DC bus capacitor ripple current and the inverter load current. The operating time for each ripple current value is determined. Finally, the heating of the DC bus capacitor is evaluated based on the operating time. This solution improves the effectiveness of evaluating the heating of the DC bus capacitor, reduces redundancy in the DC bus capacitor design, simplifies the heat dissipation design of the DC bus capacitor in the inverter system, and saves system costs.

Description

Technical Field

[0001] This invention relates to the field of capacitor heat generation calculation methods, specifically to a method for designing DC bus capacitor ripple current parameters for electric vehicles. Background Technology

[0002] As a key component of inverters for electric vehicles, the DC bus capacitor is located between the DC power supply and the power module of the inverter, and plays the role of energy storage and filtering.

[0003] In the parameter design of DC bus capacitors, the ripple current that the capacitor withstands when the inverter system outputs continuous power and continuous current is usually used to evaluate the steady-state heating of the DC bus capacitor. The steady-state temperature rise of the DC bus capacitor obtained by this method is usually relatively high and cannot meet the temperature limit of the DC bus capacitor.

[0004] To address these issues, DC bus capacitors typically employ design improvements such as increasing their capacitance or enlarging the busbar area (reducing the equivalent series resistance of the DC bus capacitor). Inverter systems also incorporate enhanced heat dissipation designs for the DC bus capacitor. However, in electric vehicle applications, the frequency and duration of continuous power and current in the inverter system are uncertain, often falling far short of the DC bus capacitor's thermal equilibrium condition. Therefore, using the aforementioned methods to calculate DC bus capacitor heat dissipation would lead to over-design of heat dissipation for both the DC bus capacitor and the inverter system, ultimately increasing the cost of inverter systems for electric vehicles.

[0005] The article "Design and Testing of Film Capacitors for Silicon Carbide Controllers" (2021.48(10)) published in the journal *Electrical Machines and Control Applications* investigated the internal temperature distribution of a film capacitor for a silicon carbide controller using a combination of finite element thermal simulation analysis and bench testing. The study focused on the temperature distribution law of the film capacitor under the rated operating conditions of a silicon carbide controller. Through comparative analysis of the design and test results, the effectiveness of using finite element thermal simulation to study film capacitors for silicon carbide controllers was demonstrated.

[0006] The article "Analysis and Optimization of the Influence of Low-Inductance Design on Converter Capacitor Ripple Current" (2021.48(11)), also published in the journal *Electrical Machines and Control Applications*, addresses the problem of excessively high effective values ​​of ripple current in the converter module's supporting capacitors due to the mismatch of stray inductance in the busbars during the design and application of high-power converter products. This leads to the capacitors operating in an overloaded state for extended periods, eventually causing damage. The proposed solution establishes corresponding mathematical models based on different busbar structures of the converter module, analyzes the root causes of excessively high supporting capacitor current, measures and extracts stray inductance for different busbar structures through experiments to guide subsequent busbar design, and eliminates the need to reduce busbar supporting capacitor ripple current by optimizing busbar design and improving control algorithms based on theoretical analysis. Finally, based on a specific converter platform, the effectiveness of the proposed improvement measures is verified by measuring capacitor current ripple under different experimental conditions and comparing and analyzing the experimental data.

[0007] The aforementioned articles all provide solutions for adjusting the bus capacitor ripple current using simulation experiments and control algorithms. However, they do not specifically explain how to address the problem of excessive heat dissipation in the DC bus capacitor and inverter system caused by this method. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the purpose of this invention is to provide a design method for DC bus capacitor ripple current parameters in electric vehicles that can solve the heat dissipation problem of DC bus capacitors in inverter systems.

[0009] To solve the above-mentioned technical problems, the technical solution provided by the present invention is: the design method for DC bus capacitor ripple current parameters for electric vehicles, comprising the following steps:

[0010] (1) Collect operating condition information of the electric vehicle;

[0011] (2) The vehicle's operating condition information is equivalent to a finite number of equivalent operating condition points, and the weight of each operating condition point in the entire operating condition cycle is calculated.

[0012] (3) Map a finite number of vehicle operating conditions to inverter loads;

[0013] (4) Apply the mapped operating point to the electrical performance simulation of the inverter system;

[0014] (5) Calculate the relationship between the DC bus capacitor ripple current and the inverter load current.

[0015] (6) The effective value of the ripple current at each operating point obtained by simulation and the weight of each operating point in the whole operating cycle are used to determine the running time of each ripple current value.

[0016] (7) Use the effective value of the ripple current to design the DC bus capacitor and evaluate the heating of the DC bus capacitor based on the operating time.

[0017] The above technical solution mainly improved three aspects: the motor phase current mapping method, the method for obtaining the DC bus capacitor ripple current coefficient, and the design method for the DC wiring capacitor ripple current. The DC bus capacitor ripple current selected by these methods is closer to practical applications, and the calculation of the DC bus capacitor's heat generation is more accurate. While reducing the redundancy in the DC bus capacitor design, it also simplifies the heat dissipation design of the DC bus capacitor in the inverter system, saving system costs.

[0018] Preferably, in step (1), several city taxis in the city are selected, and driving data of these vehicles are collected within a specified time. The driving motor speed / torque data in the driving data from a single full charge to a battery SOC of 30% are extracted. Then, the data is statistically analyzed and classified according to the range of speed and torque changes, which is equivalent to a finite number of cycles of several segments in which the driving motor works continuously.

[0019] Preferably, the whole vehicle sample can be selected based on the market launch area and usage attributes of the electric vehicle.

[0020] Preferably, in step (2), the method of segmented energy proportion statistics is adopted to convert the continuous cycle of the drive motor into regional characteristic working conditions, the equivalent speed characterizes the regional speed characteristics, the equivalent torque characterizes the regional load characteristics, and the energy proportion characterizes the weight of the region.

[0021] Preferably, each segment in step (1) is equivalent, wherein the equivalent rotational speed is... In the formula, ω is the equivalent rotational speed, ω i Let n be the rotational speed at point i within a segment, and n be the number of operating points within a segment; equivalent torque. In the formula, T is the equivalent torque, T i The torque at point i within a segment, where n is the number of operating points within a segment; the percentage of rotational speed energy. In the formula, E represents the percentage of fragment energy. j Let k be the segment energy and k be the number of segments. This gives us several equivalent operating points of the drive motor and their proportions.

[0022] Preferably, the fragment energy E j The calculation method is as follows:

[0023] Preferably, in step (3), based on the obtained equivalent operating point, the input demand to the motor, i.e. the output of the inverter, is mapped according to the external characteristic map of the drive motor. The operating point motor parameters are mapped using the MATLAB MBC Modelfitting tool, including motor power, effective value of phase current, effective value of phase voltage, power factor, and output frequency.

[0024] Preferably, in step (4), a simulation model of the inverter and its equivalent load is established based on the inverter topology and the obtained equivalent motor load.

[0025] Preferably, in step (5), the effective value of the ripple current used in the DC bus capacitor capacity design is calculated based on the relationship coefficient between the effective value of the DC bus capacitor ripple voltage and the effective value of the phase current, and the equivalent operating condition ratio. The calculation formula is as follows: And according to the formula i = I dc +i ripple The calculated current value is used to design the internal conductive copper busbar of the capacitor.

[0026] Preferably, based on the obtained DC bus capacitance, the ripple current is compared with the capacitor heating under the condition that the capacitor structure remains unchanged by using three-dimensional temperature field simulation software.

[0027] Compared with existing technologies, the significant advantages of this solution are: by adopting the motor phase current mapping method, the DC bus capacitor ripple current coefficient acquisition method, and the parametric current design method in this solution, the effectiveness of evaluating the heating of the DC bus capacitor can be improved. While reducing the redundancy of the DC bus capacitor design, it also simplifies the heat dissipation design of the DC bus capacitor in the inverter system and saves system costs. Attached Figure Description

[0028] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0029] Figure 1 This is an example of the vehicle drive motor speed / torque data operating condition selected in this embodiment;

[0030] Figure 2 This is a schematic diagram of the inverter and its equivalent complex simulation model in this embodiment. Detailed Implementation

[0031] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0032] The method for designing DC bus capacitor ripple current parameters for electric vehicles according to the present invention includes the following steps:

[0033] (1) Collect operating condition information of electric vehicles. Specifically, select several urban taxis in the city. The whole vehicle sample can be selected according to the market deployment area and usage attributes of electric vehicles. Collect driving data of these vehicles within a specified time. Extract the drive motor speed / torque data from the driving data when the battery is fully charged to 30% SOC. Then, statistically analyze and classify the data according to the range of speed and torque changes. This is equivalent to a finite number of cycles of continuous operation of several drive motors. Example of operating condition is as follows: Figure 1 As shown;

[0034] (2) The vehicle's operating condition information is equivalent to a finite number of equivalent operating condition points. The weight of each operating condition point in the entire operating condition cycle is calculated. Specifically, a block-based energy proportion statistical method is adopted to equate the continuous cycle of the drive motor's operation to regional characteristic operating conditions. The equivalent speed represents the regional speed characteristics, the equivalent torque represents the regional load characteristics, and the energy proportion represents the weight of the region. Among these, the equivalent speed... In the formula, ω is the equivalent rotational speed, ω i Let n be the rotational speed at point i within a segment, and n be the number of operating points within a segment; equivalent torque. In the formula, T is the equivalent torque, T i The torque at point i within a segment, where n is the number of operating points within a segment; the percentage of rotational speed energy. In the formula, E represents the percentage of fragment energy. j Let E be the segment energy, k be the number of segments, and thus obtain several equivalent operating points of the drive motor and their proportions. j The calculation method is as follows: The equivalent examples of vehicle operating conditions are shown in the table below:

[0035]

[0036]

[0037] (3) Mapping a finite number of vehicle operating points to the inverter load. Specifically, based on the obtained equivalent operating points, mapping the input demand of the drive motor to the inverter output according to the external characteristic map of the drive motor. The MATLAB MBC Modelfitting tool is used to map the motor parameters at the operating points, including motor power, effective value of phase current, effective value of phase voltage, power factor, and output frequency. An example of motor characteristic mapping is shown in the table below:

[0038] Torque / Nm engine speed / rpm Motor power (kW) Phase current RMS value / A Phase voltage RMS value / V Power factor 12 1158 1.455079 27.35532756 18.68027503 0.995001 34 1003 3.57089 68.75180501 18.95114278 0.967113 48 1039 5.222199 91.29017647 21.55910243 0.943639 11 3032 3.492356 25.71799719 48.18176088 0.995347 32 2871 9.620105 65.81560436 52.10477981 0.968777 10 4721 4.943455 24.03712289 74.53973712 0.995804 29 4815 14.62147 61.0635403 85.23374699 0.973788 11 6784 7.814031 26.65759921 107.6340913 0.995048 35 6808 24.95079 71.84952483 125.5825758 0.960549 14 8886 13.0266 33.53356341 142.6097689 0.995342

[0039] (4) Apply this mapped operating point to the electrical performance simulation of the inverter system. Specifically, based on the inverter topology and the obtained equivalent motor load, establish a simulation model of the inverter and its equivalent load. The simulation results of the DC bus capacitor ripple current are shown in the table below:

[0040]

[0041]

[0042] The relationship between the effective value of the DC bus capacitor ripple current and the effective value of the motor phase current obtained from the simulation is as follows: Figure 2 As shown;

[0043] (5) Calculate the relationship between the DC bus capacitor ripple current and the inverter load current. Specifically, based on the relationship coefficient between the effective value of the DC bus capacitor ripple voltage and the effective value of the phase current, and the equivalent operating condition ratio, calculate the effective value of the ripple current used in the DC bus capacitor capacity design. The calculation formula is as follows: And according to the formula i = I dc +i ripple The calculated current value is used to design the internal conductive copper busbar of the capacitor;

[0044] (6) The effective value of the ripple current at each operating point obtained by simulation and the weight of each operating point in the whole operating cycle are used to determine the running time of each ripple current value.

[0045] (7) Use the effective value of the ripple current to design the DC bus capacitor, and evaluate the heating of the DC bus capacitor based on the running time. Based on the obtained DC bus capacitor, use three-dimensional temperature field simulation software to compare the heating of the capacitor with the ripple current while keeping the capacitor structure unchanged.

[0046] Compared with existing technologies, the significant advantages of this solution are: by adopting the motor phase current mapping method, the DC bus capacitor ripple current coefficient acquisition method, and the parametric current design method in this solution, the effectiveness of evaluating the heating of the DC bus capacitor can be improved. While reducing the redundancy of the DC bus capacitor design, it also simplifies the heat dissipation design of the DC bus capacitor in the inverter system and saves system costs.

[0047] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for designing DC bus capacitor ripple current parameters for electric vehicles, characterized in that, Includes the following steps: (1) Collect operating condition information of the electric vehicle; (2) The vehicle's operating condition information is equivalent to a finite number of equivalent operating condition points, and the weight of each operating condition point in the entire operating condition cycle is calculated. (3) Map a limited number of vehicle operating conditions to the inverter load. Specifically, based on the obtained equivalent operating conditions, map the input requirements of the motor to the inverter output according to the external characteristic map of the drive motor. Use the matlabMBC Modelfitting tool to map the motor parameters of the operating conditions, including motor power, effective value of phase current, effective value of phase voltage, power factor, and output frequency. (4) Apply the mapped operating point to the electrical performance simulation of the inverter system. Specifically, establish a simulation model of the inverter and its equivalent load based on the inverter topology and the obtained equivalent motor load. (5) Calculate the relationship between the DC bus capacitor ripple current and the inverter load current. Specifically, calculate the effective value of the ripple current used in the DC bus capacitor capacity design based on the relationship coefficient between the effective value of the DC bus capacitor ripple voltage and the effective value of the phase current, as well as the equivalent operating condition ratio. (6) The effective value of the ripple current at each operating point obtained from the simulation and the weight of each operating point in the whole operating cycle are used to determine the running time of each ripple current value. (7) Use the effective value of the ripple current to design the DC bus capacitor and evaluate the heating of the DC bus capacitor based on the operating time.

2. The method for designing DC bus capacitor ripple current parameters for electric vehicles according to claim 1, characterized in that, In step (1), several city taxis in the city are selected, and driving data of these vehicles are collected within a specified time. The drive motor speed / torque data in the driving data from a single full charge to a battery SOC of 30% are extracted. Then, the data is statistically analyzed and classified according to the range of speed and torque changes, which is equivalent to a finite number of cycles of several segments in which the drive motor works continuously.

3. The method for designing DC bus capacitor ripple current parameters for electric vehicles according to claim 2, characterized in that, The whole vehicle samples were selected based on the market deployment area and usage attributes of electric vehicles.

4. The method for designing DC bus capacitor ripple current parameters for electric vehicles according to claim 1, characterized in that, In step (2), the method of segmented energy proportion statistics is adopted to convert the continuous cycle of the drive motor into regional characteristic working conditions. The equivalent speed represents the regional speed characteristics, the equivalent torque represents the regional load characteristics, and the energy proportion represents the weight of the region.

5. The method for designing DC bus capacitor ripple current parameters for electric vehicles according to claim 4, characterized in that, For each segment in step (1), perform an equivalent rotational speed, where the equivalent rotational speed is... In the formula, ω is the equivalent rotational speed, ω i Let be the rotational speed at point i within a segment, and n be the number of operating points within a segment; Equivalent torque In the formula, T is the equivalent torque, T i The torque at point i within a segment, where n is the number of operating points within a segment; the percentage of rotational speed energy. In the formula, E represents the percentage of fragment energy. j Let k be the segment energy and k be the number of segments. This gives us several equivalent operating points of the drive motor and their proportions.

6. The method for designing DC bus capacitor ripple current parameters for electric vehicles according to claim 5, characterized in that, The fragment energy E j The calculation method is as follows:

7. The method for designing DC bus capacitor ripple current parameters for electric vehicles according to claim 1, characterized in that, Based on the obtained DC bus capacitance, the ripple current and capacitor heating were compared using three-dimensional temperature field simulation software while keeping the capacitor structure unchanged.

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