Method for calculating winding temperature influence coefficient of lightweight traction transformer bushing

Abstract

The invention discloses a method for calculating a winding temperature influence coefficient of a lightweight traction transformer bushing. Comprising the following steps: establishing a lightweight traction transformer winding body and sleeve thermal test platform under an air cooling working condition, carrying out a steady thermal test under a typical traction working condition of a motor train unit, collecting temperature data, and calculating a winding temperature influence coefficient of a sleeve based on the obtained temperature data, and evaluating the negative influence degree of the bushing on the winding temperature of the lightweight traction transformer. The beneficial effects of the invention are that a test method, an evaluation coefficient and an evaluation method which consider the influence of the sleeve on the lightweight traction transformer winding are proposed for the first time, and the axial temperature change trend of the winding and the circumferential temperature distribution rule of the winding at the section where the sleeve is located are fused; objective and accurate evaluation of the negative influence degree of the prototype sleeve on the winding temperature under specific design can be realized, and a new technical means is provided for efficient design and structural optimization of the lightweight traction transformer.

Description

Technical Field

[0001] The present invention relates to the field of online monitoring, fault prediction and health management of electrical insulation of lightweight traction transformers for high-speed EMUs, and in particular to a method for calculating the winding temperature influence coefficient of a lightweight traction transformer bushing. Background Art

[0002] Temperature is a crucial parameter that cannot be ignored during the design and operation phases of high-voltage electrical equipment, as the electrical and mechanical properties and remaining life of insulation materials, primarily polymers, are strongly temperature-dependent. To meet the development requirements of higher-speed EMUs, lightweight traction transformers have recently attracted widespread attention from domestic and international rail transit vehicle manufacturers and researchers. To minimize weight and alleviate the burden of high-speed EMUs, lightweight traction transformers have transitioned from oil-immersed to dry-type insulation, using epoxy resin or silicone rubber as cast insulation and direct air cooling. This achieves a weight reduction of up to approximately 45% compared to existing oil-immersed traction transformers. During the design and verification phase, researchers discovered that the unique and uniquely designed lead bushings of lightweight traction transformers can negatively impact the axial and circumferential temperature distribution of the windings if the design and placement are poor. This can exacerbate temperature gradients within the windings, causing excessive stress within the insulation and leading to cracking. In severe cases, this can rapidly develop into localized overheating during operation, ultimately leading to thermal breakdown and threatening the safe operation of high-speed EMUs. Therefore, during the design phase of lightweight traction transformers, it is necessary to evaluate the impact of bushings on winding temperature on prototypes. Currently, there is no scientific and objective method for evaluating the impact of bushings on winding temperature, as well as the relevant parameters and coefficients. Therefore, it is necessary to propose a method for calculating the winding temperature impact coefficient of lightweight traction transformer bushings. By implementing a series of steps, namely "test platform - data collection - evaluation indicators", the negative impact of prototype bushings on winding temperature under a specific design can be objectively and accurately evaluated. This will provide new technical means for the efficient design and structural optimization of lightweight traction transformers, and have far-reaching significance for the efficient, safe, and stable operation of higher-speed EMUs. Summary of the Invention

[0003] In response to the above technical problems, the purpose of the present invention is to propose a method for calculating the winding temperature influence coefficient of the lightweight traction transformer bushing, which can achieve an objective and accurate evaluation of the negative impact of the prototype bushing on the winding temperature during the design stage of the lightweight traction transformer, and has far-reaching significance for the efficient, safe and stable operation of higher-speed EMUs.

[0004] The technical solutions for achieving the purpose of the present invention are as follows:

[0005] The first step is to establish a thermal test platform for lightweight traction transformer windings and bushings under air cooling conditions.

[0006] The thermal test platform for lightweight traction transformer winding body and bushing under air-cooling conditions includes: lightweight traction transformer winding body (1), positive bushing (2), positive bushing terminal board (3), negative bushing (4), negative bushing terminal board (5), power positive cable (6), power negative cable (7), winding axial temperature sensor (8), winding circumferential temperature sensor (9), wind speed sensor (10), axial flow fan (11), variable frequency speed regulation device (12), high-power adjustable DC power supply (13), and data acquisition and control terminal (14);

[0007] The positive electrode bushing (2) and the negative electrode bushing (4) are both fixed vertically to the ground on the upper surface of the lightweight traction transformer winding body (1); the positive electrode bushing (2) is located at one end of the lightweight traction transformer winding body (1) close to the axial flow fan (11), i.e., the cooling air flow inlet end; the negative electrode bushing (4) is located at one end of the lightweight traction transformer winding body (1) away from the axial flow fan (11), i.e., the cooling air flow outlet end; the positive electrode bushing terminal plate (3) is fixed to the upper end of the positive electrode bushing (2), and the negative electrode bushing terminal plate ( 5) is fixed to the upper end of the negative sleeve (4); the positive sleeve terminal plate (3) is connected to the positive terminal of the high-power adjustable DC power supply (13) through the positive power cable (6), and the negative sleeve terminal plate (5) is connected to the negative terminal of the high-power adjustable DC power supply (13) through the negative power cable (7); the winding axial temperature sensors (8) are evenly installed along the direction of the cooling air flow, that is, the axial direction of the winding, and the height is consistent with the center height of the lightweight traction transformer winding body (1), and the total number is recorded as N axial , N axial ≥8; all winding axial temperature sensors (8) are connected to the data acquisition and control terminal (14); winding circumferential temperature sensors (9) are installed in the circumferential direction of the lightweight traction transformer winding body (1) at the cross-section position where the positive bushing (2) and the negative bushing (4) are located, and the total number of winding circumferential temperature sensors (9) in a single cross-section is recorded as N cir , N cir≥16; all winding circumferential temperature sensors (9) are connected to the data acquisition and control terminal (14); the lightweight traction transformer winding body (1), the positive bushing (2), the positive bushing terminal board (3), the negative bushing (4), and the negative bushing terminal board (5) constitute the winding part of the test platform, and the air outlet of the axial flow fan (11) is directly opposite to the winding part of the test platform to provide the cooling airflow required during the test; the wind speed sensor (10) is installed at the air inlet of the axial flow fan (11), and all wind speed sensors (10) are connected to the data acquisition and control terminal (14) for real-time monitoring of the output wind speed of the axial flow fan (11); the axial flow fan (11) is connected to the variable frequency speed regulating device (12), and the variable frequency speed regulating device (12) is connected to the data acquisition and control terminal (14) for real-time control of the output wind speed of the axial flow fan (11);

[0008] Step 2: Conduct steady-state thermal tests under typical traction conditions of the EMU and collect temperature data

[0009] The wind speed of the axial flow fan (11) is set to v by the data acquisition and control terminal (14). set , v set ≥5m / s; when the data acquisition and control terminal (14) determines that the output airflow of the axial flow fan (11) has reached a steady state through the average wind speed value detected by all wind speed sensors (10), the output power of the high-power adjustable DC power supply (13) is set to P through the data acquisition and control terminal (14). rs , in W, and start the high-power adjustable DC power supply (13); when the temperature data collected by all the winding axial temperature sensors (8) and the winding circumferential temperature sensors (9) no longer fluctuate over time and are considered to be in a steady state, the temperature values ​​monitored by all the winding axial temperature sensors (8) from the cooling air inlet to the cooling air outlet are recorded as T sur_1 、T sur_2 ,…,T sur_i , i=N axial At the same time, the data collected by the winding circumferential temperature sensor (9) at the cross-section position of the positive sleeve (2) is recorded as T in the clockwise or counterclockwise direction along the circumferential direction of the lightweight traction transformer winding body (1) from the winding circumferential temperature sensor (9) at the highest vertical position. p_1 、T p_2 ,…,T p_j , j = N cir The data collected by the winding circumferential temperature sensor (9) at the cross-section position of the negative electrode casing (4) is sequentially recorded as T from the winding circumferential temperature sensor (9) at the highest vertical position along the circumferential direction of the lightweight traction transformer winding body (1) in the same direction as above. n_1 、T n_2 ,…,Tn_j , j = N cir ;

[0010] Step 3: Calculate the bushing winding temperature influence coefficient F based on the obtained temperature data

[0011] Calculate the bushing winding temperature influence coefficient F according to the following formula:

[0012]

[0013] Where, F p is the positive casing component of the winding temperature influence coefficient, F n is the negative bushing component of the winding temperature influence coefficient, S pn is the axial temperature uniformity coefficient;

[0014] F p The calculation method is as follows:

[0015]

[0016] Where, max(T p_i 0.7 ) is to set all T p_i (1≤i≤N cir ) is individually raised to the power of 0.7 and the maximum value is taken. min(T p_i 0.3 ) is to set all T p_i (1≤i≤N cir ) Perform the 0.3 power operation separately and take the minimum value;

[0017] F n The calculation method is as follows:

[0018]

[0019] Where, max(T n_i 0.7 ) is to set all T n_i (1≤i≤N cir ) is individually raised to the power of 0.7 and the maximum value is taken. min(T n_i 0.3 ) is to set all T n_i (1≤i≤N cir ) Perform the 0.3 power operation separately and take the minimum value;

[0020] In F p and F n In the calculation method, the value of K is obtained based on the following formula:

[0021]

[0022] In the formula is the floor rounding operator symbol;

[0023] S pn The calculation method is as follows:

[0024]

[0025] Step 4: Evaluate the negative impact of bushings on the winding temperature of lightweight traction transformers

[0026] If 0<F≤5, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is low, and the design can meet the requirements of long-term stable operation; if 5<F≤10, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is moderate, and attention needs to be paid to its thermal state during operation; if F>10, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is high, and the design cannot meet the requirements of long-term stable operation.

[0027] The beneficial effect of the present invention is that a method for calculating the winding temperature influence coefficient of a lightweight traction transformer bushing has the following advantages:

[0028] 1) This paper proposes, for the first time, a test method, evaluation coefficient, and assessment method that consider the impact of bushings on lightweight traction transformer windings. By integrating the axial temperature variation trend of the windings and the circumferential temperature distribution pattern of the windings at the cross-section where the bushings are located, it is possible to objectively and accurately assess the degree of negative impact of the bushings on the winding temperature under a specific design. This provides a new technical means for the efficient design and structural optimization of lightweight traction transformers, and has far-reaching significance for the efficient, safe, and stable operation of higher-speed EMUs.

[0029] 2) Compared with the blindness and uncertainty of temperature distribution collection and evaluation of adverse effects of temperature distribution in traditional prototype temperature rise tests, the method proposed in the present invention can realize the quantitative calculation and evaluation of the rationality of the prototype bushing position setting before the batch production of lightweight traction transformers in the design stage. It not only effectively improves the reliability of the evaluation results, but also has higher work efficiency and can effectively reduce the consumption of manpower and material resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of a method for calculating the winding temperature influence coefficient of a lightweight traction transformer bushing according to the present invention;

[0031] Figure 2 The present invention is a schematic structural diagram of a lightweight traction transformer winding body and bushing thermal test platform under air-cooling conditions. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the accompanying drawings and specific implementation processes. It should be emphasized that the specific implementation cases described herein are only used to illustrate the present invention and do not limit the scope of the present invention and its claims.

[0033] The first step is to establish a thermal test platform for lightweight traction transformer windings and bushings under air cooling conditions.

[0034] The thermal test platform for lightweight traction transformer winding body and bushing under air-cooling conditions includes: lightweight traction transformer winding body (1), positive bushing (2), positive bushing terminal board (3), negative bushing (4), negative bushing terminal board (5), power positive cable (6), power negative cable (7), winding axial temperature sensor (8), winding circumferential temperature sensor (9), wind speed sensor (10), axial flow fan (11), variable frequency speed regulation device (12), high-power adjustable DC power supply (13), and data acquisition and control terminal (14);

[0035] The positive pole bushing (2) and the negative pole bushing (4) are both fixed vertically to the ground on the upper surface of the lightweight traction transformer winding body (1). The positive pole bushing (2) and the negative pole bushing (4) are integrally cast with the lightweight traction transformer winding body (1). The temperatures of the two will affect each other. The positive pole bushing (2) and the negative pole bushing (4) reduce the effective area of ​​convective heat transfer of the winding, which has a certain degree of negative impact on the uniformity of the winding temperature distribution. The positive pole bushing (2) is located at the end of the lightweight traction transformer winding body (1) close to the axial flow fan (11), that is, the cooling air flow inlet end, and the negative pole bushing (4) is located at the end of the lightweight traction transformer winding body (1) away from the axial flow fan (11), that is, the cooling air flow outlet end. The positive pole bushing terminal plate (3) is fixed to The upper end of the positive sleeve (2) and the negative sleeve terminal plate (5) are fixed to the upper end of the negative sleeve (4); the positive sleeve terminal plate (3) is connected to the positive terminal of the high-power adjustable DC power supply (13) through the positive power cable (6), and the negative sleeve terminal plate (5) is connected to the negative terminal of the high-power adjustable DC power supply (13) through the negative power cable (7), and then the Joule loss of the winding required during the test is applied to the lightweight traction transformer winding body (1) through the high-power adjustable DC power supply (13), which is an indispensable driving force for the temperature distribution; the winding axial temperature sensors (8) are evenly installed along the cooling air flow direction, that is, the winding axial direction, and the height is consistent with the center height of the lightweight traction transformer winding body (1). The total number is recorded as N axial , N axial=10; all winding axial temperature sensors (8) are connected to the data acquisition and control terminal (14); winding circumferential temperature sensors (9) are installed in the circumferential direction of the lightweight traction transformer winding body (1) at the cross-section position where the positive pole bushing (2) and the negative pole bushing (4) are located. The total number of winding circumferential temperature sensors (9) in a single cross-section is recorded as N cir , N cir =16; all winding circumferential temperature sensors (9) are connected to the data acquisition and control terminal (14); the lightweight traction transformer winding body (1), the positive bushing (2), the positive bushing terminal board (3), the negative bushing (4), and the negative bushing terminal board (5) constitute the winding part of the test platform, and the air outlet of the axial flow fan (11) is directly opposite to the winding part of the test platform to provide the cooling airflow required during the test; the wind speed sensor (10) is installed at the air inlet of the axial flow fan (11), and all wind speed sensors (10) are connected to the data acquisition and control terminal (14) for real-time monitoring of the output wind speed of the axial flow fan (11); the axial flow fan (11) is connected to the variable frequency speed regulating device (12), and the variable frequency speed regulating device (12) is connected to the data acquisition and control terminal (14) for real-time control of the output wind speed of the axial flow fan (11);

[0036] Step 2: Conduct steady-state thermal tests under typical traction conditions of the EMU and collect temperature data

[0037] The wind speed of the axial flow fan (11) is set to v by the data acquisition and control terminal (14). set , v set =7m / s; when the data acquisition and control terminal (14) determines that the output airflow of the axial flow fan (11) has reached a steady state through the average wind speed value detected by all wind speed sensors (10), that is, when the wind speed fluctuation between two adjacent measurements is less than 0.1m / s, the output power of the high-power adjustable DC power supply (13) is set to P by the data acquisition and control terminal (14). rs =1936W, and start the high-power adjustable DC power supply (13); when the temperature data collected by all the winding axial temperature sensors (8) and the winding circumferential temperature sensors (9) no longer fluctuate over time and are considered to be in a steady state, or when the temperature fluctuation between two adjacent measurements is less than 0.5°C, the temperature values ​​monitored by all the winding axial temperature sensors (8) from the cooling air inlet to the cooling air outlet are recorded as T sur_1 、T sur_2 ,…,T sur_10 ; where T sur_1 =17.84℃、T sur_2 =19.83℃、T sur_3 =20.47℃、T sur_4 =21.73℃、T sur_5 =23.64℃、Tsur_6 =26.76℃、T sur_7 =29.64℃、T sur_8 =33.90℃、T sur_9 =35.12℃、T sur_10 =38.03°C; At the same time, the data collected by the winding circumferential temperature sensor (9) at the cross-section position of the positive sleeve (2) is recorded as T in the clockwise direction along the circumference of the lightweight traction transformer winding body (1) from the winding circumferential temperature sensor (9) at the highest vertical position. p_1 、T p_2 ,…,T p_16 ; where T p_1 =30.02℃、T p_2 =28.11℃、T p_3 =26.94℃、T p_4 =24.57℃、T p_5 =23.04℃、T p_6 =22.93℃、T p_7 =22.73℃、T p_8 =21.65℃、T p_9 =22.84℃、T p_10 =23.48℃、T p_11 =24.01℃、T p_12 =24.32℃, T p_13 =25.87℃、T p_14 =25.99℃、T p_15 =28.36℃、T p_16 =29.43℃; the data collected by the winding circumferential temperature sensor (9) at the cross-section position of the negative pole casing (4) is recorded as T in the clockwise direction from the winding circumferential temperature sensor (9) at the highest vertical position along the lightweight traction transformer winding body (1). n_1 、T n_2 ,…,T n_16 ; where T n_1 =42.27℃、T n_2 =41.29℃、T n_3 =40.50℃、T n_4 =39.41℃, T n_5 =37.22℃, T n_6 =37.12℃、T n_7 =35.88℃、T n_8 =34.95℃、T n_9 =36.01℃、T n_10 =36.68℃、T n_11 =38.76℃、T n_12 =39.86℃、Tn_13 =41.30℃、T n_14 =41.97℃、T n_15 =42.29℃、T n_16 =43.16℃;

[0038] Step 3: Calculate the bushing winding temperature influence coefficient F based on the obtained temperature data

[0039] Calculate the bushing winding temperature influence coefficient F according to the following formula:

[0040]

[0041] Where, F p is the positive bushing component of the winding temperature influence coefficient, F n is the negative bushing component of the winding temperature influence coefficient, S pn is the axial temperature uniformity coefficient;

[0042] F p The calculation method is as follows:

[0043]

[0044] Where, max(T p_i 0.7 ) is to set all T p_i (1≤i≤N cir ) is individually raised to the power of 0.7 and the maximum value is taken. min(T p_i 0.3 ) is to set all T p_i (1≤i≤N cir ) Perform the 0.3 power operation separately and take the minimum value;

[0045] F n The calculation method is as follows:

[0046]

[0047] Where, max(T n_i 0.7 ) is to set all T n_i (1≤i≤N cir ) is individually raised to the power of 0.7 and the maximum value is taken. min(T n_i 0.3 ) is to set all T n_i (1≤i≤N cir ) Perform the 0.3 power operation separately and take the minimum value;

[0048] In F p and F nIn the calculation method, the value of K is obtained based on the following formula:

[0049]

[0050] In the formula is the rounding down operator; the T collected in the second step during the experiment is p_1 、T p_2 ,…,T p_16 Substitute F p The calculation formula of F p =3.3377; the T collected in the second step during the experiment n_1 、T n_2 ,…,T n_16 Substitute F n The calculation formula of F n =3.5272;

[0051] S pn The calculation method is as follows:

[0052]

[0053] The T collected in the second step during the experiment sur_1 、T sur_2 ,…,T sur_10 Substitute S pn The calculation formula of S pn =0.4328;

[0054] Based on the third step F p 、F n and S pn , the bushing winding temperature influence coefficient F=4.2073;

[0055] Step 4: Evaluate the negative impact of bushings on the winding temperature of lightweight traction transformers

[0056] If 0<F≤5, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is low, and the design can meet the requirements of long-term stable operation; if 5<F≤10, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is moderate, and attention needs to be paid to its thermal state during operation; if F>10, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is high, and the design cannot meet the requirements of long-term stable operation.

Claims

1. A method for calculating the winding temperature influence coefficient of a lightweight traction transformer bushing, characterized in that: The following steps are involved: The first step is to establish a thermal test platform for lightweight traction transformer windings and bushings under air cooling conditions. The thermal test platform for lightweight traction transformer winding body and bushing under air-cooling conditions includes: lightweight traction transformer winding body (1), positive bushing (2), positive bushing terminal board (3), negative bushing (4), negative bushing terminal board (5), power positive cable (6), power negative cable (7), winding axial temperature sensor (8), winding circumferential temperature sensor (9), wind speed sensor (10), axial flow fan (11), variable frequency speed regulation device (12), high-power adjustable DC power supply (13), and data acquisition and control terminal (14); The positive electrode bushing (2) and the negative electrode bushing (4) are both fixed vertically to the ground on the upper surface of the lightweight traction transformer winding body (1); the positive electrode bushing (2) is located at one end of the lightweight traction transformer winding body (1) close to the axial flow fan (11), i.e., the cooling air flow inlet end; the negative electrode bushing (4) is located at one end of the lightweight traction transformer winding body (1) away from the axial flow fan (11), i.e., the cooling air flow outlet end; the positive electrode bushing terminal plate (3) is fixed to the upper end of the positive electrode bushing (2), and the negative electrode bushing terminal plate ( 5) is fixed to the upper end of the negative sleeve (4); the positive sleeve terminal plate (3) is connected to the positive terminal of the high-power adjustable DC power supply (13) through the positive power cable (6), and the negative sleeve terminal plate (5) is connected to the negative terminal of the high-power adjustable DC power supply (13) through the negative power cable (7); the winding axial temperature sensors (8) are evenly installed along the direction of the cooling air flow, that is, the axial direction of the winding, and the height is consistent with the center height of the lightweight traction transformer winding body (1), and the total number is recorded as N axial , N axial ≥8; all winding axial temperature sensors (8) are connected to the data acquisition and control terminal (14); winding circumferential temperature sensors (9) are installed in the circumferential direction of the lightweight traction transformer winding body (1) at the cross-section position where the positive bushing (2) and the negative bushing (4) are located, and the total number of winding circumferential temperature sensors (9) in a single cross-section is recorded as N cir , N cir ≥16; all winding circumferential temperature sensors (9) are connected to the data acquisition and control terminal (14); the lightweight traction transformer winding body (1), the positive bushing (2), the positive bushing terminal board (3), the negative bushing (4), and the negative bushing terminal board (5) constitute the winding part of the test platform, and the air outlet of the axial flow fan (11) is directly opposite to the winding part of the test platform to provide the cooling airflow required during the test; the wind speed sensor (10) is installed at the air inlet of the axial flow fan (11), and all wind speed sensors (10) are connected to the data acquisition and control terminal (14) for real-time monitoring of the output wind speed of the axial flow fan (11); the axial flow fan (11) is connected to the variable frequency speed regulating device (12), and the variable frequency speed regulating device (12) is connected to the data acquisition and control terminal (14) for real-time control of the output wind speed of the axial flow fan (11); Step 2: Conduct steady-state thermal tests under typical traction conditions of the EMU and collect temperature data The wind speed of the axial flow fan (11) is set to v by the data acquisition and control terminal (14). set , v set ≥5m / s; when the data acquisition and control terminal (14) determines that the output airflow of the axial flow fan (11) has reached a steady state through the average wind speed value detected by all wind speed sensors (10), the output power of the high-power adjustable DC power supply (13) is set to P through the data acquisition and control terminal (14). rs , in W, and start the high-power adjustable DC power supply (13); when the temperature data collected by all the winding axial temperature sensors (8) and the winding circumferential temperature sensors (9) no longer fluctuate over time and are considered to be in a steady state, the temperature values ​​monitored by all the winding axial temperature sensors (8) from the cooling air inlet to the cooling air outlet are recorded as T sur1 、T sur2 ,…,T suri , i=N axial At the same time, the data collected by the winding circumferential temperature sensor (9) at the cross-section position of the positive sleeve (2) is recorded as T in the clockwise or counterclockwise direction along the circumferential direction of the lightweight traction transformer winding body (1) from the winding circumferential temperature sensor (9) at the highest vertical position. p_1 、T p_2 ,…,T p_j , j = N cir The data collected by the winding circumferential temperature sensor (9) at the cross-section position of the negative electrode casing (4) is sequentially recorded as T from the winding circumferential temperature sensor (9) at the highest vertical position along the circumferential direction of the lightweight traction transformer winding body (1) in the same direction as above. n_1 、T n_2 ,…,T n_j , j = N cir ; Step 3: Calculate the bushing winding temperature influence coefficient F based on the obtained temperature data Calculate the bushing winding temperature influence coefficient F according to the following formula: Where, F p is the positive casing component of the winding temperature influence coefficient, F n is the negative bushing component of the winding temperature influence coefficient, S pn is the axial temperature uniformity coefficient; F p The calculation method is as follows: Where, max(T p_i 0.7 ) is to set all T p_i (1≤i≤N cir ) is individually raised to the power of 0.7 and the maximum value is taken. min(T p_i 0.3 ) is to set all T p_i (1≤i≤N cir ) Perform the 0.3 power operation separately and take the minimum value; F n The calculation method is as follows: Where, max(T n_i 0.7 ) is to set all T n_i (1≤i≤N cir ) is individually raised to the power of 0.7 and the maximum value is taken. min(T n_i 0.3 ) is to set all T n_i (1≤i≤N cir ) Perform the 0.3 power operation separately and take the minimum value; In F p and F n In the calculation method, the value of K is obtained based on the following formula: In the formula is the floor rounding operator symbol; S pn The calculation method is as follows: Step 4: Evaluate the negative impact of bushings on the winding temperature of lightweight traction transformers If 0<F≤5, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is low, and the design can meet the requirements of long-term stable operation; if 5<F≤10, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is moderate, and attention needs to be paid to its thermal state during operation; if F>10, the negative impact of the bushing of the lightweight traction transformer on the winding temperature is high, and the design cannot meet the requirements of long-term stable operation.

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