Method for designing voltage class of medium-voltage direct-current AT type traction power supply system

Abstract

The invention discloses a voltage class design method for a medium-voltage direct-current AT type traction power supply system, and the method comprises the steps: S1, determining a system network parameter set according to a train operation scene, building a system equivalent circuit model, and initializing a voltage class to be a minimum voltage class; s2, judging whether the voltage level is within a preset range, and if so, S3; otherwise, outputting a result; s3, calculating electrical parameters of the line based on power flow analysis, and judging whether the pantograph voltage is within a preset range, if so, S4; otherwise, S7; s4, judging whether the stray current and the steel rail potential are within a preset range or not, and if so, S5; otherwise, S7; s5, calculating the temperature of the wire, judging whether the temperature of the wire is within a preset range or not, and if so, S6; otherwise, S7; s6, calculating the total cost according to economic analysis, and returning a result under the current voltage grade; and S7, increasing the current voltage grade according to a preset gradient, and returning to S2. The method has the effect that the voltage grade design of the medium-voltage direct-current traction power supply system accessed by the direct-current AT is realized.

Description

Technical Field

[0001] The invention relates to a train traction power supply system, and in particular to a voltage level design method for a medium voltage direct current AT type traction power supply system. Background Art

[0002] With the development of high-power power electronics technology and DC transmission systems, medium voltage DC traction power supply systems (MVDCTPSS) have received more and more attention. Figure 1 Compared with the existing 25kV AC traction power supply system, the medium voltage DC traction power supply system eliminates the phase separation, improves the power supply distance and power quality, and has become an important power supply solution for electrified railways.

[0003] Introducing a DC autotransformer (DCAT) in the MVDC TPSS, the bipolar power supply scheme can further reduce the traction current and improve the system power supply capacity. Figure 2 As shown. Through the regulation of the DCAT device, the current in the rail is transferred to the overhead line and the negative feeder. As a key issue in the DC system, the system stray current and rail potential can be effectively suppressed, and its power supply capacity can be further improved. Therefore, compared with MVDC TPSS without DCAT, MVDC TPSS based on DCAT has more advantages and meets the actual application requirements. However, there is no consensus on how to choose the voltage level of MVDC TPSS connected to DCAT.

[0004] like Figure 3 As shown in the figure, most existing studies focus on the voltage level design of conventional DC power supply schemes, and the main evaluation indicators include the pantograph voltage of the train, the conductor temperature, and the rail potential. However, after the introduction of DCAT devices and negative feeders, the characteristics of MVDC TPSS will be significantly different from the direct power supply scheme, and its characteristics are reflected in:

[0005] 1) The DCAT device will regulate the system current, that is, transfer the rail current to the overhead line and negative feeder;

[0006] 2) DCAT devices introduce a large number of additional nodes;

[0007] 3) Unlike direct power supply which only has positive polarity and neutral, the negative feeder has negative polarity voltage;

[0008] 4) The negative feeder introduces an additional energy transmission branch.

[0009] Under the combined effect of the above characteristics, the circuit structure of the MVDC TPSS connected to DCAT will change, the energy flow distribution will become more complex, and the existing model will be difficult to accurately describe the system energy flow distribution characteristics, and thus it will be impossible to evaluate related indicators such as pantograph voltage and conductor temperature. Summary of the invention

[0010] In view of this, the present invention provides a voltage level design method for a medium voltage DC AT traction power supply system, taking into account the regulatory effect of DCAT on energy flow and the additional nodes introduced by it, as well as the negative polarity voltage and additional branches introduced by the negative feeder, and the bipolar output of the traction substation (TSS) is equivalent to two voltage sources in series, and an MVDCTPSS equivalent circuit model suitable for DCAT is established. By solving the model, the energy flow distribution after DCAT access is obtained, and then the corresponding pantograph voltage, conductor temperature and rail potential can be evaluated. In addition, the important indicator of stray current is introduced into the evaluation index. By modeling and evaluating it, the stray current is suppressed at a reasonable level. In the final determination link of the voltage level, an economic analysis model is established and evaluated, so as to achieve the optimal voltage level design under comprehensive consideration.

[0011] To achieve the above purpose, the specific technical solutions adopted by the present invention are as follows:

[0012] A method for designing a voltage level of a medium voltage DC AT type traction power supply system comprises the following steps:

[0013] S1: Determine the network parameter set of the medium voltage DC autotransformer (AT) traction power supply system according to the train operation scenario, establish the system equivalent circuit model, and initialize the voltage level to the minimum voltage level;

[0014] S2: Determine whether the voltage level is within a preset range. If yes, proceed to step S3; otherwise, output the results at each voltage level.

[0015] S3: Calculate the electrical parameters of the line based on the power flow analysis, and determine whether the pantograph voltage is within a preset range. If yes, proceed to step S4; otherwise, proceed to step S7;

[0016] S4: Determine whether the stray current and rail potential are within a preset range. If yes, proceed to step S5; otherwise, proceed to step S7;

[0017] S5: Calculate the conductor temperature and determine whether the conductor temperature is within a preset range. If yes, proceed to step S6; otherwise, proceed to step S7;

[0018] S6: Calculate the total cost based on economic analysis and return the result at the current voltage level;

[0019] S7: Increase the current voltage level according to a preset gradient and return to step S2.

[0020] Optionally, in step S1, network parameters are determined according to the actual operation scenario, and a system equivalent circuit model is established. When modeling the system, the substation is modeled as an ideal voltage source in series with internal resistance, the train is modeled as an ideal current source running at constant power, the resistivity of the conductors and rails is set to be constant with temperature and regarded as uniform conductors, and the DC autotransformer is modeled as a voltage-balanced capacitor with no energy loss.

[0021] Optionally, in step S1, the network parameter set for system modeling determined by the application scenario includes various conductor models, lengths and cross-sectional areas, and substation spacing; the conductors include overhead wires, catenary cables, and negative feeders.

[0022] Optionally, in step S2, the preset range of the voltage level is 10 kV to 35 kV.

[0023] Optionally, according to system modeling, the superposition theorem is used to calculate the average stray current as the stray current evaluation index in step S4.

[0024] Optionally, according to system modeling, the maximum voltage at each train-rail contact point is used as the rail potential evaluation index in step S4.

[0025] Optionally, in step S5, according to T a +ΔT calculates the conductor temperature, where:

[0026]

[0027] ΔT is the thermal equilibrium temperature of the conductor, q c is the unit convection heat dissipation, q r is the unit radiation heat dissipation, q s To absorb the sun's heat, mC p is the total heat capacity of the wire, I is the wire current, R is the unit resistance, K angle is the wind direction factor, k f is the thermal conductivity of air, T a is the ambient air temperature, T0 is the wire temperature at the previous moment, N Re is the Reynolds coefficient, D is the outer diameter of the wire, ρ f is the air density, v ω is the wind speed, μ f is the absolute viscosity of air, ε is the emissivity of the wire, α is the solar absorptivity, Q se is the corrected radiation heat intensity, θ is the effective incident angle, and A' is the projection area of ​​the wire.

[0028] Optionally, in step S6, according to C total =C inv +C opeWhen calculating the total cost, it should be noted that the calculated cost mainly includes the part that is greatly affected by the voltage level, and the part that is almost unaffected by the voltage level, such as the rail cost, is no longer considered. Among them:

[0029] Investment cost inv =N TSS C TSS +N DCAT C DCAT +C con ;

[0030] N TSS is the number of substations, the cost of a single substation C TSS =C pe,TSS +C pre,TSS +C civ,TSS ; C pe,TSS is the cost of the power electronic converter in the substation, C pre,TSS is the cost of the substation protection device, C civ,TSS The civil engineering cost of the substation;

[0031] N DCAT is the number of DC autotransformers; the cost of a single DC autotransformer C DCAT =C pe,DCAT +C pre,DCAT +C civ,DCAT , C pe,DCAT is the cost of the DC autotransformer power electronic converter, C pre,DCAT is the cost of the DC autotransformer protection device, C civ,DCAT The civil construction cost of the DC autotransformer;

[0032] Wire cost C con =L con (S tro +S mes +S neg ), L con is the wire length, S tro , S mes , S neg They correspond to the price per unit length of conductor for the overhead line, catenary and negative feeder respectively.

[0033] Operation loss cost C ope =ct(k c P con +k p P pe );

[0034] c is the electricity price, P con and P pe are line power and converter power respectively, k c is the line loss rate, k pis the power loss rate of the converter, and t is the operating time.

[0035] Optionally, according to the total cost at each voltage level, the voltage level with the lowest cost is selected as the current medium voltage DC AT traction power supply system voltage level.

[0036] The remarkable effects of the present invention are:

[0037] The present invention proposes a method for designing the voltage level of a medium-voltage DC AT traction power supply system, which takes into account the regulating effect of the DC AT device on the energy flow and the additional nodes it introduces, as well as the negative polarity voltage and additional branches introduced by the negative feeder, and establishes an equivalent circuit model of the traction power supply system suitable for DC AT. By solving the model, the energy flow distribution after the DC AT is connected can be obtained, and then the corresponding key indicators can be evaluated to realize the voltage level design of the traction power supply system with DC AT connection.

[0038] In addition, this method takes into account the stray current, an important indicator of the DC system. By establishing a stray current calculation model under DC AT access and evaluating it, the stray current can be limited to the required range, thereby ensuring the reliability of surrounding facilities. At the same time, in the process of determining the final voltage level, an important method of economic evaluation is introduced. In terms of construction cost and operating cost, a corresponding economic analysis model is established, and then an economic evaluation is performed on the voltage level that meets the requirements, thereby determining the final voltage level. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a topological diagram of the existing medium voltage DC traction power supply system;

[0040] Figure 2 It is a topological diagram of a medium voltage DC traction power supply system that introduces a DC autotransformer;

[0041] Figure 3 It is a flow chart of voltage level design of existing medium voltage DC traction power supply system;

[0042] Figure 4 is a flow chart of the method of the present invention;

[0043] Figure 5 It is a diagram of an equivalent model of a DCAT system in a specific embodiment of the present invention;

[0044] Figure 6 is a rail potential equivalent model diagram in a specific embodiment of the present invention;

[0045] Figure 7 It is an equivalent model diagram of a single train operation scenario in a specific embodiment of the present invention;

[0046] Figure 8is the relationship between the distance between substations and the cross-sectional area of ​​the contact network at different voltage levels in a specific embodiment of the present invention;

[0047] Fig. 9 is the relationship between cost and cross section at 35 kV in a specific embodiment of the present invention;

[0048] Fig.10 In the specific embodiment of the present invention, 225 mm 2 Economic index curves for different voltage levels under cross-sectional area;

[0049] Fig.11 is the experimental simulation result in a specific embodiment of the present invention, wherein Fig.11 (a) is the curve of pantograph voltage and conductor temperature changing with time. Fig.11 (b) is the rail potential and average stray current curve. DETAILED DESCRIPTION

[0050] The following specifically illustrates the implementation mode of the present invention in conjunction with the accompanying drawings. The embodiments are provided for illustrative purposes only and cannot be understood as limiting the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present invention, because many changes can be made to the present invention without departing from the spirit and scope of the present invention.

[0051] like Figure 4 As shown, the present embodiment provides a method for designing a voltage level of a medium voltage DC AT type traction power supply system, comprising the following steps:

[0052] S1: Determine the network parameter set of the medium voltage DC autotransformer (AT) traction power supply system according to the train operation scenario, establish the system equivalent circuit model, and initialize the voltage level to the minimum voltage level;

[0053] S2: Determine whether the voltage level is within a preset range. If yes, proceed to step S3; otherwise, output the results at each voltage level.

[0054] S3: Calculate the electrical parameters of the line based on the power flow analysis, and determine whether the pantograph voltage is within a preset range. If yes, proceed to step S4; otherwise, proceed to step S7;

[0055] S4: Determine whether the stray current and rail potential are within a preset range. If yes, proceed to step S5; otherwise, proceed to step S7;

[0056] S5: Calculate the conductor temperature and determine whether the conductor temperature is within a preset range. If yes, proceed to step S6; otherwise, proceed to step S7;

[0057] S6: Calculate the total cost based on economic analysis and return the result at the current voltage level;

[0058] S7: Increase the current voltage level according to a preset gradient, and return to step S2. The preset gradient here can adaptively select a suitable span for voltage design of different levels according to application requirements or design needs.

[0059] During the specific implementation, first, a parameter set is established that includes different driving organizations, overhead wire types, substation spacing and other line conditions; based on this, the grid voltage fluctuation, rail potential / stray current and line current carrying capacity under different parameter conditions and voltage levels are calculated. If the limit is not exceeded, the candidate set is entered, otherwise the voltage level is increased to obtain a higher power supply capacity; for the candidate set that meets the requirements, the economic indicators are comprehensively evaluated from aspects such as system construction and operation and maintenance.

[0060] For the calculation of the pantograph voltage, it is first necessary to model the line. In step S1, the present invention determines the network parameters according to the actual operation scenario and establishes a system equivalent circuit model. When modeling the system, the substation is modeled as an ideal voltage source with internal resistance in series, the train is modeled as an ideal current source running at constant power, the resistivity of the conductor and the rail is set to be constant with temperature and regarded as a uniform conductor, and the DC autotransformer is modeled as a voltage-balanced capacitor with no energy loss.

[0061] Based on the above modeling principles, we can get Figure 5 The system model shown.

[0062] Combination Figure 5 The system model shown can be expressed as follows based on the assumed train branch current:

[0063]

[0064] Because the circuit is connected in parallel with capacitors, the power flow equation cannot be used directly for calculation. Additional conditions are required. Due to the effect of the DC autotransformer (DCAT), the voltage on the capacitor is balanced, and U 1_i =U 2_i , and without considering energy loss, we can also get I 1_i =I 2_i , all DCATs are like this, so the conditions for using the power flow equation are met, and then the power flow analysis can be used to calculate various electrical parameters in this circuit model, including pantograph voltage, conductor current, etc.

[0065] For the calculation of the conductor temperature, it can be calculated according to T a +ΔT calculation, where:

[0066]

[0067] ΔT is the thermal equilibrium temperature of the conductor, q cis the unit convection heat dissipation, q r is the unit radiation heat dissipation, q s To absorb the sun's heat, mC p is the total heat capacity of the wire, I is the wire current, R is the unit resistance, K angle is the wind direction factor, k f is the thermal conductivity of air, T a is the ambient air temperature, T0 is the wire temperature at the previous moment, N Re is the Reynolds coefficient, D is the outer diameter of the wire, ρ f is the air density, v ω is the wind speed, μ f is the absolute viscosity of air, ε is the emissivity of the wire, α is the solar absorptivity, Q se is the corrected radiation heat intensity, θ is the effective incident angle, and A' is the projection area of ​​the wire.

[0068] For the calculation of rail potential, combined with Figure 6 It can be seen that by introducing N DCATs, the system can be divided into N+1 sections. Take two trains on a certain section as an example. Figure 6 As shown, R g is the rail-to-ground resistance, which is assumed to be large enough to ensure that the leakage current is much smaller than the traction current. tr1 , U tr2 It is the neutral point of DCAT (or TSS) and the contact point between the train and the track, which can be calculated from the pantograph voltage.

[0069] For any point on the rail, taking the calculation point as an example, the rail potential can be calculated as:

[0070]

[0071] The above formula can be selected according to the following relationship:

[0072]

[0073] Similar analysis applies to other points, so the maximum rail potential will appear at one of the train-track contact points, which can be expressed as:

[0074] U rmax =max{U tr1 ,U tr2 ,U tr3 ,…}

[0075] As the train runs, the maximum track voltage needs to be recalculated in real time, thereby evaluating the rail potential.

[0076] For stray current, take single column operation as an example. Figure 7The length of this segment is L, the unit resistance is r, l1 is the distance from the train to DCAT (or TSS), and l is the distance from the calculation point to DCAT (or TSS).

[0077] Then the stray current at the calculation point is:

[0078]

[0079] At this time, the average stray current in this section can be expressed as:

[0080]

[0081] In addition, the superposition theorem is used to evaluate the stray current by considering multi-column operation and finding the average stray current.

[0082] For economic indicators, in step S6, according to C total =C inv +C ope When calculating the total cost, it should be noted that the calculated cost mainly includes the part that is greatly affected by the voltage level, and the part that is almost unaffected by the voltage level, such as the rail cost, is no longer considered. Among them:

[0083] Investment cost inv =N TSS C TSS +N DCAT C DCAT +C con ;

[0084] N TSS is the number of substations, the cost of a single substation C TSS =C pe,TSS +C pre,TSS +C civ,TSS ; C pe,TSS is the cost of the power electronic converter in the substation, C pre,TSS is the cost of the substation protection device, C civ,TSS The civil engineering cost of the substation;

[0085] N DCAT is the number of DC autotransformers; the cost of a single DC autotransformer C DCAT =C pe,DCAT +C pre,DCAT +C civ,DCAT , C pe,DCAT is the cost of the DC autotransformer power electronic converter, C pre,DCAT is the cost of the DC autotransformer protection device, C civ,DCAT The civil construction cost of the DC autotransformer;

[0086] Wire cost C con =Lcon (S tro +S mes +S neg ), L con is the wire length, S tro , S mes , S neg They correspond to the price per unit length of conductor for the overhead line, catenary and negative feeder respectively.

[0087] Operation loss cost C ope =ct(k c P con +k p P pe );

[0088] c is the electricity price, P con and P pe are line power and converter power respectively, K c is the line loss rate, K P is the power loss rate of the converter, and t is the operating time.

[0089] Therefore, in the end, based on the total cost situation at each voltage level, the voltage level with the lowest cost can be selected as the voltage level of the current medium-voltage DC AT traction power supply system.

[0090] During implementation, the cost of power electronic converters is affected by power capacity and voltage level, expressed as: C pe =k v P ins , k v Represents the cost of unit power capacity at different voltage levels, P ins The installed capacity of each substation is determined by the driving arrangement and power supply distance.

[0091] The cost of the protective device can be borne by: C pre =n bre C bre +n dis C dis Calculate, n bre and n dis are the number of circuit breakers and disconnectors in each substation, C bre and C dis are the costs of circuit breakers and disconnectors, respectively, which are affected by the voltage level.

[0092] Similarly, the civil engineering cost is calculated as C civ =(S land +dΔS)A land Calculation, A land is the land area of ​​each station, S landis the land price, ΔS is the change in land price, and d is the uncertainty coefficient caused by geographical factors.

[0093] In order to further understand the technical effect of the present invention, the medium voltage range of the voltage level selected in this embodiment is 10kV~35kV. Assume that the total distance of a certain line is 121 kilometers. On this line, the power and speed of each train are set to 12MW and 300km / h respectively. Considering the up and down lines of the train, in order to avoid multiple trains meeting at the same time, the interval time between consecutive trains on the up and down lines is 5 minutes and 5 minutes and 30 seconds respectively. In this case, the resistivity ρ of the contact network and the negative feeder are 14.2Ω·mm 2 / km and 14.4Ω·mm 2 / km, the resistance of the wire can be obtained by Calculated, where L is the length of the conductor and S is the cross-sectional area of ​​the conductor. Due to the different cross-sectional areas of the overhead wire, catenary and negative feeder, the corresponding conductor resistance will also be different.

[0094] The thermal parameters are shown in Table 1. To simplify the analysis, the absorption of solar heat is ignored in this example. In addition, the outer diameter of the wire can be Calculate, where S is the cross-sectional area of ​​the wire:

[0095] Table 1 Thermal parameters

[0096]

[0097] The parameters of the relevant economic analysis are shown in Table 2. In this case, the configuration with the lowest cost after 10 years is considered to be the optimal option.

[0098] Table 2 Cost parameters

[0099]

[0100] Subsequently, the indicators including pantograph voltage, stray current, track potential and temperature were evaluated according to the calculation method given above. This example requires that the pantograph voltage deviation is within ±10%, the maximum rail potential is 150V, the stray current is 15A, and the maximum conductor temperature is 90℃. ΔU and DCAT spacing are set to 5kV and 40km respectively.

[0101] According to the above implementation, a configuration solution set of voltage levels under different TSS spacings can be obtained, such as Figure 8 As shown, considering factors such as cost and weight, the maximum cross-sectional area is selected as 600mm 2It can be found that under the same conductor parameters, as the voltage level increases, the spacing that TSS can supply power increases, which means that the number of TSS required at the same distance decreases. At the same time, since the traction current decreases with the increase of voltage level, the required conductor cross-section also decreases under the same TSS spacing.

[0102] Although increasing the voltage level can reduce the number of TSS, traction current and conductor cross-section, it will also lead to higher voltage stress on the converter and protection devices, and the equipment construction cost will also increase. In order to determine the optimal voltage level, an economic analysis should be conducted.

[0103] After conducting an economic analysis on the configuration solution set of voltage levels, it can be seen that when the voltage level is constant, the cost varies with the change of the cross-sectional area of ​​the conductor. Fig. 9 The example is shown for 35kV. It can be seen that the cross-sectional area of ​​the conductor has a significant impact on the investment cost. In addition, there is an optimal conductor cross-sectional area that minimizes the total cost. The optimal cross-sectional area for 35kV is 125mm 2 The same analysis can be applied to other voltage levels.

[0104] Similarly, when the conductor cross-sectional area is constant, the voltage level will also significantly affect the cost. 2 Under the cross-sectional area, examples are given for different voltage levels, such as Fig.10 As shown. Although the high voltage level has lower operating loss cost, its investment cost is higher. Therefore, a higher voltage level does not mean lower cost. In this case, considering the lowest cost in 10 years, the 15kV and 20kV voltage levels are obviously better. In addition, in the 225mm 2 In terms of cross-sectional area, the operating loss of 20kV is lower than that of 15kV, so the voltage level with the best cost in the final 10 years is 20kV.

[0105] Based on the above analysis, considering all available voltage levels and cross-sectional areas, the most economical voltage level is selected as 20kV, and the corresponding cross-sectional area in this example is 225mm 2 .

[0106] In order to verify the effectiveness of the proposed design method, a simulation model was established. Using the parameters of the optimal economic model obtained in this example, the above evaluation indicators were re-analyzed. The pantograph voltage and the output node temperature of a single TSS under the motion of a single train were taken as examples. At the same time, the maximum rail potential and stray current were given, such as Fig.11 shown.

[0107] It can be seen that under the designed voltage level, the indicators such as pantograph voltage, rail potential, stray current, and conductor temperature all meet the requirements. Therefore, the above results prove that the proposed method can reasonably design the voltage level for the medium-voltage DC traction power supply system based on DACT. In addition, it should be noted that the obtained results are only the best configuration of this example. Although the designed optimal voltage level may vary with different system parameters, the provided design method is universal and can still be used.

[0108] From the above, it can be seen that the main technical contributions made by the present invention are:

[0109] 1) A model describing the energy flow characteristics of a medium-voltage DC traction power supply system suitable for DCAT access was established. By considering the regulatory effect of the DCAT device on the energy flow and the additional nodes it introduced, as well as the negative polarity voltage and additional branches introduced by the negative feeder, and by equating the bipolar output of the traction substation to two voltage sources in series, an equivalent circuit model of a medium-voltage DC traction power supply system suitable for DCAT was established. Based on this model, the energy flow distribution after DCAT access can be obtained, and then the key indicators of stray current, economy, pantograph voltage, conductor temperature, and rail potential can be evaluated to achieve the voltage level design of the medium-voltage DC traction power supply system connected to DCAT;

[0110] 2) Further consideration is given to the stray current, a key indicator of the DC system. By establishing and evaluating the stray current calculation model under DCAT access, the stray current can be limited to the required range, thereby ensuring the reliability of surrounding facilities;

[0111] 3) In the process of determining the final voltage level, an important method of economic evaluation is introduced. In terms of construction cost and operation cost, a corresponding economic analysis model is established to conduct an economic evaluation of the voltage level that meets the requirements, so as to determine the final voltage level that meets the safety and economy requirements.

[0112] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A method for designing voltage levels of a medium voltage DC AT traction power supply system, characterized in that: The following steps are involved: S1: Determine the network parameter set of the medium voltage DC autotransformer (AT) traction power supply system according to the train operation scenario, establish the system equivalent circuit model, and initialize the voltage level to the minimum voltage level; S2: Determine whether the voltage level is within a preset range. If yes, proceed to step S3; otherwise, output the results at each voltage level. S3: Calculate the electrical parameters of the line based on the power flow analysis, and determine whether the pantograph voltage is within a preset range. If yes, proceed to step S4; Otherwise, proceed to step S7; S4: Determine whether the stray current and rail potential are within a preset range, if yes, proceed to step S5; Otherwise, proceed to step S7; S5: Calculate the conductor temperature and determine whether the conductor temperature is within a preset range. If yes, proceed to step S6; otherwise, proceed to step S7; S6: Calculate the total cost based on economic analysis and return the result at the current voltage level; S7: Increase the current voltage level according to a preset gradient and return to step S2.

2. The method for designing voltage levels of a medium voltage DC AT traction power supply system according to claim 1 is characterized in that: In step S1, the network parameters are determined according to the actual operation scenario, and a system equivalent circuit model is established. When modeling the system, the substation is modeled as an ideal voltage source in series with internal resistance, the train is modeled as an ideal current source running at constant power, the resistivity of the conductor and the rail is set to be constant with temperature and regarded as a uniform conductor, and the DC autotransformer is modeled as a voltage-balanced capacitor with no energy loss.

3. The method for designing voltage levels of a medium voltage DC AT type traction power supply system according to claim 1 or 2, characterized in that: In step S1, the network parameter set for system modeling is determined by the application scenario, including various conductor types, lengths and cross-sectional areas, and substation spacing; the conductors include overhead wires, catenary cables, and negative feeders.

4. The method for designing voltage levels of a medium voltage DC AT traction power supply system according to claim 3 is characterized in that: In step S2, the voltage level preset range is 10kV to 35kV.

5. The method for designing voltage level of medium voltage DC AT type traction power supply system according to claim 1 is characterized in that: According to the system modeling, the average stray current is calculated using the superposition theorem as the stray current evaluation index in step S4.

6. The method for designing voltage level of medium voltage DC AT type traction power supply system according to claim 1 is characterized in that: According to the system modeling, the maximum voltage at each train-rail contact point is used as the rail potential evaluation index in step S4.

7. The method for designing voltage levels of a medium voltage DC AT type traction power supply system according to claim 3 is characterized in that: In step S5, according to T a +ΔT calculates the conductor temperature, where: q s =αQ se sin(θ)A'; ΔT is the change in conductor temperature, q c is the unit convection heat dissipation, q r is the unit radiation heat dissipation, q s To absorb the sun's heat, mC p is the total heat capacity of the wire, I is the wire current, R is the unit resistance, K angle is the wind direction factor, k f is the thermal conductivity of air, T a is the ambient air temperature, T0 is the wire temperature at the previous moment, N Re is the Reynolds coefficient, D is the outer diameter of the wire, ρ f is the air density, v ω is the wind speed, μ f is the absolute viscosity of air, ε is the emissivity of the wire, α is the solar absorptivity, Q se is the corrected radiation heat intensity, θ is the effective incident angle, and A' is the projection area of ​​the wire.

8. The method for designing voltage levels of a medium voltage DC AT traction power supply system according to any one of claims 4 to 7, characterized in that: In step S6, according to C total =C inv +C ope Calculate the total cost, where: Investment cost inv =N TSS C TSS +N DCAT C DCAT +C con ; N TSS is the number of substations, the cost of a single substation C TSS =C pe,TSS +C pre,TSS +C civ,TSS ; C pe,TSS is the cost of the power electronic converter in the substation, C pre,TSS is the cost of the substation protection device, C civ,TSS The civil engineering cost of the substation; N DCAT is the number of DC autotransformers; the cost of a single DC autotransformer C DCAT =C pe,DCAT +C pre,DCAT +C civ,DCAT , C pe,DCAT is the cost of the DC autotransformer power electronic converter, C pre,DCAT is the cost of the DC autotransformer protection device, C civ,DCAT The civil construction cost of the DC autotransformer; Wire cost C con =L con (S tro +S mes +S neg ), L con is the wire length, S tro , S mes , S neg They correspond to the price per unit length of conductor for the overhead contact network, catenary and negative feeder respectively. Operation loss cost C ope =ct(k c P con +k p P pe ); c is the electricity price, P con and P pe are line power and converter power respectively, K c is the line loss rate, K p is the power loss rate of the converter, and t is the operating time.

9. The method for designing voltage levels of a medium voltage DC AT type traction power supply system according to claim 8 is characterized in that: According to the total cost at each voltage level, the voltage level with the lowest cost is selected as the voltage level of the current medium voltage DC AT traction power supply system.

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