A medium-voltage direct-current AT type traction power supply system voltage grade design method

Abstract

The application discloses a medium-voltage direct-current AT type traction power supply system voltage grade design method, which comprises the following steps: S1, determining a system network parameter set according to a train operation scene, establishing a system equivalent circuit model, and initializing a voltage grade as a minimum voltage grade; S2, judging whether the voltage grade is within a preset range, yes, then S3, otherwise, outputting a result; S3, calculating electrical parameters of a line based on a power flow analysis, and judging whether a pantograph voltage is within a preset range, yes, then S4, otherwise, S7; S4, judging whether a stray current and a rail potential are within a preset range, yes, then S5, otherwise, S7; S5, calculating a conductor temperature, and judging whether the conductor temperature is within a preset range, yes, then S6, otherwise, S7; S6, calculating a 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 of realizing voltage grade design of a medium-voltage direct-current traction power supply system with direct-current AT access.

Description

TECHNICAL FIELD

[0001] The present application relates to a train traction power supply system, in particular to a voltage level design method of a medium voltage direct current AT type traction power supply system. BACKGROUND

[0002] With the development of high-power power electronic technology and direct current transmission system, medium voltage direct current traction power supply system (MVDC TPSS) has been paid more and more attention, and its structure is shown in Figure 1 Compared with the existing 25kV alternating current traction power supply system, the medium voltage direct current traction power supply system eliminates the electric phase, improves the power supply distance and power quality, and has become an important power supply scheme for electrified railway.

[0003] The introduction of direct current autotransformer (DCAT) in MVDC TPSS can further reduce the traction current and improve the system power supply capacity, and its structure is shown in Figure 2 Through the regulation of DCAT device, the current in the rail is transmitted to the catenary and negative feeder line, as a key problem in the direct current system, the system stray current and rail potential can be effectively inhibited, and the power supply capacity can be further improved. Therefore, compared with the MVDC TPSS without DCAT, the MVDC TPSS based on DCAT has more advantages and meets the actual application requirements. However, how to select the voltage level of the MVDC TPSS with DCAT has not been determined.

[0004] As shown in Figure 3 , the existing researches mostly focus on the voltage level design of the conventional direct current power supply scheme, and the main evaluation indexes include the pantograph voltage of the train, the conductor temperature, and the rail potential. However, after the introduction of DCAT device and negative feeder line, the characteristics of MVDC TPSS will be significantly different from the direct power supply scheme, and the characteristics are reflected in:

[0005] 1) The DCAT device will regulate the system current, i.e. transfer the rail current to the catenary and negative feeder line;

[0006] 2) The DCAT device introduces a large number of additional nodes;

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

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

[0009] Under the joint action of the above characteristics, the circuit structure of the MVDC TPSS with DCAT will change, and the energy flow distribution will be more complex. The existing model is difficult to accurately describe the system energy flow distribution characteristics, and the evaluation of pantograph voltage, conductor temperature and other related indexes cannot be carried out. SUMMARY

[0010] Therefore, the application provides a medium-voltage direct-current AT type traction power supply system voltage grade design method, which considers the regulation of DCAT on energy flow and the additional nodes introduced by DCAT, as well as the negative polarity voltage introduced by the negative feedback line and the additional branch, and equivalently connects the bipolar output of the traction substation (TSS) to two voltage sources in series, establishes an equivalent circuit model of the MVDCTPSS containing DCAT, and obtains the energy flow distribution after the access of DCAT by solving the model, and then the pantograph voltage, conductor temperature and rail potential can be evaluated. In addition, the important index of stray current is introduced in the evaluation index, the stray current is suppressed at a reasonable level by modeling and evaluating, and an economic analysis model is established in the final determination of the voltage grade to evaluate the economic analysis model, and then the optimal voltage grade design under the consideration of comprehensive factors is realized.

[0011] To achieve the above object, the specific technical scheme adopted by the application is as follows:

[0012] A medium-voltage direct-current AT type traction power supply system voltage grade design method, comprising the following steps:

[0013] S1: determining the network parameter set of the medium-voltage direct-current autotransformer (AT) type traction power supply system according to the train operation scene, establishing an equivalent circuit model of the system, and initializing the voltage grade to the minimum voltage grade;

[0014] S2: determining whether the voltage grade is within a preset range, if yes, entering step S3; otherwise, outputting the results under each voltage grade;

[0015] S3: calculating the electrical parameters of the line based on the power flow analysis, and determining whether the pantograph voltage is within a preset range, if yes, entering step S4; otherwise, entering step S7;

[0016] S4: determining whether the stray current and the rail potential are within a preset range, if yes, entering step S5; otherwise, entering step S7;

[0017] S5: calculating the conductor temperature, and determining whether the conductor temperature is within a preset range, if yes, entering step S6; otherwise, entering step S7;

[0018] S6: calculating the total cost according to the economic analysis, and returning the results under the current voltage grade;

[0019] S7: increasing the current voltage grade by a preset gradient, and returning to step S2.

[0020] Optionally, in step S1, the network parameters are determined according to the actual operation scenario, and the system equivalent circuit model is established, and in the system modeling, the substation is modeled as an ideal voltage source with internal resistance in series, the train is modeled as an ideal current source operating at constant power, the resistivity of the wire and the rail is set as not changing with temperature and is regarded as a uniform conductor, and the DC autotransformer is modeled as a voltage-balanced capacitor without energy loss.

[0021] Optionally, in step S1, the network parameter set in the system modeling by applying the scenario includes various wire models, lengths and cross-sectional areas and substation spacings; the wires include the catenary, the messenger wire and the negative feeder.

[0022] Optionally, in step S2, the voltage grade preset range is 10kV-35kV.

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

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

[0025] Optionally, in step S5, the wire temperature is calculated according to T a +ΔT, wherein:

[0026]

[0027] ΔT is the wire thermal equilibrium temperature, q c is the unit convective heat dissipation, q r is the unit radiative heat dissipation, q s is the absorbed solar heat energy, 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 air heat transfer coefficient, T a is the ambient air temperature, T0 is the wire temperature at the previous time, N Re is the Reynolds number, D is the wire outer diameter, 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 radiative heat intensity, θ is the effective incidence angle, and A' is the projection area of the wire.

[0028] Optionally, in step S6, the wire temperature is calculated according to C total =C inv +C opeThe total cost is calculated, and it is noted that the calculated cost mainly includes the part affected by the voltage level, and the part such as the cost of steel rail which is hardly affected by the voltage level is no longer considered, wherein:

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

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

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

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

[0033] The cost C ope of operation loss is ct(k c P con + k p P pe );

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

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

[0036] The significant effect of the present application is:

[0037] The present application proposes a medium-voltage DC AT type traction power supply system voltage level design method, which considers the regulation of energy flow by the DC AT device, the additional nodes introduced by the DC AT device, and the negative polarity voltage and additional branches introduced by the negative feedback line. An equivalent circuit model of the traction power supply system suitable for the DC AT is established. By solving the model, the energy flow distribution after the DC AT is connected can be obtained, and the corresponding key indicators can be evaluated to realize the voltage level design of the traction power supply system after the DC AT is connected.

[0038] In addition, the present method considers the stray current, an important indicator of the DC system. By establishing a stray current calculation model under the DC AT connection and evaluating it, the stray current can be limited within the required range, thereby ensuring the reliability of the surrounding facilities. At the same time, in the final voltage level determination link, the important method of economic evaluation is introduced. For the construction cost and operation cost, the corresponding economic analysis model is established, and then the voltage level that meets the requirements is evaluated in terms of economy, thereby determining the final voltage level. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 is a topological structure diagram of an existing medium-voltage DC traction power supply system;

[0040] Figure 2 is a topological structure diagram of a medium-voltage DC traction power supply system with a DC autotransformer introduced;

[0041] Figure 3 is a voltage level design flowchart of an existing medium-voltage DC traction power supply system;

[0042] Figure 4 is a method flowchart of the present application;

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

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

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

[0046] Figure 8is the relationship between substation spacing and cross-sectional area of catenary under different voltage levels in the embodiment of the present application;

[0047] Figure 9 is the relationship between cost and cross-sectional area at 35kV in the embodiment of the present application;

[0048] Figure 10 is the relationship between cross-sectional area and economic index curve of different voltage levels in the embodiment of the present application at 225mm 2

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

[0050] The embodiments of the present application will be described in detail below with reference to the accompanying drawings. The embodiments are given only for the purpose of illustration and should not be understood as limiting the present application. The accompanying drawings are used for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present application, because many changes can be made to the present application without departing from the spirit and scope thereof.

[0051] As shown in Figure 4 , the voltage level design method of the medium-voltage DC AT type traction power supply system provided by the embodiment comprises the following steps:

[0052] S1: determining a set of network parameters of the medium-voltage DC autotransformer (AT) type traction power supply system according to the train operation scenario, establishing an equivalent circuit model of the system, and initializing the voltage level to the minimum voltage level;

[0053] S2: determining whether the voltage level is within a preset range, if yes, proceeding to step S3; otherwise, outputting the results under each voltage level;

[0054] S3: calculating the electrical parameters of the line based on the power flow analysis, and determining whether the pantograph voltage is within a preset range, if yes, proceeding to step S4; otherwise, proceeding to step S7;

[0055] S4: determining whether the stray current and the rail potential are within a preset range, if yes, proceeding to step S5; otherwise, proceeding to step S7;

[0056] S5: calculating the conductor temperature, and determining whether the conductor temperature is within a preset range, if yes, proceeding to step S6; otherwise, proceeding to step S7;

[0057] S6: calculating the total cost according to the economic analysis, and returning the results under the current voltage level;​

[0058] S7: raising the current voltage level by a preset gradient and returning to step S2, where the preset gradient can be adaptively selected according to application requirements or design needs to design different voltage levels with appropriate spans.

[0059] In a specific implementation, first, a parameter set containing different train operation organizations, contact line types, substation spacings and other line conditions is established; based on this, the network voltage fluctuation, rail potential / spread current and line current carrying capacity under different parameter conditions and voltage levels are calculated, if not out of limit, then entering the candidate set, otherwise, raising the voltage level to obtain higher power supply capacity; for the candidate set meeting the requirements, the economic indicators are comprehensively evaluated from the aspects of system construction and operation and maintenance.

[0060] For the calculation of pantograph voltage, first, the line needs to be modeled, in step S1, the network parameters are determined according to the actual operation scene, and the system equivalent circuit model is established, and 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 with constant power, the resistivity of the conductor and rail is set as not changing with temperature and is regarded as a uniform conductor, and the DC autotransformer is modeled as a capacitor with voltage balance and no energy loss.

[0061] Based on the above modeling principles, the system model shown in FIG. Figure 5 is obtained.

[0062] Combined with the system model shown in FIG. Figure 5 , the branch current of the assumed train can be expressed as follows:

[0063]

[0064] Because the capacitor is connected in parallel in the circuit, the power flow equation cannot be directly used for calculation, and additional conditions are needed, due to the action of the DC autotransformer (DCAT), the voltage balance on the capacitor is known as U 1_i = U 2_i , and without considering energy loss, I 1_i = I 2_i , each DCAT is so, thus satisfying the condition for using the power flow equation, and thus the various electrical parameters in this circuit model, including the pantograph voltage and conductor current, can be calculated using power flow analysis.

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

[0066]

[0067] ΔT is the thermal equilibrium temperature of the conductor, q cq is the unit convection heat loss r q is the unit radiation heat loss s mC is the absorbed solar heat energy p I is the wire current, R is the unit resistance, K angle k is the wind direction factor f T is the air thermal conductivity a T0 is the wire temperature at the previous time, N Re D is the wire outer diameter, p f v is the air density ω v is the wind speed, m f m is the absolute viscosity of air, e is the emissivity of the wire, a is the solar absorptivity, Q se Q is the corrected radiation heat intensity, q is the effective incident angle, A' is the projected area of the wire.

[0068] For the calculation of the rail potential, combined with Figure 6 It can be seen that by introducing N DCATs, the system can be divided into N+1 segments. Taking two trains on a certain section as an example, as shown in Figure 6 R is the rail-ground resistance, which is assumed to be large enough to ensure that the leakage current is much smaller than the traction current. U1, U2, U g , U tr1 , U tr2 are the neutral points of the DCAT (or TSS) and the train-rail contact points, 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 also applies to other points, therefore, the maximum rail potential will appear at one of the train-rail contact points, which can be represented as:

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

[0075] With the running of the train, the maximum rail voltage needs to be recalculated in real time, thereby the rail potential can be evaluated.

[0076] For stray current, taking single train operation as an example, as shown in Figure 7The length of this segment is L, the unit resistance is r, and 11 is the distance from the train to the DCAT (or TSS), and 1 is the distance from the calculation point to the DCAT (or TSS).

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

[0078]

[0079] The average stray current in this section can be expressed as:

[0080]

[0081] In addition, considering multiple trains running, the average stray current is calculated using the superposition theorem, so as to evaluate the stray current.

[0082] For economic indicators, in step S6, the C total = C inv + C ope The total cost is calculated, and it should be noted that the calculated cost mainly includes the part affected by the voltage level, and the part such as the cost of the rail which is almost not affected by the voltage level is no longer considered, wherein:

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

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

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

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

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

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

[0089] Therefore, the voltage level with the lowest cost can be selected as the current voltage level of the medium voltage DC AT traction power supply system according to the total cost under each voltage level.

[0090] In the implementation process, the cost of power electronic converter is affected by the power capacity and voltage level, which is represented as: C pe = k v P ins , k v represents the cost per unit power capacity under different voltage levels, and P ins is the installed capacity of each substation, which is determined by the train arrangement and power supply distance.

[0091] The cost of protection devices can be calculated by: C pre = n bre C bre + n dis C dis , n bre and n dis are the number of circuit breakers and disconnectors of each substation respectively, and C bre and C dis are the cost of circuit breakers and disconnectors respectively, which are affected by the voltage level.

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

[0093] To further understand the technical effects of this invention, the medium voltage range of 10kV to 35kV is selected in this embodiment. Assume a line with a total distance of 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, to avoid multiple trains meeting simultaneously, the interval between consecutive trains on the up and down lines is 5 minutes and 5 minutes 30 seconds, respectively. In this case, the resistivity ρ of the contact wire and the negative electrode feeder is 14.2Ω·mm². 2 / km and 14.4Ω·mm 2 / km, the resistance of the conductor can be determined by Calculations show that L is the length of the conductor and S is its cross-sectional area. Since the cross-sectional areas of the contact wire, catenary wire, and negative feeder are different, the corresponding conductor resistances will also be different.

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

[0095] Table 1 Thermal parameters

[0096]

[0097] The relevant economic analysis parameters are shown in Table 2. In this case, the configuration with the lowest cost over 10 years is taken as the optimal option.

[0098] Table 2 Cost Parameters

[0099]

[0100] Subsequently, parameters including pantograph voltage, stray current, rail potential, and temperature were evaluated using the calculation methods described above. This example requires a pantograph voltage deviation within ±10%, a maximum rail potential of 150V, a stray current of 15A, and a maximum conductor temperature of 90℃. The ΔU and DCAT spacings are set to 5kV and 40km, respectively.

[0101] Based on the above implementation method, the configuration solution set for 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 chosen to be 600mm². 2It can be found that, with the same conductor parameters, the distance between TSSs increases with the increase of voltage level, which means that the number of TSSs needed for the same distance decreases. At the same time, since the traction current decreases with the increase of voltage level, the cross section of the conductor needed also decreases with the increase of the distance between TSSs.

[0102] Although increasing the voltage level can reduce the number of TSSs, the traction current and the cross section of the conductor, it will also cause the converter and protection device to bear higher voltage stress, and the cost of equipment construction will also increase. In order to determine the optimal voltage level, economic analysis should be carried out.

[0103] After economic analysis of the configuration solution of voltage level, it can be found that when the voltage level is certain, the cost changes with the change of the cross section area of the conductor. Figure 9 The example is shown for 35 kV. It can be seen that the cross section area of the conductor has a significant impact on the investment cost. In addition, there is an optimal cross section area of the conductor that minimizes the total cost, and the optimal cross section area corresponding to 35 kV is 125 mm 2 . The same analysis can be applied to other voltage levels.

[0104] Similarly, when the cross section area of the conductor is certain, the voltage level will also significantly affect the cost. The example under the cross section area of 225 mm 2 is given, as shown in Figure 10 . Although the high voltage level has lower operating loss cost, it has higher investment cost. Therefore, higher voltage level does not mean lower cost. In this case, considering the cost in 10 years, the voltage levels of 15 kV and 20 kV are obviously better. In addition, under the cross section area of 225 mm 2 , the operating loss of 20 kV is lower than that of 15 kV, so the optimal voltage level in the final 10 years is 20 kV.

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

[0106] In order to verify the effectiveness of the design method proposed, a simulation model is established. Using the parameters of the optimal economic model obtained in this example, the above evaluation indexes are reanalyzed. Taking the pantograph voltage under the movement of a single train and the output node temperature of a single TSS as examples. At the same time, the maximum rail potential and the stray current are given, as shown in Figure 11 .

[0107] It can be seen that, under the designed voltage level, the pantograph voltage, rail potential, stray current, conductor temperature and other indicators meet the requirements. Therefore, the above results prove that the proposed method can be used for reasonable voltage level design of the DACT-based medium-voltage DC traction power supply system. 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] As can be seen from the above, the main technical contribution made by the present application is:

[0109] 1) An energy flow characteristic description model suitable for DCAT access is established for the medium-voltage DC traction power supply system, the regulating effect of the DCAT device on the energy flow and the additional nodes introduced by it are considered, as well as the negative polarity voltage and additional branches introduced by the negative feedback line, and the bipolar output of the traction substation is equivalent to two voltage sources in series, thereby establishing an equivalent circuit model of the medium-voltage DC traction power supply system suitable for DCAT. Based on the 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, realizing the voltage level design of the medium-voltage DC traction power supply system with DCAT access;

[0110] 2) Further considering the stray current, a key indicator of the DC system, by establishing a stray current calculation model under DCAT access and evaluating it, the stray current can be limited within the required range, thereby ensuring the reliability of the surrounding facilities;

[0111] 3) In the final voltage level determination link, the important method of economic evaluation is introduced. For the construction cost and operation cost, the corresponding economic analysis model is established, and the voltage level meeting the requirements is evaluated in terms of economy, so as to determine the final voltage level meeting the safety and economy.

[0112] The above embodiments are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above embodiments, and any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principles of the present application shall be equivalent replacement methods and shall be included in the protection scope of the present application.

Claims

1. A method for voltage level design of a medium voltage direct current (MVDC) AT-type traction power supply system, characterized in that, The method comprises the following steps: S1: determining a network parameter set of a medium-voltage direct-current autotransformer AT type traction power supply system according to a train operation scenario, establishing a system equivalent circuit model, and initializing a voltage level as a minimum voltage level; S2: judging whether the voltage level is within a preset range, if yes, entering step S3; otherwise, outputting a result under each voltage level; S3: calculating electrical parameters of a line based on a power flow analysis, and judging whether a pantograph voltage is within a preset range, if yes, entering step S4; otherwise, entering step S7; S4: judging whether a stray current and a rail potential are within a preset range, if yes, entering step S5; otherwise, entering step S7; S5: calculating a conductor temperature, and judging whether the conductor temperature is within a preset range, if yes, entering step S6; otherwise, entering step S7; S6: calculating a total cost according to economic analysis, and returning a result under a current voltage level; S7: increasing the current voltage level by a preset gradient, and returning to step S2; In step S1, the network parameters are determined according to an actual operation scenario, and the system equivalent circuit model is established, and when the system is modeled, the substation is modeled as an ideal voltage source with an internal resistance in series, the train is modeled as an ideal current source operating at a constant power, the resistivity of the conductor and the rail is set as not changing with temperature, and the conductor and the rail are regarded as uniform conductors, and the direct-current autotransformer is modeled as a capacitor with voltage balance and without energy loss. In step S1, the network parameter set when the system is modeled by applying a scenario comprises various conductor types, lengths and cross-sectional areas, and substation spacings; the conductor comprises a catenary, a load-bearing cable and a negative feedback line. In step S6, the total cost is calculated according to Total cost = Cost of the first route + Cost of the second route Investment costs ; Cost of substation, Csub ; C pe,TSS Cost of substation power electronics converter, Csub pre,TSS Cost of substation protection, Csub civ,TSS Cost of substation civil works; N = number of DC autotransformers; cost of a single DC autotransformer , C pe,DCAT N = number of DC autotransformers; cost of a single DC autotransformer pre,DCAT N = number of DC autotransformers; cost of a single DC autotransformer civ,DCAT N = number of DC autotransformers; cost of a single DC autotransformer Wire cost , L con is the wire length, S tro , S mes , S neg correspond to the price of wire per unit length for the catenary, the messenger and the negative feeder, respectively. Operating loss cost ; c is the electricity price, P con and P pe are the line 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.

2. The medium voltage DC AT-type traction power supply system voltage class design method of claim 1, wherein In step S2, the preset range of the voltage level is 10kV-35kV.

3. The medium voltage DC AT traction power supply system voltage class design method of claim 1 or 2, characterized in that, According to the system modeling, the average stray current is calculated by using the superposition theorem as the stray current evaluation index in step S4.

4. The medium voltage DC AT traction power supply system voltage class design method of claim 3, wherein, According to the system modeling, the maximum voltage at each train and rail contact point is taken as the rail potential evaluation index in step S4.

5. The medium voltage DC AT traction power supply system voltage class design method of claim 1, wherein, According to the total cost under each voltage level, the voltage level with the lowest cost is selected as the current voltage level of the medium-voltage direct-current AT type traction power supply system.

6. The medium voltage DC AT-type traction power supply system voltage class design method of claim 1, wherein ​ 7. The medium voltage DC AT traction power supply system voltage class design method of claim 3, wherein, In step S5, the following is calculated the conductor temperature, wherein: ； ； ； ； ； q is the heat loss by convection c q is the heat loss by convection r q is the heat loss by radiation s q is the heat absorbed from the sun p q is the total heat capacity of the conductor, I is the conductor current, R is the unit resistance, K angle k is the wind direction factor f k is the air thermal conductivity, T is the ambient air temperature, T is the conductor temperature at the previous time step, N Re k is the Reynolds number, D is the outer diameter of the conductor, k is the air density, k is the wind speed, k is the absolute viscosity of air, k is the emissivity of the conductor, k is the solar absorptivity, k is the corrected radiation heat intensity, k is the effective incident angle, k is the projected area of the conductor.

8. The medium voltage DC AT-type traction power supply system voltage class design method of claim 1, wherein, ​

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