An optimal site selection and sizing method for large-scale coastal flexible direct current power transmission systems

Abstract

The present invention discloses a method for optimizing the site selection and sizing of a large-scale coastal flexible direct current (DC) power transmission system, which includes two stages: preliminary screening and economic comparison. The preliminary screening stage aims to minimize the total capacity of newly built flexible DC converter stations and considers stability constraints, thereby obtaining one or more candidate solutions. The economic comparison stage calculates the additional revenue coefficient of the site and line based on the degree of sharing and expansion potential, and then estimates the equivalent static cost of each candidate solution. The optimal solution is selected based on the equivalent static cost ranking. The present invention is applicable to the planning and research of flexible DC transmission systems in scenarios such as large-scale new energy consumption, large-scale power cluster transmission, and large-scale DC power feed-in. It can also provide a reference for the planning and research of flexible DC in other scenarios such as offshore wind power collection and transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current transmission of electric power systems, and in particular relates to an optimized site selection and sizing method for a large-scale coastal flexible direct current power transmission system. Background Art

[0002] Flexible DC transmission technology specifically refers to high-voltage DC transmission technology (VSC-HVDC) based on voltage-source converters (VSCs). In my country, modular multilevel converters (MMCs) are the mainstream topology for VSCs. MMCs utilize fully controlled devices and pulse modulation technology to overcome the challenges of conventional DC transmission, such as commutation failure and the inability to connect to weak AC systems. This technology holds great promise for future development.

[0003] For many years, the development of flexible DC transmission technology has remained in the theoretical research stage, with demonstration projects only beginning to be implemented in recent years. With the continuous upgrading of equipment and breakthroughs in control and protection systems, MMC-type flexible DC transmission systems have achieved increasingly higher voltage levels and capacities, and are gradually trending from two-terminal to multi-terminal DC transmission systems. Examples include the Kunliulong UHV hybrid DC project, the Zhangbei multi-terminal MMC DC project, and the Baihetan-Jiangsu hybrid cascade DC project.

[0004] Offshore wind power boasts abundant resources, high power generation utilization hours, no land occupation, no water consumption, and suitability for large-scale development. In recent years, offshore wind power development has become a global hotspot and frontier for renewable energy development. With the implementation of the "dual carbon" strategy, the amount of offshore wind power connected to the grid in my country's eastern coastal areas will increase annually, but this may pose challenges to power transmission in coastal areas. To increase the penetration rate of offshore wind power and meet the large-scale power transmission needs of coastal areas, building a multi-terminal flexible direct current transmission system is a promising approach. However, the domestic power industry and State Grid Corporation of China have not yet conducted sufficient research on planning methods for large-scale coastal flexible direct current transmission systems, which may not be sufficient to support the optimal siting and sizing of related projects.

[0005] Site selection and capacity determination are important issues in engineering project decision-making. According to existing research and engineering experience, the method suitable for site selection and capacity determination of flexible direct current projects is mainly the technical and economic scheme demonstration method [You Guangzeng, Xiao Liang, Li Lingfang, Zhu Xinchun, Yang Jian, Wu Xinglong, Wang Guoteng, Xu Zheng. Design method for strengthening the power grid in typical high-proportion renewable energy areas using flexible DC technology [J]. Power Capacitors and Reactive Compensation, 2020, 41(01): 142-150+157. DOI: 10.14044 / j.1674-1757.pcrpc.2020.01.024] and mathematical model method [Tang Xiaojun, Han Minxiao, Xie Yan, Wang Jing, Huo Qidi, Tian Chunzheng, Zhang Xin, Shen Xuhui. Flexible DC capacity and point selection configuration method for urban power grid zone interconnection [J]. Power System Technology, 2019, 43(05): 1709-1716. DOI: 10.13335 / j.1000- 3673.pst.2018.2097], among which the technical and economic scheme demonstration method requires obtaining alternative plans in advance, while the mathematical model method is highly dependent on the ideal mathematical model and has poor practical operability. Summary of the Invention

[0006] In view of the above, the present invention provides an optimized site selection and sizing method for a large-scale coastal flexible direct current power transmission system, which is suitable for the planning of a large-scale coastal flexible direct current power transmission system, and advocates obtaining the required number of site selection and sizing optimization plans through two stages of operations: preliminary screening and economic comparison.

[0007] A method for optimizing the site selection and sizing of large-scale coastal flexible direct current (DC) power transmission systems is used to select several nodes (locations with access to power equipment, such as power plants and substations) in the regional power grid and add flexible DC converter stations of a certain capacity at these nodes. Specifically, the method includes:

[0008] (1) Preliminary screening stage: For regional power grids with low thermal power output and high wind power output, multiple candidate schemes are calculated based on the premise that no line overload occurs when the N-1 fault of the main transmission channel (usually refers to the transmission channel with large transmission capacity or high voltage level) occurs, and the goal is to minimize the total capacity of the newly built flexible direct current converter stations;

[0009] (2) Economic comparison stage: Based on the degree of sharing and expansion potential, the additional revenue coefficient of the flexible DC converter station site and the DC line is calculated, and then the equivalent static cost of each candidate scheme is estimated. One or more schemes with the lowest equivalent static cost are selected as the final scheme.

[0010] Furthermore, the specific implementation process of the preliminary screening stage is as follows:

[0011] 1.1 Based on the current status and planning prospects of the regional power grid, obtain a set of regional power grid operation data. This operation data involves n major nodes (usually large-scale substations, power plants, and other important sites) that can be selected as flexible DC converter station sites, and these nodes are connected by m lines;

[0012] 1.2 Determine the number of terminals of the newly built flexible direct current transmission system, i.e. the number of additional flexible direct current converter stations, which is n VSC ;

[0013] 1.3 Constructing n-dimensional column vector P new With y, vector P new The value of the i-th element in newi is the active power fed into the i-th node of the newly built flexible direct current transmission system, and the i-th element value y in the vector y is i Indicates whether to build a new flexible DC converter station at the i-th node, y i =0 means no, y i =1 means yes;

[0014] 1.4 Before building a new flexible direct current transmission system, use the direct current method to calculate the active power flow of the regional power grid, and then calculate the m-dimensional line load factor column vector η when any line k is disconnected. Lk0 , the element value corresponding to line k in this vector is 0, k is the line index number and 1≤k≤m;

[0015] 1.5 Establish the following objective function: the sum of the capacities of the newly built flexible DC converter stations S is less than the set threshold ε;

[0016]

[0017] Where: D i represents the capacity of the new flexible DC converter station at the i-th node;

[0018] 1.6 Based on the above objective function and related constraints, multiple groups of candidate solutions are obtained. Each group of candidate solutions contains a set of D i and y i data.

[0019] Furthermore, the relevant constraints involved in step 1.6 are as follows:

[0020]

[0021] Where: R is the m-order diagonal matrix related to the thermal stability power limit of the line, X k is the m×n sparse matrix constructed based on the line reactance after line k is disconnected, B k is the n×n-order node admittance matrix after line k is disconnected, η maxis an m-dimensional column vector of the upper limit of line load rate.

[0022] Furthermore, the expression of the diagonal matrix R is as follows:

[0023]

[0024] Where: P Lmaxk is the thermal stability power limit of line k.

[0025] Furthermore, the sparse matrix X k Each row corresponds to a line, where all elements in row k are 0, and the values of elements in row l except for columns a and b are 1 / x respectively. l and -1 / x l , the rest of the elements are 0, x l is the reactance of line l, 1≤l≤m and l≠k, a and b are the index numbers of the nodes connecting the two ends of line l respectively, and a<b.

[0026] Furthermore, the specific implementation process of the economic comparison stage is as follows:

[0027] 2.1 For the multiple sets of candidate solutions obtained, determine the site of the flexible DC converter station in each solution and the DC lines involved in the connection between the sites, and then obtain all the non-repeated sites and DC lines involved in all candidate solutions, with the number being z1 and z2 respectively;

[0028] 2.2 Calculate the additional revenue coefficient for each site;

[0029] 2.3 Calculate the equivalent converter station construction cost of each candidate solution based on the additional revenue coefficient of the site;

[0030] 2.4 Calculate the additional revenue coefficient of each DC line;

[0031] 2.5 Calculate the equivalent length of the DC line in each candidate scheme based on the additional benefit coefficient of the DC line;

[0032] 2.6 Calculate the equivalent DC line construction cost of each candidate solution based on the equivalent length of the DC line;

[0033] 2.7 Calculate the equivalent static cost of each candidate solution, which is the sum of the equivalent DC line construction cost and the equivalent converter station construction cost;

[0034] 2.8 Compare the equivalent static costs of the candidate solutions and select one or more solutions with the lowest equivalent static costs as the final solutions.

[0035] Furthermore, in step 2.2, the additional revenue coefficient of each site is calculated by the following formula:

[0036] E 1p =A1(f 1p -1)+F 1p (p=1,2,...,z1)

[0037] Where: E 1p is the additional benefit coefficient of the p-th site, A1 is the conversion factor of the given site sharing degree (with a value in the range of 0 to 1), f 1p is the number of times the pth site is selected from all candidate solutions, F 1p is the expansion potential coefficient of the p-th site (given in the range of 0 to 1).

[0038] Furthermore, in step 2.3, the equivalent converter station construction cost of each candidate solution is calculated by the following formula:

[0039]

[0040] Where: C 0zj is the basic cost of adding the jth flexible DC converter station in the zth candidate solution, S zj For the capacity of the jth flexible DC converter station added in the zth candidate solution, c ave E is the average cost required for each additional unit capacity of the flexible DC converter station, 1zj is the additional profit coefficient of adding the j-th flexible DC converter station to the corresponding site in the z-th candidate solution, and Z is the number of candidate solutions.

[0041] Furthermore, in step 2.4, the additional revenue coefficient of each DC line is calculated by the following formula:

[0042] E 2q =A2(f 2q -1),(q=1,2,...,z2)

[0043] Where: E 2p is the additional revenue coefficient of the qth DC line, A2 is the conversion factor of the given line sharing degree (with a value in the range of 0 to 1), f 2q is the number of times the qth DC line appears in all candidate solutions.

[0044] Furthermore, in step 2.5, the equivalent length of the DC line in each candidate solution is calculated by the following formula:

[0045]

[0046] Among them: z is the equivalent length of the DC line in the zth candidate solution, d zg is the aviation distance of the g-th DC line in the z-th candidate solution, λzg is the tortuosity coefficient of the gth DC line in the zth candidate solution (i.e., actual length divided by aviation distance), E 2zg is the additional revenue coefficient of the g-th DC line in the z-th candidate solution.

[0047] Furthermore, in step 2.6, the equivalent DC line construction cost of each candidate solution is calculated using the following formula:

[0048] C Lz =(c1+c2+c3)·l z

[0049] Where: C Lz is the equivalent DC line construction cost of the zth candidate solution, l z is the equivalent length of the DC line in the zth candidate solution, c1 is the tower cost per unit length of the line, c2 is the stringing cost per unit length of the line, and c3 is the civil engineering and accessories cost per unit length of the line.

[0050] The present invention is aimed at flexible direct current transmission systems for large-scale coastal power transmission. It can provide the optimal / preferred site and sizing scheme for the flexible direct current transmission system based on comprehensive consideration of factors such as construction requirements, stability constraints, scheme costs and scalability. It is suitable for the planning and research of flexible direct current transmission systems in scenarios such as large-scale new energy consumption, large-scale power cluster transmission and large-scale direct current power feed-in. It can also provide a reference for the planning and research of flexible direct current transmission in other scenarios such as offshore wind power aggregation and transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 The figure is a flow chart of the method for optimizing site selection and volume determination according to the present invention.

[0052] Figure 2 Schematic diagram of the site of candidate solutions 1 to 3 in the embodiment of the present invention.

[0053] Figure 3 Schematic diagram of the site of candidate solutions 4 to 6 in the embodiment of the present invention.

[0054] Figure 4 Schematic diagram of the DC lines involved in all candidate solutions in the embodiments of the present invention. DETAILED DESCRIPTION

[0055] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0056] The method for optimizing the site selection and sizing of a large-scale coastal flexible direct current power transmission system of the present invention includes a total of 15 steps, of which the first 7 steps belong to the preliminary screening stage, used to screen out candidate solutions that meet construction requirements and stability constraints; the last 8 steps belong to the economic comparison stage, used to select the economically optimal solution; Figure 1 The specific steps are as follows:

[0057] (1) Based on the current status and planning prospects of the regional power grid, select the grid data to be used. In this data, the proportion of regional thermal power output should be below a certain threshold value, and the regional wind power output should be above a certain threshold value. After selecting the data, determine the list of nodes in the system that can be selected as flexible DC converter station sites (assuming there are n such nodes) and the list of lines between these nodes (assuming there are m such lines).

[0058] (2) Determine the number of terminals of the planned flexible DC system (assuming the number of flexible DC terminals is n VSC ).

[0059] (3) Construct n-dimensional column vector P new with y:P new The element P in newi represents the active power fed into node i by the newly built multi-terminal flexible DC system; the element y in y i Indicates whether to build a new flexible DC converter station at node i (y i =0 means no converter station is built at node i, y i =1 indicates that a new converter station is to be built at node i).

[0060] (4) Before constructing the flexible DC system, the DC method is used to calculate the active power flow of the regional power grid under consideration; then, when any line k is disconnected, the load factor of each line is calculated and the m-dimensional line load factor column vector η is given. Lk0 , note that the element in this vector corresponding to the disconnected line k is 0.

[0061] (5) For the case of line k being disconnected, an m-order diagonal matrix R is constructed based on the line thermal stability power limit, and an m×n-order sparse matrix X is constructed based on the line reactance. k , and calculate the node admittance matrix B after line k is broken k On this basis, the following constraint conditions are added:

[0062]

[0063] In the above formula, the larger positive value M is only used in y i When it is 0, ensure P newi is 0, D i represents the capacity of the new converter station at node i, and the matrices R and X kThe calculation formulas are as follows:

[0064]

[0065]

[0066] Where: P Lmaxk represents the thermal stability power limit of line k, x ij represents the reactance of line l connecting nodes i and j (when the line is disconnected, the reactance is considered infinite).

[0067] (6) The objective function is set to minimize the sum of converter station capacities, that is,

[0068] (7) Using the constraints set in step (5) and the objective function set in step (6), candidate solutions can be screened and obtained through calculation; at this point, the preliminary screening of the site selection and capacity determination solutions has been completed.

[0069] (8) Next, the economic comparison stage is entered; assuming that step (7) has screened out a total of Z candidate schemes, the DC line direction in each candidate scheme is determined in turn, and a total of z1 station sites and z2 DC line sections are obtained for the construction of these Z schemes.

[0070] (9) Calculate the additional benefit coefficient E of building the i-th site in sequence 1i , the calculation formula is as follows:

[0071] E 1i =A1·(f 1i -1)+F 1i ,(i=1,2,...,z1)

[0072] Where: A1 is the artificially given site sharing degree conversion factor (with a value in the range of 0 to 1), f 1i is the number of times site i is selected from all Z candidate solutions, F 1i is the expansion potential coefficient of the artificially given site i (with a value in the range of 0 to 1).

[0073] (10) Calculate the equivalent converter station construction cost C of scheme k in turn Ck , the calculation formula is as follows:

[0074]

[0075] Where: C 0kj is the basic cost of converter station j in scheme k, S kj is the capacity of converter station j in solution k, c ave E is the average additional cost required for each additional unit capacity of the converter station, 1kjis the additional profit coefficient of constructing converter station j in scheme k.

[0076] (11) Calculate the additional benefit coefficient E of constructing the i-th section of the DC line 2i , the calculation formula is as follows:

[0077] E 2i =A2·(f 2i -1),(i=1,2,...,z2)

[0078] Where: A2 is the artificially given line sharing degree conversion factor (with a value in the range of 0 to 1), f 2i is the number of times DC line i is selected among all Z candidate solutions.

[0079] (12) Calculate the equivalent calculation length l of the DC line constructed by scheme k in turn k , the calculation formula is as follows:

[0080]

[0081] Where: d kj is the aviation distance of DC line j in scheme k, λ kj is the tortuosity coefficient of DC line j in scheme k, E 2kj is the additional profit coefficient of constructing DC line j under scheme k.

[0082] (13) Calculate the equivalent DC line construction cost C of the kth candidate solution in turn Lk , the calculation formula is as follows:

[0083] C Lk =(c1+c2+c3)·l k

[0084] Among them: c1 is the tower cost per unit length of line, c2 is the stringing cost per unit length of line, and c3 is the civil engineering and other accessories cost per unit length of line.

[0085] (14) Calculate the equivalent total static cost of the kth candidate solution in turn, that is, C k =C Ck +C Lk .

[0086] (15) Compare the equivalent total static costs of each candidate solution and select one or more final solutions.

[0087] The Jiangsu power grid is a typical receiving-end power grid with a high proportion of electricity receiving. With the access of large-scale wind power in the coastal areas of northern Jiangsu, the reduction in the output proportion of thermal power units in southern Jiangsu, and the continuous increase in load levels, the imbalance between power generation centers and power consumption centers within the Jiangsu power grid will be aggravated.

[0088] According to the East China Power Grid's 2025 summer high-voltage planning data, the Jiangsu power grid has 161 525kV nodes (i.e., n = 161) and 325 525kV transmission lines (i.e., m = 325). Under this planned operating mode, the Jiangsu power grid's internal cross-river sections will transmit a total of 14,293.9MW of active power from the north side of the Yangtze River to the south, and the load factors of these transmission lines are also high. Specifically, with the exception of the Qiuteng-Qinhuai cross-river section, if N-1 occurs in other cross-river sections, individual line load factors will exceed 80%, posing a significant threat to the safe power supply in southern Jiangsu. Furthermore, in addition to the cross-river sections, there are also some 525kV transmission lines with relatively high load factors, primarily located in the southern Jiangsu load center.

[0089] In order to solve the problem of insufficient transmission capacity of Jiangsu power grid cross-river section after large-scale access of wind power to coastal areas in the future, it is possible to consider using the reserved line construction space in the Sutong GIL cross-river corridor (corresponding to Figure 4 For the DC line ②), a new three-terminal flexible DC transmission system will be built in Jiangsu Province so that the 525kV grid load rate in Jiangsu Province is not higher than 80% when the 525kV transmission channel N-1 at the cross-river section or the southern Jiangsu load center is used.

[0090] Therefore, if n VSC =3 (i.e., building a three-terminal flexible DC system) and M = 3000 (i.e., requiring the power of a single converter station to not exceed 3000MW), and requiring the DC line to include the Sutong GIL cross-river corridor section (corresponding to Figure 4 For the DC line in ②), according to steps (1) to (7) of the method for optimizing the site selection and capacity determination of the present invention, 6 candidate solutions are obtained, as shown in Table 1:

[0091] Table 1

[0092]

[0093] From the table above, we can see that the rectifier stations of Schemes 1 to 6 are all located at Sunantong 51 busbar, and the converter stations of Schemes 1 to 3 have the same construction capacity, and the converter stations of Schemes 4 to 5 have the same construction capacity. Figure 2 As shown, the site selection for options 4 to 6 is as follows Figure 3 As shown in the figure, the larger black circle represents the site of the rectifier station, and the smaller gray circle represents the site of the inverter station; it is easy to obtain that z1=8.

[0094] Since the total capacity of the converter stations in each scheme is very similar, the construction costs of the converter stations in each scheme are not much different. Therefore, the cost difference between the different candidate schemes in this example mainly depends on the line construction cost. Based on the site selection results of schemes 1 to 6, the required DC lines can be planned. The planning results show that the six candidate schemes involve a total of 9 DC lines (i.e., z2 = 9). Figure 4 shown.

[0095] Let A1 = F 1i =0.1, A2=0.2 and the tortuosity coefficient is always 1.2, then the equivalent total static cost of each scheme can be calculated according to steps (8) to (14) of the method for optimizing site selection and capacity determination of the present invention. The results are shown in Table 2:

[0096] Table 2

[0097]

[0098] As can be seen from the above table, based on the equivalent total static cost ranking, it can be considered that Solution 5 is the optimal solution in this embodiment.

[0099] The above description of the embodiments is intended to facilitate understanding and application of the present invention by those skilled in the art. It is apparent that those skilled in the art can readily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without requiring creative effort. Therefore, the present invention is not limited to the above embodiments. Any improvements or modifications made by those skilled in the art based on the disclosure of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for optimizing the site selection and sizing of a large-scale coastal flexible direct current (DC) power transmission system, which is used to select several nodes in a regional power grid and add a fixed-capacity DC converter station at these nodes. The method specifically includes: (1) Preliminary screening stage: For regional power grids with low thermal power output and high wind power output, multiple candidate schemes are calculated based on the premise that no line overload occurs when the main transmission channel N-1 fails and the goal is to minimize the total capacity of the newly built flexible direct current converter stations. The specific implementation process is as follows: 1.1 Based on the current status and planning prospects of the regional power grid, obtain a set of regional power grid operation data. This operation data involves n major nodes that can be selected as sites for flexible direct current converter stations, and these nodes are connected by m lines; 1.2 Determine the number of terminals of the newly built flexible direct current transmission system, i.e. the number of additional flexible direct current converter stations, which is n VSC ; 1.3 Constructing n-dimensional column vector P new With y, vector P new The value of the i-th element in newi is the active power fed into the i-th node of the newly built flexible direct current transmission system, and the i-th element value y in the vector y is i Indicates whether to build a new flexible DC converter station at the i-th node, y i =0 means no, y i =1 means yes; 1.4 Before building a new flexible direct current transmission system, use the direct current method to calculate the active power flow of the regional power grid, and then calculate the m-dimensional line load factor column vector η when any line k is disconnected. Lk0 , the element value corresponding to line k in this vector is 0, k is the line index number and 1≤k≤m; 1.5 Establish the following objective function: the sum of the capacities of the newly built flexible DC converter stations S is less than the set threshold ε; Where: D i represents the capacity of the new flexible DC converter station at the i-th node; 1.6 Based on the above objective function and related constraints, multiple groups of candidate solutions are obtained. Each group of candidate solutions contains a set of D i and y i data; (2) Economic comparison stage: Based on the degree of sharing and expansion potential, the additional revenue coefficient of the flexible DC converter station site and the DC line is calculated, and then the equivalent static cost of each candidate scheme is estimated. One or more schemes with the lowest equivalent static cost are selected as the final scheme.

2. The method for optimizing site selection and capacity determination according to claim 1, characterized in that: The relevant constraints involved in step 1.6 are as follows: Where: R is the m-order diagonal matrix related to the thermal stability power limit of the line, X k is the m×n sparse matrix constructed based on the line reactance after line k is disconnected, B k is the n×n-order node admittance matrix after line k is disconnected, η max is an m-dimensional column vector of the upper limit of line load rate.

3. The method for optimizing site selection and volume determination according to claim 2, characterized in that: The expression of the diagonal matrix R is as follows: Where: P Lmaxk is the thermal stability power limit of line k; The sparse matrix X k Each row corresponds to a line, where all elements in row k are 0, and the values of elements in row l except for columns a and b are 1 / x respectively. l and -1 / x l , the rest of the elements are 0, x l is the reactance of line l, 1≤l≤m and l≠k, a and b are the index numbers of the nodes connecting the two ends of line l respectively, and a<b.

4. The method for optimizing site selection and capacity determination according to claim 1, characterized in that: The specific implementation process of the economic comparison stage is as follows: 2.1 For the multiple sets of candidate solutions obtained, determine the site of the flexible DC converter station in each solution and the DC lines involved in the connection between the sites, and then obtain all the non-repeated sites and DC lines involved in all candidate solutions, with the number being z1 and z2 respectively; 2.2 Calculate the additional revenue coefficient for each site; 2.3 Calculate the equivalent converter station construction cost of each candidate solution based on the additional revenue coefficient of the site; 2.4 Calculate the additional revenue coefficient of each DC line; 2.5 Calculate the equivalent length of the DC line in each candidate scheme based on the additional benefit coefficient of the DC line; 2.6 Calculate the equivalent DC line construction cost of each candidate solution based on the equivalent length of the DC line; 2.7 Calculate the equivalent static cost of each candidate solution, which is the sum of the equivalent DC line construction cost and the equivalent converter station construction cost; 2.8 Compare the equivalent static costs of the candidate solutions and select one or more solutions with the lowest equivalent static costs as the final solutions.

5. The method for optimizing site selection and capacity determination according to claim 4, characterized in that: In step 2.2, the additional revenue coefficient of each site is calculated using the following formula: Yes 1p =A1(f 1p -1)+F 1p ,p=1,2,...,z1 Where: E 1p is the additional benefit coefficient of the pth site, A1 is the conversion factor for the given site sharing degree, and f 1p is the number of times the pth site is selected from all candidate solutions, F 1p is the expansion potential coefficient of the p-th site.

6. The method for optimizing site selection and capacity determination according to claim 4, characterized in that: In step 2.3, the equivalent converter station construction cost of each candidate solution is calculated using the following formula: Where: C 0zj is the basic cost of adding the jth flexible DC converter station in the zth candidate solution, S zj For the capacity of the jth flexible DC converter station added in the zth candidate solution, c ave E is the average cost required for each additional unit capacity of the flexible DC converter station, 1zj is the additional profit coefficient of adding the j-th flexible DC converter station to the corresponding site in the z-th candidate solution, and Z is the number of candidate solutions.

7. The method for optimizing site selection and capacity determination according to claim 4, characterized in that: In step 2.4, the additional revenue coefficient of each DC line is calculated using the following formula: E 2q =A2(f 2q -1),q=1,2,...,z2 Where: E 2p is the additional revenue coefficient of the qth DC line, A2 is the conversion factor for the given line sharing degree, and f 2q is the number of times the qth DC line appears in all candidate solutions.

8. The method for optimizing site selection and capacity determination according to claim 4, characterized in that: In step 2.5, the equivalent length of the DC line in each candidate solution is calculated using the following formula: Among them: z is the equivalent length of the DC line in the zth candidate solution, d zg is the aviation distance of the g-th DC line in the z-th candidate solution, λ zg is the tortuosity coefficient of the g-th DC line in the z-th candidate solution, E 2zg is the additional revenue coefficient of the g-th DC line in the z-th candidate solution.

9. The method for optimizing site selection and capacity determination according to claim 4, characterized in that: In step 2.6, the equivalent DC line construction cost of each candidate solution is calculated using the following formula: C Lz =(c1+c2+c3)·l z Where: C Lz is the equivalent DC line construction cost of the zth candidate solution, l z is the equivalent length of the DC line in the zth candidate solution, c1 is the tower cost per unit length of the line, c2 is the stringing cost per unit length of the line, and c3 is the civil engineering and accessories cost per unit length of the line.

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