An Optimization Configuration Method for a Flexible DC Grid Current Limiting Reactor

Abstract

The present invention relates to the technical field of flexible DC power grids, and aims to provide an optimized configuration method for current-limiting reactors in flexible DC power grids. The method includes: dividing the flexible DC power grid into protection zones, establishing a fault approximate analytical model for a single protection zone, and solving the functional expression between the maximum breaking current of each DC circuit breaker and the values of each current-limiting reactor; selecting the optimization objective of the maximum breaking current of the DC circuit breaker and a reasonable value range of the current-limiting reactors; then solving the optimized value set that meets the requirements within a single protection zone; and combining the total value upper limit of the current-limiting reactors and the total breaking capacity of the DC circuit breakers, selecting the overall optimal configuration scheme from the optimized value sets in each protection zone. On the premise of ensuring that the total value of the current-limiting reactors does not exceed the upper limit, the present invention selects the configuration scheme with the minimum total capacity of the DC circuit breakers as the optimal configuration scheme, which can improve the operation reliability of the flexible DC power grid while reducing the cost of protection equipment.

Description

Technical Field

[0001] The present invention relates to the technical field of flexible DC power grids, and particularly to an optimization configuration method for current-limiting reactors in flexible DC power grids. Background Art

[0002] Modular multilevel converters (MMCs) have been widely used in flexible DC projects due to advantages such as low harmonic content, low switching frequency, and high reliability. With the continuous maturity of flexible DC transmission technology, constructing flexible DC power grids has gradually become a new development direction in the current power industry. However, flexible DC power grids have low damping characteristics, resulting in fast development speed and wide spread of DC faults, seriously threatening the safe and reliable operation of the system. Ensuring the survival ability of flexible DC power grids under faults is a key technical difficulty at present.

[0003] To ensure that flexible DC power grids still have the ability to transmit power after a fault occurs, DC circuit breakers should be configured on both sides of the line; when a fault occurs, the faulty line is disconnected, and the non-faulty part of the power grid can maintain normal operation. However, this solution will lead to too large a breaking capacity and too high a cost of DC circuit breakers, causing great economic pressure on constructing flexible DC power grids. To suppress fault currents and reduce the cost of DC circuit breakers, current flexible DC power grid projects configure certain current-limiting reactors on both sides of the transmission line to suppress fault currents.

[0004] However, in existing projects, the configured current-limiting reactors do not consider different parameters such as MMC converter stations and transmission lines, but use simple average distribution; this results in the overall current-limiting effect in flexible DC power grids not reaching the optimal, so there is still room for optimizing the total cost of DC circuit breakers. At present, there is a lack of reliable and effective theoretical methods for the optimization configuration of current-limiting reactors in flexible DC power grids. Only relying on electromagnetic transient simulation for optimization configuration takes a lot of time and has too low efficiency to ensure obtaining the global optimal result. Therefore, considering the future demand for constructing multi-terminal DC power grids, it is of great significance to study the optimization configuration strategy of current-limiting reactors. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide an optimization configuration method for current-limiting reactors in flexible DC power grids.

[0006] To solve the above technical problem, the present invention proposes the following technical solutions:

[0007] Provide an optimization configuration method for current-limiting reactors in flexible DC power grids, including:

[0008] (1) Protectively partition the flexible DC grid according to the spatial positions of the converter stations and transmission lines; each protection partition includes a single converter station, several current-limiting reactors, and DC circuit breakers, where the number of current-limiting reactors and DC circuit breakers is the same as the number of transmission lines connected to the converter station;

[0009] (2) Establish a fault approximate analytical model for a single protection partition, and solve the functional expression between the maximum breaking current of each DC circuit breaker in the partition and the values of each current-limiting reactor;

[0010] (3) According to the current-carrying capacity of the IGBT used in the DC circuit breaker, select the optimization objective of the maximum breaking current of the DC circuit breaker, and select the reasonable value range of the current-limiting reactor; then, based on the functional expression between the maximum breaking current of each DC circuit breaker and each current-limiting reactor, solve multiple optimized value sets of current-limiting reactors that meet the requirements in a single protection partition;

[0011] (4) Combine the total upper limit of the values of the current-limiting reactors in the flexible DC grid and the total breaking capacity of the DC circuit breakers, and select the overall optimal configuration scheme for the flexible DC grid from the optimized value sets in each protection partition.

[0012] Description of the invention principle:

[0013] In the fault approximate analytical model of a single protection partition established in the present invention, the converter station in the partition is equivalent to a second-order discharge circuit, which is composed of an equivalent capacitor, an equivalent inductor, and an equivalent resistor connected in series. The converter station outside the partition is equivalent to a voltage source series inductor model. Since the impedance of the transmission line is mainly inductive, it is replaced by an equivalent reactance. The value of the current-limiting reactor outside the partition is regarded as a fixed value, and the analytical mathematical expression of the breaking current of each DC circuit breaker in a single partition and the current-limiting reactor in the partition is solved based on this model.

[0014] Based on the fact that the cost of the DC circuit breaker is mainly composed of IGBT devices, in the method for optimizing the configuration of the current-limiting reactor considering the constraint of the current-carrying capacity of the IGBT proposed in the present invention, an integer multiple (4-6 times) of the current-carrying capacity of the IGBT is used as the breaking capacity target of the DC circuit breaker. The actual breaking current should be as close as possible to its breaking capacity to ensure the maximum utilization of the current-carrying capacity of the IGBT. At the same time, in a single-end converter station, a reasonable value range of the current-limiting reactor is selected as the boundary condition according to this target.

[0015] The optimal configuration method of the fault current limiting reactor for the flexible DC grid proposed by the present invention is obtained through iteration and multi-zone accumulation based on the optimal configuration scheme in each protection zone obtained in the above process. The specific method is as follows: The flexible DC grid is decomposed into several protection zones, and each protection zone includes multiple optimal configuration schemes of the fault current limiting reactor. Then, the total upper limit of the values of the fault current limiting reactors in the entire flexible DC grid is set. One scheme is selected from the schemes in each protection zone, and the total value of the fault current limiting reactors selected in each zone and the total capacity of the DC circuit breakers are added together. Through iteration, several configuration schemes of the fault current limiting reactors for the flexible DC grid can be obtained. On the premise of ensuring that the total value of the fault current limiting reactors does not exceed the upper limit, the configuration scheme that minimizes the total capacity of the DC circuit breakers is selected as the final optimal configuration scheme of the fault current limiting reactor for the flexible DC grid.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] 1. On the premise of ensuring that the total value of the fault current limiting reactors does not exceed the upper limit, the present invention selects the configuration scheme that minimizes the total capacity of the DC circuit breakers as the final optimal configuration scheme of the fault current limiting reactor for the flexible DC grid.

[0018] 2. The present invention can improve the operation reliability of the flexible DC grid while reducing the cost of protection equipment. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of a four-terminal flexible DC grid based on MMC and its protection zones.

[0020] Figure 2 It is a flowchart of the optimization configuration method of the fault current limiting reactor considering the IGBT current-carrying capacity constraint proposed by the present invention.

[0021] Figure 3 It is a fault approximate equivalent model of a single protection zone.

[0022] Figure 4 It is a schematic diagram of the mathematical relationship between the breaking current of a single DC circuit breaker and the values of multiple fault current limiting reactors in a single protection zone.

[0023] Figure 5 It is a schematic diagram of the set of values of the fault current limiting reactors in a single protection zone obtained under the condition of maximizing the utilization of the breaking capacity of each DC circuit breaker. Detailed Embodiment

[0024] First of all, it should be noted that the present invention relates to database technology, which is an application of computer technology in the field of information security technology. During the implementation process of the present invention, the application of multiple software function modules will be involved. The applicant believes that after carefully reading the application documents and accurately understanding the implementation principle and the invention purpose of the present invention, and in combination with the existing well-known technologies, those skilled in the art can fully implement the present invention by using the software programming skills they have mastered. The aforementioned software function modules include but are not limited to: fault approximation analysis models, etc. All those mentioned in the application documents of the present invention belong to this category, and the applicant will not list them one by one.

[0025] Those skilled in the art know that in addition to implementing a part of the system provided by the present invention and its various devices, modules, and units in the form of pure computer-readable program code, the method steps can be logically programmed to enable the system provided by the present invention and its various devices, modules, and units to be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers, etc. to achieve the same functions. Therefore, the system provided by the present invention and its various devices, modules, and units can be considered as a kind of hardware component, and the devices, modules, and units included therein for implementing various functions can also be regarded as the structures within the hardware component; the devices, modules, and units for implementing various functions can also be regarded as either software modules for implementing the method or the structures within the hardware component.

[0026] It should also be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising an..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the element.

[0027] The optimization configuration method of the flexible DC grid current-limiting reactor provided by the present invention includes the following steps:

[0028] (1) Protectively partition the flexible DC grid according to the spatial positions of the converter stations and transmission lines; each protection partition includes a single converter station, several current-limiting reactors, and DC circuit breakers, where the number of current-limiting reactors and DC circuit breakers is the same as the number of transmission lines connected to the converter station;

[0029] (2) Establish a fault approximate analysis model for a single protection zone, and solve the mathematical expression between the maximum breaking current of each DC circuit breaker in the zone and the values of each current-limiting reactor; specifically including:

[0030] (2.1) Set the system parameters of the flexible DC grid, including converter station parameters, transformer parameters and line parameters;

[0031] (2.2) For a single protection zone, taking a single DC circuit breaker in it as the analysis object, when a DC fault occurs at the near end of the line where the DC circuit breaker is located, the converter station in the zone is equivalent to a second-order discharge circuit, and the other converter stations connected to this converter station outside the zone are equivalent to a DC source series equivalent inductance model;

[0032] (2.3) Take the current-limiting reactor in the zone as a variable, and regard the current-limiting reactor outside the zone as a fixed value, and solve the maximum breaking current I of this DC circuit breaker f_DCCBj_areai mathematical expression, where DCCBj in the subscript represents the jth DC circuit breaker in the zone, and areai represents the ith protection zone;

[0033] When solving the maximum breaking current of the DC circuit breaker, the mathematical expression is as follows:

[0034]

[0035] Among them,

[0036]

[0037] In the above formulas, the converter station located in this zone is named the near-end converter station, and the converter station located outside this zone and connected to the near-end converter station through a line is named the adjacent-end converter station; then t represents the independent variable time, U eq1 、U eq2 represent the equivalent voltages of the near-end MMC1 and the adjacent-end MMC2 respectively, C eq1 、L eq1 、R eq1 represent the equivalent capacitance, inductance and resistance of the near-end MMC respectively, C eq2 、L eq2 、R eq2 represent the equivalent capacitance, inductance and resistance of the adjacent-end MMC respectively, L dc1 represents the current-limiting reactor at the connection between the near-end MMC and the fault point, L dc2_1 、L dc2_2 represent two groups of reactors on the line connecting the adjacent-end MMC and the near-end MMC respectively, U eq10 、U eq20 represent the initial voltages of the near-end converter station and the adjacent-end converter station respectively, I 10 、I 20respectively represent the initial currents of the proximal converter station and the adjacent converter station; e is the natural base, and a, β, γ, a, b, c, C1, C2, and C3 are all mathematical symbols without direct physical meaning, only used for facilitating the writing of mathematical expressions, and their definitions are given in the above formula.

[0038] (2.4) Repeat the above steps to obtain the mathematical expression of the maximum breaking current of N i DC circuit breakers for the i-th protection zone; where N i is the number of DC circuit breakers in the i-th protection zone, which is the same as the number of transmission lines, 1 ≤ i ≤ m, and m is the number of protection zones in the flexible DC grid, which is the same as the number of converter stations.

[0039] (3) According to the current-carrying capacity of the IGBTs used in the DC circuit breakers, select the optimization objective of the maximum breaking current of the DC circuit breakers, and select the reasonable value range of the current-limiting reactors;

[0040] The optimization objective is defined as follows: Let the rated current of the IGBT devices in the converter station and the DC circuit breaker be II GBT_N , then set the boundary of the maximum breaking current of the DC circuit breaker to 6I IGBT_N , and select 4I IGBT_N , 5I IGBT_N , 6I IGBT_N as the three groups of values for the optimization objective of the maximum breaking current of the DC circuit breaker;

[0041] Then, based on the function expressions between the maximum breaking current of each DC circuit breaker and each current-limiting reactor, solve the multiple sets of optimized values of the current-limiting reactors that meet the requirements within a single protection zone; specifically, it includes the following steps;

[0042] (3.1) Select the i-th protection zone, select the fault current breaking time according to the actual working conditions of the DC circuit breaker, and based on the N i mathematical expressions in this zone, calculate the mathematical relationship between the maximum breaking current of the N i DC circuit breakers and the values of the current-limiting reactors in this zone by iterating the values of the current-limiting reactors in this zone;

[0043] (3.2) Based on the three set optimization objectives, obtain the sets of values of the current-limiting reactors that meet the objectives from the calculation results, which are respectively represented as L dc_4IGBT_areai_DCCBj , L dc_5IGBT_areai_DCCBj , L dc_6IGBT_areai_DCCBj ; where the subscript DCCBj represents the j-th DC circuit breaker in the current protection zone corresponding to this set of values, and the subscript areai represents that this set of values is used for the i-th protection zone; the same operation is performed for each DC circuit breaker in this protection zone to obtain a total of 3N i sets of values;

[0044] (3.3) Select any one of the 3 sets corresponding to a single circuit breaker, and take the intersection among the N i sets to obtain a set of current-limiting reactor values within the current protection zone; iterate through the 3 sets corresponding to each DC circuit breaker and repeat the same operation to obtain a total of 3^N i sets of current-limiting reactor values within the current zone;

[0045] (3.4) Based on the 3^N i sets of current-limiting reactor values obtained in step (3.3), calculate the sum of the current-limiting reactor values and the total breaking capacity of the DC circuit breakers within the corresponding zones respectively;

[0046] (3.5) Sort the 3^N i sets of current-limiting reactor value sets in step (3.4) in descending order according to the total breaking capacity of the corresponding DC circuit breakers. The highest is 6N i I IGBT_N , and the lowest is 4N i I IGBT_N , with a total of 3^N i sets; for several sets with the same total breaking capacity of the DC circuit breakers, select a set with the lowest required sum of current-limiting reactor values as the representative, so as to obtain 2N i +1 sets of current-limiting reactor value sets, which can make the total breaking capacity of the DC circuit breakers within the protection zone change from 4N i I IGBT_N to 6N i I IGBT_N and be an integer multiple of I IGBT_N . These 2N i +1 current-limiting reactor values are the optimal configuration scheme within the current protection zone.

[0047] (4) Combine the total value upper limit of the current-limiting reactors in the flexible DC grid and the total breaking capacity of the DC circuit breakers, and select the overall optimal configuration scheme for the flexible DC grid from the optimized value sets in each protection zone; specifically including:

[0048] (4.1) Set the total value upper limit of the current-limiting reactors in the flexible DC grid, select one from the 2N i +1 sets of optimized configuration sets of current-limiting reactors in each protection zone, and sum the current-limiting reactor values and the total breaking capacity of the DC circuit breakers of these m sets of configuration sets;

[0049] (4.2) Repeat the process of step (4.1) until each optimized configuration scheme in each protection zone is iterated, and obtain the optimized configuration scheme of the current-limiting reactors in the flexible DC grid, with a total of types;

[0050] (4.3) Select the solution with the total value of the current-limiting reactor not exceeding the upper limit and the minimum total breaking capacity of the DC circuit breaker from all solutions as the optimal configuration solution of the current-limiting reactor for the current flexible DC power grid.

[0051] The following further describes in detail the specific implementation manners of the present invention in conjunction with the accompanying drawings and embodiments.

[0052] Take Figure 1 the MMC-based four-terminal flexible DC power grid shown as the specific embodiment 1, and combine Figure 2 the flowchart shown to describe the specific steps of the method for optimizing the configuration of the current-limiting reactor considering the IGBT current-carrying capacity constraint proposed by the present invention.

[0053] As Figure 1 shown, the flexible DC power grid is divided into 4 protection zones based on the spatial positions of the MMC converter stations and transmission lines. Each zone includes 1 converter station and 2 parts of transmission lines. One current-limiting reactor and one DC circuit breaker need to be configured on each part of the transmission line.

[0054] Take the example of solving the breaking current of DCCB1 in protection zone 1 to establish an approximate fault analysis model as Figure 3 shown. The breaking current of DCCB1 is fed by two converter stations, MMC1 and MMC3. Among them, the converter station MMC1 in protection zone 1 is replaced by a second-order RLC equivalent circuit, the converter station MMC3 outside protection zone 1 is replaced by a voltage source in series with an equivalent reactance model, and the transmission line parameters are replaced by equivalent reactances. For Figure 3 the approximate model shown, the analytical mathematical expression of the breaking current of DCCB1 can be obtained. Repeating the above steps, the analytical mathematical expressions of the breaking currents of the remaining DC circuit breakers in the flexible DC power grid can be obtained.

[0055] Set the parameters of the flexible DC power grid, including converter station parameters, transmission line parameters, etc., as shown in Table 1.

[0056] Simulation parameters Parameter value DC voltage ±535 kV Active power (MMC1 / MMC3 / MMC4) -750 MW / 1500 MW / -1500 MW <![CDATA[Sub-module capacitor C SM (P1 to P4)]]> 8 mF / 8 mF / 15 mF / 15 mF <![CDATA[Bridge arm inductor L arm (P1~P4)]]> 100 mH / 50 mH / 100 mH / 50 mH Number of sub-modules in the arm 233 <![CDATA[Average-configured DC reactor L dc > 150 mH Line length Line1 - Line4 205.9 km / 188.1 km / 208.4 km / 49.6 km Line reactance 1.049 mH / km

[0057] Suppose that the 4.5 kV 3 kA IGBT products of ABB Company are used in the flexible DC power grid, and its rated current is 3 kA. Then, the optimization objectives of the DC breaker breaking current are 12 kA, 15 kA, and 18 kA. Taking protection zone 1 as an example, it contains 2 DC breakers DCCB1 and DCCB8, and 2 current-limiting reactors Ldc1 and Ldc8. Suppose the fault current breaking time is 6 ms, that is, the DC breaker starts to cut off the fault current at the 6 ms moment after the fault occurs. Then, based on the obtained analytical expression of the DC breaker breaking current, for the 2 current-limiting reactors Ldc1 and Ldc8 in the protection zone, the breaking currents of DCCB1 and DCCB8 just do not exceed the optimization objectives of 12 kA, 15 kA, and 18 kA. Since the analytical expression is used for iteration, this process will not take too long. After the iteration is completed, the schematic diagram as shown in Figure 4 can be obtained. Each curve in it represents the value sets of Ldc1 and Ldc8 when the breaking current of DCCB1 or DCCB8 just does not exceed a certain optimization objective.

[0058] As shown in Figure 4 , there are intersections among different curves, and each intersection represents a group that can make the breaking currents of DCCB1 and DCCB8 in protection zone 1 meet the objectives. That is, there are 9 groups of optimized value sets of the current-limiting reactors in protection zone 1, and the total breaking capacity range of the DC breakers corresponding to these 9 groups of sets is 24 kA to 36 kA, with a unit of 3 kA in the middle, and there are 5 kinds of total breaking capacities in total. For each total breaking capacity, calculate the total reactance value required for the corresponding value set of the current-limiting reactor, and select the group with the smallest required reactance as the optimal value. Finally, 5 groups of optimal value sets of the current-limiting reactors in protection zone 1 can be obtained, corresponding to the total breaking capacity of the DC breaker from 24 kA to 36 kA. The specific value results are shown in Table 2.

[0059]

[0060] After obtaining the above 9 groups of optimized configuration sets of the current-limiting reactors, according to the total breaking capacity of the DCCB corresponding to different sets, the total breaking capacity of the DCCB in protection zone 1 can be specifically divided into 5 cases: 2 sets of 18 kA, 1 set of 18 kA + 1 set of 15 kA, 2 sets of 15 kA, 1 set of 18 kA + 1 set of 12 kA, 1 set of 15 kA + 1 set of 12 kA, and 2 sets of 12 kA. Select the one with the smallest total value from the current-limiting reactor sets corresponding to each case. Finally, 5 groups of optimal configuration sets of the current-limiting reactors applicable to protection zone 1 can be obtained, as shown in Table 3.

[0061] <![CDATA[L dc1 / mH]]> DCCB1 <![CDATA[L dc2 / mH]]> DCCB2 <![CDATA[DCCB all = 24 kA]]> 261 12 kA 117 12 kA <![CDATA[DCCB all = 27 kA]]> 195 12 kA 118 15 kA <![CDATA[DCCB all = 30 kA]]> 155 18 kA 119 12 kA <![CDATA[DCCB all = 33 kA]]> 153 18 kA 147 15 kA <![CDATA[DCCB all = 36 kA]]> 152 18 kA 188 18 kA

[0062] Repeat the above operations for other protection zones. Since each protection zone in the four-terminal flexible DC grid contains one converter station, two DC circuit breakers, and two current-limiting reactors, similar to protection zone 1, there are also five groups of optimal values in other protection zones.

[0063] Based on the five groups of optimal values of the current-limiting reactors in the four protection zones, select one from the optimal value sets of each protection zone, calculate the total value of the current-limiting reactors and the total breaking capacity of the DC circuit breakers in the corresponding flexible DC grid, and iterate through the sets selected for each protection zone. There are a total of 625 configuration schemes for the current-limiting reactors in the flexible DC grid. Set the upper limit of the total allowable value of the current-limiting reactors in the flexible DC grid to 1200 mH. Then, the computer selects the scheme with the total value of the current-limiting reactors not exceeding the upper limit and the minimum total breaking capacity of the DC circuit breakers from these 625 schemes as the optimal configuration scheme for the current-limiting reactors in the flexible DC grid.

Claims

1. An optimization configuration method for a flexible DC grid current-limiting reactor, characterized in that Including: (1) According to the spatial positions of the converter station and the transmission lines, the flexible DC grid is divided into protection zones; each protection zone includes a single converter station, several current-limiting reactors, and DC circuit breakers, where the number of current-limiting reactors and DC circuit breakers is the same as the number of transmission lines connected to the converter station; (2) Establish a fault approximate analytical model for a single protection zone, and solve the mathematical expression between the maximum breaking current of each DC circuit breaker in the zone and the values of each current-limiting reactor; (3) According to the current-carrying capacity of the IGBT used in the DC circuit breaker, select the optimization objective of the maximum breaking current of the DC circuit breaker, and select a reasonable value range for the current-limiting reactor; then, based on the function expression between the maximum breaking current of each DC circuit breaker and each current-limiting reactor, solve for a set of optimized values of multiple current-limiting reactors that meet the requirements within a single protection zone; (4) Considering the total upper limit of the values of the current-limiting reactors and the total breaking capacity of the DC circuit breakers in the flexible DC grid, select the overall optimal configuration plan for the flexible DC grid from the sets of optimized values in each protection zone.

2. The method according to claim 1, wherein The specific steps of step (2) include: (2.1) Set the system parameters of the flexible DC grid, including converter station parameters, transformer parameters, and line parameters; (2.2) For a single protection zone, taking a single DC circuit breaker in it as the analysis object, when a DC fault occurs at the near end of the line where the DC circuit breaker is located, the converter station in the zone is equivalent to a second-order discharge circuit, and the other converter stations connected to this converter station outside the zone are equivalent to a DC source in series with an equivalent inductance model; (2.3) Taking the in-zone current-limiting reactor as a variable and the out-of-zone current-limiting reactor as a fixed value, solve for the maximum breaking current I of this DC circuit breaker f_DCCBj_areai The mathematical expression, where DCCBj in the subscript represents the j-th DC circuit breaker in this zone, and areai represents the i-th protection zone; (2.4) Repeat the above steps to obtain the mathematical expression of the maximum breaking current of N DC circuit breakers for the i-th protection zone; where N i is the number of DC circuit breakers in the i-th protection zone, which is the same as the number of transmission lines, 1 ≤ i ≤ m, and m is the number of protection zones in the flexible DC grid, which is the same as the number of converter stations. i ​ 3. The method according to claim 1, characterized in that, In step (3), the optimization objective is defined as follows: Let the rated current of the IGBT device in the converter station and the DC circuit breaker be I IGBT_N , then set the maximum breaking current boundary of the DC circuit breaker to 6I IGBT_N , and select 4I IGBT_N , 5I IGBT_N , 6I IGBT_N Three sets of values as the optimization objectives for the maximum breaking current of the DC circuit breaker.

4. The method according to claim 3, wherein In step (3), the specific steps of solving for a set of optimized values of multiple current-limiting reactors that meet the requirements within a single protection zone are as follows; (3.1) Select the $i$-th protection partition, select the fault current interruption time according to the actual working conditions of the DC circuit breaker, and based on $N$ i mathematical expressions in this partition, calculate the mathematical relationship between the maximum interruption current of $N$ i DC circuit breakers and the value of the current-limiting reactor in this protection partition by iterating the values of the current-limiting reactors in this partition; (3.2) Based on the three optimization objectives that have been set, obtain the set of values of the current-limiting reactor that meet the objectives from the calculation results, which are respectively denoted as L dc_4IGBT_areai_DCCBj , L dc_5IGBT_areai_DCCBj , L dc_6IGBT_areai_DCCBj ; where the subscript DCCBj indicates that this set of values corresponds to the j-th DC circuit breaker in the current protection zone, and the subscript areai indicates that this set of values is used for the i-th protection zone; the same operation is performed for each DC circuit breaker in this protection zone, and a total of 3N i sets of value sets are obtained; (3.3) Select any one of the 3 sets corresponding to a single circuit breaker, and take the intersection among a total of N i sets to obtain a set of current-limiting reactor values within the current protection zone; iterate through the 3 sets corresponding to each DC circuit breaker and repeat the same operation to obtain a total of 3^N i sets of current-limiting reactor value sets within the current zone; (3.4) Based on the 3^N obtained in step (3.3) i Set of current-limiting reactor value sets, calculate the sum of the current-limiting reactor values and the total breaking capacity of the DC circuit breakers in the corresponding partition respectively; (3.5) Arrange the set of values of the 3^N i groups of current-limiting reactance in descending order according to their corresponding total breaking capacity of the DC circuit breaker, with the highest being 6N i I IGBT_N , and the lowest being 4N i I IGBT_N , for a total of 3^N i groups; For several sets with the same total breaking capacity of the DC circuit breaker, select the set with the lowest total value of the required current-limiting reactor as the representative, so as to obtain 2N i +1 sets of current-limiting reactor values, which can make the total breaking capacity of the DC circuit breaker in this protection zone change from 4N i I IGBT_N to 6N i I IGBT_N and is an integer multiple of I IGBT_N These 2N i +1 sets of current-limiting reactor values are the optimal configuration scheme in the current protection zone.

5. The method according to claim 1, characterized in that, In step (4), the specific steps of selecting the overall optimal configuration plan for the flexible DC grid from the sets of optimized values in each protection zone are as follows: (4.1) Set the upper limit of the total value of the current-limiting reactors in the flexible DC power grid, and select one from the 2N + 1 sets of optimal configurations of current-limiting reactors in each protection zone. Sum up the values of the current-limiting reactors in these m sets of configuration sets and the total breaking capacity of the DC circuit breakers. i ​ (4.2) Repeat the process in step (4.1) until each optimized configuration scheme in each protection partition is iterated, and an optimized configuration scheme of the current-limiting reactor in the flexible DC grid is obtained, with a total of types; (4.3) Select the plan with the total value of the current-limiting reactors not exceeding the upper limit and the minimum total breaking capacity of the DC circuit breakers from all the plans as the optimal configuration plan of the current-limiting reactors for the flexible DC grid.

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