Flexible DC power grid fault current limiting device optimal configuration method and system based on hybrid calculation model

Abstract

The invention provides a flexible DC power grid fault current limiting device optimal configuration method and system based on a hybrid calculation model, and relates to the technical field of flexible DC power transmission. According to the method, in a unified optimization iteration process, fault current calculation and scheme screening are carried out by differentially applying models with different calculation efficiencies. Specifically, on the basis of top layer parameters of the fault current limiting device, schemes which do not meet current limiting constraints are rapidly screened through an approximate analysis model; the rest schemes are precisely verified through a numerical calculation model in combination with the refined parameters; and finally, evaluating the total cost of the feasible scheme, and performing iterative optimization by taking the lowest total cost of equipment in the whole network as a target to obtain optimal configuration. The problem that calculation efficiency and configuration precision are difficult to consider in the prior art is solved, optimization efficiency is improved on the premise that engineering precision is guaranteed, and the method is suitable for fine design of the current limiting device of the large-scale MMC flexible direct-current power grid.

Description

Technical Field

[0001] This invention relates to the field of flexible DC transmission technology, and in particular to an optimized configuration method and system for a flexible DC power grid fault current limiting device based on a hybrid computing model. Background Technology

[0002] Flexible DC power grids, due to their low damping and rapid rise in fault current, place stringent requirements on fault current suppression technology. A scientifically sound fault current limiting scheme can effectively suppress short-circuit current and reduce fault protection costs, which is of great significance for the construction of flexible DC power grids. Existing optimization methods for fault current limiting devices in flexible DC power grids can be broadly categorized into two types: The first type is based on precise numerical simulation (such as PSCAD / EMTDC) or full numerical calculation methods. Because flexible DC power grids have complex structures and numerous lines, the number of potentially configured active current limiting devices, such as fault current limiters, is considerable. Furthermore, fault current limiters involve auxiliary components such as switching elements, auxiliary capacitors, and metal oxide surge arresters, resulting in a huge amount of overall numerical calculation and excessively long optimization time, leading to extremely low overall optimization design efficiency for current limiting devices in complex power grids. The second type uses simple empirical formulas or simplified models to approximate the fault current, but ignores the detailed parameters of active current limiting devices such as fault current limiters, resulting in insufficient accuracy. The final design may not meet safety requirements or have poor economic efficiency.

[0003] In summary, existing technologies cannot balance computational efficiency and configuration accuracy, which is a pressing problem that needs to be solved in this field. Summary of the Invention

[0004] Purpose of the invention: To propose an optimized configuration method and system for flexible DC grid fault current limiting devices based on a hybrid computing model, so as to solve the contradiction between computational efficiency and configuration accuracy in existing technologies, and to achieve fast and accurate optimized configuration of flexible DC grid fault current limiting devices.

[0005] To achieve the above objectives, a first aspect of the present invention proposes an optimized configuration method for a flexible DC grid fault current limiting device based on a hybrid computation model, comprising the following steps: S1. An optimized configuration model is constructed based on the flexible DC grid, topology, and parameter information. Its decision variables include the top-level parameters and refined parameters of the fault current limiting device. The optimized configuration model includes an approximate analytical model of fault current and a numerical calculation model of fault current. S2. Generate an initial configuration scheme for the current limiting device and input it into the optimized configuration model for iterative optimization. During a single iterative optimization process, for a set of candidate configuration schemes, the following steps are executed in sequence: a) Based on the top-level parameters, the fault current approximate analytical model is used to quickly calculate the breaking current of each DC circuit breaker, and the configuration schemes that do not meet the current limiting effect constraints are initially screened based on the calculation results. b) For the configuration schemes that pass the initial screening, the fault current numerical calculation model is used to accurately calculate the breaking current of each DC circuit breaker, and the configuration schemes that do not meet the constraints are screened out again. S3. For the remaining configuration schemes after further screening in step S2, evaluate the total cost of the corresponding fault current limiting device. S4. Set the optimization objective to minimize the total cost of fault-limiting devices across the entire network; set the constraint objective as follows: The fault current shall not exceed the maximum breaking capacity of the DC circuit breaker; The value of the current-limiting reactor shall not exceed the maximum threshold required for the stability of the DC power grid. Repeat steps S2 to S3 until the preset optimization and constraint objectives are met, and output the optimal complete configuration scheme.

[0006] As a preferred embodiment, in step S1, the flexible DC grid is a flexible DC grid based on a modular multilevel converter (MMC), and the fault current limiting device includes a current limiting reactor that is always connected in series with the DC line, and a fault current limiter with active switching capability.

[0007] As a preferred embodiment, in step S1, the top-level parameters include the values ​​of current-limiting reactors configured at various locations in the flexible DC grid, the current-limiting inductance value of the fault current limiter, and the operating time of the fault current limiter; the refined parameters include other parameters of the fault current limiters configured at various locations in the flexible DC grid, excluding the current-limiting inductance.

[0008] As a preferred embodiment, in step S2, the fault current approximate analytical model is configured to quickly evaluate the peak value of the fault current, and the fault current numerical calculation model is activated for accurate verification only when the peak value meets the constraint conditions.

[0009] As a preferred embodiment, the approximate analytical model of the fault current obtains an analytical or semi-analytical solution to the fault current by simplifying the differential equations of the power grid; the simplification includes ignoring the nonlinear characteristics of some refined parameters, equating the network to an RL circuit, and ignoring the feed current of the MMC converter station at the far end of the fault point.

[0010] As a preferred embodiment, the numerical calculation model for fault current is constructed by equating the MMC converter station during the fault period to an RLC circuit, fully considering the detailed circuit models of all converter stations and current limiting devices in the DC power grid, constructing a set of high-order differential equations, and taking into account the nonlinear characteristics of all refined parameters.

[0011] As a preferred embodiment, the fault current limiter with active switching capability uses a current-limiting inductor for current limiting, including three operating states: before switching, during switching, and after switching. Before being put into operation, the current limiter is approximately short-circuited to the outside and is considered as a conductor; During the commissioning period, the current limiter clamps the transient voltage across the current limiting inductor through the auxiliary branch. The fault current first flows through the auxiliary branch and then transfers to the current limiting inductor until the current in the auxiliary branch drops to 0. Once the circuit is operational, the fault current limiter is considered to be a current-limiting inductor connected in series in the circuit; the auxiliary branch is composed of at least one of MOV, capacitor, and power electronic device.

[0012] As a preferred embodiment, the fault current approximate analytical model iteratively calculates the breaking current of the DC circuit breaker, and the calculation process is as follows: S2-a-1, The fault current limiter starts to engage at time t1 and finishes engaging at time t2. The MMC will be used during the current limiter's engagement period. insert The discharge quantity is fitted using a quadratic function. In the first iteration, it is assumed that the branch current remains constant. Based on the electrical parameters of the model at time t1, the approximate analytical solution of the DC circuit breaker breaking current, branch current and other electrical quantities at time t2 is calculated. S2-a-2. Based on the analytical calculation results of the branch current at time t2 in the previous iteration, substitute the branch currents again to calculate the approximate analytical solutions of the DC circuit breaker breaking current, branch currents and other electrical quantities at time t2. Based on the calculated branch current, the approximate discharge amount of MMC during the current limiter activation period is further estimated; S2-a-3. Repeat the above iterative process, using the approximate analytical solution calculated in each iteration as input for the next iteration, until the difference between the input and output variables is less than the preset value. At this point, the iteration accuracy is considered to have met the requirements, and the calculation is complete.

[0013] As a preferred option, in step S3, the total cost of the fault current limiting device evaluated consists of the cost of the current limiting reactor and the cost of the fault current limiter. The current-limiting reactors are configured on both sides of each line, and their number is fixed. Their cost is related to the value of their reactance. The fault current limiter is not a necessary configuration. If configured, it is connected in series with the current-limiting reactor. Its number is not fixed, and the cost is related to the value of the current-limiting inductor, the number of power electronic devices, and passive components.

[0014] Furthermore, the present invention also proposes an optimized configuration system for a flexible DC grid fault current limiting device for implementing the method described in the first aspect, the system comprising: The preliminary screening module is configured to call the fault current approximate analytical model, calculate the breaking current of each DC circuit breaker based on the top-level parameters of the candidate configuration scheme, and perform preliminary screening. The precise verification module is configured to call the fault current numerical calculation model, and for the configuration schemes that have passed the initial screening, accurately calculate the breaking current of each DC circuit breaker based on all its decision variables, and perform a second screening. The comprehensive evaluation and optimization module is configured to evaluate the total cost of configuration schemes that pass the second screening, manage the optimization iteration process, and output the optimal complete configuration scheme.

[0015] Compared with the prior art, the present invention has the following beneficial effects: (1) In the optimization iteration process, the present invention adopts a differentiated collaborative mode of "rapid coarse screening by approximate analytical model + precise fine screening by numerical calculation model", which reduces the unnecessary calculation overhead from the source. Among them, the approximate analytical model achieves a faster calculation speed than pure numerical calculation by simplifying the differential equation of the power grid (ignoring some nonlinearity of refined parameters, equivalent to RL circuit, etc.), which can quickly eliminate obviously unqualified schemes and avoid the huge amount of calculation required by the traditional pure numerical calculation model to perform high-precision simulation of all schemes one by one.

[0016] (2) Only schemes with potential feasibility that are initially screened through an approximate analytical model will be used for precise verification in the numerical calculation model of fault current. This numerical calculation model fully considers the detailed circuit models of all converter stations (equivalent to RLC circuits) and current limiting devices in the DC power grid, takes into account the nonlinear characteristics of all refined parameters, and the calculation results are close to the accurate solution, ensuring that the final scheme strictly meets the core constraints such as "the fault current does not exceed the maximum breaking capacity of the DC circuit breaker" and "the value of the current limiting reactor does not exceed the system stability threshold". Therefore, the reliability and safety of the scheme output by this invention are comparable to those obtained by traditional pure numerical calculation methods, and far superior to pure empirical formulas or simplified model schemes that ignore refined parameters, avoiding safety hazards or economic waste caused by insufficient accuracy.

[0017] (3) This invention divides decision variables into top-level parameters (current limiting reactor values, fault current limiter current limiting inductance values ​​and operating time, etc.) and refined parameters (fault current limiter internal MOV rated voltage, number of power electronic devices, etc.), and matches appropriate calculation models for different levels of parameters. It can quickly lock the bottom line of current limiting effect through top-level parameters, and ensure the safety, volume and cost optimization of the equipment itself through refined parameters, thus realizing the collaborative design of current limiting reactor and fault current limiter.

[0018] (4) This invention aims to achieve the lowest total cost of fault current limiting equipment across the entire network. During the screening process, the cost characteristics of the current limiting device are fully considered: the number of current limiting reactors is fixed, and the cost is related to the reactor value; fault current limiters are not essential, and their cost is related to the current limiting inductor value, the number of power electronic devices, etc. After retaining feasible solutions through two rounds of screening, the total cost is accurately calculated based on all parameters. Then, the global optimal solution is iteratively approximated through optimization algorithms such as genetic algorithms to ensure that the final solution minimizes investment costs while meeting safety constraints, thus balancing technical feasibility and economic rationality. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the overall process of the method of the present invention.

[0020] Figure 2 This is a structural block diagram of the optimized configuration system of the present invention.

[0021] Figure 3 This is a schematic diagram illustrating the collaborative operation of the hybrid computing model in a single iteration.

[0022] Figure 4 This is a topology diagram of a flexible DC power grid test system according to one embodiment.

[0023] Figure 5 A fault current limiter topology and its equivalent circuit are shown in one embodiment.

[0024] Figure 6 This is a refined numerical calculation equivalent model of a test system for one embodiment, in which each MMC converter station is equivalent to a second-order RLC model after a fault, the transmission line is equivalent to an RL model, and the specific circuit model of the fault current limiter is substituted into it.

[0025] Figure 7 This is a simplified approximate analytical calculation model for a test system in one embodiment. It ignores the remote converter station connected to the fault point via more than one line, decouples the lines on both sides of the fault point, and substitutes a simplified fault current limiter model into it to obtain an approximate analytical model for the breaking current of the DC circuit breaker at a single fault point. Detailed Implementation

[0026] In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention can be practiced without one or more of these details. In other instances, certain technical features well-known in the art have not been described in order to avoid obscuring the invention.

[0027] This embodiment uses Figure 4The following is an example of a ±500kV true bipolar four-terminal flexible DC power grid. This grid includes four converter stations (MMC1-MMC4) based on half-bridge modular multilevel converters (MMC), which are connected in a ring network via DC lines. MMC1 and MMC3 are sending-end converter stations, transmitting active power of 1500MW and 750MW respectively, while MMC2 and MMC4 are receiving-end converter stations, receiving active power of 1500MW and 750MW respectively.

[0028] I. The candidate installation locations for the fault current limiting device are on both sides of each DC transmission line. The fault current limiting device consists of two parts: Current-limiting reactor: Always connected in series in the line, its inductance value L dc The decision variable is assumed to have a value range of 50mH-250mH. Fault Current Limiter: A current limiter topology using a current-limiting inductor and a metal oxide surge arrester (MOV) in parallel is employed. During activation, the MOV clamps the inductor voltage. Initially, the fault current flows through the MOV, then transfers to the current inductor until it completely flows through the current-limiting inductor. At this point, the MOV current is zero, indicating the current limiter is fully activated. Afterward, the fault current limiter can be considered as a current-limiting inductor connected in series with the faulty line. A fault current limiter is not mandatory; if configured, it is connected in series with a current-limiting reactor. The core decision variable (top-level parameter) is the current-limiting inductor. L FCL (Value range 0-200mH) and action time t FCL (Defined as the time the current limiter is put into operation minus the time the fault occurs, with a value range of 1-3ms).

[0029] Its detailed parameters include the rated voltage of the metal oxide surge arrester (MOV). U MOV (Value range 0kV-500kV) and the time required for the fault current limiter to be activated. t insert (Defined as the time required for the current limiter to be activated from start to finish, with a value range of 0.5ms-5ms) II. Specific Construction of the Optimized Configuration Model Optimization objective: Minimize the total investment cost of the fault current limiting device. C total The total cost model is as follows: C total = C reactor ( L dc ) + C FCL ( LFCL , t FCL , U MOV , C ) The cost function is determined based on market research and empirical formulas for equipment design.

[0030] Constraints: Fault current constraint: When a bipolar short-circuit fault occurs at the DC bus outlet, the peak fault current must be limited to below the breaking capacity (18kA) of the DC circuit breaker.

[0031] Stability constraint: During normal operation, the value of the current-limiting reactor shall not exceed 250mH to avoid affecting the system stability and dynamic response.

[0032] III. Implementation Details of the Hybrid Computing Model (1) Implementation of the approximate analytical model (fast model) of fault current In this embodiment, the approximate analytical model is based on the following simplification: a) Treat all MMC converter stations as equivalent to a second-order RLC discharge circuit, select one fault point, and ignore the contribution of the remote MMC to the fault current.

[0033] b) The faulty line is split off with the fault point as the boundary. Under a metallic short-circuit fault, the fault currents on both sides are considered to be approximately decoupled.

[0034] c) The fault current limiter is equivalent to a current-limiting inductor connected in parallel with a controlled voltage source and a controlled switch. t FCL At all times L FCL When the circuit is engaged, the voltage across the fault current limiter is clamped by the controlled voltage source during the engagement period. The fault current first flows entirely through the controlled source and then gradually shifts to the current-limiting inductor. When the controlled source current drops to 0, the current limiter is considered to be fully engaged.

[0035] After obtaining the calculation model based on the above simplification method, an iterative calculation method is used to analytically calculate the breaking current of the DC circuit breaker. The calculation process is as follows: 1) In the analytical model, assume the fault current limiter is in t Start investing immediately. t 2. Once the current limiter is engaged, the MMC converter station will be put into operation during the current limiter period. t insert The discharge quantity within the time period is fitted using a quadratic function. In the first iteration, it is assumed that the branch current remains constant. t Calculate the electrical parameters of the model at time 1. t DC circuit breaker breaking current at time 2 IDCCB Approximate analytical solutions for branch currents and other electrical quantities.

[0036] 2) Based on the previous iteration t The analytical calculation results of the branch currents at time 2 are used to substitute the branch currents again for calculation. t DC circuit breaker breaking current at time 2 I DCCB The approximate analytical solutions for the branch currents and other electrical quantities are calculated using the following formulas: (1) Each new variable is defined as follows: (2) Based on the calculated branch current, the approximate discharge amount of the MMC converter station during the current limiter activation period is further estimated. (3) Where k is the approximate linearized slope of the fault current during the current limiter's operation.

[0037] 3) Repeat the above iterative process, using the approximate analytical solution calculated in each iteration as input for the next iteration, until the difference between the input and output variables is very small (less than 1%), at which point the iteration accuracy is considered to have met the requirements and the calculation is complete.

[0038] Using the above approximate calculation method, the maximum breaking current of a DC circuit breaker at a single fault point, considering the operation of the fault current limiter, can be directly calculated. I DCCB (This is also the peak fault current at the fault point), and a single calculation takes approximately milliseconds. This model is primarily used for rapid assessment. L dc , L FCL , t FCL These three top-level parameters have a macroscopic impact on the fault current at different fault points and the DC circuit breaking capacity requirements.

[0039] (2) Implementation of the numerical calculation model (precise model) for fault current The numerical calculation model includes the RLC equivalent model corresponding to each MMC converter station in the DC grid and the specific circuit model of the fault current limiter. This model is used to accurately calculate the fault current waveform and evaluate the impact of the refined parameters of the fault current limiter on the breaking current of the fault DC circuit breaker. The single numerical calculation takes about hundreds of milliseconds.

[0040] IV. Specific Implementation of the Optimization Process This embodiment uses a genetic algorithm (GA) as the optimization algorithm, with a population size of 100 and a maximum number of iterations of 300. Each individual in the GA population represents a set of current limiting device configuration schemes, and the main parameters include [ L dc , L FCL1 , ..., L FCLm , t FCL , U MOV1 , ..., U MOVm [Among them, the whole network current-limiting reactor] L dc The values ​​are the same, but the parameters of the fault current limiters are different, and they coexist. m Each fault current limiter has a configurable location, and each fault current limiter only responds to faults occurring on the line it is located on.

[0041] Step S2a (rapid coarse screening): For 100 individuals in the GA population (each individual represents a set of current limiting device configuration schemes) L dc , L FCL , t FCL , U MOV First, extract only its top-level parameters. L dc , L FCL , t FCL Input an approximate analytical model for calculation. Set a threshold. I max = 20kA, and I min =15kA. If calculated... I DCCB > I max or I DCCB < I min If the individual fails to meet the basic flow restriction requirements, it will be immediately eliminated. In this embodiment, this step can quickly eliminate a large number of individuals that do not meet the basic flow restriction requirements.

[0042] Step S2b (Precise Screening): For individuals that passed the coarse screening, a precise numerical calculation method is used to perform the final screening. During the calculation, all parameters of the individual (including refinement parameters) are input. U MOV (etc.). If the simulation results show that the peak fault current still exceeds 25kA, or the energy absorbed by the MOV exceeds the limit, then the individual will be eliminated.

[0043] Step S3 (Cost Assessment): For feasible individuals selected through S2b, calculate the total cost based on all their parameters. C total .

[0044] Step S4 (Iteration): Based on the cost assessment results, GA performs selection, crossover, and mutation to generate a new generation of population, and repeats the above process.

[0045] V. Implementation Results and Comparison For the same optimization problem, both the traditional single numerical calculation model and the hybrid calculation model of this invention were used to solve it. Comparing the final optimization results and the optimization time required to achieve the optimal results reveals that both the traditional numerical calculation model and the optimization results of this invention can reach the optimal cost of the current limiting scheme, but the time required differs significantly. The traditional numerical optimization model requires 3000 seconds to converge to the optimal solution, while this invention only requires about 30 seconds, improving computational efficiency by approximately 200 times.

[0046] In summary, this embodiment fully verifies that the method described in this invention can effectively resolve the contradiction between efficiency and accuracy in the optimized configuration of fault current limiting devices for flexible DC power grids, and has significant engineering application value.

[0047] As described above, although the invention has been shown and described with reference to specific preferred embodiments, it should not be construed as limiting the invention itself. Various changes in form and detail may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. An optimization configuration method for fault current limiting devices in flexible DC power grids based on a hybrid computing model, wherein models with different computational efficiencies are applied differentially for fault current calculation and scheme selection during a unified optimization iteration process; Its features Includes the following steps: S1. An optimized configuration model is constructed based on the flexible DC grid, topology, and parameter information. Its decision variables include the top-level parameters and refined parameters of the fault current limiting device. The optimized configuration model includes an approximate analytical model of fault current and a numerical calculation model of fault current. S2. Generate an initial configuration scheme for the current limiting device and input it into the optimized configuration model for iterative optimization. During a single iterative optimization process, for a set of candidate configuration schemes, the following steps are executed in sequence: a) Based on the top-level parameters, the fault current approximate analytical model is used to quickly calculate the breaking current of each DC circuit breaker, and the configuration schemes that do not meet the current limiting effect constraints are initially screened based on the calculation results. b) For the configuration schemes that pass the initial screening, the fault current numerical calculation model is used to accurately calculate the breaking current of each DC circuit breaker, and the configuration schemes that do not meet the constraints are screened out again. S3. For the remaining configuration schemes after further screening in step S2, evaluate the total cost of the corresponding fault current limiting device. S4. Set the optimization objective to minimize the total cost of fault-limiting devices across the entire network; set the constraint objective as follows: The fault current shall not exceed the maximum breaking capacity of the DC circuit breaker; The value of the current-limiting reactor shall not exceed the maximum threshold required for the stability of the DC power grid. Repeat steps S2 to S3 until the preset optimization and constraint objectives are met, and output the optimal complete configuration scheme.

2. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 1, characterized in that, In step S1, the flexible DC grid is a flexible DC grid based on a modular multilevel converter (MMC), and the fault current limiting device includes a current limiting reactor that is always connected in series with the DC line, and a fault current limiter with active switching capability.

3. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 1, characterized in that, In step S1, the top-level parameters include the values ​​of current-limiting reactors configured at various locations in the flexible DC grid, the current-limiting inductance value of the fault current limiter, and the operating time of the fault current limiter. The refined parameters include all parameters of the fault current limiters configured in the flexible DC grid, excluding the current-limiting inductor.

4. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 1, characterized in that, In step S2, the fault current approximate analytical model is configured to quickly evaluate the peak value of the fault current, and the fault current numerical calculation model is activated for accurate verification only when the peak value meets the constraint conditions.

5. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 1, characterized in that, The approximate analytical model of the fault current obtains the analytical or semi-analytical solution of the fault current by simplifying the differential equation of the power grid. The simplification includes ignoring the nonlinear characteristics of some refined parameters, equating the network to an RL circuit, and ignoring the feed current of the MMC converter station at the far end of the fault point.

6. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 1, characterized in that, The numerical calculation model for fault current equates the MMC converter station during a fault as an RLC circuit, fully considers the detailed circuit models of all converter stations and current limiting devices in the DC grid, constructs a set of high-order differential equations, and takes into account the nonlinear characteristics of all refined parameters.

7. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 2, characterized in that, The fault current limiter with active switching capability uses a current-limiting inductor for current limiting, and includes three working states: before switching, during switching, and after switching. Before being put into operation, the current limiter is approximately short-circuited to the outside and is considered as a conductor; During the commissioning period, the current limiter clamps the transient voltage across the current limiting inductor through the auxiliary branch. The fault current first flows through the auxiliary branch and then transfers to the current limiting inductor until the current in the auxiliary branch drops to 0. Once the circuit is operational, the fault current limiter is considered to be a current-limiting inductor connected in series in the circuit; the auxiliary branch is composed of at least one of MOV, capacitor, and power electronic device.

8. The method for optimizing the configuration of fault current limiting devices for flexible DC power grids based on a hybrid computing model according to claim 1, characterized in that, The approximate analytical model for fault current iteratively calculates the breaking current of the DC circuit breaker. The calculation process is as follows: S2-a-1, The fault current limiter starts to engage at time t1 and finishes engaging at time t2. The MMC will be used during the current limiter's engagement period. insert The discharge quantity is fitted using a quadratic function. In the first iteration, it is assumed that the branch current remains constant. Based on the electrical parameters of the model at time t1, the approximate analytical solution of the DC circuit breaker breaking current, branch current and other electrical quantities at time t2 is calculated. S2-a-2. Based on the analytical calculation results of the branch current at time t2 in the previous iteration, substitute the branch currents again to calculate the approximate analytical solutions of the DC circuit breaker breaking current, branch currents and other electrical quantities at time t2. Based on the calculated branch current, the approximate discharge amount of MMC during the current limiter activation period is further estimated; S2-a-3. Repeat the above iterative process, using the approximate analytical solution calculated in each iteration as input for the next iteration, until the difference between the input and output variables is less than the preset value. At this point, the iteration accuracy is considered to have met the requirements, and the calculation is complete.

9. The method for optimizing the configuration of a flexible DC grid fault current limiting device based on a hybrid computing model according to claim 1, characterized in that, In step S3, the total cost of the fault current limiting device evaluated consists of the cost of the current limiting reactor and the cost of the fault current limiter. The current-limiting reactors are configured on both sides of each line, and their number is fixed. Their cost is related to the value of their reactance. The fault current limiter is not a necessary configuration. If configured, it is connected in series with the current-limiting reactor. Its number is not fixed, and the cost is related to the value of the current-limiting inductor, the number of power electronic devices, and passive components.

10. A flexible DC grid fault current limiting device optimization configuration system for implementing the method of any one of claims 1 to 9, characterized in that, include: The preliminary screening module is configured to call the fault current approximate analytical model, calculate the breaking current of each DC circuit breaker based on the top-level parameters of the candidate configuration scheme, and perform preliminary screening. The precise verification module is configured to call the fault current numerical calculation model, and for the configuration schemes that have passed the initial screening, accurately calculate the breaking current of each DC circuit breaker based on all its decision variables, and perform a second screening. The comprehensive evaluation and optimization module is configured to evaluate the total cost of configuration schemes that pass the second screening, manage the optimization iteration process, and output the optimal complete configuration scheme.

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