A method and system for coordinated design of injection capacity of a power flow controller and dc capacitor

Abstract

The application discloses a tide flow controller injection capacity and direct current capacitor collaborative design method and system. The method comprises the following steps: calculating a tide flow injection capacity lower limit, a floating capacitor lower limit and a blocking capacitor lower limit respectively; constructing a feasible region and a monotonic cost function about the injection capacity, the floating capacitor and the blocking capacitor to obtain a continuous optimal solution by minimum cost analysis; mapping the continuous optimal solution to a purchasable discrete specification and checking a margin, a ripple and overvoltage; adjusting the discrete combination to meet the checking requirements to output selection. Through the strictness of forward and reverse boundary working conditions, cost function monotonicity utilization and multi-scene robustness expansion, an interpretable and reviewable low-cost selection meeting performance and safety constraints is obtained.

Description

Technical Field

[0001] This invention relates to the field of power system optimization, and more specifically, to a method and system for the coordinated design of power flow controller injection capacity and DC capacitor. Background Technology

[0002] A power flow control device is a power electronic device that regulates the flow of active and reactive power. It uses a modular multilevel converter (MMC) composed of switching elements to regulate power flow. This is achieved by changing the equivalent impedance or phase angle of the tie line through a controllable series voltage. The device consists of a series-injected converter capacity, DC or floating capacitor energy storage, insulation, and a control and protection platform.

[0003] During the design process, the capacity of the series injection converter is related to the sensitivity of the device, the floating capacitor is related to the upper and lower limits of ripple and energy constraints, and the blocking capacitor is related to overvoltage. Blindly increasing the specifications of components will lead to increased costs. Using simulation methods to simulate the combination of various components and test them under various scenarios will lead to insufficient interpretability of the selection and make it difficult to determine whether the solution is cost-optimal. Summary of the Invention

[0004] This invention overcomes the shortcomings of existing equipment and provides a method and system for the collaborative design of power flow controller injection capacity and DC capacitor. It can solve the effective domain based on various indicators and determine the component specifications with the goal of minimizing costs. Under the premise of meeting the discrete constraints of performance, safety and specifications, it can obtain interpretable and verifiable low-cost selection.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for coordinating the injection capacity and DC capacitor of a power flow controller, comprising a power flow control module, a reactive power coordination module, and a reverse blocking module, including: Calculate the lower limit of power flow injection capacity, the lower limit of floating capacitance of reactive power coordination module, and the lower limit of blocking capacitance of reverse blocking module respectively; The calculation process for power flow injection capacity is as follows: Calculate the baseline active power of the line; Establish the sensitivity of line active power change to series injection voltage, wherein the series injection voltage is orthogonal to the line current; For the system's forward power flow requirements, the minimum required device capacity is calculated by back-calculating the forward sensitivity and reverse sensitivity respectively; the reverse sensitivity is an externally given value and is dated according to the energy margin of the floating capacitor of the reactive power coordination module between the highest withstand voltage and the lowest operating voltage. Take the maximum value of the minimum device capacity for both forward and reverse power flows; Construct the feasible region and cost function for power flow injection capacity, floating capacitance, and blocking capacitance; The continuous lower bounds of capacity, floating capacitor and blocking capacitor are obtained by using the minimum cost solution of the cost function, and the discrete mapping combination of the procureable capacity set that satisfies the minimum cost solution is used for verification. Adjust the discrete mapping combination to meet the verification requirements, and output the selection.

[0006] Preferably, the forward and reverse sensitivities are given externally or calculated from the line parameters according to a linear sensitivity relationship.

[0007] As a preferred option, the calculation process for the lower limit of the floating capacitor of the reactive power coordination module is as follows: The first minimum capacitance value is calculated based on the ripple of the power frequency pulsating power applied to the floating capacitor and the set allowable peak value of the ripple. Set the system's charging and discharging energy requirements, and calculate the second minimum capacitance value based on the maximum allowable voltage fluctuation range of the capacitor; The maximum of the first minimum capacitance value and the second minimum capacitance value is taken as the lower limit of the floating capacitance value.

[0008] Preferably, the calculation process for the blocking capacitance of the reverse blocking module is as follows: Calculate the peak voltage during fault interruption; Using the peak voltage as the upper limit, the lower limit of the blocking capacitor value is calculated as the lower limit of the blocking capacitor value.

[0009] As a preferred method, the verification method includes: calculating the corresponding margin, ripple value, and overvoltage value of the discrete mapping combination.

[0010] As a preferred option, the verification also includes setting multiple operating scenarios, obtaining the corresponding lower limits of capacity, floating capacitance, and blocking capacitance under each scenario, and taking the maximum value of each lower limit.

[0011] Preferably, if the reactive power coordination module and the reverse blocking module share a capacitor, the capacitance value of the shared capacitor is the maximum value of the floating capacitor and the blocking capacitor.

[0012] Preferably, the expression for the cost function is:

[0013] in, The device is injected with capacitance, floating capacitor value, and blocking capacitor value. These are the energy storage cost coefficients for fixed cost, unit capacity cost, floating capacitor, and blocking capacitor, respectively. This refers to the capacitor's withstand voltage rating. This is the average operating voltage of the floating capacitor. This is a safe voltage.

[0014] A power flow controller injection capacity and DC capacitor co-design system includes: The constraint parameter calculation module is used to calculate the lower limit of power flow injection capacity, the lower limit of floating capacitance of reactive power coordination module, and the lower limit of blocking capacitance of reverse blocking module, respectively. The calculation process for power flow injection capacity is as follows: Calculate the baseline active power of the line; Establish the sensitivity of line active power change to series injection voltage, wherein the series injection voltage is orthogonal to the line current; Based on the system's forward and reverse power flow requirements, the minimum device capacity required for each is calculated in reverse; the reverse sensitivity is an externally given value and is dated according to the energy margin of the floating capacitor of the reactive power coordination module between the highest withstand voltage and the lowest operating voltage. Take the maximum value of the minimum device capacity for both forward and reverse power flows; The cost function construction module is used to construct the feasible region and cost function for power flow injection capacity, floating capacitor and blocking capacitor; The discrete product specification mapping module is used to obtain the continuous lower limits of capacity, floating capacitor and blocking capacitor with the minimum cost solution of the cost function, and to verify the discrete mapping combination of the procureable capacity set that satisfies the minimum cost solution. The selection verification module is used to adjust the discrete mapping combination to meet the verification requirements and output the selection. Compared with the prior art, the beneficial effects of the present invention are: (1) The minimum device capacity is taken under the boundary conditions of forward and reverse power flow respectively, so as to cope with the most stringent operating conditions; (2) All terms of the cost function are positive, so the entire cost function is monotonic. Consequently, the minimum cost point, i.e. the continuous optimal point, is located at the intersection of effective constraints, and candidate solutions can be obtained without large-scale intelligent algorithm search without interpretation. (3) To cope with the maximum value in multiple scenarios, it is extended to robust selection, which can cope with the maximum value under multiple working conditions, and thus can meet the most stringent requirements under multiple working conditions. Attached Figure Description

[0015] Figure 1 This is a circuit diagram of the power flow control device of the present invention; Figure 2 This is a flowchart of the collaborative design method of the present invention; Figure 3 This is a schematic diagram of the collaborative design system of the present invention; Figure 4 This is a comparison chart of the unit injection capacity control capability of the simulation model of the verification example at different operating points; Figure 5 This is a schematic diagram of the feasible region for the injection capacity and blocking capacitance formation of the present invention. Detailed Implementation

[0016] The present disclosure will be further described below with reference to the accompanying drawings and embodiments.

[0017] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0018] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0019] In this disclosure, terms such as "upper," "lower," "left," "right," "front," "back," "vertical," "horizontal," "side," and "bottom" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are merely relational terms determined for the convenience of describing the structural relationship of the various components or elements in this disclosure, and do not specifically refer to any component or element in this disclosure, nor should they be construed as limiting this disclosure.

[0020] In this disclosure, terms such as "fixed connection," "connected," and "linked" should be interpreted broadly, indicating a fixed connection, an integral connection, or a detachable connection; a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can determine the specific meaning of these terms in this disclosure based on the specific circumstances, and they should not be construed as limitations on this disclosure.

[0021] Example: A method for co-designing power flow controller injection capacity and DC capacitor, such as Figure 1 As shown, its collaborative design object consists of a power flow control module (P module), a reactive power coordination module (Q module), and a reverse blocking module (D module) connected in series along the AC tie line. An inductor-based reactance X is also configured on the main circuit connected in series. L It connects to the power grid at the terminal voltage V2. The power flow control module and reactive power coordination module together constitute the series injection section, while the reverse blocking module is responsible for blocking fault current to prevent fault current leakage. The three core variables of the collaborative design are: P-Q series injection capacity. Q module floating capacitor and D module blocking capacitor .

[0022] The whole process is as follows Figure 2 As shown.

[0023] First, calculate the lower limit of the power flow injection capacity. The key to this step is to evaluate the unit injection capacity control capability in both the forward and reverse directions separately, and determine the capacity lower limit based on the more stringent direction. Specifically, first calculate the baseline active power of the line. Ignoring line resistance, the baseline active power flow is determined by the sending-end voltage amplitude. Receiving end voltage amplitude Line equivalent reactance X and power angle The decision is made jointly, and its expression is:

[0024] When the device injects a series voltage orthogonal to the line current into the line... At this time, its effect is equivalent to changing the equivalent reactance of the line, and the active power flow after injection is:

[0025] Within the small-signal range where the injected voltage is much smaller than the product of the line reactance and the rated current, a first-order Taylor expansion of the active power expression yields the power flow variation. With injection voltage Approximate linear relationship:

[0026] Device injection capacity Defined as the maximum permissible series injection voltage amplitude and the rated line current of the device. The product of:

[0027] Therefore, the unit injection capacity regulation capability η can be defined as the active power flow regulation that can be generated per unit injection capacity, i.e., the linear sensitivity relationship of line parameters:

[0028] This indicator is jointly determined by the sending and receiving end voltages, line reactance, rated current, and power angle, and is a core indicator for evaluating capacity utilization efficiency. Because the units in this formula are the same, This is the per-unit value (pu value).

[0029] Based on this, the above sensitivity can be directly used to calculate the positive unit capacity control capability during positive regulation (increasing current). And adjust the amplitude according to the positive current. To deduce the minimum device capacity required for forward regulation, we need to consider the physical essence of forward regulation: the device releases active power from a floating capacitor to the grid, causing the capacitor to discharge and the voltage to drop. Its limit is constrained by the lower limit of the allowable regulation voltage. However, since forward discharge is an actively controllable discharge process, the converter can be gradually adjusted through the modulation ratio. Furthermore, forward control is limited by the injected voltage capability, and energy constraints are usually not a limiting factor. Therefore, forward control only needs to be calculated based on static sensitivity. In contrast, during reverse control (downflow), in addition to static sensitivity, the floating capacitor of the Q module must also be considered during the control evaluation cycle. The available energy margin within the system. Reverse descent power requires absorbing energy from the line and storing it in a floating capacitor, whose voltage flows towards... Approaching voltage limits, this passive absorption process can easily exceed the maximum withstand voltage when the capacitor voltage is too high. Therefore, the available energy margin on the reverse side is the key constraint determining the capacitance. The integral of the actual work exchanged between the floating capacitor and the circuit within a control cycle is limited by the capacitor's allowable energy margin. This energy margin is determined by the value of the floating capacitor. and the upper and lower limits of capacitor voltage , Decision, satisfaction:

[0030] Therefore, reverse unit capacity regulation capability The adjustment sensitivity needs to be calculated under reverse boundary conditions, taking the boundary conditions corresponding to the floating capacitor at its highest withstand voltage and lowest operating voltage. This should be based on the amplitude required for reverse power flow control. The minimum device capacity required for the reverse direction is determined by reverse calculation. Finally, the maximum of the minimum device capacities required for both the forward and reverse directions is taken as the lower limit of the power flow injection capacity. :

[0031] Secondly, the lower limit of the floating capacitor of the reactive power coordination module is calculated. This calculation requires consideration of two sources of constraint: first, the steady-state ripple constraint caused by power frequency pulsating power; and second, the energy constraint required for reverse regulation. Regarding the ripple constraint, the grid fundamental frequency generates pulsating power at twice the power frequency within the device, and its amplitude... Applying a voltage ripple across a floating capacitor will cause periodic fluctuations in the capacitor voltage, and the amplitude of this voltage ripple is... With pulsating power amplitude and capacitance value Average operating voltage of floating capacitor It is related to the fundamental angular frequency ω of the power grid, and its approximate relationship is:

[0032] According to the set ripple allowable peak value This allows us to deduce the first minimum capacitance value that meets the ripple requirements. :

[0033] For energy constraints, the charging and discharging energy requirements within the reverse control cycle set by the system need to be considered. In addition to the maximum allowable voltage fluctuation range of the capacitor, a second minimum capacitance value is determined to maintain the capacitor voltage within the limit under a given energy throughput requirement. The larger of the first and second minimum capacitance values ​​is taken as the lower limit of the floating capacitor value. .

[0034] Next, calculate the lower limit of the blocking capacitance of the reverse blocking module. When a short-circuit fault occurs in the system, the D module needs to block the fault current, and the equivalent inductance in the fault loop... The stored energy will be transferred to the blocking capacitor. This causes the capacitor voltage to rise rapidly. Peak voltage during fault interruption. Approximately equal to the equivalent driving voltage before blocking The superposition of the voltage increment generated by the inductor energy transfer is expressed as follows:

[0035] The voltage increment is determined by the initial blocking current. Equivalent inductance and blocking capacitor A joint decision. Based on a safe voltage threshold. The upper limit, that is, the requirement The lower limit of the blocking capacitor value can be derived. :

[0036] After obtaining the lower limit of the current injection capacity respectively Floating capacitor lower limit and lower limit of blocking capacitance Next, construct a framework for the capacity of trend injection. Floating capacitor and blocking capacitor The three-dimensional feasible region and cost function. Design vector. feasible region Defined as a three-dimensional vector space that satisfies both the capacity lower bound constraint and the capacitance lower bound constraint:

[0037]

[0038] in, For fixed costs, it covers parts that are less related to capacity and capacitance, such as control and protection platforms, insulation, and structure; The unit capacity cost coefficient is obtained by regression analysis of the supplier's capacity quotation. The unit energy storage cost coefficient for the floating capacitor of the Q module. The unit energy storage cost coefficient for the blocking capacitor of module D is obtained by regression analysis of the corresponding capacitor module price. This is the average operating voltage of the floating capacitor; The safe voltage threshold; This is a correction factor for the capacitor's withstand voltage rating, reflecting the additional cost of high-voltage capacitors due to insulation and voltage equalization requirements. In the cost function, and The energy storage costs corresponding to floating capacitors and blocking capacitors are respectively adopted. The reason for this energy storage method is that the cost of capacitors is proportional to their energy storage capacity.

[0039] Due to the cost function , , and All are positive values, and the cost function is positive. , and The partial derivatives are all greater than zero:

[0040] That is, the cost increases monotonically with each design variable. Therefore, the continuous optimal solution that minimizes the cost within the feasible region must lie at the intersection of the effective constraint boundaries, i.e. , , This continuous optimal solution can be obtained analytically without large-scale parameter scanning or intelligent search algorithms, and has good interpretability.

[0041] After obtaining the continuous lower bound optimal solution, the discrete mapping and verification stage begins. Since the actual procurable device capacity and capacitors exist in discrete specifications, it is necessary to derive the solution from the set of procurable capacities. Q module capacitor set and D module capacitor set In this process, the smallest specification not lower than the corresponding continuous lower limit is selected to obtain the discrete rated capacity. Floating capacitor rating and blocking capacitor rating :

[0042] The discrete mapping combination is validated, and the validation content includes: Calculate discrete capacity margin That is, the relative proportion of discrete rated capacity exceeding the continuous lower limit:

[0043] Calculate the voltage safety margin of the main body That is, the ratio of the difference between the safe voltage threshold and the peak fault blocking voltage under discrete capacitance values ​​to the safe voltage threshold:

[0044] To be on the safe side, the steady-state ripple value under the discrete capacitance value was recalculated. and fault blocking peak voltage Confirm that each item meets the allowable peak ripple and overvoltage constraints. If any check item fails to meet the requirements, adjust the discrete mapping combination, select the next specification level, and re-check until all constraints meet the check requirements. Then, output the final capacity and capacitor selection.

[0045] The selection of the next specification refers to replacing any one of the capacitance, floating capacitor, and blocking capacitor one by one, and selecting a higher specification with a cost higher than the previous test but lower than other combinations, thereby pursuing the lowest cost.

[0046] In addition, the verification also includes extended multi-condition scenarios. (Settings) The system covers various operating and fault scenarios, including different load levels, renewable energy output combinations, and fault types. A corresponding capacity lower limit is calculated for each scenario. Floating capacitor lower limit and lower limit of blocking capacitance Take the maximum value of each lower bound across all scenarios as the robustness lower bound:

[0047] In one implementation, if the reactive power coordination module and the reverse blocking module share a capacitor bank, that is, the same capacitor bank simultaneously performs the functions of a floating capacitor and a blocking capacitor, then the capacitance value of the shared capacitor is taken as the maximum value of the lower limit of the floating capacitor and the lower limit of the blocking capacitor. At the same time, manufacturing deviation, aging and module voltage equalization correction coefficients are superimposed to ensure that the shared scheme meets the requirements simultaneously under the three constraints of steady-state ripple, reverse energy and fault overvoltage.

[0048] Compared with conventional solutions that only consider forward demand and ripple configuration, the method of this invention can effectively reduce redundant capacity and energy storage costs while meeting bidirectional control and intrinsic safety constraints, achieving optimal design of capacity and capacitor synergy.

[0049] Example 2 like Figure 3 As shown, a power flow controller injection capacity and DC capacitor co-design system includes: The constraint parameter calculation module is used to calculate the lower limit of power flow injection capacity, the lower limit of floating capacitance of reactive power coordination module, and the lower limit of blocking capacitance of reverse blocking module, respectively. The cost function construction module is used to construct the feasible region and cost function for power flow injection capacity, floating capacitor and blocking capacitor; The discrete product specification mapping module is used to obtain the continuous lower limits of capacity, floating capacitor and blocking capacitor with the minimum cost solution of the cost function, and to verify the discrete mapping combination of the procureable capacity set that satisfies the minimum cost solution. The verification and selection module is used to adjust the discrete mapping combination to meet the verification requirements and output the selection.

[0050] Verification example: Taking a 220kV, 70km tie line as an example, the requirement is to cover a bidirectional control range of approximately -50MW to +350MW, while limiting the peak voltage of the blocking capacitor to below the 220kV safety threshold. First, the bidirectional control capability is calculated using the voltage sensitivity under forward and reverse power flow. The lower limit of the injection capacity is obtained by choosing the stricter of the reverse and forward constraints. Then, the lower limit of the blocking capacitance is calculated based on the fault overvoltage constraint. The lower limit of the floating capacitance is calculated using ripple constraints and energy constraints. .

[0051] In one specific embodiment, the continuous optimal value maps to an injection capacity of approximately 0.90 per unit, the blocking capacitance of the D module is approximately 100 μF, and the floating capacitance of the Q module is calculated separately based on reverse regulation energy and ripple. The recalculated blocking peak is approximately 195 kV, within the 220 kV safety threshold, and the reverse regulation capability reaches -50 MW, meeting the bidirectional regulation requirements.

[0052] Test the unit injection capacity control capability and forward / reverse asymmetry at different operating points, such as Figure 4 As shown. Figure 4 The results show that, under the same injection capacity, the reverse descent flow side is constrained by the available energy of the floating capacitor earlier, and its unit capacity control capability is lower than that in the main forward operating range. This result indicates that determining the unit capacity solely based on the maximum forward control capability will introduce an underdrive risk at the -50 MW reverse boundary. Therefore, the lower limits of forward and reverse capacities must be calculated separately, and the more stringent results should be adopted.

[0053] Calculations were performed within the feasible region of the injection capacity, floating capacitance, and blocking capacitance. The results are shown below. Figure 5 As shown, only a two-dimensional graph of the injected capacity and the blocking capacitance is displayed here, omitting the feasible region of the floating capacitance. Figure 5The data shows that the power flow control range constraint forms the lower bound of the injection capacity, while the overvoltage and ripple constraints form the lower bound of the capacitance, together constituting the feasible region. Since both capacity cost and energy storage cost increase monotonically with design variables, the continuous economic optimum that satisfies all constraints is located near the intersection of the effective boundaries; in this example, the injection capacity is approximately 0.90 per unit and the blocking capacitance is approximately 100 μF.

[0054] Verification results show that the method of the present invention can achieve an interpretable and verifiable low-cost selection while satisfying bidirectional control and ontological security constraints.

[0055] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications may be made without departing from the technical solutions described in the claims.

Claims

1. A method for coordinating the injection capacity and DC capacitor design of a power flow controller, comprising a power flow control module, a reactive power coordination module, and a reverse blocking module, characterized in that, include: Calculate the lower limit of power flow injection capacity, the lower limit of floating capacitance of reactive power coordination module, and the lower limit of blocking capacitance of reverse blocking module respectively; The calculation process for power flow injection capacity is as follows: Calculate the baseline active power of the line; Establish the sensitivity of line active power change to series injection voltage, wherein the series injection voltage is orthogonal to the line current; For the system's forward power flow requirements, the minimum required device capacity is calculated by back-calculating the forward sensitivity and reverse sensitivity respectively. The reverse sensitivity is corrected based on the energy margin of the floating capacitor of the reactive power coordination module between the highest withstand voltage and the lowest operating voltage. Take the maximum value of the minimum device capacity for both forward and reverse power flows; Construct the feasible region and cost function for power flow injection capacity, floating capacitance, and blocking capacitance; The continuous lower bounds of capacity, floating capacitor and blocking capacitor are obtained by using the minimum cost solution of the cost function, and the discrete mapping combination of the procureable capacity set that satisfies the minimum cost solution is used for verification. Adjust the discrete mapping combination to meet the verification requirements, and output the selection.

2. The method for co-designing power flow controller injection capacity and DC capacitor according to claim 1, characterized in that, The forward and reverse sensitivities are given externally or calculated from the line parameters according to the linear sensitivity relationship.

3. The method for co-designing power flow controller injection capacity and DC capacitor according to claim 1, characterized in that, The calculation process for the lower limit of the floating capacitor of the reactive power coordination module is as follows: The first minimum capacitance value is calculated based on the ripple of the power frequency pulsating power applied to the floating capacitor and the set allowable peak value of the ripple. Set the system's charging and discharging energy requirements, and calculate the second minimum capacitance value based on the maximum allowable voltage fluctuation range of the capacitor; The maximum of the first minimum capacitance value and the second minimum capacitance value is taken as the lower limit of the floating capacitance value.

4. The method for co-designing power flow controller injection capacity and DC capacitor according to claim 1, characterized in that, The calculation process for the blocking capacitance of the reverse blocking module is as follows: Calculate the peak voltage during fault interruption; Using the peak voltage as the upper limit, the lower limit of the blocking capacitor value is calculated as the lower limit of the blocking capacitor value.

5. The method for co-designing power flow controller injection capacity and DC capacitor according to claim 1, characterized in that, The verification methods include: calculating the corresponding margin, ripple value, and overvoltage value of the discrete mapping combination.

6. The method for co-designing power flow controller injection capacity and DC capacitor according to claim 1, characterized in that, The verification also includes setting multiple operating scenarios, obtaining the corresponding lower limits of capacity, floating capacitance, and blocking capacitance under each scenario, and taking the maximum value of each lower limit.

7. A method for co-designing power flow controller injection capacity and DC capacitor according to claim 3 or 4, characterized in that, If the reactive power coordination module and the reverse blocking module share a capacitor, the capacitance value of the shared capacitor shall be the maximum value of the floating capacitor and the blocking capacitor.

8. The method for co-designing power flow controller injection capacity and DC capacitor according to claim 1, characterized in that, The expression for the cost function is: in, The device is injected with capacitance, floating capacitor value, and blocking capacitor value. These are the energy storage cost coefficients for fixed cost, unit capacity cost, floating capacitor, and blocking capacitor, respectively. This refers to the capacitor's withstand voltage rating. This is the average operating voltage of the floating capacitor. This is a safe voltage.

9. A power flow controller injection capacity and DC capacitor co-design system, characterized in that, include: The constraint parameter calculation module is used to calculate the lower limit of power flow injection capacity, the lower limit of floating capacitance of reactive power coordination module, and the lower limit of blocking capacitance of reverse blocking module, respectively. The calculation process for power flow injection capacity is as follows: Calculate the baseline active power of the line; Establish the sensitivity of line active power change to series injection voltage, wherein the series injection voltage is orthogonal to the line current; Based on the system's forward and reverse power flow requirements, the minimum required device capacity for each is calculated. The reverse sensitivity is an externally given value and is dated according to the energy margin of the floating capacitor of the reactive power coordination module between the highest withstand voltage and the lowest operating voltage. Take the maximum value of the minimum device capacity for both forward and reverse power flows; The cost function construction module is used to construct the feasible region and cost function for power flow injection capacity, floating capacitor and blocking capacitor; The discrete product specification mapping module is used to obtain the continuous lower limits of capacity, floating capacitor and blocking capacitor with the minimum cost solution of the cost function, and to verify the discrete mapping combination of the procureable capacity set that satisfies the minimum cost solution. The selection verification module is used to adjust the discrete mapping combination to meet the verification requirements and output the selection.

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