Alternating current and direct current hybrid section construction method and system

Abstract

The invention discloses an AC / DC hybrid section construction method and system, and the method comprises the steps: determining a combination mode of AC / DC lines in a power transmission line according to a principle of maximizing the short-circuit standard-exceeding risk alleviation capability of a power system, and determining a combination mode of AC / DC lines in the power transmission line according to a principle of maximizing the voltage supporting capability of an AC / DC hybrid power grid; determining the type of a converter station adopted by the direct-current line, determining the power transmission amount of the alternating-current line according to the power imbalance tolerance of the power system, and determining the power transmission capacity of the direct-current line according to the capacity of the alternating-current line bearing the transfer power flow of the direct-current line under the N-1 fault of the direct-current line, according to the method, the safety, the stability and the controllability of the series-parallel section are considered in a multi-dimensional mode, the requirements of a novel electric power system for a power grid structure in the future are met, and power grid structure optimization and scale expansion can be effectively promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of power systems, and in particular relates to an AC / DC hybrid section construction method and system. Background Art

[0002] Embedded DC transmission technology refers to a new type of transmission technology that "embeds" the DC transmission system at both ends into the AC grid of the power grid. By constructing an embedded DC, it is possible to effectively improve the transmission capacity of the transmission section without changing the existing AC main grid structure. There are various section transmission structures in AC / DC hybrid power grids. If there are n transmission lines in a section, including m DC transmission lines, the uncontrollable dimension of the power grid flow is nm-1. Therefore, in order to ensure the maximum controllability of the AC / DC hybrid power grid, an AC / DC hybrid transmission section structure containing a single AC line is selected to achieve complete control of the flow. How to construct a reasonable AC / DC hybrid transmission section is the current research focus of scholars. In response to several safety and stability issues that may exist in the AC / DC hybrid grid, most scholars today start from control and propose many effective improved control strategies. The fast power regulation capability of the flexible DC system was used to improve the frequency and synchronization characteristics of the AC / DC hybrid power system. By optimizing the power flow relationship between the AC / DC parallel transmission channels, the risk of AC line overload under a single fault was reduced. It was then pointed out that the disconnection of the transmission line can be used as a control method to avoid the collapse of the hybrid system. Only these control strategies can improve the operating stability of the AC / DC hybrid section to a certain extent, but they cannot completely avoid the safety and stability risks of the AC / DC hybrid. It is also necessary to propose a reliable AC / DC hybrid section construction plan from multiple perspectives.

[0003] Therefore, how to propose a fully controllable AC / DC hybrid section construction method that has both controllability, safety, and stability is an urgent problem to be solved by technical personnel in this field. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention proposes an AC / DC hybrid section construction method and system.

[0005] The technical solution of the present invention is as follows:

[0006] A method for constructing an AC / DC hybrid section, wherein the AC / DC hybrid section is provided with a transmission line having a single AC line and a plurality of DC lines, comprising:

[0007] Based on the principle of maximizing the risk mitigation capability of the power system for exceeding the short circuit limit, the combination of the AC and DC lines in the transmission line is determined, and based on the principle of maximizing the voltage support capability of the AC / DC hybrid power grid, the type of converter station used in the DC line is determined, and the power transmission capacity of the AC line is determined based on the power imbalance tolerance of the power system, and the transmission capacity of the DC line is determined based on the AC line's ability to carry the DC line transfer power flow when the DC line fails N-1.

[0008] Furthermore, the specific method for determining the combination mode of AC and DC lines in the transmission line based on the principle of maximizing the short circuit risk mitigation capability of the power system includes:

[0009] Constructing a combination mode set of AC and DC lines in a transmission line, wherein the combination mode set includes several combination modes of AC and DC lines; respectively calculating the short-circuit current margin corresponding to each combination mode of AC and DC lines in the combination mode set, and selecting the combination mode of AC and DC lines corresponding to the maximum short-circuit current margin;

[0010] The calculation formula of the short-circuit current margin is as follows:

[0011]

[0012] In the formula, K sci is the minimum margin of short-circuit current in the power system; I bmaxi is the maximum allowable short-circuit current of the ith node; I sci is the actual short-circuit current of the ith node, and min is the minimum value of the corresponding indicator among all nodes in the system.

[0013] Furthermore, the specific method for determining the type of converter station used in the DC line based on the principle of maximizing the voltage support capacity of the AC / DC hybrid section includes:

[0014] The system voltage stiffness is calculated by the following formula:

[0015] K vtg =U sys / U sys0

[0016]

[0017] Among them, λ SCR is the short-circuit ratio of the networked equipment at any point in the power grid; U sys0 It is the no-load voltage at any point when the network-connected equipment is not connected to the grid;

[0018] When the calculated system voltage stiffness is greater than 0.95, a conventional DC converter station type is selected; otherwise, a converter station type with voltage support capability is selected.

[0019] Furthermore, the specific method for determining the AC line transmission capacity based on the power imbalance tolerance of the power system includes:

[0020] Calculate the maximum power shortage that may occur in the AC / DC hybrid power grid and use the maximum power shortage as the AC line transmission capacity:

[0021]

[0022]

[0023] Where P acsum P is the total transmission power of the AC line in the AC / DC hybrid section; acj is the active power delivered by the jth AC line in the AC channel; ΔP is the maximum power shortage allowed in the section; P sum is the total power delivered to the section; P dci Transmit power for the ith DC line; n ac is the number of AC lines; m is the number of DC lines; RoCoF is the maximum frequency change rate of the system; f N is the frequency of the system under normal working conditions; H sys is the system inertia value; β is the frequency oscillation amplitude; ζ is the system damping ratio parameter; ω n is the natural oscillation angular frequency of the system; T is the equivalent inertia time constant of the system; Δf max is the maximum frequency deviation; D is the system load damping coefficient; K is the speed gain of the unit speed governor; t1 is the time from the start of the fault to the frequency deviation reaching the maximum value; Δf st It is the quasi-steady-state frequency deviation.

[0024] Furthermore, when the DC line N-1 fails, the specific method for determining the DC line transmission capacity based on the AC line's ability to carry the DC line transfer power flow includes:

[0025] The capacity constraint formula of the DC line in the AC / DC hybrid section under N-1 fault is as follows:

[0026] P dcmax =n ac ×P acmax -P acsum

[0027] Where P dcmax is the maximum transmission power of a single DC; P acmax is the maximum transmission power of a single-circuit AC line; P acsum is the maximum power shortage that may occur in the AC / DC hybrid power grid; n ac is the number of AC lines.

[0028] Furthermore, the method also includes: determining the operating state of the AC line in the hybrid section by the phase angle difference of the power grids at both ends of the transmission line.

[0029] Furthermore, the specific method for determining the operating state of the AC line in the hybrid section by the phase angle difference of the power grids at both ends of the transmission line includes:

[0030] The system adopts an active disconnection strategy to measure the phase difference of the AC power grid at both ends of the transmission line, and compares the measured phase difference between the power grids at both ends with the phase difference threshold. If the phase difference between the AC power grids at both ends of the transmission line is less than the phase difference threshold, that is, the phase difference constraint formula of the sending and receiving end power grids of the AC / DC hybrid power grid is met, the AC line operates normally and maintains a single-AC and multiple-DC hybrid structure; otherwise, the phase difference between the AC power grids at both ends of the transmission line is greater than or equal to the phase difference threshold. Based on the active disconnection strategy of the AC tie line, the AC line is disconnected, and the sending and receiving end power grids operate asynchronously to avoid instability, thereby ensuring the synchronous stability of the power grid operation.

[0031] A system for constructing an AC / DC hybrid section, wherein the AC / DC hybrid section is provided with a transmission line having a single AC line and a plurality of DC lines, and the system comprises an AC / DC line combination mode determination module, a converter station type determination module, an AC line transmission amount determination module, and a DC line transmission capacity determination module;

[0032] The AC / DC line combination mode determination module is used to determine the combination mode of the AC / DC lines in the transmission line based on the principle of maximizing the short circuit risk mitigation capability of the power system;

[0033] The converter station type determination module is used to determine the type of converter station used for the DC line based on the principle of maximizing the voltage support capability of the AC / DC hybrid power grid;

[0034] The AC line transmission capacity determination module is used to determine the AC line transmission capacity based on the power imbalance tolerance of the power system;

[0035] The DC line transmission capacity determination module is used to determine the DC line transmission capacity by using the DC line capacity constraint formula of the AC / DC hybrid section under N-1 faults.

[0036] An electronic device comprises a memory and a processor, wherein the memory stores a computer program, and the processor is used to call and run the computer program stored in the memory to execute any of the methods described above.

[0037] A computer-readable storage medium stores a computer program, wherein the computer program implements the steps of any of the above methods when executed by a processor.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] The present invention proposes a method and system for constructing an AC / DC hybrid section. The method adopts the principle of maximizing the risk mitigation capability of the power system for short circuit exceeding the standard to determine the combination mode of the AC / DC lines in the transmission line, and the principle of maximizing the voltage support capability of the AC / DC hybrid power grid to determine the type of converter station used for the DC line, and determines the transmission capacity of the AC line based on the power imbalance tolerance of the power system, and determines the transmission capacity of the DC line based on the AC line's ability to carry the DC line's transfer power flow under the condition of a DC line N-1 fault. The method considers the safety, stability, and controllability of the hybrid section in multiple dimensions, meets the requirements of the future new power system for the power grid structure, and can effectively promote the optimization of the power grid structure and the expansion of its scale. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a flow chart of the method for constructing an AC / DC hybrid section in an embodiment;

[0041] Figure 2 It is a typical structure diagram of the AC / DC hybrid section in the embodiment;

[0042] Figure 3 The characteristic curve diagram of the voltage stiffness changing with the short circuit ratio in the embodiment;

[0043] Figure 4 This is a typical structural diagram of the interconnection of three power transmission channels in the embodiment;

[0044] Figure 5 Graph showing frequency response of each unit when HVDC1 is locked in the embodiment. DETAILED DESCRIPTION

[0045] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention. After reading the present invention, various equivalent modifications of the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

[0046] Embodiment 1:

[0047] The present invention provides a method for constructing an AC / DC hybrid section, wherein the AC / DC hybrid section is provided with a transmission line having a single AC line and a plurality of DC lines, such as Figure 1 and Figure 2 As shown, the method comprises the following specific steps:

[0048] S1. Based on the principle of maximizing the risk mitigation capability of the power system for short circuit exceeding the standard, determine the combination of AC and DC lines in the transmission line;

[0049] S2. Determine the type of converter station used for the DC line based on the principle of maximizing the voltage support capacity of the AC / DC hybrid power grid;

[0050] S3. Determine the AC line transmission capacity based on the power imbalance tolerance of the power system, and determine the DC line transmission capacity based on the AC line's ability to carry the DC line's transferred power flow when the DC line fails N-1.

[0051] Embodiment 2:

[0052] This embodiment is further designed on the basis of the first embodiment in that, in this embodiment, based on the principle of maximizing the short circuit risk mitigation capability of the power system, the specific method for determining the combination mode of the AC and DC lines in the transmission line includes:

[0053] A combination mode set of AC and DC lines in a transmission line is constructed, wherein the combination mode set includes several combination modes of AC and DC lines; the short-circuit current margin corresponding to each combination mode of AC and DC lines in the combination mode set is calculated respectively, and the combination mode of AC and DC lines corresponding to the maximum short-circuit current margin is selected;

[0054] The calculation formula of short-circuit current margin is as follows:

[0055]

[0056] In the formula, K sci is the minimum margin of short-circuit current in the power system; I bmaxi is the maximum allowable short-circuit current of the ith node; I sci is the actual short-circuit current of the ith node, and min is the minimum value of the corresponding index among all nodes in the system. sci When <0, it indicates that the line short-circuit current exceeds the limit, reflecting the short-circuit current level of the AC / DC hybrid power grid.

[0057] Embodiment three:

[0058] This embodiment is further designed on the basis of the second embodiment in that, in this embodiment, based on the principle of maximizing the voltage support capacity of the AC / DC hybrid section, the specific method for determining the type of converter station used in the DC line includes:

[0059] The voltage stiffness of the system is calculated by the following formula. The voltage support strength at any point in the power grid is defined as the ability to maintain the voltage modulus at the access point close to the no-load voltage at the access point, which is called voltage stiffness K. vtg :

[0060]

[0061] Among them, λ SCR is the short-circuit ratio of the networked equipment at any point in the power grid; U sys0It is the no-load voltage at any point when the network-connected equipment is not connected to the grid;

[0062] Voltage stiffness K vtg The value range of is [0,1]. th When it is equal to zero, the voltage stiffness K vtg Equal to 1; when Z th When it is equal to infinity, the voltage stiffness K vtg is equal to zero.

[0063] Voltage stiffness K vtg Short circuit ratio λ SCR By comparison, it can be seen that K vtg The reflected power grid information and network equipment information are more SCR More comprehensive, SCR It only reflects and The modulus information does not reflect the phase angle information of the two. In addition, the short circuit ratio λ SCR The value range of is [0,∞], and K vtg The value range is [0,1].

[0064] When the calculated system voltage stiffness is greater than 0.95, a conventional DC converter station type is selected; otherwise, a converter station type with voltage support capability is selected.

[0065] like Figure 3 As shown, when the short circuit ratio λ SCR When the short-circuit ratio is 3, the voltage stiffness is 0.95. It is generally believed that when the short-circuit ratio is greater than 3, that is, the voltage stiffness is greater than 0.95, the access point SYS is a relatively strong access point, because its voltage drop after loading is less than 5% of the rated voltage, and the system voltage support capacity is strong, which can ensure the stability of the system voltage under fault conditions.

[0066] During the planning stage, by calculating the system voltage stiffness index, it can be determined whether the system voltage support capacity is sufficient to ensure the system voltage stability. If not, it is necessary to consider using a DC converter station type with voltage support capacity.

[0067] Embodiment 4:

[0068] This embodiment is further designed on the basis of the third embodiment in that the specific method for determining the AC line transmission amount based on the power imbalance tolerance of the power system in this embodiment includes:

[0069] For an AC / DC hybrid grid structure with only one AC line, when any N-1 or N-2 fault occurs in the DC transmission system, it will not cause a power shortage in the sending and receiving power grids, and the power transmitted by the DC will be transferred to the AC line. Therefore, the DC line fault will not affect the frequency stability of the AC / DC hybrid power grid; when an N-1 or N-2 fault occurs in the AC transmission line, the sending and receiving power grids will completely lose AC connection, and the power transmitted by the original AC line will not be borne by the DC system, which will cause a power shortage in the sending and receiving power grids and cause frequency fluctuations.

[0070] Taking into account the system frequency stability constraints, the system frequency stability is checked by the maximum power imbalance that may occur in the system under fault conditions, and the maximum transmission power of the AC line that meets the maximum allowable frequency deviation of the system is determined.

[0071] Calculate the maximum power shortage that may occur in the AC / DC hybrid power grid and use the maximum power shortage as the AC line transmission capacity:

[0072]

[0073] Where P acsum is the maximum power shortage that may occur in the AC / DC hybrid power grid; P acj The active power delivered to the jth AC line in the AC channel; P sum is the total power delivered to the section; P dci Transmit power for the ith DC line; n ac is the total number of AC lines in the hybrid section; m is the number of DC lines;

[0074] In order to deal with the frequency shock caused by power imbalance at both ends of the power grid under fault conditions, the maximum frequency change rate, maximum frequency deviation and quasi-steady-state frequency deviation indicators are often used in actual power grids to check frequency stability.

[0075]

[0076] Where RoCoF is the maximum frequency change rate of the system; f N is the frequency of the system under normal working conditions; H sys is the system inertia value; β is the frequency oscillation amplitude; ζ is the system damping ratio parameter; ω n is the natural oscillation angular frequency of the system; T is the equivalent inertia time constant of the system; Δf max is the maximum frequency deviation; D is the system load damping coefficient; K is the speed gain of the unit speed governor; t1 is the time from the start of the fault to the frequency deviation reaching the maximum value; Δf st It is the quasi-steady-state frequency deviation.

[0077] The maximum power shortage of the sending and receiving systems that may be caused by the AC line N-1 / N-2 failure is used to verify the system frequency stability, thereby determining the maximum allowable transmission power of the AC line, and thus determining the allocated capacity of the AC line.

[0078] Embodiment five:

[0079] This embodiment is further designed on the basis of the fourth embodiment in that, when a line fails, the transmission power of the faulty line will be transferred to the AC line. By using the power flow constraints under normal conditions and N-1 faults, a reasonable distribution relationship between the AC and DC transmission powers can be selected, thereby ensuring that the AC line will not be overloaded under the N-1 fault of the DC system, thereby determining the distribution capacity of each DC line. In an AC / DC hybrid power grid, under N-1 fault, the transmission capacity on the faulty DC line will be transferred to the AC transmission line, and the transmission capacity constraint of the DC transmission line can be determined by the power flow transfer amount. For an AC / DC hybrid cross-section structure containing one AC line, when the AC channel has only one circuit, the maximum transmission power P allowed for a single DC line must be guaranteed. dcmax Should be less than the maximum transmission power P of a single AC line acmax When the AC channel has a parallel double-circuit line, the maximum transmission power of the AC line should be twice the maximum transmission power of the single-circuit AC line. In this example, when the DC line N-1 fails, the specific method for determining the DC line transmission capacity based on the AC line's ability to carry the DC line transfer power flow includes:

[0080] The capacity constraint formula of the DC line in the AC / DC hybrid section under N-1 fault is as follows:

[0081] P dcmax =n ac ×P acmax -P acsum

[0082] Where P dcmax is the maximum transmission power of a single DC; P acmax is the maximum transmission power of a single-circuit AC line; P acsum is the maximum power shortage that may occur in the AC / DC hybrid power grid, that is, the total power transmitted by the AC line; n ac is the number of AC lines.

[0083] Embodiment six:

[0084] This embodiment is further designed on the basis of the fifth embodiment in that the method in this embodiment also includes: determining the operating state of the AC line in the hybrid section by the phase angle difference of the power grids at both ends of the transmission line.

[0085] Embodiment seven:

[0086] This embodiment is further designed on the basis of the sixth embodiment in that the specific method for determining the operating state of the AC line in the hybrid section by the phase angle difference of the power grids at both ends of the transmission line in this embodiment includes:

[0087] The system adopts an active disconnection strategy to measure the phase difference of the AC power grid at both ends of the transmission line, and compares the measured phase difference of the power grid at both ends with the phase difference threshold. If the phase difference of the AC power grid at both ends of the transmission line is less than the phase difference threshold, that is, the phase difference constraint formula of the AC / DC hybrid power grid is satisfied, the AC line operates normally and maintains a single-AC and multiple-DC hybrid structure; otherwise, the phase difference of the AC power grid at both ends of the transmission line is greater than or equal to the phase difference threshold. Based on the active disconnection strategy of the AC tie line, the AC line is disconnected, and the sending and receiving power grids operate asynchronously to avoid instability, thereby ensuring the synchronous stability of the power grid operation. The phase difference constraint formula of the sending and receiving power grids of the AC / DC hybrid power grid is: Δθ ac <θ th Among them, the AC tie line relay protection device detects the phase difference between the two ends in real time as Δθ ac , the system phase difference threshold is θ th , when the phase difference Δθ between the two ends of the AC / DC hybrid transmission system ac Reaching the threshold θ th After that, the AC circuit between the two AC power grids is disconnected to achieve asynchronous operation.

[0088] Embodiment eight:

[0089] The present invention provides an AC / DC hybrid section construction system, wherein the AC / DC hybrid section is provided with a transmission line having a single AC line and a plurality of DC lines, and the system comprises an AC / DC line combination mode determination module, a converter station type determination module, an AC line transmission amount determination module, and a DC line transmission capacity determination module;

[0090] The module for determining the combination mode of AC and DC lines is used to determine the combination mode of AC and DC lines in the transmission line based on the principle of maximizing the short-circuit risk mitigation capability of the power system;

[0091] A converter station type determination module is used to determine the type of converter station used for the DC line based on the principle of maximizing the voltage support capacity of the AC / DC hybrid power grid;

[0092] An AC line transmission capacity determination module, used to determine the AC line transmission capacity based on the power imbalance tolerance of the power system;

[0093] The DC line transmission capacity determination module is used to determine the DC line transmission capacity based on the capacity constraint of the AC / DC hybrid section DC line under N-1 faults.

[0094] Embodiment nine:

[0095] An electronic device comprises a memory and a processor, wherein the memory stores a computer program, and the processor is used to call and run the computer program stored in the memory to execute any of the methods described above.

[0096] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of any of the above methods are implemented.

[0097] Application examples:

[0098] In this example, the method of the present invention is implemented in a simulation of a certain power grid. The power grid is an IEEE two-area four-machine system, which constructs an AC / DC hybrid power grid with one AC line. The power grid structure is as follows: Figure 4 As shown, it includes synchronous machines 1 to 8.

[0099] The frequency response curves of each unit under simulated HVDC1 blocking are as follows: Figure 5 As shown. It can be seen that under this AC / DC hybrid transmission construction scheme, after the HVDC1 DC lockout, the frequency offset of each unit in the sending and receiving power grid is very small, and returns to the rated value after a small fluctuation, which can effectively avoid the impact of DC lockout on the sending and receiving system frequency, and has a higher evaluation stability. Under a short-circuit fault, the phase difference between the sending and receiving units may continue to increase and eventually lose step, but the AC line can be actively disconnected by detecting the phase difference, so as to realize asynchronous operation of the sending and receiving ends and maintain the synchronous stability of the power grid. The method of the present invention can meet the conditions for safe and stable operation of the power grid and is feasible in engineering.

[0100] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for constructing an AC / DC hybrid section, wherein the AC / DC hybrid section is provided with a transmission line having a single AC line and a plurality of DC lines, characterized in that: include: Based on the principle of maximizing the risk mitigation capability of the power system for exceeding the short circuit limit, the combination of the AC and DC lines in the transmission line is determined, and based on the principle of maximizing the voltage support capability of the AC / DC hybrid power grid, the type of converter station used in the DC line is determined, and the power transmission capacity of the AC line is determined based on the power imbalance tolerance of the power system, and the transmission capacity of the DC line is determined based on the AC line's ability to carry the DC line transfer power flow when the DC line fails N-1.

2. The AC / DC hybrid section construction method according to claim 1, characterized in that: The specific method for determining the combination mode of the AC and DC lines in the transmission line based on the principle of maximizing the short circuit risk mitigation capability of the power system includes: Constructing a combination mode set of AC and DC lines in a transmission line, wherein the combination mode set includes several combination modes of AC and DC lines; respectively calculating the short-circuit current margin corresponding to each combination mode of AC and DC lines in the combination mode set, and selecting the combination mode of AC and DC lines corresponding to the maximum short-circuit current margin; The calculation formula of the short-circuit current margin is as follows: In the formula, K sci is the minimum margin of short-circuit current in the power system; I bmaxi is the maximum allowable short-circuit current of the i-th node; I sci is the actual short-circuit current of the ith node, and min is the minimum value of the corresponding indicator among all nodes in the system.

3. The AC / DC hybrid section construction method according to claim 2, characterized in that: The specific method for determining the type of converter station used for the DC line based on the principle of maximizing the voltage support capacity of the AC / DC hybrid section includes: The system voltage stiffness is calculated by the following formula: K vtg =U sys / IN sys0 Among them, λ SCR is the short-circuit ratio of the networked equipment at any point in the power grid; U sys0 It is the no-load voltage at any point when the network-connected equipment is not connected to the grid; When the calculated system voltage stiffness is greater than 0.95, a conventional DC converter station type is selected; otherwise, a converter station type with voltage value support capability is selected.

4. The AC / DC hybrid section construction method according to claim 3, characterized in that: The specific method for determining the AC line transmission capacity based on the power imbalance tolerance of the power system includes: Calculate the maximum power shortage that may occur in the AC / DC hybrid power grid and use the maximum power shortage as the AC line transmission capacity: Where P acsum P is the total transmission power of the AC line in the AC / DC hybrid section; acj is the active power delivered by the jth AC line in the AC channel; ΔP is the maximum power shortage allowed in the section; P sum is the total power delivered to the section; P dci Transmit power for the ith DC line; n ac is the number of AC lines; m is the number of DC lines; RoCoF is the maximum frequency change rate of the system; f N is the frequency of the system under normal working conditions; H sys is the system inertia value; β is the frequency oscillation amplitude; ζ is the system damping ratio parameter; ω n is the natural oscillation angular frequency of the system; T is the equivalent inertia time constant of the system; Δf max is the maximum frequency deviation; D is the system load damping coefficient; K is the speed gain of the unit speed governor; t1 is the time from the start of the fault to the frequency deviation reaching the maximum value; Δf st It is the quasi-steady-state frequency deviation.

5. The AC / DC hybrid section construction method according to claim 4, characterized in that: When the DC line N-1 fails, the specific method for determining the DC line transmission capacity based on the AC line's ability to carry the DC line transfer power flow includes: The capacity constraint formula of the DC line in the AC / DC hybrid section under N-1 fault is as follows: P dcmax =n ac ×P acmax -P acsum Where P dcmax is the maximum transmission power of a single DC; P acmax is the maximum transmission power of a single-circuit AC line; P acsum is the maximum power shortage that may occur in the AC / DC hybrid power grid; n ac is the number of AC lines.

6. The AC / DC hybrid section construction method according to claim 5, characterized in that: The method further comprises: determining the operation state of the AC line in the hybrid section by the phase angle difference of the power grids at both ends of the transmission line.

7. The AC / DC hybrid section construction method according to claim 6, characterized in that: The specific method for determining the operating state of the AC line in the hybrid section by the phase angle difference of the power grids at both ends of the transmission line includes: The system adopts an active disconnection strategy to measure the phase difference of the AC power grids at both ends of the transmission line, and compares the measured phase difference between the power grids at both ends with the phase difference threshold. If the phase difference between the AC power grids at both ends of the transmission line is less than the phase difference threshold, that is, the phase difference constraint formula of the sending and receiving end power grids of the AC / DC hybrid power grid is met, the AC line operates normally and maintains a single-AC and multiple-DC hybrid structure; otherwise, the phase difference between the AC power grids at both ends of the transmission line is greater than or equal to the phase difference threshold. Based on the active disconnection strategy of the AC tie line, the AC line is disconnected, and the sending and receiving end power grids operate asynchronously to avoid instability, thereby ensuring the synchronous stability of the power grid operation.

8. A system for constructing an AC / DC hybrid section, wherein the AC / DC hybrid section is provided with a transmission line having a single AC line and a plurality of DC lines, characterized in that: The system includes an AC and DC line combination mode determination module, a converter station type determination module, an AC line transmission amount determination module, and a DC line transmission capacity determination module; The AC / DC line combination mode determination module is used to determine the combination mode of the AC / DC lines in the transmission line based on the principle of maximizing the short circuit risk mitigation capability of the power system; The converter station type determination module is used to determine the type of converter station used for the DC line based on the principle of maximizing the voltage support capability of the AC / DC hybrid power grid; The AC line transmission capacity determination module is used to determine the AC line transmission capacity based on the power imbalance tolerance of the power system; The DC line transmission capacity determination module is used to determine the DC line transmission capacity according to the DC line capacity constraint formula of the AC / DC hybrid section under N-1 faults.

9. An electronic device, characterized in that: The electronic device comprises a memory and a processor, wherein the memory stores a computer program, and the processor is configured to call and run the computer program stored in the memory to execute the method according to any one of claims 1 to 7.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the steps of the method as claimed in any one of claims 1 to 7 are implemented.

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