Quantification method for frequency intensity and voltage intensity of power grid nodes

Abstract

The invention discloses a power grid node frequency intensity and voltage intensity-oriented quantification method. The method comprises the following steps of: 1) respectively considering an inertia evaluation strategy of equivalent inertia of a synchronous machine, equivalent inertia of a new energy converter station and equivalent inertia of a load to be used for analyzing node-level inertia support intensity of a system; 2) proposing a power grid node level frequency intensity quantification strategy based on power grid node frequency deviation, frequency change rate and node inertia intensity; and 3) a system node voltage support strength quantification method considering node short-circuit capacity influence caused by voltage drop. And 4) proposing a power grid node-level voltage intensity quantification strategy considering the system node voltage deviation, the voltage change rate and the operation short-circuit ratio. According to the invention, through frequency voltage disturbance quantity information, inertia intensity and a short-circuit ratio, influences of different response characteristics of different inertia sources and system operation state changes on the short-circuit ratio are considered, and frequency support intensity and voltage support intensity are respectively subjected to quantitative analysis; the accuracy and reliability of frequency support strength and voltage support strength quantification are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy power systems, and specifically is a method for quantifying frequency intensity and voltage intensity of power grid nodes. Background Art

[0002] With the rapid development of renewable energy power generation systems and power grids, the scale of wind farms continues to expand (Y.Xiong, W.Yao, J.Wen, et al., "Two-Level Combined Control Scheme of VSC-MTDC IntegratedOffshore Wind Farms for Onshore System Frequency Support," IEEE Transactions on PowerSystems, vol.36, no.1, pp.781-792, Jan.2021). This makes the inertia sources in the power system more diverse, and the frequency and voltage stability issues are becoming increasingly prominent. However, existing methods only consider disturbance information (such as Δf, ΔV, rate of change of frequency (RoCoF) and rate of change of voltage (RoCoV)) to determine whether the power grid is stable. However, node inertia and short-circuit ratio have a significant effect on the strength analysis of grid-connected node frequency and voltage. Existing frequency support strength quantification methods focus on node-based generalized inertia analysis. The inertia values ​​of synchronous generator units and grid nodes can be simply calculated by the law of conservation of energy and the swing equation (J.Liu, Y.Hou, J.Guo, X.Liu, et al. "ACost-Efficient Virtual Synchronous Generator System Based on Coordinated Photovoltaic and Supercapacitor," IEEE Trans.PowerElectron., vol.38, no.12, pp.16219-16229, Dec.2023.). However, since grid nodes usually contain multiple inertia providers (such as synchronous generators, new energy converter stations, and loads), and the dynamic response characteristics of these providers are different, the centralized and unified node inertia generalized calculation method cannot capture the variability between multiple providers and cannot obtain accurate node inertia values. Therefore, the frequency strength value of the grid node should further consider the dynamic response speed of different inertia providers.On the other hand, the existing voltage support strength mainly focuses on short-circuit ratio (SCR) analysis (J. Wang, W. Huang, N. Tai, et al. "A Bidirectional Virtual Inertia Control Strategy for the Interconnected Converter of Standalone AC / DC Hybrid Microgrids," IEEE Trans. on Power Sys., vol. 39, no. 1, pp. 745-754, 2024.), and a series of SCR calculation methods are proposed for voltage strength values, such as DC SCR, MTDC SCR, renewable energy SCR, operating SCR, generalized SCR, effective SCR, equivalent SCR, etc. (H. Xiao, Y. Zhang, X. Duan, et al. "Evaluating Strength of Hybrid Multi-Infeed HVDC Systems for Planning Studies Using Hybrid Multi-Infeed Interactive Effective Short-Circuit Ratio," IEEE Trans. Power Del., vol.36, no.4, pp.2129-2144, Aug.2021.), but these methods ignore the voltage difference between the faulty node and the adjacent nodes, resulting in inaccurate short-circuit capacity calculation. Based on the above analysis, the disturbance information of the grid nodes (such as Δf, ΔV, RoCoF, RoCoV), inertia index and SCR index are comprehensively considered to establish a quantification method for the frequency and voltage intensity of the grid nodes.

[0003] Therefore, a quantification method for the frequency intensity and voltage intensity of power grid nodes is urgently needed.

[0004] However, the current wind farm consistency control method only considers the influence of rotor kinetic energy (Y.Xiong, WYao, Y.Yao, et al., "Distributed Cooperative Control of Offshore Wind Farms Integrated via MTDC System for Fast Frequency Support," IEEE Transactions on Industrial Electronics, vol. 70, no. 5, pp. 4693-4704, May 2023), ignoring the problem of wind turbine operating point offset caused by active frequency support, and cannot accurately quantify the active frequency support capability of wind turbines. In order to quantify the active frequency support capability of wind farms under the influence of wake effect, and at the same time maximize the provision of active frequency support services for wind farms;

[0005] Therefore, some people are currently applying for the following application to solve the above problems;

[0006] Application number: CN202311258475.6, Patent name: "A microgrid cascading failure simulation method based on comprehensive weather scenarios and node inertia" Technical comparison:

[0007] The technical solution of "A microgrid cascading failure simulation method that integrates weather scenarios and node inertia" involves a node inertia calculation method. The node inertia method it adopts only considers the synchronous machine (traditional power system) and calculates the node inertia through the electrical distance from each node of the system to the generator node combined with the inertia of the synchronous machine.

[0008] This article focuses on our node inertia calculation method based on the new power system, which includes multiple inertia sources, such as synchronous machines, converter stations and loads. The inertia of each inertia source is calculated and finally weighted to obtain the node inertia. There are differences in system background (new power system and traditional power system containing only synchronous generators), calculation method (considering multiple inertia sources and obtaining node inertia by capacity weighting and only considering synchronous generator inertia), and evaluation method (considering frequency / voltage deviation, change rate and inertia / operation short-circuit ratio indicators respectively).

[0009] Application number: US17584767, Patent name: "METHOD FOR MONITORING AN ELECTRICITYSUPPLY GRID" Technical comparison:

[0010] The technical solution of "METHOD FOR MONITORING AN ELECTRICITY SUPPLY GRID" involves a node voltage strength calculation method, which adopts a monitoring method for the power supply grid to infer the power flow of the measurement line based on the voltage amplitude and phase angle of the node.

[0011] This paper proposes two quantitative methods for node frequency strength and voltage strength, which are respectively obtained through the deviation, change rate and inertia / short circuit ratio of frequency and voltage, and then weighted according to the capacity of each inertia provider to obtain the frequency and voltage support strength of the node, which can more accurately reflect the frequency and voltage support strength of the node. Through the node frequency and voltage strength, the node's resistance to frequency and voltage changes can be obtained. Summary of the invention

[0012] In order to solve the above technical problems, the present invention proposes a quantification method for the frequency intensity and voltage intensity of power grid nodes. The present invention targets the development trend of new energy power systems and considers the quantitative analysis of the system frequency intensity and voltage intensity including new power systems and load characteristics.

[0013] To achieve the above object, the technical solution adopted by the present invention is:

[0014] A method for quantifying frequency intensity and voltage intensity of power grid nodes, the method comprising the following steps:

[0015] 1) An inertia assessment strategy that considers the equivalent inertia of synchronous machine, equivalent inertia of new energy converter station and equivalent inertia of load is used to analyze the inertia support strength of system node level;

[0016] 2) A grid node-level frequency strength quantification strategy based on grid node frequency deviation, frequency change rate and node inertia strength is proposed;

[0017] 3) A quantitative method for the voltage support strength of system nodes considering the impact of node short-circuit capacity caused by voltage drop;

[0018] 4) A grid node-level voltage intensity quantification strategy considering system node voltage deviation, voltage change rate and operating short-circuit ratio is proposed.

[0019] Further, step (1) is specifically as follows;

[0020] By quantifying the inertia strength of each inertia source at the grid node, as shown below:

[0021] 1. Equivalent inertia of synchronous generator;

[0022] The equivalent inertia of a synchronous generator to a certain node is affected by the inertia of the synchronous generator and the electrical distance between the synchronous generator and the grid node:

[0023]

[0024] In the formula, B ik is the susceptance between the i-th node and the k-th SG in the power grid; H k is the inertia of the kth synchronous generator; r ik is the correlation coefficient between the i-th node of the power grid and the k-th synchronous generator of the power grid; i∈k th is the corresponding element of the kth row of the voltage correlation matrix between the grid node and the synchronous generator;

[0025] 2. Equivalent inertia of new energy converter station;

[0026] Since large-scale offshore wind farms are usually connected to the land power grid through converter stations, and the converter stations use the frequency disturbance information of the land power grid for inertia support, the relationship between frequency and active power and its inertia support can be expressed by the rotor motion equation:

[0027]

[0028] In the formula, H GSVSC,i represents the inertia time constant of the i-th GSVSC, P m and P e Represent mechanical power and electromagnetic power respectively. ΔP 1 It represents the virtual kinetic energy absorbed or released by the new energy converter station inertia during the frequency support process;

[0029] The new energy converter station uses the electric energy stored in the HVDC capacitor to achieve inertia response. Therefore, in the quantitative analysis of the equivalent inertia of the new energy converter station, the response of the DC capacitor needs to be considered. The relationship between the DC voltage and the active power is expressed by the capacitor dynamic equation as follows:

[0030]

[0031] Where P in and P out Respectively represent the input and output power of the new energy converter station, C dc,i Indicates the DC capacitor capacity, S GSVSC,i represents the rated capacity of the i-th renewable energy converter station in the power grid, Ni represents the number of DC capacitors in the i-th renewable energy converter station in the power grid, ΔP 2 Indicates the dynamic electrostatic power stored or released by the HVDC capacitor;

[0032] Based on (2)-(3), the equivalent inertia of the new energy converter station at the grid node can be expressed as:

[0033]

[0034] In the formula, H GSVSC,i represents the GSVSC equivalent inertia at the i-th node of the power grid;

[0035] 3. Load equivalent inertia;

[0036] When the system frequency / voltage changes, the system's active power demand will also change, resulting in abnormal active power, which will affect the system inertia. The equivalent inertia of the load to a certain node will be affected by the load's static characteristics and the real-time frequency and voltage. The load's static characteristics are expressed by the following polynomial:

[0037]

[0038] Where a, b, and c represent the proportions of the system's constant impedance, constant current, and constant power loads, respectively;

[0039] The load active power fluctuation is expressed by solving the total differential of frequency and voltage in equation (5), and we get:

[0040]

[0041] At the same time, based on the rotor motion equation, the load active power fluctuation can also be expressed as:

[0042]

[0043] According to (6) and (7), the load equivalent inertia H load,i It can be expressed as:

[0044]

[0045] Based on formulas (1)-(8), the grid node-level inertia H i It can be expressed as:

[0046]

[0047] Where FSQ is the frequency strength value of node i, H SG,i is the equivalent inertia of SG after electrical distance conversion, H GSVSC,i is the equivalent inertia of GSVSC, H load,i is the equivalent inertia of the load, S SG,i is the rated capacity of SGs corresponding to HSG, S GSVSC,i is the rated capacity of the GSVSC station corresponding to node i, S GSVSC,i is the rated capacity of the GSVSC station corresponding to node i, Sload,i is the H of node i load,i The corresponding load rated capacity, z SG,i is a binary variable of SG status, where 0 is off and 1 is on. GSVSC,i is a binary variable representing the GSVSC state of node i, where 0 is off and 1 is on. load,i is a binary variable representing the local load status of node i, where 0 means no local load and 1 means yes local load.

[0048] Further, step (2) is as follows:

[0049] The frequency strength of the power grid node represents the ability of the system to maintain frequency stability after disturbance, which includes frequency deviation, frequency change rate and inertia index. The construction of frequency deviation and frequency change rate index needs to consider the national standard. The formula is as follows:

[0050] 1. Frequency deviation index;

[0051] The frequency deviation index is constructed according to the frequency deviation requirements, as shown in the following formula:

[0052]

[0053] In the formula, f i 、f H 、f l 、f H,max and f L,min They represent the frequency value of system node i, the upper limit of the frequency dead zone, the lower limit of the frequency dead zone, the maximum allowable frequency value of the system, and the minimum allowable frequency value of the system respectively;

[0054] 2. Frequency change rate indicator;

[0055] The frequency change rate represents the rate of change of the system at the beginning when the frequency event occurs. The smaller the value, the slower the frequency changes during disturbances and the stronger the frequency stability. The frequency change rate index is constructed based on the system node frequency.

[0056] As shown below:

[0057]

[0058] Where, RoCoF max Indicates the maximum frequency change rate allowed by the system;

[0059] 3. Quantification method of power grid node frequency intensity:

[0060] Taking into account the frequency deviation index, frequency change rate index, and inertia index of the system power grid nodes, the frequency strength of the system power grid nodes is obtained as shown below:

[0061]

[0062] Further, step (3) is as follows:

[0063] The short-circuit ratio is an important indicator to characterize the voltage strength of the system, which is defined as the ratio of the short-circuit capacity to the rated DC power:

[0064] SCR=S scc,i / P N,i (13)

[0065] In the formula, S scc,i is the short-circuit capacity of node i, P N,i is the DC power;

[0066] Short-circuit capacity is one of the important indicators of SCR. Compared with the existing short-circuit capacity calculated based on rated voltage, the operating short-circuit capacity is calculated by considering the change of voltage difference between the fault node and the adjacent fault nodes:

[0067]

[0068] Considering the joint influence of multiple DC branches of MTDC, another important factor affecting SCR is the DC power P N,i , whose expression is:

[0069]

[0070] Where P dN,i and P dN,j are the i-th and j-th rated DC powers respectively;

[0071] Based on the definition of multi-input short-circuit ratio, ΔU at different nodes can be dc The ratio is replaced by the ratio of mutual impedance to self-impedance, expressed as:

[0072]

[0073] According to (14)-(16), considering that the real-time operating state of the system is constantly changing, its operating short-circuit ratio OSCR can be expressed as:

[0074]

[0075] Further, step (3) is as follows:

[0076] The grid node-level voltage intensity quantification strategy includes voltage deviation index and voltage change rate index;

[0077] 1. Voltage deviation index;

[0078] The voltage deviation indicator is constructed based on the voltage deviation limit as follows:

[0079]

[0080] Where U i , U h , U l , U max and U min They represent the system node voltage, the upper limit of the voltage dead zone, the lower limit of the voltage dead zone, the maximum value of the system voltage and the minimum value of the system voltage respectively;

[0081] 2. Voltage change rate indicator;

[0082] The voltage change rate represents the rate of change of the system at the beginning when a voltage event occurs. The smaller the value, the better the stability of the system and the slower the voltage changes during disturbances. Based on the system node voltage, the voltage change rate indicator is constructed as follows:

[0083]

[0084] In the formula, RoCoV max Indicates the maximum rate of voltage change allowed by the system;

[0085] Taking into account the voltage deviation index, voltage change rate index, and short-circuit ratio index of the system grid nodes, the voltage strength of the system grid nodes is obtained as shown below:

[0086]

[0087] Compared with the prior art, the present invention has the following beneficial effects:

[0088] A quantification method for the frequency intensity and voltage intensity of power grid nodes constructed by the present invention, in terms of the frequency intensity quantification strategy, considers the frequency deviation index, the frequency change rate index and the inertia index, wherein the inertia index, by considering the inertia sources (synchronous generators, new energy converter stations and loads) with different response speeds respectively, to improve the accuracy and reliability of quantification. In terms of voltage intensity quantification, the voltage deviation index, the voltage change rate index and the short-circuit ratio index are considered. In terms of the short-circuit ratio index, the voltage difference between the faulty node and the faulty adjacent node is considered to improve its accuracy. The present invention has high practical value in analyzing the frequency intensity and voltage intensity of power grid nodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 is the topological structure diagram of the example;

[0090] Figure 2 It is the offshore wind farm control strategy;

[0091] Figure 3 It is the small signal stability analysis diagram of the example model;

[0092] Figure 4 It is a quantitative indicator of frequency disturbance information at different nodes in the power grid;

[0093] Figure 5 It is the frequency strength index for power grid nodes proposed by the present invention;

[0094] Figure 6 It is a quantitative indicator of voltage disturbance information at different nodes of the power grid;

[0095] Figure 7 It is the voltage strength index for power grid nodes proposed by the present invention. DETAILED DESCRIPTION

[0096] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments:

[0097] The present invention discloses a quantification method for the frequency strength and voltage strength of power grid nodes, including: calculating the inertia of each inertia source of the power grid node, and constructing the power grid node frequency strength quantification method through disturbance information (frequency deviation, frequency change rate) and inertia; constructing an operating short-circuit ratio, and constructing the power grid node voltage strength quantification method through disturbance information (voltage deviation, voltage change rate) and the operating short-circuit ratio.

[0098] The topological structure of the example is as follows Figure 1 As shown in the figure, the example system is a 3-region 5-terminal system. In this test system, there are two wind farms, each of which contains 25 wind turbines arranged in a matrix. The wind farm is connected to the onshore power grid with 10 machines (G1~G10) and 39 nodes through the VSC-MTDC system. Three land converter stations (converter station 1~converter station 3) are connected to the onshore power grid nodes 3, 6 and 23. Two offshore converter stations (converter station 4, converter station 5) integrate two wind farms. The rated frequency and voltage of the onshore power grid are 50Hz and 220kV, respectively.

[0099] like Figure 3 As shown in Figure 1, in order to analyze the influence of the DC voltage droop control parameters of the converter station 1 on the system stability, the control platform is linearized using the model linearization toolbox in MATLAB / Simulink to analyze the changes in its eigenvalues. According to the small signal analysis, the input and output points are selected to construct the zero-pole diagram of different control parameters. The root locus trend of the eigenvalues ​​in the zero-pole diagram under different control parameters is shown in Figure 1. Figure 3 As shown. It can be seen that as the DC voltage droop control parameter of converter station 1 changes between 5 and 25, the damping ratio (DR) of most critical eigenvalues ​​in its zero-pole diagram is still greater than 0.06. This proves that when the DC voltage droop control parameter of VSC1 changes, the system has strong robustness.

[0100] The technical solution of the present invention is verified by a specific example below. Figure 1 is the example structure, and according to Figure 2 The constructed wind turbine communication network, when a short circuit fault occurs in the land AC system at 10s and is eliminated 625ms after the fault, causes the frequency and voltage of the land AC system to drop. At this time, the frequency and voltage strength of the land power grid will change accordingly. In order to highlight the accuracy of the frequency and voltage strength of the power grid quantified by the present invention, this paper compares the frequency and voltage disturbance information with the frequency and voltage strength quantified by the present invention. The simulation results are as follows: Figures 4 to 7 shown.

[0101] This paper conducts a quantitative analysis of the converter station nodes in the land power grid. The frequency disturbance information after the disturbance is as follows: Figure 4 As shown, it can be seen that since the grid nodes are in the same network, the frequency strength of the system grid cannot be fully quantified only by the disturbance information. The frequency strength at different nodes is basically the same. It can still be seen that since the fault point is set at grid node 1, its strength quantified by the disturbance information is the lowest, followed by grid node 3, and the highest frequency strength is grid node 4.

[0102] Using the grid node frequency strength quantification method of the present invention, in the case Figure 5 As shown, this paper takes into account the disturbance information and node inertia of the power grid nodes. The disturbance generated at 10s is reflected in the voltage strength of each power grid node. It can be seen that since the distance between the AC power grid node and the fault point is: power grid node 1 < power grid node 2 < power grid node 3, and for the DC side, power grid node 1 < power grid node 3 < power grid node 2, therefore, the method proposed in the present invention makes the power grid node frequency strength: power grid node 1 < power grid node 2 < power grid node 3. If only the disturbance information is used to quantify the frequency strength, it lacks consideration of the impact of the HVDC side and cannot fully reflect the frequency strength of the power grid node.

[0103] The voltage intensity is constructed by using the grid voltage disturbance information (voltage deviation, voltage change rate) Figure 6 As shown, it can be seen that when the grid voltage exceeds the dead zone range, that is, [0.9, 1.1], the grid strength constructed by the disturbance information can change. From the simulation results, it can be seen that since grid node 2 is the farthest from the fault point, its voltage strength is the highest, while grid node 1 is the closest and its voltage strength is the lowest.

[0104] Using the grid node voltage intensity quantification method described in the present invention, such as Figure 7As shown in the figure, it can be seen that when the fault is restored, there is still a certain volatility. This is because the removal of the grid voltage fault will cause a certain oscillation in the system. Compared with the method that only considers the disturbance information, the voltage intensity quantification method proposed in this paper comprehensively considers the influence of the short-circuit capacity on the AC side and the power on the DC side through the operation of the short-circuit ratio index, and combines the disturbance information. The quantified voltage intensity is more accurate.

[0105] The above description is only a preferred embodiment of the present invention and does not constitute any other form of limitation to the present invention. Any modification or equivalent change made based on the technical essence of the present invention still falls within the scope of protection required by the present invention.

Claims

1. A method for quantifying frequency intensity and voltage intensity of power grid nodes, characterized in that: The method comprises the following steps: 1) An inertia assessment strategy that considers the equivalent inertia of synchronous machine, equivalent inertia of new energy converter station and equivalent inertia of load is used to analyze the inertia support strength of system node level; 2) A grid node-level frequency strength quantification strategy based on grid node frequency deviation, frequency change rate and node inertia strength is proposed; 3) A quantitative method for the voltage support strength of system nodes considering the impact of node short-circuit capacity caused by voltage drop; 4) A grid node-level voltage intensity quantification strategy considering system node voltage deviation, voltage change rate and operating short-circuit ratio is proposed.

2. The method for quantifying the frequency intensity and voltage intensity of power grid nodes according to claim 1, characterized in that: Step 1) is as follows: By quantifying the inertia strength of each inertia source at the grid node, as shown below:

1. Equivalent inertia of synchronous generator; The equivalent inertia of a synchronous generator to a certain node is affected by the inertia of the synchronous generator and the electrical distance between the synchronous generator and the grid node: In the formula, B ik is the susceptance between the i-th node and the k-th SG in the power grid; H k is the inertia of the kth synchronous generator; r ik is the correlation coefficient between the i-th node of the power grid and the k-th synchronous generator of the power grid; i∈k th is the corresponding element of the kth row of the voltage correlation matrix between the grid node and the synchronous generator; 2. Equivalent inertia of new energy converter station; Since large-scale offshore wind farms are usually connected to the land power grid through converter stations, and the converter stations use the frequency disturbance information of the land power grid for inertia support, the relationship between frequency and active power and its inertia support can be expressed by the rotor motion equation: In the formula, H GSVSC,i represents the inertia time constant of the i-th GSVSC, P m and P e Represent mechanical power and electromagnetic power respectively. ΔP1 represents the virtual kinetic energy absorbed or released by the inertia of the new energy converter station during the frequency support process; The new energy converter station uses the electric energy stored in the HVDC capacitor to achieve inertia response. Therefore, in the quantitative analysis of the equivalent inertia of the new energy converter station, the response of the DC capacitor needs to be considered. The relationship between the DC voltage and the active power is expressed by the capacitor dynamic equation as follows: Where P in and P out Respectively represent the input and output power of the new energy converter station, C dc,i Indicates the DC capacitor capacity, S GSVSC,i represents the rated capacity of the i-th renewable energy converter station in the power grid, Ni represents the number of DC capacitors in the i-th renewable energy converter station in the power grid, and ΔP2 represents the dynamic electrostatic power stored or released by the HVDC capacitor; Based on (2)-(3), the equivalent inertia of the new energy converter station at the grid node can be expressed as: In the formula, H GSVSC,i represents the GSVSC equivalent inertia at the i-th node of the power grid; 3. Load equivalent inertia; When the system frequency / voltage changes, the system's active power demand will also change, resulting in abnormal active power, which will affect the system inertia. The equivalent inertia of the load to a certain node will be affected by the load's static characteristics and the real-time frequency and voltage. The load's static characteristics are expressed by the following polynomial: Where a, b, and c represent the proportions of the system's constant impedance, constant current, and constant power loads, respectively; The load active power fluctuation is expressed by solving the total differential of frequency and voltage in equation (5), and we get: At the same time, based on the rotor motion equation, the load active power fluctuation can also be expressed as: According to (6) and (7), the load equivalent inertia H load,i It can be expressed as: Based on formulas (1)-(8), the grid node-level inertia H i It can be expressed as: Where FSQ is the frequency strength value of node i, H SG,i is the equivalent inertia of SG after electrical distance conversion, H GSVSC,i is the equivalent inertia of GSVSC, H load,i is the equivalent inertia of the load, S SG,i is the rated capacity of SGs corresponding to HSG, S GSVSC,i is the rated capacity of the GSVSC station corresponding to node i, S GSVSC,i is the rated capacity of the GSVSC station corresponding to node i, S load,i is the H of node i load,i The corresponding load rated capacity, z SG,i is a binary variable of SG status, where 0 is off and 1 is on. GSVSC,i is a binary variable representing the GSVSC state of node i, where 0 is off and 1 is on. load,i is a binary variable representing the local load status of node i, where 0 means no local load and 1 means yes local load.

3. The method for quantifying frequency intensity and voltage intensity of power grid nodes according to claim 1, characterized in that: Step 2) is as follows: The frequency strength of the power grid node represents the ability of the system to maintain frequency stability after disturbance, which includes frequency deviation, frequency change rate and inertia index. The construction of frequency deviation and frequency change rate index needs to consider the national standard. The formula is as follows:

1. Frequency deviation index; The frequency deviation index is constructed according to the frequency deviation requirements, as shown in the following formula: In the formula, f i 、f H 、f l 、f H,max and f L,min They represent the frequency value of system node i, the upper limit of the frequency dead zone, the lower limit of the frequency dead zone, the maximum allowable frequency value of the system, and the minimum allowable frequency value of the system respectively; 2. Frequency change rate indicator; The frequency change rate represents the rate of change of the system at the beginning when a frequency event occurs. The smaller the value, the slower the frequency changes during disturbances and the stronger the frequency stability. Based on the system node frequency, the frequency change rate indicator is constructed as follows: Where, RoCoF max Indicates the maximum frequency change rate allowed by the system; 3. Quantification method of power grid node frequency intensity: Taking into account the frequency deviation index, frequency change rate index, and inertia index of the system power grid nodes, the frequency strength of the system power grid nodes is obtained as shown below:

4. The method for quantifying frequency intensity and voltage intensity of power grid nodes according to claim 1, characterized in that: Step 3) is as follows: The short-circuit ratio is an important indicator to characterize the voltage strength of the system, which is defined as the ratio of the short-circuit capacity to the rated DC power: SCR=S scc,i / P N,i (13) In the formula, S scc,i is the short-circuit capacity of node i, P N,i is the DC power; Short-circuit capacity is one of the important indicators of SCR. Compared with the existing short-circuit capacity calculated based on rated voltage, the operating short-circuit capacity is calculated by considering the change of voltage difference between the fault node and the adjacent fault nodes: Considering the joint influence of multiple DC branches of MTDC, another important factor affecting SCR is the DC power P N,i , whose expression is: Where P dN,i and P dN,j are the i-th and j-th rated DC powers respectively; Based on the definition of multi-input short-circuit ratio, ΔU at different nodes can be dc The ratio is replaced by the ratio of mutual impedance to self-impedance, expressed as: According to (14)-(16), considering that the real-time operating state of the system is constantly changing, its operating short-circuit ratio OSCR can be expressed as:

5. The method for quantifying frequency intensity and voltage intensity of power grid nodes according to claim 1, characterized in that: Step 4) is as follows: The grid node-level voltage intensity quantification strategy includes voltage deviation index and voltage change rate index; 1. Voltage deviation index; The voltage deviation indicator is constructed based on the voltage deviation limit as follows: Where U i , U h , U l , U max and U min They represent the system node voltage, the upper limit of the voltage dead zone, the lower limit of the voltage dead zone, the maximum value of the system voltage and the minimum value of the system voltage respectively; 2. Voltage change rate indicator; The voltage change rate represents the rate of change of the system at the beginning when a voltage event occurs. The smaller the value, the better the stability of the system and the slower the voltage changes during disturbances. Based on the system node voltage, the voltage change rate indicator is constructed as follows: In the formula, RoCoV max Indicates the maximum rate of voltage change allowed by the system; Taking into account the voltage deviation index, voltage change rate index, and short-circuit ratio index of the system grid nodes, the voltage strength of the system grid nodes is obtained as shown below:

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