Active unloading optimal section search method considering energy storage and frequency stability constraints

Abstract

The present invention discloses a method for searching for the optimal section of active unloading considering energy storage and frequency stability constraints, and relates to the technical field of power system stability analysis. The key points of the technical solution are: inputting relevant operating data obtained when the power system is unstable into the MILP model, and solving to obtain the initial active unloading section set; constraining the charging and discharging model according to the capacity of the energy storage system in the island generated by active unloading to obtain the charging and discharging strategy, and constraining the initial active unloading section set according to the charging and discharging strategy to obtain the intermediate unloading section set; filtering the intermediate unloading section set according to the frequency change rate limit in the island generated by active unloading and the unloading model to obtain the optimal section of active unloading. The present invention simultaneously considers the frequency stability constraint and the regulating effect of the energy storage system on the unbalanced power of the subsystem, and solves to obtain the optimal section of active unloading that meets a series of constraints.

Description

Technical Field

[0001] The present invention relates to the technical field of power system stability analysis, and more specifically, to an active unloading optimal section search method considering energy storage and frequency stability constraints. Background Art

[0002] Active disconnection control of power systems is the last line of defense to prevent large-scale power outages. As an important part of the corrective control strategy, it can provide remedial measures to prevent the system from collapsing after a serious fault. Without appropriate control measures, the system may collapse after a serious disturbance and eventually cause a catastrophic accident. Active disconnection means that when the integrity of the system is difficult to maintain, the appropriate disconnection section is determined to decompose the entire system into several islands that can operate independently and stably to prevent the expansion and spread of accidents.

[0003] Determining a feasible active unloading strategy is a very complex process. It must meet the stable operating conditions and various dynamic constraints of all unloading islands to ensure that all unloading islands can operate stably. The key task is to determine the appropriate unloading section. Since the existing research results basically aim to minimize unbalanced power, the impact of the current impact on the unloading section on the subsystem after unloading is not comprehensively considered. In order to increase the frequency support capacity of the system during the unloading process and reduce the economic losses of cutting the machine and load, it is also necessary to consider the charging and discharging capacity of the energy storage system during the unloading process.

[0004] Therefore, how to study and design an active unbundling optimal section search method that takes into account energy storage and frequency stability constraints and can overcome the above-mentioned defects is a problem that we urgently need to solve. Summary of the invention

[0005] In order to address the deficiencies in the prior art, the purpose of the present invention is to provide a method for searching for the optimal section of active splitting that takes into account energy storage and frequency stability constraints. According to the set optimization objectives, the frequency stability constraints and the regulating effect of the energy storage system on the unbalanced power of the subsystem are considered at the same time, and the optimal section of active splitting that meets a series of constraints is solved.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions: an active unloading optimal section search method considering energy storage and frequency stability constraints, comprising the following steps:

[0007] The MILP model is established according to the objective function and basic constraints of the active disconnection of the power system, and the relevant operating data obtained when the power system is unstable is input into the MILP model to solve and obtain the initial active disconnection section set;

[0008] Considering the energy storage system in the isolated island, a charging and discharging model is established, and the charging and discharging strategy is obtained after constraining the charging and discharging model according to the capacity of the energy storage system in the isolated island caused by active decoupling, and the intermediate decoupling section set is obtained after constraining and screening the initial active decoupling section set according to the charging and discharging strategy;

[0009] A splitting model is established considering the frequency stability constraint in the island. According to the frequency change rate limit in the island caused by active splitting and the splitting model, the intermediate splitting section set is filtered to obtain the optimal section for active splitting.

[0010] Furthermore, the objective function is constructed by considering two factors: the flow impact of the disconnected section and the load shedding size.

[0011] Furthermore, the expression of the objective function is specifically:

[0012]

[0013] Where L represents the set of lines l; V represents the set of nodes i; x l Indicates the disconnection status of the line, x l =0, indicating that the branch is disconnected, otherwise, x l =1, indicating that the branch is connected; P l 0 Indicates the active power on line l when the system is running stably; ΔP Di represents the load shedding amount of node i; λ1 and λ2 represent the penalty coefficients for disconnecting the line and shedding the load, respectively.

[0014] Furthermore, the basic constraints include a basic operation model of the power system, a generator synchronization grouping model, and a system connectivity model after the generator synchronization grouping.

[0015] Furthermore, the basic operation model of the power system includes AC power flow constraints, node voltage amplitude and phase angle upper and lower limit constraints, power source active and reactive output upper and lower limit constraints, node balance constraints and wind farm output constraints.

[0016] Furthermore, the expression of the generator coherence clustering model is specifically:

[0017]

[0018]

[0019]

[0020] x i,k ∈{0,1},x j,k ∈{0,1},x l ∈{0,1}

[0021] Among them, i and j represent the first and last nodes of branch l respectively; x i,k 、x j,k Represent the state variables of nodes i and j respectively. If x i,k =1,x j,k =1, then the corresponding node x i and x j belongs to the point in the island k, otherwise, x i,k =0,x j,k = 0, then the corresponding node x i and x j Does not belong to an isolated island k; x l Indicates the disconnection status of the line; l,k Represents auxiliary variables, used to linearize two 0-1 variables x i,k and x j,k Multiply.

[0022] Specifically, according to the generator grouping situation after the fault occurs, a generator synchronization clustering model is established, which limits each node to belong to only one island, and converts the nonlinear product term into a linear form to show the lines that need to be disconnected due to decoupling.

[0023] Furthermore, the expression of the system connectivity model after the generator coherence grouping is specifically:

[0024]

[0025] -Mx l ≤P l ′≤Mx l

[0026] P′ ref ≥1

[0027] Where M represents a constant value; P l ′ represents the active power of the virtual branch; L Ei , L Oi Respectively represent the receiving end bus and the sending end bus of the transmission line; P′ ref Indicates the active power generated by the virtual generator, and its value is greater than or equal to 1.

[0028] Specifically, after the system is actively decoupled, the formed islands are internally connected, but not connected to each other, and each node can only belong to one island. The model can be determined and constructed by the single commodity flow method. The specific method is: the connectivity constraint can be guaranteed by constructing a virtual active power balance constraint. If the virtual active power of each island is balanced, it means that there is a path from the generator node to all other nodes in the corresponding island, which can ensure the internal connectivity of the island.

[0029] Furthermore, the expression of the charge-discharge model is specifically:

[0030]

[0031]

[0032] ΔP Gmin ≤ΔP Gi ≤ΔP Gmax ,0≤ΔP Di ≤P Dmax

[0033] Where V represents the set of nodes i; P Gi , Q Gi Respectively represent the active and reactive output of the generator; P Wi Indicates the dispatching output of the wind farm; Respectively represent the discharge and charging power of energy storage; P l , Q l Respectively represent the active and reactive power of branch l; P Di , Q Di Respectively represent the active and reactive loads of load node i; is the power factor of load node i; ΔP Gi is the amount of machine cutting or the amount of generator change; ΔP Di represents the load shedding amount of node i; L Ei , L Oi Respectively represent the receiving end bus and the sending end bus of the transmission line; ΔP Gmin Indicates the minimum value of the amount of machine cutting or generator change; ΔP Gmax Indicates the maximum value of the machine cutting amount or the generator change amount; P Dmax Indicates the maximum load that can be removed.

[0034] Specifically, according to the capacity of the energy storage system connected to the power grid, the unbalanced power generated by the island after decoupling is appropriately reduced to improve the frequency stability of the island. The constraints include the node balance conservation law of the power system with energy flow and the limitation of the size of the shedding load.

[0035] Furthermore, the expression of the decoupling model is specifically:

[0036]

[0037]

[0038]

[0039]

[0040]

[0041]

[0042]

[0043]

[0044] Where K represents the set of islands generated by decomposition; It represents the surplus of power generation over load in island k; It indicates the shortfall of power generation in island k below the load; Respectively represent the maximum power of energy storage discharge and charging; RoCoF max represents the maximum frequency change rate of the system; M represents a constant value; δ is a binary variable; H k represents the equivalent inertia of island k; H g represents the time constant of the generator; f0 represents the rated frequency.

[0045] Specifically, according to the limit of the system frequency change rate, the unbalanced power generated by the island after decoupling is kept within a safety margin to prevent unnecessary minimum frequency or overshoot frequency in each island. In order to express the relationship between the total power generation and the total load of the island when decoupling, two variables are introduced and The surplus amount of power generation exceeding the load or the deficit amount of power generation less than the load are both non-negative numbers and cannot exist at the same time; the maximum frequency change rate limits the actual unbalanced power size; the inertia of the k island is related to the online units of the island.

[0046] Furthermore, the overall optimization model expression composed of the MILP model, the charging and discharging model and the splitting model is specifically as follows:

[0047]

[0048] Among them, vector x represents the binary variables in the model; vector y represents the continuous variables in the model; a and c represent the coefficient vectors of the variables to be solved in the objective function; A, B, and C represent matrices in abstract form, and g and f represent vectors in abstract form, representing the coefficients of related constraints.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] The present invention proposes a method for searching the optimal section of active unloading considering energy storage and frequency stability constraints. Aiming at the problem of incomplete consideration of system frequency stability after unloading in existing studies, a model for searching the optimal section of active unloading of the power system is constructed. That is, an objective function considering two factors, namely, flow impact and load shedding size of the unloading section, is established. At the same time, constraints such as energy storage and frequency stability are taken into account to ensure the safe and stable operation of the system after unloading. Finally, after integrating and optimizing the objective function, charging and discharging model and unloading model, the optimal section of active unloading can be searched reliably. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0052] Figure 1 is a flow chart in an embodiment of the present invention;

[0053] Figure 2 To verify the modified New England 39-node system structure diagram adopted by the present invention;

[0054] Figure 3 1 is a comparison diagram of the transient process of the New England 39-node system decoupling after modification in Example 1 of the present invention, (a) is the generator power angle curve before decoupling, (b) is the bus voltage curve before decoupling, (c) is the generator frequency curve before decoupling, (d) is the generator power angle curve after decoupling, (e) is the bus voltage curve after decoupling, and (f) is the generator frequency curve after decoupling;

[0055] Figure 4 1 is a comparison diagram of the transient process of the New England 39-bus system decoupling after modification in Example 2 of the present invention, (a) is the generator power angle curve after decoupling, (b) is the bus voltage curve after decoupling, and (c) is the generator frequency curve after decoupling;

[0056] Figure 5 1 is a comparison diagram of the transient process of the New England 39-bus system decoupling after modification in Example 3 of the present invention, (a) is the generator power angle curve after decoupling, (b) is the bus voltage curve after decoupling, and (c) is the generator frequency curve after decoupling;

[0057] Figure 6 This is a topology diagram after system decoupling in Example 3 of the present invention. DETAILED DESCRIPTION

[0058] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0059] Embodiment 1: Active unloading optimal section search method considering energy storage and frequency stability constraints, such as Figure 1 As shown, the following steps are included:

[0060] S1: Establish a MILP model based on the objective function and basic constraints of active disconnection of the power system, and input the relevant operating data obtained when the power system is unstable into the MILP model to solve and obtain the initial active disconnection section set;

[0061] S2: Considering the energy storage system in the isolated island, a charging and discharging model is established, and the charging and discharging model is constrained according to the capacity of the energy storage system in the isolated island caused by active decoupling to obtain a charging and discharging strategy, and the initial active decoupling section set is constrained and screened according to the charging and discharging strategy to obtain an intermediate decoupling section set;

[0062] S3: Considering the frequency stability constraint in the island, a splitting model is established, and according to the frequency change rate limit in the island caused by active splitting and the splitting model, the intermediate splitting section set is filtered to obtain the optimal section for active splitting.

[0063] The objective function is constructed by considering two factors: the flow impact of the decoupling section and the load shedding size. The specific expression of the objective function is:

[0064]

[0065] Where L represents the set of lines l; V represents the set of nodes i; x l Indicates the disconnection status of the line, x l =0, indicating that the branch is disconnected, otherwise, x l =1, indicating that the branch is connected; P l 0 Indicates the active power on line l when the system is running stably; ΔP Di represents the load shedding amount of node i; λ1 and λ2 represent the penalty coefficients for disconnecting the line and shedding the load, respectively.

[0066] The basic constraints include the basic operation model of the power system, the generator synchronization grouping model and the system connectivity model after the generator synchronization grouping.

[0067] The basic operation model of the power system includes AC power flow constraints, node voltage amplitude and phase angle upper and lower limit constraints, power source active and reactive output upper and lower limit constraints, node balance constraints and wind farm output constraints.

[0068] 1. The specific AC power flow constraints are:

[0069]

[0070] Among them, g l, b l They represent the susceptance and conductance of branch l respectively; v i 、v j Represent the voltage amplitude of nodes i and j respectively; θ i ,θ j Represent the phase angles of nodes i and j respectively.

[0071] 2. The upper and lower limits of node voltage amplitude and phase angle are as follows:

[0072]

[0073]

[0074] in, Respectively represent the minimum and maximum values ​​of the voltage amplitude at node i; Respectively represent the minimum and maximum values ​​of the voltage phase angle of node i;

[0075] 3. Upper and lower limits of active and reactive power output of power supply:

[0076]

[0077] in, Respectively represent the minimum and maximum active output of the generator; Respectively represent the minimum and maximum reactive output of the generator.

[0078] 4. Node balance constraints:

[0079]

[0080]

[0081] 5. Wind farm output constraints are as follows:

[0082]

[0083] in, Represents the predicted output value of the wind farm W.

[0084] The expression of the generator coherence clustering model is as follows:

[0085]

[0086]

[0087]

[0088] x i,k ∈{0,1},x j,k ∈{0,1},x l ∈{0,1}

[0089] Among them, i and j represent the first and last nodes of branch l respectively; x i,k 、x j,k Represent the state variables of nodes i and j respectively. If x i,k =1,x j,k =1, then the corresponding node x i and x j belongs to the point in the island k, otherwise, x i,k =0,x j,k = 0, then the corresponding node x i and x j Does not belong to an isolated island k; x l Indicates the disconnection status of the line; l,k Represents auxiliary variables, used to linearize two 0-1 variables x i,k and x j,k Multiply.

[0090] Specifically, according to the generator grouping situation after the fault occurs, a generator synchronization clustering model is established, which limits each node to belong to only one island, and converts the nonlinear product term into a linear form to show the lines that need to be disconnected due to decoupling.

[0091] The expression of the system connectivity model after the generator synchronization grouping is as follows:

[0092]

[0093] -Mx l ≤P l ′≤Mx l

[0094] P′ ref ≥1

[0095] Where M represents a constant value; P l ′ represents the active power of the virtual branch; L Ei , L Oi Respectively represent the receiving end bus and the sending end bus of the transmission line; P′ ref Indicates the active power generated by the virtual generator, and its value is greater than or equal to 1.

[0096] Specifically, after the system is actively decoupled, the formed islands are internally connected, but not connected to each other, and each node can only belong to one island. The model can be determined and constructed by the single commodity flow method. The specific method is: the connectivity constraint can be guaranteed by constructing a virtual active power balance constraint. If the virtual active power of each island is balanced, it means that there is a path from the generator node to all other nodes in the corresponding island, which can ensure the internal connectivity of the island.

[0097] In step S2, the expression of the charge and discharge model is specifically:

[0098]

[0099]

[0100] ΔP Gmin ≤ΔP Gi ≤ΔP Gmax ,0≤ΔP Di ≤P Dmax

[0101] Where V represents the set of nodes i; P Gi , Q Gi Respectively represent the active and reactive output of the generator; P Wi represents the dispatching output of wind farm; P i dis , P i ch Respectively represent the discharge and charging power of energy storage; P l , Q l Respectively represent the active and reactive power of branch l; P Di , Q Di Respectively represent the active and reactive loads of load node i; is the power factor of load node i; ΔP Gi is the amount of machine cutting or the amount of generator change; ΔP Di represents the load shedding amount of node i; L Ei , L Oi Respectively represent the receiving end bus and the sending end bus of the transmission line; ΔP Gmin Indicates the minimum value of the amount of machine cutting or generator change; ΔP Gmax Indicates the maximum value of the machine cutting amount or the generator change amount; P Dmax Indicates the maximum load that can be removed.

[0102] The charging and discharging size of the energy storage device is limited to ensure that the energy storage device cannot be charged and discharged at the same time.

[0103]

[0104]

[0105] In the formula, ψ i is a binary variable. If node i has an energy storage device, then ψ i =1, otherwise 0. Here, we only consider installing energy storage devices on four nodes with large active loads, namely node 4, node 8, node 20 and node 39; Represent the state variables of charging and discharging respectively; Pi ch,max , P i dis,max The maximum charge and discharge power is set to 100MW.

[0106] Taking into account the capacity limit and initial energy level of each energy storage device, the charging and discharging of each energy storage device are restricted as follows:

[0107]

[0108] in, are the maximum and minimum capacities of energy storage device i, respectively. The maximum capacity is set to 100MWh. is the initial energy storage at i, set to 25% of the maximum capacity; are the charging and discharging efficiency of the energy storage device, respectively, and are set to 0.95.

[0109] Specifically, according to the capacity of the energy storage system connected to the power grid, the unbalanced power generated by the island after decoupling is appropriately reduced to improve the frequency stability of the island. The constraints include the node balance conservation law of the power system with energy flow and the limitation of the size of the shedding load.

[0110] The specific expression of the decoupling model is:

[0111]

[0112]

[0113]

[0114]

[0115]

[0116]

[0117]

[0118]

[0119] Where K represents the set of islands generated by decomposition; It represents the surplus of power generation over load in island k; P represents the shortfall of power generation in island k below load; i dis,max , P i ch,max Respectively represent the maximum power of energy storage discharge and charging; RoCoF maxrepresents the maximum frequency change rate of the system; M represents a constant value; δ is a binary variable; H k represents the equivalent inertia of island k; H g represents the time constant of the generator; f0 represents the rated frequency.

[0120] Specifically, according to the limit of the system frequency change rate, the unbalanced power generated by the island after decoupling is kept within a safety margin to prevent unnecessary minimum frequency or overshoot frequency in each island. In order to express the relationship between the total power generation and the total load of the island when decoupling, two variables are introduced and The surplus amount of power generation exceeding the load or the deficit amount of power generation less than the load are both non-negative numbers and cannot exist at the same time; the maximum frequency change rate limits the actual unbalanced power size; the inertia of the k island is related to the online units of the island.

[0121] The overall optimization model consisting of the MILP model, the charging and discharging model and the splitting model is expressed as follows:

[0122]

[0123] Among them, vector x represents the binary variables in the model; vector y represents the continuous variables in the model; a and c represent the coefficient vectors of the variables to be solved in the objective function; A, B, and C represent matrices in abstract form, and g and f represent vectors in abstract form, representing the coefficients of related constraints.

[0124] Embodiment 2:

[0125] Example 1: New England 39-node system as an example

[0126] The modified New England 39-node system includes 10 generators and 46 branches. Wind farms with a capacity of 150MW are connected at nodes 5 and 19 respectively. The topology of the system is as follows: Figure 2 As shown in the figure, a three-phase grounding fault occurs in branch 6-7 at 0s. After 0.3s, the fault is cleared. After another 0.05s, branch 21-22 is disconnected due to overload. Figure 3 From the generator power angle curve shown in (a), we can see that generators G1, G2, G8, and G10 are a synchronous group, and generators G3, G4, G5, G6, G7, and G9 are another synchronous group. Figure 3 (b) and Figure 3 From (c), we can see that if the system is not decoupled, the out-of-step oscillation between the two generator sets will cause a significant increase in the frequency of the G3, G4, G5, G6, G7, and G9 generators, a significant decrease in the system bus voltage, and eventually the entire system may collapse.

[0127] The decoupling section, section flow impact, unbalanced power and load shedding obtained by the S1 method are shown in Table 1. The generator power angle, bus voltage and generator frequency curves obtained after the system is decoupling are shown in Table 1. Figure 3 As shown in (d), (e), and (f), it can be seen that after the system is actively decoupled, the power angle of the generator and the system bus voltage return to the stable range, and the generator frequency of G3, G4, G5, G6, G7, and G9, which have lost stability, returns to about 1.02pu.

[0128] Embodiment 3:

[0129] Example 2, modified New England 39-node system.

[0130] The relevant data results obtained by solving the S1-S2 method are shown in Table 1. The generator power angle, bus voltage, and generator frequency curves obtained after the system is decoupled and controlled are shown in Table 1. Figure 4 It can be seen that after the system is actively decoupled, the power angle of the generator and the system bus voltage return to the stable range, the generator frequency of G3, G4, G5, G6, G7, and G9 that have lost stability returns to about 1.01pu, and the load shedding is less than that in Example 1.

[0131] Table 1 Comparison of the results of the decomposition of Example 1-2

[0132]

[0133] Implementation Example 3: Modified New England 39-node system.

[0134] According to S1-S3, the method used in the present invention solves the relevant data results as shown in Table 2. The generator power angle, bus voltage, and generator frequency curves obtained after the system is decoupled and controlled are shown in Table 2. Figure 5 As shown in (a)-(c) of FIG. 1 , the topological diagram of the system after decomposition obtained by the method proposed in the present invention is as follows: Figure 6 It can be seen that after the system is actively decoupled, the power angle of the generator and the system bus voltage return to the stable range, and the generator frequencies of G3, G4, G5, G6, G7, and G9 that have lost stability return to values ​​closer to the rated frequency.

[0135] Table 2 Solution results of Example 3

[0136]

[0137] By analyzing the above example data and simulation, it can be obtained that after the system is decoupled according to the decoupling section obtained by the present invention, the obtained generator power angle, bus voltage and generator frequency can all return to a stable level. Compared with Example 1 and Example 2, the method described in the present invention not only minimizes the load shedding amount, but also the final frequency is closer to the rated frequency. In summary, the present invention simultaneously considers the two factors of the flow impact and the load shedding size of the decoupling section, and establishes constraints such as energy storage and frequency stability to ensure the safe and stable operation of the system after decoupling.

[0138] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An active unloading optimal section search method considering energy storage and frequency stability constraints is characterized by: The following steps are involved: The MILP model is established according to the objective function and basic constraints of the active disconnection of the power system, and the relevant operating data obtained when the power system is unstable is input into the MILP model to solve and obtain the initial active disconnection section set; Considering the energy storage system in the isolated island, a charging and discharging model is established, and the charging and discharging strategy is obtained after constraining the charging and discharging model according to the capacity of the energy storage system in the isolated island caused by active decoupling, and the intermediate decoupling section set is obtained after constraining and screening the initial active decoupling section set according to the charging and discharging strategy; Considering the frequency stability constraint in the island, a splitting model is established, and according to the frequency change rate limit in the island caused by active splitting and the splitting model, the intermediate splitting section set is filtered to obtain the optimal section for active splitting. The objective function is constructed by considering two factors: the flow impact of the disconnected section and the load shedding size; The expression of the objective function is specifically: Where L represents the set of branches l; V represents the set of nodes i; x l Indicates the disconnection state of branch l, x l =0, indicating that the branch is disconnected, otherwise, x l =1, indicating that the branch is connected; P l 0 Indicates the active power on branch l when the system is running stably; ΔP Di represents the load shedding amount of node i; λ1 and λ2 represent the penalty coefficients for opening the branch and shedding the load, respectively.

2. The method for searching the optimal cross section of active unloading considering energy storage and frequency stability constraints according to claim 1 is characterized in that: The basic constraints include a basic operation model of the power system, a generator synchronization grouping model, and a system connectivity model after the generator synchronization grouping.

3. The method for searching the optimal cross section of active unloading considering energy storage and frequency stability constraints according to claim 2 is characterized in that: The basic operation model of the power system includes AC power flow constraints, node voltage amplitude and phase angle upper and lower limit constraints, power source active and reactive output upper and lower limit constraints, node balance constraints and wind farm output constraints.

4. The method for searching the optimal cross-section of active unloading considering energy storage and frequency stability constraints according to claim 2 is characterized in that: The expression of the generator coherence clustering model is specifically: x i,k ∈{0,1},x j,k ∈{0,1},x l ∈{0,1} Among them, i and j represent the first and last nodes of branch l respectively; x i,k 、x j,k Represent the state variables of nodes i and j respectively. If x i,k =1,x j,k =1, then the corresponding node x i and x j belongs to the point in the island k, otherwise, x i,k =0,x j,k = 0, then the corresponding node x i and x j Does not belong to an isolated island k; x l Indicates the disconnection status of branch l; z l,k Represents auxiliary variables, used to linearize two 0-1 variables x i,k and x j,k Multiply.

5. The method for searching the optimal cross section of active unloading considering energy storage and frequency stability constraints according to claim 2 is characterized in that: The expression of the system connectivity model after the generator coherence grouping is specifically: -Mx l ≤P l ′≤Mx l P r ′ ef ≥1 Where M represents a constant value; P l ′ represents the active power of the virtual branch; L Ei , L Oi They represent the receiving end bus and the sending end bus of the transmission line respectively; P r ' ef Indicates the active power generated by the virtual generator, and its value is greater than or equal to 1; x l Indicates the open state of branch l.

6. The method for searching the optimal cross-section of active unloading considering energy storage and frequency stability constraints according to claim 1 is characterized in that: The expression of the charge and discharge model is specifically: ΔP Gmin ≤ΔP Gi ≤ΔP Gmax ,0≤ΔP Di ≤P Dmax Where V represents the set of nodes i; P Gi , Q Gi Respectively represent the active and reactive output of the generator; P Wi represents the dispatching output of wind farm; P i dis , P i ch Respectively represent the discharge and charging power of energy storage; P l , Q l Respectively represent the active and reactive power of branch l; P Di , Q Di Respectively represent the active and reactive loads of load node i; is the power factor of load node i; ΔP Gi is the amount of machine cutting or the amount of generator change; ΔP Di represents the load shedding amount of node i; L Ei , L Oi Respectively represent the receiving end bus and the sending end bus of the transmission line; ΔP Gmin Indicates the minimum value of the amount of machine cutting or generator change; ΔP Gmax Indicates the maximum value of the machine cutting amount or the generator change amount; P Dmax Indicates the maximum load that can be removed.

7. The method for searching for the optimal cross-section of active unloading considering energy storage and frequency stability constraints according to claim 6 is characterized in that: The expression of the decoupling model is specifically: Where K represents the set of islands generated by decomposition; It represents the surplus of power generation over load in island k; P represents the shortfall of power generation in island k below load; i dis,max , P i ch,max Respectively represent the maximum power of energy storage discharge and charging; RoCoF max represents the maximum frequency change rate of the system; M represents a constant value; δ is a binary variable; H k represents the equivalent inertia of island k; H g represents the time constant of the generator; f0 represents the rated frequency.

8. The method for searching the optimal cross section of active unloading considering energy storage and frequency stability constraints according to claim 1 is characterized in that: The overall optimization model expression composed of the MILP model, the charging and discharging model and the splitting model is specifically as follows: Among them, vector x represents the binary variables in the model; vector y represents the continuous variables in the model; a and c represent the coefficient vectors of the variables to be solved in the objective function; A, B, and C represent matrices in abstract form, and g and f represent vectors in abstract form, representing the coefficients of related constraints.

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