Power distribution network flexible interconnection planning method considering network reconstruction

Abstract

The invention provides a power distribution network flexible interconnection planning method considering network reconstruction. The method comprises the following steps: (1) establishing an equivalent model of a flexible interconnection device; (2) a double-layer model considering both network operation economy and reliability is built, a planning layer carries out addressing and sizing on the flexible interconnection device for solving, an operation layer formulates a reconstruction scheme of the power distribution network and an operation strategy of the flexible interconnection device, and meanwhile, the network reliability in a fault state is evaluated; and (3) aiming at the problem that the proposed model is a large-scale mixed integer nonlinear programming problem, solving by adopting a mixed algorithm, solving the planning layer model by adopting an optimization algorithm, and solving the operation layer model by adopting second-order cone programming and utilizing a solver. According to the invention, through coordination of intra-region network reconstruction and inter-region flexible interconnection, good effects are achieved in the aspects of improving economical efficiency and reliability of network operation, reducing investment required by flexible interconnection and promoting consumption of new energy.

Description

Technical Field

[0001] The present invention relates to the technical field of power distribution network optimization, and in particular to a method for planning inter-regional flexible interconnection taking network reconstruction into account. Background Art

[0002] As global energy and environmental issues become more prominent, a large number of distributed generation (DG) are connected to the distribution network. However, with the increase in installed capacity, the impact on the distribution network is becoming increasingly significant: complex power flow, unpredictable DG output, and reduced voltage quality. Traditional distribution networks have limited means of regional power flow control, and the low flexibility of power grid operation has caused waste of resources and high costs. Therefore, flexible interconnected distribution networks have been developed. They use flexible power electronic devices to interconnect distribution networks, optimize network structure, and thus improve the controllability of distribution network power flows. New power electronic devices represented by smart soft switches (SOPs) and power flow controllers can achieve the change of power flow distribution through flexible interconnection under the conditions of existing network architecture.

[0003] From the perspective of flexible interconnection device (FID) operation strategy, current research does not cooperate with existing optimization methods when realizing flexible interconnection of the network, resulting in a large amount of power required to be transmitted by the flexible interconnection line. In the traditional distribution network, some interconnection switches are connected at fixed positions. The combination of interconnection switches and segment switches can change the topology of the network, and the network flow can be optimized without additional investment in equipment. When the distribution network realizes flexible interconnection, comprehensive consideration of network reconstruction can reduce the capacity of the required flexible interconnection equipment, thereby reducing the construction investment cost of the network, and the network flow can be further optimized. From the perspective of the optimal configuration of flexible interconnection equipment, the configuration of flexible interconnection equipment is currently mostly considered from the perspective of economy or new energy consumption rate. However, the flexible interconnection equipment is connected to realize the flexible interconnection of the distribution network, which makes the network have more power transmission paths to choose from. When a fault occurs, the load can be transferred under the condition of meeting the capacity limit. Few literatures consider network reliability when solving the problem of site selection and capacity determination of flexible interconnection equipment.

[0004] Therefore, the present invention proposes a distribution network flexible interconnection planning method taking into account network reconstruction, coordinates network reconstruction and FID flexible interconnection, comprehensively considers the economy and reliability of network operation, and solves the proposed model as a large-scale mixed integer nonlinear programming problem using a hybrid algorithm of optimization algorithm and second-order cone programming. Summary of the invention

[0005] In order to reasonably plan the flexible interconnection devices in the distribution network, the present invention proposes a distribution network flexible interconnection planning method taking into account network reconstruction, coordinating network reconstruction and flexible interconnection, and taking into account the economic efficiency and reliability of network operation. The technical solution adopted by the present invention to solve its technical problem is: to provide a new distribution network planning method taking into account the reconstruction within the network and the flexible interconnection between networks, including the following steps:

[0006] Step 1: Establish an equivalent model of the flexible interconnect device (FID) and its constraints;

[0007] Step 2: Build a two-layer model that takes into account both the economy and reliability of network operation. Among them, the planning layer uses the lowest annual comprehensive cost including line loss costs, power outage loss costs and flexible interconnection device investment costs as the objective function to select the location and capacity of the flexible interconnection device; the operation layer uses the lowest line loss cost as the objective function to reconstruct the network and optimize the operation of the flexible interconnection device, and evaluates the reliability of the network. The constraints of the planning layer include the access location constraints and capacity constraints of the flexible interconnection device; the constraints of the operation layer include flexible interconnection device constraints, power grid flow balance constraints, node voltage and line safety constraints, distributed power output constraints, network reconstruction radial constraints and switch action times constraints;

[0008] Step 3: As the proposed model is a large-scale mixed integer nonlinear programming problem, a hybrid algorithm is used to solve it. The optimization algorithm is used to solve the planning layer model, and the second-order cone programming is used to solve the operation layer model using a solver;

[0009] Specifically, the equivalent model of the flexible interconnection device and its constraints in step 1 are as follows:

[0010]

[0011]

[0012]

[0013]

[0014]

[0015] Where: P FID，i，t , P FID，j，t , Q FID，i，t With Q FID，j，t are the active power and reactive power injected into nodes i and j by the FID connected to both ends of nodes i and j at time t respectively; are the active power loss and reactive power loss of the FID connected to nodes i and j at time t respectively; A FID With B FIDis the active power loss coefficient and reactive power loss coefficient of FID; S FID，ij is the capacity of the FID connected to nodes i and j.

[0016] Specifically, in step 2, the planning layer takes the lowest annual comprehensive cost as the objective function:

[0017]

[0018] Where: It represents the annual construction investment cost converted from FID; It represents the annual operation and maintenance cost of FID; represents the annual investment cost of the flexible interconnection line; C opt It represents the cost transmitted from the operation layer to the planning layer. The calculation of each part can be obtained by the following formula:

[0019]

[0020] Where: d is the discount rate; y is the useful life of FID; c FID is the investment cost per unit capacity of FID; k is the installed capacity of the kth FID; m is the total number of installed FIDs.

[0021]

[0022] Where: ε FID is the annual operation and maintenance cost coefficient of FID.

[0023]

[0024] In the formula: c is the investment cost per unit length of the line required for flexible interconnection; l is the unit length of the line.

[0025] C opt =C loss +C fault

[0026] Where: C loss C represents the line loss cost when the network is running in steady state; fault Indicates the power outage loss in case of network failure.

[0027] Specifically, the operation layer in step 2 takes the lowest network steady-state operation as the objective function:

[0028]

[0029] Where: μ is the loss cost coefficient of the distribution network; T is the total number of time segments; n is the total number of distribution network branches; P i，t is the line loss value of branch i at time t; is the active power loss value of the kth FID at time t, and Δt is the duration of the time period.

[0030] Specifically, the planning layer constraints in step 2 are the access location and capacity constraints of the flexible interconnection device:

[0031] This article implements flexible interconnection between networks by configuring FID, so the two ends of FID should be connected to two networks to be interconnected respectively;

[0032] FID capacity constraints:

[0033] S FID，k =m k s unit

[0034] Where: m k is a positive integer; s unit Install capacity for FID units.

[0035] Specifically, the inner constraints in step 2 include flexible interconnection device constraints, power grid flow balance constraints, node voltage and line safety constraints, distributed power output constraints, network reconstruction radial constraints and switch action times constraints:

[0036] Network flow constraints:

[0037]

[0038]

[0039] Where: ψ(i) is the set of nodes connected to node i through branches; P i，t With Q i，t are the active power and reactive power injected into node i at time t; V i，t With V j，t are the node voltages of nodes i and j at time t; θ ij，t is the voltage phase difference between node i and node j at time t; G ij , B ij , G ii With B ii are the real and imaginary parts of the self-admittance and mutual-admittance of node i in the node admittance matrix, respectively.

[0040] Node balance constraints:

[0041]

[0042]

[0043]

[0044] Where: and is the active power and reactive power output by the photovoltaic power source and wind turbine connected to node i at time t; and Inject the active power and reactive power of the connected FID into node i; and is the load carried by node i at time t; I ij，t is the current value of branch ij at time t.

[0045] Network security constraints:

[0046] V i，min ≤V i，t ≤V i，max

[0047]

[0048] Where: V i，max With V i，min is the voltage limit value of node i; I ij，max is the current limit value of branch ij.

[0049] Distributed power constraints:

[0050]

[0051]

[0052]

[0053] Where: represents the active power injected into node i by DG at time t; It represents the maximum active power that DG is allowed to inject into node i at time t; represents the power factor angle of DG at node i at time t; Indicates the upper limit of the DG capacity connected to node i.

[0054] The network is reconstructed in radial constraints, and there are no loops or islands after reconstruction:

[0055]

[0056] Where n is the number of nodes in the network; N is the number of closed branches; ζ is the connection diagram of the distribution network simplified by graph theory; L(ζ) is the Laplace matrix; Λ(ζ) is the connection matrix. When there is a branch connection between two nodes ij, the corresponding α i，j is 1.

[0057] Constraints on the number of times a switch can operate during a reconstruction cycle:

[0058] H tot ≤Hmax

[0059] Where: H tot Indicates the total number of switching actions during the reconstruction cycle; H max Indicates the total number of switch actions allowed by the network.

[0060] Specifically, in step 2, the operation layer needs to evaluate the reliability of the network:

[0061] C fault =αβEENS

[0062] Where β represents the average electricity price conversion multiple; EENS represents the amount of electricity lost due to power outages in the system.

[0063] EENS=∑L i t u，i

[0064]

[0065] Where: L i is the average load of node i; t u，i is the average downtime of users in one year; a With t r，a are the failure rate and repair time of the equipment respectively; A represents the set of equipment that causes power outage at the load point.

[0066] Specifically, in step 2, a two-layer model is constructed that takes into account both the economic efficiency and reliability of network operation. The objective function of the planning layer is to minimize the annual comprehensive cost, and the generated FID installation location and capacity are transmitted to the operation layer; the operation layer includes two aspects: the normal operation state and the fault operation state of the network; the normal operation state is based on the planning results transmitted by the planning layer, and the network reconstruction plan and FID operation strategy with the minimum line loss cost are found; the fault operation state uses the Monte Carlo simulation method to conduct reliability assessment, and calculates the power outage loss of the network when a distribution network fails; the operation layer transmits the line loss cost of the network in the normal operation state and the power outage loss cost of the fault operation state to the planning layer, and finally realizes the cyclic iteration of the planning layer and the operation layer, and solves the network optimization plan that satisfies both the operation and configuration optimality.

[0067] Specifically, in step 3, the operation layer model is solved by using a solver using a second-order cone programming:

[0068] The decision variables in the FID optimization model include discrete variables: FID capacity, and continuous variables: FID operation strategy and network reconstruction plan. It mixes integer and nonlinear programming, and the solution process is very complicated. Therefore, cone relaxation constraints are used to convert it into a second-order cone programming model. First, the auxiliary variable A is introduced. i，t , C ij，t With D ij，t :

[0069]

[0070] The above network constraints can be converted into the following formula:

[0071]

[0072]

[0073]

[0074]

[0075]

[0076] The introduction of second-order rotating cone theory can transform the FID constraint into a second-order cone constraint:

[0077]

[0078]

[0079] Specifically, in step 3, the proposed model is a large-scale mixed integer nonlinear programming problem, and a hybrid algorithm is used to solve it. The planning layer model is solved by an optimization algorithm, and the operation layer model is solved by a second-order cone programming using a solver: the planning layer uses an optimization algorithm to generate FID configuration information, and the operation layer uses a second-order cone programming to solve the FID site selection and capacity determination results transmitted by the planning layer to obtain the transmission power and network reconstruction topology of the FID at different times, and then the line loss cost and fault state reliability evaluation results during steady-state operation of the network are transmitted to the planning layer, and the planning layer sorts the fitness to find the best planning method for the network.

[0080] The technical effect of the present invention is that: the inter-regional flexible interconnection planning method taking into account the reconstruction within the network can give full play to the flexibility of the network from the perspective of power grid operation, and can more effectively reduce network losses, smooth voltage fluctuations, and reduce power outage losses; from the perspective of power grid planning, the configuration of FID is optimized in conjunction with network reconstruction, which can reduce the power required to be transmitted on the flexible interconnection line and reduce the investment cost of FID. The present invention simultaneously considers the site selection, capacity determination and operation optimization of the flexible interconnection device, taking into account the economic efficiency and reliability of network operation, making the present invention more widely used, and the established double-layer model makes the planning solution more convenient, and the solution method of the second-order cone programming hybrid optimization algorithm adopted makes the planning result more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Attached Figure 1 A diagram of a flexible interconnection planning model for regions taking into account the reconstruction within the network according to the present invention;

[0082] Attached Figure 2 A system diagram of improving IEEE33 nodes and IEEE22 nodes in an embodiment of the present invention;

[0083] Attached Figure 3 The output prediction curve diagram of the photovoltaic unit in the embodiment of the present invention;

[0084] Attached Figure 4 A wind turbine output prediction curve diagram in an embodiment of the present invention;

[0085] Attached Figure 5 A typical daily load curve diagram in an embodiment of the present invention;

[0086] Attached Figure 6 This is a FID transmission power distribution curve in an embodiment of the present invention; DETAILED DESCRIPTION

[0087] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0088] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those generally understood by those of ordinary skill in the art to which the present application belongs. It should be noted that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0089] This embodiment provides a new distribution network planning method taking into account intra-network reconstruction and inter-network flexible interconnection, including the following steps:

[0090] Step 1: Establish an equivalent model of the flexible interconnect device (FID) and its constraints;

[0091]

[0092]

[0093]

[0094]

[0095]

[0096] Where: P FID，i，t , P FID，j，t , Q FID，i，t With QFID，j，t are the active power and reactive power injected into nodes i and j by the FID connected to both ends of nodes i and j at time t respectively; are the active power loss and reactive power loss of the FID connected to nodes i and j at time t respectively; A FID With B FID is the active power loss coefficient and reactive power loss coefficient of FID; S FID，ij is the capacity of the FID connected to nodes i and j.

[0097] Step 2: Build a two-layer model that takes into account both the economy and reliability of network operation. The planning layer selects the location and capacity of flexible interconnection devices with the lowest annual comprehensive cost as the objective function; the operation layer reconstructs the network and optimizes the operation of flexible interconnection devices with the lowest line loss cost as the objective function, and evaluates the reliability of the network. The constraints of the planning layer include the access location constraints and capacity constraints of the flexible interconnection devices; the constraints of the operation layer include flexible interconnection device constraints, power grid flow balance constraints, node voltage and line safety constraints, distributed power output constraints, network reconstruction radial constraints, and switch action times constraints;

[0098] The planning layer objective function is:

[0099]

[0100] Where: It represents the annual construction investment cost converted from FID; It represents the annual operation and maintenance cost of FID; represents the annual investment cost of the flexible interconnection line; C opt It represents the cost transmitted from the operation layer to the planning layer. The calculation of each part can be obtained by the following formula:

[0101]

[0102] Where: d is the discount rate; y is the useful life of FID; c FID is the investment cost per unit capacity of FID; k is the installed capacity of the kth FID; m is the total number of installed FIDs.

[0103]

[0104] Where: ε FID is the annual operation and maintenance cost coefficient of FID.

[0105]

[0106] In the formula: c is the investment cost per unit length of the line required for flexible interconnection; l is the unit length of the line.

[0107] Copt =C loss +C fault

[0108] Where: C loss C represents the line loss cost when the network is running in steady state; fault Indicates the power outage loss in case of network failure.

[0109] The objective function of the operation layer is:

[0110]

[0111] Where: μ is the loss cost coefficient of the distribution network; T is the total number of time segments; n is the total number of distribution network branches; P i，t is the line loss value of branch i at time t; is the active power loss value of the kth FID at time t, and Δt is the duration of the time period.

[0112] The reliability assessment method of the network at the operation layer:

[0113] C fault =αβEENS

[0114] Where β represents the average electricity price conversion multiple; EENS represents the amount of electricity lost due to power outages in the system.

[0115] EENS=∑L i t u，i

[0116]

[0117] Where: L i is the average load of node i; t u，i is the average downtime of users in one year; a With t r，a are the failure rate and repair time of the equipment respectively; A represents the set of equipment that causes power outage at the load point.

[0118] FID position constraints:

[0119] This article implements flexible interconnection between networks by configuring FID, so the two ends of FID should be connected to two networks to be interconnected respectively;

[0120] FID capacity constraints:

[0121] S FID，k =m k s unit

[0122] Where: m k is a positive integer; s unit Install capacity for FID units.

[0123] Network flow constraints:

[0124]

[0125]

[0126] Where: ψ(i) is the set of nodes connected to node i through branches; P i，t With Q i，t are the active power and reactive power injected into node i at time t; V i，t With V j，t are the node voltages of nodes i and j at time t; θ ij，t is the voltage phase difference between node i and node j at time t; G ij , B ij , G ii With B ii are the real and imaginary parts of the self-admittance and mutual-admittance of node i in the node admittance matrix, respectively.

[0127] Node balance constraints:

[0128]

[0129]

[0130]

[0131] Where: and is the active power and reactive power output by the photovoltaic power source and wind turbine connected to node i at time t; and Inject the active power and reactive power of the connected FID into node i; and is the load carried by node i at time t; I ij，t is the current value of branch ij at time t.

[0132] Network security constraints:

[0133] V i，min ≤V i，t ≤V i，max

[0134]

[0135] Distributed power constraints:

[0136]

[0137]

[0138]

[0139] Where: represents the active power injected into node i by DG at time t; It represents the maximum active power that DG is allowed to inject into node i at time t; represents the power factor angle of DG at node i at time t; Indicates the upper limit of the DG capacity connected to node i.

[0140] Network Reconfiguration Radial Constraints

[0141]

[0142] Step 3: As the proposed model is a large-scale mixed integer nonlinear programming problem, a hybrid algorithm is used to solve it. The optimization algorithm is used to solve the planning layer model, and the second-order cone programming is used to solve the operation layer model using a solver;

[0143] The network constraints and FID constraints can be converted into the following formula

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150]

[0151] Example

[0152] Based on the background of improving the interconnection network between IEEE33 nodes and IEEE22 nodes, the network is reconstructed by using the existing tie switches in the IEEE33 network. Figure 2 As shown in the figure, the dotted part is the tie switch. Nodes 7, 10, 24, and 27 are connected to photovoltaic units with capacities of 500kVA, 500kVA, 300kVA, and 400kVA, respectively. Nodes 13 and 30 are connected to wind turbines with capacities of 1000kVA and 1000kVA, respectively. They all use constant power factor to generate electricity, and the power factor is always equal to 0.95. The load curve of the node and the output curve of the distributed power source are shown in the attached figure. Figure 3-5 The reactance per unit length of the overhead line is approximately 0.4Ω / km, and the parameter settings are shown in the following table:

[0153] Table 1 Parameter settings

[0154]

[0155] In order to further analyze the impact of distribution network reconstruction and flexible interconnection, this paper sets up four planning schemes: Scheme 1: No network reconstruction and no FID configuration; Scheme 2: Network reconstruction but no FID configuration; Scheme 3: No network reconstruction but FID configuration; Scheme 4: Both network reconstruction and FID configuration. The planning results of different schemes are shown in Table 2.

[0156] Table 2 Planning results of four schemes

[0157]

[0158] As can be seen from the above table, the optimal strategy is Scheme 4, which takes the minimum annual comprehensive cost of the network as the objective function for planning. Network reconstruction and flexible interconnection can both reduce operating costs while improving the power supply reliability of the network. Compared with Scheme 1, Scheme 2, Scheme 3 and Scheme 4 reduce the annual comprehensive cost by 31.371%, 41.151% and 59.567% respectively. Although Scheme 3 and Scheme 4 require additional construction of lines and additional FIDs, an additional investment of 249,900 yuan and 14,500 yuan per year is required, but compared with the reduction of 276,470 yuan and 378,520 yuan in line loss costs and power outage loss costs, the total cost is still significantly reduced. From the perspective of line loss, Scheme 2, Scheme 3 and Scheme 4 reduce the annual line loss costs by 124,070 yuan, 196,360 yuan and 290,490 yuan respectively compared with Scheme 1. Scheme 4 has the most obvious loss reduction effect, followed by Scheme 3, and Scheme 2 is the smallest. Although distribution network reconstruction can find the optimal combination of network switches to reduce line losses, the use of FID to achieve network interconnection reduces the equivalent impedance of the network, and the loss reduction effect is more obvious. The mutual optimization effect of FID and reconstruction is better.

[0159] The active power and reactive power distribution curves of FID between nodes 9-46 in scheme 3 and scheme 4 within one day are shown in the attached figure. Figure 6 In Scheme 4, network reconstruction combined with flexible interconnection can reduce the power flow of FID branches and reduce the investment cost required for FID. FID can change the power flow direction of branches according to the load fluctuations of the two distribution networks, making the operation of the distribution network more flexible and controllable.

[0160] From the perspective of reliability analysis, after using FID flexible interconnection, the units of the two distribution networks serve as backup for each other. When a single fault occurs, part of the load can be transferred. Assuming that the power output of the unit under normal operation is 80% of the maximum output power, the reliability evaluation results are shown in Table 3. The average power outage frequency, average power outage duration, and power shortage expectation of the system are all optimal for Scheme 4. The use of distribution network reconstruction can further play the role of flexible interconnection in improving the reliability of the distribution network.

[0161] Table 3 Reliability evaluation of four schemes

[0162]

[0163] In summary, the flexible interconnected distribution network planning method including FID taking into account network reconstruction proposed in this paper can more effectively reduce network losses, smooth voltage fluctuations, and reduce power outage losses from the perspective of grid operation; from the perspective of grid planning, optimizing the configuration of FID in conjunction with network reconstruction can reduce the power required to be transmitted on the flexible interconnected lines and reduce the investment required for FID.

[0164] It should be understood that the size of the serial numbers of the steps in the above embodiments does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0165] The embodiments described above are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, a person skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A distribution network flexible interconnection planning method taking network reconstruction into account, characterized in that: The following steps are involved: Step 1: Establish an equivalent model of the flexible interconnect device (FID) and its constraints; Step 2: Build a two-layer model that takes into account both network operation economy and reliability; Among them, the planning layer uses the lowest annual comprehensive cost including line loss cost, power outage loss cost and flexible interconnection device investment cost as the objective function to select the location and capacity of the flexible interconnection device; the operation layer uses the lowest line loss cost as the objective function to reconstruct the network and optimize the operation of the flexible interconnection device, and evaluate the reliability of the network; the constraints of the planning layer include the access location constraint and capacity constraint of the flexible interconnection device; the constraints of the operation layer include the flexible interconnection device constraint, the power flow balance constraint, the node voltage and line safety constraint, the distributed power output constraint, the network reconstruction radial constraint and the switch action number constraint; Step 3: As the proposed model is a large-scale mixed integer nonlinear programming problem, a hybrid algorithm is used to solve it. The planning layer model is solved by the optimization algorithm, and the operation layer model is solved by the second-order cone programming solver.

2. The method for planning distribution network flexible interconnection taking into account network reconstruction according to claim 1, characterized in that: In step 1, an equivalent model of the flexible interconnection device is established as follows: Where: P FID,i,t , P FID,j,t , Q FID,i,t With Q FID,j,t are the active power and reactive power injected into nodes i and j by the FID connected to both ends of nodes i and j at time t respectively; are the active power loss and reactive power loss of the FID connected to nodes i and j at time t respectively; A FID With B FID is the active power loss coefficient and reactive power loss coefficient of FID; S FID,ij is the capacity of the FID connected to nodes i and j.

3. The method for planning distribution network flexible interconnection taking into account network reconstruction according to claim 1, characterized in that: In step 2, the planning layer takes the lowest annual comprehensive cost as the objective function: Where: It represents the annual construction investment cost converted from FID; It represents the annual operation and maintenance cost of FID; represents the annual investment cost of the flexible interconnection line; C opt Represents the cost of transmission from the operation layer to the planning layer; the calculation of each part can be obtained by the following formula: Where: d is the discount rate; y is the useful life of FID; c FID is the investment cost per unit capacity of FID; k is the installed capacity of the kth FID; m is the total number of installed FIDs; Where: ε is the annual operation and maintenance cost coefficient of FID; Where: c is the investment cost per unit length of the line required for flexible interconnection; l is the unit length of the line; C opt =C loss +C fault Where: C loss C represents the line loss cost when the network is running in steady state; fault Indicates the power outage loss in case of network failure.

4. The method for planning distribution network flexible interconnection taking into account network reconstruction according to claim 1, characterized in that: In step 2, the operation layer takes the lowest line loss cost of the network in steady state operation as the objective function: Where: μ is the loss cost coefficient of the distribution network; T is the total number of time segments; n is the total number of distribution network branches; P i,t is the line loss value of branch i at time t; is the active power loss value of the kth FID at time t, and Δt is the duration of the time period.

5. The method for planning distribution network flexible interconnection taking into account network reconstruction according to claim 1, characterized in that: In step 2, the planning layer constraints are the access location and capacity constraints of the flexible interconnection device: FID position constraints: This article implements flexible interconnection between networks by configuring FID, so the two ends of FID should be connected to two networks to be interconnected respectively; FID capacity constraints: S FID,k =m k s unit Where: m k is a positive integer; s unit Install capacity for FID units.

6. The method for planning distribution network flexible interconnection taking into account network reconstruction according to claim 1, characterized in that: In step 2, the constraints of the operation layer include flexible interconnection device constraints, power grid flow balance constraints, node voltage and line safety constraints, distributed power output constraints, network reconstruction radial constraints and switch action times constraints: Network flow constraints Where: ψ(i) is the set of nodes connected to node i through branches; P i,t With Q i,t are the active power and reactive power injected into node i at time t; V i,t With V j,t are the node voltages of nodes i and j at time t; θ ij,t is the voltage phase difference between node i and node j at time t; G ij , B ij , G ii With B ii are the real and imaginary parts of the self-admittance and mutual-admittance of node i in the node admittance matrix, respectively; Node balance constraints: Where: and is the active power and reactive power output by the photovoltaic power source and wind turbine connected to node i at time t; and Inject the active power and reactive power of the connected FID into node i; and is the load carried by node i at time t; I ij,t is the current value of branch ij at time t; Network security constraints: In i,min ≤V i,t ≤V i,max Where: V i,max With V i,min is the voltage limit value of node i; I ij,max is the current limit value of branch ij; Distributed power constraints: Where: represents the active power injected into node i by DG at time t; It represents the maximum active power that DG is allowed to inject into node i at time t; represents the power factor angle of DG at node i at time t; represents the upper limit of the DG capacity connected to node i; Network Reconfiguration Constraints: Where n is the number of nodes in the network; N is the number of closed branches; ζ is the connection diagram of the distribution network simplified by graph theory; L(ζ) is the Laplace matrix; Λ(ζ) is the connection matrix. When there is a branch connection between two nodes ij, the corresponding α i,j is 1; Constraints on the number of times a switch can operate during a reconfiguration cycle H tot ≤H max Where: H tot Indicates the total number of switching actions during the reconstruction cycle; H max Indicates the total number of switch actions allowed by the network.

7. The method for planning distribution network flexible interconnection taking into account network reconstruction according to claim 1, characterized in that: In step 2, the operation layer needs to evaluate the reliability of the network: C fault =αβEENS In the formula, β represents the average electricity price conversion multiple; EENS represents the power loss of the system due to power outage, which can be calculated by the following formula: ONCE=∑L i t u,i Where: L i is the average load of node i; t u,i is the average downtime of users in a year; λ a With t r,a are the failure rate and repair time of the equipment respectively; A represents the set of equipment that causes power outage at the load point.

8. The distribution network flexible interconnection planning method taking into account network reconstruction according to claim 1 is characterized in that: In step 2, a two-layer model is built that takes into account both the economic efficiency and reliability of network operation: The planning layer objective function is to minimize the annual comprehensive cost, and transmit the generated FID installation location and capacity to the operation layer; the operation layer includes the normal operation status and fault operation status of the network; the normal operation status is based on the planning results transmitted by the planning layer, and the network reconstruction plan and FID operation strategy with the lowest line loss cost are found; The fault operation state uses the Monte Carlo simulation method to conduct reliability assessment and calculate the power outage loss of the network when a distribution network failure occurs; The operation layer transmits the line loss costs under normal network operation and the power outage loss costs under faulty operation to the planning layer, and finally realizes the cyclic iteration of the planning layer and the operation layer to solve the network optimization solution that satisfies both operation and configuration optimality.

9. The method for planning distribution network flexible interconnection taking network reconstruction into account according to claim 1, characterized in that: In step 3, the operation layer model is solved by using a second-order cone programming solver: The decision variables in the FID optimization model include discrete variables: network reconstruction scheme and continuous variables: FID operation strategy, which is a mixture of integer and nonlinear programming. The solution process is very complicated, so cone relaxation constraints are used to convert it into a second-order cone programming model. First, the auxiliary variable A is introduced. i,t , C ij,t With D ij,t : The above network constraints can be converted into the following formula: The introduction of second-order rotating cone theory can transform the FID constraint into a second-order cone constraint:

10. The distribution network flexible interconnection planning method taking into account network reconstruction according to claim 1, characterized in that: In step 3, the proposed model is a large-scale mixed integer nonlinear programming problem, and a hybrid algorithm is used to solve it. The planning layer model is solved by an optimization algorithm, and the operation layer model is solved by a second-order cone programming solver: The planning layer uses an optimization algorithm to generate FID configuration information. The operation layer uses second-order cone programming to solve the FID site selection and capacity determination results transmitted by the planning layer, and obtains the transmission power and network reconstruction topology of the FID at different times. The line loss cost and fault state reliability assessment results during steady-state operation of the network are then transmitted to the planning layer. The planning layer sorts the fitness and finds the best planning method for the network.

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