Active power distribution network real-time safety assessment method and system based on key links

Abstract

The invention relates to a key link-based active power distribution network real-time safety assessment method and system. The method comprises the following steps of S1, acquiring basic data of a distribution network, a distributed power supply and a load; s2, performing load flow calculation through a forward-backward sweep method to obtain bus voltage values of the distribution network at all moments and active power flowing through all lines; s3, selecting key links including key nodes and key lines; and S4, based on the constructed risk assessment indexes, performing real-time safety assessment on the key links from two aspects of bus voltage and line power flow. The method and the system can effectively evaluate the risk of the distribution network and improve the operation safety of the distribution network.

Description

Technical Field

[0001] The present invention relates to the technical field of power distribution network security assessment, and in particular to a real-time security assessment method and system for an active power distribution network based on key links. Background Art

[0002] There is uncertainty in the output and load demand of distributed power sources. Their fluctuations can easily change the operating conditions of the distribution network, causing risks such as voltage over-limit and line overload. In serious cases, the protection device will trip and cause power outages to users, resulting in serious economic losses. Therefore, it is very important to conduct real-time safety assessment of the distribution network. Summary of the invention

[0003] The purpose of the present invention is to provide a real-time safety assessment method and system for an active distribution network based on key links, which can effectively assess the risks of the distribution network and improve the safety of the distribution network operation.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is: a real-time security assessment method for an active distribution network based on key links, comprising the following steps:

[0005] S1. Obtain basic data of distribution network, distributed power source and load;

[0006] S2. Calculate the power flow by forward-backward substitution method to obtain the bus voltage value of the distribution network at each moment and the active power flowing through each line;

[0007] S3. Select key links, including key nodes and key lines;

[0008] S4. Based on the constructed risk assessment indicators, real-time safety assessment of key links is carried out from the two perspectives of bus voltage and line flow.

[0009] Furthermore, the selection of key nodes is realized based on trend tracking and PageRank algorithm, and the implementation method is as follows:

[0010] First, based on the power flow calculation results, power flow tracking is achieved through power allocation analysis;

[0011]

[0012] In the formula, K=(k ij ) n×n is the distribution coefficient matrix of generator to load; P GG =diag(P G1 ,P G2 ,…,P Gn ), P G1 ,P G2 ,…,P Gn represents the active output column vector of the n-dimensional generator; PTT =diag(P T1 ,P T2 ,…,P Tn ), P T1 ,P T2 ,…,P Tn represents the total active power column vector of n-dimensional nodes; P ij is the active power transmitted from node i to node j on line ij; P Tj is the total active power injected into node j; P Gi→j For the generator G i The power distribution to load node j, P Lj is the load active power of node j; P Gi→L(s,t) For the generator G i Power distribution to line L(s,t), P L(s,t) The active power downstream distribution matrix A of line L(s,t) is constructed as:

[0013]

[0014] The contribution of the generator to the line power and node load, as well as the power drawn by the load from the generator, are obtained by calculation using formula (1);

[0015] The PageRank algorithm represents the entire network as a directed graph, where web pages represent each node and node-related edges represent links between nodes; the PR value is used to represent the importance of the power grid node to the entire network, which is obtained by combining the importance of the node itself and the importance of other nodes linked to the node; the steady-state value PR is obtained after iterative calculation of the entire network, and the larger the PR value of the node, the more important it is in the network; the iterative calculation of the PR value is shown in formula (3);

[0016]

[0017] In the formula, {p1,p2,…,p N} is the set of network nodes; M(p i ) points to node p i The node set of L(p j ) is p j The number of outgoing nodes; N is the number of all nodes in the network; σ is the damping coefficient;

[0018] Combined with the power transmission relationship, the contribution of the generator to the lines and nodes in the system is considered; all the energy in the system is provided by the generator, and the position of the generator in the system energy supply is determined according to the size of the generator power generation, and the generator importance weight η is defined Gk , as shown in formula (4);

[0019]

[0020] Where P Gk For unit G k Power generation; ∑P G is the total power generation of the system;

[0021] Combined with the importance weight of the generator, the power allocation of the generator to the line is further considered; the system node link strength based on the power flow tracking result as shown in formula (5) is used to characterize the link weight between each node;

[0022]

[0023] Where n is the number of system units; ν i , ν j For line L(ν i ,ν j ) The node numbers at both ends; P Gk→L(νi,νj) For unit G k On line L(ν i ,ν j ) power distribution; η Gk For the generator G k The importance weight of

[0024] The power allocation of the generator node to each load node needs to take into account the importance of each generator in the system and is expressed by the load weight as shown in formula (6);

[0025]

[0026] Where N is the number of generators in the system; P Gi→j For the generator G i For the power allocation of node j; η Gi For the generator G i Importance weight of

[0027] In order to prevent distortion caused by excessive node load weights, the node load weights are normalized:

[0028]

[0029] Based on the PageRank algorithm, the system link strength is used as an importance ranking factor, and the importance of system load is comprehensively considered to select key nodes:

[0030] In a power grid with n nodes, suppose node ν has links from ν1,ν2,…,ν l ,ω line (ν i ,ν) is the link node ν iand ν are the system link strength weights, then the calculation formula for node criticality is shown in formula (8);

[0031]

[0032] In the formula, PT(ν l ) is the node source ν pointing to node ν l Its own PT node key value; S out (ν l ) is the node source ν l The outgoing chain strength;

[0033] Through the above definition, the system link strength matrix is ​​obtained as shown in formula (9);

[0034]

[0035] Where L is the system line set; ω Line (ν i ,ν j ) is the link strength between two nodes. If there is no line link between the nodes, it is 0. Divide each row of the system link strength matrix W by the sum of the elements in the row and normalize it to obtain the new link strength matrix Q:

[0036]

[0037] After introducing normalization, the load importance factor matrix is:

[0038]

[0039] Combining the link strength matrix with the node load importance factor, the improved link matrix Q' is obtained:

[0040] Q'=ψ load Q (12)

[0041] According to formula (12), we get the matrix

[0042]

[0043] Where E is the unit matrix;

[0044] Therefore, the iterative formula (8) is transformed into the following form:

[0045]

[0046] If the node voltage is higher than the upper voltage limit or lower than the lower voltage limit, the risk caused by the node voltage exceeding the limit is expressed by formula (15);

[0047]

[0048] In the formula, Sev i is the voltage over-limit severity of the ith node; V i is the voltage of the ith node; V max is the maximum operating voltage limit specified, take V max =1.05pu; V min is the specified minimum operating voltage limit;

[0049] The comprehensive criticality of the node is obtained by normalizing the PT value and the voltage over-limit severity and weighting them:

[0050] COM i =ω1PT i ′+ω2Sev i ′ (16)

[0051] In the formula, COM i is the comprehensive criticality of the i-th node; PT i ' and Sev i ' are the PT value of the i-th node and the normalized value of the voltage over-limit severity; by i Sort and select key nodes from large to small.

[0052] Furthermore, the key lines are selected based on the power flow betweenness, and the implementation method is as follows:

[0053] In order to characterize the utilization and importance of each line in the entire network when power source m transmits power P(m,n) to load n, the line flow betweenness of a single power source-load node pair is defined as:

[0054]

[0055] In the formula, min(P m ,P n ) is the weight of the single power flow medium, which is the smaller value between the actual active output of power source m and the actual active load of load n; P(m,n) is the active power transmitted from power source m to load m; P ij (m,n) is the contribution power of power flow on line ij when power source m transmits active power to load m; F ij,mn Characterizes the importance of the line in the distribution network when a single power source transmits active power to a single load;

[0056] Calculate the F of each line according to formula (17): ij,mn According to the superposition principle, the power flow betweenness of all power source-load node pairs when transmitting active power is accumulated to obtain the line power flow betweenness F ij :

[0057]

[0058] In the formula, m is the power node, G is the power node set; n is the load node, L is the load node set;

[0059] In the calculation formula (18) of the line power flow betweenness, it is necessary to obtain P(m,n) and P ij (m,n); According to the principle of power flow tracking, when the power grid power flow distribution is determined, the network is simplified into a lossless network, and then the downstream distribution matrix is ​​constructed. The downstream tracking method is used to track the power flow of the power source-load node pair to determine the power component in the line ij.

[0060] Furthermore, the line power flow betweenness is calculated based on the power flow betweenness method, and its implementation method is as follows:

[0061] 1) When the operating status of the power grid is determined, the active power generated by the generator, the active power absorbed by the load, the active power flow of each line and the line loss are obtained;

[0062] 2) The active power loss of the line is regarded as an equivalent load and evenly distributed to the head and end nodes of the line to form an active power lossless network;

[0063] 3) Based on the active lossless network, construct the downstream distribution matrix A = (a ij ) r×r , r is the number of nodes in the network, and the element a in the matrix ij for:

[0064]

[0065] Where P ij is the active power flowing from node i to node j on line ij; P Tj is the total active power injected into node j;

[0066] 4) Construct the generator active output diagonal array P GG , node active load diagonal matrix P LL and the node injected active power diagonal matrix P TT :

[0067] P GG =diag[P G1 ,P G2 ,…P Gr ] (20)

[0068] P LL =diag[P L1 ,P L2 ,…P Lr ] (twenty one)

[0069] P TT =diag[P T1 ,PT2 ,…P Tr ] (twenty two)

[0070] Where P Gr is the active output of the generator at node r; P Lr is the active load of node r; P Tr is the total active power injected into node r;

[0071] 5) Calculate the coefficient matrix K for allocating active power from the power node to the load node G and the coefficient matrix K of the load node drawing active power from the power node L :

[0072] K G =P GG (P TT ) -1 A -1 (twenty three)

[0073] K L =P LL (P TT A T ) -1 (twenty four)

[0074] 6) Obtain the active power P(m,n) provided by power node m to load node n:

[0075] P(m,n)=K L (n,m)P Gm (25)

[0076] In the formula, K L (m,n) is the active power extraction coefficient matrix K L The element in the nth row and mth column of Gm is the active output of generator node m;

[0077] 7) Calculate P ij (m,n):

[0078] P ij (m,n)=K L (n,j)K G (m,i)P ij (26)

[0079] Where i is the node number where the active power of the line flows in; j is the node number where the active power of the line flows out; K L (n,j) represents the matrix K L The element in the nth row and jth column, K G (m,i) represents the matrix K G The element in the mth row and ith column ofij is the active power on line ij;

[0080] 8) According to formula (18), calculate the power flow intermediate number F of all lines in the power grid: ij .

[0081] Furthermore, the constructed risk assessment indicators include component layer risk assessment indicators and distribution network layer risk assessment indicators. The component layer risk assessment indicators include key node overvoltage risk indicators, key node low voltage risk indicators and key line overload risk indicators. The distribution network layer risk assessment indicators include distribution network overvoltage risk indicators, distribution network low voltage risk indicators and distribution network line overload risk indicators.

[0082] Furthermore, the key node overvoltage risk index reflects the risk of the voltage of each node of the distribution network exceeding the voltage upper limit under different system operating conditions, and its calculation formula is as follows:

[0083]

[0084] In the formula, R i,t vh is the overvoltage risk of the i-th key node at time t; B i,t vh Sev is a 0-1 indicator of overvoltage operation risk, indicating whether a voltage over-limit event occurs; vh is the severity function of the voltage exceeding the upper limit, specifically:

[0085]

[0086] Where V i,t is the voltage of the i-th key node at time t; V max is the specified maximum operating voltage limit;

[0087] The key node low voltage risk index reflects the risk that the voltage of each node of the distribution network is lower than the voltage lower limit under different system operating conditions. The calculation formula is as follows:

[0088]

[0089] In the formula, R i,t vl is the low voltage risk of the i-th key node at time t; B i,t vh It is a 0-1 indicator of low voltage operation risk, indicating whether a voltage crossing lower limit event occurs; Sev vl is the severity function of the voltage exceeding the lower limit, specifically:

[0090]

[0091] Where V min is the specified minimum operating voltage limit;

[0092] The critical line overload risk index is defined for each line. The percentage of the flow in each line exceeding the transmission capacity limit determines the severity of its overload. The calculation formula of the critical line overload risk index is as follows:

[0093]

[0094] In the formula, R ij,t p is the active power overload risk of the key line ij at time t; B i,t vh Sev is a 0-1 indicator of line overload risk, indicating whether a line overload event occurs; p is the severity function of line overload, specifically:

[0095]

[0096] Where P ij,max is the maximum limit of active power flowing through the key line ij; α is the set threshold.

[0097] Furthermore, the distribution network overvoltage risk index is obtained by multiplying the overvoltage risk of each key node by its criticality and then adding them together:

[0098]

[0099] In the formula, R sys,t vh is the overvoltage risk of the distribution network at time t; COM i,t is the criticality of the i-th key node at time t; N is the set of key nodes;

[0100] The distribution network low voltage risk index is obtained by multiplying the low voltage risk of each key node by its criticality and then adding them together:

[0101]

[0102] In the formula, R sys,t vl is the low voltage risk of the distribution network at time t;

[0103] The distribution network line overload risk index is obtained by adding up the overload risks of each key line and multiplying it by its criticality:

[0104]

[0105] In the formula, R sys,t pis the line overload risk of the distribution network at time t; F ij,t is the criticality of the key path ij at time t; L is the key path set.

[0106] The present invention also provides a real-time safety assessment system for an active distribution network based on key links, comprising a memory, a processor, and computer program instructions stored in the memory and capable of being executed by the processor. When the processor executes the computer program instructions, the above method can be implemented.

[0107] The present invention also provides a computer-readable storage medium on which computer program instructions are stored. When the computer program instructions are executed by a processor, the above method is implemented.

[0108] Compared with the prior art, the present invention has the following beneficial effects: the present invention proposes a real-time safety assessment method and system for an active distribution network based on key links. The method first selects key links including key nodes and key lines, and then constructs risk assessment indicators to perform real-time safety assessment on key links from the two perspectives of bus voltage and line flow. It can effectively assess the risks of the distribution network at each moment and improve the safety of distribution network operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0109] Figure 1 is a flow chart of a method implementation of an embodiment of the present invention;

[0110] Figure 2 This is a flowchart of key node selection in an embodiment of the present invention;

[0111] Figure 3 It is an IEEE33 node system containing a distributed power source in an embodiment of the present invention;

[0112] Figure 4 is the key bus selection result in the embodiment of the present invention;

[0113] Figure 5 It is a 68-node system of Chengmei Line in a certain city containing distributed power sources in an embodiment of the present invention;

[0114] Figure 6 It is the result of selecting the key busbar of the 68-node system of Chengmei Line in the embodiment of the present invention;

[0115] Figure 7 is a key line selection flow chart in an embodiment of the present invention;

[0116] Figure 8 is the key line selection result in the embodiment of the present invention;

[0117] Fig. 9 It is the result of selecting the key lines of the 68-node system of Chengmei Line in the embodiment of the present invention;

[0118] Fig.10 It is an IEEE33 node system containing a distributed power source in an embodiment of the present invention;

[0119] Fig.11 is a distributed power output curve in an embodiment of the present invention;

[0120] Fig.12 is the load demand of each busbar of the distribution network in the embodiment of the present invention;

[0121] Fig.13 is the distribution network bus voltage and line active power in the embodiment of the present invention;

[0122] Fig.14 These are the various risks of network distribution in the embodiments of the present invention. DETAILED DESCRIPTION

[0123] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0124] 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 commonly understood by those skilled in the art to which the present application belongs.

[0125] It should be noted that the terms used herein are only for 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 "comprise" and / or "include" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0126] like Figure 1 As shown, this embodiment provides a real-time security assessment method for an active distribution network based on key links, comprising the following steps:

[0127] S1. Obtain basic data of distribution network, distributed power source and load;

[0128] S2. Calculate the power flow by forward-backward substitution method to obtain the bus voltage value of the distribution network at each moment and the active power flowing through each line;

[0129] S3. Select key links, including key nodes and key lines;

[0130] S4. Based on the constructed risk assessment indicators, real-time safety assessment of key links is carried out from the two perspectives of bus voltage and line flow.

[0131] The relevant contents involved in this method are further described in detail below.

[0132] 1. Selection of key links

[0133] 1.1 Key node selection based on trend tracking and PageRank

[0134] 1.1.1 Trend Tracking and PageRank Algorithm

[0135] The purpose of power flow tracking is to find the power distribution relationship between generators, loads, and lines, and to clarify the power distribution of generators to loads (or the power extraction of loads from generators), the power distribution of generators to lines, and the power extraction of loads from lines. Power flow tracking can be achieved through power distribution analysis based on power flow calculation results.

[0136]

[0137] In the formula, K=(k ij ) n×n is the distribution coefficient matrix of generator to load; P GG =diag(P G1 ,P G2 ,…,P Gn ), P G1 ,P G2 ,…,P Gn represents the active output column vector of the n-dimensional generator; P TT =diag(P T1 ,P T2 ,…,P Tn ), P T1 ,P T2 ,…,P Tn represents the total active power column vector of n-dimensional nodes; P ij is the active power transmitted from node i to node j on line ij; P Tj is the total active power injected into node j; P Gi→j For the generator G i The power distribution to load node j, P Lj is the load active power of node j; P Gi→L(s,t) For the generator G i Power distribution to line L(s,t), P L(s,t) The active power downstream distribution matrix A of line L(s,t) is constructed as:

[0138]

[0139] The contribution of the generator to the line power and node load, as well as the power drawn by the load from the generator, can be obtained by calculating formula (1). Compared with the traditional method of directly using the flow calculation results to establish a weighted directed graph of the power system, the flow tracking technology can determine the power distribution relationship between each generator and load node, and the actual use of the generator and load on the transmission line. Using the above results, we can get evaluation indicators that are more suitable for the power system. Therefore, the flow tracking results can more effectively reflect the important links of the system.

[0140] The PageRank algorithm represents the entire network as a directed graph, where web pages represent each node and the node-related edges represent the links between nodes. The PR value represents the importance of a node, which is a combination of the importance of the node itself and the importance of other nodes linked to the node. In the PageRank algorithm, the calculation of the PR value is implemented in an iterative manner, and the specific iterative formula is shown in formula (3).

[0141]

[0142] In the formula, {p1,p2,…,p N} is the set of network nodes under study; M(p i ) points to p i The node set of L(p j ) is p j The number of outbound nodes; N is the number of all nodes in the network; σ is the algorithm damping coefficient, which is used to ensure the convergence of the algorithm. It is generally recommended to be set to 0.85.

[0143] For a network of N nodes, each node has an intrinsic importance of (1-σ) / N. If node p i The importance of P i , then the node will contribute σP i The importance of a node is shared by all nodes to which it points. The steady-state value PR is obtained after iterative calculation of the entire network. The larger the PR value of a node, the more important it is in the network.

[0144] The PageRank concept is applied to the evaluation and identification of key nodes in the power system. Since key nodes have an important impact on the power transmission capacity and operation safety of the power grid in actual power grid operation, and the PR value can represent the impact of the power grid node i on the overall network, the PR value can be used to select key nodes. i represents the impact of electric energy on the overall network at grid node i, PR i The larger the value, the more important the node i is.

[0145] 1.1.2 Establishment of weighted directed power grid model

[0146] The PageRank algorithm treats each linked node equally in the weight allocation, and does not consider the importance of each node itself. For an energy transmission system, under the current system operation, the size and direction of its flow are determined, so the allocation of node link weights should be determined by the actual line flow conditions. At the same time, in actual operation, the importance of large load nodes and power generation nodes to the power grid also needs to be considered in the setting of the calculation model. In summary, on the basis of establishing a weighted directed graph model of the power network and the importance of load nodes, the PageRank idea is combined to select key nodes for the power system.

[0147] Combined with the power transmission relationship, the contribution of the generator to the lines and nodes in the system is considered. From the energy perspective, all the energy in the system is provided by the generator. The position of the generator in the system energy supply can be determined according to the size of the generator power generation, and the importance weight of the generator η is defined Gk , as shown in formula (4).

[0148]

[0149] Where P Gk For unit G k Power generation; ∑P G is the total power generation of the system.

[0150] Combined with the importance weight of the generator, the power allocation of the generator to the line is further considered. The system node link strength based on the power flow tracing results as shown in formula (5) is used to characterize the link weight between each node.

[0151]

[0152] Where n is the number of system units; ν i , ν j For line L(ν i ,ν j ) The node numbers at both ends; P Gk→L(νi,νj) For unit G k On line L(ν i ,ν j ) power distribution; η Gk For the generator G k The importance weight of .

[0153] From the above definition, it can be concluded that the power flow components of the lines with larger link weights may come from more important units, or from multiple units. If a line has power provided by multiple units, it means that the line is on the power supply path of multiple units in the system, and disconnecting it may cause a large power flow transfer. Therefore, the link strength can reflect the energy distribution in the system in terms of both the depth and breadth of the power flow.

[0154] Similar to the definition of line link strength, the power allocation of the generator node to each load node also needs to take into account the importance of each generator in the system and is expressed by the load weight as shown in formula (6).

[0155]

[0156] Where N is the number of generators in the system; P Gi→j For the generator G i For the power allocation of node j; η Gi For the generator G i The importance weight of .

[0157] The significance of defining this indicator is that for a given load, the higher its importance means that the node draws power from important units, or draws power from multiple units. This type of load node is obviously in a more central position in the system flow. Therefore, this indicator can show the importance of the electrical position of the node load under the current system flow.

[0158] In order to prevent distortion caused by excessive node load weights, the node load weights are normalized:

[0159]

[0160] 1.1.3 Comprehensive key node selection

[0161] Through the relevant indicators defined above, drawing on the idea of ​​PageRank algorithm, the system link strength is used as the importance ranking factor, and the importance of system load is comprehensively considered. A method for selecting key nodes of the power system is proposed.

[0162] In a power grid with n nodes, suppose node ν has links from ν1,ν2,…,ν l ,ω line (ν i ,ν) is the link node ν i and ν are the system link strength weights, the formula for calculating node criticality is shown in formula (8).

[0163]

[0164] In the formula, PT(νl ) is the node source ν pointing to node ν l Its own PT node key value; S out (ν l ) is the node source ν l The chain strength.

[0165] According to the above definition, the system link strength matrix is ​​obtained as shown in formula (9).

[0166]

[0167] Where L is the system line set; ω Line (ν i ,ν j ) is the link strength between two nodes. If there is no line link between the nodes, it is 0. Divide each row of the system link strength matrix W by the sum of the elements in the row and normalize it to get the new link strength matrix Q:

[0168]

[0169] After introducing normalization, the load importance factor matrix is:

[0170]

[0171] Combining the link strength matrix with the node load importance factor, the improved link matrix Q' can be obtained:

[0172] Q'=ψ load Q (12)

[0173] According to formula (12), we get the matrix

[0174]

[0175] Where E is the unit matrix.

[0176] Therefore, the iterative formula (8) is transformed into the following form:

[0177]

[0178] In addition, in order to serve the node voltage safety assessment in the following text, it is necessary to consider the node voltage over-limit factor in the selection of key nodes. If the node voltage is higher than the voltage upper limit or lower than the voltage lower limit, the risk caused by the node voltage over-limit is expressed by formula (15).

[0179]

[0180] In the formula, Sev i is the voltage over-limit severity of the ith node; V iis the voltage of the ith node; V max is the maximum operating voltage limit specified, take V max =1.05pu; V min is the minimum operating voltage limit, take V min =0.95pu.

[0181] The comprehensive criticality of the node can be obtained by normalizing the PT value and the voltage over-limit severity and weighting them:

[0182] COM i =ω1PT i ′+ω2Sev i ′ (16)

[0183] In the formula, COM i is the comprehensive criticality of the i-th node; PT i ' and Sev i ' are the PT value and the normalized value of the voltage over-limit severity of the i-th node respectively. i Sort them so that the key nodes are selected from large to small.

[0184] Based on the above analysis of the key node selection method, the key node selection process is as follows: Figure 2 shown.

[0185] 1.1.4 Example Analysis

[0186] (1) IEEE33 node system

[0187] In order to verify the effectiveness and correctness of this method, we selected Figure 3 The IEEE 33-node system shown in the figure is used as the example system, and the distributed power supply is connected to the bus 6, 12, 18, and 33. The parameters of the distributed power supply are shown in Table 1. Among them, the distributed power supply is processed as a PQ node, the power factor is 0.95, and the balanced node voltage is 1p.u.

[0188] Table 1 Distributed power supply parameters

[0189]

[0190] Matlab programming is used to implement the proposed method, calculate the comprehensive criticality, and sort the obtained results. The first 15 critical buses of the system are shown in Table 2.

[0191] Table 2 Key bus selection results

[0192]

[0193] Table 2 can be visualized as Figure 4As shown in the figure, the red points are the first 15 key buses of the system corresponding to Table 2.

[0194] from Figure 4 It can be seen that the key buses 1, 6, 12, and 18 are all power nodes and are the main source of power for the entire network. For an actual system, as the main source of electric energy, it is reasonable for the generator set to have a higher status in energy transmission. This proves that the method proposed in this project can select nodes in key positions from the perspective of energy transmission. The reason why power bus 33 is not a key node is that the load power on its nearby buses 30, 31, and 32 is large, which leads to the small power displayed by the node. At the same time, buses 5 to 18 constitute the only path for DG1, DG2, and DG3 to transmit power to other buses. If these nodes are disconnected, it means that the distributed power connected to them is removed from the system, which will have a huge impact on the power supply safety of the system. Therefore, these buses are also key buses. In addition, because buses 17 and 18 have a voltage over-limit situation, the voltage over-limit severity is high, while other buses do not have a voltage over-limit situation, so the voltage over-limit severity is 0, which makes the comprehensive criticality of buses 17 and 18 much higher than that of other buses. This result shows that the proposed method not only selects from the perspective of energy transmission, but also comprehensively considers the node voltage over-limit situation, which will better serve the node voltage safety assessment in the following text.

[0195] Considering the randomness and intermittency of the output power of distributed power sources, the power flow of the distribution network will change with the fluctuation of the output power of distributed power sources, which will cause the selection results of key nodes to change. Assume that at a certain moment, Figure 3 The output power of distributed power sources DG1 and DG4 is reduced by 0.8 times, and the output power of DG2 and DG3 is increased by 1.2 times. The key nodes of the distribution network are reselected, and the selection results are shown in Table 3.

[0196] Table 3 Results of key bus selection considering DG output power fluctuation

[0197]

[0198] Comparing Table 2 and Table 3, it can be found that the selection results of the key bus have not changed, but the criticality of each bus will change. This is because the importance weight of the generator and the power allocation of the generator node to each load node will change with the fluctuation of the DG output power, and the criticality of each node is closely related to the two, so the criticality of each node will also change.

[0199] Considering that the distribution network is generally designed as a ring and operates in an open loop, in some extreme operating modes, such as load transfer or fault handling, the interconnection line may be connected, resulting in a short-term ring network operation state, which will also change the flow of the distribution network, thus causing the selection results of the key nodes to change. Assuming that all five interconnection lines in Table 2 are connected, the selection of key nodes of the distribution network is re-performed, and the selection results are shown in Table 4. The bold node numbers in the table indicate that the results are different from those in Table 2.

[0200] Table 4 Results of selecting key nodes considering contact line access

[0201]

[0202]

[0203] Comparing Table 2 and Table 4, it can be found that busbars 20, 21, 22, 26 and 33 are selected as key busbars, while busbars 1, 5, 7, 13 and 14 are no longer key busbars. In addition, the importance of other key busbars has also changed to varying degrees. The reason for the significant change in the selection results of key busbars is that the access of interconnection lines will greatly change the energy transmission path and line flow distribution of the distribution network, and the system connection strength will change, which will lead to a significant change in the criticality of each busbar.

[0204] (2) Field example verification in a certain city

[0205] In order to verify the effectiveness of the proposed method in a certain city, Figure 5 The simulation analysis is carried out by taking the 68-node system of Chengmei Line in a certain city as an example. Distributed power sources are connected to bus No. 48, No. 65, No. 57, and No. 15 of the 68-node system of Chengmei Line in a certain city respectively, and the parameters of distributed power sources are shown in Table 5. The distributed power source is processed as a PQ node with a power factor of 0.95; the balanced node voltage is 1p.u.

[0206] Table 5 Distributed power supply parameters

[0207]

[0208] The proposed method is also implemented using Matlab to calculate the comprehensive criticality and sort the results. The first 15 critical buses of the 68-bus system of Chengmei Line are shown in Table 6.

[0209] Table 6 Results of key bus selection for the 68-node system of Chengmei Line in a certain city

[0210]

[0211] Figure 6 The red dots in the figure are the first 15 key buses of the system corresponding to Table 6.

[0212] from Figure 6 It can be found that among the selected key buses, buses 48, 65 and 57 are all connected to distributed power sources and belong to power supply nodes. The distributed power sources connected to the three buses have a large output and are the main source of power for the entire network. Therefore, the above three nodes have a high status in the energy transmission of the entire network, which proves that the method proposed in this project can also select nodes in key positions from the perspective of energy transmission in the field system. Although buses 1 and 15 are both power supply nodes, they are not selected as key buses because the power they transmit is small and does not have a large enough impact on the entire network. At the same time, buses 36 to 48 are the main paths for DG1, DG2, and DG3 to transmit power to other buses in the entire network. If these buses are disconnected, the energy supply of distributed power sources to the entire network will be greatly weakened, which will have a huge impact on the power supply safety of the system. Therefore, these buses are also selected as key buses.

[0213] In summary, the key node selection method of the present invention takes into account the influence of power flow distribution and structure and the over-limit situation of node voltage, and can effectively select nodes that have an important impact on the power supply capacity and safe operation of the power grid in actual operation. In view of the fluctuation of the output power of the distributed power supply in the distribution network and the change of key nodes caused by the access of the interconnection line, the real-time key node selection result can be obtained through the real-time selection method.

[0214] 1.2 Key route selection based on power flow betweenness

[0215] 1.2.1 Line flow betweenness

[0216] In the power grid, the power generated by the power source node is transmitted to the load node through the line. Due to the different positions of the power source node and the load node in the network topology, the same power source node transmits power to different load nodes through different lines, and the same load node uses different lines to draw power from different power sources. Therefore, when each power source node in the power grid transmits power to each load node at the same time, the power flowing through each line is different, which makes the utilization degree of each line and its importance in the system different.

[0217] In order to characterize the utilization and importance of each line in the entire network when power source m transmits power P(m,n) to load n, the line flow betweenness of a single power source-load node pair is defined as:

[0218]

[0219] In the formula, min(P m ,P n) is the weight of the single power flow medium, which is the smaller value between the actual active output of power source m and the actual active load of load n; P(m,n) is the active power transmitted from power source m to load m; P ij (m,n) is the contribution power of power flow on line ij when power source m transmits active power to load m; F ij,mn It can characterize the importance of the line in the distribution network when a single power source transmits active power to a single load.

[0220] According to formula (17), the F of each line can be calculated ij,mn According to the superposition principle, the power flow betweenness of all power source-load node pairs when transmitting active power can be accumulated to obtain the line power flow betweenness F ij :

[0221]

[0222] Where m is the power node, G is the power node set; n is the load node, L is the load node set.

[0223] Since the line power flow betweenness F ij It is not only related to the actual output of power supply and load, but also to the system operation status and network topology. ij It reflects the contribution and importance of line ij in the power transmission process and can be used as an evaluation indicator for the selection of key lines.

[0224] 1.2.2 Calculation of line power flow betweenness based on power flow tracing method

[0225] In the expression (18) of the line power flow betweenness, it is necessary to obtain P(m,n) and P ij (m,n). According to the principle of power flow tracking, when the power grid power flow distribution is determined, the network is simplified into a lossless network, and then the downstream distribution matrix is ​​constructed. The downstream tracking method is used to track the power flow of the power source-load node pair to determine the power components in the line ij. The specific calculation method is as follows:

[0226] 1) When the operating status of the power grid is determined, the active power generated by the generator, the active power absorbed by the load, the active power flow of each line and the line loss are obtained.

[0227] 2) The active power loss of the line is regarded as an equivalent load and evenly distributed to the head and end nodes of the line to form an active and lossless network.

[0228] 3) Based on the active lossless network, construct the downstream distribution matrix A = (a ij ) r×r , r is the number of nodes in the network, and the element a in the matrix ij for:

[0229]

[0230] Where P ij is the active power flowing from node i to node j on line ij; P Tj is the total active power injected into node j.

[0231] 4) Construct the generator active output diagonal array P GG , node active load diagonal matrix P LL and the node injected active power diagonal matrix P TT :

[0232] P GG =diag[P G1 ,P G2 ,…P Gr ] (20)

[0233] P LL =diag[P L1 ,P L2 ,…P Lr ] (twenty one)

[0234] P TT =diag[P T1 ,P T2 ,…P Tr ] (twenty two)

[0235] Where P Gr is the active output of the generator at node r; P Lr is the active load of node r; P Tr is the total active power injected into node r.

[0236] 5) Calculate the coefficient matrix K for allocating active power from the power node to the load node G and the coefficient matrix K of the load node drawing active power from the power node L :

[0237] K G =P GG (P TT ) -1 A -1 (twenty three)

[0238] K L =P LL (P TT A T ) -1 (twenty four)

[0239] 6) Obtain the active power P(m,n) provided by power node m to load node n:

[0240] P(m,n)=K L (n,m)P Gm (25)

[0241] In the formula, K L (m,n) is the active power extraction coefficient matrix K L The element in the nth row and mth column of Gm is the active output of generator node m.

[0242] 7) Calculate P ij (m,n):

[0243] P ij (m,n)=K L (n,j)K G (m,i)P ij (26)

[0244] Where i is the node number where the active power of the line flows in; j is the node number where the active power of the line flows out; K L (n,j) represents the matrix K L The element in the nth row and jth column, K G (m,i) represents the matrix K G The element in the mth row and ith column of ij is the active power on line ij.

[0245] 8) According to formula (18), calculate the power flow intermediate number F of all lines in the power grid: ij .

[0246] Based on the above analysis of the key path selection method, the key path selection process is as follows: Figure 7 shown.

[0247] 1.2.3 Example Analysis

[0248] In order to verify the effectiveness and correctness of the proposed method, the same distributed power supply setting as in Section 1.1.4 is selected as the example system.

[0249] Matlab programming is used to implement the proposed method, calculate the power flow betweenness, and normalize and sort the obtained results. The first 15 key lines of the system are shown in Table 7.

[0250] Table 7 Key line selection results

[0251]

[0252] Table 7 can be visualized as Figure 8 As shown in the figure, the red lines are the first 15 key lines of the system corresponding to Table 7.

[0253] from Figure 8It can be seen that the key lines are all lines close to the power supply nodes. Since the load power on nodes 7-11 and 15-18 is small, the power generated by DG2 and DG3 can be transmitted forward more evenly and will not be consumed too much on these nodes, so the power flowing through the lines connected to these nodes is also large, so these lines are more important. Lines 3-23 and 23-24 can transmit the power from the main network and DG1 to the large load nodes 24 and 25, so lines 3-23 and 23-24 are also crucial. As for the lines near the DG4 access point, since the load power on nodes 29-32 is large, 0.12MW, 0.20MW, 0.15MW and 0.21MW respectively, and the power generated by DG4 is 0.6MW, it cannot meet the needs of the four nodes, and its power cannot be further transmitted outward. Therefore, although lines 33-32, 32-31, 31-30 are close to the power supply node, from the perspective of power flow, the criticality of these lines is relatively low. For lines 13-12 and 14-13 close to DG2, their node loads are not large, and the power flowing through the lines is relatively large, but they are still not critical lines. The reason is that according to the power flow calculation results, the power of DG2 is transmitted to the direction of nodes 11 to 7, rather than to the direction of nodes 13 to 17. Therefore, lines 13-12 and 14-13 only draw power from DG3 and only play a role in transmitting the power from DG3. According to the superposition theorem, their criticality is lower than other lines that can transmit power from multiple power sources.

[0254] Considering the fluctuation of the output power of distributed power sources, the power flow of the distribution network changes accordingly, which leads to changes in the selection of key lines. Also assume that at a certain moment, Figure 3 The output power of distributed power sources DG1 and DG4 is increased by 1.2 times, and the output power of DG2 and DG3 is reduced by 0.8 times. The key lines of the distribution network are reselected. The selection results are shown in Table 8. The bold node numbers in the table indicate differences from the results in Table 7.

[0255] Table 8 Results of key line selection considering DG output power fluctuation

[0256]

[0257] Comparing Table 7 and Table 8, it can be found that the selection results of key lines have not changed, but the criticality of each line has changed to a certain extent, proving that the fluctuation of distributed generation output power will lead to changes in the selection results of key lines. Under the fluctuation of DG output power, the power flowing through each line will change, and its contribution and importance in the power transmission process will also change accordingly. Therefore, the line criticality obtained based on the power flow betweenness method will also change accordingly.

[0258] Considering that the access of the interconnection line changes the power flow of the distribution network, the selection result of the key line will change. Figure 3 All five interconnection lines in the distribution network are connected, and the key lines of the distribution network are reselected. The selection results are shown in Table 9. The bold node numbers in the table indicate differences from the results in Table 7.

[0259] Table 9 Results of selecting key nodes considering the access of contact lines

[0260]

[0261]

[0262] Comparing Table 7 and Table 9, it can be found that all key lines have undergone significant changes. Lines 33-32, 32-31, 29-25, etc. are selected as key lines, while lines 12-11, 11-10, 10-9, etc. are no longer key lines. This is because the access of the tie line will greatly change the power flow distribution of each line in the distribution network, which will lead to a significant change in the line criticality obtained based on the power flow betweenness method.

[0263] 1.2.4 Field example verification in a certain city

[0264] In order to verify the actual effect of the proposed method in a certain city, the 68-node system of Chengmei Line in a certain city is also used for simulation analysis. The proposed method is implemented using Matlab, the power flow betweenness is calculated, and the obtained results are normalized and sorted. The first 15 key lines of the system are shown in Table 10.

[0265] Table 10 Results of key line selection for the 68-node system of Chengmei Line in a certain city

[0266]

[0267] Fig. 9 The red lines in the figure are the first 15 key lines of the system corresponding to Table 2-25.

[0268] from Fig. 9It can be found that the key lines are all lines close to the power supply nodes. Since the load power on bus No. 9-10 and No. 36-48 is small, or even mostly zero, the power generated by DG1, DG2 and DG3 can be transmitted forward more evenly and will not be consumed too much at these nodes. This will cause the power flowing through the lines connected to these nodes to be large, that is, the large amount of energy generated by the distributed power source is mainly transmitted to the entire network through these lines, so these lines are of great importance. Although lines such as No. 55-58 are also close to the power supply node, due to the large load on the bus to which they belong, and these lines are terminal lines in terms of geographical location, they cannot continue to transmit energy backward, so their criticality is relatively low.

[0269] In summary, the key line selection method proposed in the present invention can effectively select the lines that have an important impact on the power supply capacity and safe operation of the power grid in actual operation from the perspective of line flow. In view of the fluctuation of the output power of the distributed power supply in the distribution network and the change of key lines caused by the access of the interconnection line, the real-time key line selection results can also be obtained through the real-time selection method.

[0270] 2. Real-time security assessment of distribution network based on key links

[0271] 2.1 Risk Assessment Indicators

[0272] In this method, the constructed risk assessment indicators include component layer risk assessment indicators and distribution network layer risk assessment indicators. The component layer risk assessment indicators include key node overvoltage risk indicators, key node low voltage risk indicators and key line overload risk indicators. The distribution network layer risk assessment indicators include distribution network overvoltage risk indicators, distribution network low voltage risk indicators and distribution network line overload risk indicators.

[0273] 2.2 Component-level risk assessment indicators

[0274] This section considers the risks of voltage exceeding the limit at the component level and line overload caused by the uncertainty of distributed generation output and load demand, including bus overvoltage risk, bus low voltage risk and line overload risk. The corresponding risk assessment indicators are as follows.

[0275] 1) Risk of overvoltage at key nodes

[0276] The overvoltage risk index reflects the risk of the voltage at each node of the distribution network exceeding the voltage upper limit under different system operating conditions. According to the distribution network voltage standard, the voltage deviation is generally limited to less than ±5%. If it exceeds this range, it is considered to be over-limit. The overvoltage risk is as follows:

[0277]

[0278] In the formula, Ri,t vh is the overvoltage risk of the i-th key node at time t; B i,t vh It is a 0-1 indicator of overvoltage operation risk, indicating whether a voltage over-limit event occurs; Sev vh is the severity function of the voltage exceeding the upper limit, specifically:

[0279]

[0280] Where V i,t is the voltage of the i-th key node at time t; V max is the maximum operating voltage limit specified, take V max =1.05pu.

[0281] 2) Low voltage risk at key nodes

[0282] The low voltage risk index reflects the risk that the voltage at each node of the distribution network is lower than the lower voltage limit under different system operating conditions. The low voltage risk is as follows:

[0283]

[0284] In the formula, R i,t vl is the low voltage risk of the i-th key node at time t; B i,t vh It is a 0-1 indicator of low voltage operation risk, indicating whether a voltage crossing lower limit event occurs; Sev vl is the severity function of the voltage exceeding the lower limit, specifically:

[0285]

[0286] Where V min is the minimum operating voltage limit, take V min =0.95pu.

[0287] 3) Risk of overloading of key lines

[0288] The line overload risk index is defined for each line. The percentage of the flow in each line that exceeds the transmission capacity limit determines the severity of its overload. The line overload risk is as follows:

[0289]

[0290] In the formula, R ij,t p is the active power overload risk of the key line ij at time t; B i,t vhSev is a 0-1 indicator of line overload risk, indicating whether a line overload event occurs; p is the severity function of line overload, specifically:

[0291]

[0292] Where P ij,max is the maximum limit of active power flowing through the key line ij; α is the set threshold, which can be selected according to the actual requirements of the operator. In this paper, α=0.67 is taken, and it is considered that when the actual active power on the line is greater than 67% of the maximum limit of the line active power, the line overload risk occurs.

[0293] 2.3 Distribution network layer risk assessment indicators

[0294] 1) Distribution network overvoltage risk

[0295] The overvoltage risk of the distribution network is obtained by multiplying the overvoltage risk of each key node by its criticality and then adding them together:

[0296]

[0297] In the formula, R sys,t vh is the overvoltage risk of the distribution network at time t; COM i,t is the criticality of the i-th key node at time t; N is the set of key nodes.

[0298] 2) Distribution network low voltage risk

[0299] The low voltage risk of the distribution network is obtained by multiplying the low voltage risk of each key node by its criticality and then adding them together:

[0300]

[0301] In the formula, R sys,t vl is the low voltage risk of the distribution network at time t.

[0302] 3) Risk of overloading of distribution network lines

[0303] The overload risk of distribution network lines is obtained by adding the overload risks of each key line and multiplying it by its criticality:

[0304]

[0305] In the formula, is the line overload risk of the distribution network at time t; F ij,t is the criticality of the key path ij at time t; L is the key path set.

[0306] Based on the analysis of the above evaluation indicators, the process of real-time security evaluation of distribution network is as follows: Figure 1 shown.

[0307] 2.4 Example Analysis

[0308] In order to verify the effectiveness and correctness of this method, we selected Fig.10 The IEEE 33-node system shown in the figure is used as an example system, and distributed power sources are connected to nodes 6, 12, 18, and 33. The 24-hour output curve of the distributed power source and the load demand of each node in the distribution network are shown in Figure 1. Fig.11 , Fig.12 As shown. Among them, the distributed power source is the PQ node, the power factor is 0.95; the voltage of the balance node is 1p.u.. By forward-backward substitution method, the bus voltage value of the distribution network at each moment and the active power flowing through each line are obtained, as shown in Fig.13 As shown, the red and blue planes in the left figure are the specified maximum and minimum operating voltage limits respectively, and the red plane in the right figure is the line active power limit.

[0309] Based on the above-mentioned key link selection method, the key links of the distribution network are selected in real time. Matlab programming is used to implement the method in this paper, and real-time risk assessment of key nodes and key lines is carried out from the two perspectives of bus voltage and line flow. The overvoltage risk, low voltage risk and overload risk of the distribution network at each moment are as follows: Fig.14 shown.

[0310] In terms of node overvoltage, it can be found that the distribution network only has overvoltage at key nodes at 12 o'clock. Fig.11 and Fig.13 It can be found that at 12 o'clock, the distributed power sources in the distribution network are at maximum output, and because the three distributed power access points DG1, DG2 and DG3 are relatively close, the voltage of key nodes 17 and 18 is raised to above 1.05pu, resulting in node overvoltage.

[0311] In terms of node low voltage, it can be found that the distribution network has low voltage at key nodes from 4 to 7 o'clock and from 18 to 22 o'clock. This is because the output of distributed power sources in the distribution network is almost 0 at these times, so the power required in the distribution network is almost all drawn from the main network, resulting in a drop in node voltage at the end of the distribution network line, and then node low voltage occurs. In particular, the load demand in the distribution network from 18 to 22 o'clock is greater than that from 4 to 7 o'clock, so the drop in node voltage at the end of the line is greater, resulting in a higher risk of node low voltage from 18 to 22 o'clock than from 4 to 7 o'clock.

[0312] In terms of line overload, it can be found that the distribution network has key line overload at 6-7 and 19-22. This is also because the output of distributed power sources in the distribution network is almost 0 at these times, so the power required in the distribution network is almost all drawn from the main network, and the distribution network is connected to the main network by only one line, which results in a large amount of power drawn from the main network flowing through this line, so the line is prone to overload. In addition, since the load demand in the distribution network at 19-22 is greater than that at 6-7, the power drawn by the distribution network is greater, resulting in a higher risk of line overload at 19-22 than at 6-7.

[0313] In summary, the present invention conducts real-time safety assessment of the selected key links from the perspectives of overvoltage, low voltage at key nodes and overload of key lines. The assessment results can effectively reflect the risks of the distribution network at different times, and can provide managers with reasonable and accurate risk warnings to ensure the safe operation of the distribution network.

[0314] This embodiment also provides an active distribution network real-time safety assessment system based on key links, including a memory, a processor, and computer program instructions stored in the memory and capable of being executed by the processor. When the processor executes the computer program instructions, the above method can be implemented.

[0315] This embodiment further provides a computer-readable storage medium on which computer program instructions are stored. When the computer program instructions are executed by a processor, the above method is implemented.

[0316] Those skilled in the art will appreciate that the embodiments of the present application may be provided as methods, systems, or computer program products. Therefore, the present application may adopt the form of a complete hardware embodiment, a complete software embodiment, or an embodiment in combination with software and hardware. Moreover, the present application may adopt the form of a computer program product implemented in one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) that include computer-usable program code.

[0317] The present application is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present application. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1A device that provides the functions specified in a block or multiple blocks.

[0318] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0319] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0320] The above is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A real-time security assessment method for active distribution network based on key links, characterized in that: The following steps are involved: S1. Obtain basic data of distribution network, distributed power source and load; S2. Calculate the power flow by forward-backward substitution method to obtain the bus voltage value of the distribution network at each moment and the active power flowing through each line; S3. Select key links, including key nodes and key lines; S4. Based on the constructed risk assessment indicators, real-time safety assessment of key links is carried out from the two perspectives of bus voltage and line flow.

2. The real-time security assessment method for active distribution network based on key links according to claim 1 is characterized in that: The selection of key nodes is realized based on trend tracking and PageRank algorithm. The implementation method is as follows: First, based on the power flow calculation results, power flow tracking is achieved through power allocation analysis; In the formula, K=(k ij ) n×n is the distribution coefficient matrix of generator to load; P GG =diag(P G1 ,P G2 ,…,P Gn ), P G1 ,P G2 ,…,P Gn represents the active output column vector of the n-dimensional generator; P TT =diag(P T1 ,P T2 ,…,P Tn ), P T1 ,P T2 ,…,P Tn represents the total active power column vector of n-dimensional nodes; P ij is the active power transmitted from node i to node j on line ij; P Tj is the total active power injected into node j; P Gi→j For the generator G i The power distribution to load node j, P Lj is the load active power of node j; P Gi→L(s,t) For the generator G i Power distribution to line L(s,t), P L(s,t) The active power downstream distribution matrix A of line L(s,t) is constructed as: The contribution of the generator to the line power and node load, as well as the power drawn by the load from the generator, are obtained by calculation using formula (1); The PageRank algorithm represents the entire network as a directed graph, where web pages represent each node and node-related edges represent links between nodes; the PR value is used to represent the importance of the power grid node to the entire network, which is obtained by combining the importance of the node itself and the importance of other nodes linked to the node; the steady-state value PR is obtained after iterative calculation of the entire network, and the larger the PR value of the node, the more important it is in the network; the iterative calculation of the PR value is shown in formula (3); In the formula, {p1,p2,…,p N } is the set of network nodes; M(p i ) points to node p i The node set of L(p j ) is p j The number of outgoing nodes; N is the number of all nodes in the network; σ is the damping coefficient; Combined with the power transmission relationship, the contribution of the generator to the lines and nodes in the system is considered; all the energy in the system is provided by the generator, and the position of the generator in the system energy supply is determined according to the size of the generator power generation, and the generator importance weight η is defined Gk , as shown in formula (4); Where P Gk For unit G k Power generation; ∑P G is the total power generation of the system; Combined with the importance weight of the generator, the power allocation of the generator to the line is further considered; the system node link strength based on the power flow tracking result as shown in formula (5) is used to characterize the link weight between each node; Where n is the number of system units; ν i , ν j For line L(ν i ,ν j ) The node numbers at both ends; P Gk→L(νi,νj) For unit G k On line L(ν i ,ν j ) power distribution; η Gk For the generator G k The importance weight of The power allocation of the generator node to each load node needs to take into account the importance of each generator in the system and is expressed by the load weight as shown in formula (6); Where N is the number of generators in the system; P Gi→j For the generator G i For the power allocation of node j; η Gi For the generator G i The importance weight of In order to prevent distortion caused by excessive node load weights, the node load weights are normalized: Based on the PageRank algorithm, the system link strength is used as an importance ranking factor, and the importance of system load is comprehensively considered to select key nodes: In a power grid with n nodes, suppose node ν has links from ν1,ν2,…,ν l ,ω line (ν i ,ν) is the link node ν i and ν are the system link strength weights, then the calculation formula for node criticality is shown in formula (8); In the formula, PT(ν l ) is the node source ν pointing to node ν l Its own PT node key value; S out (ν l ) is the node source ν l The outgoing chain strength; Through the above definition, the system link strength matrix is ​​obtained as shown in formula (9); Where L is the system line set; ω Line (ν i ,ν j ) is the link strength between two nodes. If there is no line link between the nodes, it is 0. Divide each row of the system link strength matrix W by the sum of the elements in the row and normalize it to obtain the new link strength matrix Q: After introducing normalization, the load importance factor matrix is: Combining the link strength matrix with the node load importance factor, the improved link matrix Q' is obtained: Q'=ψ load Q (12) According to formula (12), we get the matrix Where E is the unit matrix; Therefore, the iterative formula (8) is transformed into the following form: If the node voltage is higher than the upper voltage limit or lower than the lower voltage limit, the risk caused by the node voltage exceeding the limit is expressed by formula (15); In the formula, Sev i is the voltage over-limit severity of the ith node; V i is the voltage of the ith node; V max is the maximum operating voltage limit specified, take V max =1.05pu; V min is the specified minimum operating voltage limit; The comprehensive criticality of the node is obtained by normalizing the PT value and the voltage over-limit severity and weighting them: COM i =ω1PT i ′+ω2Sev i ′ (16) In the formula, COM i is the comprehensive criticality of the i-th node; PT i ' and Sev i ' are the PT value of the i-th node and the normalized value of the voltage over-limit severity; by i Sort and select key nodes from large to small.

3. The real-time security assessment method for active distribution network based on key links according to claim 1 is characterized in that: The key lines are selected based on the power flow betweenness, and the implementation method is as follows: In order to characterize the utilization and importance of each line in the entire network when power source m transmits power P(m,n) to load n, the line flow betweenness of a single power source-load node pair is defined as: Where, min(P m ,P n ) is the weight of a single power flow betweenness, which is the smaller value between the actual active output of source m and the actual active load of load n; P(m,n) is the active power transmitted from source m to load m; P ij (m,n) is the contribution power of power flow on line ij when power source m transmits active power to load m; F ij,mn Characterizes the importance of the line in the distribution network when a single power source transmits active power to a single load; Calculate the F of each line according to formula (17): ij,mn According to the superposition principle, the power flow betweenness of all power source-load node pairs when transmitting active power is accumulated to obtain the line power flow betweenness F ij : In the formula, m is the power node, G is the power node set; n is the load node, L is the load node set; In the calculation formula (18) of the line power flow betweenness, it is necessary to obtain P(m,n) and P ij (m,n); According to the principle of power flow tracking, when the power grid power flow distribution is determined, the network is simplified into a lossless network, and then the downstream distribution matrix is ​​constructed. The downstream tracking method is used to track the power flow of the power source-load node pair to determine the power component in the line ij.

4. The real-time security assessment method for active distribution network based on key links according to claim 3 is characterized in that: Based on the power flow betweenness method, the line power flow betweenness is calculated as follows: 1) Under the condition of determining the operating status of the power grid, the active power generated by the generator, the active power absorbed by the load, the active power flow of each line and the line loss are obtained; 2) The active power loss of the line is regarded as an equivalent load and evenly distributed to the head and end nodes of the line to form an active power lossless network; 3) Based on the active lossless network, construct the downstream distribution matrix A = (a ij ) r×r , r is the number of nodes in the network, and the element a in the matrix ij for: Where P ij is the active power flowing from node i to node j on line ij; P Tj is the total active power injected into node j; 4) Construct the generator active output diagonal array P GG , node active load diagonal matrix P LL and the node injected active power diagonal matrix P TT : P GG =diag[P G1 ,P G2 ,…P Gr ] (20) P LL =diag[P L1 ,P L2 ,…P Lr ] (21) P TT =diag[P T1 ,P T2 ,…P Tr ] (22) Where P Gr is the active output of the generator at node r; P Lr is the active load of node r; P Tr is the total active power injected into node r; 5) Calculate the coefficient matrix K for allocating active power from the power node to the load node G and the coefficient matrix K of the load node drawing active power from the power node L : K G =P GG (P TT ) -1 A -1 (23) K L =P LL (P TT A T ) -1 (24) 6) Obtain the active power P(m,n) provided by power node m to load node n: P(m,n)=K L (n,m)P Gm (25) In the formula, K L (m,n) is the active power extraction coefficient matrix K L The element in the nth row and mth column of Gm is the active output of generator node m; 7) Calculate P ij (m,n): P ij (m,n)=K L (n,j)K G (m,i)P ij (26) Where i is the node number where the active power of the line flows in; j is the node number where the active power of the line flows out; K L (n,j) represents the matrix K L The element in the nth row and jth column, K G (m,i) represents the matrix K G The element in the mth row and ith column of ij is the active power on line ij; 8) According to formula (18), calculate the power flow intermediate number F of all lines in the power grid: ij .

5. The real-time security assessment method for active distribution network based on key links according to claim 1 is characterized in that: The constructed risk assessment indicators include component layer risk assessment indicators and distribution network layer risk assessment indicators. The component layer risk assessment indicators include key node overvoltage risk indicators, key node low voltage risk indicators and key line overload risk indicators. The distribution network layer risk assessment indicators include distribution network overvoltage risk indicators, distribution network low voltage risk indicators and distribution network line overload risk indicators.

6. The real-time security assessment method for active distribution network based on key links according to claim 5 is characterized in that: The key node overvoltage risk index reflects the risk of the voltage of each node of the distribution network exceeding the voltage upper limit under different system operating conditions, and its calculation formula is as follows: In the formula, R i,t vh is the overvoltage risk of the i-th key node at time t; B i,t vh Sev is a 0-1 indicator of overvoltage operation risk, indicating whether a voltage over-limit event occurs; vh is the severity function of the voltage exceeding the upper limit, specifically: Where V i,t is the voltage of the i-th key node at time t; V max is the specified maximum operating voltage limit; The key node low voltage risk index reflects the risk that the voltage of each node of the distribution network is lower than the voltage lower limit under different system operating conditions. The calculation formula is as follows: In the formula, R i,t vl is the low voltage risk of the i-th key node at time t; B i,t vh It is a 0-1 indicator of low voltage operation risk, indicating whether a voltage crossing lower limit event occurs; Sev vl is the severity function of the voltage exceeding the lower limit, specifically: Where V min is the specified minimum operating voltage limit; The critical line overload risk index is defined for each line. The percentage of the flow in each line exceeding the transmission capacity limit determines the severity of its overload. The calculation formula of the critical line overload risk index is as follows: In the formula, R ij,t p is the active power overload risk of the key line ij at time t; B i,t vh Sev is a 0-1 indicator of line overload risk, indicating whether a line overload event occurs; p is the severity function of line overload, specifically: Where P ij,max is the maximum limit of active power flowing through the key line ij; α is the set threshold.

7. The real-time security assessment method for active distribution network based on key links according to claim 5 is characterized in that: The distribution network overvoltage risk index is obtained by multiplying the overvoltage risk of each key node by its criticality and then adding them together: In the formula, R sys,t vh is the overvoltage risk of the distribution network at time t; COM i,t is the criticality of the i-th key node at time t; N is the set of key nodes; The distribution network low voltage risk index is obtained by multiplying the low voltage risk of each key node by its criticality and then adding them together: In the formula, R sys,t vl is the low voltage risk of the distribution network at time t; The distribution network line overload risk index is obtained by adding up the overload risks of each key line and multiplying it by its criticality: In the formula, R sys,t p is the line overload risk of the distribution network at time t; F ij,t is the criticality of the key path ij at time t; L is the key path set.

8. A real-time security assessment system for active distribution network based on key links, characterized in that: The method comprises a memory, a processor, and computer program instructions stored in the memory and executable by the processor. When the processor executes the computer program instructions, the method according to any one of claims 1 to 7 can be implemented.

9. A computer-readable storage medium having computer program instructions stored thereon, characterized in that: When the computer program instructions are executed by a processor, the method according to any one of claims 1 to 7 is implemented.

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