A dynamic partitioning method for distribution network based on improved VVS calculation model

Abstract

The present invention proposes a distribution network dynamic partitioning method based on an improved VVS calculation model, and the steps are as follows: first, by adding voltage reactive output perturbation to the reactive source to calculate the voltage / reactive sensitivity matrix between each node; second, fuzzifying the voltage / reactive sensitivity matrix to obtain the membership matrix between each node; and analyzing the membership relationship between reactive source nodes according to the membership matrix between each node, pre-partitioning the reactive source nodes; then, analyzing the membership relationship between each load node and reactive source node according to the membership matrix between each node, and determining the partitioning result; finally, constructing a partitioning membership index to evaluate the partitioning result, and ensuring reactive power balance within the partition. The present invention calculates the VVS of each reactive source to the remaining nodes by adding voltage reactive output perturbation to the reactive source, which can reflect the temporal changes of power supply and load, and better ensure the reasonable distribution of voltage and reactive power within each partition.

Description

Technical Field

[0001] The present invention relates to the technical field of distribution network partitioning, and in particular to a distribution network dynamic partitioning method based on an improved VVS calculation model. Background Art

[0002] Voltage and reactive power control of power systems is an important means to ensure safe, economical and stable operation of the system. The existing voltage and reactive power control modes are divided into three types: centralized control, decentralized independent control and hierarchical control. Among them, the three-level voltage control mode first proposed by the French Electricity Company has been well applied in power grids in many countries or regions. In hierarchical control, it is necessary to first divide the power grid into multiple sub-areas with strong internal coupling and weak mutual coupling, and then realize the global control of voltage and reactive power of the entire power grid through coordinated optimization. Therefore, reasonable zoning is the basis for hierarchical voltage and reactive power control, and the study of zoning methods is an important topic of voltage control.

[0003] At present, most distribution network partitions are based on electrical distance. There are many definitions of electrical distance, of which two are more common. One is to use the topological information of the power grid itself to define the electrical distance, such as the node admittance matrix, impedance matrix, etc., but this type of method cannot reflect the power flow state of the system and the advantages of reactive power in voltage control. The other is to use the voltage / reactive sensitivity (Voltage / Var Sensitivity, VVS) between nodes to define the electrical distance. Among them, the most common calculation method of the VVS matrix is ​​the Jacobian matrix of the Newton-Raphson method. The eigenvector corresponding to its minimum eigenvalue is closely related to the reactive power distribution, and can be used as the basis for regional division in secondary voltage control. The existing VVS calculation method has the following disadvantages: 1) VVS refers to the effect of reactive power output change ΔQ on the voltage change ΔU of other nodes, where ΔU and ΔQ are nonlinear and non-positive and negatively correlated. In the Jacobian matrix of power flow calculation, ΔQ is very small (usually less than 10e-5), which cannot characterize the VVS of other nodes when the reactive power source ΔQ changes greatly; 2) The sensitivity between non-adjacent nodes may be greater than the sensitivity between adjacent nodes. Therefore, the connectivity of the system cannot be guaranteed only by partitioning based on electrical distance, and isolated nodes need to be merged afterwards; 3) The VVS matrix is ​​an asymmetric matrix. The existing method uses mathematical methods to process it and obtains a symmetric electrical distance matrix, but the electrical distance cannot fully characterize the sensitivity relationship between two nodes. The existing method uses mathematical methods to process it and obtains a symmetric electrical distance matrix, but the electrical distance cannot fully characterize the sensitivity relationship between two nodes. Summary of the invention

[0004] In view of the shortcomings of the existing electrical distance, the present invention proposes a dynamic partitioning method for distribution network based on an improved VVS calculation model. The voltage reactive power sensitivity of each reactive source to the remaining nodes is calculated by adding voltage reactive power output perturbation to the reactive source. The proposed improved sensitivity calculation method can reflect the temporal changes of power supply and load, thereby better ensuring the reasonable distribution of voltage reactive power in each partition.

[0005] The technical solution of the present invention is achieved in this way:

[0006] A distribution network dynamic partitioning method based on an improved VVS calculation model, the steps are as follows:

[0007] Step 1: Calculate the voltage / reactive sensitivity matrix between nodes by adding voltage reactive output perturbation to the reactive source; wherein the nodes include reactive source nodes and load nodes;

[0008] Step 2: Fuzzify the voltage / reactive sensitivity matrix to obtain the membership matrix between each node; and analyze the membership relationship between reactive source nodes according to the membership matrix between each node, and pre-partition the reactive source nodes;

[0009] Step 3: Analyze the membership relationship between each load node and reactive source node according to the membership matrix between each node, and determine the partition result;

[0010] Step 4: Construct a partition membership index to evaluate the partition results and ensure reactive power balance within the partition.

[0011] Preferably, the voltage / reactive power sensitivity matrix is ​​calculated as follows:

[0012]

[0013] Where deltQ k is the reactive power output perturbation change vector of the kth reactive source node, S VQ is the voltage / reactive power sensitivity matrix, the element (S VQ )ij represents the voltage change of the controlled node i when the reactive output disturbance of the reactive source j changes, V is the load node voltage, and Q is the reactive power of the reactive source node.

[0014] Preferably, the membership matrix is ​​calculated as follows:

[0015]

[0016] Where G represents the number of reactive source nodes, μ A ((S VQ ) ij ) ijIt represents the membership of the jth controlled node belonging to the i-th reactive source node; when there is only one reactive source in the partition, the membership of node i to the reactive source is the membership of the partition; when there are multiple reactive sources in a partition, the sum of the memberships of node i to all reactive sources in the partition is the membership of the partition.

[0017] Preferably, the partition membership index includes a regional comprehensive membership index, a regional reactive power balance index and a sub-regional scale index.

[0018] Preferably, the expression of the regional comprehensive membership index is:

[0019]

[0020] Among them, F k represents the comprehensive regional membership index, It represents the average membership of the reactive source and the controlled node in region k multiplied by the minimum membership of the reactive source and the controlled node in the region. represents the average membership of the reactive power source in region k with the controlled nodes in the region multiplied by the minimum membership of the reactive power source not belonging to region k with the controlled nodes in region k, l k is the number of controlled nodes in region k, μ Aij represents the membership matrix;

[0021] F k The larger the value, the better the partitioning effect; conversely, the smaller the value, the better the partitioning effect. k The smaller the value, the worse the partitioning effect.

[0022] Preferably, the expression of the regional reactive power balance index is:

[0023]

[0024]

[0025] Among them, Q k is the reactive power balance of area k; is the reactive power balance index; K is the number of partitions; is the maximum reactive power in region k, is the reactive power demand value within area k.

[0026] Preferably, the expression of the sub-region size index is:

[0027]

[0028] Among them, n K is the number of internal nodes in the sub-region K, and S is the sub-region scale index.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention calculates the VVS of each reactive source to the remaining nodes by adding voltage reactive output perturbations to the reactive source. At the same time, voltage reactive power partitioning is performed based on the improved VVS calculation method. First, the VVS matrix is ​​fuzzified to obtain the membership matrix between the nodes. According to the membership relationship between the power supply nodes, the power supply nodes with strong coupling relationships are merged to determine the number of initial partitions. Then, the final partitioning result is determined based on the membership relationship between each load node and each reactive power. Finally, the partitioning result is evaluated based on the partition membership index to ensure strong coupling within the partition, weak coupling between partitions, and reactive power balance in the partition. The proposed sensitivity method reflects the temporal changes of "source-load", so as to better ensure the reasonable distribution of voltage and reactive power in various regions. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0031] Figure 1 It is a comparison diagram of the sensitivity curves calculated by the method of the present invention and the Newton method; among them, (a) the VVS of reactive source 34 node to each node, (b) the VVS of reactive source 35 node to each node, (c) the VVS of reactive source 36 node to each node, (d) the VVS of reactive source 37 node to each node, and (e) the VVS of reactive source 38 node to each node.

[0032] Figure 2 It is a partition flow chart of the present invention. DETAILED DESCRIPTION

[0033] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] VVS analysis is based on the power flow equation of the system, using the differential change relationship of certain quantities in the system to study certain characteristics of the system. The power balance equation of the power system is:

[0035] f(x,u,α)=0 (1)

[0036] Where x is the state variable, including the voltage amplitude and phase angle of the load node and the phase angle of the generator node; u is the control variable, including the generator node voltage, active output and reactive output of the compensation device; α is the relevant parameter of the power flow calculation, including the active and reactive power of the load node, because when the system is operating normally, the diversity of the load affects the system voltage.

[0037] By Taylor expanding equation (1) and ignoring its higher-order terms, we can obtain the following linear relationship:

[0038]

[0039]

[0040] Formula (3) can be simplified as follows:

[0041] △x=S xu △u+S xα △α (4)

[0042] In the formula, S xu , S xα are the sensitivities of variable x to u and α respectively.

[0043] Depending on the selected variables, different types of sensitivity indicators can be constructed, which can be applied to different scenarios in the power system to solve problems such as voltage stability assessment, weak node determination, and reactive compensation point selection. In the voltage-reactive control partition, the reactive power of the power source node and the voltage of the load node are selected to construct the VVS indicator, which is used to study the impact of the reactive power output change of the reactive source on the load node voltage. According to formula (4), the calculation formula of this indicator is as follows:

[0044] △V=S VQ △Q+S Vα △α (5)

[0045] In the formula, ΔV and ΔQ are the changes of load node voltage and power node reactive power, respectively. VQ , S Vα They are respectively the sensitivity of the load node voltage V to the power node reactive power Q and the power flow related parameter α.

[0046] From formula (5), we can see that the change of control variable Q and power flow related parameter α will directly affect the change of load node voltage V. In a fixed time section, the load and other power flow related parameters are fixed. We only focus on the impact of reactive power source output change on load node voltage. At this time, Δα=0, formula (5) can be simplified to:

[0047]

[0048] Most existing methods use the Jacobian matrix of power flow calculation to calculate the VVS between nodes. In the Newton-Raphson method power flow calculation, there is the following formula:

[0049]

[0050] Where ΔP, ΔQ, Δθ and ΔV are the changes of active power, reactive power, voltage phase angle and voltage amplitude of the node respectively; J is the Jacobian matrix, which can be obtained by dividing the matrix into blocks: Pθ , J PV , J Qθ and J QV , are the partial derivatives of active power with respect to phase angle, active power with respect to voltage, reactive power with respect to phase angle, and reactive power with respect to voltage respectively.

[0051] In order to analyze the impact of reactive power change on voltage, it is usually assumed that the load active power remains unchanged, that is, ΔP = 0. At this time, according to formula (7), VVS can be obtained as follows:

[0052]

[0053] The VVS is obtained by converging the Jacobian matrix of the power flow using the Newton method. It is currently the most commonly used method for obtaining VVS. Among them, the selection of reactive disturbance in this method is related to the criterion of power flow convergence, that is, the reactive power unbalance ΔQ in the power flow of each node. In matpower, ΔQ<10E-5 is generally assumed by default, so there are the following defects:

[0054] (1) ΔQ is very small and cannot reflect the sensitivity when ΔQ changes greatly;

[0055] (2) Sensitivity is a nonlinear, non-positive or negative correlation quantity;

[0056] (3) It cannot reflect the sensitivity changes under different load levels;

[0057] (4) The connectivity of the system cannot be guaranteed and can only be verified and merged afterwards;

[0058] (5) Existing methods usually use a logarithmic transformation function for sensitivity, thus making the electrical distance matrix a symmetric matrix. However, the sensitivity between two nodes is inherently different, so the electrical distance indicator cannot fully characterize the sensitivity relationship between the two nodes.

[0059] (6) The transmission network X>>R, so the voltage is strongly correlated with reactive power and weakly correlated with active power. The effect of active power on voltage can be ignored. However, the distribution network has a larger R / X ratio, and the effect of active power on voltage cannot be ignored.

[0060] Based on this, the embodiment of the present invention proposes a distribution network dynamic partitioning method based on an improved VVS calculation model, such as Figure 2 As shown, the specific steps are as follows:

[0061] Step 1: Calculate the voltage / reactive sensitivity matrix between nodes by adding voltage reactive output perturbation to the reactive source; wherein the nodes include reactive source nodes and load nodes;

[0062] The control capability of reactive source nodes over the voltage of controlled nodes can be characterized by the quasi-steady-state voltage control sensitivity. The sensitivity matrix is ​​obtained by considering the perturbation of reactive source output:

[0063]

[0064] Where deltQ k is the reactive power output perturbation change vector of the kth reactive source node, S VQ is the voltage / reactive power sensitivity matrix, the element (S VQ )ij represents the voltage change of the controlled node i when the reactive output disturbance of the reactive source j changes, V is the load node voltage, and Q is the reactive power of the reactive source node.

[0065] The selection of the reactive source perturbation is related to the type of reactive source. When the reactive source node is a PQ type node, the perturbation deltQ of the capacitor bank G The single group capacity can be taken, DG's deltQ G The installed capacity S DG The reactive power output perturbation of DG is an empirical value, which can be determined according to the output time series change curve of DG (daily output prediction curve). When the output changes slowly, a lower ratio is selected, such as 1% to 2%. When the output changes rapidly, a higher ratio is selected, such as 5%. When the load time series curve in the area is increasing, a positive value is selected, otherwise a negative value is taken. When DG is a PV type node, the voltage of the PV node is used as the control variable, and different voltage disturbance deltV is selected. Q changes with deltV to obtain deltQ G , and then we can get ΔV / deltQ G .

[0066] Figure 1 The following is a comparison chart of the sensitivity curves obtained by Newton-Raphson power flow calculation and reactive perturbation power flow calculation. The following conclusions can be drawn from the analysis of these curves:

[0067] (1) The VVS index curve obtained by the VVS analysis method introduced in the present invention is consistent with the physical concept and is consistent with the change trend of the VVS curve obtained by the Newton-Raphson power flow algorithm, which shows the correctness of the method introduced in this article.

[0068] (2) The method introduced in the present invention can reflect the VVS under different load levels, which is more reasonable than the sensitivity analysis of voltage and reactive power control characteristics obtained by using only the Newton-Raphson power flow algorithm.

[0069] (3) If the VVS index is obtained by adding reactive power perturbation, it will be smaller and can better reflect the sensitive characteristics between voltage and reactive power.

[0070] Step 2: Fuzzify the voltage / reactive sensitivity matrix to obtain the membership matrix between each node; and analyze the membership relationship between reactive source nodes according to the membership matrix between each node, and pre-partition the reactive source nodes;

[0071] In order to measure the closeness of the electrical connection between nodes, the VVS is fuzzy divided, and the membership function is established to solve the membership matrix between nodes. The main idea is to make the VVS objects divided into the same category have the greatest similarity, while different categories have differences, which can reflect the coupling degree between nodes more intuitively and vividly.

[0072] Basic knowledge of fuzzy sets: The membership function represents the degree to which an object x belongs to the data set A, denoted by μ A (x), object x is all data objects contained in the space where set A is located, and the membership value range is [0,1], that is, 0<=μ A (x) <= 1. μ A (x)=1 means x∈A, that is, x completely belongs to A.

[0073] The membership function defined on the space X = {x} is equivalent to defining a fuzzy set A, that is, a fuzzy subset A defined on the domain X = {x}. For a finite number of objects x1, x2, ..., x n The fuzzy set A can be expressed as:

[0074] A={μ A (x i ),x i |x i ∈X} (10)

[0075] The degree of membership of each data point to a certain class is represented by a value in the interval [0,1].

[0076] VVS-based membership matrix: The data to be classified is the required sensitivity matrix S VQ Element (S VQ ) ij The i-th row is the control node, and the j-th column is the reactive node. Then the output membership matrix A is:

[0077]

[0078] Where G represents the number of reactive source nodes, μ A ((S VQ ) ij ) ij It represents the membership of the jth controlled node belonging to the i-th reactive source node; when there is only one reactive source in the partition, the membership of node i to the reactive source is the membership of the partition; when there are multiple reactive sources in a partition, the sum of the memberships of node i to all reactive sources in the partition is the membership of the partition.

[0079] Step 3: Analyze the membership relationship between each load node and reactive source node according to the membership matrix between each node, and determine the partition result;

[0080] Step 4: Construct a partition membership index to evaluate the partition results and ensure reactive power balance within the partition.

[0081] In view of the system load, topology and DG output changes, three sub-region membership indicators were selected to evaluate the zoning scheme. The regional comprehensive membership index measures the regional coupling degree; the regional reactive balance index is used to measure the reactive regulation performance of the region and the degree of coordination of the regional "source-load" matching; and the sub-region size index evaluates the sub-region size.

[0082] Regional comprehensive membership index: The regional comprehensive membership index is constructed based on the membership matrix obtained by fuzzification of the VVS matrix. This index combines the membership within the region and the membership between regions to evaluate the coupling degree of the partition results. The membership of region k can be directly measured from the membership matrix A:

[0083]

[0084] In the formula, l k is the number of controlled nodes in region k. This formula represents the average membership of reactive sources and controlled nodes in region k multiplied by the minimum membership of reactive sources and controlled nodes in the region. The smaller the value of this formula, the weaker the coupling degree in the region.

[0085] The degree of inter-regional membership is a measure of the looseness of the connection between nodes in different regions. Therefore, formula (13) is used to measure the connection strength between regions.

[0086]

[0087] Formula (13) represents the average membership between the reactive sources in region k and the controlled nodes in the region multiplied by the minimum membership between the reactive sources not belonging to region k and the controlled nodes in region k; the larger the value of this formula, the weaker the coupling between regions.

[0088] The comprehensive membership index of weak coupling within the region and strong coupling between regions is used for evaluation. The comprehensive membership index of the region can be expressed as:

[0089]

[0090] F k The larger the value, the better the partitioning effect; conversely, the smaller the value, the better the partitioning effect. k The smaller the value, the worse the partitioning effect.

[0091] Regional reactive power balance index: In order to prevent the regional voltage from exceeding the limit, the region should have a certain voltage regulation capability. When the DG output penetration rate is the highest, the voltage range restriction index is the most serious. The reactive power in the region should meet the needs of self-sufficiency in the region and minimize the reactive power flow across regions. The present invention uses the regional reactive power balance index to measure the reactive power level in the region. The reactive power balance constraints are as follows:

[0092]

[0093]

[0094] In the formula, Q k is the reactive power balance of area k; is the reactive power balance index; K is the number of partitions; is the maximum reactive power in region k, is the reactive power demand value within area k.

[0095] Sub-region size index: The calculation speed of each sub-region depends on the size of the sub-region, and the overall calculation time is the time used to calculate the largest sub-region. Therefore, the present invention defines the sub-region size index:

[0096]

[0097] Where n K is the number of internal nodes in the sub-region K, and S is the sub-region scale index. The closer the index S is to 1, the higher the regional scale balance is, and the higher the overall computing efficiency after partitioning is.

[0098] Theoretically, an important principle of reactive power voltage control in power systems is to ensure the balance of reactive power hierarchical partitions and avoid long-distance transmission of reactive power. In order to meet multiple requirements such as "strong coupling within a region and weak coupling between regions", "each region must contain reactive source nodes and load nodes, and the reactive sources within the region must have sufficient reactive reserves". The present invention divides the secondary voltage control partition into the following stages:

[0099] (1) The first stage: The shortcomings of the existing VVS calculation method are analyzed, and an improved VVS calculation method is proposed. The VVS between each node is calculated by adding voltage and reactive power perturbations to the reactive source.

[0100] (2) The second stage: Based on the VVS calculation method proposed in the first stage, the VVS matrix is ​​fuzzified to obtain the membership matrix between each node. The membership relationship between reactive sources is analyzed, reactive source nodes with strong coupling relationships are merged, and the number of initial partitions is determined.

[0101] (3) The third stage: According to the affiliation between each load node and each reactive power, the partitioning result is determined according to the affiliation.

[0102] (4) Final stage: The partition results are evaluated based on the partition membership index to ensure strong coupling within the partition, weak coupling between partitions, and reactive power balance of the partitions.

[0103] The above method does not require any manual intervention. After reading the line load data and network topology data, dynamic partitioning can be achieved. The partitioning flow chart is as follows: Figure 2 shown.

[0104] The significant feature of the method of the present invention that distinguishes it from previous methods is that the proposed improved voltage / reactive sensitivity calculation method can reflect the changes in the timing characteristics of reactive sources and load outputs, fully demonstrating the advantages of reactive sources with adjustable reactive power in voltage control. The present invention proposes a voltage / reactive sensitivity calculation method by adding voltage reactive output perturbations to reactive sources. On the basis of the voltage / reactive sensitivity calculation method proposed by the present invention, the voltage / reactive sensitivity matrix is ​​fuzzified to obtain the membership matrix between each node, and a regional comprehensive membership index is established. The zoning model is combined with regional reactive power balance and sub-regional scale adjustment to make the zoning more reasonable.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A distribution network dynamic partitioning method based on an improved VVS calculation model, characterized in that: The steps are as follows: Step 1: Calculate the voltage / reactive power sensitivity matrix between each node by adding voltage and reactive power output perturbation to the reactive source; Among them, the nodes include reactive source nodes and load nodes; The calculation method of the voltage / reactive power sensitivity matrix is: Where deltQ k is the reactive power output perturbation change vector of the kth reactive source node, S VQ is the voltage / reactive power sensitivity matrix, the element (S VQ )ij represents the voltage change of the controlled node i when the reactive output disturbance of the reactive source j changes, V is the load node voltage, and Q is the reactive power of the reactive source node; Step 2: Fuzzify the voltage / reactive sensitivity matrix to obtain the membership matrix between each node; and analyze the membership relationship between reactive source nodes according to the membership matrix between each node, and pre-partition the reactive source nodes; The calculation method of the membership matrix is: In the formula, G represents the number of reactive source nodes, μ A ((S VQ ) ij ) ij represents the membership of the jth controlled node belonging to the i-th reactive source node; when there is only one reactive source in a partition, the membership of node i to the reactive source is the membership of the partition; when there are multiple reactive sources in a partition, the sum of the memberships of node i to all reactive sources in the partition is the membership of the partition; Step 3: Analyze the membership relationship between each load node and reactive source node according to the membership matrix between each node, and determine the partition result; Step 4: Construct a partition membership index to evaluate the partition results and ensure reactive power balance within the partition.

2. The method for dynamic partitioning of distribution network based on improved VVS calculation model according to claim 1 is characterized in that: The zoning membership index includes a regional comprehensive membership index, a regional reactive power balance index and a sub-regional scale index.

3. The method for dynamic partitioning of distribution network based on improved VVS calculation model according to claim 2 is characterized in that: The expression of the regional comprehensive membership index is: Among them, F k represents the comprehensive regional membership index, It represents the average membership of the reactive source and the controlled node in region k multiplied by the minimum membership of the reactive source and the controlled node in the region. represents the average membership of the reactive power source in region k with the controlled nodes in the region multiplied by the minimum membership of the reactive power source not belonging to region k with the controlled nodes in region k, l k is the number of controlled nodes in region k, μ Aij represents the membership matrix; F k The larger the value, the better the partitioning effect; conversely, the smaller the value, the better the partitioning effect. k The smaller the value, the worse the partitioning effect.

4. The method for dynamic partitioning of distribution network based on improved VVS calculation model according to claim 2 is characterized in that: The expression of the regional reactive power balance index is: Among them, Q k is the reactive power balance of area k; is the reactive power balance index; K is the number of partitions; is the maximum reactive power in region k, is the reactive power demand value within area k.

5. The method for dynamic partitioning of distribution network based on improved VVS calculation model according to claim 4 is characterized in that: The expression of the sub-region scale index is: Among them, n K is the number of internal nodes in the sub-region K, and S is the sub-region scale index.

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