Medium and low voltage flexible interconnection device multi-layer collaborative planning method considering low voltage transformer area source load characteristics

Abstract

The invention relates to a multi-layer collaborative planning method for medium-low voltage flexible interconnection equipment considering low-voltage transformer area source load characteristics, and the method comprises the following steps: step 1, an upper-layer low-voltage FID planning configuration model, step 2, a middle-layer medium-voltage SOP planning configuration model, step 3, a lower-layer medium-low voltage distribution network optimization operation model, step 4, a middle-layer medium-low voltage distribution network optimization configuration model, and step 5, a middle-layer medium-low voltage distribution network optimization configuration model. And 4, converting the middle-layer model and the lower-layer model into a single-layer model, and solving the single-layer model by adopting a hybrid algorithm combining an improved whale optimization algorithm and second-order cone programming. The method can make up for the defects in flexible interconnection equipment planning in an existing power distribution network, accords with the actual operation condition of the power distribution network from the source-load characteristics of a transformer area, and improves the planning efficiency. The planning scheme is more scientific and applicable, source-load complementarity in the power distribution network is deeply mined, medium and low voltage flexible interconnection is realized, and the method adapts to the development trend of the flexible power distribution network.

Description

Technical Field

[0001] The present invention relates to a planning method, in particular to a multi-layer collaborative planning method for medium and low voltage flexible interconnected equipment taking into account the source-load characteristics of a low voltage area, and belongs to the technical field of distribution network planning and optimal configuration. Background Art

[0002] Renewable energy sources such as photovoltaic power generation have the characteristics of intermittent, random and uncertain. With the increase of new energy sources, too many distributed power sources connected to the distribution network may pose challenges to the stability of the power grid, including voltage fluctuations, frequency fluctuations and current imbalance. At present, most experts and scholars at home and abroad have taken measures to reduce the impact of DG access on the power grid through the source-grid-load-storage links. For example, on the "source" side, smart inverters can be used to regulate DG output, and active, reactive or active and reactive coordinated control strategies can be formulated to regulate DG output to change the power flow distribution to adjust the node voltage; on the "grid" side, the distribution network can be adjusted by adjusting transformer taps, reconstructing the distribution network, and using reactive control devices to improve the carrying capacity of the distribution network for new energy; at the "load and storage" level, demand response, configuration of fixed or mobile energy storage and other means can be used to improve the impact of high penetration DG on the distribution network. However, with the development of power electronics technology, flexible interconnected devices that can quickly and accurately control their own power flow and thus affect the overall power flow distribution of the system are triggering a new research boom.

[0003] As a new type of power distribution equipment, flexible interconnected devices (FID) of power electronics have attracted wide attention in the medium and low voltage systems of distribution networks. Medium voltage uses intelligent soft open point as a flexible interconnection device, aiming to replace traditional circuit breaker interconnection switches with controllable power electronic equipment, realize flexible connection between feeders, and provide high-efficiency control and power flow optimization. Compared with traditional interconnection switches, SOP has the ability of continuous power control and power flow regulation, which solves problems such as power outages; at the same time, SOP changes the operation mode of the distribution network, and is expected to solve the problems of power outages and limited power supply capacity improvement. Low voltage uses low voltage FID as a flexible interconnection device to interconnect multiple substations with complementary sources and loads in the same distribution area. By balancing the load of interconnected low voltage substations, the risk of heavy load and overload of substation transformers can be effectively reduced. At the same time, stable voltage support is provided for the substation, solving the problems caused by the high proportion of intermittent distributed generation access to low voltage substations and overload operation of substation transformers.

[0004] At present, there have been many related studies on the planning model of medium and low voltage flexible interconnection equipment, but most of them only start from the perspective of pure medium voltage or low voltage, and separate the medium voltage flexible interconnection planning from the low voltage flexible interconnection planning, ignoring the correlation between medium voltage lines and low voltage substations. Flexible interconnection at the medium voltage level can improve the imbalance of source and load between feeders, improve the power quality of the distribution network, and reduce the impact of high penetration rate distributed power access on the distribution network, but there is also a widespread imbalance of source and load distribution between low voltage substations. Therefore, the joint planning of medium voltage and low voltage flexible interconnection equipment is of great significance to improving the system regulation capability, increasing the penetration rate of DG, and reducing network losses. It is necessary to carry out research on the coordinated planning of medium and low voltage flexible interconnection equipment in the distribution network. However, the problem of interconnection equipment planning at the medium voltage or low voltage level alone is already a large-scale nonlinear optimization problem, which is difficult to solve using traditional methods, and the solution process is extremely complex. At present, most studies often choose intelligent algorithms for optimization and solution in the configuration stage of flexible interconnection equipment in low-voltage areas. Flexible interconnection is planned in all low-voltage areas, and there are too many positions to be selected, ignoring the actual situation of source and load in the low-voltage area. The possibility of interconnection may not exist between low-voltage areas with similar source and load composition. To solve the above problems, the present invention considers the different source and load characteristics between low-voltage areas, and divides low-voltage areas into different types, such as resource-intensive areas, load-intensive areas (such as industrial areas), and ordinary load areas (such as residential load areas). By analyzing the source-load complementarity between different load types, as a selection scheme for low-voltage area interconnection equipment planning, the complexity of the model is reduced.

[0005] The present invention analyzes the source-load characteristics of different low-voltage substations, selects substation clusters with strong source-load complementarity as the selection scheme for low-voltage flexible interconnected equipment planning, reduces the complexity of the model, and starts from the source-load characteristics of the low-voltage substations. It can increase the scientificity and applicability of the planning scheme, make up for the shortcomings of other current flexible interconnected equipment planning and design methods, and is conducive to more comprehensive and in-depth exploration of the role of flexible interconnected equipment in the distribution network, enhance the flexibility of the distribution system, improve the power quality, and provide a reliable reference for subsequent distribution network upgrades and renovations. Summary of the invention

[0006] The present invention is aimed at the technical problems existing in the prior art, and provides a multi-layer collaborative planning method for medium and low voltage flexible interconnected equipment taking into account the source-load characteristics of low voltage areas. For the problem of collaborative planning of medium and low voltage flexible interconnected equipment in distribution networks, most studies currently often select intelligent algorithms for optimization and solution in the configuration stage of low voltage area flexible interconnected equipment, and plan flexible interconnection in all low voltage areas. There are too many positions to be selected, ignoring the actual situation of source load in low voltage areas, and some low voltage areas with similar source loads may not necessarily have the possibility of interconnection. The present invention first considers the different source-load characteristics of low voltage areas, divides low voltage areas into different types, and analyzes the source-load complementarity between areas with different load types as a candidate for low voltage area interconnected equipment planning, as a reference for low voltage flexible interconnected equipment planning, so as to reduce the complexity of the model. On this basis, a multi-layer optimization configuration method is proposed, and the middle-level model and the lower-level model are converted into a single-layer model, and a hybrid algorithm combining an improved whale optimization algorithm (IWOA) and second-order cone programming (SOCP) is used to solve.

[0007] In order to achieve the above-mentioned purpose, the technical scheme of the present invention is as follows: a multi-layer collaborative planning method for medium and low voltage flexible interconnected equipment taking into account the source-load characteristics of the low voltage area,

[0008] In order to simplify the model of optimizing the configuration of low-voltage FID equipment, the present invention first analyzes the source-load characteristics of each low-voltage area in the distribution network. The low-voltage areas are roughly divided into the following according to the source-load distribution:

[0009] ① Load-intensive areas, such as industrial load areas and office load areas.

[0010] ② Resource-intensive substations and substations with a large surplus of distributed power generation, such as substations containing small photovoltaic power stations.

[0011] ③General load substations, substations with normal load distribution, such as residential load substations.

[0012] ④ General resource type substations, substations with slightly surplus distributed power sources, such as substations containing a certain amount of distributed power sources.

[0013] In view of the relatively short flexible interconnection distance of low-voltage substations, it is stipulated that the distance between interconnected substations shall not exceed two substations. Based on the source and load characteristics of the substations, the following principles for flexible interconnection of substations are proposed:

[0014] First, screen the load-intensive and resource-intensive substations in the distribution network, analyze the source-load distribution of the two substations near the selected substation, and give priority to flexible interconnection between the load-intensive and resource-intensive substations.

[0015] Secondly, if the load-intensive area and the resource-intensive area are too far apart to be directly interconnected, priority is given to interconnecting the load-intensive area with multiple resource-general areas, or interconnecting the resource-intensive area with multiple load-general areas. Finally, select adjacent load-general areas and resource-general areas, and if they are not interconnected with other areas, interconnect the adjacent load-general areas with resource-general areas. The low-voltage substations are divided into different types. By analyzing the source-load complementarity between substations of different types, alternative plans for the planning of low-voltage substation interconnection equipment are proposed. Compared with the phenomenon that most current studies consider all substations to be interconnected, which leads to an overly complex model, the complexity of the model can be reduced and the solution speed of the model can be accelerated. Taking into account the complexity of the calculation, the complex mixed integer nonlinear model is solved in layers, and a hybrid algorithm combining the improved whale algorithm and the second-order cone programming algorithm is adopted. Under the premise of ensuring accuracy, the solution speed of the model is accelerated. The present invention can make up for the deficiencies in the planning of flexible interconnection equipment in the existing distribution network. Starting from the source-load characteristics of the substation, it conforms to the actual operation status of the distribution network, the planning scheme is more scientific and applicable, and the source-load complementarity in the distribution network is deeply excavated to realize medium and low voltage flexible interconnection and adapt to the development trend of the flexible distribution network.

[0016] A multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas, the method comprising the following steps:

[0017] Step 1: Upper layer low voltage FID planning configuration model,

[0018] Step 2: Middle-level medium voltage SOP planning configuration model,

[0019] Step 3: Optimization operation model of lower-level medium and low voltage distribution network,

[0020] Step 4: Convert the middle-level model and the lower-level model into a single-level model, and solve it using a hybrid algorithm that combines the improved whale optimization algorithm (IWOA) and second-order cone programming (SOCP).

[0021] Among them, in step 1, the upper low-voltage FID planning configuration model is as follows:

[0022] The upper configuration model established selects the installation location Z of the low voltage FID connected to the medium and low voltage distribution network F,i With installation capacity S F,i As the optimization decision variable,

[0023] (1) Objective function,

[0024] The upper-level low-voltage FID planning and configuration model aims to select the appropriate low-voltage FID equipment installation location and capacity based on the planning of the low-voltage area flexible interconnection candidate location based on the source-load characteristics, so as to minimize the system comprehensive voltage deviation rate and power supply flexibility.

[0025] The optimal goal is the objective, and the objective function expression is:

[0026] minf up =λ1f1+λ2f2(8.1)

[0027] Where: λ1, λ2 are indicator weights, and λ1+λ2=1; f1, f2 represent the minimum system comprehensive voltage deviation rate and power supply flexibility, respectively, where:

[0028] The voltage deviation rate is an important indicator for measuring the power quality of the distribution network. It can be represented by the comprehensive voltage deviation rate of the distribution network, as shown in formula (8.2):

[0029]

[0030] Where N is the total number of nodes in the system; U i,t is the voltage value of node i at time period t, U i,ref is the rated voltage of node i. The power supply capacity of the distribution network is an important indicator for the stable operation of the system, which can be expressed by the branch load margin. The branch load margin refers to the ratio between the open capacity of a branch in the distribution network and the maximum capacity of the line. The larger the branch load margin, the more open the branch capacity is to meet the actual load demand. Otherwise, it may cause branch overload. The specific calculation method is as follows:

[0031]

[0032] Where N L is the total number of branches in the system, I lmax is the upper limit of the current in branch ij, I ij,t is the effective value of the current in branch ij at time t,

[0033] (2) Constraints,

[0034] The upper-level low-voltage FID planning and configuration model aims to adjust the access location and installation capacity of low-voltage FID equipment under the premise that the number of low-voltage FID equipment access and single-point access capacity are limited, and the system comprehensive voltage deviation rate is minimized.

[0035] The optimization model expression is as follows:

[0036]

[0037] Among them, NFID,max S is the upper limit of the number of low-voltage FID devices allowed to be connected to the distribution network. FID,max The maximum capacity that can be installed on a single FID device.

[0038] Among them, step 2: middle-level medium-voltage SOP planning and configuration model, specifically as follows: the established middle-level medium-voltage SOP planning and configuration model selects the installation location Z of the medium-voltage connected in the medium- and low-voltage distribution network S,i With installation capacity S S,i As optimization decision variables, (1) objective function,

[0039] The goal of the middle-level medium-voltage SOP planning and configuration model is to provide suitable medium-voltage SOP equipment installation locations and capacities for the lower-level optimized operation layer based on the upper-level model, with the goal of minimizing the medium-voltage SOP equipment operation cost. The objective function expression is:

[0040]

[0041] Where N SOP is the number of SOPs connected to the distribution network, P i,t SOP,loss is the active power loss value of the ith SOP in period t. (2) Constraints,

[0042] The middle-level medium-voltage SOP planning configuration model provides the installation location and capacity of medium-voltage SOP equipment for the lower level based on the installation location and capacity of the upper-level low-voltage FID equipment, with the goal of minimizing the operating cost of the medium-voltage SOP equipment. The optimization model expression is:

[0043] The upper-level low-voltage FID planning and configuration model targets the access number of low-voltage FID devices and the single-point access capacity to be limited. By adjusting the access location and installation capacity of the low-voltage FID devices, and taking the minimum system comprehensive voltage deviation rate and the optimal power supply flexibility as the goal, the optimization model expression is:

[0044]

[0045] Among them, N SOP,max is the upper limit of the number of SOPs connected to the distribution network, S SOP,max The maximum capacity that can be installed on a single SOP device.

[0046] Among them, step 3: the optimization operation model of the lower medium and low voltage distribution network is as follows:

[0047] (1) Decision variables

[0048] Node voltage squared:

[0049] Branch current square:

[0050] Active power flow of branch: P = {P ij,t |ij∈[1,N L ],t∈[1,24]}

[0051] Branch reactive power flow: Q = {Q ij,t |ij∈[1,N L ],t∈[1,24]}

[0052] FID transmits active power:

[0053] FID transmits reactive power:

[0054] SOP transmits active power:

[0055] SOP transmits reactive power:

[0056] Where N represents the total number of nodes in the distribution network; i represents the node number of the distribution network; N L V represents the total number of branches in the distribution network; ij represents the ijth branch; t represents the operating time point. i,t is the square of the voltage at node i during the h period, v i,t represents the actual voltage of node i during the h period, F ij,t is the square of the current in branch ij during the time period h, f ij,t It represents the actual current on branch ij during the time period h. ij,t is the active power flowing through branch ij during time period t, Q ij,t is the reactive power flowing through branch ij during time period t, P i,t FID is the active power transmitted in the corresponding port of the FID connected to node i during time period t, Q i,t FID is the reactive power transmitted in the corresponding port of the FID connected to node i during time period t, P i,t SOP is the active power transmitted in the corresponding port of the SOP connected to node i during time period t, Q i,t SOP is the reactive power transmitted in the corresponding port of the SOP connected to node i during time period t,

[0057] (2) Objective function

[0058] The lower-level medium and low voltage distribution network optimization operation model is based on the upper-level middle-level flexible interconnection equipment planning model. After determining the installation location and capacity of the medium-voltage and low-voltage flexible interconnection equipment, it optimizes the operation of the system with the goal of minimizing the operating cost of the distribution system. The objective function is shown in formula (8.7).

[0059]

[0060] (3) Constraints

[0061] Active power constraint of st branch:

[0062] Branch reactive power constraint:

[0063] Branch terminal voltage constraint:

[0064] Branch current constraints:

[0065] Voltage square constraint: V min ≤V≤V max

[0066] Current square constraint: F min ≤F≤F max

[0067] in, is the distributed photovoltaic output connected to node j in time period t, is the active power consumed by the load at node j during time period t, is the reactive power consumed by the load at node j during time period t.

[0068] (4) Medium voltage SOP related constraints

[0069] As a flexible multi-state switch, SOP can flexibly control power flow and adjust node voltage, and can also be used for fault recovery when the distribution network fails. The power loss of SOP is expressed as:

[0070]

[0071] In the formula, is the loss of the SOP at the node i connected to the SOP during the time period t, A is the loss of the SOP at the node j connected to the SOP during the time period t, i SOP is the loss coefficient of the SOP converter connected at node i, is the active power transmitted by the SOP at node i connected to the SOP during time period t, is the reactive power transmitted by the SOP at node i connected to the SOP during time period t.

[0072] Since each port of SOP is isolated by the DC link, each port only needs to meet its own capacity constraints, that is,

[0073]

[0074] In the formula, S i SOP is the capacity of the converter at node i under SOP.

[0075] (5) Low-pressure FID-related constraints

[0076] As a low-voltage flexible interconnection, FID can transmit power between stations and provide certain reactive power support. The power loss of FID is expressed as:

[0077]

[0078] In the formula, is the loss of the FID at the node i connected to the FID during the time period t, A is the loss of SOP at node j connected to FID during time period t, i FID is the loss coefficient of the SOP converter connected at node i, is the active power transmitted by the FID at the node i connected to the FID during the time period t, is the reactive power transmitted by the FID at node i connected to the FID during time period t.

[0079] Since each port of the FID is isolated by the DC bus, each port only needs to meet its own capacity constraints, that is,

[0080]

[0081] In the formula, S i FID is the capacity of the converter at node i of FID.

[0082] (6) Second-order cone constraint

[0083] Since the underlying model problem belongs to a mixed integer non-linear programming (MINLP) problem, the constraints contain quadratic terms and integer terms, and the conventional solution method does not work well. Therefore, the second-order cone relaxation is used to transform it into a convenient planning problem. The second-order cone constraint of the line is:

[0084] Second-order cone constraints on branches:

[0085] In the formula, || ||2 is the two-norm.

[0086] Among them, step 4 is as follows:

[0087] Step 4-1: First, analyze the source-load characteristics of each low-voltage area in the distribution network, classify each low-voltage area, generate a low-voltage flexible interconnection equipment access location plan based on the area flexible interconnection division principle proposed in the present invention, and input this plan into the upper low-voltage FID planning layer.

[0088] Step 4-2: Based on the upper-layer low-voltage FID planning layer, on the basis of the planning of the candidate locations for flexible interconnection of low-voltage substations with source-load characteristics, the improved whale optimization algorithm is used to optimize the access location and installation capacity of the FID between each low-voltage substation, with the goal of minimizing the system's comprehensive voltage deviation rate and optimizing power supply flexibility, to generate the initial configuration plan for the low-voltage FID, and send this configuration plan to the middle-layer and lower-layer optimization models.

[0089] Step 4-3: The middle-level SOP planning layer, based on the import of the low-voltage FID configuration plan sent by the upper layer, generates a medium-voltage SOP configuration plan with the goal of minimizing system operation costs, and sends it to the lower-level medium and low-voltage distribution network optimization operation layer.

[0090] Step 4-4: The lower-layer medium and low voltage distribution network optimizes the operation layer. The medium and low voltage flexible interconnection configuration scheme sent by the upper and lower layers is imported, and the middle-layer model and the lower-layer model are converted into a single-layer model. According to its own load information and system voltage and flow constraints, a commercial solver is used to minimize the operating cost of the distribution system, optimize the operation of the system, and calculate various parameters of the distribution network.

[0091] Step 4-5: The operating indicators in the distribution network obtained by the lower model are sent to the upper model, and the new fitness value of each target is calculated. If the number of iterations is not met, the upper model updates the low-voltage FID configuration scheme through the improved whale optimization algorithm, and sends it to the merged single-layer model, and then goes to step 4-3.

[0092] Compared with the prior art, the present invention has the following advantages:

[0093] (1) The present invention proposes a method for planning candidate locations for flexible interconnection of low-voltage substations based on source-load characteristics. The low-voltage substations are divided into different types, and the source-load complementarity between substations of different load types is analyzed as a candidate solution for planning low-voltage substation interconnection equipment. Compared with the problem of too many candidate locations for flexible interconnection in other existing methods, the present invention can reduce the complexity of the model and speed up the solution of the model.

[0094] (2) The multi-layer collaborative planning model of medium and low voltage flexible interconnected equipment taking into account the source-load characteristics of different low voltage substations of the present invention takes into account the complexity of calculation, solves the complex mixed integer nonlinear model in layers, and adopts a hybrid algorithm combining the improved whale algorithm and the second-order cone programming algorithm. Under the premise of ensuring accuracy, the speed of solving the model is accelerated.

[0095] (3) The present invention starts from both the low-voltage and medium-voltage levels, and coordinates the planning of the installation location and installation capacity of the low-voltage FID and the medium-voltage SOP. The degree of complementarity between the equipment is higher, and the potential for flexible interconnection between distribution networks is fully explored. The configuration scheme is more reasonable, and the overall investment cost of the equipment is reduced.

[0096] (4) The present invention can make up for the deficiencies in the planning of flexible interconnection equipment in the existing distribution network. Starting from the source-load characteristics of the substation, it conforms to the actual operation conditions of the distribution network. The planning scheme is more scientific and applicable. It deeply explores the source-load complementarity in the distribution network, realizes medium and low voltage flexible interconnection, and adapts to the development trend of flexible distribution networks. BRIEF DESCRIPTION OF THE DRAWINGS

[0097] Figure 1 It is a schematic diagram of the overall process of the present invention,

[0098] Figure 2 This is the topology diagram of the distribution network containing medium and low voltage flexible interconnected equipment.

[0099] Figure 3 Solve the flow chart for the model. DETAILED DESCRIPTION

[0100] In order to deepen the understanding of the present invention, the present embodiment is described in detail below with reference to the accompanying drawings.

[0101] Embodiment 1: The present invention discloses a multi-layer collaborative planning model for medium and low voltage flexible interconnected devices taking into account the source-load characteristics of low voltage areas, the structure diagram of which is shown in the attached figure. Figure 2As shown. For the problem of coordinated planning of low-voltage flexible interconnected equipment in distribution networks, most current studies often choose intelligent algorithms for optimization and solution in the configuration stage of low-voltage flexible interconnected equipment. Flexible interconnections are planned in all low-voltage areas, and there are too many locations to be selected, ignoring the actual situation of source and load in the low-voltage area. Some low-voltage areas with similar source and load composition may not necessarily have the possibility of interconnection. The present invention first considers the source-load characteristics between low-voltage areas, divides low-voltage areas into different types, and analyzes the source-load complementarity between different types of areas as a candidate for planning low-voltage area interconnected equipment, providing a reference for low-voltage flexible interconnected equipment planning to reduce the complexity of the model. On this basis, a multi-layer optimization configuration method is proposed. The decision variables of the upper model are the installation location and installation capacity of the low-voltage FID, and the objective function is to minimize the system comprehensive voltage deviation rate and optimize the power supply flexibility; the decision variables of the middle model are the installation location and installation capacity of the medium-voltage SOP, and the objective function is to minimize the system operation cost; the decision variables of the lower model are the transmission power of the medium and low voltage flexible interconnected equipment and the distribution network operation control variables, and the goal is to minimize the distribution system operation cost and optimize the system operation. The middle model and the lower model are converted into a single-layer model, and the hybrid algorithm combining the improved whale optimization algorithm (IWOA) and the second-order cone programming (SOCP) is used to solve it.

[0102] A multi-layer collaborative planning method for medium and low voltage flexible interconnected equipment considering the source-load characteristics of low voltage areas.

[0103] In order to simplify the model of optimizing the configuration of low-voltage FID equipment, the present invention first analyzes the source-load characteristics of each low-voltage area in the distribution network. The low-voltage areas are roughly divided into the following according to the source-load distribution:

[0104] ① Load-intensive areas, such as industrial load areas and office load areas.

[0105] ② Resource-intensive substations and substations with a large surplus of distributed power generation, such as substations containing small photovoltaic power stations.

[0106] ③General load substations, substations with normal load distribution, such as residential load substations.

[0107] ④ General resource type substations, substations with slightly surplus distributed power sources, such as substations containing a certain amount of distributed power sources.

[0108] In view of the relatively short flexible interconnection distance of low-voltage substations, it is stipulated that the distance between interconnected substations shall not exceed two substations. Based on the source and load characteristics of the substations, the following principles for flexible interconnection of substations are proposed:

[0109] First, screen the load-intensive and resource-intensive substations in the distribution network, analyze the source-load distribution of the two substations near the selected substation, and give priority to flexible interconnection between the load-intensive and resource-intensive substations.

[0110] Secondly, if the load-intensive area and the resource-intensive area are too far apart to be directly interconnected, priority is given to interconnecting the load-intensive area with multiple resource-general areas, or interconnecting the resource-intensive area with multiple load-general areas. Finally, select adjacent load-general areas and resource-general areas, and if they are not interconnected with other areas, interconnect the adjacent load-general areas with resource-general areas.

[0111] A multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas, the method comprising the following steps:

[0112] Step 1: Upper layer low voltage FID planning configuration model,

[0113] Step 2: Middle-level medium voltage SOP planning configuration model,

[0114] Step 3: Optimization operation model of lower-level medium and low voltage distribution network,

[0115] Step 4: Convert the middle-level model and the lower-level model into a single-level model, and solve it using a hybrid algorithm that combines the improved whale optimization algorithm (IWOA) and second-order cone programming (SOCP).

[0116] Among them, in step 1, the upper low-voltage FID planning configuration model is as follows:

[0117] The upper configuration model established selects the installation location Z of the low voltage FID connected to the medium and low voltage distribution network F,i With installation capacity S F,i As the optimization decision variable,

[0118] (1) Objective function,

[0119] The goal of the upper-level low-voltage FID planning and configuration model is to select the appropriate low-voltage FID equipment installation location and capacity based on the planning of the low-voltage area flexible interconnection candidate location based on the source-load characteristics, with the goal of minimizing the system comprehensive voltage deviation rate and optimizing the power supply flexibility. The objective function expression is:

[0120] minf up =λ1f1+λ2f2(8.1)

[0121] Where: λ1, λ2 are indicator weights, and λ1+λ2=1; f1, f2 represent the minimum system comprehensive voltage deviation rate and power supply flexibility, respectively, where:

[0122] The voltage deviation rate is an important indicator for measuring the power quality of the distribution network. It can be represented by the comprehensive voltage deviation rate of the distribution network, as shown in formula (8.2):

[0123]

[0124] Where N is the total number of nodes in the system; U i,t is the voltage value of node i at time period t, U i,ref is the rated voltage of node i. The power supply capacity of the distribution network is an important indicator for the stable operation of the system, which can be expressed by the branch load margin. The branch load margin refers to the ratio between the open capacity of a branch in the distribution network and the maximum capacity of the line. The larger the branch load margin, the more open the branch capacity is to meet the actual load demand. Otherwise, it may cause branch overload. The specific calculation method is as follows:

[0125]

[0126] Where N L is the total number of branches in the system, I lmax is the upper limit of the current in branch ij, I ij,t is the effective value of the current in branch ij at time t,

[0127] (2) Constraints,

[0128] The upper-level low-voltage FID planning and configuration model aims to adjust the access location and installation capacity of low-voltage FID equipment under the premise that the number of low-voltage FID equipment access and single-point access capacity are limited, and the system comprehensive voltage deviation rate is minimized.

[0129] The optimization model expression is as follows:

[0130]

[0131] Among them, N FID,max S is the upper limit of the number of low-voltage FID devices allowed to be connected to the distribution network. FID,max The maximum capacity that can be installed on a single FID device.

[0132] Among them, step 2: middle-level medium-voltage SOP planning and configuration model, specifically as follows: the established middle-level medium-voltage SOP planning and configuration model selects the installation location Z of the medium-voltage connected in the medium- and low-voltage distribution network S,i With installation capacity S S,i As the optimization decision variable,

[0133] (1) Objective function,

[0134] The goal of the middle-level medium-voltage SOP planning and configuration model is to provide suitable medium-voltage SOP equipment installation locations and capacities for the lower-level optimized operation layer based on the upper-level model, with the goal of minimizing the medium-voltage SOP equipment operation cost. The objective function expression is:

[0136]

[0137] Where N SOP is the number of SOPs connected to the distribution network, P i,t SOP,loss is the active power loss value of the i-th SOP in period t.

[0138] (2) Constraints,

[0139] The middle-level medium-voltage SOP planning configuration model provides the installation location and capacity of medium-voltage SOP equipment for the lower level based on the installation location and capacity of the upper-level low-voltage FID equipment. The optimization model is aimed at minimizing the operating cost of the medium-voltage SOP equipment.

[0140] The expression is

[0141] The upper-level low-voltage FID planning and configuration model targets to adjust the access location and installation capacity of low-voltage FID equipment under the premise that the number of low-voltage FID equipment access and single-point access capacity are limited, and the system comprehensive voltage deviation is used

[0142] The goal is to minimize the power consumption and optimize the power supply flexibility. The optimization model expression is:

[0143]

[0144] Among them, N SOP,max is the upper limit of the number of SOPs connected to the distribution network, S SOP,max The maximum capacity that can be installed on a single SOP device.

[0145] Among them, step 3: the optimization operation model of the lower medium and low voltage distribution network is as follows:

[0146] (1) Decision variables

[0147] Node voltage squared:

[0148] Branch current square:

[0149] Active power flow of branch: P = {P ij,t |ij∈[1,N L ],t∈[1,24]}

[0150] Branch reactive power flow: Q = {Q ij,t |ij∈[1,N L ],t∈[1,24]}

[0151] FID transmits active power:

[0152] FID transmits reactive power:

[0153] SOP transmits active power:

[0154] SOP transmits reactive power:

[0155] Where N represents the total number of nodes in the distribution network; i represents the node number of the distribution network; N L V represents the total number of branches in the distribution network; ij represents the ijth branch; t represents the operating time point. i,t is the square of the voltage at node i during the h period, v i,t represents the actual voltage of node i during the h period, F ij,t is the square of the current in branch ij during the time period h, f ij,t It represents the actual current on branch ij during the time period h. ij,t is the active power flowing through branch ij during time period t, Q ij,t is the reactive power flowing through branch ij during time period t, P i,t FID is the active power transmitted in the corresponding port of the FID connected to node i during time period t, Q i,t FID is the reactive power transmitted in the corresponding port of the FID connected to node i during time period t, P i,t SOP is the active power transmitted in the corresponding port of the SOP connected to node i during time period t, Q i,t SOP is the reactive power transmitted in the corresponding port of the SOP connected to node i during time period t,

[0156] (2) Objective function

[0157] The lower-level medium and low voltage distribution network optimization operation model is based on the upper-level middle-level flexible interconnection equipment planning model. After determining the installation location and capacity of the medium-voltage and low-voltage flexible interconnection equipment, it optimizes the operation of the system with the goal of minimizing the operating cost of the distribution system. The objective function is shown in formula (8.7).

[0158]

[0159] (3) Constraints

[0160] Active power constraint of st branch:

[0161] Branch reactive power constraint:

[0162] Branch terminal voltage constraint:

[0163] Branch current constraints:

[0164] Voltage square constraint: V min ≤V≤V max

[0165] Current square constraint: F min ≤F≤F max

[0166] in, is the distributed photovoltaic output connected to node j in time period t, is the active power consumed by the load at node j during time period t, is the reactive power consumed by the load at node j during time period t.

[0167] (4) Medium voltage SOP related constraints

[0168] As a flexible multi-state switch, SOP can flexibly control power flow and adjust node voltage, and can also be used for fault recovery when the distribution network fails. The power loss of SOP is expressed as:

[0169]

[0170] In the formula, is the loss of the SOP at the node i connected to the SOP during the time period t, A is the loss of the SOP at the node j connected to the SOP during the time period t, i SOP is the loss coefficient of the SOP converter connected at node i, is the active power transmitted by the SOP at node i connected to the SOP during time period t, is the reactive power transmitted by the SOP at node i connected to the SOP during time period t.

[0171] Since each port of SOP is isolated by the DC link, each port only needs to meet its own capacity constraints, that is,

[0172]

[0173] In the formula, S i SOPis the capacity of the converter at node i under SOP.

[0174] (5) Low-pressure FID-related constraints

[0175] As a low-voltage flexible interconnection, FID can transmit power between stations and provide certain reactive power support. The power loss of FID is expressed as:

[0176]

[0177] In the formula, is the loss of the FID at the node i connected to the FID during the time period t, A is the loss of SOP at node j connected to FID during time period t, i FID is the loss coefficient of the SOP converter connected at node i, is the active power transmitted by the FID at the node i connected to the FID during the time period t, is the reactive power transmitted by the FID at node i connected to the FID during time period t.

[0178] Since each port of the FID is isolated by the DC bus, each port only needs to meet its own capacity constraints, that is,

[0179]

[0180] In the formula, S i FID is the capacity of the converter at node i of FID.

[0181] (6) Second-order cone constraint

[0182] Since the underlying model problem belongs to a mixed integer non-linear programming (MINLP) problem, the constraints contain quadratic terms and integer terms, and the conventional solution method does not work well. Therefore, the second-order cone relaxation is used to transform it into a convenient planning problem. The second-order cone constraint of the line is:

[0183] Second-order cone constraints on branches:

[0184] In the formula, || ||2 is the two-norm.

[0185] Among them, step 4 is as follows:

[0186] Step 4-1: First, analyze the source-load characteristics of each low-voltage area in the distribution network, classify each low-voltage area, generate a low-voltage flexible interconnection equipment access location plan based on the area flexible interconnection division principle proposed in the present invention, and input this plan into the upper low-voltage FID planning layer.

[0187] Step 4-2: Based on the upper-layer low-voltage FID planning layer, on the basis of the planning of the candidate locations for flexible interconnection of low-voltage substations with source-load characteristics, the improved whale optimization algorithm is used to optimize the access location and installation capacity of the FID between each low-voltage substation, with the goal of minimizing the system's comprehensive voltage deviation rate and optimizing power supply flexibility, to generate the initial configuration plan for the low-voltage FID, and send this configuration plan to the middle-layer and lower-layer optimization models.

[0188] Step 4-3: The middle-level SOP planning layer, based on the import of the low-voltage FID configuration plan sent by the upper layer, generates a medium-voltage SOP configuration plan with the goal of minimizing system operation costs, and sends it to the lower-level medium and low-voltage distribution network optimization operation layer.

[0189] Step 4-4: The lower-layer medium and low voltage distribution network optimizes the operation layer. The medium and low voltage flexible interconnection configuration scheme sent by the upper and lower layers is imported, and the middle-layer model and the lower-layer model are converted into a single-layer model. According to its own load information and system voltage and flow constraints, a commercial solver is used to minimize the operating cost of the distribution system, optimize the operation of the system, and calculate various parameters of the distribution network.

[0190] Step 4-5: The operating indicators in the distribution network obtained by the lower model are sent to the upper model, and the new fitness value of each target is calculated. If the number of iterations is not met, the upper model updates the low-voltage FID configuration scheme through the improved whale optimization algorithm, and sends it to the merged single-layer model, and then goes to step 4-3.

[0191] It should be noted that the above embodiments are not intended to limit the protection scope of the present invention, and equivalent changes or substitutions made on the basis of the above technical solutions all fall within the protection scope of the claims of the present invention.

Claims

1. A multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas, characterized in that: The method comprises the following steps: Step 1: Upper low-voltage FID planning configuration model based on source-load characteristics analysis, Step 2: Middle-level medium voltage SOP planning configuration model, Step 3: Optimization operation model of lower-level medium and low voltage distribution network, Step 4: Convert the middle-level model and the lower-level model into a single-level model, and solve it using a hybrid algorithm that combines the improved whale optimization algorithm (IWOA) and second-order cone programming (SOCP).

2. The multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas according to claim 1 is characterized in that: The above-mentioned substation type division and interconnection principle based on source-load characteristics and the objective function construction of the upper-level model, The principle of substation type classification based on source-load characteristics is as follows: by analyzing the source-load characteristics of each low-voltage substation in the distribution network, the low-voltage substation is roughly divided into the following types according to the source-load distribution: ① Load-intensive areas, such as industrial load areas and office load areas. ② Resource-intensive areas, areas with a large surplus of distributed power generation, including areas with small photovoltaic power stations, ③General load area, area with normal load distribution, including residential load area, ④ Areas with general resources and areas with slightly surplus distributed power sources, including areas with a certain amount of distributed power sources. The principle of interconnection between substations based on source-load characteristics is as follows: if the distance between load-intensive substations and resource-intensive substations is too far to be directly interconnected, the load-intensive substations are preferentially interconnected with multiple resource-general substations, or the resource-intensive substations are interconnected with multiple load-general substations. Finally, the adjacent load-general substations and resource-general substations are screened, and when they are not interconnected with other substations, the adjacent load-general substations are interconnected with the resource-general substations. The objective function of the upper model and the access restriction of the low-voltage FID equipment are as follows: Objective function: minf up =λ1f1+λ2f2 The goal of the upper-level low-voltage FID planning and configuration model is to select the appropriate low-voltage FID equipment installation location and capacity based on the planning of the candidate locations for flexible interconnection of low-voltage substations based on source-load characteristics, with the goal of minimizing the system's comprehensive voltage deviation rate and optimizing power supply flexibility. In the formula: λ1 and λ2 are indicator weights, and λ1+λ2=1; f1 and f2 represent the minimum system comprehensive voltage deviation rate and power supply flexibility, respectively. The voltage deviation rate is an important indicator for measuring the power quality of the distribution network, and is represented by the comprehensive voltage deviation rate of the distribution network: The power supply capacity of the distribution network is an important indicator of the stable operation of the system, which is expressed by the branch load margin. The branch load margin refers to the ratio between the open capacity of a branch in the distribution network and the maximum capacity of the line. The larger the branch load margin, the more open the branch capacity is to meet the actual load demand. Otherwise, it may cause branch overload. The specific calculation method is as follows: Where N is the total number of nodes in the system; U i,t is the voltage value of node i at time period t, U i,ref is the rated voltage of node i, N L is the total number of branches in the system, I lmax is the upper limit of the current in branch ij, I ij,t is the effective value of the current in branch ij at time t.

3. The multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas according to claim 2 is characterized in that: The goal of the middle-level medium-voltage SOP planning and configuration model is to provide suitable medium-voltage SOP equipment installation locations and capacities for the lower-level optimized operation layer based on the upper-level model, with the lowest medium-voltage SOP equipment operation cost as the goal, and to optimize the medium-voltage SOP equipment access restrictions, including: Objective function: Where N SOP is the number of SOPs connected to the distribution network, P i,t SOP,loss is the active power loss value of the ith SOP in period t, The access restrictions of the medium voltage SOP equipment should take into account the access location, access capacity and access quantity of the SOP. SOP device access restrictions: Among them, N SOP,max is the upper limit of the number of SOPs connected to the distribution network, S SOP,max The maximum capacity that can be installed on a single SOP device.

4. The multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas according to claim 1 is characterized in that: Model construction of flexible interconnected devices in the multi-layer model; As an interconnection device at the low voltage level, the low voltage FID is mainly responsible for the transmission of power between stations and certain reactive power support. The power loss of the FID is expressed as: In the formula, is the loss of the FID at the node i connected to the FID during the time period t, A is the loss of SOP at node j connected to FID during time period t, i FID is the loss coefficient of the SOP converter connected at node i, is the active power transmitted by the FID at the node i connected to the FID during the time period t, is the reactive power transmitted by the FID at the node i connected to the FID during the time period t. Since each port of the FID is isolated by the DC bus, each port only needs to meet its own capacity constraint, that is, In the formula, S i FID is the capacity of the converter at node i of FID, The medium voltage SOP is an interconnection device at the medium voltage level. As a flexible multi-state switch, it can flexibly control the power flow and adjust the node voltage. It is also used for fault recovery when the distribution network fails. The power loss of SOP is expressed as: In the formula, is the loss of the SOP at the node i connected to the SOP during the time period t, A is the loss of the SOP at the node j connected to the SOP during the time period t, i SOP is the loss coefficient of the SOP converter connected at node i, is the active power transmitted by the SOP at node i connected to the SOP during time period t, is the reactive power transmitted by the SOP at the node i connected to the SOP during the time period t. Since each port of the SOP is isolated by the DC link, each port only needs to meet its own capacity constraint, that is, 5. The multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas according to claim 3 is characterized in that: The lower-level low-voltage distribution network operation optimization model takes into account the power flow and operating parameters of the low-voltage flexible interconnected distribution network, takes the minimum operating cost of the distribution system as the goal, optimizes the operation of the system, and constructs the objective function of the lower-level model. The objective function is: The reactive power flow and voltage and current parameters in the system are selected as decision variables. The decision variables are: Node voltage squared: Branch current square: Active power flow of branch: P = {P ij,t |ij∈[1,N L ],t∈[1,24]} Branch reactive power flow: Q = {Q ij,t |ij∈[1,N L ],t∈[1,24]} FID transmits active power: FID transmits reactive power: SOP transmits active power: SOP transmits reactive power: Where N represents the total number of nodes in the distribution network; i represents the node number of the distribution network; N L represents the total number of branches in the distribution network; ij represents the ijth branch; t represents the operating time point, V i,t is the square of the voltage at node i during the h period, v i,t represents the actual voltage of node i during the h period, F ij,t is the square of the current in branch ij during the time period h, f ij,t represents the actual current on branch ij during the time period h, P ij,t is the active power flowing through branch ij during time period t, Q ij,t is the reactive power flowing through branch ij during time period t, P i,t FID is the active power transmitted in the corresponding port of the FID connected to node i during time period t, Q i,t FID is the reactive power transmitted in the corresponding port of the FID connected to node i during time period t, P i,t SOP is the active power transmitted in the corresponding port of the SOP connected to node i during time period t, Q i,t SOP It is the reactive power transmitted in the corresponding port of the SOP connected to node i during the time period t.

6. The multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas according to claim 3 is characterized in that: The multi-layer model belongs to a mixed integer non-linear programming problem (MINLP). The constraints have quadratic terms and integer terms. Conventional solution methods do not work well. Therefore, second-order cone relaxation is used to transform it into a convenient planning problem. The second-order cone constraints of the line are: The second-order cone constraints of the branch are: In the formula, || ||2 is the two-norm.

7. The multi-layer collaborative planning method for medium and low voltage flexible interconnection equipment taking into account the source-load characteristics of low voltage areas according to claim 2 is characterized in that: Step 4 is as follows: Step 4-1: First, analyze the source-load characteristics of each low-voltage area in the distribution network, classify each low-voltage area, generate a low-voltage flexible interconnection equipment access location plan based on the area flexible interconnection division principle proposed in the present invention, and input this plan into the upper low-voltage FID planning layer. Step 4-2: Based on the upper-layer low-voltage FID planning layer, on the basis of the planning of the candidate locations for flexible interconnection of low-voltage substations with source-load characteristics, the improved whale optimization algorithm is used to optimize the access location and installation capacity of the FID between each low-voltage substation, with the goal of minimizing the system's comprehensive voltage deviation rate and optimizing power supply flexibility, to generate the initial configuration plan for the low-voltage FID, and send this configuration plan to the middle-layer and lower-layer optimization models. Step 4-3: The middle-level SOP planning layer, based on the import of the low-voltage FID configuration plan sent by the upper layer, generates a medium-voltage SOP configuration plan with the goal of minimizing system operation costs, and sends it to the lower-level medium and low-voltage distribution network optimization operation layer. Step 4-4: The lower-layer medium and low voltage distribution network optimizes the operation layer. The medium and low voltage flexible interconnection configuration scheme sent by the upper and lower layers is imported, and the middle-layer model and the lower-layer model are converted into a single-layer model. According to its own load information and system voltage and flow constraints, a commercial solver is used to minimize the operating cost of the distribution system, optimize the operation of the system, and calculate various parameters of the distribution network. Step 4-5: The operating indicators in the distribution network obtained by the lower model are sent to the upper model, and the new fitness value of each target is calculated. If the number of iterations is not met, the upper model updates the low-voltage FID configuration scheme through the improved whale optimization algorithm, and sends it to the merged single-layer model, and then goes to step 4-3.

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