A method for connecting a roof photovoltaic to a grid in a low-voltage area of a rural power grid considering three-phase imbalance

Abstract

The present application relates to a kind of low-voltage substation roof photovoltaic grid-connected method considering three-phase imbalance, belong to power system technical field.The technical scheme is: the scale of roof photovoltaic development in substation is evaluated;Photovoltaic grid-connected phase optimization demand evaluation, comb the roof photovoltaic development project list, carry out grid-connected node planning, distinguish three-phase grid-connected project set and single-phase grid-connected project set, according to the installed capacity of single-phase grid-connected project set each item, form single-phase grid-connected project sequence P;Three-phase unbalanced flow is used to calculate;In order of optimization single-phase grid-connected project sequence project grid-connected phase.The present application uses low-voltage substation distribution cabinet or tower as evaluation node, combines substation topological structure and electrical history behavior to realize the scale of roof photovoltaic development evaluation, node-by-node refined grid-connected demand prediction and grid-connected planning, can minimize the influence of village roof photovoltaic development on substation three-phase unbalanced degree in planning stage, realize the decision support of actual engineering planning.

Description

TECHNICAL FIELD

[0001] The application relates to a roof photovoltaic grid-connected method for a rural power grid low-voltage transformer area, and belongs to the technical field of power systems. BACKGROUND

[0002] A new power system is a power system mainly based on new energy, and the power supply structure is changed from a coal power plant dominated by controllable and continuous power to a new energy power plant dominated by strong uncertainty and weak controllable processing. With the promotion of the construction of the new power system mainly based on new energy, more and more distributed photovoltaics are connected to rural power grids, and distributed photovoltaic power generation in rural areas in China will increase significantly year by year. The rural power distribution system will change from the original radial passive network to an active network with a large number of distributed power sources. With the large-scale development of distributed photovoltaic power generation in rural areas, the traditional rural power grid will have the following problems:

[0003] 1. The influence of large-scale distributed photovoltaic access on the power quality of the low-voltage transformer area power grid, such as three-phase imbalance and voltage deviation;

[0004] 2. In the area mainly based on the rural power grid, due to the mismatch between the regional load characteristics and the photovoltaic power output characteristics, it is difficult to consume, and it is necessary to transform and upgrade or add new transformer capacity;

[0005] 3. Influence on power grid automation and relay protection. For example, during system failure, both the system and the photovoltaic will provide short-circuit current, which may cause relay device misoperation and other problems.

[0006] At the same time, due to the imperfect management of low-voltage distribution networks and the difference in user loads, there is a problem of three-phase imbalance of system load in a large number of low-voltage distribution networks. With the widespread access of distributed photovoltaics, especially single-phase photovoltaics, the three-phase imbalance problem of low-voltage distribution networks will be further aggravated, which will bring great risks to the operation of the transformer area.

[0007] At present, there is no standard research method for the grid-connected technology of distributed photovoltaics in the low-voltage transformer area, and the problem of three-phase imbalance cannot be solved, and it cannot be applied to the roof photovoltaic grid-connected method of the rural power grid low-voltage transformer area. SUMMARY

[0008] The application aims to provide a roof photovoltaic grid-connected method for a rural power grid low-voltage transformer area, which takes the low-voltage transformer area distribution cabinet or pole tower as the evaluation node, combines the transformer area topological structure and power consumption history to realize roof photovoltaic development scale evaluation, node-by-node refined grid-connected demand prediction and grid-connected planning, minimize the influence of roof photovoltaic development on the three-phase power flow imbalance of the transformer area in the planning stage, realize decision support for actual engineering planning, and solve the problems in the background art.

[0009] The technical scheme of the application is:

[0010] A low-voltage rural grid rooftop photovoltaic grid-connected method considering three-phase imbalance, comprising the following steps:

[0011] ① Obtain the typical daily output curve of photovoltaic, and evaluate the development scale of the low-voltage rural grid rooftop photovoltaic;

[0012] ② Optimize the demand evaluation of photovoltaic grid-connected phase, sort out the rooftop photovoltaic development project list, plan the grid-connected node, distinguish the three-phase grid-connected project set and the single-phase grid-connected project set, form the single-phase grid-connected project sequence P according to the installed capacity of the single-phase grid-connected project set;

[0013] ③ Calculate by using three-phase imbalance flow;

[0014] ④ Optimize the grid-connected phase of the single-phase grid-connected project sequence P project in sequence.

[0015] The specific steps are as follows:

[0016] In the step ①, the evaluation of the typical daily output curve of photovoltaic can be based on the clustering analysis of the historical power generation data of the local distributed photovoltaic project and obtaining the standardized typical daily output curve of photovoltaic, or using professional photovoltaic resource evaluation software or platform to obtain the standardized typical daily output curve of photovoltaic according to meteorological data analysis; the standardized typical daily output curve of photovoltaic is divided into 24 periods, each period t contains a value, i.e. the average power generation of the rooftop photovoltaic project per unit installation (1kWp) in the period t p t (kW), the value range is 0 to 1.

[0017] The evaluation of the development scale of the low-voltage rural grid rooftop photovoltaic includes the estimation of the building rooftop photovoltaic installation and the distribution of the building rooftop photovoltaic grid-connected node.

[0018] The step ② evaluates the development scale of each node and the overall scale of the low-voltage rural grid rooftop photovoltaic with the low-voltage distribution cabinet or the tower as the evaluation node;

[0019] There are M evaluation nodes in the low-voltage rural grid, which constitute the evaluation node set B:

[0020] B:{B1,B2,…,B m ,…,B M}

[0021] There are X buildings to be evaluated in the low-voltage rural grid, which constitute the building set b to be evaluated:

[0022] b:{b1,b2,…,b x ,…,b X}

[0023] Use satellite high-definition map to identify the roof area of each building b x by image recognition To consider the building roof bearing construction, air conditioning, water tank and agricultural production function required area, combined with the roof photovoltaic available area coefficient σ, estimate the effective area of each building roof photovoltaic development

[0024]

[0025] Unit roof area (1m 2 ) photovoltaic installed capacity P u (kWp), thus the building b x The installed roof photovoltaic capacity of each building b

[0026]

[0027] The village photovoltaic development adopts multi-point access, and the inverter output multi-point is connected to the low-voltage line nearby. Referring to the geographical distribution of each evaluation node B m in the evaluation node set B in the low-voltage area wiring diagram, the roof photovoltaic grid-connected node of each building b x is researched, and the shortest straight-line distance between the grid-connected node and the building b x is taken as the standard to determine the corresponding grid-connected node of the roof photovoltaic of each building b x ;

[0028] After forming the grid-connected node distribution result of the roof photovoltaic of each building, it is represented by an M×X matrix MX. If the roof photovoltaic of the building b x will be connected to the node B m , the corresponding photovoltaic installed capacity is , then the m×x item in the matrix is , otherwise it is 0; the roof photovoltaic of one building b x can only have one grid-connected node.

[0029]

[0030] According to the installed capacity of the photovoltaic grid-connected project, under the condition of considering the photovoltaic installed capacity and the inverter capacity ratio, three-phase grid-connected project set and single-phase grid-connected project set can be distinguished.

[0031] The inverter capacity ratio γ is determined by the typical daily output curve:

[0032]

[0033] The roof photovoltaic component installed capacity of the building b x is , then the corresponding project inverter rated capacity is

[0034]

[0035] According to the roof photovoltaic installed capacity of each building bx Inverter rated installed capacity Projects with a power output greater than 8kW belong to the three-phase grid-connected project category. This belongs to the single-phase grid-connected project group; the grid-connected demand of three-phase grid-connected photovoltaics does not require optimization. It participates in power flow simulation calculations with a three-phase four-wire grid connection and balanced three-phase output power. In the single-phase grid-connected project group, each project will be calculated based on its inverter installed capacity. Sort the projects from largest to smallest to form a sequence P of single-phase grid-connected projects;

[0036] P:{P1,P2,…,P n ,…,P N}

[0037] Photovoltaic grid-connected optimization sequentially optimizes the photovoltaic grid-connected phases of each project in the single-phase grid-connected project sequence P in order to improve the three-phase imbalance problem in the transformer area.

[0038] In step ③, the negative sequence voltage imbalance is used. As a key indicator:

[0039]

[0040] Based on OpenDSS software, and referencing the grid topology of the distribution area, the three-phase unbalanced load curves of typical scenarios for each user, and the output curves of existing distributed generation facilities, the three-phase unbalanced power flow is used for calculation.

[0041] Scenario S is divided into 24 time periods, each lasting 1 hour, allowing for the calculation of evaluation nodes B. m Negative sequence voltage imbalance corresponding to time period t And from node B under scene S m The root mean square value of the negative sequence voltage imbalance for all time periods T is used as node B. m Negative sequence voltage imbalance

[0042]

[0043] Negative sequence voltage imbalance in scenario S

[0044]

[0045] This is equal to the sum of the negative sequence voltage imbalance of each node, where M is the total number of nodes being evaluated.

[0046] Step ④, which sequentially optimizes the grid-connected phases of the single-phase grid-connected project sequence P, is divided into two stages:

[0047] Phase one: the enumeration method is used to optimize the single-phase grid-connected project sequence P above the median installed capacity of the project grid-connected phase, and the grid-connected strategy with the lowest comprehensive imbalance degree in the scene is obtained as the corresponding project grid-connected phase;

[0048] Phase two: considering the grid-connected strategy in phase one, the enumeration method is used to optimize the single-phase grid-connected project sequence P below the median installed capacity of the project grid-connected phase, and the grid-connected strategy with the lowest comprehensive imbalance degree in the scene is obtained as the corresponding project grid-connected phase.

[0049] The beneficial effects of the present application are: taking the low-voltage distribution cabinet or tower as the evaluation node, combining the substation topological structure and the electricity consumption history to realize the roof photovoltaic development scale evaluation, node-by-node fine grid-connected demand prediction and grid-connected planning, minimize the influence of the village roof photovoltaic development on the substation three-phase power flow imbalance in the planning stage, and realize the decision support for the actual engineering planning. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 The flowchart of the present application;

[0051] Figure 2 The photovoltaic typical daily output curve diagram of the present application. DETAILED DESCRIPTION

[0052] The present application will be further described by examples in combination with the drawings.

[0053] A kind of low-voltage substation roof photovoltaic grid-connected method of rural network considering three-phase imbalance, comprising the following steps:

[0054] ①Obtain photovoltaic typical daily output curve, evaluate the scale of roof photovoltaic development in substation;

[0055] ②Photovoltaic grid-connected phase optimization demand evaluation, sort out roof photovoltaic development project list, carry out grid-connected node planning, distinguish three-phase grid-connected project set and single-phase grid-connected project set, form single-phase grid-connected project sequence P according to the installed capacity of single-phase grid-connected project set;

[0056] ③Calculate using three-phase imbalance power flow;

[0057] ④Optimize the grid-connected phase of single-phase grid-connected project sequence P project in turn.

[0058] Specific steps are as follows:

[0059] The step ①, photovoltaic typical daily output curve evaluation, can be based on local distributed photovoltaic project historical power generation data for cluster analysis and obtain standardized photovoltaic typical daily output curve, or use professional photovoltaic resource evaluation software or platform, based on meteorological data analysis to obtain standardized photovoltaic typical daily output curve; the standardized photovoltaic typical daily output curve is divided into 24 periods, each period t contains a value, that is, the average power generation of unit installation (1kWp) roof photovoltaic project in period t p t (kW), the value range is 0 to 1.

[0060] The said substation roof photovoltaic development scale evaluation includes building roof photovoltaic installation estimation and building roof photovoltaic grid-connected node distribution.

[0061] The said step ② takes low-voltage substation distribution cabinet or tower as evaluation node to evaluate the development scale of each node and the overall scale of low-voltage substation roof photovoltaic.

[0062] There are M evaluation nodes in the substation, which constitute the evaluation node set B:

[0063] B:{B1,B2,…,B m ,…,B M}

[0064] There are X buildings to be evaluated in the substation, which constitute the building set b to be evaluated:

[0065] b:{b1,b2,…,b x ,…,b X}

[0066] Using satellite high-definition map, through image recognition, the roof area of each building b x is obtained. In order to consider the area required for building roof bearing construction, air conditioning, water tank and agricultural production function, combined with the available area coefficient σ of roof photovoltaic, the effective area of each building roof photovoltaic development is estimated

[0067]

[0068] The unit roof area (1m 2 ) photovoltaic installation is P u (kWp), so the roof photovoltaic installation capacity of each building b x can be obtained.

[0069]

[0070] The whole village photovoltaic development adopts multi-point access, and the inverter outlet is connected to the low-voltage line nearby. The geographical distribution of each evaluation node B m in the evaluation node set B in the low-voltage substation geographical wiring diagram is studied to research each building bx the roof photovoltaic grid-connected node to the building b x with the shortest straight-line distance between the grid-connected node and the building b x as the standard, determine the grid-connected node corresponding to the roof photovoltaic of each building b

[0071] After the distribution result of the grid-connected node of the roof photovoltaic of each building is formed, it is represented by an MxX matrix MX, if the roof photovoltaic of the building b x is connected to the node B m , the corresponding photovoltaic installed capacity is then the mthxth item in the matrix is otherwise, it is 0; the roof photovoltaic of a building b x can only have one grid-connected node;

[0072]

[0073] According to the installed capacity of the photovoltaic grid-connected project, under the condition of considering the photovoltaic installed capacity and the inverter capacity ratio, three-phase grid-connected project set and single-phase grid-connected project set can be distinguished.

[0074] The inverter capacity ratio γ is determined by the typical daily output curve:

[0075]

[0076] b x The roof photovoltaic component installed capacity of the building b is then the corresponding project inverter rated capacity is

[0077]

[0078] According to the inverter rated installed capacity of each building b x , if the inverter rated installed capacity of the building b is greater than 8kW, it belongs to the three-phase grid-connected project set, if the inverter rated installed capacity of the building b is less than 8kW, it belongs to the single-phase grid-connected project set; the photovoltaic grid-connected demand of the three-phase grid-connected does not need to be optimized, the three-phase four-wire grid-connected and the three-phase balanced output power participate in the power flow simulation calculation, the projects in the single-phase grid-connected project set will be sorted from large to small according to the inverter installed capacity

[0079] P:{P1,P2,…,P n ,…,P N}

[0080] The photovoltaic grid-connected optimization sequentially optimizes the photovoltaic grid-connected phase of each project in the single-phase grid-connected project sequence P, in order to improve the three-phase imbalance problem of the transformer area.

[0081] In step ③, the negative sequence voltage unbalance degree As the main indicators:

[0082]

[0083] According to the OpenDSS software, the three-phase unbalanced load curve of the typical scene of each user and the output curve of the existing distributed power generation facility are referred to, and three-phase unbalanced power flow is calculated;

[0084] The scene S is divided into 24 periods, and each period is 1 hour, so that the evaluation node B m The negative sequence voltage unbalance degree corresponding to the period t And the node B m The root mean square value of the negative sequence voltage unbalance degree of all periods T is taken as the negative sequence voltage unbalance degree of the node B m

[0085]

[0086] The negative sequence voltage unbalance degree of the scene S

[0087]

[0088] The sum of the negative sequence voltage unbalance degrees of each node, and M is the total number of evaluation nodes.

[0089] The step ④ sequentially optimizes the single-phase grid-connected project sequence P and the grid-connected phase, which is divided into two stages:

[0090] Stage one: using the enumeration method to optimize the grid-connected phase of the project with installed capacity above the median in the single-phase grid-connected project sequence P, and obtaining the grid-connected strategy with the lowest comprehensive unbalance degree under the scene as the grid-connected phase of the corresponding project;

[0091] Stage two: considering the grid-connected strategy in stage one, using the enumeration method to optimize the grid-connected phase of the project with installed capacity below the median in the single-phase grid-connected project sequence P, and obtaining the grid-connected strategy with the lowest comprehensive unbalance degree under the scene as the grid-connected phase of the corresponding project.

[0092] In this embodiment, a low-voltage distribution area roof photovoltaic grid-connected method considering three-phase imbalance of a rural power grid comprises the following steps:

[0093] (1) Photovoltaic typical daily output curve evaluation

[0094] The present application does not specify the photovoltaic typical daily output curve evaluation method, and can perform cluster analysis on local distributed photovoltaic project historical power generation data to obtain a standardized photovoltaic typical daily output curve, or can use professional photovoltaic resource evaluation software or platform to analyze meteorological data to obtain a standardized photovoltaic typical daily output curve.​

[0095] The standardization photovoltaic power curve used in the application distinguishes 24 time periods, each time period t contains a value, that is, the average power p of a unit installation (1 kWp) roof photovoltaic project in time period t t (kW), ranging from 0 to 1.

[0096] (2) The list of roof photovoltaic development projects is sorted out and the grid-connected node is planned, and a single-phase grid-connected project sequence is formed

[0097] The application takes the low-voltage distribution cabinet or tower as the evaluation node to evaluate the development of roof photovoltaic in low-voltage area.

[0098] There are M evaluation nodes in the area, which form the evaluation node set B:

[0099] B:{B1,B2,…,B m ,…,B M}

[0100] There are X buildings to be evaluated in the area, which form the building set b to be evaluated:

[0101] b:{b1,b2,…,b x ,…,b X}

[0102] Using satellite high-definition maps, the roof area of each building b x is identified by image recognition In order to consider the area required for building roof bearing construction, air conditioning, water tank, agricultural production and other functions, combined with the available area coefficient σ of roof photovoltaic, the effective area of roof photovoltaic development of each building b

[0103]

[0104] The unit roof area (1m 2 ) photovoltaic installation is P u (kWp), so the roof photovoltaic installation capacity of each building b x can be obtained

[0105]

[0106] Most of the village photovoltaic development adopts multi-point access, and the inverter outlet is connected to the low-voltage line nearby. Referring to the geographical distribution of each evaluation node B m in the low-voltage area geographical wiring diagram, the roof photovoltaic grid-connected node of each building b x is studied, and the shortest straight line distance between the grid-connected node and the building b x is taken as the standard to determine the roof photovoltaic grid-connected node of each building b xRoof photovoltaic corresponding to grid-connected node.

[0107] The above method can form the roof photovoltaic installation grid-connected node distribution result of each building, which is represented by an MxX matrix MX. If the building b x 's roof photovoltaic is connected to the node B m , the corresponding photovoltaic installation capacity is , then the mthxth item in the matrix is , otherwise it is 0. The roof photovoltaic of a building b x can only have one grid-connected node.

[0108]

[0109] 8kW below grid-connected demand is connected to 220V, 8kW to 20kW grid-connected demand is connected to 380V three-phase, and single 20kW or more grid-connected demand can be considered to be connected to 380V three-phase or to select a public pole transformer low-voltage dedicated line access scheme. Therefore, according to the photovoltaic grid-connected project installation scale, considering the photovoltaic installation and inverter capacity ratio, the three-phase grid-connected project set and the single-phase grid-connected project set can be distinguished.

[0110] The local inverter capacity ratio γ is determined with reference to the typical daily output curve in (1):

[0111]

[0112] The roof photovoltaic component installation of a building b x is , then the corresponding project inverter rated capacity is

[0113]

[0114] According to the inverter rated installation capacity of each building b x , is greater than 8kW, it belongs to the three-phase grid-connected project set, , it belongs to the single-phase grid-connected project set. The three-phase grid-connected photovoltaic grid-connected demand does not need to be optimized, and is connected to three-phase four-wire grid and three-phase balanced output power participates in the power flow simulation calculation. The single-phase grid-connected project set will be sorted from large to small according to the inverter installation capacity to form a single-phase grid-connected project sequence P.

[0115] P:{P1,P2,…,P n ,…,P N}

[0116] The photovoltaic grid-connected optimization sequentially optimizes the photovoltaic grid-connected phase of each project in the single-phase grid-connected project sequence P to improve the three-phase imbalance problem of the transformer area.

[0117] (3) Method for assessing the three-phase imbalance level in transformer areas

[0118] The three-phase imbalance assessment method refers to "GB / T 15543-2008 Power Quality Three-Phase Voltage Imbalance", using negative sequence voltage imbalance degree. As a key indicator:

[0119]

[0120] The power flow calculation relies on OpenDSS software, and takes into account the grid topology of the distribution area, the three-phase unbalanced load curves of typical scenarios for each user, and the output curves of existing distributed generation facilities. The calculation is performed using three-phase unbalanced power flow.

[0121] Scenario S is divided into 24 time periods, each lasting 1 hour, allowing for the calculation of evaluation nodes B. m Negative sequence voltage imbalance corresponding to time period t And from node B under scene S m The root mean square value of the negative sequence voltage imbalance for all time periods T is used as node B. m Negative sequence voltage imbalance

[0122]

[0123] Negative sequence voltage imbalance in scenario S This is equal to the sum of the negative sequence voltage imbalance of each node, where M is the total number of nodes being evaluated.

[0124]

[0125] (4) Optimize the grid-connected phases of single-phase grid-connected projects in sequence P.

[0126] Phase 1:

[0127] For P1 to P N / 2 (Round down) Global optimization is performed on the grid connection phase of the grid connection demand. Any single-phase photovoltaic grid connection demand can be connected to phase A, phase B, or phase C. The method is to enumerate P1 to P2. N / 2 The project may have a grid connection strategy, totaling... A grid connection strategy, with strategy S z Negative sequence voltage imbalance in the following scenario As the main comparison parameter for a strategy. P1 to P N / 2 Optimal grid connection strategy S for grid connection demand opt Then, among the Z strategies The minimum strategy, thus determining P1 to P2. N / 2 The phase of the project's grid connection.

[0128] Phase Two:

[0129] Building upon the achievements of Phase One, the first step is to complete the work on P1 to P2. N / 2 (Round down) Grid-connected modeling, then... To P N Global optimization of grid connection phases is performed. Any single-phase photovoltaic grid connection requirement can be assigned to phase A, phase B, or phase C, and the method is summarized using an enumeration approach. To P N The project may have a grid connection strategy, totaling... A grid connection strategy, with strategy S z Negative sequence voltage imbalance at each node This serves as the primary comparison parameter for a strategy. To P N Optimal grid connection strategy S for grid connection demand opt Then, among the Z strategies The minimum strategy, thus completing P1 to P2. N Optimize the grid connection phase for all projects.

Claims

1. A method for grid-connecting rooftop photovoltaic systems in low-voltage rural power grid areas that takes into account three-phase imbalance, characterized in that... It includes the following steps: ① Obtain the power generation curve of a typical photovoltaic day to assess the scale of rooftop photovoltaic development in the transformer substation area; ② Photovoltaic grid-connected phase optimization demand assessment, sort out the list of rooftop photovoltaic development projects, carry out grid connection node planning, distinguish between three-phase grid-connected project sets and single-phase grid-connected project sets, and form a single-phase grid-connected project sequence P based on the installed capacity of each item in the single-phase grid-connected project set; ③ Calculations are performed using three-phase unbalanced power flow; ④ Optimize the sequence of single-phase grid-connected projects in order. Project grid connection phase; Step ② uses the low-voltage distribution cabinet or tower as the evaluation node to evaluate each node and the overall scale of rooftop photovoltaic development in the low-voltage distribution area. There are a total of The evaluation nodes constitute the evaluation node set. : ； There are a total of The buildings to be evaluated constitute a set of buildings to be evaluated. : ； Using high-resolution satellite maps, buildings are identified through image recognition. Roof area To account for the area required for building roof load-bearing structures, air conditioning, water tanks, and agricultural production functions, the usable area coefficient of the rooftop photovoltaic system is considered. Estimate the effective area of ​​rooftop photovoltaic development for each building. ; ； Photovoltaic installations per unit roof area From this, we can obtain the various buildings Available rooftop photovoltaic capacity ； The village's photovoltaic development adopts a multi-point access approach, with inverter outputs connected to low-voltage lines at multiple points of proximity. The node set is evaluated with reference to the geographical wiring diagram of the low-voltage distribution area. Each evaluation node Geographical distribution study of various buildings Rooftop photovoltaic grid-connected nodes, with grid-connected nodes and buildings The standard for determining the buildings is the shortest straight-line distance between them. Rooftop photovoltaic systems correspond to grid-connected nodes; After the allocation of grid-connected nodes for rooftop photovoltaic installations on each building is determined, matrix It indicates that if the building The rooftop solar panels will be connected to the grid at the nodes. The corresponding photovoltaic installed capacity is Then the first in the matrix Item for Otherwise, it is 0; a building Rooftop solar panels can only have one grid-connected node; ； Based on the installed capacity of grid-connected photovoltaic projects, and taking into account the photovoltaic installed capacity and inverter capacity ratio, three-phase grid-connected project sets and single-phase grid-connected project sets can be distinguished. The inverter capacity ratio Determined by typical daily power curves: ； Unit installed rooftop photovoltaic projects during the time period The average power generation within the range is 0 to 1; The rooftop photovoltaic module installation is Then the rated capacity of the inverter for the corresponding project can be determined. for ； According to each building Inverter rated installed capacity , Projects with a power output greater than 8kW belong to the three-phase grid-connected project category; otherwise... It belongs to the single-phase grid-connected project set.

2. The method for grid-connecting rooftop photovoltaic systems in low-voltage rural power grid areas considering three-phase imbalance, as described in claim 1, is characterized in that... The specific steps are as follows: In step ①, the typical daily power generation curve of photovoltaic (PV) is evaluated by performing cluster analysis on historical power generation data of local distributed PV projects to obtain a standardized typical daily power generation curve. This is done using professional PV resource assessment software or a platform, based on meteorological data analysis. The standardized typical daily power generation curve is divided into 24 time periods, each period... It contains a single numerical value.

3. The method for grid-connecting rooftop photovoltaic systems in low-voltage rural power distribution areas considering three-phase imbalance, as described in claim 2, is characterized in that: The assessment of the scale of rooftop photovoltaic development in the transformer area includes the estimation of building rooftop photovoltaic installation capacity and the allocation of building rooftop photovoltaic grid-connected nodes.

4. The method for grid-connecting rooftop photovoltaic systems in low-voltage rural power grid areas considering three-phase imbalance, as described in claim 2, is characterized in that: Three-phase grid-connected photovoltaic (PV) grid-connection requirements do not require optimization. Power flow simulation calculations are performed using a three-phase four-wire grid connection with balanced three-phase output power. For single-phase grid-connected projects, the calculations will be based on the inverter's installed capacity. Sort them from largest to smallest to form a sequence of single-phase grid-connected projects. ; ； Photovoltaic grid connection optimization for single-phase grid-connected project series The grid-connected phases of photovoltaic power in each project are optimized sequentially to improve the three-phase imbalance problem in the distribution area.

5. A method for grid-connecting rooftop photovoltaic systems in low-voltage rural power grid areas considering three-phase imbalance, as described in claim 1 or 2, characterized in that: In step ③, the negative sequence voltage imbalance is used. As an indicator: ； Based on OpenDSS software, and referencing the grid topology of the distribution area, the three-phase unbalanced load curves of typical scenarios for each user, and the output curves of existing distributed generation facilities, the three-phase unbalanced power flow is used for calculation. Scene By dividing the time into 24 time periods, each lasting one hour, the evaluation nodes can be calculated. Corresponding time period Negative sequence voltage imbalance And by scene Next node All time periods The root mean square value of the negative sequence voltage imbalance is used as the node Negative sequence voltage imbalance : ； Scene Negative sequence voltage imbalance , ； This equals the sum of the negative sequence voltage imbalances at each node. To evaluate the total number of nodes.

6. A method for grid-connecting rooftop photovoltaic systems in low-voltage rural power distribution areas considering three-phase imbalance, as described in claim 1 or 2, characterized in that: Step ④ optimizes the sequence of single-phase grid-connected projects sequentially. The grid connection phase of the project is divided into two stages: Phase 1: Optimize the grid connection phases of projects with installed capacity above the median in the single-phase grid-connected project sequence P using an enumeration method, and obtain the grid connection strategy with the lowest overall imbalance in the scenario as the corresponding project grid connection phase; Phase Two: Considering the grid connection strategy of Phase One, the grid connection phases of projects with the median installed capacity or below in the single-phase grid connection project sequence P are optimized by enumeration to obtain the grid connection strategy with the lowest overall imbalance in the scenario, which is then used as the corresponding project grid connection phase.

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