A rural low-voltage distribution area photovoltaic inverter cooperative control method and system

Abstract

The present application relates to the technical field of power distribution area operation control, in particular to a rural low-voltage power distribution area photovoltaic inverter collaborative control method and system, which calculates the adjustment weight of different photovoltaic inverters through the key coefficient in the Jacobian inverse matrix, and then iteratively optimizes the reactive power adjustment amount of each photovoltaic inverter, the goal being to use the smallest reactive power increment to maximize the reduction of the overvoltage level of the power distribution area. When the reactive power of all photovoltaic inverters in the low-voltage power distribution area reaches the maximum capacity limit, the adjustment weight is calculated through the Jacobian coefficient matrix, and the active power reduction amount of each photovoltaic inverter is iteratively optimized to make the voltage of the power distribution area not exceed the limit. By adjusting the reactive power and active power of the photovoltaic inverter most sensitive to the node voltage amplitude variation, the present application can reduce the network loss and light rejection rate of the low-voltage power distribution area, and improve the power quality of the power distribution area.

Description

Technical Field

[0001] This invention relates to the field of distribution transformer area operation control technology, specifically to a collaborative control method and system for photovoltaic inverters in rural low-voltage distribution transformer areas. Background Technology

[0002] Rural power distribution systems are a crucial component of my country's power system, closely linked to the energy supply for the production and daily lives of nearly half the nation's population. Compared to developed urban areas, rural areas possess more physical space and greater potential for distributed renewable energy development. Currently, with the continuous advancement and development of various distributed renewable energy utilization technologies, distributed photovoltaic (PV) power is experiencing rapid growth, significantly impacting the planning and operation of my country's power distribution system.

[0003] Distributed residential photovoltaic (PV) systems connected to low-voltage distribution substations are one of the important forms of rural distributed PV. Statistics show that in 2023, 43.483 million kilowatts of newly installed PV capacity in my country were distributed residential PV systems. To effectively promote the development of distributed PV, in June 2021, the National Energy Administration launched a pilot program for the development of rooftop distributed PV systems across entire counties (cities, districts).

[0004] The key characteristics of rural distribution network power supply are: generally longer distances and smaller, more dispersed loads. High-density installation of residential distributed photovoltaic systems can easily lead to the following two problems:

[0005] 1) Backflow of power and full load on distribution transformers can even damage transformers and low-voltage switches;

[0006] 2) The power quality in the distribution area deteriorates, mainly manifested as voltage deviation, which causes frequent grid disconnection of photovoltaic power and seriously affects the normal power consumption of users.

[0007] The two issues mentioned above severely hinder the further development and utilization of distributed photovoltaic (PV) power in rural areas, leading to the suspension of PV project registration in several counties (cities, districts) across the country. While backflow and overloaded distribution transformers can generally be resolved through management measures, voltage deviation requires effective technical solutions. Generally, large-scale expansion and upgrading of rural distribution substations is the most fundamental solution to addressing voltage deviation and improving the acceptance capacity of distributed residential PV systems; however, this approach involves huge investments and is not economically viable. Therefore, it is urgent to research voltage optimization control methods for rural distribution substations to improve power quality and support the sustainable development of distributed residential PV systems.

[0008] Regarding voltage deviation regulation technology research, the U.S. National Renewable Energy Laboratory has significantly reduced voltage deviation in distribution systems by optimizing the power factor and reactive power output of photovoltaic inverters. The Fraunhofer Institute for Solar Energy Research in Germany has studied voltage control issues in distribution systems after large-scale distributed photovoltaic (PV) grid integration, primarily focusing on improving voltage quality through intelligent control of distributed PV inverters. Domestic scholars have proposed numerous voltage control strategies based on reactive power regulation to address voltage quality issues caused by large-scale grid connection of distributed PV, but most of these strategies are designed for medium-voltage, three-phase symmetrical distribution systems. Reference 1, "Low Voltage Management in Distribution Substations Using On-Load Capacity and Voltage Regulating Transformers and MPC Technology" (Electric Power Construction, 2023, Vol. 44, No. 10), designs a set of low-voltage distribution substation voltage management and economic operation optimization control strategies using on-load capacity and voltage regulating transformers and model predictive control technology. Reference 2, "Reactive Power Optimization in Low-Voltage Distribution Substations Considering Distributed Photovoltaics" (Master's Thesis, 2017, South China University of Technology), establishes a robust optimization configuration model for distributed reactive power compensation, solves for distributed reactive power compensation configuration schemes, and improves the voltage quality of distribution substations with distributed photovoltaic access.

[0009] Existing methods primarily focus on reactive power optimization control strategies for medium-voltage, three-phase symmetrical distribution transformer areas, with less attention paid to voltage regulation in rural low-voltage distribution transformer areas. This is mainly because data is difficult to obtain in rural low-voltage distribution transformer areas, controllers struggle to communicate with household photovoltaic inverters, and different photovoltaic inverters are difficult to coordinate for control to suppress voltage exceedances. Furthermore, the presence of three-phase imbalance in low-voltage distribution transformer areas further complicates the coordinated control of individual photovoltaic inverters. Existing technologies addressing low-voltage issues include an invention patent with publication number CN119324479A, which discloses a four-layer joint flexible voltage control method for distributed photovoltaic low-voltage grid connection. This method consumes excess power based on predicted photovoltaic output and solar water heaters, and uses a nonlinear least squares method to fit the predicted second voltage value of each node with the power relationship. Combined with operational data, it calculates the maximum reactive power regulation of the photovoltaic inverter. However, this method is complex in principle and difficult to implement in practice.

[0010] The invention patent with publication number CN119864882A discloses an automatic control method, device and control system for a high-proportion distributed photovoltaic inverter. It controls the active power droop based on the AC side frequency of the photovoltaic inverter and controls the reactive power droop based on the AC side voltage. The droop control is a distributed control method. However, the prior art requires communication, which is somewhat redundant. Moreover, it is difficult to establish a connection between the frequency of the low-voltage distribution network and the active power. Summary of the Invention

[0011] Purpose of the invention: In order to overcome the shortcomings of the prior art, the present invention provides a collaborative control method for photovoltaic inverters in rural low-voltage distribution substations. This method solves the problems of ineffective control of reactive and active power of household photovoltaic inverters in rural low-voltage distribution substations and poor voltage quality in low-voltage distribution substations. The present invention also provides a collaborative control system for photovoltaic inverters in rural low-voltage distribution substations.

[0012] Technical solution: According to a first aspect of the present invention, a method for coordinated control of photovoltaic inverters in rural low-voltage distribution substations is provided, the method comprising:

[0013] Using the power information and network topology parameters of the nodes in the low-voltage distribution area, the unbalanced power flow of the three-phase four-wire system is solved by the Newton-Raphson method. After the power flow converges iteratively, the three-phase voltage of each node in the low-voltage distribution system is obtained. The first judgment is made as to whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, no coordinated control is required. If the voltage exceeds the limit, the node number and phase number corresponding to the maximum voltage are obtained.

[0014] The weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages at each node are obtained from the inverse matrix of the obtained Jacobian matrix.

[0015] Reactive power-voltage coordinated regulation is performed based on the weight of the reactive power regulation coefficient, and the reactive power of each photovoltaic inverter is iteratively updated.

[0016] The second step is to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues a command to update the reactive power control target of each photovoltaic inverter. If the voltage exceeds the limit, it further determines whether the reactive power setting value of the photovoltaic inverter of all nodes has reached the corresponding maximum capacity. If it has not reached the corresponding maximum capacity, the power flow is recalculated, the weight of the reactive power adjustment coefficient is updated, the reactive power adjustment amount is iterated, and reactive power-voltage coordinated adjustment is performed again. If the maximum value has been reached, it means that active power adjustment is required.

[0017] Perform active power-voltage coordinated regulation and iteratively update the active power of each photovoltaic inverter;

[0018] The third check determines whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the active power control target of each photovoltaic inverter. Otherwise, the power flow is recalculated, the weight of the active power regulation coefficient is updated, and active power-voltage coordinated regulation is performed again until the voltage does not exceed the limit.

[0019] Furthermore, including:

[0020] After the power flow iteration converges, the three-phase voltages of each node in the low-voltage power distribution system are obtained, including:

[0021] Determine the nodes corresponding to the current rural low-voltage distribution substation, and determine the set of nodes corresponding to the single-phase or three-phase household photovoltaic inverters connected to it;

[0022] In a low-voltage distribution transformer area, each node is connected to an active power source of varying capacity. i and reactive load Q i Load information from each node is collected and summarized periodically through household smart meters. Assume there are N+1 nodes in the distribution transformer area, where the secondary side of the distribution transformer is the balancing node, and the remaining N nodes are PQ nodes. The set of node numbers connected to the photovoltaic inverter is denoted as K. PV,index ;

[0023] The three-phase voltages of the nodes connected to the photovoltaic inverter are obtained by solving the unbalanced power flow of the three-phase four-wire system using the Newton-Raphson method. There are a total of 3×N voltages. In the power flow iteration, the power flow correction equations are obtained for all nodes except the balancing node. Then, the inverse matrix of the Jacobian matrix, which characterizes the changes in active and reactive power of each node, is obtained.

[0024] Furthermore, including:

[0025] The weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages at each node, obtained from the inverse of the obtained Jacobian matrix, are expressed as follows:

[0026] The weights of the active power regulation coefficients corresponding to the three-phase phase voltages at each node are expressed as follows:

[0027]

[0028] The weights of the reactive power regulation coefficients corresponding to the three-phase phase voltages at each node are expressed as follows:

[0029]

[0030] in, Let be the coefficients of the active power component corresponding to the inverse matrix of the Jacobian of node i. K represents the coefficients of the active power component corresponding to the inverse matrix of the Jacobian between nodes k and i. PV,index This is the retrieval sequence number of the node connected to the photovoltaic inverter. Let be the coefficient term of the reactive power component corresponding to the inverse matrix of the Jacobian of node i. The coefficients of the reactive power component corresponding to the inverse matrix of the Jacobian between node k and node i.

[0031] Furthermore, including:

[0032] In the reactive power-voltage coordinated regulation based on the weight of the reactive power regulation coefficient, the regulation formula corresponding to the reactive power magnitude of each photovoltaic inverter is as follows:

[0033] Q i =Q i +△Q×λ Q,i , i∈K PV,index ;

[0034] Wherein, △Q is the step size of reactive power iteration, which is set according to the adjustment capability of the photovoltaic inverter and also takes into account the iteration speed.

[0035] Furthermore, including:

[0036] The process of performing reactive power-voltage coordinated regulation based on the weights of the reactive power regulation coefficient, and iteratively updating the reactive power of each photovoltaic inverter, further includes:

[0037] After all photovoltaic inverter reactive power settings have been updated, it is determined whether the inverter's reactive power settings exceed the inverter's maximum capacity. The maximum capacity is... S i,max The rated capacity of the photovoltaic inverter is 1.1 times its rated power P. i,max If the maximum capacity is exceeded, the reactive power setting value of the photovoltaic inverter is set to Q. i =Q i,max .

[0038] Furthermore, including:

[0039] In the process of performing active power-voltage coordinated regulation and iteratively updating the active power of each photovoltaic inverter, the active power update formula for each photovoltaic inverter is expressed as follows:

[0040] P i =P i -△P×λ P,i , i∈K PV,index ;

[0041] Wherein, ΔP is the step size of the active power iteration, which is set according to the adjustment capability of the photovoltaic inverter and also takes into account the iteration speed.

[0042] On the other hand, the present invention also provides a rural low-voltage distribution area photovoltaic inverter collaborative control system, the system comprising:

[0043] The first judgment module is used to solve the unbalanced power flow of a three-phase four-wire system by using the power information and network topology parameters of the nodes in the low-voltage distribution area and the Newton-Raphson method. After the power flow converges iteratively, the three-phase voltage of each node in the low-voltage distribution system is obtained. The first judgment is made as to whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, no coordinated control is required. If the voltage exceeds the limit, the node number and phase number corresponding to the maximum voltage are obtained.

[0044] The weight calculation module is used to obtain the weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages of each node based on the inverse matrix of the obtained Jacobian matrix.

[0045] The reactive power update module is used to perform reactive power-voltage coordinated regulation according to the weight of the reactive power regulation coefficient, and iteratively update the reactive power of each photovoltaic inverter.

[0046] The second judgment module is used to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the reactive power control target of each photovoltaic inverter. If the voltage exceeds the limit, it further determines whether the reactive power setting value of the photovoltaic inverter of all nodes has reached the corresponding maximum capacity. If it has not reached the corresponding maximum capacity, the power flow is recalculated, the weight of the reactive power adjustment coefficient is updated, the reactive power adjustment amount is iterated, and reactive power-voltage coordinated adjustment is performed again. If the maximum value has been reached, it means that active power adjustment is required.

[0047] The active power update module is used to perform active power-voltage coordinated regulation and iteratively update the active power of each photovoltaic inverter.

[0048] The third judgment module is used to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the active power control target of each photovoltaic inverter. Otherwise, the power flow is recalculated, the weight of the active power regulation coefficient is updated, and active power-voltage coordinated regulation is performed again until the requirement of voltage not exceeding the limit is met.

[0049] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0050] (1) The present invention first solves the three-phase four-wire unbalanced power flow, thereby obtaining the linear relationship between the change in reactive power, the change in active power and the change in node voltage amplitude. By adjusting the reactive power and active power of the photovoltaic inverter, which is most sensitive to the change in node voltage amplitude, the node voltage amplitude is effectively reduced, the voltage limit is avoided, and the power quality of the distribution area is improved.

[0051] (2) The present invention obtains the weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltage of each node through the inverse matrix of the Jacobian matrix, and performs reactive power-voltage coordinated adjustment and active power-voltage coordinated adjustment according to the corresponding weights, thereby coordinating the control of the reactive power and active power of all photovoltaic inverters in the low-voltage distribution area, possessing global optimality, and can minimize the network loss and curtailment rate of the low-voltage distribution area.

[0052] (3) This invention performs multiple voltage limit checks. The first check is performed after the power flow iteration converges, obtaining the three-phase voltage of each node in the low-voltage distribution system. The first check determines whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, no coordinated control is required. If the voltage exceeds the limit, the node number and phase number corresponding to the highest voltage are obtained. The second check is performed after the reactive power is updated, determining whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the reactive power control target of each photovoltaic inverter. If the voltage exceeds the limit, the reactive power setting value of the photovoltaic inverters of all nodes is further determined. The system checks whether all photovoltaic inverters have reached their corresponding maximum capacity. If not, the power flow is recalculated, and the weights of the active power regulation coefficients are updated. If the maximum capacity has been reached, active power regulation is required. The third check involves updating the active power of each photovoltaic inverter and then checking whether the three-phase voltage at each node exceeds the limit. If not, the controller issues a command to update the active power control target for each photovoltaic inverter. Otherwise, the power flow is recalculated, the weights of the active power regulation coefficients are updated, and active power-voltage coordinated regulation is performed again until the voltage limit requirement is met. Therefore, this invention eliminates the need for complex multi-objective coordinated optimization control processes and adjustments to distribution transformer taps. The proposed scheme is simple, reliable, and effectively improves the power quality of the distribution transformer area. Capacity checks can increase the photovoltaic access capacity of the distribution transformer area, promote the development of distributed photovoltaics, and meet the requirements for practical application prospects.

[0053] (4) This invention addresses the voltage quality management problem in rural three-phase four-wire unbalanced low-voltage distribution substations after high-penetration household photovoltaic systems are connected. It uses a centralized coordinated control strategy to optimize the allocation of reactive and active power of photovoltaic inverters, thereby suppressing voltage over-limit problems in distribution substations, reducing the reactive power increment of photovoltaic inverters, minimizing the active power discard of photovoltaic inverters, avoiding voltage over-limits, and improving the voltage quality of low-voltage distribution substations. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of a typical topology and collaborative controller structure of a three-phase four-wire rural low-voltage distribution substation according to an embodiment of the present invention.

[0055] Figure 2This is a schematic diagram illustrating the principle of the Jacobian inverse matrix after power flow calculation in a three-phase four-wire system according to an embodiment of the present invention.

[0056] Figure 3 This is a flowchart of the collaborative control method for photovoltaic inverters in rural low-voltage distribution substations according to an embodiment of the present invention;

[0057] Figure 4 The simulation verification results of a specific embodiment of the present invention are shown in (a), which is a schematic diagram of voltage distribution before photovoltaic inverter coordinated control, (b) is a schematic diagram of voltage distribution after reactive power-voltage coordinated control, and (c) is a schematic diagram of voltage distribution after active power-voltage coordinated control. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] Example 1: This example provides a method for coordinated control of photovoltaic inverters in rural low-voltage distribution substations. This method is based on rural low-voltage distribution substations. This example provides a typical topology of an existing three-phase four-wire rural low-voltage distribution substation, as shown below. Figure 1 As shown, for a rural low-voltage distribution area with N+1 nodes, there are a total of 38 nodes in the figure, i.e., N=37. Figure 1 In node K PV,index The nodes {8,11,14,15,16,17,18,19,20,21,26,28,29,34,36,38} are connected to single-phase or three-phase residential photovoltaic inverters.

[0060] In a low-voltage distribution transformer area, each node is connected to an active power source of varying capacity. i and reactive load Q i The system collects and aggregates load information from each node at regular intervals using household smart meters and transmits it to the collaborative controller. Node K connected to a single-phase or three-phase photovoltaic inverter... PV,indexTwo-way communication is established with the collaborative controller via 4G / 5G, power line carrier, and Zigbee wireless networks. This allows the acquisition of status information such as active and reactive power of each photovoltaic inverter, and also enables the collaborative controller to issue control target commands to the inverters. The collaborative controller unit uses the collected distribution area data to solve the three-phase four-wire unbalanced power flow and formulate optimal reactive and active power control targets for the photovoltaic inverters. The collected distribution area data includes the access nodes of the photovoltaic inverters and the magnitude of their input active / reactive power, as well as the active and reactive power of the loads connected to each node in the low-voltage distribution area. The load data of the nodes is updated periodically via smart meters.

[0061] In a power system, after passing through impedance Z ij =R ij +jX ij The voltage drop after impedance is:

[0062] ;

[0063] Among them, V i and V j The voltages at nodes i and j are ΔV, respectively. ij This is due to a voltage drop.

[0064] Because three-phase imbalance exists in a three-phase four-wire low-voltage distribution area, power flow calculations need to be performed for each phase separately. The voltage drop or power flow constraint equation can be expressed as:

[0065]

[0066] In this system, superscripts a, b, and c represent phases a, b, and c of the three-phase voltage, respectively, and r and x represent the impedance between lines ij. The same superscript indicates self-impedance, while different superscripts indicate mutual impedance. In a three-phase four-wire system, coupling and decoupling between different phases need to be considered. After decoupling, power flow calculations are performed separately for phases a, b, and c. This results in three times the number of variables in the equations compared to a traditional symmetrical three-phase power flow system.

[0067] This invention employs the Newton-Raphson method to solve the asymmetrical power flow in a three-phase four-wire system. In polar coordinates, a set of corrected power flow equations is obtained for all nodes except the equilibrium node, as follows:

[0068]

[0069] Among them, J -1 V,P and J -1 V,Q The active power coefficient and reactive power coefficient terms corresponding to the inverse matrix of the Jacobian are divided into the inverse matrix of the Jacobian matrix. The structure of the Jacobian matrix is ​​as follows: Figure 2 As shown, J -1V,P and J -1 V,Q They are 3N×3N square matrices, J -1 V,P and J -1 V,Q The physical meanings of these terms are the contribution coefficients to the change in node voltage amplitude after variations in active and reactive power at each node. In a low-voltage distribution transformer area, the adjustable nodes are the set K of nodes connected to the photovoltaic inverter. PV,index Calculate the weights of the reactive power and active power regulation coefficients for the three-phase voltage at each node, i.e.:

[0070]

[0071] Among them: J -1 V,P and J -1 V,Q The coefficients of the active power component and the reactive power component corresponding to the inverse matrix of the Jacobian are K. PV,index This is the retrieval sequence number of the node connected to the photovoltaic inverter.

[0072] This invention uses key coefficients in the Jacobian inverse matrix to iteratively optimize the reactive power regulation of each photovoltaic inverter, aiming to minimize the overvoltage level of the distribution transformer area by utilizing the minimum reactive power increment. When the reactive power of all photovoltaic inverters in the low-voltage distribution transformer area reaches the maximum capacity limit, and the overvoltage level of the distribution transformer area still cannot be reduced to the specified range, the active power of each photovoltaic inverter is iteratively optimized and reduced using key coefficients in the Jacobian inverse matrix, i.e., minimum capacity curtailment is performed to ensure that the voltage of the distribution transformer area does not exceed the limit.

[0073] Therefore, as Figure 3 As shown in this embodiment, the collaborative control method for photovoltaic inverters in rural low-voltage distribution areas includes the following specific steps:

[0074] Step 1: Using the power information and network topology parameters of the low-voltage distribution transformer substation nodes, solve the unbalanced power flow of the three-phase four-wire system using the Newton-Raphson method. Assume that the distribution transformer substation has N+1 nodes, where the secondary side of the distribution transformer is the balancing node, and the remaining N nodes are PQ nodes. The set of node indices connected to the photovoltaic inverter is denoted as K. PV,index ;

[0075] Step 2: After the power flow iteration converges, determine whether the maximum voltage among the three-phase voltages of each node in the low-voltage distribution system (i.e., the voltage among the 3×N voltages) exceeds the limit. If it does not exceed the limit, no coordinated control is required, and return to step 1; if the voltage exceeds the limit, obtain the node number and phase number corresponding to the maximum voltage.

[0076] Step 3: Calculate the inverse matrix J of the Jacobian matrix. -1 And calculate the weights of the reactive power and active power adjustment coefficients corresponding to the three-phase voltages at each node, that is:

[0077]

[0078] Among them: J -1 V,P and J -1 V,Q The coefficients of the active power component and the reactive power component corresponding to the inverse matrix of the Jacobian are K. PV,index This is the retrieval sequence number of the node connected to the photovoltaic inverter;

[0079] Step 4: Perform reactive power-voltage coordinated regulation, iteratively updating the reactive power of each photovoltaic inverter, i.e.:

[0080] Q i =Q i +△Q×λ Q,i , i∈K PV,index ;

[0081] Where: △Q is the step size of reactive power iteration, which can be set according to the adjustment capability of the photovoltaic inverter and take into account the iteration speed. The preferred value is 0.5kVar.

[0082] After all photovoltaic inverter reactive power settings have been updated, it is determined whether the inverter's reactive power settings exceed the inverter's maximum capacity, which is expressed as follows: S i,max The rated capacity of the photovoltaic inverter is typically 1.1 times its rated power P. i,max If the maximum capacity is exceeded, set the reactive power setting value of the photovoltaic inverter to Q. i =Q i,max ;

[0083] Step 5: Determine if the maximum voltage among the 3×N voltages exceeds the limit. If not, the controller issues a command to update the reactive power control target of each photovoltaic inverter and returns to Step 1. If the voltage exceeds the limit, further determine if the reactive power setting value of all photovoltaic inverters at all nodes has reached the corresponding maximum capacity. If not, recalculate the power flow and update λ. Q,i Returning to step 4, if the maximum value has been reached, it means that active power regulation is required, i.e., a small amount of light curtailment, to avoid voltage exceeding the limit.

[0084] Step 6: Perform active power-voltage coordinated regulation, iteratively updating the active power of each photovoltaic inverter, i.e.:

[0085] P i=P i -△P×λ P,i , i∈K PV,index ;

[0086] Where: △P is the step size of the active power iteration, which can be set according to the adjustment capability of the photovoltaic inverter and take into account the iteration speed. The preferred value is 0.2kW.

[0087] Step 7: Determine if the maximum voltage among the 3×N voltages exceeds the limit. If not, the controller issues a command to update the active power control target for each photovoltaic inverter and returns to Step 1. If the voltage exceeds the limit, recalculate the power flow and update λ. P,i Return to step 6 until the voltage limit requirement is met.

[0088] A specific embodiment of the present invention is a collaborative control method for photovoltaic inverters in a rural low-voltage distribution substation. This low-voltage distribution substation has 33 nodes. After single-phase or three-phase photovoltaic inverters are connected to different nodes, during the midday period of intense sunlight, the voltage of some nodes exceeds the limit. The three-phase voltage distribution of each node is as follows: Figure 4 As shown in (a), voltage over-limits occurred at nodes 10-18, exceeding the specified maximum value of 1.1 pu, with the A-phase voltage over-limit at node 18 being the most severe. The collaborative control method for photovoltaic inverters in low-voltage distribution substations first performs reactive power-voltage collaborative control. Based on the unbalanced power flow calculation results of a three-phase four-wire system, the adjustment coefficient λ of the three-phase reactive power of different photovoltaic inverters is calculated. Q,i The size is determined iteratively. When iteration reaches iters = 83, the reactive power of each node reaches its maximum capacity. At this point, the three-phase voltage distribution of each node is as follows: Figure 4 As shown in (b), all node voltages do not exceed the maximum value of 1.1 pu. However, if the voltage amplitude needs to be controlled within 1.05 pu, reactive power-voltage coordinated control is no longer sufficient, and active power-voltage coordinated control is required. Similarly, based on the unbalanced power flow calculation results of a three-phase four-wire system, the adjustment coefficient λ of the three-phase active power of different photovoltaic inverters is calculated. P,i The size is determined iteratively. When iterating to iters = 24, the three-phase voltage at each node does not exceed 1.05 pu, meeting the requirement. The three-phase voltage distribution at each node at this point is as follows: Figure 4As shown in (c) above. During the reactive power-voltage coordinated control and regulation process, the total reactive power of each photovoltaic inverter in the system increased from 33.63kVar to 93.26kVar, reaching its maximum. During the active power-voltage coordinated control process, the total active power of each photovoltaic inverter decreased from 224.2kW ​​to 218.12kW, which is a reduction of about 6kW of active power. This reduced the three-phase voltage of each node in the low-voltage distribution transformer area to within 1.05pu, thus improving the voltage quality of the low-voltage distribution transformer area.

[0089] In a radial three-phase four-wire low-voltage distribution area, this invention obtains a linear relationship between changes in reactive power, active power, and node voltage amplitude by solving the three-phase four-wire asymmetrical power flow. By adjusting the reactive and active power of the photovoltaic inverter, which is most sensitive to changes in node voltage amplitude, the node voltage amplitude is effectively reduced, voltage overshoot is avoided, and the power quality of the distribution area is improved.

[0090] In summary, this invention differs from reactive power optimization control strategies for medium-voltage, three-phase symmetrical distribution systems. It addresses the voltage quality management issues in rural three-phase four-wire unbalanced low-voltage distribution areas after the integration of high-penetration household photovoltaic systems. Through a centralized, coordinated control strategy for optimizing the allocation of reactive and active power in photovoltaic inverters, it suppresses voltage exceedance issues in distribution areas, minimizing reactive power and line losses, and minimizing active power and curtailment, thus significantly improving voltage quality in low-voltage distribution areas.

[0091] Example 2: The present invention also provides a rural low-voltage distribution area photovoltaic inverter collaborative control system, the system comprising:

[0092] The first judgment module is used to solve the unbalanced power flow of a three-phase four-wire system by using the power information and network topology parameters of the nodes in the low-voltage distribution area and the Newton-Raphson method. After the power flow converges iteratively, the three-phase voltage of each node in the low-voltage distribution system is obtained. The first judgment is made as to whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, no coordinated control is required. If the voltage exceeds the limit, the node number and phase number corresponding to the maximum voltage are obtained.

[0093] The weight calculation module is used to obtain the weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages of each node based on the inverse matrix of the obtained Jacobian matrix.

[0094] The reactive power update module is used to perform reactive power-voltage coordinated regulation according to the weight of the reactive power regulation coefficient, and iteratively update the reactive power of each photovoltaic inverter.

[0095] The second judgment module is used to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the reactive power control target of each photovoltaic inverter. If the voltage exceeds the limit, it further determines whether the reactive power setting value of the photovoltaic inverter of all nodes has reached the corresponding maximum capacity. If it has not reached the corresponding maximum capacity, it recalculates the power flow, updates the weight of the reactive power adjustment coefficient, iterates the reactive power adjustment amount, and performs reactive power-voltage coordinated adjustment again. If the maximum value has been reached, it means that active power adjustment is required.

[0096] The active power update module is used to perform active power-voltage coordinated regulation and iteratively update the active power of each photovoltaic inverter.

[0097] The third judgment module is used to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the active power control target of each photovoltaic inverter. Otherwise, the power flow is recalculated, the weight of the active power regulation coefficient is updated, and active power-voltage coordinated regulation is performed again until the requirement of voltage not exceeding the limit is met.

[0098] Other technical features of the photovoltaic inverter collaborative control system for rural low-voltage distribution substations described in this embodiment are similar to those of the corresponding photovoltaic inverter collaborative control method for rural low-voltage distribution substations, and will not be repeated here.

[0099] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. "A plurality of" means two or more, unless otherwise explicitly specified.

[0100] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0101] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0102] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0103] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as will be understood by those skilled in the art to which embodiments of the invention pertain.

[0104] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (such as a computer-based system, a processor-included system, or other system that can fetch and execute instructions from, an instruction execution system, apparatus, or device). For the purposes of this specification, "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transmit programs for use by, or in conjunction with, an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of computer-readable media include: an electrical connection having one or more wires (electronic device), a portable computer disk drive (magnetic device), random access memory (RAM), read-only memory (ROM), erasable and editable read-only memory (EPROM or flash memory), fiber optic devices, and portable optical disc read-only memory (CDROM). Alternatively, the computer-readable medium may be paper or other suitable media on which the program can be printed, since the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpreting, or otherwise processing as necessary, and then stored in a computer memory.

[0105] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0106] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0107] Furthermore, the functional units in the various embodiments of the present invention can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0108] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for coordinated control of photovoltaic inverters in rural low-voltage distribution substations, characterized in that, The method includes: Using the power information and network topology parameters of the nodes in the low-voltage distribution area, the unbalanced power flow of the three-phase four-wire system is solved by the Newton-Raphson method. After the power flow converges iteratively, the three-phase voltage of each node in the low-voltage distribution system is obtained. The first judgment is made as to whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, no coordinated control is required. If the voltage exceeds the limit, the node number and phase number corresponding to the maximum voltage are obtained. The weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages at each node are obtained from the inverse matrix of the obtained Jacobian matrix. Reactive power-voltage coordinated regulation is performed based on the weight of the reactive power regulation coefficient, and the reactive power of each photovoltaic inverter is iteratively updated. The second step is to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues a command to update the reactive power control target of each photovoltaic inverter. If the voltage exceeds the limit, it further determines whether the reactive power setting value of the photovoltaic inverter of all nodes has reached the corresponding maximum capacity. If it has not reached the corresponding maximum capacity, the power flow is recalculated, the weight of the reactive power adjustment coefficient is updated, the reactive power adjustment amount is iterated, and reactive power-voltage coordinated adjustment is performed again. If the maximum value has been reached, it means that active power adjustment is required. Perform active power-voltage coordinated regulation and iteratively update the active power of each photovoltaic inverter; The third check determines whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the active power control target of each photovoltaic inverter. Otherwise, the power flow is recalculated, the weight of the active power regulation coefficient is updated, and active power-voltage coordinated regulation is performed again until the voltage does not exceed the limit.

2. The method for coordinated control of photovoltaic inverters in rural low-voltage distribution areas according to claim 1, characterized in that, After the power flow iteration converges, the three-phase voltages of each node in the low-voltage power distribution system are obtained, including: Determine the nodes corresponding to the current rural low-voltage distribution substation, and determine the set of nodes corresponding to the single-phase or three-phase household photovoltaic inverters connected to it; In a low-voltage distribution area, each node accesses active power P i and reactive power load Q i of different capacities, and the load information of each node is collected and summarized by a household smart meter at a fixed time; it is assumed that there are N+1 nodes in the distribution area, among which the secondary side of the distribution transformer is a balanced node, and the remaining N nodes are PQ nodes, and the node number set of the node accessing the photovoltaic inverter is K PV,index ; The three-phase voltages of the nodes connected to the photovoltaic inverter are obtained by solving the unbalanced power flow of the three-phase four-wire system using the Newton-Raphson method. There are a total of 3×N voltages. In the power flow iteration, the power flow correction equations are obtained for all nodes except the balancing node. Then, the inverse matrix of the Jacobian matrix, which characterizes the changes in active and reactive power of each node, is obtained.

3. The method for coordinated control of photovoltaic inverters in rural low-voltage distribution areas according to claim 2, characterized in that, The weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages at each node, obtained from the inverse of the obtained Jacobian matrix, are expressed as follows: The weights of the active power regulation coefficients corresponding to the three-phase phase voltages at each node are expressed as follows: The weights of the reactive power regulation coefficients corresponding to the three-phase phase voltages at each node are expressed as follows: in, Let be the coefficients of the active power component corresponding to the inverse matrix of the Jacobian of node i. K represents the coefficients of the active power component corresponding to the inverse matrix of the Jacobian between nodes k and i. PV,index This is the retrieval sequence number of the node connected to the photovoltaic inverter. Let be the coefficient term of the reactive power component corresponding to the inverse matrix of the Jacobian of node i. The coefficients of the reactive power component corresponding to the inverse matrix of the Jacobian between node k and node i.

4. The method for coordinated control of photovoltaic inverters in rural low-voltage distribution areas according to claim 3, characterized in that, In the reactive power-voltage coordinated regulation based on the weight of the reactive power regulation coefficient, the regulation formula corresponding to the reactive power magnitude of each photovoltaic inverter is as follows: Q i =Q i +△Q×λ Q,i ,i∈K PV,index ; Wherein, △Q is the step size of reactive power iteration, which is set according to the adjustment capability of the photovoltaic inverter and also takes into account the iteration speed.

5. The method for coordinated control of photovoltaic inverters in rural low-voltage distribution areas according to claim 1, characterized in that, The process of performing reactive power-voltage coordinated regulation based on the weights of the reactive power regulation coefficient, and iteratively updating the reactive power of each photovoltaic inverter, further includes: After all photovoltaic inverter reactive power settings have been updated, it is determined whether the inverter's reactive power settings exceed the inverter's maximum capacity. The maximum capacity is... S i,max The rated capacity of the photovoltaic inverter is 1.1 times its rated power P. i,max If the maximum capacity is exceeded, the reactive power setting value of the photovoltaic inverter is set to Q. i =Q i,max .

6. The method for coordinated control of photovoltaic inverters in rural low-voltage distribution areas according to claim 3, characterized in that, In the process of performing active power-voltage coordinated regulation and iteratively updating the active power of each photovoltaic inverter, the active power update formula for each photovoltaic inverter is expressed as follows: P i =P i -△P×λ P,i ,i∈K PV,index ; Wherein, ΔP is the step size of the active power iteration, which is set according to the adjustment capability of the photovoltaic inverter and also takes into account the iteration speed.

7. A collaborative control system for photovoltaic inverters in a rural low-voltage distribution area, characterized in that, The system includes: The first judgment module is used to solve the unbalanced power flow of a three-phase four-wire system by using the power information and network topology parameters of the nodes in the low-voltage distribution area and the Newton-Raphson method. After the power flow converges iteratively, the three-phase voltage of each node in the low-voltage distribution system is obtained. The first judgment is made as to whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, no coordinated control is required. If the voltage exceeds the limit, the node number and phase number corresponding to the maximum voltage are obtained. The weight calculation module is used to obtain the weights of the reactive power and active power adjustment coefficients corresponding to the three-phase phase voltages of each node based on the inverse matrix of the obtained Jacobian matrix. The reactive power update module is used to perform reactive power-voltage coordinated regulation according to the weight of the reactive power regulation coefficient, and iteratively update the reactive power of each photovoltaic inverter. The second judgment module is used to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the reactive power control target of each photovoltaic inverter. If the voltage exceeds the limit, it further determines whether the reactive power setting value of the photovoltaic inverter of all nodes has reached the corresponding maximum capacity. If it has not reached the corresponding maximum capacity, the power flow is recalculated, the weight of the reactive power adjustment coefficient is updated, the reactive power adjustment amount is iterated, and reactive power-voltage coordinated adjustment is performed again. If the maximum value has been reached, it means that active power adjustment is required. The active power update module is used to perform active power-voltage coordinated regulation and iteratively update the active power of each photovoltaic inverter. The third judgment module is used to determine whether the three-phase voltage of each node exceeds the limit. If it does not exceed the limit, the controller issues an instruction to update the active power control target of each photovoltaic inverter. Otherwise, the power flow is recalculated, the weight of the active power regulation coefficient is updated, and active power-voltage coordinated regulation is performed again until the requirement of voltage not exceeding the limit is met.

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