Online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination

Abstract

This invention discloses an online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination. Specifically, it proposes a multi-task collaborative management model for three-phase current and voltage imbalance in low-voltage distribution networks with a high proportion of distributed photovoltaic (PV) systems. This model designs the priority order of each control module and fully considers information interaction between interconnected microgrids and adjacent PV systems. This allows for the construction of a three-phase imbalance management model for low-voltage distribution networks with a high proportion of distributed PV systems, eliminating frequent voltage amplitude and voltage imbalance exceeding limits at various nodes in the distribution network, and significantly reducing the three-phase current imbalance of distribution transformers. In other words, this invention proposes a multi-task coordinated control technology to eliminate frequent voltage amplitude and voltage imbalance exceeding limits at various nodes in the distribution network, ensuring the safe and reliable operation of the distribution network.

Description

Technical Field

[0001] This invention relates to the field of power distribution network technology, and specifically to an online management method for three-phase imbalance in low-voltage power distribution networks based on multi-task coordination. Background Technology

[0002] Integrating a high proportion of photovoltaic (PV) systems into a low-voltage distribution network can lead to a series of new problems. For example, the timing mismatch between distributed PV power generation and load curves causes frequent voltage over-limit phenomena. Furthermore, the presence of numerous unevenly distributed single-phase loads in the low-voltage distribution network results in varying degrees of three-phase current and voltage imbalance. The randomness and dispersion of high-proportion distributed PV integration further exacerbate this imbalance, leading to the following adverse effects on the low-voltage distribution network: First, neutral line overload: Because low-voltage distribution networks are three-phase four-wire systems, after decomposing the three-phase unbalanced current into symmetrical components, the current flowing through the neutral line is three times that of the zero-sequence current component. When this current is large enough, it may cause the wires and cables to overheat and degrade, or even fail.

[0003] Second, overheating of distribution transformers: Since the walls of the fuel tank of distribution transformers are made of ferromagnetic materials, when there is a zero-sequence current inside the transformer, a zero-sequence magnetic flux will be generated in the iron core. When the zero-sequence magnetic flux passes through the tank wall, it will induce eddy currents, thereby generating losses and causing the transformer to overheat, reducing the transformer's lifespan.

[0004] Third, the utilization rate of distribution transformers is reduced: due to the uneven distribution of a large number of single-phase photovoltaic systems and loads in the phases of the low-voltage distribution network, some phases of the distribution transformer are under light load and some phases are under heavy load, which greatly reduces its utilization rate.

[0005] In addition, three-phase imbalance in the power distribution network can also lead to problems such as reduced voltage quality, increased line losses, abnormal vibration or malfunction of equipment such as induction motors, and malfunction of protection relays.

[0006] Currently, there is no mature technology for managing three-phase imbalance in low-voltage distribution networks, and research on three-phase current imbalance compensation in low-voltage distribution networks is also limited. Previous research has largely focused on voltage control, developing strategies such as local voltage control or centralized control based on optimal power flow to suppress voltage exceedances. Therefore, a multi-task coordinated control technology is urgently needed to eliminate the frequent voltage amplitude and voltage imbalance exceeding the limits at various nodes of the distribution network, ensuring the safe and reliable operation of the distribution network. Summary of the Invention

[0007] The main objective of this invention is to provide an online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination. This method aims to address the urgent need for a multi-task coordination control technology to eliminate the frequent occurrence of voltage amplitude and voltage imbalance exceeding limits at various nodes of the distribution network, thereby ensuring the safe and reliable operation of the distribution network.

[0008] The technical solution proposed in this invention is as follows: A method for online management of three-phase imbalance in low-voltage distribution networks based on multi-task coordination includes: Based on the forward-backward substitution method, a power flow calculation method suitable for three-phase unbalanced low-voltage distribution networks is constructed. Based on the power flow calculation method, the current imbalance reactive power compensation expression is derived. Based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters, a comprehensive current imbalance compensation model for multi-microgrid cooperation of distribution transformers is established. Based on the droop control function, a voltage control and voltage imbalance management model that takes into account the coordination mechanism of adjacent distributed photovoltaic systems is constructed. The current imbalance comprehensive compensation model is coordinated and integrated with the voltage control and voltage imbalance management model to construct a low-voltage distribution network current and voltage three-phase imbalance management model with a high proportion of distributed photovoltaic power based on multi-task collaboration. The aforementioned imbalance management model eliminates the over-limit phenomenon of voltage amplitude and voltage imbalance at each node of the distribution network, and reduces the three-phase current imbalance of the distribution transformer.

[0009] Preferably, the method for constructing a power flow calculation method suitable for three-phase unbalanced low-voltage distribution networks based on the forward-backward substitution method includes: Obtain nodes in an unbalanced distribution network i Three-phase voltage and through lines in unbalanced distribution networks line current and power flow Among them, three-phase voltage Line current and power flow ; Calculate the injected current: Initialize the three-phase voltage values ​​of all nodes in the unbalanced distribution network to obtain the node current in each iteration. The three-phase injection currents at the location are as follows: (1) In the formula, ; For the node in the current iteration i place Phase voltage; and we have: , ; i Representative node i , Represents a node i place a Phase line current; Represents a node i place b Phase line current; Represents a node i place c Phase line current; Represents a node i place a Phase power flow; Represents a node i place b Phase power flow; Represents a node i place c Phase power flow; For the node in the current iteration place a Phase voltage; For the node in the current iteration place b Phase voltage; For the node in the current iteration place c Phase voltage; For the node in the current iteration place Phase active power; For the node in the current iteration place Phase reactive power; The imaginary unit; Perform back-substitution calculation: Starting from the end node of the unbalanced distribution network line, calculate the current through the line using Kirchhoff's current law. current : (2) In the formula, Represented as nodes i A collection of connected downstream branches; branch road Place The current at the sending end of the phase; Perform forward calculations: Starting from the root node of the unbalanced distribution network line, move towards the terminal node, and calculate the node values ​​based on Ohm's law. voltage : (3) In the formula, ,and Indicates the line The impedance matrix is ​​a A fully symmetric complex matrix; For the node in the current iteration place a Phase voltage; For the node in the current iteration place b Phase voltage; For the node in the current iteration place c Phase voltage; Perform iterative calculations: Repeat the above iterative process until the voltage difference at all nodes of the unbalanced distribution network meets the accuracy requirements, i.e., satisfies: (4) In the formula, For unbalanced distribution network nodes i Voltage difference at the point; To iterate to the th t Next time node i The three-phase voltage at the location, To iterate to the th Next time node i The three-phase voltage at the location; To determine the accuracy of iterative convergence.

[0010] Preferably, the step of deriving the current imbalance reactive power compensation expression based on the power flow calculation method, and establishing a comprehensive current imbalance compensation model for multi-microgrid cooperation of distribution transformers based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters, includes: The three-phase voltages at the node are approximately balanced, i.e., the node... i The voltage at that point is This simplifies the original complex nonlinear optimization problem into a real linear programming problem, where... for a Phase voltage amplitude, and satisfying: ; make Indicates the phase through the distribution transformer active power, Indicates the phase through the distribution transformer The reactive power is calculated using the symmetrical component method, specifically the zero-sequence current component and the negative-sequence current component through the distribution transformer, as shown in the following equation: (5) (6) In the formula, It is the zero-sequence current component. It is the negative sequence current component; Adjust the reactive power compensation of the star-connected photovoltaic system to compensate for the zero-sequence current component of the distribution transformer, as shown in formula (7): (7) In the formula, For micro-network k Compensation of zero-sequence current component in distribution transformers by a star-connected photovoltaic system. For micro-network k The degree of compensation for the zero-sequence current component passing through the distribution transformer. This is the zero-sequence current component; make For micro-network k Phase of a star-connected photovoltaic system The injected reactive power adjustment amount, It can be represented as: (8) By combining equations (5) and (8), equation (7) can be transformed into: (9) (10) In the formula, For micro-network k The degree of compensation for the zero-sequence current component passing through the distribution transformer; Both star-connected and delta-connected photovoltaic systems can jointly compensate for the negative sequence current component, yielding formula (11): (11) In the formula, For micro-network k The degree of compensation for the negative sequence current component passing through the distribution transformer; It is the negative sequence current component; For micro-network k Compensation of negative sequence current components in distribution transformers by a star-connected photovoltaic system; For micro-network k The compensation of the negative sequence current component in the distribution transformer by the mid-delta connected photovoltaic system is explained in detail as follows: (12) (13) In the formula, For micro-network k A mid-delta connected photovoltaic system for distribution transformers a Compensation for phase current components; For micro-network kA mid-delta connected photovoltaic system for distribution transformers b Compensation for phase current components; For micro-network k A mid-delta connected photovoltaic system for distribution transformers c Compensation for phase current components; Based on Kirchhoff's laws, formula (13) can be further extended as follows: (14) In the formula, For micro-network k Mid-triangle photovoltaic system ab Phase reactive power adjustment; For micro-network k Mid-triangle photovoltaic system bc Phase reactive power adjustment; For micro-network k Mid-triangle photovoltaic system ca Phase reactive power adjustment; By combining equations (12) and (14), expanding the real and imaginary parts, equation (11) is transformed into: (15) (16) In the formula, For micro-network k The degree of compensation for the negative sequence current component passing through the distribution transformer; The reactive power compensation for the positive sequence current component in the distribution transformer, and the improvement of the power factor, are expressed as follows: (17) In the formula, It is the positive sequence component compensation degree.

[0011] Preferably, the step of establishing a comprehensive current imbalance compensation model for multi-microgrid cooperation of distribution transformers based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters further includes: Based on the aforementioned current imbalance reactive power compensation expression, the current imbalance compensation problem of the distribution transformer is transformed into a linear programming problem to establish a comprehensive current imbalance compensation model for the multi-microgrid cooperation of the distribution transformer, as shown in formulas (18)-(23): (18) (19) (20) (twenty one) (twenty two) (twenty three) Wherein, formula (18) is the objective function of the comprehensive compensation model, which is to maximize the weighted sum of the compensation degrees of each current component; formula (19) is the optimization control variable of the comprehensive compensation model; formula (20) is the reactive power compensation constraint of each current component of the distribution transformer; formula (21) specifies the constraint range of the compensation degree. For micro-network k The maximum contribution to improving current imbalance; in the comprehensive compensation model, since the compensation current vector of each microgrid is opposite to the original current vector, the negative sequence and zero sequence current components of the distribution transformer are compensated by the photovoltaic system in each microgrid in a coordinated manner, and the effects between microgrids do not cancel each other out. This refers to the reactive power adjustment of a photovoltaic system in a star-connected configuration. This refers to the reactive power adjustment of the photovoltaic system under a delta connection configuration. The weighting coefficients for the zero-sequence current component compensation degree are: The weighting coefficients for the positive sequence current component compensation degree. The weighting coefficient for the compensation degree of the negative sequence current component; This represents the upper limit of the compensation degree; This represents the upper limit of reactive power adjustment for a photovoltaic system in a star-connected configuration. This represents the lower limit of reactive power adjustment for a photovoltaic system in a star-connected configuration. This represents the upper limit of reactive power adjustment for a photovoltaic system in a delta connection configuration. This represents the lower limit of reactive power adjustment for a photovoltaic system in a delta connection configuration.

[0012] Preferably, the process of transforming the current imbalance compensation problem of the distribution transformer into a linear programming problem based on the current imbalance reactive power compensation expression, in order to establish a comprehensive current imbalance compensation model for multi-microgrid cooperation of the distribution transformer, further includes: After each microgrid obtains the total reactive power adjustment, the upward and downward adjustment ratios of the photovoltaic system under different connection methods are calculated. The adjustment ratios for the star connection method are as follows: , ; The adjustment ratios for the delta connection method are as follows: , ; In the formula, This refers to the upward adjustment ratio for photovoltaic systems in a star-connected configuration. This represents the downward adjustment ratio for photovoltaic systems in a star-connected configuration. This refers to the upward adjustment ratio of the photovoltaic system in a delta connection configuration. This refers to the downward adjustment ratio of the photovoltaic system under the delta connection method; The adjustment ratio is then transmitted to the corresponding photovoltaic system within the microgrid. The system will control each photovoltaic system to calculate the reactive power adjustment amount sequentially based on the received adjustment ratio. This allows them to work together to improve the three-phase current imbalance in the distribution transformer, where at node i... The reactive power adjustment of a star-connected photovoltaic system is calculated as shown in formula (24). : (twenty four) In the formula, This refers to the reactive power regulation capacity of the photovoltaic system. This refers to the reactive power regulation capacity of the photovoltaic system.

[0013] Preferably, the voltage control and voltage imbalance mitigation model based on the droop control function and taking into account the coordination mechanism of adjacent distributed photovoltaic systems includes: Node i Place The piecewise droop function of the phase is expressed as: (25) In the formula, For photovoltaic systems at nodes i Place Phase node voltage amplitude; For photovoltaic systems at nodes i Place Phase reactive power adjustment K To control the gain, For node voltage magnitude to node Sensitivity of reactive power injection; For nodes i Place The upper limit of phase node voltage, For nodes i Place The lower limit of the phase node voltage; The voltage imbalance of a distribution network is calculated using the IEEE 141-1993 standard, defined as follows: (26) In the formula, For nodes i The average phase voltage amplitude; For nodes iThree-phase voltage imbalance at the point; As can be seen from formula (26), when the voltage imbalance exceeds the set threshold, the electrical phase with the largest deviation from the average phase voltage amplitude is identified. Based on the droop function of formula (27), the droop function of the photovoltaic inverter at the node is calculated. i Place The reactive power adjustment of the phase, wherein the droop function is: (27) In the formula, for and Projection operator between; The reactive power adjustment is used to eliminate the three-phase voltage imbalance. For node voltage magnitude to node i Sensitivity of reactive power injection; For nodes i Place Phase voltage amplitude; For nodes i The upper limit of reactive power adjustment, For nodes i The lower limit of reactive power adjustment; When the droop control method fails to completely eliminate local voltage violations or reduce the voltage imbalance to below a set threshold, the local photovoltaic system sends voltage adjustment request information to adjacent photovoltaic systems. Wherein, when the local photovoltaic system experiences overvoltage, the following condition is met: When the local photovoltaic system is undervoltage, it satisfies the condition. When the voltage imbalance exceeds a set threshold, the condition is met. ; When node l Place When the photovoltaic system of the phase receives demand information, it calculates the required reactive power adjustment according to formula (28), thereby achieving the adjustment with the node. i Place The photovoltaic systems of the phases cooperate to adjust the node voltage amplitude and voltage imbalance, wherein formula (28) is: (28) In the formula, For nodes i Place Phase voltage magnitude to node l Place Sensitivity of reactive power injection; For nodes l Place The amount of reactive power adjustment required when a photovoltaic system receives demand information from a neighboring photovoltaic system; For nodes l The upper limit of reactive power adjustment, For nodes l The lower limit of reactive power adjustment.

[0014] Preferably, the imbalance mitigation model includes a voltage control response module, a voltage imbalance response module, and a current imbalance response module; the step of eliminating the over-limit phenomena of voltage amplitude and voltage imbalance at each node of the distribution network and reducing the three-phase current imbalance of the distribution transformer through the imbalance mitigation model includes: Construct a multi-task collaborative control strategy to avoid conflicts between various control objectives; Perform current imbalance compensation; Local voltage control and voltage imbalance management are implemented.

[0015] Preferably, the construction of a multi-task collaborative control strategy to avoid conflicts between various control objectives includes: Set voltage control to the highest priority; When a voltage over-limit phenomenon is observed, the voltage control response module is invoked to control the voltage. If the voltage amplitude is within the preset safety range and the voltage imbalance exceeds the set threshold, the voltage imbalance response module is invoked to control the voltage. When the adjustable reactive power capacity of the photovoltaic system is insufficient to eliminate local voltage violations or voltage imbalances, a cooperation request is sent to the adjacent photovoltaic system, thereby calling on the remaining reactive power regulation capacity of the adjacent photovoltaic system to maintain the voltage amplitude and imbalance within the preset safety range in this area. When the voltage amplitude and voltage imbalance both meet the requirements, and no cooperation request is received from the adjacent photovoltaic system, the photovoltaic system in each microgrid system calls the current imbalance response module to control the current. The current imbalance compensation includes: First, the measured active and reactive power information of the distribution transformer is transmitted to the control units of each microgrid; The main controller of each microgrid aggregates multiple photovoltaic systems in the area into a three-phase star connection and a three-phase delta connection photovoltaic system, which is then marked as an aggregated photovoltaic system. The reactive power regulation capacity information of the aggregated photovoltaic system is then collected. The microgrid's main controller obtains reactive power limitation information from each photovoltaic system based on the current imbalance problem of the distribution transformer, and calculates the upward / downward adjustment ratio information propagated by the photovoltaic system to obtain the reactive power regulation of the photovoltaic system. The aforementioned local voltage control and voltage imbalance mitigation include: For a single photovoltaic system, first measure the voltage amplitude of each node, and then calculate the corresponding reactive power adjustment for voltage control or voltage imbalance management. When a photovoltaic system is unable to eliminate voltage over-limits or reduce voltage imbalance to below a preset threshold, it sends the required voltage adjustment information to adjacent photovoltaic systems. The collaboration request.

[0016] Preferably, the step of eliminating the over-limit phenomenon of voltage amplitude and voltage imbalance at each node of the distribution network through the imbalance management model, and reducing the three-phase current imbalance of the distribution transformer, further includes: The aforementioned imbalance management model was used to test a standard low-voltage distribution network system, and the voltage change trend and the improvement of the three-phase imbalance of current and voltage were obtained after a high proportion of distributed photovoltaic power was connected to the low-voltage distribution network.

[0017] Preferably, the step of testing the standard low-voltage distribution network system using the imbalance management model and obtaining the voltage change trend and the improvement of the three-phase imbalance of current and voltage after a high proportion of distributed photovoltaic power is connected to the low-voltage distribution network includes: A compensation degree index is introduced to quantify the mitigation effect of current imbalance. The compensation degrees for positive-sequence, negative-sequence, and zero-sequence current components are defined as follows: (29) (30) (31) In the formula, This represents the compensation degree for the positive sequence current component. This represents the compensation degree for the negative sequence current component. The compensation degree for the zero-sequence current component; This represents the amplitude of the positive-sequence current component before reactive power compensation. This represents the amplitude of the positive sequence current component after reactive power compensation. This represents the angle between the positive-sequence current component and the voltage before reactive power compensation. This is the angle between the positive sequence current component and the voltage after reactive power compensation. This represents the amplitude of the negative sequence current component before reactive power compensation. This represents the amplitude of the negative sequence current component after reactive power compensation. The magnitude of the zero-sequence current component before reactive power compensation. This represents the amplitude of the zero-sequence current component after reactive power compensation.

[0018] The above technical solution can achieve the following beneficial effects: This invention proposes a multi-task collaborative three-phase imbalance management model for low-voltage distribution networks with a high proportion of distributed photovoltaic (PV) systems. The model prioritizes various control modules and fully considers information interaction between interconnected microgrids and adjacent PV systems. This allows for the construction of a three-phase imbalance management model for low-voltage distribution networks with a high proportion of distributed PV systems, eliminating frequent voltage amplitude and imbalance exceeding limits at various nodes in the distribution network, and significantly reducing the three-phase current imbalance of distribution transformers. In other words, this invention proposes a multi-task coordinated control technique to eliminate frequent voltage amplitude and imbalance exceeding limits at various nodes in the distribution network, ensuring the safe and reliable operation of the distribution network. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0020] Figure 1 This is a flowchart of the first embodiment of an online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention; Figure 2 This is a typical three-phase imbalance interconnected microgrid architecture diagram in the third embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 3 This is a schematic diagram of a photovoltaic system aggregated in an interconnected microgrid in the third embodiment of the online governance method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 4 This is a vector diagram of the three-phase imbalance compensation current in the third embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 5 This is a negative sequence current component compensation vector diagram in the fourth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 6 This is a zero-sequence current component compensation vector diagram in the fourth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 7 This is a schematic diagram of the local voltage droop control method in the sixth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 8This is a schematic diagram of four interconnected microgrids constructed in a standard low-voltage distribution network in the tenth embodiment of the online governance method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Figure 9 The node voltage imbalance curves under different schemes in the tenth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention. Detailed Implementation

[0021] It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0022] This invention proposes an online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination.

[0023] As attached Figure 1 As shown, in the first embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, this embodiment includes the following steps: Step S110: Based on the forward-backward substitution method, a power flow calculation method applicable to three-phase unbalanced low-voltage distribution networks is proposed.

[0024] Step S120: Based on the power flow calculation method, derive the current imbalance reactive power compensation expression. Based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters, establish a comprehensive current imbalance compensation model for multi-microgrid cooperation of distribution transformers.

[0025] Step S130: Based on the droop control function, construct a voltage control and voltage imbalance management model that takes into account the coordination mechanism of adjacent distributed photovoltaic systems.

[0026] Step S140: The current imbalance comprehensive compensation model is coordinated and integrated with the voltage control and voltage imbalance management model to construct a low-voltage distribution network current and voltage three-phase imbalance management model with a high proportion of distributed photovoltaic power generation based on multi-task collaboration.

[0027] Step S150: Eliminate the over-limit phenomenon of voltage amplitude and voltage imbalance at each node of the distribution network through the imbalance management model, and reduce the three-phase current imbalance of the distribution transformer.

[0028] This invention proposes a multi-task collaborative three-phase imbalance management model for low-voltage distribution networks with a high proportion of distributed photovoltaic (PV) systems. The model prioritizes various control modules and fully considers information interaction between interconnected microgrids and adjacent PV systems. This allows for the construction of a three-phase imbalance management model for low-voltage distribution networks with a high proportion of distributed PV systems, eliminating frequent voltage amplitude and imbalance exceeding limits at various nodes in the distribution network, and significantly reducing the three-phase current imbalance of distribution transformers. In other words, this invention proposes a multi-task coordinated control technique to eliminate frequent voltage amplitude and imbalance exceeding limits at various nodes in the distribution network, ensuring the safe and reliable operation of the distribution network.

[0029] Furthermore, this invention establishes a comprehensive compensation model for current imbalance in multi-microgrid collaboration for distribution transformers. First, based on the symmetrical component method, the expressions for the zero-sequence and negative-sequence current components of the distribution transformer are calculated. Second, the zero-sequence current component is compensated by a star-connected photovoltaic system, while the negative-sequence and positive-sequence current components are jointly compensated by star- and delta-connected photovoltaic systems, thus deriving expressions for the cooperative compensation of unbalanced current by photovoltaic systems with different connection methods. Then, considering the reactive power regulation capacity limitation of the photovoltaic system, and taking the maximum compensation of the three-phase unbalanced current of the distribution transformer as the objective function, a centralized comprehensive compensation model for unbalanced current in multi-microgrid collaboration is established. In addition, this invention proposes a reactive power regulation allocation method to enable multiple distributed photovoltaic systems in an interconnected microgrid to cooperate in improving the three-phase unbalanced current.

[0030] Furthermore, this invention proposes a voltage control and voltage imbalance mitigation model that considers the coordination mechanism between adjacent distributed photovoltaic (PV) systems. First, a voltage control model for a low-voltage distribution network with a high proportion of distributed PV is established based on the droop control function of the local voltage. Second, a three-phase voltage imbalance mitigation model is constructed based on the ability to quantify the voltage imbalance index of the distribution network using only three-phase voltage amplitudes. Finally, considering the reactive power capacity limitations of PV inverters, a coordination mechanism between adjacent distributed PV systems is developed to enable cooperation between adjacent PV systems to jointly address the problems of voltage exceeding limits and large voltage imbalances.

[0031] In the second embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the first embodiment, step S110 includes the following steps: Since most distribution network topologies are radial or weakly meshed, and the resistance and reactance of the lines are comparable, the traditional Newton-Raphson power flow method cannot converge. Therefore, to ensure accurate power flow results even in large-scale unbalanced distribution networks, step S210 is executed first.

[0032] Step S210: Obtain nodes in the unbalanced distribution network iThree-phase voltage and through lines in unbalanced distribution networks line current and power flow Among them, three-phase voltage Line current and power flow .

[0033] Step S220: Calculate the injection current: Initialize the three-phase voltage values ​​of all nodes in the unbalanced distribution network to obtain the node voltage values ​​in each iteration. The three-phase injection currents at the location are as follows: (1) In the formula, ; For the node in the current iteration i place Phase voltage; and we have: , ; i Representative node i , Represents a node i place a Phase line current; Represents a node i place b Phase line current; Represents a node i place c Phase line current; Represents a node i place a Phase power flow; Represents a node i place b Phase power flow; Represents a node i place c Phase power flow; For the node in the current iteration place a Phase voltage; For the node in the current iteration place b Phase voltage; For the node in the current iteration place c Phase voltage; For the node in the current iteration place Phase active power; For the node in the current iteration place Phase reactive power; It is the imaginary unit.

[0034] Step S230: Perform back-substitution calculation: Starting from the end node of the unbalanced distribution network line, calculate the current through the line using Kirchhoff's current law. current : (2) In the formula, Represented as nodes A collection of connected downstream branches; branch road Place The current at the sending end of the phase.

[0035] Step S240: Perform forward calculation: Starting from the root node of the unbalanced distribution network line, move towards the terminal node and calculate the node based on Ohm's law. voltage : (3) In the formula, ,and Indicates the line The impedance matrix is ​​a A fully symmetric complex matrix; For the node in the current iteration place a Phase voltage; For the node in the current iteration place b Phase voltage; For the node in the current iteration place c Phase voltage.

[0036] Step S240: Perform iterative calculations: Repeat the above iterative process until the voltage difference at all nodes of the unbalanced distribution network meets the accuracy requirements, i.e., satisfies: (4) In the formula, For unbalanced distribution network nodes i Voltage difference at the point; To iterate to the th t Next time node i The three-phase voltage at the location, To iterate to the th Next time node i The three-phase voltage at the location; To determine the accuracy of iterative convergence.

[0037] In the third embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the second embodiment, the step of deriving the reactive power compensation expression for current imbalance based on the power flow calculation method in step S120 includes the following steps: As attached Figure 2 As shown, this embodiment takes a typical three-phase unbalanced interconnected microgrid architecture as an example. Each microgrid has a main controller to control the photovoltaic system within the microgrid. The main controller also cooperates with the main controllers in other microgrids to reduce the current imbalance of the distribution transformer.

[0038] Considering that interconnected microgrids contain several different types of distributed photovoltaic systems, this invention aggregates them into an equivalent star-connected three-phase photovoltaic system and an equivalent delta-connected three-phase photovoltaic system, as detailed in the appendix. Figure 3 As shown in the figure, To pass a The active power of the phase, To pass b The active power of the phase, respectively through c The active power of the phase; To pass a The reactive power of the phase, To pass b The reactive power of the phase, To pass c The reactive power of the phase; To connect photovoltaic systems via a star topology a Phase current, To connect photovoltaic systems via a star topology b Phase current, To connect photovoltaic systems via a star topology c Phase current; To connect photovoltaic systems via a delta configuration a Phase current, To connect photovoltaic systems via a delta configuration b Phase current, To connect photovoltaic systems via a delta configuration c The current in the phase; PV stands for photovoltaic system.

[0039] Furthermore, considering the inflexibility and slow response of traditional reactive power regulation devices, this invention employs a photovoltaic inverter capable of flexibly and quickly changing photovoltaic reactive power output to compensate for three-phase current imbalance in low-voltage distribution networks. (See attached diagram) Figure 4 As shown (attached) Figure 4 The vector diagram for compensating for three-phase current imbalance is shown in the figure. For compensation of zero-sequence current in star-connected photovoltaic systems; For compensation of negative sequence current in star-connected photovoltaic systems. For compensation of negative sequence current in a delta-connected photovoltaic system; It is a negative sequence current component. This is the zero-sequence current component; The zero-sequence current component after compensation. (This refers to the compensated negative sequence current component).

[0040] Considering that the voltage imbalance rate in a typical power distribution network must be maintained within a relatively small upper limit to avoid derating of induction motors, step S301 is executed first.

[0041] Step S301: Set the three-phase voltage of the node to approximately balanced, i.e., the node... i The voltage at that point is This simplifies the original complex nonlinear optimization problem into a real linear programming problem, thereby reducing its computational complexity. for a Phase voltage amplitude, and satisfying: .

[0042] Step S302: Let Indicates the phase through the distribution transformer active power, Indicates the phase through the distribution transformer The reactive power is calculated using the symmetrical component method, specifically the zero-sequence current component and the negative-sequence current component through the distribution transformer, as shown in the following equation: (5) (6) In the formula, It is the zero-sequence current component. It is the negative sequence current component.

[0043] Reactive power compensation for zero-sequence current components includes the following steps: Step S303: Adjust the reactive power compensation of the star-connected photovoltaic system to partially or completely compensate for the zero-sequence current component of the distribution transformer, as shown in the attached diagram. Figure 4 As shown in formula (7): (7) In the formula, For micro-network k Compensation of zero-sequence current component in distribution transformers by a star-connected photovoltaic system. For micro-network k The degree of compensation for the zero-sequence current component passing through the distribution transformer. This represents the zero-sequence current component.

[0044] Step S304: Let For micro-network k Phase of a star-connected photovoltaic system The injected reactive power adjustment amount, It can be represented as: (8) Step S305: Combine formulas (5) and (8) to transform formula (7) into: (9) (10) In the formula, For micro-network k The degree of compensation for the zero-sequence current component passing through the distribution transformer.

[0045] Specifically, reactive power compensation for negative sequence current components includes the following steps: Step S306: Both star-connected and delta-connected photovoltaic systems can jointly compensate for the negative sequence current component (as shown in the attached diagram). Figure 4 As shown), formula (11) is obtained: (11) In the formula, For micro-network k The degree of compensation for the negative sequence current component passing through the distribution transformer; It is the negative sequence current component; For micro-network k Compensation of negative sequence current components in distribution transformers by a star-connected photovoltaic system; For micro-network k The compensation of the negative sequence current component in the distribution transformer by the mid-delta connected photovoltaic system is explained in detail as follows: (12) (13) In the formula, For micro-network k A mid-delta connected photovoltaic system for distribution transformers a Compensation for phase current components; For micro-network k A mid-delta connected photovoltaic system for distribution transformers b Compensation for phase current components; For micro-network k A mid-delta connected photovoltaic system for distribution transformers c Compensation for phase current components.

[0046] Step S307: Based on Kirchhoff's laws, formula (13) is further extended to: (14) In the formula, For micro-network k Mid-triangle photovoltaic system ab Phase reactive power adjustment; For micro-network k Mid-triangle photovoltaic system bc Phase reactive power adjustment; For micro-network k Mid-triangle photovoltaic system ca Phase reactive power adjustment.

[0047] Step S308: Combine equations (12) and (14), expand the real and imaginary parts, and transform equation (11) into: (15) (16) In the formula, For micro-network k The degree of compensation for the negative sequence current component passing through the distribution transformer.

[0048] Specifically, reactive power compensation for the positive sequence current component includes the following steps: Step S309: Perform reactive power compensation on the positive sequence current component in the distribution transformer and improve the power factor. The expression is: (17) In the formula, It is the positive sequence component compensation degree.

[0049] In the fourth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the third embodiment, step S120 further includes the following steps: Step S410: Based on the current imbalance reactive power compensation expression, the current imbalance compensation problem of the distribution transformer is transformed into a linear programming problem to establish a comprehensive current imbalance compensation model for multi-microgrid cooperation of the distribution transformer, as shown in formulas (18)-(23): (18) (19) (20) (twenty one) (twenty two) (twenty three) Wherein, formula (18) is the objective function of the comprehensive compensation model, which is to maximize the weighted sum of the compensation degrees of each current component; formula (19) is the optimization control variable of the comprehensive compensation model; formula (20) is the reactive power compensation constraint of each current component of the distribution transformer; formula (21) specifies the constraint range of the compensation degree. For micro-network k The maximum contribution to improving current imbalance; in the comprehensive compensation model, since the compensation current vector of each microgrid is opposite to the original current vector, the negative sequence and zero sequence current components of the distribution transformer are compensated by the photovoltaic system in each microgrid in a coordinated manner, and the effects between microgrids do not cancel each other out. This refers to the reactive power adjustment of a photovoltaic system in a star-connected configuration. This refers to the reactive power adjustment of the photovoltaic system under a delta connection.

[0050] Details are as attached Figure 5 and attached Figure 6 As shown, Figure 5 middle, The initial value of the negative sequence current component; For microgrid compensation of negative sequence current. This is for microgrid 2 to compensate for negative sequence current. This is for microgrid 3 to compensate for negative sequence current. For compensation of negative sequence current in microgrid 4; The negative sequence current component after compensation; Figure 6 middle, The initial value of the zero-sequence current component; For microgrid 1 to compensate for zero-sequence current, For microgrid 2 to compensate for zero-sequence current, For microgrid 3 to compensate for zero-sequence current, For compensation of zero-sequence current in microgrid 4; This refers to the zero-sequence current component after compensation.

[0051] In addition, the model also includes different microgrids The sum equals 1 to avoid overcompensation; Formula (22) ensures that the reactive power regulation capability of the aggregated star-connected photovoltaic system is within its capacity range; Formula (23) ensures that the reactive power regulation capability of the aggregated delta-connected photovoltaic system is within its capacity range. The weighting coefficients for the zero-sequence current component compensation degree are: The weighting coefficients for the positive sequence current component compensation degree. The weighting coefficient for the compensation degree of the negative sequence current component; This represents the upper limit of the compensation degree; This represents the upper limit of reactive power adjustment for a photovoltaic system in a star-connected configuration. This represents the lower limit of reactive power adjustment for a photovoltaic system in a star-connected configuration. This represents the upper limit of reactive power adjustment for a photovoltaic system in a delta connection configuration. This represents the lower limit of reactive power adjustment for a photovoltaic system in a delta connection configuration.

[0052] In the fifth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the fourth embodiment, after step S410, the following steps are further included: Step S510: After each microgrid obtains the total reactive power adjustment, calculate the upward and downward adjustment ratios of the photovoltaic system under different connection methods. The adjustment ratios for the star connection method are as follows: , ; The adjustment ratios for the delta connection method are as follows: , ; In the formula, This refers to the upward adjustment ratio for photovoltaic systems in a star-connected configuration. This represents the downward adjustment ratio for photovoltaic systems in a star-connected configuration. This refers to the upward adjustment ratio of the photovoltaic system in a delta connection configuration. This represents the downward adjustment ratio of the photovoltaic system in a delta connection configuration.

[0053] Step S520: The adjustment ratio is transmitted to the corresponding photovoltaic system in the microgrid via the column controller.

[0054] Step S530: Control each photovoltaic system to sequentially calculate the reactive power adjustment amount based on the received adjustment ratio. This allows them to work together to improve the three-phase current imbalance in distribution transformers, where nodes... i Place The reactive power adjustment of a star-connected photovoltaic system is calculated as shown in formula (24). : (twenty four) In the formula, This refers to the reactive power regulation capacity of the photovoltaic system. This refers to the reactive power regulation capacity of the photovoltaic system.

[0055] In the sixth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the fifth embodiment, step S130 includes the following steps: When the reactive power capacity of the photovoltaic inverter is sufficient, droop control is typically used to eliminate voltage disturbances, as shown in the attached diagram. Figure 7 As shown. (Attached) Figure 7 middle, This is the reactive power adjustment amount; This is the upper limit of reactive power adjustment. This is the lower limit of the reactive power adjustment. For nodes i Voltage amplitude at the location; This represents the upper limit of the node voltage amplitude. This represents the lower limit of the node voltage amplitude.

[0056] From the appendix Figure 7 It can be seen that when the node voltage amplitude exceeds When a voltage limit is exceeded (i.e., when this occurs), the photovoltaic inverter adjusts its reactive power to change the voltage at its node. The amount of reactive power adjustment is proportional to the voltage amplitude until the capacity limit is reached. Therefore, the following steps are performed: Step S610: Transfer the node i Place The piecewise droop function of the phase is expressed as: (25) In the formula, For photovoltaic systems at nodes i Place Phase node voltage amplitude; For photovoltaic systems at nodes i Place Phase reactive power adjustment K To control the gain, For node voltage magnitude to node Sensitivity of reactive power injection; For nodes i Place The upper limit of phase node voltage, For nodes i Place The lower limit of the phase node voltage.

[0057] Step S620: Calculate the voltage imbalance of the distribution network using the IEEE 141-1993 standard, defined as follows: (26) In the formula, For nodes i The average phase voltage amplitude; For nodes i The three-phase imbalance of the voltage at the point.

[0058] Step S630: As can be seen from formula (26), when the voltage imbalance exceeds the set threshold (e.g., 2%), find the electrical phase that deviates the largest from the average phase voltage amplitude. Based on the droop function of formula (27), the droop function of the photovoltaic inverter at the node is calculated. i Place The reactive power adjustment of the phase gradually brings the maximum / minimum phase voltage closer to the average phase voltage amplitude, thereby reducing voltage imbalance. The droop function is: (27) In the formula, for and Projection operator between; The reactive power adjustment is used to eliminate the three-phase voltage imbalance. For node voltage magnitude to node i Sensitivity of reactive power injection; For nodes i Place Phase voltage amplitude; For nodes i The upper limit of reactive power adjustment, For nodes i The lower limit of reactive power adjustment.

[0059] Step S640: Considering that when the reactive power regulation capacity of a local photovoltaic system reaches its limit, and when the droop control-based method fails to completely eliminate local voltage violations or reduce the voltage imbalance to below a set threshold (e.g., 2%), voltage adjustment demand information is sent from the local photovoltaic system to the adjacent photovoltaic system. Wherein, when the local photovoltaic system experiences overvoltage, the following condition is met: When the local photovoltaic system is undervoltage, it satisfies the condition. When the voltage imbalance exceeds a set threshold, the condition is met. .

[0060] Step S650: When node l Place When the photovoltaic system of the phase receives demand information, it calculates the required reactive power adjustment according to formula (28), thereby achieving the adjustment with the node. i Place The photovoltaic systems of the phases cooperate to adjust the node voltage amplitude and voltage imbalance, wherein formula (28) is: (28) In the formula, For nodes i Place Phase voltage magnitude to node lPlace Sensitivity of reactive power injection; For nodes l Place The amount of reactive power adjustment required when a photovoltaic system receives demand information from a neighboring photovoltaic system; For nodes l The upper limit of reactive power adjustment, For nodes l The lower limit of reactive power adjustment.

[0061] In the seventh embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the sixth embodiment, the imbalance management model includes a voltage control response module, a voltage imbalance response module, and a current imbalance response module; step S140 includes the following steps: To avoid conflicts between various control objectives, a multi-task collaborative control strategy is constructed.

[0062] Step S710: Construct a multi-task collaborative control strategy to avoid conflicts between various control objectives.

[0063] Step S720: Perform current imbalance compensation.

[0064] Step S730: Perform local voltage control and voltage imbalance mitigation.

[0065] In the eighth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the seventh embodiment, step S710 includes the following steps: Step S811: Set voltage control to the highest priority.

[0066] Specifically, considering that voltage exceeding limits is the primary factor threatening the safe and reliable operation of the distribution network, voltage control is set as the highest priority.

[0067] Step S812: When a voltage over-limit phenomenon is observed, the voltage control response module is invoked to control the voltage.

[0068] Step S813: If the voltage amplitude is within the preset safety range and the voltage imbalance exceeds the set threshold (2%), call the voltage imbalance response module to control the voltage.

[0069] Step S814: When the adjustable reactive power capacity of the photovoltaic system is insufficient to eliminate local voltage violations or voltage imbalances, a cooperation request is sent to the adjacent photovoltaic system, thereby calling upon the remaining reactive power regulation capacity of the adjacent photovoltaic system to maintain the voltage amplitude and imbalance within the preset safety range in this area.

[0070] Step S815: When the voltage amplitude and voltage imbalance both meet the requirements, and no cooperation request is received from the adjacent photovoltaic system, the photovoltaic system in each microgrid system calls the current imbalance response module to control the current.

[0071] Step S720 includes the following steps: Step S821: First, transmit the measured active and reactive power information of the distribution transformer to the control unit of each microgrid.

[0072] Step S822: The main controller of each microgrid aggregates multiple photovoltaic systems in the area into a three-phase star connection and a three-phase delta connection photovoltaic system, and marks them as aggregated photovoltaic systems. Then, the reactive power regulation capacity information of the aggregated photovoltaic system is collected.

[0073] Step S823: The microgrid's main controller obtains reactive power limitation information from each photovoltaic system based on the current imbalance problem of the distribution transformer, and calculates the upward / downward adjustment ratio information propagated by the photovoltaic system to obtain the reactive power regulation of the photovoltaic system.

[0074] Step S730 includes the following steps: Step S831: For a single photovoltaic system, first measure the voltage amplitude of each node, and then calculate the corresponding reactive power adjustment for voltage control or voltage imbalance mitigation.

[0075] Step S832: When the photovoltaic system is unable to eliminate voltage over-limit or reduce the voltage imbalance to below a preset threshold (2%), it will send the required voltage adjustment information to the adjacent photovoltaic system. The collaboration request.

[0076] In the ninth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the eighth embodiment, after step S150, the following steps are further included: Step S910: Use the imbalance management model to test the standard low-voltage distribution network system and obtain the voltage change trend after a high proportion of distributed photovoltaic power is connected to the low-voltage distribution network, as well as the improvement of the current and voltage three-phase imbalance.

[0077] In the tenth embodiment of the online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination proposed in this invention, based on the ninth embodiment, step S910 includes the following steps: Step S1010: Introduce a compensation degree index to quantify the mitigation effect of current imbalance. The compensation degrees for positive-sequence, negative-sequence, and zero-sequence current components are defined as follows: (29) (30) (31) In the formula, This represents the compensation degree for the positive sequence current component. This represents the compensation degree for the negative sequence current component. The compensation degree for the zero-sequence current component; This represents the amplitude of the positive-sequence current component before reactive power compensation. This represents the amplitude of the positive sequence current component after reactive power compensation. This represents the angle between the positive-sequence current component and the voltage before reactive power compensation. This is the angle between the positive sequence current component and the voltage after reactive power compensation. This represents the amplitude of the negative sequence current component before reactive power compensation. This represents the amplitude of the negative sequence current component after reactive power compensation. The magnitude of the zero-sequence current component before reactive power compensation. This represents the amplitude of the zero-sequence current component after reactive power compensation.

[0078] From formula (29) - formula (31), we can see that , and The values ​​are all between 0 and 100%; when the compensation degree is equal to 0, it means that the corresponding current component in the distribution transformer has not been compensated; when the compensation degree is equal to 100%, it means that the corresponding current component in the distribution transformer has been fully compensated; therefore, the higher the compensation degree, the better the effect of current imbalance control.

[0079] Four interconnected microgrids are constructed on a standard low-voltage distribution network, as shown in the attached diagram. Figure 8 As shown, the proposed method is then verified. (Appendix) Figure 8 The installation locations of all single-phase and three-phase photovoltaic systems in the system are given, with capacities of 30kVA and 150kVA, respectively. All experiments were implemented on the MATLAB platform, and the proposed optimization model was solved using the CPLEX solver.

[0080] Simulation Result Analysis: Table 1 summarizes the zero-sequence current component, negative-sequence current component, and positive-sequence current component values ​​and their corresponding compensation levels after adopting this scheme: Table 1 As shown in Table 1, the three current components in this scheme are significantly reduced compared with the initial values, and their compensation degree is all above 70%, which verifies the superiority of this scheme.

[0081] Appendix Figure 9A comparison is given between the node voltage imbalance curve using the proposed scheme (i.e., the method shown in the figure) and the node voltage imbalance curve with the initial value. It can be seen that the method proposed in this invention can reduce the voltage imbalance of all nodes in the low-voltage distribution network as a whole.

[0082] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0083] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method for online management of three-phase imbalance in low-voltage distribution networks based on multi-task coordination, characterized in that, include: Based on the forward-backward substitution method, a power flow calculation method suitable for three-phase unbalanced low-voltage distribution networks is constructed. Based on the power flow calculation method, the current imbalance reactive power compensation expression is derived. Based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters, a comprehensive current imbalance compensation model for multi-microgrid cooperation of distribution transformers is established. Based on the droop control function, a voltage control and voltage imbalance management model that takes into account the coordination mechanism of adjacent distributed photovoltaic systems is constructed. The current imbalance comprehensive compensation model is coordinated and integrated with the voltage control and voltage imbalance management model to construct a low-voltage distribution network current and voltage three-phase imbalance management model with a high proportion of distributed photovoltaic power based on multi-task collaboration. The aforementioned three-phase current and voltage imbalance mitigation model eliminates the over-limit phenomenon of voltage amplitude and voltage imbalance at each node of the distribution network, and reduces the three-phase current imbalance of the distribution transformer.

2. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 1, characterized in that, The aforementioned power flow calculation method based on forward-backward substitution, applicable to three-phase unbalanced low-voltage distribution networks, includes: Obtain nodes in an unbalanced distribution network i Three-phase voltage and through lines in unbalanced distribution networks line current and power flow Among them, three-phase voltage Line current And power flow ; Calculate the injected current: Initialize the three-phase voltage values ​​of all nodes in the unbalanced distribution network to obtain the node current in each iteration. The three-phase injection currents at the location are as follows: (1) In the formula, ; For the node in the current iteration i place Phase voltage; and we have: , ; i Representative node i , Represents a node i place a Phase line current; Represents a node i place b Phase line current; Represents a node i place c Phase line current; Represents a node i place a Phase power flow; Represents a node i place b Phase power flow; Represents a node i place c Phase power flow; For the node in the current iteration place a Phase voltage; For the node in the current iteration place b Phase voltage; For the node in the current iteration place c Phase voltage; For the node in the current iteration place Phase active power; For the node in the current iteration place Phase reactive power; The imaginary unit; Perform back-substitution calculation: Starting from the end node of the unbalanced distribution network line, calculate the current through the line using Kirchhoff's current law. current : (2) In the formula, Represented as nodes i A collection of connected downstream branches; branch road Place The current at the sending end of the phase; Perform forward calculations: Starting from the root node of the unbalanced distribution network line, move towards the terminal node, and calculate the node values ​​based on Ohm's law. voltage : (3) In the formula, ,and Indicates the line The impedance matrix is ​​a A fully symmetric complex matrix; For the node in the current iteration place a Phase voltage; For the node in the current iteration place b Phase voltage; For the node in the current iteration place c Phase voltage; Perform iterative calculations: Repeat the above iterative process until the voltage difference at all nodes of the unbalanced distribution network meets the accuracy requirements, i.e., satisfies: (4) In the formula, For unbalanced distribution network nodes i Voltage difference at the point; To iterate to the th t Next time node i The three-phase voltage at the location, To iterate to the th Next time node i The three-phase voltage at the location; To determine the accuracy of iterative convergence.

3. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 2, characterized in that, The process involves deriving the current imbalance reactive power compensation expression based on the power flow calculation method, establishing a comprehensive current imbalance compensation model for multi-microgrid collaboration of distribution transformers based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters, including: The three-phase voltages at the node are approximately balanced, i.e., the node... i The voltage at that point is This simplifies the original complex nonlinear optimization problem into a real linear programming problem, where... for a Phase voltage amplitude, and satisfies: ; make Indicates the phase through the distribution transformer active power, Indicates the phase through the distribution transformer The reactive power is calculated using the symmetrical component method, specifically the zero-sequence current component and the negative-sequence current component through the distribution transformer, as shown in the following formula: (5) (6) In the formula, This is the zero-sequence current component. It is the negative sequence current component; Adjust the reactive power compensation of the star-connected photovoltaic system to compensate for the zero-sequence current component of the distribution transformer, as shown in formula (7): (7) In the formula, For micro-network k Compensation of the zero-sequence current component in the distribution transformer by a star-connected photovoltaic system. For micro-network k The degree of compensation for the zero-sequence current component passing through the distribution transformer. This is the zero-sequence current component; make For micro-network k Phase of a star-connected photovoltaic system The injected reactive power adjustment amount, It can be represented as: (8) Combining equations (5) and (8), we can transform equation (7) into: (9) (10) In the formula, For micro-network k The degree of compensation for the zero-sequence current component passing through the distribution transformer; Both star-connected and delta-connected photovoltaic systems can jointly compensate for the negative sequence current component, yielding formula (11): (11) In the formula, For micro-network k The degree of compensation for the negative sequence current component passing through the distribution transformer; It is the negative sequence current component; For micro-network k Compensation of negative sequence current components in distribution transformers by a star-connected photovoltaic system; For micro-network k The compensation of the negative sequence current component in the distribution transformer by the mid-delta connected photovoltaic system is explained in detail as follows: (12) (13) In the formula, For micro-network k A mid-delta connected photovoltaic system for distribution transformers a Compensation for phase current components; For micro-network k A mid-delta connected photovoltaic system for distribution transformers b Compensation for phase current components; For micro-network k A mid-delta connected photovoltaic system for distribution transformers c Compensation for phase current components; Based on Kirchhoff's laws, formula (13) can be further extended as follows: (14) In the formula, For micro-network k Mid-triangle photovoltaic system ab Phase reactive power adjustment; For micro-network k Mid-triangle photovoltaic system bc Phase reactive power adjustment; For micro-network k Mid-triangle photovoltaic system ca Phase reactive power adjustment; By combining equations (12) and (14), expanding the real and imaginary parts, equation (11) is transformed into: (15) (16) In the formula, For micro-network k The degree of compensation for the negative sequence current component passing through the distribution transformer; The reactive power compensation for the positive sequence current component in the distribution transformer, and the improvement of the power factor, are expressed as follows: (17) In the formula, It is the positive sequence component compensation degree.

4. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 3, characterized in that, The comprehensive current imbalance compensation model for multi-microgrid collaboration of distribution transformers, based on the current imbalance reactive power compensation expression and the star and delta connection methods of photovoltaic inverters, also includes: Based on the aforementioned current imbalance reactive power compensation expression, the current imbalance compensation problem of the distribution transformer is transformed into a linear programming problem to establish a comprehensive current imbalance compensation model for the multi-microgrid cooperation of the distribution transformer, as shown in formulas (18)-(23): (18) (19) (20) (21) (22) (23) Wherein, formula (18) is the objective function of the comprehensive compensation model, which is to maximize the weighted sum of the compensation degrees of each current component; formula (19) is the optimization control variable of the comprehensive compensation model; formula (20) is the reactive power compensation constraint of each current component of the distribution transformer; formula (21) specifies the constraint range of the compensation degree. For micro-network k The maximum contribution to improving current imbalance; in the comprehensive compensation model, since the compensation current vector of each microgrid is opposite to the original current vector, the negative sequence and zero sequence current components of the distribution transformer are compensated by the photovoltaic system in each microgrid in a coordinated manner, and the effects between microgrids do not cancel each other out. This refers to the reactive power adjustment of a photovoltaic system in a star-connected configuration. This refers to the reactive power adjustment of the photovoltaic system under a delta connection configuration. The weighting coefficients for the zero-sequence current component compensation degree are: This represents the weighting coefficient for the positive sequence current component compensation degree. The weighting coefficient for the compensation degree of the negative sequence current component; This represents the upper limit of the compensation degree; This represents the upper limit of reactive power adjustment for a photovoltaic system in a star-connected configuration. This represents the lower limit of reactive power adjustment for a photovoltaic system in a star-connected configuration. This represents the upper limit of reactive power adjustment for a photovoltaic system in a delta connection configuration. This represents the lower limit of reactive power adjustment for a photovoltaic system in a delta connection configuration.

5. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 4, characterized in that, Based on the aforementioned reactive power compensation expression for current imbalance, the current imbalance compensation problem of the distribution transformer is transformed into a linear programming problem to establish a comprehensive current imbalance compensation model for multi-microgrid cooperation of the distribution transformer. This model further includes: After each microgrid obtains the total reactive power adjustment, the upward and downward adjustment ratios of the photovoltaic system under different connection methods are calculated. The adjustment ratios for the star connection method are as follows: ， ； The adjustment ratios for the delta connection method are as follows: ， ； In the formula, This refers to the upward adjustment ratio for photovoltaic systems in a star-connected configuration. This represents the downward adjustment ratio for photovoltaic systems in a star-connected configuration. This refers to the upward adjustment ratio of the photovoltaic system in a delta connection configuration. This refers to the downward adjustment ratio of the photovoltaic system under the delta connection method; The adjustment ratio is then transmitted to the corresponding photovoltaic system within the microgrid. The system will control each photovoltaic system to calculate the reactive power adjustment amount sequentially based on the received adjustment ratio. This allows them to work together to improve the three-phase current imbalance in the distribution transformer, where at node i... The reactive power adjustment of a star-connected photovoltaic system is calculated as shown in formula (24). : (24) In the formula, This refers to the reactive power regulation capacity of the photovoltaic system. This refers to the reactive power regulation capacity of the photovoltaic system.

6. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 5, characterized in that, The voltage control and voltage imbalance mitigation model, based on the droop control function and taking into account the coordination mechanism of adjacent distributed photovoltaic systems, includes: Node i Place The piecewise droop function of the phase is expressed as: (25) In the formula, For photovoltaic systems at nodes i Place Phase node voltage amplitude; For photovoltaic systems at nodes i Place Phase reactive power adjustment K To control the gain, For node voltage magnitude to node Sensitivity of reactive power injection; For nodes i Place The upper limit of phase node voltage, For nodes i Place The lower limit of the phase node voltage; The voltage imbalance of a distribution network is calculated using the IEEE 141-1993 standard, defined as follows: (26) In the formula, For nodes i The average phase voltage amplitude; For nodes i Three-phase voltage imbalance at the point of application; As can be seen from formula (26), when the voltage imbalance exceeds the set threshold, the electrical phase with the largest deviation from the average phase voltage amplitude is identified. Based on the droop function of formula (27), the droop function of the photovoltaic inverter at the node is calculated. i Place The reactive power adjustment of the phase, wherein the droop function is: (27) In the formula, for and Projection operator between; To eliminate reactive power adjustment for three-phase voltage imbalance; For node voltage magnitude to node i Sensitivity of reactive power injection; For nodes i Place Phase voltage amplitude; For nodes i The upper limit of reactive power adjustment, For nodes i The lower limit of reactive power adjustment; When the droop control method fails to completely eliminate local voltage violations or reduce the voltage imbalance to below a set threshold, the local photovoltaic system sends voltage adjustment request information to adjacent photovoltaic systems. Wherein, when the local photovoltaic system experiences overvoltage, the following condition is met: When the local photovoltaic system is undervoltage, it satisfies the condition. When the voltage imbalance exceeds a set threshold, the condition is met. ; When node l Place When the photovoltaic system of the phase receives demand information, it calculates the required reactive power adjustment according to formula (28), thereby achieving the adjustment with the node. i Place The photovoltaic systems of the phases cooperate to adjust the node voltage amplitude and voltage imbalance, wherein formula (28) is: (28) In the formula, For nodes i Place Phase voltage magnitude to node l Place Sensitivity of reactive power injection; For nodes l Place The amount of reactive power adjustment required when a photovoltaic system receives demand information from a neighboring photovoltaic system; For nodes l The upper limit of reactive power adjustment, For nodes l The lower limit of reactive power adjustment.

7. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 6, characterized in that, The imbalance mitigation model includes a voltage control response module, a voltage imbalance response module, and a current imbalance response module. The method of eliminating the over-limit phenomena of voltage amplitude and voltage imbalance at each node of the distribution network through the imbalance management model, and reducing the three-phase current imbalance of the distribution transformer, includes: Construct a multi-task collaborative control strategy to avoid conflicts between various control objectives; Perform current imbalance compensation; Local voltage control and voltage imbalance management are implemented.

8. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 7, characterized in that, The construction of a multi-task collaborative control strategy to avoid conflicts between various control objectives includes: Set voltage control to the highest priority; When a voltage over-limit phenomenon is observed, the voltage control response module is invoked to control the voltage. If the voltage amplitude is within the preset safety range and the voltage imbalance exceeds the set threshold, the voltage imbalance response module is invoked to control the voltage. When the adjustable reactive power capacity of the photovoltaic system is insufficient to eliminate local voltage violations or voltage imbalances, a cooperation request is sent to the adjacent photovoltaic system, thereby calling on the remaining reactive power regulation capacity of the adjacent photovoltaic system to maintain the voltage amplitude and imbalance within the preset safety range in this area. When the voltage amplitude and voltage imbalance both meet the requirements, and no cooperation request is received from the adjacent photovoltaic system, the photovoltaic system in each microgrid system calls the current imbalance response module to control the current. The current imbalance compensation includes: First, the measured active and reactive power information of the distribution transformer is transmitted to the control units of each microgrid; The main controller of each microgrid aggregates multiple photovoltaic systems in the area into a three-phase star connection and a three-phase delta connection photovoltaic system, which is then marked as an aggregated photovoltaic system. The reactive power regulation capacity information of the aggregated photovoltaic system is then collected. The microgrid's main controller obtains reactive power limitation information from each photovoltaic system based on the current imbalance problem of the distribution transformer, and calculates the upward / downward adjustment ratio information propagated by the photovoltaic system to obtain the reactive power regulation of the photovoltaic system. The aforementioned local voltage control and voltage imbalance mitigation include: For a single photovoltaic system, first measure the voltage amplitude of each node, and then calculate the corresponding reactive power adjustment for voltage control or voltage imbalance management. When a photovoltaic system is unable to eliminate voltage over-limits or reduce voltage imbalance to below a preset threshold, it sends the required voltage adjustment information to adjacent photovoltaic systems. The collaboration request.

9. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 8, characterized in that, The process of eliminating the over-limit phenomena of voltage amplitude and voltage imbalance at each node of the distribution network through the imbalance management model, and reducing the three-phase current imbalance of the distribution transformer, further includes: The aforementioned imbalance management model was used to test a standard low-voltage distribution network system, and the voltage change trend and the improvement of the three-phase imbalance of current and voltage were obtained after a high proportion of distributed photovoltaic power was connected to the low-voltage distribution network.

10. The online management method for three-phase imbalance in low-voltage distribution networks based on multi-task coordination according to claim 9, characterized in that, The process of using the aforementioned imbalance mitigation model to test a standard low-voltage distribution network system and obtaining the voltage change trend after a high proportion of distributed photovoltaic power is connected to the low-voltage distribution network, as well as the improvement in the three-phase imbalance of current and voltage, includes: A compensation degree index is introduced to quantify the mitigation effect of current imbalance. The compensation degrees for positive-sequence, negative-sequence, and zero-sequence current components are defined as follows: (29) (30) (31) In the formula, This represents the compensation degree for the positive sequence current component. This represents the compensation degree for the negative sequence current component. The compensation degree for the zero-sequence current component; This represents the amplitude of the positive-sequence current component before reactive power compensation. This represents the amplitude of the positive sequence current component after reactive power compensation. This is the angle between the positive-sequence current component and the voltage before reactive power compensation. This is the angle between the positive sequence current component and the voltage after reactive power compensation. This represents the amplitude of the negative sequence current component before reactive power compensation. This represents the amplitude of the negative sequence current component after reactive power compensation. The magnitude of the zero-sequence current component before reactive power compensation. This represents the amplitude of the zero-sequence current component after reactive power compensation.

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