Three-phase imbalance calculation and commutation treatment method, system and device for low-voltage distribution network with photovoltaic reverse feeding and medium

Abstract

The present application relates to the technical field of power system operation and control, and discloses a low-voltage distribution network three-phase imbalance calculation and commutation treatment method, system, equipment and medium considering photovoltaic reverse delivery, comprising: collecting distribution transformer outlet side data, identifying current phase quadrant by power polarity, and reconstructing three-phase current vector in combination with voltage reference phase; performing symmetrical component transformation on the reconstructed vector to construct a normalized imbalance degree index; when the index is out of limit, stripping the fixed background current from the total current, predicting the substation current under different phase sequence combinations based on the real-time voltage of each switch, solving the optimal commutation strategy with the minimum imbalance degree as the target and executing; after the action, closed-loop review is performed, and if the standard is not met, optimization is performed again based on the latest data. The present application reconstructs the current vector by power data assistance, and proposes a normalized imbalance degree representation index, so as to realize accurate evaluation and commutation treatment under all working conditions.

Description

Technical Field

[0001] This invention relates to the field of power system operation and control technology, and in particular to a method, system, equipment and medium for calculating and managing three-phase imbalance in low-voltage distribution networks that takes into account photovoltaic backfeeding. Background Technology

[0002] Against the backdrop of the rapid development of distributed photovoltaic (PV) power generation, large-scale integration of single-phase residential PV systems into low-voltage distribution networks has become commonplace. Given the inherent random fluctuations of PV power generation and the asymmetric characteristics of single-phase integration, single-phase or multi-phase power backflow frequently occurs in distribution substations. This shift fundamentally changes the nature of the three-phase imbalance problem in the substations, transforming it from the traditional "pure load-driven" type to the more complex "source-load superposition" type.

[0003] However, existing methods for calculating three-phase imbalance face limitations in practical applications. Firstly, the insufficient measurement capabilities of low-voltage distribution areas restrict the application of vector algorithms. Some low-voltage distribution areas only have current amplitude measurement capabilities, lacking the crucial phase measurement capability, making it impossible to directly implement the prescribed calculation method based on the symmetrical component method.

[0004] Secondly, the imbalance estimation formula based on current amplitude lacks rigorous physical support, especially in photovoltaic reverse-feed scenarios, where a decrease in current amplitude may mask the electrical fact that phase has already reversed. Under such conditions, simple amplitude calculations not only fail to accurately reflect the imbalance state of the transformer area but may also lead to misjudgments by the control system.

[0005] Photovoltaic backfeeding can also cause numerical singularities in traditional indicators. The traditional definition of current imbalance typically uses the positive-sequence current as the denominator. In high-penetration photovoltaic (PV) areas, the positive-sequence component of the total current in the area can approach zero, causing the calculated imbalance value to approach infinity, i.e., a singularity. This numerical divergence affects the stability of this indicator as an objective function of the control system and interferes with the normal operation of the automatic switching logic of the commutation switch. Summary of the Invention

[0006] In view of the aforementioned existing problems, the present invention provides a method, system, equipment and medium for calculating and managing three-phase imbalance in low-voltage distribution networks that takes into account photovoltaic backfeeding.

[0007] Therefore, the technical problem solved by this invention is: the lack of phase information leads to the inability to accurately calculate the imbalance, and the traditional imbalance index values ​​diverge under photovoltaic backfeed conditions. To solve the above-mentioned technical problems, the present invention provides the following technical solution, which is a method for calculating and commutating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed, including: identifying the quadrant of each phase current based on the positive and negative polarities of the active and reactive power on the distribution transformer outlet side, and reconstructing the three-phase current vector on the distribution transformer outlet side. The reconstructed three-phase current vector is subjected to symmetrical component transformation to obtain the positive-sequence current component, negative-sequence current component and zero-sequence current component, and a normalized unbalance index is constructed. A fixed background current vector is obtained based on the normalized unbalance, and the expected current when switching to different phase sequences is calculated based on the real-time voltage of each switch, and the predicted three-phase current at the beginning of the transformer area is reconstructed. With the goal of minimizing the normalized imbalance index corresponding to the predicted three-phase current at the head end of the transformer area, the optimal phase combination of the commutation switch is solved. After the commutation switch performs its action, the operating data is re-acquired and the actual normalized imbalance index is calculated. Based on the latest measured data, the commutation strategy is repeatedly generated and executed until the target is met.

[0008] As a preferred embodiment of the three-phase imbalance calculation and commutation management method for low-voltage distribution networks considering photovoltaic backfeeding described in this invention, the three-phase current vector at the output side of the reconstructed distribution transformer includes: determining the power flow direction by using the polarity of active power and determining the inductive or capacitive characteristics by using the polarity of reactive power, thereby determining the sign and value of the power factor angle of each phase current relative to the voltage, and synthesizing the three-phase current vector by combining the measured current amplitude and the voltage reference phase.

[0009] As a preferred embodiment of the method for calculating and managing three-phase imbalance in a low-voltage distribution network considering photovoltaic backfeeding described in this invention, the step of obtaining a fixed background current vector based on the normalized imbalance degree includes: when the normalized imbalance degree index exceeds a preset threshold, subtracting the current vector of the currently connected commutation switch from the total current vector at the outlet of the distribution transformer to obtain a fixed background current vector. Determining the preset threshold includes constructing a three-phase current stochastic power flow dataset containing random amplitude and random phase fluctuations, covering operating conditions from pure load to photovoltaic backfeed. For each sample, calculate the national standard equivalent imbalance index and the normalized imbalance index respectively, and establish the mapping relationship between the two. The normalized imbalance index value corresponding to the national standard imbalance limit is found based on the mapping relationship and used as the preset threshold.

[0010] As a preferred embodiment of the method for calculating and managing three-phase imbalance in a low-voltage distribution network considering photovoltaic backfeeding as described in this invention, the reorganization and prediction of the three-phase current at the head end of the transformer substation includes determining the load current amplitude and power factor angle relative to voltage of each commutation switch based on the measured voltage amplitude, active power and reactive power of each commutation switch node, and constructing the current vector of each commutation switch. The fixed background current vector is obtained by subtracting the current vectors of all currently connected phase-switching switches from the total current vector at the output of the distribution transformer. The fixed background current vector is superimposed with the expected current vector when each commutator switches to different access phases to obtain the predicted three-phase current at the head end of the transformer area. The predicted three-phase current at the head end of the transformer substation is transformed by symmetrical components, and the corresponding normalized unbalance index is calculated.

[0011] As a preferred embodiment of the method for calculating and managing three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in this invention, the method for solving the optimal access phase combination of the commutation switches includes using integer encoding to represent the access phase sequence state of each commutation switch and randomly generating different sets of switch states as the initial population. A fitness function is constructed using the normalized imbalance index, and iterative optimization is performed through selection, crossover, and mutation operations. When the number of iterations reaches the termination condition, the optimal commutation strategy is output.

[0012] As a preferred embodiment of the method for calculating and managing three-phase imbalance in a low-voltage distribution network considering photovoltaic backfeeding described in this invention, the step of re-collecting operating data and calculating the actual normalized imbalance index includes re-collecting the active power, reactive power, current amplitude, and voltage amplitude at the low-voltage side outlet of the distribution transformer, reconstructing the three-phase current vector, and calculating the actual normalized imbalance index. If the actual normalized imbalance index still exceeds the preset threshold, the latest measured data after commutation will be used as input to update the obtained fixed background current vector, predict the three-phase current of the transformer area after each switch switching, and solve for the optimal connection phase combination.

[0013] As a preferred embodiment of the method for calculating and managing three-phase imbalance in a low-voltage distribution network that considers photovoltaic backfeeding as described in this invention, the method further includes data acquisition performed by a monitoring terminal installed at the low-voltage side outlet of the distribution transformer, and the commutation switch being an intelligent commutation switch installed on the user load side.

[0014] This invention provides a system for calculating and managing three-phase imbalance in low-voltage distribution networks that takes into account photovoltaic backfeeding.

[0015] As a preferred embodiment of the three-phase imbalance calculation and commutation management system for low-voltage distribution networks considering photovoltaic backfeeding described in this invention, it includes: a data acquisition module, a current vector reconstruction module, an imbalance calculation module, a commutation strategy generation module, and a commutation execution and closed-loop correction module. The data acquisition module collects the active power, reactive power, current amplitude, and voltage amplitude at the low-voltage side outlet of the distribution transformer. The current vector reconstruction module uses the polarity of active power and reactive power to identify the current phase quadrant, and reconstructs the three-phase current vector by combining the voltage reference phase. The unbalance calculation module is used to perform symmetrical component transformation on the reconstructed current vector, construct a normalized unbalance index and compare it with a preset threshold to determine whether to trigger governance. The commutation strategy generation module strips the fixed background current and predicts the transformer area current under different phase sequence combinations based on the real-time voltage of each switch, and solves the optimal access phase combination with the goal of minimizing the imbalance. The commutation execution and closed-loop correction module issues and executes commutation commands, and after the action, it re-collects data and calculates the imbalance. If the imbalance does not meet the standard, it triggers the strategy generation module to optimize again.

[0016] The present invention provides a computer device, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps of calculating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed and the method of commutation management.

[0017] The present invention provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, the steps of calculating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed and the method of commutation management are implemented.

[0018] The beneficial effects of this invention are as follows: Addressing the common lack of synchronous phasor measurement units (PMUs) in low-voltage distribution substations, this invention utilizes the P / Q polarity characteristics of intelligent switches to inversely deduce the current phase. This achieves "soft measurement" of the current vector without upgrading expensive hardware. The normalized sequence component imbalance index effectively overcomes the numerical singularity problem of traditional indices when photovoltaic power is fully connected to the grid and the positive sequence current is close to zero. Whether under pure load conditions or under conditions of high photovoltaic power generation or even complete reverse power supply, this index can continuously and stably characterize the degree of imbalance in the distribution area, providing a reliable basis for automated control.

[0019] By incorporating measured voltage and taking into account the voltage distribution of the actual line, and combining it with the "closed-loop rolling governance" mechanism, the control deviation caused by the uncertainty of line impedance was effectively eliminated, and the success rate of commutation governance was significantly improved. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. 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 these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of a method for calculating and commutating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed, provided as an embodiment of the present invention.

[0022] Figure 2 National standard equivalent indexes for a method of calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeed, provided in an embodiment of the present invention. With normalized index A correlation diagram. Detailed Implementation

[0023] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0024] Example 1, referring to Figure 1 This is the first embodiment of the present invention, which provides a method for calculating and mitigating three-phase imbalance in a low-voltage distribution network that considers photovoltaic backfeeding, including: S1: Identify the quadrant of each phase current based on the positive and negative polarities of the active and reactive power on the distribution transformer outlet side, and reconstruct the three-phase current vector on the distribution transformer outlet side.

[0025] S2: Perform symmetrical component transformation on the reconstructed three-phase current vector to obtain the positive sequence current component, negative sequence current component and zero sequence current component, and construct a normalized unbalance index.

[0026] S3: Obtain a fixed background current vector based on the normalized unbalance, and calculate the expected current when switching to different phase sequences based on the real-time voltage of each switch, and reconstruct the predicted three-phase current at the beginning of the transformer area.

[0027] S4: With the goal of minimizing the normalized unbalance index corresponding to the predicted three-phase current at the head end of the transformer area, solve for the optimal phase combination of the switching switches.

[0028] S5: After the commutation switch performs its action, the operating data is re-acquired and the actual normalized imbalance index is calculated. Based on the latest measured data, the commutation strategy is repeatedly generated and executed until the target is met.

[0029] Example 2, refer to Figure 2 As an embodiment of the present invention, based on the above embodiment, a method for calculating and commutating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed is provided.

[0030] Furthermore, in this embodiment, step S1 identifies the quadrant of each phase current based on the positive and negative polarities of the active and reactive power at the distribution transformer outlet side, and reconstructs the three-phase current vector at the distribution transformer outlet side. Specific steps include S101-S102: S101: Real-time acquisition of three-phase operating data at the low-voltage side outlet of the distribution transformer, including active power of each phase. reactive power Current amplitude Voltage amplitude Among them, subscript Indicates the phase sequence.

[0031] S102: The polarity of active power is used to determine the power flow direction, and the polarity of reactive power is used to determine the inductive or capacitive characteristics. In this way, the sign and value of the power factor angle of each phase current relative to the voltage are determined, and the three-phase current vector is synthesized by combining the measured current amplitude and the voltage reference phase.

[0032] Utilizing the active power on the distribution transformer output side and reactive power Determine the positive and negative polarities, identify the power flow direction of each phase (positive direction is load-dominated, negative direction is photovoltaic backflow-dominated), and calculate the power factor angle of the total current relative to the voltage. .

[0033] Let the absolute value of the basic phase angle be... Determined by combining four-quadrant logic : 1) First Quadrant (Positive Action, Sensibility): If and ,but ; 2) Fourth Quadrant (Positive Active Power, Capacity): If and ,but ; 3) Second quadrant (reverse work, sensibility): If and ,but ; 4) Third quadrant (reverse active power, capacitive): If and ,but .

[0034] The three-phase voltages on the low-voltage side of the distribution transformer are set as the reference phasors, and the voltage reference phase angle vector is defined as follows: Combined with measured current amplitude With the calculated phase angle Construct the three-phase total current vector at the distribution transformer outlet side. : Furthermore, in this embodiment, step S2 performs a symmetrical component transformation on the reconstructed three-phase current vector to obtain the positive-sequence current component, negative-sequence current component, and zero-sequence current component, and constructs a normalized unbalance index. The specific steps include: S201: Regarding the traditional unbalance calculation formula... To address the issue of time-related failures, a normalized evaluation index applicable to all operating conditions is constructed.

[0035] The positive-sequence component of the total current in the distribution area is calculated using the transformation matrix of the distribution transformer outlet current vector and symmetric components. Negative order components and zero-order components : Among them, the operator .

[0036] S202: Calculate the normalized imbalance index Define an unbalance index based on current capacity normalization. The calculation formula is as follows: The denominator is the sum of the magnitudes of the three sequence components, which represents the "total apparent current capacity" of the system, thus avoiding the risk of division by zero when only I1 is used as the denominator.

[0037] S203: Construct a three-phase current stochastic power flow dataset containing random amplitude and random phase fluctuations, covering operating conditions from pure load to photovoltaic backfeed; calculate the national standard equivalent unbalance index and the normalized unbalance index for each sample, and establish a mapping relationship between the two; find the normalized unbalance index value corresponding to the national standard unbalance limit based on the mapping relationship, and use it as the preset threshold. Construct a random power flow dataset containing N samples. Generate three-phase current data with random amplitude and random phase fluctuations to cover all operating conditions from pure load to photovoltaic backfeed.

[0038] The ordinal components of each sample were calculated using the symmetric component method, and the national standard equivalent index was calculated accordingly. With normalized index .

[0039] Define national standard equivalent indicators The mean of the imbalance between zero order and negative order: Define normalization index : by x-axis Plot a comparison chart of indicator distributions on the vertical axis.

[0040] like Figure 2 As shown, the two types of indicators exhibit a significant positive correlation under all operating conditions. Based on this mapping relationship, the corresponding national standard limits (such as...) can be found. (when) Value. Set this value as the governance threshold. When the normalized index of the three-phase imbalance of the distribution transformer outlet current calculated in real time exceeds When the system imbalance exceeds the limit, the commutation control process is triggered.

[0041] Furthermore, in this embodiment, step S3 obtains a fixed background current vector based on the normalized imbalance, and calculates the expected current when switching to different phase sequences based on the real-time voltage of each switch, reconstructing the predicted three-phase current at the head end of the transformer area. Specific steps include: S301: Based on the measured voltage amplitude, active power and reactive power of each commutation switch node, determine the load current amplitude and power factor angle relative to the voltage of each commutation switch, and construct the current vector of each commutation switch. This step is triggered when the current imbalance in the transformer area exceeds the limit. To achieve precise management, the total current in the transformer area must first be decoupled into "fixed background current" and "controllable commutation current," and the operating status of the transformer area under different switch combinations should be deduced through mathematical models.

[0042] The model inputs include the measured total current vector at the head end of the transformer substation: denoted as... The active / reactive power and voltage data from the transformer substation's main meter are used to recover the data. Measured load data of the kth intelligent commutator switch: Active power reactive power Node voltage amplitude Current access phase sequence .(in (where N is the total number of commutator switches).

[0043] Calculate the load current amplitude for this user based on the power and voltage data of the k-th switch. and its power factor angle relative to voltage (The calculation method is the same as step S1).

[0044] S302: Subtract all currently connected phase-switching switch current vectors from the total current vector at the outlet of the distribution transformer to obtain a fixed background current vector; superimpose the fixed background current vector with the expected current vector when each phase-switching switch switches to different connected phases to obtain the predicted three-phase current at the head end of the distribution area. The "fixed background current" of the transformer area (i.e., the current of users and line losses without phase-switching switches) is obtained by subtracting the current generated by all current intelligent phase-switching switches from the total current at the beginning of the transformer area using vector subtraction.

[0045] Let the current k-th switch be connected to phase . ( ), and its corresponding current vector is .

[0046] Three-phase background current The calculation is as follows: in, These are the sets of commutation switches currently connected to phases A, B, and C.

[0047] Introducing decision variables If the k-th switch is switched to If they are similar, then... Otherwise, it is 0.

[0048] Constructing a new three-phase current prediction model for the first end of the transformer substation It is equal to the sum of the background current and the commutation current after reorganization: S303: Perform symmetrical component transformation on the predicted three-phase current at the head end of the transformer area and calculate the corresponding normalized unbalance index.

[0049] The predicted Substituting into the symmetric component transformation matrix, we obtain the predicted order components: Therefore, an optimization function is constructed with the objective of minimizing the normalized imbalance: The constraints include: 1) Switch state constraints: (Each switch must select one and only one phase); 2) Number of actions constraint: Limit the total number of switches in a single adjustment action to avoid excessive disturbance.

[0050] Furthermore, in this embodiment, step S4 aims to minimize the normalized imbalance index corresponding to the predicted three-phase current at the head end of the transformer substation, and solves for the optimal phase combination of the commutation switch. The specific steps include: S401: The phase sequence state of each commutator switch is represented by an integer encoding method, and M sets of switch states are randomly generated as the initial population. S402: Construct a fitness function using the normalized imbalance index, and perform iterative optimization through selection, crossover, and mutation operations; when the number of iterations reaches the termination condition, output the optimal commutation strategy.

[0051] Define a fitness function to preferentially select individuals with low imbalance: in, To prevent decimals with a denominator of 0.

[0052] Genetic operations are performed, using a roulette wheel to retain superior individuals; partial switch state fragments of two individuals are randomly swapped; the access phase sequence of a switch is randomly changed (e.g., changing phase A to phase B) to prevent the algorithm from getting trapped in local optima.

[0053] When the number of iterations reaches the preset value or the calculated optimal value is reached... When the value is less than the target threshold (e.g., 5% according to national standards), output the optimal commutation strategy. .

[0054] Furthermore, in this embodiment, after the commutation switch performs its action, step S5 involves re-collecting operating data and calculating the actual normalized imbalance index. Based on the latest measured data, the commutation strategy is repeatedly generated and executed until the target is met. Specific steps include: The active power, reactive power, current amplitude, and voltage amplitude at the low-voltage side outlet of the distribution transformer are re-acquired, the three-phase current vector is reconstructed, and the actual normalized unbalance index is calculated. If the actual normalized unbalance index still exceeds the preset threshold, the latest measured data after phase switching is used as input to update the obtained fixed background current vector, predict the three-phase current of the transformer area after each switch switching, and solve for the optimal phase combination.

[0055] Intelligent phase-switching switch receives optimal strategy Commutation is performed at the zero-crossing point. After commutation is complete, data from the beginning of the transformer area is collected again to recalculate the actual normalized imbalance. If affected by changes in line impedance or sudden load changes, resulting in If the target is still not met, repeat steps S3 to S4 based on the latest measured voltage after commutation to perform the next round of fine-tuning until the system is balanced.

[0056] Example 3 is the third embodiment of the present invention, which differs from the previous two embodiments in that: This embodiment also provides a three-phase imbalance calculation and commutation management system for low-voltage distribution networks that takes into account photovoltaic backfeeding, including: a data acquisition module, a current vector reconstruction module, an imbalance calculation module, a commutation strategy generation module, and a commutation execution and closed-loop correction module; The data acquisition module collects active power, reactive power, current amplitude, and voltage amplitude at the low-voltage side outlet of the distribution transformer. The current vector reconstruction module uses the polarity of active and reactive power to identify the current phase quadrant and reconstructs the three-phase current vector by combining the voltage reference phase. The unbalance calculation module is used to perform symmetrical component transformation on the reconstructed current vector, construct a normalized unbalance index and compare it with a preset threshold to determine whether to trigger remediation. The commutation strategy generation module strips out the fixed background current and predicts the transformer area current under different phase sequence combinations based on the real-time voltage of each switch. It solves the optimal access phase combination with the goal of minimizing the imbalance. The commutation execution and closed-loop correction module issues and executes commutation commands. After the action, it re-collects data and calculates the imbalance. If the target is not met, it triggers the strategy generation module to optimize again.

[0057] This embodiment also provides an electronic device, including: a memory and a processor; the memory is used to store computer-executable instructions, and the processor is used to execute the computer-executable instructions to realize the three-phase imbalance calculation and commutation management method for low-voltage distribution networks considering photovoltaic backfeed as proposed in the above embodiment.

[0058] This embodiment also provides a storage medium storing a computer program that, when executed by a processor, implements the three-phase imbalance calculation and commutation management method for low-voltage distribution networks considering photovoltaic backfeeding as proposed in the above embodiments.

[0059] The storage medium proposed in this embodiment and the method for calculating and commutating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed proposed in the above embodiments belong to the same inventive concept. Technical details not described in detail in this embodiment can be found in the above embodiments, and this embodiment has the same beneficial effects as the above embodiments.

[0060] Based on the above description of the implementation methods, those skilled in the art can clearly understand that the present invention can be implemented using software and necessary general-purpose hardware, and of course, it can also be implemented using hardware. Based on this understanding, the technical solution of the present invention, or the part that contributes to the prior art, can be embodied in the form of a software product. This computer software product can be stored in a computer-readable storage medium, such as a computer floppy disk, read-only memory (ROM), random access memory (RAM), flash memory, hard disk, or optical disk, etc., including several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute the methods of the various embodiments of the present invention.

[0061] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

[0062] Example 4 is an embodiment of the present invention. Based on the above embodiments, a method for calculating and commutating the three-phase imbalance of a low-voltage distribution network considering photovoltaic backfeed is provided.

[0063] This embodiment constructs a simplified low-voltage distribution substation simulation scenario to verify the calculation and mitigation methods. The rated phase voltage of this substation is 220V, and the network topology includes a transformer outlet monitoring point and five typical nodes. Node 1 connects to a three-phase balanced fixed background load, with each phase having an active power of 5kW, a power factor of 0.9, and exhibiting a lagging characteristic. Node 2 is a distributed power source access point, connecting a single-phase photovoltaic power generation system in phase A, with a power generation of 12kW and a power factor of 0.93. The photovoltaic power generation exceeds the fixed load of Node 1 in phase A, resulting in active power backflow in phase A. Nodes 3, 4, and 5 are controllable user loads equipped with intelligent phase-switching switches, with active powers of 3kW, 4kW, and 5kW, respectively, and a power factor of 0.9 for all. In the initial operating state before mitigation, the intelligent phase-switching switches of nodes 3, 4, and 5 are all connected to phase B, while phase C bears the background load of node 1.

[0064] For the initial operating conditions, the system performs current vector reconstruction based on the power data collected from the distribution transformer outlet side. The monitoring terminal detects a negative active power value in phase A. Using a phase angle identification method based on power polarity, combined with four-quadrant logic, the phase relationship between the phase A current and voltage is determined. The calculated and reconstructed three-phase current vector data at the distribution transformer outlet shows: Phase A current amplitude of 33.5A, phase angle of 161.7°, exhibiting power reversal characteristics; Phase B current amplitude of 85.9A, phase angle of -145.8°; and Phase C current amplitude of 25.3A, phase angle of 94.2°. Based on the symmetrical component method, the positive sequence current amplitude is obtained as 26.0A, the negative sequence current amplitude as 33.7A, and the zero sequence current amplitude as 35.1A. Using the traditional sequence component unbalance calculation formula, the calculation result is 129.5%. The normalized unbalance index proposed in this invention is then used. An assessment was conducted, and the calculated result was 51.33%.

[0065] The system triggers a commutation mitigation process based on real-time voltage sensing. A fixed background current is obtained by separating the intelligent commutator switch current from the total current in the transformer area, and this background current is used to normalize the unbalance index. Minimize the objective function and use a genetic algorithm to optimize the phase sequence of nodes 3, 4, and 5. The algorithm outputs a strategy that switches the phase switches of nodes 3, 4, and 5 from phase B to phase A, transferring the 12kW total load of phase B to phase A and absorbing the electricity generated by the photovoltaic system in phase A.

[0066] After implementing the optimization strategy, the changes in the three-phase current amplitude data at the transformer outlet are as follows: Phase A current amplitude is 27.7A, phase angle is -34.9°; Phase B and Phase C current amplitudes are both 25.3A, with phase angles of -145.8° and 94.2° respectively. Sequence component analysis shows that the positive sequence current is 26.0A, and the negative sequence current and zero sequence current amplitudes are both 1.6A. At this time, the traditional negative sequence unbalance index is 6.2%, while the normalized unbalance index proposed in this invention... It changed from 51.33% to 7.84%.

[0067] This embodiment verifies that the present invention can accurately characterize the system state and provide effective governance strategies under complex operating conditions of photovoltaic backfeeding and extremely uneven load, and realize the three-phase balanced optimized operation of the distribution transformer area.

Claims

1. A method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding, characterized in that: include, Identify the quadrant of each phase current based on the positive and negative polarities of the active and reactive power at the distribution transformer outlet side, and reconstruct the three-phase current vector at the distribution transformer outlet side. The reconstructed three-phase current vector is subjected to symmetrical component transformation to obtain the positive-sequence current component, negative-sequence current component and zero-sequence current component, and a normalized unbalance index is constructed. A fixed background current vector is obtained based on the normalized unbalance, and the expected current when switching to different phase sequences is calculated based on the real-time voltage of each switch, and the predicted three-phase current at the beginning of the transformer area is reconstructed. With the goal of minimizing the normalized imbalance index corresponding to the predicted three-phase current at the head end of the transformer area, the optimal phase combination of the commutation switch is solved. After the commutation switch performs its action, the operating data is re-acquired and the actual normalized imbalance index is calculated. Based on the latest measured data, the commutation strategy is repeatedly generated and executed until the target is met.

2. The method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in claim 1, characterized in that: The three-phase current vector at the output side of the reconstructed distribution transformer includes determining the power flow direction by using the polarity of active power and determining the inductive or capacitive characteristics by using the polarity of reactive power, thereby determining the sign and value of the power factor angle of each phase current relative to the voltage, and combining the measured current amplitude with the voltage reference phase to synthesize the three-phase current vector.

3. The method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in claim 2, characterized in that: The step of obtaining a fixed background current vector based on the normalized unbalance includes subtracting the current vector of the currently connected commutator switch from the total current vector at the outlet of the distribution transformer when the normalized unbalance index exceeds a preset threshold to obtain a fixed background current vector. Determining the preset threshold includes constructing a three-phase current stochastic power flow dataset containing random amplitude and random phase fluctuations, covering operating conditions from pure load to photovoltaic backfeed. For each sample, calculate the national standard equivalent imbalance index and the normalized imbalance index respectively, and establish the mapping relationship between the two. The normalized imbalance index value corresponding to the national standard imbalance limit is found based on the mapping relationship and used as the preset threshold.

4. The method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in claim 3, characterized in that: The recombined prediction of the three-phase current at the first end of the transformer area includes determining the load current amplitude and power factor angle relative to the voltage of each commutation switch based on the measured voltage amplitude, active power and reactive power of each commutation switch node, and constructing the current vector of each commutation switch. The fixed background current vector is obtained by subtracting the current vectors of all currently connected phase-switching switches from the total current vector at the output of the distribution transformer. The fixed background current vector is superimposed with the expected current vector when each commutator switches to different access phases to obtain the predicted three-phase current at the head end of the transformer area. The predicted three-phase current at the head end of the transformer substation is transformed by symmetrical components, and the corresponding normalized unbalance index is calculated.

5. The method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in claim 4, characterized in that: The method for finding the optimal access phase combination of the commutation switches includes using an integer encoding method to represent the access phase sequence state of each commutation switch and randomly generating different sets of switch states as the initial population. A fitness function is constructed using the normalized imbalance index, and iterative optimization is performed through selection, crossover, and mutation operations. When the number of iterations reaches the termination condition, the optimal commutation strategy is output.

6. The method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in claim 5, characterized in that: The process of re-collecting operating data and calculating the actual normalized imbalance index includes re-collecting the active power, reactive power, current amplitude, and voltage amplitude at the low-voltage side outlet of the distribution transformer, reconstructing the three-phase current vector, and calculating the actual normalized imbalance index. If the actual normalized imbalance index still exceeds the preset threshold, the latest measured data after commutation will be used as input to update the obtained fixed background current vector, predict the three-phase current of the transformer area after each switch switching, and solve for the optimal connection phase combination.

7. The method for calculating and mitigating three-phase imbalance in low-voltage distribution networks considering photovoltaic backfeeding as described in claim 6, characterized in that, It also includes data collection performed by a monitoring terminal installed at the low-voltage side outlet of the distribution transformer, and the phase-switching switch being an intelligent phase-switching switch installed on the user load side.

8. A system for calculating and mitigating three-phase imbalance in a low-voltage distribution network considering photovoltaic backfeed, employing the method for calculating and mitigating three-phase imbalance in a low-voltage distribution network considering photovoltaic backfeed as described in any one of claims 1 to 7, characterized in that... include: The system includes a data acquisition module, a current vector reconstruction module, an unbalance calculation module, a commutation strategy generation module, and a commutation execution and closed-loop correction module. The data acquisition module collects the active power, reactive power, current amplitude, and voltage amplitude at the low-voltage side outlet of the distribution transformer. The current vector reconstruction module uses the polarity of active power and reactive power to identify the current phase quadrant, and reconstructs the three-phase current vector by combining the voltage reference phase. The unbalance calculation module is used to perform symmetrical component transformation on the reconstructed current vector, construct a normalized unbalance index and compare it with a preset threshold to determine whether to trigger governance. The commutation strategy generation module strips the fixed background current and predicts the transformer area current under different phase sequence combinations based on the real-time voltage of each switch, and solves the optimal access phase combination with the goal of minimizing the imbalance. The commutation execution and closed-loop correction module issues and executes commutation commands, and after the action, it re-collects data and calculates the imbalance. If the imbalance does not meet the standard, it triggers the strategy generation module to optimize again.

9. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the three-phase imbalance calculation and commutation treatment method for low-voltage distribution networks considering photovoltaic backfeed, as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the three-phase imbalance calculation and commutation treatment method for low-voltage distribution networks considering photovoltaic backfeed, as described in any one of claims 1 to 7.

Images