A method for controlling three-phase load imbalance in low-voltage substations

Abstract

The present invention discloses a method for controlling three-phase load imbalance in a low-voltage substation, comprising: setting the number and installation locations of phase-changing switches in the low-voltage substation based on similarity, design load, and real-time load; and periodically executing a three-phase load imbalance commutation strategy when the low-voltage substation is in operation. The method for controlling three-phase load imbalance in a low-voltage substation of the present invention comprehensively considers similarity, design load, and real-time load, can optimize the substation to address the random increase in actual load on residential power and overload of distribution lines, and corrects this with a real-time commutation strategy, effectively solving the problem of uneven load distribution caused by three-phase imbalance.

Description

Technical Field

[0001] The present invention relates to the field of three-phase load imbalance control in power grids, and in particular to a three-phase load imbalance control method for low-voltage substations. Background Art

[0002] To maintain stable operation of three-phase AC power systems, three-phase load balance is a key indicator. Low-voltage substations typically use single-phase power, often without proper planning. This leads to a disordered number of connections and phase sequence. This imbalance in the number of users and load across the three phases results in an unbalanced load across the three phases during operation.

[0003] Existing methods for controlling three-phase imbalance vary, requiring varying equipment. This not only increases investment costs for compensation devices, but also requires consideration of labor costs and subsequent equipment maintenance. Existing methods balance the three-phase load at the low-voltage outlet of the distribution transformer by outputting a compensating current. However, the fundamental problem of three-phase imbalance lies in the uneven distribution of the three-phase load. Therefore, this method cannot fundamentally address the uneven load distribution that causes three-phase imbalance. Furthermore, existing solutions consider the number and location of phase-changing switches and the phase-changing adjustment strategy separately, lacking coordination between the three, making it impossible to achieve the desired phase adjustment. Summary of the Invention

[0004] In view of the above-mentioned defects of the prior art, an object of the present invention is to provide a method for controlling three-phase load imbalance in a low-voltage substation.

[0005] To achieve the above objectives, the present invention provides the following technical solutions:

[0006] A method for controlling three-phase load imbalance in a low-voltage substation area, comprising:

[0007] Based on similarity, design load and real-time load, set the number and installation location of phase-changing switches in the low-voltage substation area;

[0008] Periodically execute the following three-phase load unbalanced commutation strategy;

[0009] The number and installation location of the low-voltage phase-changing switches are as follows:

[0010] Step S11, topologically transforming the low-voltage area power grid wiring diagram, with users as vertices and lines between users as edges;

[0011] Step S12: Calculate the similarity between each vertex and other vertices based on the electrical distance;

[0012] Step S13: Using electrical distance as the first convergence condition, achieving minimum partition unit aggregation, partitioning based on similarity, counting the number of partitions, forming a first partition and numbering them to form a set D, and giving the number n of the first partitions;

[0013] Step S14: collect the design load of each vertex, calculate the sum of the design load of each partition according to the set in step S13, and use the second convergence condition that the sum of the design load of each partition is less than 1 / (2n) of the total load of the substation. Independently partition the partitions that do not meet the condition, return to step S11, and update the number of remaining partitions m;

[0014] Step S15: When the load of each partition is less than 1 / (2m) of the total load of the substation after partitioning, the loop ends and goes to step S16; otherwise, it returns to step S12;

[0015] Step S16: Determine the number of network breaks in the low-voltage area as m based on the final number of partitions m, and the number of phase-changing switches to be installed as m-1;

[0016] Step S17: collecting the actual load data of each partition in real time through the load metering system;

[0017] Step S18: Based on the real-time load data of the partition as the third judgment condition, when the actual load value of a partition is greater than 1 / (2m) of the total design load of the substation, return to step S12; when the actual load value of each partition is less than 1 / (2m) of the total design load of the substation, the process ends.

[0018] Furthermore, the installation position of the phase-changing switch is the vertex incoming line connection point of each partition that is closest to the electrical distance of the substation transformer.

[0019] Furthermore, the three-phase load unbalanced commutation strategy includes:

[0020] Through two measurements with a certain time interval, the maximum phase, middle phase and minimum phase of the three-phase power grid are determined, and the three-phase load current and three-phase imbalance are obtained;

[0021] When the three-phase imbalance obtained by the two calculations reaches a first threshold, commutation preparation is started;

[0022] Determining in sequence whether there are available commutation switches in the maximum phase and the middle phase, and determining whether the load difference between the maximum phase and the minimum phase or the load difference between the middle phase and the minimum phase is greater than a set second threshold, and executing commutation if the second threshold is satisfied, where the second threshold is a certain percentage of the rated load of a single phase;

[0023] After the commutation is completed, the number of available commutation switches of the largest phase or the middle phase performing the commutation is reduced by one, and the number of available commutation switches of the smallest phase receiving the commutation is increased by one.

[0024] Furthermore, after the condition that the load difference between the maximum phase and the minimum phase or the load difference between the intermediate phase and the minimum phase is greater than the set second threshold is met, a confirmation step is also included before executing the commutation: a commutation operation simulation is performed to calculate whether the difference in the three-phase load imbalance coefficient before and after the commutation is greater than the third threshold. If this condition is met, the commutation is executed.

[0025] Furthermore, the third threshold value ranges from 3% to 10%.

[0026] Furthermore, the first threshold value ranges from 10% to 20%.

[0027] Furthermore, the second threshold value is in the range of 15%-20% of the single-phase rated load.

[0028] The present invention achieves the following technical effects:

[0029] The low-voltage substation three-phase load imbalance control method of the present invention comprehensively considers similarity, design load and real-time load, can randomly increase the actual load of residents' electricity and the overload of distribution lines, optimize the substation, and make corrections with real-time phase switching strategies, which can effectively solve the problem of uneven load distribution caused by three-phase imbalance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of the low voltage substation three-phase load imbalance control method of the present invention;

[0031] Figure 2 This is an example of a low-voltage area power grid wiring topology diagram;

[0032] Figure 3 Partition diagram after taking electrical distance as the first convergence condition;

[0033] Figure 4 Partition diagram after taking design load as the second convergence condition;

[0034] Figure 5 Partition map after actual user load increases;

[0035] Figure 6 Partition diagram after taking actual load as the third convergence condition;

[0036] Figure 7 This is the commutation strategy flow chart. DETAILED DESCRIPTION

[0037] To further illustrate various embodiments, the present invention is provided with accompanying drawings. These drawings form part of the present disclosure and are primarily used to illustrate the embodiments and, in conjunction with the relevant description in the specification, to explain the operating principles of the embodiments. By referring to these drawings, one of ordinary skill in the art will understand other possible embodiments and the advantages of the present invention. The components in the figures are not drawn to scale, and similar reference numerals are generally used to represent similar components.

[0038] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0039] The present invention proposes a specific implementation of a three-phase load imbalance control method for a low-voltage substation, which includes the following steps:

[0040] (1) Based on similarity, design load and real-time load, set the number and installation location of phase-changing switches in the low-voltage substation area;

[0041] (2) When the low-voltage station is operating, a three-phase load unbalanced commutation strategy is periodically executed.

[0042] The two processes are described below.

[0043] 1. Based on similarity, design load and real-time load, set the number and installation location of phase-changing switches in the low-voltage area

[0044] 1. Topologically transform the low-voltage grid wiring diagram, that is, treat each user as a vertex and number it to form a set V, treat each line as an edge and number it to form a set E, and treat the load p of each node as a set P. Figure 2 shown.

[0045] 2. Calculate the similarity between each vertex and other vertices based on electrical distance. For vertex x, N(x) represents its neighbor set. For vertex x, the similarity of y∈V can be defined as

[0046] 3. Taking electrical distance as the first convergence condition, realize the aggregation of minimum partition units and reach the first partition number; the electrical distance from node A to neighbor node B is expressed as the distance from node A to the center of mass of neighbor node B in Euclidean space, and the formula is:

[0047] Partition according to similarity, count the number of partitions, form the first partition and number them, form a set of partitions D, and the number of partitions is n. Figure 3 As shown, the circle of one color is one partition, and the uncircled part is one partition (the same below).

[0048] 4. Collect the design load of each vertex and calculate the sum of the design load of each partition P1 according to the set D in step 3. The second convergence condition is that the sum of the design load of each partition is less than 1 / (2n) of the total load of the area, where P1i represents the load value of each partition. If P1i is greater than 1 / (2n) of the total design load of the area, as Figure 3 Within the purple circle shown, isolate the partition and return to step 1 to renumber and repartition the vertices and edges of the isolated partition. The partitions of the entire platform form a new set F, with a total number of partitions m.

[0049] 5. If the load of each partition after partitioning is less than 1 / (2m) of the total load of the substation, the loop ends and proceeds to the next step. Otherwise, return to step 2.

[0050] 6. Determine the number of network disconnections m in the low-voltage substation based on the number of partitions, and the number of phase-changing switches to be installed is m-1.

[0051] 7. The actual load data P2 of each partition is collected in real time through the load metering system.

[0052] 8. The installation position of the phase-changing switch is the vertex incoming line connection point in each partition that is closest to the electrical distance from the substation transformer.

[0053] 9. Due to the randomness of the increase in actual residential power load and the unpredictability of temporary access to low-voltage power loads, and the fact that distribution lines have a certain overload capacity, the increase in load may cause the actual load P2 of a certain partition to be greater than 1 / (2m) of the total design load of the substation, reducing the effect of phase switching. Therefore, it is necessary to add the real-time load data of the partition as the third criterion, where P2j represents the actual load value of each partition. Figure 6 As shown. If P2j is greater than 1 / (2m2) of the total design load of the substation, return to step 2, recalculate the partitions, and propose a new partitioning scheme. If the actual load value of each partition is less than 1 / (2m2) of the total design load of the substation, the process ends.

[0054] After the number and installation location of the phase-changing switches in the low-voltage substation are confirmed, the modifications are carried out and the phase-changing switches are installed.

[0055] 2. Three-phase load unbalanced commutation strategy

[0056] When operating in the low-voltage area, the three-phase load unbalanced commutation strategy is periodically executed. This strategy includes:

[0057] Through two measurements with a certain time interval, the maximum phase, middle phase and minimum phase of the three-phase power grid are determined, and the three-phase load current and three-phase imbalance δ% are obtained;

[0058] When the three-phase imbalance δ% reaches a first threshold, commutation preparation is started;

[0059] Determining in sequence whether there are available commutation switches for the maximum phase and the middle phase, and determining whether the load difference ΔP1 between the maximum phase and the minimum phase or the load difference ΔP2 between the middle phase and the minimum phase is greater than a set second threshold, where the second threshold is a certain fraction of the single-phase rated load;

[0060] After the commutation is completed, the number of available commutation switches of the largest phase or the middle phase performing the commutation is reduced by one, and the number of available commutation switches of the smallest phase receiving the commutation is increased by one.

[0061] By executing the above three-phase load unbalanced commutation strategy, the three-phase load can be kept at a lower three-phase unbalance degree.

[0062] Preferably, after the load difference between the maximum and minimum phases, or the load difference between the intermediate and minimum phases, is greater than a set second threshold, a confirmation step is included before commutation: a commutation operation simulation is performed to calculate whether the difference δ% in the three-phase load imbalance coefficient before and after commutation is greater than a third threshold. If this condition is met, commutation is performed. This approach can reduce the need for inefficient commutation and extend the service life of the commutation switch.

[0063] exist Figure 7 In this example, Na, Nb, and Nc represent the number of available commutation switches for the maximum phase, minimum phase, and intermediate phase, respectively. The interval between two measurements is 30 seconds; the first threshold is 15%; the second threshold is 1 / 6 of the single-phase rated load; and the third threshold is 5%. These parameters can be adaptively adjusted through data analysis.

[0064] After statistical analysis of experimental data and historical grid data and reports, the setting ranges of each parameter are as follows:

[0065] The value range of the first threshold is: 15%-20%;

[0066] The value range of the second threshold is: 15%-20% of the single-phase rated load;

[0067] The value range of the third threshold is: 3%-10%.

[0068] 3. Laboratory environment simulation

[0069] The steps include:

[0070] 1. Set up the experimental environment and set the input A phase load to 200W, B phase load to 100W, and C phase load to 99W.

[0071] 2. Grid data measurement: The maximum phase load was measured to be 200W, the middle phase load was 100W, and the minimum phase load was 99W. The maximum phase load current was calculated to be 0.909A, the middle phase load current was 0.455A, and the minimum phase load current was 0.450A. The three-phase load current imbalance coefficient was 50.376%. After a delay, the corresponding parameters were re-tested and the parameters remained unchanged, with the three-phase load current imbalance coefficient still at 50.376%.

[0072] 3. Set the available commutation switch conditions: Enter the number of available commutation switches for phase A as 2, the number of available commutation switches for phase B as 0, and the number of available commutation switches for phase C as 0.

[0073] 4. Output results: After executing the commutation strategy, the output load on phase A is 150W, the load on phase B is 100W, and the load on phase C is 149W. At this time, the number of commutable switches on phase A is 1, the number of commutable switches on phase B is 0, and the number of commutable switches on phase C is 0. After commutation, the unbalance coefficient of the three-phase load current is 12.782%.

[0074] Data verification shows that the three-phase load unbalanced commutation strategy can effectively reduce the imbalance of the three-phase load current.

[0075] Although the present invention has been particularly shown and described in conjunction with preferred embodiments, it will be understood by those skilled in the art that various changes in form and details may be made to the present invention without departing from the spirit and scope of the invention as defined in the appended claims, and all such changes are within the scope of protection of the present invention.

Claims

1. A method for controlling three-phase load imbalance in a low-voltage area, characterized in that: include: Based on similarity, design load and real-time load, set the number and installation location of phase-changing switches in the low-voltage substation area; When the low-voltage section is in operation, a three-phase load unbalanced commutation strategy is periodically executed; The number and installation location of the low-voltage phase-changing switches are as follows: Step S11, topologically transforming the low-voltage area power grid wiring diagram, with users as vertices and lines between users as edges; Step S12: Calculate the similarity between each vertex and other vertices based on the electrical distance; Step S13: Using the allowed electrical distance as the first convergence condition, achieving minimum partition unit aggregation, partitioning based on similarity, counting the number of partitions, forming first partitions and numbering them to form a set D, and giving the number n of first partitions; Step S14: collect the design load of each vertex, calculate the sum of the design load of each partition according to the set in step S13, and use the second convergence condition that the sum of the design load of each partition is less than 1 / (2n) of the total load of the substation. Independently partition the partitions that do not meet the condition, return to step S11, and update the number of remaining partitions m; Step S15: When the load of each partition is less than 1 / (2m) of the total load of the substation after partitioning, the loop ends and goes to step S16; otherwise, it returns to step S12; Step S16: Determine the number of network breaks in the low-voltage area as m based on the final number of partitions m, and the number of phase-changing switches to be installed as m-1; Step S17: collecting the actual load data of each partition in real time through the load metering system; Step S18: Taking the partition real-time load data as the third convergence condition, when the actual load value of a partition is greater than 1 / (2m) of the total design load of the substation, return to step S12; when the actual load value of each partition is less than 1 / (2m) of the total design load of the substation, the process ends.

2. The low voltage substation three-phase load imbalance control method according to claim 1, characterized in that: The electrical distance in step S13 is expressed as follows: the electrical distance from node A to neighbor node B is the distance from node A to the centroid of neighbor node B in the Euclidean space.

3. The low voltage substation three-phase load imbalance control method according to claim 1, characterized in that: The installation position of the phase-changing switch is the vertex incoming line connection point in each partition that is closest to the electrical distance from the substation transformer.

4. The low voltage substation three-phase load imbalance control method according to claim 1, characterized in that: The three-phase load unbalanced commutation strategy includes: Through two measurements with a certain time interval, the maximum phase, middle phase and minimum phase of the three-phase power grid are determined, and the three-phase load current and three-phase imbalance are obtained; When the three-phase imbalance obtained by the two calculations reaches a first threshold, commutation preparation is started; determining, in sequence, whether there are available commutation switches for the maximum phase and the middle phase, and determining whether the load difference between the maximum phase and the minimum phase, or the load difference between the middle phase and the minimum phase, is greater than a set second threshold, where the second threshold is a percentage of the single-phase rated load; After the commutation is completed, the number of available commutation switches of the largest phase or the middle phase performing the commutation is reduced by one, and the number of available commutation switches of the smallest phase receiving the commutation is increased by one.

5. The low voltage substation three-phase load imbalance control method according to claim 4, characterized in that: After the condition that the load difference between the maximum phase and the minimum phase or the load difference between the intermediate phase and the minimum phase is greater than the set second threshold is met, a confirmation step is also included before executing the commutation: a commutation operation simulation is performed to calculate whether the difference in the three-phase load imbalance coefficient before and after the commutation is greater than the third threshold. If this condition is met, the commutation is executed.

6. The low voltage substation three-phase load imbalance control method according to claim 5, characterized in that: The value range of the third threshold is 3%-10%.

7. The method for controlling three-phase load imbalance in a low-voltage substation area according to claim 4, characterized in that: The first threshold value ranges from 10% to 20%.

8. The method for controlling three-phase load imbalance in a low-voltage substation area according to claim 4, characterized in that: The second threshold value ranges from 15% to 20% of the single-phase rated load.

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