Three-phase unbalanced load commutation control method considering load fluctuation frequency characteristics

Abstract

The present invention relates to a three-phase unbalanced load phase change control method considering the frequency characteristics of load fluctuations, belonging to the technical field of three-phase unbalance governance in distribution networks. A phase change type load automatic balancing control system containing the frequency characteristics of load fluctuations, a phase change type load automatic balancing control method containing the frequency characteristics of load fluctuations are constructed and verified. The advantages are that the structure of the phase change type unbalanced load automatic balancing control system considering the frequency characteristics of load fluctuations is analyzed in detail, its phase change process and control strategy are designed, and the control processes of the circuit breaker and thyristor of the phase change unit based on the composite switch are systematically described. By constructing a three-phase unbalanced model of the low-voltage distribution network, it is proved that the present invention can efficiently achieve three-phase unbalance governance. After phase change by the load automatic balancing method considering the frequency characteristics of load fluctuations, the degree of three-phase unbalance is effectively reduced, and the low-voltage distribution network can operate in a relatively good state.

Description

Technical Field

[0001] The present invention relates to the technical field of three-phase imbalance control in distribution networks, and particularly to a three-phase unbalanced load phase-changing control method considering the frequency characteristics of load fluctuations, especially a control solution using a phase-changing unit circuit breaker based on a composite switch and thyristors to reduce the degree of three-phase imbalance on the load side of the distribution network. Background Art

[0002] Most of the low-voltage distribution networks in China are of the three-phase four-wire system, but each single-phase load is often connected to the low-voltage distribution network in an unbalanced form. In addition, due to factors such as the electricity consumption habits of each user and seasonal changes, the power fluctuations of each single-phase load have high randomness, which is the main reason for the three-phase imbalance in the low-voltage distribution network. As the main electrical equipment for supplying power to the load, the output, loss and other characteristics of the distribution transformer are seriously affected by the three-phase imbalance, and even accidents such as local burning may occur.

[0003] Increasing compensation devices, manual phase change, and adding automatic balancing devices are the main means to solve the three-phase imbalance in the low-voltage distribution network at present. Increasing compensation devices uses compensation devices such as active filters to perform three-phase compensation on the load, but it can only improve the three-phase imbalance of the line above the compensation point, and the three-phase imbalance state of the line below the compensation point still cannot be solved. Although the means of manual phase change and adding automatic balancing devices can solve the three-phase imbalance from the source, the manual phase change scheme requires many personnel to operate simultaneously, while adding an automatic balancing device is to collect the electrical quantity data on the load side in real time, obtain a balance control scheme according to the balance strategy, and the use of automatic control can save a large amount of labor costs.

[0004] At present, most existing studies only connect a three-phase balance controller at the low-voltage outlet of the distribution transformer. This method can only ensure that the data fed back by the controller to the upper-level power grid is three-phase balanced, but the power consumption states between each user may still be three-phase unbalanced. There are also some studies that introduce lower-level controllers connected to each load to maintain the three-phase balance between each user, but due to the lack of consideration of the frequency characteristics of load fluctuations, users with higher load fluctuation frequencies are often phase-changed, which will result in a shorter three-phase balance maintenance time and still require phase change in a short time, reducing the service life of the phase change switch. Summary of the Invention

[0005] The present invention provides a three-phase unbalanced load phase-changing control method considering the frequency characteristics of load fluctuations to solve the problem of low efficiency in existing three-phase imbalance control.

[0006] The technical solution adopted by the present invention includes the following steps:

[0007] (1) Construct a phase - conversion type load automatic balancing control system with load fluctuation frequency characteristics;

[0008] (2) A phase - conversion type load automatic balancing control method with load fluctuation frequency characteristics;

[0009] (3) Verify the above steps.

[0010] The phase - conversion type load automatic balancing control system with load fluctuation frequency characteristics in step (1) of the present invention includes one main controller at the low - voltage outlet of the distribution transformer. Each user is a single - phase user, equipped with 1 phase - conversion switch. According to the geographical distribution of each user, m users with relatively close distances can be divided into one user group, and a total of n user groups may be divided along the line. Each user group is equipped with 1 distributed controller.

[0011] Based on the electrical quantity data fed back by each phase - conversion switch in its own user group, the distributed controller calculates in real - time the imbalance factor μ describing the degree of imbalance and the load fluctuation frequency λ of each user. The calculation formulas are as follows:

[0012]

[0013]

[0014] In the formula, I m is the maximum value of the peak currents of each phase of ABC, I ’ is the average value of the peak currents of each phase of ABC, P max and P min are respectively the maximum and minimum values of the power consumption of each user within 1 hour, that is, the load fluctuation frequency λ is the average value of the percentage of load change of each user within 1 day. Based on the calculated values, the distributed controller can independently perform phase - conversion adjustment on the managed load to ensure the three - phase balance of the user group in the area.

[0015] In addition, each distributed controller needs to upload the collected and calculated data to the main controller. The main controller simultaneously collects the three - phase electrical quantities at the low - voltage side of the distribution transformer, analyzes the imbalance factor μ at the low - voltage outlet of the distribution transformer. If the obtained result is outside the specified threshold of the three - phase imbalance factor, the main controller then performs load automatic balancing control based on the load fluctuation frequency characteristics of the electrical quantities of each user fed back by each distributed controller, obtains the optimal phase - conversion adjustment plan for each user, and transmits the macro - control instruction to each distributed controller. The distributed controller controls the phase - conversion switches under its jurisdiction to perform phase - conversion adjustment to make the three - phase imbalance factor within the specified threshold.

[0016] Among them, the input terminals of each phase-change switch are all connected to the three-phase line and the neutral line, and the output terminals are connected to the user load in the way of one live wire and one neutral line. The phase of each load connection is managed by the phase-change switch, so that each local user load is evenly connected to the three-phase line.

[0017] Step (2) of the present invention includes the following steps:

[0018] 1) Phase-change type load automatic balancing control process

[0019] First, according to the electrical quantities of each user group uploaded by the distributed controller and the electrical quantities at the low-voltage outlet of the distribution transformer, calculate the unbalance factor at the low-voltage outlet of the distribution transformer, and judge whether this factor exceeds the threshold. The ideal value of this threshold is 10%, so as to determine whether the main controller needs to issue a macro three-phase unbalance control instruction, and determine the phase-change switch to be phase-adjusted through the macro three-phase unbalance control instruction;

[0020] In addition, a higher phase-change number will increase the switch loss and reduce the service life of the phase-change switch. The main controller should fully consider the load fluctuation frequency characteristic λ, select the phase-change switch corresponding to the load with the smallest load fluctuation frequency to act, and judge whether there is a risk that the number of switch actions has exceeded the limit under the action of the distributed controller of this phase-change switch. If there is a risk, other phase-change switches that meet the action conditions need to be selected to avoid reducing the life due to too high a switch action frequency;

[0021] 2) Phase-change strategy

[0022] In the phase-change type load automatic balancing control considering the load fluctuation frequency characteristic, the phase-change switch selected by the main controller for adjustment needs to consider the previous control commands of the distributed controller and the load fluctuation frequency characteristics of each;

[0023] Set the average value of the three-phase load current as the target current value, and the main controller calculates the difference between the current value of each phase and the target current value, which are respectively denoted as I a0 、I b0 、I c0 , and at the same time, each distributed controller respectively records the difference between the current value of each phase of its own user group and the target current value as I an0 、I bn0 、I cn0 ; Name the maximum and minimum values among the three current differences calculated by the main controller and each distributed controller as I max0 、I min0 or I maxn0 、I minn0, in the main controller and each distributed controller memory, the phase names corresponding to the above two currents are respectively named the max phase and the min phase. For the distributed controller, command the phase change switch to transfer the max phase load to the min phase to complete one phase change; for the main controller, subtract each load current in the max phase from I min0 Make a difference. To avoid the risk of too high switching action frequency mentioned above, select the smallest 1 / 3 difference data as the alternative. According to the past load fluctuation frequency characteristic data, finally select the user with the lowest load fluctuation frequency from the smallest 1 / 3 difference data, and send the control instruction to its corresponding distributed controller, so that the phase change switch changes the load from the max phase to the min phase to complete the first phase change. The main controller and each distributed controller continue to exchange the electrical quantity data they are responsible for. If the three-phase unbalance factor is still not within the threshold range, continue to perform phase change according to the above process. Repeat this cycle until the three-phase unbalance factor is within the threshold range;

[0024] 3) Phase change switch

[0025] To prevent the phenomenon of user power interruption during the phase change process, a phase change unit structure of a composite switch is adopted. This structure uses the form of a mechanical switch in parallel with a thyristor. When receiving the phase change command, open the mechanical breaker switch and apply a trigger pulse to the thyristor in parallel with it. According to the voltage phase of the load and the positive and negative of the current, apply a trigger pulse to the thyristor of the phase to be switched, and use the phase voltage difference to achieve forced phase change to avoid power supply interruption during the phase change process;

[0026] Taking the phase change from phase A to phase B as an example, its control strategy is as follows: First, open the mechanical switch S of phase A A , and trigger the thyristors V1 and V2 of phase A; before the user connects to phase B, confirm that the current of the mechanical switch of phase A has completely dropped to 0, otherwise there will be an inter-phase circulating current; according to the positive and negative of the current i A and the phase of u A 、u B , determine the thyristors and mechanical switches of phase B to act. There are the following several working conditions:

[0027] 1) If i A >0, u A >u B , V3 is under reverse voltage and cannot conduct; at this time, if V4 is triggered, an inter-phase circulating current will be formed. Therefore, V3 and V4 cannot be triggered at this stage;

[0028] 2) If i A >0, u A <u B , trigger V3, and at the same time cut off the trigger pulses of V1 and V2; detect that i A passes through zero, and trigger V4; delay to close S B, cut off the trigger pulse of V4;

[0029] 3) If i A < 0, u A < u B , do not trigger V3 and V4;

[0030] 4) If i A < 0, u A > u B , trigger V4, and at the same time cut off the trigger pulses of V1 and V2; detect i A passing through zero, trigger V3; delay to close S B , cut off the trigger pulse of V4.

[0031] In step (3) of the present invention, a 380V three-phase four-wire distribution network with 15 users is adopted to verify the three-phase unbalanced load phase change control method considering the frequency characteristics of load fluctuations, and the conclusion that this method can effectively reduce the degree of three-phase imbalance is obtained.

[0032] The advantages of the present invention are that the structure of the phase change type unbalanced load automatic balancing control system considering the frequency characteristics of load fluctuations is analyzed in detail, and its phase change process and control strategy are designed, and the control processes of the circuit breaker and thyristor of the phase change unit based on the composite switch are systematically described. By constructing a three-phase unbalanced model of the low-voltage distribution network, it is proved that the present invention can efficiently achieve the treatment of three-phase imbalance. After phase change by the load automatic balancing method considering the frequency characteristics of load fluctuations, the degree of three-phase imbalance is effectively reduced, and the low-voltage distribution network can operate in a relatively good state. Description of the Drawings

[0033] Figure 1 is the structure diagram of the three-phase unbalanced load automatic balancing control system of the present invention;

[0034] Figure 2 is the phase change type unbalanced load automatic balancing control diagram of the present invention considering the frequency characteristics of load fluctuations;

[0035] Figure 3 is the phase change strategy flow chart of the three-phase unbalanced load considering the frequency characteristics of load fluctuations of the present invention;

[0036] Figure 4 is the composite phase change switch diagram of the present invention;

[0037] Figure 5 is the action control flow chart of the phase change unit based on the composite switch of the present invention;

[0038] Figure 6 is the three-phase current waveform diagram before phase change of the present invention;

[0039] Figure 7 It is the waveform diagram of the three-phase current after commutation of the present invention. Detailed implementation manners

[0040] The following further describes the detailed content and specific implementation manners of the present invention in conjunction with the accompanying drawings.

[0041] See Figures 1 to 7 As shown, the purpose of the present invention is to provide a three-phase unbalanced load commutation control method considering the frequency characteristics of load fluctuations, which solves the above problems existing in the prior art. The structure of the commutation type unbalanced load automatic balancing control system considering the frequency characteristics of load fluctuations is analyzed in detail, and its commutation process and control strategy are designed, and the control processes of the circuit breaker and thyristor of the commutation unit based on the composite switch are systematically described. By constructing a three-phase unbalanced model of the low-voltage distribution network, it is proved that the present invention can efficiently achieve the governance of three-phase unbalance.

[0042] 1. Construction of a commutation type load automatic balancing control system with load fluctuation frequency characteristics

[0043] The construction scheme of the commutation type load automatic balancing control system with load fluctuation frequency characteristics of the present invention is as Figure 1 shown. Specifically, it includes one main controller at the low-voltage outlet of the distribution transformer. Each user is a single-phase user and is equipped with 1 commutation switch. According to the geographical distribution of each user, several (m) users with relatively close distances can be divided into one user group. Along the line, a total of n user groups may be divided. Each user group is equipped with 1 distributed controller.

[0044] The distributed controller calculates in real time the unbalance factor μ describing the unbalance degree and the load fluctuation frequency λ of each user according to the electrical quantity data fed back by each commutation switch in the user group. The calculation formulas are as follows:

[0045]

[0046]

[0047] In the formula, I m is the maximum value of the peak currents of each phase of ABC, I ’ is the average value of the peak currents of each phase of ABC, P max and P min are respectively the maximum value and the minimum value of the power consumption of each user within 1 hour, that is, the load fluctuation frequency λ is the average value of the percentage of load change of each user within 1 day. The distributed controller can independently perform commutation adjustment on the managed load according to the calculated value to ensure the three-phase balance of the user group in the area;

[0048] In addition, each distributed controller needs to upload the collected and calculated data to the master controller. The master controller simultaneously collects the low-voltage three-phase electrical quantities of the distribution transformer and analyzes the unbalance factor μ at the low-voltage outlet of the distribution transformer. If the obtained result is outside the specified threshold of the three-phase unbalance factor, the master controller will perform automatic load balancing control based on the fluctuation frequency characteristics of the electrical quantities of each user fed back by each distributed controller, obtain the optimal phase-changing adjustment plan for each user, and transmit the macro-control command to each distributed controller. The distributed controller controls the phase-changing switches under its jurisdiction to perform phase-changing adjustment so that the three-phase unbalance factor is within the specified threshold.

[0049] Among them, the input terminals of each phase-changing switch are all connected to the three-phase line and the neutral line, and the output terminals are connected to the user load in the way of one live wire and one neutral line. By managing the phase of each load connection through the phase-changing switch, each local user load can be evenly connected to the three-phase line.

[0050] 2. The phase-changing type load automatic balancing control method including the load fluctuation frequency characteristics comprises the following steps:

[0051] 2.1 Phase-changing type load automatic balancing control process

[0052] The automatic balancing control of the three-phase load unbalance considering the load fluctuation frequency characteristics is jointly realized in the master controller and the distributed controller, and the detailed process is given in Figure 2 First, according to the electrical quantities of each user group uploaded by the distributed controller and the low-voltage outlet electrical quantities of the distribution transformer, calculate the low-voltage outlet unbalance factor of the distribution transformer, and judge whether this factor exceeds the threshold. The present invention believes that the ideal value of this threshold is 10%, so as to determine whether the master controller needs to issue a macro three-phase unbalance control command, and determine the phase-changing switch to be phase-adjusted through the macro three-phase unbalance control command.

[0053] In addition, a higher phase-changing times will increase the switch loss and reduce the service life of the phase-changing switch. The master controller should fully consider the load fluctuation frequency characteristic λ, select the phase-changing switch corresponding to the load with the minimum load fluctuation frequency to act, and judge whether there is a risk that the number of switch actions of this phase-changing switch exceeds the limit under the action of the distributed controller. If there is a risk, other phase-changing switches meeting the action conditions need to be selected to avoid reducing the service life due to too high switch action frequency.

[0054] 2.2 Phase-changing strategy

[0055] In the phase-changing type load automatic balancing control considering the load fluctuation frequency characteristics, the phase-changing switch selected by the master controller for adjustment needs to consider the previous control commands of the distributed controller and the load fluctuation frequency characteristics of each load.

[0056] Set the average value of the three-phase load current as the target current value. The main controller calculates the difference between the current value of each phase and the target current value, denoted as I a0 、I b0 、I c0 , and at the same time, for each distributed controller, the difference between the current value of each phase of its respective user group and the target current value is denoted as I an0 、I bn0 、I cn0 ; Name the maximum and minimum values among the three current differences calculated by the main controller and each distributed controller as I max0 、I min0 or I maxn0 、I minn0 . In the memories of the main controller and each distributed controller, name the phases corresponding to the above two currents as the max phase and the min phase respectively. For the distributed controller, command the phase change switch to transfer the load of the max phase to the min phase to complete one phase change; for the main controller, subtract the load current in the max phase from I min0 respectively. To avoid the risk of too high switching action frequency mentioned above, select the smallest 1 / 3 difference data as the alternative. According to the historical load fluctuation frequency characteristic data, finally select the user with the lowest load fluctuation frequency from the smallest 1 / 3 difference data, and send the control instruction to its corresponding distributed controller to make the phase change switch change the load from the max phase to the min phase to complete the first phase change. The main controller and each distributed controller continue to exchange the electrical quantity data they are responsible for. If the three-phase unbalance factor is still not within the threshold range, continue to perform phase change according to the above process. Repeat this cycle until the three-phase unbalance factor is within the threshold range. The automatic balance control block diagram considering the load fluctuation frequency characteristic is shown in Figure 3 . The calculation method of the load fluctuation frequency λ is shown in formula (1-2).

[0057] 2.3 Phase change switch

[0058] To prevent the phenomenon of user power interruption during the phase change process, the phase change unit structure of the composite switch used in the present invention is shown in Figure 4 . This structure adopts the form of a mechanical switch in parallel with a thyristor. When the phase change command is issued, open the mechanical breaker switch and apply a trigger pulse to the thyristor in parallel with it. According to the voltage phase of the load and the positive and negative of the current, apply a trigger pulse to the thyristor of the phase to be switched, and use the phase voltage difference to achieve forced phase change and avoid power supply interruption during the phase change process.

[0059] Taking the example of changing from phase A to phase B, its control strategy is as shown in Figure 5 . First, open the mechanical switch S of phase A A, trigger thyristors V1 and V2 of phase A; before the user connects phase B, confirm that the current of the mechanical switch in phase A has completely dropped to 0, otherwise interphase circulating current will be generated; according to the positive and negative of the current i A and u A , u B , determine the thyristors and mechanical switches for operating phase B. There are the following specific operating conditions.

[0060] 1) If i A > 0 and u A > u B , V3 is under reverse voltage and cannot conduct; at this time, if V4 is triggered, interphase circulating current will be formed. Therefore, V3 and V4 cannot be triggered in this stage.

[0061] 2) If i A > 0 and u A < u B , trigger V3, and at the same time cut off the trigger pulses of V1 and V2; detect that i A passes through zero, and trigger V4; delay to close S B , and cut off the trigger pulse of V4.

[0062] 3) If i A < 0 and u A < u B , do not trigger V3 and V4.

[0063] 4) If i A < 0 and u A > u B , trigger V4, and at the same time cut off the trigger pulses of V1 and V2; detect that i A passes through zero, and trigger V3; delay to close S B , and cut off the trigger pulse of V4.

[0064] 3. Numerical Examples and Simulation Analysis

[0065] To verify the feasibility of the load automatic balancing control method considering the frequency characteristics of load fluctuations, a three-phase four-wire 380V three-phase unbalanced model was built, and 15 simulated users were configured. The user data is shown in Table 1.

[0066] Table 1 Related User Data

[0067]

[0068] Before the commutation switch operates, the three-phase currents are as Figure 6 shown. Taking I maxX as the current peak value of phase X, then I maxA is 87.76A, I maxB is 58.03A, and I maxC is 84.43A. The average value of each phase current I’ It is 76.74 A. According to formula (1-1), the unbalance factor μ = 14.36%, which indicates that the three-phase unbalance suppression of each distributed controller has not met the requirements, and the main controller must conduct macro-control.

[0069] Phase A is the max phase and Phase B is the min phase. Therefore, it is necessary to transfer the users (User 3 or User 11) corresponding to the smallest 1 / 3 difference data of Phase A to Phase B, observe the load volatility, and it is found that the μ value of User 11 is lower than that of User 3, and the probability of subsequent current fluctuations after the phase change is the smallest. So User 11 is transferred to the Phase B line. After the phase change, the three-phase current is as Figure 7 shown. I maxA is 81.49 A, I maxB is 64.58 A, I maxC is 82.57 A, I ave is 76.21 A. After the phase change, the μ value is 8.35%, within the threshold range of 10%.

[0070] After the phase change by the load automatic balancing method considering the load fluctuation frequency characteristics, the three-phase unbalance degree is effectively reduced, and the low-voltage distribution network can operate in a relatively good state.

Claims

1. A three-phase unbalanced load phase conversion control method considering the frequency characteristics of load fluctuations, characterized in that Including the following steps: (1)Construct a commutation-type load automatic balancing control system with load fluctuation frequency characteristics, including a main controller at the low-voltage outlet of the distribution transformer. Each user is a single-phase user, equipped with 1 commutation switch. According to the geographical distribution of each user, m users with relatively close distances can be grouped into one user group, and n user groups may be divided along the line in total. Each user group is equipped with 1 distributed controller; The distributed controller calculates in real time the unbalance factor describing the unbalance degree and the load fluctuation frequency of each user according to the electrical quantity data fed back by each phase conversion switch in this user group. μ and λ The calculation formula is as follows: Wherein, I m is the maximum value of the peak currents of each phase of ABC, I ’ is the average value of the peak currents of each phase of ABC, P max and P min are respectively the maximum and minimum values of the power consumption of each user within 1 hour, that is, the load fluctuation frequency λ is the average value of the percentage of load change of each user within 1 day. The distributed controller independently performs phase conversion adjustment on the managed load according to the calculated value to ensure the three-phase balance of the user group in the area; In addition, each distributed controller needs to upload the collected and calculated data to the master controller, and the master controller simultaneously collects the low-voltage three-phase electrical quantities of the distribution transformer and analyzes the unbalance factor at the low-voltage outlet of the distribution transformer. μ If the obtained result is outside the specified threshold of the three-phase unbalance factor, the master controller will perform automatic load balancing control based on the fluctuation frequency characteristics of the electrical quantities of each user fed back by each distributed controller, obtain the optimal phase change adjustment plan for each user, and transmit the macro control instruction to each distributed controller. The distributed controller controls the phase change switch under its jurisdiction to perform phase change adjustment so that the three-phase unbalance factor is within the specified threshold. Wherein, the input terminals of each phase change switch are all connected to the three-phase line and the neutral line, and the output terminals are connected to the user load in the way of one live wire and one neutral line. The phase sequence of each load connection is managed through the phase change switch, so that each local user load is evenly connected to the three-phase line; (2), Implement the phase change type load automatic balancing control method with load fluctuation frequency characteristics: First, according to the electrical quantities of each user group uploaded by the distributed controller and the electrical quantities at the low-voltage outlet of the distribution transformer, calculate the unbalance factor at the low-voltage outlet of the distribution transformer, and judge whether this factor exceeds the threshold value. The ideal value of this threshold is 10%, so as to determine whether the main controller needs to issue a macro three-phase unbalance control instruction, and determine the phase change switch to be phase-adjusted through the macro three-phase unbalance control instruction; In addition, a higher commutation frequency will increase the switching losses and reduce the life of the commutation switch. The main controller should fully consider the frequency characteristics of the load fluctuation λ , select the commutation switch corresponding to the load with the minimum load fluctuation frequency for operation, and determine whether there is a risk that the number of switch operations has exceeded the limit under the action of the distributed controller of the commutation switch. If there is a risk, other commutation switches that meet the action conditions need to be selected to avoid reducing the life due to too high a switch operation frequency; (3), Use a 380V three-phase four-wire distribution network with 15 users to verify the above steps, and obtain the conclusion that this method can effectively reduce the degree of three-phase unbalance.

2. The three-phase unbalanced load phase conversion control method considering the frequency characteristics of load fluctuations according to claim 1, wherein: The step (2) includes the following steps: In the phase change type load automatic balancing control considering the load fluctuation frequency characteristics, the phase change switch selected by the main controller for adjustment needs to consider the previous control commands of the distributed controller and the load fluctuation frequency characteristics of each; Set the average value of the three-phase load current as the target current value. The main controller calculates the difference between the current value of each phase and the target current value, which are respectively denoted as I a0 、 I b0 、 I c0 ; at the same time, for each distributed controller, the difference between the current value of each phase of its respective user group and the target current value is respectively denoted as I an0 、 I bn0 、 I cn0 ; name the maximum and minimum values among the three current differences calculated by the main controller and each distributed controller as I max0 、 I min0 or I maxn0 、 I minn0 ; in the main controller and the memory of each distributed controller, name the phases corresponding to the above two currents as the max phase and the min phase respectively. For the distributed controller, command the phase change switch to transfer the max-phase load to the min phase to complete one phase change; For the main controller, the load currents in the max phase are respectively subtracted from I min0 To avoid the risk of excessive switching action frequency mentioned above, the smallest 1 / 3 difference data is selected as an alternative. Based on the previous load fluctuation frequency characteristic data, the user with the lowest load fluctuation frequency is finally selected from the smallest 1 / 3 difference data, and the control instruction is sent to its corresponding distributed controller, so that the phase change switch changes the load from the max phase to the min phase to complete the first phase change. The main controller and each distributed controller continue to exchange the electrical quantity data they are responsible for. If the three-phase unbalance factor is still not within the threshold range, the phase change continues according to the above process. This cycle continues until the three-phase unbalance factor is within the threshold range.

3. A three-phase unbalanced load phase conversion control method considering the frequency characteristics of load fluctuations according to claim 2, characterized in that: In the step (2), in order to prevent the phenomenon of user power interruption during the phase change process, the phase change switch adopts the phase change unit structure of a composite switch, and this structure adopts the form of a mechanical switch in parallel with a thyristor. After receiving the phase change command, open the mechanical circuit breaker switch, and apply a trigger pulse to the thyristor in parallel with it. According to the voltage phase of the load and the positive and negative of the current, apply a trigger pulse to the thyristor of the phase to be switched, and use the phase voltage difference to realize forced phase change and avoid power supply interruption during the phase change process; The control strategy for the phase change from phase A to phase B is to first turn on the mechanical switch S of phase A A , triggering the thyristors V1 and V2 of phase A; before the user connects to phase B, confirm that the current of the mechanical switch of phase A has completely dropped to 0, otherwise inter-phase circulating current will be generated; according to the positive and negative of the current of phase A i A and u A , u B phase, determine the thyristors and mechanical switches of phase B to act. There are the following specific operating conditions: 1) If i A > 0, u A > u B , V3 is under reverse voltage and cannot conduct; at this time, if V4 is triggered, an inter-phase circulating current will be formed, so V3 and V4 cannot be triggered in this stage; 2) If i A > 0, u A < u B , trigger V3, and at the same time cut off the trigger pulses of V1 and V2; detect i A zero crossing, trigger V4; delay closing S B , cut off the trigger pulse of V4; 3) If i A < 0, u A < u B , then V3 and V4 are not triggered; 4) If i A < 0, u A > u B , trigger V4, and at the same time cut off the trigger pulses of V1 and V2; detect i A zero crossing, trigger V3; delay closing S B , and cut off the trigger pulse of V4.

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