A method for three-phase load imbalance treatment of a low-voltage distribution network based on multiple converters

Abstract

The application discloses a kind of based on low voltage distribution network three-phase load imbalance treatment method of multi-converter, comprising: by measuring the real-time phase current voltage of low voltage distribution network start end three-phase, calculate three-phase real-time load, and judge whether load is balanced;If unbalanced, then three-phase load sum is multiplied by power balance coefficient to obtain the active power output of each phase line after balancing;Active power output and actual active power output of each phase are corrected by PI controller to obtain active power control instruction for controlling A, B, C three-phase converter;According to single-phase capacity adjustment margin constraint condition, select corresponding control scheme to realize three-phase imbalance treatment in transformer area or collaborative treatment between transformer areas;The application reasonably utilizes converter device, realizes the flexible interconnection of three-phase and distribution station between low voltage distribution network three-phase of three-phase four-wire system without increasing short-circuit current, enhances the regulation margin of three-phase load imbalance in transformer area, and improves power quality of distribution network.

Description

Technical Field

[0001] This invention relates to the field of power distribution network management technology, specifically a method for managing three-phase load imbalance in low-voltage power distribution networks based on multi-converter converters. Background Technology

[0002] Because low-voltage distribution networks have numerous and dispersed power users, there are a large number of spatiotemporally unbalanced single-phase loads, making three-phase unbalanced operation unavoidable. With the increase in load types and electricity consumption, as well as the proportion of single-phase, nonlinear, and impulsive loads, the current and voltage imbalances caused by these loads are often accompanied by other forms of interference: harmonics, voltage drops, and voltage fluctuations. Current imbalance leads to increased active power loss in the power grid, thus reducing efficiency; furthermore, voltage imbalance negatively impacts rotating electrical machines, causing severe material damage and reducing their lifespan. Transformers, capacitor banks, and some protection systems are also affected by unbalanced power supply current and voltage.

[0003] The three-phase imbalance problem on the low-voltage distribution side is becoming increasingly serious and has become a prominent issue that urgently needs to be addressed in the operation of low-voltage distribution networks. If a low-voltage distribution network operates with three-phase imbalance for a long period of time, it will have a significant negative impact on the economic, safe, and stable operation of the network. Summary of the Invention

[0004] The purpose of this section is to outline some aspects of embodiments of the present invention and to briefly describe some preferred embodiments. Simplifications or omissions may be made in this section, as well as in the abstract and title of this application, to avoid obscuring the purpose of these documents; however, such simplifications or omissions should not be construed as limiting the scope of the invention.

[0005] In view of the above-mentioned problems, the present invention is proposed.

[0006] A first aspect of this invention provides a method for managing three-phase load imbalance in a low-voltage distribution network based on multiple converters, comprising: calculating the real-time three-phase load by measuring the real-time phase current and voltage of the three phases at the beginning of the low-voltage distribution network, and determining whether the real-time three-phase load is balanced; if unbalanced, multiplying the sum of the three-phase loads by a power balance coefficient to obtain the active power output of each phase line after balancing; correcting the active power output of each phase line with the actual active power output of each phase through a PI controller to obtain an active power control command for controlling the three-phase converters A, B, and C; and, based on the active power control command and according to the single-phase capacity adjustment margin constraint, selecting a corresponding control scheme to achieve self-management of three-phase imbalance within the distribution area or coordinated management between distribution areas.

[0007] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the low-voltage distribution network includes a multi-port converter system, DC interconnection, a multi-port converter control system, and a three-phase four-wire low-voltage distribution substation.

[0008] The multi-port converter system consists of six single-phase full-bridge sub-converters connected back-to-back along a common DC bus. The three-phase four-wire low-voltage distribution substation is flexibly interconnected within the multi-port converter system via a common DC bus.

[0009] The multi-port converter control system selects the corresponding control scheme based on the single-phase capacity adjustment margin constraint to achieve self-control of three-phase imbalance within the transformer area or coordinated control between transformer areas.

[0010] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the flexible interconnection includes:

[0011] A multi-port converter device is embedded in a three-phase four-wire low-voltage distribution network. The six ports of the multi-port converter device are respectively connected to the A, B, and C phases and the neutral line of the three-phase four-wire low-voltage distribution area through L filters and switch groups.

[0012] An active power transmission channel is formed between the three phases A, B, and C, as well as between the distribution substations. At the same time, a reactive power transmission channel is formed between the converter and the AC side. The reactive power transmission channel adjusts the reactive power command of the control port converter according to the AC reactive power measurement, so as to realize the reactive power mutual assistance between the port converter and the low-voltage distribution network side.

[0013] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the step of determining whether the three-phase real-time load is balanced includes:

[0014] The three-phase real-time load P is calculated by measuring the real-time phase current and real-time phase voltage of each phase at the beginning of the low-voltage distribution network. a P b P c ;

[0015] The three-phase real-time load balancing refers to the sum of the active power vector values ​​transmitted through the converter to the active power command value of the regulating converter control system being zero, expressed as:

[0016]

[0017] in, These represent the three-phase active power command value of the regulating converter control system and the three-phase active power vector value transmitted through the converter.

[0018] Determine whether the three-phase real-time load is balanced. If the three-phase real-time load is balanced, disconnect the port switch S of the three-port converter. If the three-phase real-time load is unbalanced, select the corresponding control scheme according to the single-phase capacity adjustment margin constraint to achieve self-control of the three-phase imbalance within the transformer area or coordinated control between transformer areas.

[0019] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, wherein: the acquisition of the active power output of each phase line includes,

[0020] The calculated three-phase real-time load P a P b P c Summing yields the total three-phase load P. sum Then sum the three-phase loads P sum Multiplying the power balance factor by the power balance factor gives the balanced active power output P of each phase of the line. av Among them, the power balance coefficient

[0021] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the acquisition of active power control commands for the A, B, and C three-phase converters includes:

[0022] The active power output of each phase line and the actual active power output of each phase are corrected by a PI controller to obtain the active power control command for controlling the three-phase converters A, B, and C.

[0023] To prevent damage caused by the power output of the converter exceeding the limit, an amplitude limiter is connected to the output of the PI controller, and the amplitude is set to the power range that each converter can transmit.

[0024] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the selection of the control scheme includes:

[0025] If the single-phase capacity regulation constraint is met, the multi-port converter is activated with active power modulation technology based on load balancing, and control scheme one is adopted to achieve self-control of three-phase load imbalance in the transformer area.

[0026] If the single-phase capacity regulation constraint is not met, then the multi-port converter is used to implement control scheme two to control the three-phase imbalance in a coordinated and complementary manner.

[0027] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, wherein: the port converter connected to phase A of distribution areas 1 and 2 is designated as A. 1or2 Port converter, the port converter connected to B in transformer areas 1 and 2 is called B. 1or2 The port converter connected to C in both transformer substations 1 and 2 is C. 1or2 Port converter.

[0028] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the control scheme one includes:

[0029] The A 1or2 Port converter and the B 1or2 All port converters adopt active power-reactive power control, that is, the outer loop adopts PQ control, the inner loop adopts current loop control, and the controller adopts a traditional PI controller.

[0030] The C 1or2 The port converter adopts DC bus voltage-reactive power control, that is, the outer loop uses V dc -Q control, the inner loop uses current loop control, and the controller uses a traditional PI controller.

[0031] As a preferred embodiment of the three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter described in this invention, the control scheme two includes:

[0032] The three-phase imbalance between the transformers is jointly managed through coordinated efforts. Based on the power flow direction, the transformers are divided into sending-end converters and receiving-end converters, which operate in rectification and inversion states, respectively.

[0033] The sending-end converter is set as the main converter, which is responsible for stabilizing the DC-side voltage at a fixed value. A general droop control method is used to set the active power droop coefficient K. p =0, the general droop control is reduced to a DC voltage control strategy;

[0034] The receiving-end converter is set as a slave converter; all converters except the master converter are slave converters. The slave converter absorbs the active power of the DC system, and a general droop control method is used to set the DC voltage droop coefficient K. d =0, the general droop control is reduced to a constant active power control strategy.

[0035] The beneficial effects of this invention are as follows: This invention provides a method for managing three-phase load imbalance in low-voltage distribution networks based on multi-port converters. It uses an embedded multi-port converter to address the unavoidable three-phase imbalance problem in three-phase four-wire low-voltage distribution networks, offering flexibility, speed, and controllability. This is mainly manifested in the flexible interconnection between three phases within a distribution area and between distribution areas via a common DC bus using multi-port single-phase converters. The integration of multi-port converters increases the flexibility of three-phase load imbalance regulation within a distribution area, while simultaneously achieving energy exchange between distribution areas. The management of three-phase load imbalance is not limited to a single distribution area, effectively improving the limitation of individual phase regulation capacity within a single distribution area. Coordinated cooperation and energy exchange between distribution areas provide sufficient regulation space to address three-phase imbalance problems caused by random and nonlinear loads and impulsive loads in the connection of single-phase loads between phases of the low-voltage distribution network. Furthermore, this invention utilizes an embedded multi-port converter device to achieve flexible interconnection between phases A, B, and C of a three-phase four-wire low-voltage distribution network; it leverages the controllability of the power electronics of the converter device to achieve load balancing between the three phases of the three-phase four-wire low-voltage distribution network and connection between distribution substations; it also constitutes a reactive power transmission channel between the converter and the AC side, realizing reactive power mutual assistance between the port converter and the low-voltage distribution network side; this invention achieves flexible interconnection between the three phases of a three-phase four-wire low-voltage distribution network without increasing short-circuit current, improving the load balance between the three phases, reducing line losses, and improving the power quality of the distribution network. Attached Figure Description

[0036] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0037] Figure 1 A schematic diagram of the overall topology of a method for managing three-phase load imbalance in low-voltage distribution networks based on multi-converter provided by the present invention;

[0038] Figure 2 A schematic diagram of a three-phase unbalanced distribution area self-control scheme based on load balancing power modulation technology, which is provided by the present invention for a method of managing three-phase load imbalance in low-voltage distribution networks based on multi-converters;

[0039] Figure 3 A schematic diagram of a control scheme for joint management of three-phase load imbalance in a low-voltage distribution network based on a multi-converter, which involves inter-station coordination and energy mutual assistance, provided by the present invention.

[0040] Figure 4 A block diagram of an active power modulation strategy for load balancing in a three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter provided by the present invention.

[0041] Figure 5 The present invention provides a control flowchart for balancing three-phase loads using a multi-port converter in a low-voltage distribution network based on a multi-converter method for addressing three-phase load imbalance.

[0042] Figure 6 This is a verification diagram of an embodiment of the method for managing three-phase load imbalance in low-voltage distribution networks based on multi-converter provided by the present invention. Detailed Implementation

[0043] 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.

[0044] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0045] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0046] This invention is described in detail with reference to the schematic diagrams. When detailing the embodiments of this invention, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not be construed as limiting the scope of protection of this invention. In actual fabrication, the three-dimensional spatial dimensions of length, width, and depth should be included.

[0047] Furthermore, in the description of this invention, it should be noted that the terms "upper," "lower," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are used solely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In addition, the terms "first," "second," or "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0048] Unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" in this invention should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; similarly, they can refer to mechanical connections, electrical connections, or direct connections, or indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0049] Example 1

[0050] Reference Figures 1-5 As one embodiment of the present invention, a method for mitigating three-phase load imbalance in a low-voltage distribution network based on a multi-converter is provided, comprising:

[0051] S1: By measuring the real-time phase current and voltage of the three phases at the beginning of the low-voltage distribution network, the real-time three-phase load is calculated, and it is determined whether the real-time three-phase load is balanced. It should be noted that:

[0052] Low-voltage distribution networks include multi-port converter systems, DC interconnection, multi-port converter control systems, and three-phase four-wire low-voltage distribution substations.

[0053] Specifically, the multi-port converter system consists of six single-phase full-bridge sub-converters connected back-to-back along a common DC bus. The three-phase four-wire low-voltage distribution substations within the multi-port converter system are flexibly interconnected via a common DC bus. The DC side of the multi-port converter system is connected to achieve interconnection between substations. Based on load balancing active power modulation technology, the multi-port converter is used to achieve self-management of three-phase imbalance within the substation and joint management of three-phase imbalance through coordinated cooperation and energy sharing between substations.

[0054] It should be noted that flexible interconnection includes,

[0055] A multi-port converter device is embedded in a three-phase four-wire low-voltage distribution network. The six ports of the multi-port converter device are connected to the three phases A, B, and C and the neutral line of the three-phase four-wire low-voltage distribution substation through L filters and switch groups, respectively. An active power transmission channel is formed between the three phases A, B, and C and between the distribution substations. At the same time, a reactive power transmission channel is formed between the converter and the AC side. The reactive power transmission channel adjusts the reactive power command of the control port converter according to the reactive power measured by the AC, so as to realize the reactive power mutual assistance between the port converter and the low-voltage distribution network side.

[0056] The multi-port converter control system selects the corresponding control scheme based on the single-phase capacity regulation margin constraint to achieve self-control of three-phase imbalance within the transformer area or coordinated control between transformer areas.

[0057] Furthermore, determining whether the three-phase real-time load is balanced includes,

[0058] The three-phase real-time load P is calculated by measuring the real-time phase current and real-time phase voltage of each phase at the beginning of the low-voltage distribution network. a P b P c ;

[0059] Specifically, three-phase real-time load balancing refers to the sum of the active power vector values ​​transmitted through the converter to the active power command value of the regulating converter control system being zero, expressed as:

[0060]

[0061] in, These represent the three-phase active power command value of the regulating converter control system and the three-phase active power vector value transmitted through the converter.

[0062] like Figure 5 As shown, it is necessary to determine whether the three-phase real-time load is balanced. If the three-phase real-time load is balanced, the port switch S of the three-port converter is disconnected. If the three-phase real-time load is unbalanced, the corresponding control scheme needs to be selected according to the single-phase capacity adjustment margin constraint to achieve self-control of the three-phase imbalance within the transformer area or coordinated control between transformer areas.

[0063] S2: If unbalanced, multiply the sum of the three-phase loads by the power balance coefficient to obtain the balanced active power output of each phase. It should be noted that:

[0064] The acquisition of active power output for each phase of the line includes,

[0065] like Figure 4 As shown, the calculated three-phase real-time load P a P b P c Summing yields the total three-phase load P.sum Then sum the three-phase load P sum Multiplying the power balance factor by the power balance factor gives the balanced active power output P of each phase of the line. av Among them, the power balance coefficient

[0066] S3: The active power output of each phase line is corrected against the actual active power output of each phase using a PI controller to obtain the active power control command for controlling the A, B, and C three-phase converters. It should be noted that:

[0067] The acquisition of active power control commands for the three-phase converters A, B, and C includes:

[0068] The active power output of each phase line and the actual active power output of each phase are corrected by a PI controller to obtain the active power control command for controlling the three-phase converters A, B, and C.

[0069] To prevent damage caused by the power output of the converter exceeding the limit, an amplitude limiter is connected to the output of the PI controller, and the amplitude is set to the power range that each converter can transmit.

[0070] S4: Based on active power control commands and according to single-phase capacity adjustment margin constraints, select the corresponding control scheme to achieve self-control of three-phase imbalance within the transformer area or coordinated control between transformers. It should be noted that:

[0071] The selection of control schemes includes,

[0072] If the single-phase capacity regulation constraint is met, the multi-port converter is activated with active power modulation technology based on load balancing, and control scheme one is adopted to achieve self-control of three-phase load imbalance in the transformer area.

[0073] If the single-phase capacity regulation constraint is not met, then the three-phase imbalance is jointly addressed by using a multi-port converter and control scheme two to coordinate and complement each other in the control area.

[0074] Specifically, for ease of description, the converters are labeled as follows: the converter connected to transformer area 1 is represented by the subscript "1", and the converter connected to transformer area 2 is represented by the subscript "2". Therefore, the port converter connected to phase A of transformer areas 1 and 2 is A. 1or2 Port converter, the port converter connected to B in transformer areas 1 and 2 is called B. 1or2 The port converter connected to C in both transformer substations 1 and 2 is C. 1or2 Port converter, overall topology as follows Figure 1 As shown;

[0075] Furthermore, such as Figure 2As shown, control scheme one includes self-control of three-phase imbalance within the transformer area. Based on the function of each port converter, the specific control strategy for each port converter is as follows:

[0076] ①A 1or2 Port converter and B 1or2 All port converters adopt active power-reactive power control, that is, the outer loop adopts PQ control, the inner loop adopts current loop control, and the controller adopts a traditional PI controller.

[0077] It should be noted that the active power command value of the outer loop is obtained through the active power modulation link of the load balancing. A positive active power command value indicates that energy flows from the grid side to the converter, and conversely, a negative active power command value indicates that energy flows from the converter to the grid side. In this way, by adjusting the sign and magnitude of the active power command value, the power flow to the balanced three-phase load can be controlled through the three-port converter system.

[0078] It should be noted that the reactive power command value is determined by the reactive power situation on the grid side. When the converter needs to compensate the grid side for reactive power, the reactive power command value is set to negative; conversely, when the converter needs to absorb reactive power on the grid side, the reactive power command value is set to positive. In this way, by adjusting the sign and magnitude of the reactive power command value, the reactive power situation on the grid side can be adjusted through the three-port converter.

[0079] Specifically, to simplify controller design, a coordinate transformation is introduced to convert the AC quantity in the stationary coordinate system into a DC quantity in the dq coordinate system. However, coordinate transformation requires two orthogonal AC components, which a single-phase system clearly does not meet. Since coordinate transformation is merely a mathematical transformation and has no practical significance, an AC quantity with a 90° phase difference from that in the single-phase system can be artificially constructed to achieve the dq coordinate transformation. In fact, simply delaying the actual AC quantity by 90° in the single-phase system can virtually create an orthogonal component. Due to the coupling relationship between the dq axes, a feedforward decoupling control strategy is used to achieve active and reactive power decoupling control, i.e.:

[0080] d-axis control: Active power command value P kref (k = a, b) and real-time load value P k The difference between (k = a, b) is used by the PI controller to form the command value i for the inner current loop. kref (k = a, b), its instruction value i kref (k = a, b) and then compared with the real-time current value i dk The difference between (k = a, b) is passed through a PI regulator and its feedforward decoupling circuit to generate a PWM modulation signal. The SPWM modulation technology is used to generate a control signal to control the converter to control the direction of active power flow.

[0081] q-axis control: reactive power command value Q kref(k = a, b) and real-time reactive power value Q k The difference between (k = a, b) is used by the PI controller to form the command value i for the inner current loop. kref (k = a, b), its instruction value i kref (k = a, b) and then compared with the real-time current value i qk The difference between (k=a, b) is processed by a PI regulator and its feedforward decoupling circuit to generate a PWM modulation signal. SPWM modulation technology is used to generate a control signal to control the converter to control the reactive power exchange between the converter and the grid side.

[0082] ②C 1or2 The port converter adopts DC bus voltage-reactive power control, that is, the outer loop uses V dc -Q control, the inner loop uses current loop control, and the controller uses a traditional PI controller;

[0083] It should be noted that the DC voltage command value of the outer loop is set to a constant value of 10kV to achieve the purpose of controlling the DC side voltage of the three-port converter to stabilize at 10kV.

[0084] For the active power control channel, the voltage control loop is considered as the outer loop, i.e.: d-axis control: DC voltage command value V dcref With feedback DC voltage value V dc The difference is compared and then processed by a PI controller to generate the current command value i for the inner current loop. drefc Its instruction value i drefc Then compared with the real-time current value i dc The difference is processed by a PI regulator and its feedforward decoupling circuit to generate a PWM modulation signal. SPWM modulation technology is used to generate a control signal to control the converter to stabilize the DC side voltage.

[0085] For the reactive power control channel, i.e., q-axis control: reactive power command value Q kref (k=c) and real-time reactive power value Q k The difference between (k=c) is used by the PI regulator to form the command value i for the inner current loop. qrefc Its instruction value i qrefc Then compared with the real-time current value i qc The difference is processed by a PI regulator and its feedforward decoupling circuit to generate a PWM modulation signal. SPWM modulation technology is used to generate a control signal to control the converter and control the reactive power exchange between the converter and the grid side.

[0086] Furthermore, control scheme two includes,

[0087] The three-phase imbalance between transformer substations is managed collaboratively. Based on the power flow direction, the substations are divided into sending-end and receiving-end converters, which operate in rectification and inversion states, respectively. The control strategy is as follows:

[0088] ① Set the sending-end converter as the main converter. The main converter is responsible for stabilizing the DC side voltage at a fixed value. Use a general droop control method to set the active power droop coefficient K. p =0, the general droop control is reduced to a DC voltage control strategy;

[0089] It should be noted that the main converter supplies active power to the DC system; the main converter is a rectifier. When the capacity regulation margin is met, its operating mechanism is that if power fluctuations occur in the main converter, it adjusts its output power to compensate for the unbalanced power in the system, thereby stabilizing the DC voltage. The stable operating point is... Figure 3 (a) will follow V dcref The horizontal line has shifted.

[0090] ② Set the receiving-end converter as a slave converter. All converters except the master converter are slave converters. The slave converters absorb the active power of the DC system. Use a general droop control method to set the DC voltage droop coefficient K. d =0, the general droop control is reduced to a constant active power control strategy, and the control block diagram is as follows. Figure 3 As shown in (b).

[0091] It should be noted that this invention provides a method for managing three-phase load imbalance in low-voltage distribution networks based on multi-port converters. It uses an embedded multi-port converter to address the unavoidable three-phase imbalance problem in three-phase four-wire low-voltage distribution networks, offering flexibility, speed, and controllability. This is mainly manifested in the flexible interconnection between three phases within a distribution area and between distribution areas via a common DC bus using multi-port single-phase converters. The integration of multi-port converters increases the flexibility of three-phase load imbalance regulation within a distribution area, while simultaneously enabling energy exchange between distribution areas. The management of three-phase load imbalance is not limited to a single distribution area, effectively mitigating the limitations imposed by the regulation capacity of each phase within a single distribution area. Coordination and energy exchange between distribution areas provide sufficient regulation space to address three-phase imbalance problems caused by random and nonlinear loads and impulsive loads on the single-phase load connections between different phases of the low-voltage distribution network. Furthermore, this invention utilizes an embedded multi-port converter device to achieve flexible interconnection between phases A, B, and C of a three-phase four-wire low-voltage distribution network; it leverages the controllability of the power electronics of the converter device to achieve load balancing between the three phases of the three-phase four-wire low-voltage distribution network and connection between distribution substations; it also constitutes a reactive power transmission channel between the converter and the AC side, realizing reactive power mutual assistance between the port converter and the low-voltage distribution network side; this invention achieves flexible interconnection between the three phases of a three-phase four-wire low-voltage distribution network without increasing short-circuit current, improving the load balance between the three phases, reducing line losses, and improving the power quality of the distribution network.

[0092] Example 2

[0093] Reference Figure 6 This is the second embodiment of the present invention. Unlike the first embodiment, this embodiment provides a verification test of a three-phase load imbalance mitigation method for low-voltage distribution networks based on multi-converter. To verify and explain the technical effects of the method, this embodiment mainly verifies the three-phase imbalance mitigation under two operating conditions.

[0094] Operating Condition 1: Self-management within the transformer area is achieved using Control Scheme 1;

[0095] The data in this embodiment comes from a low-voltage distribution substation in a certain region. This substation often experiences three-phase imbalance in summer, and its power supply load consists of old residential buildings, all of which are single-phase loads, making it a typical substation area; the grid voltage V abc =220, and the load conditions of each phase at a certain moment are shown in Table 1.

[0096] Table 1: Load data for each phase at a certain moment.

[0097]

[0098] Figure 6 (a) shows the current situation of each phase when the three-phase load is unbalanced. It can be clearly seen from the figure that the three-phase current is in an unbalanced state, with an imbalance degree δ = 66.7%. After adjustment by the self-regulating control strategy within the transformer area, the current of each phase is as follows: Figure 6 As shown in (b), the three-phase imbalance is greatly improved, with an imbalance degree of δ = 1.5%, which verifies the accuracy of the multi-converter in managing three-phase imbalance.

[0099] Operating Condition 2: Control Scheme 2 is adopted to achieve coordinated management between the substations;

[0100] The information of the transformer area is the same as that of operating condition 1, and the load situation of the transformer area is the same as that of operating condition 1. However, the control scheme adopts control scheme 2, which uses a multi-port converter to extract 10kW of active power from transformer area 2 to provide mutual support to transformer area 1.

[0101] Figure 6 (c) This figure shows the current situation of each phase in transformer substation 1 when the three-phase load is unbalanced. It is clear from the figure that the three-phase current is in an unbalanced state, with an imbalance degree δ = 66.7%. After adjustment through a coordinated management strategy between transformer substations, the current of each phase in transformer substation 1 is as follows: Figure 6 As shown in (d), the three-phase imbalance is greatly improved, with an imbalance degree δ = 1.3%. Simultaneously, a comparison is made... Figure 6 (b) Figure 6 (d) It can be found that the power transmission from transformer 2 to transformer 1 effectively improves the output of transformer 1, prevents overload or long-term heavy load, and improves the safety and stability of the power distribution network. This verifies the accuracy of the collaborative management of three-phase imbalance between multiple converter transformer substations.

[0102] In summary, the above steps demonstrate that the present invention can rationally utilize converter devices to achieve flexible interconnection between the three phases and between distribution stations in a three-phase four-wire low-voltage distribution network without increasing short-circuit current. It also enables coordinated cooperation between distribution stations to address three-phase load imbalance, enhances the adjustment margin of three-phase load imbalance within distribution stations, strengthens the connection between distribution stations, reduces line losses, achieves economical operation of the distribution network, and improves the power quality of the distribution network.

[0103] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended 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.

Claims

1. A method for mitigating three-phase load imbalance in low-voltage distribution networks based on multi-converter converters, characterized in that, include: The real-time phase current and voltage of the three phases at the beginning of the low-voltage distribution network are measured to calculate the real-time load of the three phases and determine whether the real-time load of the three phases is balanced. If the three-phase load is unbalanced, the sum of the three-phase loads is multiplied by the power balance coefficient to obtain the active power output of each phase line after balance. The active power output of each phase line and the actual active power output of each phase are corrected by a PI controller to obtain the active power control command for controlling the three-phase converters A, B, and C. Based on the active power control command, and according to the single-phase capacity adjustment margin constraint, the corresponding control scheme is selected to achieve self-control of three-phase imbalance within the transformer area or coordinated control between transformer areas. The selection of the control scheme includes, If the single-phase capacity regulation constraint is met, the multi-port converter is activated with active power modulation technology based on load balancing, and control scheme one is adopted to achieve self-control of three-phase load imbalance in the transformer area. If the single-phase capacity regulation constraint is not met, then the three-phase imbalance is jointly addressed by using a multi-port converter and control scheme two to coordinate and complement each other in the control area. Configure the port converter connected to phase A of transformer substations 1 and 2 as follows: The port converter connected to phase B of transformer substations 1 and 2 is... The port converter, connected to phase C of transformer substations 1 and 2, is... Port converter; The first control scheme includes, The Port converter and the All port converters adopt active power-reactive power control, that is, the outer loop adopts PQ control, the inner loop adopts current loop control, and the controller adopts a traditional PI controller. The The port converter adopts DC bus voltage-reactive power control, i.e., the outer loop uses... -Q control, the inner loop uses current loop control, and the controller uses a traditional PI controller.

2. The method for mitigating three-phase load imbalance in low-voltage distribution networks based on multi-converter as described in claim 1, characterized in that: The low-voltage distribution network includes a multi-port converter system, DC interconnection, a multi-port converter control system, and a three-phase four-wire low-voltage distribution substation. The multi-port converter system consists of six single-phase full-bridge sub-converters connected back-to-back along a common DC bus. The three-phase four-wire low-voltage distribution substation is flexibly interconnected within the multi-port converter system via a common DC bus. The multi-port converter control system selects the corresponding control scheme based on the single-phase capacity adjustment margin constraint to achieve self-control of three-phase imbalance within the transformer area or coordinated control between transformer areas.

3. The method for managing three-phase load imbalance in low-voltage distribution networks based on multi-converter as described in claim 2, characterized in that: The flexible interconnection includes, A multi-port converter device is embedded in a three-phase four-wire low-voltage distribution network. The six ports of the multi-port converter device are respectively connected to the A, B, and C phases and the neutral line of the three-phase four-wire low-voltage distribution area through L filters and switch groups. An active power transmission channel is formed between the three phases A, B, and C, as well as between the distribution substations. At the same time, a reactive power transmission channel is formed between the converter and the AC side. The reactive power transmission channel adjusts the reactive power command of the control port converter according to the AC reactive power measurement, so as to realize the reactive power mutual assistance between the port converter and the low-voltage distribution network side.

4. The method for mitigating three-phase load imbalance in low-voltage distribution networks based on multi-converter as described in claim 1 or 3, characterized in that: The determination of whether the three-phase real-time load is balanced includes, The three-phase real-time load is calculated by measuring the real-time phase current and real-time phase voltage of each phase at the beginning of the low-voltage distribution network. ; The three-phase real-time load balancing refers to the sum of the active power vector values ​​transmitted through the converter to the active power command value of the regulating converter control system being zero, expressed as: in, These represent the three-phase active power command value of the regulating converter control system and the three-phase active power vector value transmitted through the converter. Determine whether the three-phase real-time load is balanced. If the three-phase real-time load is balanced, disconnect the port switch S of the three-port converter. If the three-phase real-time load is unbalanced, select the corresponding control scheme according to the single-phase capacity adjustment margin constraint to achieve self-control of the three-phase imbalance within the transformer area or coordinated control between transformer areas.

5. The method for managing three-phase load imbalance in low-voltage distribution networks based on multi-converter as described in claim 4, characterized in that: The acquisition of the active power output of each phase line includes, The calculated three-phase real-time load Summing yields the total three-phase load. Then sum the three-phase loads. Multiplying the power balance factor by the power balance factor gives the balanced active power output of each phase of the line. Among them, the power balance coefficient .

6. The method for managing three-phase load imbalance in low-voltage distribution networks based on multi-converter as described in claim 5, characterized in that: The acquisition of active power control commands for the A, B, and C three-phase converters includes, The active power output of each phase line and the actual active power output of each phase are corrected by a PI controller to obtain the active power control command for controlling the three-phase converters A, B, and C. To prevent damage caused by the power output of the converter exceeding the limit, an amplitude limiter is connected to the output of the PI controller, and the amplitude is set to the power range that each converter can transmit.

7. The method for mitigating three-phase load imbalance in low-voltage distribution networks based on multi-converter as described in claim 6, characterized in that: The second control scheme includes, The three-phase imbalance between the transformers is jointly managed through coordinated efforts. Based on the power flow direction, the transformers are divided into sending-end converters and receiving-end converters, which operate in rectification and inversion states, respectively. The sending-end converter is set as the main converter, which is responsible for stabilizing the DC-side voltage at a fixed value. A general droop control method is used to adjust the active power droop coefficient. The general droop control is downgraded to a DC voltage control strategy; The receiving-end converter is set as a slave converter; all converters except the master converter are slave converters. The slave converters absorb the active power of the DC system, and a general droop control method is used to adjust the DC voltage droop coefficient. The general droop control is reduced to a constant active power control strategy.

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