Power distribution network flexible interconnection device and control method thereof

Abstract

The application discloses a power distribution network flexible interconnection device and a control method thereof. The device comprises a first AC / DC converter, a second AC / DC converter and a controller. The AC port of the first AC / DC converter is connected with the shunt expansion transformer secondary side of a power distribution area, the AC port of the second AC / DC converter is connected with the main transformer secondary side of the power distribution area, and the DC port of the second AC / DC converter is connected with the DC port of the first AC / DC converter. The controller is used for collecting the voltage and current parameters of the main transformer, judging the operation condition of the main transformer based on the voltage and current parameters, and controlling the working mode of the first AC / DC converter and the second AC / DC converter according to the operation condition. The application has the functions of power mutual aid of the power distribution area, emergency power supply of the distributed power supply, overvoltage and undervoltage control of the main transformer, three-phase unbalanced load control, and harmonic control, thereby improving the power supply reliability of the power distribution network.

Description

Technical Field

[0001] This invention relates to the field of AC power distribution network technology, and specifically to a flexible interconnection device for power distribution networks and its control method. Background Technology

[0002] At present, most low-voltage distribution areas adopt a single transformer and single line power supply form, and each distribution area is independent and lacks unified management and control. Due to open-loop operation, they cannot share the remaining capacity, which makes the load imbalance between adjacent distribution areas increasingly serious. How to effectively utilize the available resources of adjacent distribution areas, achieve reasonable power distribution, and improve the load imbalance of distribution areas is the current problem.

[0003] The current state of research is as follows: (1) The patent "A fan cooling method for a power electronic transformer" applied for by XJ Group Co., Ltd. - This patent proposes a method to achieve air cooling using NTC resistors with negative temperature coefficients. It does not require a dedicated sampling circuit to collect the temperature of the IGBT module. The fan is directly controlled based on the NTC temperature, which is simple in structure and reduces costs.

[0004] Disadvantages: It requires high temperature sampling accuracy for NTC resistors and cannot monitor the temperature of IGBT modules in real time, which is not conducive to over-temperature protection logic and poses an over-temperature risk.

[0005] (2) The patent "A Power Electronic Transformer Module and Transformer Based on Transformer Shell Insulation" applied for by Tian Xing et al. of TBEA Xi'an Electric Technology Co., Ltd. - This patent proposes a power electronic transformer container model and air-cooling equipment installation, which realizes heat dissipation of several power electronic transformer modules through an independent air duct, and has the advantages of high power density, easy disassembly and maintenance.

[0006] Limitations: The internal space of this model cannot guarantee uniform temperature, and there may be problems with excessively high local temperatures.

[0007] (3) The patent "A Microgrid System for Distribution Areas Based on Flexible DC Interconnection and Its Control Method" applied for by Zhang Xiaoyan et al. of NARI Technology Co., Ltd. - This patent proposes several solutions to distribution area faults, can realize coordinated operation and power flow control of multiple distribution areas, realize cross-distribution area consumption of new energy, and improve the reliability of distribution area power supply and load power supply capacity.

[0008] Disadvantages: Multiple DC / DC devices are required to connect new energy sources to the grid, resulting in higher costs.

[0009] (4) The patent "A flexible interconnection device for distribution network and its control method" applied for by Bi Cheng et al. of Shandong Taikai DC Technology Co., Ltd. - This patent proposes a flexible interconnection device based on the improvement of the traditional distribution network interconnection switch, which can realize the power flow between the distribution network bus and feeder and improve energy utilization.

[0010] Shortcomings: It cannot solve the problems of phase-to-phase load imbalance, and the overvoltage and undervoltage problems of the main transformer.

[0011] (5) The patent "Control method, device, electronic equipment and storage medium of flexible interconnection system of transformer substation" applied for by Wang Cheng et al. of Nanjing Nanrui Relay Protection Electric Co., Ltd. - This patent proposes a device for flexible interconnection system of transformer substation and its control method, electronic equipment and storage medium, which can control the output power of flexible interconnection converter according to the active power demand of each transformer substation, which is conducive to the economical, safe and efficient operation of flexible interconnection converter of transformer substation.

[0012] Shortcomings: It cannot solve the problems of phase-to-phase load imbalance, and the overvoltage and undervoltage problems of the main transformer.

[0013] In summary, existing research has not addressed issues such as phase-to-phase current imbalance on the load side, overvoltage caused by renewable energy integration, and undervoltage due to high load demand. It is evident that while existing flexible interconnection devices can achieve power distribution between distribution areas, problems such as main transformer overvoltage caused by renewable energy integration, main transformer undervoltage caused by load overload, three-phase load imbalance due to uneven three-phase load distribution, and harmonic mitigation remain to be solved.

[0014] In addition, existing flexible interconnection devices mostly adopt a three-phase three-bridge AC / DC topology, which has insufficient zero-sequence component processing capability and cannot independently control the zero-sequence voltage. When the load is unbalanced, the zero-sequence current can only flow through the neutral loop, resulting in excessive neutral current. Furthermore, existing devices cannot effectively suppress zero-sequence harmonics, and the harmonic mitigation range is limited. Summary of the Invention

[0015] The technical problem to be solved by this invention is to provide a flexible interconnection device for distribution networks and its control method, which, in addition to meeting the basic functions of power mutual assistance between distribution areas and emergency power supply of distributed sources, adopts a three-phase four-bridge arm structure to additionally provide the ability to manage overvoltage and undervoltage of the main transformer, manage three-phase unbalanced loads, and manage harmonics, thereby improving the power supply reliability of the distribution network.

[0016] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A flexible interconnection device for a power distribution network includes a first AC / DC converter, a second AC / DC converter, and a controller. The AC port of the first AC / DC converter is connected to the secondary side of a parallel expansion transformer in the distribution substation. The AC port of the second AC / DC converter is connected to the secondary side of a main transformer in the distribution substation. The primary side of the parallel expansion transformer is connected in parallel with the primary side of the main transformer and connected to the power grid. The DC port of the second AC / DC converter is connected to the DC port of the first AC / DC converter. Both the first AC / DC converter and the second AC / DC converter adopt a three-phase four-bridge arm structure, including an A-phase bridge arm, a B-phase bridge arm, a C-phase bridge arm, and an N-phase bridge arm. The N-phase bridge arm is used to independently adjust the zero-sequence voltage and provide a zero-sequence current path. The controller is used to collect the voltage and current parameters of the main transformer, determine the operating condition of the main transformer based on the voltage and current parameters, and control the operating mode of the first AC / DC converter and the second AC / DC converter according to the operating condition. The operating conditions include three-phase balanced heavy load, three-phase unbalanced heavy load, voltage overvoltage, voltage undervoltage, and current harmonic difference. The working modes include: Power sharing mode is used to transfer active power from heavily loaded transformer areas to lightly loaded transformer areas; The overvoltage and undervoltage management modes are used to calculate the required capacity of the device based on the reactive power status of the main transformer and provide reactive power compensation to restore the main transformer voltage to the normal range. The three-phase unbalanced load management mode is used to compensate for zero-sequence current through the N-arm bridge, and combined with phase-to-phase active power transfer and reactive power compensation to achieve dynamic balance of the three-phase load. Harmonic mitigation mode is used to detect harmonic currents and generate a reverse compensation signal to inject into the system.

[0017] Preferably, the controller determines overload based on the capacity of the main transformer. When the demand power exceeds 80% of the capacity of the main transformer, it is determined to be overloaded. When the current of any one phase is greater than the overload phase current threshold and the currents of the three phases are not simultaneously greater than the threshold, it is determined to be a three-phase unbalanced overload.

[0018] Preferably, in the power mutual assistance mode, the controller generates a power adjustment command based on the heavy load detection result, controls the operating status of the first AC / DC converter and the second AC / DC converter, and transfers the active power of the heavy load area to the light load area.

[0019] Preferably, in the overvoltage management mode, the controller controls the device to output inductive reactive power to balance the capacitive reactive power excess based on the required increase in capacity calculated by the reactive power excess calculation device of the main transformer; in the undervoltage management mode, the controller controls the device to output reactive power to supplement the system reactive power based on the voltage drop of the main transformer and the required increase in capacity calculated by the current system load rate calculation device.

[0020] Preferably, in the three-phase unbalanced load management mode, the controller performs three stages: three-phase unbalance detection, power regulation strategy generation, and power execution. The three-phase unbalance detection stage directly compares the amplitude differences of the three-phase currents and calculates the unbalance degree. The power regulation strategy generation stage combines active power transfer and reactive power compensation to generate regulation commands. The power execution stage achieves precise power transfer and compensation through the first AC / DC converter and the second AC / DC converter.

[0021] Preferably, in the harmonic mitigation mode, the controller performs three stages: harmonic detection, compensation signal generation, and compensation signal injection. The harmonic detection stage uses an adaptive filtering algorithm to detect harmonic current in real time. The compensation signal generation stage uses feedback control to generate a compensation signal that is opposite to the system harmonics. The compensation signal injection stage injects the compensation signal into the system.

[0022] Preferably, the DC ports of the first AC / DC converter and / or the second AC / DC converter are connected to distributed new energy sources.

[0023] The present invention also proposes a control method for the aforementioned flexible interconnection device in the power distribution network, comprising: 1) Monitor and collect voltage and current data on the secondary side of the main transformer and transmit them to the controller of the flexible interconnection device; 2) Based on the collected data, the operating status of the main transformer is divided into six conditions: three-phase balanced light load, three-phase balanced heavy load, three-phase unbalanced heavy load, voltage overvoltage, voltage undervoltage, and current harmonic difference. For each condition, a corresponding flexible interconnection device working mode is formulated. 3) The controller selects the working mode of the low-voltage flexible interconnection device according to the working condition of the main transformer and puts it into operation on the power grid. Under the three-phase unbalanced heavy load condition, the controller controls the N-arm to independently output zero-sequence compensation current to directly compensate for the zero-sequence current component. 4) When the transformer in the distribution area fails, the distributed power source will connect to the grid via the DC port to supply AC400V power to the distribution area to meet the power supply requirements.

[0024] Compared with the prior art, the advantages of the present invention are as follows: This invention provides functions for overvoltage and undervoltage control of main transformers, three-phase imbalance control (single-phase heavy load control), and harmonic control, thereby improving the reliability of power supply in the distribution network. Attached Figure Description

[0025] Figure 1 This is a structural diagram of a flexible interconnect device system.

[0026] Figure 2 This is the operating logic for flexible interconnect devices. Detailed Implementation

[0027] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0028] Current low-voltage flexible interconnect devices have the following problems: 1. Unable to resolve issues such as phase-to-phase load imbalance and single-phase overload in the main transformer; 2. Overvoltage and undervoltage issues of the main transformer; 3. Online harmonic mitigation is not possible.

[0029] This embodiment proposes a flexible interconnection device for power distribution networks, which can directly compensate for zero-sequence current, improve the ability to manage unbalanced loads (single-phase heavy loads), alleviate overvoltage and undervoltage problems of main transformers, and realize online harmonic control functions.

[0030] like Figure 1 As shown, the flexible interconnection device for the distribution network consists of three parts: AC / DC converter 1, AC / DC converter 2, and a controller. The AC port of AC / DC converter 1 is connected to the secondary side of the parallel expansion transformer of the AC400V power grid, and its DC port is connected to the DC port of AC / DC converter 2. The AC port of AC / DC converter 2 is connected to the main transformer of the AC400V power grid. Based on the power grid load requirements, the controller controls the operating status of AC / DC converter 1 and AC / DC converter 2 to meet the power demands of the power grid load.

[0031] like Figure 1 As shown, the main transformer is located between 10kV and 400V, and the "parallel capacity expansion transformer" refers to this main transformer. The primary side of the parallel capacity expansion transformer is connected in parallel with the primary side of the main transformer to the same 10kV busbar, and its secondary side outputs 400V AC power, which is connected to the AC port of AC / DC converter 1. The AC port of AC / DC converter 2 is directly connected to the AC 400V distribution network. The basic working principle of the system is as follows: In the flexible interconnection device of the distribution network, the AC ports of the two AC / DC converters are connected to two AC400V transformer substations, and the DC ports can be connected to distributed new energy sources (such as energy storage batteries). The controller collects the voltage, current and other parameters of the main transformer and classifies the main transformer operating conditions into six conditions based on the voltage and current parameters: three-phase balanced light load, three-phase balanced heavy load, three-phase unbalanced heavy load, voltage overvoltage, voltage undervoltage, and current harmonic difference. The controller selects the corresponding working mode for the low-voltage flexible interconnection device according to the different main transformer operating conditions, so as to realize the functions of power mutual assistance between the two transformer substations, overvoltage and undervoltage management of the main transformer, three-phase unbalanced load management, and harmonic management.

[0032] In this embodiment, both AC / DC converter 1 and AC / DC converter 2 adopt a three-phase four-arm structure, including phase A arm, phase B arm, phase C arm, and N arm. Compared with the traditional three-phase three-arm structure, the N arm added in this embodiment can independently adjust the zero-sequence voltage, effectively suppressing the zero-sequence current. When the three-phase load is severely unbalanced, the N arm can actively inject zero-sequence compensation current into the system to ensure phase voltage balance.

[0033] Taking phases A and B each at 10kW and phase C at 50kW as an example, the total power demand is 70kW. The total power does not reach the level of heavy overload, but phase C is already in a single-phase heavy overload state. Traditional three-phase three-arm topologies can only achieve overall power regulation and cannot perform separate compensation for phase C. In this embodiment, after the controller detects the three-phase unbalanced heavy load condition, it starts the three-phase unbalanced load management program. It generates a zero-sequence compensation current command independently through the N-arm and injects the zero-sequence current into the system. At the same time, combined with inter-phase active power transfer and reactive power compensation, the excess load of phase C is transferred to other phases or another transformer area through power mutual assistance, achieving precise management of single-phase heavy load.

[0034] In this embodiment, the controller of the flexible interconnection device of the distribution network collects the phase voltages UA, UB, and UC of the secondary side of the main transformer, as well as the currents IA, IB, IC, and IN of ABCN. Based on the voltage and current data and in conjunction with the standard, the operating conditions are divided. 1) Three-phase balanced light load and heavy load: 80% of the main transformer capacity S is used as the boundary between light load and heavy load. If the required power is greater than 80% of S, it is considered heavy load, and if it is less than 80% of S, it is considered light load. Three-phase balanced light load is the normal operating condition and no action is required. 2) Three-phase unbalanced overload: It is known that when the power reaches 80%S, it is a heavy load. At this time, the phase current is Iz. When the current of any one phase is greater than Iz (the three phase currents are not greater than Iz at the same time), it is a three-phase unbalanced overload. 3) Overvoltage and undervoltage: The rated phase voltage is 220V. When the voltage is more than 220V±10%, it is judged as overvoltage or undervoltage. 4) Current harmonic difference: When the harmonics exceed the set threshold, it is judged as current harmonic difference; In this embodiment, the flexible interconnection device for the power distribution network has the following functions under different operating conditions: Power mutual assistance function between distribution transformers: In the power mutual assistance mode, the controller generates a power adjustment command based on the overload detection result, controls the operating status of the first AC / DC converter and the second AC / DC converter, and transfers the active power of the overloaded distribution transformer to the lightly loaded distribution transformer. In this embodiment, the flexible interconnection device of the distribution network connects two distribution transformers. If at a certain moment the load rate of the first distribution transformer is low and the load of the second distribution transformer is high, and the transformer is in a state of heavy overload, the controller controls the power to flow from the end with lower load power to the end with higher load power by adjusting the operating status of AC / DC converter 1 and AC / DC converter 2, thereby realizing the power mutual assistance function between the two distribution transformers and solving the problem of transformer heavy overload.

[0035] In power sharing mode, the device supports both automatic and manual control. In automatic control mode, the device automatically analyzes data collected from both power grids and makes logical judgments based on the load rates of the two busbars: when the first transformer area's busbar is heavily loaded and the second transformer area's busbar is lightly loaded, the control device transfers power from the second transformer area to the first; conversely, it does the opposite. When both busbars are under light or heavy load conditions, the device automatically analyzes and compares the load rates of both busbars and controls the automatic power transfer from the less loaded side to the more loaded side. In manual control mode, the device no longer operates according to automatic logic but strictly follows dispatch instructions to achieve power transfer between the two transformer areas.

[0036] Based on the power balance function, heavy load and overload management can be achieved: a main transformer overload refers to a load exceeding 80% of its capacity or even exceeding its rated capacity; this device reduces the load on the main transformer by transferring active power between different transformer substations (power balance), the specific process of which is as follows: 1) Overload detection: Real-time monitoring of the main transformer's output power; when it exceeds 80% of its capacity, overload and overload mitigation measures are implemented. 2) Power Regulation Strategy: Based on the heavy load and overload detection results, power regulation commands are generated to perform power adjustment and active power transfer. Active power from heavily loaded and overloaded transformer areas is transferred to lightly loaded transformer areas to balance supply and demand. Through power adjustment, the heavy load and overload situation is improved. 3) Power regulation: This device enables bidirectional transfer of active power between different stations.

[0037] Overvoltage and undervoltage mitigation of the main transformer: If, at a certain moment, the controller detects that the voltage of the main transformer reaches the overvoltage or undervoltage threshold, the low-voltage flexible interconnection device provides reactive power compensation to restore the main transformer voltage to the normal voltage range, thus achieving the purpose of overvoltage and undervoltage mitigation. This includes: Overvoltage management: The essence of overvoltage is that there is too much capacitive reactive power and insufficient inductive reactive power in the system. Therefore, when the main transformer is judged to be in an overvoltage condition, the flexible interconnection device calculates the additional capacity required by the device based on the excess reactive power of the main transformer, and controls the device to output the corresponding reactive power, thereby increasing the inductive reactive power demand of the system, balancing the excess capacitive reactive power, and achieving the purpose of overvoltage management. Undervoltage management: The essence of undervoltage is insufficient capacitive reactive power and excessive inductive reactive power in the system. Therefore, when the main transformer is judged to be undervoltage or overvoltage, it is necessary to improve the reactive power supply capacity of the system and supplement the reactive power of the system. The flexible interconnection device calculates the required additional capacity based on the voltage drop of the main transformer and the current system load rate, and controls the device to output the corresponding reactive power to achieve the purpose of undervoltage management.

[0038] Therefore, in the overvoltage management mode, the controller controls the device to output inductive reactive power to balance the capacitive reactive power excess based on the required increase in capacity calculated by the reactive power excess calculation device of the main transformer; in the undervoltage management mode, the controller controls the device to output reactive power to supplement the system reactive power based on the voltage drop of the main transformer and the required increase in capacity calculated by the current system load rate calculation device.

[0039] Three-phase unbalanced load management: If, at a certain moment, a three-phase imbalance of voltage and current occurs due to the connection of large single-phase equipment or uneven load distribution, such as a single-phase heavy load, the low-voltage flexible interconnection device will activate the unbalanced working condition management program to provide active power support and reactive power compensation, and adjust the voltage and current to a three-phase balanced state. Three-phase unbalanced load management achieves dynamic balance of the three-phase load of the main transformer through phase-to-phase active power transfer and reactive power compensation, involving three stages: 1) Three-phase imbalance detection process: Directly compare the amplitude differences of the three-phase currents to calculate the degree of imbalance; 2) Power regulation strategy generation stage: Combining active power transfer and reactive power compensation to achieve the desired governance effect; 3) Power execution stage: This device enables precise power transfer and compensation, and can directly compensate for zero-sequence current, thereby improving the ability to manage three-phase imbalance. Therefore, in the three-phase unbalanced load management mode, the controller performs three stages: three-phase unbalance detection, power regulation strategy generation, and power execution. The three-phase unbalance detection stage directly compares the amplitude differences of the three-phase currents and calculates the unbalance degree. The power regulation strategy generation stage combines active power transfer and reactive power compensation to generate regulation commands. The power execution stage injects zero-sequence compensation current into the system through the N-arm and achieves precise power transfer and compensation through the first AC / DC converter and the second AC / DC converter.

[0040] Harmonic mitigation: When the controller detects that the current harmonics have reached the compensation threshold, the low-voltage flexible interconnect device performs harmonic compensation to improve the harmonic situation; Harmonic mitigation consists of three stages: harmonic detection, compensation signal generation, and compensation signal injection.

[0041] 1) Harmonic detection: The device adopts an adaptive filtering algorithm to automatically track the harmonic changes in the system and detect the harmonic current in the system in real time. When it exceeds the set threshold, it is determined to be a current harmonic difference. 2) Compensation signal generation: Feedback control is used to generate a compensation signal that is opposite to the system harmonics based on the detected harmonic current; 3) Compensation signal injection: The device injects the compensation signal through the four bridge arms A, B, C, and N, thereby simultaneously suppressing positive sequence harmonics, negative sequence harmonics, and zero sequence harmonics, and improving current harmonics. Therefore, in the harmonic mitigation mode, the controller performs three stages: harmonic detection, compensation signal generation, and compensation signal injection. The harmonic detection stage uses an adaptive filtering algorithm to detect harmonic current in real time. The compensation signal generation stage uses feedback control to generate a compensation signal opposite to the system harmonics. The compensation signal injection stage injects the compensation signal into the system through the A-phase bridge arm, B-phase bridge arm, C-phase bridge arm, and N-phase bridge arm to simultaneously suppress positive-sequence harmonics, negative-sequence harmonics, and zero-sequence harmonics.

[0042] like Figure 1 As shown, the DC ports of AC / DC converter 1 and AC / DC converter 2 in the low-voltage flexible interconnection device are configured with distributed new energy access ports, which can input new energy power into the grid. Depending on whether there is a distributed power source on site, it can be decided whether to retain the DC port connected to the distributed power source.

[0043] like Figure 2 As shown, the operating logic of the flexible interconnection device for the distribution network in this embodiment is as follows: 1) Monitor and collect voltage and current data on the secondary side of the main transformer and transmit them to the controller of the flexible interconnection device; 2) Based on the collected data, the operating status of the main transformer is divided into six conditions: three-phase balanced light load, three-phase balanced heavy load, three-phase unbalanced heavy load, voltage overvoltage, voltage undervoltage, and current harmonic difference. For each condition, a corresponding flexible interconnection device working mode is formulated. 3) The controller selects the working mode of the low-voltage flexible interconnection device according to the working condition of the main transformer and puts it into operation on the power grid. Under the three-phase unbalanced heavy load condition, the controller controls the N-arm to independently output zero-sequence compensation current to directly compensate for the zero-sequence current component. 4) When the transformer in the distribution area fails, the distributed power source will connect to the grid via the DC port to supply AC400V power to the distribution area to meet the power supply requirements.

[0044] In another preferred embodiment, the flexible interconnection device for the distribution network is also equipped with a DC-side ultra-fast solid-state protection subsystem. This subsystem includes a SiC MOSFET solid-state circuit breaker (SSCB) connected in series with the positive (P) terminal of the DC bus, a mechanical bypass contactor (KM_BP) connected in parallel with the solid-state circuit breaker, and an ultra-fast fuse (FU) connected in series between the DC bus and the DC capacitor bank. The protection detection circuit includes two independent hardware detection channels: Channel A is for DC voltage transient detection, consisting of a high-speed resistor divider and a dual-threshold voltage comparator; Channel B is for current rate of change (di / dt) detection, consisting of a Rogowski coil and a passive differentiating circuit. The output signals of the two detection channels are ORed to directly drive the gate drive circuit of the solid-state circuit breaker and the hard-blocking pins of all IGBTs in the two converters.

[0045] During normal operation, the mechanical bypass contactor closes to reduce conduction losses. When a short circuit occurs, the voltage transient detection circuit detects the DC voltage drop within 0.5μs, or the di / dt detection circuit detects a change rate exceeding 5kA / μs. The SiC MOSFET gate is directly driven via hard-wired logic without the need for ADC sampling or software interruption. The total detection time is << 1μs, and the solid-state circuit breaker is completely turned off within 3-5μs, which is much shorter than the short-circuit withstand time of the IGBT (5-15μs), achieving non-destructive removal.

[0046] In addition, the device can be configured with a graded derating thermal management subsystem based on junction temperature monitoring. A junction temperature monitoring unit is integrated within each IGBT module. An independent MCU reads the junction temperature and performs graded derating (100%→75%→50%→25%→shutdown) via hardware power limiting circuitry. Direct junction temperature monitoring response is <<1ms, providing an earlier warning than radiator monitoring; it is independent of the main controller, so derating protection remains effective even if the main controller crashes; the five-level derating allows the device to continue operating at reduced power under overheating conditions instead of shutting down directly, significantly improving power supply continuity.

[0047] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-readable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create a machine for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to operate in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0048] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A flexible interconnection device for power distribution networks, characterized in that, It includes a first AC / DC converter, a second AC / DC converter, and a controller. The AC port of the first AC / DC converter is connected to the secondary side of the parallel expansion transformer of the distribution substation. The AC port of the second AC / DC converter is connected to the secondary side of the main transformer of the distribution substation. The primary side of the parallel expansion transformer is connected in parallel with the primary side of the main transformer and connected to the power grid. The DC port of the second AC / DC converter is connected to the DC port of the first AC / DC converter. Both the first AC / DC converter and the second AC / DC converter are three-phase four-bridge-arm structures, including three-phase bridge arms A, B, and C and an N-bridge arm. The N-bridge arm is used to independently adjust the zero-sequence voltage and compensate for the zero-sequence current. The controller is used to collect the voltage and current parameters of the main transformer, determine the operating condition of the main transformer based on the voltage and current parameters, and control the operating mode of the first AC / DC converter and the second AC / DC converter according to the operating condition. The operating conditions include three-phase balanced heavy load, three-phase unbalanced heavy load, voltage overvoltage, voltage undervoltage, and current harmonic difference. The working modes include: Power sharing mode is used to transfer active power from heavily loaded transformer areas to lightly loaded transformer areas; Overvoltage and undervoltage management modes are used to calculate the required capacity of the device based on the reactive power status of the main transformer and provide reactive power compensation to restore the main transformer voltage to the normal range. The three-phase unbalanced load management mode is used to compensate for zero-sequence current through the N-arm bridge, and combined with phase-to-phase active power transfer and reactive power compensation to achieve dynamic balance of the three-phase load. Harmonic mitigation mode is used to detect harmonic currents and generate a reverse compensation signal to inject into the system.

2. The flexible interconnection device for power distribution networks according to claim 1, characterized in that, The controller determines overload based on the main transformer capacity. When the demand power exceeds 80% of the main transformer capacity, it is determined to be overloaded. When any phase current is greater than the overload phase current threshold and the three phase currents are not simultaneously greater than the threshold, it is determined to be a three-phase unbalanced overload.

3. The flexible interconnection device for power distribution networks according to claim 1, characterized in that, In the power mutual assistance mode, the controller generates a power adjustment command based on the heavy load detection result, controls the operating status of the first AC / DC converter and the second AC / DC converter, and transfers the active power of the heavy load area to the light load area.

4. The flexible interconnection device for power distribution networks according to claim 1, characterized in that, In the overvoltage management mode, the controller, based on the required increase in capacity calculated by the main transformer's reactive power calculation device, controls the device to output inductive reactive power to balance the capacitive reactive power excess; in the undervoltage management mode, the controller, based on the voltage drop of the main transformer and the required increase in capacity calculated by the current system load rate, controls the device to output reactive power to supplement the system's reactive power.

5. The flexible interconnection device for power distribution networks according to claim 1, characterized in that, In the three-phase unbalanced load management mode, the controller performs three stages: three-phase unbalance detection, power regulation strategy generation, and power execution. The three-phase unbalance detection stage directly compares the amplitude differences of the three-phase currents and calculates the unbalance degree. The power regulation strategy generation stage combines active power transfer and reactive power compensation to generate regulation commands. The power execution stage achieves precise power transfer and compensation through the first AC / DC converter and the second AC / DC converter.

6. The flexible interconnection device for power distribution networks according to claim 1, characterized in that, In the harmonic mitigation mode, the controller performs three stages: harmonic detection, compensation signal generation, and compensation signal injection. The harmonic detection stage uses an adaptive filtering algorithm to detect harmonic current in real time. The compensation signal generation stage uses feedback control to generate a compensation signal that is opposite to the system harmonics. The compensation signal injection stage injects the compensation signal into the system.

7. The flexible interconnection device for power distribution networks according to claim 1, characterized in that, The DC ports of the first AC / DC converter and / or the second AC / DC converter are connected to distributed new energy sources.

8. The control method for the flexible interconnection device of the distribution network according to any one of claims 1 to 7, characterized in that, include: 1) Monitor and collect voltage and current data on the secondary side of the main transformer and transmit them to the controller of the flexible interconnection device; 2) Based on the collected data, the operating status of the main transformer is divided into six conditions: three-phase balanced light load, three-phase balanced heavy load, three-phase unbalanced heavy load, voltage overvoltage, voltage undervoltage, and current harmonic difference. For each condition, a corresponding flexible interconnection device working mode is formulated. 3) The controller selects the working mode of the low-voltage flexible interconnection device according to the working condition of the main transformer and puts it into operation on the power grid. Under the three-phase unbalanced heavy load condition, the controller controls the N-arm to independently output zero-sequence compensation current to directly compensate for the zero-sequence current component. 4) When the transformer in the distribution area fails, the distributed power source will connect to the grid via the DC port to supply AC400V power to the distribution area to meet the power supply requirements.

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