Transformer area flexible interconnection device with three-phase imbalance treatment function and electric energy quality comprehensive treatment method

Abstract

The invention discloses a transformer area flexible interconnection device with a three-phase imbalance treatment function and an electric energy quality comprehensive treatment method. The transformer area flexible interconnection device comprises two three-bridge-arm converter modules, a common all-solid-state energy storage battery, two alternating current ports, a power grid voltage synchronous acquisition unit and an energy management system which are integrated. During normal operation, the two converter modules are controlled by the energy management system to realize mutual assistance of active power and reactive voltage optimization between stations; and meanwhile, the converter module can be controlled to output specific compensation current, and three-phase current imbalance of a single transformer area is dynamically treated. When a power grid breaks down, the device can switch the converter module into a network construction control mode, the converter module and the all-solid-state energy storage battery cooperate to form an autonomous micro-grid, and power supply of a key load is guaranteed; and automatic synchronous grid connection can be realized after the power grid is recovered. According to the invention, flexible interconnection of transformer areas, comprehensive treatment of electric energy quality and uninterrupted power supply of islands are realized by using a single device, and the balance, reliability and electric energy quality of a power distribution network are improved.

Description

Technical Field

[0001] This invention relates to the field of power distribution network technology, specifically to a flexible interconnection device for distribution transformer areas with three-phase imbalance management function and a comprehensive power quality management method, which is particularly suitable for flexible interconnection systems for low-voltage distribution transformer areas to achieve power mutual assistance, load balancing, reactive power and voltage optimization, and three-phase imbalance management. Background Technology

[0002] The power distribution network is a crucial link in energy production, conversion, and consumption. It also serves as a support platform for renewable energy consumption, a platform for diverse and massive information data, and a trading platform for multiple market participants, occupying a core position in the entire smart grid system. With economic growth and improved living standards, the increasing electricity consumption in rural areas and the uneven development of distributed photovoltaic power generation have resulted in uneven loads on transformers in different areas, with some transformers being overloaded and others underloaded.

[0003] In response to the increasingly complex electricity demand and the integration of new energy sources, the government is increasing investment in the construction and upgrading of traditional power distribution networks. However, the investment in transformer expansion is huge, and it is necessary to economically solve the problem of uneven load on transformers in different areas.

[0004] Distributed energy storage is an effective way to solve problems such as renewable energy consumption, power quality, and grid congestion. With the continuous development of all-solid-state battery technology and products, facilities such as distribution area energy storage and home energy storage based on all-solid-state batteries are expected to be widely used, giving rise to new distribution area operation modes. Distributed energy storage serves as the main power source to meet the main power demand of the distribution area, while the distribution network plays a backup role. However, existing energy storage systems mostly focus on single functions, such as peak shaving and valley filling or local voltage support, lacking a systematic solution for multi-distribution area coordination, flexible mutual assistance, and comprehensive management. Therefore, there is an urgent need for a flexible interconnection device and control method that can achieve flexible power allocation between distribution areas, support autonomous operation of distribution areas, and have the ability to manage three-phase imbalances, in order to improve the operating efficiency, reliability, and power quality of the distribution network. Summary of the Invention

[0005] To address the aforementioned issues, this invention proposes a flexible interconnection device for transformer substations with three-phase imbalance mitigation capabilities and its control technology. The invention provides a corresponding topology and control method. Through the AC port of this device connected to the low-voltage side outlet of the transformer substation, load balancing and mutual assistance of multiple transformers are achieved. In the event of a grid fault, the autonomous operation of the transformer substation is realized through the grid control mode of the converter module of this device.

[0006] The technical solution of the present invention is as follows: On the one hand, the present invention provides a flexible interconnection device for transformer substations with three-phase imbalance mitigation function, characterized in that it includes: Two three-arm converter modules are connected to the low-voltage side feeders of the first and second area transformers respectively through the first AC port and the second AC port. A shared all-solid-state energy storage battery is connected in parallel with the DC side of the two converter modules via a DC bus; a grid voltage synchronization signal acquisition unit has its input terminal connected to the low-voltage side of the first or second zone transformer. An energy management system is communicatively connected to the two converter modules, the management system of the all-solid-state energy storage battery, and the grid voltage synchronization signal acquisition unit, respectively. The two three-bridge converter modules, the all-solid-state energy storage battery, and the energy management system are integrated in the same cabinet, forming an integrated flexible interconnection device for the transformer substation with bidirectional power flow control, power quality management, and islanding operation capabilities.

[0007] Furthermore, the input terminal of the grid voltage synchronization signal acquisition unit is connected to the power supply side of the low-voltage side main switch of the transformer through a voltage transformer, and is used to acquire the amplitude, frequency and phase information of the grid voltage, and serve as the reference for the grid-connected synchronization or self-synchronization of the converter module.

[0008] Furthermore, the energy management system is configured to perform coordinated control in at least one of the following modes: The power mutual assistance and voltage optimization mode is used to control the two converter modules to perform active power transfer and reactive power dynamic support between the two transformer modules in grid-connected state. The three-phase unbalanced active compensation mode is used to control the converter module connected to the target transformer to output a specific compensation current to offset the unbalanced components of the three-phase load of the transformer. The grid-type islanded operation mode is used to switch the two converter modules to voltage source mode in the event of a grid failure. Together with the all-solid-state energy storage battery, it provides autonomous and stable voltage and frequency to the connected transformer areas, forming an autonomous microgrid.

[0009] Furthermore, under the scheduling of the energy management system, the all-solid-state energy storage battery performs at least one of the following functions: As a real-time energy buffer unit for power exchange between substations, it absorbs or releases instantaneous power differences to maintain a constant DC bus voltage. As the power source for three-phase imbalance mitigation, it provides the necessary instantaneous active and reactive power support for the compensation current output by the converter module; As the main supporting power source in the grid islanding mode, it provides continuous power to all or part of the critical loads during grid failures.

[0010] On the other hand, the present invention also provides a control system for the above-mentioned flexible interconnection device for transformer substations, characterized in that it includes: The mode switching module is used to seamlessly switch between power mutual assistance mode, three-phase imbalance management mode and grid islanding mode based on grid status, transformer area load and power quality information; The power control module is used to control the converter module to achieve precise power transmission in power mutual assistance mode based on the dual closed-loop vector control algorithm and according to the active and reactive power commands issued by the energy management system. The unbalanced compensation calculation module is used to calculate the three-phase compensation current command to offset the negative sequence and zero sequence components in real time based on the collected three-phase voltage and current of the transformer under the three-phase unbalanced management mode, through instantaneous power theory and coordinate transformation. The grid control module is used to autonomously generate the amplitude, frequency and phase signals of the converter module output voltage based on the active power-frequency droop and reactive power-voltage droop control algorithms in the grid island mode, so as to maintain the stable operation of the island microgrid. The self-synchronizing grid connection module is used to automatically adjust the output of the grid control module based on the information from the grid voltage synchronization signal acquisition unit after the grid is restored, so as to synchronize it with the grid voltage and complete the shockless grid connection operation.

[0011] Third, this invention provides a method for flexible interconnection of power distribution areas and comprehensive power quality management, characterized in that it is applied to the above-mentioned device and includes the following steps: S1: Real-time acquisition of load data, power quality data, and status data of the all-solid-state energy storage battery from the two interconnected transformer areas; S2: Based on the collected data, the energy management system determines the current power grid status and optimization objectives, and selects and enters one of the following dominant operation modes: power mutual assistance mode, three-phase imbalance management mode, or grid islanding mode. S3: Executes the control algorithm corresponding to the selected dominant operating mode and coordinates other auxiliary functions, including: When the power mutual assistance mode is executed, the power transmission command between the two converter modules is dynamically adjusted, and the voltage level at the grid connection point is monitored, supplemented by reactive power regulation. When the three-phase imbalance mitigation mode is executed, the control current output of the current-straining module is adjusted to compensate for the current, while monitoring and maintaining the DC bus voltage stability. When the grid islanding mode is executed, the control converter module and energy storage battery work together to maintain the stability of the island voltage frequency, while monitoring the grid side status and preparing for synchronous grid connection. S4: Based on changes in system status, the energy management system dynamically switches or mixes control commands of different modes to achieve the composite functional objectives of load balancing in transformer areas, three-phase imbalance management, reactive power and voltage optimization, and uninterrupted power supply.

[0012] Furthermore, the specific implementation steps of the three-phase imbalance control mode include: S31: Sample the three-phase voltage and current of the target transformer and decompose them into positive-sequence, negative-sequence, and zero-sequence components using α-β coordinate transformation; S32: Based on the components, the instantaneous negative sequence power and zero sequence power characterizing the three-phase imbalance are calculated; S33: Combine the negative sequence power and zero sequence power with the power loss of the converter module and the power adjustment required to maintain the stability of the DC bus voltage, and calculate the reference value of the three-phase compensation current that the converter module should output. S34: Using current tracking control technology, the converter module is driven to output the three-phase compensation current, thereby dynamically offsetting the unbalanced current component on the load side of the transformer.

[0013] Furthermore, the switching process between the network islanding mode and the self-synchronized grid connection includes: S32: When an abnormal grid voltage is detected, immediately switch the control strategy of the two converter modules from grid current control to grid voltage control, and start the active-frequency and reactive-voltage droop control algorithms. S33: During islanded operation, the all-solid-state energy storage battery serves as the main power source, providing power support to the converter module through the DC bus, and dynamically adjusting the output according to the droop coefficient and load demand; S34: When a stable and qualified grid voltage is detected again through the grid voltage synchronization signal acquisition unit, the self-synchronization process is started: the voltage amplitude and frequency setpoint in the grid control are gradually adjusted to approach the grid voltage value, while phase closed-loop tracking is performed at the same time. S35: When the voltage amplitude difference, frequency difference and phase difference are all less than the preset threshold, control the closing of the low-voltage side grid connection switch of the corresponding transformer, and then smoothly switch the control strategy of the converter module back to the grid-connected power control mode.

[0014] The present invention also provides a distribution substation system, characterized in that it includes at least two distribution substation transformers and the above-described flexible interconnection device for substations. The device is connected between the low-voltage sides of the transformers in the at least two distribution substations to achieve flexible interconnection, comprehensive power quality management, and island autonomy between the at least two distribution substations.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) By combining dual converter modules, all-solid-state energy storage and a unified energy management system, a single device can simultaneously achieve four major functions: flexible interconnection of distribution areas, power quality management, islanded autonomy and smooth grid-connected switching. The system has a high degree of integration, reducing redundant investment in equipment and installation space.

[0016] 2) Employing instantaneous power theory and α The detection and compensation algorithm based on β coordinate transformation can quickly and accurately separate unbalanced components and achieve dynamic and active compensation through the converter module, directly improving power quality from the source with significant governance effects.

[0017] 3) Achieve load balance and mutual assistance between transformers, self-governance of transformer areas, and optimization of reactive power-voltage; and manage three-phase imbalance. Attached Figure Description

[0018] Figure 1 This is a structural diagram of the topology device of the present invention.

[0019] Figure 2 This is a block diagram of the device power control under normal operating conditions of the present invention.

[0020] Figure 3 This is a block diagram of the network control of the device of the present invention.

[0021] Figure 4 This is a block diagram of the three-phase imbalance control device of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to the embodiments and accompanying drawings, but this should not be construed as limiting the scope of protection of the present invention.

[0023] A flexible interconnection device for transformer substations with three-phase imbalance mitigation function mainly includes two converter modules, an all-solid-state energy storage battery, two AC ports, a voltage acquisition port, and an energy management system (EMS). The AC terminals of the two converter modules are connected to the back ends of the low-voltage side switches of two transformer substations in the substation area via their respective AC ports. The DC terminals of the two converter modules are connected in parallel and connected to the output terminal of the all-solid-state battery. The voltage acquisition port is connected to the front end of the low-voltage side switch of one transformer substation in the substation area. The energy management system is connected to the signal terminals of the two converter modules and the all-solid-state energy storage battery via its corresponding communication ports, and collects external transformer substation power information through its corresponding communication ports. The flexible interconnection device and its control technology are characterized by the following steps: 1) Two converter modules: both are three-arm topologies, each arm consists of two power electronic switching silicon carbide devices connected in series, and the two converter modules are connected through a DC bus; 2) Power control technology at the AC output end of the device Control the active and reactive power on the AC side of the converter module: i) Outer loop power PI control First, calculate the reference values ​​of the d-axis and q-axis currents on the AC side of the converter module. , : (1) in, P ref , P These are the given and actual measured values ​​of the active power on the AC side of the converter module, respectively. Q ref , Q These are the given and actual measured values ​​of reactive power on the AC side of the converter module, respectively. k p1 , k i1 The outer loop power PI control coefficient; ii) Inner loop current PI control Then calculate the d-axis and q-axis voltage control quantities of the converter module. u d , u q (2) in, i d , i q These are the actual measured values ​​of the converter module current along the d and q axes, respectively. oh s For grid voltage frequency, u s This refers to the amplitude of the AC line voltage on the converter module. i s The AC side voltage phase of the converter module. L This refers to the filter inductance value of the converter module; k p2 , k i2 This refers to the inner loop current control coefficient. iii) Pulse width modulation signal Based on the above steps, the d-axis and q-axis voltage control quantities of the AC side of the converter module are obtained. u d , u q This leads to the three vector pulse width control signals for the three arms of the converter module. d a , d b , d c ; Right now (3) Based on the active power setpoints of the two converter modules provided by the energy management system. P ref With reactive power setpoint Q ref Calculate the 6 PWM pulse vector control signals of the two converter modules corresponding to the above formula (3). The two converter modules of the present invention output the corresponding active power and reactive power through their corresponding AC ports to adjust the load power of the two transformers and realize the load balance of the transformers and the reactive power-voltage optimization. 3) Three-phase imbalance control The collected three-phase AC voltage of the low-voltage side transformer in the distribution area u a , u b , u c Taiwan Transformer Current i a , i b , i c ,through α-β Coordinate transformation, to obtain α-β Transformer voltage components in coordinate system v α , v β , v 0 and transformer current component i α , i β , i 0: (4) (5) Calculator converts active power p reactive power q and the zero-sequence component generated by imbalance p 0: (6) Active power p After high-pass filtering, the high-frequency component of the active power is obtained. ; Let the operating loss of the converter module be... p loss ,but (7) In the formula, U dcref , Udc These are the given and measured values ​​of the DC bus voltage, respectively. k p , k i The PI control coefficient for the DC bus voltage; then (8) Then it can be calculated using the following formula. α-β Current reference value in coordinate system , : (9) Therefore, through α-β Inverse coordinate transformation yields the current reference value in the abc coordinate system. , , (10) The pulse width modulation signal of the three bridge arms of the converter module of this device can be obtained by passing the three current reference quantities through the current hysteresis comparator. The converter module of this invention outputs the corresponding compensation current through the AC port to achieve three-phase balance of the transformer. 4) Grid control mode under power grid faults Switch both converter modules to grid-connected control mode. First, establish droop control for the active power and frequency, and reactive power and voltage of the converter modules, as shown in the following formula: (11) The voltage phase angle is obtained as follows: (12) In the above formula, E , f , i These represent the voltage amplitude, frequency, and phase angle under the converter module network control mode. f s0 50Hz m p , n q These are the droop coefficients for active power and frequency, and reactive power and voltage, respectively. oh =2π f ; Therefore, three pulse vector control signals for the three arms of the converter module are obtained. d a , d b , d c : (13) In the above formula, the phase of the grid voltage is i 0 o'clock, time t Start timing from 0; Before the low-voltage side switch of the distribution area is disconnected, the phase of the pulse control signal is kept synchronized with the grid voltage. Therefore, this device outputs an AC voltage synchronized with the grid voltage, providing reliable power to the load of the power supply line; After the fault ends, the device performs a synchronization operation based on the grid voltage sensed by the voltage acquisition port before the switch, so that the voltage output by the device is synchronized with the grid. Then the power supply line switches to grid power supply, and the device switches to power control mode. 5) Energy Management System (EMS) Functions: EMS collects power information from the two converter modules, the all-solid-state battery, and the transformer via its corresponding communication ports to manage the energy of the entire device. Its strategy is as follows: i) Transformer load balancing and mutual assistance strategy Based on the load difference of the transformers, EMS calculates the adjustment amount and sends active power setpoints to the two converter modules, so as to balance the active power of the transformer with excessive load to the transformer with light load, thereby achieving load balancing and mutual assistance between transformers. ii) Reactive power-voltage optimization Based on the grid voltage, EMS sends reactive power setpoints to the two converter modules to perform reactive power-voltage optimization; iii) Three-phase imbalance management strategy for transformers If the three phases of the transformer are unbalanced, the converter module connected to the transformer is commanded to perform the corresponding three-phase imbalance mitigation according to step 3). iv) Power grid fault strategies When EMS detects a grid fault, it sends a command to all converter modules in the device. After receiving the command, the converter modules switch to grid-based control mode to achieve autonomous operation of the distribution area. When the device senses that the grid has resumed normal power supply through its voltage acquisition port, it sends a command to the converter modules to automatically synchronize with the grid. After the synchronization is completed, the device returns to normal operation mode.

[0024] See Figure 1 , Figure 1 This is a topology diagram of the invention. As shown in the diagram, the flexible interconnection device for the transformer substation mainly includes two converter modules, an all-solid-state energy storage battery, two AC ports, a voltage acquisition port, and an energy management system (EMS). The low-voltage sides of the two transformers in the substation are flexibly interconnected through the AC ports of this device. The AC ports of the device are connected to the downstream end of the low-voltage side switch of the transformer, and the voltage acquisition port of the device is connected to the upstream end of the low-voltage side switch of the transformer to acquire the grid voltage synchronization signal. The EMS performs unified energy management of the source, grid, load, and storage.

[0025] Figure 2 This is a block diagram of the device power control under normal operating conditions of the present invention. As shown in the figure, the EMS determines the active power of the device converter module based on the load condition and voltage level of the transformer in the distribution area. P ref reactive power Q ref The system generates dq-axis current reference commands through outer-loop power control, then generates dq-axis voltage commands through inner-loop current control, and finally generates PWM signals to control the converter module. Therefore, by providing active power, load balancing and mutual assistance are achieved in the transformer. Reactive power-voltage optimization is achieved by providing reactive power.

[0026] Figure 3 This is a control block diagram of the grid-connected control mode of the converter module in this invention. Under grid fault conditions, the converter module of this device switches to grid-connected control mode. Through the droop control of active power-frequency and reactive power-voltage, the amplitude and phase of the converter module's output voltage are obtained, thereby synthesizing a voltage output by the converter module that is synchronized with the grid voltage.

[0027] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention. Where there is no conflict, the above embodiments and features described therein can be combined with each other.

Claims

1. A distribution area flexible interconnection device with three-phase imbalance governance function, characterized in that, include: Two three-arm converter modules are connected to the low-voltage side feeders of the first and second area transformers respectively through the first AC port and the second AC port. A shared all-solid-state energy storage battery is connected in parallel with the DC side of the two converter modules via a DC bus; a grid voltage synchronization signal acquisition unit has its input terminal connected to the low-voltage side of the first or second zone transformer. An energy management system is communicatively connected to the two converter modules, the management system of the all-solid-state energy storage battery, and the grid voltage synchronization signal acquisition unit, respectively. The two three-bridge converter modules, the all-solid-state energy storage battery, and the energy management system are integrated in the same cabinet, forming an integrated flexible interconnection device for the transformer substation with bidirectional power flow control, power quality management, and islanding operation capabilities.

2. The distribution area flexible interconnection device with three-phase imbalance governance function according to claim 1, characterized in that, The input terminal of the power grid voltage synchronization signal acquisition unit is connected to the power supply side of the low-voltage side main switch of the transformer through a voltage transformer. It is used to acquire the amplitude, frequency and phase information of the power grid voltage and serve as the reference for the grid-connected synchronization or self-synchronization of the converter module.

3. The distribution area flexible interconnection device with three-phase imbalance governance function according to claim 1, characterized in that, The energy management system is configured to perform coordinated control in at least one of the following modes: The power mutual assistance and voltage optimization mode is used to control the two converter modules to perform active power transfer and reactive power dynamic support between the two transformer modules in grid-connected state. The three-phase unbalanced active compensation mode is used to control the converter module connected to the target transformer to output a specific compensation current to offset the unbalanced components of the three-phase load of the transformer. The grid-type islanded operation mode is used to switch the two converter modules to voltage source mode in the event of a grid failure. Together with the all-solid-state energy storage battery, it provides autonomous and stable voltage and frequency to the connected transformer areas, forming an autonomous microgrid.

4. The distribution area flexible interconnection device with three-phase imbalance governance function according to claim 1 or 3, characterized in that, Under the scheduling of the energy management system, the all-solid-state energy storage battery performs at least one of the following functions: As a real-time energy buffer unit for power exchange between substations, it absorbs or releases instantaneous power differences to maintain a constant DC bus voltage. As the power source for three-phase imbalance mitigation, it provides the necessary instantaneous active and reactive power support for the compensation current output by the converter module; As the main supporting power source in the grid islanding mode, it provides continuous power to all or part of the critical loads during grid failures.

5. A control system for the flexible interconnection device of any one of claims 1 to 4, characterized in that, include: The mode switching module is used to seamlessly switch between power mutual assistance mode, three-phase imbalance management mode and grid islanding mode based on grid status, transformer area load and power quality information; The power control module is used to control the converter module to achieve precise power transmission in power mutual assistance mode based on the dual closed-loop vector control algorithm and according to the active and reactive power commands issued by the energy management system. The unbalanced compensation calculation module is used to calculate the three-phase compensation current command to offset the negative sequence and zero sequence components in real time based on the collected three-phase voltage and current of the transformer under the three-phase unbalanced management mode, through instantaneous power theory and coordinate transformation. The grid control module is used to autonomously generate the amplitude, frequency and phase signals of the converter module output voltage based on the active power-frequency droop and reactive power-voltage droop control algorithms in the grid islanding mode, so as to maintain the stable operation of the islanded microgrid. The self-synchronizing grid connection module is used to automatically adjust the output of the grid control module based on the information from the grid voltage synchronization signal acquisition unit after the grid is restored, so as to synchronize it with the grid voltage and complete the shockless grid connection operation.

6. A method for flexible interconnection of distribution transformer areas and comprehensive power quality management, characterized in that, Applied to the apparatus as described in any one of claims 1 to 4, comprising the following steps: S1: Real-time acquisition of load data, power quality data, and status data of the all-solid-state energy storage battery from the two interconnected transformer areas; S2: Based on the collected data, the energy management system determines the current power grid status and optimization objectives, and selects and enters one of the following dominant operation modes: power mutual assistance mode, three-phase imbalance management mode, or grid islanding mode. S3: Executes the control algorithm corresponding to the selected dominant operating mode and coordinates other auxiliary functions, including: When the power mutual assistance mode is executed, the power transmission command between the two converter modules is dynamically adjusted, and the voltage level at the grid connection point is monitored, supplemented by reactive power regulation. When the three-phase imbalance mitigation mode is executed, the control current output of the current-straining module is adjusted to compensate for the current, while monitoring and maintaining the DC bus voltage stability. When the grid islanding mode is executed, the control converter module and energy storage battery work together to maintain the stability of the island voltage frequency, while monitoring the grid side status and preparing for synchronous grid connection. S4: Based on changes in system status, the energy management system dynamically switches or mixes control commands of different modes to achieve the composite functional objectives of load balancing in transformer areas, three-phase imbalance management, reactive power and voltage optimization, and uninterrupted power supply.

7. The method according to claim 6, characterized in that, The specific implementation steps of the three-phase imbalance control mode include: S31: Sample the three-phase voltage and current of the target transformer and decompose them into positive-sequence, negative-sequence, and zero-sequence components using α-β coordinate transformation; S32: Based on the components, the instantaneous negative sequence power and zero sequence power characterizing the three-phase imbalance are calculated; S33: Combine the negative sequence power and zero sequence power with the power loss of the converter module and the power adjustment required to maintain the stability of the DC bus voltage, and calculate the reference value of the three-phase compensation current that the converter module should output. S34: Using current tracking control technology, the converter module is driven to output the three-phase compensation current, thereby dynamically offsetting the unbalanced current component on the load side of the transformer.

8. The method according to claim 6, wherein, The switching process between the isolated network mode and the self-synchronized grid connection includes: S32: When an abnormal grid voltage is detected, immediately switch the control strategy of the two converter modules from grid current control to grid voltage control, and start the active power-frequency and reactive power-voltage droop control algorithms. S33: During islanded operation, the all-solid-state energy storage battery serves as the main power source, providing power support to the converter module through the DC bus, and dynamically adjusting the output according to the droop coefficient and load demand; S34: When a stable and qualified grid voltage is detected again through the grid voltage synchronization signal acquisition unit, the self-synchronization process is started: the voltage amplitude and frequency setpoint in the grid control are gradually adjusted to approach the grid voltage value, while phase closed-loop tracking is performed at the same time. S35: When the voltage amplitude difference, frequency difference and phase difference are all less than the preset threshold, control the closing of the low-voltage side grid connection switch of the corresponding transformer, and then smoothly switch the control strategy of the converter module back to the grid-connected power control mode.

9. A power distribution network system, characterized by It includes at least two distribution transformer substations, and a flexible interconnection device for the distribution substations as described in any one of claims 1 to 4; The device is connected between the low-voltage sides of the transformers in the at least two distribution substations to achieve flexible interconnection, comprehensive power quality management, and island autonomy between the at least two distribution substations.

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