A flexible loop closing device and system for interconnecting different feeders

Abstract

The application discloses a kind of to realize different feeder interconnection flexible loop device and system, including first AC / DC converter, second AC / DC converter, DC bus and controller.Two sides AC / DC converter AC port is connected two sides AC380V resident side load with three-phase four-line mode respectively, and DC port is connected by DC bus;The controller is used to execute common-mode current suppression algorithm, multi-node voltage constant regulation based on model predictive control, fault side power fast recovery algorithm, three-phase unbalanced load management and harmonic management.The application adjusts different grid-connected point voltage vector by small amplitude regulation converter output voltage, realizes the loop of small amplitude phase deviation power grid, realizes the low-cost composite device with load voltage regulation, low voltage management and flexible interconnection function.

Description

Technical Field

[0001] This invention relates to the field of flexible power transmission and distribution, and specifically to a flexible loop-connecting device and system for interconnecting different feeders. Background Technology

[0002] The distribution network is a crucial component of the power system. Its primary function is to safely and reliably transmit electrical energy from the high-voltage transmission network to industrial, commercial, and residential users through voltage reduction, distribution, and regulation. It also undertakes tasks such as voltage regulation, power quality optimization, and fault isolation. In recent years, users have increasingly demanded higher power supply reliability, better power quality, and more diversified services. Low-voltage flexible loop-closing devices can significantly improve the reliability of the power grid. When necessary, flexible loop-closing devices (such as B2B and UPQC) can mitigate three-phase imbalance in the power grid to a certain extent. Furthermore, low-voltage flexible loop-closing devices can eliminate the need for transformers, greatly reducing device size and increasing power density.

[0003] Existing technical solutions generally have the following problems: 1. Traditional solutions use power frequency transformers, which are large in size and weight and cannot meet the requirements of compact scenarios; 2. Currently, flexible loop closing devices suffer from low efficiency; the multi-stage AC / DC conversion architecture results in system efficiency generally below 92%. 3. Traditional loop closing equipment uses dynamic switches, mechanical switches and other equipment, which can only close the loop when the voltage amplitude and phase of the two grids are basically the same. This requires high grid synchronization and has a long adjustment time. Summary of the Invention

[0004] The technical problem to be solved by this invention is to provide a flexible loop-closing device and system for interconnecting different feeders, which adjusts the voltage vector of different grid connection points by slightly adjusting the output voltage of the converter, thereby achieving loop closing of the grid with small phase deviation. With the help of the loop-closing switch, the power consumption of the system under static conditions is reduced, and a low-cost composite device solution with on-load voltage regulation, low voltage management and flexible interconnection functions is realized.

[0005] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A flexible loop interconnection device for different feeders includes a first AC / DC converter, a second AC / DC converter, a DC bus, and a controller, wherein: The AC ports of the first AC / DC converter and the second AC / DC converter are respectively connected to the corresponding AC380V residential loads, and the DC ports of the first AC / DC converter and the second AC / DC converter are connected through the DC bus. Both the first AC / DC converter and the second AC / DC converter are three-phase four-wire topologies, with the N-phase serving as the path for zero-sequence current. The controller is used for: The common-mode current suppression algorithm is executed. The common-mode current is calculated by collecting the currents of phases A, B, C, and N. When the common-mode current exceeds the set value, a modulation wave is generated and superimposed on the N-arm to suppress the common-mode current. Multi-node voltage constant regulation based on model predictive control is implemented. The voltage amplitude / phase angle and injected power of each node are collected by smart meters. The voltage fluctuation is predicted by a small-signal state-space model and the voltage deviation is minimized to maintain the voltage constant of different nodes. The fault-side power fast recovery algorithm is implemented. When a grid fault is detected on a certain side, the control device switches to power transfer mode within 20ms, transferring power from the non-faulty side to the faulty side to ensure seamless power outage on the residential load side.

[0006] Furthermore, the controller is also used to perform three-phase unbalanced load management: when the output current imbalance is detected to reach a set threshold, the imbalance management function is activated to separate the positive and negative sequences of the output current, and then the negative sequence current is generated by control and superimposed on each bridge arm of the inverter side to adjust the three-phase current to a balanced state by supplementing reactive current.

[0007] Furthermore, in the common-mode current suppression algorithm, the device adopts a non-isolated topology and both N phases are grounded simultaneously. The controller calculates the common-mode current by collecting the currents of phases A, B, C, and N. When the common-mode current exceeds the set value, the common-mode current suppression algorithm is activated. The device is equipped with a common-mode current suppression converter with adjustable capacity on the N phase. The controller superimposes the generated modulation wave onto the N bridge arm to achieve the effect of suppressing the common-mode current.

[0008] Furthermore, in the multi-node voltage constant regulation based on model predictive control, the controller processes the meter data according to the different voltage levels of different nodes in order to simultaneously stabilize the constant voltage of multiple nodes.

[0009] Furthermore, the controller is also used to perform harmonic mitigation, specifically harmonic compensation when a current harmonic is detected to reach a compensation threshold value.

[0010] Furthermore, under normal operating conditions, the first AC / DC converter is in a voltage stabilization state, and the second AC / DC converter is in a power distribution state; the DC bus is used for the connection of energy storage systems and photovoltaic systems to achieve peak shaving and valley filling functions.

[0011] The present invention also proposes a flexible loop-connecting system for different feeders, including the aforementioned flexible loop-connecting device for different feeders, and corresponding AC380V residential-side loads connected to the AC ports of the first AC / DC converter and the second AC / DC converter, respectively.

[0012] Compared with the prior art, the advantages of the present invention are as follows: 1. By using a low-voltage flexible interconnection device, the load rate of the transformers on both sides can be balanced; 2. Multi-node power control, voltage regulation, and rapid fault response functions were achieved by using a model predictive control algorithm; 3. The device adopts a modular design, which improves its scalability and reduces the replacement time in case of device failure; 4. By adding a capacity-adjustable converter to the N-phase and superimposing a common-mode current suppression algorithm, the device can achieve unbalanced operation and can be organically integrated with the power grid. 5. The device has a fault-side power fast recovery algorithm. When a fault occurs on one side of the power grid, the device can quickly support and transfer power within 20ms, ensuring seamless power outages for residential loads. Attached Figure Description

[0013] Figure 1 This is an overall topology diagram of a flexible loop system for interconnecting different feeders, as described in an embodiment of the present invention.

[0014] Figure 2 This is a topology diagram of the main circuit of the flexible loop-connecting device for different feeders according to an embodiment of the present invention. Detailed Implementation

[0015] 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.

[0016] This embodiment proposes a flexible loop-connecting system for interconnecting different feeders, including transformer substations connected to two AC10kV power grids on both sides, and a flexible loop-connecting device connecting the AC380V residential loads on both sides. The AC10kV power grids on both sides are converted to AC400V (actual operating voltage approximately AC380V) via the transformer substations, and the AC380V on both sides is connected to the flexible interconnection device in a three-phase four-wire configuration. The device converts the AC400V of the two power grids to DC750V through two back-to-back AC / DC modules. The two back-to-back modules achieve active power flow interconnection between the two power grids via a DC bus, completing the flexible interconnection of the two power grids. The DC bus can be connected to photovoltaic or energy storage devices to achieve peak shaving and valley filling, and to increase economic benefits on the user side.

[0017] like Figure 1As shown, the flexible loop-closing device in this embodiment includes a first AC / DC converter on the left side of the figure, a second AC / DC converter on the right side of the figure, a DC bus, and a controller. Wherein: The AC port of the first AC / DC converter is connected to the AC 380V residential load on the left, and the AC port of the second AC / DC converter is connected to the AC 380V residential load on the right. The DC ports of the first and second AC / DC converters are connected via the DC bus. Both the first and second AC / DC converters are three-phase four-wire topologies, with the N-phase serving as the path for zero-sequence current. The device adopts a modular design, with each back-to-back module having a capacity of 100kVA. It can be expanded to different capacities for different application scenarios, up to a maximum of 400kVA. When a module fails and needs replacement, the replacement can be completed and the device restarted within half an hour, improving the device's fault tolerance.

[0018] like Figure 2 As shown, the AC 380V grid on both sides is converted into DC output through AC / DC converters. The DC voltage is adjustable and can be connected to photovoltaic, energy storage, and other devices. The DC buses on both sides are connected through DC / DC converters on both sides, enabling AC-side power sharing, DC-side energy storage charging, and DC-side power support. In terms of energy coupling, under normal operating conditions, the left AC / DC converter is in voltage stabilization mode, while the right AC / DC converter is in power distribution mode. The DC output can be used for energy storage systems and photovoltaic systems to achieve peak shaving and valley filling functions.

[0019] The flexible loop-closing device in this embodiment uses low-voltage flexible interconnection as the basic unit. By cooperating with the transformer, it forms a comprehensive power distribution network management equipment. Combined with nearby energy storage or new energy vehicle resources, it adjusts the voltage vector of different grid connection points by slightly adjusting the output voltage of the converter, thereby achieving loop closing of the power grid with small phase deviation. With the addition of a loop-closing switch, it reduces the system power consumption under static conditions, realizing a low-cost composite device solution that combines on-load voltage regulation, low-voltage management, and flexible interconnection functions.

[0020] In terms of the control system, the flexible loop-closing device in this embodiment supports rapid switching between multiple operating modes (such as grid-connected, islanded, and hybrid modes) to adapt to complex and ever-changing grid demands. It also incorporates intelligent coordinated control, employing a model predictive control-based algorithm to achieve multi-node voltage constant regulation, ensuring stable operation at different voltage levels across different nodes. The device uses the N-phase as a path for zero-sequence current, and incorporates a capacity-adjustable converter in the N-phase, employing a common-mode current suppression algorithm to achieve unbalanced operation and good integration with the grid. The device uses a fault-side power fast recovery algorithm. Through innovation and optimization of the control algorithm and communication rate, the device can rapidly support and transfer power within 20ms when a grid fault occurs on one side, ensuring seamless power outages for residential loads.

[0021] The main functions of the flexible ring-closing device in this embodiment include: Overvoltage and undervoltage management of main transformer: If the controller detects that the voltage of the main transformer reaches the overvoltage or undervoltage threshold at a certain moment, the low-voltage flexible interconnection device provides reactive power compensation to restore the voltage of the main transformer to the normal voltage range, thereby achieving the purpose of overvoltage and undervoltage management.

[0022] Three-phase unbalanced load management: If, at a certain moment, a three-phase voltage and current imbalance occurs due to the connection of large single-phase equipment or uneven load distribution, resulting in a single-phase heavy load, the low-voltage flexible interconnection device initiates the unbalanced operating condition management program. Through the underlying controller, the output current is separated into positive and negative sequences. Then, the negative-sequence current is modulated and superimposed on each bridge arm of the inverter side, providing active power support and reactive power compensation, thus adjusting the voltage and current to a three-phase balanced state. Specifically, when the device detects that the output current imbalance reaches 50%, the underlying controller activates the unbalance management function, separating the positive and negative sequences of the output current. The negative-sequence current is then modulated and superimposed on each bridge arm of the inverter side, adjusting the three-phase current to a balanced state by supplementing reactive current.

[0023] Common-mode current suppression: Due to the TN-C wiring configuration of the power grid, the device's ground wire will experience a common-mode current with an amplitude of approximately 120A. The device uses a non-isolated topology with both N-phase lines grounded simultaneously. Due to modulation imbalances and high-frequency components inherent in the semiconductor converter, a voltage difference exists at the midpoint of the three-phase voltages on both sides. Since the midpoint of the three-phase voltages is connected to the N-phase line, a voltage difference also exists between the two N-phase lines. This voltage difference is forced to zero through the ground wire, resulting in a large common-mode current on the ground wire. To suppress this common-mode current, this embodiment incorporates a common-mode current suppression converter with adjustable capacity in the N-phase. When the common-mode current exceeds a set value, the converter activates a common-mode current suppression algorithm: by collecting the currents of phases A, B, C, and N and calculating the common-mode current through the controller, when the common-mode current exceeds 10A, the algorithm is activated, and the generated modulation wave is superimposed onto the N-arm to suppress the common-mode current. Using the common-mode current suppression algorithm can significantly suppress the common-mode current on the ground wire, ensuring the safety of the device and its surrounding environment.

[0024] Harmonic mitigation: When the controller detects that the current harmonics have reached the compensation threshold, the flexible loop closing device performs harmonic compensation to improve the harmonic situation.

[0025] Multi-node voltage constant regulation: Smart meters collect data on voltage amplitude / phase angle and injected power at each node. A small-signal state-space model is then used to predict voltage fluctuations and minimize voltage deviations, maintaining constant voltage at different nodes. Constraints are added to ensure voltage safety. The device also processes the meter data to address the different voltage levels at each node, enabling simultaneous stabilization of varying voltages at multiple nodes.

[0026] Three-phase unbalanced voltage suppression: When a large three-phase unbalanced voltage occurs in the power grid, and the worst case is a single-phase short circuit, the device can suppress the three-phase unbalanced voltage from the power grid to a certain extent by changing the control strategy, so as to ensure the power supply quality and reliability of the load side.

[0027] Balanced load rate of transformers on both sides: When the load rates of transformers on both sides are unbalanced, the active and reactive power can be allocated by the main control, so that the lightly loaded transformer can provide a part of the power to the heavily loaded transformer, thereby reducing the load rate of the heavily loaded transformer.

[0028] Rapid Power Recovery on Fault Side: The device employs a rapid power recovery algorithm on the fault side, enabling rapid power support and transfer within 20ms in the event of a grid fault on one side, ensuring seamless power outages for residential loads. Specifically, when a fault occurs on the first busbar while the second busbar is functioning normally, power is transferred from the second busbar to the first busbar to ensure power continuity and provide emergency power. Conversely, when a fault occurs on the second busbar while the first busbar is functioning normally, power is transferred from the first busbar to the second busbar. After the faulty busbar heals itself, the device can exit the fault transfer mode and return to the power mutual assistance state.

[0029] An alternative solution is to add a transformer on the DC side. Adding a transformer provides electrical isolation between the two devices and avoids ground loop current issues caused by AC-side networking. However, it's important to note that while adding a transformer reduces harmonic effects, its larger size and higher cost lead to reduced power density and increased overall cost.

[0030] In another preferred embodiment, the device can replace the aforementioned non-isolated back-to-back topology with a topology based on a single high-frequency isolated LLC resonant converter. This topology uses a single high-frequency isolation transformer to achieve complete electrical isolation between the AC systems on both sides, blocking fault propagation across regions; it utilizes the soft-switching characteristics of LLC to achieve zero-voltage turn-on (ZVS) of the primary-side switch and zero-current turn-off (ZCS) of the secondary-side rectifier across the entire load range, significantly reducing switching losses and increasing operating frequency and power density; it reduces voltage stress on power devices through transformer turns ratio and LLC voltage divider design, so that the switching devices do not need to withstand the full DC bus voltage; at the same time, it achieves decoupling of AC / DC control on both sides, so that DC voltage fluctuations do not directly affect the AC side, improving system stability and dynamic response speed.

[0031] In another embodiment, the device can also be configured with a fault ride-through and full hardware protection system to improve fault response speed and operational safety. The protection system may include: a DC-side ultra-fast solid-state protection subsystem, employing a SiCMOSFET-based solid-state circuit breaker connected in parallel with a mechanical bypass contactor, combined with dual hardware criteria of voltage transients and current change rates, to achieve microsecond-level detection and solid-state interruption of DC short-circuit faults; an AC-side ground fault detection and selective isolation subsystem, employing a zero-sequence current transformer and insulation monitoring instrument for redundant detection, combined with a switchable grounding resistance network and a four-pole circuit breaker, to achieve single-phase ground fault location and selective isolation; a passive hardware circulating current suppression network, employing an N-line common-mode magnetic ring, a DC bus differential-mode reactor, and IGBT parallel RC buffer circuit, to fully suppress high-frequency circulating currents within the back-to-back structure; and a graded derating thermal management subsystem based on junction temperature monitoring, employing a junction temperature monitoring unit integrated within the IGBT module and an independent MCU, performing five-level derating through a hardware power limiting circuit, allowing the device to continue operating with reduced power under overheating conditions instead of direct shutdown.

[0032] In summary, this invention proposes a flexible loop-closing device and system for interconnecting different feeders, which is a flexible loop-closing solution with high efficiency, good compatibility, and stable reliability. Firstly, in terms of the overall solution, the AC10kV power grid is converted to AC400V via a transformer substation. The AC400V power from both sides is then connected to a flexible interconnection device using a three-phase four-wire configuration. This device uses two back-to-back AC / DC modules to convert the AC400V from both power grids to DC750V. The two back-to-back modules achieve active power flow interconnection between the two power grids via a DC bus, completing the flexible interconnection of the two grids. The DC bus can be connected to photovoltaic or energy storage devices to achieve peak shaving and valley filling, and to increase economic benefits on the user side.

[0033] Secondly, in terms of the control system, the device supports rapid switching between multiple operating modes (such as grid-connected, islanded, and hybrid modes) to adapt to complex and ever-changing power grid demands. It also incorporates intelligent coordinated control, employing a model predictive control-based algorithm to achieve constant voltage regulation across multiple nodes, ensuring stable operation at different voltage levels at different nodes.

[0034] Furthermore, the device adopts a modular design, with each back-to-back module having a capacity of 100kVA. This capacity can be expanded to different levels for different application scenarios, up to a maximum of 400kVA, enhancing the device's scalability. When a module fails and needs replacement, the replacement can be completed and the device restarted within half an hour, improving its fault tolerance.

[0035] Meanwhile, the device uses the N phase as the path to achieve zero-sequence current. It also adds a converter with adjustable capacity to the N phase and adopts a common-mode current suppression algorithm, which enables the device to operate unbalancedly and integrate well with the power grid.

[0036] Finally, the device uses a fault-side power fast recovery algorithm. Through innovation and optimization of the control algorithm and communication rate, the device can achieve rapid power support and transfer within 20ms when a fault occurs on one side of the power grid, ensuring seamless power outages for residential loads.

[0037] 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.

[0038] 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 loop-closing device for interconnecting different feeders, characterized in that, It includes a first AC / DC converter, a second AC / DC converter, a DC bus, and a controller, wherein: The AC ports of the first AC / DC converter and the second AC / DC converter are respectively connected to the corresponding AC380V residential loads, and the DC ports of the first AC / DC converter and the second AC / DC converter are connected through the DC bus. Both the first AC / DC converter and the second AC / DC converter are three-phase four-wire topologies, with the N-phase serving as the path for zero-sequence current. The controller is used for: The common-mode current suppression algorithm is executed. The common-mode current is calculated by collecting the currents of phases A, B, C, and N. When the common-mode current exceeds the set value, a modulation wave is generated and superimposed on the N-arm to suppress the common-mode current. Multi-node voltage constant regulation based on model predictive control is implemented. The voltage amplitude / phase angle and injected power of each node are collected by smart meters. The voltage fluctuation is predicted by a small-signal state-space model and the voltage deviation is minimized to maintain the voltage constant of different nodes. The fault-side power fast recovery algorithm is implemented. When a grid fault is detected on a certain side, the control device switches to power transfer mode within 20ms, transferring power from the non-faulty side to the faulty side to ensure seamless power outage on the residential load side.

2. The flexible loop-connecting device for different feeders according to claim 1, characterized in that, The controller is also used to perform three-phase unbalanced load management: when the output current imbalance is detected to reach a set threshold, the unbalance management function is activated to separate the positive and negative sequence of the output current, and then the negative sequence current is generated by control and superimposed on each bridge arm of the inverter side to adjust the three-phase current to a balanced state by supplementing reactive current.

3. The flexible loop-closing device for interconnecting different feeders according to claim 1, characterized in that, In the common-mode current suppression algorithm, the device adopts a non-isolated topology and both N phases are grounded simultaneously. The controller calculates the common-mode current by collecting the currents of phases A, B, C, and N. When the common-mode current exceeds the set value, the common-mode current suppression algorithm is activated. The device is equipped with a common-mode current suppression converter with adjustable capacity on the N-phase. The controller superimposes the generated modulation wave onto the N-bridge arm to achieve the effect of suppressing common-mode current.

4. The flexible loop-connecting device for interconnecting different feeders according to claim 1, characterized in that, In the multi-node voltage constant regulation based on model predictive control, the controller processes the meter data according to the different voltage levels of different nodes in order to simultaneously stabilize the constant voltage of multiple nodes.

5. The flexible loop-connecting device for interconnecting different feeders according to claim 1, characterized in that, The controller is also used to perform harmonic mitigation, specifically harmonic compensation when a current harmonic is detected to reach a compensation threshold.

6. The flexible loop-closing device for interconnecting different feeders according to claim 1, characterized in that, Under normal operating conditions, the first AC / DC converter is in voltage stabilization mode, and the second AC / DC converter is in power distribution mode; the DC bus is used for the connection of energy storage systems and photovoltaic systems to realize peak shaving and valley filling functions.

7. A flexible loop interconnection system for different feeders, characterized in that, It includes the flexible loop-connecting device for different feeders as described in any one of claims 1 to 6, and the corresponding AC380V residential loads respectively connected to the AC ports of the first AC / DC converter and the second AC / DC converter.

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