Symmetrical split H-bridge cascade topology circuit and medium-voltage flexible interconnection control method

Abstract

The invention discloses a symmetric split H-bridge cascade topology circuit and a medium-voltage flexible interconnection control method, and the circuit comprises a symmetric split series branch, a series-parallel topology series part is symmetrically split into two sub series units with the same structure, each sub series unit is cascaded by N H-bridge modules, a parallel branch is connected, the primary side of a phase-shifting transformer is connected with a power distribution network bus, and the secondary side of a phase-shifting transformer is connected with a power distribution network bus. And the secondary side is connected with the H-bridge DC side and the system control unit through the three-phase rectifier bridge. The module number and the device type are dynamically adjusted according to the voltage difference, which side of the symmetric H bridge is put into work is dynamically selected according to the power flow direction, and generation of power circulation is reduced; network following / network construction collaboration: running in a series branch current source / voltage source mode, and performing pre-synchronization control to realize 15ms fast smooth switching; and fault isolation: active isolation and cooperative isolation are combined, hot switching is carried out on a standby module, and millisecond-level fault processing is realized. According to the invention, the reliability, the multi-scene adaptability and the new energy consumption capability of the power distribution network are improved, and the equipment cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of power distribution technology, and particularly relates to a symmetrical split H-bridge cascaded topology circuit and a medium-voltage flexible interconnection control method. Background Technology

[0002] In flexible interconnection scenarios of medium-voltage distribution networks, traditional topologies struggle to simultaneously meet the demands for efficiency, cost, and scalability, exhibiting significant technical limitations. Current mainstream topologies are primarily categorized into series, parallel, and series-parallel types: Series topologies (such as TCSC and SSSC) suffer from low power flow control accuracy, non-expandable AC ports, and a response time as slow as 2 seconds, making them unsuitable for rapidly changing grid conditions; Parallel topologies (such as MMC and BTB-VSC) offer high control accuracy and support for multi-port expansion, but require handling all dispatched power, resulting in relatively low efficiency, and require more than twice the number of power devices compared to series-parallel topologies, leading to high equipment costs; Series-parallel topologies (such as UPFC and DPFC) handle only a portion of the power, offering significant efficiency and cost advantages, but impose strict limitations on voltage amplitude and phase difference, making it prone to circulating currents in active power, leading to decreased operating efficiency, and their non-expandable AC ports hinder adaptability to various application scenarios.

[0003] Existing technologies have shortcomings in efficiency optimization and coordinated network control: First, they lack dynamic adjustment mechanisms to address the impact of voltage amplitude and phase differences on efficiency, resulting in large efficiency fluctuations due to fixed power unit operation modes. Second, network switching in series-parallel topologies relies on complex control logic, with switching times often exceeding 20ms, failing to meet the grid's requirements for rapid fault response. Third, fault isolation largely relies on single active isolation or traditional relay protection, lacking a coordinated mechanism and resulting in insufficient reliability in fault handling. For example, while the flexible interconnection topology disclosed in CN120638344A optimizes power flow control, it does not involve symmetrical split-structure design; CN118739303B proposes a flexible interconnection switch topology and control method for medium-voltage distribution networks, but does not consider efficiency improvement. Therefore, there is an urgent need for a topology and control method that can balance efficiency, cost, and scalability, and possesses both rapid network switching and reliable fault isolation capabilities. Summary of the Invention

[0004] This invention aims to solve the problems of active power circulation, low efficiency, slow network switching, and unreliable fault isolation in traditional topologies of medium-voltage flexible interconnection devices. It provides a symmetrical split H-bridge cascaded topology circuit and a medium-voltage flexible interconnection control method to achieve high reliability, high efficiency, and flexible interconnection in multiple scenarios for distribution networks. The specific technical solution is as follows:

[0005] A symmetrical split H-bridge cascaded topology circuit includes symmetrical split series branches, parallel branches, and a system control unit;

[0006] The symmetrical split series branch is composed of multiple power units directly connected in series to achieve voltage superposition. The series section is symmetrically split into a first sub-series unit and a second sub-series unit. The two sub-series units have the same structure and are symmetrically distributed on both sides. Each sub-series unit is composed of N power units cascaded together. Both sub-series units support the flexible operation of some or all power units. The first or second sub-series unit is selected to operate according to the power flow direction to reduce active circulating current. Each power unit includes an interconnected H-bridge and a three-phase rectifier bridge. The system control unit can independently control the H-bridge of each power unit.

[0007] The parallel branch uses a phase-shifting transformer and a three-phase rectifier bridge. The primary side of the phase-shifting transformer is connected to the distribution network bus, the secondary side of the phase-shifting transformer is connected to the AC side of the three-phase rectifier bridge, and the DC side of the three-phase rectifier bridge is connected to the DC side of the H-bridge of the power unit for decoupling control of active and reactive power.

[0008] The system control unit independently controls the H-bridge of each power unit and is responsible for global power flow allocation, network mode decision-making, and fault command issuance.

[0009] Preferably, the H-bridge of the power unit adopts a hybrid structure of SiC-MOSFET and IGBT. In each of the N power units in the sub-series unit, a portion of the H-bridges are configured as SiC-MOSFETs and the other portion are configured as IGBTs.

[0010] A medium-voltage flexible interconnection control method, which adopts a symmetrical split H-bridge cascaded topology circuit, specifically includes the following steps:

[0011] After power-on initialization, data acquisition and status detection are performed first, while external control commands are received to obtain control targets.

[0012] When a fault is detected, a lockout command is issued to lock out the power unit of the faulty branch, cut off the fault current path, and quickly inject reverse voltage through the series branch to cancel the voltage at the fault point, force the fault current to cross zero, and put the standby module into place to replace the faulty module.

[0013] The number of power units is adjusted according to the voltage difference between the two sides. When the voltage difference is small, the number of participating units is reduced, and when the voltage difference is large, the number of participating units is increased.

[0014] In grid-connected mode, the series branch power unit operates in current source mode, and the parallel branch participates in reactive power regulation. If switching to grid-connected mode, the difference between the grid-connected parameters and the grid is adjusted, the control loop is switched, and a second fine-tuning is performed when the voltage and frequency deviations exceed the limits.

[0015] In grid-connected mode, the series branch power unit operates in voltage source mode, simulating the characteristics of a synchronous generator. If switching to grid connection, the grid connection parameters need to be adjusted to match the grid, the control loop needs to be switched, and secondary fine-tuning is required when the voltage and frequency deviations exceed the limits.

[0016] Preferably, the number of power units is controlled based on the voltage difference. Specifically, when the voltage difference is small and does not exceed the withstand voltage of a set of power units, only one set of power units is activated. As the voltage difference increases, the number of activated power units is gradually increased to ensure the withstand voltage requirement while minimizing the number of power units activated and reducing losses.

[0017] Preferably, when the number of power units is small, the switching frequency is increased and the SiC module is enabled; when the number of power units is large, the switching frequency is reduced to balance losses and efficiency.

[0018] Preferably, when power flows from distribution network 1 to distribution network 2, the first sub-series unit is activated, and when power flows from distribution network 2 to distribution network 1, the second sub-series unit is activated, so that the energy of the parallel branch always comes from the power supply side, reducing power circulation.

[0019] Preferably, the number of secondary windings of the phase-shifting transformer matches the total number of power units.

[0020] Preferably, when a short-circuit fault in a series branch is detected, the series unit is first triggered to limit the current and suppress the fault current. Then, an active isolation command is issued to lock the H-bridge of the power unit of the fault branch and inject a reverse voltage to cancel the voltage at the fault point, force the fault current to cross zero, and achieve arc-free isolation.

[0021] As can be seen from the above embodiments, the advantages of the present invention compared with the prior art are as follows:

[0022] (A) Significantly improved efficiency: By dynamically adjusting the number of power units and differentiating SiC devices, the maximum efficiency of the whole machine is improved, achieving higher efficiency than traditional series-parallel topologies;

[0023] (B) Fast Network Switching: Employing pre-synchronization control improves network switching time, resulting in faster switching speeds compared to traditional topologies. This enables rapid response to scenarios such as single-line power quality anomalies and multi-device collaborative power supply to faulty lines, enhancing the distribution network's resilience to disturbances.

[0024] (C) Reliable fault isolation: The "active isolation + collaborative isolation" scheme achieves millisecond-level fault handling, and with the hot switching of backup modules, it reduces the failure rate of the device and improves the reliability of the power distribution network operation;

[0025] (D) Strong adaptability to multiple scenarios: The symmetrical split structure supports the expansion of AC ports, and the active and reactive power are completely decoupled. It can be flexibly connected to distributed power sources and energy storage, improve the new energy consumption capacity, and the number of power devices is less than that of the parallel topology, reducing equipment costs. Attached Figure Description

[0026] Figure 1 The control strategy flowchart provided in this application;

[0027] Figure 2 The optimized symmetric split-cascade H-bridge topology provided in this application. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other. To achieve the above objectives, this invention adopts the following technical solution.

[0029] This invention is achieved using the following technical solution:

[0030] like Figure 1 As shown, the present invention provides a medium-voltage flexible interconnection control method, which specifically includes the following steps:

[0031] After the system is powered on and initialized, it first performs data acquisition and status detection, and at the same time receives external control commands to obtain control targets.

[0032] When the system detects a fault, the system control unit issues a lockout command to lock out the power unit of the faulty branch, cut off the fault current path, and quickly inject reverse voltage through the series branch to cancel the voltage at the fault point, force the fault current to cross zero, and put the backup module into operation to replace the faulty module.

[0033] The number of power units is adjusted according to the voltage difference between the two sides. When the voltage difference is small, the number of participating units is reduced, and when the voltage difference is large, the number of participating units is increased.

[0034] In grid-connected mode, the series branch power units operate as current source units, while the parallel branches participate in reactive power regulation. If switching to grid-connected mode is required, the grid-connected parameters and their difference from the grid must be adjusted, the control loop switched, and secondary fine-tuning performed when voltage and frequency deviations exceed limits. In grid-connected mode, the series branch power units operate as voltage source units, simulating synchronous generator characteristics. If switching to grid-connected mode is required, the grid-connected parameters must be adjusted to match the grid, the control loop switched, and secondary fine-tuning performed when voltage and frequency deviations exceed limits.

[0035] Symmetrical split H-bridge cascaded topology circuit, such as Figure 2 As shown, the core of this topology includes symmetrical split series branches, parallel branches, and a system control unit. Each phase of the three phases in the figure has the same circuit structure, and the specific architectural features are as follows:

[0036] (1) Symmetrical split series branch: The series-parallel topology is usually composed of multiple power units directly connected in series or in parallel to achieve voltage superposition. The series part of the series-parallel topology is symmetrically split into a first sub-series unit and a second sub-series unit. The two sub-series units have the same structure and are symmetrically distributed on both sides. Each sub-series unit is composed of N power units cascaded together. Both sub-series units support the flexible operation of some or all power units. The first sub-series unit or the second sub-series unit is selected to work according to the power flow direction to reduce the generation of active circulating current. Each power unit includes an interconnected H-bridge and a three-phase rectifier bridge. The system control unit can independently control the H-bridge of each power unit.

[0037] (2) Parallel branch: A phase-shifting transformer and a three-phase rectifier bridge are used. The primary side of the phase-shifting transformer is connected to the distribution network bus, the secondary side of the phase-shifting transformer is connected to the AC side of the three-phase rectifier bridge, and the DC side of the three-phase rectifier bridge is connected to the DC side of the H-bridge of the power unit, which is responsible for the decoupling control of active and reactive power.

[0038] (3) System control unit: The system control unit can independently control the H-bridge of each power unit and is responsible for global power flow distribution, network mode decision-making and fault command issuance.

[0039] An efficiency optimization control strategy based on a symmetric split-cascade topology is designed to improve overall system efficiency through dynamic adjustment and differentiated application of components. The specific strategy is as follows:

[0040] (1) Dynamically adjust the number of power units

[0041] Status acquisition: Real-time acquisition of the voltage difference k in the distribution network;

[0042] Module activation logic: When the voltage difference is small and does not exceed the withstand voltage of a group of power units, only one group of power units is activated. As the voltage difference increases, the number of activated power units is gradually increased to ensure the withstand voltage requirement while minimizing the number of power units activated, thus reducing losses.

[0043] (2) Differentiated applications of SiC devices

[0044] Device configuration: The H-bridge of the power unit adopts a hybrid structure of SiC-MOSFET and IGBT. In the N power units of each sub-series unit, some H-bridges are configured as SiC-MOSFETs and others are configured as IGBTs to balance losses and efficiency.

[0045] Switching frequency adjustment: When the number of power units is small, increase the switching frequency and enable the SiC module; when the number of power units is large, decrease the switching frequency.

[0046] A network-connected / network-structured collaborative control mechanism is constructed to achieve rapid mode switching. The specific logic is as follows:

[0047] (1) Follow-up Network (GFL) Control Strategy

[0048] Core logic: The power units in the series branch operate in current source mode and track the grid phase through a phase-locked loop (PLL), while the parallel branch is responsible for reactive power regulation;

[0049] Control loop design: Only the "current inner loop control loop" is enabled. The system control unit issues the current setpoint according to the power flow demand of the distribution network, and the power unit achieves accurate current tracking through PI regulation.

[0050] Applicable scenarios: Strong power grid environment, where only power flow distribution is required, such as normal power supply in industrial parks.

[0051] (2) Network-based (GFM) control strategy

[0052] Core logic: The power unit of the series branch switches to voltage source mode, simulates the rotor motion equation of the synchronous generator, and autonomously constructs voltage and phase through droop control;

[0053] Control loop design: The inner current loop is used as the execution link, and an active / reactive power control loop and a virtual impedance control loop are added externally to generate the setpoint of the inner current loop; the virtual impedance control loop can suppress voltage fluctuations caused by sudden load changes;

[0054] Applicable scenarios: weak grids, islanded operation scenarios, when voltage / frequency support is required, such as power supply after a distribution network line failure.

[0055] (3) Fast switching mechanism

[0056] Pre-synchronization control: Pre-adjusting the voltage amplitude and phase in the grid configuration mode;

[0057] Switching execution: The system control unit controls each power unit to lock the grid control loop and activate the grid control loop, and the series branch modules synchronously complete the current source / voltage source mode switching;

[0058] Post-switch verification: After the switch is completed, the system control unit monitors the voltage and frequency deviations in real time. If the deviation exceeds the limit, a secondary fine-tuning is triggered to ensure a smooth switch.

[0059] A fault isolation scheme combining "active isolation" and "collaborative isolation" is formed to achieve millisecond-level fault handling. The specific scheme is as follows:

[0060] (1) Active isolation based on power electronic devices

[0061] Power unit lockout: When a fault occurs (such as a short circuit fault), the system control unit issues a lockout command, and the H-bridge IGBT of the power unit in the faulty branch is immediately locked out, cutting off the fault current path;

[0062] Reverse voltage injection: Rapidly injecting reverse voltage into the series branch to cancel the fault point voltage, forcing the fault current to cross zero quickly, and achieving arc-free isolation;

[0063] Hot switching of backup power units: Each sub-series unit (power unit) is equipped with 1-2 backup power units. The temperature and capacitor voltage of the main power unit are monitored in real time. When the main power unit fails, the backup power unit is put into operation to replace it, ensuring the continuity of the output voltage waveform.

[0064] (2) Collaborative isolation combined with traditional protection

[0065] System-level linkage: The device establishes a communication connection with the power distribution system master station and the power distribution automation master station, and uploads data such as fault current and voltage in real time. The master station locates the fault section through data analysis.

[0066] Action logic coordination: When a fault occurs, the relay protection system first triggers current limiting measures (such as current limiting of series units) to suppress the fault current. Then the device starts the active isolation strategy. The two work together to achieve precise isolation of the fault section.

[0067] Recovery control: After the fault is cleared, the device gradually restores grid connection through pre-synchronization control to avoid inrush current.

[0068] The specific implementation method is as follows:

[0069] Taking a medium-voltage flexible interconnection device with a rated power of 500kW and an AC port voltage of 10kV as an example, the present invention will be described in detail as follows:

[0070] Topology parameter configuration: The first and second sub-series units of the symmetrical split series branch are each configured with 8 power units (N=8). The first two modules of each sub-series unit use 1200V SiC-MOSFETs, and the remaining 6 modules use 1200V VIGBTs. The parallel branch uses two phase-shifting transformers (the number of secondary windings matches the total number of H-bridge modules, adopting a 6-winding phase-shifting design with a phase difference of 60°). The primary windings are connected in parallel to the 10kV distribution network bus, and each secondary winding is connected to the DC side of the corresponding H-bridge module through a rectifier circuit. Through the isolation characteristics of the secondary windings of the phase-shifting transformers and the coordinated control of the series and parallel branches, power fluctuation smoothing and mode switching support are achieved.

[0071] Control execution: When the device is connected to the 10kV distribution network, the power flows from distribution network 1 to distribution network 2. When the voltage difference k=10%, the system control unit activates the first sub-series unit, puts in 2 power units, and activates SiC-MOSFET, with the switching frequency set to 10kHz. When the voltage difference k=15%, the system control unit adds an IGBT module, and the switching frequency drops to 8kHz.

[0072] Grid-following / Grid-connection switching execution: Under normal circumstances, the device operates in grid-following mode, tracking the distribution network voltage (10kV±5%) and frequency (50Hz±0.2Hz). When a line in the distribution network experiences a sudden voltage drop to 9.2kV (fluctuation -8%) and a frequency drop to 49.6Hz due to a load change, the system control unit captures the abnormal signal and initiates pre-synchronization control. Simultaneously, it adjusts the voltage amplitude in grid-connection mode to 9.3kV and the frequency to 49.7Hz, with a difference from the grid of ≤1%. Subsequently, a switching command is issued, and the grid-following / grid-connection switching is completed within 15ms. 0.5Mvar reactive power and 0.45MW active power are injected into the series branch, and the line voltage and frequency gradually return to normal. Once the parameters return to the threshold, the device automatically switches back to grid-following mode.

[0073] Fault isolation execution: When a short circuit fault in the series branch is detected, the system control unit immediately triggers the current limiting of the series unit to suppress the fault current to 1.2 times the rated current; the system control unit issues an active isolation command, the IGBT of the power unit of the fault branch is locked, and the series branch is injected with reverse voltage to force the fault current to cross zero quickly; at the same time, a backup power unit is put into operation to replace the fault module, and the output voltage waveform is not interrupted.

Claims

1. A symmetrical split-type H-bridge cascaded topology circuit, characterized in that, Includes symmetrical split series branches, parallel branches, and system control unit; The symmetrical split series branch is composed of multiple power units directly connected in series to achieve voltage superposition. The series section is symmetrically split into a first sub-series unit and a second sub-series unit. The two sub-series units have the same structure and are symmetrically distributed on both sides. Each sub-series unit is composed of N power units cascaded together. Both sub-series units support the flexible operation of some or all power units. The first or second sub-series unit is selected to operate according to the power flow direction to reduce active circulating current. Each power unit includes an interconnected H-bridge and a three-phase rectifier bridge. The system control unit can independently control the H-bridge of each power unit. The parallel branch uses a phase-shifting transformer and a three-phase rectifier bridge. The primary side of the phase-shifting transformer is connected to the distribution network bus, the secondary side of the phase-shifting transformer is connected to the AC side of the three-phase rectifier bridge, and the DC side of the three-phase rectifier bridge is connected to the DC side of the H-bridge of the power unit for decoupling control of active and reactive power. The system control unit independently controls the H-bridge of each power unit and is responsible for global power flow allocation, network mode decision-making, and fault command issuance.

2. The symmetrical split H-bridge cascaded topology circuit according to claim 1, characterized in that, The H-bridge of the power unit adopts a hybrid structure of SiC-MOSFET and IGBT. In each of the N power units in the sub-series unit, some H-bridges are configured as SiC-MOSFETs and others are configured as IGBTs to balance losses and efficiency.

3. A medium-voltage flexible interconnection control method, which adopts the symmetrical split H-bridge cascaded topology circuit as described in claim 1 or 2, characterized in that, Specifically, the following steps are included: After power-on initialization, data acquisition and status detection are performed first, and external control commands are received to obtain control targets. When a fault is detected, a lockout command is issued to lock out the power unit of the faulty branch, cut off the fault current path, and quickly inject reverse voltage through the series branch to cancel the voltage at the fault point, force the fault current to cross zero, and put the standby module into place to replace the faulty module. The number of power units is adjusted according to the voltage difference between the two sides. When the voltage difference is small, the number of participating units is reduced, and when the voltage difference is large, the number of participating units is increased. In grid-connected mode, the series branch power unit operates in current source mode, and the parallel branch participates in reactive power regulation. If switching to grid-connected mode, the difference between the grid-connected parameters and the grid is adjusted, the control loop is switched, and a second fine-tuning is performed when the voltage and frequency deviations exceed the limits. In grid-connected mode, the series branch power unit operates in voltage source mode, simulating the characteristics of a synchronous generator. If switching to grid connection, the grid connection parameters need to be adjusted to match the grid, the control loop needs to be switched, and secondary fine-tuning is required when the voltage and frequency deviations exceed the limits.

4. A medium-voltage flexible interconnection control method according to claim 3, characterized in that, The number of power units is dynamically adjusted based on the voltage difference between the two sides. Specifically, when the voltage difference is small and does not exceed the withstand voltage of a set of power units, only one set of power units is activated. When the voltage difference increases, the number of power units put into operation is gradually increased so that the withstand voltage requirement can be guaranteed while minimizing the power unit input and reducing losses.

5. A medium-voltage flexible interconnection control method according to claim 4, characterized in that, SiC devices are partially used to reduce switching losses. When the number of power units is small, the switching frequency is increased and the SiC module is enabled. When the number of power units is large, the switching frequency is reduced to balance losses and efficiency.

6. A medium-voltage flexible interconnection control method according to claim 3, characterized in that, When power flows from distribution network 1 to distribution network 2, the first sub-series unit is activated; when power flows from distribution network 2 to distribution network 1, the second sub-series unit is activated, ensuring that the energy of the parallel branch always comes from the power supply side, thus reducing power circulation.

7. A medium-voltage flexible interconnection control method according to claim 3, characterized in that, The number of secondary windings of the phase-shifting transformer is matched with the total number of power units.

8. A medium-voltage flexible interconnection control method according to claim 3, characterized in that, When a short-circuit fault in a series branch is detected, the series unit is first triggered to limit the current and suppress the fault current. Then, an active isolation command is issued to lock the H-bridge of the power unit of the fault branch and inject reverse voltage to cancel the voltage at the fault point, force the fault current to cross zero, and achieve arc-free isolation.

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