A three-level cascaded solid state transformer topology and control method

Abstract

The application relates to a three-level cascaded solid-state transformer topology and a control method, which comprises the following steps: collecting system operation parameters and states of each modular power unit in real time, distributing power instructions and dynamically adjusting individualized power reference values of each modular power unit, realizing voltage self-adaptive balance among the modular power units; after each module controller receives the individualized power reference values, completing the whole-process control of internal rectification, isolation conversion and output adjustment of the module; cooperatively monitoring faults, isolating fault units, and realizing power reconstruction by redistributing the individualized power reference values of normal modular power units. The application constructs a hybrid SST topology with a three-level cascade as the core, has the advantages of multi-level low stress and modularity easy expansion, realizes coordinated optimization at the system level and autonomous control at the module level, each power unit has independent voltage support and fault isolation capability, can realize modular autonomy, and the system has high reliability.

Description

Technical Field

[0001] This application relates to the field of energy storage system technology, specifically to a three-level cascaded solid-state transformer topology and control method. Background Technology

[0002] Solid-state transformers, also known as power electronic transformers, are a new type of power conversion device based on power electronics technology and the principle of high-frequency electromagnetic induction. Compared with traditional power frequency transformers, solid-state transformers achieve energy transmission and electrical isolation through power electronic conversion and high-frequency transformers, offering advantages such as small size, light weight, high power density, flexible control, and bidirectional power flow. Existing solid-state transformers typically employ a multi-stage power conversion structure, using power electronic devices to complete AC-DC-DC or AC-DC-AC energy conversion and relying on high-frequency transformers for electrical isolation. In existing technologies, the high-voltage side power conversion topologies of solid-state transformers mainly include schemes based on modular multilevel converters and schemes based on cascaded H-bridges.

[0003] However, existing technologies have obvious drawbacks: although the modular multilevel converter-based scheme has excellent harmonic performance, the system is complex and costly; while the two-level cascaded H-bridge scheme has problems such as high device voltage stress and large switching losses.

[0004] Therefore, this invention proposes a three-level cascaded solid-state transformer topology and control method. Summary of the Invention

[0005] To address the technical challenges of existing solid-state transformers in medium- and high-voltage energy storage and DC microgrid applications, such as high voltage stress on power devices, large switching losses, low system efficiency, difficulty in voltage balancing during multi-module cascade operation, high control complexity, and inability to achieve flexible bidirectional power transmission, this invention proposes a three-level cascaded solid-state transformer topology and control method. Through topology innovation and hierarchical collaborative control strategies, it achieves a balance between low device stress, high system efficiency, flexible scalability, and high reliability.

[0006] This application provides a three-level cascaded solid-state transformer topology and control method, specifically adopting the following technical solution.

[0007] This application provides a three-level cascaded solid-state transformer topology. The overall structure of the solid-state transformer (SST) of this invention adopts a three-level topology, consisting of a high-voltage AC input stage, an isolation conversion stage, and a low-voltage DC output stage connected in series. These three stages form a complete energy transmission link, realizing the entire process of power conversion from high-voltage AC input to low-voltage DC output. The entire system consists of several completely identical modular power units. All modular power units have the same structure, parameters, and control methods, possessing excellent interchangeability and ease of maintenance. Each modular power unit independently completes the complete process of AC-DC rectification, DC-DC isolation conversion, and DC output regulation, and can operate independently without relying on other modules. Multiple modular power units are connected in series on the high-voltage side and in parallel on the low-voltage side, forming a scalable architecture that adapts to different input voltage levels and output power levels. When it is necessary to increase the input voltage level, the number of modular power units connected in series in the same phase link can be increased. When it is necessary to increase the output power level, the number of modular power units connected in parallel or the configuration of phase units can be increased, without requiring significant modifications to the overall topology, thus offering extremely high flexibility.

[0008] The high-voltage AC input stage consists of three identical phase units (phase A, phase B, and phase C). These three units are not directly electrically connected to each other, forming three independent and symmetrical input links, corresponding to phases A, B, and C of the three-phase high-voltage AC power grid, respectively. Each phase unit comprises several modular power units connected in series to the grid side of the corresponding phase. One end of the series-connected phase unit is connected to the high-voltage AC input port of the corresponding phase, and the other end is connected to the common ground. The series connection of modular power units achieves graded processing of the high-voltage AC, reducing the input voltage pressure on individual modules. Each modular power unit's front end uses a diode-clamped three-level full-bridge topology as an AC-DC rectifier circuit to convert high-voltage AC to high-voltage DC. The specific implementation scheme is as follows.

[0009] (1) In the NPC three-level full-bridge topology of each modular power unit, there are two high-voltage DC support capacitors connected in series. The two high-voltage DC support capacitors have the same capacity and are connected in series between the positive and negative poles of the DC bus. Their connection point forms the midpoint potential of the DC bus. This midpoint potential provides a zero-level reference for the three-level output and also plays a role in voltage division, distributing the DC bus voltage equally to the two high-voltage DC support capacitors and reducing the voltage stress of a single capacitor.

[0010] (2) Each NPC three-level full-bridge topology contains two bridge arms with identical structures. Each bridge arm consists of four power switching devices and two clamping diodes. The four power switching devices are connected in series between the positive and negative terminals of the DC bus. The two clamping diodes are connected between the midpoint of the bridge arm and the midpoint O of the DC bus. The midpoint potential constructed by the two high-voltage DC support capacitors is used to realize three levels of output: positive level, negative level and zero level.

[0011] (3) The NPC three-level full-bridge topology with multiple modular power units is connected in series on the grid side of the corresponding phase. Each modular power unit only bears 1 / N of the input high voltage, where N is the number of modular power units connected in series in that phase. This greatly reduces the voltage stress of a single power switching device. Compared with the existing two-level cascaded H-bridge topology, the device voltage stress is reduced by 50%, and the switching loss is effectively reduced. In addition, the output voltage waveform of the three-level topology is closer to a sine wave, which can significantly improve the grid side harmonic performance and reduce EMI interference. It can meet the grid harmonic standards without adding a complex filter circuit.

[0012] Isolation converter stage: Located inside each modular power unit, between the high-voltage AC input stage and the low-voltage DC output stage. Each modular power unit's AC-DC output terminal is connected to a set of three-phase interleaved series resonant dual active bridges as an isolated DC-DC converter to realize the conversion of high-voltage DC to low-voltage DC, electrical isolation, and bidirectional power transmission. Its specific structure and characteristics are as follows.

[0013] (1) The primary side of the three-phase interleaved series resonant dual active bridge DAB-SRC converter consists of three sets of series resonant dual active bridge power conversion branches with identical structures. The three primary active bridge power conversion branches are independent of each other and electrically decoupled. During operation, they operate interleaved with a fixed phase difference of 120° to achieve the superposition of multi-phase power and stable energy transmission, effectively reduce the current ripple of the primary DC bus, and improve the stability of power transmission.

[0014] (2) Both the primary and secondary sides of this converter adopt the same single-phase half-bridge unit structure. The three single-phase half-bridge units on the primary side are connected in parallel to the high-voltage DC bus of the output of the front-stage NPC three-level full-bridge topology, sharing the same high-voltage DC power supply to ensure that the input voltage of the three power conversion branches is consistent. The three single-phase half-bridge units on the secondary side are connected in parallel to the output DC bus of the isolation stage to realize the convergence of low-voltage DC power. The primary and secondary sides achieve electrical isolation and voltage level conversion through a high-frequency isolation transformer. The high-frequency isolation transformer uses high-frequency magnetic core material, which is small in size, light in weight, and has high power density. Compared with the traditional power frequency transformer, it greatly reduces the overall size and weight of the solid-state transformer.

[0015] (3) A series resonant network is connected in series between the bridge arm of each primary active bridge power conversion branch and the primary winding of the high-frequency isolation transformer. The series resonant network is composed of a resonant inductor and a resonant capacitor connected in series. The parameters of the resonant inductor and the resonant capacitor are matched to form a series resonant circuit. The resonant inductor can be implemented by an independent inductor device or can be formed by the leakage inductance of the high-frequency isolation transformer. There is no need to add an independent inductor device, which simplifies the hardware structure. The series resonant network enables the converter to still have good soft switching characteristics under high-frequency operating conditions, greatly reducing the switching loss of power switching devices, improving system efficiency, and reducing EMI interference.

[0016] (4) The three-phase interleaved series resonant dual active bridge DAB-SRC converter has bidirectional power flow regulation capability. By controlling the switching sequence of the primary and secondary half-bridge units, it can flexibly realize the forward transmission of electrical energy from the grid to the load, or the reverse transmission from the load back to the grid. It is suitable for various application scenarios such as energy storage systems, DC microgrids, and new energy grid connection, and is compatible with the bidirectional energy interaction requirements of future smart grids.

[0017] A bidirectional power transmission model for the DAB-SRC converter is established to quantify its power transmission capability.

[0018] The power transfer model from the grid to the load is as follows: . .

[0019] The power transmission model from the load to the grid in the reverse direction is as follows.

[0020] In the formula, and These represent the forward and reverse transmission power, respectively. This is the high voltage DC voltage on the primary side of the isolation stage. The voltage is the low-voltage DC voltage on the secondary side of the isolation stage, and k is the turns ratio of the high-frequency isolation transformer. The phase difference between the primary and secondary half-bridge switching signals, 0 < < , The reverse transmission coupling coefficient is determined by the coupling characteristics of the three primary windings and secondary windings of the high-frequency isolation transformer, and its value ranges from 0.95 to 1. The DAB-SRC converter of this invention has similar forward and reverse transmission power magnitudes and symmetrical control logic, ensuring both flexibility and stability in bidirectional energy interaction. This is the resonant frequency of the series resonant network. This is the value of the resonant inductance. This is the value of the resonant capacitor.

[0021] (5) The high-frequency isolation transformer includes three primary windings and three corresponding secondary windings. The three primary windings are connected in series with the three power conversion branches on the primary side, and the three secondary windings are connected in series with the three power conversion branches on the secondary side. The three primary windings and the three secondary windings are all coupled on the same high-frequency isolation transformer core, realizing the electromagnetic coupling and superposition of the three power sources. Thanks to the structural symmetry of the three power conversion branches and their design of being coupled to the same high-frequency isolation transformer core, in the series resonance operation mode, the branches exhibit natural power sharing characteristics. There is no need to add an additional current sharing control loop to achieve current balance of the three branches, simplifying the control logic and improving the stability of the system.

[0022] Low-voltage DC output stage: The low-voltage DC output stage of each modular power unit adopts a three-phase interleaved parallel DC-DC topology. It is located downstream of the isolation converter stage and connected between the secondary DC bus of the DAB and the low-voltage DC output bus. It completes the final regulation and voltage stabilization of the low-voltage DC output of the isolation converter stage, ensuring the accuracy and stability of the output voltage and meeting the power demand of the load. The specific scheme is as follows.

[0023] (1) This low-voltage DC output stage is composed of three bidirectional half-bridge converters with identical structures, which are connected in parallel with alternating phases. The input terminals of the three bidirectional half-bridge converters are all connected in parallel to the DC bus on the secondary side of the front-end DAB to receive the low-voltage DC power output from the isolation converter stage. The output terminals are all connected in parallel to the low-voltage DC output terminal of the modular power unit to realize the collection and output of low-voltage DC power. Each bidirectional half-bridge converter includes two series-connected power switching devices and a filter inductor. The filter inductor is used to suppress the ripple of the output current and improve the output power quality.

[0024] (2) The three bidirectional half-bridge conversion phases are operated by a fixed phase difference interleaved modulation method. The phase interleaving makes the output current ripple of the three phases appear in staggered time dimension, and they cancel each other after superposition, which greatly reduces the total output current ripple.

[0025] To achieve bidirectional power transfer and adapt to the bidirectional power flow characteristics of the isolation stage DAB-SRC, the bidirectional half-bridge conversion phase of this low-voltage DC output stage adopts symmetrical switching control logic, clearly defining the switching control and current flow design during reverse power transfer: During forward power transfer from the isolation stage to the load, the module controller controls the switching transistors of each bidirectional half-bridge conversion phase to conduct alternately. Current flows from the DC bus on the secondary side of the DAB into the bidirectional half-bridge conversion phase, and after filtering by the filter inductor, it is output to the load. The current flow direction is: DAB secondary side → bidirectional half-bridge switch → filter inductor → load; During reverse transmission from the load to the isolation stage, the module controller switches the switching transistor drive timing, making the switching transistor conduction logic reversed compared to the forward transmission. At this time, the load-side electrical energy flows into the bidirectional half-bridge phase-changing circuit through the filter inductor, and then converges to the DC bus on the secondary side of the DAB. The current flow direction is load → filter inductor → bidirectional half-bridge switch → DAB secondary side, and then feeds back to the high-voltage AC grid through the bidirectional transmission function of the isolation stage DAB-SRC, realizing bidirectional energy interaction. This forms a good match with the bidirectional power flow of the isolation stage, meeting the application requirements of energy storage system charging and discharging, load energy feedback, and other scenarios.

[0026] (3) This low-voltage DC output stage has excellent fault tolerance capability. When any phase of the three bidirectional half-bridge conversion phases fails, the control system can quickly detect the fault signal and isolate the fault phase by controlling the switching device to prevent the fault from spreading to other phases and other modular power units. The other two normal phases can still maintain the modular power unit to continue operating in derating mode, ensuring that the module can still output a certain amount of power, guaranteeing the continuous power supply capability of the system, and significantly improving the overall reliability of the solid-state transformer.

[0027] This application provides a topology control method for a three-level cascaded solid-state transformer: In order to achieve stable operation of each level and module of the solid-state transformer, solve the problems of voltage and current balance in multi-module cascade, and improve system efficiency and reliability, this invention adopts a hierarchical collaborative control strategy, combining system-level coordinated control and module-level autonomous control, to achieve unified coordinated management of power units at each level, while having fault response and reconfiguration capabilities, reducing device stress, improving system efficiency, and ensuring high reliability and flexible adaptability.

[0028] The control architecture of this invention is divided into two layers: a system-level coordination control layer and a module-level autonomous control layer. The two control layers cooperate and coordinate with each other to achieve global power scheduling and parameter balancing, while ensuring the independent and stable operation of each modular power unit, thereby reducing the control complexity of the system.

[0029] (1) System-level coordination and control layer: Implemented by a system-level controller, responsible for global power scheduling, command allocation and system operation status monitoring. It collects the input voltage, input current, output voltage and output current of the entire solid-state transformer system in real time, as well as the DC side voltage, power output and operation status of each modular power unit. Based on the collected parameters, it performs global analysis and decision-making, generates global power commands, and decomposes them to each phase unit and each modular power unit. At the same time, it realizes voltage balance control between modules and fault response and reconfiguration.

[0030] (2) Module-level autonomous control layer: Implemented by the module controller of each modular power unit. Each module controller operates independently, receives individualized power reference values ​​allocated by the system-level coordination control layer, and autonomously completes all control functions of the high-voltage AC input stage, isolation conversion stage, and low-voltage DC output stage within the module without relying on real-time intervention from the system controller. This includes NPC three-level rectification control, DAB-SRC isolation conversion control, and interleaved parallel DC-DC output regulation control. At the same time, it feeds back the operating status parameters of the module to the system-level controller in real time, realizing the autonomous operation of the module, reducing the control burden of the system-level controller, and improving the system's response speed and reliability.

[0031] The core of system-level coordinated control is to achieve global energy management, voltage balancing between modules, and fault response and reconfiguration. The specific implementation methods are as follows.

[0032] (1) Global energy management: During steady-state operation, the three-level cascaded solid-state transformer of this invention strictly follows the following power balance relationship.

[0033] ;in, Indicates the first The first The active power of each modular power unit This represents the total active power exchanged between the entire unit and external systems. Based on the aforementioned power balance relationship, the system control layer sets power reference values ​​for each module. Unified planning and dynamic adjustments are carried out to ensure the overall energy consistency of the cascaded system under multi-module conditions.

[0034] The system-level controller collects the output current and output voltage of each modular power unit in real time, calculates the active power of each modular power unit, and then sums them to obtain the actual total active power of the system. The actual total active power is compared with the system's total power target value to obtain the power deviation. Combined with the current number of effective modules and operating status of the system, the total power command of the system is generated through proportional-integral (PI) regulator calculation. Subsequently, based on the above power balance relationship, the total power command is evenly decomposed to each phase unit, and then further distributed to each modular power unit to obtain the individualized power reference value of each module. This ensures that the power output of each module matches the total system demand, guarantees the overall energy consistency of the cascaded system under multi-module conditions, and avoids system instability caused by power imbalance.

[0035] The model of the proportional-integral controller is: . .

[0036] In the formula, This is the total system power command. This is the proportional gain of the PI controller. The integral coefficient of the PI controller. Let T be the power deviation, T be the settling time, and t be the integral variable. The target value for total system power. This represents the actual total active power of the system.

[0037] This model allows for precise design of PI regulator parameters, ensuring rapid convergence of power deviations and achieving stable global power scheduling, which differs from existing simple power allocation methods.

[0038] (2) Adaptive Voltage Balancing Control: To address the issue of inconsistent voltage distribution that easily occurs when multiple modular power units are connected in series during three-level cascaded operation, this invention introduces an adaptive voltage balancing strategy at the system level. This strategy eliminates the need for additional voltage equalization circuits and achieves automatic balancing of the DC-side voltage of the series modules solely through software control, simplifying the hardware structure and reducing costs. Specifically, the system-level controller monitors the DC-side voltage of each modular power unit in real time, calculates the deviation between the DC-side voltage of each module and the average DC-side voltage of all series modules, and dynamically adjusts the individualized power reference value of the module in real time based on the magnitude and direction of the voltage deviation. For modules with high DC-side voltage, the power reference value is appropriately reduced to decrease the power output of the module, thereby lowering the DC-side voltage of the module. For modules with low DC-side voltage, the power reference value is appropriately increased to increase the power output of the module, thereby raising the DC-side voltage of the module. Through this closed-loop adjustment, the DC-side voltage of all series modules automatically tends to be consistent, achieving voltage balancing between modules and avoiding damage to power devices or system failures caused by voltage imbalance.

[0039] (3) Fault response and reconfiguration control: This control system has a complete fault response and reconfiguration capability, which can quickly detect and isolate faulty units and realize the derating operation of the system to ensure uninterrupted power supply to critical loads. The specific implementation method is as follows: The system-level controller monitors the operating status of each modular power unit and each power conversion phase in real time. By collecting the current and voltage signals of the power switching devices, it determines whether there are faults such as device damage, short circuit, and overcurrent. When an abnormality is detected in an individual modular power unit or an individual power conversion phase in the system, the system-level controller immediately issues a control command to quickly cut off the input power of the faulty unit or faulty phase, isolate it, and prevent the fault from spreading to other normal units or phases. At the same time, the system-level controller automatically adjusts the power distribution of the remaining normal modular power units and redistributes the total power command Pref so that the normal modules can bear more power output, realize continuous power supply in derating mode, ensure uninterrupted operation of critical loads, and improve the reliability and fault tolerance of the system.

[0040] Module-level autonomous control means that the module controller of each modular power unit receives its own individualized power reference value. Then, it independently completes all control functions of the three-stage conversion circuit within this module, specifically including input stage control, isolation stage control, and output stage control. The three work together to ensure that the module's power output, voltage accuracy, and operating efficiency meet the requirements.

[0041] The technical solution of this application has achieved the following beneficial effects.

[0042] 1. By combining the NPC three-level full-bridge, DAB-SRC, and interleaved parallel DC-DC three-level, a hybrid SST topology with three-level cascade as the core is constructed, which has the advantages of multi-level low stress and modular easy expansion.

[0043] 2. A hierarchical control method was proposed to support this, realizing system-level coordination optimization and module-level autonomous control, including key control objectives such as three-level midpoint potential balance and output current sharing.

[0044] 3. Each power unit has independent voltage support and fault isolation capabilities, enabling modular autonomy and high system reliability. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the overall architecture of the three-phase SST of the three-level cascaded solid-state transformer topology in the embodiments of this application.

[0046] Figure 2 This is a modular power unit structure diagram of a three-level cascaded solid-state transformer topology in an embodiment of this application.

[0047] Figure 3 This is the NPC three-level circuit structure of the solid-state transformer topology with three-level cascades in the embodiments of this application.

[0048] Figure 4 This is a three-phase interleaved DAB-SRC topology structure for a three-level cascaded solid-state transformer topology in the embodiments of this application.

[0049] Figure 5 This is an interleaved parallel DC / DC converter with a three-level cascaded solid-state transformer topology in the embodiments of this application.

[0050] Figure 6 This is a block diagram of system power coordination and module voltage equalization control for a three-level cascaded solid-state transformer topology in an embodiment of this application.

[0051] Figure 7 This is an NPC rectifier topology control block diagram of the three-level cascaded solid-state transformer topology in the embodiments of this application.

[0052] Figure 8 The diagram shows a typical current and voltage waveform for the three-phase interleaved DAB-SRC implementation of ZVZCS in the three-level cascaded solid-state transformer topology in this application embodiment.

[0053] Figure 9 This is a timing diagram of the driving of each arm of the three-phase interleaved DAB-SRC in the three-level cascaded solid-state transformer topology in the embodiments of this application.

[0054] Figure 10 This is a block diagram of the interleaved parallel DC-DC control of the three-level cascaded solid-state transformer topology in the embodiments of this application.

[0055] Figure 11 The image shows the interleaved parallel DC-DC output current waveform of the three-level cascaded solid-state transformer topology in this embodiment of the application. Detailed Implementation

[0056] This invention provides a three-level cascaded solid-state transformer topology and control method. The topology adopts a modular and scalable three-level architecture, and the control method realizes unified coordination and management of power units at each level. While reducing device stress and improving system efficiency, it ensures high reliability and flexible adaptability.

[0057] The present application will now be further described with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application and should not be construed as limiting the scope of protection of the present application. It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present application.

[0058] like Figure 1As shown, the three-level cascaded solid-state transformer topology provided in this embodiment mainly includes a three-phase high-voltage AC input port, three phase units with identical structures (phase A unit, phase B unit, and phase C unit), and a unified low-voltage DC output port. Its core lies in the use of a modular three-level power unit cascaded topology architecture, combined with a hierarchical collaborative control method, to achieve efficient and reliable power conversion, adapting to the application requirements of medium-voltage and medium-power scenarios. Specific parameters can be adjusted according to the actual application scenario, such as an input voltage level of 10kV, an output voltage level of 750V, with 5 modular power units connected in series in each phase unit, and a total system power of 500kW.

[0059] This invention provides a method for controlling a three-level cascaded solid-state transformer topology, comprising the following steps.

[0060] S1: System initialization, completing control parameter presets and status self-checks. After the system is powered on, the system-level controller and the module controllers of all modular power units are synchronously initialized, loading preset control parameters, including the total system power target value, rated parameters of each module, resonant frequency, PI regulator parameters, etc. Subsequently, the system-level controller performs a status self-check on the entire system, and each module controller performs a self-check on its own three-stage conversion circuit. After passing the self-check, it enters standby mode, waiting to receive power commands. If the self-check fails, an alarm is immediately triggered and the faulty unit is isolated.

[0061] S2: System-level coordinated control to achieve global energy balance and adaptive voltage equalization between modules. Executed by the system-level controller, it collects real-time input and output parameters of the entire system and the operating status of each module. Based on the power balance relationship, it plans global power commands and distributes them evenly to each phase unit and each modular power unit. Simultaneously, it monitors the DC-side voltage of each series-connected module in real time and achieves adaptive voltage equalization between modules by dynamically adjusting the power reference values ​​of each module, ensuring that the voltage deviation of modules in the same phase is controlled within the allowable range.

[0062] S3: Module-level autonomous control enables autonomous operation of modules, reducing the burden on the system-level controller and eliminating the need for real-time intervention. Each modular power unit's module controller receives individualized power reference values ​​allocated by the system and autonomously controls its internal three-stage conversion circuitry, including NPC three-level rectification control, three-phase interleaved DAB-SRC isolation conversion control, and interleaved parallel DC-DC output regulation control. Simultaneously, it feeds back its own operating status parameters to the system-level controller in real time.

[0063] S4: Fault Response and Reconfiguration. This enables rapid fault isolation and power reconfiguration to ensure continuous system power supply. The system-level controller and module controllers work together to monitor faults across the entire system, including module faults, phase faults, and system-level faults. Upon detection, the faulty unit is immediately isolated to prevent fault propagation. Subsequently, the power reference values ​​of normal modules are reallocated to achieve system power reconfiguration. If it is a single phase fault, the module operates at a reduced rate; if it is a module fault, the power is taken over by a normal module in the same phase, ensuring uninterrupted power supply to critical loads.

[0064] S5: Shutdown control for smooth system shutdown. When a shutdown command is received or an unrepairable serious fault is detected, the system-level controller sends a shutdown signal. Each module controller responds synchronously, gradually reducing its own power output to zero at a preset rate and shutting down the switching devices of the three-stage conversion circuit. Subsequently, the soft-start circuit is shut down and the input / output circuit breakers are disconnected in a preset sequence. After the system-level controller confirms that all modules have safely shut down, it cuts off the main power supply to the system and saves the operating parameters and fault records before shutdown, thus completing the shutdown process.

[0065] Example 1: As Figure 1 and Figure 2 As shown, a three-level cascaded solid-state transformer topology is disclosed. In this embodiment, the three-phase high-voltage AC input ports A, B, and C are respectively connected to the corresponding phase units (phase A unit, phase B unit, and phase C unit) via three independent input links. The structures of the three input links are completely identical, ensuring symmetrical input and transformation of three-phase electrical energy.

[0066] Taking the A-phase input link as an example, from the AC grid side to the A-phase unit, an input circuit breaker QS11, a soft starter circuit, and an input inductor L1 are sequentially installed. The soft starter circuit consists of a pre-charging resistor R11 and a bypass relay QS21 connected in parallel with it. The input circuit breaker QS11 is a high-voltage vacuum circuit breaker with a rated voltage matching the input high-voltage level and has overcurrent and short-circuit protection functions. The pre-charging resistor R11 is a power resistor with a resistance value set according to the system capacity and is used to limit the starting inrush current. The bypass relay QS21 is a high-voltage relay with high-frequency conduction characteristics. The input inductor L1 is an iron-core inductor with an inductance value set according to the grid harmonic standard and is used to suppress grid-side harmonic current and limit current.

[0067] Similarly, the B-phase input link is equipped with an input inductor L2, an input circuit breaker QS12, and a soft-start circuit consisting of a pre-charging resistor R12 and a bypass relay QS22; the C-phase input link is equipped with an input inductor L3, an input circuit breaker QS13, and a soft-start circuit consisting of a pre-charging resistor R13 and a bypass relay QS23. All component parameters of the B-phase and C-phase input links are completely identical to those of the A-phase, and the structure of each phase is completely symmetrical, avoiding imbalance interference between the three phases and ensuring stable system operation. The above input terminal structure design provides the necessary overcurrent limiting, fault isolation, and start-up process protection functions for this embodiment of the invention, preventing input-side faults or inrush currents from damaging subsequent power modules.

[0068] like Figure 2 As shown, each phase unit is composed of multiple modular power units connected in series. In this embodiment, taking phase A as an example, it includes several modular power units A1 to AN. Each modular power unit is connected in series to the phase A high-voltage AC bus. The input terminal of the first modular power unit A1 is connected to the input inductor L1 of the phase A input link, and the output terminal of the last modular power unit A5 is connected to the common ground terminal, realizing natural voltage division under the action of input high voltage. Each modular power unit only bears 1 / N of the phase A input high voltage, which greatly reduces the voltage stress of a single module.

[0069] Phase B and Phase C units are structurally identical to Phase A units, each consisting of several modular power units B1 to BN connected in series with C1 to CN. The first end of a Phase B unit is connected to the input inductor L2 of the Phase B input link, and the last end is connected to the common ground. The first end of a Phase C unit is connected to the input inductor L3 of the Phase C input link, and the last end is connected to the common ground. The three phase units are not directly electrically connected to each other, forming three independent and symmetrical input links to ensure independent conversion of three-phase power. All modular power units have identical structure, parameters, and control methods, offering good scalability and ease of maintenance. When the input voltage level needs to be increased, the number of modular power units connected in series in each phase unit can be increased; when the output power level needs to be increased, the number of modular power units in each phase unit or the configuration of the phase units can be increased.

[0070] To further enhance the anti-interference capability of the high-voltage AC input stage and resist overvoltage and surge impacts caused by grid fluctuations and lightning strikes, protecting downstream power modules and devices from damage, this embodiment adds overvoltage surge protection modules to all three-phase input links. The specific connection sequence is: input grid → surge protector → overvoltage suppressor → input circuit breaker → soft starter circuit → input inductor → phase unit, ensuring consistent and robust protection logic. This overvoltage surge protection module adopts a series combination structure of surge protector and overvoltage suppressor. The surge protector uses a zinc oxide arrester with a rated voltage compatible with the 10kV input high-voltage level, used to suppress lightning surge interference from the grid side. The overvoltage suppressor uses a metal oxide varistor (MOV), connected in parallel downstream of the surge protector, used to absorb instantaneous overvoltage pulses from the grid, preventing overvoltage signals from being transmitted to downstream power modules. The parameters of the three-phase overvoltage surge protection modules are uniform and matched to the input high-voltage level, effectively improving the system's anti-interference capability and ensuring stable operation of the system in complex grid environments.

[0071] Example 2: A three-level cascaded solid-state transformer topology is disclosed, such as... Figure 2 As shown, the internal topology of the modular power unit is as follows: Each modular power unit contains three stages of conversion circuits connected in series: a high-voltage AC input stage, an isolation conversion stage, and a low-voltage DC output stage. These three stages sequentially realize the complete process of AC-DC rectification, DC-DC isolation conversion, and DC output regulation. Each modular power unit independently completes the entire power conversion process without relying on other modules. The high-voltage AC input stage is an NPC three-level rectifier circuit; the isolation conversion stage is a three-phase interleaved DAB-SRC converter; and the low-voltage DC output stage is a three-phase interleaved parallel DC-DC topology.

[0072] Further, such as Figure 3 As shown, the high-voltage AC input stage—three-level rectification is implemented as follows: This stage adopts a diode midpoint clamped (NPC) three-level full-bridge circuit. This circuit is the core of realizing the conversion of high-voltage AC to high-voltage DC. Its specific component configuration and connection relationship are as follows.

[0073] The diode-neutral-clamped three-level full-bridge circuit includes eight main switching transistors, four clamping diodes, and two series-connected high-voltage DC support capacitors. All eight main switching transistors are IGBT devices, with models selected according to the system voltage and current ratings, possessing high-frequency switching characteristics and high voltage withstand capability. Each IGBT device is connected in reverse parallel with a fast recovery diode for freewheeling and IGBT protection. The four clamping diodes D1-D4 are all fast recovery diodes, with reverse voltage withstand values ​​matched to the main switching transistors, used to achieve midpoint clamping and reduce voltage stress on the main switching transistors. The two high-voltage DC support capacitors C1 and C2 have identical capacitances of 1000μF, with a rated voltage of 1 / 10 of the input high voltage. They are connected in series between the positive and negative terminals of the DC bus, and their connection point forms the DC bus midpoint O, enabling each bridge arm to stably output... Three levels, their Therefore, the total high-voltage DC bus voltage output by the NPC three-level rectifier circuit is approximately 1.414 times the module input AC voltage. The eight main switching transistors are S11, S12, S13, S14, S21, S22, S23, and S24; the four clamping diodes are D1, D2, D3, and D4; and the two series-connected high-voltage DC support capacitors are C1 and C2.

[0074] The first bridge arm consists of four main switching transistors S11, S12, S13, and S14 connected in series between the positive and negative terminals of the DC bus. The collector of S11 is connected to the positive terminal of the DC bus, and the emitter of S11 is connected to the collector of S12 at node A. The emitter of S12 is connected to the collector of S13 at the midpoint O of the bridge arm. The emitter of S13 is connected to the collector of S14 at node B. The emitter of S14 is connected to the negative terminal of the DC bus. The anode of clamping diode D1 is connected to the midpoint O of the bridge arm, which is the connection point of the two high-voltage DC support capacitors C1 and C2, and its cathode is connected to node A. The anode of clamping diode D2 is connected to node B, and its cathode is connected to the midpoint O of the bridge arm.

[0075] The structure and connection method of the second bridge arm are exactly the same as those of the first bridge arm. The main switching transistors S21, S22, S23, and S24 are connected in series between the positive and negative terminals of the DC bus. The collector of S21 is connected to the positive terminal of the DC bus, and the emitter of S24 is connected to the negative terminal of the DC bus. The connection point of S21 and S22 is node C, and the connection point of S23 and S24 is node D. The anode of the clamping diode D3 is connected to the midpoint O of the bridge arm, and the cathode is connected to node C. The anode of the clamping diode D4 is connected to node D, and the cathode is connected to the midpoint O of the bridge arm.

[0076] This stage of the circuit operates as a rectifier. Its input is connected to the high-voltage AC input of the modular power unit, receiving the high-voltage AC power after series distribution. Through the drive signal output by the module controller, it controls the conduction and cutoff of the eight main switching transistors. Combined with the midpoint potential balance control, it converts the input high-voltage AC power into stable high-voltage DC power and outputs it to the primary side of the isolation converter stage, providing a stable high-voltage DC input power supply for the isolation converter stage.

[0077] Furthermore, such as Figure 4 As shown, the isolation converter stage—three-phase interleaved DAB-SRC—is implemented as follows: The isolation converter stage adopts a three-phase interleaved series resonant dual active bridge (DAB-SRC) topology. This converter is the core for realizing the conversion of high voltage DC to low voltage DC, electrical isolation, and bidirectional power transmission. Its specific component configuration and connection relationship are as follows.

[0078] This three-phase interleaved series resonant dual active bridge (DAB-SRC) converter consists of three identical series resonant power transmission branches connected together. The three branches are arranged in parallel and coupled to the same high-frequency isolation transformer. During operation, they operate with a 120° phase difference, achieving superposition and smooth transmission of the three power sources. The three identical series resonant power transmission branches are branch A1, branch B1, and branch C1.

[0079] The input side consists of three single-phase half-bridge power conversion branches A1, B1 and C1, each of which includes two power switching devices (IGBTs) connected in series.

[0080] The input side of branch A1 includes switching transistors S31 and S32, the input side of branch B1 includes switching transistors S41 and S42, and the input side of branch C1 includes switching transistors S51 and S52. All switching transistors are IGBT devices, with models matched to the main switching transistors in the input stage, and fast recovery diodes are connected in reverse parallel. The two switching transistors of each single-phase half-bridge power conversion branch are connected in series to the high-voltage DC bus output from the preceding NPC three-level rectifier circuit. The three single-phase half-bridge branches are connected in parallel, sharing the same high-voltage DC power supply to ensure that the input voltage of the three power conversion branches is consistent. The input side is the primary side to the high-voltage side.

[0081] Each power transmission branch is equipped with an independent series resonant network, which consists of a resonant inductor Lr and a resonant capacitor Cr connected in series. The resonant inductor Lr has an inductance of 10μH and can be implemented by an independent inductor or formed by the leakage inductance of a high-frequency isolation transformer. In this embodiment, an independent inductor is used to facilitate parameter adjustment. The resonant capacitor Cr has a capacitance of 1μF. The parameters of the resonant inductor Lr and the resonant capacitor Cr are matched to form a series resonant circuit. The resonant frequency is set to 10kHz to ensure that the converter can achieve soft-switching operation under high-frequency operating conditions.

[0082] A series resonant network is connected in series with the primary winding of a high-frequency isolation transformer to construct a resonant energy transmission channel for the isolation stage. The high-frequency isolation transformer uses a high-frequency ferrite core, which is small in size, light in weight, and has high power density. Its turns ratio is set according to the ratio of the high-voltage DC bus voltage to the low-voltage DC bus voltage. This high-frequency isolation transformer includes three primary windings and three corresponding secondary windings. The three primary windings are connected in series with the series resonant network of the three power transmission branches, and the three secondary windings are connected in series with the three single-phase half-bridge power conversion branches on the secondary side. All three primary windings and three secondary windings are coupled to the same high-frequency isolation transformer core, realizing the electromagnetic coupling and superposition of the three power paths, and ensuring the natural power balance of the three power transmission branches.

[0083] The output side is equipped with three single-phase half-bridge power conversion branches, A2, B2, and C2, corresponding to those on the input side. Their structure is identical to the single-phase half-bridge power conversion branches on the input side. Each single-phase half-bridge power conversion branch includes two IGBT power switching devices connected in series. The three single-phase half-bridge branches are connected in parallel to the secondary DC bus of the DAB, realizing the convergence of low-voltage DC power and outputting it to the low-voltage DC output stage. The output side is the secondary side-low-voltage side.

[0084] This stage of the circuit is mainly responsible for converting the high-voltage DC power provided by the preceding NPC three-level rectifier circuit into a stable low-voltage DC power output that meets the system requirements through a series resonant network and a high-frequency isolation transformer. It realizes voltage level conversion and electrical isolation between the high and low voltage sides, and also has bidirectional power flow regulation capability, which can flexibly realize the forward and reverse transmission of electrical energy, and is suitable for application scenarios such as energy storage systems.

[0085] Furthermore, such as Figure 5 As shown, the low-voltage DC output stage—interleaved parallel DC-DC converter—is implemented as follows: This stage is used to perform final regulation of the low-voltage DC output from the isolation converter stage, ensuring stable output voltage and low output current ripple. Its topology consists of multiple bidirectional half-bridge converter phases with the same structure connected in an interleaved parallel configuration. In this embodiment, three bidirectional half-bridge converter phases with the same structure are used: phase 1, phase 2, and phase 3. The specific component configuration and connection relationship are as follows.

[0086] Each bidirectional half-bridge converter phase includes two series-connected power switching devices (IGBTs) and a filter inductor L: Phase 1 includes switching transistors S61 and S62 and filter inductor L1; Phase 2 includes switching transistors S63 and S64 and filter inductor L2; and Phase 3 includes switching transistors S65 and S66 and filter inductor L3. All switching transistors are IGBT devices, with the model determined according to the low-voltage side current rating, and fast recovery diodes are connected in reverse parallel. The three filter inductors L1, L2, and L3 have identical parameters, with an inductance value of 50μH, used to suppress output current ripple and improve output power quality.

[0087] The input terminals of all bidirectional half-bridge conversion phases are connected in parallel to the DC bus on the secondary side of the preceding DAB stage to receive the low-voltage DC power output from the isolation conversion stage. The output terminals of all bidirectional half-bridge conversion phases are connected in parallel to the low-voltage DC output terminal of this modular power unit, realizing the collection and output of low-voltage DC power. In terms of control, the three bidirectional half-bridge conversion phases operate in an interleaved modulation mode with a fixed 120° phase difference. The drive signal output by the module controller controls the on and off of the switching devices of each phase, so that the output current ripple of the three phases appears staggered in the time dimension, and cancels each other out after superposition, significantly reducing the total output current ripple.

[0088] Meanwhile, the output stage has excellent fault tolerance. When any phase fails, the module controller can quickly detect the fault signal and immediately shut down all switching devices in that phase to isolate the faulty phase. The other two normal phases can still maintain the modular power unit in derating mode, with the output power reduced to 2 / 3 of the normal power, ensuring that the module can still output a certain amount of power and avoiding the entire module from shutting down due to a single phase failure, thus enhancing the fault tolerance and reliability of the system.

[0089] Example 3: A three-level cascaded solid-state transformer topology is disclosed, wherein the system connection and energy collection are implemented as follows: The solid-state transformer of the present invention realizes the collection and output of electrical energy through two-stage busbars, ensuring stable output voltage and sufficient power. The specific implementation method is as follows.

[0090] The first-stage merging: The low-voltage DC output terminals of all modular power units within the same phase link are connected in parallel to form the DC output port of that phase. Taking the A-phase link as an example, the low-voltage DC output terminals of multiple modular power units (A1-AN) of the A-phase unit are connected in parallel to form the A-phase DC output port, realizing the collection of the output power of all modular power units in the A-phase. Similarly, the low-voltage DC output terminals of multiple modular power units (B1-BN) of the B-phase unit are connected in parallel to form the B-phase DC output port, and the low-voltage DC output terminals of multiple modular power units (C1-CN) of the C-phase unit are connected in parallel to form the C-phase DC output port. The voltages of the three-phase DC output ports are completely consistent, ensuring the stability of the subsequent second-stage merging.

[0091] Second-level convergence: such as Figure 1 As shown, the DC output ports of phases A, B, and C are further connected in parallel on the low-voltage side, and after merging, they form a unified low-voltage DC bus to realize the aggregated output of three-phase electrical energy. This low-voltage DC bus is equipped with output circuit breakers (QS31, QS32), filter inductors (L4, L5), and a DC-side soft-start circuit composed of soft-start resistors (R21, R22) and bypass relays (QS41, QS42).

[0092] Among them, the output circuit breakers QS31 and QS32 are low-voltage circuit breakers with rated voltages matched to the low-voltage DC bus voltage. They have overcurrent and short-circuit protection functions and are used to realize the on-off control and fault isolation of the low-voltage DC output. The filter inductors L4 and L5 have the same parameters and an inductance value of 10μH. They are used to further suppress the current ripple of the low-voltage DC bus, improve the output power quality, and ensure that the output ripple meets the GB / T 14549-1993 power quality public power grid harmonic standard. The DC side soft starter circuit consists of soft starter resistors R21 and R22 and bypass relays QS41 and QS42. The soft starter resistors R21 and R22 are power resistors with resistance values ​​set according to the low-voltage side load capacity. They are used to limit the inrush current when the load starts and avoid damage to the output stage power devices. The bypass relays QS41 and QS42 are low-voltage relays. When the system is running normally, the bypass relays are closed, short-circuiting the soft starter resistors to ensure system operating efficiency.

[0093] The unified low-voltage DC bus output is connected to the load, such as energy storage batteries, DC microgrids, and low-voltage electrical equipment, to provide stable and high-quality low-voltage DC power to the load and complete the entire power conversion process of the solid-state transformer.

[0094] Example 4: A control method for a three-level cascaded solid-state transformer is disclosed, wherein step S1 includes the following steps.

[0095] S1.1 After the system is powered on, the system-level controller and all module controllers are initialized synchronously and loaded with preset control parameters, including: the total power target value of the system, the rated voltage / current parameters of each modular power unit, the resonant frequency of the series resonant network, the proportional coefficient and integral coefficient of the PI regulator, the voltage balance deviation threshold, the fault judgment threshold, and the three-phase interleaved phase difference.

[0096] S1.2 The system-level controller performs a comprehensive self-check of the entire solid-state transformer system, including: the status of the three-phase high-voltage input port, the operating status of each modular power unit, the on / off status of the power switching devices, the working status of the sensors, and the coupling status of the high-frequency isolation transformer.

[0097] S1.3 Each module controller performs a self-test on its internal three-level conversion circuit, including: the midpoint potential of the NPC three-level rectifier circuit, the resonant current of the DAB-SRC isolation converter circuit, and the current ripple of the interleaved parallel DC-DC output circuit, to ensure that each component is fault-free and the parameters are normal.

[0098] S1.4 After the self-test passes, the system-level controller sends a synchronization signal, and all module controllers enter standby mode, waiting to receive power commands. If the self-test fails, the system immediately issues an alarm signal and cuts off the power supply to the corresponding faulty unit to prevent the fault from spreading.

[0099] Example 5: Figures 6-11 As shown, a control method for a three-level cascaded solid-state transformer is disclosed. The control method adopts a hierarchical collaborative control strategy, which is divided into system-level coordinated control and module-level autonomous control. The two layers of control work together to achieve stable operation, voltage balancing, current balancing, and fault response of the solid-state transformer. The system-level coordinated control in step S2 includes the following steps.

[0100] System Coordination and Control Implementation: System-level coordination and control is implemented by a system-level controller. The system controller uses a high-performance DSP chip, which has high-speed computing power and multi-channel data acquisition capabilities. It collects the operating parameters of each module in real time to realize global power scheduling, module voltage adaptive balancing, and fault response and reconfiguration. The specific implementation steps are as follows.

[0101] (1) Parameter acquisition: The system controller acquires the three-phase high-voltage AC input voltage and input current in real time through voltage sensors and current sensors, as well as the high-voltage DC side voltage of each modular power unit in each phase unit. Low-voltage DC side output voltage Output current It also receives the operating status signal of each power conversion phase; simultaneously, it receives the operating status and power output value fed back by the module controller of each modular power unit through the communication interface. Parameters such as midpoint potential deviation ΔVc are collected to comprehensively acquire all system operating parameters, providing data support for subsequent control decisions. Communication interface includes a CAN bus. Operating status is indicated as normal / fault.

[0102] (2) Global power scheduling: The system controller sets the total power target value of the system based on the collected load demand signal. Simultaneously, it summarizes the actual power output of all normal modular power units. The actual total active power of the system was calculated. ;Will and By comparison, the power deviation ΔP is obtained. The deviation is input into the PI controller to generate the total system power command. Subsequently, based on the number of valid modules in each phase unit, faulty modules are eliminated, and... The power is evenly distributed to the three phase units A, B, and C, resulting in power commands for each phase unit. _l; then transfer the power commands of each phase unit The power is evenly distributed to each normal modular power unit within this phase unit, resulting in an individualized power reference value for each module. _lk ensures that the power output of each module matches the total system demand, maintaining the system's energy balance.

[0103] (3) Adaptive equalization adjustment of module voltage: The system controller calculates the average voltage of the high voltage DC side of all series-connected modular power units in each phase unit in real time. ; , where n is the number of normal modular power units in that phase; for each modular power unit, calculate its voltage deviation. ; ;like >0, this module's voltage is too high, so it will be... _lk decrease , and Positive correlation, calculated by a preset algorithm, reduces the module's power output, causing its DC-side voltage to drop; if If the voltage is less than 0, the module voltage is too low, so it should be... _lk increases This increases the module's power output, causing its DC-side voltage to rise; through this closed-loop adaptive adjustment, it is updated every 10ms. _lk ensures that the voltage deviation on the high-voltage DC side of all modular power units within the same phase unit is controlled within ±5%, thereby achieving voltage balance between modules.

[0104] (4) Fault Response and Reconfiguration: The system controller monitors the operating status of each modular power unit and each power conversion phase in real time. First, it clarifies the specific judgment criteria for serious system-level faults to ensure that fault identification is accurate and practical. The specific judgment thresholds are as follows: 1) Abnormal input voltage: According to GB / T 12325-2022 "Power Quality Supply Voltage Deviation", if the three-phase high-voltage AC input voltage deviates from the rated value by ±15% and the duration is ≥50ms, it is judged as an abnormal input voltage; 2) Short circuit current judgment: According to the safety standards for power electronic equipment, if the short circuit current on the input side is ≥3 times the rated input current, or the short circuit current on the output side is ≥2.5 times the rated output current, a short circuit fault judgment is triggered instantaneously; 3) Batch module failure: ≥30% of the modular power units in the same phase unit fail, or all three phase units have faulty modules and the total proportion of faulty modules is ≥20%; 4) Failure of key components: Magnetic saturation of the high-frequency isolation transformer and batch damage of power switching devices. When a fault is detected in a modular power unit, a command is immediately issued to open the input circuit breaker of this module, isolating the faulty module and marking it as faulty. Simultaneously, the number of effective modules in that phase unit is recalculated, and the individualized power reference value Pref_lk of the remaining normal modules in that phase unit is adjusted to distribute the power of the faulty module to the normal modules, ensuring that the power output of that phase unit remains stable. If a fault occurs in a power conversion phase, the module controller immediately feeds back the fault signal to the system controller, which then controls the module to enter derating operation mode, adjusting the module's... The power allocation is adjusted from 0.1k to 2 / 3 of the normal power, while the power distribution of other normal modules is also adjusted. For power reallocation after fault reconstruction, a priority strategy is adopted to ensure operability and power supply reliability, prioritizing the power supply to critical loads. The specific priorities are as follows: Level 1 Priority - Mandatory: Critical loads such as energy storage battery charging and discharging circuits and core control power supplies are given priority to allocate sufficient power to ensure uninterrupted operation; Level 2 Priority - Guaranteed: DC microgrid main circuit loads are allocated remaining power after meeting Level 1 priority requirements; Level 3 Priority - Adaptable: Ordinary low-voltage electrical equipment is flexibly allocated based on remaining power, and power supply can be suspended when power is insufficient. During power reallocation, the system controller prioritizes allocating available power to Level 1 priority loads, then sequentially to Level 2 and Level 3 loads, ensuring stable power supply to critical loads and improving the operability and practicality of fault reconstruction. If a severe system-level fault is detected, an emergency shutdown procedure is immediately triggered, cutting off the input power to all modules and issuing an alarm signal to prevent the fault from escalating and causing equipment damage, ensuring the safety of personnel and equipment.

[0105] Example 6: Figures 6-11As shown, a control method for a three-level cascaded solid-state transformer is disclosed, wherein in step S3, the module-level autonomous control implementation: the module controller of each modular power unit adopts an independent MCU chip, which communicates with the system-level controller via a CAN bus to receive individualized power reference values. After _lk, the module autonomously completes the control of the three-level conversion circuit within the module without the need for real-time intervention from the system-level controller. The specific implementation steps are as follows.

[0106] a) Input-level NPC three-level rectification control implementation: The module controller collects the high-voltage AC input voltage and input current of this module in real time, as well as the voltage of the high-voltage DC support capacitors C1 and C2. and The specific steps are as follows: The voltage and current dual closed-loop control based on the dq axis and the midpoint potential balance control are performed.

[0107] (1) Coordinate transformation: The collected three-phase high-voltage AC input voltage and input current are converted into voltage in a two-phase stationary coordinate system (α-β axis) through Clark transformation. , and current , Then, it is converted into voltage in a rotating coordinate system (dq axis) through Park transformation. , and current , This provides a foundation for dual closed-loop control.

[0108] (2) Voltage loop adjustment: Set the target value of the high voltage DC bus voltage of this module. According to the system design, in this embodiment, the AC input voltage of the module is 1.414 times, and the voltage deviation is calculated. ; ; To measure the voltage of the high-voltage DC bus, Input to the PI regulator, calculate and output active current command. Reactive current command Set to 0 to achieve unity power factor operation.

[0109] (3) Current loop adjustment: Calculate the current deviation; , In the formula, The active current command value on the d-axis is the difference between the actual and the commanded active current value; the d-axis is the coordinate axis in phase with the grid voltage in a rotating coordinate system. The q-axis represents the difference between the commanded and measured reactive current values; the q-axis is a coordinate axis in a rotating coordinate system that is perpendicular to the grid voltage. , Two independent PI controllers are input separately, and the output dq-axis voltage commands are calculated and output. , ; Calculate voltage deviation , ;Will , Converted into an α-β axis voltage modulation signal via inverse Park transform and Then, a three-phase modulated wave is generated through the inverse Clark transform. , , .

[0110] (4) Midpoint potential balance adjustment: Calculate capacitor voltage deviation , ,like If the voltage is >5V, exceeding the allowable range, the zero-sequence voltage compensation amount will be calculated using a preset algorithm. A positive compensation amount is injected into the three-phase modulated wave, increasing the charging and discharging time of C2 and decreasing the charging and discharging time of C1; if If the voltage is less than -5V, then inject negative compensation. The charging and discharging times are reversed; the compensated modulation wave is modulated by carrier phase shift to generate PWM drive signals for the eight main switches (S11~S14, S21~S24) of the NPC full-bridge, controlling the switching devices to turn on and off, ensuring... and The deviation is controlled within ±5V, while achieving sinusoidal input current, unity power factor, and DC bus voltage stability.

[0111] b) Implementation of three-phase interleaved DAB-SRC control in the isolation stage: The module controller collects the high-voltage DC voltage on the primary side and the low-voltage DC voltage on the secondary side of the isolation stage in real time, as well as the resonant current of the three power transmission branches, and performs soft switching control, three-phase interleaved drive control and power regulation control. The specific steps are as follows.

[0112] (1) Soft switching control: The resonant frequency of the series resonant network is set to 10kHz, which matches the parameters Lr=10μH and Cr=1μF in this embodiment. The module controller controls the switching transistors (S31~S32, S41~S42, S51~S52) of the three power transmission branches to always operate at a fixed resonant frequency, ensuring that the resonant current starts from zero in each switching cycle, and naturally decays to zero after half a resonant cycle to complete the energy transfer, thereby realizing zero current switching (ZCS) of the power switching device and reducing switching losses.

[0113] (2) Three-phase staggered drive control: The module controller generates drive signals for three power transmission branches, so that the drive signals of branches A1, B1 and C1 are staggered by 120° phase. For example, the drive signal of branch A1 is 0°, branch B1 is 120° and branch C1 is 240°, ensuring that the three resonant currents are evenly distributed on the time axis. After superposition, they cancel the DC bus current ripple on the primary and secondary sides, and improve the stability of power transmission.

[0114] (3) Power regulation control: According to this module _lk, calculate the required output power for the isolation stage. This method abandons the traditional power regulation method of adjusting the switching frequency and instead uses the adjustment of the phase difference φ (0 < φ < π) between the primary and secondary half-bridge switching signals to achieve precise power regulation. This method does not change the operating frequency of the series resonant network and can always maintain the resonant state, ensuring that the soft-switching characteristics do not fail and the switching losses are kept at a low level, which is consistent with the core goal of reducing switching losses and improving system efficiency. The specific regulation logic is as follows: when When the output power is greater than the current output power, appropriately increase the phase difference φ to enhance the effective transmission component of the resonant current and increase the energy transmission amount; when When the output power is less than the current output power, the phase difference φ is appropriately reduced to decrease the effective transmission component of the resonant current and reduce the amount of energy transmitted. At the same time, the structural symmetry and magnetic coupling characteristics of the three power transmission branches are used to achieve natural power balance between the branches, ensuring that the output power deviation of the three branches is controlled within ±3%.

[0115] c) Output stage - interleaved parallel DC-DC control implementation: The module controller acquires the low-voltage DC output voltage of this module in real time. The current of the filter inductor in the three bidirectional half-bridge phase transformations. , and The current and voltage dual-loop control and interleaved modulation control are executed, and the specific steps are as follows.

[0116] (1) Voltage loop adjustment: Set the target value of the low-voltage DC output voltage of this module. In this embodiment, the voltage is 750V. The voltage deviation is calculated. , ,Will Input to the PI regulator to generate the total output current command. .

[0117] (2) Current loop adjustment: Inductor current reference evenly distributed across three phases ; Calculate the current deviation for each phase , , The three current deviations are input into the PI regulator, which outputs the corresponding modulation signals.

[0118] (3) Interleaved modulation control: The module controller generates three phase PWM drive signals, so that the drive signals of phase 1, 2 and 3 are staggered by 120° phase, and control the switching transistors (S61~S66) of each phase to turn on and off; through phase interleaving, the inductor current ripple of the three phases is staggered and superimposed and canceled out, ensuring that the peak value of the module output current ripple is ≤5% of the rated current, which meets the high power quality requirements.

[0119] (4) Fault-tolerant control: The module controller monitors the operating status of the three bidirectional half-bridge conversion phases in real time. When an abnormal inductor current or a switching transistor drive failure is detected in a certain phase, the two switching transistors of that phase are immediately turned off to isolate the faulty phase; at the same time, the voltage loop output is adjusted. The power is reduced to 2 / 3 of the normal value and redistributed to the remaining two normal phases, allowing the module to continue operating in derating mode, ensuring stable module output power and preventing the fault from escalating.

[0120] Furthermore, to ensure the coordinated operation of system-level coordinated control and module-level autonomous control and to avoid control conflicts, a unified control timing and synchronization mechanism is established to coordinate control timing and synchronization.

[0121] (1) Timing synchronization: The system-level controller and all module controllers use the same clock source (100MHz) to ensure consistent control timing; the system-level controller sends a timer to all module controllers every 10ms. Upon receiving the synchronization signal, the module controller immediately updates its own control parameters to complete power regulation and equalization control.

[0122] (2) Communication synchronization: The system-level controller and the module controller communicate via CAN bus. The communication baud rate is set to 1Mbps to ensure the real-time transmission of parameters. The module controller feeds back its own operating status parameters to the system-level controller every 5ms. The system-level controller adjusts the global power scheduling and voltage balancing strategy in real time based on the feedback parameters.

[0123] (3) Fault synchronization: When the module controller detects its own fault, it immediately sends a fault signal to the system controller. The system controller responds within 1ms and completes fault isolation and power reconfiguration. At the same time, the system controller feeds back the fault information to the module controller to guide the faulty module to enter a safe state and avoid the spread of the fault.

[0124] The three-phase AC power is connected to independent cascaded power units of phases A, B, and C. Each modular power unit independently completes the complete power conversion process from AC rectification and isolation to DC output. After being converted by the power module cluster within each phase, the electrical energy first converges at the DC output port of that phase, and then the DC outputs of the three phases converge again on a unified low-voltage DC bus, ultimately forming a modular, highly reliable solid-state transformer topology with electrical isolation characteristics.

[0125] Example 7: A control method for a three-level cascaded solid-state transformer is disclosed, wherein step S4 includes the following steps.

[0126] S4.1 Fault Detection and Identification: The system-level controller identifies fault types in real time based on its own collected parameters and fault signals fed back by the module controllers, including: modular power unit faults, power conversion phase faults, and system-level faults. Modular power unit faults include abnormal high-voltage DC side voltage and zero power output; power conversion phase faults include damaged switching transistors and abnormal inductor current; system-level faults include abnormal input voltage and short circuits.

[0127] S4.2 Fault isolation.

[0128] S4.2.1 If a fault is detected in a modular power unit, the system-level controller immediately issues a command to control the input circuit breaker of the module to open, isolate the faulty module, and mark it as a fault state to prevent the fault from spreading to other modules.

[0129] S4.2.2 If a fault is detected in a power conversion phase, the module controller immediately shuts down the switch of that phase, isolates the faulty phase, and sends a fault signal to the system-level controller.

[0130] S4.3 System Reconfiguration and Power Redistribution.

[0131] S4.3.1 If the fault is a modular power unit, the system-level controller recalculates the number of valid modules in that phase unit and adjusts the individualized power reference values ​​of the remaining normal modules in that phase unit. _lk distributes the power of the faulty module to the normal module, ensuring that the power output of that phase unit remains stable.

[0132] S4.3.2 If the fault is a power conversion phase fault, the system-level controller will control the module to enter derating operation mode and adjust the module's... Reduce the power output of the system to 2 / 3 of its normal power, and simultaneously adjust the power distribution of the remaining normal modules to ensure that the overall power output of the system meets the basic load requirements.

[0133] S4.4 Fault Alarm and Recording: The system-level controller issues a fault alarm signal and records the fault type, fault location, fault occurrence time, and system parameters at the time of the fault for subsequent maintenance. If the fault is a serious system-level fault, the power supply to the entire system is immediately cut off to avoid equipment damage and safety accidents.

[0134] Example 8: A control method for a three-level cascaded solid-state transformer is disclosed, wherein step S5 includes the following steps.

[0135] S5.1 When the system receives a shutdown command or detects an unrepairable serious fault, the system-level controller sends a shutdown signal, and all module controllers respond synchronously.

[0136] S5.2 The module controller gradually reduces its own power output at a rate of 10% of rated power per second. After the power output drops to 0, it shuts down the switching devices of the three-stage conversion circuit and stops energy transmission. Then, it shuts down the soft start circuit first, and then disconnects the input circuit breaker and the output circuit breaker in sequence to avoid damage to the devices by inrush current.

[0137] S5.3 The system-level controller monitors the shutdown status of all modules. After confirming that all modules have been safely shut down, it cuts off the main power supply to the system and completes the shutdown process.

[0138] S5.4 Save the system operating parameters and fault records before shutdown to provide a reference for subsequent system startup and maintenance.

[0139] The above are merely preferred embodiments of this application. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A three-level cascaded solid-state transformer topology, characterized in that: It includes three access circuits, three phase units, and a common combiner terminal. The three access circuits correspond one-to-one with the three phases of the AC power grid and one-to-one with the three phase units. The input terminal of the access circuit is connected to the corresponding phase of the AC power grid, and the output terminal is connected to the corresponding phase unit. Each phase unit contains at least two power units, which include a high-voltage AC circuit, an isolation conversion circuit, and a low-voltage DC circuit. The input terminal of the high-voltage AC circuit is connected to the output terminal of the access circuit, the output terminal of the high-voltage AC circuit is connected to the input terminal of the isolation conversion circuit, the output terminal of the isolation conversion circuit is connected to the input terminal of the low-voltage DC circuit, and the output terminal of the low-voltage DC circuit is connected to the common combiner terminal. Within the same phase unit, the high-voltage AC circuits of each power unit are connected in series, and the low-voltage DC circuits of each power unit are connected in parallel and then connected to the common combiner terminal. The low-voltage DC circuits of all three phase units are connected to the common combiner terminal, which is connected to the DC power grid.

2. The three-level cascaded solid-state transformer topology according to claim 1, characterized in that: The solid-state transformer adopts a three-stage scalable topology, consisting of a high-voltage AC input stage, an isolation conversion stage, and a low-voltage DC output stage connected in series. The high-voltage AC input stage includes three independent phase units, which are connected to a three-phase high-voltage AC power grid. Each phase unit is composed of several modular power units connected in series. The modular power units of the entire system have the same structure and are connected in series on the high-voltage side and in parallel on the low-voltage side.

3. The three-level cascaded solid-state transformer topology according to claim 2, characterized in that... Each modular power unit independently integrates a three-stage conversion circuit, specifically including: The input side adopts a diode-midpoint clamped three-level full-bridge topology as the rectifier circuit, which includes two series-connected high-voltage DC support capacitors and a power switch bridge arm with clamping diodes. The middle side uses a three-phase interleaved series resonant dual active bridge as the isolation converter circuit; The output side adopts a three-phase interleaved parallel DC-DC converter topology, which consists of three bidirectional half-bridge conversion phases.

4. A method for controlling a three-level cascaded solid-state transformer topology as described in any one of claims 1-3, characterized in that, include: Initialization, parameter loading, and status self-checking are completed synchronously for solid-state transformers, system-level controllers, and module controllers of each modular power unit; System-level coordinated control: Real-time acquisition of system operating parameters and the status of each modular power unit, power allocation commands, and dynamic adjustment of the individualized power reference values ​​of each modular power unit to achieve adaptive voltage balance among modular power units; Module-level autonomous control: After receiving individualized power reference values, each module controller completes the entire process control of rectification, isolation conversion and output regulation within its module; The system-level controller and module controller work together to monitor faults, isolate faulty units, and achieve power reconfiguration by reallocating individualized power reference values ​​for normal modular power units.

5. The three-level cascaded solid-state transformer topology control method according to claim 4, characterized in that... System-level coordination and control includes the following steps: Parameter acquisition involves collecting operating parameters of the system and each modular power unit through sensors, and receiving feedback parameters from the controllers of each module. Global power scheduling sets a total power target value based on load demand, generates a total power command through proportional-integral regulator calculation, and distributes it to each normal modular power unit to obtain an individualized power reference value. The module voltage adaptive equalization adjustment dynamically adjusts the individualized power reference value to keep the voltage between the modular power units balanced. Fault response and reconfiguration: Monitor faults and isolate faulty units, and reallocate individualized power reference values ​​for each normal modular power unit to achieve power reconfiguration.

6. The three-level cascaded solid-state transformer topology control method according to claim 4, characterized in that... Module-level autonomous control includes the following steps: The input stage features three-level rectification control. The module controller acquires the high-voltage AC input voltage, input current, and voltages of the two high-voltage DC supporting capacitors of this module. It executes voltage and current dual closed-loop control and midpoint potential balance control based on a rotating coordinate system, including coordinate transformation, voltage loop adjustment, current loop adjustment, and midpoint potential balance adjustment. The isolation stage three-phase interleaved isolation conversion control module controller collects the DC voltage of the high and low voltage sides of the isolation stage and the resonant current of the three power transmission branches, and executes soft switching control, three-phase interleaved drive control and power regulation control; The output stage features interleaved parallel DC-DC converter control. The module controller collects the low-voltage DC output voltage of this module and the filter inductor current of the three bidirectional half-bridge conversion phases, and performs current and voltage dual-loop control and interleaved modulation control, including voltage loop regulation, current loop regulation, interleaved modulation control and fault-tolerant control.

7. The three-level cascaded solid-state transformer topology control method according to claim 6, characterized in that: The input-level three-level rectification control includes: the module controller acquires the high-voltage AC input voltage, input current and high-voltage DC support capacitor voltage of this module, executes voltage and current dual closed-loop control and midpoint potential balance control based on a rotating coordinate system, generates drive signals to control the switching devices to turn on and off, and realizes sinusoidal input current, unity power factor and DC bus voltage stability.

8. The method for controlling the topology of a three-level cascaded solid-state transformer according to claim 6, characterized in that: The isolation stage three-phase interleaved isolation conversion control includes: the module controller collects the DC voltage of the high and low voltage sides of the isolation stage and the resonant current of the three power transmission branches, and executes soft switching control, three-phase interleaved drive control and power regulation control to achieve precise power regulation and natural power balance between branches.

9. The method for controlling the topology of a three-level cascaded solid-state transformer according to claim 6, characterized in that: The output stage interleaved parallel DC-DC converter control includes: the module controller collects the low-voltage DC output voltage of this module and the filter inductor current of the three bidirectional half-bridge conversion phases, performs current and voltage dual-loop control and interleaved modulation control, and has fault tolerance capability, isolates fault phases and makes the module operate at a reduced rate.

10. The three-level cascaded solid-state transformer topology control method according to claim 4, characterized in that... Establish a unified control timing and synchronization mechanism, including timing synchronization, communication synchronization, and fault synchronization.

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