A solid state transformer start-up and module bypass switching control method and system

Abstract

The application provides a solid-state transformer starting and module bypass switching control method and system, relates to the field of power electronic converters, and solves the problem that in the prior art, when a redundant standby module of a solid-state transformer is put into operation, a large current overshoot is easily generated during charging of a DC bus capacitor, which leads to device damage and unreliable bypass switching. The method comprises: a starting control strategy, a module rotation rest control strategy and a fault switching control strategy; the starting control strategy divides the starting process into an internal DC link uncontrolled charging stage, a power module AC / DC boosting stage, a load side DC charging stage and a load side DC / DC H-bridge unlocking stage; under the module rotation rest control strategy, the control system controls the power modules to rotate and rest in turn, so that any one of the power modules is periodically in a redundant standby state; and the fault switching control strategy is used to remove a faulty module and put a module in a redundant standby state into operation when a fault occurs in the power module. The application is used for solid-state transformer starting and module bypass switching control.

Description

Technical Field

[0001] This application relates to the field of power electronic converter technology, and in particular to a solid-state transformer start-up and module bypass switching control method and system. Background Technology

[0002] Solid-state transformers (SSEs) are key equipment in power distribution networks and play a vital role in renewable energy integration and smart microgrid construction. Currently, SSEs often employ modular structures such as series-input parallel-output (ISOP) and are configured with redundant modules to address the failure issues of devices like SiC MOSFETs. Existing redundancy backup schemes are mainly divided into cold standby and hot standby. However, in cold standby schemes, the initial voltage of the bus capacitor of the redundant module is zero, requiring a long charging time through current-limiting resistors upon activation, leading to power deficit during fault switching and necessitating a large design margin for non-faulty modules. While hot standby schemes eliminate charging delays, all modules operate continuously, making it impossible to verify the backup function before a fault, posing a safety hazard. Furthermore, regardless of whether it's cold or hot standby, the module switching timing, carrier modulation coordination, and current phase control are complex during startup and fault switching, easily leading to overcurrent or system oscillations, affecting power supply reliability. Therefore, existing technologies suffer from the technical problem of large current overshoot during the charging of the DC bus capacitor when the redundant standby module of the SSE is activated, causing device damage and hindering reliable bypass switching. Summary of the Invention

[0003] This application provides a method and system for starting up and switching the module bypass of a solid-state transformer, which solves the technical problem in the prior art where the DC bus capacitor charging is prone to large current overshoot when the redundant backup module of the solid-state transformer is put into operation, resulting in device damage and unreliable bypass switching.

[0004] To achieve the above objectives, this application adopts the following technical solution: Firstly, a method for starting up and switching modules of a solid-state transformer is provided, including: a start-up control strategy, a module rotation control strategy, and a fault switching control strategy. The start-up control strategy divides the start-up process into an internal DC-link uncontrolled charging stage, a power module AC / DC boost stage, a load-side DC charging stage, and a load-side DC / DC H-bridge unlocking stage. Under the module rotation control strategy, the control system controls the power modules to rotate in turn, so that any one power module is periodically in a redundant standby state. The fault switching control strategy is used to disconnect the faulty module and put the module in the redundant standby state into operation when a power module fails. In conjunction with the first aspect mentioned above, in one possible implementation, the uncontrolled charging phase of the internal DC link includes: closing the soft-start switch to charge the DC link capacitors of n+1 power modules, including redundant backup modules, through the soft-start resistor; after the voltage reaches the allowable level, closing the grid-connected switch bypass soft starter to continue charging until the voltage stabilizes.

[0005] In conjunction with the first aspect above, in one possible implementation, the module rotation control strategy includes: during the i-th rotation, closing the bypass switch of power module i, blocking its load-side DC / DC circuit, and disconnecting power module i; unlocking the load-side DC / DC circuit of the power module that was in standby status in the previous rotation; adjusting the carrier phase shift angle of the remaining working modules; after the system stabilizes, disconnecting the bypass switch of power module i, making it a new redundant standby module, and restoring the carrier phase configuration.

[0006] In conjunction with the first aspect above, in one possible implementation, the fault switching control strategy includes: when power module x fails, closing the bypass switch of power module x and locking the load-side DC / DC circuit of power module x; unlocking the load-side DC / DC circuit of power module y, which is currently in redundant standby state; adjusting the carrier phase shift angle of the remaining working modules and exiting the rotation function.

[0007] In conjunction with the first aspect mentioned above, in one possible implementation, during the execution of the module rotation control strategy and the fault switching control strategy, the control system calculates the target current amplitude and phase after switching based on the changes in grid voltage, reactor inductance and module chain output voltage, and adjusts the phase current magnitude and phase to bring the system into a steady state.

[0008] In conjunction with the first aspect mentioned above, in one possible implementation, the carrier phase shift angle of the remaining working modules is adjusted as follows: when all n+1 modules are running, the carrier phase shift angle is adjusted to 2π / (n+1); when n modules are running, the carrier phase shift angle is adjusted to 2π / n.

[0009] Secondly, a solid-state transformer starting and module bypass switching control system is provided, including: a main power circuit and a control system; a soft starter is provided in the main power circuit, the soft starter is composed of a soft starter switch and a high-power resistor connected in series, and the soft starter is connected in parallel with the grid-connected switch in the main power circuit; a power module valve group is provided in the main power circuit, each phase power module valve group contains several power modules, and the power modules are provided with parallel static equalizing resistors and bypass switches, with both ends connected to AC lines; the control system is used to execute the starting control strategy, the module rotation control strategy, and the fault switching control strategy.

[0010] In conjunction with the second aspect above, in one possible implementation, one end of the soft starter is connected between the reactor and the grid-connected switch, and the other end is connected between the AC voltage sampling sensor and the fuse group, in order to limit the charging current during the startup phase.

[0011] In conjunction with the second aspect above, in one possible implementation, the power module contains three H-bridge circuits. The first H-bridge circuit is used for AC / DC conversion on the rectifier side, forming an independent internal DC link. The second and third H-bridge circuits are respectively connected to the primary and secondary sides of the isolation transformer for DC / DC power transmission on the load side. A voltage sampling sensor is provided in the internal DC link.

[0012] In conjunction with the second aspect mentioned above, in one possible implementation, the control system includes a three-level architecture of a main controller, phase controllers, and module controllers; the main controller is responsible for system-level scheduling and electrical quantity control; the phase controllers are responsible for the coordinated control of each power module in their respective phases; and the module controllers are responsible for the bypass switch control, AC / DC modulation, and DC / DC power transmission control of their respective modules.

[0013] This application provides a method and system for starting up and switching modules of a solid-state transformer. By setting a soft starter in the main power circuit and a bypass switch in the power module, along with a three-level control architecture, it realizes current-limiting charging during the startup process and module rotation during normal operation. Through the module rotation control strategy, each power module can periodically operate as a backup module, thereby verifying the effectiveness of the backup module and its switching control logic without affecting the normal operation of the system. This solves the problems of traditional cold standby being unverifiable and hot standby being costly and unable to verify the switching logic. Simultaneously, in the event of a fault, the fault switching control strategy can quickly disconnect the faulty module and activate the verified backup module. Combined with dynamic adjustment of the carrier phase shift angle and precise calculation of the current phase, seamless switching under fault conditions is achieved, greatly improving the power supply reliability and operational stability of the solid-state transformer system. This solves the technical problem in existing technologies where the DC bus capacitor charging is prone to large current overshoot when the redundant backup module of the solid-state transformer is activated, leading to device damage and unreliable bypass switching.

[0014] It should be understood that the descriptions of technical features, technical solutions, beneficial effects, or similar language in this application do not imply that all features and advantages can be achieved in any single embodiment. Rather, it is understood that the description of a feature or beneficial effect means that a specific technical feature, technical solution, or beneficial effect is included in at least one embodiment. Therefore, the descriptions of technical features, technical solutions, or beneficial effects in this specification do not necessarily refer to the same embodiment. Furthermore, the technical features, technical solutions, and beneficial effects described in this embodiment can be combined in any suitable manner. Those skilled in the art will understand that embodiments can be implemented without one or more specific technical features, technical solutions, or beneficial effects of a particular embodiment. In other embodiments, additional technical features and beneficial effects may be identified in specific embodiments that do not embody all embodiments. Attached Figure Description

[0015] Figure 1 A system architecture diagram of a solid-state transformer startup and module bypass switching control system provided in this application embodiment; Figure 2 A schematic diagram of the connection of a module controller interface, a driver adapter circuit, and a module power circuit is provided for an embodiment of this application; Figure 3 This is a schematic diagram of a main controller interface structure provided in an embodiment of this application; Figure 4 This is a schematic diagram of a phase controller interface structure provided in an embodiment of this application; Figure 5 A flowchart illustrating a solid-state transformer startup and module bypass switching control method provided in an embodiment of this application; Figure 6 A flowchart illustrating another solid-state transformer startup and module bypass switching control method provided in this application embodiment; Figure 7 A flowchart illustrating another solid-state transformer startup and module bypass switching control method provided in this application embodiment; Figure 8 A flowchart illustrating another solid-state transformer startup and module bypass switching control method provided in this application embodiment; Figure 9 A phasor diagram of voltages on the rectifier side before and after bypass switch switching is provided in an embodiment of this application; Figure 10 This application provides a schematic diagram of the modulation principle when power modules are connected in series. Figure 11 This is a flowchart illustrating another solid-state transformer startup and module bypass switching control method provided in an embodiment of this application. Detailed Implementation

[0016] In the description of this application, unless otherwise stated, " / " means "or," for example, A / B can mean A or B. The "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, and B alone. Furthermore, "at least one" means one or more, and "multiple" means two or more. The terms "first," "second," etc., do not limit the quantity or order of execution, and "first," "second," etc., do not necessarily imply differences.

[0017] It should be noted that, in this application, the terms "exemplary" or "for example" are used to indicate that something is being described as an example, illustration, or illustration. Any embodiment or design described as "exemplary" or "for example" in this application should not be construed as being more preferred or advantageous than other embodiments or design solutions. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0018] Example 1: This embodiment provides a solid-state transformer start-up and module bypass switching control system. For example... Figure 1 As shown, the system includes a main power circuit and a control system.

[0019] A soft starter is installed in the main power circuit. The soft starter consists of a soft starter switch and a high-power resistor connected in series, and the soft starter is connected in parallel with the grid-connected switch in the main power circuit.

[0020] Specifically, one end of the soft starter is connected between the reactor and the grid-connected switch, and the other end is connected between the AC voltage sampling sensor and the fuse group. During the initial startup phase, the grid-connected switch is open, and the grid voltage, after being current-limited by the high-power resistor in the soft starter, charges the subsequent circuits. This parallel structure design, compared to the scheme where a series resistor in the main circuit needs to be physically removed, automatically bypasses the soft starter by closing the grid-connected switch, achieving seamless removal of the current-limiting resistor. This ensures the safety of the startup process and simplifies the operation. It should be understood that although... Figure 1 The specific connection positions of the soft starter and the grid-connected switch are shown. However, in other embodiments, as long as the soft starter can be used to limit current in the circuit during startup and be bypassed during normal operation, the specific connection nodes can be adjusted according to the circuit layout.

[0021] The main power circuit is equipped with a power module valve group. Each phase power module valve group contains several power modules. The power modules are equipped with parallel static equalizing resistors and bypass switches, and both ends are connected to AC lines.

[0022] Specifically, the static equalizing resistor is used to force the voltage across each module to be evenly distributed when the module is in a high-resistance state or not conducting, preventing voltage unevenness caused by device parameter dispersion from damaging the device. The bypass switch serves as the physical channel during module failure or rotational shutdown. When the switch is closed, the two ends of the module are short-circuited, and the current flows directly through the switch without passing through the module's internal circuitry. This structure, which connects the static equalizing resistor and the bypass switch in parallel, cleverly reuses the connection point of the equalizing resistor, saving space and ensuring that the voltage across the module is balanced by the equalizing resistor at the moment the bypass switch closes, avoiding transient overvoltage surges during switch operation. Figure 1 In the example, the power module valve stack contains three phases, A, B, and C, with each phase consisting of multiple power modules connected in series to withstand medium and high voltage grid voltages.

[0023] The control system is used to execute startup control strategies, module rotation control strategies, and fault switching control strategies.

[0024] Specifically, the control system, acting as the "brain" of the entire system, is responsible for acquiring electrical signals such as voltage and current in the main power circuit and issuing control commands according to preset logic. The startup control strategy manages the process of the solid-state transformer from shutdown to grid connection; the module rotation control strategy periodically switches standby modules during normal operation to verify their availability; and the fault switching control strategy quickly isolates faulty modules and activates standby modules in the event of a sudden fault. Through this hardware architecture, the control system achieves refined management of the entire lifecycle of the solid-state transformer.

[0025] Through the above scheme, this embodiment constructs the hardware foundation for the solid-state transformer startup and module bypass switching control system. The parallel structure of the soft starter and grid-connected switch provides a physical path for current limiting during the startup phase, while the parallel structure of the static voltage equalizing resistor and bypass switch in the power module provides an execution unit for module-level bypass operations. The control system coordinates the execution of these three strategies. This architecture design not only solves the problems of large startup impact and difficult fault clearing in traditional solid-state transformers, but also provides the necessary hardware support for the rotation verification and seamless switching described in subsequent embodiments, ensuring the high reliability and high availability of the system.

[0026] Example 2: Based on Example 1, this embodiment provides a detailed description of the specific hardware connection relationships and internal structure of the solid-state transformer startup and module bypass switching control system.

[0027] One end of the soft starter is connected between the reactor and the grid-connected switch, and the other end is connected between the AC voltage sampling sensor and the fuse group. It is used to limit the charging current during the startup phase.

[0028] Specifically, combined Figure 1As shown, during the initial power-on phase, the grid-connected switch is in the open state. The grid voltage, after passing through the reactor, first flows through the soft starter, then through the fuse bank to the power module valve bank. The high-power resistor connected in series inside the soft starter plays a major voltage-dividing role at this moment, effectively limiting the charging current amplitude flowing through the capacitors inside the subsequent power modules, preventing overcurrent damage caused by instantaneous short-circuit effects in the capacitors. It should be understood that placing the soft starter at this location, compared to placing it after the fuse bank, provides more comprehensive protection for the fuse bank and all subsequent power devices. If the soft starter were placed at the grid-side inlet, it would be unable to limit the inrush current of the reactor itself; if placed inside the power module, it would be unable to uniformly limit the charging current of the rectifier module. Therefore, this location choice achieves global current-limiting protection for the rectifier-side charging circuit.

[0029] The power module contains three H-bridge circuits. The first H-bridge circuit is used for AC / DC conversion on the rectifier side, forming an independent internal DC link. The second and third H-bridge circuits are connected to the primary and secondary sides of the isolation transformer, respectively, for DC / DC power transmission on the load side. A voltage sampling sensor is installed in the internal DC link.

[0030] Specifically, combined Figure 2 As shown, the power module, as the core conversion unit of the solid-state transformer, is designed with a three-stage conversion structure. The first H-bridge circuit, as the rectifier stage, is directly connected to the high-voltage AC line. It converts AC to DC using pulse width modulation (PWM) technology and establishes a stable DC voltage on its DC-side capacitor; this DC link is called the internal DC chain. This internal DC chain is a key node for energy storage and buffering within the power module. A voltage sampling sensor monitors this voltage value in real time, providing feedback signals to the control system. The second H-bridge circuit is connected to the primary winding of the isolation transformer, and the third H-bridge circuit is connected to the secondary winding of the isolation transformer. Energy is transferred between them through a high-frequency transformer, achieving electrical isolation and voltage matching between the high-voltage and low-voltage sides. This cascaded "AC / DC-DC / DC" structure decouples the control of the rectifier side from that of the load side. The rectifier side focuses on maintaining DC chain voltage stability and power factor correction, while the load side focuses on output voltage regulation and energy transfer, greatly improving the system's control flexibility and stability.

[0031] The control system comprises a three-tier architecture: a main controller, phase controllers, and module controllers. The main controller is responsible for system-level scheduling and electrical quantity control. The phase controllers are responsible for the coordinated control of each power module in their respective phases. The module controllers are responsible for the bypass switch control, AC / DC modulation, and DC / DC power transmission control of their respective modules.

[0032] Specifically, combined Figure 2 Schematic diagram of module controller interface, driver adapter circuit and module power circuit connection. Figure 3 Main controller interface structure diagram and Figure 4 As shown in the schematic diagram of the phase controller interface structure, the control system adopts a hierarchical distributed architecture to cope with the complex control requirements of multiple modules and multiple variables of the solid-state transformer. The main controller, as the top-level core, is as follows: Figure 3 As shown, it communicates with each phase controller via fiber optic interfaces, responsible for receiving global electrical quantity signals such as grid voltage and current, executing system-level scheduling commands, such as grid connection and shutdown, and performing overall closed-loop control of electrical quantities. The phase controller acts as an intermediate layer, such as... Figure 4 As shown, it is responsible for coordinating the operation of all series-connected power modules on a single-phase bridge arm and handling issues such as phase-to-phase balancing. The module controller, as the underlying execution unit, is as follows: Figure 2 As shown, the controller is directly embedded within each power module, responsible for executing the lowest-level switching actions, including driving the bypass switch to open and close, generating PWM modulation signals for the AC / DC H-bridge, and controlling power transmission at the DC / DC level. This three-tier architecture decomposes complex control tasks: the main controller handles slow global variables, the module controllers handle fast local variables, and the phase controllers act as a bridge between the main and module controllers. This ensures both unified global scheduling of the system and real-time response of each power module, avoiding control delays caused by excessive computational load on a single controller. It also provides computational power support for the rapid fault switching and rotation control described in subsequent embodiments.

[0033] Example 3: This embodiment provides a solid-state transformer startup and module bypass switching control method, applicable to solid-state transformer startup and module bypass switching control systems as described in Embodiment 1 or 2. Figure 5 As shown, the method includes a startup control strategy, a module rotation control strategy, and a fault switching control strategy: S501, Execute the startup control strategy, dividing the startup process into the internal DC link uncontrolled charging stage, the power module AC / DC boost stage, the load-side DC charging stage, and the load-side DC / DC H-bridge unlocking stage.

[0034] The startup control strategy refers to a series of logical steps by which the control system smoothly transitions the solid-state transformer from a shutdown state to a normal operating state.

[0035] In this embodiment, the control system first executes an uncontrolled charging phase of the internal DC link, using the grid voltage through a current-limiting circuit to charge the DC link capacitor inside the power module. Then, it enters the AC / DC boost phase of the power module, actively controlling the rectifier-side H-bridge circuit to boost the DC link voltage to the target value. Next, it enters the load-side DC charging phase, charging the output capacitor on the load side. Finally, it enters the load-side DC / DC H-bridge unlocking phase, releasing the lockout on the DC / DC circuit and allowing it to begin transmitting power. It should be understood that although this embodiment divides the startup process into four phases, in other embodiments, depending on the system capacity or load characteristics, certain phases may be combined or further subdivided, as long as a smooth establishment from zero voltage to the rated operating state can be achieved.

[0036] It should be noted that the phased startup design is intended to cope with the huge inrush current generated by the large capacitive load in the solid-state transformer at the moment of startup, and to prevent damage to the power devices.

[0037] As an example, in a 10KV solid-state transformer system, the control system triggers the switching of the above four stages sequentially according to preset time and voltage thresholds.

[0038] Based on the above steps, the phased startup control strategy effectively limits the inrush current during startup, protects the capacitors and switching devices inside the power module, and ensures safe startup of the equipment.

[0039] S502, Execute the module rotation control strategy, the control system controls the power modules to rotate in turn, so that any one power module is in a redundant standby state periodically.

[0040] Among them, the module rotation control strategy refers to the control logic in which, during the normal operation of the system, the control system periodically designates one power module to exit the working state and switch to the standby state, while the original standby module is put into operation.

[0041] In this embodiment, the control system sequentially performs a "cut-off-start" operation on each power module according to a preset rotation cycle. During the i-th rotation, the control system cuts off the currently operating power module i, putting it into a redundant standby state, while simultaneously starting the power module that was in standby mode in the previous cycle. This rotation mechanism ensures that every power module in the system is rotated to a standby position within a certain period. It should be understood that the rotation cycle can be adjusted according to the module's reliability indicators or maintenance requirements, for example, set to rotate every 24 hours or 168 hours.

[0042] It should be noted that in traditional hot standby schemes, the standby module is always online, making it impossible to verify whether it can successfully take over the work in the event of a failure; while in cold standby schemes, the standby module is idle for a long time, and its functional availability is unknown. This embodiment uses a rotation strategy to force each module to periodically switch between "standby" and "operating" states, thereby verifying the hardware integrity of the standby module and the effectiveness of the switching logic in real time without affecting the normal operation of the system.

[0043] As an example, suppose the system has 11 power modules, and the control system controls the 1st to 11th modules to take turns resting, with each module taking turns to run as a backup module for a period of time.

[0044] Based on the above steps, the module rotation control strategy solves the industry pain point that the backup function cannot be verified in the existing technology, realizes the normalized "check-up" of the backup module and its switching control logic, and greatly improves the reliability and security of the system.

[0045] S503. Execute the fault switching control strategy to disconnect the faulty module and put the module in redundant standby mode into operation when the power module fails.

[0046] Among them, the fault switching control strategy refers to a series of emergency operations in which the control system responds quickly, isolates the faulty module and activates the backup module when a fault is detected in a power module.

[0047] In this embodiment, when power module x fails, the control system immediately closes the bypass switch of the module, physically disconnecting it from the main circuit, and simultaneously locks its internal circuitry to prevent the fault from escalating. At the same time, the control system unlocks power module y, which is currently in redundant standby mode, and immediately puts it into operation to replace the faulty module. It should be understood that the response speed requirement for fault switching is much higher than that for rotation switching, typically needing to be completed within milliseconds to ensure uninterrupted power supply.

[0048] It should be noted that since the system has verified the availability of the backup module through a rotation strategy in normal times, it can be assured that the backup module can be reliably put into operation when a failure occurs, thus avoiding the risk of secondary failure caused by "the backup module may be damaged" in the traditional solution.

[0049] As an example, when an overcurrent fault is detected in power module 5, the control system blocks its drive signal within tens of microseconds and closes its bypass switch within milliseconds, while simultaneously unlocking the drive signal of the backup module.

[0050] Based on the above steps, a fault switching control strategy was implemented to achieve rapid and seamless switching in case of a fault, ensuring the continuous power supply capability of the solid-state transformer system under single-module failure.

[0051] Based on the above technical solutions, this embodiment constructs a control logic framework for the entire lifecycle of a solid-state transformer. The startup control strategy ensures the safe startup of the equipment, the module rotation control strategy innovatively solves the problem of verifying backup functions, and the fault switching control strategy ensures the fault-tolerant operation capability of the system. The three work together to achieve the high-reliability operation of the solid-state transformer.

[0052] Example 4: This embodiment is based on embodiment 3, such as Figure 6 As shown, the specific implementation process of the uncontrolled charging phase of the internal DC link in the startup control strategy is explained in detail: S601. Close the soft start switch to charge the DC link capacitors of the n+1 power modules, including the redundant backup modules, through the soft start resistor.

[0053] Among them, the soft start switch refers to the switching device connected in series inside the soft starter, and the soft start resistor refers to the current limiting element connected in series with the high-power resistor.

[0054] In this embodiment, after detecting a start command, the control system first closes the soft-start switch, while the grid-connected switch remains open. The grid voltage, after passing through the reactor, flows through the closed soft-start switch and the series-connected high-power resistor, then through the fuse group into the power module valve group stack. Due to the presence of the soft-start resistor, the loop impedance increases significantly, thereby limiting the charging current amplitude flowing through the DC link capacitor inside the power module. This charging current flows through the anti-parallel diode of the rectifier-side AC / DCH bridge circuit, charging the DC link capacitor in an uncontrolled rectification manner. It should be understood that "uncontrolled charging" here means that the switching devices (such as SiC MOSFETs) in the AC / DC H-bridge circuit are in a latched state, using only the unidirectional conductivity of their body diodes or anti-parallel diodes for rectification, without any PWM modulation control. Figure 1 As shown, the current path is sequentially: power grid, reactor, soft starter, fuse group, and power module valve group.

[0055] It should be noted that if the grid-connected switch is directly closed to charge the capacitor, the voltage across the capacitor cannot change abruptly, initially equivalent to a short circuit. With only a small line impedance and reactor impedance in the circuit, a huge inrush current will be generated, easily damaging the rectifier diodes and capacitor. This step limits the inrush current to the safe range allowed by the device by inserting a soft-start resistor in series.

[0056] As an example, in a 10kV solid-state transformer system, assuming a soft-start resistor of 100 ohms, the initial charging current peak is limited to about 100A, which is far lower than the thousands of amperes of inrush current that may be generated by direct closing.

[0057] Based on the above steps, by closing the soft-start switch and limiting the current through a resistor, safe pre-charging of the DC link capacitor of the power module is achieved, effectively preventing current overshoot during the initial startup and protecting the safety of the power devices.

[0058] S602. After the voltage reaches the allowable level, close the grid-connected switch bypass soft starter and continue charging until the voltage stabilizes.

[0059] The allowable level refers to a preset threshold where the DC link capacitor voltage is charged to near the peak value of the grid voltage.

[0060] In this embodiment, the control system monitors the DC-link voltage value within each power module in real time. When the voltage rises to an allowable level, the control system issues a command to close the grid-connected switch. Since the grid-connected switch is connected in parallel with the soft starter, after the switch is closed, the current will mainly flow through the grid-connected switch branch with extremely low impedance, and will no longer flow through the soft starter resistor, thus bypassing the soft starter. At this time, the soft starter switch can be opened to reduce losses, or it can remain closed without affecting the conduction of the main circuit. Subsequently, the grid voltage continues to charge the DC-link capacitor through the grid-connected switch. Since the capacitor voltage is close to the peak grid voltage at this time, the charging current naturally decays to a minimum value until the DC-link voltage stabilizes near the peak grid voltage.

[0061] It should be noted that the design logic of staged charging is as follows: the first stage uses a resistor to limit the current, sacrificing charging speed for safety; the second stage uses a bypass resistor to restore charging efficiency and establish a stable DC voltage. This "current limiting first, then direct connection" strategy balances device safety and startup efficiency.

[0062] As an example, when the DC link voltage is charged to approximately 8.2 kV (approximately 90% of the peak voltage of the 10 kV grid), the control system determines that the voltage has reached the allowable level and triggers the grid-connected switch to close.

[0063] Based on the above steps, by bypassing the soft starter after the voltage reaches the target, the power loss caused by the current limiting resistor is eliminated, and the DC link voltage is steadily built up to the maximum value of the uncontrolled charging stage, laying the voltage foundation for the subsequent AC / DC boost stage.

[0064] Example 5: This embodiment is based on embodiment 3, such as Figure 7 The following is a detailed explanation of the specific implementation steps of the module rotation control strategy. This strategy, through periodic switching operations, allows each power module to experience transitions between "operating" and "standby" states, implemented via steps S701 to S704, which are explained in detail below: S701. During the i-th rotation, close the bypass switch of power module i, block its load-side DC / DC circuit, and disconnect power module i.

[0065] In this context, "removal" refers to isolating the power module from the electrical connection of the main power circuit, so that it no longer undertakes the task of power transmission.

[0066] In this embodiment, the control system selects the power module i that needs to be rotated according to a preset rotation cycle. First, the control system issues a command to close the bypass switch inside the module. After the bypass switch is closed, the current will mainly flow through the bypass switch branch with extremely low impedance, thereby bypassing the H-bridge circuit inside the power module. Subsequently, the control system blocks the PWM drive signals of the two H-bridge circuits in the DC / DC circuit on the load side of the module, causing it to stop working. At this time, although the power module i may still be conducting on the rectifier side through the anti-parallel diode, the energy transmission on the load side has been completely cut off, achieving logical disconnection. It should be understood that the disconnection operation is an orderly process. Closing the bypass switch first to establish a current path and then blocking the control signal can avoid overvoltage caused by sudden circuit disconnection.

[0067] It should be noted that by closing the bypass switch, the voltage across the power module i is forced to near zero potential, which creates a safe electrical environment for its subsequent use as a backup module.

[0068] As an example, assuming the rest period is set to 24 hours and the current rest period is the 5th, the control system controls the bypass switch of the 5th power module to close and locks out its DC / DC circuit.

[0069] Based on the above steps, by closing the bypass switch and locking the circuit, a smooth transition from the working state to the physical bypass state of power module i is achieved, avoiding current surges during the disconnection process.

[0070] S702. Unlock the load-side DC / DC circuit of the power module that was in standby mode in the previous round.

[0071] Unlocking refers to restoring the PWM drive signal of the DC / DC circuit, enabling it to regain its energy transmission capability.

[0072] In this embodiment, simultaneously with or immediately after power module i is disconnected, the control system unlocks the load-side DC / DC circuit of the power module that was placed in standby mode during the previous round (i.e., the (i-1)th off-cycle). In the previous round, only the rectifier side of this module was online, while the load side was locked. Upon unlocking, its load-side H-bridge circuit begins operation, taking over the original load-side power transmission task of power module i. This coordinated "disconnect and reconnect" action ensures a constant total system output power.

[0073] It should be noted that since the rectifier side of the backup module is always online during the standby period and the DC link voltage is maintained at the rated value, there is no need to go through a charging process when unlocking its load side, thus achieving zero-delay commissioning.

[0074] As an example, if the 5th module is removed, the DC / DC circuit of the 4th module (assuming the 4th module in the previous round was a spare module) is unlocked.

[0075] Based on the above steps, the power transmission task was transferred by unlocking the backup module, verifying the ability of the backup module to be put into operation immediately when needed.

[0076] S703. Adjust the carrier phase shift angle of the remaining working modules.

[0077] Among them, the carrier phase shift angle refers to the phase difference between the carriers of each module in multi-level modulation, which is used to optimize the output voltage waveform quality.

[0078] In this embodiment, when power module i is removed, the number of modules actually participating in series operation changes. To ensure optimal harmonic performance of the output voltage waveform, the control system needs to recalculate and allocate the carrier phase of the remaining working modules. Specifically, the control system adjusts the carrier generator parameters in each module controller according to the current number of online modules, so that the carriers of each module are evenly distributed in phase.

[0079] It should be noted that the real-time performance of carrier adjustment is crucial. If the adjustment is not timely or the angle is calculated incorrectly, it may lead to an increase in the harmonic distortion rate of the output voltage, or even cause system oscillation.

[0080] As an example, if the system originally had 11 modules running, and after removing one, 10 modules remain, then the carrier phase shift angle needs to be adjusted accordingly.

[0081] Based on the above steps, by dynamically adjusting the carrier phase shift angle, it is ensured that the output voltage waveform of the solid-state transformer remains of high quality and meets the grid connection requirements even after the number of modules changes.

[0082] S704. After the system stabilizes, disconnect the bypass switch of power module i to serve as a new redundant backup module, and restore the carrier phase configuration.

[0083] System stability refers to the attenuation of electrical quantity (such as voltage and current) fluctuations to within the allowable range, and the newly added modules being able to properly handle the load.

[0084] In this embodiment, the control system monitors the system's operating status. Once it determines that the system has been operating stably for a period of time, it issues a command to disconnect the bypass switch of power module i. At this time, power module i reconnects to the main circuit, but its load-side DC / DC circuit remains locked and does not participate in power transmission; therefore, its role changes to that of a new redundant standby module. Simultaneously, the control system restores the carrier phase configuration to the state containing all modules (n+1), preparing for the next rotation.

[0085] It should be noted that the bypass switch is disconnected after the system has stabilized. At this point, the voltage across module i will be evenly distributed by the static voltage equalizing resistor, preventing drastic voltage spikes. This step marks the end of the i-th cycle and the completion of preparations for the (i+1)-th cycle.

[0086] As an example, after the system has been running stably for 10 minutes, the control system disconnects the bypass switch of the 5th module, and the 5th module officially enters standby mode, waiting to be put into use during the next rotation.

[0087] Based on the above steps, by disconnecting the bypass switch and restoring the carrier configuration, the module role conversion was completed, giving each module the opportunity to operate as a backup module, thereby achieving comprehensive verification of the backup function.

[0088] Based on the above technical solution, this embodiment demonstrates in detail the specific execution process of the module rotation control strategy. Through the closed-loop operation of "cut-on-adjust-restore", not only is the periodic rotation of power modules realized, but more importantly, the commissioning process of the backup module is practiced in the field during normal operation, verifying the hardware integrity of the backup module and the effectiveness of the control logic, thus solving the hidden danger of traditional backup schemes being "backed up but not used, and unresponsive when needed".

[0089] Example 6: This embodiment, based on Embodiment 3, provides a detailed explanation of the specific implementation steps of the fault-switching control strategy. Unlike the periodic, planned rotation control strategy described in Embodiment 5, the fault-switching control strategy is a non-periodic emergency response mechanism triggered by sudden events, such as... Figure 8 As shown, this is achieved through the following steps S801, S802, and S803, which will be explained in detail below: S801. When power module x fails, close the bypass switch of power module x to lock the load-side DC / DC circuit of power module x.

[0090] Among them, a fault refers to an abnormal situation such as overcurrent, overvoltage, overtemperature or communication interruption that occurs inside the power module, causing the module to be unable to continue to work normally.

[0091] In this embodiment, the control system monitors the operating status of each power module in real time. Once a fault is detected in power module x, the control system immediately triggers fault protection logic. First, the control system issues a command to close the bypass switch inside power module x, physically short-circuiting the faulty module from the main circuit to prevent further escalation of the fault. Simultaneously or immediately afterward, the control system blocks the PWM drive signal of the DC / DC circuit on the load side of that module, cutting off its energy transmission channel. It should be understood that the response speed of fault switching is crucial, typically requiring completion within milliseconds or even microseconds to minimize the impact on the system power supply. Unlike a rotation strategy, fault switching does not require waiting for a specific period but responds immediately.

[0092] As an example, when power module 5 detects an internal IGBT overcurrent fault, the module controller locks out the drive within tens of microseconds and triggers the bypass switch.

[0093] Based on the above steps, the faulty module is quickly isolated by rapidly closing the bypass switch and the interlocking circuit, preventing the fault from spreading and creating conditions for the deployment of the backup module.

[0094] S802. Unlock the load-side DC / DC circuit of the power module y, which is currently in redundant standby mode.

[0095] Among them, power module y refers to the module that is in standby mode during the rotation cycle before the fault occurs, with its rectifier side running online and its load side in a locked state.

[0096] In this embodiment, while isolating the faulty module x, the control system quickly locates the power module y, which is currently in redundant standby mode. Since power module y maintains an online rectifier side and stable DC-link voltage under the rotational rotation strategy, the control system only needs to unlock the PWM drive signal of its load-side DC / DC circuit to immediately put it into power transmission operation. This process eliminates the need for charging waiting time, achieving a "hot-swappable" deployment of the standby module. It should be understood that due to the rotational rotation strategy, the availability of standby module y has been verified in previous rotational rotation cycles, thus ensuring a high success rate for fault switching.

[0097] It should be noted that if the system does not adopt a rotation strategy, the backup module may not be powered on or operated for a long time, and its capacitor status and device health status are unknown, posing a great risk of failure during fault switching.

[0098] As an example, if the current power module 11 is in standby mode, the control system unlocks its DC / DC circuit, allowing it to take over the power transmission task originally performed by the faulty module 5.

[0099] Based on the above steps, by unlocking the backup module, seamless power takeover in case of failure was achieved, ensuring the continuity of the system's external power supply.

[0100] S803. Adjust the carrier phase shift angle of the remaining working modules and exit the rest function.

[0101] Among them, the "exit rotation" function refers to the logic of pausing or terminating the periodic module rotation operation of the control system.

[0102] In this embodiment, since the faulty module x has been removed, the number of modules actually participating in series operation in the system is reduced. The control system needs to recalculate and allocate the carrier phase shift angle of each module based on the number of remaining online modules to ensure the quality of the output voltage waveform. Simultaneously, the control system issues a command to exit the rotation function. This is because after a fault occurs, the system is already in a non-perfect state; redundant backup modules have been consumed, and there is no longer the redundancy margin required for rotation. If rotation is forcibly continued at this time, the system may lose its last backup guarantee, or even trigger a secondary fault. Therefore, exiting the rotation function is a necessary measure to ensure the stable operation of the system after a fault. It should be understood that after exiting rotation, the system will enter a "single-module fault operation mode," at which time the system can issue an alarm signal to prompt maintenance personnel to repair or replace the faulty module as soon as possible.

[0103] It should be noted that the carrier phase shift angle adjustment algorithm is the same as the algorithm in the rotation strategy, both of which are dynamically calculated based on the number of online modules, ensuring the consistency of the control logic.

[0104] As an example, before the fault, the system had 11 modules running with a carrier phase shift angle of 2π / 11; after the fault, 10 modules remained running with the carrier phase shift angle adjusted to 2π / 10, and the system stopped the rotation timer.

[0105] Based on the above steps, by adjusting the carrier phase shift angle and disabling the rotation function, it is ensured that the system can still operate stably in a non-perfect state after a fault, thus avoiding the risk of system collapse caused by blind rotation.

[0106] Based on the above technical solution, this embodiment details the execution process of the fault switching control strategy. Compared with the rotation strategy in Embodiment 5, fault switching is characterized by its suddenness and urgency. Its core lies in using pre-verified backup modules from the rotation strategy for rapid replacement and actively exiting the rotation logic after a fault to protect the system. This strategy effectively solves the problems of unreliable fault switching and unknown status of backup modules in traditional solid-state transformers, greatly improving the system's fault tolerance.

[0107] Example 7: This embodiment, based on embodiments 5 and 6, provides a detailed explanation of the specific algorithms for electrical quantity calculation and carrier adjustment during the switching process. Regardless of whether it's a module rotation control strategy or a fault switching control strategy, when performing module removal or activation operations, the system needs to calculate the target current amplitude and phase after switching based on changes in grid voltage, reactor inductance, and module chain output voltage, and adjust the phase current magnitude and phase to bring the system to a steady state. The details are as follows: Specifically, combined Figure 9 As shown, during steady-state operation of the solid-state transformer system, the grid voltage U and the inductor voltage U on the reactor are... L The total output voltage U of the power module chain S These three components form a closed voltage triangle. When a module in the system is bypassed, the amplitude and phase of the total output voltage of the power module chain will change abruptly, disrupting the original balance. To achieve seamless switching, the control system needs to recalculate the inductor voltage U based on the target state after the switch. L The amplitude and phase of the target current I are then derived from the amplitude and phase of the current. The control system first obtains the steady-state parameters before switching, including the grid voltage U and the inductor voltage U. L1 Module chain output voltage U S1 And the voltage Ux of the module to be disconnected. Since the amplitude and phase of the grid voltage U remain constant, and the voltage Ux of the disconnected module is known, according to geometric relationships, the output voltage U of the switched module chain is... S2 In terms of amplitude, it is greater than U S1 This reduces Ux. The control system uses the triangle cosine theorem to calculate the inductor voltage U after the switch. L2 The amplitude is calculated using the following formula: , where β is U L1 The angle between Ux and the current phase. Subsequently, the control system uses the triangle sine theorem to calculate the current phase offset angle α, with the following formula: Ultimately, the control system, based on U... L2 Given the amplitude and offset angle α, determine the target current amplitude I2 = U after switching. L2 The current loop setpoint of the controller is adjusted based on the formula (ω * L) and its phase, where ω is the angular frequency and L is the inductance of the reactor. It should be understood that this real-time calculation method based on phasor geometry can accurately predict the system state after switching, thereby proactively adjusting control parameters. This avoids the current surges and power fluctuations caused by relying on a PID controller for slow tracking, achieving true "seamless switching."

[0108] When adjusting the carrier phase shift angle of the remaining working modules, the control system dynamically calculates the carrier phase shift angle based on the number of modules currently running online.

[0109] Specifically, the rectifier side of the solid-state transformer employs carrier phase-shift SPWM modulation technology. By shifting the carrier phases of each module by a certain angle, specific harmonics in the output voltage are canceled, thereby obtaining a high-quality stepped waveform voltage. When all n+1 modules are running, to achieve optimal harmonic cancellation, the control system adjusts the carrier phase shift angle to 2π / (n+1), meaning the phase difference between the carriers of two adjacent modules is 360° / (n+1). When one module is bypassed and the remaining n modules are running, the control system adjusts the carrier phase shift angle to 2π / n. This adjustment process is achieved by modifying the phase register of the carrier generator in each module controller. It should be noted that real-time adjustment of the carrier phase shift angle is crucial for maintaining the output voltage waveform quality. If the phase shift angle is not adjusted in time after the number of modules decreases, the carrier will no longer be uniformly distributed, resulting in low-order harmonics in the output voltage, increasing the filter load, and even causing system resonance. Through the above carrier adjustment strategy, this invention ensures that the waveform quality of the solid-state transformer output voltage remains at an optimal level during dynamic changes in the number of modules, further enhancing the stability of the system during switching.

[0110] As an example, the rectifier-side AC / DC H-bridge current uses a carrier horizontal phase-shifted SPWM modulation strategy with a modulation frequency of 50Hz and a period of 20ms. Under normal operating conditions, 11 power modules are connected in series, with the phase shift angle θ = 2π / 11 between the 11 triangular carriers, a carrier frequency of 400Hz, and a period of 2.5ms. When one power module is idle or fails, the bypass switch in the corresponding power module closes, and 10 power modules are connected in series. The phase shift angle θ = 2π / 10 between the 10 triangular carriers, a carrier frequency of 400Hz, and a period of 2.5ms. The SPWM modulation principle when 10 power modules are connected in series is as follows: Figure 10 As shown, the trend of carrier 1 to carrier 10 is included. These 10 carriers have the same frequency but their phases are staggered. Specifically, the phase difference between two adjacent carriers is 2π / 10 (i.e., 36°).

[0111] Based on the above technical solution, this embodiment provides underlying control logic support for "seamless switching" through specific mathematical models and carrier adjustment algorithms. Electrical quantity calculations ensure a smooth transition of power flow before and after switching, while carrier adjustment ensures consistently excellent waveform quality before and after switching. Together, these two aspects ensure high-performance operation of the solid-state transformer under module rotation and fault switching scenarios.

[0112] Example 8: This embodiment uses a specific 10kV solid-state transformer system as an example to illustrate the application effect of the above system architecture in detail. This system aims to verify the feasibility and superiority of the aforementioned embodiment in actual high-voltage, high-power scenarios.

[0113] In this embodiment, the solid-state transformer is connected to a 10kV AC power grid, using a three-phase three-wire connection. The power module valve stack in the main power circuit contains n+1 power modules per phase, where n=11, meaning each phase consists of 12 power modules connected in series. The H-bridge circuit within each power module uses 1200V SiC MOSFET power devices to meet the requirements of high switching frequency and high efficiency. It should be understood that although this embodiment selects specific voltage levels and device parameters, the control strategy of this invention is equally applicable in other medium- and high-voltage application scenarios by adjusting the number of modules or the voltage rating of the devices.

[0114] During the execution of the startup control strategy, the system exhibited stable voltage build-up characteristics. Specifically, in the uncontrolled charging phase of the internal DC link, the control system closed the soft-start switch, and the grid voltage charged the DC link capacitors of the 12 power modules through the current-limiting resistors. Due to the suppression effect of the current-limiting resistors, the initial charging current peak was effectively limited within the safe range of the devices. When the DC link voltage of each module rose to approximately 8.2KV (approximately 90% of the grid voltage peak), the control system determined that the voltage had reached the allowable level and immediately closed the grid-connected switch bypass soft starter. Subsequently, the system entered the AC / DC boost phase, where the control system unlocked the rectifier-side H-bridge circuits of all 12 modules and adopted a carrier phase-shift SPWM modulation strategy to further boost and stabilize the DC link voltage at the rated value of approximately 10KV. During this process, due to the adoption of a phased startup strategy, no significant current overshoot was observed, verifying the safety of the startup control strategy.

[0115] During normal operation, the system implements a module rotation control strategy. The control system sets the rotation cycle T to 168 hours (one week), with each phase power module rotating sequentially. Taking the first rotation of phase A as an example, the control system closes the bypass switch of power module 1, blocking its load-side DC / DC circuit, while simultaneously unlocking the load-side DC / DC circuit of power module 12, which was in standby mode in the previous round. At this time, the number of modules actually participating in power transmission in phase A changes from 11 to 10 (plus one bypassed module, a total of 11 modules online, one of which is in bypass standby mode). The control system immediately adjusts the carrier phase shift angle of the remaining 10 working modules from 2π / 11 to 2π / 10, or 36 degrees. Observation with an oscilloscope shows that the output voltage waveform has only a slight fluctuation at the moment of switching and recovers stability within tens of milliseconds, verifying that the rotation strategy has minimal impact on the output power quality, and successfully verifying the reliability of power module 12 as a standby module.

[0116] To further verify the effectiveness of the fault switching control strategy, this embodiment simulates a scenario of a sudden overcurrent fault in power module 5. At the moment of the fault, the control system detects the fault signal within microseconds and immediately closes the bypass switch of power module 5, locking its internal circuitry to prevent the fault from spreading. Simultaneously, the control system unlocks power module 1, which is currently in standby mode (assuming power module 1 is a standby module during the current off-cycle). Since the DC link voltage of power module 1 remains at its rated value during standby, its activation process achieves zero delay. The control system synchronously adjusts the carrier phase shift angle to 2π / 10 and immediately exits the off-cycle function, with the system entering a non-perfect state of operation. From the occurrence of the fault to the system's return to stable operation, the entire process takes less than 10 milliseconds, and the load-side voltage does not show a significant drop, achieving truly seamless switching.

[0117] Example 9: This implementation example Figure 11 As shown, the complete execution flow of the solid-state transformer startup and module bypass switching control method of this application is described, covering the entire process control logic of system startup self-test, normal operation, module rotation and fault handling. Upon system startup, initialization is performed first, followed by sequential completion of multiple self-tests: First, the system self-test is checked for normality; if the self-test fails, the system directly enters the shutdown maintenance process; if the self-test is normal, the uncontrolled charging process of the internal DC link is checked; if the uncontrolled charging is abnormal, the startup process is terminated and shutdown maintenance is initiated; after uncontrolled charging is normal, the AC / DC boost process of the power module is checked for normality; if the boost is abnormal, the system enters the shutdown maintenance process; after the boost is normal, the load-side DC charging process is checked for normality; if the charging is abnormal, the startup is terminated; after the load-side DC charging is normal, the H-bridge unlocking process of the load-side DC / DC is checked for normality; if the unlocking is abnormal, the system enters the shutdown maintenance process. Once all self-tests are normal, the system enters normal operation. After entering normal operation, the system continuously performs module status monitoring and rotation control: First, it checks for module faults. If a fault is detected, the faulty module is bypassed and a redundant backup module is activated. The system then runs with n modules and enters a shutdown maintenance process. If no fault is detected, it checks if the preset rotation period T has been reached. If not, it continues monitoring the module status. If the rotation period has been reached, it performs a module rotation operation, rotating module 1 and activating module n+1. After the first rotation, it monitors the module status again to check for faults. If a fault is detected, the faulty module is bypassed and a redundant backup module is activated. The system then runs with n modules and enters a shutdown maintenance process. If no fault is detected, it checks again if the rotation period T has been reached. If the period has been reached, it rotates module 2 and activates module 1. This process continues, continuously looping module status monitoring, fault handling, and rotation operations.

[0118] Based on the complete process description of System Embodiment 8 and Method Embodiment 9 above, the solid-state transformer start-up and module bypass switching control method and system provided by the present invention are verified through parameter setting and operation effect demonstration. It can effectively solve key technical problems such as start-up impact, backup function verification and seamless fault switching in high voltage and high power situations, and has extremely high engineering practical value.

[0119] Although this application has been described herein in conjunction with various embodiments, those skilled in the art, by reviewing the accompanying drawings, disclosure, and appended claims, will understand and implement other variations of the disclosed embodiments in carrying out the claimed application. In the claims, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple instances. A single processor or other unit can implement several functions listed in the claims. While different dependent claims may recite certain measures, this does not mean that these measures cannot be combined to produce good results.

[0120] Although this application has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made thereto without departing from the spirit and scope of this application. Accordingly, this specification and drawings are merely exemplary illustrations of this application as defined by the appended claims, and are considered to cover any and all modifications, variations, combinations, or equivalents within the scope of this application. Clearly, those skilled in the art can make various alterations and modifications to this application without departing from the spirit and scope of this application. Thus, if such modifications and modifications of this application fall within the scope of the claims of this application and their equivalents, this application is also intended to include such modifications and modifications.

Claims

1. A method for starting up and switching the bypass of a solid-state transformer, characterized in that, The method includes: a startup control strategy, a module rotation control strategy, and a fault switching control strategy; The startup control strategy divides the startup process into an internal DC link uncontrolled charging stage, a power module AC / DC boost stage, a load-side DC charging stage, and a load-side DC / DC H-bridge unlocking stage. Under the module rotation control strategy, the control system controls the power modules to rotate in turn, so that any one power module is in a redundant standby state periodically. The fault switching control strategy is used to disconnect the faulty module and put the module in redundant standby mode into operation when the power module fails.

2. The method according to claim 1, characterized in that, The uncontrolled charging phase of the internal DC link includes: When the soft-start switch is closed, the DC link capacitors of the n+1 power modules, including the redundant backup modules, are charged through the soft-start resistor. Once the voltage reaches the allowable level, close the grid-connected switch and bypass the soft starter to continue charging until the voltage stabilizes.

3. The method according to claim 1, characterized in that, The module rotation control strategy includes: During the i-th rotation, the bypass switch of power module i is closed, its load-side DC / DC circuit is locked, and power module i is disconnected; Unlock the load-side DC / DC circuit of the power module that was in standby mode in the previous round; Adjust the carrier phase shift angle of the remaining working modules; After the system stabilizes, disconnect the bypass switch of power module i to serve as a new redundant backup module, and restore the carrier phase configuration.

4. The method according to claim 1, characterized in that, The fault switching control strategy includes: When power module x fails, the bypass switch of power module x is closed, and the load-side DC / DC circuit of power module x is locked. Unlock the load-side DC / DC circuit of power module y, which is currently in redundant standby mode; Adjust the carrier phase shift angle of the remaining working modules and exit the rest period function.

5. The method according to claim 1, characterized in that, During the execution of the module rotation control strategy and fault switching control strategy, the control system calculates the target current amplitude and phase after switching based on the changes in grid voltage, reactor inductance and module chain output voltage, and adjusts the phase current magnitude and phase to bring the system into steady state.

6. The method according to claim 4, characterized in that, The adjustment of the carrier phase shift angle of the remaining working modules includes: When all n+1 modules are running, the carrier phase shift angle is adjusted to 2π / (n+1); When n modules are running, the carrier phase shift angle is adjusted to 2π / n.

7. A solid-state transformer start-up and module bypass switching control system, used to implement the method described in any one of claims 1-6, characterized in that, The system includes: a main power circuit and a control system; The main power circuit is equipped with a soft starter, which consists of a soft starter switch and a high-power resistor connected in series. The soft starter is connected in parallel with the grid-connected switch in the main power circuit. The main power circuit is equipped with a power module valve group, and each phase power module valve group contains several power modules. Each power module is equipped with a static equalizing resistor and a bypass switch connected in parallel, and both ends are connected to the AC line. The control system is used to execute startup control strategy, module rotation control strategy and fault switching control strategy.

8. The solid-state transformer start-up and module bypass switching control system according to claim 7, characterized in that, One end of the soft starter is connected between the reactor and the grid-connected switch, and the other end is connected between the AC voltage sampling sensor and the fuse group, which is used to limit the charging current during the startup phase.

9. The solid-state transformer start-up and module bypass switching control system according to claim 7, characterized in that, The power module contains three H-bridge circuits. The first H-bridge circuit is used for AC / DC conversion on the rectifier side, forming an independent internal DC link. The second and third H-bridge circuits are respectively connected to the primary and secondary sides of the isolation transformer for DC / DC power transmission on the load side. The internal DC link is equipped with a voltage sampling sensor.

10. The solid-state transformer start-up and module bypass switching control system according to claim 7, characterized in that, The control system comprises a three-level architecture consisting of a main controller, a phase controller, and a module controller. The main controller is responsible for system-level scheduling and electrical quantity control; The phase controller is responsible for the coordinated control of each power module in its phase; The module controller is responsible for the bypass switch control, AC / DC modulation, and DC / DC power transmission control of the module it belongs to.

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