A single-stage power electronic transformer three-phase string system and a control method thereof

Abstract

The application discloses a single-stage power electronic transformer three-phase group string system and a control method thereof, and relates to the technical field of power electronics and electric energy conversion. The system comprises three single-stage AC-AC conversion modules with the same structure, the input ends of the modules are connected with the phase live wires and neutral lines of a three-phase power grid respectively, the three modules are connected with each other through a three-phase four-wire output structure, each module comprises a single-stage energy transmission branch which is sequentially connected by a folded bridge unit, a resonance cavity, a high-frequency isolation transformer and a flip bridge unit, and a control system captures the phase of input voltage and controls the switching frequency to realize single-stage conversion. The system realizes three-phase decoupling voltage stabilization and neutral point potential offset suppression through independent frequency modulation of each phase and cooperation of the neutral line circuit. Through independent frequency modulation of each phase and cooperation of the neutral line circuit, three-phase decoupling voltage stabilization and neutral point potential offset suppression are realized without an additional compensation circuit.

Description

Technical Field

[0001] This application relates to the field of power electronics and power conversion technology, and in particular to a three-phase string system of a single-stage power electronic transformer and its control method. Background Technology

[0002] Single-stage AC-AC power electronic transformers offer advantages over traditional multi-stage conversion structures (AC-DC-DC-AC) such as higher efficiency, smaller size, and simpler control. However, when extending single-stage conversion modules to three-phase power supply systems, the following technical challenges remain:

[0003] First, there is the issue of startup inrush current. Single-stage converter modules typically contain a resonant cavity (such as an LLC structure) and a high-frequency isolation transformer. During startup, due to capacitor charging or transient core saturation, a current surge several times the rated value can easily occur, leading to overstress of the power switching transistors or triggering overcurrent protection.

[0004] Second, the three-phase decoupling control problem. When three single-stage conversion modules share a neutral point to form a three-phase four-wire output, the asymmetrical load will cause the neutral point potential to shift. If the control of each phase is coupled to each other, it is difficult to achieve independent voltage regulation of each phase output voltage.

[0005] Third, the fault redundancy capability is insufficient. In existing three-phase power electronic transformer systems, the failure of any module often leads to the shutdown of the entire system, making it unsuitable for scenarios with high requirements for continuous power supply, such as coal mines and data centers.

[0006] Fourth, the protection mechanism is simplistic. Existing systems mostly employ software overcurrent protection, with response times exceeding milliseconds, which cannot protect power devices from short-circuit damage within microseconds. Summary of the Invention

[0007] To solve the above problems, the first technical solution adopted in this application is: to provide a single-stage power electronic transformer three-phase string system, including: Three identical single-stage AC-AC converter modules, with their input terminals connected to the live wires and neutral wires of each phase of a three-phase power grid, respectively; The three-phase four-wire output structure is configured to interconnect the neutral points of the output terminals of the three conversion modules; each conversion module includes a single-stage energy transfer branch consisting of a folded bridge unit, a resonant cavity, a high-frequency isolation transformer, and a flip-bridge unit connected in sequence. The control system is configured to achieve a single-stage energy conversion from the input side to the output side by capturing the phase of the input voltage and controlling the switching frequency of the folded bridge unit and the flip bridge unit. The system achieves decoupled voltage regulation control of the three-phase output voltage and suppression of neutral point potential drift under asymmetrical load conditions by independently modulating the phase-by-phase energy conversion branches and cooperating with the neutral line current loop.

[0008] Furthermore, the control system is equipped with a soft-start program: During the system startup phase, the control system outputs a drive pulse with an initial frequency higher than the rated resonant frequency, and controls the frequency of the drive pulse to decrease to the rated resonant frequency at a preset slope, so as to limit the charge injection current at startup by increasing the equivalent inductive reactance of the resonant cavity.

[0009] Furthermore, the initial frequency of the driving pulse is configured to be 3 to 5 times the rated resonant frequency.

[0010] Furthermore, the control system includes a central controller and slave controllers respectively located in each of the conversion modules; the central controller is connected to each of the slave controllers via a communication bus, and is used to redistribute the operating power of the remaining two conversion modules when any conversion module fails.

[0011] Furthermore, the controller incorporates multi-level protection logic based on response time scales: Level 1 protection: The power device current is monitored by a hardware comparator, and the drive signal is blocked when the current exceeds the first threshold, with a response time in microseconds. Second-level protection: When an abnormal resonant current or a load short circuit is detected, the flip-bridge unit is blocked and the folded bridge unit enters a direct-through mode, with a response time in milliseconds.

[0012] Furthermore, it also includes: A line temperature detection unit is disposed on the surface of a magnetic element inside the conversion module; A leakage current detection unit is configured between the input and output terminals of the conversion module; the control system is configured to perform a shutdown or derating operation when the feedback value of any detection unit exceeds a preset safety threshold.

[0013] Furthermore, the folded bridge unit includes a bridge circuit composed of controlled power switching transistors; the control system tracks the phase of the input voltage through a phase-locked loop and controls the bridge circuit to reverse polarity at the voltage zero-crossing point.

[0014] The second technical solution adopted in this application is: providing a control method for a system applied to any of the foregoing claims, comprising: S1: Each of the aforementioned conversion modules independently performs input voltage phase-locking and captures the voltage zero-crossing point; S2: Start at an initial frequency higher than the rated resonant frequency, and control the frequency to decrease linearly over time to suppress the starting current surge; S3: Centered on the rated resonant frequency, the switching frequency is finely adjusted in real time according to the feedback voltage at the output terminal to achieve output voltage regulation.

[0015] Furthermore, in step S3, the range of variation of the switching frequency is limited to between 0.8 and 1.2 times the rated resonant frequency.

[0016] Furthermore, it also includes a redundancy switching step: when the central controller receives a fault alarm signal from any slave controller, it disconnects the faulty module from the output bus and adjusts the power reference value of the remaining normally operating modules so that the system continues to supply power in derating mode.

[0017] Due to the adoption of the above technical solution, this application has at least one of the following beneficial effects compared with the prior art:

[0018] 1. By combining phase-separated independent frequency modulation and neutral line circuit, three-phase decoupling voltage regulation and neutral point potential deviation suppression are achieved without the need for additional compensation circuits.

[0019] 2. The high-frequency soft-start method utilizes the high impedance characteristics of the high-frequency region of the resonant cavity to effectively suppress the starting current surge.

[0020] 3. The central controller and slave controllers work together to quickly disconnect faulty modules and redistribute power to remaining modules, thereby improving system fault tolerance.

[0021] 4. Multi-dimensional monitoring of line temperature and leakage current improves safety in high-risk scenarios. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein: Figure 1 This is a schematic diagram of the structure of a three-phase string system of a single-stage power electronic transformer provided in an embodiment of this application; Figure 2 This is a schematic diagram of a multi-level protection logic based on a response time scale provided in an embodiment of this application; Figure 3 A waveform diagram provided for one embodiment of this application; Figure 4 This is a flowchart illustrating a control method provided in an embodiment of this application. Detailed Implementation

[0023] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. It is understood that the specific embodiments described herein are only for explaining this application and not for limiting it. Furthermore, it should be noted that, for ease of description, only the parts related to this application are shown in the accompanying drawings, not all structures. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0024] The terms "first," "second," etc., used in this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0025] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0026] The existing three-phase power electronic transformer systems have the following drawbacks: First, most adopt a multi-stage conversion structure (AC-DC-DC-AC), resulting in low efficiency, large size, and complex control. Second, when extending a single-stage conversion module to a three-phase system, there is a lack of effective decoupling methods between the outputs of each phase. When the load is unbalanced, it is easy to cause neutral point potential shift, resulting in three-phase output voltage imbalance and affecting power supply quality. Third, for special explosion-proof environments such as coal mines, the existing systems cannot effectively cope with complex working conditions and lack good adaptability to non-uniform loads.

[0027] In view of this, this application sets up three identical single-stage AC-AC conversion modules connected to the three-phase live wires and the neutral wire respectively, and adopts a phase-independent frequency modulation strategy in conjunction with the neutral wire current loop—this enables decoupled voltage regulation control of the three-phase output voltage, effectively suppressing neutral point potential shift under asymmetrical load conditions, thereby simplifying the system structure while improving power supply quality and load adaptability; Figure 1 As shown, Figure 1 A schematic diagram of a three-phase string system of a single-stage power electronic transformer provided in an embodiment of this application includes: Three identical single-stage AC-AC converter modules, with their input terminals connected to the live wires and neutral wires of each phase of a three-phase power grid, respectively; The three-phase four-wire output structure is configured to interconnect the neutral points of the output terminals of the three conversion modules; each conversion module includes a single-stage energy transfer branch consisting of a folded bridge unit, a resonant cavity, a high-frequency isolation transformer, and a flip-bridge unit connected in sequence. The control system is configured to achieve a single-stage energy conversion from the input side to the output side by capturing the phase of the input voltage and controlling the switching frequency of the folded bridge unit and the flip bridge unit. The system achieves decoupled voltage regulation control of the three-phase output voltage and suppression of neutral point potential drift under asymmetrical load conditions by independently modulating the phase-by-phase energy conversion branches and cooperating with the neutral line current loop.

[0028] In this embodiment, the single-stage power electronic transformer three-phase string system includes three identical single-stage AC-AC conversion modules, denoted as module A, module B, and module C. The input terminal of module A is connected to the A-phase live wire and the neutral line N of the three-phase power grid; the input terminal of module B is connected to the B-phase live wire and the neutral line N; and the input terminal of module C is connected to the C-phase live wire and the neutral line N. The output terminals of the three modules are connected to the same neutral point, forming a three-phase four-wire output structure.

[0029] Each conversion module internally comprises a folded bridge unit, a resonant cavity, a high-frequency isolation transformer, and a flip-bridge unit, which are electrically connected in sequence to form a single-stage energy transfer branch. The folded bridge unit, composed of controlled power switches, converts the input high-voltage AC power into a high-frequency square wave signal. The resonant cavity, consisting of a resonant inductor and a resonant capacitor connected in series, filters the high-frequency square wave signal, outputting a quasi-sine wave signal. The high-frequency isolation transformer provides electrical isolation and voltage level conversion. The flip-bridge unit restores the high-frequency quasi-sine wave signal to a power frequency AC signal, serving as the module's low-voltage AC output.

[0030] The control system includes slave controllers within each conversion module and a central controller. Each module's slave controller independently captures the phase of its input voltage and achieves single-stage energy conversion from the input to the output by controlling the switching frequencies of the folded bridge and flip-bridge units. Each module employs a phase-independent frequency modulation strategy, meaning that the control loops of each module are decoupled and each independently regulates its own phase output voltage. Simultaneously, the outputs of the three modules share the same neutral point, providing a return path for unbalanced current. When the system is under asymmetrical load conditions, each module independently adjusts its switching frequency through its own voltage feedback loop to maintain the stability of its own phase output voltage. The vector sum of the currents on the neutral line represents the three-phase unbalanced current. This loop decouples the regulation of each phase's output voltage, effectively suppressing neutral point potential shifts without requiring additional compensation circuitry.

[0031] To address the current surge issue during startup, the control system is equipped with a soft-start program: during system startup, the control system outputs a drive pulse with an initial frequency higher than the rated resonant frequency, and controls the frequency of the drive pulse to decrease to the rated resonant frequency at a preset slope, thereby limiting the charge injection current at startup by increasing the equivalent inductive reactance of the resonant cavity; the initial frequency of the drive pulse is configured to be 3 to 5 times the rated resonant frequency.

[0032] In one specific implementation, the slave controller of each module tracks the phase of the input voltage of its own module in real time via a phase-locked loop before startup and detects the positive zero-crossing point of the voltage. When a positive zero-crossing point is detected, a drive pulse is immediately generated.

[0033] The initial frequency of the driving pulse is set to be higher than the rated resonant frequency of the resonant cavity. Preferably, the initial frequency is configured to be 3 to 5 times the rated resonant frequency, i.e. = ,in The value range is 3 to 5. The principle is that the resonant cavity exhibits high impedance characteristics in the high-frequency region, and the equivalent inductive reactance increases with the increase of the switching frequency. Using a higher initial switching frequency causes the resonant cavity to exhibit a larger equivalent inductive reactance at startup, thereby limiting the rate of change of the charge injection current, di / dt, to a lower level. The formula is expressed as di / dt = / ,in Input voltage, Switching frequency The equivalent input inductance is as follows.

[0034] Subsequently, at the preset ramp time Inside, the control system controls the frequency of the drive pulses according to a preset slope. The frequency decreases linearly until it reaches the rated resonant frequency. The frequency decrease follows this pattern: = ,in = ( - ) / During the frequency drop-off process, the voltage gain of the resonant cavity gradually increases from a low value to 1, and the output voltage is smoothly established without overshoot or spikes.

[0035] The control system includes a central controller and slave controllers located in each conversion module. The central controller is connected to each slave controller via a communication bus and is used to redistribute the operating power of the remaining two conversion modules when any conversion module fails.

[0036] In one specific implementation, the control system adopts a master-slave architecture. The central controller is connected to the slave controllers within each conversion module via a communication bus (e.g., CAN bus or RS485 bus). Each slave controller is located within its corresponding conversion module and is responsible for the independent control, data acquisition, and fault detection of its module. The central controller is responsible for system-level coordination and management. It sends power reference values, start / stop commands, and other control commands to each slave controller via the communication bus and receives operating status and fault information uploaded by each slave controller. When any conversion module fails, the slave controller of that module sends a fault alarm signal to the central controller. Based on the received fault information, the central controller recalculates the power allocation scheme for the remaining two normally operating modules and sends the updated power reference value to the corresponding slave controller via the communication bus, thereby realizing the redistribution of operating power in the event of a fault.

[0037] The controller has built-in multi-level protection logic based on response time scale: Level 1 protection: The power device current is monitored by a hardware comparator, and the drive signal is blocked when the current exceeds the first threshold, with a response time in microseconds. Second-level protection: When an abnormal resonant current or load short circuit is detected, the flip-over bridge unit is blocked and the folded bridge unit enters the pass-through mode, with a response time in milliseconds.

[0038] like Figure 2 As shown, Figure 2 This is a schematic diagram of a multi-level protection logic based on response time scale provided in an embodiment of this application; in this embodiment, the controller has a built-in multi-level protection logic based on response time scale. This protection logic is divided into the following five levels according to the fault response time: Level 1 (Microsecond Level): Instantaneous Overcurrent / Short Circuit Protection for Power Devices. A high-speed hardware comparator is integrated into the drive circuit of each power switch, with its input connected to a current sampling signal or desaturation detection circuit. The first protection threshold is set slightly higher than the maximum normal operating peak current, for example, 1.2 to 1.5 times the rated peak current. When the hardware comparator detects that the current exceeds the first threshold, it directly blocks the drive signal of the switch for one clock cycle (typically less than 200 nanoseconds) through hardware logic gates, without software intervention. The overall response time of this level of protection is controlled in the microsecond range (e.g., 1 microsecond to 5 microseconds), enabling the power device to be shut down before it suffers short-circuit damage energy.

[0039] Level 2 (100 microseconds to 1 millisecond): Load short circuit and LLC resonant cavity overcurrent protection. This level of protection is implemented by software logic in the slave controller, but uses a fast analog-to-digital converter and a high-speed interrupt service routine to improve response speed. Level 2 protection is triggered when the slave controller detects one of the following two situations: (a) the amplitude of the resonant current exceeds a preset threshold or the waveform exhibits abnormal distortion; (b) a short circuit is determined at the output terminal based on load current estimation. After triggering, the slave controller performs the following protection actions: First, it immediately blocks all drive signals of the flip-bridge unit, making its output high-impedance state to block the short-circuit current from flowing back from the load side; second, it controls the folded bridge unit to enter the shoot-through mode. The specific implementation of the shoot-through mode is as follows: during the positive half-cycle of the resonant current, a pair of diagonal switches in the folded bridge unit are turned on; during the negative half-cycle of the resonant current, another pair of diagonal switches are turned on, thereby forming a local circulation loop in the resonant cavity and rapidly dissipating the energy. The response time range of this level of protection is 100 microseconds to 1 millisecond.

[0040] Level 3 (1 ms to 10 ms): Overvoltage, undervoltage, and module-level overheat protection. The controller monitors the DC bus voltage and output voltage in real time via a voltage sampling circuit, and the temperature of the power switching transistors and magnetic components in real time via temperature sensors. When the detected voltage exceeds the overvoltage protection threshold, falls below the undervoltage protection threshold, or the temperature exceeds the alarm threshold, Level 3 protection is triggered. This level of protection performs actions including reducing output power (derating operation) or bypassing the faulty module (by controlling the bypass switch to take the module out of operation). The response time for this level of protection ranges from 1 ms to 10 ms.

[0041] Level 4 (10 ms to 100 ms): Input / Output Leakage Current Protection. Leakage current sensors are installed on both the input and output sides of each converter module. The leakage current detection value is read in real time from the controller via the analog-to-digital conversion channel. When the effective value of the leakage current exceeds a preset safety threshold (e.g., 30 mA or 50 mA for electrical equipment in underground coal mines), Level 4 protection is triggered. This level of protection performs the following actions: issuing an audible and visual alarm, and disconnecting the input or output relay of the corresponding module to safely isolate the faulty module from the system. The response time range for this level of protection is 10 ms to 100 ms.

[0042] Level 5 (100 milliseconds to 1 second): System-level coordinated protection and redundancy switching. This level of protection is executed by the central controller. When the central controller receives any fault alarm signal sent by a slave controller via the communication bus, it determines the fault type and severity. For permanent faults, the central controller performs the following operations: First, it displays the fault information on the touchscreen and issues an alarm; second, it instructs the faulty module to disconnect from the output bus; third, it recalculates the power reference value of the remaining normally operating modules and distributes the load, allowing the system to continue supplying power in derating mode. The response time range for this level of protection is 100 milliseconds to 1 second.

[0043] The system also includes: A line temperature detection unit is disposed on the surface of a magnetic component inside the conversion module; in one embodiment, one or more temperature sensors are attached to or embedded in the coil surface or core of the high-frequency isolation transformer and resonant inductor. The temperature sensor can be a PT100 platinum resistance temperature sensor or an NTC negative temperature coefficient thermistor.

[0044] Each slave controller reads the temperature detection value in real time through the analog-to-digital conversion channel. The control system has at least two preset temperature thresholds: the first alarm threshold... (e.g., 105 degrees Celsius) and the first trip threshold (For example, 125 degrees Celsius). When ≥ When the temperature rise is limited, the control system performs a derating operation, that is, reduces the output power to limit the temperature rise; when ≥ When this happens, the control system performs a shutdown operation, which means blocking all drive signals and removing the module from the system.

[0045] A leakage current detection unit is configured between the input and output terminals of the conversion module. The control system is configured to perform a shutdown or derating operation when the feedback value of any detection unit exceeds a preset safety threshold. Leakage current sensors are installed on both the input side (high-voltage grid side) and the output side (low-voltage load side) of each module. These leakage current sensors can be magnetically modulated or zero-flux Hall effect current sensors. The control system continuously monitors the effective value of the leakage current. The control system has a preset leakage current safety threshold. This threshold is set according to the safety standards of the application scenario (for example, for electrical equipment in underground coal mines, it can be set to 30 mA or 50 mA). When any detection unit... Exceed When this happens, the control system issues an alarm signal and disconnects the input or output relays of the corresponding module, isolating the faulty module from the system.

[0046] All detection data (including voltage, current, temperature, leakage current, switching frequency, and fault status) are collected by each slave controller and uploaded to the central controller via the communication bus. The central controller then forwards the data to the touch screen for real-time display.

[0047] The folded bridge unit includes a bridge circuit composed of controlled power switching transistors; the control system tracks the phase of the input voltage through a phase-locked loop and controls the bridge circuit to reverse polarity at the zero-crossing point of the voltage.

[0048] In one specific implementation, the folded bridge unit consists of a full-bridge topology composed of four controlled power switches; the phase-locked loop in the control system outputs a phase signal synchronized with the input voltage in real time. When the phase signal indicates that the input voltage has crossed zero, the control system determines the switch combination that needs to be switched based on the current voltage polarity, and issues a precise drive pulse at the zero-crossing moment to synchronously switch the two sets of diagonal switches in the folded bridge. This zero-crossing switching method helps reduce switching losses and electromagnetic interference.

[0049] like Figure 3 As shown, Figure 3 This is a waveform diagram provided in an embodiment of the present application; wherein, the upper waveform diagram is a single-phase input / output voltage waveform diagram, which shows the input voltage of a single AC-AC converter module. and the output voltage obtained after single-stage conversion As can be seen from the graph, the output voltage The waveform basically remained consistent with the input voltage. The sinusoidal characteristics of the same phase, and the fact that its amplitude is stabilized at the preset target value, verify that the single-stage conversion control strategy proposed in this application can achieve accurate voltage regulation output.

[0050] The middle waveform diagram is the resonant cavity current waveform diagram, which shows the current waveform at high frequency switching (switching frequency). Approaching the resonant frequency Driven by the input voltage, a high-frequency quasi-sinusoidal resonant current flows through the LLC resonant cavity. The frequency of this current is much higher than the power frequency, and its amplitude is modulated by the envelope, the shape of which is proportional to the instantaneous value of the input / output voltage.

[0051] The lower waveform diagram shows the three-phase input / output voltage waveforms. The three upper curves (Vin-a, Vin-b, Vin-c) represent the input phase voltages of the three-phase power grid, while the three lower curves (Vo-a, Vo-b, Vo-c) represent the output phase voltages of the three-phase four-wire system. The diagram shows that the output three-phase voltages have correct phase, balanced amplitude, and good waveform quality. This demonstrates that the system, through independent phase control of each module, can successfully convert three-phase input electrical energy into stable and balanced three-phase output electrical energy, providing high-quality power to the downstream loads.

[0052] This application also provides a control method for a three-phase string system of a single-stage power electronic transformer applied to any of the above embodiments, such as... Figure 4 As shown, Figure 4 A flowchart illustrating a control method provided in an embodiment of this application includes: S1: Each conversion module independently performs input voltage phase-locking and captures the voltage zero-crossing point; S2: Start at an initial frequency higher than the rated resonant frequency, and control the frequency to decrease linearly over time to suppress the starting current surge; S3: Centered on the rated resonant frequency, the switching frequency is finely adjusted in real time according to the feedback voltage at the output terminal to achieve output voltage regulation; specifically, when the frequency drops to... Afterward, the system enters steady-state operation. The effective value of the output voltage is then acquired from the controller at a preset sampling frequency (e.g., 10 kHz to 100 kHz). and with reference voltage By comparison, the error signal is obtained. The error signal is input to a proportional-integral (PI) controller, and the output of the PI controller serves as a correction factor for the switching frequency. The calculation formula is as follows: .

[0053] in, This is the proportionality coefficient. is the integral coefficient.

[0054] In step S3, the range of switching frequency variation is limited to between 0.8 and 1.2 times the rated resonant frequency.

[0055] The control method also includes a redundancy switching step: when the central controller receives a fault alarm signal from any slave controller, it disconnects the faulty module from the output bus and adjusts the power reference value of the remaining normally operating modules so that the system continues to supply power in derating mode.

[0056] In one specific implementation, the system operates according to the following steps when performing redundancy switching. When the central controller receives any fault alarm signal sent by the controller via the communication bus, it first determines the fault type and severity. If it is confirmed to be a permanent fault (e.g., power switch failure, drive circuit failure, etc.), the central controller immediately issues a cut-off command to control the AC contactor or solid-state relay at the output of the faulty module to disconnect, thereby severing the electrical connection between the faulty module and the output bus.

[0057] Next, the central controller recalculates the power reference value based on the system's current total load demand and the capacity of the two remaining normal modules. Assuming the three modules have equal rated capacity, each module handles approximately 33.3% of the total power under rated operating conditions. When one phase module is disconnected, the central controller sends the updated power reference value to the two remaining normally operating slave controllers, increasing their respective power handling capacity to approximately 50% of their original rated value, thus bringing the system's total output power to approximately 66.7% of its original rated capacity.

[0058] Finally, the slave controllers of the remaining two modules adjust their respective frequency converter control strategies based on the received new power reference value to stabilize the corresponding voltage and power output. Through the above redundancy switching steps, the system can continue to supply power in derating mode in the event of partial module failure, avoiding a complete system shutdown.

[0059] In the several embodiments provided in this application, it should be understood that the disclosed methods and devices can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For instance, the division of modules or units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed.

[0060] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment, depending on actual needs.

[0061] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0062] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A three-phase string system for a single-stage power electronic transformer, characterized in that, include: Three identical single-stage AC-AC converter modules, with their input terminals connected to the live wires and neutral wires of each phase of a three-phase power grid, respectively; The three-phase four-wire output structure is configured to interconnect the neutral points of the output terminals of the three conversion modules; each conversion module includes a single-stage energy transfer branch consisting of a folded bridge unit, a resonant cavity, a high-frequency isolation transformer, and a flip-bridge unit connected in sequence. The control system is configured to achieve a single-stage energy conversion from the input side to the output side by capturing the phase of the input voltage and controlling the switching frequency of the folded bridge unit and the flip bridge unit. The system achieves decoupled voltage regulation control of the three-phase output voltage and suppression of neutral point potential drift under asymmetrical load conditions by independently modulating the phase-by-phase energy conversion branches and cooperating with the neutral line current loop.

2. The system according to claim 1, characterized in that, The control system is equipped with a soft-start program: During the system startup phase, the control system outputs a drive pulse with an initial frequency higher than the rated resonant frequency, and controls the frequency of the drive pulse to decrease to the rated resonant frequency at a preset slope, so as to limit the charge injection current at startup by increasing the equivalent inductive reactance of the resonant cavity.

3. The system according to claim 2, characterized in that, The initial frequency of the driving pulse is configured to be 3 to 5 times the rated resonant frequency.

4. The system according to claim 1, characterized in that, The control system includes a central controller and slave controllers respectively located in each of the conversion modules; the central controller is connected to each of the slave controllers via a communication bus and is used to redistribute the operating power of the remaining two conversion modules when any conversion module fails.

5. The system according to claim 1, characterized in that, The controller has built-in multi-level protection logic based on response time scale: Level 1 protection: The power device current is monitored by a hardware comparator, and the drive signal is blocked when the current exceeds the first threshold, with a response time in microseconds. Second-level protection: When an abnormal resonant current or a load short circuit is detected, the flip-bridge unit is blocked and the folded bridge unit enters a direct-through mode, with a response time in milliseconds.

6. The system according to claim 1, characterized in that, Also includes: A line temperature detection unit is disposed on the surface of a magnetic element inside the conversion module; A leakage current detection unit is configured between the input and output terminals of the conversion module; the control system is configured to perform a shutdown or derating operation when the feedback value of any detection unit exceeds a preset safety threshold.

7. The system according to claim 1, characterized in that, The folded bridge unit includes a bridge circuit composed of controlled power switching transistors; the control system tracks the phase of the input voltage through a phase-locked loop and controls the bridge circuit to reverse polarity at the zero-crossing point of the voltage.

8. A control method applied to the system according to any one of claims 1 to 7, characterized in that, include: S1: Each of the aforementioned conversion modules independently performs input voltage phase-locking and captures the voltage zero-crossing point; S2: Start at an initial frequency higher than the rated resonant frequency, and control the frequency to decrease linearly over time to suppress the starting current surge; S3: Centered on the rated resonant frequency, the switching frequency is finely adjusted in real time according to the feedback voltage at the output terminal to achieve output voltage regulation.

9. The method according to claim 8, characterized in that, In step S3, the range of variation of the switching frequency is limited to between 0.8 and 1.2 times the rated resonant frequency.

10. The system according to claim 1, characterized in that, It also includes a redundancy switching step: when the central controller receives a fault alarm signal from any slave controller, it disconnects the faulty module from the output bus and adjusts the power reference value of the remaining normally operating modules so that the system continues to supply power in derating mode.

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