A unidirectional solid state transformer structure and a control method thereof

Abstract

The application discloses a one-way solid-state transformer structure and a control method thereof, and relates to the technical field of semiconductor switching devices. The one-way solid-state transformer structure comprises a plurality of phase units with the same structure, a system control unit and a system output capacitor. The plurality of phase units with the same structure comprise a plurality of one-way sub-modules with the same structure and a reactor. The alternating-current input ports of the plurality of one-way sub-modules with the same structure are connected in series after being cascaded in sequence, and then are connected in series with the reactor, and then serve as a three-phase medium-voltage alternating-current input interface of the one-way solid-state transformer structure. The direct-current output ports of the plurality of one-way sub-modules with the same structure are all connected in parallel with the system output capacitor, and then serve as system low-voltage direct-current output ports of the one-way solid-state transformer structure. The one-way solid-state transformer structure can realize stable control of system low-voltage direct-current output voltage. The one-way solid-state transformer structure and the control method thereof can be widely applied to the technical field of semiconductor switching devices.

Description

Technical Field

[0001] This invention relates to the field of semiconductor switching device technology, and in particular to a unidirectional solid-state transformer structure and its control method. Background Technology

[0002] Traditional transformers, as the cornerstone of the power grid, have inherent limitations such as bulky size, limited functionality, and inability to directly convert AC to DC power. With the rapid development of renewable energy, AI data centers, and DC supercharging for electric vehicles, the power grid's demand for equipment with millisecond-level response, AC / DC power conversion, high power density, and intelligent interaction capabilities is increasingly urgent. Thanks to technological breakthroughs in power semiconductors and high-performance magnetic core materials, solid-state transformers can achieve flexible power quality adjustment, direct DC port access, and real-time status monitoring while reducing size. They are not only an upgrade and replacement for traditional transformers but also a core intelligent interface connecting the physical power grid and the digital world, crucial for building a flexible, efficient, and zero-carbon power grid for the future.

[0003] Existing solid-state transformers primarily use cascaded full-bridge converters (CHBs) as their basic structure. Each phase of the system consists of multiple AC / DC power modules with identical structure and function, connected in series on the AC side and in parallel on the DC side. Each AC / DC power module comprises a front-end rectifier unit and a rear-end DC / DC isolation unit, both of which include at least three four-switch full-bridge circuits. This necessitates at least 12 fully controlled semiconductor switching devices per AC / DC power module, along with corresponding drive and control circuitry. Since the entire solid-state transformer consists of dozens of AC / DC power modules, the required number of fully controlled semiconductor switching devices and corresponding drive and control circuitry can reach hundreds. This not only significantly increases system costs but also adds to the complexity of system control and management, severely hindering the development and application of solid-state transformers. Summary of the Invention

[0004] To address the aforementioned technical problems, the present invention aims to provide a unidirectional solid-state transformer structure and its control method, which can achieve stable control of the system's low-voltage DC output voltage.

[0005] The first technical solution adopted in this invention is: a unidirectional solid-state transformer structure, comprising several phase units with identical structures, a system control unit, and a system output capacitor. The several phase units with identical structures include several unidirectional sub-modules and reactors with identical structures. The AC input ports of the several unidirectional sub-modules with identical structures are cascaded sequentially and then connected in series with the reactors, serving as the three-phase medium-voltage AC input interface of the unidirectional solid-state transformer structure. The DC output ports of the several unidirectional sub-modules with identical structures are all connected in parallel with the system output capacitor, serving as the system low-voltage DC output port of the unidirectional solid-state transformer structure. Wherein: Several of the aforementioned phase units are used to realize the power conversion between the input three-phase medium-voltage AC power and low-voltage DC power; The unidirectional submodule is used to convert the input three-phase medium-voltage AC power into medium-voltage DC power and perform power conversion with the low-voltage DC side to obtain low-voltage DC power; The reactor is used to filter out high-frequency harmonics in three-phase medium-voltage AC power. The system control unit is used to generate switching control signals based on voltage and current sampling signals through a voltage and current dual closed-loop controller, and control the operation of the phase unit. The system output capacitor is used to filter out high-frequency harmonics in the low-voltage DC side voltage and output the final low-voltage DC power.

[0006] Furthermore, the unidirectional submodule specifically includes a three-level rectifier unit, a DC capacitor, and an isolated DC transformer. The input terminal of the three-level rectifier unit serves as the AC input port of the unidirectional submodule, and the output terminal of the three-level rectifier unit is connected to the input terminals of the DC capacitor and the isolated DC transformer. The output terminal of the isolated DC transformer serves as the DC output port of the unidirectional submodule. The three-level rectifier unit is used to convert the input three-phase medium-voltage AC power into medium-voltage DC power; The DC capacitor is used to filter out high-frequency harmonics of the medium-voltage DC side voltage and output filtered medium-voltage DC power. The isolated DC transformer is used to convert the filtered medium-voltage DC power to the low-voltage DC power to obtain low-voltage DC power.

[0007] Furthermore, the three-level rectifier unit is configured with three optional connection topologies: a dual-switch non-half-bridge structure, a dual-switch half-bridge structure, and a single-switch structure.

[0008] Furthermore, the dual-switch non-half-bridge structure specifically includes a first fully controlled semiconductor switch, a second fully controlled semiconductor switch, a first power diode, and a second power diode. The first fully controlled semiconductor switch and the first power diode are connected in series to form a first half-bridge, and the second fully controlled semiconductor switch and the second power diode are connected in series to form a second half-bridge. The connection point between the first power diode and the first fully controlled semiconductor switch, and the connection point between the second power diode and the second fully controlled semiconductor switch, serve as the input terminal of the three-level rectifier unit. The first half-bridge and the second half-bridge are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is connected in parallel with the DC capacitor in the unidirectional submodule. When the first fully controlled semiconductor switch is turned off and the second fully controlled semiconductor switch is turned on, the dual-switch non-half-bridge structure is in the positive input state. The current of the AC input port of the unidirectional submodule flows into the DC capacitor through the first power diode and the second fully controlled semiconductor switch. The voltage of the AC input port of the unidirectional submodule is positively clamped to the DC capacitor voltage. When the first fully controlled semiconductor switch is turned on and the second fully controlled semiconductor switch is turned off, the dual-switch non-half-bridge structure is in a negative input state. The current of the AC input port of the unidirectional submodule flows into the DC capacitor through the second power diode and the first fully controlled semiconductor switch. The voltage of the AC input port of the unidirectional submodule is reverse clamped to the DC capacitor voltage. When the first fully controlled semiconductor switch and the second fully controlled semiconductor switch are simultaneously turned on, the dual-switch non-half-bridge structure is in the cut-off state. The current at the AC input port of the unidirectional submodule is controlled by the first and second fully controlled semiconductor switches to prevent it from flowing into the DC capacitor, and the voltage at the AC input port of the unidirectional submodule is clamped to 0.

[0009] Furthermore, the dual-switch half-bridge structure specifically includes a third fully controlled semiconductor switch, a fourth fully controlled semiconductor switch, a third power diode, and a fourth power diode. The third and fourth fully controlled semiconductor switches are connected in series to form the first half-bridge, and the third and fourth power diodes are connected in series to form the second half-bridge. The connection point between the third and fourth fully controlled semiconductor switches and the connection point between the third and fourth power diodes serve as the input terminal of the three-level rectifier unit. The first and second half-bridges are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is connected in parallel with the DC capacitor in the unidirectional submodule. When the third fully controlled semiconductor switch is turned off and the fourth fully controlled semiconductor switch is turned on, the dual-switch half-bridge structure is in a positive input state or a negative cut-off state. If it is in the positive input state, the current of the AC input port of the unidirectional submodule flows into the DC capacitor through the third power diode and the fourth fully controlled semiconductor switch, and the voltage of the AC input port of the unidirectional submodule is positively clamped to the DC capacitor voltage. If it is in the negative cut-off state, the current of the AC input port of the unidirectional submodule is controlled by the fourth power diode and the fourth fully controlled semiconductor switch and does not flow into the DC capacitor. The voltage of the AC input port of the unidirectional submodule is clamped to 0. When the third fully controlled semiconductor switch is turned on and the fourth fully controlled semiconductor switch is turned off, the dual-switch half-bridge structure is in a positive cut-off state or a negative input state. If it is in the positive cut-off state, the current of the AC input port of the unidirectional submodule is controlled by the third power diode and the third fully controlled semiconductor switch and does not flow into the DC capacitor. The voltage of the AC input port of the unidirectional submodule is clamped to 0. If it is in a negative input state, the current of the AC input port of the unidirectional submodule flows into the DC capacitor under the control of the third power diode and the third fully controlled semiconductor switch, and the voltage of the AC input port of the unidirectional submodule is reverse clamped to the DC capacitor voltage.

[0010] Furthermore, the single-switch structure specifically includes a fifth fully controlled semiconductor switch, a fifth power diode, a sixth power diode, a seventh power diode, an eighth power diode, and a ninth power diode. The fifth and sixth power diodes are connected in series to form a first half-bridge, and the seventh and eighth power diodes are connected in series to form a second half-bridge. The connection point between the fifth and sixth power diodes and the connection point between the seventh and eighth power diodes serve as the input terminal of the three-level rectifier unit. The first half-bridge, the second half-bridge, and the fifth fully controlled semiconductor switch are connected in parallel and then in series with the ninth power diode, and then in parallel with the DC capacitor in the unidirectional submodule. The two ends of the DC capacitor serve as the output terminal of the three-level rectifier unit, wherein: When the fifth fully controlled semiconductor switch is turned off and the input current of the three-level rectifier unit flows into the first half-bridge and out of the second half-bridge, the fifth power diode, the eighth power diode and the ninth power diode are turned on, and the single-switch structure is in the activated state. When the fifth fully controlled semiconductor switch is turned off and the input current of the three-level rectifier unit flows out of the first half-bridge and into the second half-bridge, the sixth, seventh, and eighth power diodes are turned on, and the single-switch structure is in the cut-out state. When the fifth fully controlled semiconductor switch is turned on, the input terminal of the three-level rectifier unit presents a 0 level.

[0011] Furthermore, the isolated DC transformer includes a high-frequency inverter unit, a high-frequency isolation transformer, and a high-frequency rectifier unit. The high-frequency inverter unit consists of a full-bridge inverter circuit composed of several fully controlled semiconductor switches, an inverter controller, and a damping circuit. The DC side of the full-bridge inverter circuit serves as the input terminal of the isolated DC transformer and is connected to the DC capacitor in the unidirectional submodule. The AC side of the full-bridge inverter circuit is connected to the primary winding of the high-frequency isolation transformer through the damping circuit. The damping circuit is composed of a capacitor or an inductor, or a series or parallel connection of a capacitor and an inductor.

[0012] Furthermore, the isolated DC transformer is configured with two optional connection topologies: a asynchronous rectification structure and a synchronous rectification structure, wherein: The high-frequency rectifier unit of the asynchronous rectifier structure is a full-bridge rectifier circuit composed of several power diodes. The AC side of the full-bridge rectifier circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side of the full-bridge rectifier circuit serves as the output terminal of the isolation DC transformer and is connected to the system output capacitor. The high-frequency rectifier unit of the synchronous rectifier structure consists of a synchronous rectifier full-bridge circuit composed of several fully controlled semiconductor switches and a rectifier controller. The AC side of the synchronous rectifier full-bridge circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side of the synchronous rectifier full-bridge circuit serves as the output terminal of the isolation DC transformer and is connected to the system output capacitor. The rectifier controller takes the secondary winding current of the high-frequency isolation transformer as input and generates control signals for the fully controlled semiconductor switches in the synchronous rectifier full-bridge circuit.

[0013] Furthermore, the voltage and current sampling signals of the system control unit include sampling signals of the three-phase medium-voltage AC input voltage and current, sampling signals of the system low-voltage DC output voltage and current, and sampling signals of the DC capacitor voltage in all unidirectional submodules. The voltage and current dual closed-loop controller uses the system low-voltage DC output voltage or the average value of the DC capacitor voltage in all unidirectional submodules as the control target, the voltage outer loop with the three-phase AC input current reference value as the output, the three-phase AC input current as the control target, and the current inner loop with the rectifier switch control signal as the output. The rectifier switch control signal is the switching control signal of the three-level rectifier unit in all unidirectional submodules. The switching control signal is used to control the working mode of the three-level rectifier unit.

[0014] The second technical solution adopted in this invention is: a control method for a unidirectional solid-state transformer structure, comprising the following steps: The three-phase medium-voltage AC power is obtained, and after high-frequency harmonics are filtered out by the reactors of each phase, it is input into the AC ports of all unidirectional sub-modules cascaded in the corresponding phase. Each submodule rectifies the medium-voltage AC power into medium-voltage DC power through a three-level rectifier unit, and then sends it to an isolation DC transformer. Through a fixed turns ratio, electrical isolation and DC-DC step-down conversion are completed to output low-voltage DC power. The low-voltage DC outputs from all submodules are connected in parallel and then filtered out by the system output capacitor to remove high-frequency ripple on the DC side, ultimately outputting stable low-voltage DC. The DC capacitor voltage of each submodule is collected, and combined with the three-phase medium-voltage AC power and the stable output low-voltage DC power, a switching control signal is generated through the voltage and current dual closed-loop controller to regulate the operating status of all three-level rectifier units. By adjusting the amplitude of the three-phase input AC current, the stable control of the system output DC voltage is achieved.

[0015] The beneficial effects of the structure and control method of this invention are as follows: The isolated DC transformer of this invention maintains a fixed input-output voltage ratio only under the regulation of its contained inverter controller and rectifier controller, that is, it appears to the outside as an isolated DC transformer with a fixed turns ratio; Under the action of its voltage and current dual closed-loop controller, the system control unit adjusts the magnitude of the three-phase input AC current by controlling the operating state of all three-level rectifier units in the UDSM, thereby realizing the control of the low-voltage DC output voltage of the system. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a unidirectional solid-state transformer structure according to the present invention; Figure 2 This is a flowchart illustrating the control method for a unidirectional solid-state transformer structure according to the present invention. Figure 3 This is a schematic diagram of the system structure of UDSM in a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 4 This is a schematic diagram of the dual-switch non-half-bridge structure and state circuit of the three-level rectifier unit in a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 5 This is a schematic diagram of the dual-switch half-bridge structure and state circuit of the three-level rectifier unit in a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 6 This is a schematic diagram of the single-switch structure and state circuit of the three-level rectifier unit in a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 7 This is a schematic diagram of the circuit structure of an isolated DC transformer in a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 8 This is a simplified system simulation model diagram of a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 9 This is a schematic diagram of a system simulation model of a unidirectional solid-state transformer provided in a specific embodiment of the present invention; Figure 10 This is a schematic diagram of the simulation waveform of a unidirectional solid-state transformer provided in a specific embodiment of the present invention. Detailed Implementation

[0017] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The step numbers in the following embodiments are only for ease of explanation and do not limit the order of the steps. The execution order of each step in the embodiments can be adapted according to the understanding of those skilled in the art.

[0018] Reference Figure 1This invention provides a unidirectional solid-state transformer structure, which includes several identical phase units, a system control unit, and a system output capacitor. The several identical phase units include several identical unidirectional sub-modules and reactors. The AC input ports of the several identical unidirectional sub-modules are cascaded sequentially and then connected in series with the reactors, serving as the three-phase medium-voltage AC input interface of the unidirectional solid-state transformer structure. The DC output ports of the several identical unidirectional sub-modules are all connected in parallel with the system output capacitor, serving as the system low-voltage DC output port of the unidirectional solid-state transformer structure. Wherein: Several of the aforementioned phase units are used to realize the power conversion between the input three-phase medium-voltage AC power and low-voltage DC power; The unidirectional submodule is used to convert the input three-phase medium-voltage AC power into medium-voltage DC power and perform power conversion with the low-voltage DC side to obtain low-voltage DC power; Specifically, the unidirectional submodule includes a three-level rectifier unit, a DC capacitor, and an isolated DC transformer. The input terminal of the three-level rectifier unit serves as the AC input port of the unidirectional submodule, and the output terminal of the three-level rectifier unit is connected to the input terminals of the DC capacitor and the isolated DC transformer. The output terminal of the isolated DC transformer serves as the DC output port of the unidirectional submodule.

[0019] In this embodiment, as Figure 3 The diagram shows the structure of a UDSM, consisting of a three-level rectifier unit for converting medium-voltage AC to medium-voltage DC, a DC capacitor for filtering high-frequency harmonics from the medium-voltage DC side, and an isolation DC transformer for power conversion between the medium-voltage and low-voltage DC sides. The input of the three-level rectifier unit serves as the AC input port of the UDSM, and its output is connected to the input of the DC capacitor and the isolation DC transformer. The output of the isolation DC transformer serves as the DC output port of the UDSM.

[0020] The three-level rectifier unit is used to convert the input three-phase medium-voltage AC power into medium-voltage DC power; More specifically, the three-level rectifier unit is configured with three optional connection topologies: a dual-switch non-half-bridge structure, a dual-switch half-bridge structure, and a single-switch structure.

[0021] The dual-switch non-half-bridge structure specifically includes a first fully controlled semiconductor switch, a second fully controlled semiconductor switch, a first power diode, and a second power diode. The first fully controlled semiconductor switch and the first power diode are connected in series to form a first half-bridge, and the second fully controlled semiconductor switch and the second power diode are connected in series to form a second half-bridge. The connection point between the first power diode and the first fully controlled semiconductor switch, and the connection point between the second power diode and the second fully controlled semiconductor switch, serve as the input terminal of the three-level rectifier unit. The first half-bridge and the second half-bridge are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is connected in parallel with the DC capacitor in the unidirectional submodule. When the first fully controlled semiconductor switch is off and the second fully controlled semiconductor switch is on, the dual-switch non-half-bridge structure is in the positive input state, and the AC input port of the unidirectional submodule... The current flows into the DC capacitor through the first power diode and the second fully controlled semiconductor switch, and the voltage at the AC input port of the unidirectional submodule is positively clamped to the DC capacitor voltage. When the first fully controlled semiconductor switch is turned on and the second fully controlled semiconductor switch is turned off, the dual-switch non-half-bridge structure is in a negative input state, and the current at the AC input port of the unidirectional submodule flows into the DC capacitor through the second power diode and the first fully controlled semiconductor switch, and the voltage at the AC input port of the unidirectional submodule is reversely clamped to the DC capacitor voltage. When both the first and second fully controlled semiconductor switches are turned on, the dual-switch non-half-bridge structure is in a cut-off state, and the current at the AC input port of the unidirectional submodule is controlled by the first and second fully controlled semiconductor switches to not flow into the DC capacitor, and the voltage at the AC input port of the unidirectional submodule is clamped to 0.

[0022] In this embodiment, the dual-switch non-half-bridge structure and operating state circuit of the UDSM are as follows: Figure 4 As shown. The circuit consists of fully controllable semiconductor switches S1 and S2, and power diodes D1 and D2. D1 and S1 are connected in series to form the first half-bridge, and D2 and S2 are connected in series to form the second half-bridge. The connection point of D1 and S1 and the connection point of D2 and S2 serve as the input terminal of the three-level rectifier unit. The first and second half-bridges are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is also connected in parallel with the DC capacitor in the UDSM. By controlling the on and off states of the fully controllable semiconductor switches S1 and S2, the input terminal of the three-level rectifier unit can present three levels: +Uc, -Uc, and 0 (where Uc is the voltage of the DC capacitor), corresponding to... Figure 4 The states are: positive input state, negative input state, and cut-off state. Specifically, when S1 is off and S2 is on, the corresponding state is... Figure 4 When S1 is in the positive state, the current at the AC input port flows into the DC capacitor through diode D1 and switch S2. At this time, the voltage at the AC input port is positively clamped to the DC capacitor voltage, i.e., uac = +Uc. When S1 is on and S2 is off, the corresponding... Figure 4In the neutral-load state, the current at the AC input port flows into the DC capacitor through diode D2 and switch S1. At this time, the voltage at the AC input port is reverse-clamped to the DC capacitor voltage, i.e., uac = -Uc. When S1 and S2 are both on, the corresponding... Figure 4 In the cut-out state, the current at the AC input port flows through switches S1 and S2 and does not flow into the DC capacitor. At this time, the voltage at the AC input port is clamped to 0, i.e., uac=0.

[0023] The dual-switch half-bridge structure specifically includes a third fully controlled semiconductor switch, a fourth fully controlled semiconductor switch, a third power diode, and a fourth power diode. The third and fourth fully controlled semiconductor switches are connected in series to form the first half-bridge, and the third and fourth power diodes are connected in series to form the second half-bridge. The connection point between the third and fourth fully controlled semiconductor switches and the connection point between the third and fourth power diodes serves as the input terminal of the three-level rectifier unit. The first and second half-bridges are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is also connected in parallel with the DC capacitor in the unidirectional submodule. When the third fully controlled semiconductor switch is off and the fourth fully controlled semiconductor switch is on, the dual-switch half-bridge structure is in a positive input state or a negative cut-off state. If it is in a positive input state, the current at the AC input port of the unidirectional submodule flows into the DC capacitor through the third power diode and the fourth fully controlled semiconductor switch. In the DC capacitor state, the voltage at the AC input port of the unidirectional submodule is positively clamped to the DC capacitor voltage. In the negative cut-off state, the current at the AC input port of the unidirectional submodule is controlled by the fourth power diode and the fourth fully controlled semiconductor switch, and does not flow into the DC capacitor; the voltage at the AC input port of the unidirectional submodule is clamped to 0. When the third fully controlled semiconductor switch is on and the fourth fully controlled semiconductor switch is off, the dual-switch half-bridge structure is either in the positive cut-off state or the negative on state. In the positive cut-off state, the current at the AC input port of the unidirectional submodule is controlled by the third power diode and the third fully controlled semiconductor switch, and does not flow into the DC capacitor; the voltage at the AC input port of the unidirectional submodule is clamped to 0. In the negative on state, the current at the AC input port of the unidirectional submodule is controlled by the third power diode and the third fully controlled semiconductor switch to flow into the DC capacitor; the voltage at the AC input port of the unidirectional submodule is reversely clamped to the DC capacitor voltage.

[0024] In this embodiment, the dual-switch half-bridge structure and operating state circuit of the UDSM are as follows: Figure 5As shown. The circuit consists of fully controllable semiconductor switches S3 and S4, and power diodes D3 and D4. S3 and S4 are connected in series to form the first half-bridge, and D3 and D4 are connected in series to form the second half-bridge. The connection point of S3 and S4 and the connection point of D3 and D4 serve as the input terminal of the three-level rectifier unit. The first and second half-bridges are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is also connected in parallel with the DC capacitor in the UDSM. By controlling the on and off states of the fully controllable semiconductor switches S3 and S4, the input terminal of the three-level rectifier unit can present three voltage levels: +Uc, -Uc, and 0 (where Uc is the voltage of the DC capacitor), corresponding to... Figure 5 The system includes positive input, negative input, and positive / negative output states. When S3 is off and S4 is on, the current at the AC input port flows into the DC capacitor through diode D3 and switch S4. At this time, the voltage at the AC input port is positively clamped to the DC capacitor voltage, i.e., uac = +Uc. Figure 5 When the circuit is in the neutral input state; with S3 off and S4 on, the current at the AC input port can also flow through diode D4 and switch S4, without flowing into the DC capacitor. At this time, the voltage at the AC input port is clamped to 0, i.e., uac=0. Figure 5 In the neutral-to-negative cutoff state, when S3 is on and S4 is off, the current at the AC input port can flow through diode D3 and switch S3, but does not flow into the DC capacitor. At this time, the voltage at the AC input port is clamped to 0, i.e., uac=0. Figure 5 In the neutral cut-off state; when S3 is on and S4 is off, the current at the AC input port can also flow into the DC capacitor through diode D4 and switch S3. At this time, the voltage at the AC input port is reverse clamped to the DC capacitor voltage, i.e., uac = -Ucp. Figure 5 Medium-to-negative input status.

[0025] The single-switch structure specifically includes a fifth fully controlled semiconductor switch, a fifth power diode, a sixth power diode, a seventh power diode, an eighth power diode, and a ninth power diode. The fifth and sixth power diodes are connected in series to form a first half-bridge, and the seventh and eighth power diodes are connected in series to form a second half-bridge. The connection point between the fifth and sixth power diodes and the connection point between the seventh and eighth power diodes serve as the input terminal of the three-level rectifier unit. The first half-bridge, the second half-bridge, and the fifth fully controlled semiconductor switch are connected in parallel and then in series with the ninth power diode, and then in parallel with the DC capacitor in the unidirectional submodule. The two ends of the DC capacitor serve as the output terminals of the three-level rectifier unit. When the fifth fully controlled semiconductor switch is off and the input current of the three-level rectifier unit flows into the first half-bridge and out of the second half-bridge, the fifth, eighth, and ninth power diodes are turned on, and the single-switch structure is in the active state. When the fifth fully controlled semiconductor switch is off and the input current of the three-level rectifier unit flows out of the first half-bridge and into the second half-bridge, the sixth, seventh, and eighth power diodes are turned on, and the single-switch structure is in the off state. When the fifth fully controlled semiconductor switch is on, the input terminal of the three-level rectifier unit is at a 0 level.

[0026] In this embodiment, the single-switch structure and operating state of the UDSM are as follows: Figure 6 As shown in the diagram, the circuit consists of a fully controllable semiconductor switch S5 and power diodes D5, D6, D7, D8, and D9. D5 and D6 are connected in series to form the first half-bridge, and D7 and D8 are connected in series to form the second half-bridge. The connection point between D5 and D6 and the connection point between D7 and D8 serve as the input terminal of the three-level rectifier unit. The first half-bridge, the second half-bridge, and the fully controllable semiconductor switch S5 are connected in parallel, then connected in series with the power diode D9, and finally connected in parallel with the DC capacitor in the UDSM. The two ends of the DC capacitor serve as the output terminal of the three-level rectifier unit. By controlling the on and off states of the fully controllable semiconductor switch S5, the input terminal of the three-level rectifier unit can present three voltage levels: +Uc, -Uc, and 0 (where Uc is the voltage of the DC capacitor), corresponding to... Figure 6 The input and output states are defined as follows: When S5 is off and the input current iL flows into the first half-bridge and out of the second half-bridge, diodes D5, D9, and D8 are turned on, and the input terminal presents a +Uc level; when S5 is off and the input current iL flows out of the first half-bridge and into the second half-bridge, diodes D6, D7, and D8 are turned on, and the input terminal presents a -Uc level; when S5 is on, the input terminal presents a 0 level.

[0027] The DC capacitor is used to filter out high-frequency harmonics of the medium-voltage DC side voltage and output filtered medium-voltage DC power. The isolated DC transformer is used to convert the filtered medium-voltage DC power to the low-voltage DC power to obtain low-voltage DC power.

[0028] Specifically, the isolated DC transformer includes a high-frequency inverter unit, a high-frequency isolation transformer, and a high-frequency rectifier unit. The high-frequency inverter unit consists of a full-bridge inverter circuit composed of several fully controlled semiconductor switches, an inverter controller, and a damping circuit. The DC side of the full-bridge inverter circuit serves as the input terminal of the isolated DC transformer and is connected to the DC capacitor in the unidirectional submodule. The AC side of the full-bridge inverter circuit is connected to the primary winding of the high-frequency isolation transformer through the damping circuit. The damping circuit is composed of a capacitor or an inductor, or a capacitor and an inductor connected in series or in parallel.

[0029] More specifically, the isolated DC transformer is configured with two optional connection topologies: a asynchronous rectification structure and a synchronous rectification structure. In the asynchronous rectification structure, the high-frequency rectification unit is a full-bridge rectifier circuit composed of several power diodes. The AC side of the full-bridge rectifier circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side of the full-bridge rectifier circuit serves as the output terminal of the isolated DC transformer and is connected to the system output capacitor. In the synchronous rectification structure, the high-frequency rectification unit consists of a synchronous full-bridge rectifier circuit composed of several fully controlled semiconductor switches and a rectifier controller. The AC side of the synchronous full-bridge rectifier circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side of the synchronous full-bridge rectifier circuit serves as the output terminal of the isolated DC transformer and is connected to the system output capacitor. The rectifier controller takes the secondary winding current of the high-frequency isolation transformer as input and generates control signals for the fully controlled semiconductor switches in the synchronous full-bridge rectifier circuit.

[0030] In this embodiment, the isolated DC transformer in the UDSM can be configured as follows: Figure 7The asynchronous and synchronous rectification topologies shown mainly consist of a high-frequency inverter unit, which inverts the DC capacitor voltage into medium-voltage side high-frequency AC, a high-frequency isolation transformer, which converts the medium-voltage side high-frequency AC into low-voltage side high-frequency AC, and a high-frequency rectifier unit, which rectifies the low-voltage side high-frequency AC into low-voltage DC. The high-frequency inverter unit is a full-bridge inverter circuit composed of fully controlled semiconductor switches T1~T4. It is used for DC-to-high-frequency AC power conversion and serves as an inverter controller. This controller controls the on / off state of switches T1~T4 and includes a damping circuit. It filters harmonics from the medium-voltage side high-frequency AC and suppresses the current conversion rate. The DC side of the full-bridge inverter circuit serves as the input terminal of the isolated DC transformer and is connected to the DC capacitor in the UDSM. The AC side of the full-bridge inverter circuit is connected to the primary winding of the high-frequency isolation transformer via a damping circuit. The inverter controller takes the primary winding current sampling signal of the high-frequency isolation transformer as input and generates control signals for the fully controlled semiconductor switches T1~T4 in the full-bridge inverter circuit. The damping circuit is composed of a capacitor Cr, an inductor Lr, or a series or parallel combination of Cr and Lr. Figure 7 As shown in the left sub-figure, the high-frequency rectifier unit of the asynchronous rectifier structure is a full-bridge rectifier circuit composed of power diodes Dr1~Dr4, wherein: the AC side of the full-bridge rectifier circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side serves as the output terminal of the isolation DC transformer and is connected to the system output capacitor Co. Figure 7 As shown in the right sub-figure, the high-frequency rectifier unit of the synchronous rectifier structure consists of a synchronous rectifier full-bridge circuit composed of fully controlled semiconductor switches Dr1~Dr4 and a rectifier controller. The AC side of the synchronous rectifier full-bridge circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side serves as the output terminal of the isolation DC transformer and is connected to the system output capacitor Co. The rectifier controller takes the secondary winding current of the high-frequency isolation transformer as input and generates control signals for the fully controlled semiconductor switches Dr1~Dr4 in the synchronous rectifier full-bridge circuit.

[0031] The reactor is used to filter out high-frequency harmonics in three-phase medium-voltage AC power. The system control unit is used to generate switching control signals based on voltage and current sampling signals through a voltage and current dual closed-loop controller, and control the operation of the phase unit. Specifically, the voltage and current sampling signals of the system control unit include sampling signals of the three-phase medium-voltage AC input voltage and current, sampling signals of the system low-voltage DC output voltage and current, and sampling signals of the DC capacitor voltage in all unidirectional submodules. The voltage and current dual closed-loop controller uses the system low-voltage DC output voltage or the average value of the DC capacitor voltage in all unidirectional submodules as the control target, the voltage outer loop with the three-phase AC input current reference value as the output, the three-phase AC input current as the control target, and the current inner loop with the rectifier switch control signal as the output. The rectifier switch control signal is the switching control signal of the three-level rectifier unit in all unidirectional submodules. The switching control signal is used to control the working mode of the three-level rectifier unit.

[0032] In this embodiment, the system control unit takes voltage and current sampling signals as input and generates rectifier switch control signals as output through a voltage and current dual-loop controller. The voltage and current sampling signals include: sampling signals of the three-phase medium-voltage AC input voltage and current, sampling signals of the system low-voltage DC output voltage and current, and sampling signals of the DC capacitor voltage in each UDSM. The voltage and current dual-loop controller includes: a voltage outer loop with the system low-voltage DC output voltage or the average value of the DC capacitor voltages in all UDSMs as the control target and the three-phase AC input current reference value as the output; and a current inner loop with the three-phase AC input current as the control target and the rectifier switch control signal as the output. The rectifier switch control signal is the switching control signal of the three-level rectifier unit in all UDSMs, used to control the operating mode of the three-level rectifier unit.

[0033] The system output capacitor is used to filter out high-frequency harmonics in the low-voltage DC side voltage and output the final low-voltage DC power.

[0034] In summary, such as Figure 1The diagram shows the structure of a unidirectional solid-state transformer used to convert three-phase medium-voltage AC power into low-voltage DC power for power supply to data centers or electric vehicle charging stations. The unidirectional solid-state transformer consists of three identical phase units for power conversion between medium-voltage AC and low-voltage DC, a system control unit for stable operation of the phase units, and a system output capacitor Co for filtering high-frequency harmonics from the low-voltage DC side. Each phase unit comprises n identical unidirectional submodules (UDSMs) to share the medium-voltage AC side of the phase unit and the power conversion between the phase unit and the low-voltage DC, and a reactor L for filtering high-frequency harmonics from the medium-voltage AC side current. The AC input ports of the n UDSMs are cascaded and then connected in series with the reactor L, serving as the three-phase medium-voltage AC input interface of the unidirectional solid-state transformer. The DC output port of each UDSM is connected in parallel with the system output capacitor Co, serving as the system low-voltage DC output port of the unidirectional solid-state transformer.

[0035] Reference Figure 2 A control method for a unidirectional solid-state transformer structure includes the following steps: S100: Obtain three-phase medium-voltage AC power, filter out high-frequency harmonics through reactors in each phase, and input it into the AC ports of all cascaded unidirectional sub-modules in the corresponding phase. S200 and each sub-module rectify medium-voltage AC power into medium-voltage DC power through a three-level rectifier unit, and then send it to an isolation DC transformer. Through a fixed ratio, electrical isolation and DC-DC step-down conversion are completed to output low-voltage DC power. S300 connects all the low-voltage DC outputs from the sub-modules in parallel, and then filters out the high-frequency ripple on the DC side through the system output capacitor, finally outputting a stable low-voltage DC. The S400 collects the DC capacitor voltage of each submodule, combines the three-phase medium-voltage AC power with the stable output low-voltage DC power, and generates a switching control signal through a voltage and current dual closed-loop controller to regulate the operating status of all three-level rectifier units. By adjusting the amplitude of the three-phase input AC current, it achieves stable control of the system output DC voltage.

[0036] The system control unit of the unidirectional solid-state transformer in this embodiment of the invention takes the sampling signals of the three-phase current on the medium-voltage AC side, the low-voltage DC side voltage, and the system given reference voltage value as inputs, and the control signal of the semiconductor switch in the three-level rectifier unit as output. The difference between the system given reference voltage value and the sampling signal of the low-voltage DC side voltage is used to generate a current reference signal through the voltage outer loop controller. The difference between the current reference signal and the sampling signal of the three-phase current on the medium-voltage AC side is used to generate the control signal of the semiconductor switch in the three-level rectifier unit through the current inner loop controller.

[0037] The proposed control strategy for the single-phase solid-state transformer is as follows: the isolated DC transformer maintains a fixed input-output voltage ratio only under the regulation of its contained inverter controller and rectifier controller, that is, it appears to the outside as an isolated DC transformer with a fixed turns ratio; the system control unit, under the action of its voltage and current dual closed-loop controller, controls the operation status of all three-level rectifier units in the UDSM and adjusts the magnitude of the three-phase input AC current to achieve the control of the system's low-voltage DC output voltage.

[0038] Compared to existing solid-state transformers with bidirectional energy transfer, the proposed unidirectional solid-state transformer is structurally simpler, requiring fewer fully controlled semiconductor switching devices and their drive circuits. In terms of control, the proposed unidirectional solid-state transformer only needs real-time control of the three-level rectifier unit, while the isolated DC transformer provides a fixed-ratio DC voltage regulation capability to the system under its own controller, making its control simpler than existing solid-state transformers. This gives the proposed unidirectional solid-state transformer advantages in lower cost and control complexity for applications such as data centers and electric vehicle charging stations where bidirectional energy transfer is not required.

[0039] Furthermore, to verify the feasibility of the proposed unidirectional solid-state transformer structure and control strategy, a simulation system was built in the simulation software. Figure 8 The system simulation model shown has three-phase 10kV AC inputs Ua, Ub, and Uc, an output capacitor Co of 10mF, and a load resistance of 0.27Ω. In the system control unit, the difference between the 800V reference voltage of the outer-loop voltage controller and the system output voltage is used by a PI control module to generate the current reference value for the inner-loop current controller. The inner-loop current controller generates a rectifier unit control signal with a frequency of 20kHz. The simulation circuit structure for each phase unit is as follows: Figure 9 As shown, each DC capacitor has a value of 2mF, and the isolation DC transformer uses the following... Figure 7 The left sub-diagram shows the current structure and provides a 2:1 voltage ratio for the system.

[0040] Figure 10 The simulation waveforms show that the system output DC voltage eventually stabilizes at 800V, the three-phase input current amplitude is approximately 200A, and the voltage of the DC capacitor in each of the three phase units eventually stabilizes at around 1600V. The simulation results effectively verify that the proposed unidirectional solid-state transformer can effectively convert 10kV medium-voltage AC power into 800V low-voltage DC power.

[0041] In summary, compared with existing solid-state transformers with bidirectional energy transmission, the proposed unidirectional solid-state transformer is structurally simpler, requiring fewer fully controlled semiconductor switching devices and their drive circuits; it is also simpler to control. This gives the proposed unidirectional solid-state transformer advantages in lower cost and control complexity for applications such as data centers and electric vehicle charging stations that do not require bidirectional energy transmission.

[0042] The preferred embodiments of the present application have been described above with reference to the accompanying drawings, but this is not intended to limit the scope of the embodiments of the present application. Any modifications, equivalent substitutions, and improvements made by those skilled in the art without departing from the scope and substance of the embodiments of the present application shall be within the scope of the embodiments of the present application.

Claims

1. A unidirectional solid-state transformer structure, characterized in that, The system includes several identical phase units, a system control unit, and a system output capacitor. Each identical phase unit comprises several identical unidirectional submodules and reactors. The AC input ports of these identical unidirectional submodules are cascaded sequentially and then connected in series with the reactors, serving as the three-phase medium-voltage AC input interface of the unidirectional solid-state transformer structure. The DC output ports of these identical unidirectional submodules are all connected in parallel with the system output capacitor, serving as the system low-voltage DC output port of the unidirectional solid-state transformer structure. Several of the aforementioned phase units are used to realize the power conversion between the input three-phase medium-voltage AC power and low-voltage DC power; The unidirectional submodule is used to convert the input three-phase medium-voltage AC power into medium-voltage DC power and perform power conversion with the low-voltage DC side to obtain low-voltage DC power; The reactor is used to filter out high-frequency harmonics in three-phase medium-voltage AC power. The system control unit is used to generate switching control signals based on voltage and current sampling signals through a voltage and current dual closed-loop controller, and control the operation of the phase unit. The system output capacitor is used to filter out high-frequency harmonics in the low-voltage DC side voltage and output the final low-voltage DC power.

2. The unidirectional solid-state transformer structure according to claim 1, characterized in that, The unidirectional submodule specifically includes a three-level rectifier unit, a DC capacitor, and an isolated DC transformer. The input terminal of the three-level rectifier unit serves as the AC input port of the unidirectional submodule. The output terminal of the three-level rectifier unit is connected to the input terminals of the DC capacitor and the isolated DC transformer. The output terminal of the isolated DC transformer serves as the DC output port of the unidirectional submodule. The three-level rectifier unit is used to convert the input three-phase medium-voltage AC power into medium-voltage DC power; The DC capacitor is used to filter out high-frequency harmonics of the medium-voltage DC side voltage and output filtered medium-voltage DC power. The isolated DC transformer is used to convert the filtered medium-voltage DC power to the low-voltage DC power to obtain low-voltage DC power.

3. The unidirectional solid-state transformer structure according to claim 2, characterized in that, The three-level rectifier unit is configured with three optional connection topologies: a dual-switch non-half-bridge structure, a dual-switch half-bridge structure, and a single-switch structure.

4. The unidirectional solid-state transformer structure according to claim 3, characterized in that, The dual-switch non-half-bridge structure specifically includes a first fully controlled semiconductor switch, a second fully controlled semiconductor switch, a first power diode, and a second power diode. The first fully controlled semiconductor switch and the first power diode are connected in series to form a first half-bridge, and the second fully controlled semiconductor switch and the second power diode are connected in series to form a second half-bridge. The connection point between the first power diode and the first fully controlled semiconductor switch, and the connection point between the second power diode and the second fully controlled semiconductor switch, serve as the input terminal of the three-level rectifier unit. The first half-bridge and the second half-bridge are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is connected in parallel with the DC capacitor in the unidirectional submodule. When the first fully controlled semiconductor switch is turned off and the second fully controlled semiconductor switch is turned on, the dual-switch non-half-bridge structure is in the positive input state. The current of the AC input port of the unidirectional submodule flows into the DC capacitor through the first power diode and the second fully controlled semiconductor switch. The voltage of the AC input port of the unidirectional submodule is positively clamped to the DC capacitor voltage. When the first fully controlled semiconductor switch is turned on and the second fully controlled semiconductor switch is turned off, the dual-switch non-half-bridge structure is in a negative input state. The current of the AC input port of the unidirectional submodule flows into the DC capacitor through the second power diode and the first fully controlled semiconductor switch. The voltage of the AC input port of the unidirectional submodule is reverse clamped to the DC capacitor voltage. When the first fully controlled semiconductor switch and the second fully controlled semiconductor switch are simultaneously turned on, the dual-switch non-half-bridge structure is in the cut-off state. The current at the AC input port of the unidirectional submodule is controlled by the first and second fully controlled semiconductor switches to prevent it from flowing into the DC capacitor, and the voltage at the AC input port of the unidirectional submodule is clamped to 0.

5. The unidirectional solid-state transformer structure according to claim 4, characterized in that, The dual-switch half-bridge structure specifically includes a third fully controlled semiconductor switch, a fourth fully controlled semiconductor switch, a third power diode, and a fourth power diode. The third and fourth fully controlled semiconductor switches are connected in series to form the first half-bridge, and the third and fourth power diodes are connected in series to form the second half-bridge. The connection point between the third and fourth fully controlled semiconductor switches and the connection point between the third and fourth power diodes serves as the input terminal of the three-level rectifier unit. The first and second half-bridges are connected in parallel, and the parallel connection point serves as the output terminal of the three-level rectifier unit, and is also connected in parallel with the DC capacitor in the unidirectional submodule. When the third fully controlled semiconductor switch is turned off and the fourth fully controlled semiconductor switch is turned on, the dual-switch half-bridge structure is in a positive input state or a negative cut-off state. If it is in the positive input state, the current of the AC input port of the unidirectional submodule flows into the DC capacitor through the third power diode and the fourth fully controlled semiconductor switch, and the voltage of the AC input port of the unidirectional submodule is positively clamped to the DC capacitor voltage. If it is in the negative cut-off state, the current of the AC input port of the unidirectional submodule is controlled by the fourth power diode and the fourth fully controlled semiconductor switch and does not flow into the DC capacitor. The voltage of the AC input port of the unidirectional submodule is clamped to 0. When the third fully controlled semiconductor switch is turned on and the fourth fully controlled semiconductor switch is turned off, the dual-switch half-bridge structure is in a positive cut-off state or a negative input state. If it is in the positive cut-off state, the current of the AC input port of the unidirectional submodule is controlled by the third power diode and the third fully controlled semiconductor switch and does not flow into the DC capacitor. The voltage of the AC input port of the unidirectional submodule is clamped to 0. If it is in a negative input state, the current of the AC input port of the unidirectional submodule flows into the DC capacitor under the control of the third power diode and the third fully controlled semiconductor switch, and the voltage of the AC input port of the unidirectional submodule is reverse clamped to the DC capacitor voltage.

6. The unidirectional solid-state transformer structure according to claim 5, characterized in that, The single-switch structure specifically includes a fifth fully controlled semiconductor switch, a fifth power diode, a sixth power diode, a seventh power diode, an eighth power diode, and a ninth power diode. The fifth and sixth power diodes are connected in series to form a first half-bridge, and the seventh and eighth power diodes are connected in series to form a second half-bridge. The connection point between the fifth and sixth power diodes and the connection point between the seventh and eighth power diodes serve as the input terminal of the three-level rectifier unit. The first half-bridge, the second half-bridge, and the fifth fully controlled semiconductor switch are connected in parallel and then in series with the ninth power diode, and then in parallel with the DC capacitor in the unidirectional submodule. The two ends of the DC capacitor serve as the output terminal of the three-level rectifier unit. When the fifth fully controlled semiconductor switch is turned off and the input current of the three-level rectifier unit flows into the first half-bridge and out of the second half-bridge, the fifth power diode, the eighth power diode and the ninth power diode are turned on, and the single-switch structure is in the activated state. When the fifth fully controlled semiconductor switch is turned off and the input current of the three-level rectifier unit flows out of the first half-bridge and into the second half-bridge, the sixth, seventh, and eighth power diodes are turned on, and the single-switch structure is in the cut-out state. When the fifth fully controlled semiconductor switch is turned on, the input terminal of the three-level rectifier unit presents a 0 level.

7. The unidirectional solid-state transformer structure according to claim 6, characterized in that, The isolated DC transformer includes a high-frequency inverter unit, a high-frequency isolation transformer, and a high-frequency rectifier unit. The high-frequency inverter unit consists of a full-bridge inverter circuit composed of several fully controlled semiconductor switches, an inverter controller, and a damping circuit. The DC side of the full-bridge inverter circuit serves as the input terminal of the isolated DC transformer and is connected to the DC capacitor in the unidirectional submodule. The AC side of the full-bridge inverter circuit is connected to the primary winding of the high-frequency isolation transformer through the damping circuit. The damping circuit is composed of a capacitor or an inductor, or a series or parallel connection of a capacitor and an inductor.

8. The unidirectional solid-state transformer structure according to claim 7, characterized in that, The isolated DC transformer is configured with two optional connection topologies: asynchronous rectification and synchronous rectification. The high-frequency rectifier unit of the asynchronous rectifier structure is a full-bridge rectifier circuit composed of several power diodes. The AC side of the full-bridge rectifier circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side of the full-bridge rectifier circuit serves as the output terminal of the isolation DC transformer and is connected to the system output capacitor. The high-frequency rectifier unit of the synchronous rectifier structure consists of a synchronous rectifier full-bridge circuit composed of several fully controlled semiconductor switches and a rectifier controller. The AC side of the synchronous rectifier full-bridge circuit is connected to the secondary winding of the high-frequency isolation transformer, and the DC side of the synchronous rectifier full-bridge circuit serves as the output terminal of the isolation DC transformer and is connected to the system output capacitor. The rectifier controller takes the secondary winding current of the high-frequency isolation transformer as input and generates control signals for the fully controlled semiconductor switches in the synchronous rectifier full-bridge circuit.

9. The unidirectional solid-state transformer structure according to claim 8, characterized in that, The voltage and current sampling signals of the system control unit include sampling signals of the three-phase medium-voltage AC input voltage and current, sampling signals of the system low-voltage DC output voltage and current, and sampling signals of the DC capacitor voltage in all unidirectional submodules. The voltage and current dual closed-loop controller uses the system low-voltage DC output voltage or the average value of the DC capacitor voltage in all unidirectional submodules as the control target, the voltage outer loop with the three-phase AC input current reference value as the output, the three-phase AC input current as the control target, and the current inner loop with the rectifier switch control signal as the output. The rectifier switch control signal is the switching control signal of the three-level rectifier unit in all unidirectional submodules. The switching control signal is used to control the working mode of the three-level rectifier unit.

10. A control method for a unidirectional solid-state transformer structure, characterized in that, Includes the following steps: The three-phase medium-voltage AC power is obtained, and after high-frequency harmonics are filtered out by the reactors of each phase, it is input into the AC ports of all unidirectional sub-modules cascaded in the corresponding phase. Each submodule rectifies the medium-voltage AC power into medium-voltage DC power through a three-level rectifier unit, and then sends it to an isolation DC transformer. Through a fixed turns ratio, electrical isolation and DC-DC step-down conversion are completed to output low-voltage DC power. The low-voltage DC outputs from all submodules are connected in parallel and then filtered out by the system output capacitor to remove high-frequency ripple on the DC side, ultimately outputting stable low-voltage DC. The DC capacitor voltage of each submodule is collected, and combined with the three-phase medium-voltage AC power and the stable output low-voltage DC power, a switching control signal is generated through the voltage and current dual closed-loop controller to regulate the operating status of all three-level rectifier units. By adjusting the amplitude of the three-phase input AC current, the stable control of the system output DC voltage is achieved.

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