Three-phase direct-current transformer

Abstract

The invention relates to a three-phase direct-current transformer which comprises a first full-bridge circuit, an isolation transformation module and a second full-bridge circuit. The first full-bridge circuit is connected with the DC input port. The primary side of the isolation transformation module is connected with the output end of the first full-bridge circuit. The input end of the second full-bridge circuit is connected with the secondary side of the isolation transformation module, and the output end of the second full-bridge circuit is connected with the direct current output port; the second full-bridge circuit comprises a plurality of switch modules and a plurality of buffer capacitors; the switch modules are connected with the corresponding buffer capacitors in parallel, and the capacitance values of the buffer capacitors are far larger than the capacitance values of parasitic capacitors of the switch modules, so that the dynamic voltages at the two ends of the switch modules in the on or off process are determined by the buffer capacitors, and the dynamic voltages at the two ends of each buffer capacitor are the same. Therefore, the dynamic voltage balance between the switch modules can be realized, and the reliability of the three-phase direct current transformer is improved.

Description

Technical Field

[0001] This disclosure relates to the field of DC transformer technology, and more particularly to a three-phase DC transformer. Background Technology

[0002] With the global energy crisis worsening and photovoltaic conversion efficiency continuously improving, the installed capacity of photovoltaic power plants is growing rapidly. Against this backdrop, large-scale photovoltaic DC collection systems, due to their simple structure, lack of reactive power compensation, and ability to transmit power over long distances, have become an important technological direction for future photovoltaic collection. The key to realizing this system lies in developing DC transformers with low cost, high efficiency, high power output, and high step-up capability to complete voltage transformation from low to medium and even high voltage.

[0003] In related technologies, isolated DC-DC converters can effectively separate and decouple the large current on the low-voltage side from the high voltage on the high-voltage side through electrical isolation. Therefore, isolated DC-DC converters are the optimal solution for achieving a high step-up ratio. However, isolated DC transformers have difficulty ensuring the dynamic voltage balance of the secondary-side switching devices, thus affecting the reliability of the transformer system. Summary of the Invention

[0004] To solve or at least partially solve the above-mentioned technical problems, this disclosure provides a three-phase DC transformer. In this disclosure, since the capacitance of the buffer capacitor is much larger than the capacitance of the parasitic capacitance of the switching module, the dynamic voltage across the switching module during the turn-on or turn-off process is determined by the buffer capacitor. Furthermore, since the dynamic voltage across each buffer capacitor is the same, the dynamic voltage across each switching module is also the same. Thus, this disclosure can achieve dynamic voltage balance among the switching modules, thereby improving the reliability of the three-phase DC transformer.

[0005] This disclosure provides a three-phase DC transformer, which includes: a first full-bridge circuit, an isolation transformer module, and a second full-bridge circuit; The first full-bridge circuit is connected to the DC input port and is used to convert the first DC signal input to the DC input port into a first three-phase AC signal. The primary side of the isolation transformer module is connected to the output terminal of the first full-bridge circuit; the isolation transformer module is used to convert the first three-phase AC signal into a second three-phase AC signal; wherein the voltages of the first three-phase AC signal and the second three-phase AC signal are different; The input terminal of the second full-bridge circuit is connected to the secondary side of the isolation transformer module, and the output terminal of the second full-bridge circuit is connected to the DC output port. The second full-bridge circuit is used to convert the second three-phase AC signal into a second DC signal; wherein, the voltage of the first DC signal and the second DC signal are different. The second full-bridge circuit includes multiple switching modules and multiple buffer capacitors; the switching modules are connected in parallel with their corresponding buffer capacitors. The capacitance of the buffer capacitor is much larger than that of the parasitic capacitance of the switching module.

[0006] Optionally, the multiple switch modules include at least one first switch unit, at least one second switch unit, at least one third switch unit, at least one fourth switch unit, at least one fifth switch unit, and at least one sixth switch unit; The second full-bridge circuit includes a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm, and a supporting capacitor; The first bridge arm includes at least one first switch unit, the second bridge arm includes at least one second switch unit, the third bridge arm includes at least one third switch unit, the fourth bridge arm includes at least one fourth switch unit, the fifth bridge arm includes at least one fifth switch unit, and the sixth bridge arm includes at least one sixth switch unit. The first end of the first bridge arm is connected to the first end of the DC output port; the second end of the first bridge arm is connected to the first output terminal of the isolation transformer module; the second end of the second bridge arm is connected to the second end of the DC output port; the first end of the third bridge arm is connected to the first end of the DC output port; the second end of the third bridge arm is connected to the first end of the fourth bridge arm; the second end of the fourth bridge arm is connected to the second end of the DC output port; the first end of the fifth bridge arm is connected to the first end of the DC output port; the second end of the fifth bridge arm is connected to the first end of the sixth bridge arm; the second end of the sixth bridge arm is connected to the second end of the DC output port; the first end of the supporting capacitor is connected to the first end of the DC output port; the second end of the supporting capacitor is connected to the second end of the DC output port. The first switch unit, the second switch unit, the third switch unit, the fourth switch unit, the fifth switch unit, and the sixth switch unit are connected in parallel with their respective buffer capacitors; The capacitance of the buffer capacitor is much larger than that of the parasitic capacitance of the first, second, third, fourth, fifth, and sixth switching units.

[0007] Optionally, the first bridge arm includes multiple first switch units connected in series, the second bridge arm includes multiple second switch units connected in series, the third bridge arm includes multiple third switch units connected in series, the fourth bridge arm includes multiple fourth switch units connected in series, the fifth bridge arm includes multiple fifth switch units connected in series, and the sixth bridge arm includes a sixth switch unit connected in series.

[0008] Optionally, the first switching unit includes a first diode, the second switching unit includes a second diode, the third switching unit includes a third diode, the fourth switching unit includes a fourth diode, the fifth switching unit includes a fifth diode, and the sixth switching unit includes a sixth diode.

[0009] Optionally, the second full-bridge circuit also includes multiple voltage-equalizing resistors; the voltage-equalizing resistors are connected in parallel with the corresponding switching modules.

[0010] Optionally, the isolation transformer module includes a first inductor, a second inductor, a third inductor, a first capacitor, a second capacitor, a third capacitor, a first isolation transformer, a second isolation transformer, and a third isolation transformer; The first terminal of the first inductor is connected to the first output terminal of the first full-bridge circuit. The second terminal of the first inductor is connected to the first terminal of the first isolation transformer through the first capacitor. The first terminal of the second inductor is connected to the second output terminal of the first full-bridge circuit. The second terminal of the second inductor is connected to the first terminal of the second isolation transformer through the second capacitor. The first terminal of the third inductor is connected to the third output terminal of the first full-bridge circuit. The second terminal of the third inductor is connected to the first terminal of the third isolation transformer through the third capacitor. The second terminal of the first isolation transformer is connected to the second terminal of the second isolation transformer and the second terminal of the third isolation transformer. The third terminal of the first isolation transformer is connected to the first output terminal of the isolation transformer module, the third terminal of the second isolation transformer is connected to the second output terminal of the isolation transformer module, the third terminal of the third isolation transformer is connected to the third output terminal of the isolation transformer module, and the fourth terminal of the first isolation transformer is connected to the fourth terminals of the second and third isolation transformers.

[0011] Optionally, the first inductor is the leakage inductance of the first isolation transformer; The second inductor is the leakage inductance of the second isolation transformer; The third inductor is the leakage inductance of the third isolation transformer.

[0012] Optionally, the first full-bridge circuit includes a seventh switching unit, an eighth switching unit, a ninth switching unit, a tenth switching unit, an eleventh switching unit, a twelfth switching unit, and an input capacitor; The first end of the seventh switch unit is connected to the first end of the DC input port, the second end of the seventh switch unit is connected to the first end of the eighth switch unit at the first node, and the second end of the eighth switch unit is connected to the second end of the DC input port. The first end of the ninth switch unit is connected to the first end of the DC input port, the second end of the ninth switch unit is connected to the first end of the tenth switch unit at the second node, and the second end of the tenth switch unit is connected to the second end of the DC input port. The first end of the eleventh switch unit is connected to the first end of the DC input port, the second end of the eleventh switch unit is connected to the first end of the twelfth switch unit at the third node, and the second end of the twelfth switch unit is connected to the second end of the DC input port. The first node is connected to the first end of the first inductor, the second node is connected to the first end of the second inductor, and the third node is connected to the first end of the third inductor. The first end of the input capacitor is connected to the first end of the DC input port, and the second end of the input capacitor is connected to the second end of the DC input port.

[0013] Optionally, the seventh switching unit includes a first IGCT device and a seventh diode, the eighth switching unit includes a second IGCT device and an eighth diode, the ninth switching unit includes a third IGCT device and a ninth diode, the tenth switching unit includes a fourth IGCT device and a tenth diode, the eleventh switching unit includes a fifth IGCT device and an eleventh diode, and the twelfth switching unit includes a sixth IGCT device and a twelfth diode. The positive terminal of the first IGCT device is connected to the first end of the DC input port, the negative terminal of the first IGCT device is connected to the positive terminal of the second IGCT device at the first node, and the negative terminal of the second IGCT device is connected to the second end of the DC input port. The positive terminal of the third IGCT device is connected to the first end of the DC input port, the negative terminal of the third IGCT device is connected to the positive terminal of the fourth IGCT device at the second node, and the negative terminal of the fourth IGCT device is connected to the second end of the DC input port. The positive terminal of the fifth IGCT device is connected to the first end of the DC input port, the negative terminal of the fifth IGCT device is connected to the positive terminal of the sixth IGCT device at the third node, and the negative terminal of the sixth IGCT device is connected to the second end of the DC input port. The positive terminal of the first IGCT device is connected to the negative terminal of the seventh diode, and the negative terminal of the first IGCT device is connected to the positive terminal of the seventh diode; the positive terminal of the second IGCT device is connected to the negative terminal of the eighth diode, and the negative terminal of the second IGCT device is connected to the positive terminal of the eighth diode; the positive terminal of the third IGCT device is connected to the negative terminal of the ninth diode, and the negative terminal of the third IGCT device is connected to the positive terminal of the ninth diode; the positive terminal of the fourth IGCT device is connected to the negative terminal of the tenth diode, and the negative terminal of the fourth IGCT device is connected to the positive terminal of the tenth diode; the positive terminal of the fifth IGCT device is connected to the negative terminal of the eleventh diode, and the negative terminal of the fifth IGCT device is connected to the positive terminal of the eleventh diode; the positive terminal of the sixth IGCT device is connected to the negative terminal of the twelfth diode, and the negative terminal of the sixth IGCT device is connected to the positive terminal of the twelfth diode.

[0014] Optionally, the three-phase DC transformer may also include a protection circuit; The protection circuit includes a current limiting unit and a voltage clamping unit; The first terminal of the current limiting unit is connected to the first terminal of the DC input port, and the second terminal of the current limiting unit is connected to the first terminal of the first full-bridge circuit; the first terminal of the voltage clamping unit is connected to the first terminal of the current limiting unit, the second terminal of the voltage clamping unit is connected to the second terminal of the current limiting unit, and the third terminal of the voltage clamping unit is connected to the second terminal of the DC input port and the second terminal of the first full-bridge circuit. The current limiting unit is used to limit the rate of change of current when the switching unit of the first full-bridge circuit is turned on, and the voltage clamping unit is used to limit the voltage across the first full-bridge circuit.

[0015] This disclosure provides a three-phase DC transformer, comprising: a first full-bridge circuit, an isolation transformer module, and a second full-bridge circuit; the first full-bridge circuit is connected to a DC input port and is used to convert a first DC signal input to the DC input port into a first three-phase AC signal; the primary side of the isolation transformer module is connected to the output terminal of the first full-bridge circuit; the isolation transformer module is used to convert the first three-phase AC signal into a second three-phase AC signal; wherein the voltages of the first three-phase AC signal and the second three-phase AC signal are different; the input terminal of the second full-bridge circuit is connected to the secondary side of the isolation transformer module, and the output terminal of the second full-bridge circuit is connected to a DC output port; the second full-bridge circuit is used to convert the second three-phase AC signal into a second DC signal; wherein the voltages of the first DC signal and the second DC signal are different; the second full-bridge circuit includes multiple switching modules and multiple buffer capacitors; the switching modules are connected in parallel with corresponding buffer capacitors; wherein the capacitance of the buffer capacitor is much larger than the capacitance of the parasitic capacitance of the switching module. Since the capacitance of the buffer capacitor is much larger than that of the parasitic capacitance of the switching module, the voltage change across the buffer capacitor during the switching module's turn-on or turn-off process is much greater than the voltage change caused by the switching module's own parasitic capacitance. In other words, the dynamic voltage across the buffer capacitor is much greater than the dynamic voltage of the switching module. Therefore, the dynamic voltage across the switching module during the turn-on or turn-off process is determined by the buffer capacitor. Furthermore, since the dynamic voltage across each buffer capacitor is the same, the dynamic voltage across each switching module is also the same. Thus, this disclosure can achieve dynamic voltage balance among the various switching modules, thereby improving the reliability of the three-phase DC transformer. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments of this application 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.

[0017] Figure 1 This is a schematic diagram of the structure of a three-phase DC transformer provided in an embodiment of this disclosure.

[0018] Figure 2 This is a schematic diagram of another three-phase DC transformer provided in the embodiments of this disclosure.

[0019] Figure 3 This is a schematic diagram of another three-phase DC transformer provided in the embodiments of this disclosure.

[0020] Figure 4 This is a schematic diagram illustrating the current changes flowing through each IGCT in a three-phase DC transformer, as provided in an embodiment of this disclosure. Detailed Implementation

[0021] The features and exemplary embodiments of various aspects of this application will now be described in detail. Numerous specific details are set forth in the following detailed description in order to provide a comprehensive understanding of this application. However, it will be apparent to those skilled in the art that this application can be implemented without some of these specific details. The following description of embodiments is merely intended to provide a better understanding of this application by illustrating examples thereof.

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The embodiments will now be described in detail with reference to the accompanying drawings.

[0023] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes said element.

[0024] It should be understood that when describing the structure of a component, when referring to a layer or region as being "above" or "on top of" another layer or region, it can mean that it is directly above the other layer or region, or that it contains other layers or regions between it and the other layer or region. Furthermore, if the component is flipped over, that layer or region will be located "below" or "under" the other layer or region.

[0025] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0026] In the embodiments of this application, the term "electrical connection" can refer to a direct electrical connection between two components, or it can refer to an electrical connection between two components via one or more other components.

[0027] In the embodiments of this application, the first node, the second node, and the third node are defined only for the convenience of describing the circuit structure, and the first node, the second node, and the third node are not actual circuit units.

[0028] Various modifications and variations can be made to this application without departing from its spirit or scope, which will be apparent to those skilled in the art. Therefore, this application is intended to cover modifications and variations falling within the scope of the corresponding claims (the claimed technical solutions) and their equivalents. It should be noted that the implementation methods provided in the embodiments of this application can be combined with each other without contradiction.

[0029] With the global energy crisis worsening and photovoltaic conversion efficiency continuously improving, the installed capacity of photovoltaic power plants is growing rapidly. Against this backdrop, large-scale photovoltaic DC collection systems, due to their simple structure, lack of reactive power compensation, and ability to transmit power over long distances, have become an important technological direction for future photovoltaic collection. The key to realizing this system lies in developing DC transformers with low cost, high efficiency, high power output, and high step-up capability to complete voltage transformation from low to medium and even high voltage.

[0030] In related technologies, isolated DC-DC converters can effectively separate and decouple the large current on the low-voltage side from the high voltage on the high-voltage side through electrical isolation. Therefore, isolated DC-DC converters are the optimal solution for achieving a high step-up ratio. However, isolated DC transformers have difficulty ensuring the dynamic voltage balance of the secondary-side switching devices, thus affecting the reliability of the transformer system.

[0031] Therefore, this disclosure provides a three-phase DC transformer. Since the capacitance of the buffer capacitor is much larger than the capacitance of the parasitic capacitance of the switching module, the voltage change across the buffer capacitor during the switching module's on or off process is much greater than the voltage change caused by the switching module's own parasitic capacitance. That is, the dynamic voltage across the buffer capacitor is much greater than the dynamic voltage of the switching module. Therefore, the dynamic voltage across the switching module during the switching process is determined by the buffer capacitor. Since the dynamic voltage across each buffer capacitor is the same, the dynamic voltage across each switching module is also the same. Thus, this disclosure can achieve dynamic voltage balance among the switching modules, thereby improving the reliability of the three-phase DC transformer.

[0032] The embodiments will now be described in detail with reference to the accompanying drawings.

[0033] Figure 1 This is a schematic diagram of the structure of a three-phase DC transformer provided in an embodiment of the present disclosure, as shown below. Figure 1 As shown, the three-phase DC transformer includes: a first full-bridge circuit 10, an isolation transformer module 20, and a second full-bridge circuit 30.

[0034] The first full-bridge circuit 10 is connected to the DC input port 41. The first full-bridge circuit 10 is used to convert the first DC signal input to the DC input port 41 into a first three-phase AC signal.

[0035] For example, the first full-bridge circuit 10 serves as an inverter unit. Based on control signals provided by an external control module, it controls the conduction state of each switching device within the first full-bridge circuit 10, converting the first DC signal provided by the DC input port 41 into a first three-phase AC signal. The switching devices in the three arms of the first full-bridge circuit 10 each receive control signals with a phase difference of 120°, achieving phase shift control with a 120° phase difference. This results in the first full-bridge circuit 10 having higher power density and core utilization, and also improves the power rating of the three-phase DC transformer.

[0036] The primary side of the isolation transformer module 20 is connected to the output terminal of the first full-bridge circuit 10; the isolation transformer module 20 is used to convert the first three-phase AC signal into a second three-phase AC signal; wherein the voltages of the first three-phase AC signal and the second three-phase AC signal are different.

[0037] For example, the primary side of the isolation transformer module 20 receives a first three-phase AC signal provided by the first full-bridge circuit 10, and converts the first three-phase AC signal into a second three-phase AC signal with a different voltage, which is then output from the secondary side of the isolation transformer module 20. Thus, this disclosure enables AC voltage conversion through the isolation transformer module 20.

[0038] The input terminal of the second full-bridge circuit 30 is connected to the secondary side of the isolation transformer module 20, and the output terminal of the second full-bridge circuit 30 is connected to the DC output port 42. The second full-bridge circuit 30 is used to convert the second three-phase AC signal into a second DC signal; wherein the voltage of the first DC signal and the second DC signal are different.

[0039] For example, the second full-bridge circuit 30 serves as a rectifier unit, converting the second three-phase AC signal provided by the secondary side of the isolation transformer module 20 into a second DC signal, and outputting the second DC signal to the outside through the DC output port 42. Thus, this disclosure can realize the conversion from the first DC signal to the second DC signal through the first full-bridge circuit 10, the isolation transformer module 20, and the second full-bridge circuit 30, thereby realizing DC voltage conversion.

[0040] The second full-bridge circuit 30 includes multiple switching modules and multiple buffer capacitors; the switching modules are connected in parallel with their corresponding buffer capacitors; wherein the capacitance of the buffer capacitor is much larger than the capacitance of the parasitic capacitance of the switching module.

[0041] For example, using a buffer capacitor can influence the soft-switching state of the switching module to reduce its switching losses. This reduction in switching losses also enhances the heat dissipation capacity of the entire DC transformer system. Furthermore, since the capacitance of the buffer capacitor is much larger than the parasitic capacitance of the switching module, the voltage change across the buffer capacitor during the switching process is much greater than the voltage change caused by the switching module's own parasitic capacitance. In other words, the dynamic voltage across the buffer capacitor is much greater than the dynamic voltage of the switching module. Therefore, the dynamic voltage across the switching module during the switching process is determined by the buffer capacitor. And because the dynamic voltage across each buffer capacitor is the same, the dynamic voltage across each switching module is also the same. Thus, this disclosure enables dynamic voltage balance among the switching modules, thereby improving the reliability of the three-phase DC transformer. Furthermore, this disclosure achieves dynamic voltage balance between various switching modules using only a buffer capacitor. Compared to conventional technologies that use series capacitors and resistors to achieve dynamic voltage balance between various switching modules, this disclosure, because it only has a buffer capacitor, does not suffer additional losses due to the resistors in the circuit during the switching module's on or off process. Therefore, this disclosure can achieve low-loss dynamic voltage balance between various switching modules.

[0042] In some embodiments, Figure 2 A schematic diagram of another three-phase DC transformer provided in this disclosure embodiment is shown below. Figure 2As shown, the multiple switch modules include at least one first switch unit 311, at least one second switch unit 312, at least one third switch unit 313, at least one fourth switch unit 314, at least one fifth switch unit 315, and at least one sixth switch unit 316.

[0043] The second full-bridge circuit 30 includes a first bridge arm 301, a second bridge arm 302, a third bridge arm 303, a fourth bridge arm 304, a fifth bridge arm 305, a sixth bridge arm 306, and a supporting capacitor C1.

[0044] The first bridge arm 301 includes at least one first switch unit 311, the second bridge arm 302 includes at least one second switch unit 312, the third bridge arm 303 includes at least one third switch unit 313, the fourth bridge arm 304 includes at least one fourth switch unit 314, the fifth bridge arm 305 includes at least one fifth switch unit 315, and the sixth bridge arm 306 includes at least one sixth switch unit 316.

[0045] The first end of the first bridge arm 301 is connected to the first end of the DC output port 42; the second end of the first bridge arm 301 is connected to the first end of the second bridge arm 302 and is connected to the first output terminal of the isolation transformer module 20; the second end of the second bridge arm 302 is connected to the second end of the DC output port 42; the first end of the third bridge arm 303 is connected to the first end of the DC output port 42; the second end of the third bridge arm 303 is connected to the first end of the fourth bridge arm 304 and is connected to the second output terminal of the isolation transformer module 20; the second end of the fourth bridge arm 304 is connected to the second end of the DC output port 42; the first end of the fifth bridge arm 305 is connected to the first end of the DC output port 42; the second end of the fifth bridge arm 305 is connected to the first end of the sixth bridge arm 306 and is connected to the third output terminal of the isolation transformer module 20; the second end of the sixth bridge arm 306 is connected to the second end of the DC output port 42; the first end of the supporting capacitor C1 is connected to the first end of the DC output port 42; the second end of the supporting capacitor C1 is connected to the second end of the DC output port 42.

[0046] The first switch unit 311, the second switch unit 312, the third switch unit 313, the fourth switch unit 314, the fifth switch unit 315, and the sixth switch unit 316 are connected in parallel with their respective buffer capacitors C2.

[0047] Among them, the capacitance of the buffer capacitor C2 is much larger than the capacitance of the parasitic capacitors of the first switch unit 311, the second switch unit 312, the third switch unit 313, the fourth switch unit 314, the fifth switch unit 315 and the sixth switch unit 316.

[0048] For example, by controlling the first switch unit 311 on the first bridge arm 301, the fourth switch unit 314 on the fourth bridge arm 304, and the fifth switch unit 315 on the fifth bridge arm 305 to be turned on, the second switch unit 312 on the second bridge arm 302, the third switch unit 313 on the third bridge arm 303, and the sixth switch unit 316 on the sixth bridge arm 306 are turned off; and by controlling the first switch unit 311 on the first bridge arm 301, the fourth switch unit 314 on the fourth bridge arm 304, and the fifth switch unit 315 on the fifth bridge arm 305 to be turned off, the second switch unit 312 on the second bridge arm 302, the third switch unit 313 on the third bridge arm 303, and the sixth switch unit 316 on the sixth bridge arm 306 are turned on, thereby enabling the second full-bridge circuit 30 to act as a rectifier unit, converting the second three-phase AC signal provided by the first output terminal and the second output terminal of the isolation transformer module 20 into a second DC signal, and outputting the second DC signal to the outside through the DC output port 42. Each of the first switching unit 311, the second switching unit 312, the third switching unit 313, the fourth switching unit 314, the fifth switching unit 315, and the sixth switching unit 316 is connected in parallel with a corresponding buffer capacitor C2. Since the capacitance of the buffer capacitor C2 is much larger than the parasitic capacitance of the corresponding first switching unit 311, second switching unit 312, third switching unit 313, fourth switching unit 314, fifth switching unit 315, and sixth switching unit 316, the voltage change across the buffer capacitor C2 during the on / off process of these switching units is much greater than the voltage change of the corresponding switching module due to its own parasitic capacitance. In other words, the dynamic voltage across the buffer capacitor C2 is much greater than the dynamic voltage of the corresponding switching module. Therefore, the first switching unit... During the conduction or cutoff of the first switch unit 311, the second switch unit 312, the third switch unit 313, the fourth switch unit 314, the fifth switch unit 315, and the sixth switch unit 316, the dynamic voltage across each switch unit is determined by its respective connected buffer capacitor C2. Since the capacitance values ​​of each buffer capacitor C2 are the same, the dynamic voltage across each buffer capacitor C2 is the same, which in turn makes the dynamic voltage across each switch module the same. Thus, this disclosure can achieve dynamic voltage balance among the first switch unit 311, the second switch unit 312, the third switch unit 313, the fourth switch unit 314, the fifth switch unit 315, and the sixth switch unit 316, thereby improving the reliability of the three-phase DC transformer.Furthermore, this disclosure achieves dynamic voltage balance between various switching modules using only a buffer capacitor C2. Compared to conventional technologies that use series capacitors and resistors to achieve dynamic voltage balance between various switching modules, this disclosure, because it only has a buffer capacitor C2, does not suffer additional losses due to the resistors in the circuit during the switching module's on or off process. Therefore, this disclosure can achieve low-loss dynamic voltage balance between various switching modules.

[0049] It should be noted that, Figure 2 The diagram only illustrates, by way of example, at least one first switch unit 311 including a first switch unit 311, at least one second switch unit 312 including a second switch unit 312, at least one third switch unit 313 including a third switch unit 313, at least one fourth switch unit 314 including a fourth switch unit 314, at least one fifth switch unit 315 including a fifth switch unit 315, and at least one sixth switch unit 316 including a sixth switch unit 316.

[0050] In some embodiments, Figure 3 A schematic diagram of another three-phase DC transformer provided in this disclosure embodiment is shown below. Figure 3 As shown, the first bridge arm 301 includes multiple first switch units 311 connected in series, the second bridge arm 302 includes multiple second switch units 312 connected in series, the third bridge arm 303 includes multiple third switch units 313 connected in series, the fourth bridge arm 304 includes multiple fourth switch units 314 connected in series, the fifth bridge arm 305 includes multiple fifth switch units 315 connected in series, and the sixth bridge arm 306 includes a sixth switch unit 316 connected in series.

[0051] For example, the number of multiple first switching units 311 and multiple second switching units 312, the number of multiple third switching units 313 and multiple fourth switching units 314, and the number of multiple fifth switching units 315 and multiple sixth switching units 316 are determined by the system voltage of the three-phase DC transformer and the rated operating voltage of each switching unit. By connecting multiple first switching units 311, multiple second switching units 312, multiple third switching units 313, multiple fourth switching units 314, multiple fifth switching units 315, and multiple sixth switching units 316 in series, the voltage is divided among the switching units, thereby improving the high voltage withstand capability of the second full-bridge circuit.

[0052] In some embodiments, see continue to see Figure 2 and Figure 3The first switching unit 311 includes a first diode D1, the second switching unit 312 includes a second diode D2, the third switching unit 313 includes a third diode D3, the fourth switching unit 314 includes a fourth diode D4, the fifth switching unit 315 includes a fifth diode D5, and the sixth switching unit 316 includes a sixth diode D6.

[0053] For example, since each switching unit uses diodes as switching devices, the conduction and turn-off of the diodes are determined by external circuits. The diodes are switched on and off by adjusting the voltage across them, rather than by applying a control signal. Therefore, diodes are uncontrolled devices that cannot be actively controlled to turn on and off. As uncontrolled devices, when a diode turns on or off, the voltage in the circuit usually needs to meet its conduction condition. Since the voltage in the circuit changes slowly rather than increasing instantaneously, the buffer capacitor C2, connected in parallel with the first diode D1, second diode D2, third diode D3, fourth diode D4, fifth diode D5, and sixth diode D6, also experiences a slow change in voltage across it when these diodes turn on or off. Therefore, using diodes as switching devices avoids inrush current in the branch containing buffer capacitor C2, thus protecting the entire DC transformer system.

[0054] In some embodiments, see continue to see Figure 2 and Figure 3 The second full-bridge circuit also includes multiple voltage-equalizing resistors R1; the voltage-equalizing resistors R1 are connected in parallel with the corresponding switching modules.

[0055] For example, the switching module includes a first switching unit 311, a second switching unit 312, a third switching unit 313, a fourth switching unit 314, a fifth switching unit 315, and a sixth switching unit 316. Each of the first switching unit 311, second switching unit 312, third switching unit 313, fourth switching unit 314, fifth switching unit 315, and sixth switching unit 316 is connected in parallel with a voltage equalizing resistor R1. Taking the example that the first switching unit 311, second switching unit 312, third switching unit 313, fourth switching unit 314, fifth switching unit 315, and sixth switching unit 316 are all diodes, when each of the first switching unit 311, second switching unit 312, third switching unit 313, fourth switching unit 314, fifth switching unit 315, and sixth switching unit 316 is in an open state, each switching unit is in a reverse blocking state, and each switching unit should share the same reverse voltage. However, due to manufacturing errors between the various switching units, their performance parameters may differ, resulting in some switching units having larger leakage currents and others smaller leakage currents. Therefore, a voltage-equalizing resistor R1 needs to be connected in parallel with each switching unit. The resistance of the voltage-equalizing resistor R1 is much smaller than the reverse resistance of each switching unit, and all voltage-equalizing resistors R1 have the same resistance value. Thus, a stable current shunt path is established for each switching unit through the voltage-equalizing resistor R1, ensuring that the current mainly flows through each voltage-equalizing resistor R1. Furthermore, because the resistance of the voltage-equalizing resistor R1 is the same, the current flowing through each voltage-equalizing resistor R1 is essentially the same, thereby ensuring that the voltage across each switching unit is the same in the off state, i.e., the static voltage across each switching unit is the same. Therefore, this disclosure, by setting the voltage-equalizing resistor R1 in parallel with the corresponding switching module, enables the second full-bridge circuit to achieve static voltage balance when all switching modules are off.

[0056] In some embodiments, see continue to see Figure 2 and Figure 3 The isolation transformer module 20 includes a first inductor L1, a second inductor L2, a third inductor L3, a first capacitor C3, a second capacitor C4, a third capacitor C5, a first isolation transformer U1, a second isolation transformer U2, and a third isolation transformer U3.

[0057] The first terminal of the first inductor L1 is connected to the first output terminal of the first full-bridge circuit 10. The second terminal of the first inductor L1 is connected to the first terminal of the first isolation transformer U1 through the first capacitor C3. The first terminal of the second inductor L2 is connected to the second output terminal of the first full-bridge circuit 10. The second terminal of the second inductor L2 is connected to the first terminal of the second isolation transformer U2 through the second capacitor C4. The first terminal of the third inductor L3 is connected to the third output terminal of the first full-bridge circuit 10. The second terminal of the third inductor L3 is connected to the first terminal of the third isolation transformer U3 through the third capacitor C5. The second terminal of the first isolation transformer U1 is connected to the second terminal of the second isolation transformer U2 and the second terminal of the third isolation transformer U3. The third terminal of the first isolation transformer U1 is connected to the first output terminal of the isolation transformer module, the third terminal of the second isolation transformer U2 is connected to the second output terminal of the isolation transformer module, the third terminal of the third isolation transformer U3 is connected to the third output terminal of the isolation transformer module, and the fourth terminal of the first isolation transformer U1 is connected to the fourth terminals of the second isolation transformer U2 and the fourth terminal of the third isolation transformer U3.

[0058] For example, the primary side of the first isolation transformer U1 is connected to the first full-bridge circuit 10 through the first inductor L1 and the first capacitor C3. After the primary side of the first isolation transformer U1 receives the first three-phase AC signal provided by the first full-bridge circuit 10, the first isolation transformer U1 converts the first three-phase AC signal into a second three-phase AC signal by relying on its internal coils and magnetic core. The ratio of the number of turns of the primary coil to the number of turns of the secondary coil of the first isolation transformer U1, as well as the capacity of the first isolation transformer U1, are adjustable.

[0059] The primary side of the second isolation transformer U2 is connected to the first full-bridge circuit 10 through the second inductor L2 and the second capacitor C4. After the primary side of the second isolation transformer U2 receives the first three-phase AC signal provided by the first full-bridge circuit 10, the second isolation transformer U2 converts the first three-phase AC signal into a second three-phase AC signal by means of its internal coils and magnetic core. The ratio of the number of turns of the primary coil to the number of turns of the secondary coil of the second isolation transformer U2, as well as the capacity of the second isolation transformer U2, are adjustable.

[0060] The primary side of the third isolation transformer U3 is connected to the first full-bridge circuit 10 through the third inductor L3 and the third capacitor C5. After the primary side of the third isolation transformer U3 receives the first three-phase AC signal provided by the first full-bridge circuit 10, the third isolation transformer U3 converts the first three-phase AC signal into a second three-phase AC signal by means of its internal coils and magnetic core. The ratio of the number of turns of the primary coil to the number of turns of the secondary coil of the third isolation transformer U3, as well as the capacity of the third isolation transformer U3, are adjustable.

[0061] The ratio of the number of turns in the primary coil to the number of turns in the secondary coil and the capacity can be different among the first isolation transformer U1, the second isolation transformer U2 and the third isolation transformer U3, thereby achieving electrical isolation and output of different voltage levels and power.

[0062] In some embodiments, the operating frequencies of the first isolation transformer U1, the second isolation transformer U2, and the third isolation transformer U3 are all 1000Hz.

[0063] For example, since the operating frequency of each switching device in the first full-bridge circuit 10 in the DC transformer is equal to the operating frequency of the corresponding isolation transformer, when the operating frequencies of the first isolation transformer U1, the second isolation transformer U2 and the third isolation transformer U3 are all 1000Hz, the operating frequency of each switching device in the first full-bridge circuit 10 is also 1000Hz. Therefore, it can meet the operating requirements of the kilovolt megawatt level isolation transformer, thereby providing space for the application of new high-capacity switching devices.

[0064] When the first isolation transformer U1, the second isolation transformer U2, and the third isolation transformer U3 operate at a frequency of 1000Hz, using amorphous materials as the magnetic cores of these transformers significantly reduces the size and manufacturing difficulty of the high-voltage, high-capacity isolation transformers. Furthermore, by employing an inductor integration method that adds an additional magnetic core, which is coupled only to one side of the coil and not to the other, precise integration of millihenry-level inductance can be achieved.

[0065] In some embodiments, the first inductor L1 is the leakage inductance of the first isolation transformer U1, the second inductor L2 is the leakage inductance of the second isolation transformer U2, and the third inductor L3 is the leakage inductance of the third isolation transformer U3.

[0066] For example, the first inductor L1 can be the leakage inductance of the first isolation transformer U1 itself, the second inductor L2 can be the leakage inductance of the second isolation transformer U2 itself, and the third inductor L3 can be the leakage inductance of the third isolation transformer U3 itself. Therefore, the first isolation transformer U1, the second isolation transformer U2, and the third isolation transformer U3 in this disclosure do not need to be equipped with additional inductors, thereby reducing the cost and size of the first isolation transformer U1, the second isolation transformer U2, and the third isolation transformer U3.

[0067] In some embodiments, see continue to see Figure 2 and Figure 3The first full-bridge circuit 10 includes a seventh switch unit 101, an eighth switch unit 102, a ninth switch unit 103, a tenth switch unit 104, an eleventh switch unit 105, a twelfth switch unit 106, and an input capacitor C6.

[0068] The first end of the seventh switch unit 101 is connected to the first end of the DC input port 41, the second end of the seventh switch unit 101 is connected to the first end of the eighth switch unit 102 at the first node A, and the second end of the eighth switch unit 102 is connected to the second end of the DC input port 41.

[0069] The first end of the ninth switch unit 103 is connected to the first end of the DC input port 41, the second end of the ninth switch unit 103 is connected to the first end of the tenth switch unit 104 at the second node B, and the second end of the tenth switch unit 104 is connected to the second end of the DC input port 41.

[0070] The first end of the eleventh switch unit 105 is connected to the first end of the DC input port 41, the second end of the eleventh switch unit 105 is connected to the first end of the twelfth switch unit 106 at the third node C, and the second end of the twelfth switch unit 106 is connected to the second end of the DC input port 41.

[0071] The first node A is connected to the first end of the first inductor L1, the second node B is connected to the first end of the second inductor L2, and the third node C is connected to the first end of the third inductor L3.

[0072] The first end of the input capacitor C6 is connected to the first end of the DC input port 41, and the second end of the input capacitor C6 is connected to the second end of the DC input port 41.

[0073] For example, the first full-bridge circuit 10, acting as an inverter unit, simultaneously turns on or off the seventh switch unit 101, the tenth switch unit 104, and the eleventh switch unit 105, and simultaneously turns on or off the eighth switch unit 102, the ninth switch unit 103, and the twelfth switch unit 106, based on the pulse width modulation signal provided by the external control module. The seventh switch unit 101, the tenth switch unit 104, and the eleventh switch unit 105 are alternately turned on and off with the eighth switch unit 102, the ninth switch unit 103, and the twelfth switch unit 106, thereby converting the first DC signal provided by the DC input port 41 into a first three-phase AC signal. The input capacitor C6 is used to filter out interference signals and absorb high-frequency noise in the first DC signal input to the DC input port 41.

[0074] In some embodiments, see continue to see Figure 2 and Figure 3Each switching unit in the first full-bridge circuit 10 includes an integrated gate-commutated thyristor (IGCT) device.

[0075] The seventh switching unit 101 includes a first IGCT device Q1 and a seventh diode D7; the eighth switching unit 102 includes a second IGCT device Q2 and an eighth diode D8; the ninth switching unit 103 includes a third IGCT device Q3 and a ninth diode D9; the tenth switching unit 104 includes a fourth IGCT device Q4 and a tenth diode D10; the eleventh switching unit 105 includes a fifth IGCT device Q5 and an eleventh diode D11; and the twelfth switching unit 106 includes a sixth IGCT device Q6 and a twelfth diode D12.

[0076] The positive terminal of the first IGCT device Q1 is connected to the first end of the DC input port 41, the negative terminal of the first IGCT device Q1 is connected to the positive terminal of the second IGCT device Q2 at the first node A, and the negative terminal of the second IGCT device Q2 is connected to the second end of the DC input port 41.

[0077] The positive terminal of the third IGCT device Q3 is connected to the first end of the DC input port 41, the negative terminal of the third IGCT device Q3 is connected to the positive terminal of the fourth IGCT device Q4 at the second node B, and the negative terminal of the fourth IGCT device Q4 is connected to the second end of the DC input port 41.

[0078] The positive terminal of the fifth IGCT device Q5 is connected to the first end of the DC input port 41, the negative terminal of the fifth IGCT device Q5 is connected to the positive terminal of the sixth IGCT device Q6 at the third node C, and the negative terminal of the sixth IGCT device Q6 is connected to the second end of the DC input port 41.

[0079] The positive terminal of the first IGCT device Q1 is connected to the negative terminal of the seventh diode D7, and the negative terminal of the first IGCT device Q1 is connected to the positive terminal of the seventh diode D7; the positive terminal of the second IGCT device Q2 is connected to the negative terminal of the eighth diode D8, and the negative terminal of the second IGCT device Q2 is connected to the positive terminal of the eighth diode D8; the positive terminal of the third IGCT device Q3 is connected to the negative terminal of the ninth diode D9, and the negative terminal of the third IGCT device Q3 is connected to the positive terminal of the ninth diode D9; the positive terminal of the fourth IGCT device Q4 is connected to the negative terminal of the tenth diode D10, and the negative terminal of the fourth IGCT device Q4 is connected to the positive terminal of the tenth diode D10; the positive terminal of the fifth IGCT device Q5 is connected to the negative terminal of the eleventh diode D11, and the negative terminal of the fifth IGCT device Q5 is connected to the positive terminal of the eleventh diode D11; the positive terminal of the sixth IGCT device Q6 is connected to the negative terminal of the twelfth diode D12, and the negative terminal of the sixth IGCT device Q6 is connected to the positive terminal of the twelfth diode D12.

[0080] For example, Figure 4 This is a timing diagram illustrating the current changes through each IGCT in a three-phase DC transformer, as provided in an embodiment of this disclosure. Figure 4As shown, taking the first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5 as examples, at time t0, the first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5 are triggered to conduct according to the pulse width modulation signal provided by the external control module, and current flows through the first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5. The time t1-t2 is the main resonance process of the DC transformer. After the main resonance process ends, the first inductor L1, the second inductor L2, and the third inductor L3 in the isolation transformer module 20 resonate with the buffer capacitor C2 in the second full-bridge circuit 30, thereby causing the first diode D1, the fourth diode D4, and the fifth diode D5 corresponding to the conducting first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5 to conduct. At time t3, when the first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5 are triggered to turn off according to the pulse width modulation signal provided by the external control module, the current in the first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5 is 0. Therefore, the first IGCT device Q1, the fourth IGCT device Q4, and the fifth IGCT device Q5 are turned off with zero current. At time t4, the current in the first diode D1, the fourth diode D4, and the fifth diode D5 decays to 0, resulting in natural turn-off. At this time, the parasitic capacitance in the first full-bridge circuit 10 resonates with the first inductor L1, the second inductor L2, and the third inductor L3. Since the parasitic capacitance in the first full-bridge circuit 10 is small, the current in the first full-bridge circuit 10 is approximately 0, so that at time t5, the second IGCT device Q2, the third IGCT device Q3, and the sixth IGCT device Q6 are turned on with zero current. Therefore, the three-phase DC transformer provided in this disclosure can achieve zero-current turn-off and zero-current turn-on under full load, thereby effectively reducing the switching losses of each device and reducing the device temperature rise.

[0081] Furthermore, since all switching devices in the first full-bridge circuit 10 are IGCT devices, and since IGCT devices can achieve a power output of 3.3MW with a single switching device, there is no need to set multiple switching devices to achieve high power output, which significantly reduces the number of switching devices and lowers the circuit complexity and cost of the first full-bridge circuit 10. In addition, each of the first IGCT device Q1, the second IGCT device Q2, the third IGCT device Q3, the fourth IGCT device Q4, the fifth IGCT device Q5, and the sixth IGCT device Q6 is connected in reverse parallel with a diode. The diode can prevent damage to the IGCT device from the induced voltage spikes generated when the IGCT device is turned off, and can also provide freewheeling current to the circuit when the IGCT device is turned off.

[0082] It should be noted that, Figure 4The diagram only illustrates the proportional relationships between time nodes t0, t1, t2, t3, t4, and t5. The specific proportional relationships should be determined based on the actual situation and are not specifically limited here.

[0083] In some embodiments, see continue to see Figure 2 and Figure 3 The three-phase DC transformer also includes a protection circuit 50.

[0084] The protection circuit 50 includes a current limiting unit and a voltage clamping unit.

[0085] The first end of the current limiting unit is connected to the first end of the DC input port 41, and the second end of the current limiting unit is connected to the first end of the first full-bridge circuit 10; the first end of the voltage clamping unit is connected to the first end of the current limiting unit, the second end of the voltage clamping unit is connected to the second end of the current limiting unit, and the third end of the voltage clamping unit is connected to the second end of the DC input port 41 and the second end of the first full-bridge circuit 10. The current limiting unit is used to limit the rate of change of current when the switching unit of the first full-bridge circuit 10 is turned on, and the voltage clamping unit is used to limit the voltage across the first full-bridge circuit 10.

[0086] For example, the current limiting unit includes a fourth inductor L4. The voltage clamping unit includes a clamping diode D13, a clamping resistor R2, and a clamping capacitor C7.

[0087] The first terminal of the fourth inductor L4 is connected to the first terminal of the DC input port 41, and the second terminal of the fourth inductor L4 is connected to the first terminal of the first full-bridge circuit 10. The first terminal of the clamping resistor R2 is connected to the first terminal of the fourth inductor L4, and the second terminal of the clamping resistor R2 is connected to the negative terminal of the clamping diode D13. The positive terminal of the clamping diode D13 is connected to the second terminal of the fourth inductor L4. The first terminal of the clamping capacitor C7 is connected to the negative terminal of the clamping diode D13, and the second terminal of the clamping capacitor C7 is connected to the second terminal of the DC input port 41 and the second terminal of the first full-bridge circuit 10. The fourth inductor L4 is used to limit the rate of change of current when the switching unit of the first full-bridge circuit 10 is conducting, to prevent the first full-bridge circuit 10 from being damaged by excessively rapid current changes. The clamping diode D13, the clamping resistor R2, and the clamping capacitor C7 are used to limit the voltage across the first full-bridge circuit 10, to prevent the first full-bridge circuit 10 from being damaged by excessively high voltage.

[0088] The above are merely specific embodiments of this disclosure, enabling those skilled in the art to understand or implement this disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not to be limited to these embodiments, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A three-phase DC transformer, characterized by The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter.

2. The three-phase DC transformer according to claim 1, characterized in that The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. The application relates to a DC-DC converter. 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The application relates to a DC-DC The buffer capacitor has a capacitance value far greater than that of the parasitic capacitance of the first switch unit, the second switch unit, the third switch unit, the fourth switch unit, the fifth switch unit and the sixth switch unit. 3.The three-phase DC transformer of claim 2, wherein, The first bridge arm comprises a plurality of the first switch units connected in series, the second bridge arm comprises a plurality of the second switch units connected in series, the third bridge arm comprises a plurality of the third switch units connected in series, the fourth bridge arm comprises a plurality of the fourth switch units connected in series, the fifth bridge arm comprises a plurality of the fifth switch units connected in series, and the sixth bridge arm comprises the sixth switch unit connected in series.

4. The three-phase DC transformer according to any of claims 2-3, characterized in that, The first switch unit comprises a first diode, the second switch unit comprises a second diode, the third switch unit comprises a third diode, the fourth switch unit comprises a fourth diode, the fifth switch unit comprises a fifth diode, and the sixth switch unit comprises a sixth diode.

5. The three-phase DC transformer according to any of claims 1-3, characterized in that The second full-bridge circuit further comprises a plurality of voltage-sharing resistors, and the voltage-sharing resistors are connected in parallel with the corresponding switch modules.

6. The three-phase DC transformer of claim 2, wherein, The isolation transformer module comprises a first inductor, a second inductor, a third inductor, a first capacitor, a second capacitor, a third capacitor, a first isolation transformer, a second isolation transformer and a third isolation transformer. The first end of the first inductor is connected to the first output end of the first full-bridge circuit, the second end of the first inductor is connected to the first end of the first isolation transformer through the first capacitor, the first end of the second inductor is connected to the second output end of the first full-bridge circuit, the second end of the second inductor is connected to the first end of the second isolation transformer through the second capacitor, the first end of the third inductor is connected to the third output end of the first full-bridge circuit, and the second end of the third inductor is connected to the first end of the third isolation transformer through the third capacitor; the second end of the first isolation transformer is connected to the second end of the second isolation transformer and the second end of the third isolation transformer. The third end of the first isolation transformer is connected to the first output end of the isolation transformer module, the third end of the second isolation transformer is connected to the second output end of the isolation transformer module, the third end of the third isolation transformer is connected to the third output end of the isolation transformer module, and the fourth end of the first isolation transformer is connected to the fourth end of the second isolation transformer and the fourth end of the third isolation transformer. 7.The three-phase DC transformer of claim 6, wherein, The first inductor is a leakage inductance of the first isolation transformer. The second inductor is a leakage inductance of the second isolation transformer. The third inductor is a leakage inductance of the third isolation transformer.

8. The three-phase DC transformer of claim 6, wherein, The first full-bridge circuit comprises a seventh switch unit, an eighth switch unit, a ninth switch unit, a tenth switch unit, an eleventh switch unit, a twelfth switch unit and an input capacitor. The first end of the seventh switch unit is connected with the first end of the direct current input port, the second end of the seventh switch unit is connected with the first end of the eighth switch unit at a first node, and the second end of the eighth switch unit is connected with the second end of the direct current input port; The first end of the ninth switch unit is connected with the first end of the direct current input port, the second end of the ninth switch unit is connected with the first end of the tenth switch unit at a second node, and the second end of the tenth switch unit is connected with the second end of the direct current input port; The first end of the eleventh switch unit is connected with the first end of the direct current input port, the second end of the eleventh switch unit is connected with the first end of the twelfth switch unit at a third node, and the second end of the twelfth switch unit is connected with the second end of the direct current input port; The first node is connected with the first end of the first inductor, the second node is connected with the first end of the second inductor, and the third node is connected with the first end of the third inductor; The first end of the input capacitor is connected with the first end of the direct current input port, and the second end of the input capacitor is connected with the second end of the direct current input port.

9. The three-phase DC transformer according to claim 8, characterized in that The seventh switch unit comprises a first IGCT device and a seventh diode, the eighth switch unit comprises a second IGCT device and an eighth diode, the ninth switch unit comprises a third IGCT device and a ninth diode, the tenth switch unit comprises a fourth IGCT device and a twelfth diode, the eleventh switch unit comprises a fifth IGCT device and an eleventh diode, and the twelfth switch unit comprises a sixth IGCT device and a tenth diode; The positive end of the first IGCT device is connected with the first end of the direct current input port, the negative end of the first IGCT device is connected with the positive end of the second IGCT device at the first node, and the negative end of the second IGCT device is connected with the second end of the direct current input port; The positive end of the third IGCT device is connected with the first end of the direct current input port, the negative end of the third IGCT device is connected with the positive end of the fourth IGCT device at the second node, and the negative end of the fourth IGCT device is connected with the second end of the direct current input port; The positive end of the fifth IGCT device is connected with the first end of the direct current input port, the negative end of the fifth IGCT device is connected with the positive end of the sixth IGCT device at the third node, and the negative end of the sixth IGCT device is connected with the second end of the direct current input port; The positive terminal of the first IGCT device is connected with the negative terminal of the seventh diode, and the negative terminal of the first IGCT device is connected with the positive terminal of the seventh diode; the positive terminal of the second IGCT device is connected with the negative terminal of the eighth diode, and the negative terminal of the second IGCT device is connected with the positive terminal of the eighth diode; the positive terminal of the third IGCT device is connected with the negative terminal of the ninth diode, and the negative terminal of the third IGCT device is connected with the positive terminal of the ninth diode; the positive terminal of the fourth IGCT device is connected with the negative terminal of the twelfth diode, and the negative terminal of the fourth IGCT device is connected with the positive terminal of the twelfth diode; the positive terminal of the fifth IGCT device is connected with the negative terminal of the eleventh diode, and the negative terminal of the fifth IGCT device is connected with the positive terminal of the eleventh diode; the positive terminal of the sixth IGCT device is connected with the negative terminal of the tenth diode, and the negative terminal of the sixth IGCT device is connected with the positive terminal of the tenth diode.

10. The three-phase DC transformer of claim 1, wherein, The three-phase DC transformer further comprises a protection circuit; The protection circuit comprises a current limiting unit and a voltage clamping unit; The first end of the current limiting unit is connected with the first end of the DC input port, and the second end of the current limiting unit is connected with the first end of the first full-bridge circuit; the first end of the voltage clamping unit is connected with the first end of the current limiting unit, the second end of the voltage clamping unit is connected with the second end of the current limiting unit, and the third end of the voltage clamping unit is connected with the second end of the DC input port and the second end of the first full-bridge circuit. The current limiting unit is used to limit the current change rate under the conduction condition of the switching unit of the first full-bridge circuit, and the voltage clamping unit is used to limit the voltage across the first full-bridge circuit.

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