Direct current transformer, electric energy conversion device and control method thereof

Abstract

The invention belongs to a transformer structure, and provides a direct-current transformer, a power transmission system and a control method of the power transmission system in order to solve the technical problems that a three-level circuit topology in a power module in an existing direct-current transformer is large in number of power devices, complex in structure and large in layout and wiring difficulty. Each power module comprises a first-stage circuit, a second-stage circuit, two first direct-current bus capacitors and two second direct-current bus capacitors, the two first direct-current bus capacitors are connected in series to form a first capacitor circuit, and the two second direct-current bus capacitors are connected in series to form a second capacitor circuit. The first-stage circuit comprises two first half-bridge circuits which are connected in series, and the second-stage circuit is a dual-active bridge converter based on a series half-bridge. The number of power devices is remarkably reduced, meanwhile, the layout and wiring difficulty of the whole circuit is simplified, and popularization and application of the direct-current transformer are easier.

Description

Technical Field

[0001] This application belongs to a transformer structure, and particularly relates to a DC transformer, a power conversion device, and a control method thereof. Background Art

[0002] A DC transformer is a core equipment in a DC distribution network, which is used to realize the access of renewable energy and DC loads, and the connection between DC buses of different voltage levels.

[0003] DC transformers usually adopt a circuit topology structure with multiple power modules connected in series on the high-voltage side and in parallel on the low-voltage side. Inside each power module is an isolated bidirectional DC-DC (Direct Current to Direct Current) converter, mostly a two-level DAB (Dual Active Bridge) or CLLC (a resonant network containing capacitor-inductor-inductor-capacitor) circuit topology. In order to increase the voltage of each power module and thus reduce the number of power modules, a three-level DAB or CLLC circuit topology is gradually applied. However, the three-level circuit topology in the form of NPC (Neutral Point Clamped) or ANPC (Active Neutral Point Clamped) requires a large number of power devices, and the circuit topology structure is complex, making the layout and wiring difficult. Summary of the Invention

[0004] In view of the technical problems of the three-level circuit topology inside the power module in the current DC transformer, such as a large number of power devices, complex structure, and difficult layout and wiring, this application provides a DC transformer, a power conversion device, and a control method thereof.

[0005] To achieve the above object, this application adopts the following technical solutions: In a first aspect, this application provides a DC transformer, which includes a plurality of power modules, and the high-voltage sides of the plurality of power modules are connected in series and the low-voltage sides are connected in parallel; The power module includes a first-stage circuit, a second-stage circuit, two first DC bus capacitors, and two second DC bus capacitors; the two first DC bus capacitors are connected in series to form a first capacitor path; the two second DC bus capacitors are connected in series to form a second capacitor path; The first-stage circuit includes two first half-bridge circuits connected in series, and the midpoints of the bridge arms of the two first half-bridge circuits are led out as the high-voltage side input port of the power module; both ends of the first-stage circuit are respectively connected to both ends of the first capacitor path; The second - stage circuit is a dual - active - bridge converter based on a series - connected half - bridge; both ends of the high - voltage side of the second - stage circuit are respectively connected to both ends of the first capacitor circuit, and both ends of the low - voltage side are respectively connected to both ends of the second capacitor circuit; The input port of the high - voltage side of the power module, between the two first DC - bus capacitors, and the mid - point of the series - connected half - bridge on the high - voltage side of the dual - active - bridge converter are connected; the mid - point of the series - connected half - bridge on the low - voltage side of the dual - active - bridge converter and between the two second DC - bus capacitors are connected.

[0006] Further, the second - stage circuit includes two series - connected second half - bridge circuits on the high - voltage side, and a DC - blocking capacitor on the high - voltage side, a high - frequency transformer, a DC - blocking capacitor on the low - voltage side, and two series - connected third half - bridge circuits on the low - voltage side; Both ends of the two series - connected second half - bridge circuits on the high - voltage side serve as both ends of the high - voltage side of the second - stage circuit, and both ends of the two series - connected third half - bridge circuits on the low - voltage side serve as both ends of the low - voltage side of the second - stage circuit; both ends of the high - voltage side of the second - stage circuit are respectively connected to both ends of the first capacitor circuit, and both ends of the low - voltage side of the second - stage circuit are respectively connected to both ends of the second capacitor circuit; the mid - points of the bridge arms of the two second half - bridge circuits are respectively connected to both ends of the primary side of the high - frequency transformer, and the mid - points of the bridge arms of the two third half - bridge circuits are respectively connected to both ends of the secondary side of the high - frequency transformer; the DC - blocking capacitor on the high - voltage side is connected in series with the primary side of the high - frequency transformer, and the DC - blocking capacitor on the low - voltage side is connected in series with the secondary side of the high - frequency transformer.

[0007] Further, the power - switching devices in the first half - bridge circuit, the second half - bridge circuit, and the third half - bridge circuit adopt MOSFETs.

[0008] In a second aspect, the present application proposes an electric - energy conversion device, including the above - mentioned DC transformer; The low - voltage sides of multiple power modules are connected in parallel to form a low - voltage DC port, which is connected to a DC load or a low - voltage DC distribution network; The high - voltage sides of multiple power modules are connected in series to form a medium - voltage DC port, which is connected to a medium - voltage DC distribution network or a medium - voltage DC load.

[0009] Further, it further includes two filter inductors; The two filter inductors are respectively connected in series to the low - voltage DC port and the medium - voltage DC port.

[0010] Further, by changing the transmission power of the second - stage circuit, closed - loop control of the medium - voltage DC distribution network or the medium - voltage DC load is achieved.

[0011] Further, the method for changing the transmission power of the second - stage circuit includes: Adjusting the phase difference between the first square - wave voltage and the second square - wave voltage; the first square - wave voltage is the voltage applied to the primary winding of the high - frequency transformer in the second - stage circuit, and the second square - wave voltage is the voltage applied to the secondary winding of the high - frequency transformer in the second - stage circuit.

[0012] Further, it further includes: by adjusting the number of power modules, controlling the phase difference of the square-wave voltages output by two adjacent power modules as a high-voltage side carrier phase-shifting adjustment strategy.

[0013] Further, the phase difference of the square-wave voltages output by two adjacent power modules is 2π / N; where N is the number of power modules.

[0014] Further, it further includes: by adjusting the phase-shift angles of the primary side and the secondary side of the high-frequency transformer in the second-stage circuit, controlling the magnitude and direction of the power transmitted by the power module.

[0015] Compared with the prior art, the present application has the following beneficial effects: The present application proposes a DC transformer, which includes a plurality of power modules. Each power module includes a first-stage circuit, a second-stage circuit, two first DC bus capacitors, and two second DC bus capacitors. The two first DC bus capacitors are connected in series to form a first capacitor path, and the two second DC bus capacitors are connected in series to form a second capacitor path. The first-stage circuit includes two first half-bridge circuits connected in series, and the second-stage circuit is a dual-active-bridge converter based on a series half-bridge. Compared with the technical solutions of traditional DC transformers with power modules based on neutral-point clamping or active neutral-point clamping, the DC transformer of the present application adopts power modules based on series half-bridges, significantly reducing the number of power devices. At the same time, it also simplifies the layout and wiring difficulty of the entire circuit, making it easier to promote and apply the DC transformer.

[0016] The present application also proposes an electric energy conversion device, which has all the advantages of the above DC transformer. In addition, the control method of the proposed electric energy conversion device, based on the structure of the above DC transformer, can achieve more flexible and convenient control. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0018] Figure 1 It is a schematic diagram of an electric energy conversion device; Figure 2 It is a schematic diagram of the high-frequency AC waveform of the dual-active-bridge converter based on a series half-bridge in the embodiment of the present application; Figure 3 It is a schematic diagram of the modulation result of the high-voltage side carrier phase-shifting of multiple power modules in the embodiment of the present application; Figure 4It is the simulation waveform diagram of the second-stage circuit in the embodiment of the present application; Figure 5 It is the simulation waveform diagram of the high-voltage side port voltages and the total output voltage of four power modules in the embodiment of the present application; Figure 6 It is the simulation waveform diagram of the total output voltage of the high-voltage side of the power module and the output voltage of the medium-voltage DC port in the embodiment of the present application. Detailed implementation manners

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are some, rather than all, of the embodiments of the present application. The components of the embodiments of the present application usually described and illustrated in the accompanying drawings here can be arranged and designed in various different configurations.

[0020] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application claimed, but merely represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts shall fall within the protection scope of the present application.

[0021] It should be noted that similar reference numerals and letters denote similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0022] In the description of the embodiments of the present application, it should be noted that if terms such as "upper", "lower", "horizontal", "inner", etc. are used to indicate the orientation or positional relationship, it is based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship when the inventive product is normally placed. It is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present application. In addition, terms such as "first", "second", etc. are only used for descriptive distinction and cannot be understood as indicating or implying relative importance.

[0023] In addition, if the term "horizontal" appears, it does not mean that the component is required to be absolutely horizontal, but it can be slightly inclined. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but it can be slightly inclined.

[0024] In the description of the embodiments of the present application, it should also be noted that unless otherwise clearly specified and limited, if the terms "set", "install", "connected", and "coupled" are used, they should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific situations.

[0025] Due to significant advantages such as facilitating the access of renewable energy, medium-voltage DC distribution networks are increasingly becoming a research hotspot. Compared with traditional AC distribution networks, medium-voltage DC distribution networks exhibit significant characteristics in terms of transmission efficiency, with lower power losses, especially suitable for long-distance power transmission scenarios. At the same time, the inherent non-harmonic characteristic of direct current fundamentally avoids the common harmonic pollution problems in AC systems, ensuring the stability of the distribution network operation and the purity of power quality. In addition, the high compatibility of medium-voltage DC distribution networks with renewable energy sources such as solar and wind makes them a key infrastructure for promoting the transformation of the energy structure.

[0026] Among the core equipment of medium-voltage DC distribution networks, DC transformers play a crucial role. They not only undertake the task of efficiently connecting renewable energy generation units and DC loads but also shoulder the important responsibility of energy transmission and conversion between DC buses of different voltage levels. Through their unique power conversion mechanism, DC transformers achieve the optimal energy scheduling of each link in the medium-voltage DC distribution network and are an indispensable part of building a flexible and efficient DC distribution network.

[0027] DC transformers generally adopt a topology structure with multiple power modules connected in series on the high-voltage side and in parallel on the low-voltage side. Through modular design, it not only improves the scalability of the system but also enhances the fault isolation ability. The core of each power module is an isolated bidirectional DC-DC (Direct Current to Direct Current) converter, whose design purpose is to achieve bidirectional energy flow to meet the needs of flexible access of distributed energy. In the initial design, the two-level DAB (Dual Active Bridge) was widely used due to its simple control and high reliability. However, with the increase in the voltage level of the distribution network, the limitations of the two-level structure in voltage handling capacity have gradually emerged.

[0028] To break through this bottleneck, three-level DAB or CLLC (a resonant network consisting of capacitor-inductor-inductor-capacitor) circuit topologies emerged. By increasing the number of power devices, the voltage level of a single power module was effectively increased, thereby reducing the total number of required modules, lowering the system cost, and improving the integration. However, when using three-level DAB in the form of NPC (Neutral Point Clamped) or ANPC (Active Neutral Point Clamped), the system complexity increased significantly. On the one hand, the increase in the number of power devices brought higher costs and control difficulties; on the other hand, the complex circuit topology imposed higher requirements on layout and wiring, not only increasing the design difficulty but also potentially affecting the system's thermal management and electromagnetic compatibility.

[0029] Based on the above situation, this application proposes a DC transformer, a power conversion device, and its control method. The following will make a detailed description of this application in combination with embodiments and drawings.

[0030] As an embodiment of a DC transformer in this application, it includes multiple power modules. The high-voltage sides of the multiple power modules are connected in series, and the low-voltage sides are connected in parallel.

[0031] A power module includes a first-stage circuit, a second-stage circuit, two first DC bus capacitors, and two second DC bus capacitors. The two first DC bus capacitors are connected in series to form a first capacitor path. The two second DC bus capacitors are connected in series to form a second capacitor path.

[0032] The first-stage circuit includes two series-connected first half-bridge circuits. The midpoints of the bridge arms of the two first half-bridge circuits are led out as the high-voltage side input port of the power module. The two ends of the first-stage circuit are respectively connected to the two ends of the first capacitor path. The first-stage circuit realizes the preliminary distribution and isolation of electrical energy on the high-voltage side. By controlling the switches of the first half-bridge circuit, the waveforms of the input current and input voltage can be adjusted.

[0033] The second-stage circuit is a dual-active-bridge converter based on a series half-bridge. The two ends of the high-voltage side of the second-stage circuit are respectively connected to the two ends of the first capacitor path, and the two ends of the low-voltage side are respectively connected to the two ends of the second capacitor path. The second-stage circuit adopts a dual-active-bridge converter based on a series half-bridge, which can achieve electrical isolation through a high-frequency transformer, ensure system safety, and support bidirectional energy flow between the high-voltage side and the low-voltage side, adapting to the access of renewable energy and the charge and discharge requirements of energy storage systems. In practical applications, the output voltage can be precisely controlled by adjusting the switching frequency and duty cycle.

[0034] Each power module integrates a first-stage circuit and a second-stage circuit internally to form a cascaded structure. The first-stage circuit is responsible for preliminary power conversion, and the second-stage circuit realizes high-efficiency isolation and fine regulation. The first capacitor circuit and the second capacitor circuit play the roles of voltage division and energy storage, absorb high-frequency harmonics in the circuit, reduce voltage fluctuations, and stabilize the operation of the first-stage circuit and the second-stage circuit.

[0035] The high-voltage side input port of the power module, between two first DC bus capacitors, and the series half-bridge midpoint on the high-voltage side of the dual-active bridge converter are connected. The series half-bridge midpoint on the low-voltage side of the dual-active bridge converter and between two second DC bus capacitors are connected. This helps balance the capacitor voltages and improve the stability of the system.

[0036] The DC transformer of this application consists of multiple power modules. The connection method of series connection on the high-voltage side and parallel connection on the low-voltage side can achieve voltage superposition on the high-voltage side to meet the requirements of high-voltage input, and current shunting on the low-voltage side, enabling a larger output current to be provided, thereby realizing power transmission and conversion.

[0037] This application can solve the problems of the large number of power devices required by the NPC or ANPC three-level circuit topology in the aforementioned DC transformer and the great difficulty in layout and wiring. The first capacitor circuit and the second capacitor circuit form a DC bus with twice the voltage, serving as the connection ports for the first-stage circuit and the second-stage circuit.

[0038] In some embodiments of this application, the second-stage circuit includes two series-connected second half-bridge circuits on the high-voltage side, as well as a high-voltage side DC-blocking capacitor, a high-frequency transformer, a low-voltage side DC-blocking capacitor, and two series-connected third half-bridge circuits on the low-voltage side. The two ends of the two series-connected second half-bridge circuits on the high-voltage side serve as the two ends of the high-voltage side of the second-stage circuit, and the two ends of the two series-connected third half-bridge circuits on the low-voltage side serve as the two ends of the low-voltage side of the second-stage circuit. The two ends of the high-voltage side of the second-stage circuit are respectively connected to the two ends of the first capacitor circuit, and the two ends of the low-voltage side of the second-stage circuit are respectively connected to the two ends of the second capacitor circuit. The midpoints of the bridge arms of the two second half-bridge circuits are respectively connected to the two ends of the primary side of the high-frequency transformer, and the midpoints of the bridge arms of the two third half-bridge circuits are respectively connected to the two ends of the secondary side of the high-frequency transformer. The high-voltage side DC-blocking capacitor is connected in series with the primary side of the high-frequency transformer, and the low-voltage side DC-blocking capacitor is connected in series with the secondary side of the high-frequency transformer. In the second-stage circuit, the circuit topologies on both sides of the high-frequency transformer are completely symmetrical. The two series-connected second half-bridge circuits and the two series-connected third half-bridge circuits are used to generate square-wave voltages with DC components. After being blocked by the high-voltage side DC-blocking capacitor and the low-voltage side DC-blocking capacitor respectively, the voltages applied to the primary side and the secondary side of the high-frequency transformer are standard square-wave voltages without DC components. The magnitude and direction of the transmitted power can be adjusted by adjusting the phase difference between the two square-wave voltages on the primary side and the secondary side of the high-frequency transformer.

[0039] In addition, this application also proposes an electric energy conversion device, such as Figure 1As shown in the figure, it is a schematic diagram of a power conversion device. The power switch devices in the first half-bridge circuit, the second half-bridge circuit, and the third half-bridge circuit adopt MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). There are a total of N power modules, which are respectively denoted as power module 1 to power module N. The turns ratio of the high-frequency transformer is n:1. The midpoints of the bridge arms of the two second half-bridge circuits are point A and point B respectively, and the midpoints of the bridge arms of the two third half-bridge circuits are point C and point D respectively. The point E of the primary side of the high-frequency transformer is near the high-voltage side DC-blocking capacitor ( Figure 1 the capacitor C1 in Figure 1 ), and the point F of the secondary side of the high-frequency transformer is near the low-voltage side DC-blocking capacitor ( Figure 1 the capacitor C2 in

[0040] After the low-voltage sides of multiple power modules are connected in parallel, a low-voltage DC port of the DC transformer is formed, and then a filter inductor is connected in series to connect to a DC load or a renewable energy source or a low-voltage DC distribution network. After the high-voltage sides of multiple power modules are connected in series, a filter inductor is connected in series again to form a medium-voltage DC port of the DC transformer, which is connected to a medium-voltage DC distribution network or a medium-voltage DC load.

[0040] Taking the connection to the medium-voltage DC distribution network as an example, the first-stage circuit in the power module serves as the interface circuit of the medium-voltage DC distribution network, and the duty cycle can be adjusted in real time according to the voltage of the medium-voltage DC distribution network, so as to control the DC current incorporated into the medium-voltage DC distribution network, and further control the transmitted power. Therefore, by changing the duty cycle, the magnitude of the DC current flowing into the power module from the medium-voltage DC distribution network can be accurately controlled. In addition, based on the adjustment of the duty cycle, the system can achieve fine control of the transmitted power, which can not only ensure the stability of power transmission, but also flexibly adjust the power output according to the requirements of the medium-voltage DC distribution network, improving the flexibility and efficiency of the system.

[0041] As Figure 2 shown in the figure, it is a schematic diagram of the high-frequency AC waveform of a dual-active-bridge converter based on a series half-bridge. The output voltage of the two series-connected second half-bridge circuits on the high-voltage side is a square-wave voltage of 0 to , and the duty cycle is 50%. is the input port voltage of the high-voltage side of the power module. After DC blocking by the high-voltage side DC-blocking capacitor ( Figure 1 the capacitor C1 in ), the voltage applied to the primary winding of the high-frequency transformer becomes a square-wave voltage with positive and negative symmetry and an amplitude of 0.5 . Similarly, the output voltage of the two series-connected third half-bridge circuits on the low-voltage side is a square-wave voltage of 0 to , and the duty cycle is 50%. After DC blocking by the low-voltage side DC-blocking capacitor ( Figure 1After the DC blocking of the capacitor C2), the voltage applied to the secondary winding of the high-frequency transformer becomes a square wave voltage with positive and negative symmetry and an amplitude of 0.5 . is the low-voltage DC port voltage. There is a certain phase difference between the two square wave voltages of the primary winding and the secondary winding of the high-frequency transformer, and a voltage will be applied to the leakage inductance of the high-frequency transformer, thereby exciting a trapezoidal wave current flowing through the transformer i p . By adjusting the phase shift angle, the size of the trapezoidal wave current waveform can be adjusted, and then the size and direction of the power transmitted by the power module can be adjusted, so as to achieve power closed-loop control. In addition, it should be noted that Figure 2 in, is the DC blocking capacitor voltage on the high-voltage side, is the DC blocking capacitor voltage on the low-voltage side.

[0042] As Figure 3 shown, it is a schematic diagram of the high-voltage side carrier phase-shifted modulation results of multiple power modules. Each power module outputs a square wave voltage of 0~ on the high-voltage side. The square wave voltages output by two adjacent power modules have a phase difference of 2π / N, where N is the number of power modules. By adopting this carrier phase-shifted modulation strategy, the fluctuation frequency of the total output voltage of the power module can be N times the fluctuation frequency of the output voltage of each power module, thereby reducing the ripple of the grid-connected current and improving the power quality.

[0043] The technical effects of this application are further described below through the embodiments of this application. When the number of power modules N is 4, and are both 1700V, and the turns ratio of the high-frequency transformer is 1, and simulation verification is carried out. As Figure 4 shown, it is the simulation waveform diagram of the second-stage circuit. From Figure 4 it can be seen that the two series-connected second half-bridge circuits on the high-voltage side and the two series-connected third half-bridge circuits on the low-voltage side in the second-stage circuit output a square wave voltage of 0V~1700V. After passing through the DC blocking capacitor, the voltages on the primary and secondary sides of the high-frequency transformer become square waves with positive and negative symmetry, and the amplitude is 850V. This result is consistent with the foregoing theoretical analysis. From Figure 4 it can be seen that the primary current waveform of the high-frequency transformer is a trapezoidal wave, and the size of the current waveform can be adjusted by adjusting the phase shift angle, and then the size and direction of the power transmitted by the power module can be adjusted.

[0044] As Figure 5 shown, it is the simulation waveform diagram of the high-voltage side port voltage and the total output voltage of four power modules. Figure 5Among them, UH1 to UH4 respectively correspond to the high-voltage side port voltages of four power modules. The high-voltage side port voltages of the four power modules are square waves of 0V to 1700V. The duty cycles of the four voltage square wave waveforms are exactly the same, and the phases differ by 90° in sequence. The total output voltage UH of the power module is the superposition of the high-voltage side port voltages of the four power modules, and the square wave frequency is four times the frequency of the high-voltage side port voltage of the power module. Thus, it can be verified that the carrier phase-shifted control is achieved by two series-connected second half-bridge circuits on the high-voltage side of the power module.

[0045] As Figure 6 shown, it is the simulation waveform diagram of the total output voltage on the high-voltage side of the power module and the output voltage of the medium-voltage DC port. From Figure 6 it can be seen that the fluctuation frequency of the current IH waveform at the medium-voltage DC port is consistent with the fluctuation frequency of the total output voltage UH of the power module. The current IH waveform at the medium-voltage DC port will fluctuate up and down with the fluctuation of the total output voltage UH of the power module. The ripple of the output voltage Uout at the medium-voltage DC port is very small, and the ripple frequency is consistent with the fluctuation frequency of the current IH at the medium-voltage DC port, which also verifies the correctness of the circuit topology of this application.

[0046] Therefore, this application actually also proposes a control method for an electric energy conversion device based on the foregoing circuit topology, and realizes the closed-loop control of a medium-voltage DC distribution network or a medium-voltage DC load by changing the transmission power of the second-stage circuit.

[0047] The above is only the preferred embodiment of this application and is not used to limit this application. For those skilled in the art, this application can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this application shall be included within the protection scope of this application.

Claims

1. A DC transformer includes multiple power modules, with the high-voltage sides of the multiple power modules connected in series and the low-voltage sides connected in parallel. It is characterized in that: The power module includes a first-stage circuit, a second-stage circuit, two first DC bus capacitors, and two second DC bus capacitors; the two first DC bus capacitors are connected in series to form a first capacitor path; the two second DC bus capacitors are connected in series to form a second capacitor path. The first-stage circuit includes two series-connected first half-bridge circuits, and the midpoints of the bridge arms of the two first half-bridge circuits are led out as the high-voltage side input port of the power module; both ends of the first-stage circuit are respectively connected to both ends of the first capacitor path. The second-stage circuit is a dual-active-bridge converter based on a series half-bridge; both ends of the high-voltage side of the second-stage circuit are respectively connected to both ends of the first capacitor path, and both ends of the low-voltage side are respectively connected to both ends of the second capacitor path. The high-voltage side input port of the power module, the connection between the two first DC bus capacitors, and the midpoint of the series half-bridge on the high-voltage side of the dual-active-bridge converter are connected; the midpoint of the series half-bridge on the low-voltage side of the dual-active-bridge converter and the connection between the two second DC bus capacitors are connected.

2. The DC transformer according to claim 1, characterized in that: The second-stage circuit includes two series-connected second half-bridge circuits on the high-voltage side, as well as a high-voltage side DC-blocking capacitor, a high-frequency transformer, a low-voltage side DC-blocking capacitor, and two series-connected third half-bridge circuits on the low-voltage side. Both ends of the two series-connected second half-bridge circuits on the high-voltage side serve as both ends of the high-voltage side of the second-stage circuit, and both ends of the two series-connected third half-bridge circuits on the low-voltage side serve as both ends of the low-voltage side of the second-stage circuit; both ends of the high-voltage side of the second-stage circuit are respectively connected to both ends of the first capacitor path, and both ends of the low-voltage side of the second-stage circuit are respectively connected to both ends of the second capacitor path; the midpoints of the bridge arms of the two second half-bridge circuits are respectively connected to both ends of the primary side of the high-frequency transformer, and the midpoints of the bridge arms of the two third half-bridge circuits are respectively connected to both ends of the secondary side of the high-frequency transformer; the high-voltage side DC-blocking capacitor is connected in series with the primary side of the high-frequency transformer, and the low-voltage side DC-blocking capacitor is connected in series with the secondary side of the high-frequency transformer.

3. The DC transformer according to claim 2, wherein: The power switching devices in the first half-bridge circuit, the second half-bridge circuit, and the third half-bridge circuit adopt MOSFETs.

4. A power conversion device, characterized in that: Including the DC transformer according to any one of claims 1 to 3; The low-voltage sides of multiple power modules are connected in parallel to form a low-voltage DC port, which is connected to a DC load or a low-voltage DC distribution network. The high-voltage sides of multiple power modules are connected in series to form a medium-voltage DC port, which is connected to a medium-voltage DC distribution network or a medium-voltage DC load.

5. The power conversion device according to claim 4, characterized in that: It further includes two filter inductors; The two filter inductors are respectively connected in series to the low-voltage DC port and the medium-voltage DC port.

6. A control method for the power conversion device according to claim 4 or 5, characterized in that: By changing the transmission power of the second-stage circuit, closed-loop control of the medium-voltage DC distribution network or the medium-voltage DC load is achieved.

7. The control method of the power conversion device according to claim 6, characterized in that: The method for changing the transmission power of the second-stage circuit includes: Adjusting the phase difference between the first square-wave voltage and the second square-wave voltage; the first square-wave voltage is the voltage applied to the primary winding of the high-frequency transformer in the second-stage circuit, and the second square-wave voltage is the voltage applied to the secondary winding of the high-frequency transformer in the second-stage circuit.

8. The control method of the power conversion device according to claim 6, wherein, It further includes: By adjusting the number of power modules, controlling the phase difference of the square-wave voltages output by two adjacent power modules as a high-voltage side carrier phase-shift adjustment strategy.

9. The control method of the power conversion device according to claim 8, characterized in that, The phase difference between the square-wave voltages output by two adjacent power modules is 2π / N, where N is the number of power modules.

10. The control method of the power conversion device according to claim 6, wherein It further includes: By adjusting the phase shift angle between the primary side and the secondary side of the high-frequency transformer in the second-stage circuit, the magnitude and direction of the power transmitted by the power module are controlled.

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