Modularized middle-high voltage direct current transformer

Abstract

The invention relates to the technical field of direct-current transformers, in particular to a modularized middle-high voltage direct-current transformer which comprises m direct-current transformation modules. The two ends of the series-connected inputs of the direct-current transformation modules are used as the inputs of the modularized middle-high voltage direct-current transformer; and the n paths of outputs of the direct current transformation modules are correspondingly connected in parallel and then are used as the n paths of outputs of the modularized middle-high voltage direct current transformer. The topological structure is reasonable in design, can realize single-machine direct-hanging middle-high voltage bus and multi-path direct current bus output, has the capacity of more than ten megawatts, and has the characteristics of modularization, high efficiency and simplicity in control.

Description

Technical Field

[0001] The present invention relates to the technical field of direct current transformers, and in particular to a modular medium- and high-voltage direct current transformer. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] The large-capacity of renewable energy power generation has led to an increase in the capacity of the converter, which requires the use of power devices with larger power levels, or the use of a topological structure of multiple low-voltage converters in parallel to meet the requirements of high-power conversion. Among them, the use of multiple low-voltage converters in parallel has flexible redundant control characteristics, which can effectively improve the reliability of the system. However, when the load is low, its efficiency is much lower than the efficiency under rated conditions. It is necessary to adopt a control strategy to optimize and improve the overall efficiency of the parallel system. At the same time, the number of devices required for multiple low-voltage converters in parallel is large, the cost is high, and the floor space is large. At present, the power of a single low-voltage converter is generally less than 500kW. In addition, as the power consumption of data centers continues to increase, the capacity of the existing DC bus voltage converter will not be able to meet the future high-power conversion needs.

[0004] The existing conventional DC power supply has small capacity, large quantity and large floor space, which cannot meet the needs of compact large-capacity DC bus power conversion, large-scale renewable energy DC aluminum and hydrogen production in future data centers, nor can it be applied to medium- and high-voltage large-capacity scenarios. At the same time, a single machine often cannot be directly connected to a medium- and high-voltage DC bus. In view of the above defects, the present invention has made improvements. Summary of the invention

[0005] In order to overcome the shortcomings of the background technology, the present invention provides a modular medium- and high-voltage DC transformer with a reasonably designed topological structure, which can realize single-machine direct hanging of medium- and high-voltage busbars and multi-channel DC bus output, with a capacity of more than ten megawatts, and has the characteristics of modularity, high efficiency and simple control.

[0006] To achieve the above-mentioned purpose, the present invention provides a modular medium- and high-voltage DC transformer, comprising m DC transformer modules; two ends of the inputs of each of the DC transformer modules are connected in series as the inputs of the modular medium- and high-voltage DC transformer; and n outputs of each of the DC transformer modules are connected in parallel as n outputs of the modular medium- and high-voltage DC transformer.

[0007] Further,

[0008] The DC transformer module includes a DC input capacitor, an inverter circuit and a multi-isolation transformation rectifier circuit;

[0009] The input end of the inverter circuit is connected in parallel with the DC input capacitor and serves as the input of the DC transformer module;

[0010] The output end of the inverter circuit is connected to the multi-isolation conversion rectifier circuit;

[0011] The multi-isolation conversion and rectification circuit has n output terminals, and the n output terminals are used as n outputs of the DC transformer module.

[0012] Further,

[0013] The multi-isolation conversion rectifier circuit includes n isolation conversion rectifier circuits;

[0014] The input side of each of the isolated conversion and rectification circuits is connected in parallel to the output end of the inverter circuit, and the output sides of each of the isolated conversion and rectification circuits are independent of each other and form n output ends of the multiple isolated conversion and rectification circuits.

[0015] Further,

[0016] The isolated conversion rectifier circuit includes a resonant inductor, a resonant capacitor, a magnetizing inductor, a step-down AC transformer, a rectifier circuit and an output capacitor;

[0017] One end of the resonant inductor and one end of the resonant capacitor are respectively connected to two ends of the output side of the inverter circuit, the other end of the resonant inductor and the other end of the resonant capacitor are respectively connected to two ends of the input side of the step-down AC transformer, and the magnetizing inductor is connected in parallel to the input side of the step-down AC transformer;

[0018] The output side of the step-down AC transformer is connected to the input side of the rectifier circuit, and the output side of the rectifier circuit is connected in parallel with the output capacitor to serve as the output of the isolation conversion rectifier circuit.

[0019] Further,

[0020] The inverter circuit is a four-arm switch;

[0021] The bridge arm switch adopts a gate turn-off thyristor;

[0022] The DC transformer module further includes a clamping circuit, which is arranged between the DC input capacitor and the inverter circuit.

[0023] Further,

[0024] The inverter circuit is a four-arm switch;

[0025] The bridge arm switch adopts a high-power transistor, a power field effect transistor or an insulated gate bipolar transistor.

[0026] Further,

[0027] The DC transformer module further includes a clamping circuit, which is arranged between the DC input capacitor and the inverter circuit.

[0028] Further,

[0029] The input end of the clamp circuit is connected in parallel with the DC input capacitor, and the output end of the clamp circuit is connected to the input end of the inverter circuit;

[0030] The input end of the clamp circuit includes a positive input end and a negative input end, and the output end of the clamp circuit includes a positive output end and a negative output end;

[0031] The clamping circuit includes an anode reactance, a clamping diode, a clamping resistor and a clamping capacitor;

[0032] The first end of the anode reactance is connected to the anode of the clamping diode, and a path is drawn from the first end of the anode reactance and the anode of the clamping diode as the positive output end of the clamping circuit, the clamping resistor is connected in parallel between the second end of the anode reactance and the cathode of the clamping diode, and the second end of the anode reactance is used as the positive input end of the clamping circuit;

[0033] The first end of the clamping capacitor is connected to the cathode of the clamping diode, and the second end of the clamping capacitor is used as the negative input terminal and the negative output terminal of the clamping circuit.

[0034] Further,

[0035] In each of the DC transformer modules, the switch tubes in the four-arm switches are all controlled by the same fixed duty cycle.

[0036] Further,

[0037] The switch tube driving signals between the DC transformer modules are staggered and phase-shifted.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] 1. The modular medium- and high-voltage DC transformer consists of m DC transformer modules connected in series and multiple outputs connected in parallel. This multi-module combination can reduce the current stress of the low-voltage bus in medium- and high-voltage large-capacity scenarios. The above-mentioned modular medium- and high-voltage DC transformer is suitable for medium- and high-voltage large-capacity scenarios, and can realize single-machine direct connection to the medium- and high-voltage bus, and multiple DC bus outputs, with a capacity of more than ten megawatts. It has the characteristics of modularity, high efficiency, and simple control. At the same time, it avoids the difficulty of manufacturing large-capacity AC transformers and greatly increases the capacity of DC transformers.

[0040] 2. The rectifier circuit in the isolated conversion rectifier circuit adopts a bridge rectifier circuit or a full-wave rectifier circuit, which can reduce the current stress of the rectifier diode and the transformer winding;

[0041] 3. The step-down AC transformer in each isolated conversion and rectification circuit can adopt different transformation ratios and capacities to achieve outputs of different voltage levels and powers;

[0042] 4. When the DC transformer module adopts a clamping circuit, the clamping circuit can limit the current change rate when the switch tube in the inverter circuit is turned on, and on the other hand, it can limit the oscillation amplitude of the AC output voltage of the inverter circuit;

[0043] 5. The switch tubes in each of the four bridge arm switches of the modular medium and high voltage DC transformer are all controlled by the same fixed duty cycle, which can realize the fixed ratio conversion from the medium and high voltage DC bus to the low voltage bus; at the same time, the driving signals between each DC transformer module are staggered and phase-shifted, which can effectively reduce the input capacitance and output capacitance of each DC transformer module.

[0044] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings.

[0045] The present invention will be further described below in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0047] Figure 1 A schematic diagram of the topological structure of a modular medium- and high-voltage DC transformer according to an embodiment of the present invention;

[0048] Figure 2 for Figure 1 A partial schematic diagram of a multi-isolation conversion rectifier circuit in module #1;

[0049] Figure 3 This is a partial schematic diagram of an embodiment of the present invention in which the four-bridge-arm switch tubes of the inverter circuit adopt insulated gate bipolar transistors and the clamping circuit is removed;

[0050] Figure 4 This is a waveform diagram of the basic working principle of a modular medium and high voltage DC transformer according to an embodiment of the present invention. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solution and advantages of the implementation of the present invention clearer, the technical solution in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is only a part of the embodiment of the present invention, not all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] like Figures 1 to 3 As shown, an embodiment of the present invention provides a modular medium- and high-voltage DC transformer, comprising m DC transformer modules; two ends of the inputs of each of the DC transformer modules are connected in series as the inputs of the modular medium- and high-voltage DC transformer; and n outputs of each of the DC transformer modules are connected in parallel as n outputs of the modular medium- and high-voltage DC transformer.

[0053] In the above technical scheme, the modular medium- and high-voltage DC transformer is composed of m DC transformer modules with inputs connected in series and multiple outputs connected in parallel, which is different from the single-input series and single-output parallel connection in the prior art; the m DC transformer modules are connected in series on the input side to achieve voltage division; the multi-module combination method proposed in the embodiment of the present invention can reduce the current stress of the low-voltage bus in medium- and high-voltage large-capacity scenarios; the above-mentioned modular medium- and high-voltage DC transformer is suitable for medium- and high-voltage large-capacity scenarios, and can realize single-machine direct hanging of the medium- and high-voltage bus, and multiple DC bus outputs, with a capacity of more than ten megawatts, and has the characteristics of modularity, high efficiency, and simple control, while avoiding the difficulty of producing and manufacturing large-capacity AC transformers and greatly improving the capacity of DC transformers.

[0054] In this embodiment, due to the modular input series connection (i.e., m DC transformer modules are connected in series on the input side), the proposed busbar transformer (i.e., modular medium and high voltage DC transformer) can be connected to the medium and high voltage DC busbar. Traditional semiconductor devices have a low withstand voltage level and cannot be directly used in medium and high voltage applications.

[0055] Here, the input capacity of the converter can be calculated according to the input voltage x input current of the DC transformer. The capacity of a single DC transformer module can be 3MW. Taking 4 modules as an example, the input capacity of more than 12MW can be achieved by using 4 DC transformer modules. Therefore, the capacity of the proposed modular medium and high voltage DC transformer can reach more than 10 megawatts.

[0056] The input voltage level used by a single DC transformer module is limited by the withstand voltage of a single device. For example, if 4500VIGCT is used, the input voltage of a single module is below 2.5kV, and it can only be applied to medium and low voltage and large capacity scenarios. The multi-module solution of the embodiment of the present invention (i.e., including m DC transformer modules) can achieve an exponential increase in the input voltage. For example, 4 modules can reach an input voltage level of 10kV, which can be well applied to medium and high voltage and large capacity scenarios.

[0057] In this embodiment, for m DC transformer modules, each DC transformer module has n output ports A1B1~AnBn; the n outputs of each DC transformer module are connected in parallel, that is, the output end A1 of the DC transformer module #1 is connected to the output end A1 of the DC transformer module #2, the output end B1 of the DC transformer module #1 is connected to the output end B1 of the DC transformer module #2, and so on. Therefore, after the output sides of all DC transformer modules are connected respectively, there are a total of n output ports isolated from each other, that is, n outputs A1B1~AnBn of the modular medium and high voltage DC transformer.

[0058] As a preferred technical solution, the DC transformer module includes a DC input capacitor, an inverter circuit and a multi-isolation conversion rectifier circuit; the input end of the inverter circuit is connected in parallel with the DC input capacitor as the input of the DC transformer module; the output end of the inverter circuit is connected to the multi-isolation conversion rectifier circuit; the multi-isolation conversion rectifier circuit has n output ends, and the n output ends are used as n outputs of the DC transformer module. In the above technical solution, the DC input capacitor plays the role of DC voltage support and filtering, the inverter circuit is used to convert the input DC power into AC power, and the multi-isolation conversion rectifier circuit has n output ends, and the output voltages are V1~Vn, that is, the n-way voltage output of the DC transformer module.

[0059] As a preferred technical solution, the multi-isolated conversion rectifier circuit includes n isolated conversion rectifier circuits; the input side of each of the isolated conversion rectifier circuits is connected in parallel to the output end of the inverter circuit, and the output sides of each of the isolated conversion rectifier circuits are independent of each other and form n output ends of the multi-isolated conversion rectifier circuit.

[0060] In this embodiment, n isolated conversion rectifier circuits are arranged in the multi-isolated conversion rectifier circuit, the input sides of the n isolated conversion rectifier circuits are connected in parallel, and the output sides are independent of each other; these isolated conversion rectifier circuits are connected in parallel to the output end of the inverter circuit to form n output ends of the multi-isolated conversion rectifier circuit, and the output voltages are V1~Vn (i.e., n voltage outputs).

[0061] As a preferred technical solution, the isolated conversion rectifier circuit includes a resonant inductor Lr, a resonant capacitor Cr, a magnetizing inductor Lm, a step-down AC transformer, a rectifier circuit and an output capacitor; one end of the resonant inductor Lr and one end of the resonant capacitor Cr are respectively connected to the two ends of the output side of the inverter circuit, the other end of the resonant inductor Lr and the other end of the resonant capacitor Cr are respectively connected to the two ends of the input side of the step-down AC transformer, and the magnetizing inductor Lm is connected in parallel to the input side of the step-down AC transformer; the output side of the step-down AC transformer is connected to the input side of the rectifier circuit, and the output side of the rectifier circuit is connected in parallel with the output capacitor as the output of the isolated conversion rectifier circuit. In specific implementation, the resonant inductor can be externally matched, or replaced by the leakage inductance of the step-down AC transformer itself to reduce system cost and volume; the resonant capacitor can be placed on the primary or secondary side of the step-down AC transformer; the magnetizing inductor can be externally matched, or the magnetizing inductance of the step-down AC transformer itself can be used to reduce system cost and volume. The isolated conversion rectifier circuit of this embodiment can realize the isolation function and the conversion function through the step-down AC transformer, and can realize the rectification function through the rectification circuit.

[0062] In some embodiments, the rectifier circuit adopts a bridge rectifier circuit or a full-wave rectifier circuit. In this embodiment, the rectifier circuit adopts a bridge rectifier circuit or a full-wave rectifier circuit, which can reduce the current stress of the rectifier diode and the transformer winding.

[0063] The rectifier circuit of this embodiment adopts a bridge rectifier circuit. The resonant inductor Lr and the resonant capacitor Cr form an LC resonant circuit. The magnetizing inductance Lm helps the switch tube in the inverter circuit to achieve zero voltage turn-on. The step-down AC transformer plays the role of electrical isolation and voltage reduction. The full-bridge rectifier circuit composed of diodes Dr1 to Dr4 plays the role of AC / DC conversion, realizing the conversion of AC voltage into DC voltage.

[0064] like Figure 2 As shown, in the multi-isolation conversion rectifier circuit of the present embodiment, the primary side of each step-down AC transformer has a resonant network (also called a resonant cavity); the resonant network is composed of Lr / Cr / Lm, so the embodiment of the present invention actually provides a multi-resonant cavity type DC transformer module.

[0065] In some embodiments, the step-down AC transformers in each of the isolated conversion and rectification circuits use different transformation ratios and capacities. In this embodiment, the step-down AC transformers in each of the isolated conversion and rectification circuits can use different transformation ratios and capacities to achieve outputs of different voltage levels and powers.

[0066] As a preferred technical solution, the inverter circuit is a four-arm switch; the arm switch adopts a gate-turn-off thyristor; the DC transformer module also includes a clamping circuit, and the clamping circuit is arranged between the DC input capacitor and the inverter circuit. When the arm switches in the four-arm switch of this embodiment adopt various thyristors (including gate-turn-off thyristors), the DC transformer module is additionally provided with a clamping circuit; the clamping circuit is arranged in the DC transformer module to suppress the oscillation amplitude of the AC voltage output by the inverter circuit.

[0067] As a preferred technical solution, the inverter circuit is a four-arm switch; the arm switch adopts a high-power transistor, a power field effect transistor or an insulated gate bipolar transistor. In this embodiment, when a high-power transistor, a power field effect transistor or an insulated gate bipolar transistor device is adopted, the DC transformer module may not be provided with a clamping circuit, that is, it is understood that the clamping circuit is removed; here, taking the insulated gate bipolar transistor device as an example, after removing the clamping circuit, Figure 3 As shown, the driving circuit needs to be controlled to limit the turn-on speed of the transistor, thereby reducing the oscillation amplitude of the AC voltage output by the inverter circuit.

[0068] In some embodiments, the DC transformer module further includes a clamping circuit, and the clamping circuit is arranged between the DC input capacitor and the inverter circuit. In this embodiment, when the bridge arm switch in the four-arm switch adopts a high-power transistor, a power field effect transistor or an insulated gate bipolar transistor device, the DC transformer module can be provided with a clamping circuit. By providing the clamping circuit in the DC transformer module, the oscillation amplitude of the AC voltage output by the inverter circuit can be suppressed, and the opening speed limit of the transistor can be realized by controlling the driving circuit, thereby further reducing the oscillation amplitude of the AC voltage output by the inverter circuit.

[0069] It can be seen that when the bridge arm switches in the four-arm switches are high-power transistors, power field effect transistors or insulated gate bipolar transistor devices, the DC transformer module of this embodiment can use a clamping circuit or remove the clamping circuit.

[0070] When the inverter circuit uses four-arm switches, the arm switches can be fully controlled power switches such as gate turn-off thyristors, high-power transistors, power field effect transistors or insulated gate bipolar transistors. In specific implementation, the switch tubes of the four-arm switches (i.e., fully controlled power switches) are all provided with anti-parallel diodes. The anti-parallel diodes belong to the prior art and are not described in detail here.

[0071] In some embodiments, the inverter circuit may also adopt a three-level inverter circuit to further reduce the voltage stress of the power device.

[0072] As a preferred technical solution, the input end of the clamp circuit is connected in parallel with the DC input capacitor, and the output end of the clamp circuit is connected to the input end of the inverter circuit; the input end of the clamp circuit includes a positive input end and a negative input end, and the output end of the clamp circuit includes a positive output end and a negative output end; the clamp circuit includes an anode reactance L buf , clamping diode D buf , Clamping resistor R buf and the clamping capacitor C buf The anode reactance L buf The first end of the clamping diode D buf The positive electrode and the anode reactance L buf The first terminal and the clamping diode D buf A path is drawn between the positive electrodes as the positive output terminal of the clamp circuit, and the anode reactance L buf The second end of the clamping diode D buf The clamping resistor R is connected in parallel between the negative electrodes of buf and the anode reactance L buf The second end of is used as the positive input end of the clamping circuit; the clamping capacitor C buf The first end of the clamping diode D buf the negative terminal and the clamp capacitor C buf The second end serves as the negative input terminal and the negative output terminal of the clamping circuit.

[0073] In this embodiment, if Figure 1 As shown, the clamp circuit is set between the DC input capacitor and the inverter circuit. The clamp circuit consists of an anode reactance L buf , clamping diode D buf , Clamping resistor R buf and the clamping capacitor C buf On the one hand, the clamping circuit can limit the current change rate when the switch tube (here the switch tube refers to the four-bridge switch tube in the inverter circuit) is turned on; on the other hand, it can limit the AC input voltage of the inverter circuit (that is, the AC output voltage V AB ) of the oscillation amplitude. Among them, the anode reactance L buf Limit the current rise rate when the switch is turned on, clamping diode D buf Used to provide a branch for the anode reactance to continue current and block the clamping capacitor C buf Discharge the inverter circuit; clamping capacitor C buf Used to buffer the anode reactance L buf The energy stored in.

[0074] As a preferred technical solution, in each of the DC transformer modules, the switch tubes in the four-arm switches are all controlled by the same fixed duty cycle. In this embodiment, the four-arm switch tubes of each DC transformer module are all controlled by a fixed duty cycle (preferably 0.48 in this embodiment, and can be between 0.45 and 0.49 in specific implementation, and the specific value can be determined according to the circuit parameter debugging test), which can realize the fixed ratio conversion from the medium and high voltage DC bus to the low voltage bus. For the above-mentioned fixed ratio conversion, for example, when the number of DC transformer modules is 5, the primary and secondary turns ratio N1: S1 of the AC isolation transformer (i.e., the step-down AC transformer) in the first isolation transformation and rectification circuit of the module #1 multi-isolation transformation and rectification circuit is 2: 0.75, and the total input voltage Vin is 10kV, and the output voltage V1 is 750V.

[0075] As a preferred technical solution, the switch tube driving signals between the DC transformer modules are staggered and phase-shifted. In this embodiment, the duty cycle of the driving signals of each DC transformer module remains consistent, but the driving signals between the modules in the multi-DC transformer module control scheme can be phase-shifted, which can equivalently increase the input capacitor and output capacitor AC current frequency, thereby reducing the input / output capacitance value by multiples; the driving signals between the modules in the embodiment of the present invention are staggered and phase-shifted, which can effectively reduce the input capacitance and output capacitance value of each module.

[0076] based on Figure 1 The multi-resonant cavity DC transformer module shown in the embodiment of the present invention also provides Figure 4 The basic working principle waveform diagram is shown in the figure. The meaning of the symbols in the figure refers to Figure 1 , 2 As shown; this waveform diagram can intuitively demonstrate the basic working principle of the embodiment of the present invention.

[0077] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0078] Parts not involved in the above embodiments are the same as the prior art or can be implemented by using the prior art, and will not be further described here.

[0079] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modular medium and high voltage DC transformer, characterized in that: It includes m DC transformer modules; The two ends of the inputs of the DC transformer modules connected in series serve as the inputs of the modular medium and high voltage DC transformer; The n-way outputs of each of the DC transformer modules are connected in parallel to serve as the n-way outputs of the modular medium- and high-voltage DC transformer.

2. A modular medium and high voltage DC transformer according to claim 1, characterized in that: The DC transformer module includes a DC input capacitor, an inverter circuit and a multi-isolation transformation rectifier circuit; The input end of the inverter circuit is connected in parallel with the DC input capacitor and serves as the input of the DC transformer module; The output end of the inverter circuit is connected to the multi-isolation conversion rectifier circuit; The multi-isolation conversion and rectification circuit has n output terminals, and the n output terminals are used as n outputs of the DC transformer module.

3. A modular medium and high voltage DC transformer according to claim 2, characterized in that: The multi-isolation conversion rectifier circuit includes n isolation conversion rectifier circuits; The input side of each of the isolated conversion and rectification circuits is connected in parallel to the output end of the inverter circuit, and the output sides of each of the isolated conversion and rectification circuits are independent of each other and form n output ends of the multiple isolated conversion and rectification circuits.

4. A modular medium and high voltage DC transformer according to claim 3, characterized in that: The isolated conversion rectifier circuit includes a resonant inductor, a resonant capacitor, a magnetizing inductor, a step-down AC transformer, a rectifier circuit and an output capacitor; One end of the resonant inductor and one end of the resonant capacitor are respectively connected to two ends of the output side of the inverter circuit, the other end of the resonant inductor and the other end of the resonant capacitor are respectively connected to two ends of the input side of the step-down AC transformer, and the magnetizing inductor is connected in parallel to the input side of the step-down AC transformer; The output side of the step-down AC transformer is connected to the input side of the rectifier circuit, and the output side of the rectifier circuit is connected in parallel with the output capacitor to serve as the output of the isolation conversion rectifier circuit.

5. A modular medium and high voltage DC transformer according to claim 2, characterized in that: The inverter circuit is a four-arm switch; The bridge arm switch adopts a gate turn-off thyristor; The DC transformer module further includes a clamping circuit, which is arranged between the DC input capacitor and the inverter circuit.

6. A modular medium and high voltage DC transformer according to claim 2, characterized in that: The inverter circuit is a four-arm switch; The bridge arm switch adopts a high-power transistor, a power field effect transistor or an insulated gate bipolar transistor.

7. A modular medium and high voltage DC transformer according to claim 6, characterized in that: The DC transformer module further includes a clamping circuit, which is arranged between the DC input capacitor and the inverter circuit.

8. A modular medium and high voltage DC transformer according to claim 5 or 7, characterized in that: The input end of the clamp circuit is connected in parallel with the DC input capacitor, and the output end of the clamp circuit is connected to the input end of the inverter circuit; The input end of the clamp circuit includes a positive input end and a negative input end, and the output end of the clamp circuit includes a positive output end and a negative output end; The clamping circuit includes an anode reactance, a clamping diode, a clamping resistor and a clamping capacitor; The first end of the anode reactance is connected to the anode of the clamping diode, and a path is drawn from the first end of the anode reactance and the anode of the clamping diode as the positive output end of the clamping circuit, the clamping resistor is connected in parallel between the second end of the anode reactance and the cathode of the clamping diode, and the second end of the anode reactance is used as the positive input end of the clamping circuit; The first end of the clamping capacitor is connected to the cathode of the clamping diode, and the second end of the clamping capacitor is used as the negative input terminal and the negative output terminal of the clamping circuit.

9. A modular medium- and high-voltage DC transformer according to any one of claims 5 to 7, characterized in that: In each of the DC transformer modules, the switch tubes in the four-arm switches are all controlled by the same fixed duty cycle.

10. A modular medium and high voltage DC transformer according to claim 9, characterized in that: The switch tube driving signals between the DC transformer modules are staggered and phase-shifted.

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