Resonant modular multi-level medium-voltage direct-current solid-state transformer and control method

Abstract

The invention relates to a resonance type modularized multi-level medium-voltage direct-current solid-state transformer and a control method, the transformer comprises an input end, an output end and a medium-frequency transformer connected with the input end and the output end, the input end comprises an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are formed by a plurality of sub-modules of half-bridge structures; a resonant cavity circuit is also arranged between the input end and the medium-frequency transformer and is used for providing a bypass channel for direct current of the input end and carrying out sine processing on alternating current flowing into the input end and the medium-frequency transformer; and the output end comprises an uncontrolled rectifying circuit formed by a plurality of diodes and is used for rectifying the induction current of the intermediate-frequency transformer into constant load current. According to the multi-level medium-voltage direct-current solid-state transformer, a bypass channel is provided for direct current through the resonant cavity circuit, and the current flowing through the power device and the medium-frequency transformer is shaped, so that the stability and the reliability of the multi-level medium-voltage direct-current solid-state transformer are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of direct current transformers, and in particular relates to a resonant modular multi-level medium-voltage direct current solid-state transformer and a control method. Background Art

[0002] The medium voltage DC power supply architecture is widely used in DC microgrids, charging stations and large-scale energy storage due to its advantages of high efficiency, low cost and high stability. The medium voltage DC converter is a key device in the medium voltage DC power supply architecture. Its function is to convert one medium voltage DC voltage into another medium voltage DC voltage to form a power supply bus, or convert it into low voltage DC for load use, while also achieving electrical isolation.

[0003] The traditional industrial frequency transformer is a passive component with a simple structure, but it can only achieve isolation and voltage conversion from AC to AC. In order to achieve isolated voltage conversion from DC to DC, the basic design idea is to first use power semiconductors to form the "DC-AC" power conversion link, then use a medium frequency (or high frequency) transformer for electrical isolation, and finally use the power conversion link to achieve "AC-DC" conversion. Since the converted AC is medium frequency (or high frequency) AC, the size of the AC transformer can be significantly reduced. In addition, since power semiconductor devices are used for power conversion, port voltage, current, power and other controls can be achieved.

[0004] In the field of low-voltage conversion, the Dual Active Bridge (DAB) is the main circuit to achieve the above functions. However, in the field of medium-voltage conversion, due to the limitation of the withstand voltage of a single device, a large number of DABs need to be operated in series. This solution has a large number of discrete transformers and needs to consider the coordinated operation between DABs, making the system more complex. The Modular Multilevel Conversion (MMC) technology is a structure that uses a bridge circuit with floating capacitors as the basic module and connects a large number of modules in series for power conversion. The Modular Medium DC (MMDC) constructed using MMC technology has the advantages of unified module control and the use of a single Medium Frequency Transformer (MFT) for isolation.

[0005] However, traditional MMDC has a large number of modules, which leads to high system cost, more control lines, and difficulty in further improving system power density. These shortcomings limit its use in some applications with strict requirements on cost and space.

[0006] Compact MMDC is an innovative power electronics technology that overcomes the above shortcomings. It achieves the goal of reducing the number of submodules and reducing costs and volume by optimizing the power loop in traditional MMDC. Under the same conversion requirements, the number of submodules required for compact MMC is only half of that of traditional MMDC, which brings obvious advantages such as halving the insulation distance between modules, halving the number of optical fibers between modules and controllers, halving the number of auxiliary power and water cooling lines, and halving the number of mechanical structural parts.

[0007] The main problem with compact MMCs is that there is a DC flux component when the medium-frequency transformer is working, which increases the maximum magnetic density of the core design and the core volume, resulting in asymmetric operation of the positive and negative half-cycles of the core, insufficient core utilization, and uneven magnetization. Improper control during operation will also cause the accumulation of DC flux, which may lead to saturation of the transformer core, which will eventually cause transformer vibration, heating and other problems. If not controlled, it may affect the long-term stability and reliability of the transformer. Eliminating the DC flux of compact MMDCs is a key technical challenge to improve the operating performance and reliability of compact MMDCs. Summary of the invention

[0008] In order to solve the stability and reliability problems of the compact MMC mentioned in the background technology, a resonant modular multi-level medium-voltage DC solid-state transformer is provided in the first aspect of the present invention, comprising an input end, an output end and an intermediate frequency transformer respectively connected to the input end and the output end, wherein the input end comprises an upper bridge arm and a lower bridge arm both formed by a plurality of half-bridge structure sub-modules; a resonant cavity circuit is also provided between the input end and the intermediate frequency transformer, wherein the resonant cavity circuit is used to provide a bypass channel for the DC current at the input end and to sinusoid the AC current flowing into the input end and the intermediate frequency transformer; the output end comprises an uncontrolled rectifier circuit composed of a plurality of diodes, which is used to rectify the induced current of the intermediate frequency transformer into a constant load current.

[0009] In some embodiments of the present invention, the resonant cavity circuit includes at least: a resonant inductor, a DC filter inductor, a resonant capacitor and an excitation inductor.

[0010] Furthermore, the resonant inductor and the resonant capacitor are both located on the trunk line of the resonant cavity circuit, and the DC filter inductor and the excitation inductor are located on the branch line.

[0011] Furthermore, the resonant inductor, the DC filter inductor, the resonant capacitor and the excitation inductor form an LLC circuit.

[0012] Furthermore, the DC filter inductor is located at the front end of the upper bridge arm or the lower bridge arm.

[0013] In some embodiments of the present invention, the DC filter inductor is used as a bridge arm inductor of an upper bridge arm or a lower bridge arm.

[0014] In some embodiments of the present invention, the constant load current is expressed as: , in I 0 means output current, i pri represents the current at the input terminal, n It represents the ratio of the number of coil turns at the output end to the number of coil turns at the input end. w s Indicates the phase of the voltage at the input terminal.

[0015] In some embodiments of the present invention, all submodules of the upper bridge arm are put into operation or bypassed at the same time, with a duty cycle of 50%.

[0016] In some embodiments of the present invention, each submodule is driven by a voltage balancing method.

[0017] The second aspect of the present invention provides a control method for a resonant modular multi-level medium-voltage DC solid-state transformer, comprising: driving each sub-module in the modular medium-voltage DC converter by a voltage equalization method in each fundamental wave cycle; the output current of all sub-modules passes through the resonance of the resonant cavity to achieve soft switching within a preset load range; the output current after soft switching passes through a medium-frequency transformer and is output to the load after uncontrolled rectification at the output end.

[0018] The beneficial effects of the present invention are: The present invention adds a series resonant cavity to the transformer path. The addition of the resonant cavity provides a bypass channel for DC current, so that DC current no longer flows through the transformer, thereby solving the DC bias problem; the resonant cavity "shapes" the current flowing through the power device and the intermediate frequency transformer, and the current is closer to a sine wave, so that the power device and the intermediate frequency transformer are at a better working point; under the action of the resonant cavity, the converter is also easier to achieve soft switching when operating under light load conditions. Soft switching significantly reduces switching losses, reduces electromagnetic interference, and improves operating efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A circuit diagram of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Figure 2 This is one of the specific circuit diagrams of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Figure 3 The second schematic diagram of a specific circuit of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Figure 4 The third schematic diagram of a specific circuit of a modal modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Figure 5 This is a fourth schematic diagram of a specific circuit of a modal modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Figure 6 is a circuit diagram of a submodule structure in some embodiments of the present invention; Figure 7 is a schematic diagram of an equivalent circuit of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Figure 8 A schematic diagram of a Thevenin equivalent circuit of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Fig. 9 Schematic diagram of an equivalent circuit of a controlled power supply of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Fig.10 Schematic diagram of an AC equivalent circuit of a resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention; Fig.11 is a schematic diagram of a secondary DC equivalent circuit in some embodiments of the present invention; Fig.12 is a schematic diagram of a primary DC equivalent circuit in some embodiments of the present invention; Fig.13 1 is a voltage gain curve under different parameter Q conditions in some embodiments of the present invention; Fig.14 One of the steady-state simulation waveforms of the resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention when it is fully loaded; Fig.15 The second steady-state simulation waveform of the resonant modular multi-level medium voltage DC solid-state transformer in some embodiments of the present invention when it is fully loaded; Fig.16 The waveforms of bridge arm voltage and bridge arm current of a resonant modular multi-level medium voltage DC solid-state transformer in no-load state in some embodiments of the present invention; Fig.17 1 and 2 are steady-state bridge arm voltage and bridge arm current waveforms of a resonant-cavity-free MMDC when fully loaded in some embodiments of the present invention.

[0020] Reference numerals 1.MMC submodule; 2.Resonant inductor; 3.DC filter inductor; 4.Resonant capacitor; 5.Excitation inductor; 6.Intermediate frequency transformer; 7.Diode; 8.DC voltage stabilizing capacitor; 9.Secondary side; 10.Primary side; 11.Resonant cavity circuit. DETAILED DESCRIPTION

[0021] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0022] refer to Figures 1 to 5 In the first aspect of the present invention, a resonant modular multi-level medium voltage DC solid-state transformer is provided, comprising an input terminal 10, an output terminal 9, and an intermediate frequency transformer 6 connected to the input terminal 10 and the output terminal 9 respectively, wherein the input terminal 10 comprises an upper bridge arm and a lower bridge arm both formed by a plurality of half-bridge structure submodules 1; a resonant cavity circuit 11 is further provided between the input terminal and the intermediate frequency transformer, wherein the resonant cavity circuit 11 is used to provide a bypass channel for the DC current of the input terminal 10, and to sinusoid the AC current flowing into the input terminal 10 and the intermediate frequency transformer 6; the output terminal 9 comprises an uncontrolled rectifier circuit composed of a plurality of diodes, which is used to rectify the inductive current of the intermediate frequency transformer into a constant load current. The resonant cavity circuit 11 comprises: a resonant inductor 2, a DC filter inductor 3, a resonant capacitor 4, and an excitation inductor 5. The output terminal 9 comprises four diodes 7 and a DC voltage stabilizing capacitor 8.

[0023] It can be understood that for the transformer, the input end corresponds to the primary side or primary coil, and the output end corresponds to the secondary side or secondary coil; the ratio of the number of turns of the primary coil to the number of turns of the secondary coil is also called the transformation ratio. In circuit design, the primary side and the secondary side are two key components, which are connected through a medium frequency transformer (MFT). This circuit structure not only realizes the conversion from medium voltage to low voltage, but also ensures that the withstand voltage of a single device does not exceed the rated value. In the converter of the present invention, since there is also a resonant cavity circuit between the primary side and the medium frequency transformer, the circuit provides a path for the DC current component, solves the DC bias problem of the medium frequency transformer MFT, and at the same time makes the AC current sinusoidal, which is convenient for soft switching.

[0024] In some embodiments of the present invention, the input end 10 (primary side) is composed of an upper bridge arm, a resonant cavity, and a lower bridge arm. The upper and lower bridge arms are each composed of N sub-modules SMn connected in series. Sub-module 1 is a half-bridge structure (it can also be a more complex structure). The AC port of the half-bridge sub-module generates 0 and V cap According to Kirchhoff's voltage law, when the voltage of all submodules is 0, the AC port voltage V between the upper and lower bridge arms is ab is the voltage Vm. When the voltage of all submodules is V cap When the voltage between the upper and lower bridge arms is V abBy switching on and off the submodules, the primary side realizes the voltage conversion from medium voltage DC to medium voltage AC. The medium frequency transformer MFT realizes the transformation of the AC voltage level between the primary side and the secondary side (such as n:1) and energy transmission.

[0025] refer to Figures 2 to 5 In some embodiments of the present invention, the resonant cavity circuit 11 at least includes: a resonant inductor 2, a DC filter inductor 3, a resonant capacitor 4 and an excitation inductor 5.

[0026] Specifically, the resonant cavity circuit consists of a resonant inductor L r , DC filter inductor L dc , resonant capacitor C r One end of the resonant cavity is connected to the upper and lower bridge arms, and the other end is connected to MFT The primary side of the capacitor is connected. C r Series medium frequency transformer MFT . Resonant Inductor L r and DC filter inductor L dc Equivalent to inductance L eq .inductance L eq , resonant capacitor C r and magnetizing inductance L m Equivalent to LLC structure.

[0027] Flow through the resonant inductor L r The current in the circuit has both direct current and alternating current components. L dc For DC, it is a short circuit and the capacitor C r For DC, it is an open circuit, so the DC current is L dc Completely bypassed, it will not flow through the transformer MFT, which avoids the DC bias problem of the transformer.

[0028] Understandably, Figures 3 to 5 An improvement of the above embodiment is shown, where the DC filter inductor is located at the front end of the upper bridge arm or the lower bridge arm; or, the DC filter inductor is used as the bridge arm inductor of the upper bridge arm or the lower bridge arm; in this way, the number of components of the resonant modular multi-level medium voltage DC solid-state transformer can be further simplified and the cost can be reduced.

[0029] The high-voltage side bridge arm submodule uses all submodules of the same bridge arm to be put into operation and bypassed at the same time, with a duty cycle of 50%. Considering the average effect of a fundamental wave cycle, the average value of the capacitor voltage of the submodule satisfies: ,in, is the average value of the submodule capacitor voltage, M is the total number of submodules connected in series in the primary MMC bridge arm. The submodule voltage equalization method here adopts a rotating modulation method, and the drive is given to each submodule in turn. Assuming that after adopting this voltage equalization method, the voltage of all submodules can be equalized, ; The bridge arm voltage is: , refer to Fig. 9 , the bridge arm voltage contains common-mode DC and medium-frequency AC components, based on which the equivalent circuit of the controlled power supply of the topology can be obtained as follows: Fig.10 As shown. It can be seen that the circuit contains two types of voltage sources, DC and medium frequency AC. Therefore, the voltage applied to the resonant cavity is The amplitude is , AC voltage square wave with a duty cycle of 50%. The traditional LLC resonant converter has good soft switching characteristics and has high regulation accuracy and efficiency in the full power range. Similar to the LLC topology, the RMMDC also applies an AC square wave voltage to the resonant cavity, so the traditional LLC analysis method, such as the fundamental wave analysis method, can be used for analysis in the RMMDC topology.

[0030] The final AC equivalent circuit is as follows Fig.10 As shown, it can be seen that compared with the traditional LLC resonant cavity, this resonant cavity has an additional DC filter inductor . Continue to refer to Fig.11 and Fig.12 It can be seen from the primary side equivalent circuit that the DC filter inductor provides a DC path, and the DC current does not pass through the medium frequency transformer MFT, thus solving the DC bias problem of the compact MMDC.

[0031] On the other hand, this circuit increases the resonant inductance and resonant capacitor Passive components, and magnetizing inductance Together they form an LLC resonant cavity. The LLC resonant cavity achieves soft switching under light load and no-load conditions, which improves the efficiency and reliability of the circuit compared to the traditional DAB-type MMDC.

[0032] Since the secondary side is uncontrolled rectifier, the output current is rectified to obtain a constant load current. , whose expression is: , in I 0 means output current, ipri represents the current at the input terminal, n It represents the ratio of the number of coil turns at the output end to the number of coil turns at the input end. w s Indicates the phase of the voltage at the input terminal.

[0033] Fundamental component of transformer primary voltage for: , It can be seen from the above formula that if the primary voltage and current are in phase, the entire secondary circuit can be equivalent to a pure resistive load. , its expression is as follows: , refer to Figure 7 , which shows the equivalent circuit diagram of RMMDC, according to Thevenin equivalence theorem, Figure 7 The equivalent circuit can be equivalent to Figure 8 ; Therefore, the definition k is the magnetizing inductance With equivalent inductance The ratio, Q is the quality factor under equivalent inductance, P is the ratio of DC filter inductance to resonant inductance: , , , , The transfer function of the circuit is: , The voltage gain can be obtained M The expression is: .

[0034] refer to Fig.13 When the switching frequency is to the right of the resonant peak frequency, it is in the inductive range, that is, it can be in the soft switching range, and the high-voltage side MMC can achieve soft switching. Q As it gets larger, the peak gain becomes smaller, but the light-load modulation phenomenon will be alleviated, that is, the slope of the curve on the right side of the resonant frequency increases.

[0035] In some embodiments of the present invention, the output end includes an uncontrolled rectifier circuit composed of multiple diodes, which is used to rectify the induced current of the medium frequency transformer into a constant load current.

[0036] Specifically, the secondary side (output end) is composed of diodes D1-D4. Diodes D1-D4 form an uncontrolled rectifier. The uncontrolled rectifier converts AC power into DC power. The DC port capacitor Cdc realizes the voltage stabilization of the low-voltage DC port.

[0037] Example 2 In a second aspect of the present invention, a control method for a resonant modular multi-level medium voltage DC solid-state transformer is provided, comprising: S100. Drive each submodule in the modular medium voltage DC converter by voltage balancing method in each fundamental wave cycle; S200. The output current of all submodules passes through the resonance of the resonant cavity to achieve soft switching within the preset load range; S300. The output current after soft switching passes through the medium frequency transformer and is output to the load after uncontrolled rectification at the output end.

[0038] In order to verify the basic principle of RMMC, a simulation model of RMMC was built in PLCES. Assuming that the circuit operation background is that the input medium voltage 2600V supplies power to 800V users, P =4, when the resonant frequency is equal to the switching frequency, the gain is constant at 4 / 5, so the transformer turns ratio can be selected as 2.6:1. Take the full load power as 64kW, then the load R is 10 ohms. According to the gain curve, select the quality factor Q =0.3, it can be seen that the gain curve works in the inductive range above the normalized angular frequency of 0.5. The resonant frequency is selected as 30kHz, so the resonant inductance can be obtained L r 100uH, resonant capacitor C r is 0.2814uF. P =4, so the DC filter inductor is 400uH, and the excitation inductor is selected to be 10 times the resonant inductor, that is, 1mH, to ensure soft switching and good transmission of resonant energy to the secondary side. The number of submodules is selected to be 10 half-bridge submodules in series. The specific circuit parameters are shown in Table 1.

[0039] Table 1 RMMDC circuit simulation parameters

[0040] It can be seen that the resonant modular multi-level medium voltage DC solid-state transformer provided by the present invention can filter out the DC current component and solve the problem of DC bias magnetism.

[0041] Reference Fig.14 The primary bridge arm voltage, primary resonant current, secondary current and voltage applied to the resonant cavity are given. Vab It can be seen that the primary resonant current, i.e. the bridge arm current, contains a DC component. Through the Scope measurement in the simulation, we can get 25.87A, which is exactly the DC current on the medium voltage side corresponding to the transmitted active power, i.e. At the same time, it can be seen that when the bridge arm submodules are put into operation at the same time, that is, when the upper tube in the submodule is turned on, the bridge arm current is positive at this time, so the soft switching of the upper tube can be realized. When the bridge arm voltage is 0, that is, all submodules are turned off at the same time, the bridge arm current is negative at this time, and the soft switching of the lower tube can be realized.

[0042] like Fig.15 As shown, from top to bottom are the resonant capacitors Cr The voltage of the resonant capacitor Cr Current and magnetizing inductance L m Current, DC filter inductance L dc Current, DC filter inductor L dc The voltage of the bridge arm current is the resonant inductance L r The current and the transformer secondary voltage. From the measured DC component, it can be seen that the resonant capacitor C r The current does not contain a DC component, and the DC filter inductor L dc Current and resonant inductance L r The current has a DC component of 25.89A. Therefore, it can be seen that the DC filter inductor in the resonant cavity can provide a DC circuit for the primary MMC, and the DC component will not flow into the transformer (here the resonant capacitor C r The DC component of the current is reflected in the topology, avoiding the inherent DC bias defect of this topology.

[0043] Verification 2: Soft switching under light load / no load can be achieved When no load is applied, the load is 1000 ohms. The simulation results are as follows: Fig.16 As shown. From top to bottom are the bridge arm voltage, bridge arm current, secondary current, V ab Voltage. It can be seen that when the bridge arm submodule is put into use, the bridge arm current is positive and soft switching can still be achieved.

[0044] like Fig.17 As shown in the figure, it shows the topological simulation waveform without the resonant cavity structure, which is the bridge arm voltage, bridge arm current, and secondary current from top to bottom. It can be seen that when the bridge arm submodule is cut off, the bridge arm current will increase from negative to positive. At this time, the lower tube of some submodules cannot achieve soft switching. Fig.12After comparison, it can be seen that the RMMDC structure proposed in this paper can realize the soft switching of the submodule. At the same time, it can be intuitively known from the waveform that the bridge arm current contains a DC component, and this bridge arm current will also directly flow into the primary side of the transformer, causing the transformer to generate DC bias magnetism.

[0045] It can be seen from the above simulation waveforms that the topology of the RMMDC proposed in this paper can solve the DC bias problem of the compact MMDC, while making the bridge arm current approximately sinusoidal and realizing soft switching in the full load range.

[0046] The flow chart and block diagram in the accompanying drawings illustrate the possible architecture, function and operation of the system, method and computer program product according to various embodiments of the present disclosure. In this regard, each box in the flow chart or block diagram can represent a module, a program segment or a part of a code, and the module, the program segment or a part of the code contains one or more executable instructions for realizing the specified logical function. It should also be noted that in some alternative implementations, the functions marked in the box can also occur in a different order from the order marked in the accompanying drawings. For example, two boxes represented in succession can actually be executed substantially in parallel, and they can sometimes be executed in the opposite order, depending on the functions involved. It should be noted that each box in the block diagram and / or flow chart, and the combination of the boxes in the block diagram and / or flow chart can be implemented with a dedicated hardware-based system that performs a specified function or operation, or can be implemented with a combination of dedicated hardware and computer instructions.

[0047] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A resonant modular multi-level medium voltage DC solid-state transformer, comprising an input end, an output end, and an intermediate frequency transformer connected to the input end and the output end respectively, characterized in that: The input end includes an upper bridge arm and a lower bridge arm, both of which are formed by a plurality of sub-modules of a half-bridge structure; A resonant cavity circuit is also provided between the input end and the intermediate frequency transformer, and the resonant cavity circuit is used to provide a bypass channel for the direct current of the input end, and to sinusoid the alternating current flowing into the input end and the intermediate frequency transformer; The output end includes an uncontrolled rectifier circuit composed of multiple diodes, which is used to rectify the induced current of the medium frequency transformer into a constant load current.

2. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 1, characterized in that: The resonant cavity circuit at least includes: a resonant inductor, a DC filter inductor, a resonant capacitor and an excitation inductor.

3. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 2, characterized in that: The resonant inductor and the resonant capacitor are both located on the main circuit of the resonant cavity circuit, and the DC filter inductor and the excitation inductor are located on the branch circuit.

4. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 3, characterized in that: The resonant inductor, the DC filter inductor, the resonant capacitor and the excitation inductor form an LLC circuit.

5. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 2, characterized in that: The DC filter inductor is located at the front end of the upper bridge arm or the lower bridge arm.

6. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 1, characterized in that: The DC filter inductor is used as a bridge arm inductor of an upper bridge arm or a lower bridge arm.

7. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 1, characterized in that: The constant load current is expressed as: ， in I 0 means output current, i pri represents the current at the input terminal, n It represents the ratio of the number of coil turns at the output end to the number of coil turns at the input end. w s Indicates the phase of the voltage at the input terminal.

8. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 1, characterized in that: All submodules of the upper bridge arm are put into operation or bypassed at the same time, with a duty cycle of 50%.

9. The resonant modular multi-level medium voltage DC solid-state transformer according to claim 1 or 8, characterized in that: Each submodule is driven by voltage balancing method.

10. A control method for a resonant modular multi-level medium voltage DC solid-state transformer, characterized in that: include: Driving each submodule in the modular medium voltage DC converter by voltage balancing method in each fundamental wave cycle; The output current of all submodules passes through the resonance of the resonant cavity to achieve soft switching within the preset load range; The output current after soft switching passes through the medium frequency transformer and is output to the load after uncontrolled rectification at the output end.

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