A single-stage multi-port high voltage DC transformer

Abstract

The application relates to the technical field of large-scale new energy direct current collection and transmission, and discloses a single-stage conversion multi-port high-voltage direct current transformer, which comprises a plurality of transformation units, each of which comprises a high-voltage bridge arm, a low-voltage bridge arm and a common bridge arm, the three bridge arms being composed of a plurality of half-bridge or full-bridge sub-modules and bridge arm inductors. The high-voltage bridge arms of all the transformation units are connected to the same high-voltage port, and the low-voltage bridge arms of the transformation units are connected to the respective low-voltage ports, so that direct current collection and voltage boosting from the low-voltage port to the high-voltage port are realized. Energy transmission and voltage conversion are realized through the plurality of transformation units and the bridge arms in each transformation unit, a single-stage conversion structure is formed, a heavy alternating current isolation transformer is not included, insulation and heat dissipation problems caused by the transformer are avoided, direct current-alternating current-direct current multi-stage conversion is not needed, conversion efficiency is improved, and engineering manufacturing difficulty and cost are reduced.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a single-stage conversion multi-port high-voltage DC transformer. Background Technology

[0002] With the continuous expansion of installed capacity of new energy sources such as photovoltaic and wind power, the large-scale collection and transmission of new energy has become an urgent problem to be solved. High-voltage direct current (HVDC) collection and transmission systems have significant advantages, including long transmission distances, large transmission capacity, high transmission efficiency, wide collection range, and no need for conventional AC power supply support. High-voltage, high-capacity DC transformers based on power electronics technology can realize large-scale DC collection and transmission of new energy, becoming a key piece of equipment in HVDC power transmission. Because DC voltage polarity is fixed, power can be rapidly reversed by controlling the current direction, making it suitable for forming multi-port DC transmission systems. Therefore, multi-port HVDC transformers have become an important part of the development of new power systems and have received widespread attention in recent years.

[0003] Currently, there are two main types of multi-port DC transformers that have been widely studied. The most common type is the multi-port transformer based on the cascaded H-bridges (CHB) structure, which consists of multiple cascaded H-bridge modules. The second type is the multi-port transformer based on the modular multilevel converter (MMC). Among them, the CBH multi-port transformer can use fewer power modules under the same voltage and power level, while the MMC multi-port transformer is more suitable for DC port applications. Reference [1] proposed the optimal topology of the modular multilevel multi-port DC substation applied to offshore DC wind farms (Proceedings of the Chinese Society for Electrical Engineering, 2016, 36(S1):61-68), which comprehensively considers factors such as turns ratio, capacity, and volume, but the engineering difficulty is relatively high. Reference [2] Synergetic Control of High-Frequency-Link Based Multi-Port Solid State Transformer (2018 IEEE Energy Conversion Congress and Exposition (ECCE) ), Portland, OR, USA, 2018, pp. 5630-5635) proposes a multi-port transformer structure with a common low-voltage high-frequency link using an MMC structure on the high-voltage side and a CHB structure on the low-voltage side. This reduces the design and engineering application difficulty of the transformer and has strong scalability, but the total cost is relatively high. Reference [3] Multi-port DC-DC Autotransformer (Proceedings of the CSEE, 2015, 35(3): 727-734) proposes a multi-port DC transformer analogous to an AC autotransformer. Part of the energy is converted in a single stage, but the transformer needs to carry DC bias, which increases the engineering difficulty. Summary of the Invention

[0004] To address the problems in the prior art, the present invention provides a single-stage transformation multi-port high-voltage DC transformer, which includes multiple transformer units with identical structures;

[0005] Each transformer unit includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm. One side of each high-voltage bridge arm, low-voltage bridge arm, and common bridge arm is connected to the midpoint M. The other side of the high-voltage bridge arm is connected to the high-voltage port, the other side of the low-voltage bridge arm is connected to the low-voltage port, and the other side of the common bridge arm is grounded.

[0006] The low-voltage bridge arm is composed of N1 full-bridge sub-modules and 1 bridge arm inductor connected in series; the common bridge arm is composed of N2 half-bridge sub-modules and 1 bridge arm inductor connected in series; and the high-voltage bridge arm is composed of N3 half-bridge sub-modules and 1 bridge arm inductor connected in series, wherein N1≥2, N2≥2, and N3≥2.

[0007] The N1st full-bridge submodule of the low-voltage bridge arm of each transformer unit is connected to the low-voltage port, and the first full-bridge submodule is connected to the bridge arm inductor and then to the midpoint M; the first half-bridge submodule of the common bridge arm is connected to the bridge arm inductor and then to the midpoint M, and the N2nd half-bridge submodule is grounded; the first half-bridge submodule of the high-voltage bridge arm is connected to the high-voltage port, and the N3rd half-bridge submodule is connected to the bridge arm inductor and then to the midpoint M.

[0008] The high-voltage arms of all transformer units are connected to the same high-voltage port, while the low-voltage arms of each transformer unit are connected to their respective low-voltage ports.

[0009] On the other hand, the present invention also provides a voltage modulation method based on the DC transformer, comprising:

[0010] 1) Set the high-voltage port voltage to V H The low-voltage port voltages are V L_1 V L_2 ...V L_n The high-voltage port power requirement is P. H The low-voltage port powers are P1, P2...P n P H =P1+P2+…+P n The subscript n represents the total number of transformer units.

[0011] 2) Based on the port voltage and power requirements set above, for any transformer unit, the bridge arm voltage requirements of each bridge arm are obtained in each control cycle T; the bridge arm voltage v1 of the low-voltage bridge arm is a sinusoidal AC voltage:

[0012] v1 = V 0_n sin(ωt)

[0013] Among them, V 0_n This represents the amplitude of the sinusoidal AC voltage of the transformer unit. t represents time;

[0014] The bridge arm voltage v2 of the common arm is a DC voltage superimposed with a sinusoidal AC voltage:

[0015] v2=V L_n +V 0_n sin(ωt)

[0016] The high-voltage bridge arm voltage v3 is a DC voltage superimposed with a sinusoidal AC voltage:

[0017] v3 = V H -V L_n -V 0_n sin(ωt)

[0018] By adjusting the number of sub-modules deployed in each bridge arm, each bridge arm can meet the aforementioned bridge arm voltage requirements.

[0019] 3) To ensure stable capacitor voltage in each bridge arm's submodule, based on step 2), a PI controller is used to change the switching of some submodules in each bridge arm, thereby controlling the current in each bridge arm; ensuring that the low-voltage bridge arm current i1 is a DC current.

[0020] i1 = I L _ n

[0021] Among them, I L_n The low-voltage port current of this transformer unit is:

[0022] I L_n =P n / V L_n

[0023] The bridge arm current i2 of the common bridge arm is a DC current superimposed with a sinusoidal AC current:

[0024] i2 = I L_n -I H_n -I0sin(ωt)

[0025] Among them, I H_n The high-voltage port current of this transformer unit is:

[0026] I H_n =P n / V H

[0027] The high-voltage bridge arm current i3 is a DC current superimposed with a sinusoidal AC current:

[0028] i3 = I H_n +I0sin(ωt)

[0029] The amplitude I0 of the sinusoidal AC current of the transformer is determined by any selected reference transformer unit and should satisfy the following formula:

[0030]

[0031] Where V L_base P is the low-voltage port voltage of the selected reference transformer unit. base V represents the port power of the selected reference transformer unit. 0_baseThe AC voltage amplitude of the selected reference transformer unit should satisfy the following conditions:

[0032] V 0_base ≤min(V L V H -V L )

[0033] In addition to the selected reference transformer unit, the sinusoidal AC voltage amplitude V of each transformer unit 0_n The following formula should be satisfied:

[0034]

[0035] The selected reference transformer unit should guarantee the sinusoidal AC voltage amplitude V of each transformer unit. 0_n satisfy:

[0036] V 0_n ≤min(V L V H -V L )

[0037] Where min(a, b) represents the minimum value between a and b.

[0038] 4) When the bridge arm voltage and bridge arm current meet the above requirements, the DC transformer can realize the voltage transformation from low-voltage port voltage to high-voltage port voltage or the high-voltage port voltage to low-voltage port voltage, and at the same time realize the power collection from multiple low-voltage ports and transmission to the high-voltage port, or the power distribution from the high-voltage port to multiple low-voltage ports.

[0039] Based on the above technical solution, the beneficial effects of the present invention compared with the prior art include:

[0040] 1) The multi-port DC transformer described in this invention is a single-stage conversion topology, which eliminates the bulky isolation transformer and large energy storage inductor, reduces insulation and heat dissipation requirements, thereby achieving lightweight and low cost;

[0041] 2) The multi-port DC transformer described in this invention is easy to control, and the multiple ports are independent of each other, which facilitates decoupling control and greatly improves engineering feasibility;

[0042] 3) The power supply described in this invention avoids the technical challenges of high-voltage devices or devices connected in series by using sub-module cascading technology, and reduces the voltage stress bearing requirements of individual devices. Attached Figure Description

[0043] Figure 1 This is a circuit topology diagram of the single-stage conversion multi-port high-voltage DC transformer of the present invention;

[0044] Figure 2This is a circuit topology diagram of a half-bridge sub-module according to an embodiment of the present invention;

[0045] Figure 3 This is a circuit topology diagram of a full-bridge submodule according to an embodiment of the present invention;

[0046] Figure 4 This is a circuit topology diagram of an embodiment of the present invention. This embodiment includes three transformer units, which are connected to one high-voltage port and three low-voltage ports.

[0047] Figure 5 In one embodiment of the present invention, the current ports 1, 2, and 3 of the high-voltage and low-voltage ports respectively represent the conversion units 1, 2, and 3 where ports 1, 2, and 3 are located;

[0048] Figure 6 The voltage and current waveforms of the low-voltage bridge arm according to an embodiment of the present invention are shown below.

[0049] Figure 7 The voltage and current waveforms of the common bridge arm according to an embodiment of the present invention are shown below.

[0050] Figure 8 The voltage and current waveforms of the high-voltage bridge arm are shown in an embodiment of the present invention. Detailed Implementation

[0051] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0052] Given that the isolation transformers in existing multi-port DC transformers introduce heat dissipation and insulation problems, and that the topologies are costly and difficult to engineer, this invention provides a single-stage conversion multi-port high-voltage DC transformer.

[0053] like Figure 1 The diagram shows the circuit topology of a single-stage multi-port high-voltage DC transformer provided in this embodiment. This transformer can be used for large-scale new energy collection and transmission, and it includes n identical transformer units, where n≥2. Figure 1 One transformer unit was selected through bidding. Each transformer unit includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm. The high-voltage bridge arms of n transformer units are connected to the same high-voltage port, and the n low-voltage bridge arms are connected to n low-voltage ports. FBSM is a full-bridge submodule, and HBSM is a half-bridge submodule.

[0054] One side of each of the high-voltage bridge arm, low-voltage bridge arm, and common bridge arm is connected to the midpoint M. The other side of the high-voltage bridge arm is connected to the high-voltage port, the other side of the low-voltage bridge arm is connected to the low-voltage port, and the other side of the common bridge arm is grounded. The low-voltage bridge arm is composed of N1 full-bridge sub-modules connected in series with one bridge arm inductor. The common bridge arm is composed of N2 half-bridge sub-modules connected in series with one bridge arm inductor. The high-voltage bridge arm is composed of N3 half-bridge sub-modules connected in series with one bridge arm inductor. Wherein, N1≥2, N2≥2, and N3≥2.

[0055] The N1st full-bridge submodule of the low-voltage bridge arm of each transformer unit is connected to the low-voltage port, and the first full-bridge submodule is connected to the bridge arm inductor and then to the midpoint M; the first half-bridge submodule of the common bridge arm is connected to the bridge arm inductor and then to the midpoint M, and the N2nd half-bridge submodule is grounded; the first half-bridge submodule of the high-voltage bridge arm is connected to the high-voltage port, and the N3rd half-bridge submodule is connected to the bridge arm inductor and then to the midpoint M.

[0056] like Figure 2 The diagram shows the circuit topology of a half-bridge submodule (HBSM) in a specific embodiment of the present invention. The half-bridge submodule circuit contains two IGBT power devices (upper and lower) and an energy storage capacitor. During one switching cycle, the drive signals of the upper and lower switching transistors of each half-bridge submodule are complementary.

[0057] like Figure 3 The diagram shows the circuit topology of a full-bridge submodule (FBSM) in a specific embodiment of the present invention. The full-bridge submodule circuit contains four IGBT power devices and one energy storage capacitor. The four IGBTs, from top to bottom and left to right, are upper switch a, upper switch b, lower switch a, and lower switch b. Within one switching cycle, each full-bridge submodule in the cascaded valve string of port arms 1 and 2 has three operating states. When the voltage between the upper and lower ports of the full-bridge submodule is positive, the drive signals of upper switch a and lower switch b are 1, and the drive signals of other switches are 0; when the voltage between the upper and lower ports of the full-bridge submodule is negative, the drive signals of upper switch b and lower switch a are 1, and the drive signals of other switches are 0; when the voltage between the upper and lower ports of the full-bridge submodule is zero, the drive signals of upper switch a and lower switch a are 1 and the drive signals of other switches are 0, or the drive signals of upper switch b and lower switch b are 1 and the drive signals of other switches are 0.

[0058] The DC transformer of this invention can realize voltage transformation from low-voltage port voltage to high-voltage port voltage or vice versa, while simultaneously enabling power to be collected from multiple low-voltage ports and transmitted to the high-voltage port, or power to be distributed from the high-voltage port to multiple low-voltage ports. The voltage modulation method of the DC transformer of this invention is described below, including:

[0059] 1) First, based on the specific application scenario, set the high-voltage port voltage to V. H The low-voltage port voltages are V L_1 V L_2 ...V L_n The high-voltage port power requirement is P. H The power requirements for the low-voltage ports are P1, P2...P n P H =P1+P2+…+P n ;

[0060] 2) Based on the port voltage and power requirements set above, for any transformer unit, the bridge arm voltage requirements of each bridge arm are obtained in each control cycle T; the bridge arm voltage requirement v1 of the low-voltage bridge arm is a sinusoidal AC voltage:

[0061] v1 = V 0_n sin(ωt)

[0062] Among them, V 0_n This represents the amplitude of the sinusoidal AC voltage of the transformer unit. t represents time;

[0063] The common arm's arm voltage requirement v2 is a DC voltage superimposed with a sinusoidal AC voltage:

[0064] v2=V L_n +V 0_n sin(ωt)

[0065] The high-voltage bridge arm voltage requirement V3 is a DC voltage superimposed with a sinusoidal AC voltage:

[0066] v3 = V H -V L_n -V 0_n sin(ωt)

[0067] By adjusting the number of sub-modules deployed in each bridge arm, each bridge arm can meet the aforementioned bridge arm voltage requirements.

[0068] 3) To ensure stable capacitor voltage in each bridge arm's submodule, based on step 2), a PI controller is used to change the switching of some submodules in each bridge arm, thereby controlling the current in each bridge arm; ensuring that the low-voltage bridge arm current i1 is a DC current.

[0069] i1 = I L_n

[0070] Among them, I L_n The low-voltage port current of this transformer unit is:

[0071] I L_n =P n / V L_n

[0072] The bridge arm current i2 of the common bridge arm is a DC current superimposed with a sinusoidal AC current:

[0073] i2 = I L_n -I H_n -I0sin(ωt)

[0074] Among them, I H_n The high-voltage port current of this transformer unit is:

[0075] I H_n =P n / V H

[0076] The high-voltage bridge arm current i3 is a DC current superimposed with a sinusoidal AC current:

[0077] i3 = I H_n +I0sin(ωt)

[0078] The amplitude I0 of the sinusoidal AC current of the transformer is determined by any selected reference transformer unit and should satisfy the following formula:

[0079]

[0080] Where V L_base P is the low-voltage port voltage of the selected reference transformer unit. base V represents the port power of the selected reference transformer unit. 0_base The AC voltage amplitude of the selected reference transformer unit should satisfy the following conditions:

[0081] V 0_base ≤min(V L V H -V L )

[0082] In addition to the selected reference transformer unit, the sinusoidal AC voltage amplitude V of each transformer unit 0_n The following formula should be satisfied:

[0083]

[0084] The selected reference transformer unit should guarantee the sinusoidal AC voltage amplitude V of each transformer unit. 0_n satisfy:

[0085] V 0_n ≤min(V L V H -V L )

[0086] Where min(a, b) represents the minimum value between a and b.

[0087] 4) When the bridge arm voltage and bridge arm current meet the above requirements, the DC transformer can realize the voltage transformation from low-voltage port voltage to high-voltage port voltage or the high-voltage port voltage to low-voltage port voltage, and at the same time realize the power collection from multiple low-voltage ports and transmission to the high-voltage port, or the power distribution from the high-voltage port to multiple low-voltage ports.

[0088] In the DC transformer of this invention, there should be a phase difference between the AC currents of any two transformer units.

[0089]

[0090] Where n≥2, is the number of transformer units in the DC transformer.

[0091] like Figure 4 The diagram shown is a topology circuit diagram of a specific embodiment of the present invention. This embodiment's single-stage conversion multi-port high-voltage DC transformer consists of three transformer units. Taking one transformer unit as an example, the directions of the bridge arm voltage and bridge arm current are indicated. The high-voltage port voltage V0 is shown below. H The voltage is 300kV, and the low-voltage port 1 voltage is V. L1 The voltage is 100kV, the power P1 is 0.3GW, and the voltage at low-voltage port 2 is V. L2 The voltage is 150kV, the power P2 is 0.3GW, and the low-voltage port 3 voltage is V. L3 It is 120kV, and the power T3 is 0.2GW.

[0092] like Figure 5 The figure shows the port current waveform of this embodiment, where the low-voltage port 1 current i L1 -3kA, low-voltage port 2 current i L2 -2kA, low-voltage port 3 current i L3 The current at the high-voltage port is -1.67kA, i. H With a voltage rating of -2.67kA, it enables DC-DC conversion and the collection and transmission of energy from the low-voltage port to the high-voltage port.

[0093] Selecting transformer unit 1 as the reference transformer unit, its AC voltage amplitude is set to -80kV, AC frequency to 200Hz (i.e., period T is 5ms). The calculated AC current amplitude of the transformer is -5kA, the AC voltage amplitude of transformer unit 2 is -60kV, and the AC voltage amplitude of transformer unit 3 is -48kV. Further, the voltage and current waveforms of each bridge arm can be obtained. For example... Figure 6 The diagram shows the voltage and current waveforms of the low-voltage bridge arm. Figure 7 The diagram shows the voltage and current waveforms of the common bridge arm. Figure 8 The diagram shows the voltage and current waveforms of the high-voltage bridge arm. (From...) Figure 6 As can be seen, the low-voltage bridge arm voltage and current of transformer unit 1 are as follows:

[0094] v 1_1 = -80000sin(400πt)

[0095] i 1_1 =-3000

[0096] The low-voltage bridge arm voltage and current of transformer unit 2 are as follows:

[0097]

[0098] i 12 =-2000

[0099] The low-voltage bridge arm voltage and current of transformer unit 3 are as follows:

[0100]

[0101] It meets the low-voltage bridge arm voltage and current requirements of each transformer unit.

[0102] Depend on Figure 7 As can be seen, the common arm voltage and current of transformer unit 1 are as follows:

[0103] v2_1=100000-80000sin(400πt)

[0104] i 21 =2000+5000sin(400πt)

[0105] The common arm voltage and current of transformer unit 2 are as follows:

[0106]

[0107] The common arm voltage and current of transformer unit 3 are as follows:

[0108]

[0109] It meets the common arm voltage and current requirements of each transformer unit.

[0110] Depend on Figure 8 As can be seen, the voltage and current of the high-voltage bridge arm of transformer unit 1 are as follows:

[0111] v3_1=200000+80000sin(400πt)

[0112] i3_1 = -1000 + 5000sin(400πt)

[0113] The voltage and current of the high-voltage bridge arm of transformer unit 2 are as follows:

[0114]

[0115] The voltage and current of the high-voltage bridge arm of transformer unit 3 are as follows:

[0116]

[0117]

[0118] It meets the voltage and current requirements of the high-voltage bridge arm of each transformer unit.

[0119] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A voltage modulation method for a multi-port high-voltage DC transformer based on a single-stage conversion, characterized in that, The single-stage conversion multi-port high-voltage DC transformer includes multiple transformer units with identical structures. Each transformer unit includes a high-voltage bridge arm, a low-voltage bridge arm, and a common bridge arm. One side of each high-voltage bridge arm, low-voltage bridge arm, and common bridge arm is connected to the midpoint M. The other side of the high-voltage bridge arm is connected to the high-voltage port, the other side of the low-voltage bridge arm is connected to the low-voltage port, and the other side of the common bridge arm is grounded. The low-voltage bridge arm is composed of N1 full-bridge submodules connected in series with one bridge arm inductor; the common bridge arm is composed of N2 half-bridge submodules connected in series with one bridge arm inductor; and the high-voltage bridge arm is composed of N3 half-bridge submodules connected in series with one bridge arm inductor. Wherein, N1 2, N2 2, N3 2; The N1st full-bridge submodule of the low-voltage bridge arm of each transformer unit is connected to the low-voltage port, and the first full-bridge submodule is connected to the bridge arm inductor and then to the midpoint M; the first half-bridge submodule of the common bridge arm is connected to the bridge arm inductor and then to the midpoint M, and the N2nd half-bridge submodule is grounded; the first half-bridge submodule of the high-voltage bridge arm is connected to the high-voltage port, and the N3rd half-bridge submodule is connected to the bridge arm inductor and then to the midpoint M. The high-voltage bridge arm of all transformer units is connected to the same high-voltage port, and the low-voltage bridge arm of each transformer unit is connected to its own low-voltage port. The voltage modulation method includes the following steps: 1) Based on the actual application scenario, set the high-voltage port voltage as follows: The low-voltage port voltages are respectively , … The power requirement of the high-voltage port is The power requirements for the low-voltage ports are as follows: , … , The subscript n represents the total number of transformer units. 2) Based on the aforementioned port voltage and power requirements, for any transformer unit, within each control cycle T, the arm voltage requirements of each bridge arm are obtained; the arm voltage of the low-voltage bridge arm is... For a sinusoidal alternating voltage: ； in, This represents the amplitude of the sinusoidal AC voltage of the transformer unit. , For time; Common arm voltage A DC voltage is superimposed with a sinusoidal AC voltage: ； High-voltage bridge arm voltage A DC voltage is superimposed with a sinusoidal AC voltage: ； By adjusting the number of sub-modules deployed in each bridge arm, each bridge arm can meet the aforementioned bridge arm voltage requirements. 3) To ensure stable capacitor voltage in each bridge arm's submodules, based on step 2), a PI controller is used to change the switching of some submodules in each bridge arm, thereby controlling the current in each bridge arm; this ensures that the bridge arm current of the low-voltage bridge arm is stable. For a direct current: ； in, The low-voltage port current of this transformer unit is: ； Bridge arm current of common bridge arm A direct current superimposed with a sinusoidal alternating current: ； in, The high-voltage port current of this transformer unit is: ； Bridge arm current of high voltage bridge arm A direct current superimposed with a sinusoidal alternating current: ； The amplitude of the sinusoidal alternating current of the transformer Determined by any selected reference transformer unit, it should satisfy the following formula: ； in The low-voltage port voltage of the selected reference transformer unit. The port power of the selected reference transformer unit. The AC voltage amplitude of the selected reference transformer unit should satisfy the following conditions: ； In addition to the selected reference transformer unit, the sinusoidal AC voltage amplitude of each transformer unit The following formula should be satisfied: ； The selected reference transformer unit should guarantee the sinusoidal AC voltage amplitude of each transformer unit. satisfy: ； in express and The minimum value in; 4) When the bridge arm voltage and bridge arm current meet the above requirements, the DC transformer can realize the voltage transformation from low-voltage port voltage to high-voltage port voltage or the high-voltage port voltage to low-voltage port voltage, and at the same time realize the power collection from multiple low-voltage ports and transmission to the high-voltage port, or the power distribution from the high-voltage port to multiple low-voltage ports.

2. The method according to claim 1, characterized in that, The full-bridge submodule includes four power devices and one energy storage capacitor, while the half-bridge submodule consists of two power devices and one energy storage capacitor.

3. The method according to claim 2, characterized in that, All power devices are fully controllable devices, including IGBTs, MOSFETs, or IGCTs.

4. The method as described in claim 1, characterized in that, There should be a phase difference between the AC currents of any two transformer units. °: ； in , where is the number of transformer units in the DC transformer.

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