Power electronic transformer based on MMDTC and modulation method thereof

Abstract

The present invention discloses a power electronic transformer based on MMDTC and a modulation method thereof. The power electronic transformer includes a T-type structure, a modular multi-level DC link module, and a dual active bridge module, and forms a low-voltage DC port and a medium-voltage DC port. The T-type structure is formed by three-phase input terminals A, B, and C and a controllable switch module. The modular multi-level DC link module is composed of an upper bridge arm and a lower bridge arm connected in series, and each bridge arm is composed of multiple sub-modules connected in cascade. The output sides of all DAB units in the dual active bridge module are connected in parallel to the low-voltage DC port, and the medium-voltage DC port is connected to multiple DAB units in the dual active bridge module in a series input and parallel output form. The power electronic transformer and the modulation method thereof of the present invention realize the medium-voltage DC port output of the transformer with a smaller number of power modules, thereby improving the power density and transmission efficiency of the power electronic transformer.

Description

Technical Field

[0001] The present invention belongs to the field of power electronics technology, and specifically relates to a power electronic transformer, and more specifically to a power electronic transformer based on an MMDTC (Modular Multilevel DC-Link Based T-type Converter) and a modulation method thereof. Background Art

[0002] The Power Electronic Transformer (PET) is an energy conversion device built based on modern power electronics technology. Essentially, it is a new type of flexible interconnected device consisting of a power electronic converter and a high-frequency isolation transformer. Traditional transformers can only perform conversion between different AC voltage levels and provide electrical isolation. In addition to these basic functions, the PET also enables AC / DC voltage conversion, direct DC power supply / load connection, and reactive power compensation. Future distribution networks will feature complex interconnections involving "source, grid, load, and storage," with multiple voltage levels and AC / DC hybrid configurations. PETs, with their extensive AC and DC ports, are particularly well-suited to this distribution network configuration. In practical PET applications, distributed power sources, energy storage, and loads can flexibly connect to either a low-voltage DC bus or a low-voltage AC bus, while large-scale centralized photovoltaic power generation, offshore wind power generation, and centralized energy storage can flexibly connect to a medium-voltage DC bus. PETs can actively control the energy flow at each port, optimizing system power flow and efficiently integrating new energy.

[0003] In the prior art, CHB (Cascaded H-Bridge) and MMC (Modular Multilevel Converter) PETs are classic multilevel PET topologies, both employing a modular structure. For CHB PETs, each submodule requires a DAB (Double Active Bridge) unit, resulting in a large number of DAB units required, increasing the cost, volume, weight, and control complexity of the PET device. Furthermore, another drawback of CHB PETs is their three independent phases and lack of a common medium-voltage DC-Bus (MVDC) port, hindering direct connection to MVDC power sources / loads. MMC PETs have a common MVDC bus port, allowing for direct serial connection of DAB units, reducing the number of DAB units required. However, the excessive number of IGBTs, DC capacitors, and high-frequency transformers in MMC PETs reduces the PET's power density and transmission efficiency. Furthermore, their high cost makes MMC PETs cost-effective and difficult to popularize. Summary of the Invention

[0004] The object of the present invention is to provide a power electronic transformer based on MMDTC and a modulation method thereof, so as to realize the medium voltage DC port output of the transformer with a smaller number of power modules and improve the power density and transmission efficiency of the power electronic transformer.

[0005] To achieve the above-mentioned purpose, the power electronic transformer based on MMDTC provided by the present invention is implemented by the following technical solutions:

[0006] A power electronic transformer based on MMDTC, comprising:

[0007] Three-phase input terminal, including A, B, and C three-phase input terminals;

[0008] The controllable switch module includes three groups of switch units: upper, middle and lower. Each group of switch units includes three high-voltage switches.

[0009] The three-phase input terminal and the controllable switch module form a T-shaped structure; each phase of the three-phase input is respectively connected to one end of a high-voltage switch in the upper group of switch units, and the other ends of the three high-voltage switches in the upper group of switch units are connected in parallel to form a U output end of the DC link; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the middle group of switch units, and the other ends of the three high-voltage switches in the middle group of switch units are connected in parallel to form an N output end; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the lower group of switch units, and the other ends of the three high-voltage switches in the lower group of switch units are connected in parallel to form an L output end of the DC link;

[0010] A modular multilevel DC link module, comprising an upper bridge arm and a lower bridge arm connected in series, wherein the connection point between the upper bridge arm and the lower bridge arm is connected to the N output terminal; the upper bridge arm comprises a cascaded upper bridge arm full-bridge submodule and a plurality of upper bridge arm half-bridge submodules, wherein the upper bridge arm full-bridge submodule is located at the top of the upper bridge arm, and an input terminal thereof is connected to the U output terminal of the DC link; the lower bridge arm comprises a cascaded lower bridge arm full-bridge submodule and a plurality of lower bridge arm half-bridge submodules, wherein the lower bridge arm full-bridge submodule is located at the bottom of the lower bridge arm, and an input terminal thereof is connected to the L output terminal of the DC link;

[0011] A dual-active bridge module, comprising two dual-active bridge units and a dual-active bridge group, wherein the input of a first dual-active bridge unit in the dual-active bridge unit is connected to the output of the upper-arm full-bridge submodule, the input of a second dual-active bridge unit in the dual-active bridge unit is connected to the output of the lower-arm full-bridge submodule, and the output of the first dual-active bridge unit, the output of the second dual-active bridge unit, and the output of the dual-active bridge group are connected in parallel; the dual-active bridge group comprises dual-active bridge units with series input and parallel output, one input end of the dual-active bridge group is connected to the connection point between the upper-arm full-bridge submodule and the upper-arm half-bridge submodule, and the other input end of the dual-active bridge group is connected to the connection point between the lower-arm full-bridge submodule and the lower-arm half-bridge submodule;

[0012] A low-voltage DC port connected to the two parallel output terminals of the dual-active bridge module;

[0013] The medium voltage DC port is connected to the two input ends of the dual active bridge group respectively.

[0014] In some embodiments of the present application, the high-voltage switch is an IGBT or a thyristor.

[0015] To achieve the aforementioned object of the invention, the power electronic transformer modulation method based on MMDTC provided by the present invention is implemented by the following technical solution:

[0016] A modulation method for a power electronic transformer based on MMDTC, the transformer comprising:

[0017] Three-phase input terminal, including A, B, and C three-phase input terminals;

[0018] The controllable switch module includes three groups of switch units: upper, middle and lower. Each group of switch units includes three high-voltage switches.

[0019] The three-phase input terminal and the controllable switch module form a T-shaped structure; each phase of the three-phase input is respectively connected to one end of a high-voltage switch in the upper group of switch units, and the other ends of the three high-voltage switches in the upper group of switch units are connected in parallel to form a U output end of the DC link; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the middle group of switch units, and the other ends of the three high-voltage switches in the middle group of switch units are connected in parallel to form an N output end; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the lower group of switch units, and the other ends of the three high-voltage switches in the lower group of switch units are connected in parallel to form an L output end of the DC link;

[0020] A modular multilevel DC link module, comprising an upper bridge arm and a lower bridge arm connected in series, wherein the connection point between the upper bridge arm and the lower bridge arm is connected to the N output terminal; the upper bridge arm comprises a cascaded upper bridge arm full-bridge submodule and a plurality of upper bridge arm half-bridge submodules, wherein the upper bridge arm full-bridge submodule is located at the top of the upper bridge arm, and an input terminal thereof is connected to the U output terminal of the DC link; the lower bridge arm comprises a cascaded lower bridge arm full-bridge submodule and a plurality of lower bridge arm half-bridge submodules, wherein the lower bridge arm full-bridge submodule is located at the bottom of the lower bridge arm, and an input terminal thereof is connected to the L output terminal of the DC link;

[0021] A dual-active bridge module, comprising two dual-active bridge units and a dual-active bridge group, wherein the input of a first dual-active bridge unit in the dual-active bridge unit is connected to the output of the upper-arm full-bridge submodule, the input of a second dual-active bridge unit in the dual-active bridge unit is connected to the output of the lower-arm full-bridge submodule, and the output of the first dual-active bridge unit, the output of the second dual-active bridge unit, and the output of the dual-active bridge group are connected in parallel; the dual-active bridge group comprises dual-active bridge units with inputs connected in series and outputs connected in parallel, one input end of the dual-active bridge group is connected to the connection point between the upper-arm full-bridge submodule and the upper-arm half-bridge submodule, and the other input end of the dual-active bridge group is connected to the connection point between the lower-arm full-bridge submodule and the lower-arm half-bridge submodule;

[0022] A low-voltage DC port connected to the two parallel output terminals of the dual-active bridge module;

[0023] The medium - voltage DC port is respectively connected to two input ends of the dual - active - bridge group;

[0024] The modulation method includes:

[0025] Using V uF，ref (t) as the modulation wave to modulate the full - bridge sub - module of the upper arm:

[0026]

[0027] Using V lF，ref (t) as the modulation wave to modulate the full - bridge sub - module of the lower arm:

[0028]

[0029] Using V uH，ref (t) as the modulation wave to modulate the half - bridge sub - module of the upper arm:

[0030]

[0031] Using V lH，ref (t) as the modulation wave to modulate the half - bridge sub - module of the lower arm:

[0032]

[0033] Where, U c is the capacitor voltage of the full - bridge sub - module or the half - bridge sub - module, N is the number of all sub - modules in the upper arm or the lower arm; m is a known modulation ratio, 0 < m ≤ 1; θ = ωt - floor{ωt / (2π / 3)}, ω is the angular frequency, and floor(x) represents the floor function.

[0034] In some embodiments of the present application, the number N of all sub - modules in the upper arm or the lower arm and the modulation ratio m satisfy:

[0035]

[0036] In some embodiments of the present application, the half - bridge sub - modules of the upper arm and the half - bridge sub - modules of the lower arm are modulated by the carrier - phase - shifted PWM modulation method.

[0037] In some embodiments of the present application, the full - bridge sub - modules of the upper arm and the full - bridge sub - modules of the lower arm are modulated by the unipolar double - frequency modulation method.

[0038] Compared with the prior art, the advantages and positive effects of the present invention are:

[0039] The MMDTC-based power electronic transformer provided by the present invention is composed of a T-type converter formed by a three-phase input end and a controllable switch module, a modular multilevel DC link module, and a dual-active bridge module. In the modular multilevel DC link module, a plurality of submodules are cascaded, comprising an upper bridge arm and a lower bridge arm. A full-bridge submodule is provided at the top of the upper bridge arm and the bottom of the lower bridge arm, respectively. A dual-active bridge unit is connected to the output end of each full-bridge submodule. The other dual-active bridge units in the dual-active bridge module form a series input and parallel output structure. The outputs of all the dual-active bridge units are connected in parallel, and the two parallel output ends form a low-voltage DC port for outputting low-voltage DC. The two input ends of the dual-active bridge group formed by the series-connected dual-active bridge units form a medium-voltage DC port. Thus, the power electronic transformer can achieve medium-voltage DC port output using a smaller number of power modules. The modulation method provided by the present invention is used to modulate the submodules of the modular multilevel DC link module, thereby obtaining a constant DC voltage at the medium-voltage DC port, facilitating direct access to a medium-voltage DC load / power source and improving the performance of the power electronic transformer.

[0040] Other features and advantages of the present invention will become more apparent after reading the detailed description of the present invention in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following is a brief introduction to the drawings required for use in the embodiments. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0042] Figure 1 Schematic diagram of the topology of an embodiment of a power electronic transformer based on MMDTC of the present invention;

[0043] Figure 2 A modulation waveform diagram of a half-bridge submodule and an input voltage waveform diagram of a bridge arm half-bridge submodule of an embodiment of a modulation method for a power electronic transformer based on MMDTC of the present invention;

[0044] Figure 3 The modulation waveforms and input voltage waveforms of the upper arm full-bridge submodule and the lower arm full-bridge submodule of an embodiment of the modulation method of the power electronic transformer based on MMDTC of the present invention are shown;

[0045] Figure 4 The upper bridge arm input voltage waveform, lower bridge arm input voltage waveform, and medium voltage DC port output voltage waveform of an embodiment of the modulation method of the power electronic transformer based on MMDTC of the present invention are shown. DETAILED DESCRIPTION

[0046] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0047] It should be noted that the technical solutions between the various embodiments of the present invention can be combined with each other, but it must be based on the fact that ordinary technicians in this field can implement it. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0048] Commonly used multi-level PET topologies in the prior art include CHB-type PET and MMC-type PET. CHB-type PET requires a large number of DAB units and lacks a medium-voltage DC port, hindering direct access to a medium-voltage DC power source / load. MMC-type PET requires a large number of power modules, such as capacitors and IGBTs, resulting in low power density and transmission efficiency. In view of the shortcomings of existing PETs, the present invention creatively proposes a novel PET topology and modulation method. Based on MMDTC, this PET not only provides a medium-voltage DC port for direct access to a medium-voltage DC power source / load, but also offers advantages in terms of the number of components, capacitors, DABs, and capacitance values. This MMDTC-based PET exhibits higher power density and transmission efficiency, is relatively low-cost, and is therefore readily adaptable for widespread use.

[0049] Figure 1 FIG. 1 is a schematic diagram of the topological structure of an embodiment of a power electronic transformer based on MMDTC according to the present invention.

[0050] like Figure 1 As shown, the power electronic transformer of this embodiment includes a T-shaped structure, a modular multi-level DC link module, and a dual active bridge module, forming a low-voltage DC port and a medium-voltage DC port. The T-shaped structure is formed by the three-phase input terminals A, B, and C and the controllable switch module; the modular multi-level DC link module is composed of an upper bridge arm and a lower bridge arm connected in series, and each bridge arm is composed of multiple cascaded sub-modules; the output sides of all DAB units in the dual active bridge module are connected in parallel to the low-voltage DC-bus (LVDC) port, and the medium-voltage DC port MVDC is connected to the multiple DAB units in the dual active bridge module with series input and parallel output.

[0051] Specifically, the controllable switch module in the T-type structure includes three groups of switch units: upper, middle, and lower. Each group of switch units includes three high-voltage switches. The A, B, and C three-phase input terminals in the T-type structure are respectively connected to one end of a high-voltage switch in each group of switch units. Figure 1 As shown, the upper switch unit includes a high voltage switch T au 、T bu 、T cu, the middle switch unit includes high voltage switch T an 、T bn 、T cn , the lower switch unit includes a high voltage switch T al 、T bl 、T cl The high voltage switch can be composed of a common power semiconductor switch device. In some embodiments, the high voltage switch is an IGBT or a thyristor. The three-phase inputs A, B, and C are connected to the high voltage switch T in the upper switch unit. au 、T bu 、T cu Connection, high voltage switch T au 、T bu 、T cu The other end is connected in parallel to form the U output end of the DC link DC-Link; the A, B, and C three-phase inputs are connected to the high-voltage switch T in the middle group switch unit respectively. an 、T bn 、T cn Connection, high voltage switch T an 、T bn 、T cn The other end of the N output terminal is connected in parallel to form the N output terminal, which is connected to the connection point between the upper bridge arm and the lower bridge arm of the modular multi-level DC link module; the A, B, and C three-phase inputs are respectively connected to the high-voltage switch T in the lower group of switch units. al 、T bl 、T cl Connection, high voltage switch T al 、T bl 、T cl The other end of the L output terminal of the DC link is connected in parallel.

[0052] The modular multilevel DC link module includes an upper arm and a lower arm connected in series. The upper arm is connected between the U output terminal and the N output terminal, and the lower arm is connected between the N output terminal and the L output terminal. The upper arm is composed of N cascaded submodules, including an upper arm full-bridge submodule 11 (Full-Bridge Submodule, FBSM) and (N-1) half-bridge submodules (HBSM). The upper arm full-bridge submodule 11 is located at the top of the upper arm, with one input terminal connected to the DC link's U output terminal and the other input terminal connected in series with the adjacent upper arm half-bridge submodule 12. The connection point between the upper arm full-bridge submodule 11 and the upper arm half-bridge submodule 12 is marked as U1. The lower arm is composed of N cascaded submodules, including a lower arm full-bridge submodule 13 and (N-1) half-bridge submodules HBSM. Among them, the lower bridge arm full-bridge submodule 13 is located at the bottom end of the lower bridge arm, one input end of which is connected to the L output end of the DC link, and the other input end is connected in series with the adjacent lower bridge arm half-bridge submodule 14. The connection point between the lower bridge arm full-bridge submodule 13 and the lower bridge arm half-bridge submodule 14 is marked as L1.

[0053] The dual active bridge (DAB) module includes two dual active bridge units and a dual active bridge group. The input of the first dual active bridge unit 15 in the dual active bridge unit is connected to the output of the upper arm full bridge submodule 11, and the input of the second dual active bridge unit 16 in the dual active bridge unit is connected to the output of the lower arm full bridge submodule 13. The dual active bridge group includes multiple dual active bridge units DAB ( Figure 1 In the embodiment, all DABs except the first dual-active bridge unit 15 and the second dual-active bridge unit 16 are DABs in the dual-active bridge group, forming a structure with series input and parallel output. One input end of the dual-active bridge group is connected to the connection point U1 between the upper bridge arm full-bridge submodule 11 and the upper bridge arm half-bridge submodule 12, and the other input end is connected to the connection point marked L1 between the lower bridge arm full-bridge submodule 13 and the lower bridge arm half-bridge submodule 14. In addition, the output of the first dual-active bridge unit 15 and the output of the second dual-active bridge unit 16 are also connected in parallel with the output of the dual-active bridge group.

[0054] The low-voltage DC port LVDC is connected to the two parallel outputs of the dual-active bridge module. This means that all DAB unit outputs in the dual-active bridge module are connected in parallel to the low-voltage DC port LVDC. The medium-voltage DC port MVDC is connected to the two inputs of the dual-active bridge group. This means that the U1-L1 port forms the medium-voltage DC port of the MMDTC power electronic transformer.

[0055] Among them, the specific circuit structures of the full-bridge sub-module FBSM, the half-bridge sub-module HBSM and the dual active bridge DAB are all existing technologies and will not be specifically presented and described here.

[0056] In this embodiment, although the MMDTC has a common DC link (DC-Link), after analysis, the average value of the voltage V on this DC link UL will change with the change of the modulation depth of the sub-module, and moreover, the voltage V UL also has a multi-frequency fluctuation characteristic. Therefore, the voltage V UL is not a constant DC voltage. So, the port U-L of the DC link DC-Link is not suitable as a medium-voltage DC port. And through the topological structure constructed in this embodiment, the U1-L1 port can provide a constant-voltage medium-voltage DC voltage and can be used as the medium-voltage DC port of the power electronic transformer.

[0057] Based on Figure 1 the topological structure of the embodiment, the present invention also provides a modulation method for a power electronic transformer based on MMDTC to achieve a constant-voltage output at the medium-voltage DC port.

[0058] Specifically, the modulation method includes:

[0059] Using V uF，ref (t) as the modulation wave to modulate the upper-bridge-arm full-bridge sub-module:

[0060]

[0061] Using V lF，ref (t) as the modulation wave to modulate the lower-bridge-arm full-bridge sub-module:

[0062]

[0063] Using V uH，ref (t) as the modulation wave to modulate the upper-bridge-arm half-bridge sub-module:

[0064]

[0065] Using V lH，ref (t) as the modulation wave to modulate the lower-bridge-arm half-bridge sub-module:

[0066]

[0067] where, U c is the capacitor voltage of the full-bridge sub-module or the half-bridge sub-module, and all the sub-modules have the same capacitor voltage; N is the number of all sub-modules in the upper-bridge arm or the lower-bridge arm; m is a known modulation ratio, 0 < m ≤ 1; θ = ωt - floor{ωt / (2π / 3)}, ω is the angular frequency, and floor(x) represents the floor function. The specific modulation method can be implemented by using the modulation means in the prior art, and this embodiment does not specifically limit the modulation means.

[0068] The above modulation wave is used to modulate each submodule in the power electronic transformer. c When it remains unchanged, the U1-L1 port will output a constant DC voltage.

[0069] In some other embodiments, to avoid overmodulation, the number N of all submodules in the upper bridge arm or the lower bridge arm and the modulation ratio m satisfy:

[0070]

[0071] Given the number N of cascaded bridge arm submodules, the operating range of the modulation ratio m can be calculated. For example, when N = 9, the modulation ratio m ranges from m∈[0.78,0.95].

[0072] In other preferred embodiments, the upper-arm half-bridge submodule and the lower-arm half-bridge submodule are modulated using a carrier phase-shifted PWM modulation method. All half-bridge submodules in each arm share the same modulation wave, and the phases of the triangular carriers differ by a fixed angle. The upper-arm full-bridge submodule and the lower-arm full-bridge submodule are modulated using a unipolar frequency-multiplication modulation method. By applying this hybrid PWM modulation method to the half-bridge and full-bridge submodules, a relatively balanced switching pattern and device power loss distribution can be achieved, while also doubling the equivalent switching frequency.

[0073] Figures 2 to 4 The waveform diagram of an embodiment of the modulation method of the power electronic transformer based on MMDTC of the present invention is shown. Figure 1 The topology structure of the MMDTC-based power electronic transformer shown is a waveform diagram of a power electronic transformer with N=9 sub-modules in the upper bridge arm or the lower bridge arm modulated by the aforementioned modulation waves, wherein the modulation ratio m=0.85, the half-bridge sub-module is modulated by the carrier phase-shifted PWM modulation method, and the full-bridge sub-module is modulated by the unipolar frequency multiplication modulation method.

[0074] Figure 2 (a) shows the modulation waveform of the half-bridge submodule HBSM. The modulation waveform of the upper arm half-bridge submodule V uH，ref (t) and the modulation wave V of the lower bridge arm half-bridge submodule lH，ref (t) triangular carriers V with the same amplitude and frequency as (N-1) groups but with a phase shift of 2π / (N-1) Z1 By comparison, modulation generates (N-1) groups of PWM modulation signals to drive the on and off of the corresponding IGBT of the half-bridge sub-module. The input voltages of each half-bridge sub-module in the upper and lower bridge arms are superimposed to obtain the input voltage V of all half-bridge sub-modules in the upper bridge arm. uH And the input voltage of all half-bridge sub-modules in the lower bridge arm V lH, its waveform is as follows Figure 2 (b) shown.

[0075] Figure 3 (a) shows the modulation waveform of the upper arm full bridge submodule. uF，ref (t), and the carrier V Z2 , the upper arm full bridge submodule is modulated using a unipolar frequency multiplication modulation method to obtain Figure 3 (b) The upper arm full-bridge submodule input voltage V shown in the waveform uF .

[0076] Figure 3 (c) shows the modulation waveform of the lower arm full bridge submodule. lF，ref (t) and carrier V Z3 , the lower arm full bridge submodule is modulated using a unipolar frequency multiplication modulation method to obtain Figure 3 (d) The waveform of the lower bridge arm full-bridge submodule input voltage V lF .

[0077] Figure 4 (a) shows the total input voltage V obtained by adding the input voltage of the upper arm full-bridge submodule and the input voltage of all half-bridge submodules of the upper arm. UN The waveform diagram and the total input voltage V of the lower bridge arm obtained by superimposing the input voltage of the lower bridge arm full-bridge sub-module and the input voltage of all half-bridge sub-modules of the lower bridge arm NL The waveform diagram of . Figure 4 (a) shows that the bridge arm input voltage is still a step multi-level waveform. Figure 4 (b) shows the output voltage V of the medium voltage DC port MVDC U1L1 The waveform diagram of . Figure 4 (b) It can be seen that the voltage at the MVDC port is a constant DC voltage with an amplitude of (N-1)U c .

[0078] The following is a comprehensive comparative analysis of three types of PETs: MMDTC-based power electronic transformers (denoted as MMDTC-type PETs in the table below), CHB-type PETs, and MMC-type PETs. First, the following assumptions are made:

[0079] (1) The grid voltage is 10kV.

[0080] (2) The submodule capacitor voltage rating is U c =1600V, IGBT voltage level is 3300V.

[0081] (3) The primary rated voltage of the DAB is 1600V, the secondary rated voltage is 800V, and the high-frequency transformer ratio is 2:1.

[0082] (4) Redundant submodule configuration is not considered.

[0083] Based on the above settings, the comparison results of the three PETs are shown in Table 1 below.

[0084] Table 1 Comparison results of three types of PET

[0085]

[0086] In the above table, the capacitance values are based on CHB type PET.

[0087] From the above table, we can see that both MMDTC PET and MMC PET have MVDC ports. However, MMDTC PET has obvious advantages over the other two types of PET in terms of the number of components, number of capacitors, number of DABs, and capacitance value.

[0088] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for a person skilled in the art to modify the technical solutions described in the aforementioned embodiments, or to replace some of the technical features therein with equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions claimed to be protected by the present invention.

Claims

1. A power electronic transformer based on MMDTC, characterized in that: include: Three-phase input terminal, including A, B, and C three-phase input terminals; The controllable switch module includes three groups of switch units: upper, middle and lower. Each group of switch units includes three high-voltage switches. The three-phase input terminal and the controllable switch module form a T-shaped structure; each phase of the three-phase input is respectively connected to one end of a high-voltage switch in the upper group of switch units, and the other ends of the three high-voltage switches in the upper group of switch units are connected in parallel to form a U output end of the DC link; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the middle group of switch units, and the other ends of the three high-voltage switches in the middle group of switch units are connected in parallel to form an N output end; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the lower group of switch units, and the other ends of the three high-voltage switches in the lower group of switch units are connected in parallel to form an L output end of the DC link; A modular multilevel DC link module, comprising an upper bridge arm and a lower bridge arm connected in series, wherein the connection point between the upper bridge arm and the lower bridge arm is connected to the N output terminal; the upper bridge arm comprises a cascaded upper bridge arm full-bridge submodule and a plurality of upper bridge arm half-bridge submodules, wherein the upper bridge arm full-bridge submodule is located at the top of the upper bridge arm, and an input terminal thereof is connected to the U output terminal of the DC link; the lower bridge arm comprises a cascaded lower bridge arm full-bridge submodule and a plurality of lower bridge arm half-bridge submodules, wherein the lower bridge arm full-bridge submodule is located at the bottom of the lower bridge arm, and an input terminal thereof is connected to the L output terminal of the DC link; A dual-active bridge module, comprising two dual-active bridge units and a dual-active bridge group, wherein the input of a first dual-active bridge unit in the dual-active bridge unit is connected to the output of the upper-arm full-bridge submodule, the input of a second dual-active bridge unit in the dual-active bridge unit is connected to the output of the lower-arm full-bridge submodule, and the output of the first dual-active bridge unit, the output of the second dual-active bridge unit, and the output of the dual-active bridge group are connected in parallel; the dual-active bridge group comprises dual-active bridge units with inputs in series and outputs in parallel, one input end of the dual-active bridge group is connected to the connection point between the upper-arm full-bridge submodule and the upper-arm half-bridge submodule, and the other input end of the dual-active bridge group is connected to the connection point between the lower-arm full-bridge submodule and the lower-arm half-bridge submodule; A low-voltage DC port connected to the two parallel output terminals of the dual-active bridge module; The medium voltage DC port is connected to the two input ends of the dual active bridge group respectively.

2. The power electronic transformer based on MMDTC according to claim 1, characterized in that: The high-voltage switch is an IGBT or a thyristor.

3. A modulation method for a power electronic transformer based on MMDTC, characterized in that: The transformer comprises: Three-phase input terminal, including A, B, and C three-phase input terminals; The controllable switch module includes three groups of switch units: upper, middle and lower. Each group of switch units includes three high-voltage switches. The three-phase input terminal and the controllable switch module form a T-shaped structure; each phase of the three-phase input is respectively connected to one end of a high-voltage switch in the upper group of switch units, and the other ends of the three high-voltage switches in the upper group of switch units are connected in parallel to form a U output end of the DC link; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the middle group of switch units, and the other ends of the three high-voltage switches in the middle group of switch units are connected in parallel to form an N output end; each phase of the three-phase input is also respectively connected to one end of a high-voltage switch in the lower group of switch units, and the other ends of the three high-voltage switches in the lower group of switch units are connected in parallel to form an L output end of the DC link; A modular multilevel DC link module, comprising an upper bridge arm and a lower bridge arm connected in series, wherein the connection point between the upper bridge arm and the lower bridge arm is connected to the N output terminal; the upper bridge arm comprises a cascaded upper bridge arm full-bridge submodule and a plurality of upper bridge arm half-bridge submodules, wherein the upper bridge arm full-bridge submodule is located at the top of the upper bridge arm, and an input terminal thereof is connected to the U output terminal of the DC link; the lower bridge arm comprises a cascaded lower bridge arm full-bridge submodule and a plurality of lower bridge arm half-bridge submodules, wherein the lower bridge arm full-bridge submodule is located at the bottom of the lower bridge arm, and an input terminal thereof is connected to the L output terminal of the DC link; A dual-active bridge module, comprising two dual-active bridge units and a dual-active bridge group, wherein the input of a first dual-active bridge unit in the dual-active bridge unit is connected to the output of the upper-arm full-bridge submodule, the input of a second dual-active bridge unit in the dual-active bridge unit is connected to the output of the lower-arm full-bridge submodule, and the output of the first dual-active bridge unit, the output of the second dual-active bridge unit, and the output of the dual-active bridge group are connected in parallel; the dual-active bridge group comprises dual-active bridge units with inputs in series and outputs in parallel, one input end of the dual-active bridge group is connected to the connection point between the upper-arm full-bridge submodule and the upper-arm half-bridge submodule, and the other input end of the dual-active bridge group is connected to the connection point between the lower-arm full-bridge submodule and the lower-arm half-bridge submodule; A low-voltage DC port connected to the two parallel output terminals of the dual-active bridge module; a medium voltage DC port, connected to the two input terminals of the dual active bridge group respectively; The modulation method comprises: Using V uF，ref (t) as a modulation wave to modulate the upper arm full-bridge submodule: Using V lF，ref (t) as a modulation wave to modulate the lower arm full-bridge submodule: Using V uH，ref (t) as a modulation wave to modulate the upper arm half-bridge submodule: Using V lH，ref (t) as a modulation wave to modulate the lower arm half-bridge submodule: where, U c is the capacitor voltage of the full-bridge sub-module or the half-bridge sub-module, N is the number of all sub-modules in the upper arm or the lower arm; m is the known modulation ratio, 0 < m ≤ 1; θ = ωt - floor{ωt / (2π / 3)}, ω is the angular frequency, and floor(x) represents the floor function.

4. The modulation method of the power electronic transformer based on MMDTC according to claim 3, characterized in that: The number N of all submodules in the upper bridge arm or the lower bridge arm and the modulation ratio m satisfy:

5. The modulation method of the power electronic transformer based on MMDTC according to claim 3 or 4, characterized in that: The upper arm half-bridge submodule and the lower arm half-bridge submodule are modulated using a carrier phase-shift PWM modulation method.

6. The modulation method of the power electronic transformer based on MMDTC according to claim 3 or 4, characterized in that: The upper arm full-bridge submodule and the lower arm full-bridge submodule are modulated using a unipolar frequency multiplication modulation method.

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