Non-isolated power electronic transformer

Abstract

This invention discloses a non-isolated power electronic transformer, comprising N cascaded full-bridge rectifier modules, a first port, and a second port, where N is a positive integer ≥ 2; each full-bridge rectifier module includes a full-bridge switching unit and a DC capacitor, the full-bridge switching unit having a first and a second AC terminal; the second AC terminal of the preceding module is connected to the first AC terminal of the following module between adjacent modules; the DC capacitor of the first full-bridge rectifier module is connected in parallel to the first arm of a half-bridge structure, and the DC capacitor of the Nth full-bridge rectifier module is connected in parallel to the second arm of a half-bridge structure; the first positive terminal of the first port is connected to the first AC input positive terminal of the full-bridge switching unit of the first full-bridge rectifier module via a first reactor, and the first negative terminal of the first port is connected to the Nth AC input negative terminal of the full-bridge switching unit of the Nth full-bridge rectifier module; the second positive terminal of the second port is connected to the midpoint of the first arm via a second reactor, and the second negative terminal of the second port is connected to the midpoint of the second arm.

Description

Technical Field

[0001] This invention relates to the field of power electronic conversion technology, and more particularly to a non-isolated power electronic transformer. Background Technology

[0002] In fields such as high-voltage power conversion, traction grid power supply, and microgrids, existing medium- and high-voltage AC-AC power converters often employ isolation transformers and complex multi-stage structures. Currently, the common AC-AC conversion structures in high-voltage, high-power applications are mainly divided into power frequency transformers and power electronic transformers. The principle of a power frequency transformer is based on electromagnetic induction. When power frequency AC current is applied to the primary winding, an alternating magnetic field is generated. This magnetic field is efficiently guided and coupled to the secondary winding through a magnetically conductive material (such as silicon steel sheets), inducing an electromotive force on the secondary side. Power frequency transformers are technologically mature and highly reliable, but they are large in size and dissipate power to some extent. Most power frequency transformers have difficult-to-adjust turns ratios, and the magnitude of transmitted power and the phase of voltage and current are uncontrollable, making them unsuitable for complex operating conditions.

[0003] Most power electronic transformers are isolated power electronic transformers that contain power frequency or high frequency transformers, and their overall structure is as follows: Figure 1 As shown, Figure 1 A traditional power electronic transformer structure diagram, such as Figure 1 As shown in section (a), the high-frequency isolated power electronic transformer consists of cascaded rectifiers and inverters, and a DC-DC converter unit; in recent years, non-isolated structures have also been developed, such as... Figure 1 Part (b) of the non-isolated power electronic transformer is shown. It consists of cascaded rectifiers and cascaded inverters. It can regulate the DC-side capacitor voltage, the amplitude of the input and output voltages, and the amplitude and phase of the input and output currents through closed-loop control. The detailed circuit diagrams of both are shown in the figure. Figure 2 As shown, Figure 2 The diagrams show traditional power electronic transformer circuits, with part (a) representing an isolated power electronic transformer and part (b) representing a non-isolated power electronic transformer circuit. Power electronic transformers, due to their use of pulse width modulation (PWM) control, can control the input and output voltage and current waveforms and can be connected to the grid at two ports. However, their complex structure and the retention of power frequency or high-frequency transformers result in drawbacks such as large size and low power factor. While existing non-isolated power electronic transformers eliminate the need for an isolation transformer, they still suffer from problems such as a large number of switching devices, complex structure, high cost, and the potential for short circuits between inter-module capacitors.

[0004] Therefore, the problem with the existing technology is that in order to optimize the configuration of isolated power electronic transformers in some application scenarios where isolation is not required, reduce some of the transformer structure to reduce the use of switching devices, reduce the transformer size, and improve transformer efficiency, there is an urgent need for an improved non-isolated power electronic transformer to effectively reduce the cost and size of non-isolated power electronic transformers. Summary of the Invention

[0005] This invention provides a non-isolated power electronic transformer to solve the problem of effectively reducing the cost and size of non-isolated power electronic transformers.

[0006] In a first aspect, embodiments of the present invention provide a non-isolated power electronic transformer, comprising: N cascaded full-bridge rectifier modules, a first port, and a second port, where N is a positive integer greater than or equal to 2; each full-bridge rectifier module includes a full-bridge switching unit and a DC capacitor, the full-bridge switching unit having a first AC terminal and a second AC terminal, wherein in two adjacent full-bridge rectifier modules, the second AC terminal of the preceding full-bridge rectifier module is connected to the first AC terminal of the following full-bridge rectifier module; a first bridge arm is connected in parallel across the DC capacitor of the first full-bridge rectifier module, and the Nth full-bridge... A second bridge arm is connected in parallel across the DC capacitor of the rectifier module. The first bridge arm and the second bridge arm form a half-bridge structure. The first positive terminal of the first port is connected to the first AC input positive terminal of the full-bridge switching unit of the first full-bridge rectifier module via a first reactor. The first negative terminal of the first port is connected to the Nth AC input negative terminal of the full-bridge switching unit of the Nth full-bridge rectifier module. The second positive terminal of the second port is connected to the midpoint of the first bridge arm via a second reactor. The second negative terminal of the second port is connected to the midpoint of the second bridge arm.

[0007] In conjunction with the first aspect, the present invention provides a first possible implementation of the first aspect, wherein the power electronic transformer includes 4N+4 switching transistors and N DC capacitors, wherein the N full-bridge switching units include 4N switching transistors, the first bridge arm includes 2 switching transistors and the second bridge arm includes 2 switching transistors.

[0008] In conjunction with the first aspect, embodiments of the present invention provide a second possible implementation of the first aspect, wherein the port voltage of the first port and the port voltage of the second port satisfy: ;in, The port voltage of the first port. This is the port voltage of the second port. This represents the DC capacitor voltage of a single full-bridge rectifier module.

[0009] In conjunction with the first aspect, this invention provides a third possible implementation of the first aspect, further comprising: a modulation unit, wherein the modulation unit employs two columns of triangular carriers, a positive triangular carrier and a negative triangular carrier, to simultaneously modulate the first modulation wave corresponding to the first port and the second modulation wave corresponding to the second port, respectively; during modulation, the high level of the port voltage of the first port is aligned with the center of the high level of the port voltage of the second port, and the low level of the port voltage of the first port is aligned with the center of the low level of the port voltage of the second port; the difference between the first modulation wave and the second modulation wave is not greater than [value missing]. ,in, N represents the DC capacitor voltage of a single full-bridge rectifier module, and N is the number of the cascaded full-bridge rectifier modules.

[0010] In conjunction with the first aspect, this invention provides a fourth possible implementation of the first aspect, wherein when the first port and the second port operate in the same voltage amplitude phase modulation mode, the modulation index of the first modulation wave and the second modulation wave are both m, and their phases are respectively... and The maximum value of the difference in voltage amplitude between the two modulated waves is at Acquired at any time, and satisfying the following conditions: When N=3, the maximum phase difference between the first port and the second port satisfies: When the number of the full-bridge rectifier modules is N, the following condition is met: ;in, The voltage amplitude of the first modulated wave. Where m is the voltage amplitude of the second modulated wave, and m is the modulation index. , These are the phases of the first modulated wave and the second modulated wave, respectively. The modulation index of the two modulation waves is N when the number of full-bridge rectifier modules is N, and E is the DC capacitor voltage of a single full-bridge rectifier module.

[0011] In conjunction with the first aspect, this invention provides a fifth possible implementation of the first aspect, wherein when the first port and the second port operate in in-phase voltage modulation mode, the modulation intensities of the first modulation wave and the second modulation wave are respectively and And the two have a common phase. The maximum value of the difference in voltage amplitude between the two modulated waves is at Acquired at any time, and satisfying the following conditions: When N=3, the following conditions are met: When the number of the full-bridge rectifier modules is N, the following condition is met: ;in, The voltage amplitude of the first modulated wave. E is the voltage amplitude of the second modulated wave, and E is the DC capacitor voltage of a single full-bridge rectifier module.

[0012] In conjunction with the first aspect, the present invention provides a sixth possible implementation of the first aspect, wherein the power electronic transformer changes the contact point of the DC capacitor of the full-bridge rectifier module at each port and the edge by means of the difference in the switching states of the first bridge arm, the second bridge arm and the bridge arm connected to the first port and the second port, so as to synthesize a level at the first port and the second port; wherein the levels of the first port and the second port are 3E and E, 2E and 0 or E and -E, respectively, and E is the DC capacitor voltage of a single full-bridge rectifier module.

[0013] In conjunction with the first aspect, the present invention provides a seventh possible implementation of the first aspect, wherein when the input voltage and current of the first port contain harmonics and reactive components, the ripple power is consumed on each of the DC capacitors, so that the second port outputs a voltage and current with unity power factor and no harmonics.

[0014] In conjunction with the first aspect, the present invention provides an eighth possible implementation of the first aspect, wherein both the first port and the second port are configured to operate in parallel with an AC power supply to achieve bidirectional power transmission, and the voltage of each DC capacitor and the amplitude of the output voltage, the amplitude of the output current, and the phase of the first port and the second port are adjusted by closed-loop control to control the active power, reactive power, and harmonics on the input and output sides.

[0015] In conjunction with the first aspect, embodiments of the present invention provide a ninth possible implementation of the first aspect, wherein the N sequentially cascaded full-bridge rectifier modules are expanded by increasing the number of cascaded full-bridge switching units.

[0016] The non-isolated power electronic transformer provided by this invention simplifies the topology of non-isolated power electronic transformers. Compared with traditional non-isolated power electronic transformers, the number of switching transistors is reduced from 8N ​​to 4N+4, resulting in cost and size advantages. Furthermore, this invention proposes a modulation method suitable for the proposed topology, achieving the maximum operating range limited by the topology structure, while successfully completing voltage and phase regulation operation while maintaining DC-side voltage balance. Moreover, the topology proposed in this invention enables bidirectional energy transfer and allows control of key physical quantities such as active and reactive power, and harmonics at the input and output. The topology proposed in this invention can improve the overall withstand voltage and output port power quality by expanding the number of cascaded structures.

[0017] Other features and advantages of the invention will be set forth in the following description. The objects and other advantages of the invention are realized and obtained through the structures particularly pointed out in the description, claims, and drawings.

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a structural diagram of a traditional power electronic transformer; Figure 2 This is a traditional power electronic transformer circuit diagram; Figure 3 The topology diagram of the non-isolated power electronic transformer provided in Embodiment 1 of this invention; Figure 4 This is a comparison diagram of three operating modes of the non-isolated power electronic transformer topology provided in Embodiment 1 of the present invention; Figure 5 This is a table showing the voltage status of two ports under different switching states of a non-isolated power electronic transformer topology provided in Embodiment 1 of the present invention. Figure 6 This is a comparison diagram of the topology of the non-isolated power electronic transformer provided in Embodiment 2 of the present invention with two modules, three modules, and N modules. Figure 7 This is a schematic diagram of the modulation method for achieving the maximum operating range of the non-isolated power electronic transformer topology according to the present invention; Figure 8 This is a schematic diagram of the voltage phase modulation port level synthesis of the non-isolated power electronic transformer topology of the present invention; Figure 9 This is a schematic diagram of the in-phase voltage port level synthesis of the non-isolated power electronic transformer topology of the present invention; Figure 10 This is a schematic diagram of the operating range of the non-isolated power electronic transformer topology of the present invention; Figure 11 This is a schematic diagram of the simulated waveform of the non-isolated two-module power electronic transformer of the present invention with a phase shift of 77°; Figure 12 This is a schematic diagram of the simulated waveform of the non-isolated four-module power electronic transformer of the present invention with a phase shift of 36°; Figure 13 This is a schematic diagram of the simulated waveform of the non-isolated six-module power electronic transformer of the present invention with a phase shift of 24°; Figure 14 This is a schematic diagram of the experimental waveform of the non-isolated three-module power electronic transformer of the present invention with a 45° phase shift. Figure 15 This is a schematic diagram of the experimental waveforms for the non-isolated three-module power electronic transformer with a turns ratio of 1:0.5 according to the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] To effectively reduce the cost and size of non-isolated power electronic transformers, the non-isolated power electronic transformer provided by this invention simplifies the topology of non-isolated power electronic transformers. Compared with traditional non-isolated power electronic transformers, the number of switching transistors is reduced from 8N ​​to 4N+4, thus offering advantages in cost and size. The following detailed description of several embodiments of this invention is provided.

[0023] Example 1: Embodiment 1 of the present invention provides a non-isolated power electronic transformer with relatively few switches. In this embodiment, the N-module converter consists of 4N+4 switching transistors and N DC-side support capacitors. Compared with traditional power electronic transformers, it has fewer switching transistors and does not have a large isolation transformer, effectively reducing the cost and size of the power electronic transformer.

[0024] The non-isolated power electronic transformer topology of this embodiment is based on a single-phase cascaded rectifier, with an additional bridge arm added to the DC capacitors of the two edge rectifier modules as the output AC terminals, such as... Figure 3 As shown, Figure 3This invention provides a topology diagram for a non-isolated power electronic transformer according to Embodiment 1. The positive terminal of the single-phase input is connected via reactor L1 to the positive AC input terminal A1P of the full-bridge rectifier in module 1. The negative AC input terminal A1N of the full-bridge rectifier is connected to A2P of module 2, and so on up to module n. The positive terminal of the single-phase output is connected via reactor L2 to the midpoint B1P of the half-bridge in the rectifier unit of module 1, and the negative terminal is connected via reactor to the midpoint BnP of the half-bridge. The function of this topology in Embodiment 1 is that the input and output can operate in parallel with the AC power supply, achieving phase and voltage regulation within a certain range, controlling reactive power and harmonics, while retaining the AC-DC conversion capability of cascaded rectifiers.

[0025] In this embodiment, the topology generates different voltage levels at the input and output through the different switching states of the four half-bridges connected to the input and output ports. For example... Figure 4 As shown, Figure 4 This is a comparison diagram of three operating modes of the non-isolated power electronic transformer topology provided in Embodiment 1 of the present invention. Figure 4 In part (a), the input and output share the same switching transistor connection method. The two half-bridges connected to the two input ports have the upper and lower transistors respectively conducting, while the two half-bridges connected to the two output ports have the lower and upper transistors respectively conducting, thus changing the contact points between the ports and the edge module capacitors. From the simplified circuit diagram, it can be seen that the input and output port voltage levels of the converter are 3E and E, respectively. Similarly, Figure 4 In part (b) of the text, Figure 4 In part (c), the input and output ports also share the same set of capacitors. By changing the switching state of the half-bridge connected to the port, the outputs of the two ports are 2E, 0 and E, -E respectively.

[0026] Statistics on the port voltage corresponding to the states of all switching transistors, as follows: Figure 5 As shown, Figure 5 This is a table showing the voltage status of two ports under different switching states of a non-isolated power electronic transformer topology provided in Embodiment 1 of the present invention. Figure 5 In this topology, there are only five possible differences between the two ports: 2E, E, 0, -E, and -2E (where E is the DC-side voltage of a single full-bridge rectifier module). It is clear that the proposed topology fundamentally limits the input and output port voltages. The converter can synthesize all voltage levels within this range. As long as the synthesized range is not exceeded, the input and output ports can be considered as two separate cascaded rectifier-inverters without considering isolation, and controlled independently. When the input has harmonics and reactive power, the ripple power is consumed on the DC-side capacitor, and the output unity power factor can be controlled to be harmonic-free. When the input and output are simultaneously connected to the grid or have a load, the circuit, viewed individually from the input or input port, is a cascaded rectifier or inverter, corresponding to multiple DC output ports, and functions the same as a regular cascaded rectifier.

[0027] Example 2: The non-isolated power electronic transformer in this invention is a cascaded structure. This second embodiment, based on the non-isolated power electronic transformer topology of the first embodiment, further increases the total number of cascaded full-bridge rectifier modules, thereby increasing the number of cascaded full-bridge rectifiers. Figure 6 As shown. Figure 6 This is a comparison diagram of the topology of the non-isolated power electronic transformer provided in Embodiment 2 of the present invention with two modules, three modules, and N modules.

[0028] In other embodiments of the present invention, to ensure that the converter operates within its maximum range, the high-level and low-level voltages of the input and output ports need to be aligned at their centers during modulation of the two ports of the topology. This embodiment uses two positive and negative triangular carrier waves to simultaneously modulate the input and output modulation waves to achieve this purpose. Figure 7 As shown. Figure 7 This diagram illustrates the modulation method for achieving the maximum operating range of the non-isolated power electronic transformer topology of the present invention. Under this modulation method, the proposed topology can operate within its maximum range. It can be deduced that for an N-module converter, the difference between the modulated waves at the two ports cannot exceed [a certain value]. The operating range of the topology will be explained below for two typical cases: phase modulation with the same voltage and voltage modulation with the same phase.

[0029] Phase adjustment with the same voltage amplitude: such as Figure 8 As shown, Figure 8 This is a schematic diagram of the voltage phase modulation port level synthesis of the non-isolated power electronic transformer topology of the present invention; m represents the modulation index of the two modulation waves. and These represent the phases of the two modulated waves, respectively. The maximum difference between the input and output modulated waves is... When it is obtained, for The maximum phase difference between the input and output of the three-module converter is calculated as follows: For module N, there are .

[0030] In-phase voltage regulation: such as Figure 9 As shown, Figure 9 This is a schematic diagram of the in-phase voltage port level synthesis of the non-isolated power electronic transformer topology of the present invention; using , This indicates the modulation index of the two modulated waves. They are in common phase. The maximum difference between the input and output modulated waves is at... When it is obtained, for The maximum voltage difference between the input and output of the three-module converter is calculated as follows: For module N, there are Plot the phase modulation range and voltage regulation range of the converter for different numbers of modules, as shown below. Figure 10 As shown, Figure 10 This is a schematic diagram of the operating range of the non-isolated power electronic transformer topology of the present invention.

[0031] Finally, in other embodiments of the present invention, the feasibility and working capability of the non-isolated power electronic transformer topology of the present invention under different numbers of modules can be verified by simulation and experiment, and simulation and experimental analysis can be performed. Figure 11 This is a schematic diagram of the simulated waveform of the non-isolated two-module power electronic transformer of the present invention with a phase shift of 77°; Figure 12 This is a schematic diagram of the simulated waveform of the non-isolated four-module power electronic transformer of the present invention with a phase shift of 36°; Figure 13 This is a schematic diagram of the simulated waveform of the non-isolated six-module power electronic transformer of the present invention with a phase shift of 24°. Figures 11-13 The simulated waveforms of the non-isolated 2, 4, and 6-module power electronic transformer topologies of this invention are shown at the boundary of the phase-shifting operating range. The input and output modulation indices are all 0.8, and the phase shift angles are 77°, 36°, and 24°, respectively. The voltage and current waveforms are good, achieving unity power factor, verifying the multi-module scalability of the proposed topology.

[0032] Figure 14 This is a schematic diagram of the experimental waveform of the non-isolated three-module power electronic transformer of the present invention with a 45° phase shift. Figure 14 The experimental waveform of the non-isolated three-module power electronic transformer of the present invention in phase-shift mode with a modulation degree of 0.75 and a phase shift of 45 degrees is shown. It can be seen that the input and output phase shift is successful and the voltage and current are in phase. The current THD is low, which verifies the phase shift characteristics of the topology.

[0033] Figure 15 This is a schematic diagram of the experimental waveforms for the non-isolated three-module power electronic transformer with a turns ratio of 1:0.5 according to the present invention. Figure 15 The experimental waveforms of the non-isolated three-module power electronic transformer topology of the present invention are shown in the transformation mode with a high-voltage side regulation of 0.8 and a low-voltage side regulation of 0.4. It can be seen that the input and output are successfully transformed and the voltage and current are in phase. The current THD is maintained at a low level, which verifies the transformation characteristics of the topology.

[0034] The above experimental results show that the non-isolated power electronic transformer of the present invention not only achieves flexible voltage and phase regulation functions in non-isolated scenarios, but also effectively reduces the number of active switching devices, thereby reducing the size and cost of the device.

[0035] The technical achievements of the embodiments of the present invention include: (1) The embodiments of the present invention simplify the topology of non-isolated power electronic transformers. Compared with traditional non-isolated power electronic transformers, the number of switching transistors is reduced from 8N ​​to 4N+4, which has cost and volume advantages.

[0036] (2) The present invention proposes a modulation method applicable to the topology of non-isolated power electronic transformers, which can achieve the maximum operating range limited by the topology, and successfully complete voltage regulation and phase regulation operation while maintaining DC side voltage balance.

[0037] (3) The non-isolated power electronic transformer topology of the present invention can realize bidirectional energy transmission and control key physical quantities such as active and reactive power and harmonics of input and output.

[0038] (4) The non-isolated power electronic transformer topology of the present invention can improve the overall withstand voltage and output port power quality by expanding the number of cascaded structures.

[0039] Those skilled in the art will recognize that the embodiments described herein are intended to help them understand the implementation methods of the present invention, and should be understood that the scope of protection of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of the present invention.

Claims

1. A non-isolated power electronic transformer, characterized in that, include: N cascaded full-bridge rectifier modules, first port and second port, where N is a positive integer greater than or equal to 2; Each of the full-bridge rectifier modules includes a full-bridge switching unit and a DC capacitor. The full-bridge switching unit has a first AC terminal and a second AC terminal. In two adjacent full-bridge rectifier modules, the second AC terminal of the preceding full-bridge rectifier module is connected to the first AC terminal of the following full-bridge rectifier module. The first bridge arm is connected in parallel across the DC capacitor of the first full-bridge rectifier module, and the second bridge arm is connected in parallel across the DC capacitor of the Nth full-bridge rectifier module. The first bridge arm and the second bridge arm are half-bridge structures. The first positive terminal of the first port is connected to the first AC input positive terminal of the full-bridge switching unit of the first full-bridge rectifier module via the first reactor, and the first negative terminal of the first port is connected to the Nth AC input negative terminal of the full-bridge switching unit of the Nth full-bridge rectifier module. The second positive terminal of the second port is connected to the midpoint of the first bridge arm via the second reactor, and the second negative terminal of the second port is connected to the midpoint of the second bridge arm.

2. The non-isolated power electronic transformer according to claim 1, characterized in that, The power electronic transformer includes 4N+4 switching transistors and N DC capacitors, wherein the N full-bridge switching units include 4N switching transistors, the first bridge arm includes 2 switching transistors and the second bridge arm includes 2 switching transistors.

3. The non-isolated power electronic transformer according to claim 1, characterized in that, The port voltage of the first port and the port voltage of the second port satisfy the following: ； in, The port voltage of the first port. This is the port voltage of the second port. This represents the DC capacitor voltage of a single full-bridge rectifier module.

4. The non-isolated power electronic transformer according to claim 1, characterized in that, Also includes: The modulation unit employs two triangular carrier waves, a positive triangular carrier wave and a negative triangular carrier wave, to simultaneously modulate the first modulation wave corresponding to the first port and the second modulation wave corresponding to the second port. During modulation, the high level of the port voltage at the first port is aligned with the high level of the port voltage at the second port, and the low level of the port voltage at the first port is aligned with the low level of the port voltage at the second port. The difference between the first modulation wave and the second modulation wave is no greater than [value missing]. ,in, N represents the DC capacitor voltage of a single full-bridge rectifier module, and N represents the number of cascaded full-bridge rectifier modules.

5. The non-isolated power electronic transformer according to claim 4, characterized in that, When the first port and the second port operate in the same voltage amplitude phase modulation mode, the modulation index of the first modulation wave and the second modulation wave are both m, and their phases are respectively... and The maximum value of the difference in voltage amplitude between the two modulated waves is at Acquired at any time, and satisfying the following conditions: ； When N=3, the maximum phase difference between the first port and the second port satisfies: ; When the number of the full-bridge rectifier modules is N, the following condition is met: ; in, The voltage amplitude of the first modulated wave. Where m is the voltage amplitude of the second modulated wave, and m is the modulation index. , These are the phases of the first modulated wave and the second modulated wave, respectively. The modulation index of the two modulation waves is N when the number of full-bridge rectifier modules is N, and E is the DC capacitor voltage of a single full-bridge rectifier module.

6. The non-isolated power electronic transformer according to claim 4, characterized in that, When the first port and the second port operate in in-phase voltage modulation mode, the modulation intensities of the first modulation wave and the second modulation wave are respectively... and And the two have a common phase. The maximum value of the difference in voltage amplitude between the two modulated waves is at Acquired at any time, and satisfying the following conditions: ； When N=3, the following conditions are met: ; When the number of the full-bridge rectifier modules is N, the following condition is met: ; in, The voltage amplitude of the first modulated wave. E is the voltage amplitude of the second modulation wave, and E is the DC capacitor voltage of a single full-bridge rectifier module.

7. The non-isolated power electronic transformer according to claim 1, characterized in that, The power electronic transformer changes the contact point of the DC capacitor of the full-bridge rectifier module at each port and the edge by the difference in the switching states of the first bridge arm, the second bridge arm, and the bridge arm connected to the first port and the second port, so as to synthesize the voltage level at the first port and the second port; wherein the voltage levels of the first port and the second port are 3E and E, 2E and 0, or E and -E, respectively, and E is the DC capacitor voltage of a single full-bridge rectifier module.

8. The non-isolated power electronic transformer according to claim 1, characterized in that, When the input voltage and current of the first port contain harmonics and reactive components, the ripple power is consumed on each of the DC capacitors, so that the second port outputs a voltage and current with unity power factor and no harmonics.

9. The non-isolated power electronic transformer according to claim 1, characterized in that, Both the first port and the second port are configured to operate in parallel with the AC power supply to achieve bidirectional power transmission. The voltage of each DC capacitor, as well as the amplitude of the output voltage, the amplitude of the output current, and the phase of the first port and the second port, are adjusted through closed-loop control to control the active power, reactive power, and harmonics on the input and output sides.

10. The non-isolated power electronic transformer according to claim 1, characterized in that, The N cascaded full-bridge rectifier modules are expanded by increasing the number of cascaded full-bridge switching units.

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