Cross-voltage-level asymmetric flexible interconnected AC / DC hybrid power distribution network

Abstract

The invention belongs to the technical field of medium-voltage power distribution network flexible interconnection, and aims to solve the problem that in the prior art, a medium-voltage flexible interconnection system which is large in capacity and stable in operation and has different voltage grades is lacked. The invention provides a cross-voltage-level asymmetric flexible interconnection alternating-current and direct-current hybrid power distribution network, which realizes asymmetric flexible interconnection of a power distribution network line at a lower voltage side and a power distribution network line at a higher voltage side by adopting a method of hybrid use of a power electronic device and a traditional transformer. Due to the adoption of the technical scheme, aiming at the requirement of asymmetric flexible interconnection of a 6kV line and a 10kV line, the cost and the benefit of the flexible interconnection device are well considered through organic fusion and cooperative cooperation of the two topological structures, higher reliability and better economical efficiency are achieved, the output capacity is greatly improved on the premise of ensuring stable operation, and the service life of the flexible interconnection device is prolonged. And the development requirements of the AC-DC hybrid power distribution network are met.

Description

Technical Field

[0001] The present invention relates to a medium voltage distribution network flexible interconnection technology, and in particular to an asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels. Background Art

[0002] Compared with traditional DC transmission technology, the flexible DC transmission technology that has emerged in recent years uses a voltage source converter composed of switchable power electronic devices to achieve AC / DC power conversion using pulse width modulation control theory. It has many advantages: no need to consider commutation failure, no need for reactive power compensation, can supply power to weak power grids or island systems, active and reactive power can be decoupled and quickly controlled separately, has good compatibility with loads, is convenient for distributed energy access, and has low output voltage and current harmonic content. So far, dozens of flexible DC transmission and distribution projects have been planned and put into operation at home and abroad. my country has established a number of flexible DC transmission demonstration projects in Nan'ao Island, Shantou, Guangdong, Dalian City, Liaoning, Zhoushan Islands, Zhejiang, Xiamen, Fujian, etc. The Zhangbei Flexible DC Project and Wudongde UHV Hybrid Multi-Terminal DC Transmission Project have been successfully put into operation.

[0003] With the rapid growth of urban scale and the rapid development of information technology, the number of sensitive loads, important loads and nonlinear loads in the power grid is increasing. The AC distribution network is facing a series of power quality problems such as large short-circuit current, tight line corridors, instantaneous voltage drop, voltage fluctuation, excessive harmonics, and intensified three-phase imbalance. It is urgent to change the existing distribution network structure and power supply mode. Compared with the traditional AC distribution network, the DC distribution network has the advantages of strong power conversion and distribution capabilities, excellent power quality, and easy asynchronous interconnection. It can better adapt to the extensive and flexible access of distributed power sources, electric vehicle charging and swapping facilities, distributed energy storage facilities and various DC loads, and has good development prospects and significant technical advantages.

[0004] At present, my country's medium-voltage AC distribution network can be divided into 6kV, 10kV, and 20kV according to the nominal system voltage. The transmission power and distance of different voltage levels are very different. In actual projects, due to the influence of line angle difference and closed-loop current, 6kV and 10kV lines, 10kV and 20kV lines are usually not directly connected, which restricts the further improvement of power supply reliability in many scenarios. With the development of power electronics and computer control technology, technicians have also tried to flexibly interconnect distribution lines of different voltage levels. For example Figure 5What is shown is an existing flexible interconnection method, which adopts a low-voltage back-to-back topology structure, uses two distribution transformers for step-down connection, and performs electrical isolation; the converter module adopts a conventional three-phase full-bridge topology, and the power of each module group generally does not exceed 200kW. Multiple groups of converter modules are connected in parallel to increase the interconnection power. Due to the use of low-voltage topology, this solution is more mature, but due to the limitation of the number of converters in parallel, the maximum capacity generally does not exceed 2MVA. The existing technology lacks medium- and large-capacity medium-voltage flexible interconnection topology structures. Summary of the invention

[0005] Based on the above problems, the present invention is proposed to provide a system that overcomes the above problems or at least partially solves the above problems, especially can be used in an asymmetric medium-voltage flexible interconnected distribution network, provides a larger output capacity, and operates stably.

[0006] In this regard, the present invention provides an asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels, which adopts a method of hybrid use of power electronic devices and traditional transformers to achieve asymmetric flexible interconnection between the distribution network lines on the lower voltage side and the distribution network lines on the higher voltage side.

[0007] One implementation method is: the higher voltage side passes through a multi-winding phase-shifting transformer to form multiple secondary low-voltage windings, and then forms a three-phase output through an H-bridge cascade, which is connected to the grid on the lower voltage side. The input transformer adopts the extended triangle connection method, with a certain electrical angle difference, to achieve the purpose of suppressing input harmonic current.

[0008] Preferably, the cascade H-bridge is formed by interconnecting a plurality of H-bridge sub-modules.

[0009] Furthermore, each submodule includes a diode uncontrolled rectifier bridge composed of six diodes and a full-bridge inverter circuit composed of four IGBTs. The output of the low-voltage winding is converted into DC through the diode uncontrolled rectifier bridge, and then converted into single-phase AC through the full-bridge inverter circuit.

[0010] Preferably, the diode uncontrolled rectifier bridge and the full-bridge inverter circuit are connected via a DC voltage stabilizing capacitor.

[0011] Preferably, the output on the lower voltage side adopts carrier phase-shift PWM technology, and the triangular carriers of different cascade units in the same phase are phase-shifted to increase the output voltage step and improve the equivalent switching frequency, thereby improving the output voltage waveform.

[0012] Another implementation is: both the higher voltage side and the lower voltage side are MMC cascade topology composed of half-bridge sub-modules, and are connected back-to-back through the DC side.

[0013] Preferably, the half-bridge submodule is composed of two IGBTs with anti-parallel diodes and a DC capacitor, which are connected in series to form a bridge arm with the series inductor and are respectively connected to the positive and negative electrode loops.

[0014] Preferably, the single MMC bridge arm is composed of 20 half-bridge sub-modules, and each half-bridge sub-module is configured with a 1200V or 1700V IGBT.

[0015] Preferably, the number of half-bridge sub-modules in the MMC cascade topology on the higher voltage side and the lower voltage side is equal.

[0016] Due to the adoption of the technical solution, in view of the demand for asymmetric flexible interconnection between the lower voltage side line and the higher voltage side line, an asymmetric flexible interconnection device is designed by using a method of mixing power electronic devices with traditional transformers; through the organic integration and coordinated cooperation of the two topological structures, the cost and benefit of the flexible interconnection device are well taken into account, and it has high reliability and good economy, and has good promotion and application value. Compared with the existing technology, under the premise of ensuring stable operation, the output capacity is greatly improved to meet the development needs of AC / DC hybrid distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of a flexible closed-loop topology structure of a medium-voltage distribution network based on D-UPFC adopted in the prior art;

[0018] Figure 2 A vector diagram of a flexible closed-loop control strategy for a medium-voltage distribution network based on D-UPFC used in the prior art;

[0019] Figure 3 A schematic diagram of a topological structure based on a controllable phase shifting technology used in the prior art;

[0020] Figure 4 A schematic diagram of a topological structure based on a flexible AC controllable interconnection technology used in the prior art;

[0021] Figure 5 It is a schematic diagram of a low voltage back-to-back topology structure for medium voltage flexible interconnection adopted in the prior art;

[0022] Figure 6 A schematic diagram of the topological structure of a multi-winding transformer combined with a cascaded H-bridge solution used in an asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels of the present invention;

[0023] Figure 7 This is a schematic diagram of the topological structure of the MMC back-to-back topology solution adopted by an asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels in the present invention. DETAILED DESCRIPTION

[0024] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0025] First of all, it should be noted that the present invention is aimed at a medium voltage distribution network, that is, a distribution network with a system voltage between 6 kV and 10 (20) kV.

[0026] The rapid development and widespread application of power electronics technology have significantly improved the control and management capabilities of power systems. FACTS technology has been able to achieve flexible control of the voltage, impedance, phase, and power flow of the transmission system. DC distribution networks have the advantages of strong power distribution capabilities, power quality, and easy asynchronous interconnection. Compared with traditional AC distribution networks, they are more adaptable to the wide and flexible access of distributed power sources, electric vehicle charging and swapping facilities, distributed energy storage facilities, and various DC loads. With the introduction and development of the concept of smart distribution networks, AC and DC flexible control technology applied to medium and low voltage distribution networks has become a research hotspot in recent years and has received widespread attention in the industry.

[0027] At present, the flexible interconnection technologies of medium-voltage distribution networks mainly include modular multi-level MMC, series compensation D-UPFC, and boosted back-to-back voltage source B2B VSC; in addition, there are also technical solutions such as high-frequency magnetic isolation, CNPC back-to-back, controllable phase shift, and flexible AC controllable interconnection, which have high application feasibility.

[0028] Especially the series compensation D-UPFC technology. D-UPFC is a new type of power flow control device composed of a parallel compensation static synchronous compensator (STATCOM) and a series compensation static synchronous series compensator (SSSC). Its structure is mainly composed of two voltage source converters, and also includes DC capacitors, series transformers, parallel transformers and control systems. It connects an AC voltage generated by the converter in series to the transmission line. Through the change of the control law, the amplitude and phase angle of the additional potential can be continuously changed, so that different functions such as parallel compensation, series compensation, and phase shift can be realized separately or simultaneously. It can greatly change the line flow, enhance system damping, suppress subsynchronous oscillations, and improve the static stability and transient stability of the system. The performance is significantly better than that of FACTS devices with single functions. The flexible loop principle of the medium-voltage distribution network based on D-UPFC is as follows: Figure 1 shown.

[0029] The control strategy of D-UPFC is to detect the voltage difference ΔU at both ends of the line, generate a voltage with the same amplitude and phase as ΔU through a bidirectional full-bridge converter, and couple it to the series transformer, so as to effectively control the circulating current within a lower range, realize closed-loop grid-connected operation, and avoid line overload or tripping. The converter output voltage ΔU can be decomposed into two orthogonal vectors to realize four-quadrant voltage modulation. Its vector diagram is as follows: Figure 2 shown.

[0030] And controllable phase shifting technology: the topology of the controllable phase shifter is as follows Figure 3 As shown in the figure, it consists of a phase-shifting transformer and a thyristor control circuit. By controlling different combinations of thyristor switches, a controllable compensation voltage is superimposed on the input voltage to achieve voltage phase angle adjustment. However, there are two problems: 1) The hierarchical control compensation is discontinuous, and the power control cannot be 360 ​​degrees linearly adjusted without difference; 2) The active power and reactive power are not decoupled, and the transmission of active power will affect the reactive power distribution, which is easy to affect the stability of the power grid, and reactive power compensation cannot be achieved.

[0031] There is also flexible AC controllable interconnection technology: the flexible AC controllable interconnection device is a hybrid series-parallel structure that combines the advantages of DPFC and high-voltage inverter topology, such as Figure 4 As shown in the figure, the series part compensates for the voltage difference of the power supply on both sides, and the parallel part provides an energy channel. In the actual distribution network, the voltage angle difference and amplitude difference on both sides of the interconnection point are very small, usually the angle difference is ≤30° and the amplitude difference is ≤0.17pu. Therefore, under the condition of transmitting the same power, the actual operating capacity of the device is very small.

[0032] However, these technologies are limited to scenarios where the voltage levels on both sides are the same (generally 10kV). For scenarios where the voltage levels on both sides are different, the existing technology uses Figure 5 The low-voltage back-to-back topology shown in the figure uses two distribution transformers for step-down connection and electrical isolation; the converter module adopts a conventional three-phase full-bridge topology, and multiple sets of converter modules are connected in parallel to increase the interconnection power. Due to the use of low-voltage topology, this solution needs to step down both sides to the same voltage level (generally 690V or 400V) to meet the conditions for low-voltage flexible interconnection. This technical solution is highly mature, but due to the limitation of the number of converter modules in parallel, the maximum capacity generally does not exceed 2MVA, which cannot meet the needs of large-capacity flexible interconnection of medium-voltage distribution networks.

[0033] The present invention provides an asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels, which uses a method of mixing power electronic devices with traditional transformers to achieve asymmetric flexible interconnection between radial distribution network lines on the lower voltage side and distribution network lines on the higher voltage side. Through the organic integration and coordinated cooperation of the two topological structures, the cost and benefit of the flexible interconnection device are well taken into account, and it has high reliability and good economy, and has good promotion and application value.

[0034] In order to clearly explain the technical solutions described in the present invention, some abbreviations of the terms are first explained:

[0035] H-bridge cascade: H-bridge is a power electronic circuit composed of four power semiconductor devices, shaped like the letter "H" and used to control the forward and reverse rotation and speed regulation of motors or other loads. H-bridge cascade is to connect multiple H-bridge circuits in sequence. When working, multiple H-bridges jointly control the direction and size of current flow. For example, in DC motor drive, each H-bridge can change the output voltage polarity and amplitude according to the control signal. After cascading, the output voltage is the sum of the output voltages of each H-bridge. In the field of power electronics, H-bridge cascade is often used in high-voltage inverters to meet the high voltage required for high-voltage motor drive; in renewable energy power generation systems, such as photovoltaic inverters or wind power inverters, when DC power needs to be converted into high-voltage, high-quality AC power, H-bridge cascade is also used to increase the output voltage level, and through reasonable control strategies to improve the output voltage waveform quality and reduce harmonic content, so as to achieve efficient conversion and transmission of electric energy.

[0036] In the present invention, multiple H bridges are interconnected to form an H bridge cascade topology structure.

[0037] IGBT: IGBT is the abbreviation of Insulated Gate Bipolar Transistor, which refers to insulated gate bipolar transistor. It is a power semiconductor device widely used in the field of power electronics. It combines the advantages of metal-oxide-semiconductor field-effect transistor (MOSFET) and bipolar junction transistor (BJT), and has the characteristics of high input impedance, low driving power, high switching rate and high voltage resistance. It can be used as a key switching element to realize current conduction, cutoff and size control in many links such as power system power conversion (rectification, inversion), motor control (motor variable frequency speed drive), power transmission and distribution (flexible DC transmission and distribution), etc., effectively improving the operating efficiency, reliability and power quality of the power system, and can work stably under various complex working conditions such as high voltage and high current, playing a vital role in promoting the development of power technology in the directions of high voltage, high frequency and high efficiency.

[0038] Diode uncontrolled rectifier bridge: a device used to convert AC to DC, which mainly uses the unidirectional conductivity of the diode to achieve the rectification function. In the circuit, when the AC voltage is in the positive half cycle, the diode is turned on, allowing current to pass, and when the AC voltage is in the negative half cycle, the diode is cut off, preventing the current from flowing in the opposite direction, thereby integrating the positive and negative voltage and current waveforms of the AC input into a DC output that flows in only one direction. This rectification process is carried out naturally, and no additional control signal is required to actively control the on and off of the diode. It plays a role in some power application scenarios where the DC voltage accuracy requirements are not particularly high and the load is relatively stable, such as some simple DC power supply, DC power supply of some motors, or DC power consumption of some industrial equipment. It is a basic and economical power electronic device for realizing AC-DC conversion.

[0039] Full-bridge inverter circuit: An important circuit topology in power electronics technology, mainly used to convert direct current into alternating current. The circuit consists of four power semiconductor devices (such as IGBT, MOSFET, etc.) to form a bridge arm. By controlling the four switching devices to turn on and off in a certain order and frequency, direct current forms an alternating current path in the circuit, and then obtains an alternating voltage at the output end. In practical applications, the full-bridge inverter circuit can adjust the frequency, amplitude and phase of the output alternating current according to different control strategies. It is widely used in many power fields that need to convert direct current into alternating current, such as uninterruptible power supply (UPS), photovoltaic power generation system, motor drive, etc., and can effectively improve the efficiency and quality of power conversion and provide a stable AC power supply for the load.

[0040] PWM: It is the abbreviation of Pulse Width Modulation, which is an effective technology for controlling power semiconductor devices (such as IGBT, MOSFET, etc.) in the power field. It controls the average value of the output voltage or current by changing the width (duty cycle) of the pulse signal, and is widely used in power conversion occasions such as rectification and inversion. Taking the inverter as an example, PWM technology can convert DC input into AC output with adjustable amplitude and frequency, and use a series of variable-width pulses to simulate sinusoidal output. By adjusting the pulse width, the effective value of the output voltage can be accurately adjusted; in addition, PWM technology can also be used in application scenarios such as motor speed regulation to control the speed and torque of the motor, and it also helps to reduce harmonic content and improve power quality. PWM technology plays a key control role in power electronic equipment, allowing equipment to convert and control power more efficiently and flexibly.

[0041] MMC: is the abbreviation of Modular Multilevel Converter. It is an advanced power electronic commutation device, mainly composed of a large number of identical sub-modules connected in series, which usually contain IGBT, capacitors and other components. MMC can achieve efficient conversion between AC and DC, and plays a key role in many high-voltage and high-power power application scenarios such as flexible DC transmission and distribution systems and high-voltage variable frequency drives. It can flexibly synthesize and output multi-level voltage waveforms by controlling the switching of sub-modules, effectively reduce the harmonic content of the output voltage, and improve the power quality. It also has a high degree of modularity, which facilitates system expansion, redundant design and fault handling, and improves the flexibility, reliability and stability of power system operation.

[0042] For example, the higher voltage side is 10kV and the lower voltage side is 6kV. Figure 6 The figure shows one embodiment of the present invention: the 10kV side forms multiple secondary low-voltage windings through a multi-winding phase-shifting transformer, and then forms a three-phase output through an H-bridge cascade, which is connected to the 6kV side power grid. The input transformer adopts the extended triangle connection method, which is mutually offset by a certain electrical angle to achieve the purpose of suppressing the input harmonic current. The cascaded H-bridge is composed of multiple H-bridge sub-modules connected in series. Each H-bridge sub-module contains a diode uncontrolled rectifier bridge composed of six diodes and a full-bridge inverter circuit composed of 4 IGBTs. The output of the low-voltage winding is converted into direct current through a diode uncontrolled rectifier bridge, and then converted into single-phase alternating current through a full-bridge inverter circuit. The diode uncontrolled rectifier bridge and the full-bridge inverter circuit are connected through a DC voltage stabilizing capacitor. The output of the 6kV side adopts carrier phase-shifted PWM technology, and the triangular carriers of different cascade units in the same phase are mutually offset by a certain phase to increase the output voltage step and increase the equivalent switching frequency, thereby improving the output voltage waveform. This topology has the following advantages: the DC side uses isolated DC power supplies, so there is no voltage balance problem; the power unit has a simple structure, which is basically the same as the low-voltage frequency conversion, and is easy to design and manufacture in a modular manner; when a power unit fails, it can be bypassed and the remaining power units can continue to work, which improves system reliability; the input transformer adopts a multiple design to reduce the input harmonic current and there is no need to add an input filter; the inverter adopts multi-level PWM output, and the output voltage waveform is very close to a sine wave, which is beneficial to improving the quality of grid-connected power; the system is highly efficient, and the overall efficiency at full load can reach more than 95%.

[0043] As another embodiment of the present invention: Figure 7The figure shows an MMC cascade topology in which both the 10kV side and the 6kV side are composed of half-bridge submodules, which are connected back-to-back through the DC side. The half-bridge submodule consists of two IGBTs with anti-parallel diodes and a DC capacitor. After being connected in series, they form a bridge arm with a series reactor and are connected to the positive and negative loops respectively. The single MMC bridge arm consists of 20 half-bridge submodules, each of which is equipped with a 1200V or 1700V IGBT. The number of half-bridge submodules in the MMC cascade topology on the 10kV side and the 6kV side is equal. This technical solution can realize flexible access to distribution lines of different voltage levels on both sides through a control strategy without the need for additional transformers. At the same time, the multi-level characteristics of MMC are similar to those of the cascade H-bridge, which can well realize high power quality grid connection. Compared with the first topology, this solution is more flexible, and the larger number of modules makes its expandable power range much larger than the first, but the relative cost is higher.

[0044] The following table compares the relevant parameters of the two embodiments of the present invention and the prior art:

[0045]

[0046] It can be seen that the output capacity upper limit of the first embodiment of the present invention, i.e., the multi-winding transformer + cascaded H-bridge topology solution, can reach up to 10MVA, which is 5 times that of the low-voltage back-to-back technical solution in the prior art. It can basically meet the conventional medium-voltage flexible interconnection requirements and has a low implementation cost.

[0047] In some occasions where the output capacity requirement is very high, it is necessary to adopt the MMC cascade topology solution described in Example 2. The upper limit of the output capacity of this solution is as high as 50MVA, which can meet the needs of high-power flexible interconnection, but its implementation cost is high and it does not have electrical isolation capability. However, this is currently the only technical solution that can reliably achieve high-power flexible interconnection.

[0048] It should be noted that, although the embodiment of the present invention only provides a specific implementation method of 6kV as the lower voltage side and 10kV as the higher voltage side, the technical solution of the present invention is still feasible for the scenario of interconnection between 6kV line and 20kV line, or interconnection between 10kV line and 20kV line.

[0049] In the description provided herein, a large number of specific details are described. However, it is understood that embodiments of the present invention can be practiced without these specific details. In some instances, well-known methods, structures and techniques are not shown in detail so as not to obscure the understanding of this description.

[0050] Similarly, it should be understood that in order to streamline the present disclosure and aid in understanding one or more of the various inventive aspects, in the above description of exemplary embodiments of the present invention, various features of the present invention are sometimes grouped together into a single embodiment, figure, or description thereof.

[0051] Those skilled in the art will appreciate that the modules or units or components of the devices in the examples disclosed herein may be arranged in the devices described in the embodiment, or alternatively may be located in one or more devices different from the devices in the examples. The modules in the foregoing examples may be combined into one module or may be divided into multiple submodules.

[0052] Those skilled in the art will appreciate that the modules in the devices in the embodiments may be adaptively changed and arranged in one or more devices different from the embodiments. The modules or units or components in the embodiments may be combined into one module or unit or component, and furthermore may be divided into a plurality of submodules or subunits or subcomponents.

[0053] Furthermore, those skilled in the art will appreciate that although some embodiments described herein include certain features included in other embodiments but not other features, the combination of features from different embodiments is meant to be within the scope of the present invention and to form different embodiments.

[0054] In addition, some of the embodiments are described herein as methods or combinations of method elements that can be implemented by a processor of a computer system or by other devices that perform the functions. Therefore, a processor with necessary instructions for implementing the method or method elements forms a device for implementing the method or method elements. In addition, the elements described herein of the device embodiments are examples of devices for implementing the functions performed by the elements for the purpose of implementing the invention.

[0055] As used herein, unless otherwise specified, the use of ordinal numbers "first," "second," "third," etc. to describe common objects merely indicates that different instances of similar objects are involved, and is not intended to imply that the objects so described must have a given order in time, space, order, or in any other manner.

[0056] Although the present invention has been described according to a limited number of embodiments, it will be apparent to those skilled in the art, with the benefit of the above description, that other embodiments may be envisioned within the scope of the invention thus described. In addition, it should be noted that the language used in this specification is primarily selected for readability and instructional purposes, rather than for explaining or limiting the subject matter of the present invention.

Claims

1. An asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels, characterized in that: The method of using power electronic devices in combination with traditional transformers is used to achieve asymmetric flexible interconnection between the radial distribution network lines on the lower voltage side and the distribution network lines on the higher voltage side.

2. The asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels as claimed in claim 1, characterized in that: The higher voltage side is formed into multiple secondary low-voltage windings through a multi-winding phase-shifting transformer, and then a three-phase output is formed through an H-bridge cascade connection, which is connected to the power grid on the lower voltage side.

3. The asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to claim 2 is characterized in that: The cascade H-bridge is formed by interconnecting a plurality of H-bridge sub-modules.

4. The asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to claim 3 is characterized in that: Each submodule includes a diode uncontrolled rectifier bridge composed of six diodes and a full-bridge inverter circuit composed of four IGBTs. The output of the low-voltage winding is converted into DC through the diode uncontrolled rectifier bridge, and then converted into single-phase AC through the full-bridge inverter circuit.

5. The asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to claim 4, characterized in that: The diode uncontrolled rectifier bridge and the full-bridge inverter circuit are connected via a DC voltage stabilizing capacitor.

6. An asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to any one of claims 2-5, characterized in that: The output on the lower voltage side adopts carrier phase-shift PWM technology, and the triangular carriers of different cascade units in the same phase are phase-shifted to increase the output voltage step and improve the equivalent switching frequency, thereby improving the output voltage waveform.

7. The asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels as claimed in claim 1, characterized in that: Both the higher voltage side and the lower voltage side are MMC cascade topologies composed of half-bridge sub-modules, which are connected back-to-back through the DC side.

8. The asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to claim 7, characterized in that: The half-bridge submodule is composed of two IGBTs with anti-parallel diodes and a DC capacitor, which are connected in series to form a bridge arm with the series inductor and are respectively connected to the positive and negative pole loops.

9. An asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to claim 7 or 8, characterized in that: The single MMC bridge arm is composed of 20 half-bridge sub-modules, and each half-bridge sub-module is configured with 1200V or 1700V IGBT.

10. An asymmetric flexible interconnected AC / DC hybrid distribution network across voltage levels according to any one of claims 7 to 9, characterized in that: The number of half-bridge submodules in the MMC cascade topology on the higher voltage side and the lower voltage side is equal.

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