CN112383229A - Multi-port power electronic transformer topological structure and alternating current-direct current micro-grid system thereof - Google Patents

Abstract

The invention relates to the technical field of power equipment, in particular to a multi-port power electronic transformer topological structure and an alternating current-direct current micro-grid system thereof, wherein the multi-port power electronic transformer topological structure comprises at least two alternating current input ends; the voltage conversion unit comprises a preset number of voltage conversion subunits, each voltage conversion subunit outputs direct-current voltages of at least two voltage classes, and direct-current voltage output ends of the same voltage class in each group of voltage conversion units are connected in parallel to form a first direct-current voltage output end of each group of voltage conversion units; the at least two groups of direct current voltage output ends comprise second direct current voltage output ends after the first direct current voltage output ends of the at least two groups of voltage conversion units are connected in parallel; and the at least one group of alternating voltage output ends are connected with the second direct voltage output end. The system has various ports of high voltage, low voltage, alternating current and direct current, and can realize flexible networking.

Description

Multi-port power electronic transformer topological structure and alternating current-direct current micro-grid system thereof

Technical Field

The invention relates to the technical field of power equipment, in particular to a multi-port power electronic transformer topological structure and an alternating current-direct current micro-grid system.

Background

The basic idea of an Electronic Power Transformer (EPT) is to use a high-frequency Transformer to replace a Power frequency Transformer, and introduce Power Electronic conversion technology to the source side and the secondary side of the high-frequency Transformer, so as to realize flexible regulation of voltage, current and Power at both sides of the Transformer through proper control. At present, power electronic transformers have been widely used in power distribution networks.

Most of the existing power electronic transformers are high-voltage and low-voltage ports. However, with the high-proportion new energy access and the large-scale application of multi-load, the direct current feature is obvious, and the number of the alternating current and direct current ports in the distribution network is gradually increased. The power electronic transformer is inflexible in networking and has more complex requirements on the structure and the transformation times of power electronic equipment.

Disclosure of Invention

In view of this, the embodiment of the invention provides a multi-port power electronic transformer topological structure and an alternating current-direct current micro-grid system, so as to solve the problem that the networking of the existing power electronic transformer is not flexible.

According to a first aspect, embodiments of the present invention provide a multi-port power electronic transformer topology, comprising:

at least two AC inputs;

the voltage conversion units in each group comprise a preset number of voltage conversion subunits, each voltage conversion subunit outputs direct-current voltages of at least two voltage grades, and direct-current voltage output ends of the same voltage grade in each group of voltage conversion units are connected in parallel to form a first direct-current voltage output end of each group of voltage conversion units;

at least two groups of direct current voltage output ends, including a second direct current voltage output end after the first direct current voltage output ends of the at least two groups of voltage conversion units are connected in parallel;

and the at least one group of alternating current voltage output ends are connected with the second direct current voltage output end.

According to the multi-port power electronic transformer topological structure provided by the embodiment of the invention, alternating current-direct current conversion is realized by utilizing at least two groups of voltage conversion units, and the voltage conversion units comprise a preset number of voltage conversion subunits. Each voltage conversion subunit can output direct-current voltages of at least two voltage grades, and the direct-current voltages of the same voltage grade in the same voltage conversion unit are connected in parallel to obtain a direct-current voltage output end of the same voltage conversion unit; and meanwhile, the direct-current voltage output ends of at least two groups of voltage conversion units are connected in parallel again, so that direct-current voltages with different voltage grades can be output, and the alternating-current voltage output ends can be obtained after the direct-current voltages are inverted. Therefore, the multi-port power electronic transformer topological structure has various high-voltage, low-voltage, alternating current and direct current ports, and flexible networking can be realized by utilizing the multi-port power electronic transformer topological structure so as to meet the requirements of different voltage grades and alternating current and direct current voltages.

With reference to the first aspect, in a first implementation manner of the first aspect, the voltage conversion subunit includes:

the input end of the voltage conversion submodule is connected with the alternating voltage input end or the last voltage conversion submodule in cascade connection;

and the input end of the high-frequency transformer module is connected with the output end of the voltage conversion submodule, and the output end of the high-frequency transformer module outputs direct-current voltages of at least two voltage grades.

With reference to the first embodiment of the first aspect, in a second embodiment of the first aspect, the high-frequency transformer module includes:

the input end of the high-frequency transformer is connected with the output ends of the voltage conversion sub-modules, and at least two output ends of the high-frequency transformer output alternating-current voltages of at least two voltage grades;

and the at least two AC/DC conversion units are respectively correspondingly connected with the at least two output ends of the high-frequency transformer and are used for outputting direct-current voltages of at least two voltage grades.

According to the multi-port power electronic transformer topological structure provided by the embodiment of the invention, the high-frequency transformer can couple and add instantaneous power output by the three-phase voltage change subunit to eliminate fluctuation of instantaneous power in a single phase.

With reference to the second embodiment of the first aspect, in a third embodiment of the first aspect, the high frequency transformer includes:

the transformer comprises at least one primary winding, an iron core and at least two secondary winding units, wherein the at least two secondary winding units output alternating-current voltages of at least two voltage grades.

According to the multi-port power electronic transformer topological structure provided by the embodiment of the invention, a multi-winding high-frequency transformer is adopted, so that on one hand, the fluctuation power of a voltage conversion submodule is effectively counteracted, and the voltage fluctuation is reduced; on the other hand, the alternating-current voltage of at least two voltage grades can be output through at least two secondary side units of the same high-frequency transformer, the number of the high-frequency transformers is reduced, the circuit structure is more compact, and a plurality of ports with different low-voltage grades can be output.

With reference to the second implementation manner of the first aspect, in a fourth implementation manner of the first aspect, the voltage conversion sub-module includes:

an AC/DC power module;

a bypass module connected with the AC/DC power module;

and the first DC/AC conversion module is connected with the output end of the AC/DC power module, and the output end of the DC/AC conversion module is connected with the input end of the high-frequency transformer.

According to the multi-port power electronic transformer topological structure provided by the embodiment of the invention, a corresponding bypass module is arranged in each AC/DC power module, and when a circuit fails, the whole chain link can be protected through the bypass module; in addition, the first DC/AC conversion module, the high-frequency transformer and the AC/DC conversion unit which are connected are utilized to realize bidirectional flow of energy, so that the flexibility of the topological structure of the multiport power electronic transformer is improved.

With reference to the fourth implementation manner of the first aspect, in the fifth implementation manner of the first aspect, the AC/DC power module is a full-bridge power module, the bypass module includes two sets of controllable switches connected in parallel, and two ends of the bypass module are respectively connected to half-bridges of the full-bridge power module.

With reference to the first aspect, in a sixth implementation manner of the first aspect, the multi-port power electronic transformer topology further includes:

and the input end of the at least one second DC/AC conversion module is respectively connected with the corresponding first DC voltage output end or the second DC voltage output end.

According to the multi-port power electronic transformer topological structure provided by the embodiment of the invention, the corresponding second DC/AC conversion module is arranged at the first DC voltage output end or the second DC voltage output end, so that the multi-port power electronic transformer topological structure can output AC voltage with corresponding voltage grade, and the output ports of the multi-port power electronic transformer topological structure are diversified.

According to a second aspect, an embodiment of the present invention further provides an ac/dc microgrid system, including:

in the first aspect of the present invention or the multi-port power electronic transformer topology according to any embodiment of the first aspect, the at least two ac input terminals are connected to a corresponding first ac bus;

and the first alternating current buses corresponding to the at least two alternating current input ends are connected through the first bus coupler switch.

According to the alternating current-direct current micro-grid system provided by the embodiment of the invention, a multi-port power electronic transformer topological structure is used as an electric energy transmission, transformation and control node, so that the output voltage can be accurately controlled, the electric energy quality interference is effectively isolated, and the voltage harmonic is suppressed; at least two first alternating current buses are connected through a first bus coupler switch, at least 2 paths of alternating current power supply loop closing control and tide flexible adjustment can be achieved, tides of 2 lines can be balanced, the lines are prevented from being overloaded, the operation level of a distribution network is improved, and more distributed energy sources can be received. Meanwhile, at least two alternating current input ends in the power electronic switch are mutually standby, the characteristic that the topological structure of the multi-port power electronic transformer is flexible, fast and flexible is utilized, a load can receive high-quality power, the pressure for adjusting the fluctuation of new energy is reduced, when one path of power supply fails or is powered off, the other path of power supply passes through the fast transmission of signals, seamless switching can be carried out, the disturbance time is reduced, and the reliability of power supply is ensured.

With reference to the second aspect, in a first embodiment of the second aspect, the system further comprises:

the system comprises a first circuit breaker, a second circuit breaker and a third circuit breaker, wherein at least one group of direct-current voltage output ends of the multi-port power electronic transformer topological structure are connected with a first energy storage system through the first circuit breaker;

and/or the presence of a gas in the gas,

and at least one group of alternating voltage output ends of the multi-port power electronic transformer topological structure are connected into a second alternating current bus, and the second alternating current bus is connected with a second energy storage system through the second circuit breaker.

According to the alternating current and direct current microgrid system provided by the embodiment of the invention, the energy storage system is connected with the corresponding direct current voltage output end and/or alternating current voltage output end through the circuit breaker, so that when the direct current output by the direct current voltage output end and/or the alternating current output by the alternating current voltage output end are abnormal, the energy storage system is reversely charged to ensure the normal operation of the alternating current and direct current microgrid system.

With reference to the second aspect and the second embodiment, in a third embodiment of the second aspect, the system further comprises:

and the input end of the dry-type transformer is connected with the corresponding first alternating-current bus, the output end of the dry-type transformer is connected with a third alternating-current bus, and the third alternating-current bus is connected with the second alternating-current bus through a second bus coupler switch.

According to the alternating current-direct current micro-grid system provided by the embodiment of the invention, the output end of the dry-type transformer is connected into the third alternating current bus, the third alternating current bus is connected with the second alternating current bus through the second bus-coupled switch, and when the topological structure of the multi-port power electronic transformer is abnormal, the voltage of the third alternating current bus can be used for supplying power to the topological structure of the multi-port power electronic transformer through the second bus-coupled switch, so that the normal operation of the alternating current-direct current micro-grid system is ensured.

According to a third aspect, an embodiment of the present invention further provides a control method for a ac/dc micro-grid system, where the ac/dc micro-grid system is configured according to the second aspect of the present invention, or according to any embodiment of the second aspect, and the control method includes:

acquiring the working states of the at least two alternating current input ends;

and switching the working modes of the alternating current-direct current micro-grid system according to the working states of the at least two alternating current input ends, wherein the working modes comprise at least one of a grid-connected mode, an off-grid mode and a disconnection mode.

According to the control method of the alternating current and direct current micro grid system, the working modes of the alternating current and direct current micro grid system are switched according to the working states of the at least two alternating current input ends, so that the alternating current and direct current micro grid system is flexible and controllable, and can adapt to different power grid environments.

With reference to the third aspect, in a first embodiment of the third aspect, the switching the operation mode of the ac/dc microgrid system according to the operation states of the at least two ac input terminals includes:

when the constant first alternating current bus works normally, controlling the alternating current input end connected to the flexible first alternating current bus to work in a V/F mode, and controlling the direct current voltage output end and the alternating current voltage output end to work in a voltage stabilization mode, so that the alternating current-direct current micro-grid system works in the grid connection mode.

With reference to the first embodiment of the third aspect, in a second embodiment of the third aspect, the switching the operation mode of the ac/dc microgrid system according to the operation states of the at least two ac input terminals includes:

when the constant first alternating current bus is in power failure, controlling an alternating current input end connected to the flexible first alternating current bus to work in a V/F mode, controlling a direct current voltage output end to provide a stable power supply, and controlling the alternating current voltage output end to work in a stable voltage mode, so that the alternating current-direct current microgrid system works in the off-grid mode.

With reference to the first embodiment of the third aspect, in the third embodiment of the third aspect, the switching the operation mode of the ac/dc microgrid system according to the operation states of the at least two ac input terminals includes:

when the first alternating-current buses are all in fault, the first alternating-current bus switch is controlled to be closed, the alternating-current and direct-current microgrid system is disconnected into an isolated microgrid, and therefore the alternating-current and direct-current microgrid system works in the disconnection mode.

With reference to the third aspect, or any one of the first to third embodiments of the third aspect, in a fourth embodiment of the third aspect, the method further comprises:

when the alternating current-direct current micro-grid system is started, judging whether a third alternating current bus voltage corresponding to the dry-type transformer is normal;

when the voltage of a third alternating current bus corresponding to the dry-type transformer is normal, controlling the second bus-coupled switch to be closed so as to uncontrollably charge the alternating current voltage output end;

controlling the alternating current-direct current micro-grid system to unlock from the alternating current voltage output end to the alternating current input end step by step;

and controlling the second bus-bar switch to be switched off.

According to the control method of the alternating current and direct current micro grid system, when the alternating current and direct current micro grid system is started, the dry type transformer is used for supplying power to the alternating current output end of the alternating current and direct current micro grid system, so that gradual reverse unlocking is achieved, and low-voltage starting is achieved.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.

FIG. 1 is a schematic structural diagram of a multi-port power electronic transformer topology according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a voltage converting subunit according to the present invention;

fig. 3 is a schematic structural diagram of a high-frequency transformer module according to an embodiment of the present invention;

fig. 4 is a schematic structural diagram of a high-frequency transformer according to an embodiment of the present invention;

FIG. 5 is a schematic diagram of a voltage conversion sub-module according to an embodiment of the invention;

FIG. 6 is a schematic diagram of a voltage conversion sub-module according to an embodiment of the invention;

FIG. 7 is a schematic diagram of a DAB conversion circuit according to an embodiment of the present invention;

FIG. 8 is a schematic diagram of a multi-port power electronic transformer topology according to an embodiment of the present invention;

FIG. 9 is a schematic structural diagram of an AC/DC microgrid system according to an embodiment of the present invention;

FIG. 10 is a schematic diagram of a configuration of an AC/DC microgrid system according to an embodiment of the present invention;

fig. 11 is a schematic structural diagram of an ac/dc microgrid system according to an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

An embodiment of the present invention provides a multi-port power electronic transformer topology, as shown in fig. 1, the multi-port power electronic transformer topology includes at least two ac input terminals, at least two sets of voltage transformation units, at least two sets of dc voltage output terminals, and at least one set of ac voltage output terminals. The voltage conversion unit is arranged corresponding to the alternating current input end. For example, when the multi-port power electronic transformer topology has two ac inputs, there are two sets of voltage converting units accordingly; when the multi-port power electronic transformer topology has three alternating current input ends, three groups of voltage transformation units are correspondingly arranged. It should be noted that the AC input terminals in fig. 1 are all represented by AC, but not all the AC input terminals are connected to the same AC power source. In fig. 1, at least two groups of DC voltage outputs are denoted DC1 and DC2, respectively, and the ac voltage outputs are not shown in fig. 1.

Wherein, each said ac input terminal may be a single-phase ac input terminal or a three-phase ac input terminal. When the alternating current input end is a three-phase alternating current input end, each alternating current input end is connected with one group of voltage transformation units.

The input end of the voltage conversion unit is connected with the corresponding alternating current input end and is used for converting the input alternating current voltage and outputting at least two groups of direct current voltage output ends and at least one group of alternating current voltage. The voltage conversion unit is correspondingly connected with the alternating current input port, each group of voltage conversion units comprises a preset number of voltage conversion subunits, each voltage conversion subunit outputs direct current voltages of at least two voltage classes, and direct current voltage output ends of the same voltage class in each group of voltage conversion units are connected in parallel to form a first direct current voltage output end of each group of voltage conversion units.

Specifically, each group of voltage conversion units comprises N voltage conversion subunits, and each voltage conversion subunit outputs direct-current voltages of at least two voltage levels by performing alternating-current/direct-current conversion on input alternating-current voltages. For example, each voltage conversion subunit can output two dc voltages with different voltage levels, namely a first voltage level and a second voltage level; because each group of voltage conversion units comprises N voltage conversion subunits, the direct-current voltage output ends of the voltage conversion subunits outputting the first voltage grade are connected in parallel in each group of voltage conversion units to form a first direct-current voltage output end of one voltage grade of the voltage conversion unit; and the direct-current voltage output ends of the voltage conversion sub-units in each group of voltage conversion units, which output the second voltage level, are connected in parallel to form a first direct-current voltage output end of the voltage conversion unit with the other voltage level. It should be noted that, the number of the voltage converting sub-units included in each group of voltage converting units may be set according to practical situations, and is not limited herein.

For the multi-port power electronic transformer topological structure, first direct-current voltage output ends with the same voltage grade in each group of voltage conversion units are connected in parallel to form a second direct-current voltage output end of the multi-port power electronic transformer topological structure. Therefore, the dc voltage output terminals of the multi-port power electronic transformer topology may include a first dc voltage output terminal of at least two voltage levels and a second dc voltage output terminal of at least two voltage levels of each group of voltage conversion units.

For example, in fig. 1, the second DC voltage output terminals of two voltage levels are denoted by DC1 and DC2, respectively. Further, the DC voltage output of the multi-port power electronic transformer topology may further include a first DC voltage output in each group of voltage conversion units, which is denoted by DC3 in fig. 1. However, the dc voltage output terminal of the multi-port power electronic transformer topology according to the embodiment of the present invention is not limited to that shown in fig. 1, and may be a dc voltage output terminal of another voltage class.

Details about the structure of the voltage conversion unit will be described in detail below.

Further, after the multi-port power electronic transformer topology is formed with the above-mentioned dc voltage output terminals, the dc voltage may be converted into the ac voltage by using the voltage inversion unit based on the corresponding dc voltage output terminals, so as to obtain at least one group of ac voltage output terminals.

It should be noted that, the topology structure of the multi-port power electronic transformer specifically adopts which voltage class or classes of dc voltage output terminals or ac voltage output terminals when applied, and may be set correspondingly according to actual situations, which is not limited herein.

The multi-port power electronic transformer topology structure provided by the embodiment utilizes at least two groups of voltage conversion units to realize alternating current-direct current conversion, and the voltage conversion units comprise a preset number of voltage conversion subunits. Each voltage conversion subunit can output direct-current voltages of at least two voltage grades, and the direct-current voltages of the same voltage grade in the same voltage conversion unit are connected in parallel to obtain a direct-current voltage output end of the same voltage conversion unit; and meanwhile, the direct-current voltage output ends of at least two groups of voltage conversion units are connected in parallel again, so that direct-current voltages with different voltage grades can be output, and the alternating-current voltage output ends can be obtained after the direct-current voltages are inverted. Therefore, the multi-port power electronic transformer topological structure has various high-voltage, low-voltage, alternating current and direct current ports, and flexible networking can be realized by utilizing the multi-port power electronic transformer topological structure so as to meet the requirements of different voltage grades and alternating current and direct current voltages.

In some optional implementations of this embodiment, the voltage converting subunit includes a voltage converting submodule and a high-frequency transformer module. The input terminal of the voltage conversion submodule is connected to the AC voltage input terminal (as shown in fig. 2), or to the last voltage conversion submodule in the cascade. The input end of the high-frequency transformer module is connected with the output end of the voltage conversion submodule, and the output end of the high-frequency transformer module outputs direct-current voltage DC of at least two voltage grades. In fig. 2, DC is used to indicate only a DC voltage, and does not indicate a voltage level of all DC voltages.

Specifically, fig. 3 outputs a schematic structural diagram of the high-frequency transformer module. The high frequency transformer module includes a high frequency transformer, and at least two AC/DC converting units. In fig. 3, only two AC/DC conversion units are output, but the scope of the present invention is not limited thereto, and the number of AC/DC conversion units may be set according to actual situations.

And the input end of the high-frequency transformer is connected with the output end of the voltage conversion submodule. The high-frequency transformer is provided with at least two output ends and is used for outputting alternating voltage with at least two voltage grades. And the output ends corresponding to the high-frequency transformers are connected with an AC/DC conversion unit respectively and used for outputting direct-current voltages of at least two voltage levels. The AC/DC conversion unit is arranged corresponding to the alternating current voltage output end of the high-frequency transformer. For example, the output of the high frequency transformer is used to output ac power of a first voltage level and ac power of a second voltage level. The alternating current of the first voltage class is connected into an AC/DC conversion unit and is used for outputting direct current of one voltage class; and the alternating-current point of the second voltage level is connected into another AC/DC conversion unit for outputting direct current of another voltage level.

The output end of the high-frequency transformer can be led out from different taps of the high-frequency transformer, and can also be led out by using a winding mode, so that the high-frequency transformer can output alternating-current voltages of at least two voltage levels. For example, the high-frequency transformer includes at least one primary winding, an iron core, and at least two secondary winding units, each of which is used for outputting an ac voltage of a corresponding voltage level. The number of windings included in each secondary winding unit can be set correspondingly according to actual conditions.

By adopting the multi-winding high-frequency transformer, on one hand, the fluctuation power of the voltage conversion submodule is effectively counteracted, and the voltage fluctuation is reduced; on the other hand, the alternating-current voltage of at least two voltage grades can be output through at least two secondary side units of the same high-frequency transformer, the number of the high-frequency transformers is reduced, the circuit structure is more compact, and a plurality of ports with different low-voltage grades can be output.

As an alternative to this embodiment, as shown in fig. 4, the high frequency transformer is a 9-winding transformer, the primary side of which has 3 windings and the secondary side of which has 6 windings. 3 windings on the primary side correspond to three-phase alternating current points respectively, 6 windings on the secondary side are divided into 2 secondary side winding units, and each secondary side winding unit outputs alternating current with a voltage grade. The high frequency transformer can add instantaneous power coupling output by the three-phase voltage change subunit to eliminate fluctuation of instantaneous power in a single phase.

Specifically, as described above, each group of voltage conversion units includes a preset number of voltage conversion subunits, and the voltage conversion submodules in each voltage conversion subunit are cascaded. For example, the ac input terminal is connected to a first voltage conversion subunit in a group of voltage conversion units, and the voltage conversion subunit has a first voltage conversion submodule; the second voltage conversion subunit of the group of voltage conversion units is provided with a second voltage conversion submodule, wherein the second voltage conversion submodule is cascaded with the first voltage conversion submodule. Thus, for each voltage conversion submodule, its input is connected to the ac voltage input or to the last voltage conversion submodule in the cascade.

Further optionally, as shown in fig. 5, the voltage conversion sub-module includes an AC/DC power module, a bypass module, and a first DC/AC conversion module. The input end of the AC/DC power module is connected with the alternating current input end, or is connected with the output end of the AC/DC power module in the last voltage conversion submodule in the cascade connection.

The bypass module is connected with the AC/DC power module, and when a circuit fails, the whole chain link can be protected through the bypass module. The bypass module may be a bypass thyristor, or may be another module capable of achieving the same function, and may be selected according to the actual situation, without any limitation.

In some optional embodiments of this embodiment, as shown in fig. 6, the AC/DC power module is a full-bridge power module, and the bypass module includes two sets of controllable switches connected in parallel, and two ends of the bypass module are respectively connected to the half-bridges of the full-bridge power module. Specifically, the AC/DC power module is an H full bridge, wherein the switching tubes T1-T4 form a full bridge, the switching tube T1 is connected in series with the switching tube T3 to form a half bridge, the switching tube T2 is connected in series with the switching tube T4 to form a half bridge, and the two half bridges are connected in parallel to form a full bridge. The bypass module comprises two bypass thyristors which are connected in parallel and are respectively T11 and T12.

Further, as described above, each voltage conversion subunit includes a voltage conversion sub-module and a high frequency transformer module. The voltage conversion submodule comprises a first DC/AC conversion module, and the high-frequency transformer module comprises a high-frequency transformer and at least two AC/DC conversion units. Then, from the viewpoint of circuit structure, the first DC/AC conversion module, the high frequency transformer and the AC/DC conversion unit can realize bidirectional flow of energy, improving the flexibility of the multi-port power electronic transformer topology. For convenience of the following description, a circuit formed by connecting the first DC/AC conversion module, the high frequency transformer, and the AC/DC conversion unit may be referred to as a conversion circuit. The converter circuit may be a DAB converter circuit, or may be another converter circuit capable of realizing bidirectional energy flow, which is not limited herein. In the following description, a DAB conversion circuit is taken as an example.

Specifically, fig. 7 shows a schematic diagram of the structure of the DAB conversion circuit. The switching tubes S1-S4 form the primary side of the high-frequency transformer, namely the first DC/AC conversion module; the switching tubes Q1-Q4 form the secondary side of the high-frequency transformer, namely the AC/DC conversion unit; the two full bridges together form a DCDC part through a high frequency transformer.

The multi-port power electronic transformer topology structure further comprises at least one second DC/AC conversion module, and the input end of the second DC/AC conversion module is connected with the corresponding first DC voltage output end or the second DC voltage output end respectively. And a corresponding second DC/AC conversion module is arranged at the first DC voltage output end or the second DC voltage output end, so that the multi-port power electronic transformer topological structure can output AC voltage with corresponding voltage grade, and the output port of the multi-port power electronic transformer topological structure is diversified.

As a specific application example of the embodiment, fig. 8 shows a schematic structural diagram of a multi-port power electronic transformer topology. In the multiport power electronic transformer topology shown in fig. 8, it has two three-phase AC inputs, AC1 and AC2 respectively; corresponding to each alternating current input end, a voltage conversion unit is respectively connected; the multi-port power electronic transformer topology has an alternating voltage output terminal, and two direct voltage output terminals DC1 and DC2 with different voltage levels. The medium-voltage alternating current AC1 and the medium-voltage alternating current AC2 adopt a consistent topological structure, at least two groups of low-voltage direct current output ports of the topological structure of the multiport power electronic transformer output direct current ports DC1 and DC2 in parallel according to the required voltage grade and output capacity, and the low-voltage direct current is converted into alternating current output signals through an inverter circuit.

As shown in fig. 8, each group of voltage converting units includes 12 voltage converting subunits, each of which includes a voltage converting submodule and a high-frequency transformer module. The voltage conversion sub-module comprises an AC/DC power module, a bypass module (not shown in FIG. 8) and a first DC/AC conversion module; the high-frequency transformer module comprises a high-frequency transformer and two AC/DC conversion units.

From the viewpoint of circuit topology, the multi-port power electronic transformer topology is divided into a medium-high voltage input stage converter, a medium isolation stage converter and a low voltage output stage converter.

The medium-high voltage input converter comprises at least one group of full-bridge cascade type AC/DC power modules, and a corresponding bypass module is arranged in each power module to realize the protection of the whole chain link.

The intermediate isolation stage comprises at least one group of DAB conversion circuits, the DAB conversion circuits comprise first DC/AC conversion modules, high-frequency transformers and AC/DC conversion units, and the DAB conversion circuits output ports with different voltage levels.

The low-voltage direct current sides are connected in parallel, at least one group of medium-voltage alternating current ports are connected with another medium-voltage alternating current port in parallel, at least two groups of direct current sides are connected in parallel, the direct current output ports are connected with the direct current ports DC1 and DC2 in parallel according to the required voltage grade and the output capacity, and the multi-port power electronic transformer topological structure simultaneously provided with medium-voltage alternating current AC 1/medium-voltage alternating current AC 2/low-voltage direct current DC 1/low-voltage direct current DC 2/low-voltage alternating current AC is realized.

Specifically, the medium-high voltage input stage converter adopts at least one group of full-bridge cascaded AC/DC power modules, each group of full-bridge cascaded AC/DC power modules comprises N voltage conversion sub-modules, wherein N is 1, 2 and 3 … …, each phase of the N voltage conversion sub-modules is cascaded, namely, the input end of the next voltage conversion sub-module is connected with the output end of the previous voltage conversion sub-module. The input points of the N voltage conversion sub-modules are connected with one end of the medium-high voltage input side, and the output points and one end of the intermediate isolation level converter are connected to different ends of the primary side of the high-frequency transformer; the medium-high voltage input side, the converter subunit and the high-frequency transformer form a medium-high voltage side high-frequency signal.

The middle isolation stage converter comprises a DAB conversion circuit topology, the DAB conversion circuit mainly comprises a first DC/AC conversion unit, a high-frequency transformer and an AC/DC conversion unit, the bidirectional flow of energy is realized, and the flexibility of the system is improved. The first DC/AC conversion units are respectively connected with the direct current sides of the voltage conversion sub-modules; the high-frequency transformer adds the instantaneous power coupling output by the first DC/AC conversion unit in each phase to eliminate the instantaneous power fluctuation in a single term; and the AC/DC conversion unit is connected with the output end of the high-frequency transformer.

The low-voltage output stage converter comprises a low-voltage direct current DC1, a low-voltage direct current DC2, a low-voltage alternating current AC port and a second DC/AC conversion unit. The second direct-current voltage output ends of the two groups of voltage conversion units on the input side of the low-voltage direct-current DC1 port are connected in parallel; the input side of the low-voltage direct current DC2 port is connected with a first direct current voltage output end. And the voltage conversion subunit and the secondary side of the high-frequency transformer form a low-voltage side high-frequency signal transmission loop.

The multi-port power electronic transformer topology structure shown in fig. 8 is equivalent to two multi-port power electronic transformer topology structures, specifically, the AC1 and the AC/dc output terminal form a first multi-port power electronic transformer topology structure, and the AC2 and the AC/dc output terminal form a second multi-port power electronic transformer topology structure.

The multi-port power electronic transformer topological structure provided by the embodiment has multiple ports of high voltage, low voltage, alternating current and direct current, and 2 alternating current ports are arranged on the high-voltage side, so that loop closing control and power flow flexible adjustment of 2 paths of alternating current power supplies are realized. The multi-port power electronic transformer topological structure has short-circuit current limiting and high-load reliable power supply, and the output voltage is accurately controlled by using the multi-port power electronic transformer topological structure technology as an important electric energy transmission, transformation and control node in a power system, so that electric energy quality interference is effectively isolated, and voltage harmonic waves are inhibited; the multi-path power supply can realize seamless load transfer and reduce disturbance duration through flexible alternating current-direct current interconnection. Furthermore, the multi-winding high-frequency transformer is adopted, so that the effective cancellation of the fluctuation power of the three-phase voltage conversion subunit is ensured, the voltage fluctuation is reduced, the number of the high-frequency transformers is reduced, the circuit structure is more compact, and a plurality of ports with different low voltage grades can be output.

The embodiment of the invention also provides an alternating current and direct current micro-grid system which comprises a multi-port power electronic transformer topological structure and at least one bus coupler switch. For the structural details of the multi-port power electronic transformer topology, please refer to the above description, which is not repeated herein.

At least two alternating current input ends in the multi-port power electronic transformer topological structure are respectively connected to corresponding first alternating current buses. That is, the ac input terminals of the multi-port power electronic transformer topology may correspond to the first ac busbars one to one, or two or more ac input terminals may correspond to one first ac busbar, where the specific connection manner is not limited, and it is only required to ensure that the ac input terminals of the multi-port power electronic transformer topology are connected to at least two first ac busbars.

Furthermore, first alternating current buses corresponding to at least two alternating current input ends of the multi-port power electronic transformer topological structure are connected through a first bus coupler switch. Wherein, the first bus-bar switch is in a normally open state. The at least two alternating current input ends are mutually standby, and when one of the at least two alternating current input ends fails, the first bus-bar switch is closed, so that the alternating current-direct current micro-grid system can work normally.

For example, as shown in fig. 9, the multi-port power electronic transformer topology has 2 AC inputs, AC1 and AC2 respectively. The AC1 and the AC2 are connected to the AC power supply 1 and the AC power supply 2 via different first AC buses, respectively, and the two first AC buses are connected to each other via a first bus tie switch.

According to the alternating current-direct current micro-grid system, the topological structure of the multi-port power electronic transformer is used as a power transmission, transformation and control node, so that the output voltage can be accurately controlled, the power quality interference can be effectively isolated, and the voltage harmonic can be inhibited; at least two first alternating current buses are connected through a first bus coupler switch, at least 2 paths of alternating current power supply loop closing control and tide flexible adjustment can be achieved, tides of 2 lines can be balanced, the lines are prevented from being overloaded, the operation level of a distribution network is improved, and more distributed energy sources can be received. Meanwhile, at least two alternating current input ends in the power electronic switch are mutually standby, the characteristic that the topological structure of the multi-port power electronic transformer is flexible, fast and flexible is utilized, a load can receive high-quality power, the pressure for adjusting the fluctuation of new energy is reduced, when one path of power supply fails or is powered off, the other path of power supply passes through the fast transmission of signals, seamless switching can be carried out, the disturbance time is reduced, and the reliability of power supply is ensured.

In some optional embodiments of this embodiment, the system further comprises a first circuit breaker, and at least one group of dc voltage outputs of the multi-port power electronic transformer topology is connected to the first energy storage system through the first circuit breaker.

Further, the system may further include a second circuit breaker, at least one set of ac voltage output terminals of the multi-port power electronic transformer topology is connected to a second ac bus, and the second ac bus is connected to a second energy storage system through the second circuit breaker.

The energy storage system is connected with the corresponding direct current voltage output end and/or the alternating current voltage output end through the circuit breaker, and therefore when the direct current output by the direct current voltage output end and/or the alternating current output by the alternating current voltage output end are abnormal, the energy storage system can be charged reversely to guarantee normal operation of the alternating current and direct current micro-grid system.

As an optional implementation manner of this embodiment, the system further includes a dry-type transformer, an input end of which is connected to the corresponding first ac bus; the output end of the dry-type transformer is connected with the third alternating current bus, and the third alternating current bus is connected with the second alternating current bus through the second bus coupler switch.

When the topological structure of the multi-port power electronic transformer is abnormal, the voltage of the third alternating current bus can be used for supplying power to the topological structure of the multi-port power electronic transformer through the second bus coupler switch so as to ensure the normal operation of the alternating current and direct current micro-grid system.

As a specific implementation manner of this embodiment, as shown in fig. 10, the first AC busbars corresponding to the medium-voltage AC1 and the AC2 are connected through the first buscouple switch. The AC1 and the AC2 are mutually standby, magnetic isolation is realized between buses at two ends in a normal operation mode, zero sequence faults are not interfered with each other, and greater flexibility is provided for the grounding mode and operation of the two systems. The access to a distributed power supply, energy storage, electric vehicle charging, an alternating current/direct current power grid and the like is facilitated. Under the support of the low-voltage energy storage system, voltage sags on two sides are not interfered with each other, and the quality of electric energy on two sides is improved.

The alternating current-direct current micro-grid system can be suitable for supplying power to multiple paths of power supplies, meanwhile, a multifunctional alternating current-direct current interconnection networking mode is provided, a distributed power supply, energy storage, electric vehicle charging, an alternating current/direct current power grid and the like can be conveniently accessed, the redundant quantity of a multi-port power electronic transformer topological structure is reduced, the power density is improved, the distributed renewable energy consumption capability is promoted, and the power supply reliability of an alternating current-direct current hybrid micro-grid is improved.

Specifically, the alternating current-direct current microgrid system is provided with 2 medium-high voltage alternating current grid-connected ports, and is connected with a medium-high voltage main power supply and a standby power supply. Before the standby power supply is put into operation, the two medium-high voltage grid-connected ports are mutually standby or one of the two medium-high voltage grid-connected ports is selected as a system power supply node. Therefore, two sections of middle-high voltage buses are designed, an alternating current bus on one side of the middle-high voltage bus is a power supply inlet wire bus, and a standby bus on the other side of the middle-high voltage bus is arranged. The multi-port power electronic transformer topological structure with double medium-high voltage Alternating Current (AC) ends, a low-voltage Direct Current (DC) end and a low-voltage AC end is adopted to realize the integrated connection of all the ports.

The medium-high voltage alternating current AC1 is connected with a first alternating current bus on one side of the medium-high voltage through a breaker, the medium-high voltage alternating current AC2 is connected with a first alternating current bus on the other side of the medium-high voltage through a breaker, a first bus coupler switch is bridged between two sections of buses, and the buses are normally opened. The DC ends of the medium-high voltage Alternating Current (AC) 1 and the AC2 are connected with the DC/AC direct current side of the topological structure of the multiport power electronic transformer after passing through an internal breaker, and the public direct current end is connected with a low-voltage direct current bus through the breaker; the multi-port power electronic transformer topology DC/AC is connected to the second AC bus via a circuit breaker.

Further, the alternating current-direct current micro-grid system also comprises a transformer, such as a dry-type transformer. The input end of the transformer is connected with a first alternating current bus corresponding to the medium-high voltage alternating current AC1, and the output end of the transformer is connected with a third alternating current bus. And the third alternating current bus is connected with a second alternating current bus corresponding to the alternating current output end of the multi-port power electronic transformer topological structure through a second bus coupler switch. When the topological structure of the multi-port power electronic transformer fails, the 380V alternating current output by the transformer can be utilized to realize low-voltage starting, gradual charging, gradual unlocking and charging from low voltage to a high-voltage capacitor, so that a 10kV current-limiting resistor is omitted, and the occupied area is saved. Specifically, 380V alternating current output by the transformer supplies power to the multi-port power electronic transformer topological structure through the second bus coupler switch, so that reverse and gradual upward charging is realized.

The 240V or 750V direct current bus corresponding to the multi-port power electronic transformer topological structure can be connected with a direct current load, a direct current energy storage system and a roof photovoltaic. The low-voltage alternating current 380V corresponding to the transformer can be connected with a low-voltage alternating current load, a fan and an alternating current energy storage system. And a second alternating current bus corresponding to the alternating current output end of the multi-port power electronic transformer topological structure is connected with a third alternating current bus corresponding to the low-voltage alternating current 380V through a second bus coupler switch, and the second bus coupler switch is in a disconnected state. The microgrid is connected to the power grid through a 10kV power supply, when the breaker is disconnected or fails, the microgrid is in an off-grid mode, poverty-relieving photovoltaic and 10kV alternating current loads still access the microgrid, a stable working power supply is provided by low-voltage direct current energy storage, and the energy storage can be automatically charged and discharged when working in a voltage-stabilizing current-limiting mode.

The voltage class corresponding to the alternating-current and direct-current micro-grid system comprises 10kV AC1/10kV AC2/750V DC/380V AC. The medium-voltage input 10kV AC1 end of the multi-port power electronic transformer topological structure is connected to an upper-level power grid through a grid-connected point, and an incoming wire of the medium-voltage input 10kV AC2 end is led from a MW-level renewable energy source or a power supply system; alternating current 10kV AC1 and AC2 buses are connected through a first bus coupler switch, AC1 and AC2 are mutually standby, two paths of alternating current are connected to achieve power flow balance, power supply reliability is improved, and new energy is flexibly connected to the power supply. The 750V direct current of this alternating current-direct current microgrid system inserts the energy storage, fills electric pile, and 380V alternating current side is for office lighting, fan and other alternating current load power supplies. The system can supply power to a traffic hub through 10kV alternating current, so that a low-voltage alternating current and direct current hybrid micro-grid is jointly constructed, the dry-type transformer is connected to a superior grid through a grid-connected point through a high-voltage end, and the low-voltage end is connected to a third alternating current bus and is connected with a low-voltage alternating current end of a multi-port power electronic transformer topological structure through a second bus coupler switch.

The alternating current-direct current micro-grid system is suitable for multi-path power supply, meanwhile, a multifunctional alternating current-direct current interconnection networking mode is provided, a distributed power supply, energy storage, electric vehicle charging, an alternating current/direct current power grid and the like can be conveniently accessed, the redundant quantity of a multi-port power electronic transformer topological structure is reduced, the power density is improved, the distributed renewable energy consumption capability is promoted, and the power supply reliability of an alternating current-direct current hybrid micro-grid is improved.

The embodiment of the invention also provides a control method of the topological structure of the multi-port power electronic transformer, which comprises the following steps:

and S11, acquiring the working states of at least two alternating current input ends.

For the working state of the alternating current input end of the alternating current and direct current micro-grid system, whether the alternating current input end works normally or not, whether the alternating current input end loses power or not and the like can be judged by monitoring the port voltage of the alternating current input end. For example, a voltage measuring device may be provided at each ac input terminal to measure the port voltage, so as to obtain the operating states of at least two ac input terminals.

And S12, switching the working modes of the alternating current and direct current micro grid system according to the working states of the at least two alternating current input ends.

The working mode comprises at least one of a grid-connected mode, an off-grid mode and a disconnection mode.

When the working state of each alternating current input end is obtained, the working mode of the whole alternating current-direct current micro-grid is switched based on the difference of the working states. The switching of the working modes is determined by controlling the working modes of all the alternating current/direct current input/output ports of the alternating current/direct current microgrid.

Details about this step will be described later.

According to the control method of the multi-port power electronic transformer topological structure, the working modes of the alternating current and direct current micro grid system are switched according to the working states of at least two alternating current input ends, so that the alternating current and direct current micro grid system is flexible and controllable, and can adapt to different power grid environments.

As an optional implementation manner of this embodiment, when the constant first ac bus operates normally, the ac input terminal connected to the flexible first ac bus is controlled to operate in the V/F mode, and the dc voltage output terminal and the ac voltage output terminal are controlled to operate in the voltage stabilization mode, so that the ac-dc microgrid system operates in the grid-connected mode.

As shown in fig. 11, a stable power supply is provided by a 10kV alternating current in a grid-connected mode, a flexible 10kV alternating current port AC2 works in a V/F mode, lean photovoltaic maximum power generation (MPPT), a low-voltage 750V direct current, 380V alternating current port works in a voltage-stabilizing mode, and power balance of a microgrid is coordinately controlled.

Optionally, when the constant first alternating current bus is in power failure, the alternating current input end connected into the flexible first alternating current bus is controlled to work in a V/F mode, the direct current voltage output end is controlled to provide a stable power supply, and the alternating current voltage output end is controlled to work in a voltage stabilizing mode, so that the alternating current-direct current microgrid system works in an off-grid mode.

As shown in fig. 11, when the 10kV AC power supply loses power, the AC/dc microgrid system operates in an off-grid mode, the low-voltage 750V dc side energy storage is used as a stable power supply, the flexible 10kV AC port AC2 operates in a V/F mode, the poverty-reduced photovoltaic maximum power generation is performed, and the 380V AC port operates in a voltage-stabilizing mode, so as to ensure source-storage-load power balance.

Optionally, when the first alternating-current buses are all in fault, the first alternating-current bus switch is controlled to be closed, the alternating-current and direct-current microgrid system is disconnected into an isolated microgrid, and therefore the alternating-current and direct-current microgrid system works in a disconnection mode.

As shown in fig. 11, when both the flexible-to-dual ports are in fault, the alternating-current/direct-current micro-grid system works in the disconnection mode, the flexible-to-10 kV bus tie switch is closed, the load of the junction station is supplied by the 10kV alternating-current power supply, and the poverty-relieving photovoltaic power generation is normal; the low-voltage alternating current and direct current micro-grid is decomposed into isolated micro-grids which are respectively powered by energy storage serving as a support source.

In some optional embodiments of this embodiment, the ac/dc microgrid system further provides low-voltage startup, and specifically includes the following steps:

(1) and when the voltage of a third alternating current bus corresponding to the dry-type transformer is normal, controlling the second bus-coupled switch to be closed so as to uncontrollably charge the alternating current voltage output end.

(2) And controlling the AC/DC micro-grid system to unlock from the AC voltage output end to the AC input end step by step.

(3) And controlling the second bus-bar switch to be switched off.

As shown in fig. 10, when the ac/dc micro grid system is started, the low-voltage start of the multi-port power electronic transformer can be realized by the low-voltage ac output by the dry-type transformer on the left side of fig. 10. That is, for the ac/dc power electronic transformer, the ac voltage output terminal is first started, and the voltage is gradually increased until the ac input terminal is unlocked. After the unlocking is completed, i.e. after the multi-port power electronic transformer is started, the second bus tie switch between the ac output of the dry-type transformer and the ac output of the multi-port power electronic transformer is opened.

As a specific embodiment of this embodiment, as shown in fig. 11, when the 10kV bus voltage is normal, the ac/dc microgrid has a start-up condition. The standby branch of the low-voltage side of the dry transformer is used for starting, if the low-voltage equipment is abnormal, the low-voltage equipment can be started by using stored energy, and the charging starting mode of the low-voltage AC side is preferentially considered. Note that, in fig. 11, the second bus tie switch is replaced with two circuit breakers 401 and 402.

During the starting process, if the topology of the multi-port power electronic transformer is abnormal (AC1 or AC2 fails), the starting is stopped, and the multi-port power electronic transformer returns to the standby state.

The start-up procedure may comprise the steps of:

1. the voltage of a bus at the 400V side of the dry-change side is normal, the energy storage locking state is realized, 312 is switched on,

2.401, 402, 301, DC/AC uncontrolled charging,

3. flexible AC1 isolation level unlock, AC2 isolation level unlock,

4.201, the switch is closed,

5.202, the switch is closed,

6. and unlocking the flexible AC1, controlling the direct current voltage stabilization, stabilizing the voltage of the module capacitor at 900V +/-10%, stabilizing the low-voltage direct current at 750V, and returning to the hot standby state if the low-voltage direct current is not stabilized at 750V.

7. Disconnect 401, 402, 301.

8. Flexible AC2 unlock, V/f control, 203 close,

9.211 is closed, the switch is turned on,

10. unlocking flexible DC/AC, controlling V/F, closing 301, automatically judging and charging stored energy, closing 311 and 313, and loading 750V direct current; and 380V alternating current is loaded, and the grid connection state is entered.

Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art may make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations fall within the scope defined by the appended claims.

Claims (15)

1. A multi-port power electronic transformer topology, comprising:

at least two AC inputs;

the voltage conversion units in each group comprise a preset number of voltage conversion subunits, each voltage conversion subunit outputs direct-current voltages of at least two voltage grades, and direct-current voltage output ends of the same voltage grade in each group of voltage conversion units are connected in parallel to form a first direct-current voltage output end of each group of voltage conversion units;

at least two groups of direct current voltage output ends, including a second direct current voltage output end after the first direct current voltage output ends of the at least two groups of voltage conversion units are connected in parallel;

and the at least one group of alternating current voltage output ends are connected with the second direct current voltage output end.

2. The multi-port power electronic transformer topology of claim 1, wherein the voltage translation subunit comprises:

the input end of the voltage conversion submodule is connected with the alternating current input end or the last voltage conversion submodule in cascade connection;

and the input end of the high-frequency transformer module is connected with the output end of the voltage conversion submodule, and the output end of the high-frequency transformer module outputs direct-current voltages of at least two voltage grades.

3. The multi-port power electronic transformer topology of claim 2, wherein the high frequency transformer module comprises:

the input end of the high-frequency transformer is connected with the output ends of the voltage conversion sub-modules, and at least two output ends of the high-frequency transformer output alternating-current voltages of at least two voltage grades;

and the at least two AC/DC conversion units are respectively correspondingly connected with the at least two output ends of the high-frequency transformer and are used for outputting direct-current voltages of at least two voltage grades.

4. The multi-port power electronic transformer topology of claim 3, wherein the high frequency transformer comprises:

the transformer comprises at least one primary winding, an iron core and at least two secondary winding units, wherein the at least two secondary winding units output alternating-current voltages of at least two voltage grades.

5. The multi-port power electronic transformer topology of claim 3, wherein the voltage translation sub-module comprises:

an AC/DC power module;

a bypass module connected with the AC/DC power module;

and the first DC/AC conversion module is connected with the output end of the AC/DC power module, and the output end of the DC/AC conversion module is connected with the input end of the high-frequency transformer.

6. The multi-port power electronic transformer topology of claim 5, wherein the AC/DC power module is a full-bridge power module, the bypass module comprises two sets of controllable switches connected in parallel, and two ends of the bypass module are respectively connected to half-bridges of the full-bridge power module.

7. The multi-port power electronic transformer topology of claim 1, further comprising:

and the input end of the at least one second DC/AC conversion module is respectively connected with the corresponding first DC voltage output end or the second DC voltage output end.

8. An AC/DC microgrid system, comprising:

the multi-port power electronic transformer topology of any one of claims 1-7, the at least two AC inputs being connected to respective first AC buses;

and the first alternating current buses corresponding to the at least two alternating current input ends are connected through the first bus coupler switch.

9. The AC/DC microgrid system of claim 8, further comprising:

the system comprises a first circuit breaker, a second circuit breaker and a third circuit breaker, wherein at least one group of direct-current voltage output ends of the multi-port power electronic transformer topological structure are connected with a first energy storage system through the first circuit breaker;

and/or the presence of a gas in the gas,

and at least one group of alternating voltage output ends of the multi-port power electronic transformer topological structure are connected into a second alternating current bus, and the second alternating current bus is connected with a second energy storage system through the second circuit breaker.

10. The AC/DC microgrid system of claim 9, further comprising:

and the input end of the dry-type transformer is connected with the corresponding first alternating-current bus, the output end of the dry-type transformer is connected with a third alternating-current bus, and the third alternating-current bus is connected with the second alternating-current bus through a second bus coupler switch.

11. A method for controlling a direct current/direct current microgrid system, wherein the direct current/direct current microgrid system is arranged according to any one of claims 8-10, and the method comprises:

acquiring the working states of the at least two alternating current input ends;

and switching the working modes of the alternating current-direct current micro-grid system according to the working states of the at least two alternating current input ends, wherein the working modes comprise at least one of a grid-connected mode, an off-grid mode and a disconnection mode.

12. The control method according to claim 11, wherein the at least two ac input terminals are respectively connected to a first constant ac bus and a first flexible ac bus, and the switching the operation mode of the ac-dc microgrid system according to the operation states of the at least two ac input terminals comprises:

when the constant first alternating current bus works normally, controlling the alternating current input end connected to the flexible first alternating current bus to work in a V/F mode, and controlling the direct current voltage output end and the alternating current voltage output end to work in a voltage stabilization mode, so that the alternating current-direct current micro-grid system works in the grid connection mode.

13. The control method according to claim 12, wherein the switching the operating mode of the ac-dc microgrid system according to the operating states of the at least two ac input terminals comprises:

when the constant first alternating current bus is in power failure, controlling an alternating current input end connected to the flexible first alternating current bus to work in a V/F mode, controlling a direct current voltage output end to provide a stable power supply, and controlling the alternating current voltage output end to work in a stable voltage mode, so that the alternating current-direct current microgrid system works in the off-grid mode.

14. The control method according to claim 12, wherein the switching the operating mode of the ac-dc microgrid system according to the operating states of the at least two ac input terminals comprises:

when the first alternating-current buses are all in fault, the first alternating-current bus switch is controlled to be closed, the alternating-current and direct-current microgrid system is disconnected into an isolated microgrid, and therefore the alternating-current and direct-current microgrid system works in the disconnection mode.

15. The control method according to any one of claims 11 to 14, characterized in that the method further comprises:

when the alternating current-direct current micro-grid system is started, judging whether a third alternating current bus voltage corresponding to the dry-type transformer is normal;

when the voltage of a third alternating current bus corresponding to the dry-type transformer is normal, controlling the second bus-coupled switch to be closed so as to uncontrollably charge the alternating current voltage output end;

controlling the alternating current-direct current micro-grid system to unlock from the alternating current voltage output end to the alternating current input end step by step;

and controlling the second bus-bar switch to be switched off.

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