Multi-port power electronic transformer topology and its AC / DC microgrid system

Abstract

The present invention relates to the technical field of power equipment, and specifically to a multi-port power electronic transformer topology structure and an AC / DC microgrid system thereof, wherein the multi-port power electronic transformer topology structure comprises at least two AC input terminals; at least two groups of voltage conversion units, which are connected to the respective AC input ports, each group of voltage conversion units comprises a preset number of voltage conversion sub-units, each voltage conversion sub-unit outputs at least two voltage levels of DC voltage, and the DC voltage output terminals of the same voltage level in each group of voltage conversion units are connected in parallel to form a first DC voltage output terminal of each group of voltage conversion units; at least two groups of DC voltage output terminals, including a second DC voltage output terminal after the first DC voltage output terminals of at least two groups of voltage conversion units are connected in parallel; and at least one group of AC voltage output terminals is connected to a second DC voltage output terminal. With high and low voltage, AC and DC multiple ports, flexible networking can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of power equipment, and in particular to a multi-port power electronic transformer topology structure and an AC / DC microgrid system. Background Art

[0002] The basic idea of ​​the power electronic transformer (EPT) is to replace the industrial frequency transformer with a high-frequency transformer, and introduce power electronic conversion technology on the source and secondary sides of the high-frequency transformer, so as to achieve flexible adjustment of the voltage, current and power on both sides of the transformer through appropriate control. At present, power electronic transformers have been widely used in distribution networks.

[0003] Among them, the existing power electronic transformers are mostly high and low voltage ports. However, with the high proportion of new energy access and the large-scale application of multiple loads, the DC characteristics are obvious, and the number of AC and DC ports in the distribution network is gradually increasing. The networking of the above-mentioned power electronic transformers is not flexible, and the structure and conversion times of power electronic equipment are more complicated. Summary of the invention

[0004] In view of this, an embodiment of the present invention provides a multi-port power electronic transformer topology structure and an AC / DC microgrid system to solve the problem of inflexible networking of existing power electronic transformers.

[0005] According to a first aspect, an embodiment of the present invention provides a multi-port power electronic transformer topology structure, comprising:

[0006] At least two AC input terminals;

[0007] At least two groups of voltage conversion units are connected to the respective AC input ports, each group of the voltage conversion units includes a preset number of voltage conversion sub-units, each of the voltage conversion sub-units outputs DC voltages of at least two voltage levels, and the DC voltage output ends of the same voltage level in each group of the voltage conversion units are connected in parallel to form a first DC voltage output end of each group of the voltage conversion units;

[0008] At least two groups of DC voltage output terminals, including second DC voltage output terminals of the at least two groups of voltage conversion units connected in parallel;

[0009] At least one set of AC voltage output terminals is connected to the second DC voltage output terminal.

[0010] The multi-port power electronic transformer topology structure provided in the embodiment of the present invention realizes AC-DC conversion by using at least two groups of voltage conversion units, and includes a preset number of voltage conversion sub-units in the voltage conversion unit. Each voltage conversion sub-unit can output DC voltages of at least two voltage levels, and the DC voltages of the same voltage level in the same voltage conversion unit are connected in parallel to obtain the DC voltage output end of the same voltage conversion unit; at the same time, the DC voltage output ends of at least two groups of voltage conversion units are connected in parallel again, so that DC voltages of different voltage levels can be output, and the AC voltage output end can be obtained by inverting the DC voltage. Therefore, the multi-port power electronic transformer topology structure has high and low voltage, AC and DC ports. The multi-port power electronic transformer topology structure can be used to realize flexible networking to meet the needs of different voltage levels and AC and DC voltages.

[0011] In combination with the first aspect, in a first implementation of the first aspect, the voltage conversion subunit includes:

[0012] A voltage conversion submodule, whose input end is connected to the AC voltage input end, or the previous voltage conversion submodule in cascade connection;

[0013] The input end of the high-frequency transformer module is connected to 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 levels.

[0014] In combination with the first implementation of the first aspect, in the second implementation of the first aspect, the high-frequency transformer module includes:

[0015] A high-frequency transformer, whose input end is connected to the output end of the voltage conversion submodule, and at least two output ends of the high-frequency transformer output AC voltages of at least two voltage levels;

[0016] At least two AC / DC conversion units are respectively connected to at least two output ends of the high-frequency transformer, and the at least two AC / DC conversion units are used to output direct current voltages of at least two voltage levels.

[0017] The multi-port power electronic transformer topology structure provided by the embodiment of the present invention can couple and add the instantaneous power output by the three-phase voltage variation subunits to eliminate the fluctuation of the instantaneous power in a single phase.

[0018] In combination with the second implementation of the first aspect, in a third implementation of the first aspect, the high-frequency transformer includes:

[0019] 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 voltages of at least two voltage levels.

[0020] The multi-port power electronic transformer topology structure provided in the embodiment of the present invention adopts a multi-winding high-frequency transformer, which, on the one hand, ensures that the fluctuating power of the voltage conversion sub-module is effectively offset, thereby reducing the voltage fluctuation; on the other hand, at least two secondary side units of the same high-frequency transformer can output AC voltages of at least two voltage levels, thereby reducing the number of high-frequency transformers, making the circuit structure more compact, and being able to output multiple ports of different low-voltage levels.

[0021] In combination with the second implementation manner of the first aspect, in a fourth implementation manner of the first aspect, the voltage conversion submodule includes:

[0022] AC / DC power modules;

[0023] A bypass module connected to the AC / DC power module;

[0024] The first DC / AC conversion module is connected to the output end of the AC / DC power module, and the output end of the DC / AC conversion module is connected to the input end of the high-frequency transformer.

[0025] The multi-port power electronic transformer topology structure provided by the embodiment of the present invention has a corresponding bypass module set in each AC / DC power module, and the bypass module can protect the entire chain link when a circuit fault occurs; and by using the connected first DC / AC conversion module, high-frequency transformer and AC / DC conversion unit, bidirectional flow of energy can be achieved, thereby improving the flexibility of the multi-port power electronic transformer topology structure.

[0026] In combination with the fourth embodiment of the first aspect, in the fifth embodiment of the first aspect, the AC / DC power module is a full-bridge power module, the bypass module includes two groups of controllable switches in parallel, and the two ends of the bypass module are respectively connected to the half bridge of the full-bridge power module.

[0027] In combination with the first aspect, in a sixth implementation of the first aspect, the multi-port power electronic transformer topology structure further includes:

[0028] At least one second DC / AC conversion module has an input end connected to the corresponding first DC voltage output end or the second DC voltage output end.

[0029] The multi-port power electronic transformer topology structure provided in an embodiment of the present invention has a corresponding second DC / AC conversion module set at the first DC voltage output end or the second DC voltage output end, so that the multi-port power electronic transformer topology structure can output an AC voltage of a corresponding voltage level, thereby making the output port of the multi-port power electronic transformer topology structure diverse.

[0030] According to a second aspect, an embodiment of the present invention further provides an AC / DC microgrid system, comprising:

[0031] In the multi-port power electronic transformer topology structure described in the first aspect of the present invention, or in any one of the embodiments of the first aspect, the at least two AC input terminals are connected to the corresponding first AC busbars;

[0032] At least one first bus tie switch, the first AC busbars corresponding to the at least two AC input terminals are connected via the first bus tie switch.

[0033] The AC / DC microgrid system provided by the embodiment of the present invention utilizes a multi-port power electronic transformer topology as a power transmission, conversion and control node, can accurately control the output voltage, effectively isolate power quality interference, and suppress voltage harmonics; at least two first AC busbars are connected through a first bus tie switch, which can realize at least two AC power closed-loop control and flexible flow regulation, balance the flow of two lines, prevent line overload, improve the distribution network operation level, and accommodate more distributed energy. At the same time, at least two AC input terminals in the power electronic switch are mutually standby, and the flexible, fast and flexible characteristics of the multi-port power electronic transformer topology are utilized to enable the load to receive high-quality power, reduce the regulation pressure of new energy fluctuations, and when one power supply fails or loses power, the other power supply can be seamlessly switched through the rapid transmission of signals, reduce disturbance time, and ensure power supply reliability.

[0034] In conjunction with the second aspect, in a first implementation of the second aspect, the system further includes:

[0035] A first circuit breaker, through which at least one group of DC voltage output terminals of the multi-port power electronic transformer topology structure is connected to a first energy storage system;

[0036] and / or,

[0037] A second circuit breaker, at least one group of AC voltage output terminals of the multi-port power electronic transformer topology structure is connected to a second AC bus, and the second AC bus is connected to a second energy storage system through a second circuit breaker.

[0038] In the AC / DC microgrid system provided in the embodiment of the present invention, the energy storage system is connected to the corresponding DC voltage output terminal and / or AC voltage output terminal through a circuit breaker, so that when the DC output of the DC voltage output terminal and / or the AC output of the AC voltage output terminal is abnormal, the reverse charging of the energy storage system is utilized to ensure the normal operation of the AC / DC microgrid system.

[0039] In combination with the second embodiment of the second aspect, in a third embodiment of the second aspect, the system further includes:

[0040] A dry-type transformer, whose input end is connected to the corresponding first AC bus, and whose output end is connected to the third AC bus, and the third AC bus is connected to the second AC bus through a second bus tie switch.

[0041] In the AC / DC microgrid system provided by the embodiment of the present invention, the output end of a dry-type transformer is connected to a third AC bus in the system, and the third AC bus is connected to the second AC bus through a second bus tie switch. When an abnormality occurs in the topology structure of the multi-port power electronic transformer, the voltage of the third AC bus can be used to supply power to the multi-port power electronic transformer topology structure through the second bus tie switch to ensure the normal operation of the AC / DC microgrid system.

[0042] According to a third aspect, an embodiment of the present invention further provides a control method for an AC / DC microgrid system, wherein the AC / DC microgrid system is provided according to the second aspect of the present invention, or the AC / DC microgrid system described in any one embodiment of the second aspect, and the control method comprises:

[0043] Acquiring the working status of the at least two AC input terminals;

[0044] According to the working status of the at least two AC input terminals, the working mode of the AC / DC microgrid system is switched, and the working mode includes at least one of a grid-connected mode, an off-grid mode and a disconnected mode.

[0045] The control method of the AC / DC microgrid system provided in the embodiment of the present invention switches the working mode of the AC / DC microgrid system through the working status of at least two AC input terminals, so that the AC / DC microgrid system is flexible and controllable and can adapt to different power grid environments.

[0046] In combination with the third aspect, in a first implementation manner of the third aspect, the at least two AC input terminals are respectively connected to a constant first AC bus and a flexible first AC bus, and the switching of the working mode of the AC / DC microgrid system according to the working states of the at least two AC input terminals includes:

[0047] When the constant first AC bus operates normally, the AC input terminal connected to the flexible first AC bus is controlled to operate in V / F mode, and the DC voltage output terminal and the AC voltage output terminal are controlled to operate in voltage stabilization mode, so that the AC / DC microgrid system operates in the grid-connected mode.

[0048] In combination with the first implementation manner of the third aspect, in the second implementation manner of the third aspect, switching the working mode of the AC / DC microgrid system according to the working status of the at least two AC input terminals includes:

[0049] When the constant first AC bus loses power, the AC input terminal connected to the flexible first AC bus is controlled to operate in V / F mode, the DC voltage output terminal is controlled to provide a stable power supply, and the AC voltage output terminal is controlled to operate in a voltage stabilization mode, so that the AC / DC microgrid system operates in the off-grid mode.

[0050] In combination with the first implementation manner of the third aspect, in the third implementation manner of the third aspect, switching the working mode of the AC / DC microgrid system according to the working status of the at least two AC input terminals includes:

[0051] When all of the first AC busbars fail, the first AC busbar switches are controlled to close, and the AC / DC microgrid system is disconnected into an isolated microgrid, so that the AC / DC microgrid system operates in the disconnected mode.

[0052] In combination with 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 includes:

[0053] When the AC / DC microgrid system is started, determining whether the third AC bus voltage corresponding to the dry-type transformer is normal;

[0054] When the voltage of the third AC bus corresponding to the dry-type transformer is normal, controlling the second bus tie switch to close so as to charge the AC voltage output terminal without control;

[0055] Controlling the AC / DC microgrid system to unlock step by step from the AC voltage output end to the AC input end;

[0056] Control the second bus tie switch to open.

[0057] The control method of the AC / DC microgrid system provided in the embodiment of the present invention uses a dry-type transformer to supply power to the AC output end of the AC / DC microgrid system when the AC / DC microgrid system is started, so as to achieve step-by-step reverse unlocking and realize low-voltage starting. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0059] Figure 1 is a schematic diagram of a topological structure of a multi-port power electronic transformer according to an embodiment of the present invention;

[0060] Figure 2 is a schematic structural diagram of a voltage conversion subunit implemented according to the present invention;

[0061] Figure 3 is a structural schematic diagram of a high-frequency transformer module according to an embodiment of the present invention;

[0062] Figure 4 is a schematic structural diagram of a high-frequency transformer according to an embodiment of the present invention;

[0063] Figure 5 is a schematic structural diagram of a voltage conversion submodule according to an embodiment of the present invention;

[0064] Figure 6 is a schematic structural diagram of a voltage conversion submodule according to an embodiment of the present invention;

[0065] Figure 7 is a structural schematic diagram of a DAB conversion circuit according to an embodiment of the present invention;

[0066] Figure 8 is a schematic diagram of a topological structure of a multi-port power electronic transformer according to an embodiment of the present invention;

[0067] Fig. 9 is a structural schematic diagram of an AC / DC microgrid system according to an embodiment of the present invention;

[0068] Fig.10 is a structural schematic diagram of an AC / DC microgrid system according to an embodiment of the present invention;

[0069] Fig.11 is a structural schematic diagram of an AC / DC microgrid system according to an embodiment of the present invention. DETAILED DESCRIPTION

[0070] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0071] The embodiment of the present invention provides a multi-port power electronic transformer topology structure, such as Figure 1As shown, the multi-port power electronic transformer topology structure includes at least two AC input terminals, at least two groups of voltage conversion units, at least two groups of DC voltage output terminals and at least one group of AC voltage output terminals. The voltage conversion units are arranged corresponding to the AC input terminals. For example, when the multi-port power electronic transformer topology structure has two AC input terminals, it has two groups of voltage conversion units accordingly; when the multi-port power electronic transformer topology structure has three AC input terminals, it has three groups of voltage conversion units accordingly. It should be noted that Figure 1 The AC input terminals in the diagram are all represented by AC, but this does not mean that all AC input terminals are connected to the same AC power supply. Figure 1 In the example, at least two groups of DC voltage output terminals are represented by DC1 and DC2 respectively, and the AC voltage output terminals are represented by Figure 1 Not shown.

[0072] Wherein, each of the AC input terminals can be a single-phase AC input terminal or a three-phase AC input terminal. When the AC input terminal is a three-phase AC input terminal, each phase AC input terminal is connected to a group of voltage conversion units.

[0073] The input end of the voltage conversion unit is connected to the corresponding AC input end, and is used to convert the input AC voltage and output at least two groups of DC voltage output ends and at least one group of AC voltage. The voltage conversion unit is connected to the AC input port accordingly, and each group of the voltage conversion units includes a preset number of voltage conversion sub-units, each voltage conversion sub-unit outputs at least two voltage levels of DC voltage, and the DC voltage output ends of the same voltage level in each group of voltage conversion units are connected in parallel to form the first DC voltage output end of each group of voltage conversion units.

[0074] Specifically, each group of voltage conversion units includes N voltage conversion sub-units, and each voltage conversion sub-unit outputs at least two voltage levels of DC voltage by performing AC-DC conversion on the input AC voltage. For example, each voltage conversion sub-unit can output two different voltage levels of DC voltage, namely, a first voltage level and a second voltage level; since each group of voltage conversion units includes N voltage conversion sub-units, then in each group of voltage conversion units, the DC voltage output ends of each voltage conversion sub-unit outputting the first voltage level are connected in parallel to form a first DC voltage output end of one voltage level of the voltage conversion unit; in each group of voltage conversion units, the DC voltage output ends of each voltage conversion sub-unit outputting the second voltage level are connected in parallel to form a first DC voltage output end of another voltage level of the voltage conversion unit. It should be noted that the number of voltage conversion sub-units included in each group of voltage conversion units can be set accordingly according to actual conditions, and no limitation is made here.

[0075] Wherein, each group of voltage conversion units can output DC voltages of at least two voltage levels. For a multi-port power electronic transformer topology structure, the first DC voltage output terminals with the same voltage level in each group of voltage conversion units are connected in parallel to form a second DC voltage output terminal of the multi-port power electronic transformer topology structure. Therefore, the DC voltage output terminals of the multi-port power electronic transformer topology structure can include the first DC voltage output terminals of at least two voltage levels of each group of voltage conversion units, and the second DC voltage output terminals of at least two voltage levels.

[0076] For example, in Figure 1 In the figure, DC1 and DC2 are used to represent the second DC voltage output terminals of two voltage levels. Further, the DC voltage output terminals of the multi-port power electronic transformer topology structure may also include the first DC voltage output terminals in each group of voltage conversion units. Figure 1 However, the DC voltage output terminal of the multi-port power electronic transformer topology structure in the embodiment of the present invention is not limited to Figure 1 As shown, it can also be a DC voltage output terminal of other voltage levels.

[0077] The structural details of the voltage conversion unit will be described in detail below.

[0078] Furthermore, after forming the above-mentioned DC voltage output terminals, the multi-port power electronic transformer topology structure can convert DC into AC based on the corresponding DC voltage output terminals and using a voltage inverter unit, thereby obtaining at least one group of AC voltage output terminals.

[0079] It should be noted that the DC voltage output terminal or AC voltage output terminal of the multi-port power electronic transformer topology structure can be configured accordingly according to actual conditions when it is used, and no limitation is imposed here.

[0080] The multi-port power electronic transformer topology structure provided in this embodiment uses at least two groups of voltage conversion units to realize AC-DC conversion, and the voltage conversion unit includes a preset number of voltage conversion sub-units. Each voltage conversion sub-unit can output DC voltages of at least two voltage levels, and the DC voltages of the same voltage level in the same voltage conversion unit are connected in parallel to obtain the DC voltage output end of the same voltage conversion unit; at the same time, the DC voltage output ends of at least two groups of voltage conversion units are connected in parallel again, so that DC voltages of different voltage levels can be output, and the AC voltage output end can be obtained by inverting the DC voltage. Therefore, the multi-port power electronic transformer topology structure has high and low voltage, AC and DC ports. The multi-port power electronic transformer topology structure can be used to achieve flexible networking to meet the needs of different voltage levels and AC and DC voltages.

[0081] In some optional implementations of this embodiment, the voltage conversion subunit includes a voltage conversion submodule and a high-frequency transformer module. The input end of the voltage conversion submodule is connected to the AC voltage input end AC (such as Figure 2 The input end of the high frequency transformer module is connected to the output end of the voltage conversion submodule, and the output end outputs a DC voltage of at least two voltage levels. Figure 2 The term DC is used here only to indicate direct current voltage, and does not indicate the voltage level of all direct current voltages.

[0082] Specifically, Figure 3 The high-frequency transformer module includes a high-frequency transformer and at least two AC / DC conversion units. Figure 3 Only two AC / DC conversion units are output, but the protection scope of the present invention is not limited to this, and the number of AC / DC conversion units can be set according to actual conditions.

[0083] The input end of the high-frequency transformer is connected to the output end of the voltage conversion submodule. The high-frequency transformer has at least two output ends for outputting AC voltages of at least two voltage levels. Corresponding to the output end of the high-frequency transformer, an AC / DC conversion unit is connected for outputting DC voltages of at least two voltage levels. The AC / DC conversion unit is arranged corresponding to the AC voltage output end of the high-frequency transformer. For example, the output end 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 AC power of the first voltage level is connected to an AC / DC conversion unit for outputting DC power of one voltage level; the AC power of the second voltage level is connected to another AC / DC conversion unit for outputting DC power of another voltage level.

[0084] The output end of the high-frequency transformer can be drawn from different taps of the high-frequency transformer, or can be drawn by means of windings, so that the high-frequency transformer can output at least two voltage levels of AC voltage. 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 to output an AC voltage of a corresponding voltage level. The number of windings included in each secondary winding unit can be set accordingly according to actual conditions.

[0085] The use of a multi-winding high-frequency transformer, on the one hand, ensures that the fluctuating power of the voltage conversion sub-module is effectively offset, thereby reducing voltage fluctuations; on the other hand, at least two secondary side units of the same high-frequency transformer can output AC voltages of at least two voltage levels, reducing the number of high-frequency transformers, making the circuit structure more compact, and being able to output multiple ports of different low-voltage levels.

[0086] As an optional implementation of this embodiment, Figure 4 As shown, the high-frequency transformer is a 9-winding transformer, with 3 windings on the primary side and 6 windings on the secondary side. The 3 windings on the primary side correspond to the three-phase AC points respectively, and the 6 windings on the secondary side are divided into 2 secondary winding units, each of which outputs an AC power of a voltage level. The high-frequency transformer can couple and add the instantaneous power output by the three-phase voltage change subunit to eliminate the fluctuation of the instantaneous power in a single phase.

[0087] Specifically, as described above, each group of voltage conversion units includes a preset number of voltage conversion sub-units, and the voltage conversion sub-modules in each voltage conversion sub-unit are cascaded. For example, the AC input terminal is connected to the first voltage conversion sub-unit in a group of voltage conversion units, and the voltage conversion sub-unit has a first voltage conversion sub-module; the second voltage conversion sub-unit in the group of voltage conversion units has a second voltage conversion sub-module, wherein the second voltage conversion sub-module is cascaded with the first voltage conversion sub-module. Therefore, for each voltage conversion sub-module, its input terminal is connected to the AC voltage input terminal, or is connected to the previous voltage conversion sub-module in the cascade.

[0088] Further optionally, if Figure 5 As shown, the voltage conversion submodule includes an AC / DC power module, a bypass module and a first DC / AC conversion module, wherein the input end of the AC / DC power module is connected to the AC input end, or is connected to the output end of the AC / DC power module in the previous cascaded voltage conversion submodule.

[0089] The bypass module is connected to the AC / DC power module, and the bypass module can protect the entire chain link when a circuit fails. The bypass module can be a bypass thyristor or other modules that can achieve the same function. The specific selection can be made according to the actual situation, and no limitation is made here.

[0090] In some optional implementations of this embodiment, such as Figure 6As shown, 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 the two ends of the bypass module are respectively connected to the half-bridge of the full-bridge power module. Specifically, the AC / DC power module is an H full-bridge, wherein the switch tubes T1-T4 constitute a full bridge, the switch tube T1 and the switch tube T3 are connected in series to form a half bridge, the switch tube T2 and the switch tube T4 are connected in series to form a half bridge, and the two half bridges are connected in parallel to form a full bridge. The bypass module includes two bypass thyristors connected in parallel, namely T11 and T12.

[0091] Further, as described above, each voltage conversion sub-unit includes a voltage conversion sub-module and a high-frequency transformer module. Among them, the voltage conversion sub-module includes a first DC / AC conversion module, and the high-frequency transformer module includes a high-frequency transformer and at least two AC / DC conversion units. Then, from the perspective of circuit structure, the first DC / AC conversion module, the high-frequency transformer and the AC / DC conversion unit can realize the bidirectional flow of energy, thereby improving the flexibility of the topological structure of the multi-port power electronic transformer. For the convenience of the following description, the circuit formed by connecting the first DC / AC conversion module, the high-frequency transformer and the AC / DC conversion unit can be referred to as a conversion circuit. The conversion circuit can be a DAB conversion circuit, or it can be other conversion circuits that can realize the bidirectional flow of energy, and no limitation is made here. In the following description, the DAB conversion circuit is taken as an example.

[0092] Specifically, Figure 7 The schematic diagram of the structure of the DAB conversion circuit is shown. Among them, the switch tubes S1-S4 constitute the primary side of the high-frequency transformer, that is, the first DC / AC conversion module; the switch tubes Q1-Q4 constitute the secondary side of the high-frequency transformer, that is, the AC / DC conversion unit; these two full bridges together constitute the DCDC part through the high-frequency transformer.

[0093] The multi-port power electronic transformer topology structure also includes at least one second DC / AC conversion module, whose input end is respectively connected to the corresponding first DC voltage output end or the second DC voltage output end. 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 topology structure can output an AC voltage of a corresponding voltage level, so as to achieve the diversity of the output ports of the multi-port power electronic transformer topology structure.

[0094] As a specific application example of this embodiment, Figure 8 The schematic diagram of the multi-port power electronic transformer topology is shown in FIG. Figure 8In the multi-port power electronic transformer topology structure shown, it has two three-phase AC input terminals, AC1 and AC2 respectively; corresponding to each AC input terminal, a voltage conversion unit is connected respectively; the multi-port power electronic transformer topology structure has an AC voltage output terminal, and two DC voltage output terminals DC1 and DC2 of different voltage levels. Among them, the medium voltage AC AC1 and the medium voltage AC AC2 adopt the same topology structure, and at least two groups of low voltage DC output ports of the multi-port power electronic transformer topology structure are connected in parallel to output DC ports DC1 and DC2 according to the required voltage level and output capacity, and the low voltage DC is then converted into an AC output signal through an inverter circuit.

[0095] like Figure 8 As shown, each group of voltage conversion units includes 12 voltage conversion sub-units, and each voltage conversion sub-unit includes a voltage conversion sub-module and a high-frequency transformer module. The voltage conversion sub-module includes an AC / DC power module, a bypass module ( Figure 8 (not shown) and a first DC / AC conversion module; the high-frequency transformer module includes a high-frequency transformer and two AC / DC conversion units.

[0096] From the perspective of circuit topology, the multi-port power electronic transformer topology is divided into a medium- and high-voltage input stage converter, an intermediate isolation stage converter, and a low-voltage output stage converter.

[0097] The medium and high voltage input converter includes at least one group of full-bridge cascaded AC / DC power modules, and a corresponding bypass module is arranged in each power module to realize the protection of the entire chain link.

[0098] The intermediate isolation stage includes at least one set of DAB conversion circuits, wherein the DAB conversion circuits include a first DC / AC conversion module, a high-frequency transformer and an AC / DC conversion unit, and the DAB conversion circuits output ports of different voltage levels.

[0099] The low-voltage DC sides are connected in parallel, and at least one group of medium-voltage AC ports is connected in parallel with another medium-voltage AC port. At least two groups of DC sides are connected in parallel, and the DC output ports output DC ports DC1 and DC2 in parallel according to the required voltage level and output capacity, thereby realizing a multi-port power electronic transformer topology structure with medium-voltage AC1 / medium-voltage AC2 / low-voltage DC1 / low-voltage DC2 / low-voltage AC.

[0100] Specifically, the medium and high voltage input stage converter adopts at least one group of full-bridge cascaded AC / DC power modules, and one group of full-bridge cascaded AC / DC power modules includes N voltage conversion submodules, N=1, 2, 3..., and the N voltage conversion submodules per phase are cascaded, that is, the input end of the latter voltage conversion submodule is connected to the output end of the previous voltage conversion submodule. The input points of the N voltage conversion submodules are connected to one end of the medium and high voltage input side, and the output points and one end of the intermediate isolation stage converter are connected to different ends of the primary side of the high-frequency transformer; the medium and high voltage input side, the commutation subunit and the high-frequency transformer form a medium and high voltage side high-frequency signal.

[0101] The intermediate isolation level converter includes a DAB conversion circuit topology, and the DAB conversion circuit mainly includes a first DC / AC conversion unit, a high-frequency transformer and an AC / DC conversion unit, which realizes a bidirectional flow of energy and improves the flexibility of the system. The first DC / AC conversion unit is respectively connected to the DC side of the voltage conversion submodule; the high-frequency transformer couples and adds the instantaneous power output by the first DC / AC conversion unit in each phase to eliminate the instantaneous power fluctuation in a single phase; the AC / DC conversion unit is connected to the output end of the high-frequency transformer.

[0102] The low-voltage output stage converter includes a low-voltage DC1, a low-voltage DC2, a low-voltage AC port, and a second DC / AC conversion unit. The first DC voltage output terminals of two groups of voltage conversion units on the input side of the low-voltage DC1 port are connected in parallel to the second DC voltage output terminals; the input side of the low-voltage DC2 port is connected to the first DC voltage output terminal. The voltage conversion subunit and the secondary side of the high-frequency transformer constitute a low-voltage side high-frequency signal transmission loop.

[0103] in, Figure 8 The multi-port power electronic transformer topology structure is equivalent to two multi-port power electronic transformer topology structures, specifically, AC1 and the AC / DC output terminals constitute the first multi-port power electronic transformer topology structure, and AC2 and the AC / DC output terminals constitute the second multi-port power electronic transformer topology structure.

[0104] The multi-port power electronic transformer topology structure provided in this embodiment has high and low voltage, AC and DC ports, and there are 2 AC ports on the high voltage side to realize closed-loop control of 2-way AC power supplies and flexible power flow regulation. The multi-port power electronic transformer topology structure has short-circuit current limiting and high-reliability power supply for loads. It uses the multi-port multi-port power electronic transformer topology structure technology as an important power transmission, conversion and control node in the power system to accurately control the output voltage, effectively isolate power quality interference, and suppress voltage harmonics; multiple power supplies are interconnected through flexible AC and DC to achieve seamless load transfer and reduce the duration of disturbances. Furthermore, the use of a multi-winding high-frequency transformer ensures that the fluctuating power of the three-phase voltage conversion sub-unit is effectively offset, the voltage fluctuation is reduced, the number of high-frequency transformers is reduced, the circuit structure is more compact, and multiple ports of different low-voltage levels can be output.

[0105] The embodiment of the present invention further provides an AC / DC microgrid system, which includes a multi-port power electronic transformer topology structure and at least one bus tie switch. For details of the structure of the multi-port power electronic transformer topology structure, please refer to the above description and will not be repeated here.

[0106] Among them, at least two AC input terminals in the multi-port power electronic transformer topology are respectively connected to the corresponding first AC busbars. 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. The specific connection method is not limited here, and it is only necessary to ensure that the AC input terminals of the multi-port power electronic transformer topology are connected to at least two first AC busbars.

[0107] Furthermore, the first AC busbars corresponding to at least two AC input terminals of the multi-port power electronic transformer topology structure are connected via a first bus tie switch. The first bus tie switch is in a normally open state. The at least two AC input terminals are mutually backup, and when one of them fails, the closing of the first bus tie switch enables the AC / DC microgrid system to operate normally.

[0108] For example, Fig. 9 The multi-port power electronic transformer topology structure has two AC input terminals, AC1 and AC2. AC1 and AC2 are respectively connected to AC power source 1 and AC power source 2 through different first AC busbars, and the two first AC busbars are connected through a first bus tie switch.

[0109] The AC / DC microgrid system provided in this embodiment uses a multi-port power electronic transformer topology as a power transmission, conversion and control node, which can accurately control the output voltage, effectively isolate power quality interference, and suppress voltage harmonics; at least two first AC busbars are connected through a first bus tie switch, which can realize at least two AC power supply closed-loop control and flexible flow adjustment, balance the flow of two lines, prevent line overload, improve the distribution network operation level, and accommodate more distributed energy. At the same time, at least two AC input terminals in the power electronic switch are standby for each other, and the flexible, fast and flexible characteristics of the multi-port power electronic transformer topology structure are used to enable the load to receive high-quality power, reduce the regulation pressure of new energy fluctuations, and when one power supply fails or loses power, the other power supply can be seamlessly switched through the rapid transmission of signals, reduce disturbance time, and ensure power supply reliability.

[0110] In some optional implementations of this embodiment, the system further includes a first circuit breaker, and at least one group of DC voltage output terminals of the multi-port power electronic transformer topology structure is connected to the first energy storage system through the first circuit breaker.

[0111] Furthermore, the system may also include a second circuit breaker, at least one group of AC voltage output terminals of the multi-port power electronic transformer topology structure is connected to a second AC bus, and the second AC bus is connected to a second energy storage system through a second circuit breaker.

[0112] The energy storage system is connected to the corresponding DC voltage output terminal and / or AC voltage output terminal through a circuit breaker, so that when the DC output from the DC voltage output terminal and / or the AC output from the AC voltage output terminal is abnormal, the reverse charging of the energy storage system is utilized to ensure the normal operation of the AC / DC microgrid system.

[0113] As an optional implementation of this embodiment, the system also includes a dry-type transformer, whose input end is connected to the corresponding first AC bus; the output end of the dry-type transformer is connected to the third AC bus, and the third AC bus is connected to the second AC bus through a second bus tie switch.

[0114] In the system, the output end of the dry-type transformer is connected to the third AC bus, and the third AC bus is connected to the second AC bus through a second bus tie switch. When an abnormality occurs in the multi-port power electronic transformer topology structure, the voltage of the third AC bus can be used to supply power to the multi-port power electronic transformer topology structure through the second bus tie switch to ensure the normal operation of the AC / DC microgrid system.

[0115] As a specific implementation of this embodiment, Fig.10As shown in the figure, the first AC busbars corresponding to the medium voltage AC1 and AC2 are connected through the first bus tie switch. AC1 and AC2 are backup for each other. Under normal operation, magnetic isolation is achieved between the busbars at both ends, and zero-sequence faults do not interfere with each other, providing greater flexibility for the grounding method and operation of the two systems. It is convenient to access distributed power sources, energy storage, electric vehicle charging, AC / DC power grids, etc. With the support of the low-voltage energy storage system, the voltage drops on both sides do not interfere with each other, which improves the power quality on both sides.

[0116] This AC / DC microgrid system can be applied to power supply from multiple power sources, and provide a multifunctional AC / DC interconnection networking mode, which is convenient for access to distributed power sources, energy storage, electric vehicle charging, AC / DC power grids, etc., reduces the number of redundant topological structures of multi-port power electronic transformers, improves power density, promotes the absorption capacity of distributed renewable energy, and improves the power supply reliability of AC / DC hybrid microgrids.

[0117] Specifically, the AC / DC microgrid system has two medium and high voltage AC grid-connected ports, which are connected to the medium and high voltage main power supply and the backup power supply. Before the backup power supply is put into use, the two medium and high voltage grid-connected ports are backup for each other, or one of them is selected as the system power supply node. Based on this, two sections of medium and high voltage busbars are designed, the AC busbar on the medium and high voltage side is the power supply incoming busbar, and the backup busbar on the medium and high voltage side is the backup busbar. A multi-port power electronic transformer topology structure with dual medium and high voltage AC (AC) terminals, low voltage DC (DC) terminals, and low voltage AC (AC) terminals is adopted to realize the integrated connection of each port.

[0118] The medium-high voltage AC1 is connected to the first AC busbar on the medium-high voltage side through a circuit breaker, and the medium-high voltage AC2 is connected to the first AC busbar on the other side of the medium-high voltage through a circuit breaker. The first bus tie switch is connected across the two busbars, and the switch is normally open. The DC ends of the medium-high voltage AC1 and AC2 are connected to the DC side of the multi-port power electronic transformer topology structure DC / AC through an internal circuit breaker, and the common DC end is connected to the low-voltage DC busbar through a circuit breaker; the multi-port power electronic transformer topology structure DC / AC is connected to the second AC busbar through a circuit breaker.

[0119] Furthermore, the AC / DC microgrid system also includes a transformer, for example, a dry-type transformer. The input end of the transformer is connected to the first AC bus corresponding to the medium and high voltage AC AC1, and the output end of the transformer is connected to the third AC bus. Among them, the third AC bus is connected to the second AC bus corresponding to the AC output end of the multi-port power electronic transformer topology structure through a second bus tie switch. When the multi-port power electronic transformer topology structure fails, the 380V AC power output by the transformer can be used to realize the low-voltage start-up of the multi-port power electronic transformer topology structure, step-by-step charging, hierarchical unlocking, and charging from low voltage to high voltage capacitors, eliminating the 10kV current limiting resistor and saving space. Specifically, the 380V AC power output by the transformer supplies power to the multi-port power electronic transformer topology structure through the second bus tie switch, thereby realizing reverse and step-by-step upward charging.

[0120] Among them, the 240V or 750V DC bus corresponding to the multi-port power electronic transformer topology can be connected to DC loads, DC energy storage systems, and rooftop photovoltaics. The low-voltage AC 380V corresponding to the transformer can be connected to low-voltage AC loads, fans, and AC energy storage systems. The second AC bus corresponding to the AC output end of the multi-port power electronic transformer topology is connected to the third AC bus corresponding to the low-voltage AC 380V through the second bus tie switch, and the second bus tie switch is in the disconnected state. The microgrid is connected to the grid through a 10kV power supply. When the circuit breaker is disconnected or fails, the microgrid is in off-grid mode, and poverty alleviation photovoltaics and 10kV AC loads are still connected to the microgrid, and the low-voltage DC energy storage provides a stable working power supply. The energy storage can automatically charge and discharge when working in the voltage stabilization and current limiting mode.

[0121] The voltage levels corresponding to this AC / DC microgrid system include 10kV AC1 / 10kV AC2 / 750V DC / 380VAC. Among them, the medium-voltage input 10kV AC1 end of the multi-port power electronic transformer topology structure is connected to the upper power grid through the grid connection point, and the medium-voltage input 10kV AC2 end is drawn from the MW-level renewable energy or power system; the AC 10kV AC1 and AC2 busbars are connected through the first bus tie switch, AC1 and AC2 are standby for each other, and the two AC accesses realize the flow balance, improve the power supply reliability, and flexibly access new energy. The 750V DC of this AC / DC microgrid system is connected to the energy storage and charging piles, and the 380V AC side is used to supply power to office lighting, fans and other AC loads. The system can supply power to the transportation hub through 10kV AC, so as to jointly form a low-voltage AC / DC hybrid microgrid. The dry-type transformer is connected to the upper power grid through the grid connection point through the high-voltage end, and the low-voltage end is connected to the third AC busbar, which is connected to the low-voltage AC end of the multi-port power electronic transformer topology structure through the second bus tie switch.

[0122] The AC / DC microgrid system provided in this embodiment is suitable for power supply by multiple power sources, and at the same time provides a multifunctional AC / DC interconnection networking mode, which is convenient for access to distributed power sources, energy storage, electric vehicle charging, AC / DC power grids, etc., reduces the number of redundant topological structures of multi-port power electronic transformers, improves power density, promotes the capacity of distributed renewable energy consumption, and improves the power supply reliability of AC / DC hybrid microgrids.

[0123] The embodiment of the present invention further provides a method for controlling a multi-port power electronic transformer topology structure, the method comprising the following steps:

[0124] S11, obtaining the working status of at least two AC input terminals.

[0125] For the working state of the AC input terminal of the AC / DC microgrid system, the port voltage of the AC input terminal can be monitored to determine whether it is working normally, whether it is power-off, etc. For example, a voltage measuring device can be set at each AC input terminal to measure the port voltage, so as to obtain the working state of at least two AC input terminals.

[0126] S12, switching the working mode of the AC / DC microgrid system according to the working status of at least two AC input terminals.

[0127] The working mode includes at least one of a grid-connected mode, an off-grid mode and a disconnected mode.

[0128] When the working state of each AC input terminal is known, the working mode of the entire AC / DC microgrid is switched based on the different working states. The switching of the working mode is determined by controlling the working mode of each AC / DC input and output port of the AC / DC microgrid.

[0129] This step will be described in detail below.

[0130] The control method of the multi-port power electronic transformer topology structure provided in this embodiment switches the working mode of the AC / DC microgrid system through the working status of at least two AC input terminals, so that the AC / DC microgrid system is flexible and controllable and can adapt to different power grid environments.

[0131] As an optional implementation 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 V / F mode, and the DC voltage output terminal and the AC voltage output terminal are controlled to operate in voltage stabilization mode, so that the AC / DC microgrid system operates in the grid-connected mode.

[0132] like Fig.11As shown, in the grid-connected mode, 10kV AC provides a stable power supply, the flexible 10kV AC port AC2 works in V / F mode, poverty alleviation photovoltaic maximum power generation (MPPT), and the low-voltage 750V DC and 380V AC ports work in the voltage stabilization mode to coordinate and control the power balance of the microgrid.

[0133] Optionally, when the constant first AC bus loses power, the AC input end connected to the flexible first AC bus is controlled to operate in V / F mode, the DC voltage output end is controlled to provide a stable power supply, and the AC voltage output end is controlled to operate in a voltage stabilization mode, so that the AC / DC microgrid system operates in an off-grid mode.

[0134] like Fig.11 As shown, when the 10kV AC power supply loses power, the AC / DC microgrid system operates in off-grid mode, with the low-voltage 750V DC side energy storage serving as a stable power source. The flexible 10kV AC port AC2 operates in V / F mode, generating maximum power for poverty alleviation photovoltaic power generation, and the 380V AC port operates in voltage stabilization mode to ensure source-storage-load power balance.

[0135] Optionally, when all first AC busbars fail, the first AC busbar switches are controlled to close, and the AC / DC microgrid system is disconnected into an isolated microgrid, so that the AC / DC microgrid system operates in a disconnected mode.

[0136] like Fig.11 As shown in the figure, when both ports of the flexible transformer fail, the AC / DC microgrid system operates in the decoupling mode, the 10kV bus tie switch of the flexible transformer is closed, the hub station load is powered by the 10kV AC power supply, and the poverty alleviation photovoltaic system generates electricity normally; the low-voltage AC / DC microgrid is decoupled into an isolated microgrid, and is powered by energy storage as the supporting source.

[0137] In some optional implementations of this embodiment, the AC / DC microgrid system further provides low voltage startup, specifically, including the following steps:

[0138] (1) When the voltage of the third AC bus corresponding to the dry-type transformer is normal, the second bus tie switch is controlled to be closed so as to charge the AC voltage output terminal without control.

[0139] (2) Control the AC / DC microgrid system to unlock from the AC voltage output end to the AC input end step by step.

[0140] (3) Control the second bus tie switch to disconnect.

[0141] like Fig.10 As mentioned above, when the AC / DC microgrid system is started, Fig.10The low-voltage AC output of the dry-type transformer on the left realizes the low-voltage startup of the multi-port power electronic transformer. That is, for the AC / DC power electronic transformer, it starts from the AC voltage output end and goes up step by step until the AC input end is unlocked. After the unlocking is completed, that is, after the multi-port power electronic transformer is started, the second bus tie switch between the AC output end of the dry-type transformer and the AC output end of the multi-port power electronic transformer is disconnected.

[0142] As a specific implementation of this embodiment, Fig.11 As shown in the figure, when the 10kV bus voltage is normal, the AC / DC microgrid is ready for startup. The standby branch on the low-voltage side of the dry transformer is used for startup. If there is an abnormality in the low-voltage equipment, energy storage can be used for startup, and the low-voltage AC side charging startup method is given priority. Fig.11 In the embodiment, two circuit breakers 401 and 402 are used to replace the second bus tie switch.

[0143] During the startup process, if the topology of the multi-port power electronic transformer is abnormal (AC1 or AC2 fault), the startup is stopped and the system returns to the standby state.

[0144] The startup process may include the following steps:

[0145] 1. The 400V bus voltage on the dry transformer side is normal, the energy storage is locked, and 312 is closed.

[0146] 2.401, 402, 301 are closed, DC / AC is not controlled for charging,

[0147] 3. The AC1 isolation level of the flexible transformer is unlocked, and the AC2 isolation level is unlocked.

[0148] 4.201 closing,

[0149] 5.202 closing,

[0150] 6. The flexible AC1 is unlocked, the DC voltage is controlled, the module capacitor voltage is stabilized at 900V±10%, and the low voltage DC is stabilized at 750V, otherwise it returns to the hot standby state.

[0151] 7. Disconnect 401, 402, and 301.

[0152] 8. AC2 of the flexible transformer is unlocked, V / f control is performed, 203 is closed,

[0153] 9.211 closing,

[0154] 10. Unlock the soft DC / AC, V / F control, close 301, the energy storage automatically determines charging, close 311 and 313, 750V DC with load; 380V AC with load, enter the grid-connected state.

[0155] 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 present invention, and such modifications and variations are all within the scope defined by the appended claims.

Claims

1. A multi-port power electronic transformer topology structure, characterized in that: include: At least two AC input terminals; At least two groups of voltage conversion units are connected to the respective AC input terminals, each group of the voltage conversion units includes a preset number of voltage conversion sub-units, each of the voltage conversion sub-units outputs DC voltages of at least two voltage levels, and the DC voltage output terminals of the same voltage level in each group of the voltage conversion units are connected in parallel to form a first DC voltage output terminal of each group of the voltage conversion units; At least two groups of DC voltage output terminals, including second DC voltage output terminals of the at least two groups of voltage conversion units connected in parallel; At least one set of AC voltage output terminals connected to the first DC voltage output terminal, or connected to the second DC voltage output terminal; Wherein, the voltage conversion subunit comprises: A voltage conversion submodule, whose input end is connected to the AC input end, or the previous voltage conversion submodule in cascade connection; A high-frequency transformer module, whose input end is connected to the output end of the voltage conversion submodule, and the output end of the high-frequency transformer module outputs a DC voltage of at least two voltage levels; The high frequency transformer module comprises: A high-frequency transformer, whose input end is connected to the output end of the voltage conversion submodule, and at least two output ends of the high-frequency transformer output AC voltages of at least two voltage levels; At least two AC / DC conversion units are respectively connected to at least two output ends of the high-frequency transformer, and the at least two AC / DC conversion units are used to output direct current voltages of at least two voltage levels; The high frequency 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 AC voltages of at least two voltage levels; The voltage conversion submodule comprises: AC / DC power modules; A bypass module connected to the AC / DC power module; The first DC / AC conversion module is connected to the output end of the AC / DC power module, and the output end of the DC / AC conversion module is connected to the input end of the high-frequency transformer.

2. The multi-port power electronic transformer topology structure according to claim 1, characterized in that: The AC / DC power module is a full-bridge power module, the bypass module includes two groups of controllable switches connected in parallel, and two ends of the bypass module are respectively connected to the half bridge of the full-bridge power module.

3. The multi-port power electronic transformer topology structure according to claim 1, characterized in that: The multi-port power electronic transformer topology structure also includes: At least one second DC / AC conversion module has an input end connected to the corresponding first DC voltage output end or the second DC voltage output end.

4. An AC / DC microgrid system, characterized in that: include: The multi-port power electronic transformer topology structure according to any one of claims 1 to 3, wherein the at least two AC input terminals are connected to the corresponding first AC busbars; At least one first bus tie switch, the first AC busbars corresponding to the at least two AC input terminals are connected via the first bus tie switch; A dry-type transformer, whose input end is connected to the corresponding first AC bus, and whose output end is connected to the third AC bus, and the third AC bus is connected to the second AC bus through a second bus tie switch.

5. The AC / DC microgrid system according to claim 4, characterized in that: The system further comprises: A first circuit breaker, through which at least one group of DC voltage output terminals of the multi-port power electronic transformer topology structure is connected to a first energy storage system; and / or, A second circuit breaker, at least one group of AC voltage output terminals of the multi-port power electronic transformer topology structure is connected to a second AC bus, and the second AC bus is connected to a second energy storage system through a second circuit breaker.

6. A control method for an AC / DC microgrid system, characterized in that: The AC / DC microgrid system is configured according to any one of claims 4 to 5, and the control method comprises: Acquiring the working status of the at least two AC input terminals; According to the working status of the at least two AC input terminals, the working mode of the AC / DC microgrid system is switched, and the working mode includes at least one of a grid-connected mode, an off-grid mode and a disconnected mode.

7. The control method according to claim 6, characterized in that: The at least two AC input terminals are respectively connected to a constant first AC bus and a flexible first AC bus, and the switching of the working mode of the AC / DC microgrid system according to the working states of the at least two AC input terminals includes: When the constant first AC bus operates normally, the AC input terminal connected to the flexible first AC bus is controlled to operate in V / F mode, and the DC voltage output terminal and the AC voltage output terminal are controlled to operate in voltage stabilization mode, so that the AC / DC microgrid system operates in the grid-connected mode.

8. The control method according to claim 7, characterized in that: The step of switching the working mode of the AC / DC microgrid system according to the working states of the at least two AC input terminals comprises: When the constant first AC bus loses power, the AC input terminal connected to the flexible first AC bus is controlled to operate in V / F mode, the DC voltage output terminal is controlled to provide a stable power supply, and the AC voltage output terminal is controlled to operate in a voltage stabilization mode, so that the AC / DC microgrid system operates in the off-grid mode.

9. The control method according to claim 7, characterized in that: The step of switching the working mode of the AC / DC microgrid system according to the working states of the at least two AC input terminals comprises: When all of the first AC busbars fail, the first AC busbar switches are controlled to close, and the AC / DC microgrid system is disconnected into an isolated microgrid, so that the AC / DC microgrid system operates in the disconnected mode.

10. The control method according to any one of claims 6 to 9, characterized in that: The method further comprises: When the AC / DC microgrid system is started, determining whether the third AC bus voltage corresponding to the dry-type transformer is normal; When the voltage of the third AC bus corresponding to the dry-type transformer is normal, controlling the second bus tie switch to close so as to charge the AC voltage output terminal without control; Controlling the AC / DC microgrid system to unlock step by step from the AC voltage output end to the AC input end; Control the second bus tie switch to open.

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