A three-circulation mode AC exchanger and its control method

Abstract

The present invention discloses a novel three-circulation mode AC converter, comprising a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm, and a seventh bridge arm; one end of the first bridge arm is simultaneously connected to one end of the second bridge arm and one end of the seventh bridge arm, the other end of the second bridge arm is connected to one end of the third bridge arm, the other end of the third bridge arm is connected to one end of the fourth bridge arm, the other end of the fourth bridge arm is simultaneously connected to one end of the fifth bridge arm and the other end of the seventh bridge arm, the other end of the fifth bridge arm is connected to one end of the sixth bridge arm, and the other end of the sixth bridge arm is connected to the other end of the first bridge arm. The present invention utilizes seven bridge arms to form an AC converter with three circulation channels and provides a corresponding control method. Compared with existing single-circulation converters, the present invention can significantly reduce the volume and cost of the converter system. There are three power transmission channels, and during normal operation, there is no need to meet the strict constraints of the single circulation channel.

Description

Technical Field

[0001] The present invention relates to the technical field of topological structures of power conversion devices, and in particular to a three-circulation mode AC converter and a control method thereof. Background Art

[0002] High-voltage, high-power AC converters are widely used in practical engineering applications, playing a crucial role in asynchronous interconnection, offshore wind power, long-distance frequency-splitting power transmission, and offshore oil and gas extraction. High-power AC converters are also indispensable in power transmission applications such as rail transit, mining, and ship propulsion, as well as in high-voltage, high-power power supply industries.

[0003] AC converters are the core components of flexible frequency-dividing power transmission systems. Currently, mainstream converter topologies all utilize modular multilevel technology. This technology enables rapid and independent control of active and reactive power within the operating range. The receiving system can be a passive network, offering advantages such as low losses, high waveform quality, and robust fault handling capabilities.

[0004] However, existing multi-loop converters, such as the 12-arm back-to-back MMC and the 9-arm M3C, require a large number of IGBTs (insulated gate bipolar transistors) due to their large number of arms, resulting in high investment costs. Single-loop converters, such as hexagonal converters, have fewer arms but only a single circulation channel. To ensure proper operation and circulation control, the reactive power on both sides of the converter must meet strict constraints. Otherwise, reactive power compensation equipment must be added to the converter, causing inconvenience. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a three-circulation mode AC converter and a control method thereof, which has low cost and is not subject to the constraints of a single-circulation converter in operation.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] A three-circulation mode AC exchanger, comprising a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and a seventh bridge arm;

[0008] One end of the first bridge arm is connected to one end of the second bridge arm and one end of the seventh bridge arm at the same time, and forms a first node;

[0009] The other end of the second bridge arm is connected to one end of the third bridge arm and forms a second node, and the other end of the third bridge arm is connected to one end of the fourth bridge arm and forms a third node;

[0010] The other end of the fourth bridge arm is connected to one end of the fifth bridge arm and the other end of the seventh bridge arm, and forms a fourth node. The other end of the fifth bridge arm is connected to one end of the sixth bridge arm, and forms a fifth node.

[0011] The other end of the sixth bridge arm is connected to the other end of the first bridge arm and serves as a sixth node;

[0012] The first node, the third node, and the fifth node are commonly used to connect a first three-phase AC system, and the second node, the fourth node, and the sixth node are commonly used to connect a second three-phase AC system.

[0013] In order to solve the above technical problems, another technical solution adopted by the present invention is:

[0014] A control method for a three-circulation mode AC exchanger is applied to the above-mentioned three-circulation mode AC exchanger, comprising the following steps:

[0015] S1. Take the first, third, and fifth nodes as the output side, and the second, fourth, and sixth nodes as the input side, and list the loop voltage equation and the node current equation;

[0016] S2. Decomposing the loop voltage equation and the node current equation according to the commutation function of the seventh bridge arm to obtain a system differential mode output equation;

[0017] S3. Extract the frequency components of the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm according to the equal power conversion and the synchronous rotating coordinate conversion, and obtain the output side voltage equation and the input side voltage equation;

[0018] S4. Obtaining a frequency component control strategy for the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm based on grid voltage-oriented feedforward decoupling control, the output-side voltage equation, and the input-side voltage equation;

[0019] S5. Establish a circulation channel relationship between the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm, and the seventh bridge arm, and implement circulation control according to the circulation channel relationship.

[0020] The beneficial effects of the present invention are: providing a three-circulation mode AC converter and a control method thereof, utilizing a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and a seventh bridge arm to form an AC converter having three circulation channels and providing a corresponding control method, for realizing commutation between two three-phase AC systems. Compared with the existing single-circulation converter, the volume and cost of the commutation system can be significantly reduced. There are three power transmission channels, and the strict constraints of the single circulation channel do not need to be met during normal operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A circuit diagram of a three-circulation mode AC converter according to an embodiment of the present invention;

[0022] Figure 2 A circuit diagram of a three-circulation mode AC converter in another phase sequence according to an embodiment of the present invention;

[0023] Figure 3 This is a schematic diagram of a scenario application of a three-circulation mode AC exchanger involved in an embodiment of the present invention;

[0024] Figure 4 A circuit diagram of a full-bridge submodule of a three-circulation mode AC converter according to an embodiment of the present invention;

[0025] Figure 5 Schematic diagram of the steps of a method for controlling a three-circulation mode AC exchanger according to an embodiment of the present invention;

[0026] Figure 6 A parameter identification diagram of an AC converter in a method for controlling a three-circulation mode AC converter according to an embodiment of the present invention;

[0027] Figure 7 This is a regulation control diagram of an input terminal component on the input side of a control method for a three-circulation mode AC converter according to an embodiment of the present invention;

[0028] Figure 8 A schematic diagram of the circulation of an AC converter according to a control method of a three-circulation mode AC converter according to an embodiment of the present invention.

[0029] Description of labels:

[0030] C1, capacitor;

[0031] D1, reverse voltage diode;

[0032] L, inductance;

[0033] G, full bridge submodule;

[0034] T1, first fully controlled switch; T2, second fully controlled switch; T3, third fully controlled switch; T4, fourth fully controlled switch;

[0035] R, equivalent resistance. DETAILED DESCRIPTION

[0036] To illustrate the technical content, achieved objectives and effects of the present invention in detail, the following description is given in conjunction with the embodiments and accompanying drawings.

[0037] Please refer to Figures 1 to 4 , a three-circulation mode AC exchanger, comprising a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and a seventh bridge arm;

[0038] One end of the first bridge arm is connected to one end of the second bridge arm and one end of the seventh bridge arm at the same time, and forms a first node;

[0039] The other end of the second bridge arm is connected to one end of the third bridge arm and forms a second node, and the other end of the third bridge arm is connected to one end of the fourth bridge arm and forms a third node;

[0040] The other end of the fourth bridge arm is connected to one end of the fifth bridge arm and the other end of the seventh bridge arm, and forms a fourth node. The other end of the fifth bridge arm is connected to one end of the sixth bridge arm, and forms a fifth node.

[0041] The other end of the sixth bridge arm is connected to the other end of the first bridge arm and serves as a sixth node;

[0042] The first node, the third node, and the fifth node are commonly used to connect a first three-phase AC system, and the second node, the fourth node, and the sixth node are commonly used to connect a second three-phase AC system.

[0043] As can be seen from the above description, the beneficial effects of the present invention are: an AC converter with three circulation channels is composed of the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm, which is used to realize commutation between two three-phase AC systems. Compared with the existing single-circulation converter, the volume and cost of the commutation system can be significantly reduced. There are three power transmission channels, and the strict constraints of the single circulation channel do not need to be met during normal operation.

[0044] Furthermore, the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm are respectively composed of an inductor L, an equivalent resistor R and two or more full-bridge sub-modules G connected in series.

[0045] As can be seen from the above description, each bridge arm is composed of an inductor L, an equivalent resistor R, and two or more full-bridge sub-modules G in series. It has few components and low cost. Different numbers of full-bridge sub-modules G can be selected according to the actual configuration, which is flexible and convenient to use.

[0046] Furthermore, the full-bridge submodule G includes a first capacitor C1, a first fully-controlled switch device T1, a second fully-controlled switch device T2, a third fully-controlled switch device T3 and a fourth fully-controlled switch device T4;

[0047] One end of the first capacitor C1 is connected to one end of the first fully-controlled switch device T1 and one end of the second fully-controlled switch device T2, and the other end of the first capacitor C1 is connected to one end of the third fully-controlled switch device T3 and one end of the fourth fully-controlled switch device T4.

[0048] The other end of the first fully-controlled switch element T1 is connected to the other end of the third fully-controlled switch element T3 and serves as a series connection terminal of the full-bridge submodule G;

[0049] The other end of the second fully-controlled switch element T2 is connected to the other end of the fourth fully-controlled switch element T4 and serves as a series connection terminal of the full-bridge sub-module G.

[0050] As can be seen from the above description, the full-bridge submodule G is composed of the first capacitor C1 and the fully-controlled switch element, and has a simple structure, strong versatility, and is easy to control.

[0051] Furthermore, the first fully-controlled switch element T1 includes a transistor and a reverse voltage diode D1;

[0052] The collector of the transistor is connected to one end of the first capacitor C1, the cathode of the reverse voltage diode D1 and one end of the second fully-controlled switch T2, and the emitter of the transistor is connected to the other end of the third fully-controlled switch T3.

[0053] Furthermore, the first fully-controlled switch is a field effect transistor or a bipolar transistor.

[0054] From the above description, it can be seen that the first fully-controlled switch can be a field-effect transistor or a bipolar transistor, and can be reasonably set according to actual needs, with strong applicability.

[0055] Please refer to Figures 5 to 8 A control method for a three-circulation mode AC exchanger is applied to the above-mentioned three-circulation mode AC exchanger, comprising the following steps:

[0056] S1. Take the first, third, and fifth nodes as the output side, and the second, fourth, and sixth nodes as the input side, and list the loop voltage equation and the node current equation;

[0057] S2. Decomposing the loop voltage equation and the node current equation according to the commutation function of the seventh bridge arm to obtain a system differential mode output equation;

[0058] S3. Extract the frequency components of the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm according to the equal power conversion and the synchronous rotating coordinate conversion, and obtain the output side voltage equation and the input side voltage equation;

[0059] S4. Obtaining a frequency component control strategy for the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm based on grid voltage-oriented feedforward decoupling control, the output-side voltage equation, and the input-side voltage equation;

[0060] S5. Establish a circulation channel relationship between the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm, and the seventh bridge arm, and implement circulation control according to the circulation channel relationship.

[0061] As can be seen from the above description, the beneficial effects of the present invention are: an AC converter with three circulation channels is composed of the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm, which is used to realize commutation between two three-phase AC systems. Compared with the existing single-circulation converter, the volume and cost of the commutation system can be significantly reduced. There are three power transmission channels, and the strict constraints of the single circulation channel do not need to be met during normal operation.

[0062] Furthermore, the step S1 further includes:

[0063] The full-bridge submodule G is equivalent to an ideal controlled voltage source.

[0064] From the above description, it can be seen that when analyzing the component circuit of the converter, equating the full-bridge sub-module G to an ideal controlled voltage source can reduce the control complexity.

[0065] Furthermore, before step S1, the following steps are further included:

[0066] S0. Adjust the number of full-bridge sub-modules G on each bridge arm according to the system voltage and modulation index of the first three-phase AC system and the second three-phase AC system and the withstand voltage level of the fully-controlled switch element.

[0067] From the above description, it can be seen that different numbers of full-bridge sub-modules G can be selected according to actual configuration, which is flexible and convenient to use.

[0068] The three-circulation mode AC exchanger and its control method of the present invention are applicable to the design and application scenarios of AC exchangers, and are specifically described as follows:

[0069] According to the different types of input and output side systems, the application scenarios of the new three-circuit mode AC exchanger can be divided into:

[0070] 1. Active system to passive system

[0071] In this scenario, the frequency conversion system based on the new three-circuit mode AC converter is suitable for power transmission and serves as a high-voltage and high-power power supply. The first three-phase AC system is a distribution station and / or substation and / or power station, and the second three-phase AC system is a passive load such as passive power grid, rail transportation, mining, smelting, and steel rolling.

[0072] 2. Passive system to active system

[0073] In this scenario, the frequency conversion system based on the new three-circuit mode AC converter is suitable for scenarios where frequency-divided new energy power sources such as frequency-divided offshore wind power, frequency-divided onshore wind power, and frequency-divided photovoltaic power are connected to the power grid.

[0074] 3. Active system to active system

[0075] In this scenario, the frequency conversion system based on the new three-circuit mode AC converter is suitable for scenarios such as the interconnection of asynchronous AC systems, the access of new energy systems including power supplies, and the interconnection of frequency-divided power systems and industrial frequency power systems.

[0076] Among them, the specific implementation case for application scenario 2 is as follows: Figure 3 The figure shows a scenario where a new three-circuit AC converter is used in a frequency-splitting offshore wind farm grid-connected. The frequency-splitting offshore wind turbine generates 50 / 3Hz low-frequency power, which is collected via a collection line to a frequency-splitting offshore booster station. It is then transmitted to land via a frequency-splitting submarine cable. The three-circuit AC converter converts the frequency-splitting power into power-frequency power, completing the grid connection of the frequency-splitting offshore wind farm. On the rectifier side, since the offshore wind farm uses maximum power point tracking (MPPT), the converter is required to provide voltage to the wind farm. Constant voltage and frequency control (V / f control) or droop control can be used to provide voltage to the wind farm. To implement V / f control, a shunt capacitor must be connected at the frequency-splitting grid connection point PCC1. On the inverter side, since the power-frequency side is directly connected to the power grid, constant DC voltage and constant AC voltage control methods can be used.

[0077] The following is an explanation through specific implementation methods:

[0078] Please refer to Figures 1 to 4 , embodiment 1 of the present invention is:

[0079] A three-circuit mode AC exchanger, such as Figure 1 As shown, it includes a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and a seventh bridge arm.

[0080] Among them, one end of the first bridge arm is simultaneously connected to one end of the second bridge arm and one end of the seventh bridge arm, and is a first node; the other end of the second bridge arm is connected to one end of the third bridge arm, and is a second node, and the other end of the third bridge arm is connected to one end of the fourth bridge arm, and is a third node; the other end of the fourth bridge arm is simultaneously connected to one end of the fifth bridge arm and the other end of the seventh bridge arm, and is a fourth node, the other end of the fifth bridge arm is connected to one end of the sixth bridge arm, and is a fifth node; the other end of the sixth bridge arm is connected to the other end of the first bridge arm, and is a sixth node.

[0081] In this embodiment, if Figure 1 As shown in the figure, the new three-circulation AC converter consists of seven arms: the first, second, third, fourth, fifth, sixth, and seventh arms. The six arms are connected end-to-end in the order AW, WB, BU, UC, CV, and VA, forming a hexagonal structure. Accordingly, the arms are named AW, WB, BU, UC, CV, and VA. U, V, and W represent the first, third, and fifth nodes, respectively; the first, third, and fifth nodes are used to connect the first three-phase AC system. C, B, and A represent the second, fourth, and sixth nodes, respectively; the second, fourth, and sixth nodes are used to connect the second three-phase AC system.

[0082] like Figure 2 As shown, without loss of generality, the phase sequence of the two sides of the system can also be Figure 2 If the configuration is performed as shown, the first, second, third, fourth, fifth, sixth, and seventh arms can be named in the order of AU, UB, BW, WC, CV, VA, and BV. Changes in the phase sequence and initial phase of the two systems will change the withstand voltage of each arm, thereby affecting the number of components required.

[0083] In this embodiment, if Figure 1 As shown, the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm are respectively composed of an inductor L, an equivalent resistor R and two or more full-bridge sub-modules G connected in series.

[0084] Among them, such as Figure 4As shown, the full-bridge sub-module G includes a first capacitor C1, a first fully-controlled switch device T1, a second fully-controlled switch device T2, a third fully-controlled switch device T3 and a fourth fully-controlled switch device T4; one end of the first capacitor C1 is simultaneously connected to one end of the first fully-controlled switch device T1 and one end of the second fully-controlled switch device T2, and the other end of the first capacitor C1 is simultaneously connected to one end of the third fully-controlled switch device T3 and one end of the fourth fully-controlled switch device T4; the other end of the first fully-controlled switch device T1 is connected to the other end of the third fully-controlled switch device T3, and serves as a series connection terminal of the full-bridge sub-module G; the other end of the second fully-controlled switch device T2 is connected to the other end of the fourth fully-controlled switch device T4, and serves as a series connection terminal of the full-bridge sub-module G.

[0085] In this embodiment, the first fully-controlled switching element T1 includes a transistor and a reverse voltage diode D1; the collector of the transistor is simultaneously connected to one end of the first capacitor C1, the cathode of the reverse voltage diode D1, and one end of the second fully-controlled switching element T2, and the emitter of the transistor is connected to the other end of the third fully-controlled switching element T3.

[0086] Furthermore, the transistor may be an insulated gate bipolar transistor (IGBT). In other equivalent embodiments, the fully controlled switch may also be a field effect transistor or a bipolar transistor, which can be reasonably configured as required.

[0087] Please refer to Figures 5 to 8 , the second embodiment of the present invention is:

[0088] A control method for a three-circulation mode AC converter is applied to a three-circulation mode AC converter of embodiment 1, such as Figure 5 As shown, the following steps are included:

[0089] S0. Adjust the number of full-bridge sub-modules G on each bridge arm according to the system voltage and modulation index of the first three-phase AC system and the second three-phase AC system and the withstand voltage level of the fully-controlled switch element.

[0090] S1. Take the first, third, and fifth nodes as the output side, and the second, fourth, and sixth nodes as the input side, and list the loop voltage equation and the node current equation;

[0091] In this embodiment, if Figure 6 As shown, taking the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm as an example, UA, AV, VB, BW, WC and UB respectively:

[0092] Each bridge arm is equivalent to a series connection of an equivalent resistor R, an inductor L and an ideal controlled voltage source. The voltage and current on the input side are , the voltage and current on the output side are The ideal controlled voltage source voltage and current of the 7 bridge arm branches are .

[0093] Therefore, the loop voltage equation is listed according to Kirchhoff's circuit theorem, and its expression is as follows:

[0094]

[0095] The node current equation is as follows:

[0096]

[0097] In addition, the node current equation can be rewritten as:

[0098]

[0099] Where V ON Indicates the voltage between the neutral points of the first three-phase AC system and the second three-phase AC system.

[0100] In this embodiment, if the input and output systems are three-phase three-wire, that is, there is no electrical connection between the neutral points of the input and output systems, and , then the node current equation can be obtained:

[0101]

[0102] S2. Based on the commutation function of the seventh bridge arm, the loop voltage equation and the node current equation are decomposed to obtain the system differential mode output equation;

[0103] In this embodiment, since the function of bridge arm 7 is to control the converter circulation current, its output current is the common mode current between the bridge arms and is not output to the systems on both sides. Therefore, the system output is differential mode. The loop voltage equation and the node current equation are decomposed into the following two equations:

[0104]

[0105]

[0106] Since the current of the seventh bridge arm Only circulating current is generated and not output to the systems on both sides. Therefore, the converter differential mode output control strategy can adopt the control method of the hexagonal converter.

[0107] S3. Extract the frequency components of the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm according to the equal power conversion and the synchronous rotating coordinate conversion, and obtain the output side voltage equation and the input side voltage equation;

[0108] In this embodiment, since the bridge arm voltage and where is the sum of the two frequency components of the input and output, meaning that a single bridge arm primarily contains two frequency components. These components need to be extracted through equal power conversion and synchronous rotating coordinate transformation for further control. This yields four sets of voltage equations in the dq coordinate system: the output-side voltage equation and the input-side voltage equation.

[0109] The input voltage equation includes the input terminal component on the input side:

[0110]

[0111] Output components on the input side:

[0112]

[0113] The output voltage equation includes the input component on the output side:

[0114]

[0115] Output components on the output side:

[0116]

[0117] in, 、 They are The dq axis components in the input side frequency and output side frequency rotation coordinate system, 、 They are The dq-axis components in the input-side frequency and output-side frequency rotation coordinate systems.

[0118] S4. Based on grid voltage-oriented feedforward decoupling control, the output side voltage equation, and the input side voltage equation, a frequency component control strategy for the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm is obtained;

[0119] In this embodiment, the grid voltage-oriented feedforward decoupling control setting expression is as follows:

[0120]

[0121]

[0122] In this embodiment, combined with Figure 7 As shown, taking the input terminal component of the input side as an example, the closed-loop control of active and reactive currents is achieved through PI regulation. 、 、 、 and The control strategies for other frequency components can be derived similarly.

[0123] S5. Establish a circulation channel relationship between the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm, and the seventh bridge arm, and implement circulation control according to the circulation channel relationship.

[0124] In this embodiment, combined with Figure 8 As shown, the expression of the circulation channel relationship is as follows:

[0125] .

[0126] Where, etc. represent the common mode voltage of the corresponding bridge arm; Represent the sizes of the three circulations respectively.

[0127] In this embodiment, by controlling two of the three circulations, e.g. and , the common-mode voltage in the third circulating channel can be obtained through the current constraint condition in the circulating channel relationship, thereby realizing circulating current control.

[0128] In summary, the present invention provides a three-circulation mode AC converter and a control method thereof, which utilizes a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm, and a seventh bridge arm to form an AC converter having three circulation channels and provides a corresponding control method for realizing commutation between two three-phase AC systems. Compared with the existing single-circulation converter, the volume and cost of the commutation system can be significantly reduced. There are three power transmission channels, and the strict constraints of the single circulation channel do not need to be met during normal operation.

[0129] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the present invention's description and drawings, or directly or indirectly applied in related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A three-circulation mode AC exchanger, characterized in that: including a first bridge arm, a second bridge arm, a third bridge arm, a fourth bridge arm, a fifth bridge arm, a sixth bridge arm and a seventh bridge arm; One end of the first bridge arm is connected to one end of the second bridge arm and one end of the seventh bridge arm at the same time, and forms a first node; The other end of the second bridge arm is connected to one end of the third bridge arm and forms a second node, and the other end of the third bridge arm is connected to one end of the fourth bridge arm and forms a third node; The other end of the fourth bridge arm is connected to one end of the fifth bridge arm and the other end of the seventh bridge arm, and forms a fourth node. The other end of the fifth bridge arm is connected to one end of the sixth bridge arm, and forms a fifth node. The other end of the sixth bridge arm is connected to the other end of the first bridge arm and serves as a sixth node; The first node, the third node, and the fifth node are commonly used to connect a first three-phase AC system, and the second node, the fourth node, and the sixth node are commonly used to connect a second three-phase AC system.

2. The three-circulation mode AC exchanger according to claim 1, characterized in that: The first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm and the seventh bridge arm are respectively composed of an inductor, an equivalent resistor and two or more full-bridge sub-modules connected in series.

3. The three-circulation mode AC exchanger according to claim 2, characterized in that: The full-bridge submodule includes a first capacitor, a first fully-controlled switch element, a second fully-controlled switch element, a third fully-controlled switch element, and a fourth fully-controlled switch element; One end of the first capacitor is connected to one end of the first fully-controlled switch device and one end of the second fully-controlled switch device, and the other end of the first capacitor is connected to one end of the third fully-controlled switch device and one end of the fourth fully-controlled switch device. The other end of the first fully-controlled switch element is connected to the other end of the third fully-controlled switch element and serves as a series connection terminal of the full-bridge submodule; The other end of the second fully-controlled switch element is connected to the other end of the fourth fully-controlled switch element and serves as a series connection terminal of the full-bridge sub-module.

4. The three-circulation mode AC exchanger according to claim 3, characterized in that: The first fully controlled switch element includes a transistor and a reverse voltage diode; The collector of the transistor is simultaneously connected to one end of the first capacitor, the cathode of the reverse voltage diode and one end of the second fully-controlled switch, and the emitter of the transistor is connected to the other end of the third fully-controlled switch.

5. The three-circulation mode AC exchanger according to claim 3, characterized in that: The first fully-controlled switch is a field effect transistor or a bipolar transistor.

6. A control method for a three-circulation mode AC exchanger, applied to a three-circulation mode AC exchanger according to any one of claims 1 to 5, characterized in that: The steps include: S1. Take the first, third, and fifth nodes as the output side, and the second, fourth, and sixth nodes as the input side, and list the loop voltage equation and the node current equation; S2. Decomposing the loop voltage equation and the node current equation according to the commutation function of the seventh bridge arm to obtain a system differential mode output equation; S3. Extract the frequency components of the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm according to the equal power conversion and the synchronous rotating coordinate conversion, and obtain the output side voltage equation and the input side voltage equation; S4. Obtaining a frequency component control strategy for the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, and the sixth bridge arm based on grid voltage-oriented feedforward decoupling control, the output-side voltage equation, and the input-side voltage equation; S5. Establish a circulation channel relationship between the first bridge arm, the second bridge arm, the third bridge arm, the fourth bridge arm, the fifth bridge arm, the sixth bridge arm, and the seventh bridge arm, and implement circulation control according to the circulation channel relationship.

7. A three-circulation mode AC exchanger control method according to claim 6, characterized in that: The step S1 further includes: The full-bridge submodule is equivalent to an ideal controlled voltage source.

8. The method for controlling a three-circulation mode AC exchanger according to claim 6, characterized in that: Before step S1, the following steps are also included: S0. Adjust the number of full-bridge sub-modules on each bridge arm according to the system voltage and modulation index of the first three-phase AC system and the second three-phase AC system and the withstand voltage level of the fully-controlled switch element.

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