Multiphase-multiphase direct AC-AC converter based on MMC and control method thereof

Abstract

The invention discloses a multi-phase-multi-phase direct-type AC-AC converter based on an MMC and a control method of the multi-phase-multi-phase direct-type AC-AC converter. The AC-AC converter provided by the invention comprises a modular multi-level converter and N single-phase three-winding transformers, the modularized multi-level converter comprises X phase units, the input side is provided with m ports and connected with a first alternating current system, and the output side is provided with N ports and connected with a second alternating current system. Each single-phase three-winding transformer comprises a first winding, a second winding and a third winding, the first windings of the N single-phase three-winding transformers are respectively connected with N phases of a second alternating current system, and the second windings are connected with N output side ports of an upper bridge arm of the modular multilevel converter; and the third winding is connected with N output side ports of the lower bridge arm of the modular multilevel converter. The AC-AC converter is based on the principle of synthesis of differential-mode and common-mode voltage components of MMC bridge arms, and is combined with a three-winding transformer with an anti-dotted terminal, so that the number of sub-modules of the M3C topology can be further reduced, and the AC-AC converter has good expansibility and is suitable for application scenes with various special requirements.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronic AC-AC converters, in particular to an MMC-based multi-phase-multi-phase direct AC-AC converter and a control method thereof. Background Art

[0002] With the development of power electronic converter technology, high-voltage and large-capacity AC-AC converters are increasingly used in renewable energy grid-connected power generation, long-distance power transmission and other fields. Especially in scenarios such as asynchronous power grid interconnection, offshore wind power transmission, railway traction power supply, and high-power variable frequency speed regulation, in order to achieve economic and technical improvements, the power conversion link has new requirements for the voltage amplitude, frequency, phase, and number of phases. It is urgent to study new topologies of AC-AC converters with flexible conversion, strong scalability, and excellent performance.

[0003] At present, high-voltage and large-capacity AC-AC converters are mainly divided into two topology types: indirect and direct. The indirect AC-AC converter consists of two back-to-back modular multilevel converters (MMC). The intermediate DC link enables the AC systems on both sides to be completely decoupled, but the sub-modules of this topology are large and the cost is high. The topology of the direct AC-AC converter is mainly based on the modular multilevel matrix converter (M3C). Each bridge arm is directly connected to the AC systems on both sides. When the three-phase-to-three-phase conversion is carried out, the sub-module usage is reduced by 25% compared with the back-to-back MMC. As the voltage level and conversion capacity increase, the cost of the sub-module is still high. Some studies have proposed the Hexverter topology and the Y-type topology. When the three-phase-to-three-phase conversion is carried out, the sub-module usage is reduced by 50% compared with the back-to-back MMC. However, these topologies have strong coupling between the multiple bridge arms, complex control, and poor scalability, which makes it difficult to apply to the changing power conversion requirements in actual applications.

[0004] Therefore, it is of great significance to study new AC-AC converter topologies with low sub-module usage, good scalability and flexible conversion. Summary of the invention

[0005] In order to solve the problems existing in the above-mentioned prior art, the present invention proposes a multi-phase-multi-phase direct AC-AC converter based on MMC. The AC-AC converter is based on the principle of synthesis of differential-mode and common-mode voltage components of the MMC bridge arm, combined with a three-winding transformer with anti-homophone terminals, which can further reduce the amount of sub-modules used in the M3C topology and has good scalability to be suitable for application scenarios with various special needs.

[0006] To this end, the present invention adopts the following technical solution: a multi-phase-multi-phase direct AC-AC converter based on MMC, which comprises a modular multi-level converter andN A single-phase three-winding transformer; The modular multilevel converter comprises X phase units, each phase unit consists of an upper bridge arm and a lower bridge arm; the input side has m ports connected to the first AC system, each input port connected to at least one phase unit midpoint; the output side has 2 N ports, X The upper bridge arms of the phase units are connected in parallel to form N Output side ports, X The lower bridge arms of the phase units are connected in parallel to form N output side ports, each pair of output side ports being connected to two ends of at least one phase unit; The single-phase three-winding transformer comprises a first winding, a second winding and a third winding, and the transformation ratio is K T :1:1, N The first winding of a single-phase three-winding transformer is connected to the second AC system N The second winding is connected to the upper bridge arm of the modular multilevel converter. N The output side port is connected, the third winding is connected to the lower bridge arm of the modular multi-level converter N Output side port connection; The first AC system is m Phase system, consisting of a, b, c, …, m phases; the second AC system is N Phase system, consisting of A, B, C, …, N phases; m ≥2, N ≥1.

[0007] Furthermore, the modular multilevel converter i The input side ports are Y i Phase units are connected, where Y i ≥1, i =a,b,c,…,m; the first j The output port and Z j Phase units are connected, where Z j ≥1, j =A,B,C, …,N; total number of phase units X ≥ m , X ≥ N , and satisfy: .

[0008] Furthermore, in the modular multi-level converter, the upper bridge arm and the lower bridge arm of each phase unit have the same structure, both comprising a plurality of full-bridge sub-modules connected in series and a bridge arm inductor.

[0009] Furthermore, in the single-phase three-winding transformer, the same-name end of the first winding and the same-name end of the second AC system j The same-name end of the second winding and the first end of the upper bridge arm of the modular multilevel converter are connected. i The output side port is connected, and the other end is grounded; the same-name end of the third winding is connected to the first end of the lower bridge arm of the modular multi-level converter. i The other end is connected to the output port and the other end is grounded.

[0010] Furthermore, the modular multilevel converter, k Phase unit upper bridge arm reference voltage u topk and the lower bridge arm reference voltage u btmk The expression is:

[0011] in, u i is the differential mode component of the bridge arm reference voltage, i =a,b,c,…,m, u j is the common-mode component of the bridge arm reference voltage, j =A,B,C,…,N. k The range can be written as k =(1,2,…, Y a ),( Y a +1, Y a +2,…, Y a + Y b ), …, ( X - Y m +1, X - Y m +2,…, X ), k The range can also be written as k =(1,2,…, Z A ),( Z A +1, Z A +2,…, ZA + Z B ),…, ( X - Z N +1, X - Z N +2,…, X ).

[0012] Furthermore, when the voltage drop on the bridge arm inductance is ignored, k The differential component of the reference voltage of the bridge arm of each phase unit u i and common mode component u j The expression is:

[0013] in, U 1 corresponds to the first AC system phase voltage amplitude, U 2 corresponds to the second AC system phase voltage amplitude, ω 1 is the first AC system angular frequency, ω 2 is the second AC system angular frequency, θ i is the first AC system phase shift angle, i =a, b, c,…,m, θ j is the second AC system phase shift angle, j =A, B, C,…, N.

[0014] Furthermore, the phase of each phase voltage of the first AC system needs to satisfy that the sum of all voltage phasors is equal to 0, while the phase of each phase voltage of the second AC system is not subject to such restriction.

[0015] Furthermore, in the MMC-based multi-phase-to-multi-phase direct AC-AC converter, power flows from the first AC system side to the second AC system side, or from the second AC system side to the first AC system side.

[0016] The present invention also provides a control method for the above-mentioned multi-phase-multi-phase direct AC-AC converter, the content of which is as follows: selecting the input side component, output side component and circulating current component in the bridge arm voltage and current as state variables to design a control strategy, and dividing the control strategy based on the bridge arm current component into two parts: outer loop control and inner loop control. The inner loop control uses the bridge arm voltage decoupling transformation to control the input side component, output side component and circulating current component of the bridge arm current respectively, wherein the input side component and the output side component each have two modes of voltage control and current control, and the corresponding control mode is selected according to specific needs; the circulating current component has only one mode of current control; when the inner loop control adopts AC voltage control, outer loop control is not required; when the inner loop control adopts AC current control, the outer loop control adopts active control or reactive control.

[0017] Furthermore, for the outer loop control of the input side component or the output side component, the active control adopts capacitor voltage mean control or active power control, and the reactive control adopts reactive power control; for the outer loop control of the circulating current component, the active control adopts bridge arm balance control, and the reactive control makes the q-axis reactive component reference value of the circulating current 0.

[0018] The present invention has the following beneficial effects: 1. The multi-phase-multi-phase direct AC-AC converter of the present invention constructs the differential-mode component of the input voltage of each phase and the common-mode component of the output voltage of each phase in the upper and lower bridge arms, and then offsets the differential-mode component and synthesizes the common-mode component through the single-phase three-winding transformer at the opposite same-name end, thereby realizing the AC-AC conversion requirements of any voltage amplitude, frequency, phase, and number of phases. This topology scheme has good scalability and can adapt to various power conversion requirements.

[0019] 2. The multi-phase-multi-phase direct AC-AC converter of the present invention can effectively reduce the number of bridge arm submodules. Although a three-winding transformer is required to realize the voltage conversion function, it also plays an isolation role, which reduces the construction cost of the converter station as a whole and reduces the design difficulty of the control algorithm. In the case of three-phase-three-phase conversion, the proposed converter topology reduces the number of submodules by 50% compared with the back-to-back MMC topology and 33% compared with the M3C topology; in the case of three-phase-single-phase conversion, the proposed converter topology reduces the number of submodules by 40% compared with the back-to-back MMC topology.

[0020] 3. The multi-phase-multi-phase direct AC-AC converter of the present invention has the advantages of MMC topology modular design, low harmonic content, bidirectional energy flow, high output level, controllable power factor, fast dynamic response, high reliability, etc. It can be used in AC-AC conversion fields such as low-frequency transmission of offshore wind power, island power supply, asynchronous interconnection of urban power grids, high-power variable frequency speed regulation, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a topological structure diagram of the multi-phase-multi-phase direct AC-AC converter based on MMC of the present invention; Figure 2 It is a block diagram of a control method of a multi-phase-multi-phase direct AC-AC converter based on MMC of the present invention; Figure 3 This is a topological structure diagram of the three-phase-three-phase direct AC-AC converter based on MMC of the present invention ( m =3, N =3, X =3, Y a = Y b = Y c =1, Z A = Z B = Z C =1); Figure 4 yes Figure 3 The voltage simulation waveform diagram of the input side and output side of the three-phase-three-phase direct AC-AC converter; Figure 5 This is another topological structure diagram of the three-phase to single-phase direct AC-AC converter based on MMC of the present invention ( m =3, N =1, X =3, Y a = Y b = Y c =1, Z A =3); Figure 6 yes Figure 5 The voltage simulation waveform diagram of the input side and output side of the three-phase-three-phase direct AC-AC converter; Figure 7 This is another topological structure diagram of the three-phase to two-phase direct AC-AC converter based on MMC of the present invention ( m =3, N =2, X =4, Y a =2, Y b = Y c =1, Z A = Z B =2); Figure 8 yes Figure 7 The voltage simulation waveforms on the input and output sides of the three-phase-three-phase direct AC-AC converter. DETAILED DESCRIPTION

[0022] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0023] like Figure 1 As shown, the present invention is a multi-phase-multi-phase direct AC-AC converter based on MMC, which is composed of a modular multi-level converter and N The AC-AC converter input side is connected to the m The first AC system of the phase is connected, and the first AC system includes phases a, b, c, ..., m; the output side of the AC-AC converter is connected to N The second AC system of the first and second AC systems is connected to the second AC system of the first and second AC systems, and the second AC system includes phases A, B, C, …, N. The number of phases of the first AC system and the second AC system satisfies m ≥2, N ≥1.

[0024] The modular multilevel converter comprises X Phase units, each phase unit has the same upper bridge arm and lower bridge arm structure, both of which contain multiple full-bridge sub-modules connected in series and a bridge arm inductor; the input side has m ports connected to the first AC system, each input port connected to the midpoint of at least one phase unit; the output side has 2 N ports, X The upper bridge arms of the phase units are connected in parallel to form N Output side ports, X The lower bridge arms of the phase units are connected in parallel to form N output side ports, and each pair of output side ports is connected to two ends of at least one phase unit.

[0025] The modular multilevel converter i The input side ports are Y i Phase units are connected, where Y i ≥1, i =a,b,c,…,m; the first j The output port and Z j Phase units are connected, where Z j ≥1, j=A,B,C,…,N. Total number of phase units X ≥ m , X ≥ N , and satisfy: . The single-phase three-winding transformer comprises a first winding, a second winding and a third winding, and the transformation ratio is K T :1:1, N The first winding of a single-phase three-winding transformer is connected to the second AC system N The second winding is connected to the upper bridge arm of the modular multilevel converter. N The output side port is connected, the third winding is connected to the lower bridge arm of the modular multi-level converter N The same-name end of the first winding and the second AC system j The same-name end of the second winding and the first end of the upper bridge arm of the modular multilevel converter are connected. i The output side port is connected, and the other end is grounded; the same-name end of the third winding is connected to the first end of the lower bridge arm of the modular multi-level converter. i The other end is connected to the output port and the other end is grounded.

[0026] The modular multilevel converter, k Phase unit upper bridge arm reference voltage u topk and the lower bridge arm reference voltage u btmk The expression is:

[0027] in, u i is the differential mode component of the bridge arm reference voltage, i =a, b, c,…, m, u j is the common-mode component of the bridge arm reference voltage, j =A, B, C,…, N. k The range can be written as k =(1,2,…, Y a ),( Y a +1, Y a +2,…, Y a + Y b ),…, ( X - Ym +1, X - Y m +2,…, X ), k The range can also be written as k =(1,2,…, Z A ),( Z A +1, Z A +2,…, Z A + Z B ),…, ( X - Z N +1, X - Z N +2, …, X ).

[0028] When the voltage drop on the bridge arm inductance is ignored, k The differential component of the reference voltage of the bridge arm of each phase unit u i and common mode component u j The expression is:

[0029] in, U 1 corresponds to the first AC system phase voltage amplitude, U 2 corresponds to the second AC system phase voltage amplitude, ω 1 is the first AC system angular frequency, ω 2 is the second AC system angular frequency, θ i is the first AC system phase shift angle, i =a, b, c,…,m, θ j is the second AC system phase shift angle, j =A, B, C,…, N. Phase of each phase voltage of the first AC system θ i It is required that the sum of all voltage phasors is equal to 0, but there is no such restriction on the phase of each phase voltage of the second AC system.

[0030]

[0031] The bridge arm current components of the modular multilevel converter can be divided into three parts: input side components i i , output side component ij , circulation component i cirk . Reference Figure 1 The positive direction of the current in k For example, the phase unit k The phase units are connected to the input side a phase and the output side A phase. The relationship between the three current components and the upper and lower bridge arm currents is:

[0032] in, i a Indicates the current of the a-phase port on the input side; i A Indicates the current of the A-phase port on the output side; i topk Indicates k The upper arm current of each phase unit; i btmk Indicates k The lower arm current of each phase unit; the a-phase port on the input side and Y a The output side A phase port is connected to Z A Phase unit connection.

[0033] Input side component i i The differential mode component of the bridge arm current, the output side component i j , circulation component i cirk They all belong to the common mode component of the bridge arm current. i cirk The calculation method is to subtract the output side component from the common mode component of the bridge arm current i j The remaining part after.

[0034] Then the dynamic equations of each component of the bridge arm current are:

[0035] in, L is the bridge arm inductance, u a Indicates the voltage of the a-phase port on the input side, u A Indicates the voltage of the A-phase port on the output side.

[0036] The input component, output component, and circulating current component of the bridge arm current are selected as state variables to design the control strategy of the AC-AC converter. Note that not all current components are selected, because some current components satisfy Kirchhoff's current law and they are not independent of each other. The bridge arm voltage is selected as the control variable, then the voltage of the first AC system on the input side and the voltage of the second AC system on the output side can be regarded as interference, and the decoupling control of each component of the bridge arm current is achieved by controlling the bridge arm voltage.

[0037] The bridge arm of the modular multilevel converter is composed of full-bridge modules connected in series, and the voltage balance of the sub-module capacitors in a single bridge arm is solved by the modulation link. At the level of converter control, it is also necessary to ensure the stability of the sum of the capacitor voltages in each bridge arm. According to the different ranges of the dynamic processes involved, it can be divided into two types of control: one is the control of the mean value of the converter capacitor voltage, which involves the power balance between the input port and the output port of the converter; the other is the control to make the mean value of the capacitor voltage of each bridge arm consistent, which involves exchanging the energy of each bridge arm inside the converter.

[0038] like Figure 2 As shown, each bridge arm is connected to the first AC system on the input side and the second AC system on the output side, so the bridge arm current contains input side components and output side components; when different bridge arms are connected, the bridge arm current also contains circulating current components. The input side component, output side component and circulating current component in the bridge arm current are selected as state variables to design the control strategy, and the control strategy based on the bridge arm current component is divided into two parts: outer loop control and inner loop control. The inner loop control uses the bridge arm voltage decoupling transformation to control the input side component, output side component and circulating current component of the bridge arm current respectively, wherein the input side component and the output side component each have two modes of voltage control and current control, and the corresponding control mode is selected according to specific needs; the circulating current component has only one mode of current control; when the inner loop control adopts AC voltage control, the outer loop control is not required; when the inner loop control adopts AC current control, the outer loop control adopts active control or reactive control.

[0039] For the outer loop control of the input side component or the output side component, the active type control adopts capacitor voltage average control or active power control, and the reactive type control adopts reactive power control; for the outer loop control of the circulating current component, the active type control adopts bridge arm balance control, and the reactive type control makes the q-axis reactive component reference value of the circulating current 0.

[0040] Application Examples like Figure 3 As shown, m =3, N =3, the first AC system and the second AC system are both three-phase systems, with three single-phase three-winding transformers. X =3, the modular multilevel converter has three phase units, Ya = Y b = Y c =1, Z A = Z B = Z C =1, the connection method of each phase unit is as follows Figure 3 shown. Figure 4 This is the voltage simulation waveform diagram of the input and output sides of the three-phase-three-phase direct AC-AC converter based on MMC in this application example. The voltage frequency of the first AC system f 1=50Hz, voltage amplitude U 1=1kV, initial phase θ a =0°, the second AC system voltage frequency f 2=20Hz, voltage amplitude U 2=0.7kV, initial phase θ A =10°, the first AC system and the second AC system are three-phase symmetrical, and the difference between them is 120°. At this time, the voltages of the six bridge arms can be approximately expressed as: .

[0041] like Figure 5 As shown, m =3, N =1, the first AC system is a three-phase system, the second AC system is a single-phase system, and there is a single-phase three-winding transformer. X =3, the modular multilevel converter has three phase units, Y a = Y b = Y c =1, Z A =3, the connection method of each phase unit is as follows Figure 5 shown. Figure 6 This is the voltage simulation waveform diagram of the input and output sides of the three-phase to single-phase direct AC-AC converter based on MMC in this application example. The voltage frequency of the first AC system f 1=50Hz, voltage amplitude U 1=1kV, initial phase θ a =0°, the second AC system voltage frequency f 2=20Hz, voltage amplitude U 2=1.2kV, initial phase θ A=30°, the first AC system has three phases that are symmetrical, each phase is 120° apart, and the initial phase of the second AC system θ A Can be set arbitrarily. At this time, the six bridge arm voltages can be approximately expressed as: .

[0042] like Figure 7 As shown, m =3, N =1, the first AC system is a three-phase system, the second AC system is a two-phase system, and there are two single-phase three-winding transformers. X =4, the modular multilevel converter has three phase units, Y a =2, Y b = Y c =1, Z A = Z B =2, the connection method of each phase unit is as follows Figure 7 shown. Figure 8 This is the voltage simulation waveform diagram of the input and output sides of the three-phase to two-phase direct AC-AC converter based on MMC in this application example. The voltage frequency of the first AC system f 1=50Hz, voltage amplitude U 1=1kV, initial phase θ a =0°, the second AC system voltage frequency f 2=20Hz, voltage amplitude U 2=1kV, initial phase θ A =0°, the first AC system has three symmetrical phases, each phase is 120° apart from each other, and the second AC system has two orthogonal phases, each phase is 90° apart from each other. At this time, the six bridge arm voltages can be approximately expressed as: .

[0043] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some of the technical features may be replaced by equivalents. Such modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A multi-phase to multi-phase direct AC-AC converter based on MMC, characterized in that: Contains a modular multilevel converter and N A single-phase three-winding transformer; The modular multilevel converter comprises X phase units, each phase unit consists of an upper bridge arm and a lower bridge arm; the input side has m ports connected to the first AC system, each input port connected to at least one phase unit midpoint; the output side has 2 N ports, X The upper bridge arms of the phase units are connected in parallel to form N Output side ports, X The lower bridge arms of the phase units are connected in parallel to form N output side ports, each pair of output side ports being connected to two ends of at least one phase unit; The single-phase three-winding transformer comprises a first winding, a second winding and a third winding, and the transformation ratio is K T :1:1, N The first winding of a single-phase three-winding transformer is connected to the second AC system N The second winding is connected to the upper bridge arm of the modular multilevel converter. N The output side port is connected, the third winding is connected to the lower bridge arm of the modular multi-level converter N Output side port connection; The first AC system is m Phase system, consisting of a, b, c, ..., m phases; the second AC system is N Phase system, consisting of A, B, C, …, N phases; m ≥2, N ≥1.

2. The MMC-based multi-phase to multi-phase direct AC-AC converter according to claim 1, characterized in that: The modular multilevel converter i The input side ports are Y i Phase units are connected, where Y i ≥1, i =a,b,c, …,m; the first j The output port and Z j Phase units are connected, where Z j ≥1, j =A,B,C, …,N; total number of phase units X ≥ m , X ≥ N , and satisfy: 。 3. The MMC-based multi-phase to multi-phase direct AC-AC converter according to claim 2, characterized in that: In the modular multi-level converter, the upper bridge arm and the lower bridge arm of each phase unit have the same structure, both comprising a plurality of full-bridge sub-modules connected in series and a bridge arm inductor.

4. The MMC-based multi-phase to multi-phase direct AC-AC converter according to claim 1, characterized in that: The single-phase three-winding transformer, the same-name end of the first winding and the same-name end of the second AC system j The same-name end of the second winding and the first end of the upper bridge arm of the modular multilevel converter are connected. i The output side port is connected, and the other end is grounded; the same-name end of the third winding is connected to the first end of the lower bridge arm of the modular multi-level converter. i The other end is connected to the output port and the other end is grounded.

5. The MMC-based multi-phase to multi-phase direct AC-AC converter according to claim 2, characterized in that: The modular multilevel converter, k Phase unit upper bridge arm reference voltage u topk and the lower bridge arm reference voltage u btmk The expression is: in, u i is the differential mode component of the bridge arm reference voltage, i =a,b,c, …,m, u j is the common-mode component of the bridge arm reference voltage, j =A,B,C, …,N.

6. The MMC-based multi-phase to multi-phase direct AC-AC converter according to claim 5, characterized in that: When the voltage drop on the bridge arm inductance is ignored, k The differential component of the reference voltage of the bridge arm of each phase unit u i and common mode component u j The expression is: in, U 1 corresponds to the first AC system phase voltage amplitude, U 2 corresponds to the second AC system phase voltage amplitude, ω 1 is the first AC system angular frequency, ω 2 is the second AC system angular frequency, θ i is the first AC system phase shift angle, i =a,b,c,…,m, θ j is the second AC system phase shift angle, j =A,B,C, …,N.

7. The MMC-based multi-phase to multi-phase direct AC-AC converter according to claim 6, characterized in that: The phase of each phase voltage of the first AC system needs to satisfy that the sum of all voltage phasors is equal to 0, while the phase of each phase voltage of the second AC system has no such restriction.

8. The MMC-based multi-phase to multi-phase direct AC-AC converter according to any one of claims 1 to 7, characterized in that: Power flows from the first AC system side to the second AC system side, or from the second AC system side to the first AC system side.

9. The control method of a multi-phase to multi-phase direct AC-AC converter according to any one of claims 1 to 8, characterized in that: The input side component, output side component and circulating current component of the bridge arm voltage and current are selected as state variables to design the control strategy, and the control strategy based on the bridge arm current component is divided into two parts: outer loop control and inner loop control. The inner loop control uses the bridge arm voltage decoupling transformation to control the input side component, output side component and circulating current component of the bridge arm current respectively, where the input side component and the output side component each have two modes of voltage control and current control, and the corresponding control mode is selected according to specific needs; the circulating current component has only one mode of current control; when the inner loop control adopts AC voltage control, outer loop control is not required; when the inner loop control adopts AC current control, the outer loop control adopts active control or reactive control.

10. The control method according to claim 9, characterized in that: For the outer loop control of the input side component or the output side component, the active type control adopts capacitor voltage average control or active power control, and the reactive type control adopts reactive power control; for the outer loop control of the circulating current component, the active type control adopts bridge arm balance control, and the reactive type control makes the q-axis reactive component reference value of the circulating current 0.

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