Decoupled control method for three active bridge converter, three active bridge converter and photovoltaic system

Abstract

The present disclosure provides a three-active-bridge converter decoupling control method, a three-active-bridge converter and a photovoltaic system. The first port, the second port and the third port of the three-active-bridge converter are coupled through a transformer, and each of the first port, the second port and the third port has a resonant cavity including a switch-controlled resonant capacitor with adjustable capacitance and a resonant inductor. By adjusting the capacitance of the third port, the resonant frequency of the third port is set to the operating frequency of the three-active-bridge converter, thereby realizing power decoupling between the first port and the second port. By adjusting the number of switch capacitor branches of different ports, the resonant frequency of the resonant cavity circuit of each port and the transmission power between the ports can be adjusted, thereby providing greater freedom for power regulation between the ports.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronics, and particularly relates to a three active bridge (TAB) converter decoupling control method, a TAB converter and a photovoltaic system. BACKGROUND

[0002] With the rapid development of photovoltaic power generation, wind power generation, energy storage batteries and direct current micro-grid technologies, multi-port converters are increasingly concerned. In the multi-port direct current transformer topology, the three active bridge (TAB) converter has high research value and application potential due to its electrical isolation, wide voltage range and flexible controllable power flow direction. In addition, the TAB can also realize single-stage energy exchange between three ports, and has higher power density and efficiency. Using the TAB to realize the interconnection of energy storage units, new energy generation units and micro-grids has become a research hotspot today.

[0003] However, due to the power coupling between the ports, it is difficult to directly control the transmission power between the ports. In order to solve the power coupling problem of the TAB converter, the software decoupling or hardware decoupling method is usually used. Software decoupling is to add a decoupling network in the control system to realize the decoupling of the energy control between the ports. However, the software decoupling method of the TAB converter increases the complexity of the control, which is not conducive to the stability of the system. Hardware decoupling mainly includes inductance type and resonance type, which realizes decoupling by setting inductance circuit or resonance circuit in the TAB converter. However, due to the existence of transformer leakage inductance, the inductance type TAB converter cannot truly realize topology level decoupling. Moreover, due to the current characteristics of the inductance type TAB converter, the current stress of the switching tube is large, thereby reducing the efficiency of the system. Compared with the inductance type TAB converter, the current stress of the resonance type decoupling TAB converter is small, and one resonance cavity in the circuit topology of the resonance type decoupling TAB converter usually resonates at the same frequency as the switching frequency, and the frequencies of the other two resonance cavities are slightly lower than the switching frequency. However, the resonance frequency of the decoupling resonance cavity must be consistent with the switching frequency to realize the resonance power decoupling, which puts certain requirements on the design of the resonance cavity of the TAB converter, and reduces the adaptability of the system. SUMMARY

[0004] Technical problems solved by the present application

[0005] In view of the above, the purpose of the present application is to overcome the shortcomings of the existing TAB converter power decoupling technology, and provide a TAB converter decoupling control method with adjustable resonant capacitance, a TAB converter and a photovoltaic system. By adjusting the resonant capacitance value of the three ports of the TAB converter, resonant hardware power decoupling is achieved, without the need for complex software decoupling strategies or the addition of extra inductance, and the transmission power between each port can be adjusted, improving the efficiency and stability of power transmission and enhancing the overall performance and reliability of the system.

[0006] Technical means for solving the technical problem

[0007] To solve the above technical problems, according to some exemplary embodiments of the present disclosure, a three-active-bridge converter decoupling control method is provided, the first port, the second port and the third port of the three-active-bridge converter are coupled via a transformer, each of the first port, the second port and the third port has a resonant cavity, the resonant cavity includes a switch-controlled resonant capacitance with adjustable capacitance value and a resonant inductance, by adjusting the capacitance value of the third port, the resonant frequency of the third port is the operating frequency of the three-active-bridge converter, thereby achieving power decoupling between the first port and the second port.

[0008] In some embodiments, the switch-controlled resonant capacitance includes a plurality of parallel resonant capacitors, and a switch tube connected to each resonant capacitor, in the third port, by controlling the on-off of the switch tube, the number of parallel-connected resonant capacitors in the switch-controlled resonant capacitance is changed, thereby adjusting the resonant frequency of the third port.

[0009] In some embodiments, by controlling the on-off of the switch tube in the first port or the second port, the capacitance value of the first port or the second port is adjusted, so that the resonant frequency of the first port or the second port is a predetermined specified frequency.

[0010] In some embodiments, the switch tube is composed of 2 source-source connected MOSFET tubes.

[0011] In some embodiments, the plurality of parallel resonant capacitors in each port have the same capacitance value.

[0012] In some embodiments, the inductance value of the resonant inductance is fixed.

[0013] In some embodiments, when decoupling the first port and the second port, the resonant frequency of the resonant cavity of the third port satisfies:

[0014]

[0015] f s : the operating frequency of the three-active-bridge converter;

[0016] f r3 : the resonant frequency of the resonant cavity of the third port

[0017] L r3 : the inductance value of the resonant inductor of the third port;

[0018] C r3 : the capacitance value of the resonant capacitor of the third port;

[0019] N3: the number of the resonant capacitors connected in parallel through the switch tube of the switch-controlled resonant capacitor of the third port.

[0020] In some embodiments, after the power decoupling, the transmission power of the first port is controlled by the phase angle between the input voltage of the first port and the third port, and the transmission power of the second port is controlled by the phase angle between the input voltage of the second port and the third port.

[0021] In some embodiments, the transmission power of the first port and the transmission power of the second port are closed-loop power controlled, with a specified reference power as input and the actual output power of the first port and the second port as output.

[0022] In some embodiments, the first port and the second port are located on the primary side of the transformer, and the third port is located on the secondary side of the transformer.

[0023] According to some exemplary embodiments of the present disclosure, a three-active-bridge converter is also provided, comprising a first port, a second port and a third port coupled via a transformer, the first port, the second port and the third port each having a resonant cavity, the resonant cavity comprising a switch-controlled resonant capacitor with adjustable capacitance value and a resonant inductor, when the first port and the second port are power decoupled, the capacitance value of the third port is adjusted so that the resonant frequency of the third port is the operating frequency of the three-active-bridge converter.

[0024] According to some exemplary embodiments of the present disclosure, a photovoltaic system is also provided, comprising the three-active-bridge converter as described above.

[0025] Inventive Effects

[0026] The TAB converter decoupling control method according to the present application can simplify the power decoupling process, realize topology-level power decoupling by adjusting the resonant capacitance value in the TAB converter, and avoid complex software decoupling strategies and additional inductance components. The method realizes power decoupling by adjusting the resonant capacitance of each port at a constant frequency, and changes the frequency of the resonant cavity and the transmission power between ports by adjusting the number of parallel branches of the resonant capacitance, thereby providing greater adjustment freedom and adapting to different application requirements. By controlling the switching state of the switch tube in the resonant cavity of each port, the resonant capacitance of each port can be accurately adjusted, so that the resonant cavity of each port can work at the required frequency, improving the efficiency and stability of power transmission. In addition, the present application is applicable to photovoltaic energy storage systems, can better adapt to the changing power requirements in photovoltaic power generation and energy storage systems, and improves the overall performance and reliability of the system. BRIEF DESCRIPTION OF DRAWINGS

[0027] The accompanying drawings are included to provide a further understanding of the present disclosure, and constitute a part of the specification, illustrate the present disclosure together with the embodiments thereof, and do not constitute a limitation of the present disclosure. In the drawings:

[0028] Figure 1 is the circuit topology diagram of the TAB converter of the present application embodiment.

[0029] Figure 2A is the Y-type equivalent circuit diagram of the TAB converter of the present application embodiment.

[0030] Figure 2B is the Δ-type equivalent circuit diagram of the TAB converter of the present application embodiment.

[0031] Figure 3A is the schematic block diagram of the TAB converter of the present application embodiment before power decoupling.

[0032] Figure 3B is the schematic block diagram of the TAB converter of the present application embodiment after power decoupling.

[0033] Figure 4 is the schematic diagram of the power flow direction of the TAB converter of the present application embodiment after power decoupling. DETAILED DESCRIPTION

[0034] In the following detailed description of replacement embodiments of the present disclosure, it is to be understood that not all of the benefits described herein need be achieved in every embodiment of the present disclosure. Indeed, one will appreciate that one can derive benefits in some embodiments that do not recite every one of the benefits described herein. As used herein, "consisting of" is to be interpreted as meaning excluding any element not specified in the claim. As used herein, "consisting essentially of" means excluding any elements not specified in the claim except for impurities or other such non-specified elements in trace amounts. As used herein, "comprising" means including, but not limited to, as understood by persons of ordinary skill in the art. As used herein, "connected" can be understood to be directly connected or indirectly connected through one or more intermediaries.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this present disclosure belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the articles "a", "an" and "the" are intended to include one or more items, and can be used interchangeably with "at least one" or "one or more". Unless otherwise specified, the use of the ordinal adjectives "first", "second", and "third", etc., to describe a common but distinct item, merely indicate that different instances of the same item are being referred to, and are not intended to imply that the associated claim is directed to more than one of the same type of item. The use of the terms "at least one" and "one or more" indicates that at least one of and one or more of the pertinent listed items can be included in the described implementations. The use of the term "plurality" indicates that there can be more than one of the referenced item present. The use of the term "another" indicates at least a second and a different instance of whatever element is preceded by this term. The use of the term "or" means "and / or" unless otherwise specified. The use of the term "based on" means "based, at least in part, on" unless otherwise specified.

[0036] In the present disclosure, all embodiments and preferred embodiments mentioned herein can be combined with each other to form new technical solutions, if not specifically stated. In the present disclosure, all technical features and preferred features mentioned herein can be combined with each other to form new technical solutions, if not specifically stated.

[0037] In the description of embodiments of the present disclosure, the term "and / or" is merely a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the existence of A alone, the existence of A and B together, and the existence of B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0038] The three-active-bridge converter decoupling control method provided by the embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings.

[0039] Figure 1 The circuit topology of the TAB three-port converter of the embodiment of the present application is shown. In the circuit of the TAB converter, the three ports, port 1, port 2 and port 3, are coupled via a three-winding high-frequency transformer T, and the turns ratio of each port is n1, n2 and n3, respectively. In the embodiment, the side where port 1 and port 2 are located is defined as the primary side of the transformer T, and the side where port 3 is located is defined as the secondary side of the transformer T.

[0040] In port 1, there are a DC power supply U1, an H-bridge converter composed of switching tubes Q1-Q4 connected in anti-parallel with diodes, and a resonant cavity connected between the H-bridge converter and the inductor L1 of the transformer T. The resonant cavity is composed of a switching control resonant capacitor SCC1 and a resonant inductor L r1 , wherein the switching control resonant capacitor SCC1 is composed of N resonant capacitors C r1 and 2N MOSFET tubes S 1a -S N1a and S 1b -S N1b . Each resonant capacitor C r1 is connected in series with source-source connected MOSFET tubes S 1a , S 1b (……S N1a , S N1b ) to form a branch, and N such branches are connected in parallel to form the switching control resonant capacitor SCC1.

[0041] Port 2 and port 3 also have the same structure. Port 2 includes a DC power supply U2, an H-bridge converter composed of switching tubes Q5-Q8, and a resonant cavity composed of a switching control resonant capacitor SCC2 and a resonant inductor L r2 . The switching control resonant capacitor SCC2 is also composed of N parallel branches connected by 1 resonant capacitor C r2 and 2 MOSFET tubes S 2a , S 2b (……S N2a , S N2b ).

[0042] Port 3 includes a DC power supply U3, an H-bridge converter composed of switching tubes Q9-Q 12 , and a resonant cavity composed of a switching control resonant capacitor SCC3 and a resonant inductor L r3 . The switching control resonant capacitor SCC3 is also composed of N resonant capacitors C r3 and 2N MOSFET tubes S 3a -S N3a and S 3b -S N3bEach resonant capacitor C r3 MOSFET S 3a , S 3b (…)

[0043] S N3a , S N3b are connected in series to form a branch, and N such branches are connected in parallel to form a switch-controlled resonant capacitor SCC3.

[0044] Here, Figure 1 The switch-controlled resonant capacitors SCC1, SCC2, and SCC3 of the three ports of the TAB converter shown in FIG. 1 are each provided with N parallel resonant capacitor branches, but this is merely an example and is not limited thereto, and the number of parallel branches in the resonant cavities of the respective ports can also be different from one another.

[0045] According to Figure 1 , by controlling the on-off of the MOSFETs S 1a -S N1a , S 1b -S N1b , S 2a -S N2a , S 2b -S N2b , S 3a -S N3a , and S 3b -S N3b in each of the parallel branches of the switch-controlled resonant capacitors SCC1, SCC2, and SCC3, the number of resonant capacitors C r1 , C r2 , and C r3 connected in parallel in the three ports can be changed, thereby changing the resonant capacitor values of the respective ports.

[0046] As shown in FIG. 2, the respective capacitor values of the switch-controlled resonant capacitors SCC1, SCC2, and SCC3 of the respective ports are respectively: Figure 1 C SCC1 =N1C r1 , C SCC2 =N2C r2 , and C SCC3 =N3C r3 (1)

[0047] wherein N1, N2, and N3 respectively represent the number of resonant capacitors C r1 , C r2 , and C r3 connected in parallel by the MOSFETs in the respective ports.

[0048] For

[0049] Figure 1 ​The circuit structure of the TAB converter shown is simplified, and Figure 2A The Y-type equivalent circuit diagram shown. The equivalent reactance Z1, Z2 and Z3 of the three ports can be represented as:

[0050]

[0051] Therefore, the resonant frequency f r1 , f r2 and f r3 of the resonant cavity of each port can be represented as:

[0052]

[0053] In addition, in order to analyze the power transmission between each port, through Y-△ transformation, the △ type equivalent circuit shown as Figure 2B can be obtained. At this time, the TAB converter can be equivalent to 3 independent DAB (Dual Active Bridge: Dual Active Bridge) converters for analysis. The equivalent reactance Z 12 between port 1 and port 2, the equivalent reactance Z 13 between port 1 and port 3, and the equivalent reactance Z 23 between port 2 and port 3 can be represented as follows:

[0054]

[0055] Based on the Δ model shown in Figure 2B , the fundamental components v 1p , v 2p and v 3p of the three-port input voltage v1, v2 and v3 are represented as:

[0056]

[0057] wherein, φ1 represents the phase angle between the center of the voltage v1 and v2 of the port 1, 2, φ2 represents the phase angle between the center of the voltage v1 and v3 of the port 1, 3, and α1, α2 and α3 represent the internal phase shift angle of the port 1, 2, 3, ω = 2πf s , f s represent the working frequency of the TAB converter, and U1, U2, U3 represent the voltage of the DC power supply of the port 1, 2, 3 respectively.

[0058] From the turns ratio n1, n2, n3, the voltage gain of the port 2, 3 relative to the port 1 is

[0059]

[0060] The equivalent fundamental current from port 1 to port 2 can be expressed as:

[0061]

[0062] Accordingly, the transmission power P 12 between port 1 and port 2 can be calculated as:

[0063]

[0064] Similarly, the transmission power P 13 between port 1 and port 3 can be calculated as: 23 and the transmission power P 12 between port 2 and port 3 can be calculated as:

[0065]

[0066] Taking the case of the power supply transmitting power from port 1 and port 2 to port 3 as an example, that is, the working condition of the TAB converter as a double-input single-output. According to Figure 2B , although the TAB converter is equivalent to three independent DAB converters, due to the existence of the equivalent reactance Z 12 between port 1 and port 2, power will be coupled between port 1 and port 2.

[0067] To offset the power coupling component between ports, the hardware power decoupling control method is adopted in the embodiment of the present application, specifically by changing the number of parallel resonant capacitors (adjusting the values of N1, N2 and N3 in formula (1)) to adjust the resonant frequency of each port resonant cavity, so as to realize the topology-level power decoupling of the circuit at a constant frequency, and finally make each power output of the system can be adjusted by a separate control loop.

[0068] Figure 3A is a schematic block diagram of the TAB converter before power decoupling in the embodiment of the present application, that is, the control strategy block diagram of the traditional power decoupling. Figure 3B is a schematic block diagram of the TAB converter after power decoupling in the embodiment of the present application. Here, taking the decoupling between port 1 and port 2 as an example.

[0069] Figure 3A In the prior art, the basic principle of the traditional power decoupling is to output the required power P 1_ref , P 2_ref from port 1 and port 2 of the TAB converter respectively through closed-loop power control according to the given power reference value P 11 , P 22 , and in the closed-loop power control, the mutually coupled phase shift control variables H 12 , H 21H represents the power coupling relationship (coupling component) between port 1 and port 2, wherein H 12 represents the phase angle between the center of the voltage v3 of port 3 and the center of the voltage v1 of port 1 H represents the influence on the actual output power P2 of port 2 21 represents the phase angle between the center of the voltage v3 of port 3 and the center of the voltage v2 of port 2 H represents the influence on the actual output power P1 of port 1.

[0070] According to formula (8) and (9), the output power of port 1 and port 2 in the traditional power decoupling case can be represented as:

[0071]

[0072] At this time, H 12 and H 21 are not 0, indicating that there is a coupling relationship between port 1 and port 2:

[0073]

[0074] In order to realize the power decoupling between port 1 and port 2, H 12 and H 21 two coupling components need to be eliminated, so that each output of the system can be adjusted by a separate control loop. The traditional decoupling methods include software decoupling and hardware decoupling. The software decoupling method by adding a decoupling network in the control system increases the complexity of control and is not conducive to the stability of the system. The hardware decoupling method of inductance type cannot truly realize the topology-level decoupling, not only needs to add an inductor, but also reduces the efficiency of the system. Although the hardware decoupling method of resonance type can reduce the circuit stress, it needs to make the resonance frequency of the resonance cavity of one of the ports strictly consistent with the working frequency of the TAB converter, which limits the adaptability of the system and cannot meet different application requirements.

[0075] According to the power decoupling control method of the embodiment of the application, the number of resonant capacitors connected in parallel to each port can be adjusted, so that the resonant frequency of each port can also be adjusted, thereby realizing topology-level power decoupling, avoiding complex software decoupling strategies and additional inductor components, and simplifying the implementation process of power decoupling.

[0076] The specific process is as follows.

[0077] According to the Δ model of Figure 2B , when the resonant frequency f r3 of port 3 is equal to the working frequency f s of the TAB converter, Z3=0, Z 12 =∞, and P 12 =0. At this time, the output power of port 1 and port 2 can be represented as:

[0078]

[0079] The output power of port 3 is:

[0080]

[0081] The power of the three ports satisfies the following formula:

[0082] P3=P1+P2 (17)

[0083] At this time, the power coupling relationship between ports 1 and 2 is H 12 and H 21 for:

[0084]

[0085] Therefore, the coupling relationship between ports 1 and 2 is released.

[0086] according to Figure 1 The circuit topology of the TAB converter shown in the figure is combined with the above equations (1) and (3). By adjusting the resonant capacitor C connected in parallel in port 3, r3 The number N3 makes the resonant frequency f of port 3 r3 is the operating frequency f of the TAB converter s ,Right now:

[0087]

[0088] The power P1 output from port 1 can be (Right now ) is independently controlled (Equation (14)), and the output power P2 of port 2 is given by (Right now ) independent control (Equation (15)), such as Figure 3B At this time, the resonant capacitor C connected in parallel in ports 1 and 2 r1 、C r2 The number N1 and N2 can be flexibly set according to the required transmission power.

[0089] Figure 3A and Figure 3B The power decoupling between port 1 and port 2 is taken as an example, but the present invention is not limited thereto. There are three power decoupling situations in the TAB converter. Figure 4 The power flow directions under three operating modes are given.

[0090] Figure 4 The operation mode ① in FIG1 corresponds to the power decoupling between port 1 and port 2 described in this embodiment. In this case, as described above, by adjusting the resonant capacitor C connected in parallel to port 3, the power of the resonant capacitor C connected in parallel to port 3 is reduced.r3 The number N3 makes its resonant frequency f r3 Equal to the operating frequency f of the TAB converter s , thereby offsetting the power decoupling component between ports 1 and 2, achieving power decoupling between ports 1 and 2, so that each power output of the system can be regulated by a separate control loop.

[0091] Figure 4 Mode ② in the diagram corresponds to the power decoupling between port 1 and port 3. Similarly, by adjusting the resonant capacitor C connected in parallel to port 2, r2 The number N2 makes its resonant frequency f r2 Equal to the operating frequency f of the TAB converter s , thereby offsetting the power decoupling components between ports 1 and 3, achieving power decoupling between ports 1 and 3, so that each power output of the system can be regulated by a separate control loop.

[0092] Operation mode ③ corresponds to the power decoupling between port 2 and port 3. Similarly, by adjusting the resonant capacitor C connected in parallel to port 1 r1 The number N1 makes its resonant frequency f r1 Equal to the operating frequency f of the TAB converter s , thereby offsetting the power decoupling component between ports 2 and 3, achieving power decoupling between ports 2 and 3, so that each power output of the system can be regulated by a separate control loop.

[0093] In summary, the hardware power decoupling strategy of this embodiment can achieve a change in the resonant frequency of a port by changing the number of resonant capacitors connected in parallel to that port. By making the resonant frequency equal to the operating frequency of the TAB converter, the power coupling component between the other two ports is offset, achieving power decoupling, and ultimately allowing the power output of each port to be adjusted by a separate control loop. Therefore, topological-level power decoupling can be achieved by adjusting the resonant capacitor values ​​of the three ports. There is no need for complex software decoupling strategies or the addition of additional inductors. Resonant hardware decoupling can be achieved by simply controlling the number of switches connected in series with the resonant capacitor at each port according to the required transmission power.

[0094] According to the decoupling control method of the three-active bridge converter in the embodiment of the present invention, topological-level power decoupling is achieved by adjusting the resonant capacitance values ​​of the three ports. Complex software decoupling strategies and the addition of additional inductors are no longer required. Resonant hardware decoupling can be achieved by simply controlling the number of switches connected in series with the resonant capacitor at each port according to the required transmission power. This provides greater adjustment freedom and can adapt to different application requirements.

[0095] In addition, by controlling the switching state of the switch tube connected in series with the resonant capacitor in the resonant cavity of each port, the resonant capacitor of each port can be accurately adjusted, so that the resonant cavity of each port can work at the required frequency, and the efficiency and stability of power transmission are improved.

[0096] The three-active-bridge converter decoupling control method and the three-active-bridge converter of the present application are suitable for photovoltaic energy storage systems, can better adapt to the changing power demand in photovoltaic power generation and energy storage systems, and improve the overall performance and reliability of the system.

[0097] It should be understood that the above description is illustrative and not restrictive. For example, the above-described embodiments (and / or aspects thereof) can be used in combination with each other. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the various embodiments of the present disclosure without departing from the scope thereof. Although the sizes and types of materials described herein are used to define the parameters of the various embodiments of the present disclosure, the various embodiments are not meant to be limiting, but are exemplary embodiments. Many other embodiments will be apparent to those skilled in the art upon reading the above description. Accordingly, the scope of the various embodiments of the present disclosure should be determined by reference to the appended claims and the full scope of equivalents for which they are entitled.

Claims

1. A method for decoupling control of a three-active-bridge converter, characterized in that, a first port, a second port and a third port of the three-active-bridge converter are coupled via a transformer, each of the first port, the second port and the third port has a resonant tank, the resonant tank comprising a switch-controlled resonant capacitance and a resonant inductance, the switch-controlled resonant capacitance having an adjustable capacitance value, the resonant frequency of the third port is set to the operating frequency of the three-active-bridge converter by adjusting the capacitance value of the third port, so as to realize power decoupling between the first port and the second port. 2.The method of claim 1, characterized in that, the switch-controlled resonant capacitance comprises: a plurality of parallel resonant capacitors; and a switch connected to each of the resonant capacitors, in the third port, the resonant frequency of the third port is adjusted by controlling the on-off of the switch and changing the number of the parallel resonant capacitors connected in the switch-controlled resonant capacitance. 3.The method of claim 2, characterized in that, the capacitance value of the first port or the second port is adjusted by controlling the on-off of the switch in the first port or the second port, so that the resonant frequency of the first port or the second port is set to a predetermined specified frequency. 4.The method of any one of claims 1 to 3, characterized in that, the switch is composed of two source-source connected MOSFETs. 5.The method of any one of claims 1 to 3, characterized in that, the plurality of parallel resonant capacitors in each port have the same capacitance value. 6.The method of any one of claims 1 to 3, characterized in that, the inductance value of the resonant inductance is fixed. 7.The method of claim 6, characterized in that, when decoupling the first port and the second port, the resonant frequency of the resonant tank of the third port satisfies: wherein f s : the operating frequency of the three-active-bridge converter; f r3 : the resonant frequency of the resonant cavity of the third port L r3 : an inductance value of the resonant inductance of the third port; C r3 : a capacitance value of the resonant capacitor of the third port; N3: the number of the parallel resonant capacitors connected in the switch-controlled resonant capacitance of the third port through the switch. 8.The method of any one of claims 1 to 3, characterized in that, after the power decoupling, the transmission power of the first port is controlled by the phase angle between the input voltage of the first port and the third port, and the transmission power of the second port is controlled by the phase angle between the input voltage of the second port and the third port. 9.The method of claim 8, characterized in that, the transmission power of the first port and the transmission power of the second port are closed-loop power controlled, with a predetermined reference power as input and the actual output power of the first port and the second port respectively as output. 10.The method of claim 9, characterized in that, The first port and the second port are located at the primary side of the transformer, and the third port is located at the secondary side of the transformer.

11. A three-active-bridge converter, characterized in that, comprising a first port, a second port and a third port coupled via a transformer, the first port, the second port and the third port each have a resonant cavity comprising a switch-controlled resonant capacitance and a resonant inductance with adjustable capacitance value, when power is decoupled from the first port and the second port, the capacitance value of the third port is adjusted so that the resonant frequency of the third port is the operating frequency of the three-active-bridge converter.

12. The three-active-bridge converter of claim 11, characterized in that, the switch-controlled resonant capacitance comprises: a plurality of parallel resonant capacitors; and a switch connected to each of the resonant capacitors, in the third port, by controlling the on-off of the switch, the number of resonant capacitors connected in parallel in the switch-controlled resonant capacitance is changed, thereby adjusting the resonant frequency of the third port.

13. The three-active-bridge converter of claim 12, characterized in that, by controlling the on-off of the switch in the first port or the second port, the capacitance value of the first port or the second port is adjusted so that the resonant frequency of the first port or the second port is a predetermined specified frequency.

14. A photovoltaic system, characterized in that, comprising the three-active-bridge converter of any one of claims 11 to 13.

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