Port coupling type multi-port capacitive energy transfer DC / DC converter and control method thereof

Abstract

The invention discloses a port coupling type multi-port capacitive energy transfer DC / DC converter and a control method thereof, and relates to the field of power electronic technology and flexible DC power transmission. The multi-port DC-DC converter aims to solve the problem that an existing multi-port DC-DC converter cannot have the advantages of being large in transmission power, high in transmission efficiency and high in integration level while achieving interconnection of different voltage levels. The port coupling type multi-port capacitive energy transfer DC / DC converter is composed of three power conversion units of the same structure, and each power conversion unit comprises three converter valve groups and two energy storage bridge arms. Through the coordinated control of the converter valve group and the energy storage bridge arm, the energy storage bridge arm in each power conversion unit alternately supports three different DC ports, the charging and discharging power of half-bridge sub-module capacitors in the energy storage bridge arm is equal, and the energy stored in the capacitors in the converter is integrally kept balanced. Therefore, energy conversion among three direct current ports with different direct current voltage levels can be realized.

Description

Technical Field

[0001] The invention belongs to the field of power electronics technology and flexible direct current transmission, and in particular relates to a three-port flexible direct current transmission DC / DC converter. Background Art

[0002] High-voltage direct current transmission technology can effectively transmit electricity generated by renewable energy to power consumption centers, so it is widely used in the process of renewable energy power transmission. With the proposal and successful application of modular multi-level converter technology, high-voltage direct current transmission technology has developed rapidly, and the grid structure has developed from two-terminal point-to-point transmission to multi-terminal ring mesh transmission. However, due to factors such as the scale of renewable energy, the current high-voltage direct current transmission lines have different voltage levels such as ±160kV, ±320kV, ±500kV, ±800kV and ±1100kV. Unlike the AC transformer in the AC power grid, the DC power grid cannot use the principle of electromagnetic induction to transmit electric energy. It is necessary to use a high-power DC-DC converter as a connecting device to convert between different voltage levels. Therefore, the multi-port DC / DC converter is a key component to solve the interconnection of multiple voltage levels.

[0003] A single two-port DC / DC converter can only realize the interconnection of two different voltage levels. If a two-port DC / DC converter is used to realize the interconnection of multiple voltage levels, the voltage level needs to be converted into a unified voltage level before connection. In this power grid structure, multiple high-power DC / DC converters are required. At the same time, DC power flow controllers and DC circuit breakers and other equipment are also required in the connection line. The power transmission efficiency is low, and a large number of power electronic equipment are required, which has low stability, high cost and large floor space.

[0004] Multi-port DC / DC converters can realize the interconnection of multiple voltage levels. Compared with the use of two-port DC / DC converters to realize the interconnection of different voltage levels, only one high-power multi-port DC / DC converter is needed to realize the interconnection of multi-voltage level circuits, which significantly improves the integration and economic benefits of the converter. Therefore, the DC power grid needs a multi-port DC-DC converter that can realize the interconnection of different voltage levels while also having the characteristics of high transmission power, high transmission efficiency, and high integration, but there are currently few feasible technical solutions.

[0005] The patent "A three-port DC power electronic transformer structure" with authorization announcement number CN209030095U proposes a new three-port DC power electronic transformer architecture. This structure realizes the interconnection of three DC voltage levels in a specific connection method by combining multiple high-frequency isolated DC / DC converters, multiple H-bridge inverter units and a non-isolated DC / DC converter. Although this design is suitable for the voltage level of the DC distribution network, its voltage level and power capacity are limited, and it is difficult to meet the technical requirements of high-voltage DC transmission.

[0006] Patent "A Multi-port Back-to-Back DC-DC Converter" with authorization announcement number CN104022499A proposes a multi-port DC-DC converter structure that is connected back-to-back to the same AC end through multiple voltage source converters. This design can achieve interconnection of multiple DC voltage levels, but it uses a two-stage power transmission method with an intermediate AC transformer, resulting in low energy transmission efficiency, large size and high cost of the equipment, which limits its applicability in certain application scenarios. Summary of the invention

[0007] The present invention aims to solve the problem that the existing multi-port DC-DC converter cannot realize the interconnection of different voltage levels while having the characteristics of large transmission power, high transmission efficiency and high integration. A port-coupled multi-port capacitive energy transfer DC / DC converter and a control method thereof are provided.

[0008] In order to realize the interconnection of three different DC voltage levels in a DC power grid, the present invention provides a port-coupled multi-port capacitive energy transfer DC / DC converter, comprising: three power conversion units connected in parallel, each power conversion unit comprising three converter valve groups and two energy storage bridge arms, the two energy storage bridge arms are respectively an upper bridge arm and a lower bridge arm, and the three converter valve groups are respectively a first converter valve group, a second converter valve group and a third converter valve group;

[0009] The upper bridge arm, the lower bridge arm and the first converter valve group are connected in series, and the ends serve as the first port;

[0010] The upper bridge arm, the lower bridge arm and the second converter valve group are connected in series, and the ends serve as the second port;

[0011] The lower bridge arm and the third converter valve group are connected in series, and the end portion serves as the third port.

[0012] Furthermore, in each power conversion unit,

[0013] One end of the first converter valve group and one end of the second converter valve group are both connected to the input end of the upper bridge arm, the output end of the upper bridge arm is connected to the input end of the lower bridge arm, and the connection end of the upper bridge arm and the lower bridge arm is connected to one end of the third converter valve group;

[0014] The other end of the first converter valve group and the output end of the lower bridge arm serve as the first port, the other end of the second converter valve group and the output end of the lower bridge arm serve as the second port, and the other end of the third converter valve group and the output end of the lower bridge arm serve as the third port.

[0015] Furthermore, the structures of the three converter valve groups are the same, and each converter valve group includes a plurality of converter valves connected in series.

[0016] Furthermore, the above-mentioned converter valve is an IGBT anti-parallel diode, an IGCT switch tube or a bidirectional thyristor.

[0017] Furthermore, the structures of the two energy storage bridge arms are the same, and each energy storage bridge arm includes a plurality of half-bridge sub-modules connected in series and a bridge arm reactor, the bridge arm reactor in the upper bridge arm is connected to the input ends of the plurality of half-bridge sub-modules, and the bridge arm reactor in the lower bridge arm is connected to the output ends of the plurality of half-bridge sub-modules;

[0018] The structures of the multiple half-bridge sub-modules are all the same, and each half-bridge sub-module includes two IGBT switch tubes connected in series and a capacitor connected in parallel to the ends of the two IGBT switch tubes.

[0019] The control method of the above-mentioned port-coupled multi-port capacitive energy transfer DC / DC converter comprises:

[0020] First port control:

[0021] Triggering the opening of the first converter valve group and keeping the second converter valve group and the third converter valve group closed, so that the current I1 of the first port flows through the two energy storage bridge arms, adjusting the sum of the output voltages of all the half-bridge sub-modules in the two energy storage bridge arms, so that the current flowing through the two energy storage bridge arms is a trapezoidal wave, and synchronously charging or discharging the half-bridge sub-modules in the two energy storage bridge arms, so that the two energy storage bridge arms jointly bear the DC voltage U1 of the first port;

[0022] The second and third ports control:

[0023] The second converter valve group and the third converter valve group are triggered to open, and the first converter valve is kept closed, so that the current I2 of the second port flows through the second converter valve group and the two energy storage bridge arms, and the current I3 of the third port flows through the third converter valve group and the lower bridge arm, and the sum of the currents flowing through the lower bridge arm is I2+I3, and the sum of the output voltages of all half-bridge sub-modules in the two energy storage bridge arms is adjusted so that the current flowing through the two energy storage bridge arms is a trapezoidal wave, and the half-bridge sub-modules in the two energy storage bridge arms are synchronously charged or discharged, so that the two energy storage bridge arms jointly bear the DC voltage U2 of the second port, and the lower bridge arm bears the DC voltage U3 of the third port.

[0024] Furthermore, the current control timings of the three power conversion units are sequentially staggered with a phase of 120°.

[0025] Furthermore, the first port control includes:

[0026] Triggering to open the first converter valve group and keeping the second converter valve group and the third converter valve group closed, so that the current I1 of the first port flows through the two energy storage bridge arms;

[0027] Adjust the sum of the output voltages of all half-bridge submodules in the upper bridge arm and the lower bridge arm to u pj1 and u nj1 , so that the current flowing through the upper bridge arm and the lower bridge arm is I1÷T c The speed changes from 0 to I1;

[0028] When the current flowing through the upper bridge arm and the lower bridge arm rises to I1, the sum of the output voltages of all half-bridge sub-modules in the upper bridge arm and the lower bridge arm is U1, and the current flowing through the upper bridge arm and the lower bridge arm is maintained at I1;

[0029] u pj1 =U pj -L1×I1÷T c ,

[0030] u nj1 =U nj -L2×I1÷T c ,

[0031] Among them, T c is the current rise or fall time of the current trapezoidal wave, U pj is the sum of the output voltages of all half-bridge submodules in the upper bridge arm, U nj It is the sum of the output voltages of all half-bridge sub-modules in the lower bridge arm, and L1 and L2 are the bridge arm reactances of the upper bridge arm and the lower bridge arm respectively.

[0032] Furthermore, the second port and the third port control include:

[0033] Triggering the opening of the second converter valve group and the third converter valve group, and keeping the first converter valve closed, so that the current I2 of the second port flows through the second converter valve group and the two energy storage bridge arms, the current I3 of the third port flows through the third converter valve group and the lower bridge arm, and the sum of the currents flowing through the lower bridge arm is I2+I3;

[0034] Adjust the sum of the output voltages of all half-bridge submodules in the upper bridge arm and the lower bridge arm to u pj2 and u nj2 , so that the current flowing through the upper bridge arm is I2÷T c The speed changes from 0 to I2, and the current flowing through the lower bridge arm is (I2+I3)÷T cThe speed changes from 0 to I2+I3;

[0035] When the current flowing through the upper bridge arm and the lower bridge arm rises to I2 and I2+I3 respectively, the sum of the output voltages of all half-bridge submodules in the upper bridge arm is U2-U3, the sum of the output voltages of all half-bridge submodules in the lower bridge arm is U3, the current flowing through the upper bridge arm is maintained at I2, and the current flowing through the lower bridge arm is maintained at I2+I3;

[0036] u pj2 =(U2-U3)-L1×I2÷T c ,

[0037] u nj2 =U3-L2×(I2+I3)÷T c .

[0038] Further, after the sum of the output voltages of all the half-bridge submodules in the upper bridge arm and the lower bridge arm is U1, the sum of the output voltages of all the half-bridge submodules in the upper bridge arm and the lower bridge arm is adjusted to be u pj3 and u nj3 , so that the current flowing through the upper bridge arm and the lower bridge arm is I1÷T c The speed changes from I1 to 0;

[0039] When the current flowing through the upper and lower bridge arms drops to 0, keep u pj3 and u nj3 , realize the shut-off of the converter valve;

[0040] u pj3 =U pj +L1×I1÷T c ,

[0041] u nj3 =U nj +L2×I1÷T c ;

[0042] After the sum of the output voltages of all half-bridge submodules in the upper bridge arm is U2-U3 and the sum of the output voltages of all half-bridge submodules in the lower bridge arm is U3, the sum of the output voltages of all half-bridge submodules in the upper bridge arm and the lower bridge arm are adjusted to be u pj4 and u nj4 , so that the current flowing through the upper bridge arm is I2÷T c The speed changes from I2 to 0, and the current flowing through the lower bridge arm is (I2+I3)÷T c The speed changes from I2+I3 to 0;

[0043] When the current flowing through the upper bridge arm and the lower bridge arm drops to 0, keep u pj4 and u nj4 , to realize the shut-off of the converter valve,

[0044] u pj4 =(U2-U3)+L1×I2÷T c ,

[0045] u nj4 =U3+L2×(I2+I3)÷T c .

[0046] Compared with the prior art, the present invention has the following advantages:

[0047] The port-coupled multi-port capacitive energy transfer DC / DC converter described in the present invention can realize energy transfer between three ports of different DC voltage levels without the need for an AC transformer, and has the characteristics of light weight, small size and high efficiency; it is suitable for realizing the interconnection of three DC systems of different voltage levels. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is the circuit schematic diagram of the port-coupled multi-port capacitive energy transfer DC / DC converter;

[0049] Figure 2 This is the circuit schematic diagram of the converter valve;

[0050] Figure 3 This is the circuit schematic diagram of the half-bridge submodule;

[0051] Figure 4 It is a schematic diagram of the working principle of the power conversion unit of the DC-DC converter;

[0052] Figure 5 It is a schematic diagram of the current interleaving control of the energy storage bridge arm of the DC-DC converter;

[0053] Figure 6 The schematic diagram of the circuit in which the IGBT anti-parallel diode in the converter valve of the DC-DC converter is replaced by a bidirectional thyristor;

[0054] Figure 7 The schematic diagram of the circuit is that the IGBT anti-parallel diode in the converter valve of the DC-DC converter is replaced by the IGCT switch tube. DETAILED DESCRIPTION

[0055] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work belong to the scope of protection of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0056] Specific implementation method 1: refer to Figures 1 to 3 Specifically describe this embodiment, the port-coupled multi-port capacitive energy transfer DC / DC converter described in this embodiment is as follows: Figure 1 As shown, it consists of three power conversion units with exactly the same structure. The three power conversion units are connected in parallel and work between three voltage ports. Each power conversion unit includes three converter valve groups with the same structure and two energy storage bridge arms with the same structure. The two energy storage bridge arms are an upper bridge arm and a lower bridge arm respectively. The three converter valve groups are a first converter valve group, a second converter valve group and a third converter valve group, which correspond to the first port, the second port and the third port respectively.

[0057] Each power conversion unit includes two energy storage bridge arms, namely an upper bridge arm and a lower bridge arm, and the bridge arm reactors of the upper bridge arm and the lower bridge arm are L1 and L2 respectively. The output end of the upper bridge arm is connected to the input end of the lower bridge arm, and the output end of the lower bridge arm is connected to the ground. One end of the first and second converter valve groups are connected to the input end of the upper bridge arm, and the other ends of the first and second converter valve groups serve as the first and second ports of the port-coupled multi-port capacitive energy transfer DC / DC converter respectively; one end of the third converter valve group is connected to the connection point of the upper and lower bridge arms, and the other end serves as the third port of the port-coupled multi-port capacitive energy transfer DC / DC converter.

[0058] The three converter valve groups in each power conversion unit have the same structure, and each converter valve group is composed of multiple converter valves connected in series. Figure 2 As shown, the structure of the converter valve is an IGBT parallel anti-diode. The IGBT parallel anti-diode can be replaced by an IGCT switch tube or a bidirectional thyristor, such as Figure 6 and 7 shown.

[0059] The energy storage bridge arm is composed of a plurality of half-bridge sub-modules and a bridge arm reactor L connected in series. Figure 3 As shown, the circuit structures of the half-bridge submodules are the same, and each half-bridge submodule includes two IGBT switches and a capacitor; the output end of each half-bridge submodule is connected to the input end of the next half-bridge submodule, wherein the input end of the first half-bridge submodule of the upper bridge arm is connected to one end of the bridge arm inductor, and the other end of the bridge arm inductor serves as the input end of the energy storage bridge arm, and the output end of the last half-bridge submodule serves as the output end of the energy storage bridge arm, and the input end of the first half-bridge submodule of the lower bridge arm serves as the input end of the energy storage bridge arm, and the output end of the last half-bridge submodule is connected to one end of the bridge arm inductor, and the other end of the bridge arm inductor serves as the output end of the energy storage bridge arm.

[0060] Specific implementation method 2: refer to Figure 4 and Figure 5Specifically describing this embodiment, the control method of the port-coupled multi-port capacitive energy transfer DC / DC converter described in this embodiment includes the following steps:

[0061] Step 1: Trigger and open the first converter valve group, and keep the other two converter valve groups closed. At this time, the upper and lower bridge arms are connected to the first port together, and the current I1 of the first port flows through the first converter valve and the upper and lower bridge arms. By controlling the sum of the output voltages of all half-bridge sub-modules of the upper and lower bridge arms, it is controlled to be a trapezoidal wave, and the half-bridge sub-modules in the energy storage bridge arm are synchronously charged or discharged. In this process, the upper and lower bridge arms jointly bear the DC voltage U1 of the first port. The specific method is as follows:

[0062] (1) Trigger the opening of the first converter valve group, and at the same time, the sum of the output voltages of all half-bridge sub-modules of the upper bridge arm u pj Control is U pj -L1×I1÷T c , the sum of the output voltages of all half-bridge submodules in the lower bridge arm u nj Control is U nj -L2×I1÷T c , at this time, the current i flowing through the upper and lower bridge arms pj I1÷T c The speed changes from 0 to I1, where T c is the current rise or fall time of the current trapezoidal wave, U pj and U nj are the terminal voltages of the upper bridge arm and the lower bridge arm, respectively, and L1 and L2 are the bridge arm reactances of the upper bridge arm and the lower bridge arm, respectively;

[0063] (2) When the current flowing through the upper and lower bridge arms rises to I1, the sum of the output voltages of all half-bridge sub-modules of the upper and lower bridge arms u pj +u nj The control is U1, and the sum of the output voltages of all half-bridge sub-modules of the upper bridge arm is u pj Control is U pj , the sum of the output voltages of all half-bridge submodules in the lower bridge arm u nj Control is U nj , then the current i flowing through the upper and lower bridge arms pj Maintain at I1;

[0064] (3) The sum of the output voltages of all half-bridge sub-modules in the upper bridge arm is u pj Control is U pj +L1×I1÷T c , the sum of the output voltages of all half-bridge submodules in the lower bridge arm u nj Control is U nj +L2×I1÷T c , at this time, the current i flowing through the upper and lower bridge armspj I1÷T c The speed changes from I1 to 0;

[0065] (4) When the current i flowing through the upper and lower bridge arms pj After it becomes 0, the sum of the output voltages of all half-bridge sub-modules of the upper bridge arm u pj For U pj +L1×I1÷T c , the sum of the output voltages of all half-bridge submodules in the lower bridge arm u nj For U nj +L2×I1÷T c A period of time is required to realize the reliable shutdown of the converter valve, and then the sum of the output voltages of all half-bridge sub-modules of the upper bridge arm u pj Becomes U2-U3, the sum of the output voltages of all half-bridge sub-modules in the lower bridge arm is u nj It changes to U3 to prepare for the zero-voltage opening of the converter valve in step 2.

[0066] Step 2: trigger and open the second converter valve group and the third converter valve group, and keep the first converter valve closed. At this time, the upper bridge arm and the lower bridge arm are connected in parallel to the second port, and the lower bridge arm is connected in parallel to the third port. The current I2 of the second port flows through the second converter valve group and the upper and lower bridge arms, and the current I3 of the third port flows through the third converter valve group and the lower bridge arm. The sum of the currents flowing through the lower bridge arm is I2+I3. By controlling the sum of the output voltages of all half-bridge sub-modules of the upper and lower bridge arms, the currents flowing through the upper and lower bridge arms are controlled to be trapezoidal waves, and the half-bridge sub-modules in the energy storage bridge arm are synchronously charged or discharged. In this process, the upper and lower bridge arms jointly bear the DC voltage U2 of the second port, and the lower bridge arm bears the DC voltage U3 of the third port. The specific method is as follows:

[0067] (1) Trigger the opening of the second and third converter valve groups, and at the same time, the sum of the output voltages of all half-bridge sub-modules of the upper bridge arm u pj Control is (U2-U3)-L1×I2÷T c , the sum of the output voltages of all half-bridge submodules in the lower bridge arm u nj Control is U3-L2×(I2+I3)÷T c , at this time the current i flowing through the upper arm pj I2÷T c The speed changes from 0 to I2, and the current i flowing through the lower bridge arm nj (I2+I3)÷T c The speed changes from 0 to I2+I3, where T c is the current rise or fall time of the current trapezoidal wave;

[0068] (2) When the current flowing through the upper and lower bridge arms rises to I2 and I2+I3 respectively, the sum of the output voltages of all half-bridge sub-modules of the upper bridge arm u pj Control is U2-U3, the sum of the output voltages of all half-bridge submodules in the lower bridge arm is u nj If the control is U3, the current i flowing through the upper bridge arm pj Maintained at I2, the current i flowing through the lower bridge arm nj Maintain at I2+I3;

[0069] (3) The sum of the output voltages upj of all half-bridge submodules in the upper bridge arm is controlled to (U2-U3)+L1×I2÷T c , the sum of the output voltages of all half-bridge submodules in the lower bridge arm u nj Control is U3+L2×(I2+I3)÷T c , at this time the current i flowing through the upper arm pj I2÷T c The speed changes from I2 to 0, and the current i flowing through the lower bridge arm nj (I2+I3)÷T c The speed changes from I2+I3 to 0;

[0070] (4) When the current i flowing through the upper and lower bridge arms pj 、i nj After all become 0, continue to maintain the sum of the output voltages u of all half-bridge sub-modules in the upper bridge arm pj =(U2-U3)+L1×I2÷T c , the sum of the output voltages of all half-bridge submodules in the lower bridge arm is U3+L2×(I2+I3)÷T c After a period of time, the converter valve is reliably turned off, and then the sum of the output voltages of all half-bridge sub-modules in the upper bridge arm gradually changes to U pj , the sum of the output voltages of all half-bridge submodules in the lower bridge arm gradually becomes U nj , preparing for the zero-voltage opening of the converter valve in step one of the next cycle.

[0071] By executing the above two steps in a loop, the energy storage bridge arm in each power conversion unit alternately supports three different DC ports through the above method, the charging and discharging power of the half-bridge sub-module capacitor in the energy storage bridge arm is equal, and the energy stored in the capacitor in the converter is kept balanced as a whole, thereby realizing the mutual energy transfer between the DC ports of three different DC voltage levels.

[0072] Furthermore, the control methods of the three power conversion units are exactly the same, but the energy storage bridge arm current control timing is staggered by 120° phases, such as Figure 5 As shown, the DC current at the three ports of the DC-DC converter is continuous.

[0073] In summary, the present invention discloses a port-coupled multi-port capacitive energy transfer DC / DC converter. Through the coordinated control of the converter valve group and the energy storage bridge arm, the energy storage bridge arm in each power conversion unit alternately supports three different DC ports, the charging and discharging power of the half-bridge submodule capacitor in the energy storage bridge arm is equal, and the energy stored in the capacitor in the converter is balanced as a whole, so that energy conversion between DC ports of three different DC voltage levels can be achieved. The port-coupled multi-port capacitive energy transfer DC / DC converter of the present invention can realize energy transfer between three ports of different DC voltage levels, and does not require the configuration of an AC transformer, and has the characteristics of small footprint, high transmission efficiency, light weight, etc.; it is suitable for realizing the interconnection of DC systems of three different voltage levels.

[0074] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that the features described in conjunction with a single embodiment may be used in other described embodiments.

Claims

1. A port-coupled multi-port capacitive energy transfer DC / DC converter, characterized in that: include: Three power conversion units connected in parallel, each power conversion unit includes three converter valve groups and two energy storage bridge arms, the two energy storage bridge arms are respectively an upper bridge arm and a lower bridge arm, and the three converter valve groups are respectively a first converter valve group, a second converter valve group and a third converter valve group; The upper bridge arm, the lower bridge arm and the first converter valve group are connected in series, and the ends serve as the first port; The upper bridge arm, the lower bridge arm and the second converter valve group are connected in series, and the ends serve as the second port; The lower bridge arm and the third converter valve group are connected in series, and the end portion serves as the third port.

2. The port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 1, characterized in that: In each power conversion unit, One end of the first converter valve group and one end of the second converter valve group are both connected to the input end of the upper bridge arm, the output end of the upper bridge arm is connected to the input end of the lower bridge arm, and the connection end of the upper bridge arm and the lower bridge arm is connected to one end of the third converter valve group; The other end of the first converter valve group and the output end of the lower bridge arm serve as the first port, the other end of the second converter valve group and the output end of the lower bridge arm serve as the second port, and the other end of the third converter valve group and the output end of the lower bridge arm serve as the third port.

3. The port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 1 or 2, characterized in that: The three converter valve groups have the same structure, and each converter valve group includes a plurality of converter valves connected in series.

4. The port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 3, characterized in that: The converter valve is an IGBT anti-parallel diode, an IGCT switch tube or a bidirectional thyristor.

5. The port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 1 or 2, characterized in that: The structures of the two energy storage bridge arms are the same, and each energy storage bridge arm includes a plurality of half-bridge sub-modules connected in series and a bridge arm reactor, the bridge arm reactor in the upper bridge arm is connected to the input ends of the plurality of half-bridge sub-modules, and the bridge arm reactor in the lower bridge arm is connected to the output ends of the plurality of half-bridge sub-modules; The structures of the multiple half-bridge sub-modules are all the same, and each half-bridge sub-module includes two IGBT switch tubes connected in series and a capacitor connected in parallel to the ends of the two IGBT switch tubes.

6. The control method of the port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 5, characterized in that: include: First port control: Triggering the opening of the first converter valve group and keeping the second converter valve group and the third converter valve group closed, so that the current I1 of the first port flows through the two energy storage bridge arms, adjusting the sum of the output voltages of all the half-bridge sub-modules in the two energy storage bridge arms, so that the current flowing through the two energy storage bridge arms is a trapezoidal wave, and synchronously charging or discharging the half-bridge sub-modules in the two energy storage bridge arms, so that the two energy storage bridge arms jointly bear the DC voltage U1 of the first port; The second and third ports control: The second converter valve group and the third converter valve group are triggered to open, and the first converter valve is kept closed, so that the current I2 of the second port flows through the second converter valve group and the two energy storage bridge arms, and the current I3 of the third port flows through the third converter valve group and the lower bridge arm, and the sum of the currents flowing through the lower bridge arm is I2+I3, and the sum of the output voltages of all half-bridge sub-modules in the two energy storage bridge arms is adjusted so that the current flowing through the two energy storage bridge arms is a trapezoidal wave, and the half-bridge sub-modules in the two energy storage bridge arms are synchronously charged or discharged, so that the two energy storage bridge arms jointly bear the DC voltage U2 of the second port, and the lower bridge arm bears the DC voltage U3 of the third port.

7. The control method of the port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 6, characterized in that: The current control timings of the three power conversion units are staggered by 120° phase.

8. The control method of the port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 7, characterized in that: The first port control comprises: Triggering to open the first converter valve group and keeping the second converter valve group and the third converter valve group closed, so that the current I1 of the first port flows through the two energy storage bridge arms; Adjust the sum of the output voltages of all half-bridge submodules in the upper bridge arm and the lower bridge arm to u pj1 and u nj1 , so that the current flowing through the upper bridge arm and the lower bridge arm is I1÷T c The speed changes from 0 to I1; When the current flowing through the upper bridge arm and the lower bridge arm rises to I1, the sum of the output voltages of all half-bridge sub-modules in the upper bridge arm and the lower bridge arm is U1, and the current flowing through the upper bridge arm and the lower bridge arm is maintained at I1; in pj1 =U pj -L1×I1÷T c , in nj1 =U nj -L2×I1÷T c , Among them, T c is the current rise or fall time of the current trapezoidal wave, U pj is the sum of the output voltages of all half-bridge submodules in the upper bridge arm, U nj It is the sum of the output voltages of all half-bridge sub-modules in the lower bridge arm, and L1 and L2 are the bridge arm reactances of the upper bridge arm and the lower bridge arm respectively.

9. The control method of the port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 8, characterized in that: The second port and the third port control include: Triggering the opening of the second converter valve group and the third converter valve group, and keeping the first converter valve closed, so that the current I2 of the second port flows through the second converter valve group and the two energy storage bridge arms, the current I3 of the third port flows through the third converter valve group and the lower bridge arm, and the sum of the currents flowing through the lower bridge arm is I2+I3; Adjust the sum of the output voltages of all half-bridge submodules in the upper bridge arm and the lower bridge arm to u pj2 and u nj2 , so that the current flowing through the upper bridge arm is I2÷T c The speed changes from 0 to I2, and the current flowing through the lower bridge arm is (I2+I3)÷T c The speed changes from 0 to I2+I3; When the current flowing through the upper bridge arm and the lower bridge arm rises to I2 and I2+I3 respectively, the sum of the output voltages of all half-bridge submodules in the upper bridge arm is U2-U3, the sum of the output voltages of all half-bridge submodules in the lower bridge arm is U3, the current flowing through the upper bridge arm is maintained at I2, and the current flowing through the lower bridge arm is maintained at I2+I3; u pj2 =(U2-U3)-L1×I2÷T c , u nj2 =U3-L2×(I2+I3)÷T c 。 10. The control method of the port-coupled multi-port capacitive energy transfer DC / DC converter according to claim 9, characterized in that: After the sum of the output voltages of all the half-bridge submodules in the upper bridge arm and the lower bridge arm is U1, the sum of the output voltages of all the half-bridge submodules in the upper bridge arm and the lower bridge arm is adjusted to be u pj3 and u nj3 , so that the current flowing through the upper bridge arm and the lower bridge arm is I1÷T c The speed changes from I1 to 0; When the current flowing through the upper and lower bridge arms drops to 0, keep u pj3 and u nj3 , realize the shut-off of the converter valve; in pj3 =U pj +L1×I1÷T c , in nj3 =U nj +L2×I1÷T c ; After the sum of the output voltages of all half-bridge submodules in the upper bridge arm is U2-U3 and the sum of the output voltages of all half-bridge submodules in the lower bridge arm is U3, the sum of the output voltages of all half-bridge submodules in the upper bridge arm and the lower bridge arm are adjusted to be u pj4 and u nj4 , so that the current flowing through the upper bridge arm is I2÷T c The speed changes from I2 to 0, and the current flowing through the lower bridge arm is (I2+I3)÷T c The speed changes from I2+I3 to 0; When the current flowing through the upper bridge arm and the lower bridge arm drops to 0, keep u pj4 and u nj4 , realize the shut-off of the converter valve, u pj4 =(U2-U3)+L1×I2÷T c , u nj4 =U3+L2×(I2+I3)÷T c 。

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