Photovoltaic medium-voltage direct-current collecting device with active balancing capability and control method of photovoltaic medium-voltage direct-current collecting device

Abstract

The invention belongs to the technical field of power electronics, and discloses a photovoltaic medium-voltage direct-current collecting device with active balance capability and a control method, the photovoltaic medium-voltage direct-current collecting device comprises a medium-voltage direct-current collecting bus and a coordination controller, and the medium-voltage direct-current collecting bus is electrically connected with a plurality of series capacitors which are connected in series. A three-active bridge converter is electrically connected between two adjacent series capacitors, and the three-active bridge converter is electrically connected with a photovoltaic panel. The three active bridge converters provide power flow paths between the medium-voltage side series capacitors while achieving photovoltaic power generation control, and the coordination controller can achieve photovoltaic side power generation control and medium-voltage side series capacitor voltage-sharing control. Compared with a traditional low-voltage power distribution framework, a power conversion link is reduced, the power density and the operation efficiency of the photovoltaic direct-current collecting device are improved, and compared with an existing medium-voltage direct-hanging type photovoltaic collecting device, voltage balance of series capacitors on the input side can be achieved under any illumination distribution; and the stability and the utilization rate of photovoltaic power generation are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and particularly relates to a photovoltaic medium-voltage DC collection device with active balancing ability and a control method therefor. Background Art

[0002] With the rapid development of photovoltaic power generation technology, large-scale photovoltaic power stations have become an important direction for the development of clean energy. In recent years, medium-voltage DC collection technology has been gradually applied to large-scale photovoltaic power stations. Compared with the traditional AC collection method, medium-voltage DC collection has advantages such as high efficiency and low cost.

[0003] However, in actual operation, due to the influence of factors such as weather conditions, cloud shading, and dust pollution, the photovoltaic array is often in an uneven illumination state, which poses a severe challenge to the efficient collection and stable operation of the photovoltaic power station. In the traditional series-type DC collection method, when some components are shaded, a "short-board effect" will occur, resulting in a significant reduction in the output power of the entire photovoltaic array. The existing series-type DC collection devices lack an effective power balance mechanism, and it is difficult to achieve voltage equalization control between each branch. At the same time, under dynamic illumination change conditions, it is difficult for the collection device to achieve maximum power point tracking, affecting the power generation efficiency of the system. Therefore, the present invention proposes a photovoltaic medium-voltage DC collection device with active balancing ability and a control method therefor. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a photovoltaic medium-voltage DC collection device with active balancing ability and a control method therefor, which solves the problem that the existing photovoltaic medium-voltage DC collection device in the prior art cannot solve the problem of unbalanced photovoltaic power generation.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] A photovoltaic medium-voltage DC collection device with active balancing ability includes a medium-voltage DC collection bus and a coordination controller. A plurality of series-connected series capacitors are electrically connected to the medium-voltage DC collection bus. A three-active-bridge converter is electrically connected between adjacent two series capacitors, and an isolated power path is generated between the series capacitors through the three-active-bridge converter to achieve active equalization of the series capacitors. A photovoltaic panel is electrically connected to the three-active-bridge converter, and the coordination controller is connected to the three-active-bridge converter through sensors and drivers to achieve signal transmission;

[0007] The three-active-bridge converter includes three full-bridge modules and a three-winding transformer. The AC sides of the three full-bridge modules are respectively connected to the three windings of the three-winding transformer, and the DC sides of the three full-bridge modules are respectively connected to the positive and negative poles of the sub-module capacitor.

[0008] Further, the three-active-bridge converter is set as a dual-transformer three-active-bridge converter, a three-active half-bridge converter, a dual-transformer three-active half-bridge converter, or an isolated three-port resonant converter.

[0009] Further, the sub-module capacitors of the three full-bridge modules in the three-active-bridge converter have their positive and negative electrodes led out to generate three DC ports, namely port ①, port ②, and port ③. Port ① and port ② are respectively connected to the positive and negative electrodes of two adjacent series capacitors to construct a power path between the series capacitors, and port ③ is connected to the photovoltaic panel.

[0010] Further, a number of adjacent three-active-bridge converters are connected in parallel through port ③ for current sharing output.

[0011] A control method for a photovoltaic medium-voltage DC collection device with active balancing ability uses the above-mentioned photovoltaic medium-voltage DC collection device with active balancing ability, as follows:

[0012] The three-active-bridge converter adopts single-phase-shift modulation to generate two control degrees of freedom, namely the phase-shift angle between port ① and port ② of the full-bridge sub-module, which is responsible for photovoltaic power generation control, and the phase-shift angle between port ② and port ③ of the full-bridge sub-module, which is responsible for series capacitor voltage equalization control;

[0013] The photovoltaic power generation control adopts maximum power point tracking control or constant power control. Among them, the maximum power point tracking control uses a method based on a mathematical model, a method based on perturbation optimization, or a method based on artificial intelligence to generate control variables. The constant power control restricts the photovoltaic output according to the superior dispatching instruction and controls the total output power of the photovoltaic to a certain reference value;

[0014] The series capacitor voltage equalization control adopts constant power control or adjacent module voltage equalization control. Among them, the constant power control statistically calculates the power generation power of all photovoltaic panels through a centralized controller, calculates the compensation power instruction that each three-active-bridge converter needs to transmit between modules to equalize the series capacitors, and then issues it to each three-active-bridge converter for execution. The adjacent module voltage equalization control is to use each three-active-bridge converter to control the voltage balance of the series capacitors connected to its port ② and port ③.

[0015] Further, in the photovoltaic power generation control, the method of maximum power point tracking control is adopted, as follows: The method based on perturbation optimization is adopted, specifically the perturbation observation method. For the i-th three-active-bridge, by changing the phase-shift angle between the full-bridge module of port ① and the full-bridge module of port ②, a perturbation is applied to the working voltage of the photovoltaic panel, and the change in the generated power is measured, so as to adjust the working point to approach the maximum power point.

[0016] Further, in photovoltaic power generation control, a constant power control method is adopted, which is specifically as follows: The PI control method is used. For the i-th three-active-bridge, the output power deviation is obtained by subtracting the photovoltaic power generation power command from the actual power generation power and sent to the input end of the PI regulator. The phase shift angle between the full-bridge module at the output port ① and the full-bridge module at the output port ② of the PI regulator

[0017] Further, in the series capacitor voltage equalization control, a method for calculating the inter-module compensation power command of the constant power control is adopted, which is specifically as follows:

[0018] For a three-active-bridge converter, the input power P Ti1 at port ① is equal to the sum of the output powers of port ② and port ③ (P Ti2 +P Ti3 ), and the expression is:

[0019]

[0020] In the formula, P Ti1 represents the input power of port ① of the i-th three-active-bridge. Taking the power flowing into the three-active-bridge from port ① as the positive direction, P Ti2 represents the output power of port ② of the i-th three-active-bridge. Taking the power flowing out of the three-active-bridge from port ② as the positive direction, P Ti3 represents the output power of port ③ of the i-th three-active-bridge. Taking the power flowing out of the three-active-bridge from port ③ as the positive direction;

[0021] The charging power P SCi of each series capacitor is 1 / N of the total power generation power, and the expression is:

[0022]

[0023] In the formula, P SCi is the output power of the i-th series capacitor, P PVi is the power generation power of the i-th photovoltaic panel, and P PVi is equal to the input power P Ti1 of port ① of the i-th three-active-bridge;

[0024] At the same time, P SCi is also determined by the connected port ② and port ③, and the expression is:

[0025]

[0026] Combining the above three formulas to obtain the inter-module compensation power command, that is, the power command of port ③ of the three-active-bridge converter, and the expression is:

[0027]

[0028] Wherein, P IMiref is the compensation power command of the i-th three-active-bridge module.

[0029] Further, in the series capacitor voltage balancing control, an adjacent-module voltage balancing control method is adopted, which is specifically as follows: for the i-th three-active-bridge converter, subtract the voltage of the series capacitor connected between its port ② and port ③, and send the deviation to the input end of the PI regulator. The PI regulator outputs the phase shift angle

[0030] Further, the three-active-bridge converter adopts one of the single-phase-shift modulation strategy, extended-phase-shift modulation strategy, dual-phase-shift modulation strategy or triple-phase-shift modulation strategy.

[0031] Advantages of the present invention:

[0032] 1. Compared with the traditional multi-stage photovoltaic DC collection system, the present invention can reduce the power conversion link, reduce costs and improve the power density and operation efficiency of the photovoltaic DC collection system at the same time;

[0033] 2. Compared with the existing single-stage medium-voltage directly-connected photovoltaic DC collection system, the present invention can achieve the balance of the series capacitor voltage on the medium-voltage side under any unbalanced light distribution, break the "short-board effect" of the series photovoltaic power generation structure, and improve the photovoltaic power generation efficiency. Description of the Drawings

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0035] Figure 1 is the structure diagram of a photovoltaic medium-voltage DC collection device with active balancing ability;

[0036] Figure 2 is the control strategy block diagram of a photovoltaic medium-voltage DC collection with active balancing ability. Detailed Embodiments

[0037] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a 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 those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0038] In the description of the present invention, it should be understood that terms such as "opening", "upper", "lower", "thickness", "top", "middle", "length", "inner", "periphery", etc. indicating orientation or positional relationships are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the components or elements referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention.

[0039] Embodiment 1:

[0040] Please refer to Figure 1 - Figure 2 , the present invention provides a technical solution: a photovoltaic medium-voltage DC collection device with active balancing ability, including a group of medium-voltage DC collection buses, N series capacitors, (N - 1) triple active bridge converters (TABs), (N - 1) photovoltaic panels, and a coordination controller;

[0041] N series-connected series capacitors are electrically connected to the medium-voltage DC collection bus, and a triple active bridge converter is electrically connected between adjacent two series capacitors, generating an isolated power path between the series capacitors to achieve active equalization of the series capacitors. Each triple active bridge converter is electrically connected to a photovoltaic panel, and the coordination controller is connected to the triple active bridge converter through sensors and drivers to achieve signal transmission;

[0042] The triple active bridge converter includes three full-bridge modules and a three-winding transformer. The AC sides of the three full-bridge modules are respectively connected to the three windings of the three-winding transformer, generating coupling through the magnetic circuit to transmit power, and the DC sides of the three full-bridge modules are respectively connected to the positive and negative poles of the sub-module capacitors.

[0043] While the triple active bridge converter realizes photovoltaic power generation control, it provides a power flow path between the series capacitors on the medium-voltage side, and the coordination controller can realize photovoltaic-side power generation control and medium-voltage-side series capacitor voltage equalization control.

[0044] Regarding the technical solution of this embodiment, the positive and negative poles of the sub-module capacitors of the three full-bridge modules in the triple active bridge converter are led out to generate three DC ports, namely port ①, port ②, and port ③. Port ① and port ② are respectively connected to the positive and negative poles of adjacent two series capacitors, constructing a power path between the series capacitors and providing the possibility for equalizing the voltage of the series capacitors. Port ③ is connected to the photovoltaic panel.

[0045] Specifically, the port ① of the TAB i is connected to the photovoltaic panel i , which can realize the power generation control of the photovoltaic panel. Port ② is connected to the series capacitor i , and port ③ is connected to the series capacitor i+1, a power path is constructed between the series capacitors, making it possible to equalize the voltage of the series capacitors.

[0046] It should be noted that the three-active-bridge converter can also be replaced by fully isolated three-port DC / DC converters such as the dual-transformer three-active-bridge converter, the three-active half-bridge converter, the dual-transformer three-active half-bridge converter, and the isolated three-port resonant converter. This embodiment does not make specific limitations on this and can be selected according to actual situations.

[0047] Furthermore, several adjacent three-active-bridge converters can be connected in parallel through interface ③ for current sharing output, expanding the ability to output power. Since multiple ports of the TAB are cross-connected in series capacitors on the medium-voltage side, it can also be called the cross-connected three-active-bridge (C2TAB) structure.

[0048] Due to the limitation of the requirement for equalizing the voltage of the series capacitors on the medium-voltage side, the traditional series-type photovoltaic DC collection device cannot cope with the uneven illumination situation. However, the photovoltaic medium-voltage DC collection device with active balancing ability in this embodiment can achieve active balancing of the series capacitors because an isolated power path is generated between the series capacitors through the TAB, and it has better power supply flexibility and power generation efficiency.

[0049] Figure 2 It is the control strategy block diagram of the photovoltaic medium-voltage DC collection device with active balancing ability. In this embodiment, single-phase-shift modulation is preferred for the TAB, and a total of two control degrees of freedom are generated, denoted as the phase-shift angle i between the full-bridge sub-module ① and the full-bridge sub-module ② and the phase-shift angle i between the full-bridge sub-module ② and the full-bridge sub-module ③ where is used for photovoltaic power generation control, is used for equalizing the voltage of the series capacitors.

[0050] In this embodiment, the photovoltaic power generation control includes two working modes: maximum power point tracking (MPPT) control and constant power control. As shown in Figure 2 (a) and (b), among which the MPPT control can generate control variables by methods based on mathematical models, methods based on perturbation optimization, methods based on artificial intelligence, etc. The constant power control needs to limit the photovoltaic output according to the superior dispatching instruction and control the total output power of the photovoltaic to a certain reference value, usually using PI control.

[0051] The equalizing control of the series capacitors includes two modes: constant power control and adjacent-module equalizing control. As shown in Figure 2As shown in (c) and (d), the constant power control means that the centralized controller calculates the total power generation of all photovoltaic panels, and then calculates the compensation power command that each TAB needs to transmit between modules to equalize the series capacitors, and then issues it to each TAB for execution. The adjacent module equalizing control mode of the series capacitor voltage equalizing control means that each TAB controls the voltages of the series capacitors connected to its port ② and port ③ to be the same, and PI control is adopted.

[0052] In the series capacitor voltage equalizing control, the calculation method of the inter-module compensation power command for constant power control is as follows. For TAB i the input power P Ti1 at port ① is equal to the sum of the output powers of port ② and port ③ (P Ti2 +P Ti3 ), and the expression is:

[0053]

[0054] The charging power P SCi of each series capacitor is 1 / N of the total power generation, and the expression is:

[0055]

[0056] At the same time, P SCi is also determined by the connected TAB i-1 port ② and TAB i port ③, and the expression is:

[0057]

[0058] Combining the above three equations to obtain the inter-module compensation power command, that is, the power command of TAB port ③, the expression is:

[0059]

[0060] In summary, compared with the traditional multi-stage photovoltaic DC collection system, the present invention can reduce the power conversion link, reduce costs and improve the power density and operation efficiency of the photovoltaic DC collection system; compared with the existing single-stage medium-voltage direct-connected photovoltaic DC collection system, the present invention can achieve the balance of the series capacitor voltages on the medium-voltage side under any unbalanced light distribution, breaking the "short board effect" of the series photovoltaic power generation structure and improving the photovoltaic power generation efficiency.

[0061] Embodiment 2:

[0062] This embodiment provides a control method for a photovoltaic medium-voltage DC collection device with active balancing ability, using the photovoltaic medium-voltage DC collection device with active balancing ability described in Embodiment 1, specifically as follows:

[0063] The three-active-bridge converter adopts single-phase-shift modulation, generating two degrees of control freedom, namely the phase-shift angle between port ① and port ② of the full-bridge sub-module, which is responsible for photovoltaic power generation control, and the phase-shift angle between port ② and port ③ of the full-bridge sub-module, which is responsible for series-capacitor voltage equalization control;

[0064] The photovoltaic power generation control adopts maximum power point tracking control or constant power control. Among them, the maximum power point tracking control uses methods based on mathematical models, perturbation optimization, or artificial intelligence to generate control variables, and the constant power control restricts the photovoltaic output according to the superior dispatching instruction and controls the total output power of the photovoltaic to a certain reference value;

[0065] The series-capacitor voltage equalization control adopts constant power control or adjacent-module voltage equalization control. Among them, the constant power control statistically calculates the power generation power of all photovoltaic panels through a centralized controller, calculates the compensation power instruction that each three-active-bridge converter needs to transmit between modules to equalize the series capacitors, and then issues it to each three-active-bridge converter for execution. The adjacent-module voltage equalization control is to use each three-active-bridge converter to control the voltage balance of the series capacitors connected to its port ② and port ③.

[0066] Further, in the photovoltaic power generation control, the method of maximum power point tracking control is adopted, specifically as follows: The method based on perturbation optimization is adopted, specifically the perturbation observation method. For the i-th three-active-bridge, by changing the phase-shift angle between the full-bridge module at port ① and the full-bridge module at port ②, a perturbation is applied to the working voltage of the photovoltaic panel, and the change in the generated power is measured, so as to adjust the working point to approach the maximum power point. In the photovoltaic power generation control, the maximum power point tracking control method can also be replaced by methods based on mathematical models and artificial intelligence.

[0067] Further, in the photovoltaic power generation control, the method of constant power control is adopted, specifically as follows: The PI control method is adopted. For the i-th three-active-bridge, the output power deviation is obtained by subtracting the photovoltaic power generation power instruction from the actual generated power and sent to the input end of the PI regulator. The PI regulator outputs the phase-shift angle

[0068] Further, in the series-capacitor voltage equalization control, the calculation method of the module-to-module compensation power instruction for constant power control is as follows: For the three-active-bridge converter, the input power P Ti1 at port ① is consistent with the sum of the output powers at port ② and port ③ (P Ti2 +P Ti3 ), and the expression is:

[0069]

[0070] In the formula, PTi1 Denote the input power of port ① of the i-th three-active-bridge as \(P_{in,i}\), taking the power flowing into the three-active-bridge from port ① as the positive direction, \(P\) Ti2 Denote the output power of port ② of the i-th three-active-bridge as \(P_{out2,i}\), taking the power flowing out of the three-active-bridge from port ② as the positive direction, \(P\) Ti3 Denote the output power of port ③ of the i-th three-active-bridge as \(P_{out3,i}\), taking the power flowing out of the three-active-bridge from port ③ as the positive direction.

[0071] The charging power \(P\) of each series capacitor SCi is \(1 / N\) of the total power generation, and the expression is:

[0072]

[0073] In the formula, \(P\) SCi is the output power of the i-th series capacitor, \(P\) PVi is the power generation of the i-th photovoltaic panel, \(P\) PVi is equal to the input power \(P_{in,i}\) of port ① of the i-th three-active-bridge Ti1 .

[0074] Meanwhile, \(P\) SCi is also determined by the connected port ② and port ③, and the expression is:

[0075]

[0076] Combining the above three formulas to obtain the inter-module compensation power command, that is, the power command of port ③ of the three-active-bridge converter, and the expression is:

[0077]

[0078] In the formula, \(P\) IMiref is the compensation power command of the i-th three-active-bridge module.

[0079] Furthermore, in the series capacitor voltage equalization control, the method of adjacent module voltage equalization control is adopted, specifically as follows: for the i-th three-active-bridge converter, subtract the voltage of the series capacitor connected between its port ② and port ③, and send the deviation to the input end of the PI regulator. The PI regulator outputs the phase shift angle

[0080] In some embodiments, the three-active-bridge converter adopts one of the single-phase-shift modulation strategy, extended phase-shift modulation strategy, double phase-shift modulation strategy or triple phase-shift modulation strategy.

[0081] In the description of this specification, the descriptions referring to terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0082] The above has shown and described the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments, and the above embodiments and the descriptions in the specification only illustrate the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed.

Claims

1. A photovoltaic medium-voltage DC collection device with active balancing ability, comprising a medium-voltage DC collection bus and a coordination controller, characterized in that, A number of series-connected capacitors are electrically connected to the medium-voltage DC collection bus. A three-active-bridge converter is electrically connected between two adjacent series-connected capacitors, creating an isolated power path between the series-connected capacitors to achieve active equalization of the series-connected capacitors. The three-active-bridge converter is electrically connected to a photovoltaic panel, and the coordination controller is connected to the three-active-bridge converter through sensors and drivers to achieve signal transmission; The three-active-bridge converter includes three full-bridge modules and a three-winding transformer. The AC sides of the three full-bridge modules are respectively connected to the three windings of the three-winding transformer, and the DC sides of the three full-bridge modules are respectively connected to the positive and negative poles of the sub-module capacitor.

2. The photovoltaic medium-voltage DC collection device with active balancing ability according to claim 1, wherein, The three-active-bridge converter is set as a dual-transformer three-active-bridge converter, a three-active half-bridge converter, a dual-transformer three-active half-bridge converter, or an isolated three-port resonant converter.

3. The photovoltaic medium-voltage DC collection device with active balancing ability according to claim 1, characterized in that, The positive and negative poles of the sub-module capacitors of the three full-bridge modules in the three-active-bridge converter are led out to generate three DC ports, namely port ①, port ②, and port ③. Port ① and port ② are respectively connected to the positive and negative poles of two adjacent series-connected capacitors to construct a power path between the series-connected capacitors, and port ③ is connected to the photovoltaic panel.

4. The photovoltaic medium-voltage DC collection device with active balancing ability according to claim 3, characterized in that, A number of adjacent three-active-bridge converters are connected in parallel through port ③ for current sharing output.

5. Control method of a photovoltaic medium-voltage DC collection device with active balancing ability, using the photovoltaic medium-voltage DC collection device with active balancing ability according to any one of claims 1 to 4, characterized in that, Specifically as follows: The three-active-bridge converter adopts single-phase-shift modulation, generating two control degrees of freedom, namely the phase-shift angle between port ① and port ② of the full-bridge sub-module, which is responsible for photovoltaic power generation control, and the phase-shift angle between port ② and port ③ of the full-bridge sub-module, which is responsible for equalizing the voltage of the series-connected capacitors; Photovoltaic power generation control adopts maximum power point tracking control or constant power control. Among them, the maximum power point tracking control uses a method based on perturbation optimization to generate control variables, and the constant power control restricts the photovoltaic output according to the superior dispatching instruction, controlling the total output power of the photovoltaic to a certain reference value; The equalizing control of the series-connected capacitors adopts constant power control or adjacent module equalizing control. Among them, the constant power control statistically calculates the power generation power of all photovoltaic panels through the centralized controller, calculates the compensation power instruction that each three-active-bridge converter needs to transmit between modules to equalize the voltage of the series-connected capacitors, and then issues it to each three-active-bridge converter for execution. The adjacent module equalizing control is to use each three-active-bridge converter to control the voltage balance of the series-connected capacitors connected to its port ② and port ③.

6. The control method of the photovoltaic medium-voltage DC collection device with active balancing ability according to claim 5, characterized in that, In photovoltaic power generation control, the maximum power point tracking control method is adopted, which is specifically as follows: The method based on perturbation optimization is adopted, specifically the perturbation observation method. For the i-th three-active-bridge, by changing the phase shift angle between the full-bridge module at port ① and the full-bridge module at port ② Perturb the working voltage of the photovoltaic panel, measure the change in power generation, and thus adjust the working point to approach the maximum power point.

7. The control method of the photovoltaic medium-voltage DC collection device with active balancing ability according to claim 5, characterized in that, In photovoltaic power generation control, a constant power control method is adopted, which is specifically as follows: The PI control method is used. For the i-th three-active-bridge, the output power deviation is obtained by subtracting the photovoltaic power generation power command from the actual power generation power, and is sent to the input end of the PI regulator. The phase shift angle between the full-bridge module at port ① and the full-bridge module at port ② of the output port of the PI regulator 8. The control method of the photovoltaic medium-voltage DC collection device with active balancing ability according to claim 5, characterized in that, In the equalizing control of the series-connected capacitors, the calculation method of the compensation power instruction between modules using constant power control is specifically as follows: For a three-active-bridge converter, the input power P at port ① Ti1 is equal to the sum of the output powers at ports ② and ③ (P Ti2 + P Ti3 ), and the expression is as follows: wherein, P Ti1 represents the input power of port ① of the i-th three-active-bridge, with the power flowing into the three-active-bridge from port ① being the positive direction, and P Ti2 represents the output power of port ② of the i-th three-active-bridge, with the power flowing out of the three-active-bridge from port ② being the positive direction, and P Ti3 represents the output power of port ③ of the i-th three-active-bridge, with the power flowing out of the three-active-bridge from port ③ being the positive direction; The charging power P of each series capacitor SCi is 1 / N of the total power generation, and the expression is: Wherein, P SCi is the output power of the i-th series capacitor, and P PVi is the power generation power of the i-th photovoltaic panel. P PVi is equal to the input power P Ti1 at port ① of the i-th three-active-bridge; Meanwhile, P SCi is also determined by the connected port ② and port ③, and the expression is: Combining the above three formulas to obtain the compensation power instruction between modules, that is, the power instruction of port ③ of the three-active-bridge converter, the expression is: Wherein, P IMiref is the compensation power command of the i-th three-active-bridge module.

9. The control method of the photovoltaic medium-voltage DC collection device with active balancing ability according to claim 5, characterized in that, In the series capacitor voltage sharing control, the method of adjacent module voltage sharing control is adopted, which is specifically as follows: for the i-th three-active-bridge converter, subtract the voltage of the series capacitor connected between its port ② and port ③, and send the deviation to the input end of the PI regulator. The PI regulator outputs the phase shift angle between the full-bridge module at port ② and the full-bridge module at port ③ 10. The control method of the photovoltaic medium-voltage DC collection device with active balancing ability according to claim 5, characterized in that, The three-active-bridge converter adopts one of the single-phase-shift modulation strategy, extended phase-shift modulation strategy, double phase-shift modulation strategy, or triple phase-shift modulation strategy.

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