CN116154861A

CN116154861A - Photovoltaic system based on direct connection and power conversion dual-mode MLPE component

Info

Publication number: CN116154861A
Application number: CN202310431556.5A
Authority: CN
Inventors: 陈维; 宋悦
Original assignee: Jiangsu Xumax Power Technology Co ltd; Shenzhen Zhongxu New Energy Co ltd
Current assignee: Jiangsu Xumax Power Technology Co ltd; Shenzhen Zhongxu New Energy Co ltd
Priority date: 2023-04-21
Filing date: 2023-04-21
Publication date: 2023-05-23
Anticipated expiration: 2043-04-21
Also published as: CN116154861B

Abstract

The invention discloses a photovoltaic system based on a direct-through and power conversion dual-mode MLPE component, and belongs to the technical field of photovoltaic grid-connected power generation systems. The MLPE component is provided with the power optimization module capable of running in a through mode or a power conversion mode, and is automatically controlled to switch between the two modes according to the characteristics of local parameter information, meanwhile, the post-stage conversion equipment can promote and keep a certain number of MLPE components in the through mode in a mode of controlling the voltage of the input side, so that the high-frequency switch of the power optimization module and the loss of an energy storage inductor are reduced by utilizing the MPPT function of the post-stage conversion equipment and the local mode control logic of the power optimization module, the energy coupling of a photovoltaic system is simplified, the power generation efficiency of the photovoltaic system is improved, and the advantages of reducing LCOE and not needing high-stability communication are achieved.

Description

Photovoltaic system based on direct connection and power conversion dual-mode MLPE component

Technical Field

The disclosure relates to the technical field of photovoltaic grid-connected power generation systems, in particular to a photovoltaic system based on a direct-through and power conversion dual-mode MLPE component.

Background

The Photovoltaic grid-connected power generation system mainly comprises a plurality of Photovoltaic modules (PV) and a grid-connected Photovoltaic inverter. The photovoltaic module is connected in series to form a photovoltaic group string, and then the photovoltaic group string is connected to the direct current input side of the inverter to form a photovoltaic array. In a system with conventional components without a photovoltaic Power optimizer, the inverter itself or the DC-DC conversion front stage of the inverter can control the Maximum Power Point Tracking (MPPT) of the string of photovoltaic groups. However, under a variety of factors, individual components in a string of photovoltaic strings are often difficult to configure under consistent power conditions, and therefore some of the components will lose power.

The MLPE is short for Module-Level Power Electronics, and in a photovoltaic system, an MLPE Module refers to Power Electronics equipment capable of performing fine control on a single or a plurality of photovoltaic modules, and mainly achieves functions of inversion, monitoring, power optimization, turn-off and the like. In order to improve the efficiency of acquiring photovoltaic power, in the existing photovoltaic system, a photovoltaic group string is formed by connecting a plurality of MLPE components in series. Generally, the MLPE component comprises a photovoltaic panel and a power optimizer, wherein the power optimizer configures output power of the photovoltaic panel at a maximum power point by means of a power conversion function of the power optimizer, so that power generation efficiency of the whole photovoltaic system is effectively improved. Meanwhile, the inverter will lose the function of the MPPT function on the dc input side due to the function of the power optimizer. In other words, the inverter controls the dc input side voltage over a range with MPPT as the destination, the total power of the string or array of photovoltaic cells will remain unchanged because it is already at the maximum power point. Meanwhile, the MPPT function of the inverter may cause a control disturbance of the power optimizer. For example, the inverter controls the dc input side voltage to misdirect the power optimizer into power limit. Therefore, in the current market, some inverters have removed the MPPT function in the photovoltaic system with the MLPE component.

Meanwhile, although the MLPE component can obtain photovoltaic efficiency improvement, the energy storage inductance in the power optimizer and the switching device increase the power loss of the system. Moreover, the switching devices will have a limited lifetime when operated under high frequency switching control for a long period of time, which will also increase the cost of system operation and maintenance. In addition, in order to meet the high-frequency use requirement, the internal resistance of the finally selected device is increased, and the system loss is further increased.

In summary, there is a need to improve the control strategy of power optimizers and inverters in photovoltaic systems incorporating MLPE components to solve the above-described problems of the prior art.

Disclosure of Invention

In view of the problems existing in the prior art, the main objective of the present invention is to provide a photovoltaic system based on a through and power conversion dual-mode MLPE component, which realizes that a power optimization module is automatically and intelligently switched between a through mode and a power conversion mode without directly interfering with the operation of the power optimization module by means of high-stability communication on the premise of reducing the leveling electric cost (LCOE), and maintains the optimizers of the through mode in a certain number, so that the loss of the optimizers is reduced, the service life of the optimizers is prolonged, and the MPPT function of a post-stage conversion device is utilized.

Based on the same inventive concept, the invention also aims to provide a control method for the photovoltaic system and a photovoltaic inverter for the photovoltaic system, which can realize the technical aim of the photovoltaic system based on the direct-through and power conversion dual-mode MLPE component.

In a first aspect, in order to achieve the above object, the present invention adopts the following technical scheme:

a photovoltaic system, comprising: the post-stage conversion equipment is connected with at least one photovoltaic group string on the input side of the post-stage conversion equipment; the photovoltaic group string comprises a plurality of MLPE components, the MLPE components comprise photovoltaic units and power optimization modules, the output ends of the photovoltaic units are coupled with the input ends of the power optimization modules, each MLPE component is connected in series with each other through the output ends of the power optimization modules to form the photovoltaic group string, wherein,

the power optimization module is provided with a local control module; the local control module is used for automatically controlling the power optimization module to operate in a direct-through mode or a power conversion mode according to the local parameter state, the power optimization module in the direct-through mode is used for directly conducting the power of the photovoltaic unit to the photovoltaic string, and the power optimization module in the power conversion mode is used for providing the power of the photovoltaic unit, which is subjected to power conversion and/or maximum power point tracking, to the photovoltaic string;

The rear-stage conversion equipment is provided with a central control module; the central control module is used for acquiring the number of the power optimization modules in the through mode in a communication mode, and controlling the input side voltage of the post-stage conversion equipment by taking the number of the power optimization modules in the through mode not lower than a preset number threshold as a destination, so that the state of the local parameter of the power optimization module is indirectly changed to switch from the power conversion mode to the through mode;

the central control module is also used for controlling the input side voltage of the post-stage conversion equipment by taking the maximum power point of the tracking photovoltaic group string as a destination, so that the power optimization module in the through mode is configured at the maximum power point.

Optionally, the local control module automatically controls the power optimization module to operate in a pass-through mode or a power conversion mode according to the local parameter state, and the method includes: the local control module determines that the power optimization module is in a power conversion mode, and the local control module controls whether the power optimization module is switched to a pass-through mode according to the state of the local duty ratio: if the operation duty ratio of the power optimization module exceeds a preset duty ratio threshold, switching the power optimization module from a power conversion mode to a through mode, and if the operation duty ratio of the power optimization module does not exceed the preset duty ratio threshold, maintaining the power optimization module to operate in the power conversion mode;

The central control module controls the input side voltage of the post-stage conversion equipment to change so as to cause the string voltage of the photovoltaic string and the output voltage of the power optimization module of each MLPE component to change accordingly, thereby indirectly causing the local control module to control the power optimization module to switch from the power conversion mode to the through mode due to the fact that the operation duty ratio is changed and exceeds a preset duty ratio threshold value.

The photovoltaic system is optional, the central control module is used for obtaining the output voltage of each power optimization module in the through mode on the same photovoltaic group string, calculating the average value of the output voltage, and sending a mode changing instruction to the power optimization module of which the difference between the output voltage and the average value exceeds a preset difference value;

the local control module automatically controls the power optimization module to operate in a pass-through mode or a power conversion mode according to the local parameter state, and the method comprises the following steps: the local control module determines that the power optimization module is in a through mode, and the local control module controls whether the power optimization module is switched to a power conversion mode according to the local change mode instruction state: and if the change mode instruction is acquired, switching the power optimization module from the pass-through mode to the power conversion mode, and if the change mode instruction is not acquired, maintaining the power optimization module in the pass-through mode.

Optionally, the local control module automatically controls the power optimization module to switch between the power conversion mode and the pass-through mode according to the local parameter state, and the switching comprises: the local control module determines that the power optimization module is in a pass-through mode, and the local control module controls whether the power optimization module is switched to a power conversion mode according to a local pass-through mode timing state:

and if the time of the power optimization module in the pass-through mode does not exceed the time threshold, maintaining the power optimization module to operate in the pass-through mode.

The photovoltaic system is optional, and the power optimization module comprises a direct current conversion circuit and a direct current circuit;

the local control module is controlled and connected to the direct current conversion circuit and is used for controlling the direct current conversion circuit to provide power tracked by the photovoltaic unit through power conversion and/or a maximum power point to the photovoltaic group string in a power conversion mode;

the local control module is in control connection with the through circuit and is used for controlling the through circuit to directly conduct the power of the photovoltaic unit to the photovoltaic string when in the through mode.

The photovoltaic system may be selected, the dc conversion circuit includes an input port coupled to the photovoltaic unit, an output port coupled to the photovoltaic string, a first switch and a second switch, a first end of the first switch is connected to an anode of the input port, a second end of the first switch is connected to a first end of the second switch, a second end of the second switch is connected to a cathode of the input port and a cathode of the output port, and an intermediate port between the first switch and the second switch is used for coupling to the anode of the output port;

the through circuit comprises a third switch, a first end of the third switch is connected with the positive electrode of the input port, and a second end of the third switch is connected with the positive electrode of the output port;

the local control module is respectively connected with the first switch, the second switch and the third switch in a control manner, and is used for controlling the high-frequency on-off of the first switch through a duty ratio signal capable of tracking the maximum power point in a power conversion mode, controlling the high-frequency on-off of the second switch through a complementary signal of the duty ratio signal and controlling the disconnection of the third switch through a switching signal; and the first switch is controlled to be kept on or off in the through mode, the second switch is controlled to be kept off, and the third switch is controlled to be on by an on signal.

The photovoltaic system may be selected, wherein the preset number threshold is 2% to 10% of the total number of MLPE components in a photovoltaic string.

The photovoltaic system is optional, the power optimization module comprises a direct current conversion circuit with a BUCK circuit topological structure, the rear-stage conversion equipment is an inverter for photovoltaic with a DC/DC front stage, the output end of the photovoltaic string is connected to the input side of the DC/DC front stage, and the inverter is provided with the central control module.

In a second aspect, in order to achieve the above object, the present invention adopts the following technical scheme: the input side of the photovoltaic inverter is connected with at least one photovoltaic group string, the photovoltaic group string comprises a plurality of MLPE components, the MLPE components comprise photovoltaic units and power optimization modules, the output ends of the photovoltaic units are coupled with the input ends of the power optimization modules, each MLPE component is connected in series with each other through the output ends of the respective power optimization modules to form the photovoltaic group string, the local control module is used for automatically controlling the power optimization modules to operate in a through mode or a power conversion mode according to the local parameter state,

the inverter is provided with a central control module, the central control module is used for acquiring the number of power optimization modules in a through mode in a communication mode, and controlling the input side voltage of the post-stage conversion equipment by taking the number of the power optimization modules in the through mode not lower than a preset number threshold as a destination, so that the state of local parameters of the power optimization modules is indirectly changed to switch from a power conversion mode to the through mode;

In a third aspect, in order to achieve the above object, the present invention adopts the following technical scheme: a control method for the photovoltaic system,

the MLPE component side is provided with the following control steps:

the power optimization module can be controlled to operate in a direct mode of directly conducting the power of the photovoltaic unit to the photovoltaic string, or in a power conversion mode of providing the power of the photovoltaic unit, which is subjected to power conversion and/or maximum power point tracking, to the photovoltaic string;

when in the power conversion mode, judging whether to control the power optimization module to switch from the power conversion mode to the through mode according to comparison of the local duty ratio and the duty ratio threshold;

when in the through mode, judging whether to control switching from the through mode to the power conversion mode according to whether the time in the through mode exceeds a time threshold value, and/or judging whether to control switching from the through mode to the power conversion mode according to whether a mode changing instruction is received;

transmitting the operation power conversion mode and the output voltage of the power optimization module to the rear-stage conversion equipment in a communication mode;

The following control steps are arranged on the side of the post-stage conversion equipment:

the method comprises the steps of obtaining operation power conversion modes of all power optimization modules in a communication mode, determining the number of the power optimization modules in a straight-through mode, and controlling input side voltage change of the post-stage conversion equipment by a first preset amplitude value until the number of the MLPE components is not lower than a preset number threshold; meanwhile, controlling the voltage change of the input side of the post-stage conversion equipment by a second preset amplitude value so as to track the maximum power point of the photovoltaic string;

if the MLPE component side judges whether to control switching from the through mode to the power conversion mode according to whether a change mode instruction is received in the through mode, the output voltage of each power optimization module in the through mode is acquired in a communication mode at the later stage conversion equipment side, the average value of the output voltages is calculated, and the change mode instruction is sent to the power optimization module of which the difference between the output voltage and the average value exceeds a preset difference value.

Compared with the prior art, the invention has the following beneficial effects:

in the photovoltaic system, the MLPE component is provided with the power optimization module capable of running in a through mode or a power conversion mode, and is automatically controlled to switch between the two modes according to the local parameter state, meanwhile, the central control module can indirectly prompt a certain number of power optimization modules to be in the through mode in a mode of controlling the voltage of the input side of the rear-stage conversion equipment, so that a part of power optimization modules with higher power in the photovoltaic string continuously run in the through mode, the rear-stage conversion equipment realizes power optimization on the maximum power point tracking function of the input side, and the power optimization modules with the rest part in the power conversion mode realize power optimization by utilizing the local maximum power point tracking, thereby realizing the reduction of the high-frequency switch and the loss of the energy storage inductor of the power optimization module by utilizing the MPPT function of the rear-stage conversion equipment, further simplifying the energy coupling control of the photovoltaic system, improving the power generation efficiency of the photovoltaic system, and further having the advantages of reducing the leveling electric cost (LCOE) and not needing to be by means of high-stability communication.

The invention is further described below with reference to the accompanying drawings.

Drawings

FIG. 1 is a structural and logical diagram of a photovoltaic system based on a pass-through and power conversion dual mode MLPE assembly in accordance with an embodiment of the application;

FIG. 2 is a schematic diagram of control logic on the MLPE component side according to a first embodiment of the application;

fig. 3 is a schematic diagram of control logic of an inverter side according to a first embodiment of the present application;

FIG. 4 is a schematic diagram of control logic on the MLPE component side according to a second embodiment of the application;

fig. 5 is a schematic diagram of control logic of an inverter side according to a second embodiment of the present application;

FIG. 6 is a schematic circuit diagram of an MLPE assembly according to a third embodiment of the application;

fig. 7 is a schematic view of a mounting structure of a photovoltaic string according to an application embodiment of the present application.

Reference numerals: 10. a photovoltaic unit; 11. a photovoltaic panel; 20. a power optimization module; 21. a DC conversion circuit; 22. a pass-through circuit; 30. a local control module; 31. a communication module; 40. a string of photovoltaic strings; 50. an inverter; 60. a central control module; q, number; q_ref, number threshold; D. a duty cycle; d_ref, duty cycle threshold; t, time; t_ref, time threshold; u_out, output voltage; u_in, input voltage; u_pv, input side voltage.

Detailed Description

In order to solve the problems of power loss and switch service life of an MLPE component and the problem of underutilization of inverter functions in the prior art, the embodiment of the invention provides a photovoltaic system based on a through and power conversion dual-mode MLPE component.

For the purpose of the invention, the photovoltaic system comprises an MLPE module which can be controlled to operate in a pass-through mode or a power conversion mode, a photovoltaic inverter which can cause the MLPE module to switch the power conversion mode and maintain a certain number Q of the MLPE modules in the pass-through mode in the photovoltaic string 40, and a control method for controlling the photovoltaic system. For a better illustration of the objects, technical solutions and advantages of the present invention, the following detailed description of the present invention will be given with reference to the accompanying drawings and examples.

As shown in fig. 1, a photovoltaic system according to an embodiment of the present invention includes one or more strings of photovoltaic strings 40, and a post-conversion device. Wherein the photovoltaic string 40 is formed by the output of n MLPE components in series with each other (where n.gtoreq.2). Specifically, the MLPE assembly is made up of a photovoltaic unit 10 and a power optimization module 20. In each MLPE module, the output end of the photovoltaic unit 10 is connected to the input end of the power optimization module 20, and the output end of the power optimization module 20 is used as the output end of the MLPE module and is connected in series with the output ends of other MLPE modules to form a photovoltaic string 40. The output of the string of photovoltaic modules 40 is connected to the input side of the post-conversion device. In the present embodiment, the post-stage conversion device may be a DC/DC pre-stage of the inverter 50. Wherein the inverter 50 is provided with DC/DC front and rear inverter circuits. The photovoltaic string 40 is connected to an input side of a DC/DC front stage, and the DC/DC front stage power-converts output power of the photovoltaic string 40 and supplies the power to a rear inverter circuit, which further inverts the power to ac power and outputs the ac power. In other alternative embodiments, the post-stage conversion device may also be an inverter circuit in a photovoltaic inverter. In various embodiments, the photovoltaic unit 10 may be the entire photovoltaic panel 11, which is packaged separately, or may be a partial string of cells in the photovoltaic panel 11.

In terms of MLPE components, for the technical purpose of the present application, the power optimization module 20 is provided with a local control module 30, and the power optimization module 20 may operate in a pass-through mode or in a power conversion mode under the control of the local control module 30. The power optimization module 20 operates in a power conversion mode, and can perform power conversion and maximum power point tracking on the power input to the photovoltaic unit 10 and then provide the power to the photovoltaic string 40. The power optimization module 20 operates in a pass-through mode and can direct power from its input photovoltaic unit 10 to the photovoltaic string 40 without power conversion. The local control module 30 automatically controls the power optimization module 20 to operate in either a pass-through mode or a power conversion mode based on the local parameter status. The local parameter state includes the measured electric parameter of the power optimization module 20 and the reference value or threshold value for evaluating the state thereof, or the electric parameter of the independent metering element arranged in the local control module 30 and the reference value or threshold value for evaluating the state thereof, or the electric parameter of the command signal acquired through the communication mode and the state of whether the command signal is acquired. It should be noted that, the maximum power point tracking needs to be implemented through power conversion, and the power conversion may also implement other functions of the MLPE component, such as power limiting.

In terms of the post-stage conversion apparatus, for the technical purpose of the present application, the inverter 50 is provided with a central control module 60. Wherein, the central control module 60 establishes a wired or wireless communication connection with the local control module 30 of each MLPE component in the photovoltaic string 40, and the central control module 60 can obtain the electrical parameter information in the local control module 30. Specifically, in one aspect, the electrical parameter information may be the current operating mode of the individual MLPE components in each photovoltaic string 40, such that the central control module 60 can determine the number Q of MLPE components in the pass-through mode. If the number Q is lower than the preset number threshold q_ref, the central control module 60 controls the DC/DC front stage to adjust the input side voltage u_pv thereof to rise. If the number Q is not lower than the preset number threshold Q_ref, stopping regulating the DC/DC front stage input side voltage U_pv with the number Q of the MLPE components controlling the pass-through mode as a destination.

It should be noted that, the central control module 60 controls the DC/DC front stage input side voltage u_pv to change, so that the string voltage of the photovoltaic string 40 changes accordingly, and the output voltage u_out of each MLPE component connected in series changes accordingly. Based on the adjustment of the DC/DC front-stage input side voltage u_pv by the central control module 60, the local control module 30 controls the power conversion module to enter the pass-through mode from the power conversion mode by satisfying the preset state condition, so that the central control module 60 indirectly switches the MLPE components, and finally, the number Q of the MLPE components in the pass-through mode in the photovoltaic system is maintained at a state not lower than the preset number threshold q_ref.

On the basis that the number Q of MLPE components is not lower than the preset number threshold q_ref, the central control module 60 will continue to regulate the input side voltage u_pv of the control DC/DC front stage until the maximum power point of the connected photovoltaic string 40 is tracked. In other words, the through-mode MLPE components are configured at the maximum power point by the inverter 50, while the power conversion mode MLPE components in the photovoltaic string 40 are configured at the maximum power point under the control of the local control module 30. It will thus be appreciated that, by the central control module 60, the accessed photovoltaic string 40 remains with the MLPE component in pass-through mode that is not lower than the quantity threshold q_ref; at the same time, these MLPE components in pass-through mode will operate at a maximum power point as the photovoltaic string 40 is configured at the maximum power point.

Therefore, the photovoltaic system and the control method thereof can continuously keep a certain number of MLPE components to operate in the through mode in the operation process. The MLPE component in the pass-through mode does not perform power conversion any more, and the switching element in the power optimization module 20 can suspend high-frequency on-off, so that the influence of high-frequency on-off on the service life of the switch is reduced. At the same time, the energy storage element or the filter element in the power optimization module 20 can also be stopped, so that the power loss during the operation of the power optimization module 20 is reduced. Meanwhile, the through-mode MLPE component can still be at the maximum power point based on the MPPT function of the inverter 50. In addition, the photovoltaic system has the characteristics of low leveling degree electricity cost, no dependence on high-stability communication and the like. In the whole, the photovoltaic system can realize simplification of energy coupling control and improvement of the power generation efficiency of the photovoltaic system.

First embodiment

Referring to fig. 1, a photovoltaic system of a first embodiment includes the MLPE assembly and the photovoltaic inverter described in the above embodiments. The output ends of the n MLPE components are connected in series to form a photovoltaic string 40, and the output end of the photovoltaic string 40 is connected to the input side of the inverter.

As shown in fig. 2 and 3, in a first specific embodiment, the MLPE assembly includes a photovoltaic unit 10, a power optimization module 20, and a local control module 30. The power optimization module 20 has a dc conversion circuit 21 with a BUCK circuit topology. As a feature of the BUCK circuit topology, the output voltage u_out of the power optimization module 20 will drop relative to the input voltage u_in. Meanwhile, in general, the higher the output voltage u_out, the closer the duty ratio D is to 1. In addition, a communication connection is established between the local control module 30 and the central control module 60 provided in the inverter.

In one aspect, the first embodiment may implement a through-mode MLPE component in a photovoltaic system to meet a preset requirement.

Referring specifically to fig. 2, on the side of the MLPE component, in order to implement automatic control of the MLPE component from the pass-through mode to the power conversion mode, the local control module 30 adopts a control method comprising: s101, determining whether the current direct mode or the power conversion mode exists; upon determining that the MLPE component is operating in the power conversion mode, the local control module 30 will perform step S102 of detecting the local duty cycle electrical parameter, determining whether the local duty cycle satisfies a preset state condition, and determining whether a switch from the power conversion mode to the pass-through mode is required.

In particular, step S102, the local control module 30 obtains the duty cycle D of the current power optimization module. Specifically, the local control module 30 is capable of detecting the input voltage u_in and the output voltage u_out of the corresponding power optimization module 20, and calculating the operation duty ratio D (i.e., u_out/u_in) according to the output voltage u_out compared to the input voltage u_in. Further, the local control module 30 will determine whether the operating duty cycle D of the power optimization module 20 exceeds a preset duty cycle threshold d_ref. If the determination result is yes, the local control module 30 switches the power optimization module 20 from the power conversion mode to the pass-through mode, and if the determination result is no, the local control module 30 maintains the power optimization module 20 to operate in the power conversion mode.

Referring specifically to fig. 3, on the photovoltaic inverter side, in order to keep the number of MLPE components in the through mode constant, the central control module 60 adopts a control method comprising: step S111, regulating the input side voltage U_pv of the DC/DC front stage with the MLPE component in the through mode reaching the target; step S112 adjusts the input side voltage of the DC/DC front stage with the tracking of the maximum power point of the photovoltaic string 40 as a destination.

In step S111, the central control module 60 may obtain the mode status information of each MLPE component, and calculate the number Q of MLPE components currently in the pass-through mode. For example, acquiring the duty cycle information of an MLPE component, if the duty cycle is greater than a duty cycle threshold, the mode in which the MLPE component is located may be confirmed. For another example, the MLPE component sends a preset pass-through mode flag signal and/or a power conversion mode flag signal to the central control module 60. The central control module 60 determines whether the number Q of through-mode MLPE components is above a number threshold Q _ ref. If the determination result is negative, the central control module 60 controls the input side voltage u_pv of the DC/DC front stage to rise with the first amplitude Δu1. If the determination result is yes, the central control module 60 stops controlling the input side voltage u_pv of the DC/DC front stage to rise by the first amplitude Δu1. Eventually, the photovoltaic system will remain in a state in which the number Q of MLPE components in the pass-through mode is not lower than the number threshold q_ref.

Specifically, in step S112, the central control module 60 obtains the input side power of the DC/DC front stage, controls the change of the input side voltage u_pv of the DC/DC front stage with the second amplitude Δu2, determines whether the input side power of the DC/DC front stage increases, if so, continues to control the change of the input side voltage u_pv of the DC/DC front stage with the second amplitude Δu2, and if not, stops controlling the change of the input side voltage u_pv of the DC/DC front stage with the second amplitude Δu2. Eventually, both the photovoltaic string 40 and the through-mode MLPE components will operate at the maximum power point.

It can be seen that the MLPE component of the present embodiment automatically controls whether to switch from the power conversion mode to the pass-through mode based on the local duty cycle state. Meanwhile, the photovoltaic inverter indirectly prompts part of the MLPE components to automatically switch into the through mode by controlling the input side voltage U_pv to meet the condition in the duty ratio state, so that the through mode MLPE components reach the quantity threshold Q_ref. In addition, each local control module 30 only transmits the status information to the central control module through communication, so that the photovoltaic system has no high dependence on the communication stability.

It should be noted that, the input side voltage u_pv of the DC/DC front stage rises, the voltage of the photovoltaic string 40 will rise, and the output voltage u_out of each MLPE device will rise accordingly. Meanwhile, as the MLPE component adopts the BUCK circuit, the duty ratio D of the MLPE component also rises along with the BUCK circuit, so that the duty ratio D is closer to the preset duty ratio threshold D_ref. In the BUCK circuit, since the photovoltaic string 40 is formed by serially connecting the MLPE components, the output current of each MLPE component is the same according to the characteristics of serial connection, and the output voltage u_out of the MLPE component is proportional to the maximum power. Meanwhile, because of the environmental differences of irradiation, temperature and the like and the factors of the differences of the service life and specification of the photovoltaic hardware, the maximum power of each MLPE component in the photovoltaic string 40 will also be different. In combination, the higher the maximum power, the higher the output voltage U_out of the MLPE device, and the more preferentially the through mode is entered. In other words, the timing at which the MLPE component switches to the pass-through mode will not be the same. Thus, as the input side voltage U_pv rises, a portion of the MLPE components switch to pass-through mode such that the number Q meets the requirement of not less than the number threshold Q_ref, the central control module 60 will cease to control the rise of the DC/DC preceding stage input side voltage U_pv at the first magnitude ΔU1.

It should be further noted that, in performing steps S111 and S112, the central control module 60 may be performed synchronously, as long as the number Q of MLPE components that finally achieve the pass-through mode is not lower than the number threshold q_ref, and the photovoltaic string 40 operates at the maximum power point. Whereas the adjustment of Δu1 and Δu2 may be performed at the same or different frequencies, may be set to the same or different amplitudes.

It will be appreciated that in other embodiments according to the present invention, the local control module 30 may automatically control the power optimization module 20 to switch from the power conversion mode to the pass-through mode, where the difference between the output voltage u_out and the input voltage u_in is higher than the preset value. A feature of these embodiments is that the input-output voltage of the power optimization module 20 is very close, and even if the through mode is entered to change the photovoltaic panel 11 to directly communicate with the photovoltaic string 40, the output voltage u_out of the photovoltaic panel 11 is located near the maximum power point, and the inverter 50 can quickly track the maximum power point of the photovoltaic string 40.

In another aspect, the present first embodiment may implement a re-switching of the MLPE component in the through mode to the power optimized mode in the photovoltaic system.

Referring specifically to fig. 2, on the MLPE component side, the local control module 30 adopts a control method further including: upon determining that the MLPE component is operating in the pass-through mode, the local control module 30 will execute step S103, i.e., detect the local time t of the pass-through mode, determine whether the time t satisfies a preset status condition, determine whether a switch from the power conversion mode to the pass-through mode is required, and determine whether a switch from the pass-through mode to the power conversion mode is required.

In particular, in step S103, a clock circuit is configured in the local control module 30, and when the MLPE component is switched from the power conversion mode to the pass-through mode, the clock circuit starts to count and obtain a time t when the MLPE component is in the pass-through mode. The local control module 30 obtains the time t calculated by the clock circuit when the MLPE component is in the pass-through mode, and whether the time t calculated by the clock circuit when the MLPE component is in the pass-through mode exceeds a preset time threshold t_ref. If the determination result is yes, the local control module 30 switches the power optimization module 20 from the pass-through mode to the power conversion mode, and if the determination result is no, the local control module 30 maintains the power optimization module 20 to operate in the pass-through mode.

It should be further noted that since the environmental state of the MLPE device will change over time, the maximum power that the MLPE device can obtain will also change, and thus the MLPE device in the pass-through mode will likely not be adapted to the pass-through mode any more. At this time, the input voltage u_in of the power optimization module may rise, resulting in a decrease in the duty ratio D. The local control module 30 again switches the MLPE component from the pass-through mode to the power conversion mode within a predetermined fixed time t_ref. At the same time, the local control module 30 will again perform power conversion and maximum power tracking on the power optimization module, and re-perform step S102 described above. If the input voltage u_in does rise and results in the duty cycle D being below the duty cycle threshold d_ref, then remain in the power conversion mode; if the duty cycle D is not lower than the duty cycle threshold d_ref, the through mode is switched again.

It follows that, with time and environmental changes, although the number of MLPE components in pass-through mode remains at q_ref, it is unchanged; but the specific which MLPE components are configured in pass-through mode varies. Therefore, different MLPE components can obtain the time for suspending high-frequency on-off. However, the first embodiment is to switch from the pass-through mode to the power conversion mode in a passive manner, and cannot quickly determine the MLPE component that needs to return to the power conversion mode.

Second embodiment

Referring to fig. 1, a photovoltaic system of a second embodiment includes the MLPE assembly and the photovoltaic inverter described in the above embodiments. The output ends of the n MLPE components are connected in series to form a photovoltaic string 40, and the output end of the photovoltaic string 40 is connected to the input side of the inverter.

As shown in fig. 4, in the second embodiment, on the side of the MLPE component, in order to implement automatic control of the MLPE component between the pass-through mode and the power conversion mode, the local control module 30 adopts a control method including: s201, determining whether the current direct mode or the power conversion mode is adopted; after determining that the MLPE component is operating in the power conversion mode, the local control module 30 will execute step S202, i.e. detect the local duty cycle electrical parameter, determine whether the local duty cycle satisfies the preset state condition, and determine whether the power conversion mode needs to be switched to the pass-through mode; upon determining that the MLPE component is operating in the pass-through mode, the local control module 30 will perform step S203, i.e. the central control module 60 sends the output voltage u_out of the power optimization module 20, and detects whether a mode change instruction is locally acquired, to determine whether a switch from the pass-through mode to the power conversion mode is required.

Specifically, in step S203, the local control module 30 sends the output voltage u_out of the local power optimization module 20 to the central control module 60 via the communication connection, and the local control module 30 determines whether to receive the change mode instruction from the central control module 60. If the determination result is yes, the local control module 30 switches the power optimization module 20 from the pass-through mode to the power conversion mode, and if the determination result is no, the local control module 30 maintains the power optimization module 20 to operate in the pass-through mode.

In a second embodiment, as shown in fig. 5, on the photovoltaic inverter side, the MLPE components that need to be switched to pass-through mode are determined quickly for overall statistics. Similar to the first embodiment, in this embodiment, the central control module 60 adopts a control method including: step S211, regulating the input side voltage U_pv of the DC/DC front stage by taking the MLPE component in the through mode as a destination; step S212 adjusts the input side voltage of the DC/DC front stage with the tracking of the maximum power point of the photovoltaic string 40 as a destination. Wherein, the central control module 60 adopts a control method further comprising: step S213, determining whether to send a mode change instruction according to the output voltage U_out state of the power optimization module in the through mode.

In step S213, the central control module 60 collects the output voltages u_out of each MLPE component in the pass-through mode of the same photovoltaic string 40 through the communication connection, for example, the photovoltaic string 40 has 30 MLPE components, the central control module 60 will obtain the output voltage data, determine the output voltage set { u_out } of the MLPE components in the pass-through mode, calculate the average value of the output voltages of the MLPE components in the pass-through mode of the photovoltaic string 40 according to the set { u_out }, compare the difference value between the output voltages u_out of each MLPE component and the average value of the output voltages one by one, and determine whether the difference value exceeds the preset value. If the result of the determination is yes, the central control module 60 sends a mode changing instruction to the MLPE component, and if the result of the determination is no, the central control module 60 does not send a mode changing instruction to the MLPE component.

It should be further noted that, in the same photovoltaic string 40, the output voltage u_out of the MLPE device is proportional to the maximum power point, and when the output voltage u_out of the MLPE device is higher than the average value, this portion of the maximum power point is also higher than the average value of the power. At the same time, the photovoltaic volt-watt characteristics show that the power difference has a low influence on the peak voltage variation, i.e. the input voltage u_in of the MLPE component is approximately equal. Therefore, when the output voltage u_out of the MLPE component is higher than the average value, it is indicated that the input and output voltages u_out of the MLPE component are very close, and the condition that the duty cycle D exceeds the preset duty cycle threshold d_ref is most likely to be satisfied.

Therefore, in the second embodiment, the MLPE component with weak power generation capacity is actively identified and is prompted to return to the power conversion mode, so as to quickly determine the MLPE component needing to be switched to the pass-through mode; the MLPE component with high power generation capacity can continuously operate in the through mode, so that frequent mode switching is avoided. It will be appreciated that the MLPE component that receives the change mode command will switch to the power conversion mode, and perform the maximum power tracking locally, and will switch to the pass-through mode again if it is determined that the duty cycle D exceeds the preset duty cycle threshold d_ref again.

It can be understood that, as shown in the first embodiment and the second embodiment, the embodiments according to the present invention may be respectively embodied as the control method of the photovoltaic system, which can both implement the number of MLPE components in the through mode in the photovoltaic system to be maintained above the threshold q_ref. In other embodiments, a combination of the first embodiment and the second embodiment may also be employed as to how to switch from the through mode to the power conversion mode. In short, if the time t of the MLPE component in the pass-through mode is less than the time threshold t_ref, but the mode change command is received, the MLPE component can be switched from the pass-through mode to the power conversion mode in advance without waiting for the time t to reach t_ref.

Third embodiment

As shown in fig. 6, a third embodiment provides an MLPE assembly that can be used in the photovoltaic system of each of the above embodiments and achieve the effects described in the embodiments. Specifically, the MLPE assembly includes a photovoltaic unit 10, a power optimization module 20, and a local control module 30. In one aspect, the local control module 30 is connected to a communication module 31 that can communicate with the central control module 60. The local control module 30 may transmit the local output voltage u_out to the central control module 60 through the communication module 31, and may receive a mode change instruction from the central control module 60. On the other hand, the power optimization module 20 is constituted by a direct current conversion circuit 21 and a pass-through circuit 22. The local control module 30 is respectively connected to the dc conversion circuit 21 and the pass circuit 22.

Specifically, the dc conversion circuit 21 is a BUCK circuit. The dc conversion circuit 21 has an input terminal v_i, an output terminal v_o, a first switch M1, a second switch M2, an inductance L, an input capacitance C1, an output capacitance C2, and a bypass diode D1. Wherein input v_i is available for connection to an output of photovoltaic unit 10 and output v_o is available for coupling to photovoltaic string 40. The first end of the first switch M1 is connected to the positive electrode of the input end V_i, the second end of the first switch M1 is connected to the first end of the second switch M2, the second end of the second switch M2 is connected to the negative electrode of the input end V_i and the negative electrode of the output end V_o, the intermediate port between the first switch M1 and the second switch M2 is connected to the first end of the inductor L, and the second end of the inductor L is connected to the positive electrode of the output end V_o. The two ends of the input capacitor C1 are respectively connected between the positive electrode and the negative electrode of the input end V_i, and the two ends of the output capacitor C2 are respectively connected between the positive electrode and the negative electrode of the output end V_o. The anode of the bypass diode D1 is connected to the cathode of the output terminal v_o, and the cathode of the bypass diode D1 is connected to the output terminal v_o.

The local control module 30 is connected to the control end of the first switch M1, and is controlled by the pwm_tg signal of the local control module 30, so as to perform the functions of power conversion and maximum power point tracking. The control end of the second switch M2 is connected to the local control module 30, and is controlled by a pwm_bg signal of the local control module 30, where the pwm_bg signal is a complementary signal of the pwm_tg signal, and starts the synchronous freewheel function. The inductor L functions as energy storage, while the input capacitor C1 and the output capacitor C2 function as filtering. Bypass diode D1 bypasses the role of conducting when the MLPE component is disconnected from communication with the photovoltaic string 40. In other circuit configurations, the input terminal v_i and the output terminal v_o may be also denoted as a ground terminal. In an alternative embodiment, the local control module 30 may detect the input voltage u_in and the output voltage u_out of the dc conversion circuit 21, and may calculate the duty cycle D of the dc conversion circuit 21 based thereon. In another alternative embodiment, the local control module 30 may detect the output current i_out and the output voltage u_out of the dc conversion circuit 21, and calculate the output power of the dc conversion circuit 21 therefrom, so as to perform the maximum power point tracking. It should be noted that PWM is a pulse width modulation signal to control high frequency on-off of the switch to implement power conversion.

The pass-through circuit 22 includes a third switch M3, a first end of the third switch M3 is connected to the positive electrode of the input terminal v_i, a second end of the third switch M3 is connected to the positive electrode of the output terminal v_o, and a control end of the third switch M3 is connected to the local control module 30. In the power conversion mode, the local control module 30 controls the first switch M1 and the second switch M2 to be turned on and off at high frequency to realize power conversion and maximum power tracking, and the local control module 30 controls the third switch M3 to be turned off continuously, so that the power of the photovoltaic unit 10 subjected to power conversion and/or maximum power point tracking is provided to the photovoltaic string 40. In the pass-through mode, the local control module 30 controls the third switch M3 to be continuously turned on, and the local control module 30 controls the first switch M1 to be continuously turned on or turned off and controls the second switch M2 to be continuously turned off, so that the power of the photovoltaic unit 10 is directly turned on to the photovoltaic string 40.

It should be noted that, the resistance of the third switch M3 may be lower than that of the first switch M1, so that the power loss of the MLPE component in the through mode may be greatly reduced. In a preferred embodiment, in the pass-through mode, the third switch M3 and the first switch M1 are turned on simultaneously, that is, the internal resistances of the third switch M3 and the first switch M1 are parallel to each other, so that the power consumption of directly turning on the photovoltaic unit 10 to the photovoltaic string 40 is lower.

In other embodiments, the scheme of implementing the switching of the MLPE component between the pass-through mode and the power conversion mode may also be that the power optimization module 20 is provided with only the dc conversion circuit 21, and no pass-through circuit 22. In the through mode, the local control module 30 controls the first switch M1 to be continuously turned on and controls the second switch M2 to be continuously turned off, so that the power of the photovoltaic unit 10 can be directly turned on to the photovoltaic string 40. In the power conversion mode, the local control module 30 controls the first switch M1 and the second switch M2 to be turned on and off at high frequency so as to realize power conversion and maximum power tracking.

As shown in fig. 7, according to the features of the embodiments of the present invention, in an application example of the photovoltaic system according to the second embodiment described above, each photovoltaic unit 10 is an individually packaged photovoltaic panel 11, and each photovoltaic panel 11 is equipped with the power optimizing module 20 described in the second embodiment described above. The power optimization module 20 is provided with the local control module 30 described in the second embodiment above to constitute an MLPE component. The n MLPE components are connected in series as a photovoltaic string 40, with the output of the photovoltaic string 40 connected to the input side of the DC/DC front stage of the inverter 50. The inverter 50 is provided with the central control module 60 described in the second embodiment.

The photovoltaic system is configured at a photovoltaic power station in Qinghai. The quantity threshold q_ref is set between 3% and 10% depending on the environment of the photovoltaic power plant and the layout characteristics of the photovoltaic panels 11. If the photovoltaic string 40 is provided with 30 MLPE components, the quantity threshold q_ref may be set to 2. The duty ratio threshold d_ref is set to 97%, and the preset value is set to 1V different from the average value. Typically, the photovoltaic string 40 has a 3% operational variance from photovoltaic panel 11 due to device aging, panel uniformity dust coverage, cloud shading, and the like. In other words, even in the scenario where the uniformity of environmental factors such as irradiation and temperature is good, there is a 3% power difference between the highest power and lowest power MLPE components in a string 40.

At the beginning of operation, the local control module 30 of each MLPE device will be in a power conversion mode, such as when the peak power of each MLPE device is in the range of 500+ -7.5 w, the maximum power point voltage of the photovoltaic panel 11 is assumed to be 45V, if the input side voltage U_pv set by the DC/DC front stage is 1200V, the output voltage U_out of each MLPE device is about 40V, and the duty cycle D is about 89%. Due to the 3% difference, the actual individual u_out may be distributed over the interval 39.4V to 40.6V. At this time, the central control module 60 will acquire the number Q of MLPE components of the photovoltaic string 40 in the pass-through mode as 0, and will control the input side voltage u_pv to rise at the first amplitude of 5V.

When the input side voltage u_pv rises to 1310V. The output voltage u_out of at least some of the MLPE components will reach 43.65V, while the duty cycle D reaches 97%. At this point, the local control module 30 of this portion of the MLPE assembly will control the power optimization module 20 to enter a pass-through mode with its photovoltaic panel 11 directly connected to the photovoltaic string 40. At this time, the central control module 60 will acquire that the number Q of MLPE components of the photovoltaic string 40 in the pass-through mode is greater than or equal to 2, and then stop controlling the input side voltage u_pv to rise at the first amplitude of 5V.

At the same time, the central control module 60 will control the input side voltage U_pv conversion at the second magnitude of 1V and track the maximum power point of the photovoltaic string 40. When the photovoltaic string 40 reaches the maximum power point, the output voltage u_out of the MLPE device in the pass-through mode will be configured at 45V, which is the maximum power point voltage of the corresponding photovoltaic panel 11. In other words, the inverter 50 completes maximum power tracking of the through-mode MLPE component. Meanwhile, the MLPE component in the power conversion mode will be correspondingly controlled by the local control module 30 to perform power conversion and maximum power point tracking by the direct current conversion circuit 21 thereof.

Over time, the central control module 60 will also periodically obtain the output voltage u_out of the MLPE component that enters the pass-through mode, calculate an average value, such as 45V, and send a change mode command to the MLPE component whose output voltage u_out is lower than 45V. The local control module 30 of these MLPE components has obtained a change mode instruction to switch from pass-through mode to power conversion mode. On the premise that the number Q continuously exceeds the number threshold Q_ref, each MLPE component is subjected to maximum power tracking, wherein the output voltage U_out of part of the MLPE components is configured to be lower than 43.65V under the action of environmental factors, and the MLPE components are kept in a power conversion mode due to the fact that the duty ratio D is lower than 97%; wherein another portion of the MLPE components are configured to have an output voltage u_out above 43.65V under an environmental factor change and return to the pass-through mode. A portion of the average out MLPE component may be continuously maintained in the pass-through mode.

It can be seen that, according to the photovoltaic system and the control method thereof in the embodiment of the present invention, the MLPE component can automatically operate in the pass-through mode or the power conversion mode, and the central control module 60 can indirectly cause the power optimization modules 20 with a certain number Q to be in the pass-through mode by controlling the voltage of the input side of the post-stage conversion device, so that the MLPE component with higher power in the photovoltaic string 40 can be continuously operated in the pass-through mode. The MLPE component in the through mode realizes power optimization on the maximum power point tracking function of the input side by the post-stage conversion equipment, and the power optimization module 20 with the rest part in the power conversion mode realizes power optimization by utilizing the local maximum power point tracking, so that the loss of a high-frequency switch and an energy storage inductor of the power optimization module 20 is reduced by utilizing the MPPT function of the post-stage conversion equipment and local mode control logic of the MLPE component, and further, the energy coupling control of a photovoltaic system is simplified and the power generation efficiency of the photovoltaic system is improved.

Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features Q indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present invention, the meaning of "plurality" means at least two, for example, two, three, etc., unless specifically defined otherwise.

The foregoing embodiments have described primarily the basic principles, principal features and advantages of the invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, and that the above embodiments and descriptions are merely illustrative of the principles of the present invention, and various changes and modifications may be made without departing from the spirit and scope of the invention, which is defined in the appended claims.

Claims

1. A photovoltaic system, comprising: a rear-stage conversion device, and at least one photovoltaic string (40) connected to an input side of the rear-stage conversion device; the photovoltaic string (40) comprises a plurality of MLPE components, the MLPE components comprise photovoltaic units (10) and power optimization modules (20), the output ends of the photovoltaic units (10) are coupled with the input ends of the power optimization modules (20), each MLPE component is connected with each other in series through the output ends of the power optimization modules (20) to form the photovoltaic string (40), and the photovoltaic string is characterized in that,

the power optimization module (20) is provided with a local control module (30); the local control module (30) is used for automatically controlling the power optimization module (20) to operate in a through mode or a power conversion mode according to the local parameter state, the power optimization module (20) in the through mode is used for directly conducting the power of the photovoltaic unit (10) to the photovoltaic group string (40), and the power optimization module (20) in the power conversion mode is used for providing the power of the photovoltaic unit (10) which is subjected to power conversion and/or maximum power point tracking to the photovoltaic group string (40);

The latter-stage conversion device is provided with a central control module (60); the central control module (60) is used for acquiring the number of the power optimization modules (20) in the through mode in a communication mode, and controlling the input side voltage of the post-stage conversion equipment by taking the number of the power optimization modules (20) in the through mode not lower than a preset number threshold as a destination, so that the local parameter state of the power optimization modules (20) is indirectly changed to switch from the power conversion mode to the through mode;

the central control module (60) is further configured to control the input side voltage of the post-stage conversion device with the aim of tracking the maximum power point of the photovoltaic string (40) so that the through-mode power optimization module (20) is configured at the maximum power point.

2. The photovoltaic system of claim 1, wherein the local control module (30) automatically controlling the power optimization module (20) to operate in either the pass-through mode or the power conversion mode based on the local parameter status comprises: the local control module (30) determines that the power optimization module (20) is in a power conversion mode, and the local control module (30) controls whether the power optimization module (20) is switched to a pass-through mode according to the state of the local duty cycle: if the operation duty cycle of the power optimization module (20) exceeds a preset duty cycle threshold, switching the power optimization module (20) from a power conversion mode to a through mode, and if the operation duty cycle of the power optimization module (20) does not exceed the preset duty cycle threshold, maintaining the power optimization module (20) to operate in the power conversion mode;

The central control module (60) controls the input side voltage of the post-stage conversion equipment to change, so that the string voltage of the photovoltaic string (40) and the output voltage of the power optimization module (20) of each MLPE component are changed, and the local control module (30) controls the power optimization module (20) to switch from the power conversion mode to the through mode in an indirect mode due to the fact that the operation duty ratio is changed and exceeds a preset duty ratio threshold value.

3. The photovoltaic system according to claim 1, wherein the central control module (60) is configured to obtain an output voltage of each power optimization module (20) in a pass-through mode on the same photovoltaic string (40) and calculate an average value thereof, and send a mode change instruction to the power optimization module (20) whose output voltage differs from the average value by more than a preset difference value;

the local control module (30) automatically controls the power optimization module (20) to operate in a pass-through mode or a power conversion mode according to the local parameter state, and the method comprises the following steps: the local control module (30) determines that the power optimization module (20) is in a pass-through mode, and the local control module (30) controls whether the power optimization module (20) is switched to a power conversion mode according to a local change mode instruction state: if a change mode instruction is acquired, the power optimization module (20) is switched from the pass-through mode to the power conversion mode, and if no change mode instruction is acquired, the power optimization module (20) is maintained in the pass-through mode.

4. The photovoltaic system of claim 1, wherein the local control module (30) automatically controlling the power optimization module (20) to switch between the power conversion mode and the pass-through mode based on the local parameter state comprises: the local control module (30) determines that the power optimization module (20) is in a pass-through mode, and the local control module (30) controls whether the power optimization module (20) is switched to a power conversion mode according to a local pass-through mode timing state:

and if the time of the power optimization module (20) in the pass-through mode exceeds the time threshold, switching the power optimization module (20) from the pass-through mode to the power conversion mode, and if the time of the power optimization module (20) in the pass-through mode does not exceed the time threshold, maintaining the power optimization module (20) to operate in the pass-through mode.

5. The photovoltaic system according to claim 1, characterized in that the power optimization module (20) comprises a direct current conversion circuit (21) and a pass-through circuit (22);

the local control module (30) is in control connection with the direct current conversion circuit (21) and is used for controlling the direct current conversion circuit (21) to provide power of the photovoltaic unit (10) subjected to power conversion and/or maximum power point tracking to the photovoltaic group string (40) in a power conversion mode;

The local control module (30) is controllably connected to the pass-through circuit (22) and is configured to control the pass-through circuit (22) to conduct power of the photovoltaic unit (10) directly to the photovoltaic string (40) in the pass-through mode.

6. The photovoltaic system according to claim 5, wherein the direct current conversion circuit (21) comprises an input port (v_i) coupled to the photovoltaic unit (10), an output port (v_o) coupled to the string of photovoltaic groups (40), a first switch (M1) and a second switch (M2), a first end of the first switch (M1) being connected to the positive pole of the input port (v_i), a second end of the first switch (M1) being connected to a first end of the second switch (M2), a second end of the second switch (M2) being connected to the negative pole of the input port (v_i) and the negative pole of the output port (v_o), an intermediate port between the first switch (M1) and the second switch (M2) being for coupling to the positive pole of the output port (v_o);

the through circuit (22) comprises a third switch (M3), a first end of the third switch (M3) is connected with the positive electrode of the input port (V_i), and a second end of the third switch (M3) is connected with the positive electrode of the output port (V_o);

the local control module (30) is respectively connected with the first switch (M1), the second switch (M2) and the third switch (M3) and is used for controlling the high-frequency on-off of the first switch (M1) through a duty ratio signal capable of tracking the maximum power point in a power conversion mode, controlling the high-frequency on-off of the second switch (M2) through a complementary signal of the duty ratio signal and controlling the disconnection of the third switch (M3) through a closing signal; and is used for controlling the first switch (M1) to be kept on or off in the through mode, controlling the second switch (M2) to be kept off, and controlling the third switch (M3) to be turned on by an on signal.

7. The photovoltaic system of claim 1, wherein the predetermined number threshold is 2% to 10% of the total number of MLPE components in a photovoltaic string (40).

8. The photovoltaic system according to claim 1, characterized in that the power optimization module (20) comprises a direct current conversion circuit (21) of a BUCK circuit topology, the latter conversion device being an inverter (50) for photovoltaic with a DC/DC front stage, the output of the string of photovoltaic modules (40) being connected to the input side of the DC/DC front stage, the inverter (50) being provided with the central control module (60).

9. A photovoltaic inverter for a photovoltaic system according to any one of claims 1 to 8, the inverter (50) having at least one photovoltaic string (40) connected to the input side, the photovoltaic string (40) comprising a number of MLPE components, the MLPE components comprising a photovoltaic unit (10) and a power optimization module (20), the photovoltaic unit (10) output being coupled to the input of the power optimization module (20), each MLPE component constituting the photovoltaic string (40) by the outputs of the respective power optimization modules (20) being connected in series with each other, the local control module (30) being adapted to automatically control the power optimization module (20) to operate in a pass-through mode or to operate in a power conversion mode depending on a local parameter state, characterized in that the inverter (50) is provided with a central control module (60), the central control module (60) being adapted to communicatively obtain the number of power optimization modules (20) in the pass-through mode and to control the input side of the post-pass level device for a destination of no less than a preset number threshold, whereby the local parameter state is switched from the local parameter state to the power optimization module (20) to the local parameter state;

10. A control method for a photovoltaic system according to any one of claims 1 to 8, characterized in that,

the MLPE component side is provided with the following control steps:

the power optimization module (20) is controllable to operate in a pass-through mode in which power of the photovoltaic unit (10) is directly conducted to the photovoltaic string (40), or in a power conversion mode in which power of the photovoltaic unit (10) that is power converted and/or maximum power point tracked is provided to the photovoltaic string (40);

when in the power conversion mode, judging whether to control the power optimization module (20) to switch from the power conversion mode to the through mode according to the comparison of the local duty ratio and the duty ratio threshold value;

transmitting the operating power conversion mode and the output voltage of the power optimization module (20) to the rear-stage conversion device in a communication manner;

the method comprises the steps of obtaining operation power conversion modes of all power optimization modules (20) in a communication mode, determining the number of the power optimization modules (20) in a through mode, and controlling input side voltage change of the post-stage conversion equipment by a first preset amplitude value until the number of the MLPE components is not lower than a preset number threshold; meanwhile, controlling the voltage change of the input side of the post-stage conversion equipment by a second preset amplitude value so as to track the maximum power point of the photovoltaic group string (40);

if the MLPE component side judges whether to control switching from the through mode to the power conversion mode according to whether a change mode instruction is received in the through mode, the output voltage of each power optimization module (20) in the through mode is acquired in a communication mode at the later stage conversion equipment side, the average value of the output voltage is calculated, and the change mode instruction is sent to the power optimization module (20) of which the difference between the output voltage and the average value exceeds a preset difference value.