Composite photovoltaic inverter and control method thereof

Abstract

The application discloses a composite photovoltaic inverter, which comprises a DC-DC boost conversion module, a DC-AC inversion module and a three-port circuit. The three-port circuit comprises a first port, a second port and a third port. The first port is used for being connected with a first photovoltaic component string group in a photovoltaic array. The second port is an input port of the DC-DC boost conversion module and is used for being connected with a second photovoltaic component string group in the photovoltaic array. The third port is an input port of the DC-AC inversion module. The composite photovoltaic inverter is further provided with a second control module. A bypass diode or a relay is connected in parallel on the DC-DC boost conversion module. The control mode of a DC-DC sub-control module of the second control module comprises two working modes, i.e. a boost compensation mode and a bypass mode. The application can maximize the overall efficiency of the photovoltaic array under a complex distributed photovoltaic power station scene, reduces the cost and solves the mismatching problem of the component level and the string level.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic power generation technology, and in particular to a composite photovoltaic inverter and its control method. Background Technology

[0002] A photovoltaic (PV) inverter is a device that converts direct current (DC) generated by solar panels into alternating current (AC). The maximum power point (MPPT) is the most important function of an inverter. During the operation of a PV power plant, factors such as clouds, shade, and dust can affect the output power. The MPPT plays a crucial role in identifying the maximum power point. Inverters come in various forms, with centralized inverters and string inverters currently dominating the market. Centralized inverters offer advantages such as lower cost and higher conversion efficiency, making them suitable for various types of unobstructed PV power plants with minimal mismatch.

[0003] The introduction of photovoltaic power optimizers allows each module to achieve MPPT (Multi-Level Testing) independently. Buck-type power optimizers, in particular, offer advantages such as simple structure, low cost, and high conversion efficiency. They can solve module-level mismatch problems in photovoltaic power plants caused by factors such as the module itself, uneven pollution, and uneven shading, effectively compensating for the limited number of MPPT channels in centralized inverters. However, Buck-type power optimizers are buck converters, meaning they cannot boost the voltage. Mismatched photovoltaic modules will reduce their output voltage to maintain consistency between the optimizer's output current and the string current. If a multi-channel series-parallel Buck power optimizer structure is connected to a centralized photovoltaic inverter photovoltaic system, only a few photovoltaic modules may experience module-level mismatch issues due to the module itself, uneven pollution, or uneven shading. The resulting problems are not significant. The multi-channel series-parallel Buck power optimizer system can maintain the system at a high efficiency by coordinating and adjusting the bus voltage of the centralized photovoltaic inverter. However, the site conditions for distributed photovoltaic projects are generally more complex than those for large-scale ground-mounted photovoltaic power stations. There are more objects that may cause shading that need to be considered. In addition to the stairwells and parapet walls on the roof, the ventilation and electromechanical facilities on the roof can also affect photovoltaic power generation. When a string in a multi-string parallel Buck power optimizer system experiences significant shading, it can cause mismatch between the parallel strings. String-level mismatch refers to the situation where the output characteristics of each string are inconsistent due to module-level mismatch. In order to maintain the consistency of the string's output current, the Buck power optimizer connected to the shaded photovoltaic module will step down and boost the current. This will result in a significant voltage mismatch between the output voltage of this string and other strings. This string will lower the common output voltage of the entire parallel multi-string system, causing the voltage conversion ratio (duty cycle) of the Buck power optimizers in other strings that have not experienced significant shading to deviate from 1, thus reducing the overall efficiency of the photovoltaic system.

[0004] String photovoltaic inverters are mainly in the form of two-stage grid-connected inverters, consisting of a front-end boost circuit and a back-end inverter circuit. The front-end adopts a modular design, with each or several photovoltaic module strings corresponding to an MPPT boost module, thus possessing string-level maximum power point tracking (MPPT) functionality. The back-end inverter circuit converts DC to AC for grid connection. Its advantage is that it is not affected by differences between modules in the string or by shading. However, compared to centralized photovoltaic inverters, it has a higher cost and lower conversion efficiency.

[0005] A string-type two-stage grid-connected photovoltaic (PV) inverter consists of a front-end boost circuit and a rear-end inverter circuit. A large electrolytic capacitor is typically used as a bus capacitor to connect the two stages, serving as an energy buffer and decoupling the influence of the front-end boost circuit on the rear-end inverter control. When the PV cells output low voltage, the front-end boost circuit increases the voltage across the bus capacitor to meet the minimum voltage requirement for grid connection of the rear-end inverter. When the PV cells output high voltage, the front-end boost circuit stops working, and the current flows directly to the bus through the boost inductor and diodes. This results in losses in the front-end circuit, including diode conduction losses and inductor resistance losses. Therefore, installing a front-end boost circuit even on PV module strings that are not subject to large-area shading not only increases system cost but also reduces system efficiency.

[0006] However, the aforementioned conventional two-stage photovoltaic grid-connected inverter control method cannot solve the problems existing in the current application of composite photovoltaic inverters. After a large area of ​​shading disappears, if the voltage mismatch problem at the string level is resolved, and the DC-DC boost compensation circuit continues to operate, the current will not only need to flow through the boost inductor and diode of the DC-DC boost compensation circuit to reach the DC-AC bus, but also incur losses from diode conduction and inductor power loss. At the same time, the photovoltaic power optimizer on this string may operate with a low duty cycle, which will not only reduce the efficiency of the composite photovoltaic inverter but also reduce the overall system efficiency. Summary of the Invention

[0007] To overcome the problems existing in the prior art, this invention provides a composite photovoltaic inverter and control method, which enables the photovoltaic inverter applied in the photovoltaic system of the Buck power optimizer to maximize the overall efficiency of the photovoltaic array in the complex distributed photovoltaic power station scenario, not only reducing costs but also solving the mismatch problem at the module level and string level.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A composite photovoltaic inverter, characterized in that: the composite photovoltaic inverter includes a DC-DC boost converter module, a DC-AC inverter module, and a three-port circuit. The three-port circuit includes a first port, a second port, and a third port. The first port is used to connect to a first photovoltaic module string in a photovoltaic array. The second port is the input port of the DC-DC boost converter module and is used to connect to a second photovoltaic module string in the photovoltaic array. The third port is the input port of the DC-AC inverter module. The output terminal of the first photovoltaic module string, the output terminal of the DC-DC boost converter module, and the input terminal of the DC-AC inverter module are all connected. The output port of the DC-AC inverter module is connected to the AC power grid via a DC bus connection. The composite photovoltaic inverter also includes a second control module, which comprises a DC-AC sub-control module and a DC-DC sub-control module, a data acquisition unit, a processing unit, a data storage unit, and a communication unit. A bypass diode or relay is connected in parallel to the DC-DC boost converter module. The positive terminal of the bypass diode or relay is connected to the positive input terminal of the second photovoltaic module string, and the negative terminal is connected to the positive terminal of the DC bus capacitor. The control modes of the DC-DC sub-control module of the second control module include two operating modes: boost compensation mode and bypass mode.

[0010] Both the first and second ports are connected to a Buck power optimizer. The Buck power optimizer includes a Buck DC-DC power conversion circuit and a first control module. The first control module includes a sampling unit, a processing unit, a driving unit, and a communication unit. The sampling unit is used to collect the input voltage, current, and output voltage and current parameters of the Buck DC-DC power conversion circuit. The processing unit tracks the maximum power point of the photovoltaic module according to the changes in the electrical parameters of its corresponding power conversion circuit and independently sets the duty cycle of the pulse modulation switch signal to generate a control signal for the Buck DC-DC power conversion circuit. The driving unit is used to amplify the control signal to drive and control the Buck DC-DC power conversion circuit. The communication unit uploads the input voltage, current, and output voltage and current parameters of the Buck DC-DC power conversion circuit collected by the sampling unit and the duty cycle data of the Buck DC-DC power conversion circuit calculated by the processing unit to the composite photovoltaic inverter.

[0011] The DC-AC sub-control module is used to variably adjust the input voltage of the DC-AC inverter module so that the duty cycle reference value of the first control module of the first photovoltaic module string does not exceed the optimized reference range, including: when the duty cycle reference value exceeds the optimized reference range, the DC-AC sub-control module adjusts the input voltage of the DC-AC inverter module by correspondingly increasing or decreasing the first amplitude; when the duty cycle reference value does not exceed the optimized reference range, the DC-AC sub-control module maintains the input voltage of the DC-AC inverter module unchanged;

[0012] The DC-DC sub-control module is used to variably adjust the input voltage of the DC-DC boost converter module so that the duty cycle reference value of the first control module of the second photovoltaic module string does not exceed the optimized reference range. This includes: when the duty cycle reference value exceeds the optimized reference range, the DC-DC sub-control module adjusts the input voltage of the DC-DC boost converter module by correspondingly increasing or decreasing the first amplitude; when the duty cycle reference value does not exceed the optimized reference range, the DC-DC sub-control module maintains the input voltage of the DC-DC boost converter module unchanged.

[0013] A control method based on the aforementioned composite photovoltaic inverter includes the following steps: The acquisition unit of the second control module of the composite photovoltaic inverter acquires electrical parameters such as the input voltage and bus voltage of the DC-DC boost converter module; the communication unit of the second control module collects duty cycle data sent by the communication units of the first and second photovoltaic module strings; the processing unit of the second control module sorts the duty cycle data acquired by the communication unit and calculates the average value of the duty cycle data for the first and second photovoltaic module strings according to a preset ratio; after the composite photovoltaic inverter starts, the control mode of the DC-DC sub-control module operates in boost compensation mode, where the DC-DC boost converter module controls the boost power conversion in a closed-loop manner to ensure that the output voltage of the second photovoltaic module string reaches the DC bus voltage after DC-DC boost power conversion, while simultaneously enabling the Buck power conversion of the second photovoltaic module string... The duty cycle data of the first control module of the power optimizer meets the duty cycle requirements, thereby variably setting the voltage parameters of the DC input side of the DC-DC boost converter module. The processing unit of the composite photovoltaic inverter compares the voltage values ​​of the input voltage and bus voltage of the DC-DC boost converter module acquired by the acquisition unit. When the difference between the two is less than the preset voltage deviation threshold stored in the data storage unit, the DC-DC sub-control module switches the control operation mode to the bypass mode; otherwise, it maintains the voltage compensation mode. In the bypass mode controlled by the DC-DC boost converter module, the DC-DC boost converter module is in a bypass state, and the bypass diode or relay on the bypass path will be in a conducting state. The power obtained from the first photovoltaic module string will be directly input to the DC bus side of the DC-AC inverter module. The output voltage of the second photovoltaic module string is the DC bus voltage. At the same time, the duty cycle data of the first control module of the Buck power optimizer of the second photovoltaic module string meets the duty cycle requirements, thereby variably setting the voltage parameters of the DC bus side of the DC-AC inverter module.The processing unit of the second control module of the composite photovoltaic inverter sorts the duty cycle data collected by the communication unit and selects the top-ranked duty cycle data of a preset proportion to calculate the average value as the duty cycle sampling set average value. The second control module obtains the duty cycle sampling set average value of the first control module of the Buck power optimizer of the first photovoltaic module string and the second photovoltaic module string. The processing unit of the composite photovoltaic inverter compares the duty cycle sampling set average value of the first control module of the Buck power optimizer of the first photovoltaic module string and the second photovoltaic module string. When the difference between the two is greater than the preset duty cycle deviation threshold stored in the data storage unit, the DC-DC sub-control module switches the control operation mode to the boost compensation mode; otherwise, it maintains the bypass mode.

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0015] This invention enables the switching between boost compensation and bypass modes in the boost compensation conversion circuit of a composite photovoltaic inverter, thereby improving the overall conversion efficiency. In particular, it solves the module-level mismatch problem by connecting a Buck power optimizer to achieve module-level MPPT. Furthermore, it solves the voltage mismatch problem between strings caused by BUCK power optimization by performing boost compensation on photovoltaic module strings that experience large-area shading.

[0016] This invention, through innovative control methods, enables the switching between boost compensation and bypass operating modes of the boost compensation converter circuit. After the large area of ​​shading affecting the photovoltaic module string connected to the Buck DC-DC boost converter circuit of the composite photovoltaic inverter disappears, the Buck DC-DC boost converter circuit can quickly switch to bypass mode. In this way, not only can the power optimizers of most of the photovoltaic modules connected to the Buck DC-DC boost converter circuit operate at a duty cycle close to 1, but the composite photovoltaic inverter as a whole also has a higher conversion efficiency after bypassing. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the frame of the composite photovoltaic inverter of the present invention;

[0018] Figure 2 This is a schematic diagram of the boost compensation mode of the composite photovoltaic inverter of the present invention;

[0019] Figure 3 This is a schematic diagram of the bypass mode of the composite photovoltaic inverter of the present invention;

[0020] Figure 4 This is a schematic diagram of the frame of the composite photovoltaic inverter of the present invention connected with a Buck-type power optimizer;

[0021] Figure 5This is a schematic diagram of an application embodiment of the present invention (I);

[0022] Figure 6 This is a schematic diagram (II) of an application embodiment of the present invention. Detailed Implementation

[0023] The following explanation, in conjunction with the accompanying drawings, provides further details.

[0024] like Figures 1-4 The composite photovoltaic inverter of this invention includes a DC-DC boost converter module, a DC-AC inverter module, and a three-port circuit. The three-port circuit includes a first port, a second port, and a third port. The first port is used to connect to the first photovoltaic module string in the photovoltaic array. The second port is the input port of the DC-DC boost converter module and is used to connect to the second photovoltaic module string in the photovoltaic array. The third port is the input port of the DC-AC inverter module. The output terminals of the first photovoltaic module string, the output terminal of the DC-DC boost converter module, and the input terminal of the DC-AC inverter module are connected through a DC bus. The output port of the DC-AC inverter module is connected to the AC power grid. The composite photovoltaic inverter of this invention also includes a second control module, which comprises a DC-AC sub-control module and a DC-DC sub-control module, a data acquisition unit, a processing unit, a data storage unit, and a communication unit. A bypass diode or relay is connected in parallel to the DC-DC boost converter module. The positive terminal of the bypass diode or relay is connected to the positive input terminal of the second photovoltaic module string, and the negative terminal is connected to the positive terminal of the DC bus capacitor. The control modes of the DC-DC sub-control module of the second control module include two working modes: boost compensation mode and bypass mode.

[0025] like Figure 4The area within the wavy line refers to the second photovoltaic module string affected by shading or other factors. Both the first and second ports are connected to a Buck-type power optimizer. The Buck-type power optimizer includes a Buck-type DC-DC power conversion circuit and a first control module. The first control module includes a sampling unit, a processing unit, a drive unit, and a communication unit. The sampling unit collects the input voltage, current, and output voltage and current parameters of the Buck-type DC-DC power conversion circuit. The processing unit tracks the maximum power point of the photovoltaic module based on the changes in the electrical parameters of its corresponding power conversion circuit and independently sets the duty cycle of the pulse modulation switch signal to generate a control signal for the Buck-type DC-DC power conversion circuit. The drive unit amplifies the control signal to drive and control the Buck-type DC-DC power conversion circuit. The communication unit uploads the input voltage, current, and output voltage and current parameters of the Buck-type DC-DC power conversion circuit collected by the sampling unit and the duty cycle data of the Buck-type DC-DC power conversion circuit calculated by the processing unit to the composite photovoltaic inverter.

[0026] The second control module is used to variably set the input voltage of the DC bus of the DC-AC inverter module and the DC input voltage of the DC-DC boost converter module, so that the duty cycle data of the first control module of the Buck power optimizer of the first and second photovoltaic module strings respectively meet the duty cycle optimization requirements; the second control module is used to variably adjust the input voltage of the second converter stage to ensure that the duty cycle reference value of the first converter stage does not exceed the optimization reference range, including: when the duty cycle reference value exceeds the optimization reference range, the second control module adjusts the input voltage of the second converter stage by correspondingly increasing or decreasing the first amplitude; when the duty cycle reference value does not exceed the optimization reference range, the second control module maintains the input voltage of the second converter stage unchanged.

[0027] The DC-AC sub-control module is used to variably adjust the input voltage of the DC-AC inverter module so that the duty cycle reference value of the first control module of the first photovoltaic module string does not exceed the optimized reference range. This includes: when the duty cycle reference value exceeds the optimized reference range, the DC-AC sub-control module adjusts the input voltage of the DC-AC inverter module by correspondingly increasing or decreasing a first amplitude; when the duty cycle reference value does not exceed the optimized reference range, the DC-AC sub-control module maintains the input voltage of the DC-AC inverter module unchanged.

[0028] The DC-DC sub-control module is used to variably adjust the input voltage of the DC-DC boost converter module so that the duty cycle reference value of the first control module of the second photovoltaic module string does not exceed the optimized reference range. This includes: when the duty cycle reference value exceeds the optimized reference range, the DC-DC sub-control module adjusts the input voltage of the DC-DC boost converter module by correspondingly increasing or decreasing a first amplitude; when the duty cycle reference value does not exceed the optimized reference range, the DC-DC sub-control module maintains the input voltage of the DC-DC boost converter module unchanged.

[0029] like Figure 2 , Figure 3The present invention relates to a control method for the composite photovoltaic inverter, specifically comprising: a data acquisition unit of the second control module of the composite photovoltaic inverter acquiring electrical parameters such as the input voltage and bus voltage of the DC-DC boost converter module; a communication unit of the second control module of the composite photovoltaic inverter collecting duty cycle data sent by the communication units of the first control module of the first photovoltaic module string and the second photovoltaic module string; a processing unit of the second control module of the composite photovoltaic inverter sorting the duty cycle data acquired by the communication unit and averaging the top-ranked duty cycle data as the average value of the duty cycle sampling set; after the composite photovoltaic inverter is started, the control mode of the DC-DC sub-control module operates in boost compensation mode, at which time the DC-DC boost converter module is used to control the boost power conversion in a closed-loop manner so that the output voltage of the second photovoltaic module string reaches the DC bus voltage after DC-DC boost power conversion, and simultaneously enabling the first control module of the Buck power optimizer of the second photovoltaic module string to... The duty cycle data of the module satisfies the duty cycle requirement, thereby variably setting the voltage parameters of the DC input side of the DC-DC boost converter module. The processing unit of the composite photovoltaic inverter compares the voltage values ​​of the input voltage and bus voltage of the DC-DC boost converter module acquired by the acquisition unit. When the difference between the two is less than the preset voltage deviation threshold stored in the data storage unit, the DC-DC sub-control module switches the control operation mode to the bypass mode; otherwise, it maintains the voltage compensation mode. In the bypass mode controlled by the DC-DC boost converter module, the DC-DC boost converter module is in a bypass state, and the bypass diode or relay on the bypass path will be in a conducting state. The power obtained from the first photovoltaic module string will be directly input to the DC bus side of the DC-AC inverter module. The output voltage of the second photovoltaic module string is the DC bus voltage. At the same time, the duty cycle data of the first control module of the Buck power optimizer of the second photovoltaic module string satisfies the duty cycle requirement, thereby variably setting the voltage parameters of the DC bus side of the DC-AC inverter module. The processing unit of the second control module of the composite photovoltaic inverter sorts the duty cycle data collected by the communication unit and selects the top-ranked duty cycle data of a preset proportion to calculate the average value as the duty cycle sampling set average value. The second control module obtains the duty cycle sampling set average value of the first control module of the Buck power optimizer of the first photovoltaic module string and the second photovoltaic module string. The processing unit of the composite photovoltaic inverter compares the duty cycle sampling set average value of the first control module of the Buck power optimizer of the first photovoltaic module string and the second photovoltaic module string. When the difference between the two is greater than the preset duty cycle deviation threshold stored in the data storage unit, the DC-DC sub-control module switches the control operation mode to the boost compensation mode; otherwise, it maintains the bypass mode.

[0030] like Figure 5 , Figure 6 Here is a schematic diagram illustrating a specific application of the present invention:

[0031] The project involves a flat cement roof, with a 4.5-meter-high staircase structure on the west side of the roof, and no obstructions on the surrounding area.

[0032] The project utilizes 166-72-450W monocrystalline silicon photovoltaic (PV) modules. These modules have a power output of 450Wp, a peak power voltage of 41.4V, a peak power current of 10.87A, an open-circuit voltage of 49.6V, and a short-circuit current of 11.58A. A total of 198 modules of this specification are used. These modules are connected to the PV inverter in strings via a Buck power optimizer. The Buck power optimizer provides maximum power point tracking for each module, ensuring its output current matches the current of the entire module string. The total installed capacity is 89.1kW. The project is implemented in 9 strings, with 22 modules per string connected to the PV inverter. In this project, photovoltaic module strings 1, 2, 3, 4, 5, 6, 7, and 8 are all free from large-area shading, meaning they are unobstructed. Only photovoltaic module string 9 is obstructed by a 4.5-meter-high staircase structure on the west side of the flat roof every afternoon, meaning it is an obstruction.

[0033] In the Buck power-optimized composite photovoltaic inverter power generation system, due to the use of the 75kW composite photovoltaic inverter described in this invention, the project capacity ratio is 1.18. The composite photovoltaic inverter of this invention includes a DC-DC boost converter module, a DC-AC inverter module, a three-port circuit, and a second control module. The three-port circuit includes a first port, a second port, and a third port. The first port is used to connect to the first photovoltaic module string in the photovoltaic array. The second port is the input port of the DC-DC boost converter module and is used to connect to the second photovoltaic module string in the photovoltaic array. The third port is the input port of the DC-AC inverter module. The output port of the first photovoltaic module string... The output of the DC-DC boost converter module and the input of the DC-AC inverter module are connected via a DC bus. The output port of the DC-AC inverter module is connected to the AC power grid. The second control module includes a DC-AC sub-control module and a DC-DC sub-control module, specifically composed of a data acquisition unit, a processing unit, a data storage unit, a drive unit, and a communication unit. A bypass diode or relay is connected in parallel with the DC-DC boost converter module. The positive terminal of the bypass diode or relay is connected to the positive input terminal of the second photovoltaic module string, and the negative terminal is connected to the positive terminal of the DC bus capacitor. The control modes of the DC-DC sub-control module of the second control module include boost compensation mode and bypass mode. In this way, photovoltaic module strings 1 to 8 without large-area shading are connected to the DC-AC inverter module of the composite photovoltaic inverter through a combiner box. Photovoltaic module string 9, which is affected by the shading of a 4.5-meter-high accessible staircase structure on the west side of the flat roof, is connected to the DC-AC inverter module of the composite photovoltaic inverter through the DC-DC converter of the aforementioned composite photovoltaic inverter.

[0034] During the operation of the Buck power-optimized hybrid photovoltaic inverter system, for most of the morning and midday hours, the sun is located to the southeast or south of the flat-roofed stairwell building, and the aforementioned photovoltaic module string 9 is not significantly shaded. At this time, the control mode of the DC-DC sub-control module of the second control module of the hybrid photovoltaic inverter's DC-DC converter operates in bypass mode to improve system efficiency. The average duty cycle sampling set of the first control module of the Buck power optimizer for the first photovoltaic module string group (1-8) is compared with the average duty cycle sampling set of the first control module of the Buck power optimizer for the second photovoltaic module string group (9). If the difference between the two is less than or equal to the preset duty cycle deviation threshold of 3% stored in the data storage unit, the DC-DC sub-control module switches its operating mode to maintain bypass mode. Simultaneously, the composite photovoltaic inverter bus voltage regulation function ensures that the real-time data of the duty cycle of the first conversion stage of each Buck-type photovoltaic power optimizer meets the requirement that the power optimizer of each photovoltaic module can operate within a high conversion efficiency range with a duty cycle close to 1, resulting in high overall system efficiency. However, in the afternoon, when the sun is located to the southwest or west of the accessible stairwell on the flat roof, the aforementioned photovoltaic module string 9 will experience large-area shading. This invention addresses this by connecting a Buck-type power optimizer to each photovoltaic module, ensuring that each photovoltaic module in the photovoltaic module string 9 operates near its maximum power point under the current conditions, thereby maximizing the power output of the photovoltaic array. This solves the module-level mismatch problem of traditional photovoltaic power generation technology. Then, the Buck-type photovoltaic power optimizer, while performing maximum power point tracking, will step down and boost current to maintain consistent output current for the shaded photovoltaic modules. This results in the output voltage of photovoltaic module string 9 being lower than the output voltage of other photovoltaic module strings 1-8 that are not heavily shaded. In a centralized photovoltaic inverter system with multiple parallel strings, the mismatched string will lower the common output voltage of the entire parallel multi-string system, causing the voltage conversion ratio (duty cycle) of the Buck-type power optimizer in other strings that are not heavily shaded to deviate from 1, thus reducing operating efficiency. At this point, the average duty cycle sampling set of the first control module of the Buck-type power optimizer for the first photovoltaic module string group (1-8) is compared with the average duty cycle sampling set of the first control module of the Buck-type power optimizer for the second photovoltaic module string group (9). If the difference between the two is greater than the preset duty cycle deviation threshold of 3% stored in the data storage unit, the DC-DC sub-control module switches its operating mode to the boost compensation mode.

[0035] The second control module of the present invention is used to variably set the input voltage of the DC-DC boost converter module so that the duty cycle data of the first control module of the Buck power optimizer in the photovoltaic module string 9 meets the duty cycle optimization requirements.

[0036] To cope with changes in solar shading caused by external conditions, including the incident angle and amount of solar irradiance, the processing unit of the composite photovoltaic inverter continuously compares the average duty cycle sampling set of the first control module of the Buck power optimizer of the first photovoltaic module string 1-8 and the second photovoltaic module string 9. When the difference between the two is greater than the preset duty cycle deviation threshold of 3% stored in the data storage unit, the DC-DC sub-control module switches the operation mode to the boost compensation mode; otherwise, it maintains the bypass mode.

[0037] It should be stated that the above specific embodiments are only preferred embodiments of the present invention and the technical principles used. Within the scope of the technology disclosed in the present invention, any changes or substitutions that can be easily conceived by those skilled in the art should be covered within the protection scope of the present invention.

Claims

1. A composite photovoltaic inverter, characterized in that, The composite photovoltaic inverter includes a DC-DC boost converter module, a DC-AC inverter module, and a three-port circuit. The three-port circuit includes a first port, a second port, and a third port. The first port is used to connect to the first photovoltaic module string in the photovoltaic array. The second port is the input port of the DC-DC boost converter module and is used to connect to the second photovoltaic module string in the photovoltaic array. The third port is the input port of the DC-AC inverter module. The output terminals of the first photovoltaic module string, the output terminal of the DC-DC boost converter module, and the input terminal of the DC-AC inverter module are connected through a DC bus. The output port of the DC-AC inverter module is connected to the AC power grid. The composite photovoltaic inverter also includes a second control module, which includes a DC-AC sub-control module and a DC-DC sub-control module. Specifically, it consists of a data acquisition unit, a processing unit, a data storage unit, a drive unit, and a communication unit. A bypass diode or relay is connected in parallel to the DC-DC boost converter module. The positive terminal of the bypass diode or relay is connected to the positive input terminal of the second photovoltaic module string, and the negative terminal is connected to the positive terminal of the DC bus capacitor. The control modes of the DC-DC sub-control module of the second control module include two working modes: boost compensation mode and bypass mode. When the control mode of the DC-DC sub-control module is running in boost compensation mode, the DC-DC boost converter module is used to control the boost power conversion in a closed loop so that the output voltage of the second photovoltaic module string reaches the DC bus voltage after the DC-DC boost power conversion. At the same time, the duty cycle data of the first control module of the Buck power optimizer of the second photovoltaic module string meets the duty cycle requirements, and the voltage parameters of the DC input side of the DC-DC boost converter module can be variably set. When the control mode of the DC-DC sub-control module is running in bypass mode, the DC-DC boost converter module is in bypass state, and the bypass diode or relay on the bypass path will be in the conducting state. The power obtained from the first photovoltaic module string will be directly input to the DC bus side of the DC-AC inverter module. The output voltage of the second photovoltaic module string is the DC bus voltage. At the same time, the duty cycle data of the first control module of the Buck power optimizer of the second photovoltaic module string meets the duty cycle requirements, so the voltage parameters of the DC bus side of the DC-AC inverter module can be variably set.

2. The composite photovoltaic inverter according to claim 1, characterized in that: Both the first and second ports are connected to a Buck power optimizer. The Buck power optimizer includes a Buck DC-DC power conversion circuit and a first control module. The first control module includes a sampling unit, a processing unit, a driving unit, and a communication unit. The sampling unit is used to collect the input voltage, current, and output voltage and current parameters of the Buck DC-DC power conversion circuit. The processing unit tracks the maximum power point of the photovoltaic module according to the changes in the electrical parameters of its corresponding Buck DC-DC power conversion circuit and independently sets the duty cycle of the pulse modulation switch signal to generate a control signal for the Buck DC-DC power conversion circuit. The driving unit is used to amplify the control signal to drive and control the Buck DC-DC power conversion circuit. The communication unit uploads the input voltage, current, and output voltage and current parameters of the Buck DC-DC power conversion circuit collected by the sampling unit and the duty cycle data of the Buck DC-DC power conversion circuit calculated by the processing unit to the composite photovoltaic inverter.

3. The composite photovoltaic inverter according to claim 2, characterized in that: The DC-AC sub-control module is used to variably adjust the input voltage of the DC-AC inverter module so that the duty cycle reference value of the first control module of the first photovoltaic module string does not exceed the optimized reference range, including: when the duty cycle reference value exceeds the optimized reference range, the DC-AC sub-control module adjusts the input voltage of the DC-AC inverter module by correspondingly increasing or decreasing the first amplitude; when the duty cycle reference value does not exceed the optimized reference range, the DC-AC sub-control module maintains the input voltage of the DC-AC inverter module unchanged; The DC-DC sub-control module is used to variably adjust the input voltage of the DC-DC boost converter module so that the duty cycle reference value of the first control module of the second photovoltaic module string does not exceed the optimized reference range. This includes: when the duty cycle reference value exceeds the optimized reference range, the DC-DC sub-control module adjusts the input voltage of the DC-DC boost converter module by correspondingly increasing or decreasing the first amplitude; when the duty cycle reference value does not exceed the optimized reference range, the DC-DC sub-control module maintains the input voltage of the DC-DC boost converter module unchanged.

4. A control method based on the composite photovoltaic inverter of claim 1, characterized in that: The acquisition unit of the second control module of the composite photovoltaic inverter acquires the input voltage and bus voltage electrical parameters of the DC-DC boost converter module; The communication unit of the second control module of the composite photovoltaic inverter collects the duty cycle data sent by the communication unit of the first control module between the first photovoltaic module string and the second photovoltaic module string. The processing unit of the second control module of the composite photovoltaic inverter sorts the duty cycle data collected by the communication unit, and calculates the average value of the duty cycle data ranked first in the first photovoltaic module string group and the second photovoltaic module string group according to the selected preset proportion as the average value of the duty cycle sampling set. After the composite photovoltaic inverter is started, the control mode of the DC-DC sub-control module operates in the boost compensation mode; the processing unit of the composite photovoltaic inverter compares the voltage values ​​of the input voltage of the DC-DC boost converter module and the bus voltage collected by the acquisition unit. When the difference between the two is less than the preset voltage deviation threshold stored in the data storage unit, the control operation mode of the DC-DC sub-control module switches to the bypass mode; otherwise, the voltage compensation mode is maintained. The processing unit of the second control module of the composite photovoltaic inverter sorts the duty cycle data collected by the communication unit and selects the top-ranked duty cycle data of a preset proportion to calculate the average value as the duty cycle sampling set average value. The second control module obtains the duty cycle sampling set average value of the first control module of the Buck power optimizer of the first photovoltaic module string and the second photovoltaic module string. The processing unit of the composite photovoltaic inverter compares the duty cycle sampling set average value of the first control module of the Buck power optimizer of the first photovoltaic module string and the second photovoltaic module string. When the difference between the two is greater than the preset duty cycle deviation threshold stored in the data storage unit, the DC-DC sub-control module switches the control operation mode to the boost compensation mode; otherwise, it maintains the bypass mode.

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