A Maximum Power Point Tracking Control Method for High-Power String-Type Photovoltaic Inverters

Abstract

The present invention belongs to the technical field of photovoltaic inverter control algorithms and discloses a maximum power point tracking control method for a high-power string photovoltaic inverter. The method is characterized by first detecting the DC input voltage of each photovoltaic string of the inverter. When the light intensity is low, the open-circuit voltage of the photovoltaic string is low. As the light intensity increases and the maximum voltage of the photovoltaic string exceeds the startup voltage threshold, all photovoltaic strings are subjected to DC / DC converter chopping boost and MPPT control, so that the input circuits of all photovoltaic strings operate at the maximum power point. When the photovoltaic string reaches a sufficiently high voltage, the DC / DC boost and MPPT tracking control of the photovoltaic string with the higher voltage are stopped, while the boost control of the photovoltaic string with the lower voltage is maintained. This method not only reduces the majority of the boost losses but also allows all photovoltaic strings to operate at the MPPT point. This method avoids the prior art practice of performing DC / DC conversion on all photovoltaic strings, avoiding unnecessary DC / DC converter boost losses and improving the overall power generation efficiency of the inverter.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic inverter control algorithms, and in particular to a maximum power point tracking control method for a high-power string photovoltaic inverter. Background Art

[0002] High-power string photovoltaic inverters are generally used on the roofs of large factories or public buildings, and operate on the user side, mainly for self-use. Excess electricity is fed into the public grid. Most of the strings in these larger distributed power stations have a large number of photovoltaic panels in series, and the array voltage is high. Due to the relatively consistent environmental parameters such as orientation, light, and temperature, the output characteristics of each string are also relatively consistent.

[0003] MPPT (Maximum Power Point Tracking), or maximum power point tracking, is a key technology for improving the conversion efficiency of string inverters. Generally, there are two control methods. The first is the MPPT control of the DC / DC converter boost part, which will work continuously, that is, constantly tracking the maximum power point of each photovoltaic string. In this solution, the power loss caused by the DC / DC converter boost is relatively large; the second is that in the initial stage, all photovoltaic strings DC / DC converters boost work. After reaching a sufficiently high voltage or energy, all photovoltaic strings DC / DC converters stop boosting and MPPT tracking control, and the strings enter the bypass working state to reduce the loss caused by the DC / DC chopping boost.

[0004] With the second method, if all DC / DC converter boosting is stopped, while energy losses from DC / DC converter boosting are saved, if there are individual PV strings with a small number of series-connected PV panels, they will not operate at their maximum power point, or even have no output current, clearly not enabling all PV strings to operate at their optimal power points. In light of this, a maximum power point tracking control method for a high-power string-type PV inverter has been invented. When the PV strings reach a sufficiently high voltage, the DC / DC boost and MPPT tracking control for the higher-voltage PV strings are stopped, while boost control for some lower-voltage PV strings is maintained. This method reduces most of the boost losses while enabling all PV strings to operate at their MPPT points, achieving optimal overall power generation efficiency.

[0005] CN116014805A discloses a photovoltaic system and control method. The photovoltaic system includes a controller and an inverter. The inverter input is used to connect multiple photovoltaic strings, which are connected in parallel. Each photovoltaic string includes a photovoltaic panel and an optimizer. The controller is used to control the inverter input voltage to be less than or equal to the MPPT voltage of a first photovoltaic string, where the first photovoltaic string is the photovoltaic string with the lowest open-circuit voltage among the multiple photovoltaic strings. The inverter is used to perform MPPT scanning on the multiple photovoltaic strings. The optimizer is used to obtain the MPPT voltage corresponding to each photovoltaic panel and maintain the photovoltaic panels operating at the corresponding photovoltaic panel MPPT voltage. This method reduces power loss caused by connecting multiple photovoltaic strings in parallel and increases the output power of the photovoltaic system.

[0006] CN115987207A discloses a photovoltaic string energy management system, method, and device. The system includes: a photovoltaic string formed by multiple photovoltaic modules connected in series with a smart junction box, and a smart repeater connected to each smart junction box; the smart junction box obtains data information of the corresponding photovoltaic modules and itself; the smart repeater calculates the baseline data of each photovoltaic module using a preset algorithm based on the data information; determines whether the difference between the data information and the baseline data exceeds a preset threshold; if so, controls the smart junction box to activate a preset optimization algorithm to optimize the output power; if not, controls the smart junction box to operate in a direct state to maintain the output power. This system can ensure that the output power of the photovoltaic modules is always maintained at the maximum power point when there is obstruction, and the photovoltaic modules can still operate at the optimal operating point when there is no obstruction.

[0007] However, in the above-mentioned prior art, if the number of solar panels configured for each input of a high-power string photovoltaic inverter is different, some circuits will not operate at the maximum power point or even the voltage will be too low to output current, resulting in a problem of reduced overall efficiency. Summary of the Invention

[0008] To solve the above problems, the purpose of the present invention is to disclose a maximum power point tracking control method for a high-power string photovoltaic inverter, which is implemented by adopting the following technical solutions.

[0009] A maximum power point tracking control method for a high-power string photovoltaic inverter is used in a photovoltaic power generation system. The photovoltaic power generation system has multiple photovoltaic strings, all of which have an equal number of photovoltaic panels connected in series. The output of the photovoltaic strings serves as the photovoltaic string DC input of the inverter. The method is characterized by first detecting the DC input voltage of each photovoltaic string of the inverter. When the light intensity is low, the open-circuit voltage of the photovoltaic strings is low. As the light intensity increases, when the maximum photovoltaic string voltage exceeds the startup voltage threshold, all photovoltaic strings are subjected to DC / DC converter chopping and boosting and MPPT control. The DC / DC converter chopping and boosting and MPPT control ensure that all photovoltaic string input circuits operate at the maximum power point and control the inverter DC bus voltage to a stable target value. In this case, MPPT control is implemented by the DC / DC converter of each photovoltaic string of the inverter.

[0010] The above-mentioned maximum power point tracking control method for a high-power string photovoltaic inverter is characterized in that: if the sunlight increases and the photovoltaic string voltage continues to rise, if the photovoltaic string input voltage exceeds a preset threshold, all string DC / DC boost control is stopped and switched to bypass mode to reduce energy loss generated by the operation of the boost loop components. At this time, MPPT is switched to inverter control by the DC / AC converter.

[0011] The maximum power point tracking control method of the high-power string photovoltaic inverter described above is characterized by: further comprising a photovoltaic string with a smaller number of photovoltaic panels connected in series, wherein the output of the photovoltaic string with a smaller number of photovoltaic panels connected in series also serves as the photovoltaic string DC input of the inverter; the photovoltaic string with a smaller number of photovoltaic panels connected in series is referred to as a low-voltage photovoltaic string, and the photovoltaic string with a larger number of photovoltaic panels connected in series is referred to as a high-voltage photovoltaic string; the boost control method of the low-voltage photovoltaic string is changed to: setting the boost target of the low-voltage photovoltaic string to the open-circuit voltage value of the high-voltage string, and setting the boost duty upper limit threshold D of the low-voltage photovoltaic string. LIMT , and the low-voltage PV string circuit no longer performs MPPT tracking.

[0012] The above-mentioned maximum power point tracking control method of a high-power string photovoltaic inverter is characterized in that: due to the low voltage photovoltaic string boost duty upper limit threshold D LIMT When the current output increases, the maximum duty cycle of the boost is limited to D LIMT , when the duty cycle reaches D LIMT After that, the duty cycle no longer increases, and the boosted voltage is U OHAssuming the same external conditions such as light and temperature, and ignoring the differences between photovoltaic panels, at a certain moment, after the output of the low-voltage photovoltaic string is boosted, the output optimal working MPPT voltage point overlaps with the MPPT voltage point of the high-voltage photovoltaic string. The DC / DC converter boost duty cycle formula is: LIMT =U I / U OH -1, where U OH is the open circuit voltage of the high voltage photovoltaic string, U I is the open circuit voltage of the low voltage photovoltaic string, the ratio U I / U OH The data is collected when the inverter is put into operation, U OH =UI / (1+D LIMT ).

[0013] The above-described maximum power point tracking control method for a high-power string photovoltaic inverter is characterized in that: for high-voltage photovoltaic strings, the MPPT control of the DC / AC converter is used to enable all input photovoltaic strings to operate at the maximum power operating point. For low-voltage photovoltaic strings, although their respective maximum power operating points are not tracked, because the output voltage curve after boosting overlaps with the voltage of the high-voltage photovoltaic strings at the maximum power operating point, the MPPT control of the DC / AC inverter part indirectly controls the output voltage of the low-voltage photovoltaic strings, enabling the low-voltage photovoltaic strings to operate at or near the maximum power point, thereby achieving efficient power generation with all photovoltaic strings at the optimal operating point.

[0014] The maximum power point tracking control method of the high-power string photovoltaic inverter described above is characterized in that: the boost duty upper limit threshold D LIMT After setting, speed and accuracy adjustment can be achieved by adding PI adjustment. PI adjustment includes: target voltage is U IST , actual voltage U REL , difference U E =U IST –U REL , voltage proportional adjustment value Duty_P=KP×U E , KP is set to 18, the role of the ratio adjustment is to control the voltage to quickly reach the target value, the voltage integral adjustment Duty_I=KI×U E , and in each integration period, U I Accumulate, the integral period is 55 milliseconds, KI is 0.0009, the integral regulation is used to eliminate the static error and stabilize the voltage at the target value; finally, the sum of the voltage PI regulation Duty_PI = Duty_P + Duty_I is obtained, and then the upper limit value D is set for the duty cycle value Duty_PI obtained by the regulation LIMT, so that the voltage control target does not exceed the upper limit; control multiple low-voltage photovoltaic strings to reach the target voltage value through the PI adjustment algorithm. When the output voltage drops after the DC / AC converter works, the duty cycle Duty_PI reaches the boost duty upper limit threshold D through PI adjustment. LIMT , the low-voltage PV strings are passively controlled to the maximum power point or near the maximum power point.

[0015] The above-described maximum power point tracking control method for a high-power string photovoltaic inverter is characterized in that: upon determining that the DC voltage is greater than a certain threshold, the DC / DC converter boost is automatically started; if the light intensity decreases and the photovoltaic string voltage or current drops to a certain value, the state automatically changes to a shutdown state and stops the DC / DC converter boost control, thereby reducing unnecessary losses.

[0016] The above-mentioned maximum power point tracking control method for a high-power string photovoltaic inverter is characterized in that: the boost target value and duty cycle upper limit D of the multi-channel MPPT inverter are LIMT , proportional coefficient KP, integral coefficient KI, all parameters are configurable, and the best control effect can be achieved by adjusting the appropriate PI parameter values.

[0017] The method of the present invention avoids the prior art of performing DC / DC conversion on all photovoltaic strings, avoids unnecessary DC / DC converter boost losses, and improves the overall power generation efficiency of the inverter. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the MPPT block diagram of the multi-channel photovoltaic string inverter of the present invention.

[0019] Figure 2 Flowchart of the inverter MPPT control method in an embodiment of the present invention.

[0020] Figure 3 Schematic diagram of the boost and bypass modes of the DC / DC converter of the present invention.

[0021] Figure 4 Schematic diagram of power-voltage curves before and after voltage boosting of photovoltaic strings according to the present invention. DETAILED DESCRIPTION

[0022] The following will be combined with the accompanying drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention so that the advantages and features of the present invention can be understood and implemented by those skilled in the art. Figure 1 and Figure 3As shown, the number of photovoltaic panels connected in series from the 1st photovoltaic string to the n-1th photovoltaic string is equal, which can be called a high-voltage photovoltaic string. Since the number of photovoltaic panels connected in series is equal, the output voltage V1 of the 1st photovoltaic string to the output voltage V of the n-1th photovoltaic string is equal. n-1 The value is relatively close. The number of photovoltaic panels in the nth photovoltaic string is less than the number of photovoltaic panels in the n-1th photovoltaic string. It can be called a low-voltage photovoltaic string. This is just an example. According to the actual situation, there may be a combination of strings with different numbers of photovoltaic panels connected in series. The output voltages of the photovoltaic string from the 1st to the nth photovoltaic string are V1 to V n-1 , V1 to V n-1 A DC / DC converter boost module is connected in series to each module, a bypass diode is connected in series in the forward direction to each DC / DC converter boost module, and the output of each DC / DC converter boost module is connected to an MPPT control input terminal of the DC / AC conversion module, that is, the DC bus terminal. The output terminal of the DC / AC conversion module generates electricity and is connected to the grid.

[0023] The present invention provides a maximum power point tracking control method for a high-power string photovoltaic inverter, characterized in that the tracking control method comprises the following steps:

[0024] S101, measuring the input voltage values of all photovoltaic strings and obtaining the maximum value;

[0025] When performing the measurement, the input voltage values of all PV strings at the inverter end are collected, the maximum voltage value is obtained through judgment, and the next step is entered;

[0026] S102, determine whether the maximum voltage of the photovoltaic string exceeds the power-on voltage threshold, if so, proceed to the next step, otherwise proceed to step S107;

[0027] The power-on voltage threshold must be greater than the minimum voltage at which the inverter auxiliary power supply and other equipment can operate normally, and at the same time, it must be as low as possible so that power can be generated even in low-voltage conditions with weak sunlight, thereby improving power generation efficiency.

[0028] S103: Start DC / DC boost control for all PV strings, and start MPPT control for each PV string, and then proceed to the next step;

[0029] As attached Figure 1, all photovoltaic string DC / DC converters implement a boost control algorithm. Each photovoltaic string circuit will set an equal boost target voltage value to obtain a stable bus voltage value. The maximum power point of each photovoltaic string itself is searched by adjusting and controlling the output current of the photovoltaic string. The controller in the MPPT inverter sends a pulse width modulation signal to the control end of each switch tube in the MPPT inverter through the drive circuit in the multi-channel MPPT inverter, thereby controlling all DC / DC converters to enter chopper boost control.

[0030] S104: If the input voltage of the PV string exceeds a preset threshold, stop the boost control of all string DC / DC converters, switch to bypass mode, and start the MPPT control of the overall DC / AC converter, and then proceed to the next step;

[0031] Generally, the loss of DC / DC converter switching elements is much greater than the loss of bypass diodes. If a relatively high PV string voltage is reached, the DC / DC boost control is stopped, such as Figure 3 As shown in the bypass diode, the current of the photovoltaic string will be output to the DC bus through the bypass diode. At this time, due to the equal number of photovoltaic panels in series in the higher voltage photovoltaic string, the external environment such as light is not much different. Figure 1 The output voltage V1 of the 1st photovoltaic string to the output voltage V of the n-1th photovoltaic string n-1 Basically the same, that is, controlling the MPPT inverter DC / AC converter to execute the MPPT algorithm can make all high-voltage strings operate near the maximum power point;

[0032] S105: The PV string with lower voltage performs DC / DC boost according to the new control mode, and then proceeds to the next step;

[0033] In specific applications, due to the application characteristics of distributed string inverters, there may be photovoltaic strings with fewer photovoltaic panels in series, and their output voltage is relatively low. When the DC / DC boost of each photovoltaic string is stopped, the low-voltage strings are collected to the DC bus through the bypass, such as Figure 4 As shown in the power-voltage curve before boosting, its maximum power point is significantly different from the maximum power point voltage of the higher PV string. This makes it impossible to obtain a maximum power point voltage that meets the characteristics of all strings. Therefore, the new control mode of the lower voltage string must be boosted.

[0034] The new boost control method is different from the S103 step method. It does not perform MPPT control on each photovoltaic string. Instead, it sets the boost target to the open circuit voltage value of the high voltage string and sets the boost duty upper limit threshold D. LIMT , due to the above low voltage string boost duty upper limit threshold D LIMTWhen the current output increases, the maximum duty cycle of the boost is limited to D LIMT , when the duty cycle reaches D LIMT After that, its duty cycle no longer increases, that is, after the boost, U OH Fixed control is 1 / (1+D LIMT ) times the PV string voltage input voltage U I Assuming the same external conditions such as light and temperature, ignoring the differences between the photovoltaic string devices themselves, at a certain moment, after the low voltage photovoltaic string output is boosted, Figure 4 As shown in the power-voltage curve after boosting, its output optimal operating MPPT voltage point will basically overlap with the MPPT voltage points of other higher-voltage PV strings. In this way, by executing the MPPT algorithm through the DC / AC converter to obtain a maximum power point voltage, all PV strings can operate at the maximum power point, or relatively close to the maximum power point. This not only reduces most of the DC / DC converter's boost loss but also allows all PV strings to basically operate near the MPPT point, achieving the best overall power generation efficiency.

[0035] S106, determining whether the DC bus voltage is lower than the preset DC / AC operating voltage, if not, returning to the previous step, if yes, proceeding to the next step;

[0036] If the light weakens the voltage, the current or voltage drops to a certain value, and the state is changed to the shutdown state and the DC / DC boost control is stopped to reduce unnecessary losses;

[0037] S107: Stop DC / DC boost and DC / AC inverter output, and return to step S101 to cyclically detect the input voltage of the photovoltaic string. If the conditions are met, restart the inverter operation.

[0038] This method avoids the need to perform DC / DC conversion on all photovoltaic strings in the prior art, avoids unnecessary DC / DC converter boost losses, and improves the overall power generation efficiency of the inverter.

[0039] The above embodiments are merely preferred technical solutions of this application and should not be construed as limiting the present invention. The scope of protection of this application shall be the technical solutions described in the claims, including equivalent alternatives to the technical features of the technical solutions described in the claims. Equivalent alternatives and improvements within this scope are also within the scope of protection of this invention.

Claims

1. A maximum power point tracking control method for a high-power string photovoltaic inverter, used in a photovoltaic power generation system having multiple photovoltaic strings, all of which have an equal number of photovoltaic panels connected in series, and the output of the photovoltaic strings serving as the photovoltaic string DC input of the inverter; characterized by: First, the DC input voltage of each photovoltaic string of the inverter is detected. When the sunlight is weak, the open-circuit voltage of the photovoltaic string is low. As the sunlight increases, when the maximum voltage of the photovoltaic string exceeds the startup voltage threshold, all photovoltaic strings are subjected to DC / DC converter chopping and boosting and MPPT control. The DC / DC converter chopping and boosting and MPPT control make all photovoltaic string input circuits operate at the maximum power point and control the inverter DC bus voltage at a stable target value. At this time, MPPT control is implemented by the DC / DC converter of each photovoltaic string of the inverter; If the sunlight increases and the PV string voltage continues to rise, if the input voltage of the PV string exceeds the preset threshold, all string DC / DC boost control will be stopped and switched to bypass mode to reduce the energy loss caused by the operation of the boost circuit components. At this time, MPPT is switched to inverter control by the DC / AC converter. The photovoltaic power generation system also includes a photovoltaic string with a small number of photovoltaic panels connected in series, and the output of the photovoltaic string with a small number of photovoltaic panels connected in series also serves as the photovoltaic string DC input of the inverter; the photovoltaic string with a small number of photovoltaic panels connected in series is called a low-voltage photovoltaic string, and the photovoltaic string with a large number of photovoltaic panels connected in series is called a high-voltage photovoltaic string. The boost control method of the low-voltage photovoltaic string is changed to: the boost target of the low-voltage photovoltaic string is set to the open-circuit voltage value of the high-voltage string, and the boost duty upper limit threshold D of the low-voltage photovoltaic string is set. LIMT , and the low-voltage PV string circuit no longer performs MPPT tracking.

2. The maximum power point tracking control method for a high-power string photovoltaic inverter according to claim 1, characterized in that: Due to the low voltage photovoltaic string boost duty upper limit threshold D LIMT When the current output increases, the maximum duty cycle of the boost is limited to D LIMT , when the duty cycle reaches D LIMT After that, the duty cycle no longer increases, and the boosted voltage is U OH Assuming the same external conditions of light and temperature, ignoring the differences of photovoltaic panels themselves, at a certain moment, after the output of the low-voltage photovoltaic string is boosted, the output optimal working MPPT voltage point overlaps with the MPPT voltage point of the high-voltage photovoltaic string. The DC / DC converter boost duty cycle formula is: D LIMT = U I / U OH -1, where U OH is the open circuit voltage of the high voltage photovoltaic string, U I is the open circuit voltage of the low voltage photovoltaic string, the ratio U I / U OH The data is collected when the inverter is put into operation, U OH = U I / (1+D LIMT ).

3. The maximum power point tracking control method for a high-power string photovoltaic inverter according to claim 2, characterized in that: For high-voltage PV strings, the MPPT control of the DC / AC converter enables all input PV strings to operate at the maximum power operating point. For low-voltage PV strings, although their respective maximum power operating points are not tracked, due to the overlap of the output voltage curve after boosting and the voltage of the high-voltage PV strings at the maximum power operating point, the MPPT control of the DC / AC inverter part indirectly controls the output voltage of the low-voltage PV strings, enabling the low-voltage PV strings to operate at or near the maximum power point, thus achieving efficient power generation of all PV strings at the optimal operating point.

4. The maximum power point tracking control method for a high-power string photovoltaic inverter according to claim 3, characterized in that: Boost duty upper limit threshold D LIMT After setting, speed and accuracy adjustment can be achieved by adding PI adjustment. PI adjustment includes: target voltage is U IST , actual voltage U REL , difference U E =U IST –U REL , voltage proportional adjustment value Duty_P=KP×U E , KP is set to 18, the role of the ratio adjustment is to control the voltage to quickly reach the target value, the voltage integral adjustment Duty_I=KI×U E , and in each integration period, U I Accumulate, the integral period is 55 milliseconds, KI is 0.0009, the integral regulation is used to eliminate the static error and stabilize the voltage at the target value; finally, the sum of the voltage PI regulation Duty_PI = Duty_P + Duty_I is obtained, and then the upper limit value D is set for the duty cycle value Duty_PI obtained by the regulation LIMT , so that the voltage control target does not exceed the upper limit; control multiple low-voltage photovoltaic strings to reach the target voltage value through the PI adjustment algorithm. When the output voltage drops after the DC / AC converter works, the duty cycle Duty_PI reaches the boost duty upper limit threshold D through PI adjustment. LIMT , the low-voltage PV strings are passively controlled to the maximum power point or near the maximum power point.

5. A maximum power point tracking control method for a high-power string photovoltaic inverter according to any one of claims 1 to 4, characterized in that: When the DC voltage is determined to be greater than a certain threshold, the DC / DC converter boost is automatically started. If the light intensity decreases and the PV string voltage or current drops to a certain value, the state automatically changes to the shutdown state and stops the DC / DC converter boost control to reduce losses.

6. A maximum power point tracking control method for a high-power string photovoltaic inverter, characterized in that The tracking control method comprises the following steps: S101, measuring the input voltage values of all photovoltaic strings and obtaining the maximum value; when performing the measurement, collecting the input voltage values of all photovoltaic strings at the inverter end, determining and obtaining the maximum voltage value, and proceeding to the next step; S102: Determine whether the maximum voltage of the photovoltaic string exceeds the power-on voltage threshold. If so, proceed to the next step; otherwise, proceed to S107. The power-on voltage threshold is greater than the minimum voltage at which the inverter auxiliary power supply can operate normally, and the power-on voltage threshold is close to the minimum voltage at which the inverter auxiliary power supply can operate normally, so that the photovoltaic string can generate electricity even under low light and low voltage conditions, thereby improving power generation efficiency. S103: Start DC / DC boost control for all photovoltaic strings, and each photovoltaic string starts its own MPPT control, and then proceed to the next step; all photovoltaic string DC / DC converters implement a boost control algorithm, and each photovoltaic string circuit sets an equal boost target voltage value to obtain a stable bus voltage value. By adjusting and controlling the output current of the photovoltaic string, the maximum power point of each photovoltaic string is searched; the controller in the MPPT inverter sends a pulse width modulation signal to the control terminal of each switch tube in the MPPT inverter through the drive circuit, thereby controlling all DC / DC converters to enter chopper boost control; S104. If the input voltage of the photovoltaic string exceeds the preset threshold, the boost control of all string DC / DC converters is stopped, the system switches to bypass mode, and the MPPT control of the overall DC / AC converter is started, and then the process proceeds to the next step. If the loss of the DC / DC converter switching element is greater than the loss of the bypass diode, and the high photovoltaic string voltage is reached, the DC / DC boost control is stopped, and the current of the photovoltaic string is output to the DC bus through the bypass diode. At this time, since the number of photovoltaic panels connected in series in the high-voltage photovoltaic string is equal and the external environment is consistent, the output voltage V1 of the first photovoltaic string is equal to the output voltage V of the n-1th photovoltaic string. n-1 Similarly, the MPPT inverter DC / AC converter is controlled to execute the MPPT algorithm, even if all high voltage strings operate near the maximum power point; S105, the low-voltage photovoltaic strings are DC / DC boosted according to the new control mode, and then proceed to the next step; when there are photovoltaic strings with fewer photovoltaic panels in series, their output voltage is low, which is called a low-voltage string. After stopping the DC / DC boost of each photovoltaic string, since the low-voltage string is connected to the DC bus through the bypass, its maximum power point is greatly different from the maximum power point voltage of the higher photovoltaic string, and it is impossible to obtain a maximum power point voltage that meets the characteristics of all strings. Therefore, the low-voltage string is operated under the new control mode for boost control; the new boost control is different from the step S103 method. Instead of performing MPPT control on each photovoltaic string, the boost target is set to the open-circuit voltage value of the high-voltage string, and the boost duty upper limit threshold D is set. LIMT , low voltage string boost duty upper limit threshold D LIMT When the current output increases, the maximum boost duty cycle is limited to D LIMT , when the duty cycle reaches D LIMT After that, its duty cycle no longer increases, that is, after the boost, U OH Fixed control is 1 / (1+D LIMT ) times the PV string voltage input voltage U I , U OH is the open-circuit voltage of the high-voltage PV string. Assuming the same external conditions of illumination and temperature, and ignoring the inherent differences among the PV string components, at a certain moment, after the low-voltage PV string output is boosted, the optimal operating MPPT voltage point of the low-voltage PV string output will overlap with the MPPT voltage point of the high-voltage PV string. The DC / AC converter executes the MPPT algorithm to obtain a maximum power point voltage, allowing all strings to operate at or near the maximum power point, reducing the boost loss of the DC / DC converter and allowing all PV strings to operate near the MPPT point, achieving optimal overall power generation efficiency. S106, determining whether the DC bus voltage is lower than the preset DC / AC operating voltage; if not, returning to the previous step; if so, proceeding to the next step; if the light weakens the voltage, and the current or voltage drops to a certain value, the state changes to the shutdown state and stops the DC / DC boost control to reduce losses; S107: Stop DC / DC boost and DC / AC inverter output, and return to step S101 to cyclically detect the input voltage of the photovoltaic string. If the conditions are met, restart the inverter operation.

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