A photovoltaic power generation system and a maximum power point tracking method thereof

Abstract

The present application provides a photovoltaic power generation system and a maximum power point tracking method thereof. The maximum power point tracking method, when the photovoltaic power generation system enters a power generation operation state, under the condition that the inverter and each optimizer are operating in the MPPT state, the inverter controls each photovoltaic string to operate in a state where the sum of the step-down ratios of each optimizer inside it is maximum, and then under the condition that the output currents of each optimizer in the same photovoltaic string are the same, each photovoltaic string reaches the highest efficiency state, thereby achieving the highest system efficiency and avoiding the impact on the actual power generation of users and the service life of the system.

Description

Technical Field

[0001] The present application relates to the technical field of photovoltaic power generation, and in particular to a photovoltaic power generation system and a maximum power point tracking method thereof. Background Art

[0002] With the technical development direction of electricity safety and power generation improvement, the use of optimizers in photovoltaic power generation systems is becoming more and more widespread. In a photovoltaic power generation system with an optimizer, each optimizer will perform MPPT (Maximum Power Point Tracking) separately, and the inverter will also perform MPPT; in this case, how to achieve the optimal overall efficiency of the system has become a technical issue, which will ultimately affect the user's actual power generation and system service life. Summary of the invention

[0003] In view of this, the present application provides a photovoltaic power generation system and a maximum power point tracking method thereof to improve system efficiency and avoid low system efficiency affecting the user's actual power generation and system service life.

[0004] To achieve the above objectives, this application provides the following technical solutions:

[0005] In a first aspect, the present application provides a maximum power point tracking method for a photovoltaic power generation system, wherein each input end of an inverter is connected to a corresponding photovoltaic string, and both ends of each photovoltaic string are formed by at least two step-down optimizers cascaded through their own output ends; the maximum power point tracking method comprises:

[0006] S101, the photovoltaic power generation system enters a power generation operation state;

[0007] S102, the inverter and each optimizer are all operated in the maximum power point tracking MPPT state;

[0008] S103, the inverter controls each photovoltaic string to operate in a state where the sum of the step-down ratios of each optimizer inside the inverter is maximum.

[0009] Optionally, step S103 includes:

[0010] For each photovoltaic string, the inverter controls its output current to be the maximum value of the input MPPT current of each internal optimizer.

[0011] Optionally, step S103 includes executing, for each photovoltaic string, when each optimizer inside the photovoltaic string is operating in a step-down mode:

[0012] S201, the inverter exits the MPPT state, and periodically disturbs the corresponding input voltage in an increasing direction until the first optimizer enters the direct-through mode, and then restores the MPPT state.

[0013] Optionally, in step S103, for each photovoltaic string, before step S201, the following steps are further included:

[0014] S301, the inverter determines whether each optimizer inside the inverter is operating in a buck mode;

[0015] If at least one optimizer is not running in the buck mode, then steps S302 and S303 are performed in sequence; if all optimizers are running in the buck mode, then step S303 is performed;

[0016] S302, the inverter exits the MPPT state, and periodically disturbs the corresponding input voltage in a decreasing direction until all optimizers operate in a buck mode, and the inverter resumes the MPPT state;

[0017] S303, waiting for all optimizers to enter the MPPT state.

[0018] Optionally, in step S103, for each photovoltaic string, after executing step S201, the following further steps are further included:

[0019] S401, the inverter determines whether the number of optimizers entering the direct mode is greater than 1, and whether the difference between the input voltages of the corresponding optimizers is greater than a preset difference;

[0020] If the number of optimizers entering the direct mode is greater than 1, and the difference between the input voltages of the corresponding optimizers is greater than the preset difference, return to step S302;

[0021] If the number of optimizers entering the direct mode is 1, or the difference between the input voltages of the optimizers entering the direct mode is less than the preset difference, step S103 is completed.

[0022] Optionally, in step S201, until the first optimizer enters the direct mode, the process includes:

[0023] Until the duty cycle of the first optimizer is greater than the preset duty cycle; or,

[0024] Until the difference between the input voltage and the output voltage of the first optimizer is less than the preset voltage difference; or,

[0025] Until the difference between the input current and the output current of the first optimizer is less than the preset current difference.

[0026] Optionally, after step S103, the method further includes:

[0027] S104, the inverter obtains input voltages of each input terminal thereof;

[0028] S105, the inverter determines whether each input voltage is greater than the voltage at the highest efficiency point of the inverter;

[0029] If there is an input terminal whose input voltage is greater than the voltage at the highest efficiency point, executing step S106;

[0030] S106, the inverter determines the voltage at the point of highest system efficiency;

[0031] S107, the inverter controls its corresponding input voltage to be the voltage at the highest efficiency point of the system.

[0032] Optionally, step S106 includes:

[0033] The inverter calculates the voltage at the highest efficiency point of the system according to the efficiency curve.

[0034] Optionally, after step S105, the method further includes:

[0035] For an input terminal whose input voltage is less than or equal to the voltage at the highest efficiency point, step S108 is executed;

[0036] S108: The inverter maintains the current operating state.

[0037] The second aspect of the present application also provides a photovoltaic power generation system, comprising: an inverter, and photovoltaic strings connected to each input end thereof;

[0038] The photovoltaic string comprises: at least two optimizers with cascaded output ends, and photovoltaic panels connected to the input ends of each of the optimizers;

[0039] The optimizer is a step-down optimizer;

[0040] The inverter is communicatively connected with each of the optimizers so that the photovoltaic power generation system executes the maximum power point tracking method of the photovoltaic power generation system as described in any one of the first aspects above.

[0041] Optionally, the inverter includes: a controller, an inverter circuit and at least two boost circuits;

[0042] The input end of each of the boost circuits serves as the corresponding input end of the inverter;

[0043] The output end of each of the boost circuits is connected to the DC bus of the inverter circuit;

[0044] The AC side of the inverter circuit serves as the output end of the inverter;

[0045] The controller is used to control the operation of each of the boost circuits and the inverter circuit, and is connected in communication with each of the optimizers.

[0046] The maximum power point tracking method of the photovoltaic power generation system provided in the present application, after the photovoltaic power generation system enters the power generation operation state, when the inverter and each optimizer are operating in the MPPT state, the inverter controls each photovoltaic string to operate in a state where the sum of the step-down ratios of each optimizer inside it is maximum, and then, when the output currents of each optimizer in the same photovoltaic string are the same, each photovoltaic string reaches the highest efficiency state, thereby achieving the highest system efficiency and avoiding the impact on the user's actual power generation and the system service life. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings to be used in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0048] Figure 1 A schematic diagram of the structure of a photovoltaic power generation system provided in an embodiment of the present application;

[0049] Figure 2 A flowchart of a maximum power point tracking method for a photovoltaic power generation system provided in an embodiment of the present application;

[0050] Figures 3 to 6 Four other flow charts of the maximum power point tracking method of the photovoltaic power generation system provided in the embodiments of the present application;

[0051] Figure 7 A schematic diagram of the structure of the inverter provided in an embodiment of the present application. DETAILED DESCRIPTION

[0052] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0053] In this application, the terms "comprises", "comprising" or any other variation thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of more restrictions, an element defined by the sentence "comprising a ..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element.

[0054] Assuming that there are 15 photovoltaic panels in the photovoltaic string and its MPPT voltage is 30V, the photovoltaic power generation system can operate in the following states:

[0055] State 1: The input voltage of each optimizer is 30V and the output voltage is 10V; the corresponding input voltage of the inverter is 150V and the bus voltage is 380V.

[0056] State 2: The input voltage of each optimizer is 30V and the output voltage is 30V; the corresponding input voltage of the inverter is 450V and the bus voltage is 450V.

[0057] State 3: The input voltage of each optimizer is 30V, but the output voltage of 2 optimizers is 10V, and the output voltage of 13 optimizers is 30V; the corresponding input voltage of the inverter is 410V, and the bus voltage is 410V.

[0058] In all the above states, the input voltage of the optimizer is the MPPT voltage point, so the photovoltaic panels are running at the maximum power point. However, the conversion efficiency of the converter in the system is different in different states, as follows:

[0059] In state 1, all optimizers are operating in the step-down state, and the corresponding boost circuit (hereinafter referred to as BOOST) in the inverter is in the boost state, and the system loss increases by two parts.

[0060] In state 2, all optimizers are running in the direct state, the corresponding BOOST in the inverter is in the off state, and the system loss does not increase.

[0061] In state 3, each optimizer is partially operating in step-down state and partially in direct-through state. The corresponding BOOST in the inverter is in the off state, and the system loss increases.

[0062] From this, it can be found that the MPPT point of a photovoltaic power generation system with an optimizer can have multiple voltage implementation methods. Different voltage implementation methods will lead to differences in the system's conversion efficiency, which will ultimately affect the user's actual power generation; different operating voltages will also lead to different system service lives, ultimately affecting the user's service life.

[0063] Therefore, the present application provides a maximum power point tracking method for a photovoltaic power generation system to improve system efficiency and avoid low system efficiency affecting the actual power generation of users and the service life of the system.

[0064] like Figure 1 As shown, in the photovoltaic power generation system, each input end of the inverter 20 is connected to the corresponding photovoltaic string 10, and both ends of each photovoltaic string 10 are formed by at least two step-down optimizers 101 cascaded through their own output ends.

[0065] See also Figure 2 The maximum power point tracking method of the photovoltaic power generation system comprises:

[0066] S101, the photovoltaic power generation system enters the power generation operation state.

[0067] S102: The inverter and each optimizer are all operating in the MPPT state.

[0068] S103, the inverter controls each photovoltaic string to operate in a state where the sum of the step-down ratios of each optimizer inside the inverter is maximum.

[0069] For the step-down type, also known as Buck optimizer, its step-down ratio is equal to output voltage / input voltage. Under the condition of constant power, the higher the step-down ratio, the higher the efficiency. Therefore, under the premise that each optimizer operates at the MPPT point, the higher the step-down ratio of the optimizer, the higher the system efficiency.

[0070] Since the output end of each optimizer string is connected in series, the output current of each optimizer in the same photovoltaic string is the same. The power of the photovoltaic panels connected to each optimizer is different, resulting in different output voltages and thus different step-down ratios; however, if the step-down ratios of each optimizer are set to their maximum values, the output currents of each optimizer cannot be guaranteed to be the same; therefore, finding the state where the sum of the step-down ratios of all optimizers is the largest is the operating state with the highest system efficiency.

[0071] The maximum power point tracking method of the photovoltaic power generation system provided in this embodiment, when the inverter and each optimizer are operating in the MPPT state, controls each photovoltaic string through the inverter to operate in a state where the sum of the step-down ratios of each optimizer inside it is maximum, and then, when the output currents of each optimizer in the same photovoltaic string are the same, each photovoltaic string reaches the highest efficiency state, thereby achieving the highest system efficiency and avoiding the impact on the user's actual power generation and the service life of the system.

[0072] In the previous embodiment, in step S103, if the step-down ratio of each optimizer is adjusted separately to maximize the sum of the step-down ratios, the adjustment process will be complicated and changeable. Therefore, this embodiment provides a more preferred implementation form, namely: for each photovoltaic string, the inverter controls its output current to be the maximum value of the input MPPT current of each optimizer inside it.

[0073] The specific principle analysis is as follows:

[0074] For the jth optimizer in the i-th PV string, its input MPPT voltage is Uij, its input MPPT current is Iij, its output current is Ipij, its output voltage is Uoij, its duty cycle is Dij, and its input power is Pij, then:

[0075] Uoij=Uij×Dij (1)

[0076] Ipij=Iij / Dij (2)

[0077] Since the output terminals of the optimizers in the same photovoltaic string are connected in series, the output currents Ipij of the optimizers are equal, which are the output currents Ipi of the photovoltaic string. Then:

[0078] Dij=Iij / Ipi (3)

[0079] For the i-th photovoltaic string, the calculation formula for its efficiency ηi is:

[0080] ηi=[Pi1×η(Di1)+Pi2×η(Di2)+Pi3×η(Di3)+…+Pin×η(Din)] / (Pi1+Pi2+Pi3+…+Pin) (4)

[0081] Where n is the number of optimizers connected in series in the PV string.

[0082] Substituting into equation (3), we have:

[0083] ηi=[Pi1×η(Ii1 / Ipi)+Pi2×η(Ii2 / Ipi)+Pi3×η(Ii3 / Ipi)+…+Pin×η(Iin / Ipi)] / (Pi1+Pi2+Pi3+…+Pin) (5)

[0084] Among them, η(Dij) is a monotonically increasing function of the duty cycle Dij. The larger the duty cycle Dij, the larger the corresponding efficiency η(Dij). Therefore, when the overall efficiency of each string optimizer (that is, the efficiency ηi of the corresponding photovoltaic string) is maximized, the sum of Dij of the photovoltaic string is maximized. Since the input MPPT current of the optimizer in the system is determined by the photovoltaic panel connected to it and is a constant under the same working condition, when the output current Ipi of the photovoltaic string is the smallest, the duty cycle Dij of each optimizer is the largest, and the corresponding efficiency η(Dij) is also the largest.

[0085] Since the output current Ipi of the photovoltaic string satisfies the following relationship:

[0086] Ipi=Ii1 / Di1=Ii2 / Di2=Ii3 / Di3=…=Iin / Din (6)

[0087] And the duty cycle Dij ranges from 0 to 1, so the minimum value of the output current Ipi of the PV string is the maximum value Iix among Ii1~Iin (the input MPPT current of the xth optimizer in the i-th PV string). At this time, the corresponding optimizer duty cycle Dix=1, that is, the xth optimizer in the i-th PV string is in the through state.

[0088] Therefore, by controlling the output current Ipi of the same PV string to be equal to the maximum input MPPT current of each optimizer inside it, the sum of the duty cycles Dij of each optimizer inside it can be maximized, that is, the sum of the step-down ratios of each optimizer can be maximized, thereby achieving optimal system efficiency and improving the overall power generation of all PV strings and the service life of the system.

[0089] In order to realize the above embodiment, for each photovoltaic string, the inverter controls its output current to be the maximum value of the input MPPT current of each optimizer inside it. This embodiment provides a specific implementation method, see Figure 3 , which includes the following operations performed for each photovoltaic string when each optimizer inside the photovoltaic string is operating in buck mode:

[0090] S201, the inverter exits the MPPT state and periodically disturbs the corresponding input voltage in an increasing direction until the first optimizer enters the direct-through mode, and then restores the MPPT state.

[0091] The buck mode means that the output voltage of the optimizer is less than the input voltage.

[0092] The pass-through mode can be specifically judged by the optimizer duty cycle being greater than the preset duty cycle, or by the difference between the input voltage and the output voltage being less than the preset voltage difference, or by the difference between the output current and the input current being less than the preset current difference. Any judgment condition being met can indicate that the optimizer has entered the pass-through mode.

[0093] This step S201 can adjust the optimizers in the photovoltaic string from all being in the step-down mode to a state where at least one optimizer is in the direct-through mode by periodically increasing the voltage of the photovoltaic string; then, the current of the photovoltaic string will be reduced to the input MPPT current of the direct-through optimizer; generally, the power of the photovoltaic panel connected to the optimizer that first enters the direct-through mode will be the largest among the optimizers in the same string, so the corresponding input MPPT current is the maximum value of the input MPPT current of each optimizer, and step S103 is completed.

[0094] Furthermore, in step S103, for each photovoltaic string, in order to avoid the optimizer not being in the buck mode, before executing step S201, Figure 4 As shown in:

[0095] S301: The inverter determines whether all optimizers inside it are operating in a buck mode.

[0096] If there is at least one optimizer that is not running in the buck mode, steps S302 and S303 are performed in sequence; if all optimizers are running in the buck mode, step S303 is performed.

[0097] S302: The inverter exits the MPPT state and periodically disturbs the corresponding input voltage in a decreasing direction until all optimizers operate in the buck mode and the inverter resumes the MPPT state.

[0098] S303, waiting for all optimizers to enter the MPPT state.

[0099] Through the above process, each optimizer in the corresponding photovoltaic string can be operated in the step-down mode, and then step S201 can be executed.

[0100] In addition, in step S103, for each photovoltaic string, after step S201, it cannot be guaranteed that each optimizer entering the direct mode is in the MPPT state, so it can be further performed. Figure 5 As shown in:

[0101] S401: The inverter determines whether the number of optimizers entering the direct-pass mode is greater than 1, and whether the difference between the input voltages of the corresponding optimizers is greater than a preset difference.

[0102] If the number of optimizers entering the pass-through mode is greater than 1, and the difference between the input voltages of the corresponding optimizers is greater than the preset difference, it means that at least one optimizer has not entered the MPPT state, and it is necessary to return to step S302.

[0103] If the number of optimizers entering the direct mode is 1, or the difference between the input voltages of the optimizers entering the direct mode is less than the preset difference, it means that step S103 is completed.

[0104] The following is a complete introduction to the specific implementation method provided in this embodiment in order of execution:

[0105] In the first step, the photovoltaic power generation system with optimizer enters the power generation operation state, and the optimizer and inverter both operate in the MPPT state.

[0106] In the second step, each optimizer is checked to see if it is running in buck mode. If at least one optimizer is not in buck mode, the third step is executed; otherwise, the fourth step is executed.

[0107] The third step is to control the inverter to be in the left disturbance mode, which means that the inverter exits the normal MPPT state, periodically disturbs the corresponding input voltage Uin in a decreasing direction, and then repeats the second step.

[0108] The fourth step is to wait for all optimizers to enter the maximum power point operation, and then proceed to the fifth step.

[0109] The fifth step is to control the inverter to be in the right disturbance mode. The right disturbance mode means that the inverter exits the normal MPPT state and periodically disturbs the corresponding input voltage Uin in the increasing direction until the first optimizer enters the direct-through mode. At this time, the inverter returns to the MPPT state again and enters the sixth step.

[0110] The sixth step is to recheck the working mode of each optimizer. If the number of optimizers entering the direct-through mode is greater than 1, and the difference in input voltages of the direct-through optimizers is greater than a preset difference, proceed to the seventh step; otherwise, the process ends.

[0111] In the seventh step, the inverter is controlled to be in the left disturbance mode until all optimizers enter the buck mode. At this time, the inverter returns to the MPPT state again and enters the fourth step.

[0112] Through the above process, the output current of each photovoltaic string can be adjusted to the maximum value of the input MPPT current of each internal optimizer; this embodiment detects the working mode of the optimizer, adjusts the operating state of the inverter, and then adjusts the output current of the optimizer, so as to achieve the optimal system conversion efficiency without affecting the MPPT tracking.

[0113] It is worth noting that after the above adjustment, the output voltage of the photovoltaic string will change, that is, the input voltage of the inverter will change, which may affect the efficiency of the corresponding BOOST in the inverter. Therefore, based on the above embodiment, this embodiment provides another maximum power point tracking method, which, after step S103, also includes Figure 6 (In Figure 2 As shown in the following example:

[0114] S104: The inverter obtains input voltages at each of its input terminals.

[0115] The input voltage Ui at the i-th input terminal of the inverter, that is, the output voltage of the i-th photovoltaic string, is calculated as follows:

[0116] Ui=Ui1×Di1+Ui2×Di2+Ui3×Di3+…+Uin×Din (7)

[0117] S105: The inverter determines whether each input voltage is greater than the voltage at the highest efficiency point of the inverter.

[0118] When a certain line of the inverter is at the highest efficiency point, its input voltage is the voltage at the highest efficiency point Um; the efficiency ηinvi of the i-th line of the inverter is also a function of the input voltage Ui. In this function: when Ui∈[0, Um], the efficiency ηinvi is monotonically increasing, and the higher the voltage, the higher the efficiency; and when Ui∈(Um, Umax], the efficiency ηinvi is monotonically decreasing, and the higher the voltage, the lower the efficiency.

[0119] For an input terminal with an input voltage less than or equal to the voltage at the highest efficiency point, it means that the input voltage will not cause the efficiency of the inverter side to decrease, and step S108 can be executed. However, if there is an input terminal with an input voltage greater than the voltage at the highest efficiency point, step S106 is executed.

[0120] S108. The inverter maintains the current operating state.

[0121] S106. The inverter determines the voltage at the point where the system efficiency is the highest.

[0122] Since the system efficiency ηsum is the product of the efficiency ηp of all PV strings and the efficiency ηinv of the inverter, we have:

[0123] ηsum=ηp×ηinv (8)

[0124] etap=(eta1×P1+eta2×P2+eta3×P3+…+etak×Pk) / (P1+P2+P3+…+Pk)(9)

[0125] Among them, P1~Pk are the power of each photovoltaic string, and k is the number of input terminals of the inverter.

[0126] ηinv=ηinv1+ηinv2+ηinv3+…+ηinvk (10)

[0127] At this time, the inverter can calculate the voltage at the highest point of system efficiency according to the efficiency curve. For each input end of the inverter, there are optimizer losses in the previous stage and BOOST losses in the subsequent stage. Therefore, the efficiency curve is composed of two functions: optimizer efficiency and inverter efficiency. When Ui∈[0, Um], the system efficiency ηsum is a monotonically increasing function. The higher the input voltage Ui, the higher the system efficiency ηsum; and when Ui∈(Um, Umax], the system efficiency ηsum is a monotonically increasing function multiplied by a monotonically decreasing function. At this time, the specific function characteristics are determined by these two functions. Therefore, a new optimal point voltage will appear, that is, the voltage Uy at the highest point of system efficiency.

[0128] S107, the inverter controls its own corresponding input voltage to be the voltage at the highest system efficiency point.

[0129] When the output voltage of a photovoltaic string is between (Um, Umax], it will change from the operation mode formed in step S103; since the corresponding input voltage of the inverter will be adjusted to the voltage Uy at the highest efficiency point of the system, each optimizer in the photovoltaic string will automatically adjust the output voltage multiplied by an additional duty cycle Dy, which is calculated as follows:

[0130] Dy=Uy / Ui (11)

[0131] Since the duty cycle Dy ranges from 0 to 1, Uy can only be less than the PV string voltage obtained in step S103; and after such conversion, the actual efficiency of the optimizer also needs to be superimposed with an ηy. At this time, the ηireal efficiency of the i-th PV string is:

[0132] ηireal=ηi×ηy (12)

[0133] ηy is a monotonically increasing function of Uy. The larger Uy is, the larger ηy is, and the range of ηy is 0 to 1. That is, through step S107, the efficiency of the corresponding BOOST of the inverter and the corresponding photovoltaic string will be changed at the same time, but the product of the two, that is, the overall efficiency of the path will be maximized.

[0134] By performing the above input voltage adjustment on each input terminal of the input voltage (Um, Umax], the overall efficiency of the system can be ensured to be optimal.

[0135] Another embodiment of the present application also provides a photovoltaic power generation system, which is as follows: Figure 1 As shown, it includes: an inverter 20, and photovoltaic strings 10 connected to each input end thereof; wherein:

[0136] The photovoltaic string 10 includes: at least two optimizers 101 with cascaded output ends, and a photovoltaic panel connected to the input end of each optimizer 101 .

[0137] The optimizer 101 is a step-down type optimizer.

[0138] The inverter 20 is in communication connection with each optimizer 101 so that the photovoltaic power generation system executes the maximum power point tracking method of the photovoltaic power generation system as described in any of the above embodiments.

[0139] The process and principle of the maximum power point tracking method can be found in the above embodiment, and will not be described in detail here.

[0140] In practical applications, the inverter 20 is Figure 7 As shown, it includes: a controller 203, an inverter circuit 202 and at least two boost circuits 201; wherein:

[0141] The input end of each boost circuit 201 serves as the corresponding input end of the inverter 20 ; the output end of each boost circuit 201 is connected to the DC bus of the inverter circuit 202 ; the AC side of the inverter circuit 202 serves as the output end of the inverter 20 .

[0142] The controller 203 is used to control the operation of each boost circuit 201 and each inverter circuit 202 , and is in communication connection with each optimizer 101 .

[0143] Other structures in the photovoltaic power generation system can refer to the prior art and will not be described here one by one. As long as the solution with the optimal system efficiency is achieved through the above-mentioned maximum power point tracking method, it is within the protection scope of this application.

[0144] The same and similar parts between the various embodiments in this specification can refer to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the system or system embodiment, since it is basically similar to the method embodiment, the description is relatively simple, and the relevant parts can refer to the partial description of the method embodiment. The system and system embodiments described above are only schematic, wherein the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or they may be distributed on multiple network units. Some or all of the modules can be selected according to actual needs to achieve the purpose of the scheme of this embodiment. Ordinary technicians in this field can understand and implement it without paying creative work.

[0145] Professionals may further appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the composition and steps of each example have been generally described in the above description according to function. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0146] With respect to the above description of the disclosed embodiments, the features described in the various embodiments in this specification may be replaced or combined with each other to enable professionals in the field to implement or use the present invention. Various modifications to these embodiments will be apparent to professionals in the field, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A maximum power point tracking method for a photovoltaic power generation system, characterized in that: In the photovoltaic power generation system, each input end of the inverter is connected to a corresponding photovoltaic string, and both ends of each photovoltaic string are formed by at least two step-down optimizers cascaded through their own output ends; the maximum power point tracking method includes: S101, the photovoltaic power generation system enters a power generation operation state; S102, the inverter and each optimizer are all operated in the maximum power point tracking MPPT state; S103, the inverter controls each photovoltaic string to operate in a state where the sum of the step-down ratios of each optimizer inside the inverter is maximum; Wherein, step S103 includes: For each photovoltaic string, the inverter controls its output current to be the maximum value of the input MPPT current of each internal optimizer; Wherein, step S103 includes executing, for each photovoltaic string, when each optimizer inside the photovoltaic string is operating in the buck mode: S201, the inverter exits the MPPT state, and periodically disturbs the corresponding input voltage in an increasing direction until the first optimizer enters the direct-through mode, and then restores the MPPT state.

2. The maximum power point tracking method of a photovoltaic power generation system according to claim 1, characterized in that: In step S103, for each photovoltaic string, before step S201, the following steps are further included: S301, the inverter determines whether each optimizer inside the inverter is operating in a buck mode; If at least one optimizer is not running in the buck mode, then steps S302 and S303 are performed in sequence; if all optimizers are running in the buck mode, then step S303 is performed; S302, the inverter exits the MPPT state, and periodically disturbs the corresponding input voltage in a decreasing direction until all optimizers operate in a buck mode, and the inverter resumes the MPPT state; S303, waiting for all optimizers to enter the MPPT state.

3. The maximum power point tracking method of a photovoltaic power generation system according to claim 2, characterized in that: In step S103, for each photovoltaic string, after executing step S201, the following steps are further included: S401, the inverter determines whether the number of optimizers entering the direct mode is greater than 1, and whether the difference between the input voltages of the corresponding optimizers is greater than a preset difference; If the number of optimizers entering the direct mode is greater than 1, and the difference between the input voltages of the corresponding optimizers is greater than the preset difference, return to step S302; If the number of optimizers entering the direct mode is 1, or the difference between the input voltages of the optimizers entering the direct mode is less than the preset difference, step S103 is completed.

4. The maximum power point tracking method of a photovoltaic power generation system according to any one of claims 1 to 3, characterized in that: In step S201, until the first optimizer enters the direct mode, the process includes: Until the duty cycle of the first optimizer is greater than the preset duty cycle; or, Until the difference between the input voltage and the output voltage of the first optimizer is less than the preset voltage difference; or, Until the difference between the input current and the output current of the first optimizer is less than the preset current difference.

5. The maximum power point tracking method of a photovoltaic power generation system according to any one of claims 1 to 3, characterized in that: After step S103, the method further includes: S104, the inverter obtains input voltages of each input terminal thereof; S105, the inverter determines whether each input voltage is greater than the voltage at the highest efficiency point of the inverter; If there is an input terminal whose input voltage is greater than the voltage at the highest efficiency point, executing step S106; S106, the inverter determines the voltage at the point of highest system efficiency; S107, the inverter controls its corresponding input voltage to be the voltage at the highest efficiency point of the system.

6. The maximum power point tracking method of a photovoltaic power generation system according to claim 5, characterized in that: Step S106 includes: The inverter calculates the voltage at the highest efficiency point of the system according to the efficiency curve.

7. The maximum power point tracking method of a photovoltaic power generation system according to claim 5, characterized in that: After step S105, the method further includes: For an input terminal whose input voltage is less than or equal to the voltage at the highest efficiency point, step S108 is executed; S108: The inverter maintains the current operating state.

8. A photovoltaic power generation system, characterized in that: include: An inverter, and, photovoltaic strings connected to each input terminal thereof; The photovoltaic string comprises: at least two optimizers with cascaded output ends, and photovoltaic panels connected to the input ends of each of the optimizers; The optimizer is a step-down optimizer; The inverter is communicatively connected with each of the optimizers so that the photovoltaic power generation system executes the maximum power point tracking method for the photovoltaic power generation system according to any one of claims 1 to 7.

9. The photovoltaic power generation system according to claim 8, characterized in that: The inverter comprises: a controller, an inverter circuit and at least two boost circuits; The input end of each of the boost circuits serves as the corresponding input end of the inverter; The output end of each of the boost circuits is connected to the DC bus of the inverter circuit; The AC side of the inverter circuit serves as the output end of the inverter; The controller is used to control the operation of each of the boost circuits and the inverter circuit, and is connected in communication with each of the optimizers.

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