Patent Publication Number: US-2022231511-A1

Title: Photovoltaic power system and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/247,379, filed on Dec. 9, 2020, which is a continuation of International Application No. PCT/CN2019/088343, filed on May 24, 2019. The International Application claims priority to Chinese Patent Application No. 201811354438.4, filed on Nov. 14, 2018. All of the afore-mentioned patent applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     This application relates to the field of photovoltaic power generation technologies, and in particular, to a photovoltaic power system and a control method thereof. 
     BACKGROUND 
     A photovoltaic power system mainly includes a solar cell module, a controller, and an inverter. The photovoltaic power system constitutes an important part of national electricity supply. At present, a process of connecting the photovoltaic power system to a power grid is: connecting the photovoltaic power system to the power grid after a direct current generated by the solar cell module is converted by a grid-tie inverter into an alternating current satisfying a requirement. Compared with a central inverter, a photovoltaic string inverter features higher efficiency and better flexibility. Therefore, the photovoltaic string inverter is more frequently selected to connect the photovoltaic power system to the power grid. 
     In the prior art, a photovoltaic virtual synchronous generator (PV-VSG) uses a photovoltaic power unit control system to implement related functions such as inertia and voltage/reactive power adjustment, by retaining an active-power reserve or configuring an energy storage element. The energy storage element needs to be added to the photovoltaic string inverter. However, this increases costs of the photovoltaic power system and requires additional installation space. Therefore, currently, it is an important research focus to use an active-power reserve to control a PV-VSG without a need to add an energy storage element to a photovoltaic string inverter. 
     The active-power reserve is an important indicator of a photovoltaic power system. Precision of the active-power reserve is highly prone to impact of changes of external factors such as illumination intensity and ambient temperature. In the prior art, controlling a PV-VSG by using an active-power reserve of the photovoltaic power system is implemented mainly in either of two control manners: an active-power reserve based on variable power point tracking, or spinning reserve capacity tracking based on a maximum power point. The foregoing two control manners cannot avoid impact of a change of an external factor on the active-power reserve, and also quite easily cause fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter. This is unfavorable to control on the virtual synchronous generator of the photovoltaic string inverter, and affects a lifespan of the photovoltaic string inverter. 
     SUMMARY 
     In view of this, embodiments of this application provide a photovoltaic power system and a control method thereof, to implement control on a virtual synchronous generator of a photovoltaic string inverter, and prolong a lifespan of the photovoltaic string inverter, without a need to add an energy storage element. 
     The embodiments of this application provide the following technical solutions. 
     A first aspect of the embodiments of this application provides a photovoltaic power system, including photovoltaic strings, a controller, a direct current-to-alternating current (DC/AC) inverter circuit, and N DC/DC converter circuits located at a previous stage of the DC/AC inverter circuit, where each DC/DC converter circuit is connected to at least one photovoltaic string, and a value of N is a positive integer greater than or equal to 2; the controller is connected to all of the DC/AC inverter circuit and the N DC/DC converter circuits, and is configured to: perform maximum power point tracking (MPPT) control on n DC/DC converter circuits, determine a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state, and control, based on the first control parameter and an active-power reserve parameter, (N−n) DC/DC converter circuits to operate in a constant power generation (CPG) mode, where a value of n is a positive integer greater than or equal to 1 and less than or equal to N−1; and the photovoltaic power system is connected to a power grid through an output end of the DC/AC inverter circuit. 
     According to an embodiment, the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit are randomly divided into the n DC/DC converter circuits and the (N−n) DC/DC converter circuits, where MPPT control is performed on the n DC/DC converter circuits, and CPG control is performed on the (N−n) DC/DC converter circuits. The N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit are controlled in the two different control manners, to implement a fast and accurate power reserve or limit of a photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter, thereby prolonging a lifespan of the photovoltaic string inverter. This further implements control on a virtual synchronous generator of the photovoltaic string inverter without a need to add an energy storage element. 
     In a possible design, the controller includes an MPPT controller, configured to: perform MPPT control on the n DC/DC converter circuits; determine the first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating apparatus; and obtain a second control parameter based on the first control parameter and the active-power reserve parameter. The controller further includes a CPG controller, which may be configured to perform CPG control on the (N−n) DC/DC converter circuits based on the second control parameter, so that the (N−n) DC/DC converter circuits operate in the CPG mode. 
     According to an embodiment, the controller includes the MPPT controller and the CPG controller; the MPPT controller is configured to perform MPPT control on the n DC/DC converter circuits; and the obtained second control parameter is used as a reference for the CPG controller to perform CPG control on the (N−n) DC/DC converter circuits. Different control may be performed on the DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter, thereby prolonging a lifespan of the photovoltaic string inverter. 
     In an embodiment, the controller includes a VSG controller, configured to calculate a VSG power parameter based on a grid connection parameter for the power grid and a VSG control algorithm. The controller further includes an MPPT controller, configured to: perform MPPT control on the n DC/DC converter circuits; determine the first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state; and obtain a second control parameter based on the first control parameter, the VSG power parameter, and the active-power reserve parameter. The controller further includes a CPG controller, configured to perform CPG control on the (N−n) DC/DC converter circuits based on the second control parameter, so that the (N−n) DC/DC converter circuits operate in the CPG mode. 
     It should be noted that the VSG controller is configured to calculate the VSG power parameter based on an actually detected current power grid frequency, a rated power grid frequency, and any virtual inertia of a constant virtual inertia, an adaptive zero virtual inertia, and an adaptive negative virtual inertia by using the VSG control algorithm, where the constant virtual inertia is a constant virtual inertia time constant in the VSG control algorithm, the adaptive zero virtual inertia is an adaptive zero virtual inertia time constant in the VSG control algorithm, and the adaptive negative virtual inertia is an adaptive negative virtual inertia time constant in the VSG control algorithm. 
     According to an embodiment, the controller includes the MPPT controller and the CPG controller; the MPPT controller is configured to perform MPPT control on the n DC/DC converter circuits; and the obtained second control parameter is used as a reference for the constant power generation CPG controller to perform CPG control on the (N−n) DC/DC converter circuits. In this way, control on the photovoltaic string inverter is implemented. 
     It should be noted that based on the foregoing possible designs, the MPPT controller may be constituted in a plurality of structures. In an embodiment, the MPPT controller includes n control circuits, a first arithmetic unit, and a second arithmetic unit, or alternatively, the MPPT controller includes n control circuits, a first arithmetic unit, and a third arithmetic unit, where each control circuit includes an MPPT processing unit and a multiplier. In an embodiment, the MPPT controller includes n MPPT processing units, a first arithmetic unit, and a fourth arithmetic unit. By using the plurality of structures disclosed in this embodiment of this application, the MPPT controller performs MPPT control on the n DC/DC converter circuits; determines the first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating apparatus; and obtains the second control parameter based on the first control parameter and the active-power reserve parameter. For a specific implementation process, refer to related descriptions in this specification. 
     A second aspect of the embodiments of this application provides a photovoltaic power system control method, which is applicable to the photovoltaic power system provided in the first aspect of the embodiments of this application. The control method includes: 
     performing MPPT control on n DC/DC converter circuits, and determining a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state, where a value of n is a positive integer greater than or equal to 1 and less than or equal to N−1; and 
     controlling, based on the first control parameter and an active-power reserve parameter, (N−n) DC/DC converter circuits to operate in a constant power generation CPG mode. 
     According to the solution, N DC/DC converter circuits are controlled in the two different control manners, to implement a fast and accurate power reserve or limit of a photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. This further implements control on the virtual synchronous generator of the photovoltaic string inverter without a need to add the energy storage element. 
     In an embodiment, the controlling, based on the first control parameter and an active-power reserve parameter, (N−n) DC/DC converter circuits to operate in a CPG mode includes: 
     obtaining a second control parameter based on the first control parameter and the active-power reserve parameter; and 
     controlling, based on the second control parameter, the (N−n) DC/DC converter circuits to operate in the CPG mode. 
     In an embodiment, the controlling, based on the first control parameter and an active-power reserve parameter, (N−n) DC/DC converter circuits to operate in the CPG mode includes: obtaining a VSG power parameter based on a grid connection parameter for a power grid and a VSG control algorithm; obtaining a second control parameter based on the first control parameter, the VSG power parameter, and the active-power reserve parameter; and controlling, based on the second control parameter, the (N−n) DC/DC converter circuits to operate in the CPG mode. 
     In an embodiment, the obtaining a VSG power parameter based on a grid connection parameter for a power grid and a VSG control algorithm includes: 
     calculating the VSG power parameter based on an actually detected current power grid frequency, a rated power grid frequency, and any one of a constant virtual inertia, an adaptive zero virtual inertia, and an adaptive negative virtual inertia by using the VSG control algorithm, where 
     the constant virtual inertia is a constant virtual inertia time constant in the VSG control algorithm, the adaptive zero virtual inertia is an adaptive zero virtual inertia time constant in the VSG control algorithm, and the adaptive negative virtual inertia is an adaptive negative virtual inertia time constant in the VSG control algorithm. 
     A third aspect of the embodiments of this application provides a controller, including a memory and a processor that communicates with the memory, where 
     the memory is configured to store program code for controlling a photovoltaic string inverter; and 
     the processor is configured to invoke the program code, in the memory, for controlling the photovoltaic string inverter, to perform a photovoltaic string inverter control method provided in the second aspect of the embodiments of this application. 
     A fourth aspect of the embodiments of this application provides a nonvolatile computer-readable storage medium, configured to store a computer program, where the computer program includes an instruction used to perform the method in any possible design of the second aspect of the embodiments of this application. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic structural diagram of a photovoltaic power system according to an embodiment of this application; 
         FIG. 2  is a schematic structural diagram of a controller according to an embodiment of this application; 
         FIG. 3  is a schematic diagram of an execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 4  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 5  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 6  is a schematic structural diagram of another controller according to an embodiment of this application; 
         FIG. 7  is a schematic diagram of an execution principle of executing a VSG control algorithm by a VSG controller according to an embodiment of this application; 
         FIG. 8  is a schematic diagram of an execution principle of executing a VSG control algorithm by a VSG controller according to an embodiment of this application; 
         FIG. 9  is a schematic diagram of an execution principle of executing a VSG control algorithm by a VSG controller according to an embodiment of this application; 
         FIG. 10  is a schematic diagram of an execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 11  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 12  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 13  is a schematic flowchart of a photovoltaic power system control method according to an embodiment of this application; 
         FIG. 14  is a schematic flowchart of a control method performed by a controller according to an embodiment of this application; 
         FIG. 15  is a schematic flowchart of an execution method for executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 16  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 17  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 18  is a schematic flowchart of another control method performed by controllers according to an embodiment of this application; 
         FIG. 19  is a schematic flowchart of an execution method for executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 20  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; 
         FIG. 21  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application; and 
         FIG. 22  is a schematic structural diagram of a controller according to an embodiment of this application. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes the technical solutions in the embodiments of this application with reference to the accompanying drawings in the embodiments of this application. In description of this application, “I” means “or” unless otherwise specified. For example, A/B may represent A or B. In this specification, “and/or” describes only an association relationship for describing associated objects and represents that three relationships may exist. For example, A and/or B may represent the following three cases: Only A exists, both A and B exist, and only B exists. In addition, in the descriptions of this application, “a plurality of” means two or more than two. In addition, for clear description of the technical solutions of the embodiments of this application, in the embodiments of this application, terms such as “first” and “second” are used to distinguish between same or similar objects having a basically same function and effect. A person skilled in the art can understand that the terms such as “first” and “second” are not used to limit a quantity and an execution sequence, and that the terms such as “first” and “second” are unnecessarily limited to be different. 
     Moreover, terms “include” and “have” in the embodiments of this application, the claims, and the accompanying drawings are inclusive. For example, a process, a method, a system, a product, or a device including a series of operations or units is not limited to the listed operations or units, and may further include operations or units that are not listed. 
     A photovoltaic power system is a power generation system including devices such as photovoltaic modules, an inverter, a cable, and a transformer, and can convert solar energy into usable electrical energy and output the electrical energy to a power grid or an off-grid system. 
     The photovoltaic modules are direct current power supplies formed after solar cells are connected in series and in parallel and then are packaged. 
     In the embodiments of this application, the inverter is a photovoltaic string inverter. A direct current side of the photovoltaic string inverter may be connected to a plurality of photovoltaic strings that are not in a parallel connection. The photovoltaic string inverter may use two levels of power conversion: conversion from a direct current to a direct current and conversion from a direct current to an alternating current. 
     A photovoltaic string is a direct current power supply formed through end-to-end series connection of positive and negative electrodes of a plurality of photovoltaic modules. 
       FIG. 1  is a schematic structural diagram of a photovoltaic power system according to an embodiment of this application. The photovoltaic power system includes a photovoltaic string inverter, photovoltaic strings  11 , and a controller  12 . The photovoltaic string inverter mainly includes a DC/AC inverter circuit  101  and a total of N DC/DC converter circuits, that is, a DC/DC converter circuit  1021  to a DC/DC converter circuit  102 N. A value of N is a positive integer greater than or equal to 2, that is, N≥2. 
     The N DC/DC converter circuits are located at a previous stage of the DC/AC inverter circuit  101 . Each DC/DC converter circuit is connected to at least one photovoltaic string  11 . 
     In an embodiment, a connection relationship between a DC/DC converter circuit and a photovoltaic string is as follows: 
     A positive electrode of an input port of each DC/DC converter circuit is connected to a positive electrode of a photovoltaic string that is in a same group as the DC/DC converter circuit, and a negative electrode of the input port of the DC/DC converter circuit is connected to a negative electrode of the photovoltaic string that is in the same group as the DC/DC converter circuit. 
     Each DC/DC converter circuit and a photovoltaic string connected to the DC/DC converter circuit are considered as being in a same group. Photovoltaic strings in a same group are in a parallel connection relationship. 
     In an embodiment, a connection relationship between the DC/AC inverter circuit  101  and the N DC/DC converter circuits that are located at the previous stage of the DC/AC inverter circuit  101  is as follows: 
     A positive electrode of an output port of each DC/DC converter circuit is connected in parallel to a positive electrode of an input port on a direct current side of the DC/AC inverter circuit  101 , and a negative electrode of the output port of the DC/DC converter circuit is connected in parallel to a negative electrode of the input port on the direct current side of the DC/AC inverter circuit  101 . 
     It should be noted that the string inverter may be applied to photovoltaic power generation scenarios, such as an application scenario of a large-sized photovoltaic station, application scenarios of small and medium-sized distributed power stations, and an application scenario of a residential photovoltaic power system. 
     An alternating current cable outlet terminal of the DC/AC inverter circuit  101  is used as an output port of the string inverter, and is connected to a power grid through a cable. Specifically, the alternating current cable outlet terminal may be connected to a transformer, or may directly be connected to a single-phase or three-phase alternating current power grid. 
     The controller  13  is connected to the DC/AC inverter circuit  101  and the N DC/DC converter circuits. 
     In an embodiment of this application, the controller  13  is configured to: perform maximum power point tracking (MPPT) control on n DC/DC converter circuits, determine a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state, and control, based on the first control parameter and an active-power reserve parameter, (N−n) DC/DC converter circuits to operate in a constant power generation (CPG) mode, where a value of n is a positive integer greater than or equal to 1 and less than or equal to N−1, that is, 1≤n≤N−1. 
     In an embodiment, the N DC/DC converter circuits are randomly divided into the n DC/DC converter circuits and the (N−n) DC/DC converter circuits in advance, provided that N≥2 and 1≤n≤N−1 are satisfied. 
     In an embodiment, the controller  13  is further configured to: collect parameters such as an input voltage and an input current that are of each DC/DC converter circuit, a direct current bus voltage, and an alternating current power grid voltage and an alternating current output current that are of the DC/AC inverter circuit in real time; provide a pulse-width modulation (PWM) control signal of each DC/DC converter circuit in real time according to a control policy of the DC/DC converter circuit; and provide a PWM control signal of the DC/AC inverter circuit in real time according to a control policy of the DC/AC inverter circuit. 
     In the photovoltaic power system disclosed in an embodiment of this application, master control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, that is, MPPT control is performed, so that the n DC/DC converter circuits operate in an MPPT mode; and slave control is performed on the (N−n) DC/DC converter circuits, that is, CPG control is performed, so that the (N−n) DC/DC converter circuits operate in a CPG mode. In this embodiment of this application, master-slave control is implemented on the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, to reduce impact of illumination intensity and ambient temperature on an active-power reserve of the photovoltaic string inverter, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     In the photovoltaic power system shown in  FIG. 1 , there may be a plurality of control manners in which the controller  13  performs MPPT control on the n DC/DC converter circuits in the N DC/DC converter circuits and performs CPG control on the (N−n) DC/DC converter circuits in the N DC/DC converter circuits. This embodiment of this application provides detailed descriptions by using the following embodiments. 
       FIG. 2  is a schematic structural diagram of a controller  13  according to an embodiment of this application. The controller  13  includes an MPPT controller  201  and a CPG controller  202 . 
     The MPPT controller  201  is configured to: perform MPPT control on n DC/DC converter circuits; determine a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating apparatus; and obtain a second control parameter based on the first control parameter and an active-power reserve parameter. 
     The n DC/DC converter circuits are master-controlled DC/DC converter circuits. 
     The CPG controller  202  is configured to perform CPG control on (N−n) DC/DC converter circuits based on the second control parameter, so that the (N−n) DC/DC converter circuits operate in a constant power generation CPG mode. 
     The (N−n) DC/DC converter circuits are slave-controlled DC/DC converter circuits. 
     In specific implementation, there are a plurality of manners in which the MPPT controller  201  performs MPPT control on the n DC/DC converter circuits. 
       FIG. 3  is a schematic diagram of an execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application. The MPPT controller  201  includes a total of n control circuits, that is, a control circuit  3011  to a control circuit  301   n , a first arithmetic unit  302 , and a second arithmetic unit  303 . 
     Each control circuit includes an MPPT processing unit and a multiplier. 
     The MPPT processing unit is configured to: detect a first input voltage and a first input current of a corresponding DC/DC converter circuit; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     The multiplier is connected to the maximum power point tracking MPPT processing unit, and is configured to calculate a product of the first input voltage and the second input current to obtain a maximum output active-power parameter. 
     The first arithmetic unit  302  connected to the n control circuits is configured to: determine a maximum output active-power parameter output by each control circuit; perform summation and averaging operations on n maximum output active-power parameters; and use an obtained maximum output active-power average value as a first control parameter. 
     The second arithmetic unit  303  is configured to: determine a power parameter based on an active-power reserve parameter and the first control parameter; and use the power parameter as a second control parameter. 
     Correspondingly, 
     a CPG controller is configured to control, based on the second control parameter, (N−n) DC/DC converter circuits to operate in a power adjustment-based constant power generation CPG mode. 
     The first control circuit and the n th  control circuit shown in  FIG. 3  are used as an example for description, and a same processing manner is also applied to other middle control circuits. 
     For the first control circuit, the MPPT processing unit is configured to: detect a first input voltage v pv1  and a first input current i pv1  of a first master-controlled DC/DC converter, and obtain an input current i MPPT1  at a maximum power point through MPPT control; and 
     the multiplier is configured to perform a multiplication operation on the first input voltage v pv1  of the first master-controlled DC/DC converter and the second input current i MPPT1  at the maximum power point, to obtain a maximum output active-power parameter P MPPT1 . 
     For the n th  control circuit, the MPPT processing unit is configured to: detect a first input voltage v pvn  and a first input current i pvn  of the first master-controlled DC/DC converter, and obtain an input current i MPPTn  at a maximum power point through MPPT control; and the multiplier is configured to perform a multiplication operation on the first input voltage v pvn  of the first master-controlled DC/DC converter and the second input current i MPPTn  at the maximum power point, to obtain a maximum output active-power parameter P MPPTn . 
     The first arithmetic unit  302  is configured to: perform summation on obtained maximum output active-power parameters P (MPPT1-MPPTn)  of n master-controlled DC/DC converters; perform an averaging operation on a value obtained through summation; and use an obtained maximum output active-power average value as an active-power reference parameter P ref1  of (N−n) slave-controlled DC/DC converters. 
     The second arithmetic unit  303  is configured to: calculate the active-power reference parameter P ref1  and an active-power reserve parameter ΔP according to formula (1); and use an obtained power parameter as a second control parameter P ref , where 
         P   ref   =P   ref1   −ΔP   (1)
 
     The active-power reserve parameter ΔP is an active-power reserve/limit parameter ΔP. 
     The power parameter P ref  calculated according to formula (1) is used as a power parameter for CPG control on the (N−n) DC/DC converter circuits. 
     Correspondingly, the CPG controller  202  shown in  FIG. 2  is configured to control, based on the power parameter P ref  for CPG control obtained by the MPPT processing unit, the (N−n) DC/DC converter circuits to operate in a power adjustment-based CPG (P-CPG) mode, to obtain a PWM signal corresponding to each DC/DC converter circuit. 
     The PWM control signal is used as a modulation signal for driving an action of a switching transistor. 
     In an embodiment, the CPG controller  202  compares the obtained power parameter P ref  with output active-power P pv_m  of an m th  slave-controlled DC/DC converter, and a proportional integral PI controller obtains a PWM control signal of the m th  slave-controlled DC/DC converter based on an obtained power comparison result, where m=n+1, n+2, . . . , N. 
     It should be noted that, a control manner in which the (N−n) DC/DC converter circuits are controlled to operate in a power adjustment-based CPG (P-CPG) mode may be a power control manner such as proportional integral control, direct power control, and model prediction control. Details are not described in this embodiment of this application. 
     A photovoltaic string inverter disclosed in this embodiment of this application does not need to be provided with a solar radiant intensity detection apparatus. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on n DC/DC converter circuits in N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
       FIG. 4  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller  201  according to an embodiment of this application. The MPPT controller  201  includes a total of n control circuits, that is, a control circuit  4011  to a control circuit  401   n , a first arithmetic unit  402 , and a third arithmetic unit  403 . 
     Each control circuit includes an MPPT processing unit and a multiplier. 
     The MPPT processing unit is configured to: detect a first input voltage and a first input current of a corresponding DC/DC converter circuit; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     The second input current at the maximum power point is obtained based on the MPPT control algorithm. 
     The multiplier is connected to the MPPT processing unit, and is configured to calculate a product of the first input voltage and the second input current to obtain a maximum output active-power parameter. 
     The first arithmetic unit  402  connected to the n control circuits is configured to determine, as a first control parameter, an average value of maximum output active-power output by the control circuits. 
     The third arithmetic unit  403  is configured to: determine a current parameter based on an active-power reserve parameter and the first control parameter; and use the current parameter as a second control parameter. 
     Correspondingly, a CPG controller is configured to control, based on the second control parameter, (N−n) DC/DC converter circuits to operate in a current adjustment-based constant power generation CPG mode. 
     The first control circuit and the n th  control circuit shown in  FIG. 4  are used as an example for description, and a same processing manner is also applied to other middle control circuits. 
     For the first control circuit, the MPPT processing unit is configured to: detect a first input voltage v pv1  and a first input current i pv1  of a first master-controlled DC/DC converter, and obtain an input current i MPPT1  at a maximum power point through MPPT control; and the multiplier is configured to perform a multiplication operation on the first input voltage v pv1  of the first master-controlled DC/DC converter and the second input current i MPPT1  at the maximum power point, to obtain maximum output active-power P MPPT1 . 
     For the n th  control circuit, the MPPT processing unit is configured to: detect a first input voltage v pvn  and a first input current i pvn  of the first master-controlled DC/DC converter, and obtain an input current i MPPTn  at a maximum power point through MPPT control; and the multiplier is configured to perform a multiplication operation on the first input voltage v pvn  of the first master-controlled DC/DC converter and the second input current i MPPTn  at the maximum power point, to obtain a maximum output active-power parameter P MPPTn . 
     The first arithmetic unit  402  is configured to: perform summation on obtained maximum output active-power parameters P (MPPT1-MPPTn)  of n master-controlled DC/DC converters; perform an averaging operation on a value obtained through summation; and use an obtained maximum output active-power average value as an active-power reference P ref1  of (N−n) slave-controlled DC/DC converters. 
     The third arithmetic unit  403  is configured to: calculate the active-power reference P ref1  and an active-power reserve parameter ΔP according to formula (1), to obtain a power parameter P ref ; process the power parameter P ref  according to formula (2); and use an obtained current parameter I ref  as a second control parameter, where 
         I   ref   =P   ref   /v   pv   (2)
 
     v pv  is an input voltage of a slave-controlled DC/DC converter, that is, a direct-current voltage of a photovoltaic string. 
     The current parameter I ref  calculated according to formula (2) is used as a current instruction for CPG control on the (N−n) DC/DC converter circuits. 
     Correspondingly, the CPG controller  202  shown in  FIG. 2  is configured to control, based on the current instruction I ref  for CPG control obtained by the MPPT processing unit, the (N−n) DC/DC converter circuits to operate in a current adjustment-based CPG (I-CPG) mode, to obtain a PWM signal corresponding to each DC/DC converter circuit. 
     The PWM control signal is used as a modulation signal for driving an action of a switching transistor. 
     In an embodiment, the CPG controller  202  compares the obtained current instruction I ref  with an input current I pv_m  of an m th  slave-controlled DC/DC converter, and a proportional integral PI controller obtains a PWM control signal of the m th  slave-controlled DC/DC converter based on an obtained current comparison result, where m=n+1, n+2, . . . , N. 
     It should be noted that, a control manner in which the (N−n) DC/DC converter circuits are controlled to operate in a power adjustment-based CPG (I-CPG) mode may be a power control manner such as proportional integral control, direct power control, and model prediction control. Details are not described in this embodiment of this application. 
     A photovoltaic string inverter disclosed in an embodiment of this application does not need to be provided with a solar radiant intensity detection apparatus. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on n DC/DC converter circuits in N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
       FIG. 5  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application. The MPPT controller  201  includes a total of n MPPT processing units, that is, an MPPT processing unit  5011  to an MPPT processing unit  501   n , a first arithmetic unit  502 , and a fourth arithmetic unit  503 . 
     Each MPPT processing unit is configured to: detect a first input voltage and a first input current of a corresponding DC/DC converter circuit; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input voltage at a maximum power point of the DC/DC converter circuit by using a maximum power point tracking MPPT control algorithm. 
     The first arithmetic unit  502  connected to the n MPPT processing units is configured to determine a second input voltage output by each MPPT processing unit; perform summation and averaging operations on n second input voltages; and use an obtained second input voltage average value as a first control parameter. 
     The fourth arithmetic unit  503  is configured to: perform calculation based on an active-power reserve parameter and the first control parameter; and use an obtained voltage parameter as a second control parameter. 
     Correspondingly, a CPG controller is configured to control, based on the second control parameter, (N−n) DC/DC converter circuits to operate in a voltage adjustment-based constant power generation CPG mode. 
     The MPPT processing unit  5011  and the MPPT processing unit  501   n  shown in  FIG. 5  are used as an example for description, and a same processing manner is also applied to other MPPT processing units. 
     For the MPPT processing unit  5011 , the MPPT processing unit  5011  is configured to: detect a first input voltage v pv1  and a first input current i pv1  of a first master-controlled DC/DC converter, and obtain an input voltage v MPPT1  at a maximum power point through MPPT control. 
     For the MPPT processing unit  501   n , the MPPT processing unit  501   n  is configured to: detect a first input voltage v pvn  and a first input current i pvn  of an n th  master-controlled DC/DC converter, and obtain an input voltage v MPPTn  at a maximum power point through MPPT control. 
     The first arithmetic unit  502  is configured to: perform summation on obtained input voltages I (MPPT1-MPPTn)  of n master-controlled DC/DC converters; perform an averaging operation on a value obtained through summation; and use an obtained input voltage average value as a voltage reference V ref1  of (N−n) slave-controlled DC/DC converters. 
     The fourth arithmetic unit  503  is configured to: process an active-power reserve parameter ΔP and the voltage reference V ref1  according to formula (3); and use an obtained voltage parameter V ref  as a second control parameter, where 
         V   ref   =V   ref1   +ΔP/V   ref1   (3)
 
     The voltage parameter V ref  calculated according to formula (3) is used as a voltage instruction for CPG control on the (N−n) DC/DC converter circuits. 
     Correspondingly, the CPG controller  202  shown in  FIG. 2  is configured to control, based on the voltage parameter V ref  for CPG control obtained by the MPPT processing unit, the (N−n) DC/DC converter circuits to operate in a voltage adjustment-based CPG (V-CPG) mode, to obtain a PWM signal corresponding to each DC/DC converter circuit. 
     The PWM control signal is used as a modulation signal for driving an action of a switching transistor. 
     In an embodiment, the CPG controller  202  compares the obtained voltage parameter V ref  with an input current v pv_m  of an m th  slave-controlled DC/DC converter, and a proportional integral PI controller obtains a PWM m  control signal of the m th  slave-controlled DC/DC converter based on an obtained voltage comparison result, where m=n+1, n+2, . . . , N. 
     A photovoltaic string inverter disclosed in an embodiment of this application does not need to be provided with a solar radiant intensity detection apparatus. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on n DC/DC converter circuits in N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     As disclosed in an embodiment of this application, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits. In this way, based on master-slave control on the N DC/DC converters located at the previous stage of the DC/AC inverter circuit, PV-VSG control by using an active-power reserve can be implemented; problems of deterioration in PV-VSG output performance and even system instability that are caused by illumination intensity and ambient temperature changes can be resolved; and an inertia support capability of a photovoltaic power system can be enhanced, and fluctuation of the direct current bus voltage and the output power that are of the string inverter in the power control process can be eliminated. 
     Based on the photovoltaic string inverter shown in  FIG. 1  in the foregoing embodiment of this application, in the process in which PV-VSG control by using the active-power reserve is implemented based on master-slave control on the N DC/DC converters located at the previous stage of the DC/AC inverter circuit, there may be a plurality of control manners in which a controller  103  performs MPPT control on the n DC/DC converter circuits in the N DC/DC converter circuits and performs CPG control on the (N−n) DC/DC converter circuits in the N DC/DC converter circuits. This embodiment of this application provides detailed descriptions by using the following embodiments. 
       FIG. 6  is a schematic structural diagram of another controller  13  according to an embodiment of this application. The controller  13  includes a VSG controller  601 , an MPPT controller  602 , and a CPG controller  603 . 
     The VSG controller  601  is configured to calculate a VSG power parameter based on a grid connection parameter for a power grid and a VSG control algorithm. 
     The MPPT controller  602  is configured to: perform MPPT control on n DC/DC converter circuits; determine a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state; and obtain a second control parameter based on the first control parameter, the VSG power parameter, and an active-power reserve parameter. 
     The n DC/DC converter circuits are master-controlled DC/DC converter circuits. 
     The CPG controller  603  is configured to perform CPG control on (N−n) DC/DC converter circuits based on the second control parameter, so that the (N−n) DC/DC converter circuits operate in a constant power generation CPG mode. 
     The (N−n) DC/DC converter circuits are slave-controlled DC/DC converter circuits. 
     In an embodiment, there are a plurality of manners in which the VSG controller  601  obtains the VSG power parameter based on the power grid parameter and the VSG control algorithm. 
       FIG. 7  is a schematic diagram of an execution principle of executing a VSG control algorithm by a VSG controller according to an embodiment of this application. 
     The VSG controller  601  is configured to calculate a VSG power parameter based on an actually detected current power grid frequency, a rated power grid frequency, and a constant virtual inertia by using the virtual synchronous generator VSG control algorithm. 
     The constant virtual inertia is a constant virtual inertia time constant in the VSG control algorithm. 
     In an embodiment, the VSG controller  601  performs VSG control calculation according to formula (4) based on the power grid frequency f pll  actually detected by a phase-locked loop, the rated power grid frequency fief, and the constant virtual inertia; and uses an obtained VSG power parameter P VSG  as the VSG power parameter. The VSG power parameter P VSG1  is a parameter having the constant virtual inertia, where 
     
       
         
           
             
               
                 
                   
                     P 
                     VSG 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
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                             f 
                           
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                     ⁢ 
                     
                       
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                           ref 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     f ref  is the rated power grid frequency, P N  is rated power of a string inverter, K f  is a primary frequency modulation coefficient, and T j1  is a constant inertia time constant. 
     In the process of performing VSG control calculation according to formula (4), a VSG power reference P VSG1  may be obtained; and the VSG power reference P VSG1  is limited to be within a preset range, where the range is a range [−0.2 P N , 0.1 P N ]. By using the VSG power parameter P VSG  obtained by dividing the VSG power reference P VSG1  by (N−n), formula (4) may also be expressed as P VSG −P VSG1 /(N−n). 
       FIG. 8  is a schematic diagram of another execution principle of executing a VSG control algorithm by a VSG controller according to an embodiment of this application. 
     The VSG controller  601  is configured to calculate a VSG power parameter based on an actually detected current power grid frequency, a rated power grid frequency, and an adaptive zero virtual inertia by using the VSG control algorithm. 
     The adaptive zero virtual inertia is an adaptive zero virtual inertia time constant in the VSG control algorithm. 
     In an embodiment, the VSG controller  601  performs VSG control calculation according to formula (5) based on the power grid frequency f pll  actually detected by a phase-locked loop, the rated power grid frequency fief, and the adaptive zero virtual inertia; and uses an obtained VSG power parameter P VSG  as the VSG power parameter. The VSG power parameter P VSG  is a parameter having the adaptive zero virtual inertia, where 
     
       
         
           
             
               
                 
                   
                     P 
                     VSG 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
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     f ref  is the rated power grid frequency, P N  is rated power of a string inverter, K f  is a primary frequency modulation coefficient, and T j2  is an adaptive inertia time constant. T j2  satisfies formula (6), where 
     
       
         
           
             
               
                 
                   
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     Δf=f ref −f pll , T j_min  is an allowed minimum inertia time constant, and T j_max  is an allowed maximum inertia time constant. 
     In the process of performing VSG control calculation according to formula (5), a VSG power reference P VSG1  may be obtained; and the VSG power reference P VSG1  is limited to be within a preset range, where the range is a range [−0.2 P N , 0.1 P N ]. By using the VSG power parameter P VSG  obtained by dividing the VSG power reference P VSG1  by (N−n), formula (5) may also be expressed as P VSG =P VSG1 /(N−n). 
       FIG. 9  is a schematic diagram of another execution principle of executing a VSG control algorithm by a VSG controller according to an embodiment of this application. 
     The VSG controller  601  is configured to calculate a VSG power parameter based on an actually detected current power grid frequency, a rated power grid frequency, and an adaptive negative virtual inertia by using the VSG control algorithm. 
     The adaptive negative virtual inertia is an adaptive negative virtual inertia time constant in the VSG control algorithm. 
     In an embodiment, the VSG controller  601  performs VSG control calculation according to formula (7) based on the power grid frequency f pll  actually detected by a phase-locked loop, the rated power grid frequency f ref , and the adaptive negative virtual inertia; and uses an obtained VSG power parameter P VSG  as the VSG power parameter. The VSG power parameter P VSG  is a parameter having the adaptive zero virtual inertia, where 
     
       
         
           
             
               
                 
                   
                     P 
                     VSG 
                   
                   = 
                   
                     
                       [ 
                       
                         
                           
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                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     f ref  is the rated power grid frequency, P N  is rated power of a string inverter, K f  is a primary frequency modulation coefficient, and T j3  is an adaptive inertia time constant. T j3  satisfies formula (8), where 
     
       
         
           
             
               
                 
                   
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                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Δf=f ref −f pll , T j_min  is an allowed minimum inertia time constant, and T j_max  is an allowed maximum inertia time constant. 
     In the process of performing VSG control calculation according to formula (7), a VSG power reference P VSG1  may be obtained; and the VSG power reference P VSG1  is limited to be within a preset range, where the range is a range [−0.2 P N , 0.1 P N ]. By using the VSG power parameter P VSG  obtained by dividing the VSG power reference P VSG1  by (N−n), formula (7) may also be expressed as P VSG =P VSG1 /(N−n). 
     In an embodiment of this application, based on the VSG power parameters generated in  FIG. 7  to  FIG. 9 , there are a plurality of manners in which the MPPT controller  602  performs MPPT control on the n DC/DC converter circuits. A specific principle of generating a second control parameter by the MPPT controller  602  may be described with reference to content about generating the second control parameter by the MPPT controller  201  in  FIG. 3 ,  FIG. 4 , and  FIG. 5 . 
     With reference to the embodiment as shown in  FIG. 3 ,  FIG. 10  is a schematic diagram of an execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application. The MPPT controller  602  includes a total of n control circuits, that is, a control circuit  3011  to a control circuit  301   n , a first arithmetic unit  302 , and a second arithmetic unit  303 . 
     Each control circuit includes a maximum power point tracking MPPT processing unit and a multiplier. 
     The MPPT processing unit is configured to: detect a first input voltage and a first input current of a corresponding DC/DC converter circuit; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     The multiplier is connected to the MPPT processing unit, and is configured to calculate a product of the first input voltage and the second input current to obtain a maximum output active-power parameter. 
     The first arithmetic unit  302  connected to the n control circuits is configured to: determine a maximum output active-power parameter output by each control circuit; perform summation and averaging operations on n maximum output active-power parameters; and use an obtained maximum output active-power average value as a first control parameter. 
     Parameters used for performing calculation by the second arithmetic unit shown in  FIG. 10  are different from the parameters used for performing calculation by the second arithmetic unit shown in  FIG. 3 . The second arithmetic unit  303  is configured to: determine a power parameter based on an active-power reserve parameter, the first control parameter, and a VSG power parameter; and use the power parameter as a second control parameter. 
     Correspondingly, a CPG controller is configured to control, based on the second control parameter, (N−n) DC/DC converter circuits to operate in a power adjustment-based constant power generation CPG mode. 
     The first control parameter is obtained by executing the MPPT control algorithm by the MPPT controller shown in  FIG. 3  in the embodiment of this application. The VSG power parameter is obtained by executing a VSG control algorithm by the VSG controller shown in  FIG. 7  to  FIG. 9  in the embodiments of this application. 
     In an embodiment, the first control circuit and the n th  control circuit shown in  FIG. 10  are used as an example for description, and a same processing manner is also applied to other middle control circuits. 
     For the first control circuit, the MPPT processing unit is configured to: detect a first input voltage v pv1  and a first input current i pv1  of a first master-controlled DC/DC converter, and obtain an input current i MPPT1  at a maximum power point through MPPT control; and 
     the multiplier is configured to perform a multiplication operation on the first input voltage v pv1  of the first master-controlled DC/DC converter and the second input current i MPPT1  at the maximum power point, to obtain a maximum output active-power parameter P MPPT1 . 
     For the n th  control circuit, the MPPT processing unit is configured to: detect a first input voltage v pvn  and a first input current i pvn  of the first master-controlled DC/DC converter, and obtain an input current i MPPTn  at a maximum power point through MPPT control; and the multiplier is configured to perform a multiplication operation on the first input voltage v pvn  of the first master-controlled DC/DC converter and the second input current i MPPTn  at the maximum power point, to obtain a maximum output active-power parameter P MPPTn . 
     The first arithmetic unit  302  is configured to: perform summation on obtained maximum output active-power parameters P (MPPT1-MPPTn)  of n master-controlled DC/DC converters; perform an averaging operation on a value obtained through summation; and use an obtained maximum output active-power average value as an active-power reference parameter P ref1  of (N−n) slave-controlled DC/DC converters. 
     The second arithmetic unit  303  is configured to: calculate the active-power reference parameter P ref1 , an active-power reserve parameter ΔP, and a VSG power parameter P VSG  according to formula (9); and use an obtained power parameter as a first control parameter P ref , where 
         P   ref   =P   ref1   −ΔP+P   VSG   (9)
 
     The active-power reserve parameter ΔP is an active-power reserve/limit parameter ΔP. 
     The power parameter P ref  calculated according to formula (9) is used as a power parameter for CPG control on the (N−n) DC/DC converter circuits. 
     Correspondingly, the CPG controller  603  shown in  FIG. 10  is configured to control, based on the power parameter P ref  for CPG control obtained by the MPPT processing unit, the (N−n) DC/DC converter circuits to operate in a power adjustment-based CPG (P-CPG) mode. 
     A PWM control signal is used as a modulation signal for driving an action of a switching transistor. 
     In an embodiment, the CPG controller  603  compares the obtained power parameter P ref  with output active-power P pv_m  of an m th  slave-controlled DC/DC converter, and a proportional integral PI controller obtains a PWM control signal of the m th  slave-controlled DC/DC converter based on an obtained power comparison result, where m=n+1, n+2, . . . , N. 
     It should be noted that, a control manner in which the (N−n) DC/DC converter circuits are controlled to operate in a power adjustment-based CPG (P-CPG) mode may be a power control manner such as proportional integral control, direct power control, and model prediction control. Details are not described in this embodiment of this application. 
     A photovoltaic string inverter disclosed in this embodiment of this application does not need to be provided with a solar radiant intensity detection apparatus. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on n DC/DC converter circuits in N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using an active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     With reference to the embodiment as shown in  FIG. 4 ,  FIG. 11  is a schematic diagram of an execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application. The MPPT controller  602  includes a total of n control circuits, that is, a control circuit  4011  to a control circuit  401   n , a first arithmetic unit  402 , and a third arithmetic unit  403 . 
     Each control circuit includes an MPPT processing unit and a multiplier. 
     The MPPT processing unit is configured to: detect a first input voltage and a first input current of a corresponding DC/DC converter circuit; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     The multiplier is connected to the MPPT processing unit, and is configured to calculate a product of the first input voltage and the second input current to obtain a maximum output active-power parameter. 
     The first arithmetic unit  402  connected to the n control circuits is configured to: determine a maximum output active-power parameter output by each control circuit; perform summation and averaging operations on n maximum output active-power parameters; and use an obtained maximum output active-power average value as a first control parameter. 
     Parameters used for performing calculation by the third arithmetic unit shown in  FIG. 11  are different from the parameters used for performing calculation by the third arithmetic unit shown in  FIG. 4 . The third arithmetic unit  403  is configured to: determine a current parameter based on an active-power reserve parameter, the first control parameter, and a VSG power parameter; and use the current parameter as a second control parameter. 
     Correspondingly, 
     a CPG controller is configured to control, based on the second control parameter, (N−n) DC/DC converter circuits to operate in a current adjustment-based constant power generation CPG mode. 
     The second control parameter is obtained by executing the MPPT control algorithm by the MPPT controller shown in  FIG. 4  in the embodiment of this application. The VSG power parameter is obtained by executing a VSG control algorithm by the VSG controller shown in  FIG. 7  to  FIG. 9  in the embodiments of this application. 
     In an embodiment, the first control circuit and the n th  control circuit shown in  FIG. 11  are used as an example for description, and a same processing manner is also applied to other middle control circuits. 
     For the first control circuit, the MPPT processing unit is configured to: detect a first input voltage v pv1  and a first input current i pv1  of a first master-controlled DC/DC converter, and obtain an input current i MPPT1  at a maximum power point through MPPT control; and the multiplier is configured to perform a multiplication operation on the first input voltage v pv1  of the first master-controlled DC/DC converter and the second input current i MPPT1  at the maximum power point, to obtain a maximum output active-power parameter P MPPT1 . 
     For the n th  control circuit, the MPPT processing unit is configured to: detect a first input voltage v pvn  and a first input current i pvn  of the first master-controlled DC/DC converter, and obtain an input current i MPPTn  at a maximum power point through MPPT control; and 
     the multiplier is configured to perform a multiplication operation on the first input voltage v pvn  of the first master-controlled DC/DC converter and the second input current i MPPTn  at the maximum power point, to obtain a maximum output active-power parameter P MPPTn . 
     The first arithmetic unit  402  is configured to: perform summation on obtained maximum output active-power P (MPPT1-MPPTn)  of n master-controlled DC/DC converters; perform an averaging operation on a value obtained through summation; and use an obtained maximum output active-power average value as an active-power reference parameter P ref1  of (N−n) slave-controlled DC/DC converters. 
     The third arithmetic unit  403  is configured to: calculate the active-power reference parameter P ref1 , an active-power reserve parameter ΔP, and a VSG power parameter P VSG  according to formula (9); process a power parameter P ref  according to formula (2); and use an obtained current parameter I ref  as a first control parameter. 
     The current parameter I ref  calculated according to formula (2) is used as a current instruction for CPG control on the (N−n) DC/DC converter circuits. 
     Correspondingly, the CPG controller  603  shown in  FIG. 11  is configured to control, based on the current instruction I ref  for CPG control obtained by the maximum power point tracking MPPT processing unit, the (N−n) DC/DC converter circuits to operate in a current adjustment-based CPG (I-CPG) mode. 
     A PWM control signal is used as a modulation signal for driving an action of a switching transistor. 
     In an embodiment, the CPG controller  202  compares the obtained current instruction I ref  with an input current I pv_m  of an m th  slave-controlled DC/DC converter, and a proportional integral PI controller obtains a PWM control signal of the m th  slave-controlled DC/DC converter based on an obtained current comparison result, where m=n+1, n+2, . . . , N. 
     It should be noted that, a control manner in which the (N−n) DC/DC converter circuits are controlled to operate in a power adjustment-based CPG (P-CPG) mode may be a power control manner such as proportional integral control, direct power control, and model prediction control. Details are not described in this embodiment of this application. 
     A photovoltaic string inverter disclosed in this embodiment of this application does not need to be provided with a solar radiant intensity detection apparatus. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on n DC/DC converter circuits in N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using an active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     With reference to the embodiment as shown in  FIG. 5 ,  FIG. 12  is a schematic diagram of another execution principle of executing an MPPT control algorithm by an MPPT controller according to an embodiment of this application. The MPPT controller  602  includes a total of n MPPT processing units, that is, an MPPT processing unit  5011  to an MPPT processing unit  501   n , a first arithmetic unit  502 , and a fourth arithmetic unit  503 . 
     Each control circuit includes an MPPT processing unit and a multiplier. 
     The MPPT processing unit is configured to: detect a first input voltage and a first input current of a corresponding DC/DC converter circuit; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     The multiplier is connected to the MPPT processing unit, and is configured to calculate a product of the first input voltage and the second input current to obtain maximum output active-power. 
     The first arithmetic unit  502  connected to the n MPPT processing units is configured to determine a second input voltage output by each MPPT processing unit; perform summation and averaging operations on n maximum output active-power; and use an obtained maximum output active-power average value as a first control parameter. 
     Parameters used for performing calculation by the fourth arithmetic unit shown in  FIG. 12  are different from the parameters used for performing calculation by the fourth arithmetic unit shown in  FIG. 5 . The fourth arithmetic unit  503  is configured to: determine a voltage reference value based on an active-power reserve parameter, the first control parameter, and a VSG power parameter; determine a voltage parameter based on the voltage reference value and the first control parameter; and use the voltage parameter as a second control parameter. 
     A CPG controller is configured to control, based on the second control parameter, (N−n) DC/DC converter circuits to operate in a voltage adjustment-based constant power generation CPG mode. 
     The first control parameter is obtained by executing the MPPT control algorithm by the MPPT controller shown in  FIG. 5  in the embodiment of this application. The VSG power parameter is obtained by executing a VSG control algorithm by the VSG controller shown in  FIG. 7  to  FIG. 9  in the embodiments of this application. 
     In an embodiment, the MPPT processing unit  5011  and the MPPT processing unit  501   n  shown in  FIG. 12  are used as an example for description, and a same processing manner is also applied to other MPPT processing units. 
     For the MPPT processing unit  5011 , the MPPT processing unit  5011  is configured to: detect a first input voltage v pv1  and a first input current i pv1  of a first master-controlled DC/DC converter, and obtain an input voltage v MPPT1  at a maximum power point through MPPT control. 
     For the MPPT processing unit  501   n , the MPPT processing unit  1201   n  is configured to: detect a first input voltage v pvn  and a first input current i pvn  of an n th  master-controlled DC/DC converter, and obtain an input voltage v MPPTn  at a maximum power point through MPPT control. 
     The first arithmetic unit  502  is configured to: perform summation on obtained input voltages I (MPPT1-MPPTn)  of n master-controlled DC/DC converters; perform an averaging operation on a value obtained through summation; and use an obtained input voltage average value as a voltage reference V ref1  of (N−n) slave-controlled DC/DC converters. 
     The fourth arithmetic unit  503  is configured to: calculate an active-power reserve parameter ΔP and a VSG power parameter P VSG  according to formula (10), to obtain a voltage reference value; perform calculation based on the voltage reference value and the voltage reference V ref1 ; and use an obtained voltage parameter as a second control parameter, where 
         V   ref   =V   ref1 +(Δ P−P   VSG )/ i   pv   (10)
 
     The voltage parameter V ref  calculated according to formula (10) is used as a voltage instruction for CPG control on the (N−n) DC/DC converter circuits. 
     Correspondingly, the CPG controller  603  shown in  FIG. 12  is configured to control, based on the voltage parameter V ref  for CPG control obtained by the MPPT processing unit, the (N−n) DC/DC converter circuits to operate in a voltage adjustment-based CPG (V-CPG) mode. 
     In an embodiment, the CPG controller  603  compares the obtained voltage parameter V ref  with an input current v pv_m  of an m th  slave-controlled DC/DC converter, and a proportional integral PI controller obtains a PWM control signal of the m th  slave-controlled DC/DC converter based on an obtained voltage comparison result, where m=n+1, n+2, . . . , N. 
     A photovoltaic string inverter disclosed in this embodiment of this application does not need to be provided with a solar radiant intensity detection apparatus. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on n DC/DC converter circuits in N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using an active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     A DC/AC inverter in the photovoltaic string inverter disclosed in this embodiment of this application is optional, and may be a string three-phase inverter or a string single-phase inverter. 
     In the photovoltaic string inverter disclosed in an embodiment of this application, master control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, that is, MPPT control is performed, so that the n DC/DC converter circuits operate in an MPPT mode; and slave control is performed on the (N−n) DC/DC converter circuits, that is, CPG control is performed, so that the (N−n) DC/DC converter circuits operate in a CPG mode. Through master-slave control, a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature can be implemented, and fluctuation of the direct current bus voltage and the alternating current output power that are of the photovoltaic string inverter in the control process can be eliminated. In addition, control on the photovoltaic string inverter is improved, and the lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. Further, PV-VSG control by using the active-power reserve can be implemented by using the VSG control algorithm. 
     An embodiment of this application further correspondingly discloses, based on the photovoltaic power system shown in the foregoing accompanying drawing, a control method for controlling the photovoltaic power system. The control method is detailed by using the following embodiments. 
       FIG. 13  is a schematic flowchart of a control method based on the photovoltaic power system shown in  FIG. 1  according to an embodiment of this application. With reference to  FIG. 1 , the photovoltaic power system control method includes the following operations. 
     S 1301 : Perform MPPT control on n DC/DC converter circuits in N DC/DC converter circuits; and determine a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state. 
     With reference to  FIG. 1 , when S 1301  is being performed, a controller  103  performs MPPT control on the n DC/DC converter circuits in the N DC/DC converter circuits; and determines the first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state. For a specific execution principle, refer to the description in  FIG. 1 . Details are not described herein again. 
     S 1302 : Control, based on the first control parameter and an active-power reserve parameter, (N−n) DC/DC converter circuits to operate in a constant power generation CPG mode. 
     With reference to  FIG. 1 , when S 1032  is being performed, the CPG controller  202  controls, based on a second control parameter, the (N−n) DC/DC converter circuits to operate in a CPG mode. 
     In the photovoltaic power system control method disclosed in an embodiment of this application, master control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, that is, MPPT control is performed, so that the n DC/DC converter circuits operate in an MPPT mode; and slave control is performed on the (N−n) DC/DC converter circuits, that is, CPG control is performed, so that the (N−n) DC/DC converter circuits operate in a CPG mode. In an embodiment of this application, master-slave control is implemented on the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, to reduce impact of illumination intensity and ambient temperature on an active-power reserve of a photovoltaic string inverter, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
       FIG. 14  is a schematic flowchart of a control method performed by a controller according to an embodiment of this application. With reference to  FIG. 2 , the method includes the following operations. 
     S 1401 : Perform MPPT control on n DC/DC converter circuits in N DC/DC converter circuits, and obtain a first control parameter. 
     Optionally, as shown in  FIG. 15 , a specific implementation process of S 1401  includes the following operations. 
     S 1501 : Detect a first input voltage and a first input current of each of the n DC/DC converter circuits in the N DC/DC converter circuits; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     With reference to  FIG. 3 , S 1501  is performed by the MPPT processing unit. For a specific execution principle, refer to the corresponding description in  FIG. 3 . Details are not described herein again. 
     S 1502 : Calculate a product of the first input voltage and the second input current that are corresponding to each of the n DC/DC converter circuits, to obtain maximum output active-power parameters of the n DC/DC converter circuits. 
     With reference to  FIG. 3 , S 1502  is performed by the multiplier. For a specific execution principle, refer to the corresponding description in  FIG. 3 . Details are not described herein again. 
     S 1503 : Perform summation and averaging operations on the maximum output active-power parameters of the n DC/DC converter circuits; and use an obtained maximum output active-power average value as the first control parameter. 
     With reference to  FIG. 3 , S 1503  is performed by the first arithmetic unit  302 . For a specific execution principle, refer to the corresponding description in  FIG. 3 . Details are not described herein again. 
     S 1504 : Determine a power parameter based on an active-power reserve parameter and the first control parameter; and use the power parameter as a second control parameter. 
     With reference to  FIG. 3 , S 1504  is performed by the second arithmetic unit  303 . For a specific execution principle, refer to the corresponding description in  FIG. 3 . Details are not described herein again. 
     S 1402 : Obtain the second control parameter based on the first control parameter and the active-power reserve parameter. 
     With reference to  FIG. 2 , S 1402  is performed by the MPPT controller  201 . For a specific execution principle, refer to the corresponding description in  FIG. 2 . Details are not described herein again. 
     In a photovoltaic power system control method disclosed in an embodiment of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of a photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on (N−n) DC/DC converter circuits, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     In an embodiment,  FIG. 16  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller that is correspondingly disclosed in an embodiment of this application based on  FIG. 4 . The method includes the following operations. 
     S 1601 : Detect a first input voltage and a first input current of each of n DC/DC converter circuits in N DC/DC converter circuits; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     With reference to  FIG. 4 , S 1601  is performed by the MPPT processing unit. For a specific execution principle, refer to the corresponding description in  FIG. 4 . Details are not described herein again. 
     S 1602 : Calculate a product of the first input voltage and the second input current that are corresponding to each of the n DC/DC converter circuits, to obtain maximum output active-power parameters of the n DC/DC converter circuits. 
     With reference to  FIG. 4 , S 1602  is performed by the multiplier. For a specific execution principle, refer to the corresponding description in  FIG. 4 . Details are not described herein again. 
     S 1603 : Perform summation and averaging operations on the maximum output active-power parameters of the n DC/DC converter circuits; and use an obtained maximum output active-power average value as a first control parameter. 
     With reference to  FIG. 4 , S 1603  is performed by the first arithmetic unit  402 . For a specific execution principle, refer to the corresponding description in  FIG. 4 . Details are not described herein again. 
     S 1604 : Determine a current parameter based on an active-power reserve parameter and the first control parameter; and use the current parameter as a second control parameter. 
     With reference to  FIG. 4 , S 1604  is performed by the third arithmetic unit  403 . For a specific execution principle, refer to the corresponding description in  FIG. 4 . Details are not described herein again. 
     In a photovoltaic power system control method disclosed in an embodiment of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of a photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on (N−n) DC/DC converter circuits, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
       FIG. 17  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller that is disclosed in an embodiment of the present invention based on  FIG. 5 . The method includes the following operations. 
     S 1701 : Detect a first input voltage and a first input current of each of n DC/DC converter circuits in N DC/DC converter circuits; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input voltage at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     With reference to  FIG. 5 , S 1701  is performed by the MPPT processing unit. For a specific execution principle, refer to the corresponding description in  FIG. 5 . Details are not described herein again. 
     S 1702 : Perform summation and averaging operations on second input voltages at maximum power points of the n DC/DC converter circuits; and use an obtained second input voltage average value as a first control parameter. 
     With reference to  FIG. 5 , S 1702  is performed by the first arithmetic unit  502 . For a specific execution principle, refer to the corresponding description in  FIG. 5 . Details are not described herein again. 
     S 1703 : Perform calculation based on an active-power reserve parameter and the first control parameter; and use an obtained voltage parameter as a second control parameter. 
     With reference to  FIG. 5 , S 1703  is performed by the fourth arithmetic unit  503 . For a specific execution principle, refer to the corresponding description in  FIG. 5 . Details are not described herein again. 
     In a photovoltaic power system control method disclosed in an embodiment of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of a photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on (N−n) DC/DC converter circuits, to implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     As disclosed in an embodiment of this application, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits. In this way, based on master-slave control on the N DC/DC converters located at the previous stage of the DC/AC inverter circuit, PV-VSG control by using an active-power reserve can be implemented; problems of deterioration in PV-VSG output performance and even system instability that are caused by illumination intensity and ambient temperature changes can be resolved; and an inertia support capability of a photovoltaic power system can be enhanced, and fluctuation of the direct current bus voltage and the output power that are of the string inverter in the power control process can be eliminated. 
     Based on the photovoltaic power system control method shown in  FIG. 13 , in the process in which PV-VSG control by using the active-power reserve is implemented based on master-slave control on the N DC/DC converters located at the previous stage of the DC/AC inverter circuit, there may be a plurality of control policies for providing, by a controller  103 , a PWM control signal of each DC/DC converter circuit in real time according to a control policy of the DC/DC converter circuit. This embodiment of this application provides detailed descriptions by using the following embodiments. 
       FIG. 18  is a schematic flowchart of another control method performed by controllers that is disclosed in an embodiment of the present invention in correspondence to  FIG. 6 . The method includes the following operations. 
     S 1801 : Calculate a VSG power parameter based on a grid connection parameter for a power grid and a VSG control algorithm. 
     With reference to  FIG. 6 , S 1801  is performed by the VSG controller  601 . For a specific execution principle, refer to the corresponding description in  FIG. 6 . Details are not described herein again. 
     S 1802 : Perform maximum power point tracking MPPT control on n DC/DC converter circuits in N DC/DC converter circuits; and determine a first control parameter that enables the n DC/DC converter circuits to be in a maximum power point operating state. 
     S 1803 : Obtain a second control parameter based on the first control parameter, the VSG power parameter, and an active-power reserve parameter. 
     With reference to  FIGS. 6 , S 1802  and S 1803  are performed by the MPPT controller  602 . For a specific execution principle, refer to the corresponding description in  FIG. 6 . Details are not described herein again. 
     In an embodiment, there are a plurality of manners in which the VSG controller  601  obtains the VSG power parameter based on the power grid parameter and the VSG control algorithm. Three manners are disclosed in this embodiment of this application, but are not limited thereto. 
     A first manner is performing VSG control calculation based on an actually detected power grid frequency, a rated power grid frequency, and a constant virtual inertia, to obtain the VSG power parameter. 
     A second manner is performing VSG control calculation based on an actually detected power grid frequency, a rated power grid frequency, and an adaptive zero virtual inertia, to obtain the VSG power parameter. 
     A third manner is performing VSG control calculation based on an actually detected power grid frequency, a rated power grid frequency, and an adaptive negative virtual inertia, to obtain the VSG power parameter. 
     In an embodiment of this application, based on the VSG power parameters generated in the foregoing different manners, there are a plurality of manners in which the MPPT controller  602  performs MPPT control on the n DC/DC converter circuits. A specific principle of generating the second control parameter by the MPPT controller  602  may be described with reference to content about generating the second control parameter by the MPPT controller  201  in  FIG. 15 ,  FIG. 16 , and  FIG. 17 . 
     With reference to the embodiment as shown in  FIG. 10 ,  FIG. 19  is a schematic flowchart of an execution method for executing an MPPT control algorithm by an MPPT controller that is disclosed in an embodiment of this application in correspondence to  FIG. 10 . The method includes the following operations. 
     S 1901 : Detect a first input voltage and a first input current of each of n DC/DC converter circuits in N DC/DC converter circuits; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     With reference to  FIG. 10 , S 1901  is performed by the MPPT processing unit. For a specific execution principle, refer to the corresponding description in  FIG. 10 . Details are not described herein again. 
     S 1902 : Calculate a product of the first input voltage and the second input current that are corresponding to each of the n DC/DC converter circuits, to obtain maximum output active-power parameters of the n DC/DC converter circuits. 
     With reference to  FIG. 10 , S 1902  is performed by the multiplier. For a specific execution principle, refer to the corresponding description in  FIG. 10 . Details are not described herein again. 
     S 1903 : Perform summation and averaging operations on the maximum output active-power parameters of the n DC/DC converter circuits; and use an obtained maximum output active-power average value as a first control parameter. 
     With reference to  FIG. 10 , S 1903  is performed by the first arithmetic unit  302 . For a specific execution principle, refer to the corresponding description in  FIG. 10 . Details are not described herein again. 
     S 1904 : Determine a power parameter based on an active-power reserve parameter, the first control parameter, and a VSG power parameter; and use the power parameter as a second control parameter. 
     With reference to  FIG. 10 , S 1904  is performed by the second arithmetic unit  303 . For a specific execution principle, refer to the corresponding description in  FIG. 10 . Details are not described herein again. 
     Correspondingly, operation S 1302  shown in  FIG. 13  is specifically: Control, based on the first control parameter and the active-power reserve parameter, the (N−n) DC/DC converter circuits to operate in a power adjustment-based constant power generation CPG mode. 
     In a photovoltaic power system control method disclosed in an embodiment of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of a photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using an active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     With reference to the embodiment as shown in  FIG. 11 ,  FIG. 20  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller that is disclosed in an embodiment of this application in correspondence to  FIG. 11 . The method includes the following operations. 
     S 2001 : Detect a first input voltage and a first input current of each of n DC/DC converter circuits in N DC/DC converter circuits; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     With reference to  FIG. 11 , S 2001  is performed by the MPPT processing unit. For a specific execution principle, refer to the corresponding description in  FIG. 11 . Details are not described herein again. 
     S 2002 : Calculate a product of the first input voltage and the second input current that are corresponding to each of the n DC/DC converter circuits, to obtain maximum output active-power parameters of the n DC/DC converter circuits. 
     With reference to  FIG. 11 , S 2002  is performed by the multiplier. For a specific execution principle, refer to the corresponding description in  FIG. 11 . Details are not described herein again. 
     S 2003 : Perform summation and averaging operations on the maximum output active-power parameters of the n DC/DC converter circuits; and use an obtained maximum output active-power average value as a first control parameter. 
     With reference to  FIG. 11 , S 2003  is performed by the first arithmetic unit  402 . For a specific execution principle, refer to the corresponding description in  FIG. 11 . Details are not described herein again. 
     S 2004 : Determine a current parameter based on an active-power reserve parameter, the first control parameter, and a VSG power parameter; and use the current parameter as a second control parameter. 
     With reference to  FIG. 11 , S 2004  is performed by the third arithmetic unit  403 . For a specific execution principle, refer to the corresponding description in  FIG. 11 . Details are not described herein again. 
     Correspondingly, operation S 1302  shown in  FIG. 13  is specifically: Control, based on the first control parameter, the (N−n) DC/DC converter circuits to operate in a current adjustment-based constant power generation CPG mode. 
     In a photovoltaic power system control method disclosed in an embodiment of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of a photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using an active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. In addition, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     With reference to the embodiment as shown in  FIG. 12 ,  FIG. 21  is a schematic flowchart of another execution method for executing an MPPT control algorithm by an MPPT controller that is disclosed in an embodiment of this application in correspondence to  FIG. 12 . The method includes the following operations. 
     S 2101 : Detect a first input voltage and a first input current of each of n DC/DC converter circuits in N DC/DC converter circuits; determine current input power of the corresponding DC/DC converter circuit based on the first input voltage and the first input current; and obtain a second input current at a maximum power point of the DC/DC converter circuit by using an MPPT control algorithm. 
     With reference to  FIG. 12 , S 2101  is performed by the MPPT processing unit. For a specific execution principle, refer to the corresponding description in  FIG. 12 . Details are not described herein again. 
     S 2102 : Perform summation and averaging operations on second input voltages at maximum power points of the n DC/DC converter circuits; and use an obtained second input voltage average value as a first control parameter. 
     With reference to  FIG. 12 , S 2102  is performed by a second arithmetic unit  502 . For a specific execution principle, refer to the corresponding description in  FIG. 12 . Details are not described herein again. 
     S 2103 : Determine a voltage reference value based on an active-power reserve parameter, the first control parameter, and a VSG power parameter; determine the voltage reference value based on the voltage reference value and the first control parameter; and use a voltage parameter as a second control parameter. 
     With reference to  FIG. 12 , S 2103  is performed by the fourth arithmetic unit  503 . For a specific execution principle, refer to the corresponding description in  FIG. 12 . Details are not described herein again. 
     Correspondingly, operation S 1302  shown in  FIG. 13  is specifically: Control, based on the first control parameter, the (N−n) DC/DC converter circuits to operate in a voltage adjustment-based constant power generation CPG mode. 
     In a photovoltaic power system control method disclosed in an embodiment of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of a photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at a previous stage of a DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using an active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of a direct current bus voltage and alternating current output power that are of the photovoltaic string inverter in a control process. Further, control on a virtual synchronous generator of the photovoltaic string inverter is implemented, and a lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element. 
     A specific principle and execution process of the operations performed specific to the photovoltaic string inverter disclosed in this embodiment of the present invention are the same as those of the control methods for the virtual synchronous generator of the photovoltaic string inverter disclosed in the embodiments of the present invention, and reference may be made to corresponding descriptions of the control methods for the virtual synchronous generator of the photovoltaic string inverter disclosed in the embodiments of the present invention. Details are not described herein again. 
     With reference to the photovoltaic power system control methods disclosed in the embodiments of this application, the photovoltaic power system control method disclosed in this embodiment of this application may be implemented directly by hardware, a processor executing program code in a memory, or a combination thereof. 
     As shown in  FIG. 22 , a controller  2200  includes: a memory  2201 , a processor  2202  that communicates with the memory, and a communications interface  2203 . 
     The processor  2201  is coupled to the memory  2202  through a bus, and the processor  2201  is coupled to the communications interface  2203  through the bus. 
     The processor  2202  may specifically be a central processing unit (CPU), a network processor (NP), an application-specific integrated circuit (ASIC), or a programmable logic device (PLD). The PLD may be a complex programmable logic device (CPLD), a field-programmable gate array (FPGA), or a generic array logic (GAL). 
     The memory  2201  may specifically be a content-addressable memory (CAM) or a random-access memory (RAM). The CAM may be a ternary content-addressable memory (TCAM). 
     The communications interface  2203  may be a wired interface, for example, a fiber distributed data interface (FDDI) or an Ethernet interface. 
     The memory  2201  may alternatively be integrated into the processor  2202 . If the memory  2201  and the processor  2202  are independent components, the memory  2201  is connected to the processor  2202 . For example, the memory  2201  may communicate with the processor  2202  through the bus. The communications interface  2203  may communicate with the processor  2202  through the bus, or the communications interface  2203  may be connected to the processor  2202  directly. 
     The memory  2201  is configured to store program code for controlling a photovoltaic string inverter. Optionally, the memory  2201  includes an operating system and an application program, and is configured to carry an operating program, code, or instruction used for the control methods for the virtual synchronous generator of the photovoltaic string inverter disclosed in the embodiments of this application. 
     When the processor  2202  or a hardware device needs to perform an operation related to the control methods for the virtual synchronous generator of the photovoltaic string inverter disclosed in the embodiments of this application, the processor  2202  or the hardware device can complete a process in which a base station in the embodiments of this application performs the corresponding control methods for the virtual synchronous generator of the photovoltaic string inverter, by invoking and executing the operating program, code, or instruction stored in the memory  2201 . A specific process is: The processor  2202  invokes the program code, in the memory  2201 , for controlling the photovoltaic string inverter, to perform the control methods for the virtual synchronous generator of the photovoltaic string inverter. 
     It can be understood that operations of a network device such as receiving/sending in the embodiments, shown in  FIG. 13  to  FIG. 21 , corresponding to the control methods for the virtual synchronous generator of the photovoltaic string inverter may be receiving/sending processing implemented by the processor, or may be a receiving/sending process completed by a receiver/transmitter. The receiver and the transmitter may exist alone, or may be integrated into a transceiver. In a possible implementation, the base station  2200  may further include a transceiver. 
     All or some of the foregoing embodiments may be implemented by using software, hardware, firmware, or any combination thereof. When software is used to implement the embodiments, the embodiments may be implemented completely or partially in a form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on the computer, the procedure or functions according to the embodiments of this application are all or partially generated. The computer may be a general-purpose computer, a dedicated computer, a computer network, or other programmable apparatuses. The computer instructions may be stored in a computer-readable storage medium or may be transmitted from a computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions may be transmitted from a website, computer, server, or data center to another website, computer, server, or data center in a wired (for example, a coaxial cable, an optical fiber, or a digital subscriber line (DSL)) or wireless (for example, infrared, radio, or microwave) manner. The computer-readable storage medium may be any usable medium accessible by a computer, or a data storage device, such as a server or a data center, integrating one or more usable media. The usable medium may be a magnetic medium (for example, a floppy disk, a hard disk, or a magnetic tape), an optical medium (for example, a DVD), a semiconductor medium (for example, a solid-state drive (SSD)), or the like. 
     An embodiment of this application further discloses a photovoltaic power system. The photovoltaic power system includes the photovoltaic string inverter shown in  FIG. 13  to  FIG. 22 . 
     In summary, in the photovoltaic power system and the control method thereof disclosed in the embodiments of this application, no solar radiant intensity detection apparatus needs to be provided. Therefore, costs of the photovoltaic string inverter can be reduced. Moreover, master MPPT control is performed on the n DC/DC converter circuits in the N DC/DC converter circuits located at the previous stage of the DC/AC inverter circuit, and slave CPG control is performed on the (N−n) DC/DC converter circuits, to implement PV-VSG control by using the active-power reserve, implement a fast and accurate power reserve or limit of the photovoltaic string inverter with any illumination intensity and ambient temperature, and eliminate fluctuation of the direct current bus voltage and the alternating current output power that are of the photovoltaic string inverter in the control process. Further, control on the virtual synchronous generator of the photovoltaic string inverter is implemented, and the lifespan of the photovoltaic string inverter is prolonged, without a need to add an energy storage element.