Megawatt photovoltaic grid-connected inverter system and pre-charging starting control method thereof

Abstract

The invention discloses a megawatt-level photovoltaic grid-connected inverter system and a pre-charging starting control method thereof, and belongs to the technical field of photovoltaic grid-connected inverter control, an MW-level photovoltaic grid-connected inverter adopts a modular design, and power modules are connected in series at each phase of a high-voltage alternating current output side. Stable charging of capacitors on the high-voltage side and the low-voltage side of the photovoltaic grid-connected inverter is achieved through staged control. A phase shift charging stage is started, a low-voltage side isolation switch and a combiner box load switch are switched on, and a photovoltaic panel is used for charging a low-voltage side capacitor and a high-voltage side capacitor; and in the charging completion phase shift stage, the high-voltage side isolation switch is switched on, the high-voltage module is unlocked to operate in a high-voltage side voltage control mode, and the capacitor voltage of the high-voltage module is stabilized at an instruction value. And the photovoltaic grid-connected inverter can be switched into a power control mode to complete the starting process. According to the method, the problems of over-voltage and over-current of the module in the starting process are avoided, and the problems of normal power taking of each power module control board card of the device and unlocking control of the device are solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of photovoltaic grid-connected inverter control, and in particular relates to a megawatt-class photovoltaic grid-connected inverter system and a pre-charging start-up control method thereof. Background Art

[0002] Traditional fossil energy is not only limited in resources, but also produces a large amount of carbon dioxide and other pollutants during combustion, which has a serious impact on the environment. Therefore, finding alternative energy sources has become an urgent need. Photovoltaic power stations use solar energy to generate electricity, which is an inexhaustible and clean energy. As a bridge connecting photovoltaic panels and the power grid, the importance of photovoltaic grid-connected inverters is becoming increasingly prominent. The main function of photovoltaic grid-connected inverters is to convert the direct current generated by photovoltaic panels into alternating current, and ensure that the inverter output is synchronized with the mains voltage to achieve seamless grid connection.

[0003] The megawatt (MW)-level photovoltaic grid-connected inverter device includes an input stage, an isolation stage, and an output stage. The output stage adopts a cascaded H-bridge structure, which enables it to be directly connected to a medium-voltage AC system. The isolation stage and the input stage adopt a parallel structure of multiple unidirectional LLC resonant converters to achieve photovoltaic DC access.

[0004] Pre-charge startup control is a key technology for MW-class photovoltaic grid-connected inverter devices, and its control characteristics are directly related to the safe and reliable operation of megawatt (MW)-class photovoltaic grid-connected inverter devices. The MW-class photovoltaic grid-connected inverter topology has no contactors and soft-start resistors on the high-voltage AC side and low-voltage DC side. Different from the conventional AC-side soft-start charging method and DC-side startup method, there are problems in this topology system that the module control board cannot normally draw power during startup and the module overcurrent causes startup failure. Therefore, how to realize the pre-charge startup of the power module of the MW-class photovoltaic grid-connected inverter device and ensure the normal power supply and unlocking control of each power module control board has become an urgent problem to be solved. Summary of the invention

[0005] The present invention provides a megawatt-class photovoltaic grid-connected inverter system and a pre-charging start-up control method thereof, which realizes the pre-charging start-up of the power module of the MW-class photovoltaic grid-connected inverter device without contactors and soft-start resistors on the high-voltage AC side and the low-voltage DC side. The start-up process is smooth, and the overvoltage and overcurrent problems of the power module are avoided, and the difficult problems of normal power supply of the control boards of the power modules of the device and the unlocking control of the device are solved.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a MW-class photovoltaic grid-connected inverter system, comprising a three-phase converter unit, each phase converter unit comprising a plurality of module units, each module unit comprising a plurality of H-bridge units, a high-voltage side capacitor C1 and a converter, wherein the converter comprises a rectifier circuit, a transformer T1, a resonant circuit, an inverter circuit and a low-voltage side capacitor C2;

[0008] All H-bridge units are cascaded and all inverter circuits are connected in parallel;

[0009] The H-bridge unit, the high-voltage side capacitor C1 and the rectifier circuit constitute a high-voltage module, and the resonant circuit, the inverter circuit and the low-voltage side capacitor C2 constitute a low-voltage module;

[0010] The high voltage module is connected to the AC busbar via an AC isolating switch, and the low voltage module is connected to the DC busbar via a DC isolating switch.

[0011] Furthermore, the DC bus is connected to the photovoltaic panel via a photovoltaic branch incoming line, and a DC load switch is provided on the photovoltaic branch incoming line.

[0012] Furthermore, the converter is a unidirectional series LLC resonant converter.

[0013] In a second aspect, the present invention provides a MW-class photovoltaic grid-connected inverter pre-charging start control method, comprising the following steps:

[0014] S1, close the DC isolating switch and charge the low-voltage module capacitor through the photovoltaic panel;

[0015] S2. Determine whether the low-voltage module control board has completed self-power-on reset and has no faults:

[0016] If the low-voltage module control board completes the self-power-on reset and there is no fault, unlock the low-voltage side module, and output the driving pulse with the duty cycle change slope to charge the high-voltage side capacitor of the high-voltage module. After the high-voltage side capacitor of the high-voltage module reaches the set high-voltage side capacitor voltage charging threshold, close the high-voltage side AC isolation switch and block the LLC pulse at the same time;

[0017] Otherwise, determine whether the number of low-voltage module failures exceeds the low-voltage module redundancy upper limit:

[0018] If the number of faulty low-voltage modules does not exceed the upper limit of low-voltage module redundancy, the faulty low-voltage modules are isolated;

[0019] If the number of faulty modules exceeds the number of redundant modules, disconnect the AC disconnector and DC disconnector, and close the grounding switch;

[0020] S3. Determine whether the number of high-voltage module failures exceeds the upper limit of module redundancy:

[0021] If the number of high-voltage module failures exceeds the upper limit of module redundancy, disconnect the AC disconnector and DC disconnector, and close the grounding switch;

[0022] Otherwise, determine whether the high voltage module has a module failure:

[0023] If the high-voltage module reports a module fault but does not exceed the limit, the high-voltage fault module bypass contactor is bypassed. If a downlink communication failure occurs, the high-voltage fault module is electronically bypassed, and the high-voltage module enters an uncontrolled charging mode. The uncontrolled charging voltage threshold is lower than the open-circuit voltage of the battery panel; the high-voltage module is then unlocked and operates in the high-voltage side voltage control mode, and the control target is the average voltage of the high-voltage module;

[0024] If there is no module failure in the high-voltage module, the high-voltage module enters the uncontrolled charging mode;

[0025] S4, the high-voltage side voltage stabilizes to the open-circuit voltage of the battery panel, unlocks the low-voltage module, and the duty cycle is output at the duty cycle change slope until the rated duty cycle is reached. The low-voltage module is controlled by an open-loop fixed frequency method.

[0026] Furthermore, in step S1, when the photovoltaic panel is used to charge the low-voltage module capacitor, a DC box load switch is closed first, and then the remaining combiner box load switches are closed in sequence.

[0027] Furthermore, in step S2, when the LLC pulse is blocked, the high voltage module controls the board to perform a self-power-off reset process, and the board program performs fault diagnosis and processing.

[0028] Furthermore, in step S2, the duty cycle change slope is calculated by the following formula:

[0029] K = (De - Dn) × f / M;

[0030] Where K is the duty cycle change slope, De is the rated duty cycle, Dn is the initial duty cycle, f is the switching frequency, and M is the number of cycles set in the rated duty cycle stage.

[0031] Furthermore, in step S2, the calculation formula for the high-voltage module high-side capacitor voltage charging threshold U0 is:

[0032] U0=K1×U g ×1.414 / 2 / N, where U g is the AC bus voltage, and N is the number of single-phase power modules.

[0033] Furthermore, in step S3, the electronic bypass is that the high-voltage module control board sends a bypass pulse to bypass the faulty high-voltage module.

[0034] Furthermore, in step S3, in the high-voltage side voltage control mode, the high-voltage side voltage control instruction is the product of the open-circuit voltage of the battery panel and the high-frequency transformer ratio and the voltage compensation factor.

[0035] Compared with the prior art, the present invention has at least the following beneficial technical effects:

[0036] The present invention provides a MW-class photovoltaic grid-connected inverter system, which adopts low-voltage parallel connection, and the converter adopts a unidirectional LLC resonant converter. The soft start circuit is cancelled on both the high-voltage side and the low-voltage side. The MW-class photovoltaic grid-connected inverter system directly draws power from the photovoltaic low voltage through the pre-charging start control method provided by the present invention, thereby reducing the cost, and the start process is smooth, avoiding the overvoltage and overcurrent phenomenon of the power module, and reducing the system instability factors caused by the voltage and current fluctuations during the start process, thereby protecting the safety of the photovoltaic grid-connected inverter system and related equipment.

[0037] The present invention discloses a pre-charging start-up control method for a MW-class photovoltaic grid-connected inverter system. The low-voltage DC input side and the high-voltage AC output side of the controlled object MW-class photovoltaic grid-connected inverter system are free of contactors and soft-start resistors. Phased control is adopted to realize the stable charging of the high and low voltage side capacitors of the photovoltaic grid-connected inverter. In the phase-shift charging stage of starting, the low-voltage side disconnector and the combiner box load switch are closed, and the low-voltage side capacitor and the high-voltage side capacitor are charged by the photovoltaic panel, so that the high-voltage side capacitor can be smoothly charged to avoid overvoltage; after the charging is completed in the phase-shift stage, the high-voltage side disconnector is closed, and the high-voltage module is unlocked to operate in the high-voltage side voltage control mode, and the high-voltage module capacitor voltage is stabilized at the command value. The photovoltaic grid-connected inverter can be switched to the power control mode to complete the startup process. The pre-charging start-up control method is adopted to avoid the module overvoltage and overcurrent problems during the startup process, and solves the problem of normal power supply of each power module control board of the device and the unlocking control of the device.

[0038] Furthermore, after the charging is completed, the high-voltage module is unlocked and the high-voltage side voltage control command value is set to the open-circuit voltage U OC High frequency transformer ratio K2 and voltage compensation factor K b The product of K b The value is 1.05. Due to the unidirectional conductivity of the diode, it prevents the unlocking overcurrent caused by unlocking the low-voltage side H bridge.

[0039] Furthermore, when the photovoltaic panels are used to charge the low-voltage module capacitors, a DC box load switch is closed first, and then the remaining combiner box load switches are closed in sequence to reduce the impact current of the photovoltaic panels charging the low-voltage side capacitors, thereby achieving large-capacity MW-level output. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a topology diagram of a MW-class photovoltaic grid-connected inverter system;

[0041] Figure 2 The present invention is a flowchart of a MW-class photovoltaic grid-connected inverter pre-charging start-up control method. DETAILED DESCRIPTION

[0042] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] In order to enable those skilled in the art to better understand the technical solutions in the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0044] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be another element centered thereon. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be another element centered thereon at the same time. The terms "upper", "lower", "front", "back", "left", "right", "top", "bottom", "inside", "outside", etc. used herein indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0046] like Figure 1 The megawatt (MW)-class photovoltaic grid-connected inverter device topology shown adopts a modular design. Each phase of the high-voltage AC output side adopts N identical cascaded H-bridge units, and each phase of the low-voltage DC input side adopts N unidirectional LLC resonant converters. Each cascaded H-bridge unit and the unidirectional LLC resonant converter together constitute a power module of the photovoltaic grid-connected inverter. Each power module is connected in series on each phase of the high-voltage AC output side, and is connected to the photovoltaic array through a DC disconnect switch and a confluence device in a three-phase parallel manner on the low-voltage DC input side. The high-voltage AC output side is connected to the AC grid through an AC disconnect switch.

[0047] Example 1

[0048] This embodiment provides a MW-class photovoltaic grid-connected inverter, including a three-phase converter unit, wherein the three-phase converter unit includes an A unit, a B unit, and a C unit.

[0049] Unit A includes multiple module units, each module unit includes an H-bridge unit, a high-voltage side capacitor C1 and a unidirectional series LLC resonant converter, wherein the unidirectional series LLC resonant converter includes a rectifier circuit, a transformer T1, a resonant circuit, an inverter circuit and a low-voltage side capacitor C2; wherein all H-bridge units are cascaded and all inverter circuits are paralleled;

[0050] The H-bridge unit, the high-voltage side capacitor C1 and the rectifier circuit form a high-voltage module, and the resonant circuit, the inverter circuit and the low-voltage side capacitor C2 form a low-voltage module (also called a low-voltage side H-bridge unit);

[0051] The topology of the B and C units is the same as that of the A unit, so it will not be described again;

[0052] The input terminals of unit A, unit B and unit C are connected in parallel, which is the DC input side; the input terminals of unit A, unit B and unit C are connected to the photovoltaic panel through the DC bus and the photovoltaic branch incoming line; a DC isolating switch is provided on the DC bus, and a DC load switch is provided on the photovoltaic branch incoming line.

[0053] The first terminal of the output end of the A unit is connected to the A phase AC busbar through an inductor, and the second terminal is connected to the second terminal of the output end of the B unit and the second terminal of the output end of the C unit;

[0054] The first terminal of the output end of the B unit is connected to the B phase AC bus, the first terminal of the output end of the C unit is connected to the C phase AC bus, and AC isolating switches are provided on the A phase AC bus, the B phase AC bus and the C phase AC bus.

[0055] Example 2

[0056] This embodiment provides a MW-class photovoltaic grid-connected inverter pre-charging start control method, which can be used to perform pre-charging start control on the MW-class photovoltaic grid-connected inverter provided in Example 1, and can also perform pre-charging start control on MW-class photovoltaic grid-connected inverters of other topologies, such as the topology when the high-voltage module and the low-voltage module in Example 1 adopt a three-level structure. Figure 2 As shown, the following steps are included:

[0057] S1. The control and protection system sends a command to the MW-level photovoltaic grid-connected inverter to close the DC isolation switch. After receiving the closed state of the DC isolation switch, the 1# DC box load switch is closed, and the photovoltaic panel is used to charge the low-voltage module capacitor. The low-voltage side module is charged to the current open-circuit voltage U of the photovoltaic panel. OC After that, the remaining combiner box load switches are closed in turn. Because the capacity of each combiner box is only a few hundred kilowatts, closing one line first and then closing the remaining combiner box load switches is to reduce the impact current of the photovoltaic panel charging the low-voltage side capacitor. In addition, in order to achieve large-capacity MW-level output, all combiner boxes need to be connected, that is, the load switches of the remaining combiner boxes need to be closed.

[0058] S2. Determine whether the low-voltage module control board has completed self-power-on reset and has no faults:

[0059] If the low-voltage module control board completes the self-power-on reset and there is no fault, the control and protection system unlocks the low-voltage side LLC power unit (i.e., the low-voltage module) and outputs a drive pulse with a duty cycle change slope to charge the high-voltage side capacitor of the high-voltage module. After the high-voltage side capacitor of the high-voltage module reaches the set high-voltage module high-voltage side capacitor voltage charging threshold U0, the control and protection system issues a command to close the high-voltage side AC isolation switch and blocks the LLC pulse at the same time. At the same time, the high-voltage module control board immediately performs a self-power-on reset process, and the board program performs corresponding fault judgment and processing.

[0060] Otherwise, first determine whether the number of low-voltage module failures exceeds the low-voltage module redundancy upper limit:

[0061] If the number of faulty low-voltage modules does not exceed the upper limit of low-voltage module redundancy, manually close the bypass contactor corresponding to the faulty low-voltage module to isolate the faulty low-voltage module;

[0062] If the number of faulty modules exceeds the number of redundant modules, the control and protection system disconnects the AC isolating switch and the DC isolating switch, closes the grounding switch, and enters the maintenance state;

[0063] S3: After the high-voltage module control board completes self-power-off reset, it is determined that the number of high-voltage module faults exceeds the upper limit of module redundancy:

[0064] If the number of high-voltage module failures exceeds the upper limit of module redundancy, the control and protection system disconnects the AC isolating switch and the DC isolating switch, closes the grounding switch, and the system enters the maintenance state.

[0065] Otherwise, determine whether the high voltage module has a module failure:

[0066] If the high-voltage module reports a module fault but does not exceed the limit, the control and protection system sends a bypass contactor command to bypass the high-voltage fault module. If a downlink communication failure occurs, the high-voltage fault module performs an electronic bypass action, and the high-voltage module enters the uncontrolled charging mode. The uncontrolled charging voltage threshold is lower than the open-circuit voltage of the battery panel. The high-voltage module is then unlocked and operates in the high-voltage side voltage control mode. The control targets are: the average voltage of the high-voltage module, the high-voltage side voltage control command U H_ref It is the product of the open circuit voltage UOC of the solar panel, the high frequency transformer ratio K2 and the voltage compensation factor Kb, that is, UOC×K2×Kb;

[0067] If there is no module failure in the high-voltage module, the high-voltage module enters the uncontrolled charging mode. The uncontrolled charging voltage threshold is lower than the open-circuit voltage of the battery panel. The high-voltage module is then unlocked and operates in the high-voltage side voltage control mode. The control targets are: the average voltage of the high-voltage side module, the high-voltage side voltage control instruction U H_ref is the open circuit voltage of the solar panel U OC High frequency transformer ratio K2 and voltage compensation factor K b The product of OC ×K2×K b .

[0068] S4. The high-voltage side voltage stabilizes to the open-circuit voltage of the battery panel, and the control and protection system unlocks the low-voltage side H-bridge unit. The duty cycle is output at the duty cycle change slope K until the rated duty cycle. After the duty cycle increases to the rated duty cycle, the open-loop fixed frequency control low-voltage module is used to complete the pre-charging startup control process.

[0069] In S2, the calculation formula for the duty cycle change slope K is:

[0070] K = (De - Dn) × f / M;

[0071] Wherein, De is the rated duty cycle, Dn is the initial duty cycle, f is the device switching frequency of the low voltage module, and M is the number of cycles set in the rated duty cycle stage.

[0072] In S2, the high-voltage module high-voltage side capacitor voltage charging threshold U0 is set, and the calculation formula is:

[0073] U0=K1×U g ×1.414 / 2 / N,U g is the AC bus voltage, and N is the number of single-phase power modules.

[0074] In S3, the electronic bypass action refers to the high-voltage module control board sending a bypass pulse to bypass the faulty high-voltage module.

[0075] In S3, the high-voltage side voltage control mode is used, and the control target is to stabilize the average voltage of the high-voltage module to the high-voltage side voltage control command value. The high-voltage side voltage control command U H_ref=:The open circuit voltage of the solar panel U OC High frequency transformer ratio K2 and voltage compensation factor K b The product of OC ×K2×K b .

[0076] The present invention proposes a pre-charging start-up control method for a MW-class photovoltaic grid-connected inverter system, which realizes the pre-charging start-up of the power modules of the MW-class photovoltaic grid-connected inverter device without contactors and soft-start resistors on the high-voltage AC side and the low-voltage DC side, avoids the current shock during the charging start-up process, and solves the problem of normal power supply of the control boards of the power modules of the device and the unlocking control of the device.

[0077] The term "consisting of" describing a combination shall include the identified elements, ingredients, parts or steps and other elements, ingredients, parts or steps that do not substantially affect the basic novel characteristics of the combination. The use of the terms "comprising" or "including" to describe a combination of elements, ingredients, parts or steps herein also contemplates embodiments that consist essentially of these elements, ingredients, parts or steps. By using the term "may", it is intended to illustrate that any of the attributes described that "may" be included are optional.

[0078] Multiple elements, ingredients, parts or steps can be provided by a single integrated element, ingredient, part or step. Alternatively, a single integrated element, ingredient, part or step can be divided into separate multiple elements, ingredients, parts or steps. The disclosure "one" or "an" used to describe an element, ingredient, part or step is not intended to exclude other elements, ingredients, parts or steps.

[0079] It should be understood that the above description is for illustration and not for limitation. Many embodiments and many applications beyond the examples provided will be apparent to those skilled in the art upon reading the above description. Therefore, the scope of the present teachings should not be determined with reference to the above description, but rather with reference to the foregoing claims and the full scope of equivalents to which such claims are entitled. For the purpose of comprehensiveness, all articles and references, including disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein in the foregoing claims is not intended to be a waiver of such subject matter, nor should it be considered that the applicant has not considered such subject matter to be part of the disclosed inventive subject matter.

Claims

1. A megawatt-class photovoltaic grid-connected inverter system, characterized in that: It includes a three-phase converter unit, each phase converter unit includes a plurality of module units, each module unit includes a plurality of H-bridge units, a high-voltage side capacitor C1 and a converter, and the converter includes a rectifier circuit, a transformer T1, a resonant circuit, an inverter circuit and a low-voltage side capacitor C2; All H-bridge units are cascaded and all inverter circuits are connected in parallel; The H-bridge unit, the high-voltage side capacitor C1 and the rectifier circuit constitute a high-voltage module, and the resonant circuit, the inverter circuit and the low-voltage side capacitor C2 constitute a low-voltage module; The high voltage module is connected to the AC busbar via an AC isolating switch, and the low voltage module is connected to the DC busbar via a DC isolating switch.

2. A megawatt-class photovoltaic grid-connected inverter system according to claim 1, characterized in that: The DC busbar is connected to the photovoltaic panel via a photovoltaic branch incoming line, and a DC load switch is provided on the photovoltaic branch incoming line.

3. A megawatt-class photovoltaic grid-connected inverter system according to claim 1, characterized in that: The converter is a unidirectional series LLC resonant converter.

4. The pre-charging start-up control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 1, characterized in that: The following steps are involved: S1, close the DC isolating switch and charge the low-voltage module capacitor through the photovoltaic panel; S2. Determine whether the low-voltage module control board has completed self-power-on reset and has no faults: If the low-voltage module control board completes the self-power-on reset and there is no fault, unlock the low-voltage module, and output the driving pulse with the duty cycle change slope to charge the high-voltage side capacitor of the high-voltage module. After the high-voltage side capacitor of the high-voltage module reaches the set high-voltage side capacitor voltage charging threshold, close the high-voltage side AC isolation switch and block the LLC pulse at the same time; Otherwise, determine whether the number of low-voltage module failures exceeds the low-voltage module redundancy upper limit: If the number of faulty low-voltage modules does not exceed the upper limit of low-voltage module redundancy, the faulty low-voltage modules are isolated; If the number of faulty modules exceeds the number of redundant modules, disconnect the AC disconnector and DC disconnector, and close the grounding switch; S3. Determine whether the number of high-voltage module failures exceeds the upper limit of module redundancy: If the number of high-voltage module failures exceeds the upper limit of module redundancy, disconnect the AC disconnector and DC disconnector, and close the grounding switch; Otherwise, determine whether the high voltage module has a module failure: If the high-voltage module reports a module fault but does not exceed the limit, the high-voltage fault module bypass contactor is bypassed. If a downlink communication failure occurs, the high-voltage fault module is electronically bypassed, and the high-voltage module enters an uncontrolled charging mode. The uncontrolled charging voltage threshold is lower than the open-circuit voltage of the battery panel; the high-voltage module is then unlocked and operates in the high-voltage side voltage control mode, and the control target is the average voltage of the high-voltage module; If there is no module failure in the high-voltage module, the high-voltage module enters the uncontrolled charging mode; S4, the high-voltage side voltage stabilizes to the open-circuit voltage of the battery panel, unlocks the low-voltage module, and the duty cycle is output at the duty cycle change slope until the rated duty cycle is reached. The low-voltage module is controlled by an open-loop fixed frequency method.

5. The pre-charging start-up control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 4, characterized in that: In the step S1, when the photovoltaic panel is used to charge the capacitor of the low-voltage module, a load switch of a DC box is closed first, and then the load switches of the remaining combiner boxes are closed in sequence.

6. The pre-charging start-up control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 4, characterized in that: In the step S2, when the LLC pulse is blocked, the high voltage module controls the board to perform a self-power-off reset process, and the board program performs fault judgment and processing.

7. The pre-charging start-up control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 4, characterized in that: In step S2, the duty cycle change slope is calculated by the following formula: K = (De - Dn) × f / M; Where K is the duty cycle change slope, De is the rated duty cycle, Dn is the initial duty cycle, f is the switching frequency, and M is the number of cycles set in the rated duty cycle stage.

8. The pre-charging start-up control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 4, characterized in that: In step S2, the calculation formula of the high-voltage module high-voltage side capacitor voltage charging threshold U0 is: U0 = K1 × U g ×1.414 / 2 / N, where U g is the AC bus voltage, and N is the number of single-phase power modules.

9. The pre-charging start-up control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 4, characterized in that: In step S3, the electronic bypass is that the high-voltage module control board sends a bypass pulse to bypass the faulty high-voltage module.

10. The pre-charging startup control method of a megawatt-class photovoltaic grid-connected inverter system according to claim 4, characterized in that: In step S3, in the high-voltage side voltage control mode, the high-voltage side voltage control instruction is the product of the open-circuit voltage of the battery panel and the high-frequency transformer ratio and the voltage compensation factor.

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