Inverter and inverter control method

Abstract

The invention provides an inverter and an inverter control method, the inverter comprises a plurality of direct current boost circuits, a direct current bus and an inverter circuit, the plurality of direct current boost circuits are used for receiving direct current input of an optimizer and outputting the direct current input of the optimizer to the inverter circuit through the direct current bus, the inverter circuit is used for receiving the direct current input of the plurality of direct current booster circuits and outputting the direct current input of the plurality of direct current booster circuits to a load or a power grid; when the plurality of direct-current booster circuits work in an MPPT (maximum power point tracking) state, if respective input voltages of n direct-current booster circuits in the plurality of direct-current booster circuits are greater than or equal to a first threshold value, the controller controls switching tubes in m direct-current booster circuits in the n direct-current booster circuits to be kept disconnected, m and n are integers greater than 0, and n is greater than or equal to m; the first threshold value is greater than or equal to the minimum voltage limit value of the DC bus.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to an inverter and an inverter control method. Background Technology

[0002] In traditional photovoltaic (PV) power generation systems, PV modules are connected in series to the inverter input. When one module is shaded, all the other unshaded modules in the same string are affected, causing an overall decrease in the power generation of the entire string.

[0003] In a high-power photovoltaic (PV) system, one or more photovoltaic modules are first connected to their corresponding optimizers. Multiple optimizer outputs are then connected in series to form an optimizer string, which is then connected to the inverter input. The optimizer performs Maximum Power Point Tracking (MPPT), ensuring that the connected modules maintain maximum power output regardless of shading of other modules connected in the same optimizer string. High-power photovoltaic technology effectively prevents power loss caused by partial shading, irradiance differences, and variations in module characteristics, ensuring that each module in the system always operates at its maximum output power. The inverter operating in a high-power photovoltaic system is called a high-power inverter. A two-stage high-power inverter internally includes a DC / DC power conversion stage. It only requires the optimizer to have step-down regulation capabilities, allowing it to flexibly adapt to complex operating conditions. The optimizer implementation is simple, low-cost, and highly efficient. However, a two-stage high-power inverter itself is complex to implement, costly, and inefficient. Improving the operating efficiency of two-stage high-power inverters is a pressing issue. Summary of the Invention

[0004] This application provides an inverter and an inverter control method that enable the inverter to have higher operating efficiency when it adopts a two-stage architecture and operates in a photovoltaic power generation system.

[0005] In a first aspect, this application provides an inverter, including multiple DC boost circuits, a DC bus, an inverter circuit, and a controller. Each of the multiple DC boost circuits is used to receive the DC input of an optimizer connected to one or more photovoltaic modules, and to boost the DC input of the optimizer and output it to the inverter circuit through the DC bus. The inverter circuit is used to receive the DC input of the multiple DC boost circuits and to convert the DC input of the multiple DC boost circuits into AC power for output to a load or the power grid. The controller is used to keep the switching transistors in m of the n DC boost circuits open when the multiple DC boost circuits are operating in MPPT state, if the input voltage of each of the n DC boost circuits is greater than or equal to a first threshold, where m and n are both integers greater than 0 and n ≥ m, and the first threshold is greater than or equal to the minimum voltage limit of the DC bus.

[0006] Because the inverter adopts a two-stage architecture, when the input voltage of some DC-DC boost circuits is high enough, the DC-DC boost circuits with input voltages greater than or equal to the first threshold and those with input voltages less than the first threshold are grouped together. This allows the DC-DC boost circuits with input voltages greater than or equal to the first threshold to stop power conversion, that is, the switching transistors in the DC-DC boost circuits will be disconnected, so that the DC power input to the optimizer can flow directly to the DC bus, saving the power loss of this part of the DC-DC boost circuit and improving the overall working efficiency of the inverter.

[0007] In one implementation, the m DC-DC boost circuits include a first DC-DC boost circuit and a DC-DC boost circuit among the n DC-DC boost circuits whose absolute value of the difference between its input voltage and that of the first DC-DC boost circuit is less than or equal to a second threshold; wherein, the first DC-DC boost circuit is the DC-DC boost circuit with the highest input voltage among the n DC-DC boost circuits.

[0008] When the switching transistors in several DC-DC boost circuits with the highest and similar input voltages are kept off, not only can the power loss caused by the power conversion operation of the DC-DC boost circuit be minimized, but the impact of these DC-DC boost circuits on the voltage fluctuation of the DC bus is also similar, which can make the inverter stabilize more quickly under new operating conditions.

[0009] In one implementation, the controller is further configured to control the inverter circuit to operate in MPPT state while the switching transistor in any one of the plurality of DC boost circuits remains off.

[0010] When the switching transistors in part of the DC-DC boost circuit in the inverter remain open, this part of the DC-DC boost circuit stops power conversion and stops MPPT. In order to maximize the total power input to the optimizer corresponding to this part of the DC-DC boost circuit, the inverter circuit executes MPPT at this time, so that the total power input to the optimizer corresponding to the DC-DC boost circuit continues to be at its maximum. Furthermore, during the execution of MPPT, the DC bus voltage eventually stabilizes near the minimum voltage value among the voltage values ​​corresponding to the maximum power points of each DC-DC boost circuit. At this time, the reduction of the DC bus voltage, that is, the reduction of the input voltage of the inverter circuit, reduces the power conversion loss of the inverter circuit, further improving the overall operating efficiency of the inverter.

[0011] In one implementation, the controller is configured to, when controlling the inverter circuit to operate in MPPT state, if the difference between the voltage of the DC bus and the first threshold is less than or equal to a third threshold and the duration is greater than or equal to a first time threshold, control the inverter circuit to exit MPPT state and control the m DC boost circuits to operate in MPPT state.

[0012] When the inverter circuit performs MPPT, the DC bus will eventually stabilize near the minimum voltage value among the voltage values ​​corresponding to the maximum power point in each DC boost circuit. When this minimum voltage value is less than or equal to the minimum voltage limit of the DC bus, the DC bus voltage will move towards the minimum voltage limit of the DC bus, that is, towards the first threshold. When the difference between the DC bus voltage and the first threshold is small and lasts for a relatively long time, it can be considered that the voltage input to some optimizers is low and they are no longer operating at the maximum power output point. At this time, all DC boost circuits are put back into MPPT state, and the inverter circuit is put out of MPPT, thereby ensuring that the inverter as a whole is put back into maximum power output. The controller is then put back into the judgment logic in the aforementioned embodiment to re-determine whether to put some or all DC boost circuits out of MPPT based on the latest operating conditions.

[0013] In one implementation, the controller is configured to control the absolute value of the voltage difference between the input voltage of the DC boost circuit and the voltage of the DC bus, which is operating in MPPT state, to be greater than or equal to a fourth threshold, wherein the fourth threshold is greater than or equal to the minimum voltage difference limit of the DC boost circuit.

[0014] It should be understood that DC boost circuits may have a minimum voltage difference between their input and output voltages in practical applications due to factors such as the minimum duty cycle of the power devices in the circuit. In other words, there is a minimum voltage difference between the input voltage and the DC bus voltage of the DC boost circuit; otherwise, the DC boost circuit cannot work properly.

[0015] In one implementation, the controller is configured to control the m DC boost circuits to operate in the MPPT state and control the inverter circuit to exit the MPPT state when the absolute value of the voltage difference between the input voltage of the DC boost circuit operating in the MPPT state and the voltage of the DC bus is greater than or equal to the fourth threshold and less than or equal to the fifth threshold, and the duration is greater than or equal to the second time threshold.

[0016] When the voltage difference of the DC-DC boost circuit is close to the minimum voltage difference limit and lasts for a relatively long time, it can be assumed that the maximum power point of the DC-DC boost circuit may be at a higher voltage position. However, due to the limitation of the minimum voltage difference, the voltage of the DC-DC boost circuit cannot reach the maximum power point. At this time, all DC-DC boost circuits are put back into MPPT state, and the inverter circuit is put out of MPPT. This ensures that the inverter as a whole can output maximum power again, and the controller is put back into the judgment logic in the aforementioned embodiment to re-determine whether to put some or all DC-DC boost circuits out of MPPT for the latest operating conditions, so as to ensure that the power generation efficiency is optimal in the new operating conditions.

[0017] In one implementation, when the controller controls the switching transistors in the m DC boost circuits to remain off for a duration greater than or equal to a third time threshold, it controls the m DC boost circuits to operate in MPPT state and controls the inverter circuit to exit MPPT state.

[0018] After some or all of the DC boost circuits in the inverter are out of MPPT for a period of time exceeding a preset fixed time, the controller is allowed to re-enter the aforementioned judgment logic. This allows the controller to determine whether to partially or completely exit MPPT based on the latest operating conditions, ensuring optimal power generation efficiency under the new operating conditions.

[0019] In one implementation, the operation in the MPPT state is achieved through a perturbation method.

[0020] The perturbation method is a commonly used method for performing MPPT. When performing MPPT using the perturbation method, the controller gradually reduces the voltage value input to the optimizer from the open-circuit voltage of the optimizer to find the maximum power point of the optimizer. During this process, there will be no large step jumps in the voltage value, thereby avoiding exceeding the minimum voltage limit of the DC bus or the minimum differential voltage limit of the DC-DC converter circuit under specific operating conditions. It can work efficiently with the control logic of the aforementioned embodiment.

[0021] Secondly, this application provides an inverter control method. The inverter includes multiple DC-DC boost circuits, a DC bus, and an inverter circuit. Each of the multiple DC-DC boost circuits is used to receive the DC input of an optimizer connected to one or more photovoltaic modules, and to boost the DC input of the optimizer and output it to the inverter circuit through the DC bus. The inverter circuit receives the DC input of the multiple DC-DC boost circuits and converts the DC input of the multiple DC-DC boost circuits into AC power for output to a load or the power grid. The method includes: when the multiple DC-DC boost circuits are operating in MPPT state, if the input voltage of each of the n DC-DC boost circuits is greater than or equal to a first threshold, controlling the switching transistors in m of the n DC-DC boost circuits to remain open, where m and n are both integers greater than 0 and n ≥ m, and the first threshold is greater than or equal to the minimum voltage limit of the DC bus.

[0022] In one implementation, the m DC-DC boost circuits include a first DC-DC boost circuit and a DC-DC boost circuit among the n DC-DC boost circuits whose absolute value of the difference between its input voltage and that of the first DC-DC boost circuit is less than or equal to a second threshold; wherein, the first DC-DC boost circuit is the DC-DC boost circuit with the highest input voltage among the n DC-DC boost circuits.

[0023] In one implementation, the inverter circuit is controlled to operate in MPPT state while the switching transistor in any one of the plurality of DC boost circuits remains off.

[0024] In one implementation, when the inverter circuit is controlled to operate in MPPT state, if the difference between the voltage of the DC bus and the first threshold is less than or equal to the third threshold and the duration is greater than or equal to the first time threshold, the inverter circuit is controlled to exit MPPT state, and the m DC boost circuits are controlled to operate in MPPT state.

[0025] In one implementation, the absolute value of the voltage difference between the input voltage of the DC boost circuit operating in MPPT state and the voltage of the DC bus is greater than or equal to a fourth threshold, the fourth threshold being greater than or equal to the minimum voltage difference limit of the DC boost circuit.

[0026] In one implementation, when the absolute value of the voltage difference between the input voltage of the DC boost circuit and the voltage of the DC bus is greater than or equal to the fourth threshold and less than or equal to the fifth threshold, and the duration is greater than or equal to the second time threshold, the m DC boost circuits are controlled to operate in MPPT state, and the inverter circuit is controlled to exit MPPT state.

[0027] In one implementation, when the duration for which the switching transistors in the m DC boost circuits remain off is greater than or equal to a third time threshold, the m DC boost circuits are controlled to operate in MPPT state, and the inverter circuit is controlled to exit MPPT state.

[0028] In one implementation, control operating in the MPPT state is achieved using a perturbation method.

[0029] It should be understood that the implementations and beneficial effects of the above-mentioned aspects of this application can be referenced from each other. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the connection of an optimizer string provided in this application;

[0031] Figure 2 This is a connection diagram of a single-stage inverter provided in this application;

[0032] Figure 3 This is a connection diagram of a two-stage inverter provided in this application;

[0033] Figure 4 This is a schematic diagram of a DC boost circuit topology provided in this application;

[0034] Figure 5 This is a schematic diagram of the PV curve of an optimizer output provided in this application;

[0035] Figure 6 This is a schematic diagram of the PV curve of an optimizer output provided in this application;

[0036] Figure 7 This is a schematic diagram of the PV curve of an optimizer output provided in this application;

[0037] Figure 8 This is a schematic diagram of the PV curve of an optimizer output provided in this application;

[0038] Figure 9 This is a schematic diagram of the PV curve of an optimizer output provided in this application;

[0039] Figure 10 This is a schematic diagram of an inverter control process provided in this application. Detailed Implementation

[0040] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0041] The term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of this application. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that excludes other embodiments. Unless otherwise specified, the terms "connected," "linked," and "connected" used herein to indicate electrical connection mean direct or indirect electrical connection.

[0042] In the following description, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "a plurality of" means two or more.

[0043] The technical solutions provided in this application can be applied to different application scenarios, especially to application scenarios that use photovoltaic power generation systems, such as industrial and commercial distributed power stations and residential photovoltaic power generation systems.

[0044] To facilitate understanding of the technical solutions provided in the embodiments of this application, the following section selects one scenario to illustrate the specific application of the technical solutions provided in the embodiments of this application. (Reference) Figure 1 This application provides an optimizer connection method and optimizer string connection format as an embodiment. For example... Figure 1 As shown, an optimizer is connected to a photovoltaic module, the photovoltaic module is directly connected to the input of the optimizer, and the output of the optimizer can be directly connected to the next stage of power electronic equipment, or as shown in the diagram. Figure 1 As shown, it is first connected in series with other optimizers before being connected to the next stage of power electronic equipment, such as an inverter.

[0045] refer to Figure 2 This is a connection method for a single-stage inverter and optimizer. The power conversion circuit in a single-stage inverter only includes the inverter circuit. The optimizers are first connected in an optimizer string, and then connected to the inverter. The electrical energy provided by the optimizer string is transmitted to the inverter circuit through the DC bus in the inverter, i.e., a DC / AC circuit. The AC output terminal of the inverter circuit is connected to the power grid.

[0046] refer to Figure 3This describes a two-stage inverter and optimizer connection configuration. The power conversion circuit in the two-stage inverter includes an inverter circuit and a DC / DC converter circuit. Optimizers are first connected in a string and then connected to the inverter. The power supplied by the optimizer string is transmitted to the inverter circuit via the DC / DC converter circuit within the inverter. The DC / DC converter circuit then transmits the power to the inverter circuit via a DC bus. The AC output of the inverter circuit is connected to the power grid. This two-stage architecture allows for more flexible adjustments to the power conversion within the inverter. The technical solution in this application is primarily designed based on this two-stage inverter architecture. It is understood that this two-stage inverter can also be designed as a multi-stage inverter, which has multiple DC / DC converter circuits that work together to convert the input DC power to DC / DC, and then output the power to the inverter circuit via a DC bus. The operating mode of the DC / DC conversion circuit in the two-stage inverter in this application embodiment can also be an equivalent operating mode formed by the coordinated operation of multiple DC / DC conversion circuits in a multi-stage inverter.

[0047] It should be understood that the output of the above inverter circuit can also be directly connected to the load.

[0048] This application describes an example where the optimizer uses a DC buck circuit and the inverter's DC / DC conversion circuit uses a DC boost circuit.

[0049] In one implementation, the optimizer uses a DC-DC buck converter, the inverter is a two-stage inverter, and the DC / DC conversion circuit within the inverter uses a DC-DC boost converter. This allows the optimizer and the inverter's DC / DC conversion circuit to work together to provide a suitable voltage to the DC bus and operate in MPPT mode, ensuring the inverter continuously and stably maintains maximum power output. (Reference) Figure 4 This diagram illustrates a DC-DC boost circuit topology provided in an embodiment of this application. During the power conversion process, the switching transistor R continuously switches, thereby boosting the voltage input to the optimizer and outputting it to the inverter circuit via the DC bus. The subsequent embodiments in this application are all based on this implementation method for technical solution description.

[0050] refer to Figure 5 This is an optimizer output PV curve provided in the embodiment of the application. Figure 5As shown, the output power curve of the optimizer is generally divided into a current-limiting region, a constant power region, and a buck region. In the current-limiting region, starting from 0 voltage, the higher the output voltage of the optimizer, the greater the output power. In the constant power region, the output power of the optimizer remains at its maximum value. In the buck region, starting from the open-circuit voltage, the lower the output voltage of the optimizer, the greater the output power. It can be seen that the optimizer can maintain its maximum output power as long as the output voltage is between the minimum voltage value Vmin and the maximum voltage value Vmax in the constant power region.

[0051] In one embodiment, the DC / DC converter circuit in the inverter operates in MPPT (Maximum Power Point Test) mode. In this mode, the DC / DC converter circuit adjusts its operating state in real time, thereby changing the input voltage it receives, and thus the output voltage of the optimizer connected to it, to maximize the optimizer's output power. Generally, the DC / DC converter circuit performs MPPT from startup. Specifically, the DC / DC converter circuit uses a perturbation method to perform MPPT. During this process, the DC / DC converter circuit slightly perturbs its input voltage within a left-right range, changing its perturbation direction according to changes in input power. For example, after perturbing to the left (i.e., reducing the input voltage), if the input power increases, it continues to perturb to the left; if the input power decreases or remains unchanged, it perturbs to the right. The feedback adjustment form for right perturbation is similar to that for left perturbation, until the maximum power point is found and stabilizes near the maximum power point. The DC / DC converter circuit, from startup, gradually decreases its input voltage from the open-circuit voltage until it finds and stabilizes near the maximum power point; that is, from startup, the DC / DC converter circuit gradually decreases the optimizer's input voltage from the open-circuit voltage. Figure 5 The open-circuit voltage shown, that is, the voltage corresponding to point C, gradually decreases to near the maximum power point D, thereby maintaining the maximum power output.

[0052] It should be noted that the optimizer described in the above embodiments can be a single optimizer or multiple optimizers. The DC / DC converter is connected to multiple optimizers as mentioned in the previous embodiments. Multiple optimizers are first connected into an optimizer string and then connected to the DC / DC converter circuit. The optimizers mentioned below are also understood in this way.

[0053] The following embodiments will use a DC-DC buck converter in the optimizer and a DC / DC converter in the inverter as an example. However, it should be understood that the embodiments of this application are not limited to this scenario. As long as the output PV curve of the optimizer meets similar characteristics, the DC / DC converter in the inverter can have similar MPPT functionality. Furthermore, the following embodiments will all be described using an inverter that includes four DC / DC converter circuits as an example.

[0054] refer to Figure 6This illustration shows the output PV curves of the optimizers corresponding to the four DC boost circuits in an inverter provided in this application embodiment. For simplicity, only the voltage value corresponding to the maximum power point and the PV curve with the minimum power value are labeled and explained. The maximum power point and open-circuit voltage of other PV curves can be understood by referring to the labeled PV curves. Figure 6 In the implementation scenario, the voltages corresponding to the maximum power points D1-D4 (D1-D3 not shown) of the optimizers corresponding to the four DC boost circuits are all less than the minimum voltage limit of the DC bus. Figure 6 In this implementation scenario, the DC-DC boost circuit in the inverter needs to boost the voltage to ensure that its output voltage meets the minimum voltage requirement of the DC bus, i.e., the DC bus voltage must be greater than or equal to the minimum voltage limit. Under these conditions, the DC-DC boost circuit performs MPPT starting from the open-circuit voltage. However, due to the existence of the minimum voltage limit, which is greater than the voltage corresponding to the maximum power point, the DC-DC boost circuit cannot continue to search for the maximum power point by decreasing the voltage. The input voltage of the DC-DC boost circuit will ultimately be limited to near the minimum voltage limit. It should be understood that the minimum voltage limit of the DC bus is determined by the AC voltage of the power grid or load connected to the inverter circuit.

[0055] refer to Figure 7 This application provides another inverter including four DC-DC boost circuits, and the output PV curves of the optimizers corresponding to each of the four DC-DC boost circuits. (Example:) Figure 7 As shown, the voltage values ​​corresponding to the maximum power points D1-D4 (D1-D3 not shown) are all greater than the minimum voltage limit of the DC bus. At this point, the DC boost circuit in the inverter does not need to perform boost operation to ensure that the output voltage of the DC boost circuit meets the minimum voltage requirement of the DC bus. Therefore, the DC boost circuit can be left unused for power conversion; that is, the switching transistors in the DC boost circuit stop switching and remain in an open state. Figure 7In the implementation scenario shown, the electrical energy input to the optimizer is directly transmitted to the DC bus through the DC boost circuit. Therefore, the operating mode of the DC boost circuit in this case is also called the shoot-through mode. At this time, the inverter circuit performs MPPT, meaning the controller adjusts the input voltage of the inverter circuit, i.e., adjusts the DC bus voltage, to find the maximum power point (MPPT) and maximize the sum of the output power of the four optimizers. Similarly, the controller uses a perturbation method to control the inverter circuit to search for the maximum sum of the output power of the four optimizers to the left. This means controlling the DC bus voltage to decrease from the open-circuit voltage until it drops to the voltage value corresponding to the maximum power point D4. Only after the DC bus voltage continues to decrease will the sum of the output power of the four optimizers continue to increase. Therefore, the DC bus voltage will eventually be maintained near the voltage value of the maximum power point corresponding to the optimizer with the smallest maximum output power among the four optimizers. Figure 7 As shown, the inverter circuit maintains the DC bus voltage at the maximum power point (MPPT) D4 by adjusting the DC bus voltage, thus ensuring that each optimizer operates in the constant power region and maximizing the sum of the output power of each optimizer. In this configuration, the inverter not only maintains maximum power output but also reduces losses by keeping the switching transistors in the DC-boost circuit open. It should be understood that under this condition, some DC-boost circuits—for example, one, two, or three—can be deactivated while the others continue to perform MPPT. This mode of operation, where some or all DC-boost circuits are deactivated based on the input voltage while the inverter performs MPPT, is called optimization mode.

[0056] In one implementation, the inverter controller (not shown in the accompanying drawings) will, when multiple DC-DC boost circuits in the inverter are operating in MPPT state, if the input voltage of n DC-DC boost circuits is greater than or equal to a first threshold, control the switching transistors in m of the n DC-DC boost circuits to remain open, where m and n are both integers greater than 0 and n ≥ m, and the first threshold is greater than or equal to the minimum voltage limit of the DC bus. Figure 7 As shown, taking the first threshold being equal to the minimum voltage limit of the DC bus as an example, there are four DC boost circuits in the inverter, and the input voltage of all four DC boost circuits is greater than the first threshold (n=4). At this time, the inverter controller will control one or more of the DC boost circuits to keep the switching transistors open (1≤m≤4). At this time, the input voltage of the DC boost circuit whose switching transistors are kept open is directly output to the DC bus. Since it is greater than the minimum voltage limit of the DC bus, it can meet the normal operation of the subsequent inverter circuit and save the operating loss of the DC boost circuit.

[0057] It should be understood that in practical applications, the first threshold can also be set to a value greater than the minimum voltage limit to provide some safety margin. This application does not specifically limit the value of the minimum voltage limit. For ease of explanation, this application uses the example of the first threshold being equal to the minimum voltage limit.

[0058] To determine which m DC-DC boost circuits should stop power conversion from among n DC-DC boost circuits whose input voltages are all greater than or equal to a first threshold, the following are some possible implementation methods.

[0059] In one implementation, the input voltage of n DC-DC boost circuits in the inverter is greater than or equal to a first threshold. The switches in m of these n DC-DC boost circuits are kept off. Here, the m selected DC-DC boost circuits are those whose input voltage values ​​are sorted from largest to smallest, corresponding to the first to mth DC-DC boost circuits. It is understandable that the higher the input voltage of a DC-DC boost circuit, the greater the power loss. Therefore, keeping the switches in the first m DC-DC boost circuits with the highest input voltage values ​​off can better save operating losses.

[0060] In another embodiment, the input voltage of n DC-DC boost circuits in the inverter is greater than or equal to a first threshold. The switches in m of the n DC-DC boost circuits are kept off. Here, the m selected DC-DC boost circuits are not only the top m DC-DC boost circuits in terms of input voltage value, but also the voltage difference between the DC-DC boost circuit with the highest and lowest input voltage is less than or equal to a second threshold. It is understood that when the voltage difference between the DC-DC boost circuit with the highest and lowest input voltage is relatively close, the operating conditions of the several DC-DC boost circuits are similar. Thus, the controller's simultaneous control of keeping the switches in these DC-DC boost circuits off has a more consistent impact on the DC bus voltage, making it easier for the inverter circuit to achieve a more efficient result when performing MPPT to find the maximum power point.

[0061] The above describes two operating modes. Further, after the inverter has been operating in optimized mode for a period of time, the photovoltaic power generation environment may change, and the optimized mode may no longer be the most efficient operating mode under the new operating conditions. At this time, it is necessary to make the inverter work back to normal mode, that is, to make all DC boost circuits in the inverter re-execute MPPT. The controller in the inverter will then re-determine whether to control some or all of the DC boost circuits to work in shoot-through mode based on the new operating conditions, so that the inverter can work back to optimized mode, thereby ensuring that the inverter's operating mode is the optimal mode under the latest operating conditions.

[0062] Continue to refer to Figure 8 In one possible implementation scenario, when the photovoltaic module connected to the optimizer with the lowest output power experiences a reduction in power generation due to shading or other reasons, its output PV curve will change accordingly. As shown in the figure, the output voltage corresponding to the maximum power point D4 of the PV curve for this optimizer will decrease, meaning the DC bus voltage will also decrease, causing the DC bus voltage to gradually approach the minimum voltage limit. At this point, if the inverter circuit continues to execute MPPT, keeping some or all the switching transistors in the DC boost circuit open, insufficient DC bus voltage may cause the inverter to malfunction. Therefore, the inverter needs to exit the termination optimization mode at this time.

[0063] In one implementation, when the inverter controller keeps the switches in the m DC-DC boost circuits in the inverter open and controls the inverter circuit to operate in MPPT state, if the difference between the DC bus voltage and the first threshold is less than or equal to the third threshold and the duration is greater than or equal to the first time threshold, the inverter circuit exits the MPPT state, and the m DC-DC boost circuits are controlled to operate in MPPT state again. Thus, when the DC bus voltage remains near the minimum voltage limit for an extended period, it is assumed that the voltage of the maximum power point sought by the inverter circuit is actually below the minimum voltage limit. In this case, some or all DC-DC boost circuits are no longer allowed to operate in shoot-through mode, and the inverter operates in normal mode. That is, all DC-DC boost circuits re-execute MPPT. The inverter controller determines whether to allow some or all DC-DC boost circuits to operate in shoot-through mode based on the conditions under the new operating conditions, ensuring the rationality of the inverter operating in optimized mode and thus achieving optimal efficiency.

[0064] It should be understood that in practical applications, the third threshold can also be set equal to the first threshold to increase the requirements for exiting the optimization mode and prevent frequent exits due to overly lenient exit conditions. The embodiments in this application do not impose specific restrictions on the value of the third threshold. For ease of explanation, this application uses the example of the third threshold being equal to the first threshold, i.e., equal to the minimum voltage limit.

[0065] In addition to considering the adverse effects of the DC bus voltage approaching the minimum voltage limit, under certain operating conditions, it is also necessary to consider whether the voltage difference between the input and output voltages of the DC boost circuit is sufficient for the DC boost circuit to operate normally. This is because if the voltage difference between the input and output voltages of the DC boost circuit is too small, that is, less than the minimum voltage difference limit, the controller cannot drive the switching transistor of the DC boost circuit to perform normal switching action.

[0066] In one embodiment, the inverter controller is further configured to ensure that the absolute value of the voltage difference between the input voltage of the DC-DC boost circuit operating in MPPT state and the voltage of the DC bus is greater than or equal to a fourth threshold, wherein the fourth threshold is greater than or equal to a minimum voltage difference limit of the DC-DC boost circuit, to ensure that the DC-DC boost circuit can operate normally. This application does not impose specific limitations on the value of the fourth threshold. For ease of explanation, this application uses the example of the fourth threshold being equal to the minimum voltage limit. It should be understood that in practical applications, the fourth threshold can also be set to be greater than the minimum voltage difference limit to provide some safety margin.

[0067] refer to Figure 9 In another possible implementation scenario, the optimization mode also needs to be terminated due to the minimum voltage difference limit between the input and output voltages of the DC boost circuit.

[0068] like Figure 9As shown, the voltage value corresponding to the maximum power point D4 of the optimizer connected to one of the DC boost circuits is low, which is lower than the minimum voltage limit of the DC bus voltage. At this time, the DC boost circuit needs to perform a power conversion operation to boost the input voltage of the DC boost circuit and output it to the DC bus, thereby meeting the minimum voltage limit of the DC bus. After a period of time, the power generation capacity of the photovoltaic modules connected to the optimizer corresponding to the DC-DC boost circuit with the lowest input voltage may increase. As a result, the voltage value corresponding to the maximum power point D4 of the optimizer connected to this DC-DC boost circuit will rise above the minimum voltage limit. At this point, the DC-DC boost circuit continues its power conversion operation and also performs MPPT (Multi-Level Testing). Therefore, the input voltage of the DC-DC boost circuit will gradually move from its original position towards D4 after the power generation environment improves. However, because the inverter is operating in optimization mode, some DC-DC boost circuits stop power conversion. The DC bus voltage is determined by the inverter's MPPT process. Once the DC bus voltage is determined, due to the minimum voltage difference limit of the DC-DC boost circuit, the input voltage of the DC-DC boost circuit performing MPPT will gradually move towards D4 as the power generation environment improves, as D4 moves to the right. However, due to the existence of the minimum voltage difference limit, the input voltage of the DC-DC boost circuit will be limited to a position where the difference between the DC bus voltage and the minimum voltage difference limit is exactly equal to the minimum voltage difference limit, ensuring the normal operation of the DC-DC boost circuit. Understandably, at this point, the input voltage of the DC-DC boost circuit cannot continue to move to the right following the maximum power point D4. As the power generation environment continues to improve, the input voltage of the DC-DC boost circuit will move further and further away from the maximum power point D4. At this time, the DC-DC boost circuit cannot operate at the maximum power point, and the voltage of the DC bus cannot be reduced to the voltage value corresponding to the maximum power point D4. At this time, the optimization mode is not the mode with the best inverter efficiency. Therefore, when the controller keeps the switching transistors in some DC-DC boost circuits in the inverter open and allows the inverter circuit to operate in MPPT state, if the voltage difference between the input voltage of the DC-DC boost circuit executing MPPT and the voltage of the DC bus is close to the minimum voltage difference limit for a period of time, the controller will control the inverter to exit the optimization mode and allow all DC-DC boost circuits to resume executing MPPT. After all DC-DC boost circuits are operating at the maximum power point, the controller will then control whether to keep the switching transistors open based on whether the voltage of the maximum power point of each DC-DC boost circuit meets the condition.

[0069] In one implementation for the aforementioned operating conditions, the inverter controller keeps the switching transistors in the m DC-DC boost circuits in the inverter open and controls the inverter circuits to operate in MPPT state. When the voltage difference between the input voltage of the DC-DC boost circuit and the DC bus voltage is greater than or equal to a fourth threshold and less than or equal to a fifth threshold, and the duration is greater than or equal to a second time threshold, the controller controls the m DC-DC boost circuits to operate in MPPT state and controls the inverter circuits to exit MPPT state. The fifth threshold is greater than or equal to the fourth threshold. Thus, if the voltage difference of the DC-DC boost circuits is consistently close to the minimum voltage difference limit, it is considered that the maximum power point input voltage of the DC-DC boost circuit is greater than its actual operating input voltage. The minimum voltage difference limit prevents the input voltage of the DC-DC boost circuit from further increasing. In this case, the inverter exits the optimization mode and re-operates in normal mode, allowing all DC-DC boost circuits to re-execute MPPT. Based on the conditions under the new operating conditions, it is determined whether to allow some or all DC-DC boost circuits to exit MPPT, ensuring the rationality of the grouping and achieving optimal efficiency.

[0070] It should be understood that in practical applications, the fifth threshold can also be set equal to the fourth threshold to increase the requirements for exiting the optimization mode and prevent frequent exits due to overly lenient exit conditions. The embodiments in this application do not impose specific restrictions on the value of the third threshold. For ease of explanation, this application uses the example of the fifth threshold being equal to the fourth threshold, i.e., equal to the minimum differential pressure limit.

[0071] In addition to considering the adverse effects of the DC bus voltage being close to the minimum voltage limit and the small voltage difference between the input and output voltages of the DC boost circuit, there are also environmental conditions of photovoltaic power generation that cause other changes in the output PV curves of each optimizer. This necessitates re-evaluating the inverter's operating mode to achieve optimal efficiency. However, in order for the controller to understand the latest output PV curve characteristics of each optimizer, it is necessary to re-execute MPPT for each DC boost circuit so that the output of each optimizer can better reflect the latest output PV curve characteristics.

[0072] To enable the inverter controller to periodically review the inverter's optimal operating mode, it can periodically urge the inverter to operate in normal mode. In one embodiment, when the switching transistors in m DC-DC boost circuits remain off for a duration greater than or equal to a third time threshold, the inverter controller controls the m DC-DC boost circuits to operate in MPPT state and controls the inverter circuits to exit MPPT state. Thus, if the inverter operates in optimized mode for an extended period, it is assumed that factors such as the solar power system's illumination environment have changed significantly, and the current optimized mode is no longer optimal. The inverter then reverts to normal operating mode and enters the next optimized mode based on actual operating conditions, thereby ensuring optimal efficiency.

[0073] In one implementation, it should be noted that in the above embodiments, the MPPT operating states of both the DC boost circuit and the inverter circuit can be implemented using the perturbation method. The perturbation method is a commonly used method for executing MPPT. When executing MPPT using the perturbation method, the controller gradually decreases the voltage value input to the optimizer from the open-circuit voltage of the optimizer, thereby finding the optimizer's maximum power point. During this process, large voltage jumps do not occur, thus avoiding exceeding the minimum voltage limit of the DC bus or the minimum differential voltage limit of the DC-DC converter circuit under specific operating conditions. This allows for efficient coordination with the control logic of the aforementioned embodiments. Alternatively, the MPPT operating states of the DC boost circuit and the inverter circuit can also be implemented using methods such as the incremental conductance method. This application does not limit the specific implementation method of the MPPT operating states.

[0074] refer to Figure 10 This is a schematic diagram of an inverter control process provided in an embodiment of this application.

[0075] First, execute step S101: The inverter first operates in normal mode, that is, each DC boost circuit in the inverter operates in MPPT state, and the input voltage of each DC boost circuit is stabilized at the maximum power point.

[0076] Then, step S102 is executed: it is determined whether the input voltage of the multiple DC-DC boost circuits meets the conditions for entering the optimization mode. When the inverter meets the conditions for entering the optimization mode, the inverter is controlled to operate in the optimization mode. Specifically, the input voltage of n DC-DC boost circuits in the multiple DC-DC boost circuits is greater than or equal to a first threshold. That is, whether there is a DC-DC boost circuit in the multiple DC-DC boost circuits of the inverter with an input voltage greater than or equal to the first threshold. If yes, step S103 is executed; otherwise, step S101 is executed.

[0077] Step S103: The inverter operates in optimization mode. Specifically, it controls m out of n DC boost circuits to stop power conversion, that is, it controls some or all of the switching transistors of the n DC boost circuits with input voltage greater than or equal to the first threshold to remain open.

[0078] After executing step S103, execute step S104.

[0079] Step S104: Determine whether the inverter meets the conditions for exiting the optimization mode. If the inverter meets the conditions for exiting the optimization mode, control the inverter to operate in normal mode. Specifically, determine whether the duration for which the m DC boost circuits stop power conversion is greater than or equal to the third time threshold. If yes, exit the optimization mode, that is, stop executing step S103 and execute step S101. If no, continue executing S103.

[0080] The aforementioned conditions for determining whether an inverter meets the exit optimization mode can also be as described in the previous embodiments: whether the difference between the DC bus voltage and the first threshold is less than or equal to the third threshold and the duration is greater than or equal to the first time threshold, or whether the absolute value of the voltage difference between the input voltage of the DC boost circuit and the DC bus voltage is greater than or equal to the fourth threshold.

[0081] The judgment and execution process is similar to that of the aforementioned embodiments, and will not be repeated here.

[0082] The preferred embodiments disclosed above are merely illustrative of this application. These preferred embodiments do not exhaustively describe all details, nor do they limit the application to the specific implementations described. Clearly, many modifications and variations can be made based on the embodiments of this application. These embodiments are selected and specifically described in this application to better explain the principles and practical applications of this application, thereby enabling those skilled in the art to better understand and utilize this application. This application is limited only by the claims and their full scope and equivalents.

Claims

1. An inverter, characterized in that, The device includes multiple DC boost circuits, a DC bus, an inverter circuit, and a controller. Each DC boost circuit receives the DC input from an optimizer connected to one or more photovoltaic modules, and performs power conversion on the DC input of the optimizer before outputting it to the inverter circuit via the DC bus. The inverter circuit receives the DC input from the multiple DC boost circuits and converts the DC input from the multiple DC boost circuits into AC power for output to the load or the power grid. The controller is configured to, when the plurality of DC boost circuits are operating in MPPT state, if the input voltage of each of the n DC boost circuits is greater than or equal to a first threshold, control the switching transistors in m of the n DC boost circuits to remain open, where m and n are both integers greater than 0 and n≥m, and the first threshold is greater than or equal to the minimum voltage limit of the DC bus.

2. The inverter according to claim 1, characterized in that, The m DC-DC boost circuits include a first DC-DC boost circuit and a DC-DC boost circuit among the n DC-DC boost circuits whose absolute value of the difference between its input voltage and that of the first DC-DC boost circuit is less than or equal to a second threshold; wherein, the first DC-DC boost circuit is the DC-DC boost circuit with the highest input voltage among the n DC-DC boost circuits.

3. The inverter according to claim 1 or 2, characterized in that, The controller is also configured to control the inverter circuit to operate in MPPT state when the switching transistor in any one of the plurality of DC boost circuits remains off.

4. The inverter according to claim 3, characterized in that, The controller is configured to, when controlling the inverter circuit to operate in MPPT state, if the difference between the voltage of the DC bus and the first threshold is less than or equal to a third threshold and the duration is greater than or equal to a first time threshold, control the inverter circuit to exit MPPT state and control the m DC boost circuits to operate in MPPT state.

5. The inverter according to any one of claims 1-5, characterized in that, The controller is used to control the absolute value of the voltage difference between the input voltage of the DC boost circuit and the voltage of the DC bus when the DC boost circuit is operating in MPPT state to be greater than or equal to a fourth threshold, wherein the fourth threshold is greater than or equal to the minimum voltage difference limit of the DC boost circuit.

6. The inverter according to claim 6, characterized in that, The controller is used to control the m DC boost circuits to operate in the MPPT state and control the inverter circuit to exit the MPPT state when the absolute value of the voltage difference between the input voltage of the DC boost circuit operating in the MPPT state and the voltage of the DC bus is greater than or equal to the fourth threshold and less than or equal to the fifth threshold, and the duration is greater than or equal to the second time threshold.

7. The inverter according to any one of claims 1-6, characterized in that, When the controller controls the switching transistors in the m DC boost circuits to remain off for a duration greater than or equal to a third time threshold, it controls the m DC boost circuits to operate in MPPT state and controls the inverter circuit to exit MPPT state.

8. The inverter according to any one of claims 1-7, characterized in that, The operation in MPPT state is achieved through the perturbation method.

9. An inverter control method, characterized in that, The inverter includes multiple DC boost circuits, a DC bus, and an inverter circuit. Each of the multiple DC boost circuits is used to receive the DC input of an optimizer connected to one or more photovoltaic modules, and to boost the DC input of the optimizer and output it to the inverter circuit through the DC bus. The inverter circuit is used to receive the DC input of the multiple DC boost circuits and to convert the DC input of the multiple DC boost circuits into AC power output to the load or the power grid. The method includes: When the plurality of DC boost circuits are operating in MPPT state, if the input voltage of each of the n DC boost circuits is greater than or equal to a first threshold, the switching transistors in m of the n DC boost circuits are kept off, where m and n are both integers greater than 0 and n≥m, and the first threshold is greater than or equal to the minimum voltage limit of the DC bus.

10. The inverter control method according to claim 9, characterized in that, The m DC-DC boost circuits include a first DC-DC boost circuit and a DC-DC boost circuit among the n DC-DC boost circuits whose absolute value of the difference between its input voltage and that of the first DC-DC boost circuit is less than or equal to a second threshold; wherein, the first DC-DC boost circuit is the DC-DC boost circuit with the highest input voltage among the n DC-DC boost circuits.

11. The inverter control method according to claim 9 or 10, characterized in that, With the switching transistor in any one of the plurality of DC boost circuits kept off, the inverter circuit is controlled to operate in MPPT state.

12. The inverter control method according to claim 11, characterized in that, When the inverter circuit is operating in MPPT state, if the difference between the voltage of the DC bus and the first threshold is less than or equal to the third threshold and the duration is greater than or equal to the first time threshold, the inverter circuit is controlled to exit the MPPT state, and the m DC boost circuits are controlled to operate in MPPT state.

13. The inverter control method according to any one of claims 9-12, characterized in that, The absolute value of the voltage difference between the input voltage of the DC boost circuit and the voltage of the DC bus when the circuit is operating in MPPT state is greater than or equal to a fourth threshold, wherein the fourth threshold is greater than or equal to the minimum voltage difference limit of the DC boost circuit.

14. The inverter control method according to claim 13, characterized in that, When the absolute value of the voltage difference between the input voltage of the DC boost circuit operating in MPPT state and the voltage of the DC bus is greater than or equal to the fourth threshold and less than or equal to the fifth threshold, and the duration is greater than or equal to the second time threshold, the m DC boost circuits are controlled to operate in MPPT state, and the inverter circuit is controlled to exit MPPT state.

15. The inverter control method according to any one of claims 9-14, characterized in that, When the duration for which the switching transistors in the m DC boost circuits remain off is greater than or equal to a third time threshold, the m DC boost circuits are controlled to operate in MPPT state, and the inverter circuit is controlled to exit MPPT state.

16. The inverter control method according to any one of claims 9-15, characterized in that, Control is achieved in MPPT state using the perturbation method.

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