A photovoltaic system and a method of controlling the matching ratio

Abstract

The application discloses a photovoltaic system and a capacity-ratio control method. The photovoltaic system comprises a DCAC converter, N DCDC converters and a controller, wherein N is an integer greater than or equal to 2; an input end of each DCDC converter is used for connecting a corresponding photovoltaic string, and output ends of the N DCDC converters are all connected to an input end of the DCAC converter; the controller is used for controlling M DCDC converters in the N DCDC converters to work in a conduction state when it is necessary to reduce a capacity ratio of the photovoltaic system, and stopping energy of corresponding photovoltaic strings from entering the DCAC converter; and M is an integer greater than or equal to 1 and smaller than N. After the capacity ratio of the photovoltaic system is reduced, the output power of the DCDC converter will be improved, so that the power generation capacity of the photovoltaic system is improved, and the power generation efficiency is improved.

Description

Technical Field

[0001] This application relates to the field of new energy technology, specifically to a photovoltaic system and a method for controlling the capacity ratio. Background Technology

[0002] A photovoltaic (PV) system typically consists of multiple PV strings, multiple DC-DC converters, and one DC-AC converter. The input of each DC-DC converter is connected to its corresponding PV string, and the outputs of all DC-DC converters are connected to the DC-AC converter.

[0003] Photovoltaic systems have a capacity ratio, which is the ratio of the power of the photovoltaic string to the power of the inverter, and is generally greater than 1.

[0004] In a photovoltaic (PV) system, increasing the number of PV strings increases the system's capacity ratio. However, when the ambient light intensity is high or the ambient temperature is low, the output power of each PV string increases. If the sum of the output power of the N DC-DC converters exceeds the conversion capacity of the DC-AC converter, the DC-AC converter will enter a DC high-voltage derating state, reducing its output power. At this point, the PV system's power generation is significantly reduced. Summary of the Invention

[0005] In view of this, this application provides a photovoltaic system and a method for controlling the capacity ratio, which can improve the power generation of the photovoltaic system and increase the power generation efficiency.

[0006] This application provides a photovoltaic system, including: a DC-AC converter, N DC-DC converters, and a controller; N is an integer greater than or equal to 2;

[0007] The input of each DC-DC converter is used to connect to the corresponding photovoltaic string, and the outputs of N DC-DC converters are all connected to the input of the DC-AC converter.

[0008] The controller is used to control M out of N DC-DC converters to operate in the on state when it is necessary to reduce the capacity ratio of the photovoltaic system, thereby stopping the energy of the corresponding photovoltaic string from entering the DC-AC converter; M is an integer greater than or equal to 1 and less than N.

[0009] Preferably, the controller also includes the ability to control M out of the N DC-DC converters to operate in power converter mode when it is necessary to increase the capacity ratio of the photovoltaic system.

[0010] Preferably, the controller is specifically used to control M of the N DC-DC converters to operate in the on state when the output voltage of at least one of the N DC-DC converters is greater than the corresponding maximum power point voltage, thereby reducing the capacity ratio of the photovoltaic system.

[0011] Preferably, the controller is further configured to control M out of the N DC-DC converters to operate in power converter state when the output voltage of at least one DC-DC converter is less than the corresponding maximum power point voltage.

[0012] Preferably, the controller is specifically used to control M out of N DC-DC converters to operate in the on state when the ambient temperature is lower than a preset temperature, thereby reducing the capacity ratio of the photovoltaic system.

[0013] Preferably, the N DC-DC converters are boost converters;

[0014] N DC-DC converters are located in a combiner box, and the controller is the controller of the combiner box;

[0015] or,

[0016] N DC-DC converters are located in the inverter, and the controller is the inverter's controller.

[0017] This application also provides a method for controlling the capacity ratio of a photovoltaic system. The photovoltaic system includes: a DC-AC converter, N DC-DC converters, and a controller; N is an integer greater than or equal to 2; the input terminal of each DC-DC converter is used to connect to the corresponding photovoltaic string, and the output terminals of the N DC-DC converters are all connected to the input terminal of the DC-AC converter.

[0018] Control methods include:

[0019] When it is necessary to reduce the capacity ratio of the photovoltaic system, control M of the N DC-DC converters to operate in the on state, and stop the energy of the corresponding photovoltaic string from entering the DC-AC converter; M is an integer greater than or equal to 1 and less than N.

[0020] Preferably, when it is necessary to reduce the capacity ratio of the photovoltaic system, controlling M out of the N DC-DC converters to operate in the on state specifically includes:

[0021] When the output voltage of at least one of the N DC-DC converters is greater than the corresponding maximum power point voltage, M of the N DC-DC converters are controlled to operate in the on state.

[0022] Preferably, when it is necessary to reduce the capacity ratio of the photovoltaic system, controlling M out of the N DC-DC converters to operate in the on state specifically includes:

[0023] When the ambient temperature is lower than the preset temperature, M of the N DC-DC converters are controlled to operate in the on state.

[0024] Preferably, it further includes: when it is necessary to increase the capacity ratio of the photovoltaic system, controlling M of the N DC-DC converters to operate in power converter state.

[0025] Preferably, when it is necessary to increase the capacity ratio of the photovoltaic system, controlling M out of the N DC-DC converters to operate in power converter mode specifically includes:

[0026] When the output voltage of at least one DC-DC converter in the N DC-DC converters is less than the corresponding maximum power point voltage, M DC-DC converters in the N DC-DC converters are controlled to operate in power converter state.

[0027] Therefore, this application has the following beneficial effects:

[0028] The technical solution provided in this application, when it is necessary to reduce the capacity ratio of a photovoltaic system, controls M out of N DC-DC converters to operate in the on state, stopping the energy of the corresponding photovoltaic strings from entering the DC-AC converter, thereby reducing the capacity ratio of the photovoltaic system. When the sum of the output power of the N DC-DC converters exceeds the conversion capacity of the DC-AC converter, i.e., exceeds a preset power, the capacity ratio of the photovoltaic system is reduced. Specifically, at least one DC-DC converter can be controlled to be in the on state, i.e., not in power conversion state. The energy of the photovoltaic strings at the input terminal of the on-state DC-DC converter does not enter the DC-AC converter, thus reducing the capacity ratio of the photovoltaic system. After the capacity ratio of the photovoltaic system is reduced, the output power of the DC-DC converter will increase, thereby increasing the power generation of the photovoltaic system and improving power generation efficiency. Attached Figure Description

[0029] Figure 1 A schematic diagram of a photovoltaic system provided in an embodiment of this application;

[0030] Figure 2 A schematic diagram of a DC-DC converter provided in an embodiment of this application;

[0031] Figure 3 A schematic diagram of another DC-DC converter provided in the embodiments of this application;

[0032] Figure 4 A schematic diagram of yet another DC-DC converter provided in the embodiments of this application;

[0033] Figure 5 A PV curve diagram of a photovoltaic string provided in an embodiment of this application;

[0034] Figure 6 A diagram showing the relationship between the output power of the DCAC converter or combiner box and the capacity ratio of the photovoltaic string, provided in the embodiments of this application.

[0035] Figure 7 A flowchart illustrating a method for controlling the capacity ratio of a photovoltaic system, as provided in an embodiment of this application. Detailed Implementation

[0036] To make the above-mentioned objectives, features and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods.

[0037] See Figure 1 The figure is a schematic diagram of a photovoltaic system provided in an embodiment of this application.

[0038] The photovoltaic system provided in this embodiment includes: a DCAC converter 200, N DC-DC converters and a controller (not shown in the figure); N is an integer greater than or equal to 2;

[0039] The embodiments in this application do not specifically limit the specific topology of the photovoltaic system. For example, N DC-DC converters are located in a combiner box, and the controller is the controller of the combiner box; or, N DC-DC converters are located in an inverter, and the controller is the controller of the inverter. That is, the DC-DC converters can be integrated with the DC-AC converter, or they can be set up independently.

[0040] The input of each DC-DC converter is used to connect to the corresponding photovoltaic string. The outputs of the N DC-DC converters are all connected to the input of the DC-AC converter 200. Specifically, the outputs of the N DC-DC converters are connected in parallel to the input of the DC-AC converter 200.

[0041] The embodiments in this application do not specifically limit the number of photovoltaic strings connected to the input terminal of each DC-DC converter; they can connect one photovoltaic string or multiple photovoltaic strings. This is for ease of description. Figure 1 The example used is connecting the input of a DC-DC converter to a photovoltaic string.

[0042] like Figure 1 As shown, the input of the first DC-DC101 converter is connected to the first photovoltaic string PV1. The input of the second DC-DC102 converter is connected to the second photovoltaic string PV2. The input of the Nth DC-DC10n converter is connected to the Nth photovoltaic string PVn.

[0043] The controller is used to control M out of N DC-DC converters to operate in the on state when it is necessary to reduce the capacity ratio of the photovoltaic system, thereby stopping the energy of the corresponding photovoltaic string from entering the DC-DC converter and reducing the capacity ratio of the photovoltaic system; M is an integer greater than or equal to 1 and less than N.

[0044] Because photovoltaic (PV) strings generate significant output power when sunlight intensity is high or ambient temperature is low, if the sum of the output power of the N DC-DC converters exceeds the conversion capacity of the DC-AC converter, the DC-AC converter will enter a DC high-voltage derating state if the input power of the DC-AC converter is not reduced, thus decreasing the power generation efficiency of the PV system. To address this issue, the technical solution provided in this application reduces the capacity ratio of the PV system when the sum of the output power of the N DC-DC converters exceeds the conversion capacity of the DC-AC converter (i.e., exceeds a preset power). Specifically, at least one DC-DC converter can be controlled to be in a conducting state (not in power conversion state). The energy from the PV string at the input of the conducting DC-DC converter does not enter the DC-AC converter, thereby reducing the capacity ratio of the PV system. After reducing the capacity ratio of the PV system, the output power of the DC-DC converters will increase, thereby increasing the power generation of the PV system and improving its power generation efficiency.

[0045] This application does not specifically limit the internal topology of each DC-DC converter. The following describes several common DC-DC converter topologies with reference to the accompanying drawings.

[0046] See Figure 2 The figure is a schematic diagram of a DC-DC converter provided in an embodiment of this application.

[0047] The DC-DC converter provided in this embodiment is a boost DC-DC converter, including an inductor L, a diode D, and a switch Q. This application does not specifically limit the type of switch Q, for example, it can be a MOS, an IGBT, or other types of semiconductor switching devices.

[0048] In this circuit, the first end of L is connected to the positive input terminal of the DC-DC converter, the second end of L is connected to the anode of D, the cathode of D is connected to the positive output terminal of the DC-DC converter, and the second end of L is connected to the negative input terminal of the DC-DC converter through Q.

[0049] In addition, in some photovoltaic systems, the voltage withstand capability of a single switch or diode is insufficient, so multiple devices are often connected in series, that is, in addition to the DC-DC converter... Figure 2 As shown, it can also be used for Figure 3 The topology shown includes two series-connected switching transistors Q1 and Q2, and two series-connected diodes D1 and D2. For other connection methods, please refer to [reference needed]. Figure 2 This will not be elaborated upon here.

[0050] Among them, the two diodes connected in series, besides Figure 3 The series connection can also be used for Figure 4 The series connection shown.

[0051] The operating states of a DC-DC converter include power conversion state and conduction state. In the conduction state, the switching transistors in the DC-DC converter are always on. Figure 2-4 It can be seen that when all switches are turned on, it is equivalent to short-circuiting the input terminal of the DC-DC converter through the inductor and the switches. This means that the energy at the input terminal of the DC-DC converter will not reach the output terminal, and consequently will not enter the input terminal of the DC-AC converter. In this state, the DC-DC converter does not perform power conversion. The power conversion state refers to the DC-DC converter's switches periodically turning on and off to perform power conversion, or the switches remaining in the off state.

[0052] In this embodiment, the decision on whether to reduce the capacity ratio can be made based on the relationship between the output voltage of the DC-DC converter and the maximum power point voltage.

[0053] The controller is specifically used to control M of the N DC-DC converters to operate in the on state when the output voltage of at least one DC-DC converter in the N DC-DC converters is greater than the corresponding maximum power point voltage, thereby reducing the capacity ratio of the photovoltaic system.

[0054] It should be understood that in addition to comparing voltage, the capacity ratio of a photovoltaic system can be reduced by monitoring the ambient temperature and determining whether to reduce the capacity ratio based on the ambient temperature.

[0055] The controller is specifically used to control M out of N DC-DC converters to operate in the on state when the ambient temperature is lower than the preset temperature, thereby reducing the capacity ratio of the photovoltaic system.

[0056] To enable those skilled in the art to better understand the technical solutions provided in the embodiments of this application, a detailed description is provided below with reference to graphs.

[0057] See Figure 5 The figure is a PV curve of a photovoltaic string provided in an embodiment of this application.

[0058] Figure 5 The horizontal axis represents the per-unit value of the output voltage, and the vertical axis represents the per-unit value of the output power.

[0059] The maximum power point of a photovoltaic string is called the MPPT point. When the DC-DC converter performs power conversion, the photovoltaic string operates in the region to the right of the MPPT point.

[0060] See Figure 6 The figure shows the relationship between the output power of the DCAC converter or combiner box and the capacity ratio of the photovoltaic string provided in the embodiments of this application.

[0061] Figure 6The horizontal axis represents DC voltage, and the vertical axis represents output power.

[0062] The broken line represents the output power of the DCAC converter or combiner box versus the DC voltage. Po is the maximum output power of the DCAC converter or combiner box, and Uo is the maximum operating voltage of the DCAC converter or combiner box. Due to the capacity limitations of the DCAC converter or combiner box, the output power will decrease when the DC voltage exceeds its full-load MPPT voltage range.

[0063] The curves represent the PV curves after multiple photovoltaic modules are connected in series and parallel. The three curves correspond to different capacity ratios. Curve 1 has the highest capacity ratio, while curve 3 has the lowest. The output power of the DCAC converter or combiner box for curves 1, 2, and 3 are 0.6Po, 0.7Po, and 1.0Po, respectively.

[0064] When the capacity ratio of the photovoltaic system is high, as shown in curve 1, the output power of the DC-DC converter is 0.6Po. Using the technical solution provided in the embodiments of this application, the M DC-DC converters are controlled to work in the on state, that is, in the on state, and no longer perform power conversion. Thus, the capacity ratio of the photovoltaic system can be reduced to curve 3, and the output power of the DC-DC converter is increased to Po, thereby increasing the power generation of the photovoltaic system and improving the power generation efficiency.

[0065] In addition, based on the method of reducing the capacity ratio of the photovoltaic system provided in the above embodiments, the DC-DC converter that is turned on can also be controlled to resume the power conversion state when the capacity ratio of the photovoltaic system needs to be increased.

[0066] The controller is also used to control M out of N DC-DC converters to operate in power converter mode when it is necessary to increase the capacity ratio of the photovoltaic system.

[0067] The controller is also used to control M out of N DC-DC converters to operate in power converter state when the output voltage of at least one DC-DC converter is less than the corresponding maximum power point voltage.

[0068] Based on the photovoltaic system provided in the above embodiments, this application also provides a method for controlling the capacity ratio of a photovoltaic system, which will be described in detail below with reference to the accompanying drawings.

[0069] See Figure 7 The figure is a flowchart of a method for controlling the capacity ratio of a photovoltaic system provided in an embodiment of this application.

[0070] The photovoltaic system provided in this embodiment includes a photovoltaic system comprising: a DC-AC converter, N DC-DC converters, and a controller; N is an integer greater than or equal to 2; the input terminal of each DC-DC converter is used to connect to the corresponding photovoltaic string, and the output terminals of the N DC-DC converters are all connected to the input terminal of the DC-AC converter;

[0071] Control methods include:

[0072] S701: Determine that the photovoltaic system needs to reduce its capacity ratio.

[0073] To determine whether a photovoltaic system needs to reduce its capacity ratio, one can compare the output voltage of each DC-DC converter with its maximum power point voltage; or determine the ambient temperature; or determine the input power of the DC-AC converter with its preset power.

[0074] S702: Control M of the N DC-DC converters to operate in the on state, stopping the energy of the corresponding photovoltaic string from entering the DC-DC converter, so as to reduce the capacity ratio of the photovoltaic system; M is an integer greater than or equal to 1 and less than N.

[0075] The technical solution provided in this application, when it is necessary to reduce the capacity ratio of a photovoltaic system, controls M out of N DC-DC converters to operate in the on state, thereby reducing the capacity ratio of the photovoltaic system. When the sum of the output power of the N DC-DC converters exceeds the conversion capacity of the DC-AC converter, i.e., exceeds a preset power, the capacity ratio of the photovoltaic system is reduced. Specifically, at least one DC-DC converter can be controlled to be in the on state, i.e., not in power conversion state. The energy of the photovoltaic string at the input terminal of the on-state DC-DC converter does not enter the DC-AC converter, thereby reducing the capacity ratio of the photovoltaic system. After the capacity ratio of the photovoltaic system is reduced, the output power of the DC-DC converter will increase, thereby increasing the power generation of the photovoltaic system and improving the power generation efficiency.

[0076] When it is necessary to reduce the capacity ratio of a photovoltaic system, controlling M out of N DC-DC converters to operate in the on state specifically includes:

[0077] When the output voltage of at least one of the N DC-DC converters is greater than the corresponding maximum power point voltage, M of the N DC-DC converters are controlled to operate in the on state.

[0078] When it is necessary to reduce the capacity ratio of a photovoltaic system, controlling M out of N DC-DC converters to operate in the on state specifically includes:

[0079] When the ambient temperature is lower than the preset temperature, M of the N DC-DC converters are controlled to operate in the on state.

[0080] The control method provided in this embodiment further includes: when it is necessary to increase the capacity ratio of the photovoltaic system, controlling M of the N DC-DC converters to operate in the power converter state.

[0081] When it is necessary to increase the capacity ratio of a photovoltaic system, control M out of N DC-DC converters to operate in power converter mode, specifically including:

[0082] When the output voltage of at least one DC-DC converter in the N DC-DC converters is less than the corresponding maximum power point voltage, M DC-DC converters in the N DC-DC converters are controlled to operate in power converter state.

[0083] The control method provided in this application, when it is necessary to reduce the capacity ratio, controls the switching transistors in the M DC-DC converters to be continuously turned on, i.e., enters the on state, thereby reducing the DC-side capacity ratio of the DC-AC converter or combiner box. Alternatively, the DC-DC converters in the on state can be controlled to enter a power conversion state, thereby increasing the DC-side capacity ratio of the photovoltaic inverter or combiner box.

[0084] The control method provided in this application embodiment controls the working state of the DC-DC converter to achieve dynamic control of the capacity ratio of the DC-AC converter or the DC side of the combiner box in the photovoltaic system, thereby improving the power generation of the photovoltaic system.

[0085] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A photovoltaic system, characterized in that, include: A DC-AC converter, N DC-DC converters, and a controller; where N is an integer greater than or equal to 2; The input terminal of each DC-DC converter is used to connect to the corresponding photovoltaic string, and the output terminals of the N DC-DC converters are all connected to the input terminal of the DC-AC converter. The controller is used to control M of the N DC-DC converters to operate in the on state when it is necessary to reduce the capacity ratio of the photovoltaic system, thereby stopping the energy of the corresponding photovoltaic string from entering the DC-DC converter. The DC-DC converter operating in the on state is used to instruct the switching transistors in the DC-DC converter to remain on. M is an integer greater than or equal to 1 and less than N; The requirement to reduce the capacity ratio of the photovoltaic system includes: the output voltage of at least one of the N DC-DC converters is greater than the corresponding maximum power point voltage.

2. The photovoltaic system according to claim 1, characterized in that, The controller is also configured to control M of the N DC-DC converters to operate in power converter mode when it is necessary to increase the capacity ratio of the photovoltaic system.

3. The photovoltaic system according to claim 2, characterized in that, The controller is further configured to control M of the N DC-DC converters to operate in power converter state when the output voltage of at least one of the N DC-DC converters is less than the corresponding maximum power point voltage.

4. The photovoltaic system according to claim 1 or 2, characterized in that, The controller is also used to control M of the N DC-DC converters to operate in the on state when the ambient temperature is lower than the preset temperature, thereby reducing the capacity ratio of the photovoltaic system.

5. The photovoltaic system according to claim 1, characterized in that, The N DC-DC converters are boost converters; The N DC-DC converters are located in a combiner box, and the controller is the controller of the combiner box; or, The N DC-DC converters are located in the inverter, and the controller is the controller of the inverter.

6. A method for controlling the capacity ratio of a photovoltaic system, characterized in that, The photovoltaic system includes: a DC-AC converter, N DC-DC converters, and a controller; where N is an integer greater than or equal to 2; the input terminal of each DC-DC converter is used to connect to the corresponding photovoltaic string, and the output terminals of the N DC-DC converters are all connected to the input terminal of the DC-AC converter; The control method includes: When it is necessary to reduce the capacity ratio of the photovoltaic system, M of the N DC-DC converters are controlled to operate in the on state, stopping the energy of the corresponding photovoltaic string from entering the DC-DC converter. The DC-DC converter operating in the on state is used to indicate that the switching transistors in the DC-DC converter remain on; M is an integer greater than or equal to 1 and less than N. The requirement to reduce the capacity ratio of the photovoltaic system includes: the output voltage of at least one of the N DC-DC converters is greater than the corresponding maximum power point voltage.

7. The control method according to claim 6, characterized in that, The method of controlling M of the N DC-DC converters to operate in the on state when it is necessary to reduce the capacity ratio of the photovoltaic system further includes: When the ambient temperature is lower than the preset temperature, M of the N DC-DC converters are controlled to operate in the on state.

8. The control method according to claim 6, characterized in that, Also includes: When it is necessary to increase the capacity ratio of the photovoltaic system, M of the N DC-DC converters are controlled to operate in power converter mode.

9. The control method according to claim 8, characterized in that, When it is necessary to increase the capacity ratio of the photovoltaic system, controlling M of the N DC-DC converters to operate in power converter mode specifically includes: When the output voltage of at least one of the N DC-DC converters is less than the corresponding maximum power point voltage, M of the N DC-DC converters are controlled to operate in power converter mode.

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