Power control method, inverter and photovoltaic system

Abstract

A power control method, an inverter and a photovoltaic system, which belong to the technical field of photovoltaic power generation. The inverter comprises N direct-current conversion circuits and an inverter circuit, wherein input terminals of the direct-current conversion circuits are used for connecting to M direct-current sources, and output terminals of the direct-current conversion circuits are connected in parallel to the inverter circuit. The power control method comprises: when an inverter is connected to the grid, increasing the switching duty cycles of direct-current conversion circuits, such that the input voltages of the direct-current conversion circuits are reduced to a first target voltage, wherein the first target voltage is less than the minimum open-circuit voltage of various photovoltaic strings; and using the first target voltage as a starting point to execute maximum power point tracking.

Description

Power control method, inverter and photovoltaic system

[0001] Cross-reference to related applications

[0002] The present disclosure is based on and claims priority to Chinese Patent Application No. 202410815801.7, filed on June 21, 2024, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present disclosure relates to a power control method, an inverter and a photovoltaic system. BACKGROUND

[0004] In a photovoltaic power supply system, multiple photovoltaic modules are usually selected to be connected in series to form photovoltaic strings, and multiple photovoltaic strings are connected in parallel to form a photovoltaic array to improve the output power of the photovoltaic system. Due to different numbers of photovoltaic modules connected in series in different photovoltaic strings, or factors such as partial or intermittent shading, aging, damage, etc. of photovoltaic modules in the photovoltaic string, the number of photovoltaic modules that can normally work in different photovoltaic strings is different. The photovoltaic string with relatively more photovoltaic modules that can normally work is a "long string", and the photovoltaic string with relatively fewer photovoltaic modules that can normally work is a "short string", thereby forming a long-short parallel string.

[0005] Under the working condition of the long-short parallel string, the MPPT (Maximum Power Point Tracking) voltage of the long string working at the maximum power point is higher than the open circuit voltage of the short string. After the traditional MPPT scanning logic tracks the MPPT voltage of the long string, the inverter will continuously work at the MPPT voltage of the long string according to its own MPPT algorithm. The MPPT voltage of the long string is usually higher than the open circuit voltage of the short string, which causes the components on the short string to be unable to effectively generate power, and the entire system has a large power loss.

[0006] SUMMARY

[0007] The present disclosure aims to at least solve one of the technical problems existing in the prior art. To this end, the present disclosure provides a power control method, an inverter and a photovoltaic system, which can make each direct current source have power output without detecting the current and power conditions of the direct current sources, thereby avoiding adjusting the input voltage of the direct current conversion circuit from large to small and making the direct current source with a smaller open circuit voltage unable to output power.

[0008] In a first aspect, the present disclosure provides a power control method. An inverter includes N direct current conversion circuits and an inverter circuit. The input end of each direct current conversion circuit is connected to M direct current sources. The output end of each direct current conversion circuit is connected to the inverter circuit in parallel. The output end of the inverter circuit is connected to a power grid. The power control method includes:

[0009] When the inverter is connected to the grid, the switching duty cycle of the DC conversion circuit is increased to reduce the input voltage of the DC conversion circuit to a first target voltage, which is less than the minimum open circuit voltage of each DC source;

[0010] The DC conversion circuit is controlled to perform maximum power point tracking starting from the first target voltage;

[0011] wherein N and M are natural numbers.

[0012] According to the power control method of the present disclosure, when the inverter is connected to the grid, the input voltage of the DC conversion circuit is reduced to be less than the minimum open circuit voltage of each DC source connected thereto, and at this time, maximum power point tracking is performed, so that each DC source can output power without detecting the current and power conditions of the DC source, thereby avoiding the situation that the input voltage of the DC conversion circuit is adjusted from large to small and the DC source with a smaller open circuit voltage cannot output power.

[0013] According to an embodiment of the present disclosure, the DC source is a photovoltaic string, and the first target voltage is less than the minimum MPPT voltage of each photovoltaic string.

[0014] According to an embodiment of the present disclosure, increasing the switching duty cycle of the DC conversion circuit comprises:

[0015] adjusting the switching duty cycle of the DC conversion circuit to the maximum duty cycle.

[0016] According to an embodiment of the present disclosure, the power control method further comprises:

[0017] After the input voltage of the DC conversion circuit is reduced to the first target voltage, gradually reducing the switching duty cycle of the DC conversion circuit, and determining the actual output power of the DC conversion circuit at different switching duty cycles;

[0018] taking the maximum value of each actual output power as the maximum output power, and the second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage of each photovoltaic string.

[0019] According to an embodiment of the present disclosure, taking the maximum value of each actual output power as the maximum output power comprises:

[0020] when the actual power changes from an upward trend or a stable trend to a downward trend, determining the actual power before the change as the maximum output power.

[0021] According to an embodiment of the present disclosure, after taking the maximum value of each actual output power as the maximum output power, the method further comprises:

[0022] controlling each photovoltaic string to operate at the second target voltage.

[0023] According to one embodiment of the present disclosure, when the input voltage of the DC conversion circuit is the first target voltage, the DC conversion circuit is in a current-limiting working state.

[0024] In a second aspect, the present disclosure provides an inverter, comprising an inverter circuit, at least one DC conversion circuit and a controller, an input end of the DC conversion circuit is configured to access a plurality of parallel photovoltaic strings, an output end of the DC conversion circuit is electrically connected with an input end of the inverter circuit, the controller is electrically connected with the DC conversion circuit and the inverter circuit respectively, and the controller is configured to execute the power control method described above.

[0025] According to the inverter of the present disclosure, when the inverter is connected to the grid, the input voltage of the DC conversion circuit is reduced to be less than the minimum open circuit voltage of each DC source connected thereto, at this time, the maximum power point tracking is executed, each DC source can output power without detecting the current and power conditions of the DC source, and the situation that the input voltage of the DC conversion circuit is adjusted from large to small and the DC source with a smaller open circuit voltage cannot output power is avoided.

[0026] In a third aspect, the present disclosure provides a photovoltaic system, comprising a plurality of parallel photovoltaic strings and the inverter described above, each photovoltaic string is electrically connected with the DC conversion circuit, and an AC side of the inverter is electrically connected with the grid or a load.

[0027] According to the photovoltaic system of the present disclosure, when the inverter is connected to the grid, the input voltage of the DC conversion circuit is reduced to be less than the minimum open circuit voltage of each DC source connected thereto, at this time, the maximum power point tracking is executed, each DC source can output power without detecting the current and power conditions of the DC source, and the situation that the input voltage of the DC conversion circuit is adjusted from large to small and the DC source with a smaller open circuit voltage cannot output power is avoided.

[0028] According to one embodiment of the present disclosure, each photovoltaic string electrically connected with the DC conversion circuit has a different open circuit voltage.

[0029] Additional aspects and advantages of the present disclosure will be in part apparent and in part pointed out hereinafter.

[0030] BRIEF DESCRIPTION OF DRAWINGS

[0031] The above and / or additional aspects and advantages of the present disclosure will become apparent and be readily appreciated from the following description, taken in conjunction with the accompanying drawings, in which:

[0032] FIG. 1 is a structural block diagram of a photovoltaic system according to an embodiment of the present disclosure;

[0033] FIG. 2 is a power-voltage characteristic curve diagram of a photovoltaic array in the related art;

[0034] FIG. 3 is a flow chart of a power control method according to an embodiment of the present disclosure;

[0035] FIG. 4 is a current-voltage characteristic curve of a single photovoltaic string according to an embodiment of the present disclosure;

[0036] FIG. 5 is a power-voltage characteristic curve of a single photovoltaic string according to an embodiment of the present disclosure;

[0037] FIG. 6 is a power-voltage characteristic curve of a photovoltaic array according to an embodiment of the present disclosure.

[0038] Reference signs: photovoltaic array 110, first DC source 111, second DC source 112, inverter 120, DC conversion circuit 121, inversion circuit 122. DETAILED DESCRIPTION

[0039] Embodiments of the present disclosure are described in detail below with reference to the accompanying drawings. The embodiments described below are examples for explaining the present disclosure and should not be construed as limiting the present disclosure.

[0040] In the following description, "circuitry" refers to an electrically conductive loop of at least one element or sub-circuitry, which is configured by electrical or electromagnetic connection. When it is said that an element or circuit is "coupled to" or "connected to" another element or said element / circuit is "coupled in" or "connected in" between two nodes, it can be directly coupled or connected to another element or there can be an intermediate element, and the connection between elements can be physical, logical, or a combination thereof. On the contrary, when it is said that an element is "directly coupled to" or "directly connected to" another element, it means that there is no intermediate element between the two.

[0041] In the description, the terms "first", "second", and the like are used to distinguish similar objects, and are not used to describe a particular order or sequence. It should be understood that the numerical descriptors used in this way can be interchanged under appropriate circumstances, so that the embodiments of the present disclosure can be implemented in an order other than that illustrated or described herein, and the objects distinguished by "first", "second", etc. are generally a class, and do not limit the number of objects, for example, the first object can be one or more. In addition, "and / or" in the specification and claims indicates at least one of the connected objects, and the character " / ", generally indicates that the objects before and after are in a "or" relationship.

[0042] In addition, descriptions such as "one embodiment", "some embodiments", "certain embodiments", "an example", "a specific example", or "some examples" mean that a particular feature, structure, material, or characteristic is included in at least one embodiment or example of the present disclosure. Descriptive terms such as "exemplary" in the present specification do not necessarily refer to the same embodiment or example. Moreover, the described particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments or examples.

[0043] Referring to FIG. 1, in a photovoltaic power supply system, the inverter usually includes a plurality of DC conversion circuits, and the input ends of the DC conversion circuits are used to access a plurality of DC sources to improve the output power of the photovoltaic system. The DC source in the photovoltaic power supply system usually refers to a photovoltaic string, and the photovoltaic string is formed by a plurality of photovoltaic components connected in series. Due to factors such as partial or intermittent shading, aging, damage, etc. in the string, long-short parallel strings are easily formed.

[0044] Referring to FIG. 2, FIG. 2 shows the power-voltage characteristic curve of a photovoltaic array 110 in the related art. In the long-short parallel string working condition, the voltage Vmpp1 at the maximum power point of the long string is higher than the open circuit voltage Voc2 of the short string. In the related art, after the MPPT scanning logic causes the inverter to track the voltage Vmpp1 at the maximum power point of the long string, the inverter 120 will continue to work at Vmpp1 according to its own MPPT algorithm, and the components on the short string cannot effectively generate power. The entire system has a large power loss, and the components on the short string are at risk of burning out in the long term.

[0045] The present disclosure proposes a power control method, an inverter, and a photovoltaic system. When the inverter is connected to the grid, the input voltage of the DC conversion circuit is reduced to be less than the minimum open circuit voltage in each DC source connected to the DC conversion circuit, so that the shortest string has power output, and the situation that the DC source with a smaller open circuit voltage cannot output power is avoided by adjusting the input voltage of the DC conversion circuit from large to small.

[0046] Referring to FIG. 3, FIG. 3 shows the steps of the power control method of the present disclosure. One embodiment of the present disclosure proposes a power control method. The inverter includes N DC conversion circuits 121 and an inverter circuit 122. The input ends of the DC conversion circuits 121 are used to access M DC sources. The output ends of each DC conversion circuit 121 are connected in parallel to the inverter circuit 122. The output end of the inverter circuit 122 is used to be electrically connected to the grid. The power control method includes steps 10 and 20.

[0047] Step 10: When the inverter is connected to the grid, the switching duty cycle of the DC conversion circuit 121 is increased, so that the input voltage of the DC conversion circuit 121 is reduced to a first target voltage, and the first target voltage is less than the minimum open circuit voltage in each DC source.

[0048] Step 20, controlling the direct current conversion circuit to perform maximum power point tracking starting from the first target voltage;

[0049] Wherein, N and M are natural numbers.

[0050] The inverter further comprises a controller, and the direct current conversion circuit 121 can be electrically connected with the controller. The execution subject of the power control method provided by the embodiments of the present disclosure can be the controller or a functional module or functional entity in the controller capable of implementing the power control method. That is, the power control method provided by the embodiments of the present disclosure can be executed on the controller, or the power control method provided by the embodiments of the present disclosure can be executed on the functional module, or the power control method provided by the embodiments of the present disclosure can also be executed on the functional entity. The power control method provided by the embodiments of the present disclosure is described below by taking the controller as an example of the execution subject.

[0051] The number of direct current conversion circuits 121 included in one inverter and the number of direct current sources connected to the input end of one direct current conversion circuit 121 can be selected according to the actual application scenario, which is not limited here. The direct current source can be a photovoltaic string, and the input end of one direct current conversion circuit 121 is connected to two photovoltaic strings as an example in the following description, and the photovoltaic strings connected to the input end of one direct current conversion circuit 121 together constitute a photovoltaic array 110.

[0052] The first direct current source 111 is a first photovoltaic string, and the second direct current source 112 is a second photovoltaic string. The input end of the direct current conversion circuit 121 is connected to the first photovoltaic string and the second photovoltaic string. The first photovoltaic string and the second photovoltaic string each comprise a plurality of series-connected photovoltaic components. The number of photovoltaic components in the first photovoltaic string and the second photovoltaic string can be the same or different, which can be selected according to the actual application scenario.

[0053] In some embodiments, the first photovoltaic string comprises 10 series-connected photovoltaic components, and the second photovoltaic string comprises 8 series-connected photovoltaic components. The output voltage of each photovoltaic component is 1V. Therefore, the output voltage of the first photovoltaic string is 10V, and the output voltage of the second photovoltaic string is 8V. Therefore, the first photovoltaic string is a relatively long photovoltaic string, hereinafter referred to as a "long string", and the second photovoltaic string is a relatively short photovoltaic string, hereinafter referred to as a "short string".

[0054] In some embodiments, the first photovoltaic string and the second photovoltaic string each include 10 photovoltaic components connected in series, but due to partial or intermittent shading, aging, damage, or other factors, 2 photovoltaic components in the second photovoltaic string cannot work normally during operation, so that only 8 photovoltaic components in the second photovoltaic string can effectively output power. In this case, the output voltage of the first photovoltaic string is 10 V, and the output voltage of the second photovoltaic string is 8 V, so the first photovoltaic string is a long string, and the second photovoltaic string is a short string.

[0055] The input end of the direct current conversion circuit 121 can also be connected to three or more photovoltaic strings in parallel. When the input end of the direct current conversion circuit 121 is connected to multiple photovoltaic strings, the photovoltaic string with the most photovoltaic components can be taken as a long string, and the other photovoltaic strings are short strings. For example, the photovoltaic array 110 includes a first photovoltaic string, a second photovoltaic string, and a third photovoltaic string. The first photovoltaic string includes 10 photovoltaic components connected in series, the second photovoltaic string includes 8 photovoltaic components connected in series, and the third photovoltaic string includes 6 photovoltaic components connected in series. The second photovoltaic string is a short string relative to the first photovoltaic string, and the second photovoltaic string is a long string relative to the third photovoltaic string.

[0056] Referring to FIG. 4 and FIG. 5, FIG. 3 shows the current-voltage characteristic curve of a single photovoltaic string, and FIG. 4 shows the power-voltage characteristic curve of a single photovoltaic string. The photovoltaic string can include at least one photovoltaic panel and an optimizer 113 connected in series with each other, and the photovoltaic panel is electrically connected to the direct current conversion circuit 121. When the voltage at the output end of the photovoltaic panel is large, the direct current conversion circuit 121 connected thereto is in a straight-through state, at which time the output voltage of the direct current conversion circuit 121 is equal to the output voltage of the photovoltaic panel. When the voltage at the output end of the photovoltaic panel decreases to the voltage corresponding to the maximum output power of the photovoltaic string, the photovoltaic string works in a maximum output power state. When the voltage continues to decrease, the direct current conversion circuit 121 adjusts the output voltage of the photovoltaic panel to maintain it in the maximum output power state, at which time the power of the photovoltaic string does not change, the voltage decreases, and the current rises. Since the current of the photovoltaic string cannot rise unlimitedly, when the voltage at the output end of the photovoltaic panel continues to decrease, the photovoltaic string works in a current-limited output state, at which time the current of the photovoltaic string does not change, and when the voltage decreases, the power of the photovoltaic string also decreases.

[0057] For a single photovoltaic string, the open circuit voltage of the photovoltaic string is greater than the MPPT voltage corresponding to the maximum power point when the photovoltaic string works in the maximum power point. When the output voltage of the photovoltaic array 110 is greater than the open circuit voltage of a certain photovoltaic string, the photovoltaic string cannot output power. The actual output voltage of the photovoltaic string when it works in the current-limited output state is less than the MPPT voltage corresponding to the maximum power point when the photovoltaic string works in the maximum power point.

[0058] Referring to FIG. 6, FIG. 6 shows a power-voltage characteristic curve of the photovoltaic array 110 of the present disclosure. For the photovoltaic array 110, the voltage Vmpp1 of the long string working in the maximum power output state is greater than the voltage Vmpp2 of the short string working in the maximum power output state, and generally, Vmpp1 is greater than the open circuit voltage Voc2 of the short string.

[0059] The direct current conversion circuit 121 can be a boost circuit, an input end of the boost circuit is connected to the photovoltaic array 110, and an output end of the boost circuit is connected to the direct current bus in parallel with other direct current conversion circuits 121 of the inverter. When the inverter is connected to the grid, the controller increases the switching duty cycle of the boost circuit, and the output voltage of the photovoltaic array 110 decreases, that is, the input voltage of the boost circuit decreases.

[0060] When the input voltage of the direct current conversion circuit 121 decreases to be less than the minimum open circuit voltage of each photovoltaic string, each photovoltaic string has power output, and at this time, maximum power point tracking is performed, that is, each photovoltaic string has output when the photovoltaic array 110 has maximum output power.

[0061] In some embodiments, when the first target voltage is less than the minimum open circuit voltage of each photovoltaic string and greater than the minimum MPPT voltage of each photovoltaic string, the inverter performs maximum power point tracking, and if the output voltage of the photovoltaic array 110 is increased from the starting point, the output power of the photovoltaic array 110 decreases. Therefore, the duty cycle of the switching tube of the direct current conversion circuit 121 is increased from the starting point, that is, the output voltage of the photovoltaic array 110 is decreased, and the maximum output power of the photovoltaic array 110 can be tracked.

[0062] In other embodiments, when the first target voltage is less than the minimum MPPT voltage of each photovoltaic string, the inverter performs maximum power point tracking, and the duty cycle of the switching tube of the direct current conversion circuit 121 is decreased from the starting point, that is, the output voltage of the photovoltaic array 110 is increased, and the maximum output power of the photovoltaic array 110 can be tracked.

[0063] According to the power control method of the present disclosure, when the inverter is connected to the grid, the input voltage of the direct current conversion circuit 121 decreases to be less than the minimum open circuit voltage of each direct current source connected thereto, and at this time, maximum power point tracking is performed, so that each direct current source has power output without detecting the current and power conditions of the direct current source, and the input voltage of the direct current conversion circuit 121 is adjusted from large to small, so that the direct current source with a smaller open circuit voltage cannot output power.

[0064] In some embodiments, the direct current source is a photovoltaic string, and the first target voltage is less than the minimum MPPT voltage of each photovoltaic string.

[0065] The first target voltage is less than the minimum MPPT voltage in each photovoltaic string, indicating that the actual output voltage of the photovoltaic array 110 gradually increases when the maximum power point tracking is performed. The smaller the value of the first target voltage, the greater the range that the controller can track when tracking the maximum power point. When adjusting the output voltage of the photovoltaic array 110, the initial voltage cannot be set to zero voltage due to the influence of hardware, and therefore the initial voltage is controlled to be the minimum output voltage of the photovoltaic array 110, so that the range of the controller for tracking the maximum power of the photovoltaic array 110 is the maximum adjustable range, and the tracking result is more accurate.

[0066] In some embodiments, increasing the switching duty cycle of the DC conversion circuit 121 includes adjusting the switching duty cycle of the DC conversion circuit 121 to a maximum duty cycle.

[0067] The DC conversion circuit 121 can be a boost circuit, the input end of the boost circuit is connected to the photovoltaic array 110, and the output end of the boost circuit is connected to the DC bus in parallel with other DC conversion circuits 121 of the inverter. The controller increases the switching duty cycle of the boost circuit, that is, increases the boost ratio of the boost circuit. When the bus voltage is constant, the boost ratio of the boost circuit increases, and the output voltage of the photovoltaic array 110 decreases, that is, the input voltage of the boost circuit decreases.

[0068] Adjusting the switching duty cycle of the boost circuit to the maximum duty cycle can make the first target voltage reach the minimum value within the adjustable range of the inverter hardware, so that the range of the controller for tracking the maximum power of the photovoltaic array 110 is the maximum adjustable range, and the tracking result is more accurate.

[0069] In some embodiments, the power control method further includes gradually reducing the switching duty cycle of the DC conversion circuit 121 after the input voltage of the DC conversion circuit 121 decreases to the first target voltage, and determining the actual output power of the DC conversion circuit 121 at different switching duty cycles; taking the maximum value of each actual output power as the maximum output power, and the second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage in each photovoltaic string.

[0070] Gradually reducing the switching duty cycle of the DC conversion circuit 121 can gradually increase the output voltage of the photovoltaic array 110 from the first target voltage. During the increase of the output voltage of the photovoltaic array 110, the output power of the photovoltaic array 110 also increases. During the reduction of the switching duty cycle of the DC conversion circuit 121, the output voltage of the photovoltaic array 110 increases, and the output power of the photovoltaic array 110 corresponding to each voltage value, i.e., the actual power of the photovoltaic array 110, is obtained. The controller can determine the maximum value of the actual power of the photovoltaic array 110 according to the values of the obtained actual power, and the maximum value of the actual power can be regarded as the maximum output power of the photovoltaic array 110. The actual output voltage of the photovoltaic array 110 corresponding to the maximum value of the actual power is the second target voltage.

[0071] The second target voltage is less than the minimum open-circuit voltage in each photovoltaic string, which can ensure that each photovoltaic string has power output.

[0072] In some embodiments, the maximum value of the actual output power is taken as the maximum output power, including: when the actual power changes from an upward trend or a stable trend to a downward trend, the actual power before the change is determined as the maximum output power.

[0073] At the first target voltage, the long string and the short string both work in the current-limited output state, and at this time, the long string and the short string both do not reach the maximum output power, so the output power of the photovoltaic array 110 is small. During the gradual increase of the actual output voltage of the photovoltaic array 110 from the first target voltage, the output power of the long string and the short string gradually increases, and the output power of the photovoltaic array 110 also increases, until the actual output voltage of the photovoltaic array 110 increases to the MPPT voltage of the short string. After the actual output voltage of the photovoltaic array 110 is greater than the MPPT voltage of the short string, during the increase of the actual output voltage of the photovoltaic array 110, the short string can still work in the maximum power output state within a certain range, the actual output voltage of the photovoltaic array 110 reaches the MPPT voltage of the long string, and the output power of the photovoltaic array 110 reaches the maximum.

[0074] After the actual output voltage of the photovoltaic array 110 reaches the MPPT voltage of the long string and continues to increase, it exceeds the open-circuit voltage of the short string and the long string, so that part of the photovoltaic strings cannot output power, and the output power of the photovoltaic array 110 will reach a stable state or appear a downward trend.

[0075] The controller obtains the actual power at the time point before the actual power of the photovoltaic array 110 reaches the stable state or appears the downward trend, which is the maximum output power of the photovoltaic array 110, and the actual output voltage of the photovoltaic array 110 corresponding to the maximum output power is the second target voltage.

[0076] The maximum output power of the photovoltaic array 110 can be determined when the actual power reaches a steady state or a downward trend, without tracking the power corresponding to all voltages, thereby reducing the tracking range.

[0077] In some embodiments, after taking the maximum value of the actual output powers as the maximum output power, the method further comprises: controlling each photovoltaic string to operate at a second target voltage.

[0078] The photovoltaic array 110 comprises a plurality of photovoltaic strings, each of which is electrically connected to the input end of the DC conversion circuit 121. After the controller tracks the maximum output power of the photovoltaic array 110, the controller adjusts the switching duty cycle of the DC conversion circuit 121 to control each photovoltaic string to operate at a second target voltage, so that the photovoltaic array 110 operates at the maximum power point.

[0079] In some embodiments, when the input voltage of the DC conversion circuit 121 is the first target voltage, the DC conversion circuit 121 is in a current-limiting operating state.

[0080] When the number of photovoltaic strings connected to the DC conversion circuit 121 is small, the output current of the photovoltaic array 110 is small, which may cause the DC conversion circuit 121 to be in a current-limiting operating state when the input voltage of the DC conversion circuit 121 is the first target voltage. The actual output voltage of the photovoltaic string operating in the current-limiting region is smaller than the MPPT voltage corresponding to the photovoltaic string operating at the maximum power point. At this time, gradually reducing the switching duty cycle of the DC conversion circuit 121 can track the maximum power point.

[0081] When the photovoltaic strings in the photovoltaic array 110 all operate at the second target voltage, it is possible that all the photovoltaic strings in the photovoltaic array 110 are in the maximum power output state, or it is possible that the short strings are in the maximum power output state and the long strings are in the current-limiting output state, at this time the entire photovoltaic array 110 outputs the maximum power.

[0082] Continuing to refer to FIG. 1, one embodiment of the present disclosure proposes an inverter comprising an inverter circuit 122, at least one DC conversion circuit 121, and a controller. The input end of the DC conversion circuit 121 is used to connect a plurality of photovoltaic strings, the output end of the DC conversion circuit 121 is connected in parallel to the inverter circuit 122, and the controller is electrically connected to the DC conversion circuit 121 and the inverter circuit 122. The controller is configured to perform the power control method described above.

[0083] There may be long strings and short strings in the photovoltaic strings connected to the DC conversion circuit 121. To avoid the controller from making the short strings unable to output power when performing maximum power tracking, the controller performs the power control method described above. The specific execution steps can be referred to the above-mentioned embodiments, which will not be described here.

[0084] According to the inverter of the present disclosure, when the inverter is connected to the grid, the input voltage of the DC conversion circuit 121 is reduced to be less than the minimum open circuit voltage of each DC source connected thereto, at which time maximum power point tracking is performed, and each DC source can output power without detecting the current and power conditions of the DC sources, avoiding the situation that the input voltage of the DC conversion circuit 121 is adjusted from large to small, and the DC source with a smaller open circuit voltage cannot output power.

[0085] One embodiment of the present disclosure provides a photovoltaic system, comprising: a plurality of parallel photovoltaic strings and the aforementioned inverter, each photovoltaic string is electrically connected to the DC conversion circuit 121, and the AC side of the inverter is electrically connected to the grid or a load.

[0086] The specific execution steps of the controller in the inverter to perform the aforementioned power control method can refer to the aforementioned embodiments, which will not be described here. The controller adjusts the switching duty cycle of the DC conversion circuit 121 to track the maximum power point from the input voltage of the DC conversion circuit 121 being less than the open circuit voltage of the shortest string, avoiding the situation that the controller performs maximum power tracking and the short string cannot output power.

[0087] According to the photovoltaic system of the present disclosure, when the inverter is connected to the grid, the input voltage of the DC conversion circuit 121 is reduced to be less than the minimum open circuit voltage of each DC source connected thereto, at which time maximum power point tracking is performed, and each DC source can output power without detecting the current and power conditions of the DC sources, avoiding the situation that the input voltage of the DC conversion circuit 121 is adjusted from large to small, and the DC source with a smaller open circuit voltage cannot output power.

[0088] In this document, the terms "comprise", "contain", or any other variant thereof are intended to cover a non-exclusive inclusion, so that a process, method, article, or apparatus that includes a list of elements not only includes those elements, but also includes other elements not explicitly listed, or inherent to such a process, method, article, or apparatus. Without more limitations, the element defined by the statement "comprises a" does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element. In addition, it should be noted that the scope of the methods and apparatus in the embodiments of the present disclosure is not limited to performing functions in the order shown or discussed, but can also include performing functions in a substantially simultaneous manner or in reverse order, for example, the described method can be performed in an order different from that described, and various steps can be added, omitted, or combined. In addition, the features described with reference to certain examples can be combined in other examples.

[0089] Although embodiments of the disclosure have been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications, alternatives, and variations can be made to the embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined by the claims and their equivalents.

Claims

1. A power control method, wherein, The inverter comprises N direct current conversion circuits and an inverter circuit, input ends of the direct current conversion circuits are connected to M direct current sources, output ends of the direct current conversion circuits are connected in parallel to the inverter circuit, and output ends of the inverter circuit are connected to a power grid; the power control method comprises: When the inverter is connected to the power grid, the switching duty cycle of the direct current conversion circuit is increased to reduce the input voltage of the direct current conversion circuit to a first target voltage, and the first target voltage is less than the minimum open circuit voltage in each direct current source; The direct current conversion circuit is controlled to perform maximum power point tracking with the first target voltage as a starting point; wherein N and M are natural numbers.

2. The power control method of claim 1, wherein, The direct current source is a photovoltaic string, and the first target voltage is less than the minimum MPPT voltage in each photovoltaic string.

3. The power control method of claim 2, wherein, The switching duty cycle of the direct current conversion circuit is adjusted to a maximum duty cycle. The power control method further comprises:

4. The power control method of claim 3, wherein, After the input voltage of the direct current conversion circuit is reduced to the first target voltage, the switching duty cycle of the direct current conversion circuit is gradually reduced, and the actual output power of the direct current conversion circuit at different switching duty cycles is determined; The maximum value of each actual output power is taken as a maximum output power, and a second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage in each photovoltaic string. The maximum value of each actual output power is taken as a maximum output power, and a second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage in each photovoltaic string.

5. The power control method of claim 4, wherein, The maximum value of each actual output power is taken as a maximum output power, and a second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage in each photovoltaic string. The maximum value of each actual output power is taken as a maximum output power, and a second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage in each photovoltaic string.

6. The power control method of claim 4, wherein, The maximum value of each actual output power is taken as a maximum output power, and a second target voltage corresponding to the maximum output power is less than the minimum open circuit voltage in each photovoltaic string. The input voltage of the direct current conversion circuit is the first target voltage, and the direct current conversion circuit is in a current limiting working state.

7. The power control method of claim 1, wherein, The inverter comprises an inverter circuit, at least one direct current conversion circuit, and a controller, input ends of the direct current conversion circuit are connected to a plurality of photovoltaic strings, output ends of the direct current conversion circuit are connected in parallel to the inverter circuit, and the controller is electrically connected to the direct current conversion circuit and the inverter circuit, and the controller is configured to perform the power control method according to any one of claims 1-7.

8. An inverter, wherein, Comprise:

9. A photovoltaic system, wherein, a plurality of parallel photovoltaic strings and an inverter according to claim 8, each photovoltaic string is electrically connected to the direct current conversion circuit, and an alternating current side of the inverter is electrically connected to a power grid or a load. Each photovoltaic string electrically connected to the direct current conversion circuit has a different open circuit voltage.

10. The photovoltaic system of claim 9, wherein, ​

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