Dual-mode controller, photovoltaic system and method of controlling the same

Abstract

The application discloses a dual-mode controller, a photovoltaic system and a control method thereof. The control method of the photovoltaic system comprises the following steps: controlling the photovoltaic system to start and operate in a current source mode; obtaining the output power of the photovoltaic system; when the output power of the photovoltaic system is less than a first preset power, controlling the photovoltaic system to keep operating in the current source mode and continuously obtaining the output power of the photovoltaic system; and when the output power of the photovoltaic system is greater than or equal to the first preset power, controlling the photovoltaic system to switch to operate in a voltage source mode. In the photovoltaic system starting process, the photovoltaic system is connected to the grid in the current source mode, so that the photovoltaic system can quickly respond to grid frequency fluctuation, the reliability of the photovoltaic system during starting is improved, and the photovoltaic system is connected to the grid in the voltage source mode after the output power of the photovoltaic system is stabilized, so that the stability of the photovoltaic system during normal operation is improved.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic power generation technology, and in particular to a dual-mode controller, a photovoltaic system, and a control method thereof. Background Technology

[0002] Currently, photovoltaic systems operate in voltage source mode, which enables the photovoltaic system to support the system in low short-circuit ratio and weak AC systems, avoiding problems such as frequency voltage collapse, power oscillation and unit disconnection.

[0003] However, in environments with unstable sunlight, such as in the early morning, the power output of a photovoltaic system when it starts up and connects to the grid is low and unstable. This can lead to reverse power output due to fluctuations in the grid frequency, i.e., current flowing back into the photovoltaic system, resulting in DC overvoltage, which can cause the photovoltaic system to start up repeatedly or experience other malfunctions. Summary of the Invention

[0004] The main objective of this invention is to provide a control method for a photovoltaic system, which aims to solve the problem of repeated startups of a photovoltaic system under unstable lighting conditions.

[0005] To achieve the above objectives, this invention proposes a control method for a photovoltaic system, applicable to a photovoltaic system having a voltage source mode and a current source mode. The control method for the photovoltaic system includes:

[0006] S100: Control the photovoltaic system to start and operate in grid-connected mode in current source mode;

[0007] S200: Obtain the output power of the photovoltaic system;

[0008] S301. When the output power of the photovoltaic system is less than the first preset power, control the photovoltaic system to maintain grid-connected operation in current source mode and continue to acquire the output power of the photovoltaic system;

[0009] S302. When the output power of the photovoltaic system is greater than or equal to the first preset power, control the photovoltaic system to switch to voltage source mode for grid-connected operation.

[0010] In one embodiment, step S100 includes:

[0011] The photovoltaic system is controlled to start in current source mode and operate in grid-connected mode in current source mode.

[0012] Alternatively, the photovoltaic system can be controlled to start in voltage source mode, and then switched to current source mode when the switching conditions are met, before being connected to the grid.

[0013] In one embodiment, step S100 includes:

[0014] When the photovoltaic system is connected to the grid in current source mode, maximum power point tracking of the photovoltaic strings is performed, and the photovoltaic system is controlled to operate at the tracked maximum power point voltage.

[0015] In one embodiment, step S100 includes:

[0016] After the photovoltaic system is started, the AC switch is closed according to the output voltage of the photovoltaic system to control the photovoltaic system to operate in parallel with the power grid.

[0017] In one embodiment, the following step is further included after step S301 or S302:

[0018] S400. When the photovoltaic system is connected to the grid in voltage source mode, the output power of the photovoltaic system is obtained;

[0019] S501. When the output power of the photovoltaic system is greater than the second preset power, control the photovoltaic system to maintain grid-connected operation in voltage source mode and continue to acquire the output power of the photovoltaic system;

[0020] S502. When the output power of the photovoltaic system is less than or equal to the second preset power, control the photovoltaic system to switch to current source mode for grid-connected operation;

[0021] Wherein, the second preset power is less than or equal to the first preset power.

[0022] In one embodiment, the following step is further included after step S502:

[0023] S600, Obtain the output power of the photovoltaic system;

[0024] S700: When the output power of the photovoltaic system is less than the first preset power, control the photovoltaic system to maintain grid-connected operation in current source mode, and return to the step of obtaining the output power of the photovoltaic system until the output power of the photovoltaic system is greater than or equal to the first preset power, then control the photovoltaic system to switch to grid-connected operation in voltage source mode.

[0025] In one embodiment, the following step is further included after step S301 or S302:

[0026] S800: When the photovoltaic system is connected to the grid in voltage source mode, the output power and output current of the photovoltaic system are obtained;

[0027] S901. When the output power of the photovoltaic system is less than the third preset power and the output current is less than the first preset current, control the photovoltaic system to maintain grid-connected operation in voltage source mode and continue to acquire the output power of the photovoltaic system.

[0028] S902. When the output power of the photovoltaic system is greater than or equal to the third preset power, or the output current is greater than or equal to the first preset current, control the photovoltaic system to switch to current source mode for grid-connected operation.

[0029] The third preset power is greater than the first preset power.

[0030] In one embodiment, the following step is further included after step S301 or S302:

[0031] S1000: Obtain the output power and output current of the photovoltaic system;

[0032] S1100: When the output power of the photovoltaic system is greater than the fourth preset power or the output current is greater than the second preset current, control the photovoltaic system to maintain grid-connected operation in current source mode, and return to execute the step of continuing to obtain the output power and output current of the photovoltaic system until the output power of the photovoltaic system is less than or equal to the fourth preset power and the output current is less than or equal to the second preset current, then control the photovoltaic system to switch to grid-connected operation in voltage source mode.

[0033] Wherein, the fourth preset power is less than or equal to the third preset power, and the second preset current value is less than or equal to the first preset current value.

[0034] In one embodiment, the following step is further included after step S301 or S302:

[0035] S1200: When the photovoltaic system is connected to the grid in voltage source mode, obtain the grid connection point voltage;

[0036] S1301. When the grid connection point voltage is greater than the first preset voltage and less than the second preset voltage, control the photovoltaic system to maintain grid connection operation in voltage source mode;

[0037] S1302. When the grid connection point voltage is less than or equal to the first preset voltage, or greater than or equal to the second preset voltage, control the photovoltaic system to switch to current source mode for grid connection operation.

[0038] In one embodiment, the control method of the photovoltaic system includes:

[0039] After the photovoltaic system switches from current source mode to voltage source mode, the maximum power point voltage obtained by the photovoltaic system when it is running in current source mode is used as the starting voltage to perform maximum power point tracking of the photovoltaic string, and the photovoltaic system is controlled to operate at the tracked maximum power point voltage.

[0040] In one embodiment, the control method for the photovoltaic system further includes:

[0041] After the photovoltaic system switches from voltage source mode to current source mode, the maximum power point voltage of the photovoltaic system when it is running in voltage source mode is used as the starting voltage to perform maximum power point tracking of the photovoltaic string, and the photovoltaic system is controlled to operate at the tracked maximum power point voltage.

[0042] The present invention also proposes a dual-mode controller for use in a photovoltaic system. The dual-mode controller includes a memory, a processor, and a control program for the photovoltaic system stored in the memory and executable on the processor. When the control program for the photovoltaic system is executed by the processor, it implements the steps of the aforementioned control method for the photovoltaic system.

[0043] The present invention also proposes a photovoltaic system, which includes a photovoltaic string, an inverter, and the aforementioned dual-mode controller; wherein the output terminal of the photovoltaic string is connected to the input terminal of the inverter, the output terminal of the inverter is connected to the power grid, and the controlled terminal of the inverter is connected to the dual-mode controller.

[0044] This invention addresses the issue of photovoltaic (PV) systems operating in current-source mode before grid connection. This avoids starting in low-light conditions, allowing the PV system to temporarily operate in current-source mode, mitigating power reversal caused by grid frequency fluctuations and preventing repeated starts. After grid connection, if the PV system's output power is less than a preset power, it maintains current-source mode; otherwise, it switches to voltage-source mode. This ensures the PV system operates normally in voltage-source mode, enabling it to function in low-short-circuit ratio, low-AC environments, preventing frequency and voltage collapse, power oscillations, and grid disconnection. Therefore, this invention improves the reliability of PV system startup and stability during normal operation by utilizing mode switching. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0046] Figure 1 This is a flowchart illustrating an embodiment of the control method for a photovoltaic system according to the present invention;

[0047] Figure 2This is a flowchart illustrating another embodiment of the control method for the photovoltaic system of the present invention;

[0048] Figure 3 This is a flowchart illustrating another embodiment of the control method for the photovoltaic system of the present invention;

[0049] Figure 4 This is a flowchart illustrating another embodiment of the control method for the photovoltaic system of the present invention;

[0050] Figure 5 This is a flowchart illustrating yet another embodiment of the control method for the photovoltaic system of the present invention.

[0051] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0052] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0053] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of that feature. Additionally, the technical solutions of the various embodiments can be combined with each other, but only on the basis of being achievable by those skilled in the art. When the combination of technical solutions is contradictory or impossible to implement, such a combination of technical solutions should be considered non-existent and not within the scope of protection claimed by this invention.

[0054] This invention proposes a control method for photovoltaic systems, which can be applied to photovoltaic systems to solve the problem of grid connection failure during startup of photovoltaic systems in the morning or other situations with weak and unstable sunlight.

[0055] Reference Figure 1 In one embodiment of the present invention, the photovoltaic system includes a photovoltaic string, an inverter, and a dual-mode controller for controlling the operation of the inverter, which are connected in sequence. The dual-mode controller can control the inverter to operate in either voltage source output mode or current source output mode. When the inverter operates in voltage source output mode, the photovoltaic system is also referred to as operating in voltage source mode; when the inverter operates in current source output mode, the photovoltaic system is also referred to as operating in current source mode.

[0056] Reference Figure 1In this embodiment, the control method of the photovoltaic system includes:

[0057] S100: Control the photovoltaic system to start and operate in grid-connected mode in current source mode;

[0058] Understandably, in situations where sunlight is very weak, such as in the morning, the output power of photovoltaic strings is low and unstable, resulting in low and unstable output power for the photovoltaic system. At this time, fluctuations in grid frequency may cause reverse power flow, meaning current flows back into the photovoltaic system, leading to DC overvoltage and causing the photovoltaic system to repeatedly start or experience other startup failures.

[0059] Given that current-source grid-connected mode offers faster power regulation compared to voltage-source grid-connected mode, this embodiment ensures the photovoltaic system operates in current-source mode after receiving the start-up command, and then closes the AC switch to guarantee grid-connected operation in this mode. Thus, even if the output power of the photovoltaic string is low and unstable, the fast power regulation speed of current-source mode allows the photovoltaic system to quickly respond to and adjust to grid frequency fluctuations, avoiding the impact of grid frequency fluctuations on the photovoltaic system at low output power, and enabling smooth startup and grid-connected operation.

[0060] It should be noted that the photovoltaic system in this embodiment is pre-configured to operate in current source mode upon receiving the start-up command. Compared to other schemes that detect various parameters, such as grid impedance, and select voltage source mode or current source mode for grid connection based on the grid impedance, this embodiment eliminates the need for detection and judgment steps, resulting in faster start-up speed and eliminating the possibility of misjudgment.

[0061] S200: Obtain the output power of the photovoltaic system;

[0062] The output power of a photovoltaic system can be achieved using an AC voltage sampling unit, an AC current sampling unit, and an AC power calculation unit, the details of which are not elaborated here. In this embodiment, the output power of the photovoltaic system can refer to the active power of the photovoltaic system.

[0063] S301. When the output power of the photovoltaic system is less than the first preset power, control the photovoltaic system to maintain grid-connected operation in current source mode and continue to acquire the output power of the photovoltaic system;

[0064] S302. When the output power of the photovoltaic system is greater than or equal to the first preset power, control the photovoltaic system to switch to voltage source mode operation.

[0065] The condition that the output power of the photovoltaic system is greater than or equal to the first preset power can be defined as the photovoltaic system's output power continuously being greater than or equal to the first preset power within a first preset time period. This avoids instantaneous power fluctuations that could lead to incorrect judgments or misjudgments of the photovoltaic system's output power, resulting in incorrect switching and startup failures. The first preset power can be the minimum output power required for the photovoltaic system to operate stably in voltage source mode without generating reverse power.

[0066] In this embodiment, when the photovoltaic system starts up in low-light conditions such as early morning, if the output power of the photovoltaic system is less than a first preset power, this embodiment controls the photovoltaic system to maintain current source mode operation. This avoids current backflow caused by switching to voltage source mode when the output power of the photovoltaic string is insufficient, ensuring normal operation of the photovoltaic system after grid connection. Until the sunlight stabilizes and the output power of the photovoltaic system is greater than or equal to the first preset power, that is, when the output power of the photovoltaic system meets the requirements for stable operation in voltage source mode, the system immediately switches to voltage source mode. Since voltage source mode can support the system in low short-circuit ratio, weak AC systems, it avoids frequency voltage collapse, power oscillation, and grid disconnection, thereby improving the stability of the photovoltaic system.

[0067] Of course, the photovoltaic system can also be started in a sunny environment such as midday. At this time, after the start-up and grid connection are completed, the output power of the photovoltaic system will immediately be greater than or equal to the first preset power. At this time, the photovoltaic system will switch to the voltage source grid connection mode immediately after being connected to the grid in the current source mode.

[0068] This invention addresses the issue of photovoltaic (PV) systems operating in current-source mode before grid connection. This avoids starting in low-light conditions, allowing the PV system to temporarily operate in current-source mode, mitigating power reversal caused by grid frequency fluctuations and preventing repeated starts. After grid connection, if the PV system's output power is less than a preset power, it maintains current-source mode; otherwise, it switches to voltage-source mode. This ensures the PV system operates normally in voltage-source mode, enabling it to function in low-short-circuit ratio, low-AC environments, preventing frequency and voltage collapse, power oscillations, and grid disconnection. Therefore, this invention improves the reliability of PV system startup and stability during normal operation by utilizing mode switching.

[0069] In some embodiments, the first preset power can be 5% to 10% of the rated output power of the photovoltaic system. Actual testing shows that an output power of 5% to 10% of the rated output power of the photovoltaic system is sufficient to meet its requirements for stable operation in voltage source mode. This embodiment sets the first preset power to 5% to 10% of the rated output power of the photovoltaic system. This avoids setting the first preset power too low, leading to premature switching and inability to resolve morning grid connection failures. It also avoids setting the first preset power too high, resulting in failure to switch to voltage source mode in a timely manner, ultimately causing problems such as frequency voltage collapse, power oscillation, and unit disconnection from the grid.

[0070] Reference Figure 2 In some embodiments, step S100 includes:

[0071] S101. Control the photovoltaic system to start in current source mode and operate in grid-connected mode in current source mode.

[0072] Alternatively, in S102, the photovoltaic system is controlled to start in voltage source mode, and when the switching conditions are met, it is switched to current source mode and then connected to the grid.

[0073] In this embodiment, whether the photovoltaic system is directly controlled to start and operate in current source mode or controlled to start in voltage source mode, the system switches to current source mode only when switching conditions are met, such as when the output voltage of the photovoltaic string is greater than or equal to the minimum grid-connected voltage, or when the output power is greater than or equal to the minimum grid-connected power. This ensures that the photovoltaic system operates in grid-connected mode in low-light and unstable environments such as early morning, thus avoiding the problem of repeated restarts.

[0074] Reference Figure 1 In one embodiment, step S100 includes: when the photovoltaic system is operating in grid-connected mode in current source mode, performing maximum power point tracking (MPPT) on the photovoltaic strings and controlling the photovoltaic system to operate at the tracked maximum power point voltage. This can improve the output power in current source grid-connected mode, reduce the impact of weak and unstable sunlight, and further improve the reliability of the photovoltaic system startup process.

[0075] Specifically, this step can begin after the photovoltaic system is connected to the grid, starting maximum power point tracking (MPPT) and operating at the tracked power point voltage until the maximum power point is reached and the system operates at the tracked power point voltage. The output power of the photovoltaic system can be acquired periodically during the MPPT process.

[0076] It should be noted that maximum power point tracking (MPPT) is faster in current source mode than in voltage source mode. Therefore, compared to traditional voltage source grid-connected MPT schemes, this embodiment performs MPT in current source mode, reaching the maximum power point much faster. Then, it switches to voltage source grid-connected mode to continue tracking the maximum power point. The photovoltaic system can reach the maximum power point much faster than traditional methods.

[0077] Reference Figure 1 In one embodiment, step S100 includes: after the photovoltaic system is started, controlling the AC switch to close according to the output voltage of the photovoltaic system, so as to control the photovoltaic system to operate in grid connection with the power grid.

[0078] In this embodiment, controlling the closing of the AC switch based on the output voltage of the photovoltaic system can mean that the amplitude and phase of the grid voltage of the photovoltaic system are synchronized. At this time, closing the AC switch and operating in grid-connected mode can avoid the grid power reversal caused by the voltage being out of sync with the grid, thus improving the grid-connected stability of the photovoltaic system.

[0079] Reference Figure 3 In one embodiment, the following step is further included after step S301 or S302:

[0080] S400. When the photovoltaic system is connected to the grid in voltage source mode, the output power of the photovoltaic system is obtained;

[0081] S501. When the output power of the photovoltaic system is greater than the second preset power, control the photovoltaic system to maintain grid-connected operation in voltage source mode and continue to acquire the output power of the photovoltaic system;

[0082] S502. When the output power of the photovoltaic system is less than or equal to the second preset power, control the photovoltaic system to switch to current source mode for grid-connected operation; wherein the second preset power is less than or equal to the first preset power.

[0083] In this embodiment, the output power being less than or equal to the second preset power can be understood as the output power being continuously less than or equal to the second preset power for a second preset time period. This avoids instantaneous power fluctuations that could lead to incorrect judgments or misjudgments of the output power by the photovoltaic system, resulting in incorrect switching and system failure.

[0084] It should be noted that photovoltaic (PV) systems may encounter unforeseen circumstances while operating in voltage source mode, such as a sudden cloudy day leading to weaker sunlight. In this situation, the output power of the PV strings becomes low and unstable, meaning they are operating under light load. Continuing to operate in voltage source mode, due to the low output power and slow response of the PV system, current may flow back into the PV system again due to grid frequency fluctuations, resulting in DC overvoltage and potentially causing other malfunctions.

[0085] This embodiment detects the output power of the photovoltaic system during operation in voltage source mode. When the output power of the photovoltaic system becomes low and unstable, that is, when it is less than or equal to a second preset power, it temporarily switches to current source mode to ensure the stability of the photovoltaic system during operation.

[0086] Reference Figure 3 Furthermore, the following steps are included after S502:

[0087] S600: Obtain the output power of the photovoltaic system;

[0088] S700: When the output power of the photovoltaic system is less than the first preset power, control the photovoltaic system to maintain grid-connected operation in current source mode, and return to the step of obtaining the output power of the photovoltaic system until the output power of the photovoltaic system is greater than or equal to the first preset power, then control the photovoltaic system to switch to grid-connected operation in voltage source mode.

[0089] It should be noted that after the emergency, the sunlight returns to normal. This embodiment, by detecting that the output power of the photovoltaic system is greater than or equal to a first preset power, confirms that the sunlight has returned to normal and immediately switches to voltage source mode, thereby improving the stability of the photovoltaic system during normal operation.

[0090] This embodiment's technical solution detects the output power of the photovoltaic system to determine if the system is experiencing an emergency. If such an emergency occurs, the system temporarily switches to current source mode to ensure its stability during operation. After confirming the emergency has ended based on the photovoltaic system's output power, it switches back to voltage source mode for grid-connected operation, further enhancing the system's stability.

[0091] In some embodiments, the second preset power can also be 5% to 10% of the rated output power of the photovoltaic system. Actual tests show that when the output power of the photovoltaic system is less than or equal to 5% to 10% of its rated output power, the system output power is low, and the photovoltaic system will again experience current backflow due to grid frequency fluctuations, leading to system failure. This embodiment sets the second preset power to 5% to 10% of the rated output power of the photovoltaic system. This avoids system failures caused by setting the second preset power too low and not switching in time, and also avoids instability caused by setting the second preset power too high, leading to frequency and voltage collapse, power oscillation, and grid disconnection.

[0092] Reference Figure 4 In one embodiment, the following step is further included after step S502:

[0093] S800: When the photovoltaic system is connected to the grid in voltage source mode, the output power and output current of the photovoltaic system are obtained;

[0094] S901. When the output power of the photovoltaic system is less than the third preset power and the output current is less than the first preset current, control the photovoltaic system to maintain grid-connected operation in voltage source mode and continue to acquire the output power of the photovoltaic system.

[0095] S902. When the output power of the photovoltaic system is greater than or equal to a third preset power, or the output current is greater than or equal to a first preset current, the photovoltaic system is controlled to switch to current source mode for grid-connected operation; wherein the third preset power is greater than the first preset power.

[0096] In this embodiment, the values ​​of the third preset power and the first preset current can be set according to requirements. For example, the third preset power can be the maximum power of the photovoltaic system when it is fully loaded, and the first preset current can be the maximum current of the photovoltaic system when it is fully loaded.

[0097] It should be explained that when a photovoltaic system operates continuously in voltage source mode under heavy or full load conditions, a significant transient drop in the grid frequency can cause a substantial increase in the output power and / or output current of the photovoltaic system, leading to overload.

[0098] In this embodiment, when the output power of the photovoltaic system is detected to be greater than or equal to the third preset power, or the output current is greater than or equal to the first preset current, it is confirmed that the photovoltaic system is overloaded and the grid frequency drops significantly. It immediately switches to the current source mode to track the grid frequency in a timely manner and avoid the photovoltaic system from overloading and causing failure.

[0099] It should be noted that this embodiment determines whether a transient frequency drop has occurred in the grid by measuring the output power of the photovoltaic system. The output power detection circuits used are all pre-existing circuits, meaning they are reused, thus eliminating the need for additional detection components and avoiding increases in the size and cost of the photovoltaic system. Furthermore, this embodiment determines a transient frequency drop in the grid by ensuring that either the output power or the output current meets the requirement, thereby improving detection sensitivity.

[0100] Reference Figure 4 Furthermore, the following steps are included after step S902:

[0101] S1000: Obtain the output power and output current of the photovoltaic system;

[0102] S1100: When the output power of the photovoltaic system is greater than the fourth preset power, or the output current is greater than the second preset current, control the photovoltaic system to maintain grid-connected operation in current source mode, and return to the step of continuing to obtain the output power and output current of the photovoltaic system until the output power of the photovoltaic system is less than or equal to the fourth preset power and the output current is less than or equal to the second preset current, then control the photovoltaic system to switch to grid-connected operation in voltage source mode; wherein, the fourth preset power is less than or equal to the third preset power, and the second preset current value is less than or equal to the first preset current value.

[0103] In this embodiment, the values ​​of the fourth preset power and the second preset current can be set according to requirements. For example, the fourth preset power and the second preset current can refer to the maximum output power and output current of the photovoltaic system when the grid frequency returns to normal.

[0104] When the output power of the photovoltaic system is less than or equal to the fourth preset power and the output current is less than or equal to the second preset current, and the grid frequency is confirmed to have returned to normal, the system immediately switches to voltage source mode to ensure the stability of the photovoltaic system's operation. This embodiment is configured so that both output power and output current simultaneously meet the requirements to determine a transient drop in grid frequency, thus improving detection accuracy.

[0105] Furthermore, the third preset power value can be 105% to 110% of the rated output power of the photovoltaic system. The first preset current value can be 110% to 120% of the rated output current of the photovoltaic system.

[0106] Actual tests show that when the output power of the photovoltaic system is greater than or equal to 105% to 110% of the rated output power, or when the output current of the photovoltaic system is greater than or equal to 110% to 120% of the rated output current, it can be preliminarily determined that the photovoltaic system overload is caused by a significant drop in grid frequency. In this embodiment, the range of values ​​for the third preset power and the first preset current ensures high accuracy and sensitivity in grid frequency drop detection.

[0107] Furthermore, the fourth preset power value can be 105% to 110% of the rated output power of the photovoltaic system. The second preset current value can be 110% to 120% of the rated output current of the photovoltaic system.

[0108] Actual testing shows that when the output power of the photovoltaic system is less than 105% to 110% of the rated output power, and the second preset current value can be 110% to 120% of the rated output current of the photovoltaic system, it can be preliminarily determined that the problem is due to the grid frequency and the recovery from a transient drop to a normal state. In this embodiment, the range of values ​​for the fourth preset power and the second preset current ensures high accuracy and sensitivity in grid frequency recovery detection.

[0109] Reference Figure 5 In one embodiment, the following step is further included after step S502:

[0110] S1200: When the photovoltaic system is connected to the grid in voltage source mode, obtain the grid connection point voltage;

[0111] S1301. When the grid connection point voltage is greater than the first preset voltage and less than the second preset voltage, control the photovoltaic system to maintain grid connection operation in voltage source mode;

[0112] S1302. When the grid connection point voltage is less than or equal to the first preset voltage, or greater than or equal to the second preset voltage, control the photovoltaic system to switch to current source mode for grid connection operation.

[0113] It should be noted that, according to standard requirements, when the grid connection point voltage is less than 20% of the grid rated voltage, the photovoltaic system needs to perform low-voltage ride-through, and when the grid connection point voltage is greater than 110% of the grid rated voltage, the photovoltaic system needs to perform high-voltage ride-through.

[0114] In practical applications, when a photovoltaic system is in voltage source mode, low-voltage ride-through requires a very large short-circuit capacity, involves complex control modeling, and is unstable, making it prone to failure. Conversely, when performing high-voltage ride-through in voltage source mode, there is insufficient supporting current to support the recovery of grid voltage.

[0115] Therefore, in this embodiment, when the grid connection point voltage is less than or equal to the first preset voltage, or greater than or equal to the second preset voltage, that is, when the photovoltaic system needs to perform low-voltage ride-through or high-voltage ride-through, this embodiment switches to current source mode and then executes the corresponding low-voltage / high-voltage ride-through strategy. Compared with the traditional scheme of directly using the mode for low-voltage / high-voltage ride-through or islanding protection, this embodiment can make the corresponding low-voltage ride-through strategy and control model simpler, greatly simplifying the modeling and control difficulty of low-voltage ride-through and high-voltage ride-through.

[0116] Furthermore, since the voltage source mode provides a constant output current to the grid and responds rapidly to the grid frequency, it can quickly respond to grid changes during low-voltage or high-voltage ride-throughs and generate a corresponding proportion of capacitive reactive current, effectively supporting grid voltage recovery and ensuring grid stability.

[0117] Furthermore, after step S1302, the following step is also included: when the grid connection point voltage is restored to the grid's rated voltage, the photovoltaic system is controlled to switch to voltage source mode for grid-connected operation.

[0118] Reference Figures 3 to 5 In one embodiment, the control method of the photovoltaic system includes: after the photovoltaic system switches from current source mode to voltage source mode, using the maximum power point voltage obtained in current source mode as the starting voltage, performing maximum power point tracking of the photovoltaic string, and controlling the photovoltaic system to operate at the tracked maximum power point voltage.

[0119] Compared to re-tracking the maximum power point voltage of the photovoltaic string, this embodiment uses the maximum power point voltage obtained in the current source mode before switching as the starting voltage for maximum power point tracking, thereby enabling faster tracking to the maximum power point voltage and improving tracking efficiency.

[0120] Reference Figures 3 to 5 In one embodiment, the control method for the photovoltaic system includes: after the photovoltaic system switches from voltage source mode to current source mode, using the maximum power point voltage in voltage source mode as the starting voltage, performing maximum power point tracking (MPPT) of the photovoltaic string, and controlling the photovoltaic system to operate at the tracked maximum power point voltage. This is compared to re-tracking the maximum power point voltage of the photovoltaic string. In this embodiment, using the maximum power point voltage obtained in the voltage source mode before the switch as the starting voltage for MPPT allows for faster tracking to the maximum power point voltage, improving tracking efficiency.

[0121] This invention also proposes a dual-mode controller for use in photovoltaic systems. The dual-mode controller includes a memory, a processor, and a photovoltaic system control program stored in the memory and executable on the processor. When executed by the processor, the photovoltaic system control program implements the steps of the photovoltaic system control method described above. Since this dual-mode controller employs all the technical solutions of the above embodiments, it possesses at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated upon further here.

[0122] The present invention also proposes a photovoltaic system, which includes a photovoltaic string, an inverter, and the aforementioned dual-mode controller. The specific structure of the dual-mode controller is as described in the above embodiments. Since the present photovoltaic system adopts all the technical solutions of all the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be described in detail here.

[0123] The above description is merely an optional embodiment of the present invention and does not limit the patent scope of the present invention. All equivalent structural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A control method of a photovoltaic system, applied to a photovoltaic system, characterized in that, The photovoltaic system has a voltage source mode and a current source mode, the power regulation speed of the current source mode is greater than the power regulation speed of the voltage source mode, and the control method of the photovoltaic system comprises the following steps: S100, starting the photovoltaic system and operating in the current source mode to be connected to the grid, and after the photovoltaic system operates in the current source mode, the photovoltaic system is connected to the grid; S200, acquiring the output power of the photovoltaic system; S301, when the output power of the photovoltaic system is less than a first preset power, controlling the photovoltaic system to operate in the current source mode to be connected to the grid, and continuing to acquire the output power of the photovoltaic system; S302, when the output power of the photovoltaic system is greater than or equal to the first preset power, controlling the photovoltaic system to switch to the voltage source mode to be connected to the grid.

2. The control method of a photovoltaic system according to claim 1, characterized by, The step S100 comprises the following steps: S101, starting the photovoltaic system in the current source mode and operating in the current source mode to be connected to the grid; Or, S102, starting the photovoltaic system in the voltage source mode, and when the switching condition is met, switching to the current source mode and then operating to be connected to the grid.

3. The control method of a photovoltaic system according to claim 1, characterized by, The step S100 comprises: When the photovoltaic system operates in the current source mode to be connected to the grid, performing maximum power point tracking of the photovoltaic string, and controlling the photovoltaic system to operate at the tracked maximum power point voltage.

4. The control method of a photovoltaic system according to claim 1, wherein The step S100 comprises: After the photovoltaic system is started, controlling the AC switch to be closed according to the output voltage of the photovoltaic system to control the photovoltaic system to operate to be connected to the grid.

5. The control method of a photovoltaic system according to claim 1, wherein The step S301 or S302 further comprises the following steps: S400, when the photovoltaic system operates in the voltage source mode to be connected to the grid, acquiring the output power of the photovoltaic system; S501, when the output power of the photovoltaic system is greater than a second preset power, controlling the photovoltaic system to operate in the voltage source mode to be connected to the grid, and continuing to acquire the output power of the photovoltaic system; S502, when the output power of the photovoltaic system is less than or equal to the second preset power, controlling the photovoltaic system to switch to the current source mode to be connected to the grid; Wherein, the second preset power is less than or equal to the first preset power.

6. The control method of a photovoltaic system according to claim 5, wherein The step S502 further comprises the following steps: S600, acquiring the output power of the photovoltaic system; S700, when the output power of the photovoltaic system is less than the first preset power, controlling the photovoltaic system to operate in the current source mode to be connected to the grid, and returning to execute the step of acquiring the output power of the photovoltaic system, until the output power of the photovoltaic system is greater than or equal to the first preset power, and then controlling the photovoltaic system to switch to the voltage source mode to be connected to the grid.

7. The control method of a photovoltaic system according to claim 1, wherein The step S301 or S302 further comprises the following steps: S800, when the photovoltaic system operates in the voltage source mode to be connected to the grid, acquiring the output power and output current of the photovoltaic system; S901, when the output power of the photovoltaic system is less than a third preset power, and the output current is less than a first preset current, controlling the photovoltaic system to operate in the voltage source mode to be connected to the grid, and continuing to acquire the output power of the photovoltaic system; S902, when the output power of the photovoltaic system is greater than or equal to a third preset power or the output current is greater than or equal to a first preset current, controlling the photovoltaic system to switch to the current source mode and operate in grid connection; The third preset power is greater than the first preset power.

8. The control method of a photovoltaic system according to claim 7, wherein The step S902 further includes the following steps: S1000, acquiring the output power and the output current of the photovoltaic system; S1100, when the output power of the photovoltaic system is greater than a fourth preset power or the output current is greater than a second preset current, controlling the photovoltaic system to keep operating in the current source mode and returning to execute the step of continuously acquiring the output power and the output current of the photovoltaic system, until the output power of the photovoltaic system is less than or equal to the fourth preset power and the output current is less than or equal to the second preset current, and then controlling the photovoltaic system to switch to the voltage source mode and operate in grid connection; The fourth preset power is less than or equal to the third preset power, and the second preset current is less than or equal to the first preset current.

9. The control method of a photovoltaic system according to claim 1, wherein, The step S301 or S302 further includes the following steps: S1200, when the photovoltaic system operates in the voltage source mode in grid connection, acquiring the grid connection point voltage; S1301, when the grid connection point voltage is greater than a first preset voltage and less than a second preset voltage, controlling the photovoltaic system to keep operating in the voltage source mode in grid connection; S1302, when the grid connection point voltage is less than or equal to the first preset voltage or greater than or equal to the second preset voltage, controlling the photovoltaic system to switch to the current source mode and operate in grid connection.

10. The method of controlling a photovoltaic system according to any one of claims 1 to 9, wherein The control method of the photovoltaic system includes: After the photovoltaic system is switched from the current source mode to the voltage source mode, taking the maximum power point voltage obtained when the photovoltaic system operates in the current source mode in grid connection as a starting voltage, performing maximum power point tracking of the photovoltaic string, and controlling the photovoltaic system to operate at the tracked maximum power point voltage.

11. A method of controlling a photovoltaic system as claimed in any one of claims 6 to 9, characterized in that, The control method of the photovoltaic system includes: After the photovoltaic system is switched from the voltage source mode to the current source mode, taking the maximum power point voltage obtained when the photovoltaic system operates in the voltage source mode in grid connection as a starting voltage, performing maximum power point tracking of the photovoltaic string, and controlling the photovoltaic system to operate at the tracked maximum power point voltage.

12. A dual-mode controller for use in a photovoltaic system, characterized in that, The dual-mode controller includes a memory, a processor, and a control program of the photovoltaic system stored on the memory and executable on the processor, and the control program of the photovoltaic system, when executed by the processor, implements the steps of the control method of the photovoltaic system according to any one of claims 1 to 10.

13. A photovoltaic system characterized by, The photovoltaic system includes a photovoltaic string, an inverter, and the dual-mode controller according to claim 12; wherein The output end of the photovoltaic string is connected to the input end of the inverter, the output end of the inverter is connected to the grid, and the controlled end of the inverter is connected to the dual-mode controller.

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