Power control method, photovoltaic inverter and photovoltaic grid-connected system

Abstract

The application provides a power control method, a photovoltaic inverter and a photovoltaic grid-connected system, and relates to the technical field of photovoltaic grid-connected power control. The method is applied to a photovoltaic grid-connected system, the photovoltaic grid-connected system comprises a photovoltaic inverter, and the output end of the photovoltaic inverter is connected with a power grid and a load. The output of the photovoltaic inverter is controlled through a power loop, and the power loop comprises a proportional term and an integral term. The power control method comprises the following steps: when it is detected that the required output power of the photovoltaic grid-connected system is less than a preset power threshold, a reduction operation is performed on the integral term; wherein the power threshold is not greater than the minimum active power of the load in all operating conditions of the photovoltaic grid-connected system; and the output power of the photovoltaic inverter is controlled based on the power loop after the integral term is reduced. The application can avoid the overshoot drop problem in the power rise control process after the small power control of the photovoltaic grid-connected system, and can improve the stability of the photovoltaic grid-connected system.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic grid-connected power control technology, and in particular to a power control method, a photovoltaic inverter, and a photovoltaic grid-connected system. Background Technology

[0002] A grid-connected photovoltaic (PV) system mainly consists of PV modules and an inverter. The input of the inverter is connected to the PV modules, while the output of the inverter is connected to the power grid. The PV grid-connected system allows the electrical energy converted by the PV modules to be transmitted to the grid, and the transmitted power can be regulated by controlling the output of the inverter.

[0003] According to relevant technical regulations for photovoltaic (PV) grid connection, when the grid frequency exceeds the rated frequency, the output power of the PV grid-connected system should be reduced to avoid excess active power in the grid causing the grid frequency to continue to rise. The inventors discovered that, depending on the rise in grid frequency, it may sometimes be necessary to control the output power of the PV grid-connected system to zero or near zero, i.e., to implement low-power control until the grid frequency returns to normal before increasing the system's output power. Furthermore, based on user needs, management personnel may also proactively issue dispatch instructions to implement low-power control of the PV grid-connected system for a certain period before gradually increasing the power output.

[0004] The inventors also discovered that after implementing the aforementioned low-power control on the photovoltaic grid-connected system, overshoot and fallback often occur during the subsequent power increase control process, meaning that the power increase process fluctuates, affecting the stability of the system. Summary of the Invention

[0005] This invention provides a power control method, a photovoltaic inverter, and a photovoltaic grid-connected system to solve the problem of system instability caused by fluctuations during the power increase control process after low-power control in photovoltaic grid-connected systems.

[0006] In a first aspect, embodiments of the present invention provide a power control method applied to a photovoltaic grid-connected system, the photovoltaic grid-connected system including a photovoltaic inverter, the output terminal of the photovoltaic inverter being connected to the power grid and a load; the output of the photovoltaic inverter being controlled by a power loop, the power loop including a proportional term and an integral term; the method comprising: When the required output power of the photovoltaic grid-connected system is detected to be less than a preset power threshold, the integral term is reduced; wherein, the power threshold is not greater than the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system; The output power of the photovoltaic inverter is controlled by the power loop after the integral term is reduced.

[0007] In conjunction with the first aspect, in some embodiments, the step of reducing the integral term when the required output power of the photovoltaic grid-connected system is detected to be less than a preset power threshold includes: reducing the integral term when the reference value of the power loop is detected to be less than the power threshold.

[0008] In conjunction with the first aspect, in some embodiments, the reference value of the power loop is given based on a manually issued scheduling instruction; or, the reference value of the power loop is given based on the current grid frequency.

[0009] In conjunction with the first aspect, in some embodiments, the step of reducing the integral term when the reference value of the power loop is detected to be less than the power threshold includes: When the reference value of the power loop is detected to be less than the power threshold, and the output of the power loop reaches the upper limit of saturation threshold, the integral term is reduced. Alternatively, when the reference value of the power loop is detected to be less than the power threshold and the integral term reaches a preset threshold, the integral term is reduced; wherein the preset threshold is a positive value.

[0010] In conjunction with the first aspect, in some embodiments, the step of reducing the integral term when the reference value of the power loop is detected to be less than the power threshold includes: reducing the integral term when the reference value of the power loop is detected to be less than the power threshold for a preset duration.

[0011] In conjunction with the first aspect, in some embodiments, the reduction operation on the integral term includes setting the cumulative deviation in the integral term to zero.

[0012] In conjunction with the first aspect, in some embodiments, the output of the photovoltaic inverter is also controlled via a current loop; the control of the photovoltaic inverter's output power based on the power loop reduced by the integral term includes: Obtain the voltage at the output terminal of the photovoltaic inverter; Based on the voltage and the output of the power loop after the integral term is reduced, the reference value of the current loop is determined; The output power of the photovoltaic inverter is controlled based on the output of the current loop.

[0013] Secondly, embodiments of the present invention also provide a power control method applied to a photovoltaic grid-connected system, the photovoltaic grid-connected system including a photovoltaic inverter, the output terminal of the photovoltaic inverter being connected to the power grid and a load; the output of the photovoltaic inverter is controlled by a power loop, the power loop including a proportional term and an integral term; the method includes: When the required output power of the photovoltaic grid-connected system is detected to increase from a first power value to a second power value, the integral term is reduced; the first power value is less than a preset power threshold, and the second power value is greater than the power threshold; the power threshold is not greater than the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system. The output power of the photovoltaic inverter is controlled by the power loop after the integral term is reduced.

[0014] In conjunction with the second aspect, in some embodiments, the step of reducing the integral term after detecting that the required output power of the photovoltaic grid-connected system has increased from the first power value to the second power value includes: detecting the output of the power loop after detecting that the required output power of the photovoltaic grid-connected system has increased from the first power value to the second power value; reducing the integral term when detecting that the output of the power loop has reached saturation; and controlling the output power of the photovoltaic inverter based on the power loop after the integral term has been reduced.

[0015] In conjunction with the second aspect, in some embodiments, the step of reducing the integral term after detecting that the required output power of the photovoltaic grid-connected system has increased from a first power value to a second power value includes: detecting the error of the power loop after detecting that the required output power of the photovoltaic grid-connected system has increased from the first power value to the second power value; and reducing the integral term when the error is detected to be negative for the first time; wherein the error is the difference between the reference value of the power loop and the power detected at the output of the photovoltaic inverter; and controlling the output power of the photovoltaic inverter based on the power loop after the integral term is reduced.

[0016] Thirdly, embodiments of the present invention provide a photovoltaic inverter applied to a photovoltaic grid-connected system, wherein the photovoltaic inverter controls the output power through the method described in any one of the first aspects above.

[0017] Fourthly, embodiments of the present invention provide a photovoltaic grid-connected system, including photovoltaic modules and a photovoltaic inverter, wherein the photovoltaic inverter controls its output power through the method described in any one of the first aspects above.

[0018] The beneficial effects of the embodiments of the present invention compared with related technologies are as follows: This invention provides a power control method applied to a photovoltaic grid-connected system. The system includes a photovoltaic inverter, whose output is controlled by a power loop, which includes a proportional term and an integral term. When the required output power of the photovoltaic grid-connected system is detected to be less than a preset power threshold, the integral term is reduced. Then, the output power of the photovoltaic inverter is controlled based on the power loop with the reduced integral term. Specifically, when low-power control of the photovoltaic grid connection is detected, the integral term is directly reduced, thereby preventing the integral term from accumulating and causing the power loop output to saturate. Therefore, at the end of low-power control, the integral term of the power loop remains small, and during subsequent power increase control, the power loop output is less likely to saturate. This effectively avoids power fluctuations caused by overshoot and fallback during power increase control, thereby improving the stability of the photovoltaic grid-connected system.

[0019] This invention also provides a power control method applied to a photovoltaic grid-connected system. The photovoltaic grid-connected system includes a photovoltaic inverter, and the output of the photovoltaic inverter is controlled by a power loop, which includes a proportional term and an integral term. When the required output power of the photovoltaic grid-connected system is detected to increase from a first power value to a second power value, the integral term is reduced. The first power value is less than a preset power threshold, and the second power value is greater than the power threshold. The output power of the photovoltaic inverter is controlled based on the power loop after the integral term is reduced. That is, when the photovoltaic grid-connected system is detected to change from low-power control to power-increase control, the integral term of the power loop is immediately reduced to ensure that the output of the power loop is not easily saturated during the subsequent power-increase control process. Therefore, it can effectively avoid power fluctuations caused by overshoot and fallback during the power-increase control process, thereby improving the stability of the photovoltaic grid-connected system.

[0020] It is understood that the beneficial effects of the third and fourth aspects mentioned above can be found in the relevant descriptions in the first aspect above, and will not be repeated here.

[0021] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this specification. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the topology of a photovoltaic grid-connected system provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the power waveform during the power increase control process after low power control provided in the embodiment of the present invention, showing the overshoot and fall-off phenomenon. Figure 3 This is a flowchart illustrating the implementation of the power control method provided in this embodiment of the invention. Figure 4 This is a flowchart illustrating the implementation of a power control method according to another embodiment of the present invention; Figure 5 This is a schematic diagram of the time point at which the integral term is set to zero, provided in an embodiment of the present invention. Detailed Implementation

[0023] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0024] Figure 1 This is a schematic diagram of the topology of a photovoltaic grid-connected system provided in an embodiment of the present invention. Figure 1 As shown, the photovoltaic grid-connected system 10 includes a photovoltaic module 11 and a photovoltaic inverter 12; the input terminal of the photovoltaic inverter 12 is connected to the photovoltaic module 11, and the output terminal of the photovoltaic inverter 12 is connected to the power grid 14; the connection point between the photovoltaic inverter 12 and the power grid 14 is the grid connection point; the load 13 is connected to the power grid at the grid connection point.

[0025] Here, "grid" can refer to the power system or the distribution network. "Load 13" can be a local load, such as electrical equipment like fans and air conditioners.

[0026] exist Figure 1 In the illustrated topology, the photovoltaic inverter 12 is used to transmit the electrical energy converted by the photovoltaic module 11 to the power grid 14, and the load 13 draws power from the power grid 14 via the grid connection point. In this embodiment of the invention, the photovoltaic grid connection is achieved by the photovoltaic module 11 supplying power to the power grid 14. Since the photovoltaic module 11 is on the power source side, the power transmission direction from the photovoltaic module 11 to the power grid 14 is defined as the positive direction of power. Therefore, the power transmitted by the photovoltaic inverter 12 to the power grid 14 is positive, and the power transmitted from the power grid 14 to the photovoltaic inverter 12 in the reverse direction is negative.

[0027] Both domestic and international technical standards for photovoltaic (PV) grid connection involve power regulation regulations during grid frequency regulation. For example, the domestic standard GB / T29319-2024, "Technical Regulations for Photovoltaic Power Generation Systems Access to Distribution Networks," specifies that for primary frequency regulation, "when the power system frequency is greater than 50Hz, the reduction in active power of the PV power generation system should be limited to no less than 10% of the rated active power; when the power system frequency is less than 50Hz, PV power generation systems equipped with energy storage should increase their active power, with the increase limited to no less than 6% of the rated active power." Therefore, relevant domestic technical regulations require that when the power system frequency exceeds the rated value, the active power transmitted from the PV system to the grid should be reduced; while when the frequency is below the rated value, the active power transmitted from the PV system should be appropriately increased as needed.

[0028] Similarly, relevant technical regulations exist abroad. For example, Italy has separate technical regulations for low-voltage grid connection and medium- and high-voltage grid connection. Taking the CEI 0-16 standard for medium- and high-voltage grid connection as an example, this standard requires that for every 0.1Hz deviation of the power system frequency from the rated value, the active power transmitted by photovoltaics should be adjusted by approximately 3-5% of the rated active power; when the frequency is greater than 50.05Hz, the active power should be reduced, and when the frequency is less than 49.95Hz, the active power should be increased. A power-frequency curve is also provided as a reference curve for active power adjustment with frequency. Therefore, relevant foreign technical regulations require that for every certain increase in the grid frequency relative to the rated frequency, the active power transmitted by photovoltaics to the grid should be reduced by a certain value, and for every certain decrease in the grid frequency relative to the rated frequency, the active power transmitted by photovoltaics to the grid should be increased by a certain value.

[0029] The aforementioned technical specifications require that the power delivered by photovoltaic (PV) power to the grid varies with the grid frequency. This is because fluctuations in grid frequency are primarily influenced by changes in grid load. If the grid frequency is higher than the rated frequency, it indicates a decrease in grid load, meaning there is a surplus of active power provided by the grid, which leads to an increase in grid frequency. Conversely, if the grid provides insufficient active power, the grid frequency will decrease. Therefore, when the grid frequency increases, the active power delivered by PV power to the grid needs to be reduced to prevent excess active power from causing the frequency to rise further. When the grid frequency recovers or decreases, the PV power generation capacity needs to be appropriately increased.

[0030] The inventors discovered through research that, depending on the rise in grid frequency, it may sometimes be necessary to control the output power of the photovoltaic grid-connected system to zero or near zero. This involves implementing low-power control of the photovoltaic grid-connected system until the grid frequency returns to normal, at which point the system's output power can be increased. Furthermore, based on user needs, administrators may proactively issue dispatch instructions to implement low-power control of the photovoltaic grid-connected system for a certain period before increasing the power output. For example, if a user requires that the total output power of the photovoltaic system be controlled within a low range before 7:00 AM each day, administrators will proactively issue dispatch instructions to maintain the photovoltaic grid-connected system's power within a very small range (e.g., 0-3 kW) to avoid exceeding the total power limit.

[0031] The inventors further discovered that after performing the aforementioned low-power control on the photovoltaic grid-connected system, overshoot and fallback often occur during the subsequent power increase control process, meaning that there are fluctuations in the power increase process, which affects the stability of the system.

[0032] Research has shown that there are several possible reasons for the overshoot and fallback phenomenon during power increase control: For example, the dynamic response lag and transient power overshoot caused by MPPT (Maximum Power Point Tracking) algorithm characteristics. MPPT is the core control module of a photovoltaic system, its function being to track the maximum power point of the photovoltaic array in real time and adjust the inverter input voltage / current to make the photovoltaic output power approach the MPP. When the control power increases, MPPT needs to switch from a low-power range to a high-power range, but due to algorithm characteristics and response speed limitations, this can lead to transient power overshoot, which then subsides.

[0033] For example, power overshoot can be caused by the underdamped characteristics of the output filter stage in a photovoltaic inverter. An LC or LCL filter is required at the output of a photovoltaic inverter to suppress harmonics generated by the high-frequency switching of the switching transistors, ensuring the output current meets grid connection standards. To reduce power loss, the damping resistor of the filter is typically small, with a damping ratio less than 1. This means the filter is often in an underdamped state. Underdamped operation can easily lead to current overshoot after a sudden power increase, resulting in power overshoot. The power output will then decrease once the current stabilizes.

[0034] For example, power overshoot can occur in the power or current loop due to improper design of the PI (Proportional-Integral) parameters or inherent limitations of PI control. Specifically, this could be caused by an excessively large proportional gain, making the controller overly sensitive to current or power errors during power increases. This could lead to a sudden, excessively large control output, causing the current to rapidly exceed the target value, resulting in power overshoot. Subsequently, due to the error reversal, the controller corrects, causing the power to decrease. Alternatively, the PI controller might be linear, and grid voltage fluctuations can cause problems. Linear PI controllers cannot adapt to these fluctuating time-varying conditions in real time, leading to error calculation errors and resulting in excessive control output, causing power overshoot and subsequent decline.

[0035] Some of the possible causes mentioned in the above studies lead to frequent overshoot and fallback, while others lead to occasional overshoot and fallback. Based on the above research and the specific scenarios in which these problems occur, the inventors, starting with overshoot and fallback caused by PI control, further investigated one possible reason for the frequent occurrence of overshoot and fallback after switching from low-power control to power-increase control: In this embodiment of the invention, the output of the photovoltaic inverter 12 is regulated by PI control, and the output expression of the PI control is:

[0036]

[0037] in, This is the proportional adjustment coefficient. This is the integral adjustment coefficient; For error, This is a reference value for power. The power detected at the output of photovoltaic inverter 12; For cumulative deviation, For the proportion term, This is an integral term.

[0038] based on Figure 1 In the topology shown, when the power supplied by the photovoltaic inverter 12 to the grid 14 is zero (zero power control), since the load 13 still needs to draw power from the grid 14, a current flowing out of the grid 14 can be detected at the grid connection point (the output terminal of the photovoltaic inverter 12), which is the power detected at the grid connection point. The value is negative. Similarly, when the power supplied by the photovoltaic inverter 12 to the grid 14 is very small, such as not exceeding 3kW, since the power required by the load 13 is generally greater than 3kW, when it draws power from the grid 14, the current flowing out of the grid 14 can still be detected at the grid connection point, i.e., the power... It is still negative.

[0039] When performing zero-power control, the power reference value It is 0, due to power It is always negative, hence the error. It is always positive, thus accumulating bias. The PI output will gradually increase over time, eventually reaching saturation. When the PI output reaches saturation during the zero-power control phase, during the subsequent power increase control process, if the error reverses, the PI output will still maintain a large positive adjustment due to the excessively high accumulated deviation, leading to overshoot. After overshoot, the accumulated deviation is gradually reduced by the reverse error, thus adjusting the power back to near the reference value, i.e., power drop. Similarly, when the power supplied by the photovoltaic inverter 12 to the grid 14 is very small, due to the actual power... If the value is consistently negative, the accumulated deviation will continue to increase, causing the PI output to reach saturation, which in turn leads to overshoot and drop-off problems when the power increases. Figure 2 A schematic diagram of the power waveform during overshoot fallback is shown.

[0040] To address the aforementioned problems, this invention provides a power control method applied to a photovoltaic grid-connected system 10, see [link to relevant documentation]. Figure 3 The power control method is described in detail below: Step 101: When the required output power of the photovoltaic grid-connected system is detected to be less than the preset power threshold, the integral term is reduced; wherein, the power threshold is not greater than the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system.

[0041] It should be understood that all operating conditions of a photovoltaic grid-connected system refer to all operating conditions of the photovoltaic grid-connected system when it is turned on and running normally. The minimum active power of the load under all operating conditions of the photovoltaic grid-connected system refers to the minimum load value under normal operating conditions of the photovoltaic grid-connected system.

[0042] In this embodiment of the invention, the required output power refers to the active power that the photovoltaic grid-connected system should supply to the grid. The required output power is less than a preset power threshold, and the power threshold is not greater than the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system. This means that the power that the photovoltaic grid-connected system should supply to the grid is controlled within a very small power range. Detecting whether the required output power of the photovoltaic grid-connected system is less than the preset power threshold determines whether the photovoltaic grid-connected system is currently undergoing low-power control.

[0043] In some embodiments, the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system is 3kW. Therefore, the power threshold can be set to any value between 0 and 3kW according to actual adjustment needs. For example, if the preset power threshold is 3kW, then when the required output power of the photovoltaic grid-connected system is detected to be less than 3kW, the integral term is reduced. As another example, if the preset power threshold is 1kW, then when the required output power of the photovoltaic grid-connected system is detected to be less than 1kW, the integral term is reduced. That is, when the system is detected to be performing low-power control, the integral term is reduced to avoid the power loop output reaching saturation.

[0044] In one possible implementation, the current demand output power can be determined by obtaining the current dispatch information. For example, during grid frequency regulation, the demand output power at the current grid frequency can be calculated based on the relevant technical standards for photovoltaic grid connection and the grid frequency in the dispatch information.

[0045] In another possible implementation, the required output power can also be determined based on manually issued scheduling instructions. For example, when a zero-power scheduling instruction is received, the instruction can be parsed and judged to determine that the current required output power is 0.

[0046] In another possible implementation, the required output power can also be obtained directly from the reference value of the power loop. It can be understood that the reference value of the power loop is given based on the required output power, and using the reference value of the power loop to determine whether the system is currently performing low-power control is faster and more direct. Therefore, one implementation of step 101 is: when the reference value of the power loop is detected to be less than the power threshold, the integral term is reduced.

[0047] Optionally, the reference value for the power loop is given based on a manually issued dispatch instruction; or, the reference value for the power loop is given based on the current grid frequency. For example, if the reference value for the power loop is given based on the current grid frequency, a lookup table of grid frequency and required output power can be preset according to relevant photovoltaic grid-connection technical standards. The required output power is then determined based on the current grid frequency and the preset lookup table, and used as the reference value for the power loop. Alternatively, a relationship (or curve) between grid frequency and output power adjustment can be determined according to relevant photovoltaic grid-connection technical standards. The required output power is then determined based on the current grid frequency and the relationship (or curve), and then based on the output power adjustment and the current actual output power, and used as the reference value for the power loop.

[0048] In the above embodiments, when low-power control is detected, the integral term is reduced; that is, the accumulation of the integral term is suppressed at the very beginning of low-power control and is continuously suppressed throughout the entire low-power control phase to prevent the power loop output from reaching saturation. This method is simple to instruct and has a rapid response. In other embodiments of the present invention, when low-power control is detected, the timing of reducing the integral term can be optimized by considering factors such as whether the integral term is saturated, whether the power loop output is saturated, or the duration of low-power control. This allows the low-power control to maintain a relatively precise control effect while suppressing power loop output saturation. These methods will be described in detail later through specific embodiments.

[0049] In some embodiments, the reduction operation of the integral term in step 101 can be implemented by reducing the cumulative deviation in the integral term by a preset factor. For example, the current cumulative deviation can be multiplied by a preset reduction factor, such as 1 / 100, 1 / 1000, etc.

[0050] In one possible implementation, the cumulative deviation in the integral term is reduced to a factor of 0, that is, the cumulative deviation in the integral term is set to zero.

[0051] In the above embodiments, the integral term is reduced by reducing the value of the accumulated deviation, thereby avoiding the power loop output from reaching saturation due to the excessive accumulation of the integral term during the low-power control stage.

[0052] Step 102: Control the output power of the photovoltaic inverter based on the power loop after the integral term is reduced.

[0053] In some embodiments, the output of the photovoltaic inverter is further controlled by a current loop, which can be used as an inner loop and the power loop as an outer loop. The output of the power loop can be used to determine the reference value of the current loop. This embodiment of the invention does not limit the specific implementation of the current loop. Step 102 can be implemented through steps 1021 to 1023: Step 1021: Obtain the voltage at the output terminal of the photovoltaic inverter.

[0054] Step 1022: Determine the reference value of the current loop based on the voltage at the output terminal of the photovoltaic inverter and the output of the power loop after the integral term is reduced.

[0055] In one possible implementation, the ratio of the output of the power loop after the integral term is reduced to the voltage at the output of the photovoltaic inverter can be used as a reference value for the current loop.

[0056] Step 1023: Control the output power of the photovoltaic inverter based on the output of the current loop.

[0057] In one possible implementation, a voltage reference value can be determined based on the output of the current loop, and a PWM control signal can be obtained based on the voltage reference value. Sending the PWM control signal to the photovoltaic inverter can realize the control of its output power.

[0058] In the above embodiment, when low-power control of the photovoltaic grid connection is detected, the integral term is directly reduced to avoid the power loop output from saturating due to the accumulation of the integral term. Therefore, the integral term of the power loop remains small at the end of the low-power control, and the power loop output is less likely to saturate during subsequent power ramp-up control. This effectively avoids power fluctuations caused by overshoot and fallback during power ramp-up control, thereby improving the stability of the photovoltaic grid-connected system.

[0059] This invention also provides a power control method applied to a photovoltaic grid-connected system 10, see [link to relevant documentation]. Figure 4 The power control method is described in detail below: Step 201: When the required output power of the photovoltaic grid-connected system is detected to increase from the first power value to the second power value, the integral term is reduced; the first power value is less than the preset power threshold, and the second power value is greater than the power threshold.

[0060] Step 202: Control the output power of the photovoltaic inverter based on the power loop after the integral term is reduced.

[0061] In the above embodiment, the system's required output power increases from a first power value to a second power value, which means the low-power control phase of the system ends and enters the power increase control phase. When the system is detected to have entered the power increase control phase from the low-power control phase, the integral term is reduced at the moment when the required output power increases. If the output of the power loop has already reached saturation at that moment, reducing the integral term can reduce the output of the power loop. As the error changes continuously during the power increase control process, the integral term no longer accumulates positively, and the output of the power loop is less likely to reach saturation. Therefore, overshoot can be avoided during power increase control.

[0062] In some embodiments, after detecting that the required output power of the photovoltaic grid-connected system increases from a first power value to a second power value, it is also possible to detect whether the output of the power loop has reached saturation. When the output of the power loop is detected to have reached saturation, the integral term is reduced or set to zero; or it is possible to detect whether the error is reversed. When the error is detected to be reversed for the first time, the integral term is reduced or set to zero. Subsequently, the output power of the photovoltaic inverter is controlled based on the power loop after the integral term is reduced or set to zero.

[0063] To make the power control method of the present invention clearer, several specific embodiments are illustrated below: Example 1 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: setting the integral term to zero when the reference value of the power loop is detected to be zero; and controlling the output power of the photovoltaic inverter based on the power loop after the integral term is set to zero.

[0064] This embodiment determines whether the photovoltaic grid-connected system is performing zero-power control. If zero-power control is detected, the integral term is set to zero. That is, during the low-power control phase, the integral term is continuously set to zero. When power increase control is subsequently implemented, the integral term accumulates from zero, making it less likely for the power loop output to reach saturation, thereby suppressing power fluctuations caused by power overshoot and fallback. See also Figure 5 In this embodiment, the time node at which the integral term begins to be set to zero is node A, and the operation of setting the integral term to zero is continuously performed throughout the low-power control phase.

[0065] Example 2 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: when the reference value of the power loop is detected to be zero and the output of the power loop reaches the upper limit threshold of saturation, setting the integral term to zero; and controlling the output power of the photovoltaic inverter based on the power loop control after the integral term is set to zero.

[0066] The saturation upper limit threshold is the maximum allowable output of the power loop.

[0067] This embodiment determines whether the photovoltaic grid-connected system is performing zero-power control and simultaneously checks whether the power loop output has reached the saturation upper limit threshold. The integral term is only set to zero when the power loop output reaches the saturation upper limit threshold. That is, during the low-power control phase, the power loop output is monitored, and once the power loop output reaches saturation, the integral term is set to zero. This avoids power loop output saturation at the start of power ramp-up control, thereby suppressing power fluctuations caused by power overshoot and fallback. See also... Figure 5 In this embodiment, the time node at which the integral term is set to zero is node B. It should be understood that node B is a time node within the low-power control phase, and its position in the figure is only for example.

[0068] Example 3 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: when the reference value of the power loop is detected to be zero and the integral term reaches a preset threshold, the integral term is set to zero; wherein the preset threshold is a positive value; then, the output power of the photovoltaic inverter is controlled based on the power loop after the integral term is set to zero.

[0069] The preset threshold is the upper limit of the integral term when the power loop output is about to saturate, set based on experience; it can be regarded as the saturation threshold of the integral term. By judging whether the integral term has reached the preset threshold, it is possible to indirectly predict whether the power loop output is about to saturate.

[0070] This embodiment determines whether the photovoltaic grid-connected system is performing zero-power control and simultaneously checks whether the integral term is saturated. When integral term saturation is detected, it is set to zero to avoid power loop output saturation at the start of power ramp-up control, thereby suppressing power fluctuations caused by power overshoot and fallback. See also Figure 5 In this embodiment, the time node at which the integral term is set to zero is node B.

[0071] Example 4 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: when the reference value of the power loop is detected to be zero and continues for a preset time, setting the integral term to zero; and controlling the output power of the photovoltaic inverter based on the power loop after the integral term is set to zero.

[0072] In the first possible implementation, the preset duration can be used to ensure that the system is truly in the zero-power control phase; for example, the preset duration is 5 seconds, meaning that the system can only be confirmed to be truly in the zero-power control phase after zero-power control lasts for 5 seconds; when the power loop reference value is detected to be zero for 3 seconds and then rises to the second power value, no integral term reset operation is performed. See also Figure 5 When making a judgment based on the preset duration, the time node when the integral term is set to zero is node B.

[0073] In the second possible implementation, the preset duration can be the empirically set duration for the integral term in the zero-power control phase to accumulate until the power loop output saturates. See also Figure 5 When making judgments based on a preset duration, the time node for setting the integral term to zero is node B. This embodiment predicts whether the power loop is about to reach saturation based on the duration of low-power control, thereby setting the integral term to zero in a timely manner to avoid overshoot caused by the power loop reaching saturation during subsequent power increase control.

[0074] In the third possible implementation, the current power loop reference value is based on the grid frequency. Therefore, there can be multiple preset durations, each selected according to the grid frequency corresponding to the current reference value. These preset durations can be empirically set, representing the duration of low-power control from the start of low-power control at a certain grid frequency until the grid frequency recovers and power ramp-up control begins. See [link to relevant documentation]. Figure 5 When making a judgment based on the preset duration, the time node for setting the integral term to zero is node C. This embodiment predicts whether the low-power control is about to end based on the duration of low-power control, thereby setting the integral term to zero in a timely manner to avoid overshoot caused by the power loop reaching saturation during subsequent power increase control.

[0075] Example 5 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: when the reference value of the power loop is detected to rise from zero to a second power value, the integral term is set to zero; the second power value is greater than a preset power threshold; then, the output power of the photovoltaic inverter is controlled based on the power loop after the integral term is set to zero.

[0076] This embodiment determines whether the photovoltaic grid-connected system has ended zero-power control and entered the power ramp-up control stage. Specifically, when the power ramp-up control time point is detected, the integral term is set to zero, and the integral term starts accumulating again from zero. This makes it less likely for the power loop output to reach saturation, thereby suppressing power fluctuations caused by power overshoot and fallback. See also... Figure 5 In this embodiment, the time node at which the integral term is set to zero is node D.

[0077] Example 6 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: when the reference value of the power loop is detected to rise from zero to a second power value, detecting whether the output of the power loop has reached saturation, or detecting whether the integral term of the power loop has reached a preset threshold. If it has, the integral term is set to zero. The output power of the photovoltaic inverter is controlled based on the power loop after the integral term is set to zero.

[0078] This embodiment determines whether the photovoltaic grid-connected system has ended zero-power control and entered the power ramp-up control phase. From the start of the power ramp-up control phase, it detects whether the output or integral term of the power loop is saturated. If saturated, the integral term is set to zero, thereby preventing the power loop output from reaching saturation and suppressing power fluctuations caused by power overshoot and fallback. See also... Figure 5 In this embodiment, the time point at which the integral term is set to zero is node E. It should be understood that node E is the time point after the power increase control begins; its position in the figure is for illustrative purposes only.

[0079] Example 7 This invention provides a power control method applied to a photovoltaic grid-connected system 10. The method includes: when the reference value of the power loop is detected to rise from zero to a second power value, and when the error of the power loop is detected to be negative for the first time, setting the integral term to zero; and controlling the output power of the photovoltaic inverter based on the power loop after the integral term is set to zero.

[0080] This embodiment determines whether the photovoltaic grid-connected system has ended zero-power control and entered the power ramp-up control phase. From the start of the power ramp-up control phase, it detects the first reversal of the error and sets the integral term to zero when the error first reverses. This allows the integral term to be cleared before overshoot occurs, instantly reducing the power loop output and directly avoiding power overshoot. See also... ​ In this embodiment, the time node at which the integral term is set to zero is node E.

[0081] The above seven embodiments are all illustrated using zero-power control as an example. It should be understood that other low-power control situations besides zero-power control can also refer to the same logic as above, and will not be illustrated one by one here.

[0082] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0083] This invention also provides a photovoltaic inverter for use in a photovoltaic grid-connected system. The photovoltaic inverter controls its output power through the method described in any of the above method embodiments.

[0084] This invention also provides a photovoltaic grid-connected system, including photovoltaic modules and a photovoltaic inverter, wherein the photovoltaic inverter controls the output power through the method described in any of the above method embodiments.

[0085] This invention also provides a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, it implements the methods described in the above-described method embodiments.

[0086] This invention also provides a computer program product, including a computer program. When the computer program is executed by a processor, it implements the methods described in the above-described method embodiments.

[0087] Computer programs include computer program code, which can be in the form of source code, object code, executable files, or certain intermediate forms. Computer-readable media can include: any entity or device capable of carrying computer program code, recording media, USB flash drives, portable hard drives, magnetic disks, optical disks, computer memory, read-only memory (ROM), random access memory (RAM), electrical carrier signals, telecommunication signals, and software distribution media, etc.

[0088] In the above embodiments, the descriptions of each embodiment have their own emphasis. Parts not detailed or described in a particular embodiment can be referred to in the relevant descriptions of other embodiments. Unless otherwise specified or in conflict with logic, the terminology and / or descriptions between different embodiments are consistent and can be referenced interchangeably. Technical features in different embodiments can be combined to form new embodiments based on their inherent logical relationships.

[0089] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A power control method, characterized in that, The method is applied to a photovoltaic grid-connected system, wherein the photovoltaic grid-connected system includes a photovoltaic inverter, the output of which is connected to the power grid and a load; the output of the photovoltaic inverter is controlled by a power loop, the power loop including a proportional term and an integral term; the method includes: When the required output power of the photovoltaic grid-connected system is detected to be less than a preset power threshold, the integral term is reduced; wherein, the power threshold is not greater than the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system; The output power of the photovoltaic inverter is controlled by the power loop after the integral term is reduced.

2. The power control method according to claim 1, characterized in that, When the required output power of the photovoltaic grid-connected system is detected to be less than a preset power threshold, the integral term is reduced, including: When the reference value of the power loop is detected to be less than the power threshold, the integral term is reduced.

3. The power control method according to claim 2, characterized in that, The reference value of the power loop is given based on a manually issued dispatch instruction; or, the reference value of the power loop is given based on the current grid frequency.

4. The power control method according to claim 2, characterized in that, The step of reducing the integral term when the reference value of the power loop is detected to be less than the power threshold includes: When the reference value of the power loop is detected to be less than the power threshold, and the output of the power loop reaches the upper limit of saturation threshold, the integral term is reduced. Alternatively, when the reference value of the power loop is detected to be less than the power threshold and the integral term reaches a preset threshold, the integral term is reduced; wherein the preset threshold is a positive value.

5. The power control method according to claim 2, characterized in that, The step of reducing the integral term when the reference value of the power loop is detected to be less than the power threshold includes: When the reference value of the power loop is detected to be less than the power threshold and this continues for a preset duration, the integral term is reduced.

6. The power control method according to claim 1, characterized in that, The reduction operation on the integral term includes: Set the cumulative deviation in the integral term to zero.

7. A power control method, characterized in that, The method is applied to a photovoltaic grid-connected system, wherein the photovoltaic grid-connected system includes a photovoltaic inverter, the output of which is connected to the power grid and a load; the output of the photovoltaic inverter is controlled by a power loop, the power loop including a proportional term and an integral term; the method includes: When the required output power of the photovoltaic grid-connected system is detected to increase from a first power value to a second power value, the integral term is reduced; the first power value is less than a preset power threshold, and the second power value is greater than the power threshold; wherein, the power threshold is not greater than the minimum active power of the load under all operating conditions of the photovoltaic grid-connected system; The output power of the photovoltaic inverter is controlled by the power loop after the integral term is reduced.

8. The power control method according to claim 7, characterized in that, When the required output power of the photovoltaic grid-connected system is detected to increase from a first power value to a second power value, the integral term is reduced, including: When the required output power of the photovoltaic grid-connected system is detected to increase from the first power value to the second power value, the output of the power loop is detected. When the output of the power loop is detected to be saturated, the integral term is reduced. Alternatively, when the required output power of the photovoltaic grid-connected system is detected to increase from the first power value to the second power value, the error of the power loop is detected. When the error is detected to be negative for the first time, the integral term is reduced. The error is the difference between the reference value of the power loop and the actual power, and the actual power is the power detected at the output of the photovoltaic inverter. The output power of the photovoltaic inverter is controlled by the power loop after the integral term is reduced.

9. A photovoltaic inverter, characterized in that, Applied to a photovoltaic grid-connected system, the photovoltaic inverter controls its output power using the method described in any one of claims 1 to 8.

10. A photovoltaic grid-connected system, characterized in that, It includes photovoltaic modules and photovoltaic inverters, wherein the photovoltaic inverters control the output power by means of any one of claims 1 to 8.

Images