Improved P-f droop-based island detection method and system for network construction type inverter

Abstract

The invention relates to the technical field of micro-grids, and provides a grid construction type inverter island detection method and system based on improved P-f droop, and the method comprises the steps: obtaining an output voltage and an output current of a grid construction type inverter connected to a large power grid through sampling, and calculating a power value of the grid construction type inverter; calculating the current frequency and the current voltage amplitude of the network-building type inverter based on the power value of the network-building type inverter and a droop ring connected with the output end of the network-building type inverter; calculating the frequency of the output voltage of the inverter based on an improved P-f droop equation; judging whether the calculated frequency of the output voltage of the inverter is out of limit or not, counting the out-of-limit times of the calculated frequency of the output voltage of the inverter, and when the frequency out-of-limit times exceed the set judgment times, judging that island detection succeeds and disconnecting the common coupling point of the grid-configuration inverter, so that the micro-grid is separated from the large power grid. The island detection method provided by the invention has no blind area, namely, the island can be detected under the worst working condition that the power of the tie line is zero.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrids, and in particular to a method and system for detecting islanding of a grid-connected inverter based on improved Pf droop. Background Art

[0002] As an effective carrier for absorbing distributed renewable energy, microgrids provide benefits such as better power supply, but also face many challenges in terms of safety protection. Once a microgrid is suddenly disconnected from the main power grid, the microgrid may not detect the disconnection and continue to operate, forming a self-sufficient power supply island together with the surrounding loads.

[0003] If the main grid loses power, the voltage and frequency in the islands will fluctuate significantly, damaging equipment. Maintenance workers could be electrocuted while repairing. Furthermore, once the main grid is restored, the inverter's output voltage amplitude and phase will be out of sync with the grid, potentially causing a large current surge, damaging loads and equipment, and potentially causing another main grid outage. Finally, without the support of the main grid, power quality deteriorates significantly, significantly impacting people's lives and work.

[0004] Currently, most islanding detection algorithms are designed for grid-following converters and are not suitable for grid-connected converters based on droop control. In droop control systems, passive islanding detection methods, due to the undersized droop coefficient and the inherent negative feedback characteristics of the system, can only detect islanding under conditions where the load power and inverter output power are severely mismatched. This method has a large blind spot. Conventional active islanding detection methods require the introduction of disturbances to achieve islanding detection, which significantly impacts the system's power quality. Summary of the Invention

[0005] The object of the present invention is to solve at least one technical problem in the background technology and to provide a method and system for detecting islanding of a grid-connected inverter based on improved Pf droop.

[0006] To achieve the above objectives, the present invention provides a grid-connected inverter islanding detection method based on improved Pf droop, comprising: The output voltage and output current of the grid-connected inverter connected to the large power grid are obtained by sampling, and the power value of the grid-connected inverter is calculated; Calculate the current frequency and current voltage amplitude of the grid-type inverter based on the power value of the grid-type inverter and the droop loop connected to the output terminal of the grid-type inverter; Calculate the frequency of the inverter output voltage based on the improved Pf droop equation; Determine whether the calculated frequency of the inverter output voltage exceeds the limit, and count the number of times the calculated frequency of the inverter output voltage exceeds the limit. When the number of frequency violations exceeds the set judgment number, it is determined that the islanding detection is successful and the common coupling point of the grid-configured inverter is disconnected, so that the microgrid is separated from the main grid.

[0007] According to one aspect of the present invention, the grid-type inverter is an inverter with an LC filter, and the LC filter is composed of a filter inductor with a loss resistor and a filter capacitor.

[0008] According to one aspect of the present invention, the improved Pf droop equation is to add an incremental term to the original droop equation. , where k is the incremental coefficient, E is the current inverter side output voltage d-axis amplitude, is the d-axis amplitude of the grid voltage.

[0009] According to one aspect of the present invention, the frequency of the inverter output voltage is calculated based on the improved Pf droop equation: After improving the droop equation, the frequency expression of the inverter output voltage is: , Where, f is the frequency of the current inverter output voltage; f ref is the reference frequency, set to 50Hz; m is the droop coefficient; P is the active power currently calculated; P ref is the set value active power.

[0010] According to one aspect of the present invention, when the proportion of the incremental term to the power deviation in the original droop equation is less than 10%, the influence of the incremental term on the original droop equation is negligible.

[0011] According to one aspect of the present invention, determining whether the calculated frequency of the inverter output voltage exceeds a limit includes: When unplanned islanding occurs, consider the worst case of islanding detection: when the tie line power is zero, the calculated frequency f must exceed the threshold ,Right now ; when When k takes a positive value, the frequency exceeds the upper limit when an island appears; when When , k takes a negative value, so that the frequency exceeds the lower limit when an island appears.

[0012] To achieve the above objectives, the present invention further provides a grid-connected inverter islanding detection system based on improved Pf droop, comprising: The grid-type inverter power value calculation module obtains the output voltage and output current of the grid-type inverter connected to the large power grid through sampling, and calculates the power value of the grid-type inverter; A frequency and voltage amplitude calculation module for a grid-type inverter, which calculates a current frequency and a current voltage amplitude of the grid-type inverter based on a power value of the grid-type inverter and a droop loop connected to an output terminal of the grid-type inverter; The inverter output voltage frequency calculation module calculates the inverter output voltage frequency based on the improved Pf droop equation; The islanding detection module determines whether the calculated frequency of the inverter output voltage exceeds the limit and counts the number of times the calculated frequency of the inverter output voltage exceeds the limit. When the number of frequency violations exceeds the set judgment number, the islanding detection is considered successful and the common coupling point of the grid-configured inverter is disconnected, so that the microgrid is separated from the main grid.

[0013] To achieve the above-mentioned objectives, the present invention also provides an electronic device, comprising a processor, a memory, and a computer program stored in the memory and executable on the processor. When the computer program is executed by the processor, the above-mentioned grid-connected inverter islanding detection method based on improved Pf droop is implemented.

[0014] To achieve the above objectives, the present invention also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the above-mentioned grid-connected inverter islanding detection method based on improved Pf droop is implemented.

[0015] According to the solution of the present invention, BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A flowchart schematically illustrates a method for detecting islanding of a grid-connected inverter based on improved Pf droop according to an embodiment of the present invention; Figure 2 Schematic diagram of the control structure of a three-phase LC grid-type inverter based on LADRC according to Example 1 of the present invention; Figure 3 Schematic diagram of the improved droop loop control topology of Example 1 of the present invention; Figure 4 This is a Simulink program design diagram for improving droop in Example 1 of the present invention; Figure 5 The incremental Simulink internal program design diagram of Example 1 of the present invention; Figure 6 1 is a Bode diagram of closed-loop transfer functions of three controllers according to Example 1 of the present invention; Figure 7 The output impedance Bode diagrams of three controllers according to Example 1 of the present invention are shown; Figure 8 The output impedance Bode diagram of the two controllers in Example 1 of the present invention; Figure 9 : Bode diagram of virtual impedance of two controllers in embodiment 1 of the present invention; Figure 10 4 is a Bode diagram of the total output impedance and its components according to embodiment 1 of the present invention. DETAILED DESCRIPTION

[0017] The present invention will now be discussed with reference to exemplary embodiments. It should be understood that the embodiments discussed are only for enabling those skilled in the art to better understand and implement the present invention, rather than implying any limitation on the scope of the present invention.

[0018] As used herein, the term "including" and variations thereof are to be interpreted as open-ended terms meaning "including, but not limited to." The term "based on" is to be interpreted as "based, at least in part, on." The terms "one embodiment" and "an embodiment" are to be interpreted as "at least one embodiment."

[0019] Figure 1 The flowchart of the grid-connected inverter island detection method based on improved Pf droop according to one embodiment of the present invention is schematically shown. Figure 1 As shown, in this embodiment, the grid-type inverter islanding detection method based on the improved Pf droop includes: The output voltage and output current of the grid-connected inverter connected to the large power grid are obtained by sampling, and the power value of the grid-connected inverter is calculated; Calculate the current frequency and current voltage amplitude of the grid-type inverter based on the power value of the grid-type inverter and the droop loop connected to the output terminal of the grid-type inverter; Calculate the frequency of the inverter output voltage based on the improved Pf droop equation; Determine whether the calculated frequency of the inverter output voltage exceeds the limit, and count the number of times the calculated frequency of the inverter output voltage exceeds the limit. When the number of frequency violations exceeds the set judgment number, it is determined that the islanding detection is successful and the common coupling point of the grid-configured inverter is disconnected, so that the microgrid is separated from the main grid.

[0020] Furthermore, according to an embodiment of the present invention, the grid-type inverter is an inverter with an LC filter, and the LC filter is composed of a filter inductor with a loss resistor and a filter capacitor.

[0021] Further, according to one embodiment of the present invention, the improved Pf droop equation is: ) based on the addition of incremental items , where k is the incremental coefficient, E is the current inverter side output voltage d-axis amplitude, is the d-axis amplitude of the grid voltage.

[0022] Furthermore, according to one embodiment of the present invention, the frequency of the inverter output voltage is calculated based on the improved Pf droop equation as follows: After improving the droop equation, the frequency expression of the inverter output voltage is: , Where, f is the frequency of the current inverter output voltage; f ref is the reference frequency, set to 50Hz; m is the droop coefficient; P is the active power currently calculated; P ref is the set value active power.

[0023] Further, according to an embodiment of the present invention, when the proportion of the incremental term to the power deviation in the original droop equation is less than 10%, the influence of the incremental term on the original droop equation is negligible.

[0024] Furthermore, according to an embodiment of the present invention, determining whether the calculated frequency of the inverter output voltage exceeds a limit includes: When unplanned islanding occurs, consider the worst case of islanding detection: when the tie line power is zero, the calculated frequency f must exceed the threshold ,Right now ; when When k takes a positive value, the frequency exceeds the upper limit when an island appears; when When , k takes a negative value, so that the frequency exceeds the lower limit when an island appears.

[0025] According to the above-mentioned solution of the present invention, the proposed islanding detection method has no blind spots, meaning that islanding can be detected even under the worst-case operating condition of zero tie-line power. Compared to conventional islanding detection methods, this method effectively utilizes the negative feedback characteristics of the droop equation, minimizing the impact of the added voltage difference increment on the original droop equation and negligible impact on power quality. Once the main grid loses power, the voltage increment increases instantaneously, allowing the frequency to quickly exceed the limit, successfully detecting the islanding. This method is cost-effective and highly effective in engineering applications.

[0026] Furthermore, to achieve the above-mentioned objectives, the present invention also provides a grid-connected inverter islanding detection system based on improved Pf droop, comprising: The grid-type inverter power value calculation module obtains the output voltage and output current of the grid-type inverter connected to the large power grid through sampling, and calculates the power value of the grid-type inverter; A frequency and voltage amplitude calculation module for a grid-type inverter, which calculates a current frequency and a current voltage amplitude of the grid-type inverter based on a power value of the grid-type inverter and a droop loop connected to an output terminal of the grid-type inverter; The inverter output voltage frequency calculation module calculates the inverter output voltage frequency based on the improved Pf droop equation; The islanding detection module determines whether the calculated frequency of the inverter output voltage exceeds the limit and counts the number of times the calculated frequency of the inverter output voltage exceeds the limit. When the number of frequency violations exceeds the set judgment number, the islanding detection is considered successful and the common coupling point of the grid-configured inverter is disconnected, so that the microgrid is separated from the main grid.

[0027] Furthermore, according to an embodiment of the present invention, the grid-type inverter is an inverter with an LC filter, and the LC filter is composed of a filter inductor with a loss resistor and a filter capacitor.

[0028] Further, according to one embodiment of the present invention, the improved Pf droop equation is: ) based on the addition of incremental items .

[0029] Furthermore, according to one embodiment of the present invention, the frequency of the inverter output voltage is calculated based on the improved Pf droop equation as follows: After improving the droop equation, the frequency expression of the inverter output voltage is: , Where k is the increment coefficient, is the d-axis amplitude of the grid voltage.

[0030] Further, according to an embodiment of the present invention, when the proportion of the incremental term to the power deviation in the original droop equation is less than 10%, the influence of the incremental term on the original droop equation is negligible.

[0031] Furthermore, according to an embodiment of the present invention, determining whether the calculated frequency of the inverter output voltage exceeds a limit includes: When unplanned islanding occurs, consider the worst case of islanding detection: when the tie line power is zero, the calculated frequency f must exceed the threshold ,Right now ; when When k takes a positive value, the frequency exceeds the upper limit when an island appears; when When , k takes a negative value, so that the frequency exceeds the lower limit when an island appears.

[0032] According to the above-mentioned solution of the present invention, the proposed islanding detection method has no blind spots, meaning that islanding can be detected even under the worst-case operating condition of zero tie-line power. Compared to conventional islanding detection methods, this method effectively utilizes the negative feedback characteristics of the droop equation, minimizing the impact of the added voltage difference increment on the original droop equation and negligible impact on power quality. Once the main grid loses power, the voltage increment increases instantaneously, allowing the frequency to quickly exceed the limit, successfully detecting the islanding. This method is cost-effective and highly effective in engineering applications.

[0033] Furthermore, to achieve the above-mentioned purpose, the present invention also provides an electronic device, comprising a processor, a memory, and a computer program stored in the memory and executable on the processor. When the computer program is executed by the processor, the above-mentioned grid-connected inverter island detection method based on improved Pf droop is implemented.

[0034] Furthermore, to achieve the above-mentioned purpose, the present invention also provides a computer-readable storage medium, on which a computer program is stored. When the computer program is executed by a processor, the above-mentioned grid-connected inverter island detection method based on improved Pf droop is implemented.

[0035] Based on the above solution, the technical solution of the present invention is described in detail below in the form of a specific embodiment in conjunction with the accompanying drawings.

[0036] Example 1 This embodiment discloses a grid-type inverter islanding detection system based on improved Pf droop. Figure 2 As shown, the inverter with LC filter passes the line impedance Access to the large power grid, the LC filter consists of a loss resistor filter inductor and filter capacitors Composition, load connected to PCC . 、 、 、 、 Representing the inductor current, inverter output current, bridge arm voltage, capacitor voltage, and inverter output voltage, respectively. P and Q are calculated through instantaneous power calculations, and reference values ​​for voltage amplitude and frequency are calculated in the droop loop. The control loop is a dual voltage and current loop based on the LADRC algorithm, with the capacitor voltage as the controlled variable, minimizing setpoint tracking error.

[0037] like Figure 3 As shown, based on the original droop equation, the incremental term is added as , after improving the Pf droop equation, the frequency expression of the inverter output voltage is: . Where k is the incremental coefficient, is the d-axis amplitude of the grid voltage. When the grid is stable, the added voltage deviation increment is very small and has no impact on the inverter. However, if the main grid loses power, causing the voltage deviation increment to increase instantaneously, the improved droop equation can quickly cause the frequency to cross the limit, thus enabling rapid detection of islanding.

[0038] Only when the added incremental term accounts for less than 10% of the power deviation in the original droop equation can it be said that in the grid-connected stable stage, the added incremental term has a negligible effect on the original droop equation, that is: , Among them: the maximum power fluctuation in the grid-connected stage is 10% of the set value power, and the maximum voltage fluctuation in the grid-connected stage is 7% of the maximum voltage amplitude. When the voltage deviation exceeds 7%, the island can be detected through the passive island detection voltage amplitude.

[0039] When unplanned islanding occurs, the added increment is beneficial to the frequency offset. In order to eliminate the detection blind spot, the worst working condition of islanding detection is considered: when the tie line power is zero, that is, , at this time the calculated frequency f must exceed the threshold Right now .

[0040] when When k takes a positive value, the frequency exceeds the upper limit when an island appears; when When , k takes a negative value, so that the frequency exceeds the lower limit when an island appears.

[0041] According to the above conditions, the value range of the incremental coefficient is finally calculated. In the grid-connected stable stage, the improved Pf droop equation is used and the counter is turned on. When the frequency exceeds the set number of judgment times, , then the island detection is judged to be successful and the common coupling point is disconnected, so that the microgrid is separated from the main grid.

[0042] like Figure 4 、 5 As shown in the figure, the program design obtains the inverter output voltage and output current through sampling. Active power and reactive power are obtained through instantaneous power calculation. The voltage deviation term (incremental term) is added to the traditional droop equation.

[0043] like Figure 6 、 7 As shown in the figure, the closed-loop transfer function and output impedance Bode diagram of the three controllers, dual closed-loop PI, second-order single-loop LADRC, and second-order dual-loop LADRC, are compared to ensure that the bandwidth of the three voltage controllers is the same. The dual closed-loop PI has a large resonance peak at the resonance point, which shows that LADRC is good at suppressing internal disturbances. and In the frequency band below 1kHz, the LADRC output impedance The amplitude-frequency response is significantly lower than that of the double closed-loop PI, indicating that LADRC is effective in suppressing external disturbances. and The effect is better.

[0044] Since the LADRC output impedance shows negative resistance near the fundamental frequency, it does not meet the requirements of the droop control system. Therefore, it is necessary to modify the output impedance by adding virtual impedance, such as Figure 8 、 9 As shown in the figure, after adding virtual impedance, the dual-loop LADRC compensates for the inductive nature of the inverter output impedance at the fundamental frequency. When the same virtual impedance is added, the dual-loop LADRC suppresses the magnitude of the virtual impedance compared to the dual closed-loop PI in the 100Hz-1kHz frequency range. This also demonstrates that if virtual impedance is required to modify the total output impedance to be inductive at the fundamental frequency, the LADRC algorithm is more advantageous.

[0045] Through the virtual impedance The adjustment of the inverter equivalent impedance can effectively suppress the resistive line impedance Adverse effects on inverter output power control, such as Figure 10 As shown. Original line impedance The value is , its resistance-to-inductance ratio is , and finally the total output impedance of the transformation It is inductive near the power frequency and the amplitude meets the requirements.

[0046] In this embodiment, the maximum voltage amplitude is 311 V, the set power is 4500 W, the Pf droop coefficient is 1e-3, and the set frequency offset is (-0.5 Hz, +0.5 Hz).

[0047] Calculate the value range of the incremental coefficient k: (1); In order to eliminate the detection blind area, the worst working condition of island detection is considered: when the tie line power is zero, .

[0048] (2); According to formula (1) and (2), the incremental coefficient is taken , and when When k takes a positive value, the frequency exceeds the upper limit when an island appears; when When , k takes a negative value, so that the frequency exceeds the lower limit when an island appears.

[0049] Those skilled in the art will appreciate that the modules and algorithm steps described in conjunction with the embodiments disclosed herein can be implemented using electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians may use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of the present invention.

[0050] Those skilled in the art will clearly understand that, for the convenience and brevity of description, the specific working processes of the above-described devices and equipment can refer to the corresponding processes in the aforementioned method implementation methods and will not be repeated here.

[0051] In the embodiments provided in this application, it should be understood that the disclosed devices and methods can be implemented in other ways. For example, the device embodiments described above are merely illustrative. For example, the division of the modules is merely a logical function division. In actual implementation, there may be other division methods, such as multiple modules or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, indirect coupling or communication connection of devices or modules, which can be electrical, mechanical or other forms.

[0052] The modules described as separate components may or may not be physically separate, and the components shown as modules may or may not be physical modules, that is, they may be located in one place or distributed across multiple network modules. Some or all of the modules may be selected according to actual needs to achieve the objectives of the embodiments of the present invention.

[0053] In addition, each functional module in the embodiment of the present invention may be integrated into one processing module, or each module may exist physically separately, or two or more modules may be integrated into one module.

[0054] If the functions are implemented as software modules and sold or used as standalone products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the present invention, or the portion that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes instructions for enabling a computer device (which can be a personal computer, server, or network device, etc.) to perform all or part of the steps of the energy-saving signal transmission / reception method according to various embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as a USB flash drive, a mobile hard drive, ROM, RAM, a magnetic disk, or an optical disk.

[0055] The above description is merely a preferred embodiment of the present application and an illustration of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by the specific combination of the above-mentioned technical features, but also encompasses other technical solutions formed by any combination of the above-mentioned technical features or their equivalents without departing from the inventive concept. For example, a technical solution formed by replacing the above-mentioned features with (but not limited to) technical features with similar functions disclosed in this application.

[0056] It should be understood that the size of the serial numbers of each step in the content of the invention and the implementation methods of the present invention does not absolutely mean 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 implementation methods of the present invention.

Claims

1. A grid-connected inverter islanding detection method based on improved Pf droop is characterized in that: include: The output voltage and output current of the grid-connected inverter connected to the large power grid are obtained by sampling, and the power value of the grid-connected inverter is calculated; Calculate the current frequency and current voltage amplitude of the grid-type inverter based on the power value of the grid-type inverter and the droop loop connected to the output terminal of the grid-type inverter; Calculate the frequency of the inverter output voltage based on the improved Pf droop equation; Determine whether the calculated frequency of the inverter output voltage exceeds the limit, and count the number of times the calculated frequency of the inverter output voltage exceeds the limit. When the number of frequency violations exceeds the set judgment number, it is determined that the islanding detection is successful and the common coupling point of the grid-configured inverter is disconnected, so that the microgrid is separated from the main grid.

2. The grid-connected inverter islanding detection method based on improved Pf droop according to claim 1, characterized in that: The grid-type inverter is an inverter with an LC filter, and the LC filter is composed of a filter inductor with a loss resistor and a filter capacitor.

3. The grid-connected inverter islanding detection method based on improved Pf droop according to claim 1, characterized in that: The improved Pf droop equation is to add an incremental term to the original droop equation. , where k is the incremental coefficient, E is the current inverter side output voltage d-axis amplitude, is the d-axis amplitude of the grid voltage.

4. The grid-connected inverter islanding detection method based on improved Pf droop according to claim 3, characterized in that: The frequency of the inverter output voltage is calculated based on the improved Pf droop equation: After improving the droop equation, the frequency expression of the inverter output voltage is: ， Where, f is the frequency of the current inverter output voltage; f ref is the reference frequency, set to 50Hz; m is the droop coefficient; P is the active power currently calculated; P ref is the set value active power.

5. The grid-connected inverter islanding detection method based on improved Pf droop according to claim 4, characterized in that: When the proportion of the incremental term to the power deviation in the original droop equation is less than 10%, the influence of the incremental term on the original droop equation is negligible.

6. The grid-connected inverter islanding detection method based on improved Pf droop according to claim 5, characterized in that: The determining whether the calculated frequency of the inverter output voltage exceeds a limit includes: When unplanned islanding occurs, consider the worst case of islanding detection: when the tie line power is zero, the calculated frequency f must exceed the threshold ,Right now ; when When k takes a positive value, the frequency exceeds the upper limit when an island appears; when When , k takes a negative value, so that the frequency exceeds the lower limit when an island appears.

7. The grid-connected inverter islanding detection system based on improved Pf droop is characterized by: include: The grid-type inverter power value calculation module obtains the output voltage and output current of the grid-type inverter connected to the large power grid through sampling, and calculates the power value of the grid-type inverter; A frequency and voltage amplitude calculation module for a grid-type inverter, which calculates a current frequency and a current voltage amplitude of the grid-type inverter based on a power value of the grid-type inverter and a droop loop connected to an output terminal of the grid-type inverter; The inverter output voltage frequency calculation module calculates the inverter output voltage frequency based on the improved Pf droop equation; The islanding detection module determines whether the calculated frequency of the inverter output voltage exceeds the limit and counts the number of times the calculated frequency of the inverter output voltage exceeds the limit. When the number of frequency violations exceeds the set judgment number, the islanding detection is considered successful and the common coupling point of the grid-configured inverter is disconnected, so that the microgrid is separated from the main grid.

8. An electronic device, characterized in that The invention comprises a processor, a memory and a computer program stored in the memory and executable on the processor, wherein when the computer program is executed by the processor, the method for detecting islanding of a grid-connected inverter based on improved Pf droop is implemented as described in any one of claims 1 to 6.

9. A computer-readable storage medium, characterized in that The computer-readable storage medium stores a computer program, and when the computer program is executed by the processor, the grid-connected inverter islanding detection method based on improved Pf droop is implemented according to any one of claims 1 to 6.

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