Photovoltaic grid-connected inverter island detection method, system and device based on wavelet transformation

Abstract

The invention discloses a photovoltaic grid-connected inverter island detection method, system and device based on wavelet transform, and the method comprises the steps: carrying out the discrete wavelet transform analysis of the collected voltage of a PCC grid-connected point of a photovoltaic power generation grid-connected system, obtaining a low-frequency signal approximation coefficient, and obtaining the energy entropy of the photovoltaic power generation grid-connected system; obtaining a local load reactive power reference value by adopting the frequency of the voltage of the PCC grid-connected point of the photovoltaic power generation grid-connected system; the voltage frequency f of the PCC grid-connected point of the photovoltaic power generation grid-connected system is measured again, the relation between the actual frequency and the preset first threshold value and the relation between the actual frequency and the preset second threshold value are judged, and if the remeasured voltage frequency is smaller than the first threshold value or larger than the second threshold value and the energy entropy is larger than 0, it is determined that the island phenomenon occurs in the photovoltaic power generation grid-connected system. According to the method, the frequency and the voltage of the common point are detected, the set threshold value is compared, and the entropy condition is added, so that the false island condition can be effectively avoided, and whether the island actually occurs or not is judged.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid island detection, and relates to a photovoltaic grid-connected inverter island detection method, system and device based on wavelet transform. Background Art

[0002] Solar power generation has advantages that traditional power generation methods do not have. It can truly achieve zero energy consumption and zero emissions, and it complies well with the environmental protection needs of modern society. At present, solar photovoltaic power generation technology has become increasingly mature and tends to be diversified and practical. The grid-connected photovoltaic power generation technology mainly uses the photovoltaic system to directly convert the light energy from the sun into electrical energy. There are many challenges in the photovoltaic power generation process, among which island detection and protection of unplanned islands are considered important.

[0003] The island operation of existing distributed power generation systems will involve many risks, such as impacting the power grid, damaging electrical equipment and endangering personal safety. "GB19939-2005-T Photovoltaic System Grid-connected Technical Requirements" clearly stipulates that photovoltaic grid-connected inverters must have anti-islanding functions. Therefore, the study of island analysis technology and protection measures with accurate identification and high-speed detection capabilities is of great significance for the large-scale development and application of distributed power generation. There have been many studies on island detection methods at home and abroad. However, the existing detection methods will affect the quality of the power grid due to the injection of disturbances, and the power grid load will also affect the detection blind area. Summary of the invention

[0004] The purpose of the present invention is to solve the problem in the prior art that when performing island detection, the injected disturbance affects the quality of the power grid, and the power grid load also affects the detection blind area, and to provide a photovoltaic grid-connected inverter island detection method, system and device based on wavelet transform.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] The photovoltaic grid-connected inverter island detection method based on wavelet transform includes:

[0007] Step 1: Collect the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system;

[0008] Step 2: Perform discrete wavelet transform analysis on the voltage signal to obtain the approximate coefficients of the low-frequency signal;

[0009] Step 3: Determine whether the voltage at the common coupling point exceeds the rated voltage at the common coupling point, and based on the determination result, use the frequency of the voltage at the PCC grid connection point of the photovoltaic power generation grid-connected system to obtain a reference value of reactive power of the local load;

[0010] Step 4: Based on the low-frequency signal approximation coefficient, the energy entropy of the photovoltaic power generation grid-connected system is obtained;

[0011] Step 5: With the local load reactive power reference value as a reference, re-measure the voltage frequency f of the common coupling point of the PCC grid-connected point of the photovoltaic power generation grid-connected system, and determine the relationship between the actual frequency and the preset first threshold and second threshold. If the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that an island phenomenon occurs in the photovoltaic power generation grid-connected system.

[0012] A further improvement of the present invention is:

[0013] Furthermore, the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system is specifically:

[0014]

[0015] Wherein, V is the difference between the actual positive sequence value and the nominal positive sequence value of the PCC voltage; Vpcc is the voltage of the common coupling point.

[0016] Furthermore, discrete wavelet transform analysis is performed on the voltage signal to obtain the approximate coefficients of the low-frequency signal. Specifically, discrete wavelet transform is performed on the voltage signal based on the dbN wavelet function. The signal is first decomposed into a low-frequency signal and a high-frequency signal through a low-pass filter and a high-pass filter; the low-frequency signal obtained from each decomposition is decomposed again through a low-pass filter and a high-pass filter until it is decomposed N times.

[0017] Furthermore, the support length and vanishing moment order of the dbN wavelet function are 2N-1 and N respectively. The dbN wavelet is an orthogonal wavelet basis. The larger the order N is, the better the stability and accuracy of wavelet analysis and reconstruction.

[0018] Further, it is determined whether the voltage at the common coupling point exceeds the rated voltage at the common coupling point. Based on the determination result, the frequency of the voltage at the PCC grid connection point of the photovoltaic power generation grid connection system is used to obtain the local load reactive power reference value, specifically:

[0019]

[0020] Among them, Vpcc is the voltage of the common coupling point, Vn is the rated voltage at the common coupling point; Qload is the total reactive load in the power grid, k1 and k2 are proportional coefficients; K3 is a constant; f is the voltage frequency of the common coupling point Vpcc.

[0021] Furthermore, based on the low-frequency signal approximation coefficient, the energy entropy of the photovoltaic power generation grid-connected system is obtained, specifically:

[0022]

[0023] Among them, A iIt is the approximate coefficient of the low-frequency signal after the voltage signal is decomposed N times.

[0024] Furthermore, when the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it also includes if the re-measured voltage frequency is greater than the first threshold or less than the second threshold, or the energy entropy is less than 0, repeating steps 1 to 5 until the judgment condition is met; the first threshold is less than the second threshold.

[0025] Photovoltaic grid-connected inverter island detection system based on wavelet transform, including:

[0026] A collection module, wherein the collection module collects the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system;

[0027] A wavelet transform module, wherein the wavelet transform module performs discrete wavelet transform analysis on the voltage signal to obtain an approximate coefficient of a low-frequency signal;

[0028] A first judgment module, which judges whether the voltage of the common coupling point exceeds the rated voltage at the common coupling point, and based on the judgment result, uses the frequency of the voltage at the PCC grid-connected point of the photovoltaic power generation grid-connected system to obtain a local load reactive power reference value;

[0029] An acquisition module, wherein the acquisition module obtains energy entropy of a photovoltaic power generation grid-connected system based on a low-frequency signal approximation coefficient;

[0030] The second judgment module uses the local load reactive power reference value as a reference, remeasures the voltage frequency f of the PCC grid-connected point of the photovoltaic power generation grid-connected system, and determines the relationship between the actual frequency and the preset first threshold and second threshold. If the remeasured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that an island phenomenon occurs in the photovoltaic power generation grid-connected system.

[0031] A terminal device comprises a memory, a processor and a computer program stored in the memory and executable on the processor, wherein the processor implements the steps of the above method when executing the computer program.

[0032] A computer-readable storage medium stores a computer program, and when the computer program is executed by a processor, the steps of the above method are implemented.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention performs discrete wavelet transform analysis on the voltage of the PCC grid-connected point of the photovoltaic power generation grid-connected system to obtain the low-frequency signal approximate coefficient, and then obtain the energy entropy of the photovoltaic power generation grid-connected system; and adopts the frequency of the voltage of the PCC grid-connected point of the photovoltaic power generation grid-connected system to obtain the local load reactive power reference value; re-measures the voltage frequency f of the PCC grid-connected point of the photovoltaic power generation grid-connected system, and judges the relationship between the actual frequency and the preset first threshold and second threshold. If the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that the photovoltaic power generation grid-connected system has an island phenomenon. The present invention can effectively avoid the occurrence of false islands by detecting the frequency and voltage of the common point, comparing the set thresholds, and adding entropy value conditions, and then determine whether the island actually occurs. The wavelet transform proposed by the present invention optimizes the cumbersome feature stratification and detection range, determines the shortcomings of process complexity, and shortens the detection time. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0036] Figure 1 A schematic flow chart of a photovoltaic grid-connected inverter island detection method based on wavelet transform of the present invention;

[0037] Figure 2 It is a structural schematic diagram of the photovoltaic grid-connected inverter island detection system based on wavelet transform of the present invention;

[0038] Figure 3 This is the equivalent model diagram of the photovoltaic grid-connected power generation system;

[0039] Figure 4 It is the equivalent circuit diagram of photovoltaic grid-connected power generation system;

[0040] Figure 5 It is a schematic diagram of discrete wavelet transform;

[0041] Figure 6 Another schematic flow chart of the photovoltaic grid-connected inverter island detection method based on wavelet transform of the present invention. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0043] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0044] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0045] In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0046] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0047] In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0048] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0049] See also Figure 1 The present invention discloses a photovoltaic grid-connected inverter island detection method based on wavelet transform, comprising:

[0050] S101: collecting the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system;

[0051] The voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system is specifically:

[0052]

[0053] Wherein, V is the difference between the actual positive sequence value and the nominal positive sequence value of the PCC voltage; Vpcc is the voltage of the common coupling point.

[0054] S102: Performing discrete wavelet transform analysis on the voltage signal to obtain approximate coefficients of low-frequency signals;

[0055] Based on the dbN wavelet function, the voltage signal is subjected to discrete wavelet transform. First, the signal is decomposed into low-frequency signal and high-frequency signal through low-pass filter and high-pass filter. The low-frequency signal obtained from each decomposition is decomposed again through low-pass filter and high-pass filter until it is decomposed N times.

[0056] The support length and vanishing moment order of the dbN wavelet function are 2N-1 and N respectively. The dbN wavelet is an orthogonal wavelet basis. The larger the order N, the better the stability and accuracy of wavelet analysis and reconstruction.

[0057] S103: Determine whether the voltage at the common coupling point exceeds the rated voltage at the common coupling point, and based on the determination result, use the frequency of the voltage at the PCC grid-connected point of the photovoltaic power generation grid-connected system to obtain a local load reactive power reference value;

[0058]

[0059] Among them, Vpcc is the voltage of the common coupling point, Vn is the rated voltage at the common coupling point; Qload is the total reactive load in the power grid, k1 and k2 are proportional coefficients; K3 is a constant; f is the voltage frequency.

[0060] S104: obtaining energy entropy of the photovoltaic power generation grid-connected system based on the low-frequency signal approximation coefficient;

[0061]

[0062] Among them, A i It is the approximate coefficient of the low-frequency signal after the voltage signal is decomposed N times.

[0063] S105: With the local load reactive power reference value as a reference, re-measure the voltage frequency f of the PCC grid connection point of the photovoltaic power generation grid-connected system, and determine the relationship between the actual frequency and the preset first threshold and second threshold. If the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that an island phenomenon occurs in the photovoltaic power generation grid-connected system.

[0064] When the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it also includes if the re-measured voltage frequency is greater than the first threshold or less than the second threshold, or the energy entropy is less than 0, repeating S101 to S105 until the judgment condition is met. The first threshold is less than the second threshold.

[0065] See also Figure 2 The present invention discloses a photovoltaic grid-connected inverter island detection system based on wavelet transform, comprising:

[0066] A collection module, wherein the collection module collects the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system;

[0067] A wavelet transform module, wherein the wavelet transform module performs discrete wavelet transform analysis on the voltage signal to obtain an approximate coefficient of a low-frequency signal;

[0068] A first judgment module, which judges whether the voltage of the common coupling point exceeds the rated voltage at the common coupling point, and based on the judgment result, uses the frequency of the voltage at the PCC grid-connected point of the photovoltaic power generation grid-connected system to obtain a local load reactive power reference value;

[0069] An acquisition module, wherein the acquisition module obtains energy entropy of a photovoltaic power generation grid-connected system based on a low-frequency signal approximation coefficient;

[0070] The second judgment module, which uses the local load reactive power reference value as a reference, re-measures the voltage frequency f of the PCC grid-connected point of the photovoltaic power generation grid-connected system, and determines the relationship between the actual frequency and the preset first threshold and second threshold. If the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that the photovoltaic power generation grid-connected system has an island phenomenon.

[0071] Example:

[0072] The present invention provides a photovoltaic grid-connected island detection method based on wavelet transform, which comprises the following steps:

[0073] Typical island detection methods include: Figure 3As shown. It consists of the main power grid, photovoltaic grid-connected power generation system, load and circuit breaker. Local loads are usually connected to the power generation system, and their transmission lines are short, so the line loss is also small. Through analysis, it can be seen that the blind area calculation methods for the two working conditions of multi-PV inverter grid-connected and single PV inverter grid-connected are similar.

[0074] As the load changes over time, capacitor banks can be switched on and off multiple times a day to compensate for reactive power. During the energization of the capacitor banks, unnecessary transient overvoltages and high-frequency inrush currents are generated. The process of energizing one capacitor bank while the other is still energized is called back-to-back switching. In order to study the transient phenomena involved in capacitor switching, Figure 4 The related Thevenin equivalent circuit is shown in Figure 2. This component reflects a capacitor bank connected to the PCC bus and a circuit breaker S1 is used to switch it. When S1 is closed, the capacitor will be connected.

[0075] Ignore the source resistance R g , the circuit consists of source inductance L g And capacitor C1 in series, the voltage at point PCC can be obtained:

[0076]

[0077] Through the Laplace expression, the solution of the above equation is expressed as:

[0078]

[0079] Where V m and f are the magnitude and initial phase of the PCC voltage, is the resonant frequency, V PCC (0) is the initial voltage of the capacitor when power is applied.

[0080] Assuming that the capacitor bank has been discharged before power is applied, taking the PCC voltage example involving capacitor switching, high frequency and overvoltage transient switching occurs. The PCC voltage is composed of two frequencies, representing the fundamental frequency part and the resonant frequency. If you simply rely on the frequency change of the PCC node, it may be wrong to detect whether the islanding phenomenon occurs.

[0081] The present invention discloses a photovoltaic grid-connected inverter island detection method based on wavelet transform, wherein discrete wavelet transform (DWT) is widely used in digital signal processing due to its simple implementation, no redundancy, strong data compression capability and high computational efficiency. The DWT of discrete signal x(n) is defined as:

[0082]

[0083] Among them, ψ(.) is called the mother wavelet, a0>1, b0>1 are fixed real values, m and k are positive integers, representing frequency and time parameters respectively.

[0084] like Figure 5 As shown in the figure, L(n) is a low-pass filter and H(n) is a high-pass filter. The circle with an arrow indicates downsampling by a factor of 2. i and A i They are detail coefficient and approximation coefficient respectively.

[0085] Let V be the difference between the actual positive sequence value and the nominal positive sequence value of the PCC voltage:

[0086]

[0087] The voltage V is selected as the signal for discrete wavelet transform analysis, and the db4 wavelet is selected as the mother wavelet for feature extraction.

[0088] like In the formula is the binomial coefficient, then:

[0089] in, The support length and vanishing moment order of db wavelet are 2N-1 and N respectively, the support length of db4 wavelet is 7, and the vanishing moment is 4th order. The dbN wavelet is an orthogonal wavelet basis and satisfies orthogonality. The larger the order N, the better the stability and accuracy in wavelet analysis and reconstruction, and the better the localized analysis can be performed. Therefore, db4 wavelet is selected.

[0090] The fourth-level wavelet coefficients are calculated and the energy entropy included in the decision criterion is derived using the wavelet coefficients. The proposed energy entropy is calculated from the approximate coefficients:

[0091]

[0092] As shown in the above equation, the entropy value does not exceed zero when capacitor switching or other non-islanding events occur. However, when islanding occurs, the entropy will be greater than zero. Therefore, according to the above equation, islanding events and non-islanding events can be effectively distinguished. When both frequency parameters and entropy parameters are met, an islanding event can be detected. However, if only the frequency requirement is met, the scheme will not send any trip signal to the islanding relay because the frequency deviation is caused by a normal event.

[0093] In the case of multiple PV inverters in parallel, the local load reactive power reference should be modified as follows:

[0094]

[0095] Where n is the number of distributed power sources;

[0096] Attached Figure 6 The following is a flow chart of the detection method. The specific steps are as follows:

[0097] Step 1: Detect the PCC voltage amplitude and frequency;

[0098] Step 2: Perform discrete wavelet transform analysis on the voltage signal, select db4 wavelet as the mother wavelet for feature extraction, and obtain the low-frequency component and high-frequency component of the voltage signal;

[0099] Step 3: Based on whether the voltage amplitude exceeds the threshold value, a corresponding local load reactive power reference value is adopted;

[0100] Step 4: Re-measure the voltage frequency f and use the wavelet coefficients to derive the energy entropy included in the decision criterion;

[0101] Step 5: During normal operation, determine the relationship between the actual frequency and the preset first threshold and second threshold. If the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that an island phenomenon occurs in the photovoltaic power generation grid-connected system.

[0102] The first threshold is 49.3 Hz; the second threshold is 50.5 Hz.

[0103] The terminal device provided in an embodiment of the present invention. The terminal device of this embodiment includes: a processor, a memory, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the steps in the above-mentioned method embodiments are implemented. Alternatively, when the processor executes the computer program, the functions of the modules / units in the above-mentioned device embodiments are implemented.

[0104] The computer program may be divided into one or more modules / units, and the one or more modules / units are stored in the memory and executed by the processor to accomplish the present invention.

[0105] The terminal device may be a computing device such as a desktop computer, a notebook, a PDA, a cloud server, etc. The terminal device may include, but is not limited to, a processor and a memory.

[0106] The processor can be a central processing unit (CPU), or other general-purpose processors, digital signal processors (DSP), application-specific integrated circuits (ASIC), field-programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc.

[0107] The memory may be used to store the computer program and / or module, and the processor implements various functions of the terminal device by running or executing the computer program and / or module stored in the memory and calling the data stored in the memory.

[0108] If the module / unit integrated in the terminal device is implemented in the form of a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the present invention implements all or part of the processes in the above-mentioned embodiment method, and can also be completed by instructing the relevant hardware through a computer program. The computer program can be stored in a computer-readable storage medium, and the computer program can implement the steps of the above-mentioned various method embodiments when executed by the processor. Among them, the computer program includes computer program code, and the computer program code can be in source code form, object code form, executable file or some intermediate form. The computer-readable medium may include: any entity or device capable of carrying the computer program code, recording medium, U disk, mobile hard disk, disk, optical disk, computer memory, read-only memory (ROM, Read-Only Memory), random access memory (RAM, Random Access Memory), electric carrier signal, telecommunication signal and software distribution medium. It should be noted that the content contained in the computer-readable medium can be appropriately increased or decreased according to the requirements of legislation and patent practice in the jurisdiction. For example, in some jurisdictions, according to legislation and patent practice, computer-readable media do not include electric carrier signals and telecommunication signals.

[0109] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A photovoltaic grid-connected inverter island detection method based on wavelet transform, characterized in that: include: Step 1: Collect the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system; Step 2: Perform discrete wavelet transform analysis on the voltage signal to obtain the approximate coefficients of the low-frequency signal; Step 3: Determine whether the voltage at the common coupling point exceeds the rated voltage at the common coupling point, and based on the determination result, use the frequency of the voltage at the PCC grid connection point of the photovoltaic power generation grid-connected system to obtain a reference value of reactive power of the local load; Step 4: Based on the low-frequency signal approximation coefficient, the energy entropy of the photovoltaic power generation grid-connected system is obtained; Step 5: With the local load reactive power reference value as a reference, re-measure the voltage frequency f of the common coupling point of the PCC grid-connected point of the photovoltaic power generation grid-connected system, and determine the relationship between the actual frequency and the preset first threshold and second threshold. If the re-measured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that an island phenomenon occurs in the photovoltaic power generation grid-connected system.

2. The photovoltaic grid-connected inverter island detection method based on wavelet transform according to claim 1 is characterized in that: The voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system is specifically: Wherein, V is the difference between the actual positive sequence value and the nominal positive sequence value of the PCC voltage; Vpcc is the voltage of the common coupling point.

3. The photovoltaic grid-connected inverter island detection method based on wavelet transform according to claim 2 is characterized in that: The discrete wavelet transform analysis is performed on the voltage signal to obtain the approximate coefficients of the low-frequency signal, specifically: based on the dbN wavelet function, the discrete wavelet transform is performed on the voltage signal, and the signal is first decomposed into a low-frequency signal and a high-frequency signal through a low-pass filter and a high-pass filter; the low-frequency signal obtained from each decomposition is decomposed again through a low-pass filter and a high-pass filter until it is decomposed N times.

4. The photovoltaic grid-connected inverter island detection method based on wavelet transform according to claim 3 is characterized in that: The support length and vanishing moment order of the dbN wavelet function are 2N-1 and N respectively. The dbN wavelet is an orthogonal wavelet basis. The larger the order N is, the better the stability and accuracy are during wavelet analysis and reconstruction.

5. The photovoltaic grid-connected inverter island detection method based on wavelet transform according to claim 4 is characterized in that: include: It is determined whether the voltage at the common coupling point exceeds the rated voltage at the common coupling point. Based on the determination result, the frequency of the voltage at the PCC grid connection point of the photovoltaic power generation grid-connected system is used to obtain the local load reactive power reference value, which is specifically: Wherein, Vpcc is the voltage at the common coupling point, Vn is the rated voltage at the common coupling point; Qload is the total reactive load in the power grid, k1 and k2 are proportional coefficients; K3 is a constant; f is the voltage frequency of the common coupling point Vpcc.

6. The photovoltaic grid-connected inverter island detection method based on wavelet transform according to claim 5 is characterized in that: The energy entropy of the photovoltaic power generation grid-connected system is obtained based on the low-frequency signal approximation coefficient, specifically: Among them, A i It is the approximate coefficient of the low-frequency signal after the voltage signal is decomposed N times.

7. The photovoltaic grid-connected inverter island detection method based on wavelet transform according to claim 6 is characterized in that: When the remeasured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it also includes if the remeasured voltage frequency is greater than the first threshold or less than the second threshold, or the energy entropy is less than 0, repeating steps 1 to 5 until the judgment condition is met; the first threshold is less than the second threshold.

8. Photovoltaic grid-connected inverter island detection system based on wavelet transform, characterized in that: include: A collection module, wherein the collection module collects the voltage of the PCC grid connection point of the photovoltaic power generation grid-connected system; A wavelet transform module, wherein the wavelet transform module performs discrete wavelet transform analysis on the voltage signal to obtain an approximate coefficient of a low-frequency signal; A first judgment module, which judges whether the voltage of the common coupling point exceeds the rated voltage at the common coupling point, and based on the judgment result, uses the frequency of the voltage at the PCC grid-connected point of the photovoltaic power generation grid-connected system to obtain a local load reactive power reference value; An acquisition module, wherein the acquisition module obtains energy entropy of a photovoltaic power generation grid-connected system based on a low-frequency signal approximation coefficient; The second judgment module uses the local load reactive power reference value as a reference, remeasures the voltage frequency f of the PCC grid-connected point of the photovoltaic power generation grid-connected system, and determines the relationship between the actual frequency and the preset first threshold and second threshold. If the remeasured voltage frequency is less than the first threshold or greater than the second threshold, and the energy entropy is greater than 0, it is determined that an island phenomenon occurs in the photovoltaic power generation grid-connected system.

9. A terminal device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that: When the processor executes the computer program, the steps of the method according to any one of claims 1 to 7 are implemented.

10. A computer-readable storage medium storing a computer program, characterized in that: When the computer program is executed by a processor, the steps of the method according to any one of claims 1 to 7 are implemented.

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