Island detection method and system based on characteristic frequency harmonic voltage

Abstract

The application provides an island detection method and system based on characteristic frequency harmonic voltage, which comprises the following steps: sampling voltage and current at a point of common coupling; pre-processing the sampling data, screening out data segments with a Pearson correlation coefficient between harmonic voltage and harmonic current greater than a set threshold, and calculating grid-side harmonic impedance; calculating the harmonic voltage responsibility of the grid side and the distributed power supply side as characteristic frequency, respectively selecting the characteristic frequency with the largest harmonic responsibility of both sides as the harmonic voltage frequency monitored in island detection, and obtaining the different change trends of the two characteristic frequency harmonic voltages after island occurrence; measuring the actual ratio; combining the change trend of the harmonic voltage, and comparing the actual ratio with the theoretical ratio to determine whether the power system has occurred island. The island detection method has less impact on the power grid quality, has a smaller blind area, can quickly and effectively identify island, and ensures the stability and safety of the power system.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of island detection of distributed power grid connection, and particularly relates to an island detection method and system based on characteristic frequency harmonic voltage. BACKGROUND

[0002] At present, in order to reduce carbon emissions in the power industry and promote energy structure reform, a large number of distributed generation (DG) is connected to the power grid. It can be predicted that the integration of distributed power into the power grid is an inevitable development trend in the future of the energy and power field, but there may be some problems in the operation of distributed power, one of which is the island effect. In the existing conventional technology, the island detection method cannot quickly and reliably detect the island state, thereby ensuring the safe and stable operation of the power system. The most widely used island detection method is mainly divided into passive detection method and active detection method. The blind area of the active detection method is small, but this method may affect the power quality of the power system. The passive detection method does not affect the power quality of the power grid, but has a large detection blind area. In summary, the current island detection method cannot guarantee a small impact on the power quality of the power grid while controlling the detection blind area to be relatively small, resulting in the inability to reliably detect the island state and affecting the stable operation of the power system. SUMMARY

[0003] The purpose of the present application is to provide an island detection method and system based on characteristic frequency harmonic voltage, in order to solve the problem that the current island detection method cannot guarantee a small impact on the power quality of the power grid while controlling the detection blind area to be relatively small, resulting in the inability to reliably detect the island state.

[0004] To achieve the above purpose, the application adopts the following technical scheme:

[0005] The application provides an island detection method based on characteristic frequency harmonic voltage, comprising the following steps:

[0006] S1, sampling the voltage and current at the power system point of common coupling to obtain sampling data;

[0007] S2, using the Pearson correlation coefficient method to preprocess the sampling data, screening out data segments with a Pearson correlation coefficient between harmonic voltage and harmonic current greater than a set threshold, and calculating the harmonic impedance of the power grid side according to the screened data segments;

[0008] S3, according to the harmonic impedance of the grid side, the harmonic responsibility of each order at the grid side and the distributed power side is calculated, and the maximum harmonic responsibility of the two is selected as the characteristic frequency, and the two characteristic frequencies are the frequencies with the greatest influence on the harmonic voltage change at the point of common coupling, so as to obtain the change trend of the two characteristic frequency harmonic voltages after islanding;

[0009] The theoretical ratio of the characteristic frequency harmonic voltage at the grid side and the distributed power side before and after islanding is calculated; the actual ratio of the characteristic frequency harmonic voltage at the grid side and the distributed power side before and after fluctuation is measured;

[0010] S4, according to the different change trends of the characteristic frequency harmonic voltage at the grid side and the distributed power side before and after islanding, and the comparison of the actual ratio and the theoretical value, it is judged whether the power system is islanded.

[0011] In some embodiments, in S1, the voltage and current at the point of common coupling are sampled by using real-time frequency tracking variable sampling frequency fast Fourier transform.

[0012] In some embodiments, in S2, the Pearson correlation coefficient method is calculated as follows formula (1):

[0013] (1);

[0014] Wherein, , is the two groups of data corresponding to the to-be-determined correlation degree of the i-th sampling point, is the sliding window length; and are the average values of the two groups of data, is the Pearson correlation coefficient.

[0015] In some embodiments, in S2, after the data segment with the Pearson correlation coefficient between the harmonic voltage and the harmonic current greater than the set threshold value is screened out, the harmonic impedance of the grid side is calculated by the following formula (2):

[0016] (2);

[0017] Wherein, is the harmonic voltage vector of the point of common connection, is the harmonic current vector of the point of common connection, is the h-th harmonic voltage vector generated by the distributed power side, is the h-th harmonic impedance of the grid side.

[0018] ​Further, in S3, the harmonic responsibility of each order of the grid side and the distributed power side is calculated according to formula (3) as follows:

[0019] (3);

[0020] wherein, and are the harmonic responsibility of the grid side and the distributed power side respectively, is the hth harmonic current of the distributed power side; is the hth local load equivalent harmonic impedance of the distributed power side.

[0021] Further, in S3, the characteristic frequency harmonic that has the greatest impact on the harmonic voltage at the point of common coupling is selected by formula (4) as follows:

[0022] (4);

[0023] wherein, is the maximum harmonic responsibility of the grid side, is the maximum harmonic responsibility of the distributed power side.

[0024] In some embodiments, in S3, the theoretical ratio of the harmonic voltage after islanding to the harmonic voltage before islanding of the characteristic frequency harmonic of the grid side and the distributed power side is calculated by formula (5) as follows:

[0025] (5);

[0026] wherein, is the theoretical ratio of the harmonic voltage after islanding to the harmonic voltage before islanding of the characteristic frequency harmonic of the grid side and the distributed power side, is the hth harmonic voltage after islanding, is the hth harmonic voltage before islanding.

[0027] In some embodiments, in S3, the change trend of the harmonic voltage after islanding is that when the harmonic voltage at the point of common coupling is mainly from the grid side, the harmonic voltage at the point of common coupling after islanding will decrease, and when the harmonic voltage at the point of common coupling is mainly from the distributed power side, the harmonic voltage at the point of common coupling after islanding will increase.

[0028] Further, in S4, in the case of fluctuation of the harmonic voltage, the ratio of the harmonic voltage after fluctuation to the harmonic voltage before fluctuation is calculated, and the determination rule is:

[0029] If the ratio of the harmonic voltage from the grid side before and after the fluctuation is less than the theoretical value, and the ratio of the harmonic voltage from the distributed power source side before and after the fluctuation is greater than the theoretical value, it is determined that islanding occurs, and if not, the voltage and current at the power system public connection point are continuously monitored.

[0030] The application also provides an island detection system based on characteristic frequency harmonic voltage, which comprises a sampling module, a preprocessing module, a harmonic responsibility calculation module, a theoretical ratio calculation module and an island determination module.

[0031] The sampling module is used for detecting and sampling the voltage and current at the power system public connection point.

[0032] The preprocessing module is used for screening data segments with a Pearson correlation coefficient between harmonic voltage and harmonic current greater than a set threshold value, and calculating the harmonic impedance of the grid side according to the screened data segments.

[0033] The harmonic responsibility calculation module is used for calculating and screening the maximum harmonic responsibility among the harmonic responsibilities of each order at the grid side and the distributed power source side, combining the influence degree of the harmonic frequencies of the selected grid side and distributed power source side on the harmonic voltage change at the public connection point, to obtain the change trend of the harmonic voltage after islanding occurs.

[0034] The theoretical ratio calculation module is used for calculating the theoretical ratio of the harmonic voltage before and after islanding of the characteristic frequency harmonic voltage of the grid side and the distributed power source side.

[0035] The island determination module is used for measuring the actual ratio of the harmonic voltage before and after the fluctuation of the characteristic frequency harmonic voltage of the grid side and the distributed power source side, comparing the actual ratio with the theoretical ratio, and determining whether the power system has islanding.

[0036] Compared with the prior art, the island detection method and system based on characteristic frequency harmonic voltage have the following beneficial technical effects.

[0037] The island detection method based on characteristic frequency harmonic voltage of the application comprises the following steps: S1, sampling the voltage and current at the point of common coupling of the power system to obtain sampling data; S2, using the Pearson correlation coefficient method to preprocess the sampling data, screening out the data segment with a Pearson correlation coefficient between the harmonic voltage and the harmonic current greater than a set threshold, and calculating the harmonic impedance of the grid side according to the screened data segment; S3, according to the harmonic impedance of the grid side, the harmonic responsibilities of each order at the grid side and the distributed power supply side are calculated, and the maximum harmonic responsibilities of the two are selected as the characteristic frequencies, respectively, and the two characteristic frequencies are the frequencies with the greatest influence on the harmonic voltage change at the point of common coupling, so as to obtain the change trend of the two characteristic frequency harmonic voltages after islanding; the theoretical ratio of the characteristic frequency harmonic voltage of the grid side and the distributed power supply side after islanding to that before islanding is calculated; the actual ratio of the characteristic frequency harmonic voltage of the grid side and the distributed power supply side after fluctuation to that before fluctuation is measured; S4, according to the different change trends of the characteristic frequency harmonic voltages of the grid side and the distributed power supply side before and after islanding, and the comparison of the actual ratio and the theoretical value, it is judged whether the power system has islanding. Based on the above, the harmonic impedance and the harmonic responsibility are accurately calculated, the state change of the power system before and after islanding can be deeply understood. By comparing the theoretical ratio and the actual ratio of the harmonic voltage before and after islanding, the occurrence of islanding event can be more accurately judged, and the risk of false alarm and missed alarm is greatly reduced. Through the island detection of the application, the change of harmonic voltage is monitored and analyzed in real time, which can send an early warning signal in the early stage of islanding event, support the data of power system, and effectively avoid the potential threat of islanding event to power system. The island detection method of the application has little influence on the quality of the power grid, the blind area of detection is small, the island can be quickly and effectively identified, and no misjudgment occurs in the case that the normal fluctuation of the power grid causes the change of system parameters at the PCC, which has good practicability. BRIEF DESCRIPTION OF DRAWINGS

[0038] The accompanying drawings are included to provide a further understanding of the application, and constitute a part of the application. The illustrative embodiments of the application and their description serve to explain the application without forming an improper limitation of the application.

[0039] Figure 1 The flowchart of the method in the island detection method and system based on characteristic frequency harmonic voltage of the application;

[0040] Figure 2 The model diagram of the AC distribution network with distributed power supply in the island detection method and system based on characteristic frequency harmonic voltage of the application;

[0041] Figure 3A schematic diagram of the harmonic responsibility calculation results of the grid side and the DG side in the island detection method and system based on the characteristic frequency harmonic voltage of the application;

[0042] Figure 4 A schematic diagram of the simulation results under the power matching condition in the island detection method and system based on the characteristic frequency harmonic voltage of the application;

[0043] Figure 5 A schematic diagram of the simulation results under the DG output fluctuation condition in the island detection method and system based on the characteristic frequency harmonic voltage of the application. DETAILED DESCRIPTION

[0044] In order to make the objects, technical solutions, and advantages of the embodiments of the present application clearer, the following will be combined with the accompanying drawings to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the accompanying drawings can be arranged and designed in various different configurations.

[0045] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but only represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative labor are within the scope of protection of the present application.

[0046] It should be noted that: similar reference numbers and letters represent similar items in the following drawings, therefore, once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings.

[0047] In the description of the embodiments of the present application, it should be noted that, if the orientation or position relationship indicated by the terms "upper", "lower", "horizontal", "inner", and the like is based on the orientation or position relationship shown in the drawings, or is the orientation or position relationship when the product of the present application is usually placed, and is only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application. In addition, the terms "first", "second", and the like are only used for differentiation in description, and cannot be understood as indicating or implying relative importance.

[0048] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly inclined. For example, "horizontal" only means that its direction is relatively more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly inclined.

[0049] In the description of the embodiments of the present application, it also needs to be explained that, unless explicitly defined and limited, if the terms "arrange", "install", "connect", "connect" appear, they should be understood in a broad sense, for example, they can be fixedly connected, or can be detachably connected, or integrally connected, can be mechanically connected, or can be electrically connected, can be directly connected, or indirectly connected through an intermediate medium, or the communication inside two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0050] How to design an active detection method to detect blind area, but not affect the power quality of power grid, and the cost control is better, the detection speed and quality are guaranteed.

[0051] As shown in Figure 1 The present application provides a kind of island detection method based on feature frequency harmonic voltage, comprising the following steps:

[0052] S1, the voltage and current at the point of common coupling of power system are sampled, and sampling data is obtained;

[0053] S2, the sampling data is preprocessed using Pearson correlation coefficient method, the data segment with Pearson correlation coefficient between harmonic voltage and harmonic current greater than the set threshold is screened out, and the harmonic impedance of grid side is calculated according to the screened data segment;

[0054] S3, according to the harmonic impedance of grid side, the harmonic responsibility of each order at the grid side and the distributed power side is calculated, and the maximum harmonic responsibility of the two is selected as the feature frequency, respectively, and the two feature frequencies are the frequency with the greatest influence on the harmonic voltage change at the point of common coupling, so that the change trend of the two feature frequency harmonic voltages after island occurs is obtained;

[0055] The theoretical ratio of the feature frequency harmonic voltage of grid side and distributed power side before island to the harmonic voltage before island is calculated;The actual ratio of the feature frequency harmonic voltage of grid side and distributed power side after fluctuation to the harmonic voltage before fluctuation is measured;

[0056] S4, according to the different change trend of the feature frequency harmonic voltage selected before and after island at the grid side and the distributed power side, and comparing the actual ratio and the theoretical value, whether the power system occurs island is judged.

[0057] The island detection method of the application firstly divides the harmonic responsibility on both sides of the PCC (Point of Common Coupling), and selects the characteristic frequency harmonic with the greatest influence on the harmonic voltage at the PCC on both sides according to the harmonic responsibility division result for island detection. Secondly, since the premise of reasonable division of harmonic responsibility is to estimate the harmonic impedance, the Pearson correlation coefficient method is used for data preprocessing to ensure that the system impedance solution is not affected by system fluctuations and the harmonic impedance is accurately estimated. Finally, the theoretical value of the ratio of the harmonic voltage at the PCC before and after the island is derived. When the change of the characteristic frequency harmonic voltage parameters on both sides before and after the island exceeds the threshold value, it is judged that the island occurs. The application can more accurately reflect the state change of the power system before and after the island by accurately calculating the harmonic impedance and the harmonic responsibility, can monitor the voltage and current changes at the power system common connection point in real time, and provides strong guarantee for the safe operation of the power system.

[0058] The island detection method and system based on characteristic frequency harmonic voltage will be further described in detail through specific embodiments.

[0059] Step 1: The voltage and current at the grid-connected point are sampled by using the real-time frequency tracking variable sampling frequency fast Fourier transform.

[0060] Step 2: The samples with a positive correlation between the harmonic voltage and the harmonic current are screened out by using the Pearson coefficient method, and the harmonic impedance of the power grid side is calculated. The calculation formula of the Pearson correlation coefficient is:

[0061] (1);

[0062] Wherein, , is the two groups of data corresponding to the correlation degree to be determined at the nth sampling point; m is the length of the sliding window; and are the average values of the two groups of data; is the Pearson correlation coefficient, which is between-1 and 1, and is used to measure the linear correlation between the two groups of data , The closer the value is to 1, the higher the correlation between the data and .

[0063] According to the superposition theorem, the hth harmonic voltage at the PCC is the vector sum of the hth harmonic voltage of the power grid side and the hth harmonic voltage generated by the DG side. Therefore, after screening out the data segment with the Pearson correlation coefficient between the harmonic voltage and the harmonic current greater than r>0.9, the hth system impedance The calculation is as follows:

[0064] (2);

[0065] wherein, is a harmonic voltage vector of the common grid connection point, is a harmonic current vector of the common grid connection point, is an h-th harmonic voltage vector generated on the distributed power supply side, is an h-th harmonic impedance on the grid side.

[0066] Step 3: Calculate the harmonic responsibility of each order on the grid side and the DG side according to formula (3), and select the maximum harmonic responsibility of the grid side and the DG through formula (4). The value can determine whether the h-th harmonic voltage mainly comes from the grid side or the DG side.

[0067] (3);

[0068] wherein, is an h-th background harmonic voltage on the grid side; is an h-th harmonic current on the DG side; is an h-th local load equivalent harmonic impedance on the DG side; , are respectively an h-th harmonic voltage and an h-th harmonic current at the PCC:

[0069] (4);

[0070] In formula (4), is the maximum harmonic responsibility of the grid side, indicating that the i-th harmonic mainly comes from the grid side, the grid side has a greater impact on the change of the harmonic voltage at the PCC, and the i-th harmonic voltage will decrease after the islanding occurs; is the maximum harmonic responsibility of the DG side, indicating that the j-th harmonic mainly comes from the DG side, the DG side has a greater impact on the change of the harmonic voltage at the PCC, and the j-th harmonic voltage will increase after the islanding occurs.

[0071] Step 4: Calculate the theoretical ratio of the h-th harmonic voltage after islanding to the h-th harmonic voltage before islanding :

[0072] (5);

[0073] Therefore, the theoretical ratio of the i-th harmonic voltage and the j-th harmonic voltage after islanding to the harmonic voltage before islanding and can be obtained through formula (5).

[0074] ​Step 5: measuring the i-th harmonic voltage and the j-th harmonic voltage, and if fluctuation occurs, calculating the ratio of the harmonic voltage after fluctuation to the harmonic voltage before fluctuation 、 When , and the condition is met, it is judged that islanding occurs. If any condition is not met, it is judged that the grid is normal and in grid-connected fluctuation, and the i-th harmonic voltage and the j-th harmonic voltage are continuously monitored.

[0075] The application also provides an islanding detection system based on characteristic frequency harmonic voltage, which comprises a sampling module, a preprocessing module, a harmonic responsibility calculation module, a theoretical ratio calculation module and an islanding judgment module; wherein:

[0076] The sampling module is used for detecting and sampling the voltage and current at the point of common coupling of the power system;

[0077] The preprocessing module is used for screening data segments with a Pearson correlation coefficient between harmonic voltage and harmonic current greater than a set threshold, and calculating the harmonic impedance of the grid side according to the screened data segments;

[0078] The harmonic responsibility calculation module is used for calculating and screening the maximum harmonic responsibility among the harmonic responsibilities of each order at the grid side and the distributed power supply side, and the two characteristic frequencies are the frequencies with the greatest influence on the harmonic voltage change at the point of common coupling, so as to obtain the change trend of the two characteristic frequency harmonic voltages after islanding occurs;

[0079] The theoretical ratio calculation module is used for calculating the theoretical ratio of the characteristic frequency harmonic voltage of the grid side and the distributed power supply side before islanding to the harmonic voltage before islanding;

[0080] The islanding judgment module is used for measuring the actual ratio of the characteristic frequency harmonic voltage of the grid side and the distributed power supply side after fluctuation to the harmonic voltage before fluctuation, comparing the actual ratio with the theoretical ratio, and judging whether the power system has islanding. The system can monitor the state of the power system in real time, effectively and accurately determine and warn islanding events, help to avoid unstable operation and potential safety hazards of the power system in islanding state, and ensure the overall stability of the power system.

[0081] The effectiveness of the islanding detection method and system based on characteristic frequency harmonic voltage is further verified by simulation verification.

[0082] A DG grid-connected system circuit model as shown in Figure 2 is built in the PSCAD simulation software platform, and the effectiveness of the proposed islanding detection method is verified based on the model.

[0083] In order to verify the effectiveness of the proposed grid impedance estimation method, a ±20% random fluctuation is superimposed on the DG side harmonic current to simulate the actual load fluctuation, and a ±10% normal random fluctuation is superimposed on the grid side harmonic current. At the same time, in order to reduce the accidental error caused by a single experiment, 70 experiments are conducted, and the data segments with a correlation coefficient greater than 0.9 are screened each time. After estimation, the average value is taken. The calculated grid impedance results and errors are shown in Table 1. From the system impedance estimation results in Table 1, it can be seen that the estimation error of the real part of the grid-side impedance is 3.2%, and the error of the imaginary part is 2.6%. The error between the estimated value and the actual reference value is small, which verifies the effectiveness of the method for estimating the grid-side harmonic impedance, that is, the estimation method used can accurately estimate the , thus making the harmonic responsibility division result more accurate.

[0084] Table 1 Grid-side impedance estimation results and errors

[0085]

[0086] According to formula (4), the harmonic responsibilities on both sides of the PCC are calculated, and the calculation results are as follows: Figure 3 As shown in the figure, for the 13th harmonic, the grid-side harmonic responsibility accounts for the largest proportion, making it the primary harmonic source. For the 5th harmonic, the DG-side harmonic responsibility accounts for the largest proportion, meaning it is the primary harmonic source. Therefore, the 13th harmonic voltage at the PCC is monitored as the characteristic frequency on the grid side, and the 5th harmonic voltage is monitored as the characteristic frequency on the DG side.

[0087] Since the load quality factor is related to the RLC load parameters, the larger the quality factor, the less likely the power balance in the system will be disrupted, and the more likely the voltage and frequency at the PCC after islanding will fluctuate within the normal range allowed by the grid rated frequency. , the proposed detection method is simulated and verified.

[0088] During the simulation, the islanding phenomenon is set to occur at 0.8s. The simulation results under the power matching condition are as follows: Figure 4 shown. Figure 4 (a) Figure 4 (b) The simulation results of voltage and frequency at PCC before and after islanding. Figure 4 (a) Figure 4(b) It can be seen that before 0.8s, the DG is normally connected to the grid, and the voltage and frequency at the PCC are stable. After the island occurs at 0.8s, the voltage and frequency at the PCC only change slightly because the output power of the DG matches the required power of the load. The voltage and frequency at the PCC are within the range of the OVP / UVP and OFP / UFP thresholds, so the traditional passive island detection method under the power frequency will lead to detection failure. Figure 4 (c), Figure 4 (d) are respectively the simulation results of 5th and 13th harmonic voltage before and after the island occurs. After the island occurs at 0.8s, Figure 4 (c) shows that the 5th harmonic voltage suddenly increases and exceeds the upper threshold value, and the 13th harmonic voltage suddenly decreases and exceeds the lower threshold value, so the island detection is successful. Figure 4 (d) shows that the 13th harmonic suddenly decreases and exceeds the lower threshold value, triggering the protection action, and the island detection is successful.

[0089] As shown in Figure 5 , the output of the distributed power source changes at different times due to the influence of natural factors such as weather, which will cause the parameters at the PCC to change. At this time, it should not be judged as an island state. In order to verify the effectiveness of the proposed method under this condition, the output of the DG is increased from 4MW to 5MW at 0.8s. The simulation results under this condition are shown in Figure 5 .

[0090] Figure 5 (a), Figure 5 (b) are the simulation results of the voltage and frequency at the PCC before and after the output of the DG fluctuates. From Figure 5 (a), (b), it can be seen that the output of the DG increases at 0.8s, but due to the clamping effect of the large grid, the voltage and frequency at the PCC hardly change. Figure 5 (c), Figure 5 (d) are respectively the simulation results of 5th and 13th harmonic voltage before and after the output of the DG fluctuates. From Figure 5 (c) can be seen that the 5th harmonic voltage at the PCC increases significantly, but the change of the harmonic voltage is small compared with the island condition and does not exceed the upper limit. From Figure 5 (d) it can be seen that the 13th harmonic voltage from the grid side is basically unchanged and does not exceed the lower limit. Since the two island detection criteria cannot be met at the same time, it is determined as a non-island state. This shows that the proposed island detection method can effectively distinguish the change of the harmonic voltage at the PCC caused by the harmonic source on both sides.

[0091] According to the above simulation analysis, it can be seen that the island detection method based on the characteristic frequency harmonic voltage can quickly and effectively identify the island under the extreme conditions recommended by IEEE Std. 1547.1, and can also avoid misjudgment when the system parameters at the PCC change due to normal fluctuations of the grid.

[0092] In summary, the island detection method and system based on characteristic frequency harmonic voltage can accurately determine whether the power system has occurred island through real-time sampling, Pearson correlation coefficient screening, harmonic impedance calculation and harmonic responsibility analysis. The method effectively identifies the change trend of harmonic voltage after islanding. By comparing the theoretical ratio and the actual ratio of harmonic voltage before and after islanding, the accuracy of island detection is improved. Real-time frequency tracking sampling and characteristic frequency harmonic analysis enhance the sensitivity and reliability of detection, ensuring timely determination of island state when harmonic voltage fluctuates, ensuring the safe and stable operation of the power system, and having better applicability.

[0093] Finally, it should be noted that: the above described, only for the preferred embodiments of the present application, not to the present application in any form of restrictions; for the ordinary skilled in the art can be shown in the specification and the above described smoothly implement the present application; however, without departing from the scope of the present application technical solutions, familiar with the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content of the technical personnel in the above disclosed technical content

Claims

1. A method for islanding detection based on characteristic frequency harmonic voltage, characterized in that, The method comprises the following steps: S1, sampling the voltage and current at the power system public connection point to obtain sampling data; S2, using the Pearson correlation coefficient method to preprocess the sampling data, screening out data segments with a Pearson correlation coefficient between the harmonic voltage and the harmonic current greater than a set threshold, and calculating the harmonic impedance of the grid side according to the screened data segments; After screening out the data segments with a Pearson correlation coefficient between the harmonic voltage and the harmonic current greater than a set threshold, the harmonic impedance of the grid side is calculated by the following formula (2): （2）； wherein, is the harmonic voltage vector at the point of common coupling, is the harmonic current vector at the point of common coupling, is the hth harmonic voltage vector generated at the distributed generator side, is the hth harmonic impedance at the grid side; S3, according to the harmonic impedance of the grid side, the harmonic responsibilities of each order at the grid side and the distributed power supply side are calculated, and the maximum harmonic responsibilities of the two are selected as the characteristic frequencies, respectively, and the two characteristic frequencies are the frequencies with the greatest impact on the harmonic voltage at the public connection point, so that the change trend of the two characteristic frequency harmonic voltages after islanding is obtained; The theoretical ratio of the characteristic frequency harmonic voltage at the grid side and the distributed power supply side before and after islanding is calculated; the actual ratio of the characteristic frequency harmonic voltage at the grid side and the distributed power supply side before and after fluctuation is measured; According to the following formula (3), the harmonic responsibilities of each order at the grid side and the distributed power supply side are calculated: （3）； wherein, and are the harmonic responsibilities of the grid side and the distributed generation side, respectively, is the hth harmonic current of the distributed generation side; is the hth local load equivalent harmonic impedance of the distributed generation side. S4, according to the different change trends of the characteristic frequency harmonic voltages at the grid side and the distributed power supply side before and after islanding, and the comparison of the actual ratio and the theoretical value, it is judged whether the power system has islanding or not.

2. The island detection method based on characteristic frequency harmonic voltage according to claim 1, characterized in that, In the S1, the voltage and current at the public connection point are sampled by using the real-time frequency tracking variable sampling frequency fast Fourier transform.

3. The island detection method based on characteristic frequency harmonic voltage according to claim 1, characterized in that, In the S2, the calculation of the Pearson correlation coefficient method is as follows formula (1): （1）； wherein, , is the Pearson correlation coefficient for the first group of data and the second group of data, is the length of the sliding window; and are the average values of the first and second groups of data, respectively, is the Pearson correlation coefficient.

4. The island detection method based on characteristic frequency harmonic voltage according to claim 1, characterized in that, In the S3, the characteristic frequency harmonic with the greatest impact on the harmonic voltage at the public connection point is selected by formula (4), and it is judged whether the h-order harmonic voltage mainly comes from the grid side or the distributed power supply side: （4）； wherein, is the maximum harmonic responsibility on the grid side, is the maximum harmonic responsibility on the distributed power supply side.

5. The island detection method based on characteristic frequency harmonic voltage according to claim 1, characterized in that, In the S3, the theoretical ratio of the characteristic frequency harmonic voltage at the grid side and the distributed power supply side before and after islanding is calculated by the following formula (5): （5）； wherein, is the theoretical ratio of the harmonic voltage after islanding to the harmonic voltage before islanding for the grid side and the distributed power side characteristic frequency, is the harmonic voltage of the hth order after islanding, is the harmonic voltage of the hth order before islanding.

6. The method of claim 1, wherein the island detection method based on characteristic frequency harmonic voltage is characterized by, In the S3, the change trend of the harmonic voltage after islanding is: when the harmonic voltage at the public connection point mainly comes from the grid side, the harmonic voltage at the public connection point will decrease after islanding; when the harmonic voltage at the public connection point mainly comes from the distributed power supply side, the harmonic voltage at the public connection point will increase after islanding.

7. The island detection method based on characteristic frequency harmonic voltage of claim 6, wherein, In the S4, in the case of harmonic voltage fluctuation, the ratio of the harmonic voltage before and after fluctuation is calculated, and the judgment rule is: If the ratio of the harmonic voltage before and after fluctuation from the grid side is less than the theoretical value, and the ratio of the harmonic voltage before and after fluctuation from the distributed power supply side is greater than the theoretical value, it is judged that islanding occurs; if not, the voltage and current at the public connection point of the power system are continuously monitored.

8. The island detection system of the island detection method based on characteristic frequency harmonic voltage according to any one of claims 1-7, characterized in that, The island detection system comprises a sampling module, a preprocessing module, a harmonic responsibility calculation module, a theoretical ratio calculation module and an island determination module; wherein: The sampling module is used for detecting and sampling the voltage and current at the public connection point of the power system; The preprocessing module is used for screening data segments with Pearson correlation coefficient between harmonic voltage and harmonic current greater than a set threshold, and calculating harmonic impedance of the grid side according to the screened data segments; The harmonic responsibility calculation module is used for calculating and screening the maximum harmonic responsibility among harmonic responsibilities of each order at the grid side and the distributed power supply side, the two characteristic frequencies being the frequencies with the greatest influence on the harmonic voltage change at the point of common coupling, so as to obtain the change trend of the harmonic voltage of the two characteristic frequencies after the islanding occurs; The theoretical ratio calculation module is used for calculating the theoretical ratio of the harmonic voltage of the characteristic frequency at the grid side and the distributed power supply side after the islanding to the harmonic voltage before the islanding; The islanding determination module is used for measuring the actual ratio of the harmonic voltage of the characteristic frequency at the grid side and the distributed power supply side after the fluctuation to the harmonic voltage before the fluctuation, comparing the actual ratio with the theoretical ratio, and determining whether the power system has islanded.

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