Island detection method and system based on characteristic frequency harmonic voltage

Abstract

The invention provides an island detection method and system based on characteristic frequency harmonic voltage. The method comprises the following steps: sampling voltage and current at a common connection point; preprocessing the sampled data, screening out a data segment of which the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold value, and calculating the harmonic impedance of the power grid side; calculating harmonic voltage responsibilities of the grid side and the distributed power supply side as characteristic frequencies, respectively selecting the characteristic frequencies with the maximum harmonic responsibilities as harmonic voltage frequencies monitored in island detection, and obtaining different change trends of harmonic voltages of the two characteristic frequencies after an island occurs; measuring an actual ratio; and judging whether the power system has an island or not by combining the change trend of the harmonic voltage and the comparison of the actual ratio and the theoretical ratio. According to the island detection method, the influence on the quality of a power grid is small, meanwhile, the detection blind area is small, the island can be rapidly and effectively recognized, and the stability and safety of a power system are guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of island detection for distributed power grid connection, and in particular relates to an island detection method and system based on characteristic frequency harmonic voltage. Background Art

[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) are connected to the power grid. It can be foreseen that the integration of distributed power sources into the power grid is an inevitable development trend in the energy and power field in the future, but there may be some problems in the operation of distributed power sources. One of the difficulties 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. Among them, the most widely used island detection methods are mainly divided into passive detection methods and active detection methods. The active detection method has a small blind area, but this method may affect the power quality of the power system. Although the passive detection method does not affect the power quality of the power grid, it has a large detection blind area. In summary, the current island detection method cannot ensure that the impact on the quality of the power grid is small, and at the same time control the detection blind area to be relatively small, resulting in the inability to reliably detect the island state, affecting the stable operation of the power system. Summary of the invention

[0003] The purpose of the present invention is to provide an island detection method and system based on characteristic frequency harmonic voltage to solve the problem that the current island detection method cannot ensure that the impact on the quality of the power grid is small, and at the same time control the detection blind area to be relatively small, resulting in the inability to reliably detect the island state.

[0004] In order to achieve the above object, the present invention adopts the following technical scheme: The present invention provides an island detection method based on characteristic frequency harmonic voltage, comprising the following steps: S1. Sampling the voltage and current at the common connection point of the power system to obtain sampling data; S2. Preprocess the sampled data using the Pearson correlation coefficient method, filter out data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold, and calculate the harmonic impedance on the grid side based on the filtered data segments; S3. According to the harmonic impedance on the grid side, the harmonic responsibilities of each order on the grid side and the distributed power supply side are calculated, and the largest harmonic responsibilities of the two are selected as the characteristic frequencies. The two characteristic frequencies are the frequencies with the greatest impact on the harmonic voltage change at the common connection point on the grid side and the distributed power supply side, respectively, so as to obtain the change trend of the harmonic voltage of the two characteristic frequencies after the island occurs; Calculate the theoretical ratio of the characteristic frequency harmonic voltage after islanding to the harmonic voltage before islanding on the grid side and the distributed power supply side; measure the actual ratio of the characteristic frequency harmonic voltage after fluctuation to the harmonic voltage before fluctuation on the grid side and the distributed power supply side; S4. According to the different changing trends of the characteristic frequency harmonic voltages selected on the grid side and the distributed power supply side before and after the islanding, and by comparing the actual ratio with the theoretical value, it is determined whether the power system is islanded.

[0005] In some embodiments, in S1 , a fast Fourier transform with real-time frequency tracking and variable sampling frequency is used to sample the voltage and current at the common connection point.

[0006] In some embodiments, in S2, the Pearson correlation coefficient method is calculated as follows: (1); in, , For the Each sampling point corresponds to two sets of data whose correlation degree is to be determined. is the sliding window length; and is the average of the two sets of data, is the Pearson correlation coefficient.

[0007] In some embodiments, in S2, after screening out the data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than the set threshold, the harmonic impedance on the grid side is calculated by the following formula (2): (2); in, is the harmonic voltage vector of the public grid connection point, is the harmonic current vector of the public grid connection point, is the hth harmonic voltage vector generated by the distributed power supply side, is the hth harmonic impedance on the grid side.

[0008] Furthermore, in S3, the harmonic responsibilities of each order on the grid side and the distributed power supply side are calculated according to the following formula (3): (3); in, and are the harmonic responsibilities on the grid side and the distributed generation side, is the hth harmonic current on the distributed power supply side; is the equivalent harmonic impedance of the hth local load on the distributed power supply side.

[0009] Furthermore, in S3, the characteristic frequency harmonic with the greatest impact on the harmonic voltage at the common connection point is selected by formula (4) to determine whether the hth harmonic voltage mainly comes from the grid side or the distributed power supply side: (4); in, is the maximum harmonic responsibility on the grid side, It is the maximum harmonic responsibility on the distributed power supply side.

[0010] In some embodiments, in S3, the theoretical ratio of the characteristic frequency harmonic voltage after islanding to the harmonic voltage before islanding on the grid side and the distributed power supply side is calculated by the following formula (5): (5); in, is the theoretical ratio of the characteristic frequency harmonic voltage after islanding to the harmonic voltage before islanding on the grid side and the distributed generation side, is the hth harmonic voltage after islanding, is the h-order harmonic voltage before the island.

[0011] In some embodiments, in S3, the change trend of the harmonic voltage after the islanding occurs is: when the harmonic voltage at the common connection point mainly comes from the grid side, the harmonic voltage at the common connection point will decrease after the islanding occurs; when the harmonic voltage at the common connection point mainly comes from the distributed power supply side, the harmonic voltage at the common connection point will increase after the islanding occurs.

[0012] Further, in S4, when the harmonic voltage fluctuates, the ratio of the harmonic voltage after the fluctuation to the harmonic voltage before the fluctuation is calculated, and the judgment rule is: If the ratio of the harmonic voltage from the grid side after the fluctuation to that before the fluctuation is less than the theoretical value, and the ratio of the harmonic voltage from the distributed power supply side after the fluctuation to that before the fluctuation is greater than the theoretical value, it is judged that an island has occurred. If not satisfied, the voltage and current at the common connection point of the power system continue to be monitored.

[0013] The present invention also provides an island detection system based on characteristic frequency harmonic voltage, 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 to detect and sample the voltage and current at the common connection point of the power system; The preprocessing module is used to filter out data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold, and calculate the harmonic impedance on the grid side based on the filtered data segments; The harmonic responsibility calculation module is used to calculate and select the largest harmonic responsibility among the harmonic responsibilities at the grid side and the distributed power supply side, and combine the influence of the selected harmonic frequencies at the grid side and the distributed power supply side on the harmonic voltage change at the common connection point to obtain the change trend of the harmonic voltage after the island occurs; The theoretical ratio calculation module is used to calculate the theoretical ratio of the characteristic frequency harmonic voltage after islanding and the harmonic voltage before islanding on the grid side and the distributed power supply side; The island determination module is used to measure the actual ratio of the characteristic frequency harmonic voltage after fluctuation to the harmonic voltage before fluctuation on the grid side and the distributed power supply side, compare the actual ratio with the theoretical ratio, and determine whether an island occurs in the power system.

[0014] Compared with the prior art, the island detection method and system based on characteristic frequency harmonic voltage of the present invention have the following beneficial technical effects.

[0015] The present invention provides an island detection method based on characteristic frequency harmonic voltage, comprising the following steps: S1, sampling the voltage and current at the common connection point of the power system to obtain sampling data; S2, pre-processing the sampling data using the Pearson correlation coefficient method, 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 on the grid side according to the screened data segments; S3, calculating the harmonic responsibility of each order at the grid side and the distributed power supply side according to the harmonic impedance on the grid side, and selecting the largest harmonic responsibility of the two as the characteristic frequency, and the two characteristic frequencies are respectively calculated. The characteristic frequencies are the frequencies that the grid side and the distributed power supply side have the greatest impact on the harmonic voltage change at the common connection point, so as to obtain the change trend of the two characteristic frequency harmonic voltages after the island occurs; calculate the theoretical ratio of the characteristic frequency harmonic voltage of the grid side and the distributed power supply side after the island and the harmonic voltage before the island; measure the actual ratio of the characteristic frequency harmonic voltage of the grid side and the distributed power supply side after the fluctuation and before the fluctuation; S4, according to the different change trends of the characteristic frequency harmonic voltage selected by the grid side and the distributed power supply side before and after the island, and compare the actual ratio with the theoretical value, judge whether the power system has an island. Based on the above, the present invention can deeply understand the state change of the power system before and after the island occurs by accurately calculating the harmonic impedance and harmonic responsibility. By comparing the theoretical ratio of the harmonic voltage before and after the island with the actual ratio, the occurrence of the island event can be judged more accurately, and the risk of false alarm and missed alarm can be reduced to a large extent. Through the island detection of the present invention, the change of the harmonic voltage is monitored and analyzed in real time, and an early warning signal can be issued in the early stage of the island event, which is a data support for the power system and effectively avoids the potential threat of the island event to the power system. The island detection method of the present invention has little impact on the quality of the power grid, and the detection blind area is small. The island can be identified quickly and effectively, and no misjudgment will occur when the system parameters at the PCC change due to normal fluctuations of the power grid. The method has good practicality. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The drawings in the specification are used to provide further understanding of the present invention and constitute a part of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0017] Figure 1 It is a flow chart of an island detection method based on characteristic frequency harmonic voltage and a method in the system of the present invention; Figure 2 A schematic diagram of a model of an island detection method based on characteristic frequency harmonic voltage and an AC distribution network containing distributed power sources in the system according to the present invention; Figure 3 It is a schematic diagram of an island detection method based on characteristic frequency harmonic voltage and a calculation result of harmonic responsibility on the grid side and the DG side in the system of the present invention; Figure 4 It is a schematic diagram of an island detection method based on characteristic frequency harmonic voltage and simulation results under power matching conditions in the system of the present invention; Figure 5 The present invention is a method for detecting an island based on characteristic frequency harmonic voltage and a schematic diagram of simulation results under DG output fluctuation conditions in the system. DETAILED DESCRIPTION

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] How to design an active detection method with a small detection blind area, but without affecting the power quality of the power grid, with better cost control, and guaranteed detection speed and quality.

[0025] like Figure 1 As shown, the present invention provides an island detection method based on characteristic frequency harmonic voltage, comprising the following steps: S1. Sampling the voltage and current at the common connection point of the power system to obtain sampling data; S2. Preprocess the sampled data using the Pearson correlation coefficient method, filter out data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold, and calculate the harmonic impedance on the grid side based on the filtered data segments; S3. According to the harmonic impedance on the grid side, the harmonic responsibilities of each order on the grid side and the distributed power supply side are calculated, and the largest harmonic responsibilities of the two are selected as the characteristic frequencies. The two characteristic frequencies are the frequencies with the greatest impact on the harmonic voltage change at the common connection point on the grid side and the distributed power supply side, respectively, so as to obtain the change trend of the harmonic voltage of the two characteristic frequencies after the island occurs; Calculate the theoretical ratio of the characteristic frequency harmonic voltage after islanding to the harmonic voltage before islanding on the grid side and the distributed power supply side; measure the actual ratio of the characteristic frequency harmonic voltage after fluctuation to the harmonic voltage before fluctuation on the grid side and the distributed power supply side; S4. According to the different changing trends of the characteristic frequency harmonic voltages selected on the grid side and the distributed power supply side before and after the islanding, and by comparing the actual ratio with the theoretical value, it is determined whether the power system is islanded.

[0026] The island detection method of the present invention first divides the harmonic responsibilities on both sides of the PCC (Point of Common Coupling) and selects the characteristic frequency harmonics on both sides that have the greatest impact on the harmonic voltage at the PCC for island detection according to the harmonic responsibility division results. Secondly, since the premise of reasonable division of harmonic responsibilities is to estimate the harmonic impedance, the Pearson correlation coefficient method is used to pre-process the data to ensure that the system impedance solution is not affected by system fluctuations and accurately estimate the harmonic impedance. Finally, the theoretical value of the harmonic voltage ratio at the PCC after and before the island is derived. When the changes in the characteristic frequency harmonic voltage parameters on both sides before and after the island exceed the threshold, it is judged that an island occurs. The present invention can more accurately reflect the state changes of the power system before and after the island by accurately calculating the harmonic impedance and harmonic responsibility, and can monitor the voltage and current changes at the common connection point of the power system in real time, providing a strong guarantee for the safe operation of the power system.

[0027] The following is a detailed description of an island detection method and system based on characteristic frequency harmonic voltage of the present invention through specific embodiments.

[0028] Step 1: Use fast Fourier transform with real-time frequency tracking and variable sampling frequency to sample the voltage and current at the grid connection point.

[0029] Step 2: Use the Pearson coefficient method to screen out samples with a positive correlation between harmonic voltage and harmonic current, and calculate the harmonic impedance on the grid side. The calculation formula of the Pearson correlation coefficient is: (1); in, , are two sets of data whose correlation degree is to be determined corresponding to the nth sampling point; m is the length of the sliding window; and is the average of the two sets of data; Pearson correlation coefficient, with a value between -1 and 1, is used to measure two sets of data , The linear correlation between the two is closer to 1, indicating that the data and The higher the correlation between them.

[0030] According to the superposition theorem, the hth harmonic voltage at the PCC is the hth harmonic voltage on the grid side The vector sum of the h-order harmonic voltage generated on the DG side. Therefore, after screening out the data segments with the Pearson correlation coefficient between the harmonic voltage and the harmonic current greater than r>0.9, the h-order system impedance is calculated by equation (2). Perform the calculation: (2); in, is the harmonic voltage vector of the public grid connection point, is the harmonic current vector of the public grid connection point, is the hth harmonic voltage vector generated by the distributed power supply side, is the hth harmonic impedance on the grid side.

[0031] Step 3: Calculate the harmonic responsibilities of the grid side and the DG side according to formula (3), and select the maximum harmonic responsibilities of the grid side and the DG according to formula (4). This value can determine whether the h-th harmonic voltage mainly comes from the grid side or the DG side.

[0032] (3); in, is the hth order background harmonic voltage on the grid side; is the hth harmonic current on the DG side; is the equivalent harmonic impedance of the hth local load on the DG side; , They are the hth harmonic voltage and harmonic current at PCC: (4); In formula (4), is the maximum harmonic responsibility on the grid side, indicating that the i-th harmonic mainly comes from the grid side. The grid side has a greater impact on the harmonic voltage change at the PCC. After the island occurs, the i-th harmonic voltage will decrease; It is the maximum harmonic responsibility on the DG side, indicating that the j-th harmonic mainly comes from the DG side. The DG side has a greater impact on the harmonic voltage change at the PCC. After the islanding occurs, the j-th harmonic voltage will increase.

[0033] Step 4: Calculate the hth harmonic voltage after islanding by equation (5): The hth harmonic voltage before the island Theoretical ratio : (5); Therefore, the theoretical ratio of the i-th harmonic voltage and the j-th harmonic voltage after islanding to the harmonic voltage before islanding is and , which can be obtained by formula (5).

[0034] Step 5: Measure the i-th harmonic voltage and the j-th harmonic voltage. If fluctuation occurs, calculate the ratio of the harmonic voltage before and after the fluctuation. , .when , and satisfy If any of the conditions are not met, it is judged that the power grid is in normal grid-connected fluctuation, and the i-th harmonic voltage and the j-th harmonic voltage are continuously monitored.

[0035] The present invention also provides an island detection system based on characteristic frequency harmonic voltage, the system comprising 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 to detect and sample the voltage and current at the common connection point of the power system; The preprocessing module is used to filter out data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold, and calculate the harmonic impedance on the grid side based on the filtered data segments; The harmonic responsibility calculation module is used to calculate and filter the largest harmonic responsibility among the harmonic responsibilities at the grid side and the distributed power supply side. The two characteristic frequencies are the frequencies with the greatest influence on the harmonic voltage change at the public connection point from the grid side and the distributed power supply side, so as to obtain the change trend of the harmonic voltage of the two characteristic frequencies after the island occurs. The theoretical ratio calculation module is used to calculate the theoretical ratio of the characteristic frequency harmonic voltage after islanding and the harmonic voltage before islanding on the grid side and the distributed power supply side; The island determination module is used to measure the actual ratio of the harmonic voltage after the characteristic frequency harmonic voltage fluctuation on the grid side and the distributed power supply side to the harmonic voltage before the fluctuation, compare the actual ratio with the theoretical ratio, and determine whether the power system has islanded. The system of the present invention can monitor the power system status in real time, and by effectively and accurately determining and warning of islanding events, it helps to avoid unstable operation and potential safety hazards of the power system in the island state, and ensure the overall stability of the power system.

[0036] The effectiveness of the island detection method and system based on characteristic frequency harmonic voltage of the present invention is further verified by simulation verification.

[0037] Build the following in the PSCAD simulation software platform Figure 2 The DG grid-connected system circuit model is shown in Figure 2. The effectiveness of the proposed island detection method is verified based on this model.

[0038] In order to verify the effectiveness of the proposed grid impedance estimation method, ±20% random fluctuations are superimposed on the harmonic current on the DG side to simulate the actual load fluctuations, and ±10% normal random fluctuations are superimposed on the harmonic current on the grid side. At the same time, in order to reduce the accidental errors caused by a single experiment, 70 experiments were conducted, and each time the data segment with a correlation coefficient greater than 0.9 was selected. 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 estimate the grid-side harmonic impedance more accurately. , thus making the harmonic responsibility division result more accurate.

[0039] Table 1 Grid side impedance estimation results and errors

[0040] 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. It can be seen that for the 13th harmonic, the grid-side harmonic responsibility accounts for the largest proportion, and the grid side is the main harmonic source. For the 5th harmonic, the DG side harmonic responsibility accounts for the largest proportion, that is, the DG side is the main harmonic source. Therefore, the 13th harmonic voltage at the PCC is monitored as the characteristic frequency of the grid side, and the 5th harmonic voltage is monitored as the characteristic frequency of the DG side.

[0041] 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 voltage frequency at the PCC after the islanding will fluctuate within the normal allowable range of the grid rated frequency. , the proposed detection method is simulated and verified.

[0042] The islanding phenomenon is set to occur at 0.8s during the simulation. The simulation results under power matching conditions are as follows: Figure 4 shown. Figure 4 (a) Figure 4 (b) are 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 connected to the grid normally, 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 DG output power matches the power required by 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 the simulation results of the 5th and 13th harmonic voltages before and after the islanding occurs. Figure 4 (c) It can be seen that the 5th harmonic voltage surge exceeds the upper threshold value, and Figure 4 (d) It can be seen that the 13th harmonic sudden drop exceeds the lower limit threshold, triggering the protection action and the island detection is successful.

[0043] like Figure 5 As shown in the figure, the output of distributed power sources will change 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 DG output is set to increase from 4MW to 5MW at 0.8s. The simulation results under this condition are shown in the figure. Figure 5 shown.

[0044] Figure 5 (a) Figure 5 (b) The simulation results of the voltage and frequency at the PCC before and after the DG output fluctuation. Figure 5 (a), (b), it can be seen that at 0.8s, the DG output increases, but due to the clamping effect of the large power grid, the voltage and frequency at the PCC hardly change. Figure 5 (c) Figure 5 (d) is the simulation result of the 5th and 13th harmonic voltages before and after the DG output fluctuation. Figure 5 (c) It can be seen that the 5th harmonic voltage at the PCC increases significantly, but compared with the island case, the harmonic voltage changes slightly and does not exceed the upper limit. 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 requirements of the two island detection criteria are not met at the same time, it is judged as a non-islanding state. This shows that the proposed island detection method can effectively distinguish the changes in the harmonic voltage at the PCC caused by the harmonic sources on both sides.

[0045] According to the above simulation analysis, the island detection method based on characteristic frequency harmonic voltage of the present invention can quickly and effectively identify islands under extreme conditions recommended by IEEE Std.1547.1, and can also avoid misjudgment when normal fluctuations of the power grid cause changes in system parameters at the PCC.

[0046] In summary, the present invention provides an island detection method and system based on characteristic frequency harmonic voltage. The island detection method based on characteristic frequency harmonic voltage can accurately determine whether an island occurs in the power system through real-time sampling, Pearson correlation coefficient screening, harmonic impedance calculation and harmonic responsibility analysis. The method effectively identifies the changing trend of harmonic voltage after islanding, and improves the accuracy of island detection by comparing the theoretical ratio and actual ratio of harmonic voltage before and after islanding. Real-time frequency tracking sampling and characteristic frequency harmonic analysis enhance the sensitivity and reliability of detection, ensure timely determination of the island state when the harmonic voltage fluctuates, and ensure safe and stable operation of the power system, with better applicability.

[0047] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any ordinary technician in the industry can smoothly implement the present invention as shown in the specification and described above. However, any equivalent changes, modifications and evolutions made by technicians familiar with the profession without departing from the scope of the technical solution of the present invention using the technical content disclosed above are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. An island detection method based on characteristic frequency harmonic voltage, characterized in that: The steps include: S1. Sampling the voltage and current at the common connection point of the power system to obtain sampling data; S2. Preprocess the sampled data using the Pearson correlation coefficient method, filter out data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold, and calculate the harmonic impedance on the grid side based on the filtered data segments; S3. According to the harmonic impedance on the grid side, the harmonic responsibilities of each order on the grid side and the distributed power supply side are calculated, and the largest harmonic responsibilities of the two are selected as the characteristic frequencies. The two characteristic frequencies are the frequencies with the greatest impact on the harmonic voltage change at the common connection point on the grid side and the distributed power supply side, respectively, so as to obtain the change trend of the harmonic voltage of the two characteristic frequencies after the island occurs; Calculate the theoretical ratio of the characteristic frequency harmonic voltage after islanding to the harmonic voltage before islanding on the grid side and the distributed power supply side; measure the actual ratio of the characteristic frequency harmonic voltage after fluctuation to the harmonic voltage before fluctuation on the grid side and the distributed power supply side; S4. According to the different changing trends of the characteristic frequency harmonic voltages selected on the grid side and the distributed power supply side before and after the islanding, and by comparing the actual ratio with the theoretical value, it is determined whether the power system is islanded.

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

3. The island detection method based on characteristic frequency harmonic voltage according to claim 1 is characterized in that: In S2, the Pearson correlation coefficient method is calculated as follows: （1）； in, , For the Each sampling point corresponds to two sets of data whose correlation degree is to be determined. is the sliding window length; and is the average of the two sets of data, is the Pearson correlation coefficient.

4. The island detection method based on characteristic frequency harmonic voltage according to claim 1 is characterized in that: In S2, after screening out the data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than the set threshold, the harmonic impedance on the grid side is calculated by the following formula (2): （2）； in, is the harmonic voltage vector of the public grid connection point, is the harmonic current vector of the public grid connection point, is the hth harmonic voltage vector generated by the distributed power supply side, is the hth harmonic impedance on the grid side.

5. The island detection method based on characteristic frequency harmonic voltage according to claim 4 is characterized in that: In S3, the harmonic responsibilities of each order on the grid side and the distributed power supply side are calculated according to the following formula (3): （3）； in, and are the harmonic responsibilities on the grid side and the distributed generation side, is the hth harmonic current on the distributed power supply side; is the equivalent harmonic impedance of the hth local load on the distributed power supply side.

6. The island detection method based on characteristic frequency harmonic voltage according to claim 5 is characterized in that: In S3, the characteristic frequency harmonic that has the greatest impact on the harmonic voltage at the common connection point is selected by formula (4), and it is determined whether the h-th harmonic voltage mainly comes from the grid side or the distributed power supply side: （4）； in, is the maximum harmonic responsibility on the grid side, It is the maximum harmonic responsibility on the distributed power supply side.

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

8. The island detection method based on characteristic frequency harmonic voltage according to claim 1 is characterized in that: In S3, the change trend of the harmonic voltage after the island occurs is: when the harmonic voltage at the common connection point mainly comes from the grid side, the harmonic voltage at the common connection point will decrease after the island occurs; when the harmonic voltage at the common connection point mainly comes from the distributed power supply side, the harmonic voltage at the common connection point will increase after the island occurs.

9. The island detection method based on characteristic frequency harmonic voltage according to claim 8, characterized in that: In S4, when the harmonic voltage fluctuates, the ratio of the harmonic voltage after the fluctuation to the harmonic voltage before the fluctuation is calculated, and the judgment rule is: If the ratio of the harmonic voltage from the grid side after the fluctuation to that before the fluctuation is less than the theoretical value, and the ratio of the harmonic voltage from the distributed power supply side after the fluctuation to that before the fluctuation is greater than the theoretical value, it is judged that an island has occurred; if not met, the voltage and current at the common connection point of the power system continue to be monitored.

10. The island detection system according to any one of claims 1 to 9, characterized in that: The island detection system includes 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 to detect and sample the voltage and current at the common connection point of the power system; The preprocessing module is used to filter out data segments where the Pearson correlation coefficient between the harmonic voltage and the harmonic current is greater than a set threshold, and calculate the harmonic impedance on the grid side based on the filtered data segments; The harmonic responsibility calculation module is used to calculate and filter the largest harmonic responsibility among the harmonic responsibilities at the grid side and the distributed power supply side. The two characteristic frequencies are the frequencies with the greatest influence on the harmonic voltage change at the public connection point from the grid side and the distributed power supply side, so as to obtain the change trend of the harmonic voltage of the two characteristic frequencies after the island occurs. The theoretical ratio calculation module is used to calculate the theoretical ratio of the characteristic frequency harmonic voltage after islanding and the harmonic voltage before islanding on the grid side and the distributed power supply side; The island determination module is used to measure the actual ratio of the characteristic frequency harmonic voltage after fluctuation to the harmonic voltage before fluctuation on the grid side and the distributed power supply side, compare the actual ratio with the theoretical ratio, and determine whether an island occurs in the power system.

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