Island detection method based on harmonic dynamic characteristic and reactive slip collaborative identification and detection system thereof

Abstract

The invention discloses an island detection method based on harmonic dynamic characteristics and reactive slip collaborative identification and a detection system thereof, and relates to the field of new energy island detection. In order to solve the defects of low feature extraction precision, threshold stiffness and insufficient response speed of existing island detection, an operation data set of a micro-grid system is acquired according to a topological structure of the micro-grid system; dynamic characteristic data fusion processing is implemented, and harmonic voltage is calculated; a harmonic voltage break variable is used as a starting criterion of island detection; sequentially judging whether the harmonic voltage break variable, the fundamental voltage amplitude and the offset of each harmonic voltage are out of limit or not so as to judge whether the micro-grid is in an island state or not; and constructing a virtual orthogonal signal to calculate a three-phase reactive slip ratio, judging whether the three-phase reactive slip ratio is out of limit, if not, determining that the microgrid is in a non-islanding state, and if yes, determining that the microgrid is in an islanding state. The method is mainly used for judging whether the microgrid is in the island state or not.

Description

Technical Field

[0001] The present invention relates to the field of new energy island detection, and particularly to an island detection method and a detection system based on cooperative identification of harmonic dynamic characteristics and reactive power slip. Background Art

[0002] At present, traditional island detection methods (such as over / under voltage, over / under frequency methods) have problems such as large detection blind areas, insufficient dynamic response, and poor disturbance compatibility in high-penetration new energy scenarios. Detection methods based on harmonic characteristics and reactive power dynamic characteristics have gradually become a research hotspot. Harmonic components can reflect the non-linear characteristics in the island state due to their sensitivity to changes in grid impedance; while reactive power slip (the dynamic change rate of reactive power) can capture the energy imbalance in the transient process. However, existing methods still rely only on single parameters of harmonics or reactive power, making it difficult to handle the blind area problem in complex working conditions. The traditional FFT algorithm has spectral leakage in non-steady-state harmonic analysis. Although wavelet transform has time-frequency localization ability, its resolution is insufficient, and the fixed threshold cannot adapt to the fluctuations of grid background harmonics, which easily leads to misjudgment or missed detection and other bottlenecks.

[0003] Currently implemented solutions mainly include passive detection methods based on sudden changes in harmonic voltage, harmonic-reactive power hybrid detection methods, wavelet decomposition (WPD) and threshold dynamic adjustment methods, etc. The passive detection method based on sudden changes in harmonic voltage determines the island when the voltage amplitude of specific harmonics (such as the 3rd and 5th harmonics) is monitored and exceeds the threshold. Advantages: No active disturbance is required, and it is compatible with harmonic-sensitive scenarios. Defects: It depends on the harmonic characteristics of the load and fails in low-distortion or background harmonic interference; it does not combine reactive power dynamic characteristics and cannot distinguish between islanding and normal load switching. The harmonic-reactive power hybrid detection method: jointly analyzes the harmonic distortion rate and the change in reactive power, and uses a fixed threshold double criterion. Advantages: The blind area is reduced compared with single-parameter methods. Defects: The harmonic feature extraction uses FFT, and the resolution in the transient process is insufficient; the calculation of the reactive power change rate does not introduce time-domain tracking, and the dynamic response delay is significant (>100 ms); the threshold is fixed and cannot adapt to changes in grid impedance. The wavelet decomposition (WPD) and threshold dynamic adjustment method: extracts multi-band harmonic energy using WPD and dynamically adjusts the threshold in combination with a sliding window. Advantages: Improve the accuracy of harmonic feature extraction. Defects: It does not fuse the reactive power slip parameter, and the island detection criterion is single; the adaptive mechanism is only for harmonics and ignores the spatio-temporal coupling characteristics of reactive power; the protection action delay is relatively high (>20 ms), and it cannot meet the fast disconnection requirements in high-penetration scenarios.

[0004] Although the above solutions are effective in specific scenarios, they have not achieved the spatio-temporal cooperative analysis of harmonic dynamic characteristics and reactive power slip, and have common problems such as low feature extraction accuracy, rigid thresholds, and insufficient response speed.

[0005] Therefore, there is a need for an islanding detection method and its detection system based on the collaborative identification of harmonic dynamic characteristics and reactive power slip, which can accurately judge the islanding state, have high detection accuracy, significantly reduce the detection blind area, and dynamically correct the feature threshold. Summary of the Invention

[0006] The present invention is to solve the defects of low accuracy in extracting islanding detection features, rigid thresholds, and insufficient response speed in the existing technology, and provides an islanding detection method and its detection system based on the collaborative identification of harmonic dynamic characteristics and reactive power slip, which can accurately judge the islanding state, have high detection accuracy, significantly reduce the detection blind area, and dynamically correct the feature threshold.

[0007] An islanding detection method based on the collaborative identification of harmonic dynamic characteristics and reactive power slip according to the present invention includes the following steps: S1. Collect the operation data set of the microgrid system according to the topological structure of the microgrid system; S2. Implement dynamic feature data fusion processing and calculate the harmonic voltage; Adopt a wavelet decomposition-fast Fourier transform hybrid algorithm to extract harmonic spectrum features; the harmonic spectrum features include harmonic components, harmonic voltage mutation, and reactive power slip rate; S3. Use the harmonic voltage mutation as the starting criterion for islanding detection; judge whether the harmonic voltage mutation exceeds the limit. If so, continue to the next step. If not, the microgrid system is in a non-islanding state; S4. Overvoltage / undervoltage voltage diagnosis; Judge whether the amplitude of the current fundamental voltage exceeds the limit. If not, continue to the next step. If so, the microgrid is in an islanding state; Judge whether the offset of each harmonic voltage exceeds the limit. If not, continue to the next step. If so, the microgrid is in an islanding state; S5. Reactive power slip time-domain tracking; Construct a virtual orthogonal signal to calculate the three-phase reactive power slip rate, and judge whether the three-phase reactive power slip rate exceeds the limit. If not, the microgrid is in a non-islanding state. If so, the microgrid is in an islanding state.

[0008] Further: In S1, the operation data set includes wind-storage power generation system operation data, photovoltaic-storage power generation system operation data, load real-time operation data, microgrid connection point data, and bus voltage data; the bus voltage data is collected according to different wiring methods of different microgrid topologies.

[0009] Further: In S2, the extraction steps of the harmonic spectrum features are as follows: Use wavelet decomposition for the collected sampling signal to divide the signal into different spectral bandwidths, and the spectral bandwidths include the spectral bandwidths of the fundamental wave, 3rd harmonic, 5th harmonic, 7th harmonic, 9th harmonic, and 11th harmonic; Calculate the sudden change of harmonic voltage at different frequencies at the PCC point before and after the fault; Calculate the reactive power slip rate at the target moment according to the reactive power value at the target moment obtained by sampling.

[0010] Further: In S21, the steps for obtaining the frequency spectrum bandwidth are as follows: S211. Define the sampling harmonic order, calculate the harmonic frequency according to the rated frequency, and obtain the target harmonic frequency; S212. Calculate the sub-frequency bandwidth of wavelet decomposition according to the target harmonic frequency to obtain the sub-frequency bandwidths at different sampling frequencies; S213. Extract the sub-frequency bandwidth index of the target harmonic frequency and locate the sub-frequency bandwidth; S214. According to the sub-frequency bandwidth index, perform harmonic voltage sampling of the corresponding sub-frequency bandwidth to obtain the frequency band signal of the corresponding sub-frequency bandwidth, and calculate the corresponding frequency spectrum bandwidth.

[0011] Further: In S3, the calculation steps of the sudden change of harmonic voltage include: S31. Calculate the critical factor of harmonic voltage change according to the load, large power grid, photovoltaic energy storage power generation system and wind power energy storage power generation system; S32. Calculate the harmonic voltage with an increased amplitude according to the critical factor of harmonic voltage change, so as to obtain the sudden change of harmonic voltage.

[0012] Further: In S3, the specific steps for judging whether the sudden change of harmonic voltage exceeds the limit include: If the sudden change of harmonic voltage exceeds the limit, if so, trigger the protection mechanism, if not, it is judged that the microgrid system is in a non-island state.

[0013] Further: In S4, the specific steps for judging whether the amplitude of the current fundamental voltage exceeds the limit include: When it is detected that the amplitude of the fundamental voltage exceeds the preset threshold, it is determined at this time that the microgrid system is in an island operation state and it is determined that the islanding effect has occurred; disconnect the switches of the photovoltaic energy storage power generation system and the wind power energy storage power generation system; if the amplitude of the fundamental voltage does not exceed the preset threshold, continue to the next step; When it is detected that the offset of each harmonic voltage exceeds the preset threshold, it is determined at this time that the microgrid system is in an island operation state and it is determined that the islanding effect has occurred; disconnect the switches of the photovoltaic energy storage power generation system and the wind power energy storage power generation system; if the offsets of all harmonic voltages do not exceed the preset threshold, execute S5.

[0014] Further: In S5, after the sudden change of harmonic voltage starts island detection, if the amplitude of the fundamental voltage and the offsets of each harmonic voltage do not exceed the limit, then enter the reactive power slip time-domain tracking; the reactive power slip time-domain tracking specifically includes: Construct virtual orthogonal signals to calculate the three-phase reactive power respectively; calculate the three-phase reactive power slip rate in real time by the differential method; Judge whether the reactive power slip rate exceeds the limit. If it exceeds the limit, it is judged that the microgrid system is in the islanding state, and the switches of the photovoltaic energy storage power generation system and the wind power energy storage power generation system are cut off; if it does not exceed the limit, it is determined that the microgrid system is in the non-islanding state.

[0015] Furthermore: after the main power grid fails and disconnects, if the output power of the photovoltaic grid-connected inverter does not match the power required by the local load, then according to the power flow direction during the actual operation of the microgrid system, the relationship between the grid active power and the microgrid connection point voltage is obtained during grid-connected operation and islanding constant power operation ; In the operating state where the microgrid does not have power inverted to the large power grid, when then the voltage at the PCC point decreases when the microgrid operates in islanding mode; In the case where the large power grid allows the microgrid to have power inversion, when then the voltage at the PCC point decreases when the microgrid operates in islanding mode; when then the voltage at the PCC point increases when the microgrid operates in islanding mode.

[0016] The detection system for implementing the islanding detection method based on the collaborative identification of harmonic dynamic characteristics and reactive power slip disclosed by the present invention includes a microgrid system, a data acquisition module, a data fusion module, and an islanding judgment module. The microgrid system includes a wind power energy storage power generation system, a photovoltaic energy storage power generation system, a microgrid load, and a large power grid. The intersection of the wind power energy storage power generation system, the photovoltaic energy storage power generation system, the microgrid load, and the large power grid is the microgrid connection point; The data acquisition module is used to collect the operation data set of the microgrid system according to the topological structure of the microgrid system; The data fusion module is used to perform fusion processing on the feature data to obtain a feature parameter space containing harmonic spectrum features; The islanding judgment module is used to judge whether the microgrid is in the islanding state according to the harmonic voltage mutation amount, fundamental wave voltage amplitude, harmonic voltage offset amount, and three-phase reactive power slip rate.

[0017] The beneficial effects of the present invention are: The present invention provides an islanding detection method and its detection system based on the collaborative identification of harmonic dynamic characteristics and reactive power slip for microgrid islanding detection. By using the WPD-FFT hybrid algorithm to improve the harmonic transient resolution, constructing a harmonic mutation amount - reactive power slip rate composite criterion, and introducing a parameter adaptive compensation mechanism, it fundamentally breaks through the technical bottleneck of traditional methods and provides a highly reliable and fast-response islanding detection solution for high-proportion new energy power grids.

[0018] The present invention adopts a method for constructing a characteristic parameter space. The threshold criterion can be flexibly adjusted according to the topological structure, supporting full coverage from industrial and commercial microgrids to remote off-grid systems. Without additional hardware disturbance devices, it effectively avoids unnecessary activation of the active method, reduces the adverse impact on the power quality of the system, can achieve high-precision detection through algorithm upgrade, and reduces the transformation cost by more than 60%.

[0019] The present invention constructs a composite characteristic parameter space of harmonic components, harmonic mutation variables and reactive power slip rate through the spatio-temporal coupling analysis of the harmonic dynamic characteristics and reactive power slip, and solves the failure problem of single-parameter detection in the power balance scenario. The high-frequency sensitivity of the harmonic voltage mutation variable and the transient tracking ability of the reactive power slip are complementary, significantly reducing the detection blind area, applicable to complex working conditions caused by high proportion of new energy access. Design a 3ms-level rapid voltage anomaly protection strategy, instantaneously cut off the grid connection when the voltage deviation exceeds the threshold, with a response speed 30 times faster than the traditional 100ms level, minimizing the islanding risk to the greatest extent; at the same time, for the scenario where the voltage does not exceed the limit but the harmonics exceed the limit, start the reactive power slip time-domain tracking, and avoid misoperation through the delay confirmation mechanism, realizing the dual safety guarantee of "fast truncation + fine judgment"; introduce a parameter adaptive compensation mechanism to dynamically correct the characteristic threshold, overcoming interference factors such as grid background harmonic fluctuations and load nonlinear changes; combine the wavelet decomposition-FFT hybrid algorithm to achieve high-resolution extraction of harmonic components in the time-frequency domain, with the transient harmonic detection accuracy improved by more than 40%, ensuring reliable capture of weak characteristic signals; based on the initialization of the microgrid wiring method and the dynamic deployment of data acquisition points, it adapts to multi-type new energy hybrid access scenarios such as photovoltaic energy storage and wind energy storage. The rapid protection mechanism reduces the impact of fault current on equipment, extends the life of key components, and at the same time avoids the deterioration of power quality, meeting the requirements of safe and economic operation of the power grid under the "dual carbon" goal.

[0020] The present invention aims to solve the common technical problems of islanding detection in the scenario of high proportion of new energy access, and realizes the following core objectives through technological innovation: 1. Improve detection efficiency and response speed: Through the spatio-temporal collaborative analysis of harmonic dynamic characteristics and reactive power slip, break through the timing limitation of traditional single-parameter detection, shorten the islanding identification time from the 100ms level of traditional methods to the 3ms level, significantly improve the detection efficiency, ensure millisecond-level rapid disconnection under extreme working conditions, and avoid fault spread.

[0021] 2. Reduce detection blind area and misjudgment risk: Construct a harmonic mutation variable-reactive power slip rate composite criterion, combined with a parameter adaptive compensation mechanism, dynamically correct the threshold range, reduce the detection blind area area by more than 60%, and reduce the misjudgment rate to less than 0.1%, significantly improving the detection reliability and adapting to complex power grid environments.

[0022] 3. Improve system security and equipment lifespan: Through a fast protection mechanism at the 3ms level, the grid connection is instantaneously cut off when the voltage exceeds the limit abnormally, reducing the impact of fault current on key equipment such as photovoltaic inverters and energy storage converters, avoiding equipment overload damage, extending the service life of equipment by more than 30%, and at the same time reducing the risk of electric shock to personnel.

[0023] 4. Reduce transformation costs and operation and maintenance complexity: Adopt software algorithm upgrades to replace hardware disturbance devices, eliminating the need for additional installation of harmonic filters or frequency disturbance equipment, reducing the system transformation cost by more than 50%; the topology adaptive design supports plug-and-play deployment, reducing on-site commissioning time and post-maintenance costs.

[0024] 5. Improve power quality and environmental benefits: Avoid the negative impact of active disturbance injection on the harmonic content of the power grid, control the total harmonic distortion rate (THD) of the grid connection point voltage within 3%, reducing electromagnetic pollution; at the same time, precise detection reduces unnecessary power generation system outages, improves the proportion of new energy consumption, and reduces carbon emissions by more than 15% annually.

[0025] 6. Enhance system compatibility and universality: Through microgrid topology dynamic adaptation and multi-source data fusion, it is compatible with photovoltaic energy storage, wind energy storage and hybrid energy systems, supports full coverage from kilowatt-level household microgrids to megawatt-level industrial and commercial systems, provides a standardized solution for new power systems, and promotes the large-scale application of new energy.

[0026] Through the deep integration of multi-parameter collaborative identification, adaptive compensation algorithm and millisecond-level protection mechanism, the present invention achieves breakthroughs in dimensions such as detection efficiency, security, economy and environmental protection, provides high-reliability and low-cost island detection technology support for new energy high-penetration power grids, and helps the safe, green and efficient operation of power systems under the "dual carbon" goal. Description of the Drawings

[0027] Figure 1 is a schematic structural diagram of the microgrid system; Figure 2 is the optimized flow chart of the embodiment. Detailed Embodiments

[0028] The following are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. The following embodiments are only used to explain the present invention and cannot be construed as a limitation of the present invention. The protection scope of the present invention should be subject to the protection scope of the claims. The embodiments of the present invention are described in detail below. For the convenience of describing the present invention and simplifying the description, the technical terms used in the specification of the present invention should be interpreted in a broad sense, including but not limited to the conventional substitution schemes not mentioned in this application, and including both direct implementation methods and indirect implementation methods.

[0029] Embodiment 1 Combined with Figure 1 and Figure 2 This embodiment is described. An islanding detection method based on the collaborative identification of harmonic dynamic characteristics and reactive power slip is disclosed in this embodiment, including the following steps: S1. Collect the operation data set of the microgrid system according to the topological structure of the photovoltaic microgrid system, and perform precise processing on the operation data set: As Figure 1 shown, collect the operation data set of the entire microgrid system. In addition to the operation data of the wind energy storage power generation system, the photovoltaic energy storage power generation system, and the real-time operation data of the load in the microgrid system, directly collect the voltage , current , and frequency of the point of common coupling (PCC) of the microgrid separately. At the same time, collect the bus voltage of different wiring methods according to the topological structure of different microgrid systems. Among them, for the 3rd harmonic voltage, 5th harmonic voltage, 7th harmonic voltage, 9th harmonic voltage, and 11th harmonic voltage on the PCC side, the high-frequency wavelet transform sampling method of double localization analysis is used, and the sampling frequency .

[0030] Figure 1 In it, PCC is the point of common coupling of the microgrid, is the active power transmitted from the power grid to the microgrid, is the reactive power transmitted from the power grid to the microgrid, and the inflow into the PCC is positive; is the active power transmitted from the wind energy storage power generation system to the PCC, is the reactive power transmitted from the wind energy storage power generation system to the PCC, and the inflow into the PCC is positive; is the active power transmitted from the photovoltaic energy storage power generation system to the PCC, is the reactive power transmitted from the wind energy storage power generation system to the PCC, and the inflow into the PCC is positive; is the active power absorbed by the microgrid load from the PCC, is the reactive power absorbed by the microgrid load from the PCC, and the outflow from the PCC is positive.

[0031] The precise processing includes: When acquiring sampling data, high-frequency sampling with a sampling interval of about 0.5 ms and multi-point sampling data are adopted to preliminarily filter out other noises except the harmonic frequencies required for island detection. And during the calculation process, amplification processing is carried out to improve the sampling accuracy.

[0032] S2. Implement dynamic feature data fusion processing; Adopt the wavelet packet decomposition-fast Fourier transform (WPD-FFT) hybrid algorithm to extract harmonic spectrum features and establish a feature parameter space including harmonic components, harmonic mutation amounts, and reactive power slip rates; Use wavelet packet decomposition (WPD) for the sampled signals obtained by acquisition, and divide the signals into different spectral bandwidths of fundamental wave, 3rd harmonic, 5th harmonic, 7th harmonic, 9th harmonic, and 11th harmonic through recursive filtering and downsampling.

[0033] Define the sampling harmonic order For the rated frequency The harmonic frequencies of each order can be obtained as: (1); In the formula, is the th target harmonic frequency.

[0034] According to the target harmonic frequency calculate the sub-frequency bandwidth of wavelet packet decomposition , and the decomposition layer number is: (2); In the formula, is the allowable frequency deviation, is the sampling frequency.

[0035] Thus, the sub-frequency bandwidth for different sampling frequencies is: (3); Extract the sub-band index of the target harmonic frequency and locate the sub-band. The calculation formula for the sub-band index is: (4); According to the calculated sub-band index , perform corresponding sub-band harmonic voltage sampling, and thus the frequency band signal of the corresponding sub-band can be obtained, and calculate its spectrum : (5); Wherein, is the allowable frequency deviation, is the sampling harmonic order, is the rated frequency.

[0036] Take the spectral peak in the spectrum , and calculate the harmonic voltage amplitude of the corresponding frequency according to the spectral peak and the phase angle : (6); (7); Introduce the calculation of mutation amount based on time scale, calculate the mutation amount of harmonic voltage at different frequencies at the PCC before and after the fault. In most cases, the voltage harmonics will tend to the magnitude during normal system operation after a period of time. Therefore, the detection of the mutation amount after time delay can avoid misjudgment of detection.

[0037] (8); Wherein, is the mutation amount of the nth harmonic voltage, is the nth sampling moment, where .

[0038] According to the reactive power values at the moment and the moment obtained by sampling, calculate the reactive power slip rate at the moment: (9); Wherein, and are respectively the reactive power values at the moment and the moment.

[0039] S3. Island detection start criterion for mutation amount of harmonic voltage; Traditional overvoltage / undervoltage rapid diagnosis has a certain recognition ability for islanding conditions and non-islanding conditions. However, there are harmonic distortion situations in the actual system operation when using the traditional harmonic voltage detection method. In order to further avoid the problem that the dead zone setting value of harmonic voltage distortion caused by harmonic distortion cannot be accurately set, this embodiment introduces the mutation amount criterion of harmonic voltage as the start mode of island detection.

[0040] It is assumed that the magnitude of the harmonic current output from the grid side before and after the islanding occurs can be ignored. Due to the existence of a filter inductor at the outlet of the wind-solar inverter, the harmonic current output by the wind-solar inverter cannot change suddenly at the moment of islanding. Introduce the critical factor of the harmonic voltage change in the microgrid, It can be calculated by the following formula: (12); Wherein, , respectively represent the equivalent inductance and equivalent capacitance of the load; , respectively represent the equivalent inductance and equivalent capacitance of the large power grid; , respectively represent the equivalent inductance and equivalent capacitance of the photovoltaic energy storage power generation system; , respectively represent the equivalent inductance and equivalent capacitance of the wind energy storage power generation system. The harmonic voltage order with an increasing amplitude can be expressed by the following formula: (13); Wherein, is the angular frequency of the system at time, from which the harmonic voltage amplitude of the required corresponding frequency can be further screened, and the harmonic voltage mutation amount can be calculated by formula (8). In order to reduce the repeated calculation and determination of the harmonic voltage with no obvious change after the occurrence of the island, the detection speed of the island algorithm can be effectively improved.

[0041] Regarding the harmonic voltage mutation threshold, it is necessary to focus on avoiding the fluctuations caused by normal switch operations. Normal operations such as load switching and capacitor switching in the microgrid system will cause short-term harmonic distortions. To avoid mis-starting of microgrid island detection, according to the harmonic voltage mutation amount calculated in S2, a safety factor (usually taken as 0.03 - 0.1) of the harmonic voltage mutation amount threshold is introduced. The starting threshold of the harmonic voltage mutation amount for system island state detection is: Wherein, is the starting threshold of the harmonic voltage mutation amount, with the unit (V); is the rated voltage amplitude at the PCC point during normal operation of the microgrid.

[0042] S4, Overvoltage / Undervoltage Fast Diagnosis; When the main power grid fails and is accidentally disconnected, if the output power of the photovoltaic grid-connected inverter does not match the power required by the local load, then in this case, the voltage and frequency at the PCC will also change accordingly.

[0043] As Figure 1 shown, according to the power flow direction during the actual operation of the microgrid, assuming the actual equivalent impedance during load operation is: (15); Wherein, is the system angular frequency, is the equivalent resistance of the load. When operating in parallel grid connection and island constant power operation, the relationship between the active power exchange value of the power grid and the voltage at the PCC point can be obtained, that is, the active power transmitted from the power grid to the microgrid is: (16); Wherein, is the voltage at the PCC point after the microgrid forms an island, is the voltage at the PCC point during the normal operation of the microgrid. If there is no normal operation state where power is inverted from the microgrid to the large power grid, when , then the voltage at the PCC point during the island operation of the microgrid system decreases. If the large power grid allows power inversion from the microgrid, when , then the voltage at the PCC point during the island operation of the microgrid system decreases; when , then the voltage at the PCC point during the island operation of the microgrid system increases.

[0044] The fundamental voltage threshold is directly related to equipment safety. Installing the voltage range (±10%) specified by the national standard IEC 60038 is a mandatory requirement. To further improve the operational safety of the equipment, the present invention introduces a safety factor (usually taken as 0.1~0.15) for the fundamental voltage amplitude island detection threshold.

[0045] Due to the situation of power inversion in the microgrid, when an island occurs and the voltage at the grid connection point decreases, that is, when the system is under voltage, the fundamental voltage amplitude island detection threshold is calculated by the formula: (17); Wherein, is the fundamental voltage amplitude island detection threshold, with the unit (V).

[0046] When the system is overvoltage during island occurrence, the fundamental voltage amplitude island detection threshold is calculated by the formula: (18); When the microgrid system detects a voltage deviation that exceeds its normal value range (threshold), a 3ms-level fast protection is triggered to disconnect the switches of the photovoltaic and energy storage power generation system and the wind power and energy storage power generation system, and it can be considered that the microgrid system is in an island operation state and the islanding effect has occurred.

[0047] However, when load matching occurs between the switch of the photovoltaic and energy storage power generation system, the grid-connected inverter of the wind power and energy storage power generation system, and the local load, the changes in the system voltage and frequency detected after the main grid is disconnected often cannot exceed the detection threshold. In this case, the island state cannot be detected by the system, so there is a large blind area in this detection method.

[0048] If there is a situation of energized load that does not match the large grid power supply in the system, the harmonic voltage variables obtained by calculating the 3rd harmonic voltage value, 5th harmonic voltage value, 7th harmonic voltage value, 9th harmonic voltage value, and 11th harmonic voltage value through S2 and screened through S3 are used to judge the microgrid state.

[0049] There are characteristic differences in the harmonic characteristics of different frequencies. When calculating the harmonic voltage threshold, it is necessary to consider them separately for each frequency. The 3rd harmonic is mainly generated by the transformer saturation effect. When in an island state, the LC resonance will amplify specific harmonics (such as the 3rd harmonic is easily amplified at the neutral point of the transformer). The threshold needs to be lower than the resonance danger value (generally 4%). Considering the sensitive characteristics of the 3rd harmonic, the present invention introduces the safety factor of the 3rd harmonic voltage offset threshold (usually taking 0.03 - 0.04), then the calculation formula for the 3rd harmonic voltage offset threshold is: (19); In the formula, is the harmonic voltage offset threshold and , unit (V).

[0050] Compared with the 3rd harmonic, the 5th and higher harmonics have lower sensitive characteristics. The present invention introduces the safety factor of the 5th and higher harmonic voltage offset thresholds (usually taking 0.04 - 0.08), then the calculation formula for the 5th and higher harmonic voltage offset thresholds is: (20); In the formula, is the harmonic voltage offset threshold and , unit (V).

[0051] If the harmonic voltage exceeds the limit, trigger a 3ms-level rapid protection to cut off the switches of the photovoltaic and energy storage power generation system and the wind power and energy storage power generation system. It can be considered that the system is in an island operation state and the islanding effect has occurred. Otherwise, execute S5.

[0052] S5, Reactive power slip time-domain tracking; If the harmonic characteristics of the load are very similar in the islanding situation and when connected to the grid, the change in harmonic voltage may not be obvious enough, resulting in the failure of the detection method and thus forming a blind spot. For example, if the local load still maintains the same harmonic characteristics when in islanding as when connected to the grid, then the harmonic voltage detection method may not be able to detect the islanding. In this embodiment, a reactive power slip time-domain tracking method is further introduced. After the harmonic mutation starts the islanding detection, if the harmonic voltage load harmonic characteristics are not much different from when connected to the grid and the determination condition of harmonic voltage over-limit cannot be detected, then the reactive power slip time-domain tracking is entered.

[0053] When operating in parallel with the grid, the grid, as a stable voltage source, can quickly balance the fluctuations of the system's reactive power, so the rate of change of reactive power is relatively low. When operating in islanding, without the support of the grid, the dynamic balance of reactive power between the distributed power source and the local load is broken, resulting in a significant increase in the rate of change of reactive power.

[0054] Construct virtual orthogonal signals (such as Hilbert transform or all-pass filter) to calculate the three-phase reactive power respectively: (21); In the formula, is the reactive power of phase M at the PCC point at time t; and are the voltage and current of phase M at the PCC point at time t respectively. T is the fundamental wave period (for a 50Hz system, T = 0.02 s).

[0055] Start the reactive power slip time-domain tracking, and judge whether the reactive power slip is over-limit. If it is over-limit, cut off the switches of the photovoltaic-storage power generation system and the wind-power-storage power generation system after a time delay; if it is not over-limit, then return to the non-islanding state.

[0056] Calculate the three-phase reactive power slip in real time through the difference method: (22); In the formula, Δt is the sampling time interval.

[0057] According to the maximum reactive power slip rate (unit: ) in the normal operation state of the system, the system islanding state threshold can be set as (unit: ), where is the safety factor (usually taken as 1.5 - 3).

[0058] If the harmonic voltage is over-limit, trigger the long-delay protection, calculate the monitored reactive power slip rate in real time. If the reactive power slip is still higher than the threshold after the time delay, cut off the switches of the photovoltaic-storage power generation system and the wind-power-storage power generation system, and it can be considered that the system is in the islanding operation state and the islanding effect has occurred.

[0059] Embodiment 2 Combined Figure 1 with Figure 2 To illustrate this embodiment, the detection system disclosed in this embodiment for implementing the islanding detection method based on the collaborative identification of harmonic dynamic characteristics and reactive power slip includes a microgrid system, a data acquisition module, a data fusion module, and an islanding judgment module. The microgrid system includes a wind-storage power generation system, a photovoltaic-storage power generation system, a microgrid load, and a large power grid. The intersection point of the wind-storage power generation system, the photovoltaic-storage power generation system, the microgrid load, and the large power grid is the microgrid grid connection point. The data acquisition module is used to collect the operation data set of the microgrid system according to the topological structure of the microgrid system. The data fusion module is used to perform fusion processing on the feature data to obtain a feature parameter space containing harmonic spectrum features. The islanding judgment module is used to judge whether the microgrid is in an island state according to the harmonic voltage mutation, fundamental wave voltage amplitude, harmonic voltage offset, and three-phase reactive power slip rate.

Claims

1. An islanding detection method based on collaborative identification of harmonic dynamic characteristics and reactive power slip, characterized in that, It includes the following steps: S1. Collect the operation data set of the microgrid system according to the topological structure of the microgrid system; S2. Implement dynamic feature data fusion processing and calculate the harmonic voltage; Adopt a hybrid algorithm of wavelet decomposition - fast Fourier transform for harmonic spectrum feature extraction; the harmonic spectrum features include harmonic components, harmonic voltage mutation amount, and reactive power slip rate; S3. Use the harmonic voltage mutation amount as the starting criterion for islanding detection; judge whether the harmonic voltage mutation amount exceeds the limit. If so, continue to the next step. If not, the microgrid system is in a non-islanding state; S4. Voltage diagnosis of overvoltage / undervoltage; Judge whether the fundamental wave voltage amplitude at present exceeds the limit. If not, continue to the next step. If so, the microgrid is in an islanding state; Judge whether the offset of each harmonic voltage exceeds the limit. If not, continue to the next step. If so, the microgrid is in an islanding state; S5. Reactive power slip time-domain tracking; Construct a virtual orthogonal signal to calculate the three-phase reactive power slip rate, and judge whether the three-phase reactive power slip rate exceeds the limit. If not, the microgrid is in a non-islanding state. If so, the microgrid is in an islanding state.

2. The islanding detection method based on the collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, characterized in that, In S1, the operation data set includes the operation data of the wind-storage power generation system, the operation data of the photovoltaic-storage power generation system, the real-time operation data of the load, the data of the microgrid grid connection point, and the bus voltage data; the bus voltage data is collected according to different wiring methods of different microgrid topological structures.

3. The islanding detection method based on collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, characterized in that In S2, the extraction steps of the harmonic spectrum features are as follows: Use wavelet decomposition for the sampled signal obtained by collection to divide the signal into different spectral bandwidths, and the spectral bandwidths include the spectral bandwidths of the fundamental wave, the 3rd harmonic, the 5th harmonic, the 7th harmonic, the 9th harmonic, and the 11th harmonic; Calculate the harmonic voltage mutation amount of different frequencies at the PCC point before and after the fault; Calculate the reactive power slip rate at the target moment according to the reactive power value sampled at the target moment.

4. A method for islanding detection based on cooperative identification of harmonic dynamic characteristics and reactive power slip according to claim 3, characterized in that, In S21, the obtaining steps of the spectral bandwidth are as follows: S211. Define the sampled harmonic order, calculate the harmonic frequency according to the rated frequency to obtain the target harmonic frequency; S212. Calculate the wavelet decomposition sub-frequency bandwidth according to the target harmonic frequency to obtain the sub-frequency bandwidths of different sampling frequencies; S213. Extract the sub-frequency bandwidth index of the target harmonic frequency and locate the sub-frequency bandwidth; S214. According to the sub-frequency bandwidth index, perform harmonic voltage sampling of the corresponding sub-frequency bandwidth to obtain the frequency band signal of the corresponding sub-frequency bandwidth and calculate the corresponding spectral bandwidth.

5. A method for islanding detection based on collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, characterized in that, In S3, the calculation steps of the harmonic voltage mutation amount include: S31. Calculate the harmonic voltage change critical factor according to the load, the large power grid, the photovoltaic-storage power generation system, and the wind-storage power generation system; S32. Calculate the harmonic voltage with an increased amplitude according to the harmonic voltage change critical factor, so as to obtain the harmonic voltage mutation amount.

6. The islanding detection method based on collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, wherein In S3, the specific steps for judging whether the harmonic voltage mutation amount exceeds the limit include: If the harmonic voltage mutation amount exceeds the limit, if so, trigger the protection mechanism. If not, judge that the microgrid system is in a non-islanding state.

7. The islanding detection method based on the collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, wherein In S4, the specific steps for judging whether the fundamental wave voltage amplitude at present exceeds the limit include: When it is detected that the fundamental voltage amplitude exceeds the preset threshold, it is determined at this time that the microgrid system is in the island operation state and the islanding effect has occurred; the switches of the photovoltaic energy storage power generation system and the wind power energy storage power generation system are disconnected; if the fundamental voltage amplitude does not exceed the preset threshold, the next step is continued; When it is detected that the offset of each harmonic voltage exceeds the preset threshold, it is determined at this time that the microgrid system is in the island operation state and the islanding effect has occurred; the switches of the photovoltaic energy storage power generation system and the wind power energy storage power generation system are disconnected; if the offset of each harmonic voltage does not exceed the preset threshold, S5 is executed.

8. A method for islanding detection based on collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, characterized in that In S5, after the islanding detection is started by the sudden change of the harmonic voltage, if neither the fundamental voltage amplitude nor the offset of each harmonic voltage exceeds the limit, the reactive power slip time-domain tracking is entered; the specific steps of the reactive power slip time-domain tracking are as follows: Construct virtual orthogonal signals to calculate the three-phase reactive power respectively; calculate the three-phase reactive power slip rate in real time by the difference method; Judge whether the reactive power slip rate exceeds the limit. If it exceeds the limit, it is judged that the microgrid system is in the island state, and the switches of the photovoltaic energy storage power generation system and the wind power energy storage power generation system are disconnected; if it does not exceed the limit, it is determined that the microgrid system is in the non-island state.

9. The islanding detection method based on collaborative identification of harmonic dynamic characteristics and reactive power slip according to claim 1, characterized in that After the main power grid fails and disconnects, if the output power of the PV grid-connected inverter does not match the power required by the local load, then according to the power flow direction during the actual operation of the microgrid system, calculate the active power transmitted from the power grid to the microgrid during grid-connected operation and island constant power operation ; In the operating state where there is no power reverse transmission from the microgrid system to the large power grid, when occurs, the voltage at the PCC point decreases when the microgrid operates in island mode; When the large power grid allows power reverse transmission in the microgrid system, when occurs, the voltage at the PCC point of the microgrid during islanding operation decreases; when occurs, the voltage at the PCC point of the microgrid during islanding operation increases.

10. A detection system for implementing an islanding detection method based on collaborative identification of harmonic dynamic characteristics and reactive power slip as described in any one of claims 1-9, characterized in that, It includes a microgrid system, a data acquisition module, a data fusion module and an islanding judgment module, The microgrid system includes a wind power energy storage power generation system, a photovoltaic energy storage power generation system, a microgrid load and a large power grid. The intersection point of the wind power energy storage power generation system, the photovoltaic energy storage power generation system, the microgrid load and the large power grid is the microgrid grid connection point; The data acquisition module is used to collect the operation data set of the microgrid system according to the topological structure of the microgrid system; The data fusion module is used to perform fusion processing on the feature data to obtain a feature parameter space containing harmonic spectrum features; The islanding judgment module is used to judge whether the microgrid is in the island state according to the sudden change of the harmonic voltage, the fundamental voltage amplitude, the harmonic voltage offset and the three-phase reactive power slip rate.

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