An island detection method, system, device and medium based on low-frequency pulsation of reactive current

Abstract

The application relates to the technical field of power distribution network operation and control, and discloses an island detection method, system, equipment and medium based on low-frequency pulsation of reactive current, which comprises a flexible interconnection device suitable for low-voltage ride-through capability. In the method, a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding harmonic interference is superimposed in the reactive current control instruction, a voltage response component with the same frequency as the disturbance is extracted by collecting a point of common coupling voltage signal, and the voltage response component is compared with a dynamic threshold value to determine an island state; when a voltage drop triggers low-voltage ride-through, the disturbance injection is maintained and the threshold value is adaptively adjusted; meanwhile, a multi-machine synchronization mechanism and a review process based on historical cases and operation rules are introduced, so that false judgments are effectively inhibited. The application does not require additional hardware, is compatible with the existing control architecture, and has the advantages of reliable detection, strong anti-interference capability and suitability for complex power grid working conditions.

Description

Technical Field

[0001] This invention relates to the field of power distribution network operation and control technology, and in particular to an islanding detection method, system, device and medium based on low-frequency reactive current pulsation. Background Technology

[0002] With the acceleration of the global energy transition, distributed generation systems (DERs), represented by photovoltaics and wind power, as well as various new energy storage systems, are being massively integrated into distribution networks. This is causing traditional "unidirectional radial" distribution systems to evolve into "multi-source active" flexible distribution networks. Under this trend, flexible interconnected devices (FIDs), with power electronic converters at their core, have become key equipment in modern distribution networks. Through topologies such as back-to-back voltage source converters (VSCs), FIDs can achieve closed-loop interconnection and power transfer between different feeders, voltage levels, and even different regional power grids. They play an irreplaceable role in improving the flexibility of distribution networks, optimizing power flow distribution, mitigating power fluctuations, and providing dynamic voltage support.

[0003] However, the high degree of power electronics integration in flexible distribution networks also presents serious challenges to the safe and stable operation of the power grid. One of the most prominent problems is the unplanned islanding effect. When the upstream power grid is disconnected due to natural disasters, equipment failures, or relay protection operations, causing the local distribution area where the FID (Power Distribution Controller) is located to lose its physical connection with the main grid, but the FID and distributed power sources within the area continue to supply power, an island is formed. Unplanned islanding not only leads to voltage and frequency runaway within the island, endangering the operational safety of sensitive equipment on the user side, but also poses a life-threatening risk to maintenance personnel performing live-line maintenance. More seriously, if the main grid performs automatic reclosing without disconnecting the island, the resulting huge current surge may directly damage power electronic devices such as the FID, causing significant economic losses.

[0004] For islanding detection, existing technical solutions are mainly divided into two categories: passive and active. Passive detection methods (such as over / under voltage and over / under frequency protection) identify islanding by monitoring changes in electrical quantities at the point of common coupling (PCC). While this does not affect power quality, it has a significant detection blind zone (NDZ). When the load demand within the island is highly matched with the FID output power, the electrical characteristics hardly drift, causing passive detection to completely fail. Active detection methods (such as frequency shift method (AFD) and sliding mode frequency shift method (SMS)) disrupt islanding balance by injecting small disturbances into the system. Although active methods reduce the blind zone, in a flexible interconnected grid environment with multiple parallel sources, disturbance signals easily cancel each other out, and frequent disturbances can increase the total harmonic distortion (THD) of the power grid, reducing power quality.

[0005] Of particular note is that, under current power grid operation standards, flexible interconnection devices must possess low-voltage ride-through (LVRT) capability. When a symmetrical or asymmetrical fault in the main grid causes a voltage dip, the FID cannot immediately disconnect; instead, it needs to inject a large amount of reactive current into the grid to provide voltage support, depending on the depth of the voltage drop. Under these conditions, traditional islanding detection schemes face significant technical bottlenecks: 1. Control logic conflict: The LVRT control logic is designed to maintain the voltage stability of the PCC point. This will actively compensate for or mask the disturbance characteristics required for island detection, resulting in a significant decrease in the sensitivity of detection algorithms based on voltage or frequency disturbances.

[0006] 2. Risk of false tripping and failure: During the transient process of a power grid fault, the violent fluctuations in electrical quantities can easily trigger the islanding criterion, leading to false tripping and prematurely cutting off necessary reactive power support. Conversely, if the protection threshold is relaxed to be compatible with LVRT, there will be serious detection delays or missed alarms when a real island is formed. Summary of the Invention

[0007] In view of the aforementioned existing problems, the present invention is proposed.

[0008] Therefore, this invention provides an islanding detection method, system, device, and medium based on low-frequency reactive current pulsation, which can solve the problems of blind spots in passive detection of flexible distribution networks when the load power is balanced, and the easy failure or malfunction of traditional active detection methods when the grid fault triggers the low voltage ride-through (LVRT) logic.

[0009] To solve the above-mentioned technical problems, the present invention provides the following technical solution: In a first aspect, the present invention provides an islanding detection method based on low-frequency pulsation of reactive current. The method is applied to a flexible interconnection device, which is connected to the power distribution network through a common connection point and has a low-voltage ride-through control function. The method includes: Acquire the three-phase voltage and three-phase current signals at the point of common coupling; A low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network is superimposed on the reactive current control command of the flexible interconnection device. The magnitude of the voltage synthesis vector is calculated based on the three-phase voltage signal, and a voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal is extracted from the magnitude of the voltage synthesis vector. The voltage response component is compared with a dynamic threshold, and the islanding is determined based on the comparison result. When a voltage drop occurs in the distribution network and triggers low-voltage ride-through control, the superposition of the low-frequency sinusoidal disturbance signal is maintained, and the dynamic threshold is adjusted. The islanding determination result is reviewed. If the review confirms the result, an islanding alarm signal is output and the output of the flexible interconnection device is blocked.

[0010] As a preferred embodiment of the islanding detection method based on low-frequency reactive current pulsation described in this invention, the step of superimposing a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network into the reactive current control command of the flexible interconnection device includes: The three-phase voltage signals are subjected to phase-locked loop processing to obtain the grid voltage phase angle; Based on the grid voltage phase angle, the three-phase voltage signal and the three-phase current signal are transformed into a synchronous rotating coordinate system to obtain the d-axis component and the q-axis component; The low-frequency sinusoidal disturbance signal is superimposed on the q-axis reactive current command to generate a total reactive current control command, which is then sent to the current regulator.

[0011] As a preferred embodiment of the islanding detection method based on low-frequency reactive current pulsation described in this invention, the step of extracting a voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal from the magnitude of the voltage synthesis vector includes: Construct a bandpass filter with a center frequency equal to the frequency of the low-frequency sinusoidal disturbance signal; The magnitude of the voltage synthesis vector is input into the bandpass filter; The steady-state output of the bandpass filter is used as the voltage response component.

[0012] As a preferred embodiment of the islanding detection method based on low-frequency reactive current pulsation described in this invention, the step of comparing the voltage response component with a dynamic threshold and determining whether islanding has occurred based on the comparison result includes: Calculate the effective value of the voltage response component; Determine whether the effective value continuously exceeds the dynamic threshold for a preset number of disturbance cycles; If the conditions are met, a preliminary island determination result is generated.

[0013] As a preferred embodiment of the islanding detection method based on low-frequency reactive current pulsation described in this invention, the step of maintaining the superposition of the low-frequency sinusoidal disturbance signal and adjusting the dynamic threshold when a voltage drop occurs in the distribution network and triggers low-voltage ride-through control includes: Real-time monitoring of voltage amplitude at the point of common coupling; When the voltage amplitude is lower than the normal operating voltage lower limit threshold, the low voltage ride-through control mode is activated. In the low voltage ride-through control mode, the injection of the low-frequency sinusoidal disturbance signal remains unchanged, and the dynamic threshold is raised to a level higher than the dynamic threshold under normal operating conditions.

[0014] As a preferred embodiment of the islanding detection method based on low-frequency reactive current pulsation described in this invention, when multiple flexible interconnection devices are connected in parallel to the same common connection point, each flexible interconnection device exchanges disturbance synchronization information through a communication link, and unifies the frequency, phase, and start time of the low-frequency sinusoidal disturbance signal based on the disturbance synchronization information.

[0015] As a preferred embodiment of the islanding detection method based on low-frequency reactive current pulsation described in this invention, the step of verifying the islanding determination result, and if the verification confirms it, outputting an islanding alarm signal and blocking the output of the flexible interconnection device, includes: Based on a pre-set historical islanding case library and a power grid operation rule library, the consistency of the preliminary islanding determination results is verified. If the preliminary island determination result conflicts with historical cases or operating rules, a manual review process will be initiated or the alarm output will be delayed. If there is no conflict, the island status is confirmed and the locking command is executed.

[0016] Secondly, the present invention provides an islanding detection system based on low-frequency reactive current pulsation, comprising: The data acquisition module is used to acquire the three-phase voltage and three-phase current signals at the point of common coupling. The disturbance injection module is used to generate a low-frequency sinusoidal disturbance signal and superimpose it onto the reactive current control command; The signal processing module is used to calculate the magnitude of the voltage synthesis vector and extract the voltage response component that is in the same frequency as the disturbance signal. The logic discrimination module is used to compare the voltage response component with the dynamic threshold to generate a preliminary islanding determination result; LVRT coordination module is used to maintain disturbance injection and adjust dynamic threshold during voltage dips; The communication synchronization module is used to achieve frequency and phase synchronization of disturbance signals in multi-machine parallel scenarios; The verification module is used to verify the preliminary judgment results based on the historical case library and operating rules, and output the final islanding alarm signal.

[0017] Thirdly, the present invention provides an electronic device including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the method described above.

[0018] Fourthly, the present invention provides a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method described above.

[0019] Compared with existing technologies, the advantages of this invention are that it proposes an islanding detection method based on low-frequency reactive current pulsation, applicable to flexible interconnected devices with low-voltage ride-through capability. This method superimposes a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding harmonic interference into the reactive current control command. By acquiring the voltage signal at the point of common coupling, the voltage response component with the same frequency as the disturbance is extracted and compared with a dynamic threshold to determine the islanding state. When a voltage drop triggers low-voltage ride-through, the disturbance injection is maintained and the threshold is adaptively adjusted. Simultaneously, a multi-machine synchronization mechanism and a verification process based on historical cases and operating rules are introduced to effectively suppress false judgments. This invention requires no additional hardware, is compatible with existing control architectures, and has the advantages of reliable detection, strong anti-interference capability, and applicability to complex power grid conditions. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart of an islanding detection method based on low-frequency reactive current pulsation, provided as an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram illustrating the application scenario and island formation of a flexible interconnect device in an island detection method based on low-frequency reactive current pulsation, provided as an embodiment of the present invention.

[0023] Figure 3 This is a control block diagram of an islanding detection method based on low-frequency reactive current pulsation provided in one embodiment of the present invention.

[0024] Figure 4 This is a schematic diagram of the island detection signal extraction and discrimination logic in an island detection method based on low-frequency pulsation of reactive current, provided as an embodiment of the present invention.

[0025] Figure 5 This invention provides an islanding detection method based on low-frequency reactive current pulsation, and includes a flowchart of the islanding detection coordination process for low-voltage ride-through (LVRT) control.

[0026] Figure 6The following is a waveform diagram of an islanding detection method based on low-frequency reactive current pulsation provided in an embodiment of the present invention. (a) shows the entire process of the change of the per-unit voltage value at the point of common coupling (PCC) under the grid fault condition. (b) depicts the q-axis reactive current command signal output by the flexible interconnection device. (c) shows the low-frequency voltage characteristic component of the PCC point stripped in real time by a bandpass filter. (d) records in detail the final decision-making process of the islanding determination logic.

[0027] Figure 7 This is an internal structural diagram of an electronic device for an islanding detection method based on low-frequency reactive current pulsation, provided as an embodiment of the present invention. Detailed Implementation

[0028] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0029] It should be noted in advance that the system mentioned in the embodiments as the subject of real-time operation refers to any system configured with this method.

[0030] Example 1, referring to Figure 1 This is the first embodiment of the present invention, which provides an islanding detection method based on low-frequency reactive current pulsation, comprising: This invention provides a method that can effectively solve the problems mentioned above. The following will describe in detail how to implement the islanding detection method based on low-frequency reactive current pulsation with multiple embodiments. Figure 1 A flowchart of an islanding detection method based on low-frequency reactive current pulsation is shown. The method is applied to a flexible interconnection device, which is connected to the distribution network through a point of common coupling and has low-voltage ride-through control functionality. The method includes: S101, acquire the three-phase voltage signal and three-phase current signal at the common coupling point; In practice, flexible interconnection devices are connected to the power distribution network through a common connection point. During operation, the power grid status needs to be sensed in real time to determine whether islanding has occurred. The accuracy of islanding detection is highly dependent on the accurate acquisition of electrical quantities at the common connection point.

[0031] When a flexible interconnection device is in grid-connected operation, if the upstream power grid is disconnected due to a fault, causing a local area to disconnect from the main grid, but the distributed power sources in the area continue to supply power, an unplanned island will be formed. At this time, the voltage and current characteristics of the point of common coupling will undergo a fundamental change.

[0032] Conversely, if the three-phase voltage and current signals are not acquired in a timely and accurate manner, the key feature quantities used for subsequent islanding identification cannot be effectively extracted. Especially during low voltage ride-through, voltage drops and current surges superimposed on disturbance signals, if the sampling accuracy is insufficient or the synchronization is poor, will directly lead to islanding identification failure or malfunction.

[0033] If only low sampling rate or asynchronous sampling voltage and current data is used for processing, phase error and amplitude distortion will be introduced, causing the dq transformation result based on the synchronous rotating coordinate system to deviate from the true value, which in turn affects the extraction accuracy of the disturbance response components and ultimately causes the islanding determination logic to fail.

[0034] Therefore, it is necessary to obtain high-precision, time-synchronized three-phase voltage and three-phase current signals at the common connection point to provide a reliable data foundation for subsequent phase-locking, coordinate transformation, disturbance injection, and response extraction.

[0035] S102, a low-frequency sinusoidal disturbance signal with a frequency lower than the system base frequency and avoiding the background noise band of the distribution network is superimposed on the reactive current control command of the flexible interconnection device; In an embodiment of the present invention, a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network is superimposed on the reactive current control command of the flexible interconnection device, including: Phase-locked loop processing is performed on the three-phase voltage signal to obtain the grid voltage phase angle; Based on the grid voltage phase angle, the three-phase voltage signal and the three-phase current signal are transformed into a synchronous rotating coordinate system to obtain the d-axis component and the q-axis component; The low-frequency sinusoidal disturbance signal is superimposed on the q-axis reactive current command to generate the total reactive current control command, which is then sent to the current regulator.

[0036] It is understandable that when a flexible interconnected device encounters a disconnection from the upstream power grid during grid-connected operation and forms an unplanned island, relying directly on traditional passive island detection methods will lead to an expansion of the detection blind zone or a response delay, failing to meet the requirement of quickly and accurately identifying the island status.

[0037] However, if a high-frequency disturbance injection method is used, it may overlap with the inherent harmonic or switching noise frequency bands in the distribution network, causing the disturbance signal to be masked by background interference, thereby weakening the observability of the islanding characteristics.

[0038] Therefore, the present invention superimposes a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network into the reactive current control command, so as to actively stimulate identifiable electrical response characteristics and improve the sensitivity and robustness of islanding detection.

[0039] In this embodiment of the invention, superimposing a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network into the reactive current control command of the flexible interconnection device may include the following steps: First, the three-phase voltage signal obtained at the point of common coupling is phase-locked to obtain the real-time grid voltage phase angle, ensuring that subsequent coordinate transformation and disturbance injection are strictly synchronized with the grid fundamental phase. Furthermore, based on the obtained grid voltage phase angle, the three-phase voltage signal and the three-phase current signal are mapped to the synchronous rotating coordinate system through Park transformation to obtain the d-axis voltage component, q-axis voltage component, d-axis current component and q-axis current component respectively; Furthermore, a low-frequency sinusoidal disturbance signal of a preset frequency is superimposed on the q-axis reactive current command to generate a total reactive current control command containing the disturbance component. This total reactive current control command is then sent to the current regulator to drive the flexible interconnect device to output controlled reactive current.

[0040] The aforementioned low-frequency sinusoidal disturbance signal can be understood as being used in this invention to inject a traceable and identifiable small-amplitude periodic disturbance into the reactive power channel without affecting normal grid-connected power exchange, so as to excite an observable shift in voltage frequency or phase when islanding occurs. The above-mentioned frequency being lower than the system base frequency and avoiding the background noise band of the distribution network can be understood as follows: in order to ensure that the disturbance signal is not overwhelmed by system harmonics, inverter switching ripple or other equipment operating noise, the selected disturbance frequency must be significantly lower than the 50 Hz system base frequency, while avoiding the common load fluctuation and power electronic equipment noise concentration areas between 10 Hz and 45 Hz. The frequency of the disturbance signal mentioned in this section can be set according to the actual needs of relevant technical personnel, combined with the measured noise spectrum characteristics of the distribution network and the islanding detection response time requirements. This invention does not limit it.

[0041] S103 calculates the magnitude of the voltage synthesis vector based on the three-phase voltage signal, and extracts the voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal from the magnitude of the voltage synthesis vector. In an embodiment of the present invention, extracting a voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal from the magnitude of the voltage synthesis vector includes: Construct a bandpass filter with a center frequency equal to the frequency of the low-frequency sinusoidal disturbance signal; Input the magnitude of the voltage synthesis vector into the bandpass filter; The steady-state output of the output bandpass filter is used as the voltage response component.

[0042] It should be noted that if only the magnitude of the voltage composite vector is compared or a wideband filter is used to extract the disturbance response, it may not be possible to effectively separate the specific frequency components excited by the low-frequency sinusoidal disturbance signal. That is, when the voltage magnitude changes are similar in islanded and grid-connected states, it is difficult to accurately determine the operating state through global statistical features.

[0043] If islanding detection is still based on the original voltage magnitude value without frequency selectivity processing, it may lead to misjudgment or missed judgment. Especially in the balanced islanding operation where the output power of the distributed power source is highly matched with the local load, the voltage amplitude hardly changes, and the traditional method is very easy to fail.

[0044] For example, the low-frequency sinusoidal disturbance signal injected by the flexible interconnection device has a frequency of 3 Hz, while there are a large number of random fluctuations between 5 Hz and 15 Hz in the distribution network caused by variable frequency air conditioners, electric vehicle charging equipment, etc. If the voltage magnitude after low-pass filtering is used directly as the criterion, the 3 Hz disturbance response will be submerged in the background noise and will not be able to form an effective islanding feature.

[0045] For example, in the scenario where multiple flexible interconnected devices operate in parallel, if the devices do not synchronously disturb the phase and adopt non-frequency selective detection methods, the disturbance signals of each other will interfere with each other, resulting in the voltage synthesis vector magnitude exhibiting a complex modulation pattern, which further reduces the reliability of island identification.

[0046] In this embodiment of the invention, the specific operation of extracting the voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal from the magnitude of the voltage synthesis vector may include the following steps: First, construct a bandpass filter with a center frequency equal to the frequency of the low-frequency sinusoidal disturbance signal. The passband width of this bandpass filter is set according to the frequency stability of the disturbance signal and the dynamic response characteristics of the system to ensure effective suppression of interference in adjacent frequency bands. Furthermore, the magnitude of the voltage synthesis vector obtained after coordinate transformation and vector synthesis is input into a bandpass filter so that only the periodic components consistent with the disturbance frequency are retained in the signal. Furthermore, after the bandpass filter enters the steady-state operating phase, its output signal is used as the voltage response component for subsequent islanding criterion calculations.

[0047] The voltage response component mentioned above can be understood as the ability of the common coupling point voltage to dynamically follow low-frequency sinusoidal disturbance signals in this invention. Its amplitude and phase will deviate significantly from the reference value under grid-connected state due to the sudden change in system impedance after islanding occurs, thus constituting a high-sensitivity islanding discrimination criterion. The passband width of the bandpass filter mentioned in this section can be set according to the actual needs of relevant technical personnel, combined with the frequency drift range of the disturbance signal and the measured noise distribution of the power distribution network. This invention does not limit it.

[0048] S104 compares the voltage response component with the dynamic threshold and determines whether islanding has occurred based on the comparison result; In this embodiment of the invention, comparing the voltage response component with a dynamic threshold and determining whether islanding has occurred based on the comparison result includes: Calculate the effective value of the voltage response component; Determine whether the effective value continuously exceeds the dynamic threshold for a preset number of disturbance cycles; If the conditions are met, a preliminary island determination result is generated.

[0049] In some embodiments, the voltage response component is compared with a dynamic threshold, and the islanding is determined based on the comparison result. Specifically, the following steps are included: Initially, the voltage response component output from the bandpass filter is processed by a sliding window to calculate its effective value within one disturbance period. This effective value is obtained using the root mean square algorithm to accurately reflect the energy level of the periodic signal. Furthermore, the calculated effective value is compared point by point with the dynamic threshold corresponding to the current moment. The dynamic threshold is adjusted in real time according to the system operating conditions. Its reference value is determined by the statistical distribution of the effective values ​​of the historical voltage response components under grid-connected steady state, and a safety margin is added. Furthermore, the number of disturbance cycles in which the effective value continuously exceeds the dynamic threshold is recorded. This count is reset to zero each time the limit is not exceeded, and incremented each time the limit is exceeded. Furthermore, it is determined whether the number of disturbance cycles that continuously exceed the dynamic threshold reaches a preset number of disturbance cycles. This preset number is an integer and not less than three, in order to ensure that the determination result has sufficient time continuity. Furthermore, if the continuous over-limit condition is met, a preliminary islanding determination result is generated, and a subsequent confirmation mechanism or protection action process is triggered.

[0050] It should be noted that the dynamic threshold mentioned above can be set according to the actual needs of relevant technical personnel, combined with the range of system impedance changes, disturbance injection intensity and noise tolerance, and this invention does not limit it.

[0051] S105, when a voltage drop occurs in the distribution network and triggers low voltage ride-through control, maintain the superposition of low-frequency sinusoidal disturbance signals and adjust the dynamic threshold. In this embodiment of the invention, when a voltage drop occurs in the distribution network and triggers low-voltage ride-through control, the superposition of low-frequency sinusoidal disturbance signals is maintained, and the dynamic threshold is adjusted, including: Real-time monitoring of voltage amplitude at the point of common coupling; When the voltage amplitude is lower than the normal operating voltage lower limit threshold, the low voltage ride-through control mode is activated. In the low voltage ride-through control mode, the injection of low-frequency sinusoidal disturbance signal remains unchanged, and the dynamic threshold is raised to a level higher than that under normal operating conditions.

[0052] The system cannot effectively identify potential islanding during voltage recovery; if the original dynamic threshold is kept unchanged, the voltage response component will rise abnormally due to changes in system impedance characteristics, current regulator saturation, and increased harmonic distortion during voltage dips, leading to a misjudgment of an islanding state.

[0053] Optionally, when a voltage drop occurs in the distribution network and triggers undervoltage ride-through control, maintaining the superposition of low-frequency sinusoidal disturbance signals and adjusting the dynamic threshold may include the following operations performed in sequence: First, the three-phase voltage composite vector magnitude at the point of common coupling is monitored in real time. Instantaneous voltage amplitude data is obtained at a sampling frequency of every millisecond, and high-frequency noise interference is eliminated by moving average filtering. Furthermore, the current voltage amplitude is compared with a preset lower limit threshold for normal operating voltage, which is set to 90% of the nominal voltage. Furthermore, when the voltage amplitude remains below the normal operating voltage lower limit threshold for two or more consecutive power frequency cycles, it is determined that a voltage drop event has occurred, and the low voltage ride-through control mode is immediately activated. Furthermore, after the low voltage ride-through control mode is activated, the frequency, amplitude, and phase parameters of the low-frequency sinusoidal disturbance signal superimposed in the q-axis reactive current command are kept unchanged to ensure the islanding detection mechanism continues to operate effectively. Furthermore, the dynamic threshold used for islanding determination is simultaneously raised to a level higher than the dynamic threshold under normal operating conditions. The increase is dynamically calculated based on the current voltage drop depth and the current regulator output saturation level to avoid false over-limits in the response component caused by voltage distortion.

[0054] It should be noted that the aforementioned normal operating voltage lower limit threshold can be set according to the actual needs of relevant technical personnel, combined with the power distribution network operation procedures and equipment tolerance capabilities, and this invention does not impose any limitations.

[0055] S106, verify the islanding determination result. If the verification is confirmed, output an islanding alarm signal and block the output of the flexible interconnection device.

[0056] In this embodiment of the invention, the islanding determination result is reviewed. If the review confirms the result, an islanding alarm signal is output and the output of the flexible interconnect device is blocked, including: Based on a pre-set historical islanding case library and a power grid operation rule library, the consistency of the preliminary islanding determination results is verified. If the initial island determination result conflicts with historical cases or operating rules, a manual review process will be initiated or the alarm output will be delayed. If there is no conflict, the island status is confirmed and the locking command is executed.

[0057] It is worth noting that in practice, the initial islanding determination result may be affected by non-islanding events such as voltage drops, load changes, or communication interference. Furthermore, since a misjudgment of islanding will directly lead to unplanned blocking of flexible interconnection devices, thereby interrupting regional power supply and weakening the resilience of the distribution network, if the initial determination has not undergone logical consistency verification, it means there is a risk of triggering protection actions due to a single abnormal electrical characteristic. Therefore, it is necessary to introduce a review mechanism based on historical experience and operating rules to perform multi-dimensional verification of the initial islanding determination result to improve alarm reliability.

[0058] Optionally, consistency verification can be performed on the preliminary islanding determination results based on a preset historical islanding case library and a power grid operation rule library. That is, the amplitude variation trend of the current voltage response component, the direction of disturbance phase shift, the low-voltage ride-through status indicator, and the distributed generation output information are compared with typical characteristic patterns in historical islanding cases, while simultaneously checking for violations of prohibitive clauses regarding islanding operation in the power grid dispatching regulations. For example, in a certain event, the effective value of the voltage response component exceeds the dynamic threshold for five consecutive disturbance cycles, but at this time the main protection of the upstream substation has not operated and the feeder switch is still closed. This state does not conform to the precondition of "the upstream circuit breaker has tripped" in all historical real islanding cases, thus indicating a logical conflict.

[0059] If the initial islanding determination results conflict with historical cases or operational rules, a manual review process will be initiated or alarm output will be delayed. For example, in the scenario described above, the system automatically pauses the islanding alarm signal issuance and pushes a notification of pending confirmation to the dispatch center. Simultaneously, the monitoring window is extended to continue collecting data for the next three disturbance cycles. If the new data still meets the determination criteria and the upper-level switch status is updated to open, the confirmation process will be re-entered; if the switch status remains closed, the alarm will ultimately be canceled.

[0060] After completing the consistency verification operation based on the historical case library and the operation rule library, the decision on whether to execute the interlocking instruction can be made based on the verification result, thereby avoiding malfunctions caused by a single abnormal signal and ensuring that the flexible interconnection device is only safely isolated when a real island occurs.

[0061] The aforementioned historical islanding case library refers to a structured feature set pre-stored in this invention, derived from real islanding events. This set includes voltage response component evolution trajectories, system topology states, protection action sequences, and disturbance response phase relationships. The power grid operation rule library refers to the digital expression of distribution network dispatching regulations embedded in this invention, used to determine whether the current operating state meets the logical premise for islanding. The islanding alarm signal refers to a binary status flag in this invention indicating that the point of common coupling has disconnected from the main grid and formed an independent power supply area. The blocking of flexible interconnection device output refers to the control command in this invention that cuts off the flexible interconnection device from injecting active power into the distribution network, causing it to switch to reactive power support or shutdown state to ensure equipment and personnel safety.

[0062] In an embodiment of the present invention, when multiple flexible interconnection devices are connected in parallel to the same common connection point, each flexible interconnection device exchanges disturbance synchronization information through a communication link, and unifies the frequency, phase and start time of the low-frequency sinusoidal disturbance signal based on the disturbance synchronization information.

[0063] Example 2, refer to Figures 2-6 This embodiment also provides an islanding detection system based on low-frequency reactive current pulsation, including: The data acquisition module is used to acquire the three-phase voltage and three-phase current signals at the point of common coupling. The disturbance injection module is used to generate a low-frequency sinusoidal disturbance signal and superimpose it onto the reactive current control command; The signal processing module is used to calculate the magnitude of the voltage synthesis vector and extract the voltage response component that is in the same frequency as the disturbance signal. The logic discrimination module is used to compare the voltage response components with the dynamic threshold to generate a preliminary islanding determination result. LVRT coordination module is used to maintain disturbance injection and adjust dynamic threshold during voltage dips; The communication synchronization module is used to achieve frequency and phase synchronization of disturbance signals in multi-machine parallel scenarios; The verification module is used to verify the preliminary judgment results based on the historical case library and operating rules, and output the final islanding alarm signal.

[0064] The data acquisition module is responsible for acquiring the instantaneous voltage and current at the PCC at a high sampling frequency and transmitting them to the signal processing module for Clark and Park transformations to obtain the active component on the d-axis and the reactive component on the q-axis. The disturbance injection module adds a condition satisfying the formula to the q-axis current command value. The signal, where A is the disturbance amplitude, typically taken as 3% to 5% of the rated current. To avoid low-frequency background noise from the power grid.

[0065] The LVRT coordination module monitors the d-axis voltage amplitude in real time. When a voltage drop is detected entering the LVRT range, the module prioritizes ensuring the reactive power compensation current output as required by the standard, while maintaining the superposition of low-frequency disturbances. The logic discrimination module uses a bandpass filter to lock the signal. Observe the voltage feedback signal at this frequency band. In grid-connected operation mode, the voltage response is extremely weak at this frequency because the grid equivalent impedance is close to zero; however, after islanding occurs, due to the presence of load impedance, a significant change in voltage signal will appear. Frequency component. By calculating the effective value of this component and combining it with the voltage deviation change rate, the system can accurately identify islanding in a short time and effectively avoid malfunctions during LVRT. In addition, this system also considers the parallel operation of multiple flexible interconnected devices, and synchronizes the injection frequency and phase of each device through a communication protocol, enhancing the detectability of disturbance signals in complex distribution network environments.

[0066] This method not only eliminates the detection blind zone near power balance but also resolves the functional conflict between fault support and safety protection in electronic power distribution networks. By using reactive current instead of active current for detection, the impact on user-side power quality is minimized, ensuring frequency stability.

[0067] In this embodiment, the application scenario of the present invention is a typical flexible distribution network structure. In this structure, the Flexible Interconnect Device (FID) achieves power sharing between feeder 1 and feeder 2 through two back-to-back voltage source converters (VSC1 and VSC2) and an intermediate DC bus. The point of common coupling (PCC) is located at the connection between feeder 1 and the FID, and is also a key monitoring point of this system. When the substation circuit breaker is in the open state, feeder 1 and its connected local sensitive loads will be disconnected from the main grid. At this time, if the FID fails to identify the islanding state in time and stop power supply, unplanned islanding will be formed. The islanding detection device and control logic of the present invention are integrated into the control unit of the FID, and the judgment logic is executed by collecting the electrical information of the PCC point in real time, as shown in the attached figure. Figure 2 As shown.

[0068] In this embodiment, furthermore, at the underlying implementation level of the control system, FID adopts a vector control strategy based on a synchronous rotating coordinate system. The system acquires voltage and current sampling signals at the PCC through sensors, tracks the grid phase using a phase-locked loop (PLL), and decomposes them into d-axis (active axis) and q-axis (reactive axis) components through an abc / dq transformation. The d-axis current loop is responsible for controlling the active power output according to power balance requirements. In the q-axis current control loop, this invention introduces a crucial disturbance injection stage. The normal reactive current setpoint... Disturbance signal generated by low frequency signal generator The reactive current commands are generated by superimposing the results using an adder. The disturbance frequency is... The frequency is set to 3Hz, and the amplitude A is set to 4% of the rated current. The superimposed signal is sent to the PI controller, and after coordinate inverse transformation and SVPWM modulation, it controls the power switch to operate, thereby generating a reactive current with low-frequency pulsation characteristics on the output side, as shown in the attached diagram. Figure 3 As shown.

[0069] Furthermore, in this embodiment, the system monitors the three-phase voltage of the PCC in real time for the signal extraction and recognition process. And calculate the magnitude of its composite vector. The signal is fed into a center frequency that is strictly locked at... A low-frequency bandpass filter (BPF) is proposed. This filter employs a second-order generalized integrator (SOGI) structure, exhibiting extremely strong frequency selectivity. It effectively suppresses interference from the 50Hz fundamental frequency and all high-frequency harmonics, accurately extracting the low-frequency voltage response component caused by the injected current. The extracted component, after RMS calculation, is fed into a comparator and an adaptive threshold. A comparison is performed. To prevent false alarms caused by noise interference, a delay counting logic is connected after the comparator. Only when the detected signal continuously exceeds the threshold for three preset pulse cycles will the system officially trigger the islanding determination output signal and execute the command to block the converter, as shown in the attached diagram. Figure 4 As shown.

[0070] Furthermore, in this embodiment, to ensure stability under voltage sag conditions, the present invention performs the following steps: Figure 5 The diagram illustrates the coordination process involving LVRT control. The system first continuously checks for significant voltage drops in the PCC voltage. If the voltage is within the normal range, the system operates in the "normal operation branch," maintaining regular low-frequency injection and monitoring using standard thresholds. Once LVRT is detected (i.e., the voltage drops below 0.9 pu), the system switches to the "LVRT operation branch." At this time, the FID (Fluorescent Irrigation Function) activates the reactive current support function to meet grid connection requirements, but the low-frequency injection signal generator does not stop working; instead, it continues to add pulsations to the support current. Due to the large voltage fluctuations during LVRT, the system automatically increases the detection threshold. Alternatively, the criterion can be modified by calculating the correlation of voltage / current ripples to enhance anti-interference capability.

[0071] Ultimately, the logic control unit (LVRT) summarizes the characteristic quantities under the current operating condition for a final judgment. If the extracted low-frequency voltage characteristics meet the islanding criteria, it proves that the main grid circuit breaker has tripped, and the system immediately executes the islanding protection action (Trip). If the characteristics do not meet the criteria, it is considered that only an external grid fault exists, and the system continues to execute the LVRT logic and monitor cyclically. This hierarchical and progressive logic structure ensures that the flexible interconnection device has extremely high islanding identification accuracy and reliability under various complex operating conditions.

[0072] Furthermore in this embodiment, Figure 6 (a) illustrates the entire process of voltage per-unit value change at the point of common coupling (PCC) under grid fault conditions. The figure shows that in the initial stage from 0 to 0.3 seconds, the system operates in normal grid-connected mode, with the voltage maintained at the rated level of 1.0 pu. At 0.3 seconds, an external grid fault is simulated, causing a sharp voltage drop to approximately 0.35 pu, entering a low-voltage ride-through (LVRT) region lasting 0.3 seconds. This figure details how the flexible interconnect device did not erroneously trip when faced with a voltage dip, but instead strictly followed grid connection specifications to enter the ride-through state. This provides a foundation for subsequent verification of the stability of the islanding detection logic during voltage dips, demonstrating that the device can effectively distinguish between external short-circuit faults and genuine islanding disconnections.

[0073] Furthermore in this embodiment, Figure 6 (b) The q-axis reactive current command signal output by the flexible interconnection device is characterized, intuitively reflecting the core "detection signal superposition" mechanism of this invention. In this current waveform, a persistent low-frequency sinusoidal pulsation component with a frequency set to 3Hz can be clearly observed. This component acts as an active disturbance source for islanding detection. When the system detects a reactive power compensation requirement at 0.2 seconds, the fundamental current component rapidly steps up to perform the LVRT reactive power support task, but the detection ripple remains stably above the support current and continues to be output. This feature fully embodies the patent's frequency domain decoupling control strategy, that is, by superimposing at the reactive power command end, the device can maintain the injection of characteristic signals of the grid state while fulfilling its grid connection support obligations, ensuring the continuity of the detection function.

[0074] Furthermore in this embodiment, Figure 6(c) The low-frequency voltage characteristic component of the PCC point, stripped in real time by a bandpass filter, is the key physical basis for identifying islanding. The waveform shows that during the grid connection and LVRT support phase before 1.0 second, although the system continuously injects a 3Hz probe current, the voltage response at this frequency is extremely weak and accompanied by a small amount of background noise due to the extremely small equivalent impedance of the main grid, resulting in a very low amplitude level. However, after simulating an "islanding event" at 1.0 second, as the main grid circuit breaker trips, causing a sudden change in the regional impedance, the injected 3Hz reactive current induces a significant voltage swing at the same frequency on the local load, with the amplitude increasing several times instantaneously. This waveform visually demonstrates the amplification effect of impedance characteristic changes on the probe signal, transforming the islanding state, which was originally hidden in the power grid, into an explicit frequency characteristic that can be accurately captured.

[0075] Furthermore in this embodiment, Figure 6 (d) The final decision-making process of the islanding determination logic is recorded in detail, consisting of the effective value (RMS) curve of the low-frequency voltage characteristic and the protection trip signal. The figure shows the process of the extracted 3Hz voltage component RMS value rapidly rising after islanding occurs in 1.0 seconds. When the curve exceeds the preset adaptive dynamic threshold line at about 1.025 seconds, the trip flag below immediately jumps from logic 0 to logic 1, and the trip command is officially output at about 1.03 seconds. This process completely reproduces the millisecond-level response time from the characteristic value exceeding the limit to the actuator action, proving that the present invention can still complete accurate determination and disconnection in about 30 milliseconds after islanding occurs, even under the complex working conditions of LVRT. It not only eliminates the detection blind zone, but also has a response speed far superior to the current power grid safety standards.

[0076] It should be noted that by actively injecting low-frequency reactive current pulsations into the control loop of the flexible interconnect device, the detection blind zone problem of traditional passive detection methods when the load power is balanced is fundamentally solved. This ensures that even under extreme operating conditions where the device output power is highly matched with the local load demand, the islanding state can be accurately identified by inducing observable periodic fluctuations in the voltage at the point of common coupling, significantly improving the detection range and safety.

[0077] The system successfully achieved efficient coordination between islanding detection logic and low voltage ride-through (LVRT) control function. By utilizing the decoupling characteristics of low-frequency disturbance signals and power frequency support current in the frequency domain, it ensures that the device can strictly execute reactive voltage support commands and simultaneously carry out islanding characteristic monitoring during grid faults. This effectively eliminates detection failures or protection maloperations caused by LVRT control intervention and enhances the system's operational reliability under complex grid fault conditions.

[0078] This invention uses extremely low frequency and small amplitude reactive current as the detection source. Its characteristic frequency is much lower than the system fundamental frequency and avoids the common harmonic frequency bands of the distribution network. It not only does not cause the system operating frequency to drift, but also minimizes the impact on the power quality of the point of common coupling. It ensures that a high-quality detection signal is provided without increasing the total harmonic distortion rate of the voltage, and has excellent compatibility with sensitive loads in the distribution network.

[0079] This invention combines a high-precision signal extraction algorithm with an adaptive threshold discrimination mechanism to construct a highly robust anti-interference technology system. It can accurately lock and identify voltage response components at specific frequencies, effectively suppressing the risk of false triggering caused by transient processes such as background noise in the distribution network, nonlinear load switching, and motor starting. It also shortens the islanding detection time and provides a stable, agile, and robust safety protection method for complex flexible distribution networks with multiple parallel sources.

[0080] Example 3, referring to Figure 7 The aforementioned unit modules can be embedded in the processor of the electronic device in hardware form or independent of it, or they can be stored in the memory of the electronic device in software form, so that the processor can call and execute the corresponding operations of the above modules.

[0081] This embodiment also provides an electronic device, which can be a terminal, and its internal structure diagram can be as follows: Figure 7 As shown, the electronic device includes a processor, memory, communication interface, display screen, and input device connected via a system bus. The processor provides computing and control capabilities. The memory includes a non-volatile storage medium and internal memory. The non-volatile storage medium stores the operating system and computer programs. The internal memory provides an environment for the operation of the operating system and computer programs stored in the non-volatile storage medium. The communication interface is used for wired or wireless communication with external terminals; wireless communication can be achieved through Wi-Fi, carrier networks, NFC (Near Field Communication), or other technologies. When executed by the processor, the computer program implements an islanding detection method based on low-frequency reactive current pulsation. The display screen can be an LCD screen or an e-ink screen. The input device can be a touch layer covering the display screen, buttons, a trackball, or a touchpad on the device's casing, or an external keyboard, touchpad, or mouse.

[0082] This embodiment also provides a computer-readable storage medium on which a computer program is stored, and when the computer program is executed by a processor, it performs the following steps: Acquire the three-phase voltage and three-phase current signals at the point of common coupling; A low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network is superimposed on the reactive current control command of the flexible interconnection device. The magnitude of the voltage synthesis vector is calculated based on the three-phase voltage signal, and the voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal is extracted from the magnitude of the voltage synthesis vector. The voltage response component is compared with a dynamic threshold, and the islanding is determined based on the comparison result. When a voltage drop occurs in the distribution network and triggers low-voltage ride-through control, the superposition of low-frequency sinusoidal disturbance signals is maintained, and the dynamic threshold is adjusted. The islanding determination result is reviewed. If the review confirms the result, an islanding alarm signal is output and the output of the flexible interconnection device is blocked.

[0083] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

[0084] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0085] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. An islanding detection method based on low-frequency reactive current pulsation, characterized in that, include: The method is applied to a flexible interconnection device, which is connected to the power distribution network through a common connection point and has a low voltage ride-through control function. The method includes: Acquire the three-phase voltage and three-phase current signals at the point of common coupling; A low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network is superimposed on the reactive current control command of the flexible interconnection device. The magnitude of the voltage synthesis vector is calculated based on the three-phase voltage signal, and a voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal is extracted from the magnitude of the voltage synthesis vector. The voltage response component is compared with a dynamic threshold, and the islanding is determined based on the comparison result. When a voltage drop occurs in the distribution network and triggers low-voltage ride-through control, the superposition of the low-frequency sinusoidal disturbance signal is maintained, and the dynamic threshold is adjusted. The islanding determination result is reviewed. If the review confirms the result, an islanding alarm signal is output and the output of the flexible interconnection device is blocked.

2. The islanding detection method based on low-frequency reactive current pulsation as described in claim 1, characterized in that, The superimposition of a low-frequency sinusoidal disturbance signal with a frequency lower than the system fundamental frequency and avoiding the background noise band of the distribution network into the reactive current control command of the flexible interconnection device includes: The three-phase voltage signals are subjected to phase-locked loop processing to obtain the grid voltage phase angle; Based on the grid voltage phase angle, the three-phase voltage signal and the three-phase current signal are transformed into a synchronous rotating coordinate system to obtain the d-axis component and the q-axis component; The low-frequency sinusoidal disturbance signal is superimposed on the q-axis reactive current command to generate a total reactive current control command, which is then sent to the current regulator.

3. The islanding detection method based on low-frequency reactive current pulsation as described in claim 2, characterized in that, Extracting the voltage response component with the same frequency as the low-frequency sinusoidal disturbance signal from the magnitude of the voltage synthesis vector includes: Construct a bandpass filter with a center frequency equal to the frequency of the low-frequency sinusoidal disturbance signal; The magnitude of the voltage synthesis vector is input into the bandpass filter; The steady-state output of the bandpass filter is used as the voltage response component.

4. The islanding detection method based on low-frequency reactive current pulsation as described in claim 3, characterized in that, The step of comparing the voltage response component with a dynamic threshold and determining whether islanding has occurred based on the comparison result includes: Calculate the effective value of the voltage response component; Determine whether the effective value continuously exceeds the dynamic threshold for a preset number of disturbance cycles; If the conditions are met, a preliminary island determination result is generated.

5. The islanding detection method based on low-frequency reactive current pulsation as described in claim 4, characterized in that, When a voltage drop occurs in the distribution network and triggers low-voltage ride-through control, maintaining the superposition of the low-frequency sinusoidal disturbance signal and adjusting the dynamic threshold includes: Real-time monitoring of voltage amplitude at the point of common coupling; When the voltage amplitude is lower than the normal operating voltage lower limit threshold, the low voltage ride-through control mode is activated. In the low voltage ride-through control mode, the injection of the low-frequency sinusoidal disturbance signal remains unchanged, and the dynamic threshold is raised to a level higher than the dynamic threshold under normal operating conditions.

6. The islanding detection method based on low-frequency reactive current pulsation as described in claim 5, characterized in that, When multiple flexible interconnect devices are connected in parallel to the same common connection point, each flexible interconnect device exchanges disturbance synchronization information through a communication link, and unifies the frequency, phase and start time of the low-frequency sinusoidal disturbance signal based on the disturbance synchronization information.

7. The islanding detection method based on low-frequency reactive current pulsation as described in claim 6, characterized in that, The process of verifying the islanding determination result, and if the verification confirms it, outputting an islanding alarm signal and blocking the output of the flexible interconnect device, includes: Based on a pre-set historical islanding case library and a power grid operation rule library, the consistency of the preliminary islanding determination results is verified. If the preliminary island determination result conflicts with historical cases or operating rules, a manual review process will be initiated or the alarm output will be delayed. If there is no conflict, the island status is confirmed and the locking command is executed.

8. An islanding detection system based on low-frequency reactive current pulsation, using the method described in any one of claims 1 to 7, characterized in that, include: The data acquisition module is used to acquire the three-phase voltage and three-phase current signals at the point of common coupling. The disturbance injection module is used to generate a low-frequency sinusoidal disturbance signal and superimpose it onto the reactive current control command; The signal processing module is used to calculate the magnitude of the voltage synthesis vector and extract the voltage response component that is in the same frequency as the disturbance signal. The logic discrimination module is used to compare the voltage response component with the dynamic threshold to generate a preliminary islanding determination result; LVRT coordination module is used to maintain disturbance injection and adjust dynamic threshold during voltage dips; The communication synchronization module is used to achieve frequency and phase synchronization of disturbance signals in multi-machine parallel scenarios; The verification module is used to verify the preliminary judgment results based on the historical case library and operating rules, and output the final islanding alarm signal.

9. An electronic device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the islanding detection method based on low-frequency reactive current pulsation as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the islanding detection method based on low-frequency reactive current pulsation as described in any one of claims 1 to 7.

Images