A power distribution network low-frequency load shedding device blocking method considering distributed power island operation and frequency oscillation characteristics

Abstract

The application discloses a power distribution network low-frequency load shedding device blocking method considering island operation and frequency oscillation characteristics of distributed power supply, real-time acquisition of feeder current, bus voltage, system frequency and tie line power, extraction of feeder current transient high-frequency detail coefficient based on wavelet transform, calculation of transient energy index, and statistics of oscillation count of system frequency between load shedding action threshold and normal frequency; if the transient energy index meets the fault starting condition, island operation characteristic criterion is detected according to the bus voltage, tie line power and system frequency, the specified quantity criterion is established and the duration exceeds the time threshold, and a blocking signal is sent to the low-frequency load shedding device; if the oscillation count is greater than or equal to the oscillation blocking threshold, a blocking signal is sent to the low-frequency load shedding device; when the system frequency meets the blocking release condition or the upper power grid restores power supply, the blocking is released. The application effectively avoids the load shedding misoperation under the island and oscillation scenarios, and considers power supply reliability and equipment safety.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection and power system safety and stability control technology, specifically to a method for blocking low-frequency load shedding devices in distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources. Background Technology

[0002] Low-frequency load shedding is a crucial safety measure for power systems to prevent frequency collapse. When a power deficit causes the frequency to drop to a set threshold, low-frequency load shedding devices automatically disconnect some loads to restore system power balance and frequency stability. However, with the large-scale integration of distributed power sources such as distributed photovoltaics and small hydropower into distribution networks, when a fault in the upstream grid causes the distribution network to disconnect from the main grid, distributed power sources may form unplanned islands with local loads. In the initial stage of islanded operation, due to power imbalance, the system frequency and voltage will deviate. At this time, the low-frequency load shedding device may misjudge as a system power deficit and disconnect loads. As a result, even if the upstream grid quickly restores power supply or completes the transfer operation, the disconnected loads cannot be immediately restored, severely reducing power supply reliability.

[0003] In the prior art, invention patent CN120675097A proposes using incoming current and zero-sequence voltage as blocking criteria, but this method cannot distinguish between frequency changes caused by islanding and actual system power deficits. Invention patent CN117849499A proposes an islanding detection scheme based on voltage and frequency, but does not combine the islanding detection results with the blocking control of the low-frequency load shedding device. Furthermore, existing low-frequency load shedding devices generally lack the ability to identify system oscillation scenarios. When system power oscillations cause the frequency to repeatedly cross the load shedding threshold, the load shedding device may repeatedly activate, causing unnecessary load losses. Invention patent CN120497965B proposes a low-frequency load shedding method and system based on real-time frequency tracking at zero crossing points, but this method does not address the frequency blocking issue. Summary of the Invention

[0004] The technical problems to be solved by this invention are: existing low-frequency load shedding technology has the problem of malfunction in islanded scenarios of distribution networks with distributed power sources, and the problem of repeated operation of the load shedding device caused by system oscillation.

[0005] To address the aforementioned problems in existing technologies, a method for blocking low-frequency load shedding devices in distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources is provided. Through a multi-layered protection mechanism of fault detection start-up, islanded identification blocking, and frequency oscillation counting blocking, the method effectively avoids malfunctions of the load shedding device in scenarios where there is no true power deficit.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for blocking low-frequency load shedding devices in a distribution network, considering the islanded operation and frequency oscillation characteristics of distributed generation, includes the following steps: Real-time acquisition of feeder current signals, bus voltage signals, system frequency, and tie-line power in the distribution network; extraction of transient high-frequency detail coefficients of feeder current signals based on wavelet transform; calculation of transient energy index based on the transient high-frequency detail coefficients; and counting of oscillations in the system frequency as it travels back and forth between the load shedding threshold and the normal frequency within a specified time window. If the transient energy index meets the fault initiation conditions, the corresponding islanding operation characteristic criteria are detected based on the bus voltage signal, tie line power and system frequency. If a specified number of islanding operation characteristic criteria are met and the duration of the metness exceeds the time threshold, then islanding is determined to have occurred, and a blocking signal is sent to the low-frequency load shedding device on the feeder. If the oscillation count is greater than or equal to the oscillation blocking threshold, a blocking signal is sent to the low-frequency load shedding device on the feeder. When the system frequency is detected to meet the lockout release condition, or when the upstream power grid restores power supply, a lockout release signal is sent to the low-frequency load shedding device on the feeder.

[0007] Furthermore, when extracting the transient high-frequency detail coefficients of the feeder current signal based on wavelet transform, the feeder current signal is decomposed into a specified number of wavelet levels using wavelet basis functions to obtain the high-frequency detail coefficients of each level. When calculating the transient energy index based on the transient high-frequency detail coefficients, the sum of the energy of each high-frequency detail coefficient within the sliding time window is calculated as the transient energy index.

[0008] Furthermore, when the transient energy index meets the fault initiation condition, specifically, the transient energy index is greater than the product of the sensitivity coefficient and the baseline value of the transient energy during normal operation.

[0009] Furthermore, when determining the islanding operation characteristics based on bus voltage signals, tie-line power, and system frequency, the criteria include: The frequency offset criterion is determined to be valid. The condition for the frequency offset criterion to be valid is that the absolute value of the frequency offset is greater than the frequency offset threshold. The frequency offset is the difference between the measured system frequency and the rated frequency. The frequency change rate criterion is used to determine whether the criterion is valid. The condition for the frequency change rate criterion to be valid is that the absolute value of the system frequency change rate is greater than the frequency change rate threshold. The voltage deviation criterion is checked to see if it is valid. The condition for the voltage deviation criterion to be valid is that the voltage amplitude deviation is greater than the voltage deviation threshold. The voltage amplitude deviation is the result of dividing the absolute value of the bus voltage deviation by the rated voltage. The tie-line power criterion is checked to see if it is valid. The tie-line power criterion is valid when the active power of the tie-line drops below the power threshold.

[0010] Furthermore, when counting the oscillations as the system frequency oscillates back and forth between the load reduction threshold and the normal frequency within a specified time window, the mathematical expression for the oscillation count is as follows:

[0011] in, This represents the oscillation count at time t within a specified time window. The oscillation detection function is defined as follows: when a complete frequency oscillation event is detected, δ(t) = 1; otherwise, δ(t) = 0. A complete frequency oscillation event is defined as the system frequency dropping from the normal frequency to below the load shearing threshold and then returning to the normal frequency.

[0012] Furthermore, when the system frequency meets the lockout release condition, it includes: The system frequency is lower than the lower limit of the safe frequency, and the duration of the system frequency being lower than the lower limit of the safe frequency is greater than or equal to the safe delay; Alternatively, the active power of the tie line recovers to above the power threshold, and the system frequency recovers to the normal range for a duration exceeding the specified recovery time. Alternatively, the blocking duration exceeds the maximum blocking time when the oscillation count is greater than or equal to the oscillation blocking threshold. Alternatively, the system frequency remains stable within the normal range for a specified period of time.

[0013] Furthermore, after sending a blocking signal to the low-frequency load shedding device on the feeder, the process also includes: Calculate the power balance assessment index within the island. If the power balance assessment index within the island is less than the specified warning value, the warning power is insufficient. The mathematical expression for the power balance assessment index within the island is as follows:

[0014] in, For the total output of distributed power sources; Contribute to energy storage; This represents the total load within the isolated island.

[0015] The present invention also proposes a blocking system for a distribution network low-frequency load shedding device that considers the islanded operation and frequency oscillation characteristics of distributed generation sources. The system includes a processor and a computer-readable storage medium connected to each other. The computer-readable storage medium stores a computer program, which is executed by the processor to implement the blocking method for the distribution network low-frequency load shedding device that considers the islanded operation and frequency oscillation characteristics of distributed generation sources.

[0016] The present invention also proposes a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the aforementioned method for blocking low-frequency load shedding devices in distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources.

[0017] The present invention also proposes a computer program product, including a computer program, which, when executed by a processor, implements the aforementioned method for blocking low-frequency load shedding devices in distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources.

[0018] Compared with the prior art, the advantages of the present invention are as follows: This invention addresses the problems of erroneous load shedding by low-frequency load shedding devices in distribution networks with distributed generation systems when unplanned islanding occurs after a fault in the upper-level grid, and the repetitive operation of load shedding devices due to system oscillations. It collects feeder current, bus voltage, system frequency, and tie-line power in real time. Based on wavelet transform, it extracts transient high-frequency detail coefficients of the feeder current and calculates transient energy indices as fault initiation conditions. Islanding operation status is confirmed by a specified number of islanding operation characteristic criteria being met and lasting for more than a time threshold. Independently separate from the islanding judgment logic, it counts the oscillations of the system frequency crossing between the load shedding threshold and the normal frequency. When the corresponding threshold is met, a blocking signal is sent to the low-frequency load shedding device. This constructs a four-level linkage logic of "signal acquisition, fault detection, islanding identification, and load shedding blocking," combined with a frequency oscillation counting blocking mechanism, achieving precise blocking control of the low-frequency load shedding device. This method effectively avoids erroneous load shedding in islanding and oscillation scenarios, reducing unnecessary load losses by more than 50% in oscillation scenarios, achieving a technical effect that balances power supply reliability and equipment safety. This invention is applicable to 10kV and below distribution networks with a high proportion of distributed power sources. With the continuous increase in the penetration rate of distributed power sources, it has broad engineering application prospects. Attached Figure Description

[0019] Figure 1 This is the overall flowchart of the method of the present invention.

[0020] Figure 2 These are the frequency response comparison curves of the system in the simulation of this invention.

[0021] Figure 3 These are comparison curves of the operating states of the load shearing device in the simulation of this invention.

[0022] Figure 4 This is the blocking effect curve under the frequency oscillation scenario in the simulation of this invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0024] To address the issues of erroneous load shedding by low-frequency load shedding devices in distribution networks containing distributed generation sources when islanding occurs following a fault in the upstream power grid, and the repeated operation of load shedding devices due to system oscillations, this embodiment proposes a method for blocking low-frequency load shedding devices in distribution networks that considers the islanding operation and frequency oscillation characteristics of distributed generation sources. This method can accurately identify and block low-frequency load shedding devices in islanding and frequency oscillation scenarios, avoiding load losses caused by erroneous operation of load shedding devices and improving the power supply reliability of distribution networks containing distributed generation sources.

[0025] like Figure 1 As shown, the method includes the following steps: Step 1: Real-time acquisition of feeder current signals, bus voltage signals, system frequency, and tie-line power in the distribution network. Multi-level decomposition of the feeder current signal based on wavelet transform to extract transient high-frequency detail coefficients.

[0026] When a short-circuit fault or line tripping occurs in the upstream power grid, significant transient high-frequency components will appear in the feeder current of the distribution network. This embodiment utilizes wavelet transform to extract these transient features as the initiation element for fault detection. Compared with traditional negative-sequence / zero-sequence component detection methods, wavelet transform transient energy detection is independent of the fault type and is sensitive to various faults (such as three-phase short circuits, single-phase grounding, line tripping, etc.).

[0027] When extracting transient high-frequency detail coefficients of feeder current signals based on wavelet transform, specifically, wavelet basis functions are used to analyze the feeder current signals. i (t) Perform wavelet decomposition of a specified number of layers (e.g., L layers) to obtain the high-frequency detail coefficients of each layer. d l (k). The l Layer detail factor during sliding time window T The energy within w is: (1) in, For the first l Layer detail factor energy; For the first l Layer wavelet detail coefficients; This represents the number of sampling points within the sliding window.

[0028] Step 2: Calculate the transient energy index based on the transient high-frequency detail coefficients, and determine whether the transient energy index meets the fault start-up conditions.

[0029] Specifically, when calculating the transient energy index based on the transient high-frequency detail coefficients, the sum of the energies of each layer's high-frequency detail coefficients within the sliding time window is calculated as the transient energy index. The transient energy index of the first L layers is calculated as follows; in this embodiment, L=3.

[0030] (2) When the transient energy exceeds a set multiple of the reference energy during normal operation, a fault is determined to have occurred in the upstream power grid, and a fault initiation signal is output. The fault initiation criterion is as follows: (3) in, The baseline value of transient energy during normal operation is obtained through moving average calculation; As the sensitivity coefficient, α = 5.0 is taken in this embodiment. When equation (3) is true, a fault start signal is output. Flag fault =1, triggering subsequent island detection logic. When equation (3) is not true, Flag fault =0, the low-frequency load shearing device remains in normal working condition, and the process jumps to step 1.

[0031] Step 3: If the transient energy index meets the fault initiation conditions, the corresponding islanding operation characteristic criteria are detected based on the bus voltage signal, tie line power and system frequency. If a specified number of islanding operation characteristic criteria are met and the duration of the metness exceeds the time threshold, then islanding is determined to have occurred.

[0032] Specifically, when a fault in the upstream power grid causes the distribution network to disconnect from the main grid, the frequency, voltage, and power within the island will undergo characteristic changes. In this embodiment, a fusion logic based on four criteria is used for island identification: (1) Criterion 1: Frequency offset criterion. The frequency offset criterion is valid when the absolute value of the frequency offset (the difference between the measured system frequency and the rated frequency) is greater than the frequency offset threshold. The mathematical expression is: (4) in, This is the measured frequency; The rated frequency is 50Hz; The frequency offset threshold is set to 0.5Hz in this embodiment.

[0033] (2) Criterion 2: Frequency change rate criterion. The frequency change rate criterion is valid when the absolute value of the system's frequency change rate is greater than the frequency change rate threshold. The mathematical expression is: (5) in, The frequency change rate threshold is set to 0.5 Hz / s in this embodiment. In the early stages of island formation, due to power imbalance, the frequency change rate is usually much greater than the value during normal operation.

[0034] (3) Criterion 3: Voltage deviation criterion. The voltage deviation criterion is valid when the voltage amplitude deviation (the result of dividing the absolute value of the bus voltage deviation by the rated voltage) is greater than the voltage deviation threshold. The mathematical expression is: (6) in, Rated voltage; The voltage offset threshold is set to 0.1 (per unit value) in this embodiment.

[0035] (4) Criterion 4: Tie line power criterion. The tie line power criterion is valid when the active power of the tie line drops below the power threshold. The mathematical expression is: (7) in, The active power of the tie line; The power threshold is set to 5% of the rated transmission power in this embodiment. After islanding is formed, the tie-line power drops to near zero.

[0036] The islanding confirmation logic is: after the fault starts, i.e., after the fault starts ( Flag fault Under the premise of 1), the frequency offset, frequency change rate, bus voltage amplitude offset, and tie line active power of the comprehensive detection system are used to detect whether the frequency offset criterion, frequency change rate criterion, voltage offset criterion, and tie line power criterion are valid. At least three of the above four criteria must be met simultaneously, and the duration must exceed the confirmation delay. T confirm When islanded operation is confirmed, an islanded confirmation signal is output. Flag island The value is 1, and in this embodiment, it is taken as 1. T confirm The time is 0.2s. The island confirmation condition can be expressed as: (8) in, For the first j The satisfaction flag of each criterion (1 or 0); The duration for which the criterion is continuously satisfied.

[0037] Step 4: After confirming the islanded operation status, send a blocking signal to the low-frequency load shedding device and the low-voltage load shedding device on the feeder. During the blocking period, the load shedding device will not operate, and the power supply to the load will be maintained continuously.

[0038] Specifically, when Flag islandWhen =1, the islanding blocking signal Lock island =1, the interlocking management module sends an interlocking command to all low-frequency and low-voltage load shedding devices on the feeder. During the interlocking period, even if the system frequency is below the load shedding action threshold, such as 49.0Hz, the load shedding devices will not perform load shedding actions. The interlocking signal is transmitted through the device's internal hardwiring or communication bus.

[0039] The operation logic of the unloading device after locking is modified as follows: (9) in, The threshold for low-frequency load shedding action; This is the lockout signal. When Lock=1, regardless of the frequency, Action=0, and the load shearing device will not operate.

[0040] Step 5: Within a specified time window, count the oscillations of the system frequency as it travels back and forth between the load shedding threshold and the normal frequency. If the oscillation count is greater than or equal to the oscillation blocking threshold, send a blocking signal to the low-frequency load shedding device on the feeder.

[0041] like Figure 1 As shown, step 5 addresses the issue that system frequency oscillations may cause frequent operation of the low-frequency load shedding device. Independent of the aforementioned islanding judgment logic, this embodiment sets a frequency oscillation counter to monitor the system frequency crossing behavior between the load shedding action threshold and the normal frequency in real time. After the number of oscillations reaches the threshold within a set time window, the load shedding device is locked.

[0042] When power system oscillations occur, the frequency may fluctuate repeatedly between the load shedding threshold and the normal frequency. If this is not identified, the load shedding device may shed load every time the frequency falls below the threshold, causing significant unnecessary load losses. This embodiment uses a frequency oscillation counter for identification and blocking.

[0043] A complete frequency oscillation event is defined as: the system frequency changes from the normal range ( f ≥ f normal The load dropped below the unloading action threshold. f < f UFLS ), and then returned to normal. f ≥ f normal The operating logic of the frequency oscillation counter is as follows: Set time window T window The cumulative number of oscillations within this window N osc, in this embodiment, is taken as T windowThe duration is 60 seconds. The recursive formula for oscillation counting is: (10) in, The oscillation detection function is defined as follows: δ(t) = 1 when a complete frequency crossover-recovery process is detected, otherwise δ(t) = 0.

[0044] The oscillation blocking criterion is: (11) in, As the oscillation blocking threshold, this embodiment takes... N th The value is 3. That is, within a 60-second time window, after three complete frequency dips and recovery oscillations, the system is considered to be oscillating, the load shedding device is locked, and subsequent oscillations will not result in load shedding.

[0045] Oscillation lockout duration is T lock, osc In this embodiment, the time window is 120 seconds, and it will automatically close after the timeout period. T window Upon expiration, the oscillation counter will be automatically reset to zero.

[0046] The system's integrated blocking signal is an OR logic of islanded blocking and oscillating blocking: (12) That is, if any locking condition is met, the unloading device will be locked.

[0047] Step 6: When the system frequency is detected to meet the lockout release condition, or when the upstream power grid resumes power supply, send a lockout release signal to the low-frequency load shedding device on the feeder.

[0048] In this embodiment, the power balance status within the island is continuously monitored during the lockout period. When the system frequency drops below the safety lower limit, the lockout is automatically released to protect the equipment. When the upstream power grid is detected to have resumed power supply or the system frequency is continuously stable within the normal range, the lockout signal is released and the load shedding device returns to normal operation.

[0049] When the system frequency meets the lockout release condition, the following situations apply: (1) Safety safety net mechanism.

[0050] Specifically, the purpose of blocking the low-frequency load shedding device is to prevent accidental load shedding in islanded scenarios. However, if the output of distributed power sources within the island is severely insufficient and the frequency continues to drop, the blocking needs to be released to protect the power generation equipment and the grid. This embodiment includes a safety fallback mechanism: The safety unlocking criterion is: the system frequency is lower than the safety frequency lower limit, and the duration during which the system frequency is lower than the safety frequency lower limit is greater than or equal to the safety delay, and the mathematical expression is as follows: (13) wherein, is the safety frequency lower limit, which is 47.5Hz in this embodiment; is the duration during which the frequency is lower than the safety lower limit; is the safety delay, which is 0.5s in this embodiment. When the frequency continues to be lower than 47.5Hz for more than 0.5s, it is determined that the power in the island is seriously unbalanced, the blocking is automatically released, and the under-frequency load shedding device is allowed to operate to cut off part of the load.

[0051] The power balance evaluation indicator in the island is: (14) wherein, is the total output of distributed generators; is the energy storage output; is the total load in the island. When K balance is less than 0.7, an early warning of insufficient power is issued to prepare for possible unlocking.

[0052] (2) The other two types of situations.

[0053] Specifically, in addition to the safety fallback mechanism, the unlocking conditions are divided into the following two categories: Island blocking unlocking: When the active power of the tie line recovers above the power threshold ( P tie > P tie,th ), and the duration during which the system frequency recovers to the normal range (|f-f0|<0.2Hz) exceeds the specified recovery time T restore (in this embodiment T restore takes 2.0s), it is determined that the system has recovered to grid connection, and the island blocking is released.

[0054] Oscillation blocking unlocking: The oscillation blocking duration, that is, the blocking duration when the oscillation count is greater than or equal to the oscillation blocking threshold, is automatically released after exceeding the maximum blocking time T lock, osc (120s); or, the system frequency continues to be stable in the normal range (49.5Hz<f<50.5Hz) for more than the specified stabilization time T stable to release in advance, in this embodiment T stable takes 30s.

[0055] The conditions for termination can be uniformly expressed as: (15) in, The condition for recovery is met; The duration of the lockout; This represents the maximum locking duration.

[0056] To verify the effectiveness of the method proposed in this embodiment, a simulation verification was conducted using a 10kV distribution network in a certain region as an example. The simulation system includes a 10kV feeder connected to distributed photovoltaic and small hydropower, and connected to the upstream grid via a tie line. The simulation parameters are shown in Table 1. Two scenarios were set up in the simulation: Scenario 1 (Island Scenario): At t=1s, a fault occurs in the upstream grid, the tie line trips, and the distribution network is disconnected from the main grid, forming an island. The DG output in the island is 5MW (3MW photovoltaic and 2MW small hydropower), the load is 6MW, and the power deficit is 1MW. The frequency response and load loss with and without the blocking method of this embodiment were compared. Scenario 2 (Oscillation Scenario): The system experiences power oscillation, with the frequency fluctuating repeatedly between 48.5~50.5Hz, and the oscillation period is about 15s. The operation of the load shedding device with and without oscillation blocking was compared.

[0057] Table 1 Simulation System Parameters

[0058] like Figure 2 As shown, under the non-blocking scheme, the frequency drops rapidly after islanding is formed. When t≈1.5s, it drops below 49.0Hz, triggering low-frequency load shedding. The load shedding device cuts off 1.0MW of load, and the frequency drops to a minimum of 48.9Hz before recovering. The steady-state frequency is about 50.0Hz, with obvious overshoot.

[0059] After adopting the interlocking scheme in this embodiment, fault detection is initiated at t≈1.05s, transient energy surges, islanding confirmation is completed at t≈1.20s, and the interlocking signal is issued. During the interlocking period, the low-frequency load shedding device does not operate, and the load shedding amount is 0.0MW. The frequency drops to a minimum of 48.7Hz, and then gradually stabilizes at 50.0Hz under the action of DG output and natural load regulation. Figure 3 As shown, in the non-blocking scheme, the load shedding device mistakenly cut off 1.0MW of load during the initial stage of islanding; in this embodiment, the load shedding device is blocked, maintaining power supply to all loads. When the upstream power grid restores power at t=10s, the block is automatically released, and the load shedding device returns to normal.

[0060] Simulation verification was conducted based on scenario 2, such as Figure 4As shown, in the frequency oscillation scenario, without oscillation blocking, the load shedding device operates every time the frequency drops below 49.0Hz, cumulatively shedding 12.0MW of load within 90s. After adopting the oscillation counting blocking in this embodiment, the load shedding device operates normally during the first three complete oscillations, shedding 6.0MW; after the third oscillation is completed, the blocking is triggered, and no further load shedding occurs during subsequent oscillations, effectively avoiding excessive load shedding caused by oscillation.

[0061] Furthermore, this embodiment also proposes a distribution network low-frequency load shedding device interlocking system considering the islanded operation and frequency oscillation characteristics of distributed generation sources. The system includes an interconnected processor and a computer-readable storage medium. The computer-readable storage medium stores a computer program, which is executed by the processor to implement the distribution network low-frequency load shedding device interlocking method considering the islanded operation and frequency oscillation characteristics of distributed generation sources described in this embodiment. The system includes the following functional modules: The fault detection module is used to collect feeder current signals in real time, extract transient high-frequency energy based on wavelet transform, determine whether a fault has occurred in the upstream power grid, and output a fault initiation signal. The islanding identification module is used to confirm whether the distribution network has entered an islanding operation state after receiving a fault start signal by comprehensively detecting frequency, voltage and power criteria. The frequency oscillation monitoring module is used to monitor the number of frequency oscillations in real time, and outputs an oscillation lockout signal when the number of oscillations reaches a set threshold. The lockout management module is used to send lockout commands to the low-frequency load shedding device and the low-voltage load shedding device based on the islanding confirmation signal or the oscillation lockout signal, and to monitor the power balance status during the lockout period and release the lockout when necessary. The lockout release module is used to release the lockout signal and restore the load shedding device to normal operation after the system has resumed grid connection or frequency stabilization.

[0062] This embodiment also proposes a computer-readable storage medium storing a computer program. When the computer program is executed by a processor, it implements the low-frequency load shedding device blocking method for distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources, as described in this embodiment.

[0063] This embodiment also proposes a computer program product, including a computer program, which, when executed by a processor, implements the low-frequency load shedding device blocking method for distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources, as described in this embodiment.

[0064] In summary, this invention proposes a method and system for interlocking low-frequency load shedding devices in distribution networks, considering the islanded operation and frequency oscillation characteristics of distributed generation sources. It achieves comprehensive interlocking based on a three-level linkage logic of "fault initiation, islanding identification, and load shedding interlocking" and a frequency oscillation counting interlocking mechanism. The advantages of this invention are: 1. For the first time, wavelet transform transient energy detection is used as the initiation element for the lockout judgment of low-frequency load shedding devices, realizing a precise triggering mechanism of "islanding only when there is a fault". Compared with traditional negative-sequence / zero-sequence component detection, wavelet transform transient energy detection is independent of fault type, has sensitivity to various faults, and has a fast detection speed, completing within approximately 50ms. As shown in the simulation, fault detection is initiated within approximately 0.05s after the fault occurs, and islanding confirmation is completed within 0.20s.

[0065] 2. A multi-criteria fusion-based islanding identification method is proposed. Through a combination of four criteria—frequency offset, frequency change rate, voltage offset, and tie-line power—it effectively distinguishes islanded operation from normal frequency fluctuations, avoiding misjudgments. For example... Figure 2 As shown, after adopting the interlocking scheme of the present invention, the load shedding device is correctly interlocked in the island scenario, avoiding 1.0MW of erroneous load shedding and maintaining power supply to all loads.

[0066] 3. An innovative frequency oscillation counting interlocking mechanism is proposed. This mechanism counts the number of complete oscillations between the load reduction threshold and the normal value. When the cumulative number of oscillations reaches the threshold, the load reduction device is automatically interlocked. For example... Figure 4 As shown, in the oscillation scenario without blocking, a total of 12.0MW of load was cut off. After adopting oscillation blocking, only 6.0MW was cut off, reducing unnecessary load loss by 50%. At the same time, a safety fallback mechanism is set up, which automatically releases the blocking when the frequency drops below 47.5Hz, taking into account both power supply reliability and equipment safety.

[0067] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-readable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create a machine for implementing the process. Figure 1 One or more processes and / or boxes Figure 1The computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to operate in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0068] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A method for blocking low-frequency load shedding devices in a distribution network considering the islanded operation and frequency oscillation characteristics of distributed generation sources, characterized in that, Includes the following steps: Real-time acquisition of feeder current signals, bus voltage signals, system frequency, and tie-line power in the distribution network; extraction of transient high-frequency detail coefficients of feeder current signals based on wavelet transform; calculation of transient energy index based on the transient high-frequency detail coefficients; and counting of oscillations in the system frequency as it travels back and forth between the load shedding threshold and the normal frequency within a specified time window. If the transient energy index meets the fault initiation conditions, the corresponding islanding operation characteristic criteria are detected based on the bus voltage signal, tie line power and system frequency. If a specified number of islanding operation characteristic criteria are met and the duration of the metness exceeds the time threshold, then islanding is determined to have occurred, and a blocking signal is sent to the low-frequency load shedding device on the feeder. If the oscillation count is greater than or equal to the oscillation blocking threshold, a blocking signal is sent to the low-frequency load shedding device on the feeder. When the system frequency is detected to meet the lockout release condition, or when the upstream power grid restores power supply, a lockout release signal is sent to the low-frequency load shedding device on the feeder.

2. The method for blocking low-frequency load shedding devices in distribution networks considering the islanded operation and frequency oscillation characteristics of distributed generation sources according to claim 1, characterized in that, When extracting transient high-frequency detail coefficients of feeder current signals based on wavelet transform, the feeder current signals are decomposed into wavelet coefficients of a specified number of layers using wavelet basis functions to obtain high-frequency detail coefficients of each layer. When calculating transient energy index based on the transient high-frequency detail coefficients, the sum of the energy of each layer of high-frequency detail coefficients within the sliding time window is calculated as the transient energy index.

3. The method for blocking low-frequency load shedding devices in distribution networks considering the islanded operation and frequency oscillation characteristics of distributed generation sources according to claim 1, characterized in that, When the transient energy index meets the fault initiation condition, specifically, the transient energy index is greater than the product of the sensitivity coefficient and the baseline value of the transient energy during normal operation.

4. The method for blocking low-frequency load shedding devices in distribution networks considering the islanded operation and frequency oscillation characteristics of distributed generation sources according to claim 1, characterized in that, When determining islanded operation characteristics based on bus voltage signals, tie-line power, and system frequency, the following criteria are included: The frequency offset criterion is determined to be valid. The condition for the frequency offset criterion to be valid is that the absolute value of the frequency offset is greater than the frequency offset threshold. The frequency offset is the difference between the measured system frequency and the rated frequency. The frequency change rate criterion is used to determine whether the criterion is valid. The condition for the frequency change rate criterion to be valid is that the absolute value of the system frequency change rate is greater than the frequency change rate threshold. The voltage deviation criterion is checked to see if it is valid. The condition for the voltage deviation criterion to be valid is that the voltage amplitude deviation is greater than the voltage deviation threshold. The voltage amplitude deviation is the result of dividing the absolute value of the bus voltage deviation by the rated voltage. The tie-line power criterion is checked to see if it is valid. The tie-line power criterion is valid when the active power of the tie-line drops below the power threshold.

5. The method for blocking low-frequency load shedding devices in distribution networks considering the islanded operation and frequency oscillation characteristics of distributed generation sources according to claim 1, characterized in that, When counting the oscillations of the system frequency as it travels back and forth between the load shedding threshold and the normal frequency within a specified time window, the mathematical expression for the oscillation count is as follows: in, This represents the oscillation count at time t within a specified time window. The oscillation detection function is defined as follows: when a complete frequency oscillation event is detected, δ(t) = 1; otherwise, δ(t) = 0. A complete frequency oscillation event is defined as the system frequency dropping from the normal frequency to below the load shearing threshold and then returning to the normal frequency.

6. The method for blocking low-frequency load shedding devices in distribution networks considering the islanded operation and frequency oscillation characteristics of distributed generation sources according to claim 1, characterized in that, When the system frequency meets the lockout release conditions, including: The system frequency is lower than the lower limit of the safe frequency, and the duration of the system frequency being lower than the lower limit of the safe frequency is greater than or equal to the safe delay; Alternatively, the active power of the tie line recovers to above the power threshold, and the system frequency recovers to the normal range for a duration exceeding the specified recovery time. Alternatively, the blocking duration exceeds the maximum blocking time when the oscillation count is greater than or equal to the oscillation blocking threshold. Alternatively, the system frequency remains stable within the normal range for a specified period of time.

7. The method for blocking low-frequency load shedding devices in distribution networks considering the islanded operation and frequency oscillation characteristics of distributed generation sources according to claim 1, characterized in that, After sending a blocking signal to the low-frequency load shedding device on the feeder, the process also includes: Calculate the power balance assessment index within the island. If the power balance assessment index within the island is less than the specified warning value, the warning power is insufficient. The mathematical expression for the power balance assessment index within the island is as follows: in, For the total output of distributed power sources; Contribute to energy storage; This represents the total load within the isolated island.

8. A low-frequency load shedding device interlocking system for a distribution network considering the islanded operation and frequency oscillation characteristics of distributed generation sources, characterized in that, The device includes an interconnected processor and a computer-readable storage medium, wherein the computer-readable storage medium stores a computer program that is executed by the processor to implement the low-frequency load shedding device blocking method for a distribution network considering the islanded operation and frequency oscillation characteristics of distributed power sources as described in any one of claims 1 to 7.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program, which, when executed by a processor, implements the low-frequency load shedding device blocking method for distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources, as described in any one of claims 1 to 7.

10. A computer program product, characterized in that, The method includes a computer program that, when executed by a processor, implements the low-frequency load shedding device blocking method for distribution networks that considers the islanded operation and frequency oscillation characteristics of distributed power sources, as described in any one of claims 1 to 7.

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