An active power disturbance-based active power distribution network island detection method

Abstract

The application discloses a kind of active power distribution network island detection methods based on active disturbance in the technical field of relay protection, the method utilizes periodic bidirectional small amplitude active disturbance to be injected to distributed power supply DG grid-connected inverter;By detecting the response of voltage at point of common coupling (PCC) to bidirectional disturbance, the natural deviation trend of voltage after island occurrence is judged;Then according to the trend, inject unidirectional large amplitude active disturbance in the corresponding direction, force the voltage at PCC to quickly exceed over / under voltage protection threshold, so as to realize island determination.The application predicts the voltage deviation direction by bidirectional small disturbance, solves the problem that the detection may fail due to the blind direction of traditional unidirectional active disturbance method, has the advantages of no detection blind area, fast detection speed, small power quality influence, significantly improves the reliability and rapidity of active power distribution network island detection.

Description

Technical Field

[0001] This invention relates to the field of relay protection technology and is applicable to islanding detection in active distribution networks, specifically an active distribution network islanding detection method based on active power disturbance. Background Technology

[0002] With the deepening implementation of the national "dual-carbon" strategic goal, the proportion of distributed generation (DG) in the distribution network, represented by photovoltaic and wind power, continues to rise. Islanding detection technology has become a major challenge restricting the high proportion of DG integration. Existing power disturbance detection methods lack the ability to pre-determine the direction of electrical quantity offset in the disturbance strategy design, and the disturbance injection has a certain degree of randomness, resulting in the detection speed failing to meet standard requirements, or failing to effectively identify disturbances even after the islanded network has stabilized.

[0003] To address the issue of blind disturbance injection, the offset direction can be pre-determined by the magnitude of the resistance change rate of the mean module (Gao Shuping, Guo Fangbin, Shao Meiyang, et al. Hybrid islanding detection method based on resistance change rate and active power [J]. Acta Energiae Solaris Sinica, 2024, 45(05):324-332.), and then active power can be injected in the corresponding direction to promote rapid voltage over-limit. In addition, the frequency offset direction can be determined based on the frequency change characteristics before and after islanding (Dai Zhihui, He Jingyuan, Yu Lirui, et al. Reactive power disturbance islanding detection method based on harmonic voltage mutation [J]. Journal of Electrical Engineering and Control, 2023, 27(03):10-20.), and then reactive current can be injected to enable rapid frequency over-limit. By directly injecting reactive current with periodic characteristic frequency (Zhong Cheng, Jing Tianjun, Yang Minghao. A new method for island detection based on periodic reactive current disturbance [J]. Journal of Electrical Engineering, 2014, 29(03):270-276.), and using the SDFT method to extract characteristic components, rapid detection of characteristic frequency fluctuations can be achieved.

[0004] Based on this, this invention designs an active power distribution network islanding detection method based on active power disturbance. It constructs a periodic active power disturbance function by analyzing the relationship between voltage and active power at the PCC (Power Control Center) under islanding conditions. By comparing the sum of the voltage differences between forward and reverse injection and the rated voltage, the voltage deviation direction is predicted, and then a large unidirectional active power disturbance is applied. The disturbance is gradually increased according to the voltage difference, causing the voltage to quickly exceed the limit, thus achieving islanding detection. Summary of the Invention

[0005] The purpose of this invention is to provide an active distribution network islanding detection method based on active power disturbance, which aims to solve the detection failure problem that may be caused by the blind selection of disturbance direction in the traditional active power disturbance method, and to achieve fast, reliable and blind-zone-free islanding detection.

[0006] To achieve the above objectives, the present invention provides the following technical solution: an active distribution network islanding detection method based on active power disturbance, comprising the following steps: Step S1: Detect the frequency and voltage of the common connection point at the DG output. If the frequency or voltage sampling data does not meet the preset setting threshold, directly trigger the protection action; otherwise, proceed to step S2. Step S2: Inject periodic bidirectional small-amplitude active power disturbances into the DG grid-connected inverter. If the voltage sampling data fails to meet the preset setting threshold again, the protection action will be triggered again; otherwise, proceed to step S3. Step S3: Detect the effective voltage value at the common coupling point PCC, and determine the natural offset trend of the voltage at PCC after islanding occurs based on the response of the effective voltage value to the periodic bidirectional small-amplitude active power disturbance. Step S4: Based on the natural offset trend in step S3, inject a unidirectional large-amplitude active power disturbance in the corresponding direction into the active power reference value in step S3, forcing the voltage at PCC to exceed the predetermined over / under voltage protection threshold. Step S5: When the voltage at PCC exceeds the over / under voltage protection threshold in step S4, islanding is determined to have occurred, and protection action is triggered.

[0007] Preferably, the periodic bidirectional small-amplitude active power disturbance function injected in step S2 is expressed as: in: U 0 represents the effective value of the 10kV system line voltage, which is 10kV. P ref1 This refers to periodic active power disturbances; P dis1 It represents the periodic positive active power disturbance. P dis2 It is a periodic negative active power disturbance.

[0008] Preferably, step S3, determining the natural shift trend of the voltage at PCC after islanding occurs, specifically further includes: if Δ U 1 * >△ U 2 * If the initial voltage after islanding is higher than the grid-connected voltage, then it is determined that the voltage after islanding is higher than the grid-connected voltage; if Δ U 1 * <△ U 2 * If the initial voltage after islanding is lower than the grid-connected voltage, then it is determined that the initial voltage after islanding is lower than the grid-connected voltage.

[0009] Preferably, the unidirectional large-amplitude active power disturbance function injected in step S4 is expressed as: in: P dis1To improve the periodic positive active power disturbance; P dis2 To improve the post-periodic negative active power disturbance, m It represents a one-way positive disturbance coefficient, which is negative. n It is a unidirectional negative disturbance coefficient, which is a positive number.

[0010] Preferably, the predetermined over / under voltage protection thresholds in step S5 include: a voltage upper limit threshold of 1.1 times the grid rated voltage. U 0, the lower voltage threshold is 0.88 times. U 0。

[0011] Preferably, the voltage reference value at the common coupling point PCC in step S3 U PCC * It is expressed by the following formula, and simplified to U* : in, This is the effective value of the voltage.

[0012] Preferably, the total voltage offset caused by the positive and negative active power disturbances in step S3 is Δ. U * It is expressed by the following formula: in: t 0 represents the initial time of the injected disturbance; T = t 1+ t 2, of which, t 1 represents the duration of the injected positive disturbance within one cycle; t 2 represents the duration of the injected negative disturbance within one cycle, Δ u t * for t The difference between the voltage at PCC at time t and the voltage at grid connection.

[0013] Preferably, in step S2, the active power reference value P ref The superimposed disturbance signal is determined by the relationship between the voltage at PCC and the injected active power, as shown in the following equation: in: P The initial active power of DG, Δ P To change the active power of DG, i.e., to inject active power.

[0014] Preferably, step S3 specifically involves: calculating the positive voltage offset Δ caused by the positive active power disturbance. U 1 * The negative voltage offset caused by the negative active power disturbance is ΔU 2 * If satisfied , where: △ U thr If the preset voltage offset value is 0.01, the disturbance is considered valid, and DG is in islanded state for further judgment of Δ. U 1 * With △ U 2 * If the size relationship is not clear, proceed to step S2; when △ U 1 * With △ U 2 * If they are equal, proceed to step S2; otherwise, proceed to step S4.

[0015] Compared with the prior art, the beneficial effects of the present invention are: (1) This method first injects periodic bidirectional small disturbances to actively detect the natural offset trend of the voltage after islanding, and then applies a large disturbance in the same direction according to the trend. This fundamentally solves the problem that the traditional unidirectional active power disturbance method may fail to detect due to the disturbance direction being opposite to the natural offset direction, and realizes reliable detection under any power balance conditions.

[0016] (2) This method is based on the existing active power control loop of the DG inverter and does not require additional hardware; the injected small disturbance amplitude is small and will not cause significant changes in electrical quantity due to grid clamping effect in grid-connected state, and has little impact on power quality. Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. 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.

[0018] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic diagram of the active power distribution network structure of the present invention; Figure 3 This is a schematic diagram of the island detection principle of the present invention; Figure 4 This is a diagram of the internal control structure of the DG inverter of the present invention; Figure 5 This is a schematic diagram of the voltage change at the PCC under active power disturbance in grid-connected and islanded scenarios according to the present invention; Figure 6 A schematic diagram illustrating the failure mechanism of the traditional unidirectional active power perturbation method; Figure 7The figure shows the simulation results of active power disturbance under grid-connected conditions; Figure 8 for P > P load A waveform diagram of the injected active power and the inverter output active power; Figure 9 for P > P load Time-injected perturbation U* Schematic diagram of the changes; Figure 10 for P < P load A waveform diagram of the injected active power and the inverter output active power; Figure 11 for P < P load Time-injected perturbation U* Schematic diagram of the changes. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In the description of this invention, it should be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0021] Example: like Figure 2 As shown, the active distribution network structure applied in this method includes distributed generation (DG), local loads, tie switches, and the main power grid. When the main grid is disconnected due to a fault, the DG may enter islanded operation. The islanding detection principle is as follows: Figure 3 As shown. The core idea of ​​the method proposed in this invention is: first, to determine the natural offset direction of the voltage after islanding by injecting a bidirectional small perturbation, and then to selectively inject a unidirectional large perturbation to force the voltage to exceed the limit. Specifically, it includes the following steps: Step S1: Detect the frequency and voltage of the common connection point at the DG output. If the frequency or voltage sampling data does not meet the preset setting threshold, directly trigger the protection action; otherwise, proceed to step S2. Step S2: Inject periodic bidirectional small-amplitude active power disturbances into the DG grid-connected inverter. If the voltage sampling data fails to meet the preset setting threshold again, the protection action will be triggered again; otherwise, proceed to step S3. Specifically, this invention utilizes the PQ control structure of a DG inverter, such as... Figure 4 As shown, at the active power reference value P ref The superimposed disturbance signal is determined by the relationship between the voltage at PCC and the injected active power, as shown in the following equation: in: P The initial active power of DG, Δ P To change the active power of DG, i.e., to inject active power; The injected periodic active power function is constructed as follows: in: U 0 represents the effective value of the 10kV system line voltage, which is 10kV. P ref1 This refers to periodic active power disturbances; P dis1 It represents the periodic positive active power disturbance. P dis2 To mitigate the impact of power quality and load switching fluctuations during grid connection, which could lead to misjudgments, a 2% voltage variation is designed to inject active power. Figure 5 This indicates the voltage change at PCC under active power disturbances in grid-connected and islanded scenarios. (Grid-connected scenario) U PCC * Constrained by the large power grid, it is largely unaffected by injected signals and remains at 1; after islanding... U PCC * Follow-up injection active P ref1 It changes with the environment; ideally, when the voltage before and after the islanding remains constant, the injected active power is ±0.04. P hour, U PCC * The change is (0.98~1.02), such as Figure 5 As shown in b; however, in actual operation, the voltage after islanding is affected by the load. P > P load At that time, UPCC * >1; P < P load hour, U PCC * <1, respectively as follows Figure 5 As shown in c and 5d; it can be seen from the figure that when the island U PCC * When the value is greater than 1, injecting periodic active power disturbances of equal magnitude but opposite direction reveals that the voltage deviation caused by the positive disturbance is more likely to exceed the predetermined voltage range; while islanding... U PCC * When <1, positive and negative periodic active power disturbances are injected, and the voltage deviation caused by the negative disturbance is more likely to exceed the predetermined voltage range.

[0022] Step S3: Detect the effective voltage value at the common coupling point PCC, and determine the natural offset trend of the voltage at PCC after islanding occurs based on the response of the effective voltage value to the periodic bidirectional small-amplitude active power disturbance. Specifically: Calculate the positive voltage offset caused by the positive active power disturbance as Δ U 1 * The negative voltage offset caused by the negative active power disturbance is Δ U 2 * If satisfied , where: △ U thr If the preset voltage offset value is 0.01, the disturbance is considered valid, and DG is in islanded state for further judgment of Δ. U 1 * With △ U 2 * If the size relationship is not clear, proceed to step S2; When △ U 1 * With △ U 2 * If they are equal, proceed to step S2; otherwise, proceed to step S4. During the injection of periodic disturbances, the positive voltage offset Δ caused by the positive disturbance is detected and recorded. U 1 * and the negative voltage offset Δ caused by the negative disturbance U 2 * Compare the sizes of the two; when △ U 1 * >△ U 2 *When the initial voltage after islanding is greater than the grid-connected voltage, a large positive active disturbance is injected again, causing the voltage to deviate rapidly and exceed the upper limit of the voltage threshold; when Δ U 1 * <△ U 2 * When the initial voltage after islanding is lower than the grid-connected voltage, a large negative active power disturbance is injected again, causing the voltage to deviate rapidly and exceed the lower limit of the voltage threshold. Figure 6 This visually demonstrates that blindly injecting unidirectional disturbances without determining the direction can lead to detection failure.

[0023] To quantitatively describe the above, the positive voltage offset caused by positive active power disturbance is defined as Δ. U 1 * The negative voltage offset caused by the negative active power disturbance is Δ U 2 * The total voltage deviation caused by positive and negative active power disturbances is Δ U * The relationship between the three can be described by the following formula: in: t 0 represents the initial time of the injected disturbance; T = t 1+ t 2, of which, t 1 represents the duration of the injected positive disturbance within one cycle; t 2 represents the duration of the injected negative disturbance within one cycle, Δ u t * for t The difference between the voltage at PCC at time t and the voltage at grid connection; If △ U 1 * With △ U 2 * satisfy , where: △ U thr The preset voltage offset value is 0.01, indicating that the disturbance is effective and DG is in an islanded state. The two values ​​are then compared.

[0024] Step S4: Based on the natural offset trend in step S3, inject a unidirectional large-amplitude active power disturbance in the corresponding direction into the active power reference value in step S3, forcing the voltage at PCC to exceed the predetermined over / under voltage protection threshold. Specifically: Based on the judgment result of step S3, a unidirectional large perturbation is constructed and injected, as shown in the following function: in: P dis1 To improve the periodic positive active power disturbance; Pdis2 To improve the post-periodic negative active power disturbance, m It represents a one-way positive disturbance coefficient, which is negative. n It is a unidirectional negative disturbance coefficient, which is a positive number.

[0025] Step S5: When the voltage at PCC exceeds the over / under voltage protection threshold in step S4, islanding is determined to have occurred, and protection action is triggered; Specifically: Continuously monitor the PCC voltage; when the voltage exceeds the over / under voltage protection upper limit threshold of 1.1... U 0 or below the lower threshold of 0.88 U At 0:00, an islanding event is detected, immediately triggering the DG output protection device to disconnect the DG from the line. The entire detection process is as follows: Figure 1 As shown.

[0026] Simulation verification: To verify this invention, an islanding detection module for active power disturbances was built in the electromagnetic simulation software PSCAD. The grid-connected inverter uses constant power control, and the DG and load parameters are shown in Table 1. Based on the algorithm presented in this paper, the periodic positive and negative small disturbance times are set to 0.1s, and the total disturbance period is 0.6s. The following section uses DG grid connection and OUV / OUF detection of islanding within the blind zone (…) P > P load , P < P load We will analyze three typical cases as examples.

[0027] Simulation results of grid connection status are as follows Figure 7 As shown, after the injection of a periodic active small disturbance, the voltage and current show no significant distortion, the frequency remains stable at 50Hz, and the electrical quantities are basically unaffected by the active small disturbance. U *The value is always 1, satisfying 0.88≤ U *≤1.1, △ U * The value is always 0, and no false alarm has been triggered.

[0028] P > P load Simulation results are as follows Figure 8 and Figure 9 As shown in the figure, the change in inverter output power lags slightly behind the injected signal. This is due to the common delays caused by the inner current loop and the PWM stage. (Before testing) U * It is 1.05 times the original, satisfying 0.88 ≤ U * ≤1.1; 1.5s periodic positive and negative small perturbations are injected into the DG. Pref1 And calculate △ U 1 * , △ U 2 * The value is compared with Δ after 0.2s. U 1 * , △ U 2 * All are greater than △ U thr And obtain △ U 1 * >△ U 2 * Inject positive large disturbance P dis1 ', satisfy within 0.1s U * ≥1.1, detect islands.

[0029] P < P load Simulation results are as follows Figure 10 and Figure 11 As shown, before detection U * It is 0.97 times the original value, satisfying 0.88 ≤ U * ≤1.1; 1.5s periodic positive and negative small perturbations are injected into the DG. P ref1 And calculate △ U 1 * , △ U 2 * The value is compared with Δ after 0.2s. U 1 * , △ U 2 * All are greater than △ U thr And obtain △ U 1 * <△ U 2 * Injecting a large reverse disturbance P dis1 ', satisfy within 0.1s U ** ≤0.88, island detection.

[0030] Table 1 Test Submodule Table ; Simulation analysis under different operating conditions after injecting active power disturbances demonstrates the speed and effectiveness of the proposed islanding detection algorithm. Compared with existing power disturbance islanding detection methods, the proposed method meets the detection speed requirements, accurately identifies islands after they stabilize, effectively reduces the detection blind zone, and exhibits superior overall performance.

[0031] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0032] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for detecting islanding in an active distribution network based on active power disturbance, characterized in that, Includes the following steps: Step S1: Detect the frequency and voltage of the common connection point at the DG output. If the frequency or voltage sampling data does not meet the preset setting threshold, directly trigger the protection action; otherwise, proceed to step S2. Step S2: Inject periodic bidirectional small-amplitude active power disturbances into the DG grid-connected inverter. If the voltage sampling data fails to meet the preset setting threshold again, the protection action will be triggered again; otherwise, proceed to step S3. Step S3: Detect the effective voltage value at the common coupling point PCC, and determine the natural offset trend of the voltage at PCC after islanding occurs based on the response of the effective voltage value to the periodic bidirectional small-amplitude active power disturbance. Step S4: Based on the natural offset trend in step S3, inject a unidirectional large-amplitude active power disturbance in the corresponding direction into the active power reference value in step S3, forcing the voltage at PCC to exceed the predetermined over / under voltage protection threshold. Step S5: When the voltage at PCC exceeds the over / under voltage protection threshold in step S4, islanding is determined to have occurred, and protection action is triggered.

2. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: The periodic bidirectional small-amplitude active power disturbance function injected in step S2 is expressed as: ； in: U 0 represents the effective value of the 10kV system line voltage, which is 10kV. P ref1 This refers to periodic active power disturbances; P dis1 It represents the periodic positive active power disturbance. P dis2 It is a periodic negative active power disturbance.

3. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: The step S3, determining the natural shift trend of the voltage at PCC after islanding occurs, specifically includes: if Δ U 1 * >△ U 2 * If the initial voltage after islanding is higher than the grid-connected voltage, then it is determined that the voltage after islanding is higher than the grid-connected voltage; if Δ U 1 * <△ U 2 * If the initial voltage after islanding is lower than the grid-connected voltage, then it is determined that the initial voltage after islanding is lower than the grid-connected voltage.

4. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: The unidirectional large-amplitude active power disturbance function injected in step S4 is expressed as follows: ； in: P dis1 To improve the periodic positive active power disturbance; P dis2 To improve the post-periodic negative active power disturbance, m It represents a unidirectional positive disturbance coefficient, which is negative. n It is a unidirectional negative disturbance coefficient, which is a positive number.

5. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: The predetermined over / undervoltage protection thresholds in step S5 include: an upper voltage limit threshold of 1.1 times the rated grid voltage. U 0, the lower voltage threshold is 0.88 times. U 0。 6. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: The voltage reference value at the common coupling point PCC in step S3 U PCC * It is expressed by the following formula, and simplified to U* : ； in, This is the effective value of the voltage.

7. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: The total voltage offset caused by the positive and negative active power disturbances in step S3 is Δ. U * It is expressed by the following formula: ； in: t 0 represents the initial time of the injected disturbance; T = t 1+ t 2, of which, t 1 represents the duration of the injected positive disturbance within one cycle; t 2 represents the duration of the injected negative disturbance within one cycle, Δ u t * for t The difference between the voltage at PCC at time t and the voltage at grid connection.

8. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: In step S2, the active power reference value P ref The superimposed disturbance signal is determined by the relationship between the voltage at PCC and the injected active power, as shown in the following equation: ； in: P For the initial active power of DG, Δ P To change the active power of DG, i.e., to inject active power.

9. The active distribution network islanding detection method based on active power disturbance according to claim 1, characterized in that: Step S3 specifically involves: calculating the positive voltage offset Δ caused by the positive active power disturbance. U 1 * The negative voltage offset caused by the negative active power disturbance is Δ U 2 * If satisfied , where: △ U thr If the preset voltage offset value is 0.01, the disturbance is considered valid, and DG is in islanded state for further determination of Δ. U 1 * With △ U 2 * If the size relationship is not clear, proceed to step S2; when △ U 1 * With △ U 2 * If they are equal, proceed to step S2; otherwise, proceed to step S4.

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