A hybrid island detection method based on Lissajous curve and impedance identification

Abstract

The present invention discloses a hybrid island detection method based on Lissajous curve and impedance identification, which belongs to the technical field of grid-type energy storage converter on-grid switching. The present invention first constructs a model in which a grid-type energy storage and an RLC load are connected in parallel to a common coupling point PCC, collects the voltage and current at the PCC point and calculates the area and short axis of the Lissajous figure, and then compares the island detection index with the preset threshold. When it is less than the threshold, active detection is triggered, and an 8-fold frequency disturbance is injected into the line. The line impedance is then calculated by fast Fourier decomposition and the mean of ten samples is compared and judged. When the load power and the grid-type energy storage have zero mismatch and the quality factor is 2.5, the present invention can accurately identify the island within 200ms, the detection blind area is extremely small, the anti-interference ability is strong in the non-island state, the detection index threshold is flexibly set, and it has significant universality, which has strongly promoted the development of island detection technology.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-connected and off-grid switching of a grid-connected energy storage converter, and in particular to a hybrid islanding detection method based on Lissajous curve and impedance identification. Background Art

[0002] The proportion of renewable energy inputs into the power grid has greatly increased, but this has also brought new difficulties and challenges to the stability of the power system. Grid-connected energy storage has attracted widespread attention due to its advantages in smoothing power fluctuations, shaving peaks and filling valleys, and enhancing the stability of the power grid. Grid-connected energy storage inverters need to be able to operate both in a grid-connected state and in an off-grid state. Generally, PQ control is often used in the grid-connected state to ensure maximum output power, while VF control or VSG control is often used in the off-grid state to support voltage and frequency. Accurate and effective islanding detection technology is the key to achieving smooth on-grid and off-grid switching for grid-connected energy storage inverters.

[0003] Researchers both domestically and internationally have conducted in-depth research on islanding detection technology, which is primarily categorized as remote and local. Because remote islanding detection relies on communication technology and is relatively costly, it is not widely used in practice. Local islanding detection is further subdivided into active and passive methods. Active detection involves injecting disturbances into the power grid and monitoring the resulting changes in voltage or frequency. This method has a smaller blind spot, but may negatively impact power quality. In contrast, passive detection only monitors electrical parameters such as voltage, current, and frequency in the line, analyzing these changes to determine whether islanding is present. This method does not affect power quality, but it does have a larger blind spot, especially in the case of long-term non-islanding faults, which can lead to the risk of misjudgment. A commonly used islanding detection method is a hybrid approach that combines the advantages of passive and active detection. Using passive detection to trigger active detection reduces the blind spot of passive detection and eliminates the need for continuous disturbance injection, resulting in a minimal impact on power quality. Currently available hybrid islanding detection methods mostly use absolute values ​​as islanding detection indicators. Their effectiveness is highly correlated with grid parameters and it is difficult to detect unplanned islanding phenomena when the load power and converter are fully matched. Summary of the Invention

[0004] The purpose of the present invention is to provide a hybrid islanding detection method based on Lissajous curves and impedance identification. The method uses Lissajous figures to capture changes in voltage and current. When the area and short axis relativity indicators based on the Lissajous figures exceed a threshold, it is determined to be a suspected islanding situation and triggers active detection to inject 8 times the frequency disturbance into the line for impedance identification. If the time exceeds the threshold, it is determined to be in an islanding state.

[0005] To achieve the above object, the present invention provides a hybrid island detection method based on Lissajous curve and impedance identification, comprising the following steps:

[0006] S1. Construct a model of grid-type energy storage and RLC load. The load and grid-type energy storage are connected in parallel at the common coupling point PCC. Collect the voltage V at the grid-type energy storage access point PCC. pcc and current I pcc , calculate the area and short axis of the voltage and current Lissajous figure at this moment and record them as A1 and b1;

[0007] S2. Based on the area and short axis obtained at the current moment in step 1, read the area and short axis A0 and b0 of the voltage and current Lissajous figure stored at the previous moment, and calculate its islanding detection indicators IA and Ib;

[0008] S3: Make a judgment based on the island detection indicators IA and Ib obtained in step 2 and the passive detection threshold

[0009] S4. Determine whether to trigger a disturbance based on the judgment result of step 2;

[0010] S5. Take the average value of the line impedance obtained by ten samplings as the line impedance Z1 obtained at this moment and compare this value with the line impedance Z0 obtained at the previous moment to reduce the interference of false operation.

[0011] Preferably, in step S2, the island detection indicators IA and Ib based on the area and short axis of the voltage and current Lissajous figure are respectively expressed as:

[0012]

[0013] Preferably, in step S3, if both IA and Ib are less than a set threshold, it is determined to be a suspected islanding phenomenon and active detection is triggered. Otherwise, the area and short axis of the voltage-current Lissajous figure obtained at this moment are recorded as A0 and b0 and step S1 is continued.

[0014] Preferably, in step S4, if active detection is triggered, an 8-fold frequency disturbance signal with an amplitude of 3% of the rated voltage and a duration of 200ms is injected into the line from the voltage and current double closed loop. pcc and current I pcc Perform fast Fourier decomposition to obtain the amplitudes of the 8th harmonic content of voltage and current, recorded as V8 and I8 respectively, and compare them to obtain the line impedance value measured at the common coupling point.

[0015] Preferably, in step S5, if the impedance exceeds twice the last value, an islanding condition is determined to have occurred and the perturbation injection is stopped. If the perturbation injection does not exceed the set threshold within 200ms, an islanding condition is determined not to have occurred, the line impedance obtained at this moment is recorded as Z0, and step 1 is repeated to measure the voltage and current at the PCC point.

[0016] Therefore, the present invention adopts the above-mentioned hybrid island detection method based on Lissajous curve and impedance identification, which has the following beneficial effects:

[0017] (1) The present invention addresses the problems that other hybrid islanding detection methods fail when the load power and the converter are fully matched, and that the threshold value of the islanding detection indicator is difficult to set. Moreover, the present invention can reliably identify the islanding effect within 200ms even when there is zero mismatch between the load power and the grid-type energy storage and the quality factor is 2.5.

[0018] (2) This invention improves the hybrid islanding detection method, which has a small detection blind spot and can reliably detect even when the load power is very close to the grid-type energy storage; it also does not cause false operation in non-islanding conditions such as voltage sag. In addition, it uses a relative islanding detection indicator, and the threshold value is less correlated with grid parameters, making it universally applicable.

[0019] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the system structure of the grid-type energy storage in the present invention;

[0021] Figure 2 Schematic diagram of hybrid island detection and judgment logic block based on Lissajous-impedance identification in the present invention;

[0022] Figure 3 The voltage and current Lissajous figure during normal operation of the present invention;

[0023] Figure 4 A simplified diagram of the control structure for active impedance detection in the present invention;

[0024] Figure 5 The following are waveform diagrams of detection indicators when the load power and the converter have different matching degrees in the present invention, wherein (a) is the detection indicator waveform diagram of IA; (b) is the detection indicator waveform diagram of Ib; (c) is the detection indicator waveform diagram of IZ; (d) is the detection indicator waveform diagram of the off-grid signal;

[0025] Figure 6The figures are waveform diagrams of islanding detection indicators during grid load switching in the present invention, wherein (a) is the waveform diagram of islanding detection indicators of IA during capacitor switching; (b) is the waveform diagram of islanding detection indicators of IA during resistor switching; (c) is the waveform diagram of islanding detection indicators of Ib during capacitor switching; and (d) is the waveform diagram of islanding detection indicators of Ib during resistor switching. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0027] Unless otherwise defined, the technical or scientific terms used in the present invention shall have the usual meanings understood by persons of ordinary skill in the field to which the present invention belongs. The words "first", "second" and similar terms used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "include" or "comprise" mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0028] Example 1

[0029] The present invention provides a hybrid island detection method based on Lissajous curve and impedance identification. The specific steps are as follows:

[0030] Step S101: Build a grid-connected energy storage system that can be connected to and off-grid. The schematic diagram is as follows: Figure 1 As shown, the main grid consists of a three-phase voltage source and R g 、L g The energy storage device consists of a chemical energy storage battery and an energy storage converter. The energy storage battery can be regarded as a DC power supply with a constant voltage. This voltage source provides DC power with a power factor of about 1 to the energy storage converter. The voltage and current at the PCC point are respectively satisfied as v = V pcc sin(ωt) and i=I pcc sin(ωt+θ);

[0031] Step S102: construct a voltage-current Lissajous figure, using current as the abscissa and voltage as the ordinate. The voltage-current graph relationship can be obtained as follows:

[0032]

[0033] It can be obtained that the above formula satisfies the elliptic equation Ax 2 +Bxy+Cy 2 =D, where D=-sin 2 θ. The voltage and current graphs drawn during normal operation of the grid-type energy storage are as follows: Figure 2 As shown, the major axis a, minor axis b, area A(t) and tilt angle θ0 of the ellipse can be expressed as:

[0034]

[0035] Here, θ represents the power factor angle between voltage and current.

[0036] When the grid-type energy storage system operates normally, the voltage and current hardly change, that is, the voltage and current Lissajous figure does not change. If there is unplanned islanding, the voltage and current Lissajous figure will deviate from the original trajectory. The area and short axis calculated at this moment are recorded as A1 and b1, and the area and short axis calculated at the previous moment are recorded as A0 and b0. The islanding detection indicators IA and Ib based on the area and short axis of the voltage and current Lissajous figure are set as follows:

[0037]

[0038] like Figure 3 As shown in Figure 1, during normal operation, the area and minor axis of the Lissajous figure do not change significantly, and the values ​​of the island detection indicators IA and Ib are very close to 1. When islanding occurs, the island detection indicators will be significantly less than 1. Therefore, an appropriate threshold is set. When both the island detection indicators IA and Ib are less than the threshold, it is determined to be a suspected islanding condition, triggering active detection. Otherwise, detection continues and the calculated area and minor axis are recorded as the indicators at the previous moment.

[0039] Step S103: Determine whether to trigger a disturbance based on the judgment result of step 2. Figure 4 As shown in the figure, an 8-fold frequency disturbance with an amplitude of 3% of the rated voltage is superimposed on the reference voltage generated by the power outer loop. The injected disturbance signal generates a voltage response v at the grid connection point through the control link. o (t) and the current response i o (t), the voltage harmonic component v8 and current harmonic component i8 in the response signal are extracted by discrete Fourier transform and can be expressed as follows:

[0040]

[0041] Here, F represents the Fourier operator.

[0042] The PCC point impedance Z can be obtainedPCC The expression is:

[0043]

[0044] Among them, v 8,0 Represents the voltage harmonic signal when no disturbance is injected; i 8,0 represents the current harmonic signal when no disturbance is injected. The voltage and current harmonic peaks when no disturbance is injected are much smaller than the harmonic signals of the voltage and current response after disturbance injection. Therefore, the PCC point impedance can be approximately regarded as:

[0045]

[0046] When the grid-connected energy storage is in the grid-connected state, the line impedance measured by the PCC point is the grid line impedance Z g With load impedance Z L In parallel, the expression is:

[0047]

[0048] When off-grid, the line impedance measured by the PCC point is the load impedance Z L Because the grid line impedance is much smaller than the load impedance, the line impedance measured when connected to the grid is much smaller than the line impedance measured when off-grid.

[0049] To reduce the possibility of misjudgment due to sudden interference, the average impedance value obtained from ten samplings is taken as the line impedance Z1 at this moment, and Z0 represents the line impedance measured at the previous moment. The island detection index of the active detection part can be expressed as:

[0050]

[0051] If the islanding detection indicator IZ is greater than 1 for more than 20ms, an islanding phenomenon is determined. If the islanding detection indicator IZ is less than 1 or the duration does not exceed 20ms, it is determined to be a non-islanding fault or a false trigger. Repeat step 1 to continue detection and stop injecting disturbances to minimize the impact on the power quality of the power system.

[0052] Therefore, the island detection is realized by using Lissajous figures and impedance measurement.

[0053] When the power matching between the load and the converter is 50%, 100% and 125% respectively, the detection index signals obtained by the proposed island detection method are as follows: Figure 5 shown.

[0054] Depend on Figure 5(a)(b) shows that even if the load and converter power are completely matched, when there is a small fluctuation in voltage and current, the proposed method can capture the change and trigger active detection. The greater the mismatch between the load and converter power, the more drastic the changes in IA and Ib, the faster their values ​​fall below the threshold (red straight line), and the earlier the impedance identification is triggered. When there is zero mismatch between the load power and the converter power, that is, P = 100%, the values ​​of the detection indicators SA and Sb will also be lower than the threshold, and active detection can be reliably triggered within 10ms. Within 200ms of active detection injection, the active detection indicators such as Figure 5 (c) When the relative index IZ obtained by impedance identification under the three power mismatch conditions exceeds the threshold (red straight line), it is determined that an islanding situation has occurred. Figure 5 (d) shows the detection indicator waveform under the off-grid signal condition, indicating that the proposed method can effectively and reliably detect the islanding phenomenon under different load and grid-connected energy storage power matching conditions.

[0055] Example 2

[0056] like Figure 6 (a)~ Figure 6 As shown in (d), when a non-islanding fault occurs due to capacitive or resistive load switching, the voltage and current also fluctuate. The islanding detection indicators IA and Ib both decrease slightly, but remain below the set threshold, thus not triggering active detection and not identifying an islanding condition. This indicates that the proposed islanding detection method does not malfunction during non-islanding load switching faults.

[0057] Therefore, the present invention adopts the above-mentioned hybrid island detection method based on Lissajous curve and impedance identification. It addresses the problems that other hybrid island detection methods fail when the load power and the converter are fully matched, and the threshold of the island detection index is difficult to set. It can also achieve reliable identification of the island effect within 200ms even when there is zero mismatch between the load power and the grid-type energy storage and the quality factor is 2.5. It also improves the hybrid island detection method, which has a smaller detection blind spot and can be reliably detected even when the load power is very close to the grid-type energy storage; and will not malfunction in non-islanding conditions such as voltage sag. It also adopts a relative island detection index, and the threshold is less correlated with the grid parameters, which has universal applicability.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A hybrid islanding detection method based on Lissajous curve and impedance identification, characterized by: The following steps are involved: S1. Construct a model of grid-type energy storage and RLC load, where the load and grid-type energy storage are connected in parallel at the common coupling point (PCC). Collect the voltage V at the PCC point where the grid-connected energy storage is connected to the grid pcc and current I pcc , calculate the area and short axis of the voltage and current Lissajous figure at this moment and record them as A1 and b1; S2. Based on the area and short axis obtained at the current moment in step 1, read the area and short axis A0 and b0 of the voltage and current Lissajous figure stored at the previous moment, and calculate its islanding detection indicators IA and Ib; S3: Make a judgment based on the island detection indicators IA and Ib obtained in step 2 and the passive detection threshold S4. Determine whether to trigger a disturbance based on the judgment result of step 2; S5. Take the average value of the line impedance obtained by ten samplings as the line impedance Z1 obtained at this moment and compare this value with the line impedance Z0 obtained at the previous moment to reduce the interference of false operation.

2. The hybrid island detection method based on Lissajous curve and impedance identification according to claim 1, characterized in that: In step S2, the island detection indicators IA and Ib based on the area and short axis of the voltage and current Lissajous figure are set to be expressed as:

3. The hybrid island detection method based on Lissajous curve and impedance identification according to claim 1, characterized in that: In step S3, if both IA and Ib are less than the set threshold, it is determined to be a suspected islanding phenomenon and active detection is triggered; otherwise, the area and short axis of the voltage and current Lissajous figure obtained at this moment are recorded as A0 and b0 and step S1 is continued.

4. The hybrid island detection method based on Lissajous curve and impedance identification according to claim 1, characterized in that: In step S4, if active detection is triggered, a disturbance signal with an amplitude of 3% of the rated voltage and an 8-fold frequency increase of 200ms is injected into the line from the voltage and current dual closed loop; For the common coupling point voltage U pcc and current I pcc Perform fast Fourier decomposition to obtain the amplitudes of the 8th harmonic content of voltage and current, recorded as V8 and I8 respectively, and compare them to obtain the line impedance value measured at the common coupling point.

5. The hybrid island detection method based on Lissajous curve and impedance identification according to claim 4, characterized in that: In step S5, if the impedance value exceeds twice the last obtained line impedance value, it is determined that an islanding situation has occurred and the disturbance injection is stopped. If the injected disturbance does not exceed the set threshold within 200ms, it is determined that no islanding situation has occurred, the line impedance obtained at this moment is recorded as Z0, and step 1 is repeated to measure the voltage and current at the PCC point.

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