Hybrid island detection method based on Lissajous curve and impedance recognition

Abstract

The invention discloses a hybrid island detection method based on a Lissajous curve and impedance identification, and belongs to the technical field of grid-connected and off-grid switching of a grid-forming type energy storage converter. The method comprises the following steps: firstly, constructing a model in which network-forming type energy storage and RLC loads are connected in parallel to a point of common coupling (PCC), collecting voltage and current of the PCC point, calculating the area and the short axis of a Lissajous figure, comparing an island detection index with a preset threshold value, triggering active detection when the island detection index is smaller than the threshold value, and injecting eight-frequency disturbance to a line; and calculating the line impedance through fast Fourier decomposition, and comparing and judging by taking a ten-time sampling mean value. According to the method, when the load power is in zero mismatch with the network-forming type energy storage and the quality factor is 2.5, the island can be accurately identified within 200ms, the detection blind area is extremely small, the anti-interference capability is high in a non-island state, the detection index threshold value is flexibly set, the method has remarkable universality, and the development of the island detection technology is powerfully promoted.

Description

Technical Field

[0001] The 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 island detection method based on Lissajous curve and impedance identification. Background Art

[0002] Under the background of "dual carbon", the proportion of new energy input into the power grid has greatly increased, but it has also brought new difficulties and challenges to the stability of the power system. Grid-connected energy storage has attracted widespread attention because of its advantages of 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 work both in grid-connected and off-grid states. Generally, PQ control is often used in grid-connected conditions to ensure maximum output power. VF control or VSG control is often used off-grid to support voltage and frequency. Accurate and effective island detection technology is the key to achieving smooth switching between grid-connected and off-grid energy storage inverters.

[0003] Researchers at home and abroad have conducted in-depth research on island detection technology, which is mainly divided into two categories: remote and local. Since remote island detection technology relies on communication technology and is costly, it is not common in practical applications. Local island detection technology is further divided into two detection methods: active and passive. The active detection method injects disturbances into the power grid and monitors the response changes of voltage or frequency for detection. This method has a small detection blind area, but may have a certain negative impact on the power quality. In contrast, the passive detection method only monitors electrical parameters such as voltage, current and frequency in the line, and determines whether there is an island phenomenon by analyzing the changes in these parameters. This method does not affect the power quality, but the detection blind area is large, especially in the case of long-term non-islanding faults, there is a risk of misjudgment. The commonly used island detection method is a hybrid island detection method that combines the advantages of passive detection and active detection. Passive detection is used to trigger active detection, which reduces the detection blind area of ​​passive detection and does not require constant injection of disturbances, which has little impact on the power quality of the power grid. 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 island detection method based on Lissajous curve and impedance identification, which uses Lissajous figures to capture the changes in voltage and current. When the area and short axis relativity indicators based on the Lissajous figure exceed the threshold, it is determined to be a suspected island situation and triggers active detection to inject 8 times the frequency disturbance into the line for impedance identification. If the threshold time is exceeded, it is determined to be in an island 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 at this moment obtained 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 island 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 threshold of passive detection

[0009] S4, determining 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 as 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.

[0014] Preferably, 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 is injected into the line from the voltage and current double closed loop for 200ms. pcc and current I pcc Perform fast Fourier decomposition to obtain the amplitudes of the 8th harmonic content of voltage and current, which are recorded as V8 and I8 respectively, and then compare them to obtain the line impedance value measured at the common coupling point.

[0015] Preferably, in step S5, if it exceeds twice the line impedance value obtained last time, it is determined that an islanding situation occurs and the injection of disturbance is stopped. If the injected disturbance does not exceed the set threshold within 200ms, it is determined that no islanding situation occurs, 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 of the islanding detection index is difficult to set. Moreover, the present invention can reliably identify the islanding effect within 200 ms even when there is zero mismatch between the load power and the grid-connected energy storage and the quality factor is 2.5.

[0018] (2) The present invention improves the hybrid island detection method, which has a small detection blind spot and can reliably detect even if the load power is very close to the grid-type energy storage; and will not cause false operation in non-islanding conditions such as voltage sag. In addition, the relative island detection index is adopted, and the threshold is less correlated with the grid parameters, which is universal.

[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 It is a schematic diagram of the system structure of the grid-type energy storage in the present invention;

[0021] Figure 2 It is a schematic diagram of a 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 schematic diagram of the control structure for active impedance detection in the present invention;

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

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

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

[0027] Unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words 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 described object 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, and the specific steps are as follows:

[0030] Step S101, build a grid-connected energy storage system that can be connected to and off the grid. The structural diagram is as follows: Figure 1 As shown, the main power grid consists of a three-phase voltage source and R g , L g The line impedance is composed of a chemical energy storage battery and an energy storage converter. The energy storage battery can be regarded as a DC power source with a constant voltage. This voltage source provides the energy storage converter with a power factor of about 1. 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, constructing a voltage-current Lissajous figure, taking the current as the abscissa and the voltage as the ordinate, the voltage-current plotting 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 inclination 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 will hardly change, that is, the voltage and current Lissajous figure will not change: if there is an unplanned island phenomenon, 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 island 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 the figure, during normal operation, the area and short axis of the Lissajous figure will not change significantly, and the values ​​of the island detection indicators IA and Ib are very close to 1; when an island phenomenon occurs, the island detection indicator will be significantly less than 1. Therefore, a suitable corresponding threshold is set. When the island detection indicators IA and Ib are both less than the threshold, it is determined to be a suspected island condition and active detection is triggered; otherwise, the detection continues and the area and short axis calculated this time are recorded as the indicators of 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, which 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 occurs, 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 the grid;

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

[0050]

[0051] When the island detection index IZ is greater than 1 for more than 20ms, it is judged that an island phenomenon occurs. If the island detection index IZ is less than 1 or the time does not exceed 20ms, it is judged that a non-island fault or false trigger occurs, and step 1 is repeated to continue detection and stop injecting disturbances to reduce the impact on the power quality of the power system.

[0052] Therefore, the method of using Lissajous figures and impedance measurement is realized to realize island detection.

[0053] When the power matching degree 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) It can be seen that even if the load and converter power are perfectly matched and there are small fluctuations 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) In the three power mismatch cases, the relative index IZ obtained by impedance identification exceeds the threshold (red straight line), which means that an islanding situation occurs. Figure 5 (d) shows the detection index 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 with capacitive or resistive load switching, the voltage and current will also fluctuate. It can be seen that the islanding detection indicators IA and Ib have a small decrease, but they are still not lower than the set threshold, and will not trigger active detection, that is, it will not be judged as an islanding situation. This shows that when a non-islanding fault occurs with load switching, the proposed islanding detection method will not malfunction.

[0057] Therefore, the present invention adopts the above-mentioned hybrid island detection method based on Lissajous curve and impedance identification, which is aimed at the problem 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. And the hybrid island detection method is improved, which has a smaller detection blind spot, and can be reliably detected even if the load power is very close to the grid-type energy storage; and will not malfunction in non-islanding states such as voltage sag. And using a relative island detection index, the threshold is less correlated with the grid parameters and has universal applicability.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. 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 solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A hybrid island detection method based on Lissajous curve and impedance identification, characterized in that: The following steps are involved: 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; S2. Based on the area and short axis at this moment obtained 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 island detection indicators IA and Ib; S3: Make a judgment based on the island detection indicators IA and Ib obtained in step 2 and the threshold of passive detection S4, determining 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 as 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 is injected into the line from the voltage and current double closed loop for 200ms. pcc and current I pcc Perform fast Fourier decomposition to obtain the amplitudes of the 8th harmonic content of voltage and current, which are recorded as V8 and I8 respectively, and then 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 it exceeds twice the line impedance value obtained last time, it is determined that an islanding situation occurs and the injection of disturbance is stopped. If the injection of disturbance does not exceed the set threshold within 200ms, it is determined that no islanding situation occurs, and 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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