A grid impedance on-line measurement method and device based on a three-phase LCL grid-connected inverter

Abstract

The application discloses a grid impedance on-line measurement method and device based on a three-phase LCL grid-connected inverter, wherein the method comprises the following steps: locking the phase information of grid voltage and current through an inverter control structure, and injecting a disturbance signal composed of a single pulse signal and a high-frequency sinusoidal signal at the current peak; synchronously measuring the grid-connected point voltage and current; point-by-point subtracting the data to eliminate the influence of grid background harmonics, and performing spectrum analysis; calculating the positive sequence and negative sequence components of the voltage and current; and finally calculating the positive sequence impedance and negative sequence impedance of the grid. Compared with the original single pulse signal, the present application can enhance the accuracy of medium and high frequency measurement by superimposing a high-frequency sinusoidal signal as a disturbance signal on the basis of the original single pulse signal, and solve the problem of large grid impedance measurement error caused by the spectrum attenuation of the existing single pulse signal in the medium and high frequency components.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, specifically to a method and apparatus for online measurement of grid impedance based on a three-phase LCL grid-connected inverter. Background Technology

[0002] Against the backdrop of new power system construction, with the development of applications such as grid-connected power generation from new energy sources, flexible DC transmission, reactive power compensation, and AC drives, power electronics technology is widely used in the power system field. Among these applications, grid-connected inverters, as important energy conversion interface devices, are seeing a gradual increase in their proportion. In the process of grid-connected inverters converting AC or DC power generated from new energy sources such as solar and wind power into AC power and transmitting it to the grid, the grid and the grid-connected inverter form a dynamic interactive system. They influence each other, and with the large-scale integration of new energy sources into the grid, the interaction between the grid-connected inverter and the grid is becoming increasingly frequent, and the oscillation risk of the entire system is also increasing significantly. Therefore, in order to ensure normal energy exchange between the grid-connected inverter and the grid, it is essential to analyze the interaction stability between them.

[0003] Currently, most assessments of grid-connected system stability rely on impedance analysis. Therefore, accurate inverter output impedance models and grid impedance information are crucial for system stability analysis. Since the parameters of grid-connected inverters are largely fixed after design and do not easily change, their output impedance models can be theoretically derived. However, for the power grid, many power electronic devices are located far from transformers, connected at the end of lines. From an impedance perspective, the portion outside these devices constitutes grid impedance, including numerous input filters and cables. Therefore, grid impedance cannot be simply treated as an inductive property. Furthermore, the randomness of load switching alters grid impedance, making it impossible to directly obtain it through modeling. For example, in a power system with multiple grid-connected inverters, the grid-side equivalent impedance, viewed from the output side of a single inverter, is represented as the parallel combination of grid impedance and other inverter output impedances. As inverters are switched on and off and their operating conditions change, the grid-side equivalent impedance also changes. Therefore, grid impedance cannot be directly obtained through modeling. Faced with complex and ever-changing power grid conditions, accurate power grid impedance information can only be obtained by measuring the power grid impedance online in real time. Only with accurate power grid impedance information can we provide a strong basis for subsequent stability analysis of the grid-connected system and research on measures to ensure stable system operation.

[0004] Existing measurement methods mostly use injected pulse signals to measure grid impedance because pulse signals have rich spectral content, and only one injection is needed to measure the grid impedance of a frequency range. However, since the spectral energy gradually decreases as the frequency increases, this method can only achieve accurate measurement in the low-frequency range, and the measurement error is large in the mid-to-high frequency range. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and device for online measurement of grid impedance based on a three-phase LCL grid-connected inverter, so as to reduce measurement errors in the mid-to-high frequency range and improve the accuracy of online measurement of grid impedance.

[0006] To address the aforementioned technical problems, this invention provides a method for online measurement of grid impedance based on a three-phase LCL grid-connected inverter, comprising:

[0007] Step S1: The phase information of the grid voltage and current is latched by the phase-locked loop in the inverter control structure, and a disturbance signal is injected when the current is at its peak. The disturbance signal is formed by superimposing a single pulse signal and a high-frequency sine wave signal at the reference amplitude of the inverter current control inner loop.

[0008] Step S2: After the disturbance signal is injected, the grid connection point voltage and current in the period where the disturbance signal is injected and the period where the adjacent non-injected disturbance signal is injected are simultaneously measured by voltage and current sensors.

[0009] Step S3: Subtract the measurement results of the above two cycles point by point to obtain data that eliminates the influence of power grid background harmonics, and perform spectrum analysis on the obtained data;

[0010] Step S4: Calculate the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point obtained from the spectrum analysis.

[0011] Step S5: Based on the calculated positive-sequence voltage and current and negative-sequence voltage and current, calculate the positive-sequence impedance and negative-sequence impedance of the power grid.

[0012] Preferably, the disturbance signal is composed of a negative single pulse signal with an amplitude equal to the peak value of the grid connection point current and a width of 0.01s, and a sine wave signal with a peak value 0.3 times that of the peak value of the grid connection point current and a frequency of 2kHz.

[0013] Preferably, after determining the time of the injected disturbance, the time span from the trough before the time to the trough after the time is taken as the period of the injected disturbance signal, and the period before this period is the period of the adjacent non-injected disturbance signal.

[0014] Preferably, step S4 calculates the positive and negative sequence components of the voltage and current frequency domain information at the grid connection point according to the following formula:

[0015]

[0016]

[0017] Among them, f k U represents the frequency corresponding to the spectrum obtained from the FFT analysis. pos (j2πf k () represents the frequency f k Positive sequence voltage at point, U neg (j2πf k () represents the frequency f k The negative sequence voltage at point a, where a is a 120° vector operator, ΔU POIa (j2πf k () represents the frequency f k Phase a AC side voltage at point ΔU POIb (j2πf k () represents the frequency f k Phase b AC side voltage, ΔU POIc (j2πf k () represents the frequency f k The AC side voltage of phase c at point I pos (j2πf k () represents the frequency f k The positive sequence current at the point, I neg (j2πf k () represents the frequency f k The negative sequence current at point ΔI POIa (j2πf k () represents the frequency f k The phase current at point a, ΔI POIb (j2πf k () represents the frequency f k The phase b current at point b, ΔI POIc (j2πf k () represents the frequency f k The c-phase current at point [location].

[0018] Preferably, step S5 calculates the positive-sequence impedance and negative-sequence impedance of the power grid according to the following formula:

[0019]

[0020]

[0021] Among them, |Z pos (j2πf k )|、∠Z pos (j2πf k ) are the positive sequence impedances of the power grid at frequency f kAmplitude and phase information at |U pos (j2πf k )|、∠U pos (j2πf k ) are the positive sequence voltages at frequency f k Amplitude and phase information at the location, |I pos (j2πf k )|、∠I pos (j2πf k ) are the positive sequence currents at frequency f k Amplitude and phase information at the location; |Z neg (j2πf k )|、∠Z neg (j2πf k ) are the negative sequence impedances of the power grid at frequency f k Amplitude and phase information at |U neg (j2πf k )|、∠U neg (j2πf k ) are the negative sequence voltages at frequency f k Amplitude and phase information at the location, |I neg (j2πf k )|、∠I neg (j2πf k ) are the negative sequence currents at frequency f k The amplitude and phase information at the location.

[0022] Preferably, when the grid-connected device is connected to a system with a short-circuit ratio greater than 20, the grid impedance measurement as described in steps S1-S5 is performed once a week; if it is connected to a system with a short-circuit ratio less than 20, the grid impedance measurement as described in steps S1-S5 is performed once a day.

[0023] The present invention also provides an online grid impedance measurement device based on a three-phase LCL grid-connected inverter, comprising:

[0024] The injection module is used to latch the phase information of the grid voltage and current through the phase-locked loop in the inverter control structure, and to inject a disturbance signal when the current is at its peak; the disturbance signal is formed by superimposing a single pulse signal and a high-frequency sine wave signal at the reference amplitude of the inverter current control inner loop;

[0025] The acquisition module is used to synchronously measure the grid connection point voltage and current of the period in which the injected disturbance signal is located and the period in which the adjacent non-injected disturbance signal is located through voltage and current sensors after the disturbance signal is injected.

[0026] The spectrum analysis module is used to subtract the measurement results of the above two cycles point by point to obtain data that eliminates the influence of background harmonics of the power grid, and to perform spectrum analysis on the obtained data;

[0027] The first calculation module is used to calculate the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point obtained by spectrum analysis.

[0028] The second calculation module is used to calculate the positive-sequence impedance and negative-sequence impedance of the power grid based on the calculated positive-sequence voltage and current and negative-sequence voltage and current.

[0029] Preferably, the disturbance signal is composed of a negative single pulse signal with an amplitude equal to the peak value of the grid connection point current and a width of 0.01s, and a sine wave signal with a peak value 0.3 times that of the peak value of the grid connection point current and a frequency of 2kHz superimposed. After determining the time of the injected disturbance, the time span from the trough before the time to the trough after the time is taken as the period in which the injected disturbance signal is located, and the period before this period is the period in which the adjacent uninjected disturbance signal is located.

[0030] Preferably, step S4 calculates the positive and negative sequence components of the voltage and current frequency domain information at the grid connection point according to the following formula:

[0031]

[0032]

[0033] Among them, f k U represents the frequency corresponding to the spectrum obtained from the FFT analysis. pos (j2πf k () represents the frequency f k Positive sequence voltage at point, U neg (j2πf k () represents the frequency f k The negative sequence voltage at point a, where a is a 120° vector operator, ΔU POIa (j2πf k () represents the frequency f k Phase a AC side voltage at point ΔU POIb (j2πf k () represents the frequency f k Phase b AC side voltage, ΔU POIc (j2πf k () represents the frequency f k The AC side voltage of phase c at point I pos (j2πf k () represents the frequency f k The positive sequence current at the point, I neg (j2πf k () represents the frequency f k The negative sequence current at point ΔIPOIa (j2πf k () represents the frequency f k The phase current at point a, ΔI POIb (j2πf k () represents the frequency f k The phase b current at point b, ΔI POIc (j2πf k () represents the frequency f k The c-phase current at point [location].

[0034] Preferably, step S5 calculates the positive-sequence impedance and negative-sequence impedance of the power grid according to the following formula:

[0035]

[0036]

[0037] Among them, |Z pos (j2πf k )|、∠Z pos (j2πf k ) are the positive sequence impedances of the power grid at frequency f k Amplitude and phase information at |U pos (j2πf k )|、∠U pos (j2πf k ) are the positive sequence voltages at frequency f k Amplitude and phase information at the location, |I pos (j2πf k )|、∠I pos (j2πf k ) are the positive sequence currents at frequency f k Amplitude and phase information at the location; |Z neg (j2πf k )|、∠Z neg (j2πf k ) are the negative sequence impedances of the power grid at frequency f k Amplitude and phase information at |U neg (j2πf k )|、∠U neg (j2πf k ) are the negative sequence voltages at frequency f k Amplitude and phase information at the location, |I neg (j2πf k )|、∠I neg (j2πf k ) are the negative sequence currents at frequency f k The amplitude and phase information at the location.

[0038] The present invention offers the following advantages: The disturbance signal is a high-frequency sinusoidal signal superimposed on the original single-pulse signal, enhancing the accuracy of mid-to-high frequency measurements and solving the problem of significant grid impedance measurement errors caused by the attenuation of mid-to-high frequency components in existing single-pulse signals. The combination of single-pulse and high-frequency sinusoidal signals covers a wider frequency range, enabling the system to detect more subtle impedance changes. By using a phase-locked loop (PLL) in the inverter control structure and appropriate signal processing, the invention can be implemented on existing hardware and control structures, thus reducing the overall system cost. Accurate grid impedance measurement allows for more precise control of the inverter output and grid matching, reducing system instability risks. Accurate impedance information facilitates appropriate equipment adjustments to adapt to grid changes, thereby enhancing the stability and reliability of the entire system. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art 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.

[0040] Figure 1 This is a flowchart illustrating an online grid impedance measurement method based on a three-phase LCL grid-connected inverter, according to Embodiment 1 of the present invention.

[0041] Figure 2 This is a schematic diagram illustrating the specific process of an online grid impedance measurement method based on a three-phase LCL grid-connected inverter according to Embodiment 1 of the present invention.

[0042] Figure 3 This is a schematic diagram of the topology of a three-phase LCL grid-connected inverter in an embodiment of the present invention.

[0043] Figure 4 This is a schematic diagram of the control structure and disturbance injection of a three-phase LCL grid-connected inverter in an embodiment of the present invention.

[0044] Figure 5 This is a current waveform diagram of phase a grid connection point after the injection of disturbance in an embodiment of the present invention.

[0045] Figure 6 These are the time-domain waveform and spectral distribution diagrams of a single-pulse signal and a single-pulse signal superimposed with a high-frequency sine wave, as shown in the embodiments of the present invention.

[0046] Figure 7a This is a schematic diagram of the positive sequence impedance measurement results of the power grid under traditional single-pulse disturbance injection. Figure 7bThis is a schematic diagram of the power grid positive sequence impedance measurement results under disturbance injection according to an embodiment of the present invention. Detailed Implementation

[0047] The following description of the embodiments is taken with reference to the accompanying drawings, which illustrate specific embodiments in which the invention can be implemented.

[0048] Please refer to Figure 1 and Figure 2 As shown, Embodiment 1 of the present invention provides a method for online measurement of grid impedance based on a three-phase LCL grid-connected inverter, comprising:

[0049] Step S1: The phase information of the grid voltage and current is latched by the phase-locked loop in the inverter control structure, and a disturbance signal is injected when the current is at its peak. The disturbance signal is formed by superimposing a single pulse signal and a high-frequency sine wave signal at the reference amplitude of the inverter current control inner loop.

[0050] Step S2: After the disturbance signal is injected, the grid connection point voltage and current in the period where the disturbance signal is injected and the period where the adjacent non-injected disturbance signal is injected are simultaneously measured by voltage and current sensors.

[0051] Step S3: Subtract the measurement results of the above two cycles point by point to obtain data that eliminates the influence of power grid background harmonics, and perform spectrum analysis on the obtained data;

[0052] Step S4: Calculate the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point obtained from the spectrum analysis.

[0053] Step S5: Based on the calculated positive-sequence voltage and current and negative-sequence voltage and current, calculate the positive-sequence impedance and negative-sequence impedance of the power grid.

[0054] As can be seen from the above steps, although the original single-pulse signal can achieve accurate measurement at low frequencies, its spectral energy attenuation is large in the mid-to-high frequency range, resulting in low measurement accuracy. The disturbance signal in this embodiment of the invention is a high-frequency sine wave superimposed on the original single-pulse signal, which can enhance the accuracy of mid-to-high frequency measurements and solve the problem of large measurement errors in power grid impedance caused by the attenuation of the mid-to-high frequency components of the existing single-pulse signal spectrum.

[0055] The three-phase LCL grid-connected inverter used in this embodiment of the invention is as follows: Figure 3 As shown. Figure 4 As shown, in step S1, a disturbance signal is superimposed at the reference amplitude of the d-axis of the grid-connected inverter current loop controller. To avoid the disturbance signal having an excessive impact on the normal operation of the grid-connected inverter, the disturbance signal is injected at the maximum positive amplitude of the sinusoidal signal, as shown below. Figure 5 As shown, the timing of the injected disturbance is determined using a phase-locked loop.

[0056] The disturbance signal is composed of a negative single-pulse signal with an amplitude equal to the peak value of the grid-connected current and a width of 0.01s, superimposed with a sine wave signal with a peak value 0.3 times that of the peak value of the grid-connected current and a frequency of 2kHz. This ensures accurate measurement even at mid-to-high frequencies. The single-pulse spectrum diagram and the spectrum diagram of the pulse and high-frequency sine wave signal proposed in this invention are shown below. Figure 6 As shown.

[0057] The inverter controller itself contains voltage and current sensors, which are responsible for collecting voltage and current data at the grid connection point. In step S2 of this embodiment, the voltage and current sensors are used directly to collect the voltage and current at the grid connection point.

[0058] After determining the time of the injected disturbance, the time span from the trough before (0.01s before) to the trough after (0.01s after) is taken as the period of the injected disturbance signal (one period is 0.02s). The period before this period is the period of the adjacent non-injected disturbance signal.

[0059] In step S3, the voltage and current of adjacent periods without injected perturbation are subtracted point by point from the voltage and current of the injected perturbation period, and then spectral analysis is performed on the processed voltage and current data. To improve computational efficiency, FFT is chosen for spectral analysis. FFT is common knowledge in this field and will not be elaborated further.

[0060] Step S4 calculates the positive and negative sequence components of the voltage and current at the grid connection point based on the frequency domain information obtained after spectrum analysis. The calculation formula is as follows:

[0061]

[0062]

[0063] Among them, f k U represents the frequency corresponding to the spectrum obtained from the FFT analysis. pos (j2πf k () represents the frequency f k Positive sequence voltage at point, U neg (j2πf k () represents the frequency f k The negative sequence voltage at point a, where a is a 120° vector operator, ΔU POIa (j2πf k () represents the frequency f k Phase a AC side voltage at point ΔU POIb (j2πf k () represents the frequency f k Phase b AC side voltage, ΔU POIc (j2πf k () represents the frequency f k The AC side voltage of phase c at point Ipos (j2πf k () represents the frequency f k The positive sequence current at the point, I neg (j2πf k () represents the frequency f k The negative sequence current at point ΔI POIa (j2πf k () represents the frequency f k The phase current at point a, ΔI POIb (j2πf k () represents the frequency f k The phase b current at point b, ΔI POIc (j2πf k () represents the frequency f k The c-phase current at point [location].

[0064] Step S5: Based on the calculated positive-sequence voltage and current, and negative-sequence voltage and current, calculate the positive-sequence impedance and negative-sequence impedance of the power grid. The calculation formula is as follows:

[0065]

[0066]

[0067] Among them, |Z pos (j2πf k )|、∠Z pos (j2πf k ) are the positive sequence impedances of the power grid at frequency f k Amplitude and phase information at |U pos (j2πf k )|、∠U pos (j2πf k ) are the positive sequence voltages at frequency f k Amplitude and phase information at the location, |I pos (j2πf k )|、∠I pos (j2πf k ) are the positive sequence currents at frequency f k Amplitude and phase information at the location; |Z neg (j2πf k )|、∠Z neg (j2πf k ) are the negative sequence impedances of the power grid at frequency f k Amplitude and phase information at |U neg (j2πf k )|、∠U neg (j2πf k ) are the negative sequence voltages at frequency f kAmplitude and phase information at the location, |I neg (j2πf k )|、∠I neg (j2πf k ) are the negative sequence currents at frequency f k The amplitude and phase information at the location.

[0068] Repeat the above steps at the set time intervals to measure the grid impedance. Specifically, when the grid-connected device is connected to a system with a short-circuit ratio greater than 20, the switching of equipment has little impact on system stability, and the device can be set to measure the grid impedance once a week. If connected to a system with a short-circuit ratio less than 20, the grid stability is lower and the probability of risk is higher, so the device can be set to measure the grid impedance once a day, or the time interval can be customized according to actual needs. Repeat steps S1-S5 each time a grid impedance measurement is required.

[0069] To better illustrate the present invention, a specific embodiment is provided below for detailed description:

[0070] For example Figure 2 The three-phase grid impedance of the three-phase LCL grid-connected inverter shown is measured online, and the grid impedance is equivalent to a resistance R. s and inductor L s The form of series, i.e. Z s =R s +jωL s U POI I POI The grid connection point voltage and current; where the DC bus voltage U dc =800V, grid-connected voltage is 380V, inverter-side inductance L1 = 0.8mL, grid-side inductance L2 = 0.2mL, filter capacitor C = 100μF, passive damping R d =3Ω, its function is to avoid LCL resonance, the switching frequency is 10kHz, the mains frequency is 50Hz, and the mains impedance L s =0.5mL, R g =4Ω. The designed disturbance signal is superimposed on the reference value of the d-axis of the current loop, such as... Figure 5 The figure shown is a simulated waveform of the grid connection point current under disturbance signal injection. Figure 6 The figures shown are the time-domain waveform and spectrum distribution of a single-pulse signal and a high-frequency sinusoidal signal superimposed on the single-pulse signal in an embodiment of the present invention. Then, the voltage and current of the three-phase grid connection point during the disturbance injection period and the adjacent non-injection period are obtained, and the difference between the two is ΔU. POIa , ΔU POIb ΔU POIc and ΔI POIa ΔI POIbΔI POIc The processed data is subjected to FFT analysis, and then the three-phase voltage and current are converted into positive and negative sequence voltage and current components through calculation. Finally, the positive sequence impedance of the power grid is calculated. Figure 7a This is a schematic diagram of the simulation measurement results of the positive sequence impedance of the power grid under traditional single-pulse disturbance injection. Figure 7b This is a schematic diagram of the simulation measurement results of the positive sequence impedance of the power grid under disturbance injection according to an embodiment of the present invention. As can be seen from the simulation measurement results, the disturbance signal proposed in this invention makes up for the inaccuracy of traditional single-pulse signals in mid-to-high frequency measurement. The amplitude and phase of the measured power grid impedance are basically consistent with the theoretical values.

[0071] Corresponding to the aforementioned online grid impedance measurement method based on a three-phase LCL grid-connected inverter in Embodiment 1 of the present invention, Embodiment 2 of the present invention provides an online grid impedance measurement device based on a three-phase LCL grid-connected inverter, comprising:

[0072] The injection module is used to latch the phase information of the grid voltage and current through the phase-locked loop in the inverter control structure, and to inject a disturbance signal when the current is at its peak; the disturbance signal is formed by superimposing a single pulse signal and a high-frequency sine wave signal at the reference amplitude of the inverter current control inner loop;

[0073] The acquisition module is used to synchronously measure the grid connection point voltage and current of the period in which the injected disturbance signal is located and the period in which the adjacent non-injected disturbance signal is located through voltage and current sensors after the disturbance signal is injected.

[0074] The spectrum analysis module is used to subtract the measurement results of the above two cycles point by point to obtain data that eliminates the influence of background harmonics of the power grid, and to perform spectrum analysis on the obtained data;

[0075] The first calculation module is used to calculate the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point obtained by spectrum analysis.

[0076] The second calculation module is used to calculate the positive-sequence impedance and negative-sequence impedance of the power grid based on the calculated positive-sequence voltage and current and negative-sequence voltage and current.

[0077] Preferably, the disturbance signal is composed of a negative single pulse signal with an amplitude equal to the peak value of the grid connection point current and a width of 0.01s, and a sine wave signal with a peak value 0.3 times that of the peak value of the grid connection point current and a frequency of 2kHz superimposed. After determining the time of the injected disturbance, the time span from the trough before the time to the trough after the time is taken as the period in which the injected disturbance signal is located, and the period before this period is the period in which the adjacent uninjected disturbance signal is located.

[0078] Preferably, step S4 calculates the positive and negative sequence components of the voltage and current frequency domain information at the grid connection point according to the following formula:

[0079]

[0080]

[0081] Among them, f k U represents the frequency corresponding to the spectrum obtained from the FFT analysis. pos (j2πf k () represents the frequency f k Positive sequence voltage at point, U neg (j2πf k () represents the frequency f k The negative sequence voltage at point a, where a is a 120° vector operator, ΔU POIa (j2πf k () represents the frequency f k Phase a AC side voltage at point ΔU POIb (j2πf k () represents the frequency f k Phase b AC side voltage, ΔU POIc (j2πf k () represents the frequency f k The AC side voltage of phase c at point I pos (j2πf k () represents the frequency f k The positive sequence current at the point, I neg (j2πf k () represents the frequency f k The negative sequence current at point ΔI POIa (j2πf k () represents the frequency f k The phase current at point a, ΔI POIb (j2πf k () represents the frequency f k The phase b current at point b, ΔI POIc (j2πf k () represents the frequency f k The c-phase current at point [location].

[0082] Preferably, step S5 calculates the positive-sequence impedance and negative-sequence impedance of the power grid according to the following formula:

[0083]

[0084]

[0085] Among them, |Z pos (j2πf k )|、∠Z pos (j2πf k ) are the positive sequence impedances of the power grid at frequency f kAmplitude and phase information at |U pos (j2πf k )|、∠U pos (j2πf k ) are the positive sequence voltages at frequency f k Amplitude and phase information at the location, |I pos (j2πf k )|、∠I pos (j2πf k ) are the positive sequence currents at frequency f k Amplitude and phase information at the location; |Z neg (j2πf k )|、∠Z neg (j2πf k ) are the negative sequence impedances of the power grid at frequency f k Amplitude and phase information at |U neg (j2πf k )|、∠U neg (j2πf k ) are the negative sequence voltages at frequency f k Amplitude and phase information at the location, |I neg (j2πf k )|、∠I neg (j2πf k ) are the negative sequence currents at frequency f k The amplitude and phase information at the location.

[0086] For the working principle and process of this embodiment, please refer to the description of the aforementioned Embodiment 1 of the present invention, which will not be repeated here.

[0087] As explained above, compared with the prior art, the beneficial effects of this invention are as follows: The disturbance signal of this invention is a high-frequency sinusoidal signal superimposed on the original single-pulse signal, which enhances the accuracy of mid-to-high frequency measurements and solves the problem of large measurement errors in grid impedance caused by the attenuation of the mid-to-high frequency components in the spectrum of existing single-pulse signals. Using the combination of single-pulse signals and high-frequency sinusoidal signals can cover a wider frequency range, enabling the system to detect more subtle impedance changes. By using a phase-locked loop in the inverter control structure and appropriate signal processing, it can be implemented on existing hardware and control structures, thereby reducing the overall system cost. Accurate measurement of grid impedance allows for more precise control of the inverter output and grid matching, thus reducing the risk of system instability. Accurate impedance information facilitates appropriate equipment adjustments to adapt to grid changes, thereby enhancing the stability and reliability of the entire system.

[0088] The above description is merely a preferred embodiment of the present invention and should not be construed as limiting the scope of the invention. Therefore, any equivalent variations made in accordance with the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for online measurement of grid impedance based on a three-phase LCL grid-connected inverter, characterized in that, include: Step S1: The phase information of the grid voltage and current is latched by the phase-locked loop in the inverter control structure, and a disturbance signal is injected when the current is at its peak. The disturbance signal is formed by superimposing a single pulse signal and a high-frequency sine wave signal at the reference amplitude of the inverter current control inner loop. Step S2: After the disturbance signal is injected, the grid connection point voltage and current in the period where the disturbance signal is injected and the period where the adjacent non-injected disturbance signal is injected are simultaneously measured by voltage and current sensors. Step S3: Subtract the measurement results of the above two cycles point by point to obtain data that eliminates the influence of background harmonics of the power grid, and perform spectrum analysis on the obtained data; Step S4: Calculate the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point obtained from the spectrum analysis. Step S5: Based on the calculated positive-sequence voltage and current and negative-sequence voltage and current, calculate the positive-sequence impedance and negative-sequence impedance of the power grid.

2. The method according to claim 1, characterized in that, The disturbance signal is composed of a negative single pulse signal with an amplitude equal to the peak value of the grid connection point current and a width of 0.01s, and a sine wave signal with a peak value 0.3 times that of the peak value of the grid connection point current and a frequency of 2kHz.

3. The method according to claim 1, characterized in that, After determining the time of the injected disturbance, the time span from the trough before the time to the trough after the time is taken as the period of the injected disturbance signal, and the period before this period is the period of the adjacent non-injected disturbance signal.

4. The method according to claim 1, characterized in that, Step S4 calculates the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point according to the following formula: in, The frequencies corresponding to the spectrum obtained from the FFT analysis. For frequency Positive sequence voltage at the location. For frequency The negative sequence voltage at that point a For 120° vector operators, For frequency Phase a AC side voltage at point, For frequency Phase b AC side voltage at point b For frequency The AC voltage on phase c at point c, For frequency The positive sequence current at the point, For frequency The negative sequence current at that point. For frequency The current in phase a at point , For frequency The current in phase b at point B, For frequency The c-phase current at point [location].

5. The method according to claim 4, characterized in that, Step S5 calculates the positive-sequence impedance and negative-sequence impedance of the power grid according to the following formula: in, , These are the positive sequence impedances of the power grid at different frequencies. Amplitude and phase information at the location, , The positive sequence voltage at frequency Amplitude and phase information at the location, , These are the positive sequence currents at different frequencies. Amplitude and phase information at the location; , These are the negative sequence impedances of the power grid at different frequencies. Amplitude and phase information at the location, , The negative sequence voltage at frequency Amplitude and phase information at the location, , The negative sequence currents at frequencies are respectively The amplitude and phase information at the location.

6. The method according to claim 1, characterized in that, When the grid-connected device is connected to a system with a short-circuit ratio greater than 20, the grid impedance measurement as described in steps S1-S5 is performed once a week; if it is connected to a system with a short-circuit ratio less than 20, the grid impedance measurement as described in steps S1-S5 is performed once a day.

7. An online grid impedance measurement device based on a three-phase LCL grid-connected inverter, characterized in that, include: The injection module is used to latch the phase information of the grid voltage and current through the phase-locked loop in the inverter control structure, and to inject a disturbance signal when the current is at its peak; the disturbance signal is formed by superimposing a single pulse signal and a high-frequency sine wave signal at the reference amplitude of the inverter current control inner loop; The acquisition module is used to synchronously measure the grid connection point voltage and current of the period in which the injected disturbance signal is located and the period in which the adjacent non-injected disturbance signal is located through voltage and current sensors after the disturbance signal is injected. The spectrum analysis module is used to subtract the measurement results of the above two cycles point by point to obtain data that eliminates the influence of background harmonics of the power grid, and to perform spectrum analysis on the obtained data; The first calculation module is used to calculate the positive and negative sequence components of the voltage and current frequency domain information of the grid connection point obtained by spectrum analysis. The second calculation module is used to calculate the positive-sequence impedance and negative-sequence impedance of the power grid based on the calculated positive-sequence voltage and current and negative-sequence voltage and current.

8. The apparatus according to claim 7, characterized in that, The disturbance signal is composed of a negative single pulse signal with an amplitude equal to the peak value of the grid connection point current and a width of 0.01s, and a sine signal with a peak value 0.3 times the peak value of the grid connection point current and a frequency of 2kHz. After determining the time of the injected disturbance, the time span from the trough before the time to the trough after the time is taken as the period of the injected disturbance signal, and the period before this period is the period of the adjacent non-injected disturbance signal.

9. The apparatus according to claim 7, characterized in that, The first calculation module calculates the positive and negative sequence components of the voltage and current frequency domain information at the grid connection point according to the following formula: in, The frequencies corresponding to the spectrum obtained from the FFT analysis. For frequency Positive sequence voltage at the location. For frequency The negative sequence voltage at that point a For 120° vector operators, For frequency Phase a AC side voltage at point, For frequency Phase b AC side voltage at point b For frequency The AC voltage on phase c at point c, For frequency The positive sequence current at the point, For frequency The negative sequence current at that point. For frequency The current in phase a at point , For frequency The current in phase b at point B, For frequency The c-phase current at point [location].

10. The apparatus according to claim 9, characterized in that, The second calculation module calculates the positive-sequence impedance and negative-sequence impedance of the power grid according to the following formula: in, , These are the positive sequence impedances of the power grid at different frequencies. Amplitude and phase information at the location, , The positive sequence voltage at frequency Amplitude and phase information at the location, , These are the positive sequence currents at different frequencies. Amplitude and phase information at the location; , These are the negative sequence impedances of the power grid at different frequencies. Amplitude and phase information at the location, , The negative sequence voltage at frequency Amplitude and phase information at the location, , The negative sequence currents at frequencies are respectively The amplitude and phase information at the location.

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