Method for measuring impedance of grid-connected inverter

Abstract

The invention discloses a method for measuring the impedance of a grid-connected inverter, which relates to the technical field of impedance measurement of the grid-connected inverter and is characterized in that impedance types and measurement accuracy needing to be measured are determined, the impedance types comprise output impedance and ground insulation impedance, and according to the measurement method and equipment requirements, a data acquisition module is used for setting parameters for acquiring measurement data, the parameters comprise a frequency range, a current amplitude value and a voltage amplitude value, the data processing and analysis module processes and analyzes measured data, calculates an impedance value, compares a calculation result with an expected value and a previous measured value for verification, and in the measurement process, the control and safety module controls starting, stopping and parameter setting of measurement equipment, so that the measurement accuracy is improved. And the working state of the inverter and the safety condition in the measurement process are monitored in real time, and the measurement process, the measurement result and the analysis are written into a measurement report to be uploaded to a system terminal, so that the accuracy and the reliability of impedance measurement of the grid-connected inverter are improved.

Description

Technical Field

[0001] The invention relates to the technical field of grid-connected inverter impedance measurement, and in particular to a method for measuring the impedance of a grid-connected inverter. Background Art

[0002] The output impedance of the grid-connected inverter is an important parameter for determining the stability of its grid-connected operation. Usually, this parameter is obtained by measuring the harmonic current injection method. However, since the grid-connected inverter itself has background harmonics and the grid operation frequency fluctuates within a certain range, these factors will affect the accuracy of the impedance measurement. Therefore, how to quickly and accurately obtain the impedance parameters of the grid-connected inverter has become a hot topic for domestic and foreign scholars.

[0003] At present, the impedance of grid-connected inverters is measured by harmonic current injection method. However, in the measurement process of the traditional harmonic current injection method, the background harmonics of the grid-connected inverter itself will interfere with the measurement results, resulting in large measurement errors. In addition, the operating frequency of the power grid is not constant, and its fluctuation will affect the synchronization of non-integer harmonic current injection, further increasing the measurement error. In addition, the internal control system of new energy power generation equipment such as grid-connected inverters is complex and the control characteristics vary greatly, which brings great difficulties to obtaining the impedance model through small signal modeling, and the use of harmonic current injection method. Summary of the invention

[0004] The purpose of the present invention is to provide a method for measuring the impedance of a grid-connected inverter, which solves the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solution, and the specific measurement method is as follows:

[0006] Specify the impedance type and measurement accuracy to be measured. Impedance types include output impedance and insulation impedance to ground.

[0007] According to the measurement method and equipment requirements, the data acquisition module sets the parameters for collecting the measurement data, the parameters including the frequency range, the current amplitude and the voltage amplitude;

[0008] The data processing and analysis module processes and analyzes the measured data, calculates the impedance value, and compares and verifies the calculated result with the expected value and the previous measured value. The data processing and analysis module includes a basic impedance measurement unit under the influence of harmonics, an impedance correction unit under the influence of temperature and frequency, and an impedance comprehensive evaluation unit;

[0009] During the measurement process, the control and safety module controls the start, stop and parameter setting of the measuring equipment, and monitors the working status of the inverter and the safety situation during the measurement process in real time;

[0010] Write the measurement process, results and analysis into a measurement report and upload it to the system terminal;

[0011] The equipment used in the data acquisition module includes an impedance analyzer, a harmonic generator and a voltage source;

[0012] The equipment used in the data processing and analysis module includes a computer for running measurement data processing and analysis software.

[0013] Optionally, the measurement process of the basic impedance measurement unit under the influence of harmonics is as follows:

[0014]

[0015] in:

[0016] ZK is the initial measured impedance value;

[0017] DL rms is the RMS value of current;

[0018] n is the total number of current measurements, DL i is the i-th current measurement value;

[0019] DY rms is the voltage RMS value;

[0020] m is the total number of voltage measurements, DY i is the i-th voltage measurement value;

[0021] α is the harmonic influence coefficient, which is used to quantify the influence of the harmonic components in the inverter output current on the impedance measurement;

[0022] DL 2 H is the second harmonic current value, DL 2 H extracts the second harmonic component from the current signal by Fourier transform method.

[0023] Optionally, the correction process of the impedance correction unit under the influence of temperature and frequency is as follows:

[0024] ZK adj =ZK / (1+F*(WW ref )*(PP nom ) 2 ;

[0025] in:

[0026] ZK adj is the corrected impedance value;

[0027] F is the temperature-frequency influence coefficient, which is used to quantify the combined effect of temperature and frequency changes on impedance measurement;

[0028] W is the current ambient temperature value;

[0029] W ref is the reference temperature value;

[0030] P is the current operating frequency value;

[0031] P nom is the rated operating frequency value.

[0032] Optionally, the evaluation process of the impedance comprehensive evaluation unit is as follows:

[0033]

[0034] in:

[0035] ZK new is the comprehensive impedance evaluation value;

[0036] β is the weighting factor of historical data, which is used to balance the impact of historical measurements and current measurements;

[0037] d is the total number of measured impedances, ZK adj(s) is the sth impedance measurement value;

[0038] ZK adj(avg) is the average value of the corrected impedance, v is the total number of corrected impedance measurements, ZK adj(j) is the jth impedance measurement value;

[0039] ∈ is the peak voltage influence coefficient, ∈ is used for the additional influence of the voltage peak on the impedance evaluation;

[0040] DK peak is the voltage peak value.

[0041] Optionally, based on the comprehensive impedance evaluation value ZK output by the impedance comprehensive evaluation unit new The specific analysis is as follows:

[0042] S1. First, the comprehensive impedance evaluation value ZK new The expected impedance range is based on the design specifications of the inverter, the measured data of similar equipment, and theoretical calculations. If the comprehensive impedance evaluation value ZK new If the impedance value ZK is within the expected impedance range and the previously measured impedance value, the system is considered stable, the power quality is normal, and there is no abnormality. newIf the impedance value exceeds the expected impedance range and the range of the previously measured impedance value, it is considered that the system is fluctuating and abnormal, and then the following S2-S6 requirements are checked one by one to find out the cause of the abnormality and re-measure and evaluate the parameters in the basic impedance measurement unit under the influence of harmonics, and the adjusted parameters are used in future measurements;

[0043] S2, check whether the measurement of the equipment used in the data acquisition module is calibrated correctly, whether there is noise and interference in the measurement circuit, including the frequency and time length of data acquisition;

[0044] S3. Check the working environment conditions of the inverter, including temperature, humidity and vibration, and check whether the environmental conditions are within the normal working range of the inverter. If the environmental conditions exceed the normal range, it will have a significant impact on the impedance measurement results;

[0045] S4. Check the operating status of the inverter, including load conditions, working mode, and whether there is a fault alarm;

[0046] S5. Review and analyze the corrected impedance value ZK over the past period of time adj And the comprehensive impedance evaluation value ZK new to check the measurement results to see if there are similar abnormal patterns and trends;

[0047] S6. Evaluate other factors that may affect the impedance measurement, including parasitic components inside the inverter, resistance and inductance of the connecting lines, and stability of the external grid.

[0048] Optionally, the comprehensive impedance evaluation value ZK new During the analysis, special attention should be paid to S3. The impedance correction unit based on the influence of temperature and frequency has taken the ambient temperature into consideration for measurement. Therefore, during the correction process of the impedance correction unit based on the influence of temperature and frequency, it is possible to directly find abnormal measurements caused by environmental conditions, especially temperature.

[0049] Optionally, the current root mean square value DL in the basic impedance measurement unit under the influence of harmonics rms And the voltage RMS value DY rms The calculation method is for discrete signals. If it is a continuous signal, the current RMS value DL rms And the voltage RMS value DY rms The calculation method is as follows:

[0050]

[0051] T is the period value, that is, the period of the continuous signal, and t is the time number, that is, the variable of the tth time within the period value T.

[0052] Compared with the prior art, the present invention has the following beneficial effects:

[0053] 1. The present invention adopts a non-integer harmonic offset frequency current injection method, which effectively avoids the influence of background harmonics on the measurement results, thereby improving the accuracy of impedance measurement and reducing the error caused by background harmonics.

[0054] 2. The present invention incorporates the changes in temperature and frequency into the impedance measurement consideration range based on the impedance correction unit under the influence of temperature and frequency, thereby achieving accurate correction of impedance, enhancing the adaptability of the measurement system to environmental changes, and improving the real-time and accuracy of impedance measurement.

[0055] 3. The present invention combines historical data and real-time measurement results, and comprehensively evaluates the impedance through an impedance comprehensive evaluation unit, which not only takes into account the current operating status, but also takes into account the past operating history and future change trends, providing a more comprehensive and accurate impedance evaluation result.

[0056] 4. The present invention adopts a multi-power module cascade structure to achieve wide-band and high-power impedance measurement, thereby solving the modeling problem caused by the "black box" of new energy power generation equipment and providing strong support for the stability research of new energy grid-connected systems.

[0057] 5. The present invention studies the open-loop and closed-loop control method of the impedance measurement equipment and its influence on the stability of the grid-connected system under different short-circuit ratios, and designs the switching control method of the impedance measurement equipment, so that the impedance measurement process has no influence on the working state of the grid-connected inverter, thereby ensuring the accuracy of the measurement and the stability of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 A flow chart of the method for measuring the impedance of the grid-connected inverter;

[0059] Figure 2 A structural diagram of a data processing and analysis module of the grid-connected inverter impedance measurement method;

[0060] Figure 3 The figure is a schematic diagram of the process flow of the measurement and analysis of the impedance of the grid-connected inverter. DETAILED DESCRIPTION

[0061] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] Regarding the measurement method of the impedance of the grid-connected inverter, it is different from the existing measurement method of the impedance of the grid-connected inverter. During the measurement process of the traditional harmonic current injection method, the background harmonics of the grid-connected inverter itself will interfere with the measurement results, resulting in large measurement errors, and the fluctuation affects the synchronization of non-integer harmonic current injection. In addition, the single measurement method directly leads to the inability of the measurement method of the impedance of the grid-connected inverter to fully reflect the impedance characteristics of the inverter under different working conditions. The algorithm unit adopts the non-integer harmonic offset current injection method, temperature and frequency corrected impedance measurement, impedance evaluation based on comprehensive historical data and real-time measurement results, and innovative means of wide-band impedance measurement equipment research and development to effectively solve the deficiencies in the existing technology and improve the accuracy and reliability of the impedance measurement of the grid-connected inverter.

[0063] For example, see Figures 1 to 3 This embodiment provides a method for measuring the impedance of a grid-connected inverter. The specific measurement method is as follows:

[0064] Specify the impedance type and measurement accuracy to be measured. Impedance types include output impedance and insulation impedance to ground.

[0065] According to the measurement method and equipment requirements, the data acquisition module sets the parameters for collecting the measurement data, including the frequency range, current amplitude and voltage amplitude;

[0066] The measured data is processed and analyzed through the data processing and analysis module to calculate the impedance value, and the calculated result is compared and verified with the expected value and the previous measured value. The data processing and analysis module includes a basic impedance measurement unit based on the influence of harmonics, an impedance correction unit based on the influence of temperature and frequency, and an impedance comprehensive evaluation unit;

[0067] During the measurement process, the control and safety module controls the start, stop and parameter setting of the measuring equipment, and monitors the working status of the inverter and the safety situation during the measurement process in real time;

[0068] Write the measurement process, results and analysis into a measurement report and upload it to the system terminal;

[0069] The equipment used in the data acquisition module includes impedance analyzer, harmonic generator and voltage source;

[0070] The equipment used in the data processing and analysis module includes a computer for running measurement data processing and analysis software.

[0071] The impedance analyzer is used to directly measure the impedance value of the inverter and has high-precision and wide-band measurement capabilities. The harmonic generator is used to generate harmonic currents and voltages of specific frequencies and amplitudes to simulate the harmonic conditions in the actual power grid. The voltage source is used to provide a stable voltage output for voltage perturbation method measurement. The data acquisition system includes a data acquisition card and corresponding software for real-time acquisition of voltage and current parameter data during the measurement process. The computer processes and analyzes the collected data and generates a measurement report.

[0072] In this embodiment, the system optimizes and innovates the measurement method of grid-connected inverter impedance in many aspects on the basis of background technology through the mutual cooperation of three algorithm units, solves the shortcomings of the existing technology, and combines ZK, ZK adj and ZK new The three calculation results improve the accuracy and reliability of the measurement results and provide strong support for the stable operation of the new energy power generation system. ZK is the initial impedance value, which is mainly used to preliminarily measure and calculate the impedance value of the grid-connected inverter. At the same time, the influence of harmonic current and harmonic content on impedance is considered. This is the basis of impedance measurement and provides an initial value for subsequent corrections. adj The impedance value after correction is further considered the influence of temperature and frequency on impedance. By introducing the temperature-frequency influence coefficient F, the initial impedance value ZK is corrected to obtain a more accurate impedance value, ZK new To obtain a comprehensive impedance evaluation value, it combines historical data and real-time measurement results to conduct a more comprehensive evaluation of impedance. By introducing the voltage ratio and the weighted sum of historical impedance data, it can reflect the impedance characteristics of the inverter under different operating conditions and consider the impact of real-time operating conditions on impedance. The three algorithm units start from basic impedance measurement and gradually introduce the complex influences of harmonics, temperature and frequency, as well as historical data and real-time peak voltage, forming an interrelated and in-depth impedance evaluation system. The results of each unit are used as input for subsequent formulas, reflecting the close connection and mutual influence between the algorithms.

[0073] See also Figures 1 to 3 , the measurement process of the basic impedance measurement unit under the influence of harmonics is as follows:

[0074]

[0075] in:

[0076] ZK is the initial measured impedance value;

[0077] DL rms is the RMS value of current;

[0078] n is the total number of current measurements, DL i is the i-th current measurement value;

[0079] DY rms is the voltage RMS value;

[0080] m is the total number of voltage measurements, DY i is the i-th voltage measurement value;

[0081] α is the harmonic influence coefficient, which is used to quantify the influence of the harmonic components in the inverter output current on the impedance measurement;

[0082] DL 2 H is the second harmonic current value, DL 2 H extracts the second harmonic component from the current signal by Fourier transform method.

[0083] In this embodiment: First, the current RMS value DL in this algorithm unit rms And the voltage RMS value DY rms They represent the effective values ​​of the measured current and voltage, respectively, and can truly reflect the current and voltage levels of the AC power. The harmonic influence coefficient α quantifies the influence of the harmonic components in the inverter output current on the impedance measurement, as well as the second harmonic current value DL 2 Under the consideration that H will affect the accuracy of impedance measurement, the initial impedance value ZK is output;

[0084] The correction term of directly relates the ratio of harmonic current to fundamental current, thus accurately quantifying the impact of harmonics on inverter impedance. This is particularly important for inverters operating in power grids with severe harmonic pollution, because it can reveal how harmonics change the impedance characteristics of the inverter, thereby affecting its stability and efficiency.

[0085] This algorithm unit introduces the second harmonic current value DL 2 H and The correction term can directly reflect the impact of harmonics on the inverter impedance, which helps to more accurately evaluate the performance of the inverter in a harmonic environment. As the basis of impedance measurement, the basic impedance measurement unit under the influence of harmonics provides an initial impedance value ZK, which provides a reliable data basis for subsequent corrections and evaluations. The harmonic influence coefficient α in the basic impedance measurement unit under the influence of harmonics can be adjusted according to actual conditions to adapt to the measurement requirements of different inverters and different working conditions. This flexibility makes the basic impedance measurement unit under the influence of harmonics have a wider range of applications.

[0086] See also Figures 1 to 3 , the correction process of the impedance correction unit under the influence of temperature and frequency is as follows:

[0087] ZK adj =ZK / (1+F*(WW ref )*(PPnom ) 2 ;

[0088] in:

[0089] ZK adj is the corrected impedance value;

[0090] F is the temperature-frequency influence coefficient, which is used to quantify the combined effect of temperature and frequency changes on impedance measurement;

[0091] W is the current ambient temperature value;

[0092] W ref is the reference temperature value;

[0093] P is the current operating frequency value;

[0094] P nom is the rated operating frequency value.

[0095] In this embodiment, first, W is the current ambient temperature value, W ref is the reference temperature value. Specifically, the temperature will affect the resistivity and conductivity of the components inside the inverter, thus affecting the impedance measurement. The two are usually taken as the standard temperature during inverter design and testing. P is the current operating frequency value, and P nom is the rated operating frequency value. Frequency changes will affect the working state and impedance characteristics of the inverter, and both are obtained through frequency measuring instruments or the inverter's internal control system;

[0096] This algorithm unit introduces the temperature-frequency influence coefficient F and the initial impedance value ZK and (WW ref )*(PP nom ) 2 The temperature-frequency ratio enables the impedance correction unit based on the influence of temperature and frequency to more comprehensively reflect the influence of temperature and frequency on the inverter impedance, which helps to improve the accuracy and reliability of impedance measurement. On the basis of the basic impedance measurement unit under the influence of harmonics, the impedance correction unit based on the influence of temperature and frequency further improves the accuracy of impedance measurement by considering the correction of temperature and frequency, which is particularly important for application scenarios that require high-precision impedance measurement. Since the changes in temperature and frequency are taken into account, the impedance correction unit based on the influence of temperature and frequency enables the measurement system to automatically adapt to changes in the working environment of the inverter, thereby enhancing the adaptability and stability of the system.

[0097] See also Figures 1 to 3 , the evaluation process of the impedance comprehensive evaluation unit is as follows:

[0098]

[0099] in:

[0100] ZK new is the comprehensive impedance evaluation value;

[0101] β is the weighting factor of historical data, which is used to balance the impact of historical measurements and current measurements;

[0102] d is the total number of measured impedances, ZK adj(s) is the sth impedance measurement value;

[0103] ZK adj(avg) is the average value of the corrected impedance, v is the total number of corrected impedance measurements, ZK adj(j) is the jth impedance measurement value;

[0104] ∈ is the peak voltage influence coefficient, ∈ is used for the additional influence of the voltage peak on the impedance evaluation;

[0105] DK peak is the voltage peak value.

[0106] In this embodiment, the algorithm unit firstly, ZK adj(s) is the sth impedance measurement value, ZK adj(avg) is the corrected impedance average value. The two are used to consider the impact of historical measurement data on the current impedance assessment and improve the accuracy and stability of the assessment. β is the historical data weighting factor, which is used to balance the weights of historical measurement data and current measurement data in impedance assessment. DK peak is the voltage peak value, which indicates the maximum value of the voltage signal. It is used to consider the additional impact of the voltage peak value on the impedance evaluation. The calculation is only for sinusoidal voltage, while for non-sinusoidal voltage, other methods such as peak detection are needed;

[0107] This algorithm unit conducts a more comprehensive evaluation of the impedance of the inverter by integrating historical data and real-time measurement results. This comprehensive evaluation capability helps to more accurately reflect the impedance characteristics of the inverter under different operating conditions. By introducing the correction term of the voltage ratio and the weighted sum of the historical impedance data, the accuracy of the impedance evaluation can be further improved. The impedance comprehensive evaluation unit can update the impedance evaluation results in real time according to the operating conditions and historical data of the inverter, thereby reflecting the dynamic characteristics of the inverter. This is especially important for application scenarios that require real-time monitoring and evaluation of inverter performance. Based on the impedance evaluation results of the impedance comprehensive evaluation unit, the control strategy of the inverter can be further optimized to improve the stability and efficiency of the system. For example, the output voltage and current waveforms of the inverter can be adjusted according to changes in the impedance characteristics.

[0108] See also Figures 1 to 3 , based on the comprehensive impedance evaluation value ZK output by the impedance comprehensive evaluation unit new The specific analysis is as follows:

[0109] S1. First, the comprehensive impedance evaluation value ZK new The expected impedance range is based on the design specifications of the inverter, the measured data of similar equipment, and theoretical calculations. If the comprehensive impedance evaluation value ZK new If the impedance value ZK is within the expected impedance range and the previously measured impedance value, the system is considered stable, the power quality is normal, and there is no abnormality. new If the impedance exceeds the expected range and the range of the previously measured impedance value, it is considered that the system is fluctuating and abnormal, and then the following S2-S6 requirements are checked one by one to find out the cause of the abnormality and re-measure and evaluate the parameters in the basic impedance measurement unit under the influence of harmonics, and the adjusted parameters are used in future measurements;

[0110] S2. Check whether the measurement of the equipment used in the data acquisition module is calibrated correctly, whether there is noise and interference in the measurement circuit, including the frequency and time length of data acquisition;

[0111] S3. Check the working environment conditions of the inverter, including temperature, humidity and vibration, and check whether the environmental conditions are within the normal working range of the inverter. If the environmental conditions exceed the normal range, it will have a significant impact on the impedance measurement results;

[0112] S4. Check the operating status of the inverter, including load conditions, working mode, and whether there is a fault alarm;

[0113] S5. Review and analyze the corrected impedance value ZK over the past period of time adj And the comprehensive impedance evaluation value ZK new to check the measurement results to see if there are similar abnormal patterns and trends;

[0114] S6. Evaluate other factors that may affect the impedance measurement, including parasitic elements inside the inverter, resistance and inductance of the connecting lines, and stability of the external grid;

[0115] Comprehensive impedance evaluation value ZK new During the analysis, special attention should be paid to S3. The impedance correction unit based on the influence of temperature and frequency has taken the ambient temperature into consideration for measurement. Therefore, during the correction process of the impedance correction unit based on the influence of temperature and frequency, it is possible to directly discover abnormal measurements caused by environmental conditions, especially temperature.

[0116] In this embodiment, based on the above analysis process, the algorithm unit can identify the specific cause of the impedance abnormality, such as sensor failure, inverter failure, and unsuitable environmental conditions, which helps to take repair measures in time to avoid the expansion of potential faults and cause greater losses. According to the analysis results, the operating parameters of the inverter can be adjusted and its working environment can be optimized to improve its performance and stability. At the same time, the measurement method and data processing flow can be improved to improve the accuracy and reliability of impedance measurement. By timely discovering and solving the impedance abnormality problem, the risk of inverter failure is reduced, and the reliability and stability of the entire system are enhanced. This is of great significance for ensuring the safe operation of the power grid and meeting the user's demand for power supply. If the impedance abnormality is caused by problems in the design or production process of the inverter, then the analysis results provide a valuable reference for subsequent improvement work. By continuously optimizing the design and production process, the performance and quality level of the inverter can be improved to further meet market demand and user expectations.

[0117] Through the comprehensive evaluation of the impedance comprehensive evaluation unit, errors in the measurement process can be discovered and corrected in time, and the accuracy of impedance measurement can be improved, which helps to more accurately evaluate the performance and stability of the inverter. The indirect cyclic influence of the impedance comprehensive evaluation unit on the basic impedance measurement unit under the influence of harmonics enables the measurement system to automatically adapt to changes in the working state and environmental conditions of the inverter. When the operating conditions of the inverter change, the system can adjust the measurement parameters and evaluation methods in time to ensure the real-time and effectiveness of the measurement results. Based on the comprehensive evaluation results of the impedance comprehensive evaluation unit, the control strategy of the inverter can be further optimized. For example, the output voltage and current waveforms of the inverter can be adjusted according to the changes in the impedance characteristics to improve the stability and efficiency of the system. By timely discovering and solving potential problems in the operation of the inverter, the indirect cyclic influence of the impedance comprehensive evaluation unit on the basic impedance measurement unit under the influence of harmonics helps to improve the reliability of the entire system, which helps to reduce system failures and downtime and improve the power supply quality and stability of the power grid;

[0118] In summary, in the specific implementation process, the impedance comprehensive evaluation unit has an important significance in terms of calculation purpose and beneficial effect on the indirect circulation influence of the basic impedance measurement unit under the influence of harmonics. It helps to improve measurement accuracy, enhance system adaptability, optimize inverter control strategy and improve system reliability.

[0119] For example 2, please refer to Figures 1 to 3 , based on the RMS value of the current in the basic impedance measurement unit under the influence of harmonics DL rms And the voltage RMS value DY rms The calculation method is for discrete signals. If it is a continuous signal, the current RMS value DL rms And the voltage RMS value DY rmsThe calculation method is as follows:

[0120]

[0121] T is the period value, that is, the period of the continuous signal, f represents the definite integral symbol, dt is the differential of the integral variable t, and t is the time number, that is, the variable of the tth time within the period value T.

[0122] In this embodiment, during the measurement process, the staff can perform different calculations based on the discrete signals and continuous signals of voltage and current. The continuous signal is a continuous distribution of the signal on the time axis without obvious breakpoints and sampling intervals, while the discrete signal is a signal in the form of discrete points, each point representing the sampling value of the signal at a certain moment;

[0123] When processing discrete signals, the sampling rate, that is, the number of samples per second, has a great influence on the accuracy of the results. The higher the sampling rate, the closer the discrete signal is to the original continuous signal, and the more accurate the calculated RMS value is. When calculating the RMS value, it is necessary to pay attention to whether the signal is a periodic signal. For non-periodic signals, it is necessary to select an appropriate time period for calculation;

[0124] In practical applications, sliding average method, filter and other methods can be used to further improve the accuracy and stability of RMS value calculation.

[0125] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for measuring the impedance of a grid-connected inverter, characterized in that: The specific implementation is as follows: Specify the impedance type and measurement accuracy to be measured. Impedance types include output impedance and insulation impedance to ground. According to the measurement method and equipment requirements, the data acquisition module sets the parameters for collecting the measurement data, including the frequency range, current amplitude and voltage amplitude; The measured data is processed and analyzed through the data processing and analysis module to calculate the impedance value, and the calculated result is compared and verified with the expected value and the previous measured value. The data processing and analysis module includes a basic impedance measurement unit based on the influence of harmonics, an impedance correction unit based on the influence of temperature and frequency, and an impedance comprehensive evaluation unit; During the measurement process, the control and safety module controls the start, stop and parameter setting of the measuring equipment, and monitors the working status of the inverter and the safety situation during the measurement process in real time; The measurement process, results and analysis are compiled into a measurement report and uploaded to the system terminal.

2. The method for measuring the impedance of a grid-connected inverter according to claim 1, characterized in that: The equipment used in the data acquisition module includes an impedance analyzer, a harmonic generator and a voltage source; The equipment used in the data processing and analysis module includes a computer for running measurement data processing and analysis software.

3. The method for measuring the impedance of a grid-connected inverter according to claim 2, characterized in that: The calculation formula of the basic impedance measurement unit under the influence of harmonics is as follows: in: ZK is the initial measured impedance value; DL rms is the RMS value of current; n is the total number of current measurements, DL i is the i-th current measurement value; DY rms is the voltage RMS value; m is the total number of voltage measurements, DY i is the i-th voltage measurement value; α is the harmonic influence coefficient, which is used to quantify the influence of the harmonic components in the inverter output current on the impedance measurement; DL2H is the second harmonic current value. DL2H extracts the second harmonic component from the current signal by Fourier transform method.

4. The method for measuring the impedance of a grid-connected inverter according to claim 3, characterized in that: The calculation formula of the impedance correction unit under the influence of temperature and frequency is as follows: ZK adj =ZK / (1+F*(W-W ref )*(P-P nom ) 2 ; in: ZK adj is the corrected impedance value; F is the temperature-frequency influence coefficient, which is used to quantify the combined effect of temperature and frequency changes on impedance measurement; W is the current ambient temperature value; W ref is the reference temperature value; P is the current operating frequency value; P nom is the rated operating frequency value.

5. The method for measuring the impedance of a grid-connected inverter according to claim 4, characterized in that: The calculation formula of the impedance comprehensive evaluation unit is as follows: in: ZK new is the comprehensive impedance evaluation value; β is the weighting factor of historical data, which is used to balance the impact of historical measurements and current measurements; d is the total number of measured impedances, ZK adj(s) is the sth impedance measurement value; ZK adj(avg) is the average value of the corrected impedance, v is the total number of corrected impedance measurements, ZK adj(j) is the jth impedance measurement value; ∈ is the peak voltage influence coefficient, ∈ is used for the additional influence of the voltage peak on the impedance evaluation; DK peak is the voltage peak value.

6. The method for measuring the impedance of a grid-connected inverter according to claim 5, characterized in that: Based on the comprehensive impedance evaluation value ZK output by the impedance comprehensive evaluation unit new The specific analysis is as follows: S1. First, the comprehensive impedance evaluation value ZK new The expected impedance range is based on the design specifications of the inverter, the measured data of similar equipment, and theoretical calculations. If the comprehensive impedance evaluation value ZK new If the impedance value ZK is within the expected impedance range and the previously measured impedance value, the system is considered stable, the power quality is normal, and there is no abnormality. new If the impedance value exceeds the expected impedance range and the range of the previously measured impedance value, it is considered that the system is fluctuating and abnormal, and then the following S2-S6 requirements are checked one by one to find out the cause of the abnormality and re-measure and evaluate the parameters in the basic impedance measurement unit under the influence of harmonics, and the adjusted parameters are used in future measurements; S2, check whether the measurement of the equipment used in the data acquisition module is calibrated correctly, whether there is noise and interference in the measurement circuit, including the frequency and time length of data acquisition; S3. Check the working environment conditions of the inverter, including temperature, humidity and vibration, and check whether the environmental conditions are within the normal working range of the inverter. If the environmental conditions exceed the normal range, it will have a significant impact on the impedance measurement results; S4. Check the operating status of the inverter, including load conditions, working mode, and whether there is a fault alarm; S5. Review and analyze the corrected impedance value ZK over the past period of time adj And the comprehensive impedance evaluation value ZK new to check the measurement results to see if there are similar abnormal patterns and trends; S6. Evaluate other factors that may affect the impedance measurement, including parasitic components inside the inverter, resistance and inductance of the connecting lines, and stability of the external grid.

7. The method for measuring the impedance of a grid-connected inverter according to claim 6, characterized in that: The comprehensive impedance evaluation value ZK new During the analysis, it is necessary to pay attention to S3. The impedance correction unit based on the influence of temperature and frequency has taken the ambient temperature into consideration for measurement. Therefore, during the correction process of the impedance correction unit based on the influence of temperature and frequency, it is possible to directly find abnormal measurements caused by environmental conditions, especially temperature.

8. The method for measuring the impedance of a grid-connected inverter according to claim 3, characterized in that: The current root mean square value DL in the basic impedance measurement unit under the influence of harmonics rms And the voltage RMS value DY rms The calculation method is for discrete signals.

9. The method for measuring the impedance of a grid-connected inverter according to claim 8, characterized in that: If it is a continuous signal, the current RMS value DL rms And the voltage RMS value DY rms The calculation method is as follows: T is the period value, that is, the period of the continuous signal; f represents the definite integral symbol, dt is the differential of the integral variable t, and t is the time order, that is, the variable at the tth time within the period value T.

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