Identification method and system for measuring fundamental wave pseudo oscillation by low-sampling-rate measurement and control device

Abstract

The invention provides an identification method for measuring fundamental wave pseudo oscillation by a low-sampling-rate measurement and control device, which belongs to the field of electrical measurement and comprises the following steps of: calculating an impedance deviation coefficient of a capacitor fundamental wave impedance value relative to a capacitor actual impedance value; searching effective maximum values and minimum values of the impedance deviation coefficients, taking the effective maximum values of the adjacent impedance deviation coefficients as analysis intervals, and calculating corresponding oscillation frequencies according to time differences of the analysis intervals; the maximum value and the minimum value threshold value of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval are calculated, and the authenticity of measurement oscillation of the measurement and control device is identified according to the relation between the effective maximum value and the minimum value of the impedance deviation coefficient and the corresponding threshold value; the invention further provides a fundamental wave pseudo oscillation identification system measured by the low-sampling-rate measurement and control device. The risk of misjudgment caused by measurement pseudo oscillation of a low-sampling-rate measurement and control device due to high-frequency harmonic aliasing can be solved.

Description

Technical Field

[0001] The present invention relates to the technical field of electrical measurement, and particularly to a method and system for identifying fundamental wave pseudo-oscillation in a low-sampling-rate measurement and control device. Background Art

[0002] The measurement and control device is a key device in the substation integrated automation system, undertaking many functions such as data acquisition, protection measurement, automatic control, and remote management. It is the "sensing organ" of the power grid state and the "control center" of the behavior, playing an irreplaceable role in improving the reliability, security, and intelligence of the power system. Therefore, the stability, reliability, and accuracy of the measurement data collected and sent by the measurement and control device are of great significance.

[0003] With the rapid advancement of the construction of the new power system, the rapid development of new energy power generation represented by photovoltaic and wind power, high-voltage transmission and flexible DC transmission, flexible interconnection and DC distribution networks, and electrical equipment with various converters as the power conversion interface has continuously improved the degree of power electronics in all links of power generation, transmission, distribution, and use in the power system. This brings two consequences: (1) Due to its own stability problems, the probability of low-frequency oscillation and sub- / supra-synchronous oscillation in the power system caused by power electronic devices increases, seriously endangering the operation reliability of the power system; (2) A large amount of harmonics will be generated during the power conversion process of power electronic devices, leading to the deterioration of power system harmonics and showing a high-frequency trend. The measurement and control device mainly focuses on the measurement of fundamental wave components. To reduce the data operation volume and control device cost, the existing massive measurement and control devices in the power grid have the current situation of low sampling frequency, no anti-aliasing filter or poor filtering effect, and asynchronous sampling in data acquisition. The high-frequency harmonics inevitably cause aliasing problems in the low-sampling-rate measurement and control devices. When the sampling frequency matches the harmonic frequency, it can even be aliased to the fundamental wave. Adding the current situation of the near-power-frequency time-varying of the power system fundamental wave frequency, non-uniform sampling will cause different degrees of leakage interference, resulting in pseudo-oscillation in the fundamental wave measurement results of the measurement and control device, so that relevant professionals such as power grid protection and regulation cannot distinguish whether the fundamental wave voltage and current oscillation measured by the measurement and control device is a pseudo-oscillation caused by measurement aliasing or a real oscillation caused by power electronic devices, and it is extremely easy to misjudge the power grid operation state and cause serious consequences.

[0004] In the prior art, the Chinese patent application for invention "Judgment and Avoidance Method of Pseudo-Oscillation in Power Frequency Measurement" with publication number CN105629189A discloses a method for judging pseudo-oscillation in power frequency measurement, including: calculating the oscillation frequency of the fundamental wave measurement result at a set power grid position, extracting the power grid fundamental wave frequency from the fundamental wave measurement result, and calculating the frequency deviation of the power grid fundamental wave frequency relative to a power grid rated frequency. Whether the oscillation in the fundamental wave measurement result is a pseudo-oscillation is judged based on one or more of the oscillation frequency, the frequency deviation, and the known signal harmonic order of the set power grid position. This method for judging pseudo-oscillation determines by the ratio of the oscillation frequency to the deviation of the fundamental wave frequency relative to the power frequency (50Hz) and the relationship with the aliased harmonic order, mainly identifying around the frequency characteristics. It involves the measurement of the fundamental wave frequency, but the current frequency measurement is based on the voltage waveform signal. In the case of voltage oscillation, significant errors will occur in the measurement of the fundamental wave frequency, affecting the calculation of the fundamental wave frequency deviation, resulting in the failure of the identification method or incorrect results, and it is only applicable to the case of current oscillation but no voltage oscillation. However, the conditions for meeting this scenario in the actual power grid are relatively harsh. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: how to solve the misjudgment risk caused by the measurement pseudo-oscillation due to high-frequency harmonic aliasing in low-sampling-rate measurement and control devices.

[0006] The present invention solves the above technical problem through the following technical solutions: a method for identifying fundamental wave pseudo-oscillation in a low-sampling-rate measurement and control device, the method including:

[0007] Calculating the impedance deviation coefficient of the fundamental wave impedance value of the capacitor relative to the actual impedance value of the capacitor;

[0008] Searching for the effective maximum and minimum values of the impedance deviation coefficient, using the adjacent effective maximum values of the impedance deviation coefficient as the analysis interval, and calculating the corresponding oscillation frequency based on the time difference of the analysis interval;

[0009] Calculating the maximum and minimum value thresholds of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and identifying the authenticity of the oscillation measured by the measurement and control device through the relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding thresholds.

[0010] Preferably, the calculation process of the impedance deviation coefficient is:

[0011] Using the full-wave Fourier algorithm to calculate the discrete data of the voltage and current waveforms of the capacitor branch where oscillation exists in the measurement of the measurement and control device cycle by cycle, and obtaining the fundamental wave components of voltage and current in each cycle;

[0012] Through the ratio operation of the fundamental wave components of voltage and current in each cycle, obtaining the fundamental wave impedance value of the capacitor corresponding to each cycle;

[0013] By calculating the ratio of the fundamental impedance value of the capacitor to the actual impedance value of the capacitor, the impedance deviation coefficient of each period is obtained.

[0014] Preferably, the fundamental voltage and current components of each period are:

[0015]

[0016] The fundamental impedance value Z m (k) is:

[0017]

[0018] The impedance deviation coefficient r(k) of each period is:

[0019]

[0020] Where N is the number of sampling points in one period of voltage or current, U(k) and I(k) are the fundamental voltage amplitude and fundamental current amplitude of the k-th period respectively, u k (n) and i k (n) are the n-th sampling value of the voltage waveform and the n-th sampling value of the current waveform in the k-th period respectively, k = 1, 2, 3,..., Z co is the actual impedance value of the capacitor.

[0021] Preferably, searching for the effective maximum and minimum values of the impedance deviation coefficient includes:

[0022] Searching for the points where the impedance deviation coefficient is greater than the left and right adjacent impedance deviation coefficients to obtain the maximum value r(a i ) of the impedance deviation coefficient;

[0023] Respectively between the adjacent maximum values r(a i ) and r(a i+1 ), and between the adjacent maximum values r(a i+1 ) and r(a i+2 ), searching for the points where the impedance deviation coefficient is less than the left and right adjacent impedance deviation coefficients to obtain the adjacent minimum values r(b i ) and r(b i+1 ) of the impedance deviation coefficient;

[0024] According to the relationship between the adjacent minimum values r(b i ) and r(b i+1 ) of the impedance deviation coefficient, determine the effective maximum and minimum values of the impedance deviation coefficient.

[0025] Preferably, taking the adjacent effective maximum values of the impedance deviation coefficient as the analysis interval, according to the reciprocal of the time difference t i of the analysis interval, the corresponding oscillation frequency f is obtainedi :

[0026]

[0027] Preferably, the time difference t of the analysis interval i is determined as follows:

[0028] When the minimum value r(b i ) of the impedance deviation coefficient is ≤ 1 and the minimum value r(b i+1 ) of the impedance deviation coefficient is ≤ 1, the minimum value r(b i ) of the impedance deviation coefficient is the effective minimum value of this analysis interval, and the adjacent effective maximum values of the impedance deviation coefficient are r(a i ) and r(a i+1 ). The starting point and ending point of this analysis interval are obtained, and the time difference t i of the i-th analysis interval is:

[0029]

[0030] When the minimum value r(b i ) of the impedance deviation coefficient > 1 and the minimum value r(b i+1 ) of the impedance deviation coefficient ≤ 1, or when the minimum value r(b i ) of the impedance deviation coefficient ≤ 1 and the minimum value r(b i+1 ) of the impedance deviation coefficient > 1, the minimum value of the impedance deviation coefficient less than 1 is the effective minimum value of this analysis interval, and the adjacent effective maximum values of the impedance deviation coefficient are r(a i ) and r(a i+2 ). The starting point and ending point of this analysis interval are obtained, and the time difference t i of the i-th analysis interval is:

[0031]

[0032] Among them, f0 is the power system power frequency, and a i , a i+1 , a i+2 are the number of cycles corresponding to the i-th, i + 1-th, and i + 2-th maximum values of the impedance deviation coefficient respectively, and b i , b i+1 are the number of cycles corresponding to the i-th and i + 1-th minimum values of the impedance deviation coefficient respectively, i = 1, 2, 3,....

[0033] Preferably, identifying the authenticity of the oscillation measured by the identification and measurement and control device includes:

[0034] According to the oscillation frequency of each analysis interval, when the fundamental frequency of the power grid is unknown and known, calculate the maximum value threshold and minimum value threshold of the impedance deviation coefficient respectively;

[0035] Search for all effective maximum and minimum values of the impedance deviation coefficient. When all effective maximum values of the impedance deviation coefficient are greater than the maximum value threshold and all effective minimum values are less than the minimum value threshold, it is determined that the oscillation measured by the measurement and control device is a false oscillation. When all effective maximum values of the impedance deviation coefficient are less than the maximum value threshold and all effective minimum values are greater than the minimum value threshold, it is determined that the oscillation measured by the measurement and control device is a true oscillation.

[0036] Preferably, according to the oscillation frequency of each analysis interval, the process of calculating the maximum value threshold and minimum value threshold of the impedance deviation coefficient respectively when the fundamental frequency of the power grid is unknown and known includes:

[0037] When the fundamental frequency f of the power grid within the analysis interval is unknown, the maximum value threshold p of the impedance deviation coefficient i and the minimum value threshold q i are respectively:

[0038]

[0039] When the fundamental frequency f of the power grid within the analysis interval is known and there is a positive deviation relative to the power system power frequency f0, the maximum value threshold p of the impedance deviation coefficient i and the minimum value threshold q i are respectively:

[0040]

[0041] When the fundamental frequency f of the power grid within the analysis interval is known and there is a negative deviation relative to the power system power frequency f0, the maximum value threshold p of the impedance deviation coefficient i and the minimum value threshold q i are respectively:

[0042]

[0043] Among them, Δf is the fundamental frequency deviation of the power grid, f i is the oscillation frequency, and α is the margin coefficient that comprehensively considers errors such as leakage of the full-wave Fourier algorithm and actual capacitance calculation.

[0044] Preferably, all effective maximum values of the impedance deviation coefficient being greater than the maximum value threshold and all effective minimum values being less than the minimum value threshold are expressed as:

[0045]

[0046] All effective maximum values of the impedance deviation coefficient being less than the maximum value threshold and all effective minimum values being greater than the minimum value threshold are expressed as:

[0047]

[0048] Among them, c iis the number of cycles corresponding to the effective maximum value r(c i ) of the impedance deviation coefficient within the i-th analysis interval, and d i is the number of cycles corresponding to the effective minimum value r(d i ) of the impedance deviation coefficient within the i-th analysis interval.

[0049] The present invention also provides a fundamental frequency pseudo-oscillation identification system for a low-sampling rate measurement and control device. The system includes:

[0050] An impedance deviation coefficient calculation module for calculating the impedance deviation coefficient of the fundamental impedance value of the capacitor relative to the actual impedance value of the capacitor;

[0051] An oscillation frequency calculation module for searching for the effective maximum and minimum values of the impedance deviation coefficient, using the adjacent effective maximum values of the impedance deviation coefficient as the analysis interval, and calculating the corresponding oscillation frequency based on the time difference of the analysis interval;

[0052] An oscillation authenticity identification module for calculating the thresholds of the maximum and minimum values of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and identifying the authenticity of the oscillation measured by the measurement and control device based on the relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding thresholds.

[0053] The advantages provided by the present invention are as follows:

[0054] 1. By searching for the effective maximum and minimum values of the impedance deviation coefficient, determining the analysis interval through adjacent effective maximum values, calculating the corresponding oscillation frequency based on the time difference of the analysis interval, calculating the thresholds of the maximum and minimum values of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and judging the authenticity of the oscillation measured by the measurement and control device based on the relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding thresholds, the present invention can distinguish between real oscillations and measurement pseudo-oscillations in the power grid, and solve the misjudgment risk caused by measurement pseudo-oscillations due to high-frequency harmonic aliasing in the measurement and control device.

[0055] 2. By identifying through the difference in the measured values of the capacitive reactance of the capacitor, which is caused by the difference in the oscillation degrees of voltage and current under true and false oscillations, the present invention is applicable not only to current oscillations but also to voltage oscillation situations, thus overcoming the defects of existing pseudo-oscillation determination methods that identify around frequency characteristics and may result in failure or incorrect results.

[0056] 3. According to the oscillation frequency of each analysis interval, the present invention can calculate the thresholds of the maximum and minimum values of the impedance deviation coefficient respectively when the fundamental frequency of the power grid is unknown and known. First, it can accurately calculate the different thresholds corresponding to different oscillation frequencies, avoiding the problem of inconsistent identification accuracy at different frequencies caused by a constant threshold. Second, it can meet the accurate threshold calculation in different scenarios (known fundamental frequency and unknown fundamental frequency), without always relying on the fundamental frequency, overcoming the problem that the frequency cannot be accurately measured during voltage fluctuations; through the above formula, the purpose of expanding the scenario applicability of the identification method and improving the identification accuracy is achieved.

[0057] 4. The pseudo-oscillation identification method of the present invention can be integrated into a low-sampling-rate measurement and control device to identify true and false oscillations in real time without the need for offline processing of data. The present invention is also applicable to the processing of offline exported data. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 is a flowchart of the fundamental wave pseudo-oscillation identification method measured by the low-sampling-rate measurement and control device provided in Embodiment 1 of the present invention;

[0059] FIG. 2(a) is a schematic diagram of the determination process of the effective maximum and minimum values of the impedance deviation coefficient in Case 1 in the fundamental wave pseudo-oscillation identification method measured by the low-sampling-rate measurement and control device provided in Embodiment 1 of the present invention;

[0060] FIG. 2(b) is a schematic diagram of the determination process of the effective maximum and minimum values of the impedance deviation coefficient in Case 2 in the fundamental wave pseudo-oscillation identification method measured by the low-sampling-rate measurement and control device provided in Embodiment 1 of the present invention;

[0061] FIG. 2(c) is a schematic diagram of the determination process of the effective maximum and minimum values of the impedance deviation coefficient in Case 3 in the fundamental wave pseudo-oscillation identification method measured by the low-sampling-rate measurement and control device provided in Embodiment 1 of the present invention;

[0062] Figure 3 is a schematic diagram of the fundamental wave pseudo-oscillation identification system measured by the low-sampling-rate measurement and control device provided in Embodiment 2 of the present invention;

[0063] Figure 4 is the waveform diagram of the voltage and current of the capacitor branch measured by the measurement and control device in Embodiment 3;

[0064] Figure 5 is a schematic diagram of the fundamental wave components of the oscillating voltage and current analyzed by the full-wave Fourier algorithm period by period in Embodiment 3;

[0065] Figure 6 is the information diagram of the impedance deviation coefficient and its effective maximum and minimum values in Embodiment 3. DETAILED DESCRIPTION OF THE INVENTION

[0066] To make the objectives, technical solutions, and advantages of the present invention more clear and understandable, the following describes the technical solutions of the present invention clearly and completely in conjunction with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are some but not all of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0067] Embodiment 1

[0068] See Figure 1 , this embodiment provides a method for identifying fundamental wave pseudo-oscillations measured by a low-sampling-rate measurement and control device, which is applicable to low-sampling measurement and control devices for monitoring capacitor branches. The method includes the following steps:

[0069] Step 1: Analyze the fundamental wave components of the voltage and current of the capacitor branch where the measurement and control device has oscillations cycle by cycle using the full-wave Fourier algorithm, obtain the fundamental wave impedance value of the capacitor, and then calculate the impedance deviation coefficient of the fundamental wave impedance value of the capacitor relative to the actual impedance value of the capacitor. Step 1 specifically includes the following processes:

[0070] Step 1.1: Determine the number of sampling points N within one cycle of the voltage or current according to the sampling frequency of the measurement and control device; the calculation formula for the number of sampling points N is:

[0071]

[0072] where f s is the sampling frequency of the measurement and control device, with the unit of Hz. The sampling frequency f s of the measurement and control device can be obtained through channels such as the product instruction manual or specification nameplate, and f0 is the power system power frequency, f0 = 50Hz.

[0073] Step 1.2: Use the full-wave Fourier algorithm to calculate the discrete data of the voltage and current waveforms of the capacitor branch where the measurement and control device has oscillation phenomena cycle by cycle, and obtain the fundamental wave components (amplitudes) of the voltage and current corresponding to each cycle. To ensure the reliability of the oscillation authenticity identification result, the duration of the collected data should not be too short, and it is recommended that the time is not less than 10 minutes. The calculation formulas for the fundamental wave voltage amplitude U(k) and fundamental wave current amplitude I(k) in the k-th cycle are:

[0074]

[0075] where U(k) is the fundamental wave voltage amplitude in the k-th cycle, with the unit of V, I(k) is the fundamental wave current amplitude in the k-th cycle, with the unit of A, u k (n), i k(n) is the nth sampling value of the voltage waveform and the nth sampling value of the current waveform in the kth cycle, respectively, where k = 1, 2, 3,..., and N is the number of sampling points included in the cycle.

[0076] Step 1.3: Obtain the fundamental impedance value of the capacitor corresponding to each cycle through the ratio operation of the fundamental components of voltage and current in each cycle; the fundamental impedance value Z m (k) is:

[0077]

[0078] where Z m (k) is the measured value of the fundamental impedance value of the capacitor in the kth cycle, with the unit of Ω, and k = 1, 2, 3,....

[0079] Step 1.4: Collect the rated capacity Q co of the capacitor (bank) in the capacitor branch and the rated voltage U c of the capacitor (bank), and calculate the actual impedance value Z co of the capacitor (bank); the actual impedance value Z co is:

[0080]

[0081] where the actual impedance value Z co is the actual value of the fundamental impedance value of the capacitor (bank), with the unit of Ω, Q co is the rated capacity of the capacitor (bank), with the unit of Mvar, and U c is the rated voltage of the capacitor (bank), with the unit of kV.

[0082] Step 1.5: Obtain the impedance deviation coefficient r(k) of each cycle through the ratio operation of the fundamental impedance value Z m (k) of the capacitor and the actual impedance value Z co of the capacitor; the impedance deviation coefficient r(k) of each cycle is:

[0083]

[0084] where r(k) is the impedance deviation coefficient in the kth cycle.

[0085] Step 2: Search for the effective maximum and minimum values of the impedance deviation coefficient, determine the analysis interval through adjacent effective maximum values, and calculate the corresponding oscillation frequency based on the time difference of the analysis interval. Step 2 specifically includes the following processes:

[0086] Step 2.1: Search for the points where the impedance deviation coefficient is greater than the impedance deviation coefficients of the left and right adjacent points according to the conditions of formula (6) to obtain the maximum value r(a i);

[0087]

[0088] where a i is the cycle number corresponding to the maximum value of the i-th impedance deviation coefficient, i = 1, 2, 3,....

[0089] Step 2.2: Search for points where the impedance deviation coefficient is less than the left and right adjacent impedance deviation coefficients respectively between adjacent maximum values of the impedance deviation coefficient r(a i ) and r(a i+1 ), and between adjacent maximum values of the impedance deviation coefficient r(a i+1 ) and r(a i+2 ) according to the conditions of formula (7), to obtain adjacent minimum values of the impedance deviation coefficient r(b i ) and r(b i+1 );

[0090]

[0091] where b i is the cycle number corresponding to the minimum value of the i-th impedance deviation coefficient, i = 1, 2, 3,....

[0092] Step 2.3: Determine the effective maximum and minimum values of the impedance deviation coefficient according to the relationship between adjacent minimum values of the impedance deviation coefficient r(b i ) and r(b i+1 ), determine each analysis interval through the adjacent effective maximum values of the impedance deviation coefficient, and calculate the time difference t i of the analysis interval based on the cycle numbers corresponding to each analysis interval.

[0093] Figure 2 is a schematic diagram of the determination process of the effective maximum and minimum values of the impedance deviation coefficient. Among them, referring to Figure 2(a), for Case 1: when the minimum value of the impedance deviation coefficient r(b i ) ≤ 1 and the minimum value of the impedance deviation coefficient r(b i+1 ) ≤ 1, the minimum value of the impedance deviation coefficient r(b i ) is the effective minimum value of this analysis interval, and the adjacent effective maximum values of the impedance deviation coefficient are r(a i ) and r(a i+1 ), to obtain the starting point and ending point of this analysis interval. The time difference t i of the i-th analysis interval is:

[0094]

[0095] Referring to Figure 2(b) and Figure 2(c), Figure 2(b) is for Case 2, and Figure 2(c) is for Case 3: when the minimum value of the impedance deviation coefficient r(b i) > 1 and the minimum value r(b) of the impedance deviation coefficient i+1 ) ≤ 1 (Case 2), or when the minimum value r(b) of the impedance deviation coefficient i ) ≤ 1 and the minimum value r(b) of the impedance deviation coefficient i+1 ) > 1 (Case 3), the minimum value of the impedance deviation coefficient less than 1 is the effective minimum value of this analysis interval, and the adjacent effective maximum values of the impedance deviation coefficient are r(a i ) and r(a i+2 ), obtain the start point and end point of this analysis interval, and calculate the time difference t of the i-th analysis interval according to the number of cycles corresponding to the start point and end point of this analysis interval according to formula (9) i is:

[0096]

[0097] where f0 is the power system power frequency, a i , a i+1 , a i+2 are the number of cycles corresponding to the i-th, i + 1-th, and i + 2-th maximum values of the impedance deviation coefficient respectively, b i , b i+1 are the number of cycles corresponding to the i-th and i + 1-th minimum values of the impedance deviation coefficient respectively, i = 1, 2, 3,....

[0098] Step 2.4, calculate the corresponding oscillation frequency f i according to the reciprocal of the time difference t of the analysis interval i , and the calculation formula of the oscillation frequency f i is:

[0099]

[0100] where f i is the oscillation frequency corresponding to the i-th analysis interval, i = 1, 2, 3,....

[0101] The subsequent oscillation identification process only involves the effective maximum and minimum values within the analysis interval. The number of cycles corresponding to the effective maximum value (start point of the analysis interval) and the minimum value within the i-th analysis interval are respectively denoted as c i and d i .

[0102] Step 3, calculate the maximum value threshold and minimum value threshold of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and identify the authenticity of the oscillation measured by the monitoring and control device through the relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding thresholds.

[0103] Step 3 specifically includes the following process:

[0104] Step 3.1: Calculate the maximum threshold and minimum threshold of the impedance deviation coefficient respectively when the fundamental frequency of the power grid is unknown and known, based on the oscillation frequency of each analysis interval.

[0105] Among them, when the fundamental frequency f of the power grid within the analysis interval is unknown, the maximum threshold p of the impedance deviation coefficient i and the minimum threshold q i are respectively:

[0106]

[0107] When the fundamental frequency f of the power grid within the analysis interval is known and has a positive deviation relative to the power system power frequency f0, the maximum threshold p of the impedance deviation coefficient i and the minimum threshold q i are respectively:

[0108]

[0109] When the fundamental frequency f of the power grid within the analysis interval is known and has a negative deviation relative to the power system power frequency f0, the maximum threshold p of the impedance deviation coefficient i and the minimum threshold q i are respectively:

[0110]

[0111] Among them, Δf is the deviation of the fundamental frequency of the power grid, and the upper limit of the value can be determined according to the limit specified in the standard "GB / T 15945-2008 Power Quality - Power System Frequency Deviation", and the value range is 0Hz ≤ Δf ≤ 0.2Hz; α is a margin coefficient that comprehensively considers errors such as leakage of the full-wave Fourier algorithm and actual capacitance calculation, and the value range is 1.05 ≤ α ≤ 1.2.

[0112] According to the oscillation frequency of each analysis interval, the present invention can calculate the thresholds of the maximum and minimum values of the impedance deviation coefficient respectively when the fundamental frequency of the power grid is unknown and known. First, it can accurately calculate different thresholds corresponding to different oscillation frequencies, avoiding the problem of inconsistent identification accuracy at different frequencies caused by a constant threshold. Second, it can meet the accurate threshold calculation in different scenarios (known fundamental frequency and unknown fundamental frequency), and does not always rely on the fundamental frequency, overcoming the problem that the frequency cannot be accurately measured when the voltage fluctuates; through the above formulas, the purpose of expanding the scenario applicability of the identification method and improving the accuracy of identification is achieved.

[0113] Step 3.2: Compare each of the effective maximum and minimum values of the searched impedance deviation coefficients with the threshold value one by one. According to the comparison results, judge the authenticity of the oscillation measured by the monitoring and control device. When all the effective maximum values of the impedance deviation coefficients are greater than the maximum threshold value and all the effective minimum values are less than the minimum threshold value, it is judged that the oscillation measured by the monitoring and control device is a false oscillation, that is, the oscillation measured by the monitoring and control device is an illusion caused by measurement aliasing, which is expressed by formula (17):

[0114]

[0115] When all the effective maximum values of the impedance deviation coefficients are less than the maximum threshold value and all the effective minimum values are greater than the minimum threshold value, it is judged that the oscillation measured by the monitoring and control device is a true oscillation, and there is an oscillation phenomenon in the power grid, which is expressed by formula (18):

[0116]

[0117] When there are cases where the effective maximum or minimum value of the searched impedance deviation coefficient is both greater than and less than the corresponding threshold value, the authenticity of the oscillation measured by the monitoring and control device cannot be judged.

[0118] In the present invention, by analyzing the fundamental wave components of the capacitor branch voltage and current with oscillation phenomenon in the monitoring and control device cycle by cycle, calculating the impedance deviation coefficient of each cycle, then searching for the effective maximum and minimum values of the impedance deviation coefficient, determining the analysis interval through adjacent effective maximum values, and calculating the corresponding oscillation frequency based on the time difference of the analysis interval. By the relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding threshold value, the authenticity of the oscillation measured by the monitoring and control device is judged. The present invention mainly utilizes the known relationship between the capacitive reactance of the capacitor and the harmonic frequency to identify from the difference in impedance of the capacitor under true and false oscillations. This difference is caused by the difference in the oscillation degree of voltage and current under true and false oscillations, and thus can distinguish the true oscillation in the power grid from the measurement false oscillation, and solve the misjudgment risk brought by the measurement false oscillation caused by high-frequency harmonic aliasing in the monitoring and control device.

[0119] Embodiment 2

[0120] See Figure 3 , this embodiment provides a measurement false oscillation identification system, including:

[0121] An impedance deviation coefficient calculation module, which is used to calculate the impedance deviation coefficient of the fundamental wave impedance value of the capacitor relative to the actual impedance value of the capacitor. The calculation process of the impedance deviation coefficient includes:

[0122] Adopt the full-wave Fourier algorithm to calculate the discrete data of the voltage and current waveforms of the capacitor branch with oscillation measured by the monitoring and control device cycle by cycle, and obtain the fundamental wave components of the voltage and current of each cycle;

[0123] By calculating the ratio of the fundamental voltage and current components in each cycle, the fundamental impedance value of the capacitor corresponding to each cycle is obtained;

[0124] By calculating the ratio of the fundamental impedance value of the capacitor to the actual impedance value of the capacitor, the impedance deviation coefficient for each cycle is obtained.

[0125] Among them, the fundamental voltage and current components in each cycle are:

[0126]

[0127] The fundamental impedance value Z m (k) is:

[0128]

[0129] The impedance deviation coefficient r(k) for each cycle is:

[0130]

[0131] Among them, N is the number of sampling points in one cycle of voltage or current, U(k) and I(k) are the fundamental voltage amplitude and fundamental current amplitude in the k-th cycle respectively, u k (n) and i k (n) are the n-th sampling value of the voltage waveform and the n-th sampling value of the current waveform in the k-th cycle respectively, k = 1, 2, 3,..., co is the actual impedance value of the capacitor.

[0132] The oscillation frequency calculation module is used to search for the effective maximum and minimum values of the impedance deviation coefficient, take the adjacent effective maximum values of the impedance deviation coefficient as the analysis interval, and calculate the corresponding oscillation frequency based on the time difference of the analysis interval.

[0133] Among them, searching for the effective maximum and minimum values of the impedance deviation coefficient includes:

[0134] Step 2.1: Search for the points where the impedance deviation coefficient is greater than the left and right adjacent impedance deviation coefficients to obtain the maximum value r(a i ) of the impedance deviation coefficient;

[0135] Step 2.2: Respectively search for the points where the impedance deviation coefficient is less than the left and right adjacent impedance deviation coefficients between the adjacent maximum values r(a i ) and r(a i+1 ), and between the adjacent maximum values r(a i+1 ) and r(a i+2 ) of the impedance deviation coefficient to obtain the adjacent minimum values r(b i ) and r(b i+1 ) of the impedance deviation coefficient;

[0136] Step 2.3: Determine the effective maximum and minimum values of the impedance deviation coefficient according to the relationship between the adjacent minimum values of the impedance deviation coefficient r(b i ) and r(b i+1 ).

[0137] Analyze the time difference t i of the interval. The determination process is as follows: When the minimum value of the impedance deviation coefficient r(b i ) ≤ 1 and the minimum value of the impedance deviation coefficient r(b i+1 ) ≤ 1, the minimum value of the impedance deviation coefficient r(b i ) is the effective minimum value of this analysis interval, and the adjacent effective maximum values of the impedance deviation coefficient are r(a i ) and r(a i+1 ). Obtain the starting point and ending point of this analysis interval. The time difference t i of the i-th analysis interval is:

[0138]

[0139] When the minimum value of the impedance deviation coefficient r(b i ) > 1 and the minimum value of the impedance deviation coefficient r(b i+1 ) ≤ 1, or the minimum value of the impedance deviation coefficient r(b i ) ≤ 1 and the minimum value of the impedance deviation coefficient r(b i+1 ) > 1, the minimum value of the impedance deviation coefficient less than 1 is the effective minimum value of this analysis interval, and the adjacent effective maximum values of the impedance deviation coefficient are r(a i ) and r(a i+2 ). Obtain the starting point and ending point of this analysis interval. The time difference t i of the i-th analysis interval is:

[0140]

[0141] where f0 is the power system power frequency, a i , a i+1 , a i+2 are the cycle numbers corresponding to the i-th, i + 1-th, and i + 2-th maximum values of the impedance deviation coefficient respectively, b i , b i+1 are the cycle numbers corresponding to the i-th and i + 1-th minimum values of the impedance deviation coefficient respectively, i = 1, 2, 3, ….

[0142] Step 2.4: Take the adjacent effective maximum values of the impedance deviation coefficient as the analysis interval, and obtain the corresponding oscillation frequency f i according to the reciprocal of the time difference t i of the analysis interval:

[0143]

[0144] An oscillation true / false identification module is used to calculate the maximum and minimum thresholds of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and identify the true or false oscillation measured by the measurement and control device through the relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding thresholds. Identifying the true or false oscillation measured by the measurement and control device includes:

[0145] Step 3.1: Calculate the maximum and minimum thresholds of the impedance deviation coefficient respectively when the fundamental frequency of the power grid is unknown and known according to the oscillation frequency of each analysis interval.

[0146] Among them, when the fundamental frequency f of the power grid within the analysis interval is unknown, the maximum threshold p i and the minimum threshold q i of the impedance deviation coefficient are respectively:

[0147]

[0148] When the fundamental frequency f of the power grid within the analysis interval is known and has a positive deviation relative to the power system power frequency f0, the maximum threshold p i and the minimum threshold q i of the impedance deviation coefficient are respectively:

[0149]

[0150] When the fundamental frequency f of the power grid within the analysis interval is known and has a negative deviation relative to the power system power frequency f0, the maximum threshold p i and the minimum threshold q i of the impedance deviation coefficient are respectively:

[0151]

[0152]

[0153] Among them, Δf is the fundamental frequency deviation of the power grid, and α is a margin coefficient that comprehensively considers errors such as leakage of the full-wave Fourier algorithm and actual capacitance calculation.

[0154] Step 3.2: Search for all effective maximum and minimum values of the impedance deviation coefficient. When all effective maximum values of the impedance deviation coefficient are greater than the maximum threshold and all effective minimum values are less than the minimum threshold, it is judged that the oscillation measured by the measurement and control device is a false oscillation. When all effective maximum values of the impedance deviation coefficient are less than the maximum threshold and all effective minimum values are greater than the minimum threshold, it is judged that the oscillation measured by the measurement and control device is a true oscillation.

[0155] All effective maximum values of the impedance deviation coefficient being greater than the maximum threshold and all effective minimum values being less than the minimum threshold are expressed as:

[0156]

[0157] All effective maximum values of the impedance deviation coefficients are less than the maximum threshold value and all effective minimum values are greater than the minimum threshold value, which is expressed as:

[0158]

[0159] where c i is the number of cycles corresponding to the effective maximum value r(c i ) of the impedance deviation coefficient in the i-th analysis interval, and d i is the number of cycles corresponding to the effective minimum value r(d i ) of the impedance deviation coefficient in the i-th analysis interval.

[0160] Embodiment 3

[0161] To better understand the present invention, the above process will be described in detail below with specific examples. The numerical values used in this example are only for illustration, and users can make corresponding changes according to actual needs. Oscillation phenomena occur in the measured voltage and current of a 35 kV capacitor branch measurement and control device. The sampling frequency of this measurement and control device is 1200 Hz, and the sampled oscillation waveform is as Figure 4 shown. Now, the authenticity of the oscillation is identified.

[0162] According to the sampling frequency of this measurement and control device, the number of sampling points N for each cycle of the measured voltage and current is calculated as N = 1200 / 50 = 24. Using the full-wave Fourier algorithm, the discrete data of the voltage and current waveforms of the capacitor branch with oscillation phenomena collected by the measurement and control device are calculated cycle by cycle according to formula (2), and the fundamental wave components (amplitudes) of the voltage and current corresponding to each cycle are obtained. The results are as Figure 5 shown. Then, the measured values of the fundamental wave impedance values of the capacitors corresponding to each cycle are calculated according to formula (3). In this example, the rated voltage of the capacitor bank included in the measured capacitor branch is 35 kV, and the rated capacity is 20 Mvar. According to formula (4), the actual value Z co of the fundamental wave impedance value of this capacitor (bank) is calculated as 61.25 Ω. According to formula (5), the ratio of the measured value to the actual value of the fundamental wave impedance value of the capacitor is calculated to obtain the impedance deviation coefficient for each cycle, as Figure 6 shown.

[0163] Search for Figure 6 the effective maximum and minimum values of the impedance deviation coefficients therein, obtain the number of cycles corresponding to the effective maximum and minimum values, and use the adjacent effective maximum values to form the starting point and ending point of the analysis interval. Then, calculate the time difference of each analysis interval based on the corresponding number of cycles, and finally obtain the corresponding oscillation frequency. The results are as Figure 6 and Table 1 shown.

[0164] Table 1 Effective maximum values, minimum values, corresponding cycle numbers, and analysis interval oscillation frequencies

[0165]

[0166] In this example, the threshold is calculated according to the maximum error principle. Since the fundamental frequency of the power grid is unknown, the deviation of the fundamental frequency is taken as the maximum value, Δf = 0.2 Hz; the margin coefficient is also taken as the maximum value, α = 1.2. According to the oscillation frequencies of each analysis interval in Table 1, the maximum threshold p of the impedance deviation coefficient is calculated according to formulas (11) and (12) respectively i = 1.013 and the minimum threshold q i = 0.987. By comparing with all the effective maximum and minimum values of the impedance deviation coefficient, it satisfies the condition that all the effective maximum values of the impedance deviation coefficient are greater than the corresponding thresholds and all the effective minimum values are less than the corresponding thresholds. Therefore, it can be judged that the oscillation measured by the measurement and control device is a pseudo-oscillation, which is an illusion caused by its measurement aliasing, and the authenticity of the oscillation is accurately identified

[0167] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention

Claims

1. Method for identifying fundamental wave pseudo-oscillation in low sampling rate measurement and control device, characterized in that: The method includes: Calculating an impedance deviation coefficient of a fundamental wave impedance value of a capacitor relative to an actual impedance value of the capacitor; Searching for effective maximum and minimum values of the impedance deviation coefficient, using an adjacent effective maximum value of the impedance deviation coefficient as an analysis interval, and calculating a corresponding oscillation frequency based on a time difference of the analysis interval; Calculating threshold values of maximum and minimum values of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and identifying the authenticity of the oscillation measured by the measurement and control device based on a relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding threshold values.

2. The method for identifying fundamental wave pseudo-oscillation in the low sampling rate measurement and control device according to claim 1, wherein: The calculation process of the impedance deviation coefficient is as follows: Using the full-wave Fourier algorithm to calculate discrete data of voltage and current waveforms of a capacitor branch where oscillation exists in the measurement by the measurement and control device for each period, and obtaining fundamental wave components of voltage and current for each period; Obtaining the fundamental wave impedance value of the capacitor corresponding to each period through a ratio operation of the fundamental wave components of voltage and current for each period; Obtaining the impedance deviation coefficient for each period through a ratio operation of the fundamental wave impedance value of the capacitor and the actual impedance value of the capacitor.

3. The method for identifying fundamental wave pseudo-oscillation in a low sampling rate measurement and control device according to claim 2, characterized in that: The fundamental wave components of voltage and current for each period are: The fundamental wave impedance value Z of the capacitor m (k) is as follows: The impedance deviation coefficient r(k) for each period is: Where N is the number of sampling points in one cycle of voltage or current, U(k) and I(k) are the fundamental voltage amplitude and fundamental current amplitude in the k-th cycle respectively, and u k (n) and i k (n) are the n-th sampling value of the voltage waveform and the n-th sampling value of the current waveform in the k-th cycle respectively, where k = 1, 2, 3,..., Z co is the actual impedance value of the capacitor.

4. The method for identifying fundamental wave pseudo-oscillation in a low sampling rate measurement and control device according to claim 1, characterized in that: Searching for the effective maximum and minimum values of the impedance deviation coefficient includes: Search for the points where the search impedance deviation coefficient is greater than the impedance deviation coefficients of the left and right adjacent points to obtain the maximum value r(a i ); Search for points where the impedance deviation coefficient is less than the impedance deviation coefficients of the adjacent left and right points between the adjacent maximum values r(a i ) and r(a i+1 ), and between the adjacent maximum values r(a i+1 ) and r(a i+2 ), to obtain the adjacent minimum values r(b i ) and r(b i+1 ); According to the relationship between the minimum values r(b i ) and r(b i+1 ) of adjacent impedance deviation coefficients, the effective maximum and minimum values of the impedance deviation coefficient are determined.

5. The method for identifying fundamental wave pseudo-oscillation in a low sampling rate measurement and control device according to claim 4, characterized in that: Taking the effective maximum value of the adjacent impedance deviation coefficient as the analysis interval, the corresponding oscillation frequency f is obtained according to the reciprocal of the time difference t of the analysis interval i i :​ 6. The method for identifying fundamental wave pseudo-oscillation in the low sampling rate measurement and control device according to claim 5, characterized in that: Analyze the time difference t of the interval i The determination process is as follows: When the impedance deviation coefficient minimum value r(b i ) ≤ 1 and the impedance deviation coefficient minimum value r(b i+1 ) ≤ 1, the impedance deviation coefficient minimum value r(b i ) is the effective minimum value of this analysis interval, and the adjacent impedance deviation coefficient effective maximum values are r(a i ) and r(a i+1 ). The start point and end point of this analysis interval are obtained, and the time difference t i of the i-th analysis interval is: When the impedance deviation coefficient minimum value r(b i ) > 1 and the impedance deviation coefficient minimum value r(b i+1 ) ≤ 1, or when the impedance deviation coefficient minimum value r(b i ) ≤ 1 and the impedance deviation coefficient minimum value r(b i+1 ) > 1, the impedance deviation coefficient minimum value less than 1 is the effective minimum value of this analysis interval, and the adjacent impedance deviation coefficient effective maximum values are r(a i ) and r(a i+2 ), obtaining the starting point and ending point of this analysis interval. The time difference t i of the i-th analysis interval is: Among them, f0 is the power frequency of the power system, a i 、a i+1 、a i+2 are the cycle numbers corresponding to the maximum values of the impedance deviation coefficients of the i-th, i+1-th, and i+2-th respectively, b i 、b i+1 are the cycle numbers corresponding to the minimum values of the impedance deviation coefficients of the i-th and i+1-th respectively, i = 1, 2, 3, ….

7. The method for identifying fundamental wave pseudo-oscillation in the low sampling rate measurement and control device according to claim 1, characterized in that: Identifying the authenticity of the oscillation measured by the measurement and control device includes: Calculating the threshold value of the maximum value of the impedance deviation coefficient and the threshold value of the minimum value respectively under the conditions that the fundamental wave frequency of the power grid is unknown and known based on the oscillation frequency of each analysis interval; Searching for all effective maximum and minimum values of the impedance deviation coefficient. When all effective maximum values of the impedance deviation coefficient are greater than the maximum value threshold and all effective minimum values are less than the minimum value threshold, it is determined that the oscillation measured by the measurement and control device is a false oscillation. When all effective maximum values of the impedance deviation coefficient are less than the maximum value threshold and all effective minimum values are greater than the minimum value threshold, it is determined that the oscillation measured by the measurement and control device is a true oscillation.

8. The method for identifying fundamental wave pseudo-oscillation in a low sampling rate measurement and control device according to claim 7, characterized in that: The process of calculating the threshold value of the maximum value of the impedance deviation coefficient and the threshold value of the minimum value respectively under the conditions that the fundamental wave frequency of the power grid is unknown and known based on the oscillation frequency of each analysis interval includes: When the fundamental frequency f of the power grid within the analysis interval is unknown, the maximum threshold p of the impedance deviation coefficient i , and the minimum threshold q i are respectively: When the fundamental grid frequency f within the analysis interval is known and has a positive deviation relative to the power system power frequency f0, the maximum threshold p of the impedance deviation coefficient i and the minimum threshold q i are respectively When the fundamental grid frequency f within the analysis interval is known and has a negative deviation relative to the power system power frequency f0, the maximum threshold p of the impedance deviation coefficient i , and the minimum threshold q i are respectively: where Δf is the fundamental frequency deviation of the power grid, f i is the oscillation frequency, and α is the margin coefficient comprehensively considering errors such as leakage of the full-wave Fourier algorithm and actual capacitance calculation.

9. The method for identifying fundamental wave pseudo-oscillation in the low sampling rate measurement and control device according to claim 8, characterized in that: All effective maximum values of the impedance deviation coefficient being greater than the maximum value threshold and all effective minimum values being less than the minimum value threshold is expressed as: All effective maximum values of the impedance deviation coefficient being less than the maximum value threshold and all effective minimum values being greater than the minimum value threshold is expressed as: Among them, c i is the number of cycles corresponding to the effective maximum value r(c i ) of the impedance deviation coefficient within the i-th analysis interval, and d i is the number of cycles corresponding to the effective minimum value r(d i ) of the impedance deviation coefficient within the i-th analysis interval.

10. A fundamental wave pseudo-oscillation identification system for a low sampling rate measurement and control device, characterized in that: The system includes: An impedance deviation coefficient calculation module, configured to calculate an impedance deviation coefficient of a fundamental wave impedance value of a capacitor relative to an actual impedance value of the capacitor; An oscillation frequency calculation module, configured to search for effective maximum and minimum values of the impedance deviation coefficient, use an adjacent effective maximum value of the impedance deviation coefficient as an analysis interval, and calculate a corresponding oscillation frequency based on a time difference of the analysis interval; An oscillation authenticity identification module, configured to calculate threshold values of maximum and minimum values of the impedance deviation coefficient corresponding to the oscillation frequency of each analysis interval, and identify the authenticity of the oscillation measured by the measurement and control device based on a relationship between the effective maximum and minimum values of the impedance deviation coefficient and the corresponding threshold values.

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