CN112886554A - Zero-sequence overcurrent protection misoperation prevention method based on waveform inertia - Google Patents
Zero-sequence overcurrent protection misoperation prevention method based on waveform inertia Download PDFInfo
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Abstract
The invention discloses a zero sequence overcurrent protection misoperation prevention method based on waveform inertia. The method comprises the following steps: s1, starting protection, and calculating a fundamental wave effective value and a second harmonic effective value of the zero-mode inrush current in real time; s2, judging whether the zero sequence current effective value reaches a zero sequence overcurrent protection starting threshold, if so, starting zero sequence overcurrent protection and starting time delay; s3, judging the content of the second harmonic, if the content of the second harmonic is always less than the setting value in the set time delay, the zero sequence overcurrent protection outlet acts; s4, calculating the inertia degree of the zero-mode inrush current waveform in real time from the second cycle after protection starting; and S5, if the second harmonic content is larger than the setting value, judging the zero-modulus inrush current waveform inertia degree, if the second harmonic content is larger than the setting value, braking zero-sequence overcurrent protection, and if the second harmonic content is smaller than the setting value, immediately opening protection and performing zero-sequence overcurrent protection. The invention can prevent the problem of zero sequence overcurrent protection misoperation caused by large initial value and long attenuation time of the zero sequence current in the transformer operation process under certain conditions.
Description
Technical Field
The invention relates to the technical field of power system relay protection, in particular to a zero-sequence overcurrent protection misoperation prevention method based on waveform inertia.
Background
With the development of economic society and the requirements of energy conservation and environmental protection, new energy sources and energy storage equipment in various forms appear in a power system, the capacity of a power grid is continuously increased, the impedance of the system is gradually reduced, and the problem that the short-circuit current exceeds the standard generally appears. In order to limit the short-circuit current on the low-voltage side of the transformer, high-impedance transformers are widely used. Under the novel power grid form, zero-mode inrush current with large amplitude and long attenuation time appears when a high-impedance transformer is in idle operation, so that the condition of zero-sequence overcurrent protection misoperation is caused, and serious threat is brought to the safe operation of a system. However, the zero-sequence overcurrent protection can only judge the magnitude of fundamental zero-sequence current at present, and can not effectively identify zero-mode inrush current generated when the transformer is in idle operation.
Regarding the improvement method of zero sequence overcurrent protection, documents [1,2] propose a method based on the second harmonic content and the variation trend of zero-mode inrush current, but the magnitude of the harmonic content is not demonstrated, and the author thinks that only the second harmonic content of zero-mode inrush current is slowly increased, and the zero sequence overcurrent protection is locked by taking the second harmonic content as the identification criterion. However, when a fault occurs, the Current Transformer (CT) may be saturated, so that the secondary current is distorted, a higher second harmonic is generated, and the zero sequence overcurrent protection may be rejected. On the other hand, the application of a large number of power electronic devices in the power system may lead to uncertain changes in the harmonic content, and the validity of the method is to be further checked. In addition, the applicability verification of the method does not consider the situation that the transformer is applied to faults, faults during inrush current and the like, and the application situation of the method is limited. Therefore, an improved method for zero-sequence overcurrent protection, which is strong in applicability, simple, reliable and practical, is not available at present.
Research on inrush current characteristics of a high-voltage built-in transformer and coping strategies thereof causing false operation of zero sequence current protection, in documents [1], liuyao and wuhan: university of science and technology in china, 2018.
Document [2], Zhang Pefu, jin Neng, Liu Yao, etc., a new bus-coupled zero-sequence overcurrent protection scheme [ J ] based on the second harmonic content and trend change, China Motor engineering newspaper, 2020, v.40; no.643(08) 203-.
Disclosure of Invention
The invention aims to overcome the defect that the zero-sequence overcurrent protection cannot identify the inrush current with large amplitude and long attenuation zero modulus when a transformer is in idle operation at present, and provides a zero-sequence overcurrent protection misoperation prevention method based on waveform inertia to solve the problems of incorrect action of the existing zero-sequence overcurrent protection when the transformer is in idle operation, prevent the zero-sequence overcurrent protection misoperation caused by large initial value of zero-sequence current and long attenuation time in the process of transformer operation under certain conditions, and improve the zero-sequence overcurrent protection action performance by using the refusal action problem of harmonic braking criterion when CT saturation and large system harmonic content only.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a zero sequence overcurrent protection misoperation prevention method based on waveform inertia comprises the following steps:
s1, starting protection, and calculating a fundamental wave effective value and a second harmonic effective value of the zero-mode inrush current in real time;
s2, judging whether the zero sequence current effective value reaches a zero sequence overcurrent protection starting threshold, if so, starting zero sequence overcurrent protection and starting time delay;
s3, judging the content of the second harmonic, if the content of the second harmonic is always less than the setting value in the set time delay, the zero sequence overcurrent protection outlet acts;
s4, calculating the inertia degree of the zero-mode inrush current waveform in real time from the second cycle after protection starting;
s5, if the second harmonic content is larger than the setting value, judging the zero-mode inrush current waveform inertia, and if the waveform inertia is larger than the setting value, braking zero-sequence overcurrent protection; and immediately opening protection and performing zero-sequence overcurrent protection once the inertia degree is detected to be smaller than the setting value.
Further, in step S1, A, B, C three-phase currents, i respectively, are measured by a current transformer installed at the outlet end of the primary winding of the transformerA，iB，iCCalculating the sudden change of three-phase current, starting protection operation when the sudden change reaches a threshold, and setting the zero-mode inrush current to be 3i0＝iA+iB+iCCalculating the power frequency fundamental wave effective value, namely zero sequence current 3I0Second harmonic effective value of 3i0_2ndSecond harmonic content of 3i0_2nd％。
Further, in step S2, the zero-sequence overcurrent protection is performed when the zero-sequence current exceeds the threshold value I0setAnd maintain a set delay t0setTo judge the zero sequence currentAnd if the zero sequence overcurrent protection exceeds the threshold value, starting zero sequence overcurrent protection and starting to calculate time delay.
Furthermore, in step S2, the zero sequence overcurrent protection has two thresholds and corresponding delays, such as the threshold and the delay of section I and section II are (I) respectively0set_I，t0set_I)、(I0set_II，t0set_II)。
Further, in step S4, the waveform inertia, that is, the waveform data of the half-period data window from the current sampling point and the waveform data of the half-period data window from the sampling point of the previous period from the current sampling point are two vectors, and the product of the cosine similarity of the two vectors and the relative euclidean distance subtracted by 1 is calculated in real time, where the waveform inertia calculation formula is:
G(t)＝Simi(X,Y)×[1-Dist(X,Y)] (1)
wherein X is 3i0[t-3T/2，t-T]，Y＝3i0[t-T/2，t]The method comprises the following steps of obtaining a sampling point, obtaining a sampling point time, obtaining a power frequency period, obtaining Simi (X, Y) and Dist (X, Y), obtaining a sampling point time, and obtaining a power frequency period time.
Furthermore, the cosine similarity algorithm measures the similarity between two sets of data based on the cosine values of the included angle between two vectors in the vector space, which essentially measures the difference between the two sets of data from the direction, but cannot measure the difference between the values, the cosine similarity has a value range of [ -1,1], when the waveforms represented by the two sets of data are similar, Simi is 1, the corresponding cosine similarity calculation formula is:
wherein X is (X)1,x2,x3,…,xn),Y＝(y1,y2,y3,…,yn) Is n-dimensional vector coordinate, x1,x2,x3,…,xnN sample values, y, representing the X vector1,y2,y3,…,ynRepresenting a Y vectorn sample values.
Furthermore, the relative euclidean distance algorithm reflects the similarity between two sets of data based on the relative distance between data points in the vector space, the value of the relative euclidean distance algorithm is directly related to the value of each data point in each feature dimension, and reflects the difference between the two sets of data in the overall value, when the cosine similarity is greater than 0, the range of Dist is [0,1], when the waveform data are completely consistent, Dist is 0, and the calculation formula is as follows:
wherein X is (X)1,x2,x3,…,xn),Y＝(y1,y2,y3,…,yn) Is n-dimensional vector coordinate, x1,x2,x3,…,xnN sample values, y, representing the X vector1,y2,y3,…,ynRepresenting the n sample values of the Y vector.
Compared with the prior art, the invention has the beneficial effects that:
1. the method of the invention does not need to measure any voltage quantity, only needs to process the three-phase inrush current of the primary winding, and is simple and convenient.
2. The method can prevent the malfunction of zero sequence overcurrent protection caused by large initial value and long decay time of the zero sequence current in the operation process of the transformer.
3. The method can prevent the zero sequence overcurrent protection from being refused when CT is saturated and the harmonic content of the system is large by only using the harmonic braking criterion.
Drawings
Fig. 1 is a flowchart of a zero sequence overcurrent protection anti-malfunction method according to an embodiment of the present invention;
FIG. 2 is a diagram illustrating an algorithm data window in accordance with an embodiment of the present invention;
FIG. 3 is a schematic diagram of the principle of waveform similarity (two-dimensional vector space) in an embodiment of the present invention;
fig. 4 is a graph (a) of time variation of effective values of zero-mode inrush current and zero-sequence current in a no-load switching scene of a simulation transformer, a graph (b) of time variation of second harmonic content and waveform inertia, and a graph (c) of time variation of condition of criterion satisfaction and protection actions in the specific embodiment of the present invention;
FIG. 5 is a diagram of the time-dependent changes of the effective values of zero-mode inrush current and zero-sequence current in the scenario of no-load-to-permanent fault of the simulation transformer, a diagram of the time-dependent changes of the second harmonic content and the waveform inertia, and a diagram of the time-dependent changes of the condition of the criterion satisfaction and protection actions in the specific embodiment of the present invention;
fig. 6 is a diagram (a) of time variation of effective values of zero-mode inrush current and zero-sequence current, a diagram (b) of time variation of second harmonic content and waveform inertia, and a diagram (c) of time variation of criterion satisfaction and protection action conditions in a permanent fault scenario during inrush current of a simulation transformer in the embodiment of the present invention;
FIG. 7 is a diagram of the time-dependent change of the effective values of zero-mode inrush current and zero-sequence current in a CT unsaturated scene, a diagram of the time-dependent change of the second harmonic content and the waveform inertia, and a diagram of the time-dependent change of the condition of the criterion satisfaction and protection actions in the embodiment of the present invention;
fig. 8 is a time-varying graph (a) of effective values of zero-mode inrush current and zero-sequence current, a time-varying graph (b) of second harmonic content and waveform inertia, and a time-varying graph (c) of criterion satisfaction and protection action conditions in a scenario where a simulated earth fault accompanies CT transient saturation in the embodiment of the present invention;
FIG. 9 is a graph (a) of the time-dependent change of the effective values of zero-mode inrush current and zero-sequence current of a recording I, a graph (b) of the time-dependent change of the second harmonic content and waveform inertia, and a graph (c) of the time-dependent change of the condition of criterion satisfaction and protection actions in accordance with the exemplary embodiment of the present invention;
FIG. 10 is a graph (a) of the time-dependent change of the effective values of zero-mode inrush current and zero-sequence current of record two, a graph (b) of the time-dependent change of the second harmonic content and waveform inertia, and a graph (c) of the time-dependent change of the condition of criterion satisfaction and protection actions in the embodiment of the present invention;
fig. 11 is a graph (a) of the time-dependent change of the effective value of zero-mode inrush current and zero-sequence current of record three, a graph (b) of the time-dependent change of the second harmonic content and waveform inertia, and a graph (c) of the time-dependent change of the condition of criterion satisfaction and protection actions in the embodiment of the present invention;
fig. 12 is a graph (a) of the time-dependent change of the effective value of the zero-mode inrush current and the zero-sequence current of record four, a graph (b) of the time-dependent change of the second harmonic content and the waveform inertia, and a graph (c) of the time-dependent change of the criterion satisfaction and protection action condition in the embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The embodiment is a zero sequence overcurrent protection anti-misoperation method based on waveform inertia, and referring to fig. 1, the implementation steps are as follows:
s1: and (4) starting protection, and calculating a fundamental wave effective value and a second harmonic effective value of the zero-mode inrush current in real time.
A, B, C three-phase currents are measured by a current transformer arranged at the outlet end of the primary winding of the transformer, wherein the three-phase currents are iA，iB，iC. And (4) calculating the sudden change of the three-phase current, starting protection operation when the sudden change reaches a threshold. Zero modulus inrush current of 3i0＝iA+iB+iCCalculating the power frequency fundamental wave effective value, namely zero sequence current 3I0Second harmonic effective value of 3i0_2ndSecond harmonic content of 3i0_2nd％。
S2: and judging whether the zero-sequence current reaches a zero-sequence overcurrent protection starting threshold, if so, starting zero-sequence overcurrent protection and starting time delay.
The action condition of the zero sequence overcurrent protection is that the zero sequence current exceeds a threshold value (I)0set) And maintaining a set delay (t) of a certain length0set). Judging whether the zero sequence current exceeds the doorAnd if the threshold value exceeds the threshold value, starting zero-sequence overcurrent protection and starting to calculate time delay. Generally, zero sequence overcurrent protection has two thresholds and corresponding delays, for example, the threshold and the delay of the section I and the section II are respectively (I)0set_I，t0set_I)、(I0set_II，t0set_II)。
S3: judging the content of the second harmonic, and if the content of the second harmonic is always smaller than the setting value in the set time delay, performing zero-sequence overcurrent protection outlet action.
Judging the second harmonic content 3i0_2nd% of the value of0setThe content of the internal second harmonic is always smaller than the setting value HsetI.e. 3i0_2nd％＜HsetAnd the zero sequence overcurrent protection outlet acts.
S4: and calculating the inertia degree of the zero-mode inrush current waveform in real time from the second cycle after the protection is started.
After the protection is started, starting from the second cycle, actually calculating the inertia degree of the zero-mode inrush current waveform.
The waveform inertia G is obtained by taking the waveform data of the half-period data window from the current sampling point to the half-period data window from the sampling point of the previous period of the current sampling point and the waveform data of the half-period data window from the sampling point of the previous period of the current sampling point as two vectors (see fig. 2), and calculating the product of the cosine similarity of the two vectors and the relative euclidean distance subtracted from 1 in real time. The waveform inertia degree calculation formula is as follows:
G(t)＝Simi(X,Y)×[1-Dist(X,Y)] (1)
wherein X is 3i0[t-3T/2,t-T]，Y＝3i0[t-T/2,t]. Simi (X, Y) is the cosine similarity of the two vectors, Dist (X, Y) is the relative Euclidean distance of the two vectors.
The cosine similarity algorithm measures the similarity between two sets of data based on the cosine values of the included angles between two vectors in a vector space, and is essentially to measure the difference between the two sets of data from the direction, but cannot measure the difference between the values, as shown in fig. 3, the cosine similarity has a value range of [ -1,1], and when the waveforms represented by the two sets of data are similar (the same shape and size are not necessarily the same), Simi is 1. The corresponding cosine similarity calculation formula is as follows:
the relative euclidean distance algorithm reflects the similarity between two sets of data based on the relative distance between data points in the vector space, as shown in fig. 3, the value of which is directly related to the value of each data point in each feature dimension, and can reflect the difference in the overall value of the two sets of data. When the cosine similarity is greater than 0, Dist ranges from [0,1], and when the waveform data are completely consistent, Dist is 0, and the calculation formula is as follows:
s5: if the second harmonic content is greater than the setting value, judging the zero-mode inrush current waveform inertia degree, if the second harmonic content is greater than the setting value, braking zero-sequence overcurrent protection, and immediately opening protection once the inertia degree is detected to be less than the setting value.
If the content of the second harmonic is larger than the setting value, calculating the waveform inertia G of the zero-mode inrush current, and if the waveform inertia is larger than the setting value GsetAnd braking zero sequence overcurrent protection. After the zero-modulus inrush current inertia degree braking, if the inertia degree is detected to be smaller than a setting value subsequently, the protection is opened immediately, and the zero-sequence overcurrent protection action is carried out.
When the current transformer enters saturation from saturation, the waveform will change, and the waveform is distorted. In addition, the current transformer cannot be immediately saturated deeply, the saturation degree can also deepen along with the residual magnetism accumulation of the power transformer, the shape of the secondary current can also be changed continuously, and the secondary current cannot be kept stable, so that the waveform inertia degree braking criterion can be introduced.
The method of the invention is verified below by simulation and field recording.
Simulation verification
The zero sequence overcurrent protection misoperation prevention method based on waveform inertia provided by the invention is subjected to simulation verification. According to the engineering setting method, the harmonic content is determined to be 18%. And carrying out simulation analysis on various operating scenes, wherein the scenes comprise transformer switching-on, transformer switching-on faults, faults during transformer inrush current, ground faults, unsaturated CT, transient saturation of the ground faults with the CT and the like.
Scene one: no-load switch-on of transformer
And (3) switching on the simulation transformer when t is 0s to obtain a time-varying graph (a) of the effective values of the zero-mode inrush current and the zero-sequence current, a time-varying graph (b) of the content of the second harmonic and the waveform inertia degree and a time-varying graph (c) of the condition of the criterion meeting and protecting action, which are shown in fig. 4. As can be seen from the figure, the effective value of the zero sequence current exceeds the current constant value of the bus-coupled zero sequence protection I section in about 0.03 s. When the action of the protection I section is delayed, the zero sequence current is still higher than a fixed value, if no braking measure is available, the protection outlet acts, and the transformer fails to be switched on. And judging the harmonic content and the waveform inertia criterion, and finding that the harmonic content and the waveform inertia criterion meet the conditions, so that the brake protection outlet is realized, and the switching-on of the transformer is successful.
Scene two: no-load on-fault of transformer
Under the same conditions, fig. 5 is a graph (a) of the time variation of the effective values of the zero-mode inrush current and the zero-sequence current when the transformer is combined with a permanent fault, a graph (b) of the time variation of the content of the second harmonic and the waveform inertia, and a graph (c) of the time variation of the condition of the criterion satisfaction and the protection action. As can be seen from the figure, the effective value of the zero sequence current exceeds the current constant value of the bus-coupled zero sequence protection I section in about 0.03 s. When the action delay of the protection I section is reached, the fault current is still higher than a fixed value. And judging the harmonic content and the waveform inertia criterion, and opening protection when the harmonic content and the waveform inertia criterion do not meet the conditions at the same time, so that the zero-sequence overcurrent protection acts correctly when the zero-sequence overcurrent protection reaches a delay outlet.
Scene three: fault during inrush current of transformer
Under the same conditions, fig. 6 is a graph (a) of the time variation of effective values of zero-mode inrush current and zero-sequence current in permanent fault during inrush current of a transformer, a graph (b) of the time variation of the content of second harmonic and waveform inertia, and a graph (c) of the time variation of the condition of criterion satisfaction and protection action. As can be seen from the figure, the effective value of the zero sequence current exceeds the current constant value of the bus-coupled zero sequence protection I section in about 0.03 s. When the action delay of the protection I section is reached, the fault current is still higher than a fixed value, the harmonic content and the waveform inertia criterion are judged, the condition that the harmonic content and the waveform inertia criterion are not met simultaneously can be found, the protection is opened, the action of the zero-sequence overcurrent protection is correct when the fault current reaches a delay outlet.
Scene four: ground fault and CT is not saturated
Under the same conditions, fig. 7 is a graph (a) of the time variation of effective values of zero-mode inrush current and zero-sequence current when a permanent earth fault and a CT is not saturated, a graph (b) of the time variation of the content of second harmonic and the waveform inertia, and a graph (c) of the time variation of the condition of criterion satisfaction and protection action. It can be seen from the figure that the effective value of the zero sequence current exceeds the current constant value of the bus-coupled zero sequence protection I section after the fault occurs. When the action delay of the protection I section is reached, the fault current is still higher than a fixed value, the harmonic content and the waveform inertia criterion are judged, the condition that the harmonic content and the waveform inertia criterion are not met simultaneously can be found, the protection is opened, the action of the zero-sequence overcurrent protection is correct when the fault current reaches a delay outlet.
Scene five: ground fault with CT transient saturation
Under the same conditions, fig. 8 is a graph (a) of the time variation of effective values of zero-mode inrush current and zero-sequence current when a permanent earth fault occurs and a CT transient is saturated, a graph (b) of the time variation of the content of second harmonic and the waveform inertia, and a graph (c) of the time variation of the condition of criterion satisfaction and protection action. It can be seen from the figure that the effective value of the zero sequence current exceeds the current constant value of the bus-coupled zero sequence protection I section after the fault occurs. When the action delay of the protection I section is reached, the fault current is still higher than a fixed value, and if no braking criterion exists, the protection acts correctly. When the criterion is started, the inertia braking criterion is met, and the inertia braking criterion is not met in about 0.15s, and then the protection is opened permanently, so that the protection can act correctly. If only the harmonic braking criterion is available, the second harmonic content is high, the criterion is met, and the protection will be refused to operate.
Wave recording verification
This method is verified by field recording. And still selecting four groups of wave recording files for analysis, and checking I, II action conditions of zero-sequence overcurrent protection. For the effective value of the fundamental wave, the effective value of the zero sequence current of recording one (figure 9) is about 100A, and the zero sequence protection is correct and does not act; the effective value of the zero sequence current of the recording II (figure 10) and the analyzed waveform is larger, the initial value can reach about 800A, the value is still larger than the fixed value 600A of the zero sequence I section after 0.2s, and the action outlet is protected; the effective value of the zero sequence current of the wave recording three and four (figures 11 and 12) and the analyzed waveform thereof is smaller than that of the wave recording two, the time of the wave recording three and four (figures 11 and 12) decaying to 600A is less than 0.2s, the action condition of the zero sequence I section is not satisfied, but the action condition of the zero sequence II section is satisfied (240A, 0.5s), and therefore the action is exported.
Because the circuit breaker needs time to trip the circuit, the current value in a short time after the circuit breaker is disconnected can be still acquired and used for calculating a novel braking criterion. After the zero-mode inrush harmonic content and waveform inertia based AND gate braking criterion are adopted, three groups of actual maloperation waveforms are successfully braked, and the zero-sequence overcurrent protection I section and the zero-sequence overcurrent protection II section are not exported, so that the method is effective.
Claims (7)
1. A zero sequence overcurrent protection misoperation prevention method based on waveform inertia is characterized by comprising the following steps:
s1, starting protection, and calculating a fundamental wave effective value and a second harmonic effective value of the zero-mode inrush current in real time;
s2, judging whether the zero sequence current effective value reaches a zero sequence overcurrent protection starting threshold, if so, starting zero sequence overcurrent protection and starting time delay;
s3, judging the content of the second harmonic, if the content of the second harmonic is always less than the setting value in the set time delay, the zero sequence overcurrent protection outlet acts;
s4, calculating the inertia degree of the zero-mode inrush current waveform in real time from the second cycle after protection starting;
s5, if the second harmonic content is larger than the setting value, judging the zero-mode inrush current waveform inertia, and if the waveform inertia is larger than the setting value, braking zero-sequence overcurrent protection; and immediately opening protection and performing zero-sequence overcurrent protection once the inertia degree is detected to be smaller than the setting value.
2. The zero-sequence overcurrent protection anti-misoperation method based on waveform inertia as claimed in claim 1, wherein in step S1,
a, B, C three-phase current is measured by a current transformer arranged at the outlet end of the primary winding of the transformerAre respectively iA，iB，iCCalculating the sudden change of three-phase current, starting protection operation when the sudden change reaches a threshold, and setting the zero-mode inrush current to be 3i0＝iA+iB+iCCalculating the power frequency fundamental wave effective value, namely zero sequence current 3I0Second harmonic effective value of 3i0_2ndSecond harmonic content of 3i0_2nd％。
3. The zero-sequence overcurrent protection anti-misoperation method based on waveform inertia as claimed in claim 1, wherein in step S2,
the action condition of the zero sequence overcurrent protection is that the zero sequence current exceeds a threshold value I0setAnd maintain a set delay t0setAnd judging whether the zero sequence current exceeds a threshold value, if so, starting zero sequence overcurrent protection and starting to calculate time delay.
4. The zero-sequence overcurrent protection malfunction prevention method based on waveform inertia as claimed in claim 3, wherein in step S2, the zero-sequence overcurrent protection has two thresholds and corresponding delays.
5. The zero-sequence overcurrent protection anti-misoperation method based on waveform inertia as claimed in claim 2, wherein in step S4,
the waveform inertia degree, namely, the waveform data of a half-period data window from the current sampling point to the half-period data window from the sampling point of the previous period of the current sampling point and the waveform data of the half-period data window from the sampling point of the previous period of the current sampling point are taken as two vectors, the product of the cosine similarity of the two vectors and the relative Euclidean distance subtracted from 1 is calculated in real time, and the calculation formula of the waveform inertia degree is as follows:
G(t)＝Simi(X,Y)×[1-Dist(X,Y)] (1)
wherein X is 3i0[t-3T/2，t-T]，Y＝3i0[t-T/2，t]The method comprises the following steps of obtaining a sampling point, obtaining a sampling point time, obtaining a power frequency period, obtaining Simi (X, Y) and Dist (X, Y), obtaining a sampling point time, and obtaining a power frequency period time.
6. The zero-sequence overcurrent protection anti-misoperation method based on waveform inertia as claimed in claim 5, wherein the cosine similarity algorithm measures the similarity between two sets of data based on the cosine value of the included angle between two vectors in the vector space, which essentially measures the difference between the two sets of data in the direction, but cannot measure the difference in the magnitude of the value, the cosine similarity has a value range of [ -1,1], when the waveforms represented by the two sets of data are similar, Simi ═ 1, the corresponding cosine similarity calculation formula is:
wherein X is (X)1,x2,x3,…,xn),Y＝(y1,y2,y3,…,yn) Is n-dimensional vector coordinate, x1,x2,x3,…,xnN sample values, y, representing the X vector1,y2,y3,…,ynRepresenting the n sample values of the Y vector.
7. The zero-sequence overcurrent protection anti-misoperation method based on waveform inertia according to claim 5, wherein the relative Euclidean distance algorithm reflects similarity between two sets of data based on relative distance between data points in a vector space, the value of the relative Euclidean distance algorithm is directly related to the value of each data point in each characteristic dimension, the difference between the two sets of data in the overall value is reflected, when the cosine similarity is greater than 0, the range of Dist is [0,1], when waveform data are completely consistent, Dist is 0, and the calculation formula is:
wherein X is (X)1,x2,x3,…,xn),Y＝(y1,y2,y3,…,yn) Is n-dimensional vector coordinate, x1,x2,x3,…,xnN sample values, y, representing the X vector1,y2,y3,…,ynRepresenting the n sample values of the Y vector.
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