CN110542821A - Small current line selection method using correlation analysis - Google Patents
Small current line selection method using correlation analysis Download PDFInfo
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Abstract
The invention discloses a small current line selection method by utilizing correlation analysis, which comprises the following steps of: the method comprises the following steps: after the line selection device is started, recording more zero-sequence current sampling data, namely recording sampling data of zero-sequence current of each line of 1 cycle before the fault and 10 cycles after the fault; step two: for each line fault zero sequence current, 1 st cycle sampling data after the fault occurs is correspondingly subtracted from 1 st cycle sampling data before the fault occurs, and then 10 th cycle sampling data after the fault occurs is correspondingly subtracted; step three: after the transient pure fault component of the zero sequence current is obtained, the transient pure fault component of the zero sequence current is used for replacing the zero sequence current to carry out correlation analysis, and therefore fault line selection is achieved.
Description
Technical Field
The invention relates to the technical field of small current line selection methods by using correlation analysis, in particular to a small current line selection method by using correlation analysis.
Background
When a single-phase earth fault occurs in a low-current earth power grid, the healthy line is similar to the earth capacitor in charging and discharging, so that the zero-sequence current of the healthy line has strong similarity. And the zero sequence current waveform of the fault line has the largest difference with the zero sequence current waveforms of other lines due to the existence of the additional zero sequence voltage source. Therefore, the fault line can be detected by analyzing the similarity of the zero sequence currents of all lines.
The existing line selection technology only utilizes part of useful information of faults, so that the existing line selection technology has certain limitation and cannot be applied to all fault conditions. In recent three years, the research on the line selection of the low-current ground fault raises the climax, and scholars at home and abroad introduce wavelet analysis, a Prony method, an ANN method, information fusion, a fuzzy method and the like into the low-current ground protection. However, if the mathematical analysis tool is excessively relied on and the thorough analysis of the fault characteristics is neglected, the problem is solved without leaving the beginning and ending, the existing line selection device starts a line selection program by using zero sequence voltage, a plurality of single-phase earth faults are all progressive processes, the zero sequence voltage is out of limit after the initial transient process of the fault occurs for a period of time, the recorded waveform does not meet the premise of line selection of fault transient signals, in addition, the line selection is only carried out for 1 time, the possibility of mistaken selection is increased, and therefore an improved technology is urgently needed to solve the problem existing in the prior art.
Disclosure of Invention
The present invention is directed to a low current line selection method using correlation analysis to solve the above-mentioned problems.
In order to achieve the purpose, the invention provides the following technical scheme: a small current line selection method using correlation analysis comprises the following steps:
The method comprises the following steps: after the line selection device is started, recording more zero-sequence current sampling data, namely recording sampling data of zero-sequence current of each line of 1 cycle before the fault and 10 cycles after the fault;
Step two: for each line fault zero sequence current, 1 st cycle sampling data before the fault occurs is correspondingly subtracted from 1 st cycle sampling data after the fault occurs, and then 10 th cycle sampling data after the fault occurs is correspondingly subtracted, namely, the asymmetric component of the zero sequence current before the fault occurs and the steady-state power frequency component of the zero sequence current after the fault occurs are correspondingly subtracted from the transient zero sequence current after the fault occurs, so that the transient pure fault component of the zero sequence current is obtained;
Step three: and after the transient pure fault component of the zero-sequence current is obtained, the transient pure fault component of the zero-sequence current is used for replacing the zero-sequence current for correlation analysis, so that fault line selection is realized.
preferably, the line selection device in the first step adopts a low-current line selection device.
preferably, in the second step, the formula corresponding to the transient zero-sequence current after the fault occurs by subtracting the asymmetric component of the zero-sequence current before the fault occurs and the steady-state power frequency component of the zero-sequence current after the fault occurs is given as i0jp and u0p, which are transient pure fault components of the zero-sequence current of each line and the zero-sequence voltage of the bus respectively; the frequency of the fault is I0j (1), U0(1), I0j (1), U0(1), I0j (10) and U0(10), which respectively correspond to the first cycle after the fault, the first cycle before the fault and the stable cycle after the fault of the zero-sequence current and the zero-sequence voltage of each line.
preferably, for the arc single-phase ground fault, i0j (10) and u0(10) in the formula are replaced by sampling values corresponding to a cycle after the arc is stabilized.
Compared with the prior art, the invention has the beneficial effects that:
The invention utilizes the small current line selection of the correlation analysis to detect the fault line by analyzing the similarity of the zero sequence current of each line, and has accurate line selection and high speed.
Drawings
FIG. 1 is a flow chart of correlation analysis route selection.
Fig. 2 is a schematic cross-correlation of signals.
Fig. 3 is a schematic diagram of a fault zero-sequence current.
Fig. 4 is a schematic diagram of a zero sequence current transient pure fault component in the 1 st cycle after a fault.
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.
Referring to fig. 1, the present invention provides a technical solution: a small current line selection method using correlation analysis comprises the following steps:
The method comprises the following steps: after the line selection device is started, recording more zero-sequence current sampling data, namely recording sampling data of zero-sequence current of each line of 1 cycle before the fault and 10 cycles after the fault;
Step two: for each line fault zero sequence current, 1 st cycle sampling data before the fault occurs is correspondingly subtracted from 1 st cycle sampling data after the fault occurs, and then 10 th cycle sampling data after the fault occurs is correspondingly subtracted, namely, the asymmetric component of the zero sequence current before the fault occurs and the steady-state power frequency component of the zero sequence current after the fault occurs are correspondingly subtracted from the transient zero sequence current after the fault occurs, so that the transient pure fault component of the zero sequence current is obtained;
step three: and after the transient pure fault component of the zero-sequence current is obtained, the transient pure fault component of the zero-sequence current is used for replacing the zero-sequence current for correlation analysis, so that fault line selection is realized.
Cross correlation function
The cross-correlation function of signals x (t) and y (t) is strictly defined as follows:
Where T is the average time. The cross-correlation function is understood to be the time average of the product of two signals, one of which is shifted in time (leading or lagging) by τ seconds, τ being referred to as the time difference.
The cross-correlation function of two signals is a useful statistic that can be used to understand the degree of similarity between two unknown (random or non-random) signals, or the temporal relationship between two known (similar or identical) signals. For example, two signals x (t) and y (t) in fig. 2(a), the maximum value of Rxy (τ) can be obtained by adjusting the time difference τ between them, so as to know the similarity degree between them, and if the two signals are known to be similar (for example, the emission signal and the echo signal of radar), the time delay between them can be obtained from the τ value.
If x (T) and y (T) are periodic signals with a period of T0, as shown in fig. 2(b), only correlation calculation needs to be performed in one of its periods:
Further, the formula (1-1a) cannot be used for a waveform in which energy is limited to one interval (for example, a non-periodic pulse type waveform). Because when T0 → ∞, (1/T0) → 0, the calculated Rxy (τ) always tends to disappear. In this case, the following formula is generally used:
In the ideal case of equation (1-1a), the average time T taken for the correlation operation should be infinite, but in practice, the correlation function is also related to the average time T, since a finite recording length is always used for processing, and T is usually large enough to satisfy statistical requirements. Combining the above several cases, the practical cross-correlation function is defined as:
two, auto-correlation function
If the signal is correlated with itself, i.e. x (t) ═ y (t), the autocorrelation function is obtained from equation (1-1 d):
Thus, the autocorrelation function is a special case of the cross-correlation function.
three, correlation operation
the argument of the correlation integral is t, but the resulting argument is the time difference τ instead of t, τ being only a parameter in the integral, called the parameter time. The results of the correlation integration differ from one parameter time to another. The correlation integral is consistent in form with the convolution integral, and there is a time-reversal relationship between the two, i.e., τ is of opposite sign. From this, the fourier transform of the correlation function can be obtained using the convolution theorem.
If only a certain time difference value tau is subjected to correlation integration, the phase relationship of the two signals is fixed, and the result of the integration cannot reflect the similarity degree of the two signals. For example, in fig. 2(a), if the difference is not adjusted, the value calculated by integration is small. Fig. 2(b) is an extreme example: x (t) and y (t) are signals that are completely correlated (exactly the same shape), but since the two signals are exactly opposite in phase, one signal is always 0 at any time, so taking τ to 0 results in a correlation integral of 0.
Therefore, the correlation of two signals is not performed by performing a correlation integration only once, but performed by performing a correlation integration on different parameter times (i.e. the instantaneous difference τ) to obtain a function with respect to τ (i.e. a correlation function). Only the maximum value of the correlation function is the correlation value that best reflects the degree of similarity of the signals.
Four, some basic properties of the correlation function
It can be shown that the correlation function has some important properties:
(1) the autocorrelation function being the even function of τ, i.e.
R(τ)＝R(-τ) (1-3)
While the cross-correlation function is not necessarily symmetric with respect to tau.
(2) The autocorrelation function is maximum at a point where τ is 0, i.e.
R(0)≥R(τ) (1-4)
And it is equal to the mean square value of the signal.
(3) the autocorrelation function of the periodic signal is also periodic.
(4) The autocorrelation function of the sum of two uncorrelated signals being equal to the sum of the autocorrelation functions of the two signals, i.e.
If z (t) ═ x (t) + y (t) and Rxy (τ) ═ 0 hold for all τ, then
R(τ)＝R(τ)+R(τ) (1-5)
principle of fault line selection
According to the characteristics of the zero sequence current of the single-phase earth fault of the small-current earth power grid, when the single-phase earth fault occurs in the small-current earth power grid, the charging and discharging of the earth capacitance of the sound circuit are similar, so that the zero sequence current of the sound circuit has stronger similarity. And the zero sequence current waveform of the fault line has the largest difference with the zero sequence current waveforms of other lines due to the existence of the additional zero sequence voltage source. Therefore, the fault line can be detected by analyzing the similarity of the zero sequence currents of all lines. The line selection flow chart of the line selection method is shown in fig. 1, and the specific implementation steps are as follows:
(1) Starting the line selection device, recording the starting time of the device as the time of fault occurrence, and immediately recording the zero sequence current data of each line of 1 cycle after the fault.
(2) Under a data window of a power frequency period, performing pairwise correlation analysis on waveforms of zero-sequence current transient components of each line in the 1 st cycle after the fault by using a formula (1-5), and solving pairwise correlation coefficients between the lines to form a correlation coefficient matrix M
In the formula, n is the number of system lines. The diagonal elements in the correlation coefficient matrix M are autocorrelation coefficients of zero-sequence currents of each line, the values of the autocorrelation coefficients are all 1, and the other elements are pairwise cross-correlation coefficients of the zero-sequence currents of each line.
(3) And calculating the comprehensive correlation coefficient rho i, i of each line relative to other lines according to the correlation coefficient matrix, wherein the comprehensive correlation coefficient rho i, i is 1,2, …, n. Defining the average of the correlation coefficients of the line and other lines as the integrated correlation coefficient of the line, i.e.
(4) And comparing the comprehensive correlation coefficients of all lines, and judging that the bus grounding fault occurs in the system when the difference delta rho between the maximum comprehensive correlation coefficient and the minimum comprehensive correlation coefficient is less than a threshold rho set (0.5 is taken during the simulation test of the embodiment). Otherwise, the line corresponding to the minimum comprehensive correlation coefficient is the fault line.
sixthly, improved correlation analysis fault line selection method
(1) Design of digital trap
when the neutral point arc suppression coil grounding system operates normally, due to the poor transposition condition of the conducting wires and the unequal three-phase capacitance to the ground, the neutral point has displacement voltage with a certain numerical value to the ground, and therefore the zero sequence current of each line is not zero. When a single-phase earth fault occurs in a system with a neutral point grounded through an arc suppression coil, fault transient zero-sequence currents of a fault line and a sound line are both composed of transient components and steady-state power frequency components. According to the superposition principle, the network after the fault can be equivalent to the superposition of the normal operation network and the fault additional network. The steady-state power frequency component contained in the fault transient zero-sequence current of each line is formed by superposing an asymmetric component before fault and a fault steady-state power frequency component. Due to the compensation effect of the arc suppression coil, the steady-state power frequency components and the phases in the transient zero-sequence currents of the fault line and the sound line are close to each other, so that the margin of line selection protection is small. If a power frequency digital trap can be constructed, steady-state power frequency components are filtered from transient zero-sequence currents of each line, and the obtained transient components are used for forming a line selection criterion, the margin of line selection protection is inevitably and obviously improved.
considering that the transmission characteristics of the current transformers are not necessarily symmetrical in practical engineering, the relation between the power frequency quantity and the load is large, and the practical system is asymmetrical. Research and simulation show that 3-4 cycles of actual neutral point arc suppression coil grounding system generally occur after a fault occurs, the transient component is very small, and the electromagnetic transient process can be considered to be basically finished. According to the characteristics, the following algorithm can be utilized to eliminate the asymmetric component and the steady-state power frequency component of the system, so as to obtain the transient pure fault component of the zero-sequence current and the bus zero-sequence voltage of each line.
In the formula: i0jp and u0p are transient pure fault components of zero-sequence current of each line and zero-sequence voltage of the bus respectively; the sampling signals of i0j (1), u0(1), i0j (1), u0(1), i0j (10) and u0(10) respectively correspond to the first cycle after the fault of the zero-sequence current and the zero-sequence voltage of the bus, the first cycle before the fault and the stable cycle after the fault, such as the 10 th cycle. For the arc single-phase ground fault, i0j (10) and u0(10) in the formula (1-8) need only be replaced by sampling values corresponding to a certain cycle after the arc is stabilized.
(2) Fault line selection method
The improved related analysis fault line selection method is different from the line selection method introduced in the 'five fault line selection principle' in the realization process that after a line selection device is started, more zero-sequence current sampling data need to be recorded, namely the sampling data of the zero-sequence current of each line of 1 cycle before the fault and 10 cycles after the fault are recorded, for the fault zero-sequence current of each line, the 1 st cycle sampling data before the fault is correspondingly subtracted from the 1 st cycle sampling data after the fault occurs, and then the 10 th cycle sampling data after the fault occurs are correspondingly subtracted, namely the asymmetric component of the zero-sequence current before the fault occurs and the steady-state power frequency component of the zero-sequence current after the fault occurs are correspondingly subtracted from the transient zero-sequence current after the fault occurs, so as to obtain the transient pure fault component of the zero-sequence current. And after the transient pure fault component of the zero-sequence current is obtained, the transient pure fault component of the zero-sequence current is used for replacing the zero-sequence current to carry out correlation analysis, and then fault line selection can be realized.
(3) Example analysis
At a position 5km away from the bus line on the line l1, when a fault closing angle is 45 °, phase a is grounded, transition resistance Rf is 20 Ω, and zero-sequence current waveforms of the lines are obtained through simulation, as shown in fig. 3, limited to space, and only zero-sequence current waveforms of the lines l1, l4, and l6 are shown in the figure. The zero sequence current waveforms of each line shown in fig. 3 are subjected to notch processing by using formulas (1-8), so as to obtain transient pure fault components of the zero sequence current of each line, and the waveforms are shown in fig. 4. As is apparent from fig. 3 and 4, after the transient zero-sequence current passes through the digital wave trap, the power frequency component and the asymmetric component are completely filtered out, so as to obtain the transient pure fault component of the zero-sequence current.
And performing pairwise correlation analysis by using the transient zero-sequence current of the 1 st cycle after the fault of each line to obtain a correlation coefficient matrix M1, and performing pairwise correlation analysis by using the transient component of the 1 st cycle zero-sequence current after the fault of each line to obtain a correlation coefficient matrix M2.
And calculating comprehensive correlation coefficient arrays of each line relative to other lines according to M1 and M2, and respectively recording the comprehensive correlation coefficient arrays as E1 and E2.
E＝[–0.762 0.638 0.632 0.640 0.635 0.638]
E＝[–0.893 0.589 0.557 0.589 0.578 0.587]
in E1 and E2, rho max-rho min >0.5 is established, and the comprehensive correlation coefficient rho 1 of the line l1 is minimum, so that the line l1 can be accurately judged to be a fault line. In the process of solving the E2, because extremely similar asymmetric components and power frequency steady-state components with strong similarity in the zero-sequence currents of the lines are filtered, the correlation between the robust lines in the E2 is weakened, that is, the comprehensive correlation coefficient is reduced compared with that of the robust lines in the E1. Meanwhile, the correlation between the fault line and each sound line is weakened, and the weakening amplitude is larger than that of the correlation between the sound lines, so that the difference between the comprehensive correlation coefficients of the fault line and the sound lines is enlarged, and the line selection protection margin is improved. Therefore, the two methods can be used for accurately selecting the line, but the protection margins are different.
(4) Method validity verification
The method is adopted to simulate various fault types such as different fault lines, different fault closing angles, different transition resistances and the like, and the simulation result is shown in table 1.
TABLE 1 Fault routing results
As can be seen from table 1, the line selection method can accurately detect a faulty line.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (4)
1. A small current line selection method using correlation analysis is characterized in that: the method comprises the following steps:
The method comprises the following steps: after the line selection device is started, recording more zero-sequence current sampling data, namely recording sampling data of zero-sequence current of each line of 1 cycle before the fault and 10 cycles after the fault;
Step two: for each line fault zero sequence current, 1 st cycle sampling data before the fault occurs is correspondingly subtracted from 1 st cycle sampling data after the fault occurs, and then 10 th cycle sampling data after the fault occurs is correspondingly subtracted, namely, the asymmetric component of the zero sequence current before the fault occurs and the steady-state power frequency component of the zero sequence current after the fault occurs are correspondingly subtracted from the transient zero sequence current after the fault occurs, so that the transient pure fault component of the zero sequence current is obtained;
Step three: and after the transient pure fault component of the zero-sequence current is obtained, the transient pure fault component of the zero-sequence current is used for replacing the zero-sequence current for correlation analysis, so that fault line selection is realized.
2. The small current line selection method using correlation analysis according to claim 1, wherein: and the line selection device in the first step adopts low-current line selection equipment.
3. The small current line selection method using correlation analysis according to claim 1, wherein: in the second step, the transient zero-sequence current after the fault occurs correspondingly subtracts the asymmetric component of the zero-sequence current before the fault occurs and the corresponding formula of the steady-state power frequency component of the zero-sequence current after the fault occurs, wherein i0jp and u0p are transient pure fault components of the zero-sequence current of each line and the zero-sequence voltage of the bus respectively; the frequency of the fault is I0j (1), U0(1), I0j (1), U0(1), I0j (10) and U0(10), which respectively correspond to the first cycle after the fault, the first cycle before the fault and the stable cycle after the fault of the zero-sequence current and the zero-sequence voltage of each line.
4. a small current line selection method using correlation analysis according to claim 3, wherein: for the arc single-phase earth fault, i0j (10) and u0(10) in the formula are replaced by sampling values corresponding to a certain cycle after the arc is stabilized.
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| CN111722055A (en) * | 2020-05-21 | 2020-09-29 | 昆明理工大学 | Single-pole grounding fault identification method for MMC direct current transmission line based on perceptual fuzzy identification |
| CN112083270A (en) * | 2020-08-14 | 2020-12-15 | 昆明理工大学 | Wind power plant current collection line single-phase earth fault line selection method based on correlation coefficient |
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| CN113567806A (en) * | 2021-07-02 | 2021-10-29 | 上海思源光电有限公司 | Small current fault line selection method, system, terminal and medium |
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Application publication date: 20191206 |