CN113612205A - MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference - Google Patents

MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference Download PDF

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CN113612205A
CN113612205A CN202110778189.7A CN202110778189A CN113612205A CN 113612205 A CN113612205 A CN 113612205A CN 202110778189 A CN202110778189 A CN 202110778189A CN 113612205 A CN113612205 A CN 113612205A
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fault
protection
line
lightning
frequency
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CN113612205B (en
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薛士敏
陈硕
陈萧
姚程
袁中琛
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Tianjin University
State Grid Tianjin Electric Power Co Ltd
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State Grid Tianjin Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/265—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured making use of travelling wave theory
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H1/00—Details of emergency protective circuit arrangements
    • H02H1/0007—Details of emergency protective circuit arrangements concerning the detecting means
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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Abstract

The invention relates to an MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference, which comprises the following steps: step (1): setting a protection measuring point at the initial end of a line, namely the line side of a line reactor, and realizing protection starting and fault pole selection by using the voltage variation of the protection measuring point; step (2): establishing a region internal and external fault identification method by utilizing the high-frequency attenuation action of a smoothing reactor on fault traveling waves; and (3): and fault identification is carried out by utilizing the time domain waveform area characteristics, and lightning stroke interference identification criteria are constructed by utilizing the reverse traveling wave.

Description

MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference
Technical Field
The invention relates to the field of relay protection and automation of a power system, in particular to a high-frequency transient state quantity protection method capable of identifying lightning interference.
Background
In the face of the difficult problem which can not be solved in the traditional direct current transmission, the flexible direct current transmission technology (VSC-HVDC) based on the voltage source converter is widely researched and rapidly developed. The flexible direct-current power transmission technology adopting the two-level or three-level converter cannot meet the current requirements of large-capacity and long-distance power transmission, and therefore, the development prospect of the power transmission technology is improved by one height through the Modular Multilevel Converter (MMC).
In order to ensure the speed, the current protection research is basically focused on the transient electrical quantity, and the research angle is mainly divided into two aspects of time domain and frequency domain. Although the transition resistance tolerance of the time domain protection method is limited and can be improved from the perspective of the frequency domain, it is necessary to select an appropriate signal processing method and to protect against false operation under an interference signal. Double-ended protection has good reliability, but they have difficulty meeting speed requirements in long-distance power transmission. Both time domain and frequency domain protection sacrifice reliability while achieving speed. Disturbances caused by non-fault source signals can also cause drastic sudden changes in the electrical quantities, which, if no measures are taken to identify these disturbances, can cause unnecessary malfunctions in the protection and interrupt the power transmission of the grid, which is a huge loss for high-voltage grids. Therefore, how to balance the relationship between good mobility and reliability within a short time window is crucial.
Lightning impulse is a unipolar pulse wave of extremely short duration, which is a high frequency source of interference for protection when lightning strikes do not cause flashover of the line insulation. In a long-distance power transmission system adopting an overhead line, the occurrence probability of lightning stroke is obviously improved under the influence of environmental and climatic factors, and the lightning interference becomes a common interference factor. Some learners recognize lightning interference by using the magnitude of current variation or distinguish the lightning interference and line faults by using the alternating variation rule of current waveform on a time axis, but both methods are based on whether line uniformity is damaged or not, and the quick recognition cannot be realized in a flexible and straight system under the condition that traveling waves can complete repeated refraction and reflection at two ends or fault points of a line. There is a difference in frequency distribution between the fault traveling wave and the lightning wave, and there is a method of identifying a lightning stroke using a high-low frequency ratio, but there is a study that frequency domain characteristics of both are similar in some cases and threshold determination is not easy. The mode maximum value is captured by wavelet transformation and the wave tail is fitted to eliminate lightning stroke interference, but the method needs extremely high sampling rate requirement to realize, and the wave tail of the traveling wave is difficult to extract due to the refraction and reflection influence of the traveling wave when a near-end fault occurs.
The fault development speed of the flexible direct current transmission system is rapid, so that the quick action of the traditional relay protection is challenged, and the transient state quantity protection is led to the hot tide of domestic and foreign research along with the development of the flexible direct current technology. The improvement of the protection recognition speed is necessarily accompanied by the reduction of the reliability, and the research progress of the current protection action speed approaches the limit, so how to improve the reliability of the protection under the requirement of ensuring the speed is important. Lightning strikes occur frequently in long-distance power transmission engineering, and the lightning strikes are not negligible as an interference source under the condition that flashover is not caused. Based on the reason, the method designs a high-frequency protection scheme by using Hilbert-Huang transformation, and provides a lightning stroke interference identification method by using the waveform characteristics of lightning waves. The lightning criterion is then combined with a protection criterion.
Disclosure of Invention
The invention aims to provide an MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference and improving reliability. The method comprises the following steps:
a MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference comprises the following steps:
step (1): the protection measuring point is arranged at the initial end of the line, namely the line side of the line reactor, and the electricity of the protection measuring point is utilizedThe voltage variation realizes protection starting and fault pole selection, and the delta U is the voltage variation measured in real time at the protection installation positionzFor protecting the starting threshold, when the voltage variation of two continuous sampling points is greater than the protection starting threshold, the protection is started;
let kpoleFor voltage variation of successive N sampling points, Δ UpAnd Δ UnThe voltage abrupt change of the anode and the cathode respectively,
Figure BDA0003155128860000021
let k1For the fault pole decision threshold, pole selection criteria are constructed as follows:
Figure BDA0003155128860000031
step (2): the method for identifying the faults inside and outside a fault area is formulated by utilizing the high-frequency attenuation effect of a smoothing reactor on fault traveling waves, original signals are sequentially decomposed into a series of Intrinsic Mode Functions (IMF) components according to the sequence from high frequency to low frequency through empirical mode decomposition, and after Hilbert transformation is carried out on each IMF component, an analytic signal z (t) is obtained, so that the instantaneous frequency f of each moment in the IMF is obtained.
Empirical mode decomposition is carried out on 1-mode fault voltage traveling waves to obtain a first intrinsic mode function component IMF1, absolute value summation is carried out on data with instantaneous frequency larger than 500Hz in IMF1, and high-frequency energy E is defined by the data1Then t after the protection is started1Inner E1Comprises the following steps:
Figure BDA0003155128860000032
N1is the number of data points in the criterion time window, when E1Greater than EzWhen a fault occurs in the line area, EzAnd setting according to the most serious out-of-area fault as a criterion threshold value.
And (3): fault identification is carried out by utilizing time domain waveform area characteristics, lightning stroke interference identification criteria are constructed by utilizing reverse traveling waves, and the following criteria are constructed according to discrete data obtained by sampling:
Figure BDA0003155128860000033
in the formula, the numerator represents the area of the waveform enclosed by the coordinate axes in a fixed time window, the denominator is the maximum value of the waveform, the rectangular area enclosed by the coordinate axes and the time window, and the ratio of the two is ks，B1For protecting the 1-mode reverse wave measured at the installation site, N2Is the number of data points in the criterion time window, when ks>k2When considered a line fault, k2And judging a threshold value for lightning stroke interference, otherwise, judging the threshold value for the lightning stroke interference.
Compared with the prior art, the invention has the beneficial effects that:
1. because the current-limiting reactor is a natural high-frequency boundary, the high-frequency component of the fault traveling wave can be greatly attenuated after passing through the current-limiting reactor, the high-frequency component of the 1-mode reverse traveling wave can be extracted through Hilbert-Huang conversion, the faults inside and outside the area can be accurately identified, and the high-frequency component has high transition resistance capability.
2. The method reasonably utilizes the waveform area characteristics from the time domain angle to design the lightning stroke recognition method, can accurately eliminate the lightning stroke interference of different positions of a line, prevents the protection from misoperation caused by the lightning stroke interference, and ensures that a power transmission system stably and continuously operates. Even under the condition that the lightning stroke causes line faults, the method cannot influence the normal action of protection. Although the reverse waveform in the case of a short-distance fault is affected by refraction and reflection, the determination of the method is not affected.
Drawings
FIG. 1 is a fault point voltage component waveform;
FIG. 2 is a voltage inversion waveform at a measurement point;
FIG. 3 is a standard lightning waveform;
FIG. 4 is a standard lightning wave amplitude spectrum;
FIG. 5 is a time domain waveform of a lightning wave;
fig. 6 is a fault traveling wave time domain waveform.
Detailed Description
The invention designs a high-frequency protection scheme by using Hilbert-Huang transform, and provides a lightning stroke interference identification method by using the waveform characteristics of lightning waves. And then, combining the lightning stroke criterion with the protection criterion to make up the shortage of transient protection and improve the reliability of the transient protection.
And (1) setting a protection measuring point at the initial end of a line (the line side of a line reactor), and realizing protection starting and fault pole selection by using the voltage variation of the protection measuring point. Let Δ U be the voltage variation measured in real time at the protection installation site, Δ UzIn order to protect the threshold value of starting, 10-25 kV can be selected according to the voltage fluctuation condition in normal operation. When the delta U of two continuous sampling points satisfies the following formula, protection is started, tsFor the current sampling instant, ts+1The next sample time.
ΔU(ts)＞ΔUz&ΔU(ts+1)＞ΔUz (1)
Let kpoleFor voltage variation of successive N sampling points, Δ UpAnd Δ UnAnd (4) respectively changing the voltage abrupt change of the anode and the cathode.
Figure BDA0003155128860000041
Let k1The threshold value is determined for the fault pole, and the range is 1.1-1.5. Thus, pole selection criteria can be constructed as shown in formula (3):
Figure BDA0003155128860000051
and (2) utilizing the high-frequency attenuation action of the smoothing reactor on fault traveling waves to formulate a method for identifying faults inside and outside the area. Empirical mode decomposition decomposes the original signal into a series of natural mode functions (IMF components) in order of high frequency to low frequency. After hilbert transformation is performed on each IMF component, an analytic signal z (t) can be obtained, and further, the instantaneous frequency f of each time in the IMF can be obtained.
Figure BDA0003155128860000052
Figure BDA0003155128860000053
Considering that the attenuation effect of the line on the 1-mode traveling wave component is relatively small, the empirical mode decomposition is carried out on the 1-mode fault voltage traveling wave to obtain a first intrinsic mode function component IMF 1. Summing the absolute values of the data in IMF1 with instantaneous frequency greater than 500Hz and defining high frequency energy E1. Then t after the protection is started1Inner E1Comprises the following steps:
Figure BDA0003155128860000054
N1the number of data points in the criterion time window. When E is1Greater than EzWhen the failure occurs, it can be considered that the failure has occurred in the line area. EzAnd setting the fault as a criterion threshold value according to the most serious out-of-area fault, wherein the value of the threshold value can be between 100 and 300 kV.
And (3) carrying out fault identification by utilizing the time domain waveform area characteristics. And constructing a lightning stroke interference recognition criterion by utilizing the reverse traveling wave, wherein the reverse traveling wave directly originates from a fault point or a lightning stroke point. And (4) constructing the criterion of the formula (7) according to the discrete data obtained by sampling.
Figure BDA0003155128860000055
In the formula (7), the numerator represents the area of the waveform enclosed by the coordinate axes in the fixed time window, the denominator is the maximum value of the waveform, the rectangular area enclosed by the coordinate axes and the time window, and the ratio of the two is ks。B1For protecting the 1-mode reverse wave measured at the installation site, N2As a criterion of a time windowNumber of data points in. When k iss>k2When it is considered a line fault, k2The optimal selection range is 0.40-0.55 for the threshold value of lightning interference judgment. Otherwise, the lightning stroke interference is realized.
The present invention is described in further detail below.
1, realizing protection starting and fault pole selection by using voltage variation of a protection measuring point.
Line faults are accompanied by a significant voltage drop, so that the protection can be started by using the magnitude of the voltage variation. When a fault occurs, the voltage variation of the fault pole is larger than that of the non-fault pole, the voltage variation of the positive pole and the negative pole is close to that of the bipolar fault, and the fault pole can be determined by utilizing the ratio of the voltage variation of the positive pole and the negative pole.
2, identifying method for faults inside and outside formulated area by high-frequency attenuation action of smoothing reactor on fault traveling wave
When the line is in fault, the voltage of the fault point is rapidly reduced, and under the condition of not considering the transition resistance, the voltage is equivalent to that a value of-U is superposed on the fault pointdcThe step signal (2) is as shown in fig. 1 (1.3 s at the time of occurrence of the failure).
The sudden change of voltage is also accompanied by the generation of each high-frequency component, and the low-frequency component content of the fault component is larger. The step signal will then be transmitted in the form of a travelling wave towards both ends of the line. After a short time delay, the reverse traveling wave B measured by the measuring pointmThe difference from the original step signal is significant, and due to the distribution parameters and frequency-dependent characteristics of the lines during transmission, various frequency components in the traveling wave are attenuated to different degrees, so that the wave head of the fault traveling wave is slowed down, as shown in fig. 2.
Bm(t)＝-Udc*A(t)*ε(t-τm) (8)
Where A (t) is the propagation function of the line, the higher the frequency the smaller the value, i.e. the greater the degree of attenuation. Meanwhile, the farther the fault distance is, the smaller A (t) is, namely the attenuation effect of the line on the traveling wave is greater. As can be seen from the above, when the traveling wave arrives, the frequency component of the fault voltage measured at the protection installation site is mainly concentrated on the lower frequency band. In addition, when a fault occurs outside a line area, due to the existence of the current-limiting reactors at two ends of the line, high-frequency components are greatly attenuated through the reactors, and the characteristic can be used as an important basis for constructing a protection scheme.
The fault voltage is taken as a non-stationary signal, and a proper signal processing method is adopted to extract a high-frequency component. Time-frequency analysis methods evolved based on Fourier analysis, such as windowed Fourier transform, wavelet transform, Winger-Ville distribution, S-transform and the like, are also limited by the lack of Fourier analysis. At the same time, these analysis methods are difficult to balance between time resolution and frequency resolution, limited by the heisenberg uncertainty principle.
The hilbert yellow transform performs empirical mode decomposition and hilbert transform on the original signal in sequence from the perspective of instantaneous frequency, and can reveal the characteristic of the change of the signal frequency along with time.
And 3, identifying the fault by utilizing the time domain waveform area characteristics.
The impact of lightning strike on the transmission line is mainly divided into two forms of direct lightning strike and induced lightning strike, wherein the direct lightning strike can be divided into lightning strike lightning conductor, lightning strike pole tower and lightning strike transmission line according to the lightning strike position, and the lightning strike transmission line can be called shielding failure.
The lightning stroke of the lightning conductor is the central lightning stroke of the span, but the occurrence probability is low, and the high-voltage transmission line is difficult to threaten. Lightning strikes on the tower or a lightning conductor nearby the tower are likely to cause counterattack on the transmission line, and for the ultra-high voltage transmission line, the lightning resistance level of the counterattack is very high, so that the counterattack probability is extremely low. Protection should not be activated when the lightning surge is not sufficient to cause damage to the insulation, which is a lightning strike disturbance. The interference situation can be distinguished by using the voltage variation of the 0 mode and the 1 mode, and the method mainly analyzes the lightning shielding failure situation.
When the shielding failure occurs, if the lightning current is not enough to cause the insulator to flashover, the line can still maintain normal operation, and the non-fault shielding failure is performed at the moment. When the amplitude of the lightning current is too high and the overvoltage suffered by the line is too large, the insulation of the line can be damaged, the fault shielding failure is the case, and the protection should be performed. Since non-fault detours are also accompanied by a voltage drop, a current increase, and generation of high-frequency components, it is necessary to accurately distinguish between non-fault detours and line faults in order to improve the reliability of transient protection.
The process of lightning striking the transmission line can be regarded as that lightning waves are transmitted to two ends of the line along an infinite lightning channel through a lightning strike point, and the waveform of the lightning current can be represented by a double-exponential function of an expression (9). The lightning voltage waveform is similar in that it is the lightning current waveform multiplied by the wave impedance of the lightning path.
i(t)＝AIP(e-αt-e-βt) (9)
IPThe peak value of lightning current, A, alpha and beta are waveform parameters, and different lightning current waveforms correspond to different parameters. The standard waveform of IEC 1.2/50. mu.s is shown in FIG. 3, where T11.2 μ s is the (apparent) wavefront time, T250 μ s is the (apparent) half peak time.
The expression (9) is subjected to fourier transform, and a frequency domain expression of the radar wave can be obtained.
Figure BDA0003155128860000071
Thus, a spectrogram of a 1.2/50 μ s waveform can be obtained as shown in FIG. 4, and it can be seen that the frequency components of the lightning waves are also mainly distributed in the low frequency band. It is known that lightning waves have a higher high frequency content than fault travelling waves in the relative content of high frequency components and low frequency components, so that lightning stroke disturbances can be identified by the relative size of the high and low frequencies. However, when the attenuation of traveling waves by the line is considered, when the lightning strike location is at the far end of the line, the attenuation of high frequency components is large, and the relationship of high and low frequency components detected at the protection installation may be similar to that of a fault traveling wave. And when a short-distance fault occurs, within a limited data window length, a traveling wave is subjected to a plurality of refraction and reflection processes between the smoothing reactor and a fault point, and the frequency distribution of the voltage of the measuring point is changed. The fault voltage at this time is also characterized by containing a large amount of high-frequency components, and it is difficult for the protection to determine whether the fault is a short-circuit fault or lightning interference. Therefore, the lightning stroke interference is difficult to accurately identify by using a frequency domain method, and a judgment method can only be searched from a time domain.
FIGS. 5 and 6 are time domain waveforms of a lightning wave and a fault traveling wave, respectively, a rectangular region is formed between a fixed time window (0.6 ms is adopted in the present invention), an abscissa axis and a maximum value, and a lightning wave curve and the coordinate axis constitute S in FIG. 52The fault traveling wave curve and the coordinate axis form T in FIG. 62。
Order:
Figure BDA0003155128860000081
Figure BDA0003155128860000082
as can be seen from fig. 6, M of the fault travelling wave1Is greater than 0.5. M2Can be calculated from the following formula, tsTo protect the starting moment.
Figure BDA0003155128860000083
M2The calculation result was 0.1172, M2Are all much smaller than 0.5. M1>M2And a lightning stroke interference identification method can be established through the characteristics.
The specific implementation process of the invention is as follows: firstly, whether the voltage break variable of two continuous sampling values exceeds a starting threshold value is calculated, and if the voltage break variable exceeds the starting threshold value, starting protection is carried out. And then, calculating the size of the high-frequency component by using Hilbert-Huang transform, and when the size of the high-frequency component is larger than a fault judgment threshold, determining that the line is possible to have a fault. Then calculating lightning stroke criterion ksE.g. ks>k2And if the fault is determined, subsequent pole selection judgment is carried out, and otherwise, lightning stroke interference is determined. Finally, the correct fault pole is selected according to pole selection criteria to protect the correct outlet and switchAnd (4) removing the fault.

Claims (1)

1. A MMC-HVDC transient state quantity protection method capable of identifying lightning stroke interference comprises the following steps:
step (1): setting a protection measuring point at the initial end of the line, namely the line side of the line reactor, realizing protection starting and fault pole selection by using the voltage variation of the protection measuring point, and enabling the delta U to be the voltage variation measured in real time at the protection installation position, wherein the delta U is the voltage variation measured in real timezFor protecting the starting threshold, when the voltage variation of two continuous sampling points is greater than the protection starting threshold, the protection is started;
let kpoleFor voltage variation of successive N sampling points, Δ UpAnd Δ UnThe voltage abrupt change of the anode and the cathode respectively,
Figure FDA0003155128850000011
let k1For the fault pole decision threshold, pole selection criteria are constructed as follows:
Figure FDA0003155128850000012
step (2): the method for identifying the faults inside and outside a fault formulation area by utilizing the high-frequency attenuation effect of a smoothing reactor on fault traveling waves comprises the steps of sequentially decomposing an original signal into a series of Intrinsic Mode Functions (IMF) components according to the sequence from high frequency to low frequency through empirical mode decomposition, carrying out Hilbert transform on each IMF component to obtain an analytic signal z (t), and further obtaining the instantaneous frequency f of each moment in the IMFj；
Empirical mode decomposition is carried out on 1-mode fault voltage traveling waves to obtain a first intrinsic mode function component IMF1, absolute value summation is carried out on data with instantaneous frequency larger than 500Hz in IMF1, and high-frequency energy E is defined by the data1Then t after the protection is started1Inner E1Comprises the following steps:
Figure FDA0003155128850000013
N1is the number of data points in the criterion time window, when E1Greater than EzWhen a fault occurs in the line area, EzSetting according to the most serious out-of-area fault as a criterion threshold value;
and (3): fault identification is carried out by utilizing time domain waveform area characteristics, lightning stroke interference identification criteria are constructed by utilizing reverse traveling waves, and the following criteria are constructed according to discrete data obtained by sampling:
Figure FDA0003155128850000021
in the formula, the numerator represents the area of the waveform enclosed by the coordinate axes in a fixed time window, the denominator is the maximum value of the waveform, the rectangular area enclosed by the coordinate axes and the time window, and the ratio of the two is ks，B1For protecting the 1-mode reverse wave measured at the installation site, N2Is the number of data points in the criterion time window, when ks>k2When considered a line fault, k2And judging a threshold value for lightning stroke interference, otherwise, judging the threshold value for the lightning stroke interference.
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