CN112034307A - Cable early fault detection method based on stationary wavelet transform and symmetric component method - Google Patents

Cable early fault detection method based on stationary wavelet transform and symmetric component method Download PDF

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CN112034307A
CN112034307A CN202010921225.6A CN202010921225A CN112034307A CN 112034307 A CN112034307 A CN 112034307A CN 202010921225 A CN202010921225 A CN 202010921225A CN 112034307 A CN112034307 A CN 112034307A
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瞿科
张文海
楚恬歆
肖先勇
陈琳
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Sichuan University
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    • 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
    • G01R31/088—Aspects of digital computing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
    • G01R19/02—Measuring effective values, i.e. root-mean-square values
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R29/00—Arrangements for measuring or indicating electric quantities not covered by groups G01R19/00 - G01R27/00
    • G01R29/16—Measuring asymmetry of polyphase networks
    • 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
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    • Y04S—SYSTEMS INTEGRATING TECHNOLOGIES RELATED TO POWER NETWORK OPERATION, COMMUNICATION OR INFORMATION TECHNOLOGIES FOR IMPROVING THE ELECTRICAL POWER GENERATION, TRANSMISSION, DISTRIBUTION, MANAGEMENT OR USAGE, i.e. SMART GRIDS
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Abstract

The invention discloses a cable early fault detection method based on stationary wavelet transform and a symmetric component method, which comprises the steps of reconstructing through discrete stationary wavelet transform to obtain a voltage fundamental frequency component and a current fundamental frequency component, calculating a zero-sequence current root mean square value, obtaining preliminary fault judgment according to a threshold value of the zero-sequence current root mean square value, calculating the zero-sequence voltage root mean square value, preliminarily eliminating three-phase symmetric faults, capacitor switching, large motor starting and load switching interference in a power distribution network through 0.1 time of rated phase voltage, limiting fault duration, further eliminating other interference and finally accurately obtaining the early fault.

Description

Cable early fault detection method based on stationary wavelet transform and symmetric component method
Technical Field
The invention relates to the field of power line control, in particular to a cable early fault detection method based on stationary wavelet transformation and a symmetric component method.
Background
Most of distribution networks in China operate in a neutral point ungrounded mode, and under the condition, when an early cable fault occurs, the phase current amplitude of the fault is not obviously changed, so that the early cable fault is difficult to detect and a feeder line with the early cable fault is difficult to determine.
There are currently studies on detecting and identifying early faults of cables, which can be mainly divided into signal processing-based and machine learning-based methods. Because the number of samples of early cable faults in an actual power distribution network is small, a large number of simulation samples need to be obtained by means of simulation software and used as training samples of a machine learning method. But the simulation sample is different from the actual situation, and the practicability of the simulation sample needs to be further verified.
Disclosure of Invention
Aiming at the defects in the prior art, the cable early fault detection method based on the stable wavelet transform and the symmetric component method solves the problem of cable early fault detection of a system that a neutral point is grounded through a small resistor and the neutral point is not grounded.
In order to achieve the purpose of the invention, the invention adopts the technical scheme that: a cable early fault detection method based on stationary wavelet transform and a symmetric component method comprises the following steps:
s1, collecting three-phase voltage signals and current signals in the transformer substation;
s2, carrying out discrete stationary wavelet transformation on the three-phase voltage signals and the three-phase current signals, and reconstructing to obtain voltage fundamental frequency components and current fundamental frequency components;
s3, calculating a zero sequence current root mean square value according to the current fundamental frequency component and a symmetrical component method;
s4, judging whether the maximum value of the root mean square value of the zero sequence current is larger than the zero sequence current threshold value, if so, jumping to a step S5, if not, judging that the fault is not an early fault, and jumping to a step S1;
s5, calculating zero sequence voltage for the voltage fundamental frequency component by adopting a symmetrical component method;
s6, calculating a zero sequence voltage root mean square value through half-wave RMS point-by-point sliding according to the zero sequence voltage;
s7, judging whether the maximum value of the root mean square value of the zero sequence voltage is larger than 0.1U, if so, jumping to a step S8, if not, judging that the zero sequence voltage is not an early fault, and jumping to a step S1, wherein U is a rated phase voltage;
s8, counting that the root mean square value of zero sequence voltage is more than 0.1VmaxIs recorded as the duration, wherein VmaxIs the maximum value of the zero sequence voltage;
and S9, judging whether the duration time is within the duration time interval, if so, judging the fault to be an early fault, if not, judging the fault to be not an early fault, and jumping to the step S1.
Further, the expressions of the voltage fundamental frequency component and the current fundamental frequency component reconstructed in step S2 are:
Figure BDA0002666795190000021
Figure BDA0002666795190000022
Figure BDA0002666795190000023
Figure BDA0002666795190000024
Figure BDA0002666795190000025
Figure BDA0002666795190000026
Va_cJ+1＝HJVa_cJ
Va_dJ+1＝GJVa_cJ
Vb_cJ+1＝HJVb_cJ
Vb_dJ+1＝GJVb_cJ
Vc_cJ+1＝HJVc_cJ
Vc_dJ+1＝GJVc_cJ
Ia_cJ+1＝HJIa_cJ
Ia_dJ+1＝GJIa_cJ
Ib_cJ+1＝HJIb_cJ
Ib_dJ+1＝GJIb_cJ
Ic_cJ+1＝HJIc_cJ
Ic_dJ+1＝GJIc_cJ
wherein, VaJ,nVoltage value, Vb, of the nth sampling point of the fundamental frequency component of the A-phase voltageJ,nThe voltage value Vc of the nth sampling point of the fundamental frequency component of the B-phase voltageJ,bThe voltage value at the b-th sampling point of the fundamental frequency component of the C-phase voltage, IaJ,nCurrent value of the nth sampling point, Ib, being the fundamental component of A-phase currentJ,nCurrent value, Ic, of the nth sampling point of the fundamental component of the B-phase currentJ,nThe current value of the nth sampling point of the fundamental frequency component of the C-phase current,
Figure BDA0002666795190000031
in order to be the first reconstruction operator,
Figure BDA0002666795190000032
for the second reconstruction operator, Va _ cJ,nApproximating the nth value of the coefficient, Va _ d, for the fundamental frequency component of the A-phase voltageJ,nFor the nth value, Vb c, of the detail coefficient of the fundamental component of the A-phase voltageJ,nApproximating the nth value, Vb _ d, of the coefficient for the fundamental frequency component of the B-phase voltageJ,nFor the nth value, Vc _ c, of the detail coefficient of the fundamental component of the B-phase voltageJ,nApproximating the nth value of the coefficient, Vc _ d, for the fundamental frequency component of the C-phase voltageJ,nFor details of fundamental frequency components of C-phase voltageThe nth value, Ia _ cJ,nFor the nth value of the approximation coefficient for the fundamental component of the A-phase current, Ia _ dJ,nFor the nth value of the detail coefficient of the fundamental component of the A-phase current, Ib _ cJ,nFor the nth value of the B-phase current fundamental frequency component approximation coefficient, Ib _ dJ,nFor the nth value of the detail coefficient of the fundamental component of the B-phase current, Ic _ cJ,nFor the nth value of the C-phase current fundamental frequency component approximation coefficient, Ic _ dJ,nFor the nth value, H, of the detail coefficient of the fundamental component of the C-phase currentJIs a low-pass filter, GJIs a high pass filter, and J is the level of stationary wavelet transform corresponding to the fundamental frequency component.
Further, step S3 includes the steps of:
s31, calculating zero sequence current for the current fundamental frequency component by adopting a symmetrical component method;
and S32, calculating the root mean square value of the zero sequence current through half-wave RMS point-by-point sliding according to the zero sequence current.
Further, the formula for calculating the zero-sequence current in step S31 is as follows:
Figure BDA0002666795190000041
wherein, I0(n) is the zero sequence current of the nth sampling point, IaJ,nCurrent value of the nth sampling point, Ib, being the fundamental component of A-phase currentJ,nCurrent value of the nth sampling point, Jc, of fundamental component of B-phase currentJ,nThe current value of the nth sampling point of the fundamental frequency component of the C-phase current.
Further, the zero sequence current root mean square value calculated in step S32 is represented by the following formula:
Figure BDA0002666795190000042
wherein, I0(N) is the zero sequence current of the b-th sampling point, N is the number of sampling points of half cycle, I0_rmsAnd (n) is the root mean square value of the zero sequence current of the nth sampling point.
Further, the zero-sequence current threshold value in step S4 is 10A.
The beneficial effects of the above further scheme are: since the early fault of the cable is mainly represented by a single-phase earth fault, according to the symmetrical component method, when the early fault of the cable occurs, the system generates zero-sequence voltage and zero-sequence current. For a fault feeder line, the zero sequence current flowing through the monitoring point contains the zero sequence currents of other normal feeder lines, so the amplitude of the zero sequence current of the fault feeder line is larger than that of the zero sequence current of the normal feeder line. By setting a reasonable threshold, which feeder the early fault is located on can be determined.
Further, the formula for calculating the zero sequence voltage in step S5 is as follows:
Figure BDA0002666795190000043
wherein, V0(n) zero sequence voltage of nth sampling point, VaJ,nVoltage value, Vb, of the nth sampling point of the fundamental frequency component of the A-phase voltageJ,nThe voltage value Vc of the nth sampling point of the fundamental frequency component of the B-phase voltageJ,nThe voltage value of the nth sampling point of the fundamental frequency component of the C-phase voltage is obtained.
Further, the zero sequence voltage root mean square value calculated in step S6 is represented by the following formula:
Figure BDA0002666795190000051
wherein, V0(N) is the zero sequence voltage of the nth sampling point, N is the number of sampling points of half cycle, V0_rmsAnd (n) is the root mean square value of the zero sequence voltage of the nth sampling point.
Further, the duration interval is [5ms,80ms ] in step S9.
In conclusion, the beneficial effects of the invention are as follows: a cable early fault detection method based on stationary wavelet transform and a symmetric component method comprises the steps of obtaining a voltage fundamental frequency component and a current fundamental frequency component through discrete stationary wavelet transform and reconstruction, calculating a zero sequence current root mean square value, obtaining preliminary fault judgment according to a threshold value of the zero sequence current root mean square value, calculating the zero sequence voltage root mean square value, preliminarily eliminating three-phase symmetric faults, capacitor switching, large motor starting and load switching interference in a power distribution network through 0.1-time rated phase voltage, limiting fault duration, further eliminating other interference and finally accurately obtaining the early fault.
Drawings
Fig. 1 is a flow chart of a cable early fault detection method based on stationary wavelet transform and a symmetric component method.
Fig. 2 is a waveform diagram illustrating the duration of an early fault.
Detailed Description
The following description of the embodiments of the present invention is provided to facilitate the understanding of the present invention by those skilled in the art, but it should be understood that the present invention is not limited to the scope of the embodiments, and it will be apparent to those skilled in the art that various changes may be made without departing from the spirit and scope of the invention as defined and defined in the appended claims, and all matters produced by the invention using the inventive concept are protected.
As shown in fig. 1, a method for detecting early failure of a cable based on stationary wavelet transform and symmetric component method includes the following steps:
s1, collecting three-phase voltage signals and current signals in the transformer substation;
in this embodiment: the three-phase voltage signal and the current signal can be obtained through an electric energy quality device, a fault recording device and the like which are arranged in the transformer substation.
S2, carrying out discrete stationary wavelet transformation on the three-phase voltage signals and the three-phase current signals, and reconstructing to obtain voltage fundamental frequency components and current fundamental frequency components;
the expressions of the voltage fundamental frequency component and the current fundamental frequency component reconstructed in step S2 are:
Figure BDA0002666795190000061
Figure BDA0002666795190000062
Figure BDA0002666795190000063
Figure BDA0002666795190000064
Figure BDA0002666795190000065
Figure BDA0002666795190000066
Va_cJ+1＝HJVa_cJ
Va_dJ+1＝GJVa_cJ
Vb_cJ+1＝HJVb_cJ
Vb_dJ+1＝GJVb_cJ
Vc_cJ+1＝HJVc_cJ
Vc_dJ+1＝GJVc_cJ
Ia_cJ+1＝HJIa_cJ
Ia_dJ+1＝GJIa_cJ
Ib_cJ+1＝HJIb_cJ
Ib_dJ+1＝GJIb_cJ
Ic_cJ+1＝HJIc_cJ
Ic_dJ+1＝GJIc_cJ
wherein, VaJ,nVoltage value, Vb, of the nth sampling point of the fundamental frequency component of the A-phase voltageJ,nThe voltage value Vc of the nth sampling point of the fundamental frequency component of the B-phase voltageJ,nVoltage value at the nth sampling point of fundamental frequency component of C-phase voltage IaJ,nCurrent value of the nth sampling point, Ib, being the fundamental component of A-phase currentJ,nCurrent value, Ic, of the nth sampling point of the fundamental component of the B-phase currentJ,nThe current value of the nth sampling point of the fundamental frequency component of the C-phase current,
Figure BDA0002666795190000071
in order to be the first reconstruction operator,
Figure BDA0002666795190000072
for the second reconstruction operator, Va _ cJ,nApproximating the nth value of the coefficient, Va _ d, for the fundamental frequency component of the A-phase voltageJ,nFor the nth value, Vb c, of the detail coefficient of the fundamental component of the A-phase voltageJ,nApproximating the nth value, Vb _ d, of the coefficient for the fundamental frequency component of the B-phase voltageJ,nFor the nth value, Vc _ c, of the detail coefficient of the fundamental component of the B-phase voltageJ,nApproximating the nth value of the coefficient, Vc _ d, for the fundamental frequency component of the C-phase voltageJ,nFor the nth value of the detail coefficient of the fundamental frequency component of the C-phase voltage, Ia _ CJ,nFor the nth value of the approximation coefficient for the fundamental component of the A-phase current, Ia _ dJ,nFor the nth value of the detail coefficient of the fundamental component of the A-phase current, Ib _ cJ,nFor the nth value of the B-phase current fundamental frequency component approximation coefficient, Ib _ dJ,nFor the nth value of the detail coefficient of the fundamental component of the B-phase current, Ic _ cJ,nFor the nth value of the C-phase current fundamental frequency component approximation coefficient, Ic _ dJ,nFor the nth value, H, of the detail coefficient of the fundamental component of the C-phase currentJIs a low-pass filter, GJIs a high pass filter, and J is the level of stationary wavelet transform corresponding to the fundamental frequency component.
The three-phase voltage signals and the three-phase current signals are processed by adopting discrete stationary wavelet transform, so that the approximate coefficients and the detail coefficients obtained after the wavelet transform are consistent with the lengths of the original three-phase voltage signals and the original three-phase current signals, the defect that the lengths of wavelet coefficients of each layer are inconsistent after the ordinary discrete wavelet transform is overcome, the characteristics of translation invariance and redundancy are simultaneously realized, and the phenomenon of Gibbs oscillation caused by the fact that the wavelet base does not have the translation invariance is avoided.
S3, calculating a zero sequence current root mean square value according to the current fundamental frequency component and a symmetrical component method;
s31, calculating zero sequence current for the current fundamental frequency component by adopting a symmetrical component method;
the formula for calculating the zero-sequence current in step S31 is as follows:
Figure BDA0002666795190000081
wherein, I0(n) is the zero sequence current of the nth sampling point, IaJ,nCurrent value of the nth sampling point, Ib, being the fundamental component of A-phase currentJ,nCurrent value, Ic, of the nth sampling point of the fundamental component of the B-phase currentJ,nThe current value of the nth sampling point of the fundamental frequency component of the C-phase current.
And S32, calculating the root mean square value of the zero sequence current through half-wave RMS point-by-point sliding according to the zero sequence current.
The zero sequence current root mean square value calculation formula in step S32 is:
Figure BDA0002666795190000082
wherein, I0(N) is the zero sequence current of the nth sampling point, N is the number of sampling points of half cycle, I0_rmsAnd (n) is the root mean square value of the zero sequence current of the nth sampling point.
S4, judging whether the maximum value of the root mean square value of the zero sequence current is larger than the zero sequence current threshold value, if so, jumping to a step S5, if not, judging that the fault is not an early fault, and jumping to a step S1;
the zero-sequence current threshold value in step S4 is 10A.
When the capacitive current of a cable line is large and the capacitive zero-sequence current in a 10kV system is greater than 10A in the system operation regulation, an arc suppression coil needs to be arranged to reduce the zero-sequence current. Therefore, the invention intends to adopt 10A as the basis for determining early failure, and the threshold value can be adjusted according to actual conditions.
S5, calculating zero sequence voltage for the voltage fundamental frequency component by adopting a symmetrical component method;
the formula for calculating the zero sequence voltage in step S5 is as follows:
Figure BDA0002666795190000091
wherein, V0(n) zero sequence voltage of nth sampling point, VaJ,nVoltage value, Vb, of the nth sampling point of the fundamental frequency component of the A-phase voltageJ,nThe voltage value Vc of the nth sampling point of the fundamental frequency component of the B-phase voltageJ,nThe voltage value of the nth sampling point of the fundamental frequency component of the C-phase voltage is obtained.
S6, calculating a zero sequence voltage root mean square value through half-wave RMS point-by-point sliding according to the zero sequence voltage;
the zero sequence voltage root mean square value calculation formula in step S6 is:
Figure BDA0002666795190000092
wherein, V0(N) is the zero sequence voltage of the nth sampling point, N is the number of sampling points of half cycle, V0_rmsAnd (n) is the root mean square value of the zero sequence voltage of the nth sampling point.
S7, judging whether the maximum value of the root mean square value of the zero sequence voltage is larger than 0.1U, if so, jumping to a step S8, if not, judging that the zero sequence voltage is not an early fault, and jumping to a step S1, wherein U is a rated phase voltage;
normally, the zero sequence voltage exceeds 0.15 times of rated phase voltage, namely, single-phase earth fault occurs. In order to improve the sensitivity of early fault detection, the threshold value of the zero sequence voltage is set to be 0.1 time of the rated phase voltage. In addition, the threshold value can be adjusted according to actual conditions.
S8, counting the time that the zero sequence voltage root mean square value is larger than the threshold value, recording as the duration, selecting the threshold value of the threshold value according to the actual situation, and implementing the methodExample with 0.1VmaxWherein V ismaxIs the maximum value of the zero sequence voltage;
and S9, judging whether the duration time is within the duration time interval, if so, judging the fault to be an early fault, if not, judging the fault to be not an early fault, and jumping to the step S1.
The duration interval is [5ms,80ms ] in step S9, as shown in fig. 2.
Zero sequence voltages are also generated by asymmetric permanent faults and transformer switching in the distribution network. However, due to the intermittent and self-cleaning properties of early cable faults, the duration of the cable faults is generally 1/4-4 cycles, and the duration of the cable faults is 5-80 ms for a 50Hz power distribution network. While the duration of an asymmetric permanent fault and transformer switching can typically last tens of cycles. The influence of the disturbance can be eliminated by judging the duration time of the zero sequence voltage.

Claims (9)

1. A cable early fault detection method based on stationary wavelet transform and a symmetric component method is characterized by comprising the following steps:
s1, collecting three-phase voltage signals and current signals in the transformer substation;
s2, carrying out discrete stationary wavelet transformation on the three-phase voltage signals and the three-phase current signals, and reconstructing to obtain voltage fundamental frequency components and current fundamental frequency components;
s3, calculating a zero sequence current root mean square value according to the current fundamental frequency component and a symmetrical component method;
s4, judging whether the maximum value of the root mean square value of the zero sequence current is larger than the zero sequence current threshold value, if so, jumping to a step S5, if not, judging that the fault is not an early fault, and jumping to a step S1;
s5, calculating zero sequence voltage for the voltage fundamental frequency component by adopting a symmetrical component method;
s6, calculating a zero sequence voltage root mean square value through half-wave RMS point-by-point sliding according to the zero sequence voltage;
s7, judging whether the maximum value of the root mean square value of the zero sequence voltage is larger than 0.1U, if so, jumping to a step S8, if not, judging that the zero sequence voltage is not an early fault, and jumping to a step S1, wherein U is a rated phase voltage;
s8, counting that the root mean square value of zero sequence voltage is more than 0.1VmaxIs recorded as the duration, wherein VmaxIs the maximum value of the zero sequence voltage;
and S9, judging whether the duration time is within the duration time interval, if so, judging the fault to be an early fault, if not, judging the fault to be not an early fault, and jumping to the step S1.
2. The method for detecting the early failure of the cable based on the wavelet transform and the symmetric component method as claimed in claim 1, wherein the expressions of the voltage fundamental frequency component and the current fundamental frequency component reconstructed in the step S2 are:
Figure FDA0002666795180000011
Figure FDA0002666795180000012
Figure FDA0002666795180000021
Figure FDA0002666795180000022
Figure FDA0002666795180000023
Figure FDA0002666795180000024
Va_cJ+1＝HJVa_cJ
Va_dJ+1＝GJVa_cJ
Vb_cJ+1＝HJVb_cJ
Vb_dJ+1＝GJVb_cJ
Vc_cJ+1＝HJVc_cJ
Vc_dJ+1＝GJVc_cJ
Ia_cJ+1＝HJIa_cJ
Ia_dJ+1＝GJIa_cJ
Ib_cJ+1＝HJIb_cJ
Ib_dJ+1＝GJIb_cJ
Ic_cJ+1＝HJIc_cJ
Ic_dJ+1＝GJIc_cJ
wherein, VaJ,nVoltage value, Vb, of the nth sampling point of the fundamental frequency component of the A-phase voltageJ,nThe voltage value Vc of the nth sampling point of the fundamental frequency component of the B-phase voltageJ,nVoltage value at the nth sampling point of fundamental frequency component of C-phase voltage IaJ,nCurrent value of the nth sampling point, Ib, being the fundamental component of A-phase currentJ,nCurrent value, Ic, of the nth sampling point of the fundamental component of the B-phase currentJ,nThe current value of the nth sampling point of the fundamental frequency component of the C-phase current,
Figure FDA0002666795180000025
in order to be the first reconstruction operator,
Figure FDA0002666795180000026
for the second reconstruction operator, Va _ cJ,nApproximating the nth value of the coefficient, Va _ d, for the fundamental frequency component of the A-phase voltageJ,nFor the nth value, Vb c, of the detail coefficient of the fundamental component of the A-phase voltageJ,nApproximating the nth value, Vb _ d, of the coefficient for the fundamental frequency component of the B-phase voltageJ,nFor the nth value, Vc _ c, of the detail coefficient of the fundamental component of the B-phase voltageJ,nIs a C-phase voltage baseThe nth value of the frequency component approximation coefficient, Vc _ dJ,nFor the nth value of the detail coefficient of the fundamental frequency component of the C-phase voltage, Ia _ CJ,nFor the nth value of the approximation coefficient for the fundamental component of the A-phase current, Ia _ dJ,nFor the nth value of the detail coefficient of the fundamental component of the A-phase current, Ib _ cJ,nFor the nth value of the B-phase current fundamental frequency component approximation coefficient, Ib _ dJ,nFor the nth value of the detail coefficient of the fundamental component of the B-phase current, Ic _ cJ,nFor the nth value of the C-phase current fundamental frequency component approximation coefficient, Ic _ dJ,nFor the nth value, H, of the detail coefficient of the fundamental component of the C-phase currentJIs a low-pass filter, GJIs a high pass filter, and J is the level of stationary wavelet transform corresponding to the fundamental frequency component.
3. The method for detecting the early failure of the cable based on the wavelet transform of stationary wavelet and the symmetric component method as claimed in claim 1, wherein said step S3 comprises the steps of:
s31, calculating zero sequence current for the current fundamental frequency component by adopting a symmetrical component method;
and S32, calculating the root mean square value of the zero sequence current through half-wave RMS point-by-point sliding according to the zero sequence current.
4. The method for detecting the early failure of the cable based on the stationary wavelet transform and the symmetric component method as claimed in claim 3, wherein the formula for calculating the zero sequence current in the step S31 is as follows:
Figure FDA0002666795180000031
wherein, I0(n) is the zero sequence current of the nth sampling point, IaJ,nCurrent value of the nth sampling point, Ib, being the fundamental component of A-phase currentJ,nCurrent value of the nth sampling point, Jc, of fundamental component of B-phase currentJ,nThe current value of the nth sampling point of the fundamental frequency component of the C-phase current.
5. The method for detecting the early failure of the cable based on the stationary wavelet transform and the symmetric component method as claimed in claim 3, wherein the formula for calculating the root mean square value of the zero sequence current in the step S32 is as follows:
Figure FDA0002666795180000032
wherein, I0(N) is the zero sequence current of the nth sampling point, N is the number of sampling points of half cycle, I0_rmsAnd (n) is the root mean square value of the zero sequence current of the nth sampling point.
6. The method for detecting the early failure of the cable based on the stationary wavelet transform and the symmetric component method as claimed in claim 1, wherein the zero sequence current threshold value in the step S4 is 10A.
7. The method for detecting the early failure of the cable based on the stationary wavelet transform and the symmetric component method as claimed in claim 1, wherein the formula for calculating the zero sequence voltage in the step S5 is as follows:
Figure FDA0002666795180000041
wherein, V0(n) zero sequence voltage of nth sampling point, VaJ,nVoltage value, Vb, of the nth sampling point of the fundamental frequency component of the A-phase voltageJ,nThe voltage value Vc of the nth sampling point of the fundamental frequency component of the B-phase voltageJ,nThe voltage value of the nth sampling point of the fundamental frequency component of the C-phase voltage is obtained.
8. The method for detecting the early failure of the cable based on the stationary wavelet transform and the symmetric component method as claimed in claim 1, wherein the zero sequence voltage root mean square value calculated in the step S6 is represented by the following formula:
Figure FDA0002666795180000042
wherein, V0(N) is the zero sequence voltage of the b-th sampling point, N is the number of sampling points of half cycle, V0_rmsAnd (n) is the root mean square value of the zero sequence voltage of the nth sampling point.
9. The method for detecting the early failure of the cable based on the wavelet transform and the symmetric component method as claimed in claim 1, wherein the duration interval in step S9 is [5ms,80ms ].
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