CN111323681B - Cable insulation monitoring method and system based on high-voltage power frequency and low-voltage ultralow frequency - Google Patents
Cable insulation monitoring method and system based on high-voltage power frequency and low-voltage ultralow frequency Download PDFInfo
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Abstract
The invention discloses a cable insulation monitoring method and system based on high-voltage power frequency and low-voltage ultralow frequency, belonging to the field of cable insulation monitoring and comprising the following steps: coupling a low-voltage ultralow-frequency signal in a high-voltage power frequency cable line; respectively measuring the zero-sequence ultralow frequency active power average value and the zero-sequence ultralow frequency reactive power average value of the first end and the last end of each line; calculating the average value and standard deviation of ultralow frequency dielectric loss of each line; judging whether the average value of the ultralow frequency dielectric loss and the standard deviation thereof exceed a set threshold value; calculating correlation coefficients between adjacent lines, and comparing the correlation coefficients with the average correlation coefficient; and calculating the change rate of the ultralow frequency dielectric loss average value of each line, performing normalization processing to construct a distribution function, obtaining the insulation degradation probability, and outputting the insulation degradation probability when the change rate is greater than the basic occupation ratio. The invention can utilize various data processing methods to make evaluation basis aiming at specific lines on the premise of realizing the on-line monitoring of the cable insulation, and timely and accurately evaluate the cable insulation state.
Description
Technical Field
The invention belongs to the field of cable insulation monitoring, and particularly relates to a cable insulation monitoring method and system based on high-voltage power frequency and low-voltage ultralow frequency.
Background
Most cables suffer from insulation breakdown events caused by water tree branches, which are low in the tree-start voltage at ac voltages. Impurities, protrusions and the like form high electrical stress, and insulation and moisture are further promoted to generate water branches. In fact, the growth speed of the water tree branches is slow, the partial discharge signals are weak, and the detection is not easy. Once the water tree branch induces the electrical tree branch, the insulation breakdown is accelerated, and the insulation breakdown may occur in a short time. Therefore, early detection of cable insulation defects is of great significance.
At present, cable insulation monitoring methods mainly comprise a local release method, a direct current method, a dielectric loss method and the like. The partial discharge method is small in detection range and is not suitable for detecting the cable body; the direct current method is susceptible to stray current interference; the dielectric loss method cannot ensure the accuracy of measurement because the resistive current is easily submerged by the capacitive current under the power frequency alternating current.
The ultra-low frequency signal almost has no attenuation when propagating in the cable core, and simultaneously can weaken the influence of the equivalent capacitance of the cable. The existing ultralow frequency dielectric loss measurement method is mainly based on IEEE.Std.400.2-2013, adopts a single ultralow frequency high-voltage power supply, can only carry out off-line measurement, and influences the normal operation of the system. The data processing method has the following characteristics: 1) the evaluation method is single, and misjudgment is easily caused only by threshold judgment; 2) the threshold is established based on the statistical distribution of a large number of different line data, and the fact that different lines have large differences due to different cable types, surrounding environments and end connecting equipment is not considered.
The power transmission and distribution line insulation state cluster analysis method based on modulation type injection, which is provided by Huazhong university of science and technology, can realize the online monitoring of the line insulation state. The method comprises the following steps: 1) only the direct current at the head end of the line is measured, and the measurement result is easily influenced by equipment connected with the tail end of the line and cannot accurately reflect the insulation state of the line; 2) the evaluation method only depends on the similarity of the measured values among the lines, and can not avoid the condition that the similarity of individual lines is low due to the difference of line types, connecting equipment and the like, and is easy to cause misjudgment.
Therefore, the prior art cannot meet the requirement for timely and accurately evaluating the insulation aging state of the cable, on the premise of realizing on-line monitoring of the cable insulation, how to reduce the influence of the cable type, the surrounding environment and the difference of the terminal connection equipment, how to make an evaluation basis for a specific line, and how to adopt which data processing methods to timely and accurately evaluate the insulation state of the cable are problems to be solved urgently.
Disclosure of Invention
Aiming at the defects or improvement requirements of the prior art, the invention provides a cable insulation monitoring method and system based on high-voltage power frequency and low-voltage ultralow frequency, so that the technical problem that the insulation aging state of a cable cannot be timely and accurately evaluated in the prior art is solved.
In order to achieve the above object, according to an aspect of the present invention, there is provided a cable insulation monitoring method based on high-voltage power frequency and low-voltage ultra-low frequency, comprising the steps of:
(1) in a power grid formed by a plurality of cable lines, coupling a low-voltage ultralow-frequency signal into the cable line without the low-voltage ultralow-frequency signal;
(2) acquiring zero-sequence ultralow-frequency currents at the head end and the tail end of each cable line, and calculating the average value of the zero-sequence ultralow-frequency active power and reactive power at the head end and the tail end of each cable line by combining reference voltage;
(3) calculating the ultralow frequency dielectric loss average value of each cable line by using the zero-sequence ultralow frequency active and reactive power average values at the first and last ends of each cable line, and calculating the standard deviation of the ultralow frequency dielectric loss average value of each cable line by using the ultralow frequency dielectric loss average value of each cable line;
(4) when the standard deviation of the ultralow frequency dielectric loss average value of a certain cable line is larger than a first threshold value or the ultralow frequency dielectric loss average value is larger than a second threshold value, entering the step (5);
(5) calculating the correlation coefficient of the ultralow-frequency dielectric loss average value time sequence between a certain cable line and adjacent lines in the power grid, entering the step (6) if each correlation coefficient is less than the average correlation coefficient, and returning to the step (4) if not;
(6) calculating the change rate of the ultralow-frequency dielectric loss average value of each cable line, carrying out normalization processing for constructing a distribution function, calculating the insulation degradation probability of a certain cable line by using the distribution function, outputting the insulation degradation probability of the cable line when the insulation degradation probability of the cable line is greater than or equal to the basic occupation ratio, and otherwise, returning to the step (4);
the cable line is a high-voltage power frequency cable line, the voltage range of the low-voltage ultralow-frequency signal is 0-50V, and the frequency is 0.01 Hz.
Further, the specific implementation manner of coupling the low-voltage ultralow frequency signal is as follows:
and a low-voltage ultralow-frequency voltage source is connected in parallel to a cable line through an arc suppression coil, a bus voltage transformer or an isolation inductor.
Further, the step (2) comprises:
acquiring zero-sequence ultralow frequency current i at the head end and the tail end of each cable line1And i2Combined with a first reference voltage urCalculating the zero-sequence ultralow frequency active power average value P of the first end and the last end of each cable liner1And Pr2Combined with a second reference voltage ucCalculating the zero-sequence ultralow frequency power average value P of the first end and the last end of each cable linec1And Pc2，
ur＝U0sin(ω0t+α)，uc＝U0cos(ω0t+α)
Wherein, ω is0Is 0.02 pi, omega0Corresponding to a frequency of 0.01Hz, U0Is a predetermined value, U0In the range of 0-50V, alpha ∈ (0,2 π), n0T represents n0A period of time consisting of periods T, T representing n0At a certain moment in the T time period, changing the value of alpha, and taking the value of Pr1U corresponding to the maximum valuerIs a first reference voltage, to urPerforming phase transformation to obtain a second reference voltage uc。
Further, the average value of ultralow-frequency dielectric loss of each cable line is TD:
further, the standard deviation of the average value of the ultralow frequency dielectric loss of each cable line is TDTS:
wherein, in the calculation of TDTS, the TD of multiple days is divided into n1Segment, n2As the number of TDs in each segment, TDiFor the ith value in a segment,
average for multi-day TDAnd (4) average value.
Further, the first threshold value ranges from 0.1 to 0.5, and the second threshold value ranges from 4 to 50.
Further, the adjacent lines of the certain cable line in the step (5) in the power grid are: in the grid, the cable types (PE, XLPE and TRXLPE), the surrounding environment (humidity, air temperature) and the installation environment (buried in earth, in cement ditches) are consistent with a certain cable line.
Further, the average correlation coefficient in the step (5) is
Wherein,
for cable lines i and their adjacent lines j in the network0And (4) a correlation coefficient (the correlation coefficient can be calculated by cosine distance) of the ultralow frequency dielectric loss average value time sequence, wherein n is the total number of adjacent lines in the power grid.
Further, the step (6) comprises:
calculating the change rate of the ultralow-frequency dielectric loss average value of each cable line and performing normalization treatment:
wherein k isi，jThe change rate of the ultralow-frequency dielectric loss average value of the cable line i at the moment j in a certain period of time, TDi，j+1、TDi，j-1Respectively the ultralow frequency dielectric loss average values of the cable line i at the time of j +1 and the time of j-1, delta t is a time interval,
is the average value of the change rate of the ultralow-frequency dielectric loss average value of the cable line i outside a certain period，n3K 'is the total number of the ultralow frequency dielectric loss average value change rates in other periods except a certain period'i，jThe relative change rate of the ultralow frequency dielectric loss average value of the cable line i at the moment j after normalization processing is carried out;
the ultralow-frequency dielectric loss average value relative change rate combination of all cable lines is used as a sample set S, elements in the S are used as random variables, and a distribution function is constructed: p (x is less than or equal to x)0) Calculating the insulation degradation probability P of the cable line i by using the distribution functioni：
Wherein x belongs to S and x0After the values of x and a are determined, the value range of a is 70-90% according to the distribution function; m is the total number of elements in the sample set S, miIs x > x0The number of elements belonging to the cable line i in the interval;
basic ratio biIs the ratio of elements belonging to cable line i in sample set S, when Pi≥biAt the time, output Pi。
According to another aspect of the present invention, there is provided a cable insulation monitoring system based on high voltage power frequency and low voltage ultra low frequency, comprising: VLF coupling unit, zero sequence VLF power calculating unit and information analyzing terminal,
the VLF coupling unit is used for coupling the low-voltage ultralow-frequency signals to the cable lines without the low-voltage ultralow-frequency signals in a power grid formed by a plurality of cable lines; the cable line is a high-voltage power frequency cable line, the voltage range of the low-voltage ultralow-frequency signal is 0-50V, and the frequency is 0.01 Hz;
the zero-sequence VLF power calculation unit is used for acquiring zero-sequence ultralow-frequency currents at the first end and the last end of each cable line, calculating zero-sequence ultralow-frequency active and reactive power average values at the first end and the last end of each cable line by combining reference voltages, and calculating the ultralow-frequency dielectric loss average value of each cable line by using the zero-sequence ultralow-frequency active and reactive power average values at the first end and the last end of each cable line;
the information analysis terminal is used for calculating the standard deviation of the ultralow frequency dielectric loss average value of each cable line by using the ultralow frequency dielectric loss average value of each cable line; when the standard deviation of the ultralow frequency dielectric loss average value of a certain cable line is larger than a first threshold or the ultralow frequency dielectric loss average value is larger than a second threshold, calculating the correlation coefficient of the ultralow frequency dielectric loss average value time sequence between the certain cable line and adjacent lines in the power grid, if each correlation coefficient is smaller than the average correlation coefficient, calculating the change rate of the ultralow frequency dielectric loss average value of each cable line and carrying out normalization processing for constructing a distribution function, calculating the insulation degradation probability of the certain cable line by using the distribution function, and outputting the insulation degradation probability of the certain cable line when the insulation degradation probability of the certain cable line is larger than or equal to the basic ratio.
In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:
(1) the invention can utilize various data processing methods to make evaluation basis aiming at specific lines on the premise of realizing the on-line monitoring of the cable insulation, and timely and accurately evaluate the cable insulation state.
(2) The problem of low-frequency oscillation of a power grid in the prior art is not effectively solved, and the method can be used for carrying out on-line monitoring on cable insulation by fully utilizing the existing ultralow-frequency signals in the power grid; when no ultralow frequency signal exists in the power grid, low-voltage ultralow frequency energy is coupled into the high-voltage power frequency for supplement, wherein the amplitude of the low-voltage ultralow frequency energy is small, and the normal operation of a cable line to be tested is not influenced.
(3) In the zero sequence ultralow frequency power measurement, a phase locking technology is utilized to extract reference voltage from a measured current value, and the reference voltage is subjected to phase transformation to respectively obtain active power and reactive power; the measurement error generated by multiple measurements can be reduced without adding an additional voltage measurement device.
(4) The zero-sequence ultralow frequency current is a three-phase current with the frequency of 0.01Hz, the interference of power frequency alternating current signals can be reduced by measuring the zero-sequence signal, the ultralow frequency signal can be conveniently detected, the measuring method is simple, and only a measuring device needs to be sleeved on the three phases of the cable.
(5) The invention measures the zero sequence ultralow frequency active power and reactive power average value of the first end and the last end of each line respectively, and subtracts the measured value of the tail end of each line from the measured value of the head end of each line respectively, so as to obtain the power average value consumed by the line insulation, thereby reducing the interference of the equipment connected with the tail end of the line, and simultaneously reducing the requirement on the time synchronism of the measuring devices at the first end and the last end by adopting the average value.
(6) The invention provides a comparison object under similar environment for each line to be tested by comparing the similar rules of the adjacent lines, and can reduce the influence of the change of factors such as seasons, surrounding environment and the like on the result.
(7) According to the invention, the VLF-TD (ultra-low frequency dielectric loss average value) change rate is subjected to normalization processing, the overall situation of the specific line and historical data of the specific line is compared, and the obtained VLF-TD relative change rate can reduce the influence of cable type difference of each line.
(8) The invention fully utilizes the advantages of big data, adopts a mathematical statistical method, can screen out a line with continuously high VLF-TD relative change rate, and outputs insulation degradation probability; with the increase of the line to be tested and the accumulation of data quantity, the randomness and the contingency can be reduced, and the misjudgment is reduced.
(9) The invention adopts various evaluation methods, utilizes threshold values, similarity and mathematical statistics methods to screen layer by layer, fully utilizes a large amount of data obtained by online monitoring, and can avoid misjudgment caused by judging by adopting a single method.
Drawings
Fig. 1 is a flowchart of a cable insulation monitoring method based on high-voltage power frequency and low-voltage ultra-low frequency according to an embodiment of the present invention;
FIG. 2 is a system architecture diagram provided by an embodiment of the present invention;
fig. 3(a) is a structural diagram of a first VLF coupling unit according to an embodiment of the present invention;
fig. 3(b) is a structural diagram of a second VLF coupling unit according to an embodiment of the present invention;
fig. 3(c) is a structural diagram of a third VLF coupling unit according to an embodiment of the present invention;
fig. 4 is a structural diagram of a zero-sequence VLF power calculation unit according to an embodiment of the present invention;
the same reference numbers will be used throughout the drawings to refer to the same or like elements or structures, wherein:
the device comprises a VLF coupling unit 1, a zero-sequence VLF power calculation unit 2, an information analysis terminal 3, a VLF voltage source 4, an arc suppression coil 5, a grounding transformer 6, a bus PT 7, an isolation inductor 8, a first port 9, a zero-sequence ultralow-frequency current measurement module 10, a reference voltage calculation module 11, a zero-sequence ultralow-frequency active and reactive power average value calculation module 12, a VLF-TD calculation module 13 and a second port 14.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
As shown in fig. 1, a cable insulation monitoring method based on high-voltage power frequency and low-voltage ultra-low frequency includes the following steps:
(1) in a power grid formed by a plurality of cable lines, coupling a low-voltage ultralow-frequency signal into the cable line without the low-voltage ultralow-frequency signal;
(2) acquiring zero-sequence ultralow-frequency currents at the head end and the tail end of each cable line, and calculating the average value of the zero-sequence ultralow-frequency active power and reactive power at the head end and the tail end of each cable line by combining reference voltage;
(3) calculating the ultralow frequency dielectric loss average value of each cable line by using the zero-sequence ultralow frequency active and reactive power average values at the first and last ends of each cable line, and calculating the standard deviation of the ultralow frequency dielectric loss average value of each cable line by using the ultralow frequency dielectric loss average value of each cable line;
(4) when the standard deviation of the ultralow frequency dielectric loss average value of a certain cable line is larger than a first threshold value or the ultralow frequency dielectric loss average value is larger than a second threshold value, entering the step (5);
(5) calculating the correlation coefficient of the ultralow-frequency dielectric loss average value time sequence between a certain cable line and adjacent lines in the power grid, entering the step (6) if each correlation coefficient is less than the average correlation coefficient, and returning to the step (4) if not;
(6) calculating the change rate of the ultralow-frequency dielectric loss average value of each cable line, carrying out normalization processing for constructing a distribution function, calculating the insulation degradation probability of a certain cable line by using the distribution function, outputting the insulation degradation probability of the cable line when the insulation degradation probability of the cable line is greater than or equal to the basic occupation ratio, and otherwise, returning to the step (4);
the cable line is a high-voltage power frequency cable line, the voltage range of the low-voltage ultralow-frequency signal is 0-50V, and the frequency is 0.01 Hz.
Further, the step (2) comprises:
acquiring zero-sequence ultralow frequency current i at the head end and the tail end of each cable line1And i2Combined with a first reference voltage urCalculating the zero-sequence ultralow frequency active power average value P of the first end and the last end of each cable liner1And Pr2Combined with a second reference voltage ucCalculating the zero-sequence ultralow frequency reactive power average value P of the first end and the last end of each cable linec1And Pc2，
ur＝U0sin(ω0t+α)，uc＝U0cos(ω0t+α)
Wherein, ω is0Is 0.02 pi, omega0Corresponding to a frequency of 0.01Hz, U0Is a predetermined value, U0In the range of 0-50V, alpha ∈ (0,2 π), n0T represents n0A period of time consisting of periods T, T representing n0At a certain moment in the T time period, changing the value of alpha, and taking the value of Pr1U corresponding to the maximum valuerIs a first reference voltage, to urPerforming phase transformation to obtain a second reference voltage uc。
Further, the average value of ultralow-frequency dielectric loss of each cable line is TD:
further, the standard deviation of the average value of the ultralow frequency dielectric loss of each cable line is TDTS:
wherein, in the calculation of TDTS, the TD of multiple days is divided into n1Segment, n2As the number of TDs in each segment, TDiFor the ith value in a segment,
mean TD over multiple days.
Further, the first threshold value ranges from 0.1 to 0.5, and the second threshold value ranges from 4 to 50. In the embodiment of the present invention, the first threshold is 0.1, and the second threshold is 4.
Further, the adjacent lines of the certain cable line in the step (5) in the power grid are: in the grid, the cable types (PE, XLPE and TRXLPE), the surrounding environment (humidity, air temperature) and the installation environment (buried in earth, in cement ditches) are consistent with a certain cable line.
Further, the average correlation coefficient in the step (5) is
Wherein,
for cable lines i and their adjacent lines j in the network0And (4) a correlation coefficient (the correlation coefficient can be calculated by cosine distance) of the ultralow frequency dielectric loss average value time sequence, wherein n is the total number of adjacent lines in the power grid. In the embodiment of the invention, the correlation coefficients of the ultralow-frequency dielectric loss average value time sequence of a certain cable line and adjacent lines in the power grid for 3 consecutive days are all smaller than the average correlation coefficient, the step (6) is carried out, and otherwise, the step (4) is returned.
Further, the step (6) comprises:
calculating the change rate of the ultralow-frequency dielectric loss average value of each cable line and performing normalization treatment:
wherein k isi，jIs the ultralow frequency dielectric loss average change rate of the cable line i at the moment j in a certain period (continuous 7 days in the embodiment of the invention), TDi，j+1、TDi，j-1Respectively the ultralow frequency dielectric loss average values of the cable line i at the time of j +1 and the time of j-1, delta t is a time interval,
is the average value of the change rate of the ultralow-frequency dielectric loss average value of the cable line i outside a certain period of time, n3K 'is the total number of the ultralow frequency dielectric loss average value change rates in other periods except a certain period'i，jThe relative change rate of the ultralow frequency dielectric loss average value of the cable line i at the moment j after normalization processing is carried out;
the ultralow-frequency dielectric loss average value relative change rate combination of all cable lines is used as a sample set S, elements in the S are used as random variables, and a distribution function is constructed: p (x is less than or equal to x)0) Calculating the insulation degradation probability P of the cable line i by using the distribution functioni：
Wherein x belongs to S and x0After the values of x and a are determined, the value range of a is 70-90% according to the distribution function; m is the total number of elements in the sample set S, miIs x > x0The number of elements belonging to the cable line i in the interval; in the embodiment of the invention, a is 80%;
basic ratio biIs the ratio of elements belonging to cable line i in sample set S, when Pi≥biAt the time, output Pi。
As shown in fig. 2, a cable insulation monitoring system based on high-voltage power frequency and low-voltage ultra-low frequency includes: VLF (ultra low frequency) coupling unit 1, zero sequence VLF power calculation unit 2 and information analysis terminal 3,
the VLF coupling unit is used for coupling the low-voltage ultralow-frequency signals to the cable lines without the low-voltage ultralow-frequency signals in a power grid formed by a plurality of cable lines; the cable line is a high-voltage power frequency cable line, the voltage range of the low-voltage ultralow-frequency signal is 0-50V, and the frequency is 0.01 Hz;
the zero-sequence VLF power calculation unit is used for acquiring zero-sequence ultralow-frequency currents at the first end and the last end of each cable line, calculating zero-sequence ultralow-frequency active and reactive power average values at the first end and the last end of each cable line by combining reference voltages, and calculating the ultralow-frequency dielectric loss average value of each cable line by using the zero-sequence ultralow-frequency active and reactive power average values at the first end and the last end of each cable line;
the information analysis terminal is used for calculating the standard deviation of the ultralow frequency dielectric loss average value of each cable line by using the ultralow frequency dielectric loss average value of each cable line; when the standard deviation of the ultralow frequency dielectric loss average value of a certain cable line is larger than a first threshold or the ultralow frequency dielectric loss average value is larger than a second threshold, calculating the correlation coefficient of the ultralow frequency dielectric loss average value time sequence between the certain cable line and adjacent lines in the power grid, if each correlation coefficient is smaller than the average correlation coefficient, calculating the change rate of the ultralow frequency dielectric loss average value of each cable line and carrying out normalization processing for constructing a distribution function, calculating the insulation degradation probability of the certain cable line by using the distribution function, and outputting the insulation degradation probability of the certain cable line when the insulation degradation probability of the certain cable line is larger than or equal to the basic ratio.
Fig. 3(a) shows that a low-voltage ultralow-frequency voltage source (i.e., VLF voltage source 4) is connected to a grounding transformer 6 via an arc suppression coil 5. Fig. 3(b) shows the low voltage ultra low frequency voltage source connected in parallel to the cabling via a busbar PT (voltage transformer) 7. Fig. 3(c) shows the low voltage ultra low frequency voltage source connected in parallel to the cabling through the isolating inductor 8.
As shown in fig. 4, the zero-sequence VLF power calculation unit includes:
a first port 9 for communication between the head end and the tail end of the line;
the zero-sequence ultralow frequency current measuring module 10 is used for acquiring zero-sequence ultralow frequency currents at the first end and the last end of each cable line;
a reference voltage calculation module 11 for calculating a reference voltage;
a zero-sequence ultralow-frequency active and reactive power average value calculation module 12, which is used for calculating the zero-sequence ultralow-frequency active and reactive power average values at the first and last ends of each cable line;
a VLF-TD calculating module 13, configured to calculate an average value of ultralow frequency dielectric loss (i.e., VLF-TD) of each cable line by using the average values of the zero-sequence ultralow frequency active power and reactive power at the first and last ends of each cable line;
a second port 14 for outputting the measured VLF-TD value.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (9)
1. A cable insulation monitoring method based on high-voltage power frequency and low-voltage ultralow frequency is characterized by comprising the following steps:
(1) in a power grid formed by a plurality of cable lines, coupling a low-voltage ultralow-frequency signal into the cable line without the low-voltage ultralow-frequency signal;
(2) acquiring zero-sequence ultralow-frequency currents at the head end and the tail end of each cable line, and calculating the average value of the zero-sequence ultralow-frequency active power and reactive power at the head end and the tail end of each cable line by combining reference voltage;
(3) calculating the ultralow frequency dielectric loss average value of each cable line by using the zero sequence ultralow frequency active and reactive power average values at the first and last ends of each cable line, and calculating the standard deviation of the ultralow frequency dielectric loss average value of each cable line by using the ultralow frequency dielectric loss average value of each cable line;
(4) when the standard deviation of the ultralow frequency dielectric loss average value of a certain cable line is larger than a first threshold value or the ultralow frequency dielectric loss average value is larger than a second threshold value, entering the step (5);
(5) calculating the correlation coefficient of the ultralow-frequency dielectric loss average value time sequence between a certain cable line and adjacent lines in the power grid, entering the step (6) if each correlation coefficient is less than the average correlation coefficient, and returning to the step (4) if not;
(6) calculating the change rate of the ultralow-frequency dielectric loss average value of each cable line, carrying out normalization processing for constructing a distribution function, calculating the insulation degradation probability of a certain cable line by using the distribution function, outputting the insulation degradation probability of the cable line when the insulation degradation probability of the cable line is greater than or equal to the basic occupation ratio, and otherwise, returning to the step (4);
the cable line is a high-voltage power frequency cable line, the voltage range of the low-voltage ultralow-frequency signal is 0-50V, and the frequency is 0.01 Hz;
the step (2) comprises the following steps:
acquiring zero-sequence ultralow frequency current i at the head end and the tail end of each cable line1And i2Combined with a first reference voltage urCalculating the zero-sequence ultralow frequency active power average value P of the first end and the last end of each cable liner1And Pr2Combined with a second reference voltage ucCalculating the zero-sequence ultralow frequency reactive power average value P of the first end and the last end of each cable linec1And Pc2，
ur＝U0sin(ω0t+α)，uc＝U0cos(ω0t+α)
Wherein, ω is0Is 0.02 pi, U0Is a predetermined value, U0In the range of 0-50V, alpha ∈ (0,2 π), n0T represents n0A period of time consisting of periods T, T representing n0A certain time within the T period.
2. The cable insulation monitoring method based on the high-voltage power frequency and the low-voltage ultralow frequency as claimed in claim 1, wherein the specific implementation manner of the coupled low-voltage ultralow frequency signal is as follows:
and a low-voltage ultralow-frequency voltage source is connected in parallel to a cable line through an arc suppression coil, a bus voltage transformer or an isolation inductor.
3. The cable insulation monitoring method based on high-voltage power frequency and low-voltage ultralow frequency as claimed in claim 1, wherein the ultralow-frequency dielectric loss average value of each cable line is TD:
4. the cable insulation monitoring method based on high-voltage power frequency and low-voltage ultralow frequency according to claim 3, wherein the standard deviation of the ultralow-frequency dielectric loss average value of each cable line is TDTS:
wherein, in the calculation of TDTS, the TD of multiple days is divided into n1Segment, n2As the number of TDs in each segment, TDiFor the ith value in a segment,
mean TD over multiple days.
5. The cable insulation monitoring method based on high-voltage power frequency and low-voltage ultra-low frequency as claimed in claim 1 or 2, wherein the range of the first threshold value is 0.1-0.5, and the range of the second threshold value is 4-50.
6. The cable insulation monitoring method based on high-voltage power frequency and low-voltage ultra-low frequency as claimed in claim 1 or 2, wherein the adjacent lines of a certain cable line in the step (5) in the power grid are: in the power grid, the cable type, the surrounding environment and the installation environment correspond to a certain cable line.
7. The cable insulation monitoring method based on high-voltage power frequency and low-voltage ultralow frequency as claimed in claim 6, wherein in the step (5), the average correlation coefficient is
Wherein,
for cable lines i and their adjacent lines j in the network0And the correlation coefficient of the ultralow-frequency dielectric loss average value time sequence is obtained, and n is the total number of adjacent lines in the power grid.
8. The cable insulation monitoring method based on high-voltage power frequency and low-voltage ultra-low frequency as claimed in claim 1 or 2, wherein the step (6) comprises:
calculating the change rate of the ultralow-frequency dielectric loss average value of each cable line and performing normalization treatment:
wherein k isi,jThe change rate of the ultralow-frequency dielectric loss average value of the cable line i at the moment j in a certain period of time, TDi，j+1、TDi,j-1Respectively the ultralow frequency dielectric loss average values of the cable line i at the time of j +1 and the time of j-1, delta t is a time interval,
is the average value of the change rate of the ultralow-frequency dielectric loss average value of the cable line i outside a certain period of time, n3K 'is the total number of the ultralow frequency dielectric loss average value change rates in other periods except a certain period'i,jThe relative change rate of the ultralow frequency dielectric loss average value of the cable line i at the moment j after normalization processing is carried out;
the ultralow-frequency dielectric loss average value relative change rate combination of all cable lines is used as a sample set S, elements in the S are used as random variables, and a distribution function is constructed: p (x is less than or equal to x)0) Calculating the insulation degradation probability P of the cable line i by using the distribution functioni：
Wherein x belongs to S and x0After the values of x and a are determined, the value range of a is 70-90% according to the distribution function; m is the total number of elements in the sample set S, miIs x > x0The number of elements belonging to the cable line i in the interval;
basic ratio biIs the ratio of elements belonging to cable line i in sample set S, when Pi≥biAt the time, output Pi。
9. A cable insulation monitoring system based on high-voltage power frequency and low-voltage ultralow frequency is characterized by comprising: VLF coupling unit, zero sequence VLF power calculating unit and information analyzing terminal,
the VLF coupling unit is used for coupling the low-voltage ultralow-frequency signals to the cable lines without the low-voltage ultralow-frequency signals in a power grid formed by a plurality of cable lines; the cable line is a high-voltage power frequency cable line, the voltage range of the low-voltage ultralow-frequency signal is 0-50V, and the frequency is 0.01 Hz;
the zero sequence VLF power calculation unit is used for acquiring zero sequence ultralow frequency current i at the head end and the tail end of each cable line1And i2Combined with a first reference voltage urCalculating the zero-sequence ultralow frequency active power average value P of the first end and the last end of each cable liner1And Pr2Combined with a second reference voltage ucCalculating the zero-sequence ultralow frequency reactive power average value P of the first end and the last end of each cable linec1And Pc2，
ur＝U0sin(ω0t+α)，uc＝U0cos(ω0t+α)
Wherein, ω is0Is 0.02 pi, U0Is a predetermined value, U0In the range of 0-50V, alpha ∈ (0,2 π), n0T represents n0A period of time consisting of periods T, T representing n0At a certain moment in the T time period, calculating the ultralow frequency dielectric loss average value of each cable line by using the zero sequence ultralow frequency active power and reactive power average values of the head end and the tail end of each cable line;
the information analysis terminal is used for calculating the standard deviation of the ultralow frequency dielectric loss average value of each cable line by using the ultralow frequency dielectric loss average value of each cable line; when the standard deviation of the ultralow frequency dielectric loss average value of a certain cable line is larger than a first threshold or the ultralow frequency dielectric loss average value is larger than a second threshold, calculating the correlation coefficient of the ultralow frequency dielectric loss average value time sequence between the certain cable line and the adjacent lines in the power grid, if each correlation coefficient is smaller than the average correlation coefficient, calculating the change rate of the ultralow frequency dielectric loss average value of each cable line and carrying out normalization processing for constructing a distribution function, calculating the insulation degradation probability of the certain cable line by using the distribution function, and outputting the insulation degradation probability when the insulation degradation probability of the certain cable line is larger than or equal to the basic ratio.
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