A global insulation condition assessment method for non-homogeneous power cables

Abstract

The application relates to the technical field of power cables, and particularly discloses a global insulation state evaluation method suitable for non-uniform power cables, which comprises the following steps: obtaining the input impedance and the reflection coefficient of the first end of the non-uniform power cable in the impedance matching state after correction, and calculating the attenuation slopes of all the section cables of the non-uniform power cable; calculating the reflection coefficients of all the intermediate joints of the non-uniform power cable according to the attenuation slopes of all the section cables of the non-uniform power cable; and evaluating the global insulation state of the non-uniform power cable according to the attenuation slopes of all the section cables of the non-uniform power cable and the reflection coefficients of all the intermediate joints, so as to obtain an evaluation result. The global insulation state evaluation method suitable for non-uniform power cables has the advantages of advanced technology, high accuracy, easy realization, strong practicability and the like, and realizes the insulation state diagnosis of different section cables and the quantitative calculation of the reflection intensity of the intermediate joints.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power cable, more particularly, to a global insulation state evaluation method suitable for non-uniform power cable. BACKGROUND

[0002] Among a large number of power cable (hereinafter referred to as cable) insulation defect detection methods, time domain reflectometry (TDR) and frequency domain reflectometry (FDR) based on active signal injection are the most widely used. TDR realizes effective diagnosis of cable insulation state by injecting pulse signals and using reflected signal time and intensity. However, TDR has some limitations in practical application due to its low signal energy and poor anti-interference ability. Compared with TDR, FDR has a longer test range and higher test sensitivity, and has been widely used in cable insulation state diagnosis. FDR mainly realizes effective diagnosis of cable insulation state by measuring cable reflection coefficient spectrum (RCS) and broadband impedance spectrum (BIS). The test results of FDR can effectively detect the location of insulation defects in the cable, and combined with effective attenuation compensation, the quantitative calculation of the reflection intensity of the cable intermediate joint can also be realized. However, since the existing FDR test technology only considers the cable to be uniform, when it is a non-uniform cable (different batches of cable butt joint, different operating environment cable), due to the difference in the attenuation of each cable section, the existing attenuation compensation method has certain limitations. In addition, when diagnosing the insulation state of the cable, the cable is usually evaluated as a whole, so when the cable is long, the influence of local degradation on the test results will be greatly reduced, thereby affecting the diagnosis results.

[0003] A Chinese invention patent with the application number 202210006066.6 and the title "Front-end impedance matching method suitable for FDR test and cable impedance mismatch position reflection state evaluation method" provides a front-end impedance matching and cable local defect evaluation method based on FDR. Through this method, the influence of cable attenuation on the test results can be effectively compensated, thereby realizing the quantitative calculation of the reflection intensity of the intermediate joint and transition resistance position in the cable. However, since this method considers the entire cable as a homogeneous cable and does not consider the influence of the non-uniformity of the cable on the analysis results, when there is a non-uniform section in the cable, the calculation results will have certain deviations. A Chinese invention patent with the application number 201910439801.0 and the title "Live cable insulation state diagnosis method, system and medium based on high-frequency pulse voltage" provides a cable insulation state diagnosis method based on the characteristics of high-frequency pulse voltage. Through this method, the cable insulation state can be effectively diagnosed in combination with the cable wave speed. However, since this method evaluates the cable as a whole, when the cable is long, the influence of local degradation on the test results will be greatly reduced, thereby affecting the diagnosis results. Therefore, diagnosing the insulation state of the cable from a local area or the whole alone has certain limitations, and it is necessary to consider the non-uniform characteristics of the cable in actual situations, and it is of great significance to explore the global insulation state evaluation method of power cables under non-uniform conditions. SUMMARY

[0004] In order to solve the problems existing in the prior art, the present application provides a global insulation state evaluation method suitable for non-uniform power cables, thereby realizing the insulation state diagnosis of different section cables and the quantitative calculation of the intermediate joint reflection intensity, which has the advantages of advanced technology, high accuracy, easy implementation, strong practicality and the like.

[0005] As a first aspect of the present application, a global insulation state evaluation method suitable for non-uniform power cables is provided, the non-uniform power cable comprising a plurality of section cables, adjacent section cables being connected by an intermediate joint, the global insulation state evaluation method suitable for non-uniform power cables comprising:

[0006] Step S1: obtaining the input impedance Z in (f) and the reflection coefficient S 11 (f) of the front end of the non-uniform power cable;

[0007] Step S2: calculating the input impedance Z' in (f) and the reflection coefficient S 11 (f) of the front end of the non-uniform power cable under impedance matching state according to the input impedance Z in(f) and the reflection coefficient Γ0(f), and the input impedance Z' of the non-uniform power cable head end in the impedance matching state in (f) and the reflection coefficient Γ0(f) are corrected to obtain the corrected input impedance Z' of the non-uniform power cable head end in the impedance matching state in·E (f) and the reflection coefficient Γ0(f) are corrected to obtain the corrected input impedance Z' of the non-uniform power cable head end in the impedance matching state 0·E (f);

[0008] Step S3: according to the corrected input impedance Z' of the non-uniform power cable head end in the impedance matching state in·E (f) and the reflection coefficient Γ0(f) are corrected to obtain the corrected input impedance Z' of the non-uniform power cable head end in the impedance matching state 0·E (f) and the reflection coefficient Γ0(f) are corrected to obtain the corrected input impedance Z' of the non-uniform power cable head end in the impedance matching state

[0009] Step S4: according to the attenuation slope of each section cable of the non-uniform power cable, the reflection coefficient of each intermediate joint of the non-uniform power cable is calculated;

[0010] Step S5: according to the attenuation slope of each section cable of the non-uniform power cable and the reflection coefficient of each intermediate joint, the overall insulation state of the non-uniform power cable is evaluated to obtain the overall insulation state evaluation result of the non-uniform power cable.

[0011] Further, the input impedance Z in (f) and the reflection coefficient S 11 (f) of the non-uniform power cable head end are obtained by:

[0012] Setting the sweep frequency range [f down ,2f up ] and the sweep point number 2N of the vector network analyzer or impedance analyzer, and measuring the reflection coefficient and the input impedance of the non-uniform power cable head end by the vector network analyzer or impedance analyzer to obtain the input impedance Z in (f) and the reflection coefficient S 11 (f) of the non-uniform power cable head end; wherein the vector network analyzer or impedance analyzer is connected to the head end of the non-uniform power cable through its test lead;

[0013] Wherein the input impedance Z in (f) and the reflection coefficient S 11 (f) of the non-uniform power cable head end can be converted by the following formula:

[0014]

[0015] Wherein f is the scan frequency point; R is the internal resistance of the vector network analyzer or impedance analyzer.

[0016] Furthermore, the input impedance Z at the beginning of the non-uniform power cable is... in (f) and reflection coefficient S 11 (f) Calculate the input impedance Z at the head end of the non-uniform power cable under impedance matching condition. i ′ n (f) and the reflection coefficient Γ0(f), also including:

[0017] Step S21: Use the formula y0(t)=exp(-4·ln(2)·t 2 / w 2 )·cos(2πf c ·t) generates a pulse width of w and an oscillation frequency of f c The Gaussian oscillation pulse is used as the incident signal. Then, the time-frequency domain processing method is used in combination with y0(t) to calculate the reflection coefficient Γ1(f) that characterizes the connection between the head end of the non-uniform power cable and the test lead. It is assumed that the characteristic impedance Z0 of the non-uniform power cable is 10Ω, where t is time.

[0018] Step S22: Calculate the combined impedance Z1(f) of the test lead under the characteristic impedance Z0 of the non-uniform power cable:

[0019]

[0020] Step S23: Calculate the input impedance Z′ of the non-uniform power cable head end under impedance matching condition. in (f) and reflection coefficient Γ0(f):

[0021] Z′ in (f)=Z in (f)-Z1(f)

[0022]

[0023] Step S24: Extract the input impedance Z′ in (f) The first N data Z″ in (f), and then use the transformation function f→t′ to obtain Z″ in (f) Convert to the t′ domain signal Z″ in (t′), then for Z″ in (t′) Phase Angle(Z″) in (t′) is subjected to a fast Fourier transform to obtain Angle(Z″). in The spectrum of (t′)) FFT[Angle(Z″) in (t′))], where Angle(·) is the complex phase;

[0024] Step S25: reserve the data representing the non-uniform power cable terminal position in FFT[Angle(Z in (t′)), and set other data to zero and perform inverse fast Fourier transform processing;

[0025] Step S26: compare the data from the M1th period to the M2th period obtained by the inverse fast Fourier transform processing in step S25 with the data at the corresponding positions in Angle(Z in (t′)), and calculate the Pearson correlation coefficient m k ;

[0026] Step S27: increase the assumed characteristic impedance Z0 of the non-uniform power cable by 1Ω, Z0=Z0+1; repeat steps S22 to S26 until the assumed characteristic impedance Z0 of the non-uniform power cable is 80Ω;

[0027] Step S28: obtain the entire Pearson correlation coefficient m k curve calculated in step S26 under different assumed characteristic impedances of the non-uniform power cable, and take the characteristic impedance Z0 corresponding to the maximum value of the curve m k as the estimated characteristic impedance Z 0·E of the non-uniform power cable.

[0028] The input impedance Z′ in (f) and the reflection coefficient Γ0(f) of the first end of the non-uniform power cable in the impedance matching state are corrected to obtain the corrected input impedance Z′ in·E (f) and the reflection coefficient Γ 0·E (f) of the first end of the non-uniform power cable in the impedance matching state, and further comprise:

[0029] First, the comprehensive impedance Z 0·E (f) of the test lead under the non-uniform power cable characteristic impedance Z 1·E is calculated:

[0030]

[0031] Then, the input impedance Z′ in·E (f) and the reflection coefficient Γ 0·E (f) of the first end of the non-uniform power cable in the impedance matching state are calculated:

[0032] Z′ in·E (f) = Z in (f) - Z 1·E (f)

[0033]

[0034] Wherein, the input impedance Z′ in·E (f) and reflection coefficient Γ 0·E (f) is the corrected result.

[0035] Furthermore, the input impedance Z′ of the modified non-uniform power cable head end under impedance matching state is... in·E (f) and reflection coefficient Γ 0·E (f) Calculating the attenuation slope of each section of the non-uniform power cable, further including:

[0036] Step S31: Extract Γ 0·E (f) The first N data Γ′ 0·E (f), and then use the transformation function f→t′ to obtain Γ′ 0·E (f) is converted to the t′ domain signal Γ′ 0·E (t′), then for Γ′ 0·E (t′) Real(Γ′) 0·E (t′)) is processed by Blackman window FFT, and the FFT result is compared with the DC component of the window function. The horizontal axis is converted to time to obtain the first localization map, where Real(·) is the complex real part;

[0037] Step S32: Record the times of each intermediate joint [T1, T2, ..., T] in the first positioning map in ascending order of time. n The corresponding signal strength is [p]. 1·0 ,p 2·0 ,…,p n·0 ], and let p i =p i·0 Simultaneously record the termination time T of the non-uniform power cable. end and its signal strength p end·0 And let p end =p end·0 Where i = [1, 2, ..., n];

[0038] Step S33: Extract S 11 (f) The first N data S′ 11 (f), and then use the transformation function f→t′ to obtain S′ 11 (f) Convert to the t′ domain signal S′ 11 (t′), then for S′ 11 (t′) Real part Real(S′) 11 (t′) is processed by Blackman window FFT to convert the x-axis to time to obtain the second localization map;

[0039] Step S34: record the signal intensity at each intermediate joint moment in the second positioning map in ascending order of time as [p' 1·0 ,p' 2·0 ,…,p' n·0 ], and let p' i =p' i·0 , and record the signal intensity at each intermediate joint mirror moment [2T end -T1,2T end -T2,…,2T end -T n ] in the second positioning map as [p" 1·0 ,p" 2·0 ,…,p" n·0 ], and let p" i =p" i·0 ; wherein, T end is the terminal moment of the non-uniform power cable;

[0040] Step S35: calculate the attenuation slope of each section cable;

[0041] The calculation of the attenuation slope of each section cable further comprises:

[0042] Step S351: calculate the attenuation slope k n of the last section cable, which is the cable between the last intermediate joint and the cable terminal:

[0043]

[0044] wherein, lg(·) is the common logarithm;

[0045] Step S352: let k' n =k n ;

[0046] Step S353: calculate the attenuation slope k n-1 of the second last section cable, which is the cable between the second last intermediate joint and the last intermediate joint:

[0047]

[0048] Step S354: calculate the equivalent attenuation slope k' n-1 of the cable between the second last intermediate joint and the cable terminal:

[0049]

[0050] Step S355: repeating step S353 to step S354 until the attenuation slope k1 of the cable between the first intermediate joint and the second intermediate joint and the equivalent attenuation slope k'1 of the cable between the first intermediate joint and the cable terminal are calculated;

[0051] Step S356: calculating the attenuation slope k0 of the cable between the cable head end and the first intermediate joint:

[0052]

[0053] wherein T1 is the time of the first intermediate joint of the non-uniform power cable, T end is the time of the terminal of the non-uniform power cable, p end is the signal strength at the time T end of the terminal of the non-uniform power cable.

[0054] Further, the reflection coefficient of each intermediate joint of the non-uniform power cable is calculated according to the attenuation slope of each section cable of the non-uniform power cable, and the method further comprises:

[0055] Step S41: performing attenuation compensation on the signal strength at each intermediate joint of the non-uniform power cable to obtain the signal strength p i·c at the time of the ith intermediate joint before attenuation.

[0056]

[0057] Step S42: constructing a signal y with f as the independent variable:

[0058]

[0059] wherein d is the length of the intermediate joint, v1 is the wave speed of the intermediate joint, and cos(·) is the cosine function.

[0060] Step S43: using enumeration to assign x in the range of [0, 0.5] respectively, and performing Blackman window FFT processing on the corresponding y, performing ratio processing on the FFT processing result and the direct current component of the window function, and recording the maximum value in the FFT amplitude spectrum and the x closest to p i·c as the reflection coefficient p i of the ith intermediate joint.

[0061] Step S44: repeating step S42 to step S43 until the reflection coefficients of all intermediate joints are calculated.

[0062] Step S45: correcting the signal strength at each intermediate joint and its mirror time:

[0063]

[0064]

[0065]

[0066] Step S46: Correct the signal strength at the cable terminal moment:

[0067]

[0068] Step S47: Repeat steps S45 to S46 until the number of cycles reaches 15.

[0069] Further, the global insulation state of the non-uniform power cable is evaluated according to the attenuation slope of each section cable and the reflection coefficient of each intermediate joint of the non-uniform power cable to obtain a global insulation state evaluation result of the non-uniform power cable, and the method further comprises:

[0070] Step S51: Calculate the attenuation slope k of each section cable i and the relative deviation A i·L of the initial measurement result k K·i :

[0071]

[0072] Step S52: Calculate the relative deviation A L·i of the wave speed of each section cable in the second positioning map and the actual wave speed v

[0073]

[0074] wherein L i is the actual length of the i-th section cable;

[0075] Step S53: Compare the values of A k·i and A L·i , when |A k·i |>2.5% or |A L·i |>10%, the section cable is seriously aged and needs to be focused on; when 1%≤|A k·i |≤2.5% or 3%≤|A L·i |≤10%, the section cable is moderately aged and needs to be strengthened; when |A k·i |<1% and |A L·i |<3%, the section cable is lightly aged and only needs to be generally concerned;

[0076] Step S54: Calculate the reflection coefficient p of each intermediate joint i and the initial measurement result pi·L Relative deviation A ρ·i :

[0077]

[0078] When|A ρ·i |>15%, the intermediate joint is severely aged and requires close monitoring; when 5% ≤|A ρ·i When |A| is ≤15%, the intermediate joint exhibits moderate aging and requires close monitoring; when |A| is ≤15%, the intermediate joint exhibits moderate aging and requires close monitoring. ρ·i When the aging rate is less than 5%, the intermediate joint is relatively mild and only requires general attention.

[0079] Further, in step S21, the pulse width w of the Gaussian oscillation pulse y0(t) is 2 to 4 times the reciprocal of the upper limit of the sweep frequency range, and the oscillation frequency f of the Gaussian oscillation pulse y0(t) is... c The value is half of the upper limit of the sweep frequency range.

[0080] Furthermore, in step S27, the entire Pearson correlation coefficient m is first obtained by traversing [10, 80Ω] using a first interval of 1Ω. k The curve is then traversed using a second spacing less than 1Ω of the first spacing. k Repeat steps S22 to S26 within 1 Ω before and after the maximum value of the curve to obtain m. k The maximum value in the curve is the estimated characteristic impedance Z of the cable. 0·E .

[0081] Furthermore, in steps S31 and S33, when obtaining the first and second positioning maps, the imaginary part of the corresponding data is selected to be processed. At this time, the cosine function cos(·) in step S42 is replaced with the sine function sin(·).

[0082] In step S34, the signal strength at each intermediate joint time point is recorded in the first positioning map in ascending order of time as [p′]. 1·0 ,p′ 2·0 ,…,p′ n·0 ], and let p′ i =p′ i·0 At this time, p′ is satisfied. i·0 =p i·0 Simultaneously record the mirror times of each intermediate joint in the first positioning map [2T]. end -T1,2T end -T2,…,2T end -T n Signal strength at [p″] 1·0 ,p″ 2·0 ,…,p″ n·0], and let p" i = p" i·0 ; wherein T end is the cable terminal time;

[0083] In the step S356, when the attenuation slope k0 of the cable between the cable head and the first intermediate joint is calculated, the attenuation slope of the cable between the last intermediate joint and the cable terminal is calculated by using the test data at the cable terminal, and the attenuation slope is k0.

[0084] Further, in the step S43, the enumeration step length during enumeration is set to 0.005, 0.01 or 0.02.

[0085] The global insulation state evaluation method for the non-uniform power cable provided by the application has the following advantages:

[0086] (1) The application can realize the approximate calculation of the cable characteristic impedance;

[0087] (2) The application can calculate the attenuation of different section cables, and the linear compensation in the logarithmic coordinates can realize the compensation of the signal strength of the intermediate joint and its mirror position;

[0088] (3) The application can calculate the absolute value of the reflection coefficient of each intermediate joint;

[0089] (4) The application can realize the global insulation state evaluation of the non-uniform cable;

[0090] (5) The application only needs to use the measured cable reflection coefficient or input impedance, without the need of the accurate cable model;

[0091] (6) The application considers the compensation method under the condition that multiple intermediate joints exist at the same time, and the calculation result is more accurate. BRIEF DESCRIPTION OF DRAWINGS

[0092] The accompanying drawings are included to provide a further understanding of the application, and constitute a part of the specification, and are used together with the following specific embodiments to explain the application, but do not constitute a limitation on the application.

[0093] Figure 1 The flow chart of the global insulation state evaluation method for the non-uniform power cable provided by the application.

[0094] Figure 2 The first positioning map calculated in the embodiment of the application is shown in the schematic diagram.

[0095] Figure 3 The second positioning map calculated in the embodiment of the application is shown in the schematic diagram. DETAILED DESCRIPTION

[0096] For further illustrating the technical means and effects taken by the present application to achieve the predetermined inventive objectives, the following will describe the specific implementation, structure, features and effects of the global insulation state evaluation method for non-uniform power cable according to the present application in detail with reference to the accompanying drawings and preferred embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, instead of all the embodiments. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0097] In the present embodiment, a global insulation state evaluation method for non-uniform power cable is provided, as shown in Figure 1 which includes a plurality of section cables connected by intermediate joints between adjacent section cables, and the global insulation state evaluation method for non-uniform power cable includes:

[0098] Step S1: obtaining the input impedance Z in (f) and the reflection coefficient S 11 (f) of the head end of the non-uniform power cable;

[0099] Step S2: calculating the input impedance Z' in (f) and the reflection coefficient S 11 (f) of the head end of the non-uniform power cable in the impedance matching state according to the input impedance Z in (f) and the reflection coefficient S in (f) of the head end of the non-uniform power cable, and correcting the input impedance Z' in·E (f) and the reflection coefficient S 0·E (f) of the head end of the non-uniform power cable in the impedance matching state to obtain the corrected input impedance Z' in·E (f) and the reflection coefficient S 0·E (f) of the head end of the non-uniform power cable in the impedance matching state;

[0100] Step S3: calculating the attenuation slope of each section cable of the non-uniform power cable according to the corrected input impedance Z' in·E (f) and the reflection coefficient S 0·E (f) of the head end of the non-uniform power cable in the impedance matching state;

[0101] Step S4: calculating the reflection coefficient of each intermediate joint of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable;

[0102] Step S5: evaluating the overall insulation state of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable and the reflection coefficient of each intermediate joint, to obtain an overall insulation state evaluation result of the non-uniform power cable.

[0103] The non-uniform power cable to which the embodiment is directed is a 700m ZR-YJV02 8.7 / 15 3*95mm2 power cable, and there is a heat-shrink intermediate joint with a length of 0.8m at a position of 377m.

[0104] Preferably, the input impedance Z in (f) and the reflection coefficient S 11 (f) further comprises:

[0105] The frequency scanning range [f down , 2f up ] and the frequency scanning point number 2N of the vector network analyzer or impedance analyzer are set, and the reflection coefficient and the input impedance of the non-uniform power cable head are measured by the vector network analyzer or impedance analyzer, to obtain the input impedance Z in (f) and the reflection coefficient S 11 (f) of the non-uniform power cable head; wherein the vector network analyzer or impedance analyzer is connected to the head of the non-uniform power cable through a test lead;

[0106] The input impedance Z in (f) and the reflection coefficient S 11 (f) of the non-uniform power cable head can be converted by the following formula:

[0107]

[0108] Wherein f is the frequency scanning point; R is the internal resistance 50Ω of the vector network analyzer or impedance analyzer.

[0109] In this embodiment, the reflection coefficient of the measured cable is tested by using an Agilent E5061B vector network analyzer.

[0110] Preferably, the input impedance Z in (f) and the reflection coefficient S 11 (f) of the non-uniform power cable head are calculated to obtain the input impedance Z i ′ n (f) and the reflection coefficient Γ0(f) of the non-uniform power cable head in the impedance matching state, further comprises:

[0111] Step S21: using the formula y0(t)=exp(-4·ln(2)·t 2 / w 2 )·cos(2πf c ·t) produces a Gaussian oscillation pulse with a pulse width w of 0.2 μs and an oscillation frequency f c of 5 MHz as an incident signal, then a time-frequency domain processing method is used in combination with y0(t) to calculate a reflection coefficient Γ1(f) representing the connection between the non-uniform power cable head and the test lead, and it is assumed that the characteristic impedance Z0 of the non-uniform power cable is 10 Ω, where t is time;

[0112] Step S22: calculate the comprehensive impedance Z1(f) of the test lead under the characteristic impedance Z0 of the non-uniform power cable:

[0113]

[0114] Step S23: calculate the input impedance Z′ in (f) and the reflection coefficient Γ0(f) of the non-uniform power cable head under the impedance matching state:

[0115] Z′ in (f) = Z in (f) - Z1(f)

[0116]

[0117] Step S24: extract the first 800 data Z″ in (f) in the input impedance Z′ in (f), and then convert the obtained Z″ in (f) into a t′ domain signal Z″ in (t′) by using a conversion function f→t′, then perform a fast Fourier transform (FFT) on the phase Angle(Z″ in (t′)) of Z″ in (t′)) to obtain a frequency spectrum FFT[Angle(Z″ in (t′))] of Angle(Z″ in (t′)), where Angle(·) is the phase of a complex number;

[0118] Step S25: retain the data representing the terminal position of the non-uniform power cable in FFT[Angle(Z″ in (t′))], and set other data to zero and perform an inverse fast Fourier transform (IFFT) processing;

[0119] Step S26: compare the data of the 3rd to 10th periods obtained by the inverse fast Fourier transform processing in step S25 with the data at the corresponding positions in Angle(Z″ in (t′)), and calculate the Pearson correlation coefficient mk ;

[0120] Step S27: increase the characteristic impedance Z0 of the assumed non-uniform power cable by a step of 1Ω, Z0 = Z0 + 1; repeat steps S22 to S26 until the characteristic impedance Z0 of the assumed non-uniform power cable takes a value of 80Ω;

[0121] Step S28: obtain the entire Pearson correlation coefficient m k curve calculated in step S26 under different assumed characteristic impedances of the non-uniform power cable, and take the characteristic impedance 24.34Ω corresponding to the maximum value of the curve m k as the estimated characteristic impedance Z 0·E ;

[0122] The input impedance Z in (f) and the reflection coefficient Γ0(f) of the non-uniform power cable are corrected to obtain the corrected input impedance Z in·E (f) and the reflection coefficient Γ 0·E (f) of the non-uniform power cable under the impedance matching state of the first end of the non-uniform power cable:

[0123] First, the characteristic impedance Z 0·E of the non-uniform power cable is calculated, and the comprehensive impedance Z 1·E (f) of the test lead under the characteristic impedance Z

[0124]

[0125] Then, the input impedance Z in·E (f) and the reflection coefficient Γ 0·E (f) of the non-uniform power cable under the impedance matching state of the first end of the non-uniform power cable are calculated:

[0126] Z in·E (f) = Z in (f) - Z 1·E (f)

[0127]

[0128] The input impedance Z in·E (f) and the reflection coefficient Γ 0·E (f) are the corrected results.

[0129] It should be noted that the first end impedance matching processing method in step S2 can refer to the processing method disclosed in the prior art, such as the Chinese invention patent with application number 202110541444.6 and the name “a power cable broadband impedance spectrum test method”.

[0130] Preferably, the step involves using the corrected input impedance Z′ of the non-uniform power cable head end under impedance matching conditions. in·E (f) and reflection coefficient Γ 0·E (f) Calculating the attenuation slope of each section of the non-uniform power cable, further including:

[0131] Step S31: Extract Γ 0·E (f) The first 800 data points Γ′ 0·E (f), and then use the transformation function f→t′ to obtain Γ′ 0·E (f) is converted to the t′ domain signal Γ′ 0·E (t′), then for Γ′ 0·E (t′) Real(Γ′) 0·E (t′)) is processed using a Blackman windowed FFT, and the FFT result is compared with the DC component of the window function. The horizontal axis is then converted to time to obtain the first localization map, such as Figure 2 As shown, Real(·) is the real part of the complex number;

[0132] Step S32: Record the times of each intermediate joint [T1, T2, ..., T] in the first positioning map in ascending order of time. n The corresponding signal strength is [p]. 1·0 ,p 2·0 ,…,p n·0 ], and let p i =p i·0 Simultaneously record the termination time T of the non-uniform power cable. end and its signal strength p end Where i = [1, 2, ..., n];

[0133] In this embodiment, the intermediate joint time T1 = 4.258 μs is recorded in the first positioning map, and the corresponding signal strength p1 = p 1·0 =4.065×10 -3 Simultaneously record the cable termination time T. end =7.816μs and signal strength p end =p end·0 =0.5435.

[0134] Step S33: Extract S 11 (f) The first 800 data points S′ 11 (f), and then use the transformation function f→t′ to obtain S′ 11 (f) Convert to the t′ domain signal S′ 11 (t′), then for S′ 11 (t′) Real part Real(S′)11 (t′)) is performed, and the abscissa is converted into time to obtain a second positioning map as shown in Figure 3 ;

[0135] Step S34: In the second positioning map, the signal intensity at each intermediate joint moment is recorded in ascending order of time as [p′ 1·0 ,p′ 2·0 ,…,p′ n·0 ], and p′ i =p′ i·0 , and the signal intensity at each intermediate joint mirror moment [2T end -T1,2T end -T2,…,2T end -T n ] in the second positioning map is recorded as [p″ 1·0 ,p″ 2·0 ,…,p″ n·0 ], and p″ i =p″ i·0 ; wherein T end is the terminal moment of the non-uniform power cable;

[0136] In this embodiment, the signal intensity at the intermediate joint moment in the second positioning map is recorded as p′1=p′ 1·0 =3.601×10 -3 , and the signal intensity at the intermediate joint mirror position in the second positioning map is recorded as p″1=p″ 1·0 =1.999×10 -3 .

[0137] Step S35: Calculate the attenuation slope of each section cable;

[0138] The calculation of the attenuation slope of each section cable further comprises:

[0139] Step S351: Calculate the attenuation slope k n of the last section cable, i.e., the cable between the last intermediate joint and the cable terminal:

[0140]

[0141] Wherein, lg(·) is the common logarithm;

[0142] In this embodiment, the attenuation slope k1 of the last section cable is calculated as:

[0143]

[0144] Wherein, lg(·) is the common logarithm;

[0145] Step S352: let k' n n ;

[0146] In this embodiment, let k'1=k1.

[0147] Step S353: calculate the attenuation slope k n-1 of the penultimate section cable, which is the cable between the penultimate intermediate joint and the last intermediate joint:

[0148]

[0149] Step S354: calculate the equivalent attenuation slope k' n-1 of the cable between the penultimate intermediate joint and the cable terminal:

[0150]

[0151] Step S355: repeat steps S353 to S354 until the attenuation slope k1 of the cable between the first intermediate joint and the second intermediate joint, and the equivalent attenuation slope k'1 of the cable between the first intermediate joint and the cable terminal are calculated;

[0152] Step S356: calculate the attenuation slope k0 of the cable between the cable head end and the first intermediate joint:

[0153]

[0154] In particular, the attenuation slope k0 of the cable between the cable head end and the intermediate joint is calculated:

[0155]

[0156] wherein T1 is the time of the first intermediate joint of the non-uniform power cable, T end is the time of the terminal of the non-uniform power cable, p end is the signal intensity at the time T end of the terminal of the non-uniform power cable.

[0157] Preferably, the calculation of the reflection coefficient of each intermediate joint of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable further comprises:

[0158] Step S41: perform attenuation compensation on the signal intensity at each intermediate joint of the non-uniform power cable to obtain the signal intensity p i·c at the i-th intermediate joint before attenuation:

[0159] ​

[0160] In this embodiment, the signal intensity p at the first intermediate joint moment before attenuation 1·c :

[0161] Step S42: Construct the signal y with the independent variable f:

[0162]

[0163] Wherein, d is the intermediate joint length 0.8m; v1 is the intermediate joint wave speed, taking the value of 1.72x10 8 m / s; cos(·) is the cosine function;

[0164] Step S43: Enumerate the value of x in the range of [0, 0.5] respectively, and perform Blackman window FFT processing on the corresponding y, take the ratio of the FFT processing result and the window function direct current component, record the maximum value in the FFT amplitude spectrum and the closest x as the reflection coefficient p i·c of the i-th intermediate joint, and calculate p1 as 0.036; i

[0165] Step S44: Repeat steps S42 to S43 until the reflection coefficients of all intermediate joints are calculated;

[0166] Step S45: Correct the signal intensity at the moment of each intermediate joint and its mirror image:

[0167]

[0168]

[0169]

[0170] In this embodiment, p1=p 1·0 , p'1=p' 1·0 ,

[0171] Step S46: Correct the signal intensity at the moment of the cable terminal:

[0172]

[0173] In this embodiment,

[0174] Step S47: Repeat steps S45 to S46 until the number of cycles reaches 15, and finally calculate k0=-3.2041x10 4 , k1=-3.5762x10​4 , p1 = 0.036.

[0175] Preferably, the evaluation of the global insulation state of the non-uniform power cable according to the attenuation slope of each section cable and the reflection coefficient of each intermediate joint of the non-uniform power cable to obtain the global insulation state evaluation result of the non-uniform power cable further comprises:

[0176] Step S51: calculating the attenuation slope k of each section cable i with the initial measurement k i·L : K·i :

[0177]

[0178] In this embodiment, for example, the relative deviation A 0·L of the attenuation slope k0 of the first section cable with the initial measurement k K·0 :

[0179]

[0180] In this embodiment, for example, the relative deviation A 1·L of the attenuation slope k1 of the second section cable with the initial measurement k K·1 :

[0181]

[0182] Step S52: calculating the relative deviation A L·i of the wave speed of each section cable in the second positioning map with the actual wave speed v:

[0183]

[0184] wherein L i is the actual length of the i-th section cable;

[0185] In this embodiment, for example, the relative deviation A L·1 of the wave speed of the first section cable in the second positioning map with the actual wave speed v:

[0186]

[0187] wherein L1 is the actual length of the first section cable;

[0188] In this embodiment, for example, the relative deviation A L·2 of the wave speed of the second section cable in the second positioning map with the actual wave speed v:

[0189]

[0190] L2 = actual length of the second section cable;

[0191] Step S53: compare the value of each section cable A k·i , A L·i , when |A k·i |>2.5% or |A L·i |>10%, the section cable is seriously aged and needs to be focused on; when 1%≤|A k·i |≤2.5% or 3%≤|A L·i |≤10%, the section cable is moderately aged and needs to be strengthened; when |A k·i |<1% and |A L·i |<3%, the section cable is slightly aged and only needs to be generally concerned;

[0192] In this embodiment, since |A k·0 |<1% and |A L·1 |<3%, the first section cable is slightly aged and only needs to be generally concerned; since |A k·1 |>2.5% and |A L·2 |>10%, the second section cable is seriously aged and needs to be focused on;

[0193] Step S54: calculate the relative deviation A i of the reflection coefficient p i·L of each intermediate joint from the initial measurement p ρ·i :

[0194]

[0195] When |A ρ·i |>15%, the intermediate joint is seriously aged and needs to be focused on; when 5%≤|A ρ·i |≤15%, the intermediate joint is moderately aged and needs to be strengthened; when |A ρ·i |<5%, the intermediate joint is slightly aged and only needs to be generally concerned.

[0196] In this embodiment, the relative deviation A 1·L of the reflection coefficient p ρ·1 of the first intermediate joint from the initial measurement p ρ·1 :

[0197]

[0198] Since |A ρ·1 |<5%, the first intermediate joint is slightly aged and only needs to be generally concerned.

[0199] Preferably, in the step S21, the pulse width w in the Gaussian oscillation pulse y0(t) is 2 to 4 times the reciprocal of the upper limit of the sweep range frequency, and the oscillation frequency f in the Gaussian oscillation pulse y0(t) is half of the upper limit of the sweep range frequency. c Preferably, in the step S21, the pulse width w in the Gaussian oscillation pulse y0(t) is 2 to 4 times the reciprocal of the upper limit of the sweep range frequency, and the oscillation frequency f in the Gaussian oscillation pulse y0(t) is half of the upper limit of the sweep range frequency.

[0200] It should be noted that in the step S21, the time-frequency domain processing method for calculating the reflection coefficient of the cable head test lead connection can refer to the existing disclosed technology, such as the Chinese invention patent with the application number 202110541444.6.

[0201] Preferably, in the step S27, first traverse [10, 80Ω] with a first interval 1Ω to obtain the entire Pearson correlation coefficient m k curve, and then traverse the position 1Ω before and after the maximum value in the m k curve with a second interval smaller than the first interval 1Ω to repeat the steps S22 to S26, and obtain the maximum value in the m k curve as the estimated cable characteristic impedance Z 0·E .

[0202] Preferably, in the steps S31 and S33, when obtaining the first positioning map and the second positioning map, the imaginary part of the corresponding data is selected for processing, and at this time, the cosine function cos(·) in the step S42 is replaced by the sine function sin(·);

[0203] In the step S34, in the first positioning map, the signal intensity at each intermediate joint moment is recorded as [p′ 1·0 , p′ 2·0 , …, p′ n·0 ] in ascending order of time, and p′ i = p′ i·0 is set, and at this time, p′ i·0 = p i·0 is satisfied; and the signal intensity at each intermediate joint mirror moment [2T end -T1, 2T end -T2, …, 2T end -T n ] in the first positioning map is recorded as [p″ 1·0 , p″ 2·0 , …, p″ n·0 ], and p″ i = p″ i·0 is set; wherein T end is the cable terminal moment.

[0204] In the step S356, when the attenuation slope k0 of the cable between the cable head end and the first intermediate joint is calculated, the test data at the cable terminal (under the same test conditions) can also be used to calculate the attenuation slope of the cable between the last intermediate joint and the cable terminal, which is k0.

[0205] Preferably, in the step S43, the enumeration step length during enumeration is set to 0.005, 0.01 or 0.02.

[0206] Preferably, the window function used in the FFT processing can be a Blackman window, a Gaussian window, a Hanning window or a Kaiser window.

[0207] The global insulation state evaluation method for non-uniform power cables provided by the present application can not only calculate the absolute value of the reflection coefficient of each intermediate joint, but also achieve the aging state evaluation of the cable section from the change of the section wave speed. The evaluation result is less affected by the cable length compared to the overall wave speed evaluation method, and has the advantages of simple calculation, high accuracy and easy implementation.

[0208] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Although the present application has been disclosed as above with a preferred embodiment, it is not intended to limit the present application. Any skilled person in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application, and any simple modification, equivalent change and modification of the above embodiment based on the technical essence of the present application are still within the scope of the technical solution of the present application.

Claims

1. A method for global insulation condition assessment suitable for non-homogeneous power cables, characterized in that, The non-uniform power cable comprises a plurality of section cables connected through intermediate joints between adjacent section cables, and the global insulation state evaluation method suitable for the non-uniform power cable comprises: Step S1 : obtaining an input impedance of the non-uniform power cable head and reflection coefficient ; Step S2: calculating input impedance and reflection coefficient of the non-uniform power cable head in impedance matching state according to input impedance of the non-uniform power cable head and reflection coefficient , and correcting input impedance and reflection coefficient of the non-uniform power cable head in impedance matching state to obtain corrected input impedance and reflection coefficient of the non-uniform power cable head in impedance matching state and reflection coefficient ; and and reflection coefficient ; and and reflection coefficient ; Step S3: calculating the attenuation slope of each segment cable of the non-uniform power cable according to the input impedance of the non-uniform power cable head in the impedance matching state after the correction and the reflection coefficient , calculating the attenuation slope of each segment cable of the non-uniform power cable; Step S4: calculating the reflection coefficient of each intermediate joint of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable; Step S5: evaluating the global insulation state of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable and the reflection coefficient of each intermediate joint, so as to obtain the global insulation state evaluation result of the non-uniform power cable; The input impedance of the non-uniform power cable head end is obtained and the reflection coefficient Further comprising: Setting a sweep range [f down ,2f up ] and a sweep point number 2N of a vector network analyzer or impedance analyzer, and measuring a reflection coefficient and an input impedance of a first end of the non-uniform power cable by the vector network analyzer or impedance analyzer to obtain an input impedance and a reflection coefficient of the first end of the non-uniform power cable; wherein the vector network analyzer or impedance analyzer is connected to the first end of the non-uniform power cable through a test lead thereof. wherein the input impedance of the non-uniform power cable first end and the reflection coefficient can be converted using the following equation: ； Wherein, f is a scanning frequency point; R is the internal resistance of the vector network analyzer or impedance analyzer; Wherein, the input impedance of the non-uniform power cable head is calculated according to the input impedance of the non-uniform power cable head and the reflection coefficient , the input impedance of the non-uniform power cable head in the impedance matching state is calculated and the reflection coefficient , and further comprising: Step S21: generating a Gaussian oscillation pulse with a pulse width w and an oscillation frequency f as an incident signal, and then using a time-frequency domain processing method in combination with y0(t) to calculate a reflection coefficient c characterizing the connection between the non-uniform power cable head and the test lead , and assuming that the characteristic impedance of the non-uniform power cable is 10 Ω, where t is time; Step S22: calculating the non-uniform power cable characteristic impedance The integrated impedance of the test lead described below : ； Step S23: calculating the input impedance of the non-uniform power cable head end in the impedance matching state and the reflection coefficient : ； ； Step S24: Extracting input impedance first N data Then, using the conversion function The resulting is converted to domain signal Then, the phase is subjected to a fast Fourier transform to obtain spectrum where Angle(·) is the complex phase; Step S25: Retention The data characterizing the non-uniform power cable terminal position is retained, other data is zeroed and inverse fast Fourier transform processing is performed; Step S26: comparing the data of the M1th cycle to the M2th cycle obtained by the inverse fast Fourier transform processing in step S25 with the data of the corresponding positions in the and calculating the Pearson correlation coefficient m k ; Step S27: increasing the characteristic impedance Z0 of the non-uniform power cable by a step of 1Ω, Z0 = Z0 +1; repeating steps S22 to S26 until the characteristic impedance Z0 of the non-uniform power cable takes a value of 80Ω; Step S28: obtaining the entire Pearson correlation coefficient m calculated in step S26 under different assumed characteristic impedance of the non-uniform power cable k curve, and the curve m k corresponding to the maximum as the estimated characteristic impedance of the non-uniform power cable ; the input impedance of the non-uniform power cable head in the impedance matching state and the reflection coefficient corrected to obtain the input impedance of the non-uniform power cable head in the impedance matching state after correction and the reflection coefficient further comprising: First, the non-uniform power cable characteristic impedance is calculated The following test leads are described below : ； Then, the input impedance of the non-uniform power cable head end in the impedance matching state is calculated and the reflection coefficient : ； ； wherein the input impedance and the reflection coefficient is the modified result; The input impedance of the non-uniform power cable head end in the impedance matching state is calculated according to the modified input impedance and the reflection coefficient The attenuation slope of each section of the non-uniform power cable is calculated. Step S31: extraction top N data and then using a conversion function the resulting is converted to domain signal and then the real part is subjected to a Blackman window FFT process, and the FFT process result is subjected to ratio processing with the window function DC component, and the abscissa is converted into time to obtain a first positioning map. Step S32: record each intermediate joint time [T1, T2, …, Tn] in the first positioning map in ascending order of time n , the corresponding position signal intensity is , and let At the same time, record the terminal time of the non-uniform power cable and the signal intensity , and let ; wherein i=[1, 2, …, n] Step S33: extraction top N data and then using a conversion function the resulting is converted to domain signal and then the real part is subjected to a Blackman window FFT process, converting the abscissa to time to obtain a second positioning map Step S34: record the signal intensity at each intermediate joint mirror moment in the second positioning map as , and let ; meanwhile, record the signal intensity at each intermediate joint mirror moment in the second positioning map , and let ; wherein T end is the terminal moment of the non-uniform power cable; Step S35: calculating the attenuation slope of each section cable; The calculation of the attenuation slope of each section cable further comprises: Step S351: Calculate the attenuation slope k of the last cable segment. n The last section of cable refers to the cable between the last intermediate joint and the cable terminal: ； Wherein, lg(·) is a common logarithm; Step S352: Let ; Step S353: Calculate the attenuation slope k of the penultimate section cable n-1 , which is the cable between the penultimate intermediate joint and the last intermediate joint: ； Step S354: Calculate the equivalent attenuation slope of the cable between the penultimate intermediate joint and the cable termination : ； Step S355: repeating steps S353 to S354 until the attenuation slope k1 of the cable between the first intermediate joint and the second intermediate joint and the equivalent attenuation slope k2 of the cable between the first intermediate joint and the cable termination are calculated ; Step S356: calculating the attenuation slope k0 of the cable between the cable head and the first intermediate joint: ； wherein T1 is the time of the first intermediate joint of the non-uniform power cable, is the time of the terminal of the non-uniform power cable, p end is the time of the terminal of the non-uniform power cable signal strength.

2. The global insulation condition assessment method for non-homogeneous power cables according to claim 1, characterized in that, The calculation of the reflection coefficient of each intermediate joint of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable further comprises: Step S41: performing attenuation compensation on the signal intensity at each intermediate joint moment of the non-uniform power cable to obtain the signal intensity at the i-th intermediate joint moment before attenuation : ； Step S42: constructing a signal y with f as the independent variable: ； Wherein, d is the length of the intermediate joint; v1 is the wave speed of the intermediate joint; cos(·) is a cosine function; Step S43: Enumerate x in the range of [0, 0.5] respectively, and perform Blackman window FFT processing on the corresponding y, perform ratio processing on the FFT processing result and the window function DC component, record the maximum value in the FFT amplitude spectrum and the closest x as the reflection coefficient of the ith intermediate joint most close x as the reflection coefficient of the ith intermediate joint ; Step S44: repeating steps S42 to S43 until the reflection coefficients of all intermediate joints are calculated; Step S45: correcting the signal intensity at the mirror time of each intermediate joint: ； ； ； Step S46: correcting the signal intensity at the cable terminal time: ； Step S47: repeating steps S45 to S46 until the number of cycles reaches 15.

3. The global insulation condition assessment method for non-homogeneous power cables according to claim 2, characterized in that, The evaluation of the global insulation state of the non-uniform power cable according to the attenuation slope of each section cable of the non-uniform power cable and the reflection coefficient of each intermediate joint, so as to obtain the global insulation state evaluation result of the non-uniform power cable, further comprises: Step S51: Calculate the attenuation slope k of each section cable i Relative deviation from the initial measurement :​ ； Step S52: Calculate the relative deviation of the cable wave velocity of each section in the second positioning map from the actual wave velocity v : ； wherein, is the actual length of the i-th section cable; Step S53: comparing the value of each section cable , , > 2.5% or > 10%, the section cable is seriously aged and needs to be focused on; when 1%≤ ≤ 2.5% or 3%≤ ≤ 10%, the section cable is moderately aged and needs to be strengthened; when < 1% and < 3%, the section cable is slightly aged and only needs to be generally concerned. Step S54: Calculate the reflection coefficient of each intermediate joint relative deviation from the initial measurement results : ； When > 15%, the intermediate joint is seriously aged and needs to be focused on; when 5%≤ ≤ 15%, the intermediate joint is moderately aged and needs to be strengthened; when < 5%, the intermediate joint is slightly aged and only needs to be generally concerned.

4. The global insulation condition assessment method suitable for a non-uniform power cable according to claim 1, characterized by, In the step S21, the value of the pulse width w in the Gaussian oscillation pulse y0(t) is 2 to 4 times the reciprocal of the upper limit value of the sweep range frequency, and the value of the oscillation frequency f c in the Gaussian oscillation pulse y0(t) is half the upper limit value of the sweep range frequency.

5. The global insulation condition assessment method for non-homogeneous power cables according to claim 1, characterized in that, In step S27, the first interval 1 Ω is used to traverse [10, 80 Ω] to obtain the entire Pearson correlation coefficient m k The maximum value position in the curve is obtained, and then the second interval smaller than the first interval 1 Ω is used to traverse m k The maximum value position in the curve is obtained, and then the second interval smaller than the first interval 1 Ω is used to traverse m k The maximum value position in the curve is obtained, and then the second interval smaller than the first interval 1 Ω is used to traverse m .

6. The global insulation condition assessment method suitable for a non-uniform power cable according to claim 2, characterized by, In steps S31 and S33, when the first positioning map and the second positioning map are obtained, the imaginary part of the corresponding data is selected for processing, and at this time, the cosine function cos(·) in step S42 is replaced by the sine function sin(·); The step S34, in the first positioning map according to time by small to big record each intermediate joint moment signal intensity for , and let At this time, meet ; While recording the first positioning map in each intermediate joint mirror moment Signal intensity , and let ; Wherein T end For cable terminal moment; In step S356, when the attenuation slope k0 of the cable between the cable head and the first intermediate joint is calculated, the test data at the cable terminal are used to calculate the attenuation slope of the cable between the last intermediate joint and the cable terminal, which is k0.

7. The global insulation condition assessment method suitable for a non-uniform power cable according to claim 2, characterized by, In step S43, the enumeration step is set to 0.005, 0.01 or 0.02.

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