CN111581903A - Distribution cable impedance spectrum determination method and device based on improved infinitesimal equivalent model - Google Patents
Distribution cable impedance spectrum determination method and device based on improved infinitesimal equivalent model Download PDFInfo
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Abstract
The invention discloses a distribution cable impedance spectrum determination method and a distribution cable impedance spectrum determination device based on an improved infinitesimal equivalent model, wherein the method comprises the following steps: the method comprises the steps of enabling a target distribution cable to be equivalent to a plurality of improved micro-element equivalent models, and respectively determining electrical parameters of each cable micro-element equivalent model; cascading the multiple cable micro-element equivalent models to form an equivalent circuit model corresponding to the target distribution cable, and determining the upper frequency limit of the equivalent circuit model; acquiring information of a predetermined local defect, and adjusting an electrical parameter of a corresponding improved infinitesimal equivalent model according to the information of the local defect; and respectively determining amplitude-frequency data and phase-frequency data corresponding to the target distribution cable under the constraint of the upper frequency limit by using a predetermined head end impedance calculation method, and drawing an impedance spectrum. The method utilizes the distributed model to quickly calculate the impedance spectrum of the distribution cable, has high accuracy, high speed and low requirement on calculation resources, can be applied to quickly calculate the impedance spectrum of the 10-35kV distribution cable at different aging stages on a commissioning site, and is used for the online performance inspection test of the power cable.
Description
Technical Field
The invention belongs to the technical field of power cables, and particularly relates to a distribution cable impedance spectrum determination method and device based on an improved infinitesimal equivalent model.
Background
At present, a cable line local defect positioning method mainly depends on an off-line test. The off-line test mainly comprises oscillation wave partial discharge and ultra-low frequency dielectric loss detection.
The main defect of the cable oscillatory wave partial discharge detection method is the interference of noise on a partial discharge signal, and the noise even can cause misjudgment. On the other hand, the discharge sources contained in the detected partial discharge signal may be a cable line, a cable terminal switch cabinet, a generator or a transformer connected to the cable line, and the like, and the determination of each discharge source in the partial discharge signal is one of the difficulties of the method; secondly, signals are attenuated and deformed during retransmission in the cable, so that the actual application effect of the partial discharge monitoring technology is far inferior to that of theoretical research results.
When the ultralow frequency dielectric loss is detected, the ultralow frequency voltage has certain accumulated damage risk to insulation under the ultralow frequency condition; and the method can only reflect the integral aging level of the cable insulation, is not sensitive to local insulation defects and cannot position cable faults.
Although the current impedance spectrum detection technology provides a feasible solution for realizing the local defect location of the cable line, the impedance spectrum and the characteristics (such as the initial value of the distortion) of the distribution cable at the initial stage cannot be obtained after the line is laid, so that the situation of cable state misjudgment is caused due to the lack of the initial value serving as a reference in the field detection process.
Disclosure of Invention
The invention provides a distribution cable impedance spectrum determination method and device based on an improved infinitesimal equivalent model, and aims to solve the problem that impedance spectrum data of a distribution cable line at the initial stage of operation are lacked in the prior art.
In a first aspect, the present invention provides a distribution cable impedance spectrum determination method based on an improved infinitesimal equivalent model, including:
s100, enabling a target distribution cable to be equivalent to a plurality of improved infinitesimal equivalent models, and respectively determining electrical parameters of each cable infinitesimal equivalent model;
step S200, cascading the plurality of cable infinitesimal equivalent models to form an equivalent circuit model corresponding to the target distribution cable, and determining the upper frequency limit of the equivalent circuit model;
step S300, obtaining information of a predetermined local defect, and adjusting an electrical parameter of a corresponding improved infinitesimal equivalent model according to the information of the local defect;
and S400, respectively determining amplitude-frequency data and phase-frequency data corresponding to the target distribution cable under the constraint of the upper frequency limit by using a predetermined head end impedance calculation method, and drawing an impedance spectrum.
In a second aspect, the present invention provides an apparatus for determining an impedance spectrum of a distribution cable based on an improved infinitesimal equivalent model, comprising:
the improved infinitesimal equivalent model determining unit is used for enabling the target distribution cable to be equivalent to a plurality of improved infinitesimal equivalent models and respectively determining the electrical parameters of the cable infinitesimal equivalent models;
the equivalent circuit model determining unit is used for cascading the plurality of cable infinitesimal equivalent models to form an equivalent circuit model corresponding to the target distribution cable and determining the upper frequency limit of the equivalent circuit model;
the electrical parameter adjusting unit is used for acquiring the information of the predetermined local defect and adjusting the electrical parameters of the corresponding improved infinitesimal equivalent model according to the information of the local defect;
and the impedance calculation unit is used for respectively determining amplitude-frequency data and phase-frequency data corresponding to the target distribution cable under the constraint of the upper frequency limit by using a predetermined head end impedance calculation method, and drawing an impedance spectrum.
According to the distribution cable impedance spectrum determination method and device based on the improved infinitesimal equivalent model, the impedance spectrum of the distribution cable is rapidly calculated by using the distributed model, the accuracy is high, the speed is high, and the demand on calculation resources is low; the method can be applied to rapid calculation of impedance spectrums of 10-35kV distribution cables in different aging stages on a commissioning site, and is used for online performance inspection and test of power cables.
Drawings
A more complete understanding of exemplary embodiments of the present invention may be had by reference to the following drawings in which:
fig. 1 is a schematic flow chart of a distribution cable impedance spectrum determination method based on an improved infinitesimal equivalent model according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a distribution cable impedance spectrum determination apparatus based on an improved infinitesimal equivalent model according to an embodiment of the present invention;
FIG. 3 is a schematic cross-sectional view of a single core coaxial cable;
FIG. 4 is a cross-sectional schematic view of a three-core coaxial cable;
FIG. 5 is a diagram illustrating a micro-element equivalent model of a general transmission line in the prior art;
FIG. 6 is an improved infinitesimal equivalent model according to an embodiment of the present invention;
FIG. 7 is an equivalent circuit model formed by cascading a plurality of improved infinitesimal equivalent models according to an embodiment of the present invention;
fig. 8 is a simulation result (i.e., an impedance spectrogram) of the head-end input impedance obtained by using the infinitesimal equivalent model in the embodiment of the present invention, wherein:
(a) the impedance amplitude-frequency diagram is obtained by calculation according to a general transmission line infinitesimal equivalent model;
(b) the impedance phase diagram is calculated according to a general transmission line infinitesimal equivalent model;
(c) the impedance amplitude-frequency diagram is obtained by calculation according to the improved infinitesimal equivalent model;
(c') is a partial enlarged view of (c);
(d) is an impedance phase diagram calculated according to the improved infinitesimal equivalent model.
Detailed Description
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein, which are provided for complete and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, the same units/elements are denoted by the same reference numerals.
Unless otherwise defined, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Further, it will be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
In order to solve the problem that the impedance spectrum data of a distribution cable line at the initial operation stage is lacked in the prior art, the embodiment of the invention provides a distribution cable impedance spectrum rapid calculation method which is used for calculating and obtaining the impedance spectrum data of a cable according to a cable distributed equivalent model.
According to the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model, the impedance spectrum under a normal state/different local aging degrees can be rapidly calculated by using the improved infinitesimal equivalent model cascaded along the cable under the condition of limited calculation resources on the distribution cable field by fixing the relevant electrical parameters and the calculation model of the cable.
As shown in fig. 1, the method for determining an impedance spectrum of a distribution cable based on an improved infinitesimal equivalent model according to an embodiment of the present invention includes:
s100, enabling a target distribution cable to be equivalent to a plurality of improved infinitesimal equivalent models, and respectively determining electrical parameters of each cable infinitesimal equivalent model;
step S200, cascading the plurality of cable infinitesimal equivalent models to form an equivalent circuit model corresponding to the target distribution cable, and determining the upper frequency limit of the equivalent circuit model;
step S300, obtaining information of a predetermined local defect, and adjusting an electrical parameter of a corresponding improved infinitesimal equivalent model according to the information of the local defect;
and S400, respectively determining amplitude-frequency data and phase-frequency data corresponding to the target distribution cable under the constraint of the upper frequency limit by using a predetermined head end impedance calculation method, and drawing an impedance spectrum.
Further, in the method for determining an impedance spectrum of a distribution cable based on an improved infinitesimal equivalent model, in step S100, an electrical parameter of any improved infinitesimal equivalent model includes:
resistance R of cable corecInductance L of cable corecResistance R of metal shielding layer of cablesCable metal shielding layer inductance LsCable insulation capacitor CI；
Wherein, the cable core wire resistance RcAnd a cable core inductance LcAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor CIParallel connection; resistance R of cable metal shielding layersAnd cable metal shielding layer inductance LsAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor CIAnd (4) connecting in parallel.
Further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model,
in step S100, the target distribution cable is uniformly divided into M sections of cable microelements, and each cable microelement corresponds to an improved microelement equivalent model; the sum of the lengths of the M sections of cable elements is the same as the length of the target distribution cable;
correspondingly, in step S200, the improved infinitesimal equivalent models are sequentially cascaded according to the positions of the cable infinitesimal in the cable corresponding to each improved infinitesimal equivalent model, so as to form an equivalent circuit model corresponding to the target distribution cable.
Further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model,
in the step S200, the upper frequency limit f of the equivalent circuit model is determined according to the following formulamax：
Wherein n is the number of the improved infinitesimal equivalent models;
l is the length of the cable element;
wherein the content of the first and second substances,
further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model,
in step S300, the information about the predetermined local defect includes:
the location of the local defect in the cable, the severity of the local defect, and an electrical parameter impact function corresponding to the severity of the local defect.
Further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model,
the electrical parameter impact function corresponding to the severity of the local defect comprises:
for determining the insulation capacitance C of a cableIThe following formula:
wherein r iscIs the outer radius of the cable core;
rsthe inner radius of the cable insulation metal shielding layer;
is the relative dielectric constant.
Further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model,
in the step S400, the predetermined head end impedance calculation method includes:
determining a head-end input impedance Z according to:
wherein, XCCapacitive reactance for cable insulation;
RACan alternating current resistance of a cable core;
XLis the inductive reactance of the cable core.
Further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model further includes:
using the DC resistance R of the cable core according toDCCorrection of the AC resistance R of the core wire of a cableAC：
RAC＝RDC(1+ys+yp)；
Wherein, ysIs the skin effect factor;
ypis the proximity effect factor.
Further, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model,
the target distribution cable is suitable for 10-35kV lines;
the target distribution cable is a coaxial cable and sequentially comprises a conductor layer, an inner semi-conducting layer, an insulating layer, an outer semi-conducting layer and a metal shielding layer from inside to outside.
The distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model considers the multilayer structure of the cable in the cross section and provides an improved cable distributed model, namely the improved infinitesimal equivalent model; calculating related cable electrical parameters by changing complex dielectric constants at different frequencies in each improved infinitesimal equivalent model; adjusting a calculation formula of complex dielectric constant according to the severity of the local defects to calculate the electrical parameters of the cable under the local defects with different severity; by setting the length of each improved infinitesimal equivalent model, the cable impedance under various local defects in different length ranges can be calculated.
As shown in fig. 3 and 4, the medium voltage cable used in the distribution network has a multilayer coaxial structure including, from the inside to the outside, a conductor layer (e.g., a core wire), an inner semiconductive layer, an insulating layer, an outer semiconductive layer, and a metal shield layer in this order. This structural relationship determines the series-parallel relationship between the electrical parameters in the equivalent model. The impedance spectrum describes the correspondence between the input impedance and the frequency at the head end of the cable. In specific implementation, the input impedance values at different frequencies constitute the impedance spectrum of the cable.
The cable is off-line when impedance spectroscopy testing is performed in the field. When the impedance spectrum is tested, excitation voltage is input to one end of the cable, the output current of the end is measured, and impedance data of the section of the cable is obtained by using frequency domain data through time-frequency transformation.
At present, according to the operation specification, the parameter test of the line is not required to be carried out before the distribution cable is put into operation, so that impedance spectrum data of the distribution cable before the distribution cable is put into operation or in the initial stage of the distribution cable is not recorded generally, and the impedance spectrum data can be used as a reference for carrying out local defect positioning by using the impedance spectrum subsequently.
As the commissioning time is gradually lengthened, local defects (structural damage) or local aging that gradually occur in the cable change the shape, contact relationship, dielectric constant of the insulating medium, or conductor or semiconductor, in the cable. Generally, the deeper a depression or scratch is formed in the cross-sectional direction of the cable by a local defect, the more severe the defect; in this case, the difference between the impedance characteristic of the cable in the defective section and the impedance characteristic of the cable in the non-defective section is more significant.
The insulation of distribution cables in overhead lines is gradually aged and exhibits a gradual change in relative permittivity, and, because the mechanism by which the insulation polarizes at different frequencies is different, the change in permittivity during aging of the insulation is not linear but frequency dependent. The change of parameters such as dielectric constant related to the electrical characteristics of the cable can be reflected on the change trend of the impedance spectrum or the frequency corresponding to the pole.
Dielectric aging is manifested as a change in the dielectric constant, which is a relative capability used to characterize a dielectric's ability to store electrostatic energy in an electric field. In the low frequency band (<1kHz), the relative dielectric constant remains constant; in the high frequency band (>1kHz), due to changes in the polarization mechanism, complex dielectric constants are commonly used for description. The imaginary part of the complex permittivity is used to characterize the loss in dielectric polarization.
The cable equivalent circuit model can be divided into two types, a lumped model and a distributed model. The coaxial cable transmission line equivalent model and the coaxial cable improved cable infinitesimal equivalent model are distributed models. It should be noted that, in view of the thinness of the inner and outer semiconductive layers, the effect of the inner and outer semiconductive layers on the impedance spectrum is generally ignored when building the distributed model.
In the coaxial cable transmission line infinitesimal equivalent model shown in fig. 5, the resistance (R) of the transmission line and the inductance (L) of the transmission line are connected in series and then connected in parallel with the insulation capacitance (C) of the transmission line at both ends thereof, respectively, as a whole.
In the coaxial cable modified cable infinitesimal equivalent model shown in fig. 6, the electrical parameters include: resistance R of cable corecInductance L of cable corecResistance R of metal shielding layer of cablesCable metal shielding layer inductance LsCable insulation capacitor CI. Wherein the core resistance (R) of the cablec) And cable core inductance (L)c) Connected in series as a whole and respectively connected with cable insulation capacitors (C) at two ends thereofI) Parallel connection; cable metal shielding layer resistance (R)s) And metal shielding layer inductor (L)s) Connected in series as a whole and respectively connected with cable insulation capacitors (C) at two ends thereofI) And (4) connecting in parallel.
Note that the following calculation steps are exemplified by a single core cable. The three-core cable can be regarded as three independent single-core cables, and the impedance of the cable is solved respectively, which is not described in detail here.
Taking the improved infinitesimal equivalent model of fig. 6 as an example, a method for calculating the head-end input impedance is given. The head-end input impedance (complex number) Z of the improved infinitesimal equivalent model (here, a single-core cable) is recorded as:
in the formula (1), XCCapacitive reactance for cable insulation;
wherein, CIFor cable insulation capacitance:
XL＝2πfL；
wherein, L is the inductance of the cable core and the inductance of the metal shielding layer;
L＝Lc+Ls。
RAC＝RDC(1+ys+yp)； (2)
in the formula (2), ysIs the skin effect factor.
In the formula (2), RDCCan be calculated (e.g., by a module built into MATLAB) based on the dimensions of the cable core and the resistivity of the cable core material.
Calculating the skin effect factor y according tos：
Wherein, x is more than 0s≤2.8；
ys＝-0.136-0.0177xs+0.0563xs 2，
Wherein, x is more than 2.8s≤3.8；
ys＝0.354xs+0.733；
Wherein x is more than 3.8s。
Calculating the variable x according tos：
In the above formula, ksTo calculate the dimensionless number of skin effect, the value is obtained by engineering experience and/or experiment.
In the formula (2), ypIs a proximity effect factor;
the single-core cable does not have the proximity effect factor ypI.e. ypIs 0.
For a three-core cable, its proximity effect factor ypDetermined by equation (3):
in the formula (3), xpIs determined by the following formula:
in the formula (4), kpThe numerical values are obtained through engineering experience and experiments for the dimensionless number used for calculating the proximity effect factor.
In general, in most cases, xpNot exceeding 2.8.
In formula (3), d is shown in FIG. 4cThe outer diameter of the cable core;
s is the distance between two adjacent cores.
In specific implementation, k is the insulation material of a cable made of cross-linked polyethylene (XLPE)sCan be taken to be 1, kpMay be taken to be 0.8.
The complex dielectric constant of the cable insulation is different at different aging stages. The correlation between the complex dielectric constant and the aging degree is shown in Table 1, wherein,0is a vacuum dielectric constant; ω ═ 2 π f, where f is the frequency (Hz).
TABLE 1 correlation between complex dielectric constant and aging degree
Further, from the relative permittivity/complex permittivity, the insulation capacitance C is determined according to equation (5)I：
In the formula (5), rcIs the outer radius of the cable core, rsFor the inner radius of the insulating metal shield of the cable
In specific implementation, the input impedance is calculated from the head end (left side in fig. 7) of the equivalent circuit model shown in fig. 7, for example, the head end input impedance of the modified infinitesimal equivalent model is calculated according to the following formula:
when X is presentL＝-2XCWhen the impedance spectrum appears at an extreme point;
as the number of cascaded cable infinitesimal equivalent models increases, Z can be written about XLAnd XCA high-order function of; the number of the extreme points is correspondingly increased, namely, the number of the extreme points is the same as that of the improved infinitesimal equivalent models included in the cascade circuit.
The transmission parameter matrix T of the equivalent circuit after the cascade of n infinitesimal equivalent models as shown in FIG. 7cableMultiplying the transmission parameter matrix T of each infinitesimal equivalent model in sequence, namely:
in the equation (8), the two parameters in the first column of each transmission parameter matrix are the open circuit parameters of the circuit, and the two parameters in the second column are the short circuit parameters of the circuit.
And the input impedance Z of the cascade circuit of n infinitesimal equivalent modelscableIt can be directly calculated from the transmission parameter matrix:
Zcable＝Tcable11/Tcable21(9)
in specific implementation, for the improved cable micro-element equivalent model and the cascaded circuit equivalent model, the input impedance at the head end can be calculated by an impedance calculation module built in MATLAB.
In specific implementation, the number of the cascaded models (usually, the number of the cascaded models is not less than 10 for improving the positioning effect of the local defect) is selected according to the length of the cable, and the upper limit of the frequency is determined according to the parameters of the cascaded equivalent circuit.
In specific implementation, the upper frequency limit f is determined by equation (10)max：
Wherein n is the number of the improved infinitesimal equivalent models, such as 10;
l is the length of the cable element, such as 1 m;
v is a function of the propagation velocity of the signal in the cable;
wherein the content of the first and second substances,
wherein the content of the first and second substances,
is about the frequency fmaxA function of (a);
wherein the content of the first and second substances,
is and fmaxIndependent function, can be based on cable sizeParameters, calculated using the mutualindex element in MATLAB;
Setting a simulation frequency range, wherein the lower limit of the simulation frequency range is 1kHz, namely the lowest value of the frequency is 1 kHz; changing the frequency f one by using simulation software MATLAB Simulink and a circulation instruction(i)(ii) a The frequency f is calculated according to Table 1 based on the predetermined degree of cable aging(i)Relative dielectric constant corresponding to insulation layer in lower cable(i)/Complex dielectric constant root and corresponding electrical parameter values; for example, the values of the electrical parameters of each cable infinitesimal equivalent model can be obtained by the power _ cablepram, which is an interface program built in MATLAB Simulink.
In addition, according to the skin effect factor ysAnd the proximity effect factor ypThe DC resistance R calculated according to simulation softwareDCTo AC resistance RACAnd (5) correcting:
RAC＝RDC(1+ys+yp)，
then, in simulation software MATLAB Simulink, an impedance calculation module is connected to the head end of the cascaded cable infinitesimal equivalent model, and the input impedance value Z of the head end of the cable in a set frequency range is calculateddata(i)(or within the frequency range, calculating the frequency f separately(i)Value of input impedance Z of cabledata(i))。
Finally, in the frequency range, according to f(i)And Zdata(i)And drawing the input impedance of the head end corresponding to the cascaded equivalent circuit model as the impedance spectrum of the distribution cable to be analyzed.
Specifically, an Impedance calculation module (Impedance Measurement Block) in MATLAB Simulink is connected to the head end of the cascaded equivalent circuit, and an Impedance spectrum is obtained by calculating the ratio of input voltage and output current with different frequencies.
In one embodiment of the present invention, when calculating the distributed impedance spectrum of XLPE distribution cable with a length of 10m, two types of distributed models as shown in fig. 5 and fig. 6 are respectively used to compare the calculation accuracy of the two types of models.
In specific implementation, 1m is used as the length of a cable infinitesimal, and an XLPE distribution cable with a length of 10m is equivalent to 10 sequentially cascaded cable infinitesimal equivalent models (the schematic diagram of an equivalent circuit is shown in fig. 7), that is, ten cascaded cable infinitesimal equivalent models (the schematic diagram of a circuit is shown in fig. 5 or fig. 6) are used for equivalent to an XLPE distribution cable with a length of 10 m.
Fig. 8 shows the impedance spectra of good-condition XLPE single-core distribution cables 10m long, obtained from the infinitesimal equivalent model simulations of fig. 5 and 6, respectively.
As shown in fig. 8, ten extreme points (that is, the number of the extreme points is the same as that of the cable infinitesimal equivalent model) can be observed on the impedance spectrum obtained by using the infinitesimal equivalent model before and after the improvement, wherein the positions where the extreme points appear are all at the zero crossing points of the impedance phase, which accords with the impedance spectrum theory, and therefore, a more accurate impedance spectrum can be calculated by using the infinitesimal equivalent model before and after the improvement.
In the low frequency band (A), as shown in FIG. 8(a)<1MHz), the amplitude of the impedance spectrum of the cable decreases along with the increase of the frequency, and the phase position of the impedance spectrum is 90 degrees; considering that the impedance phase spectrum (as shown in FIG. 8(d)) of the cable becomes smooth after the transverse multi-layer structure, the phase frequency curve becomes smooth from 1MHz to fmaxA regular decay of the phase frequency curve is observed in the frequency range of (a).
On the impedance amplitude-frequency diagram, the amplitude change of the extreme point before improvement (as shown in fig. 8 (a)) is irregular and has a large value; after the improvement (as shown in fig. 8 (c)), the amplitude of the extreme point gradually decreases with the increase of the frequency.
It should be noted that the number of the infinitesimal equivalent models and the cable length corresponding to the infinitesimal equivalent models determine the upper frequency limit f in the cascade circuit modelmax. For example, f of the cascaded circuit model corresponding to the XLPE distribution cable 10m long in the embodimentmaxAt 21MHz, beyond which the calculation of the impedance spectrum will no longer be able to accurately approximate the actual situation. Thus, although FIG. 8 showsThe frequency range given in (1) k-45 MHz, however, the data before 21MHz can accurately reflect the change of impedance spectrum, and the analysis mainly refers to the calculation result before the upper limit of frequency. All the calculation results are given in fig. 8 mainly for comparison with the calculation results of the improved model, and are not used for explaining the upper frequency limit.
In summary, when the method of the present embodiment is implemented in the simulation software MATLAB Simulink, the method includes the following steps:
1) calculating the upper limit f of the frequency according to the number n of the infinitesimal equivalent modelsmaxAnd setting the simulation frequency range f accordingly(i)；
2) Determining that the cable micro element with the local defect corresponds to the kth cable micro element equivalent model;
3) changing the frequency one by one and calculating the complex dielectric constant under the frequency by using a cyclic instruction to obtain the frequency f(i)Relative dielectric constant corresponding to insulating medium with local defect(i)(the severity of the local defect can be adjusted);
4) calculating and f(i)And(i)corresponding cable distribution parameters;
5) correcting resistance parameters according to the skin effect factor and the proximity effect factor, and calculating the impedance of the cable under the corresponding frequency, namely the frequency is f(i)Cable impedance value Z of timedata(i)；
6) According to f(i)And Zdata(i)And drawing a cable impedance spectrum.
In summary, the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model of the embodiment of the invention solves the practical problem of low evaluation accuracy caused by the loss of the initial state of the cable line in the impedance spectrum test process, and realizes the rapid calculation of the distributed impedance spectrum under the normal state/different aging degrees along the cable line under the condition of limited calculation resources by solidifying the relevant parameters and the calculation model.
By comparing the theoretical numerical value curve obtained by calculation with the actual test curve of the impedance spectrum test equipment on site by adopting the method, the rapid diagnosis of the insulation state of the cable line based on the distributed impedance spectrum and the defect positioning based on the distortion point can be realized, the site detection efficiency is improved, and the state detection and defect positioning accuracy is optimized.
The method realizes the rapid calculation of the impedance spectrum of the cascade cable micro-element equivalent model along the line cable under the condition of limited field available parameters under normal state/different aging degrees. The method solves the practical problem of low evaluation accuracy caused by the loss of the initial state of the cable line in the impedance spectrum test process. Furthermore, the local defects and the aging state of the distribution cable can be evaluated by comparing and analyzing the test data and the theoretical data of the impedance test instrument.
As shown in fig. 2, the distribution cable impedance spectrum determination apparatus based on the improved infinitesimal equivalent model according to the embodiment of the present invention includes:
the improved infinitesimal equivalent model determining unit 10 is used for enabling the target distribution cable to be equivalent to a plurality of improved infinitesimal equivalent models and respectively determining the electrical parameters of the cable infinitesimal equivalent models;
an equivalent circuit model determining unit 20, configured to cascade the multiple cable infinitesimal equivalent models to form an equivalent circuit model corresponding to the target distribution cable, and determine an upper frequency limit of the equivalent circuit model;
the electrical parameter adjusting unit 30 is configured to acquire information of a predetermined local defect, and adjust an electrical parameter of a corresponding improved infinitesimal equivalent model according to the information of the local defect;
and an impedance calculating unit 40, configured to determine amplitude-frequency data and phase-frequency data corresponding to the target distribution cable, respectively, under the constraint of the upper frequency limit, and draw an impedance spectrum, by using a predetermined head end impedance calculating method.
The distribution cable impedance spectrum determination device based on the improved infinitesimal equivalent model has the same conception, technical scheme and technical effect with the distribution cable impedance spectrum determination method based on the improved infinitesimal equivalent model, and the description is omitted here.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
The invention has been described above by reference to a few embodiments. However, other embodiments of the invention than the one disclosed above are equally possible within the scope of the invention, as would be apparent to a person skilled in the art from the appended patent claims.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a// the [ device, component, etc ]" are to be interpreted openly as at least one instance of a device, component, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
Claims (10)
1. A distribution cable impedance spectrum determination method based on an improved infinitesimal equivalent model is characterized by comprising the following steps:
s100, enabling a target distribution cable to be equivalent to a plurality of improved infinitesimal equivalent models, and respectively determining electrical parameters of each cable infinitesimal equivalent model;
step S200, cascading the plurality of cable infinitesimal equivalent models to form an equivalent circuit model corresponding to the target distribution cable, and determining the upper frequency limit of the equivalent circuit model;
step S300, obtaining information of a predetermined local defect, and adjusting an electrical parameter of a corresponding improved infinitesimal equivalent model according to the information of the local defect;
and S400, respectively determining amplitude-frequency data and phase-frequency data corresponding to the target distribution cable under the constraint of the upper frequency limit by using a predetermined head end impedance calculation method, and drawing an impedance spectrum.
2. The method of claim 1 wherein the distribution cable impedance spectrum determination based on the improved infinitesimal equivalent model,
in step S100, the electrical parameters of any improved infinitesimal equivalent model include:
resistance R of cable corecInductance L of cable corecElectricity, electricityCable metal shielding layer resistance RsCable metal shielding layer inductance LsCable insulation capacitor CI；
Wherein, the cable core wire resistance RcAnd a cable core inductance LcAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor CIParallel connection; resistance R of cable metal shielding layersAnd cable metal shielding layer inductance LsAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor CIAnd (4) connecting in parallel.
3. The method of claim 1 wherein the distribution cable impedance spectrum determination based on the improved infinitesimal equivalent model,
in step S100, the target distribution cable is uniformly divided into M sections of cable microelements, and each cable microelement corresponds to an improved microelement equivalent model; the sum of the lengths of the M sections of cable elements is the same as the length of the target distribution cable;
correspondingly, in step S200, the improved infinitesimal equivalent models are sequentially cascaded according to the positions of the cable infinitesimal in the cable corresponding to each improved infinitesimal equivalent model, so as to form an equivalent circuit model corresponding to the target distribution cable.
4. The method of claim 3 wherein the distribution cable impedance spectrum is determined based on an improved infinitesimal equivalent model,
in the step S200, the upper frequency limit f of the equivalent circuit model is determined according to the following formulamax：
Wherein n is the number of the infinitesimal equivalent models;
l is the length of a single cable element;
v(fmax) As a function of the propagation speed of the signal in the cable;
L(fmax)and C(fmax)Are respectively the frequency fmaxThe core wire inductance and the insulation capacitance of the cable;
wherein the content of the first and second substances,
5. the method of claim 1 wherein the distribution cable impedance spectrum determination based on the improved infinitesimal equivalent model,
in step S300, the information about the predetermined local defect includes:
the location of the local defect in the cable, the severity of the local defect, and an electrical parameter impact function corresponding to the severity of the local defect.
6. The method of claim 5 wherein the distribution cable impedance spectrum is determined based on an improved infinitesimal equivalent model,
the electrical parameter impact function corresponding to the severity of the local defect comprises:
for determining the insulation capacitance C of a cableIThe following formula:
wherein r iscIs the outer radius of the cable core;
rsthe inner radius of the cable insulation metal shielding layer;
is the relative dielectric constant.
7. The method of claim 1 wherein the distribution cable impedance spectrum determination based on the improved infinitesimal equivalent model,
in the step S400, the predetermined head end impedance calculation method includes:
determining a head-end input impedance Z according to:
wherein, XCCapacitive reactance for cable insulation;
RACan alternating current resistance of a cable core;
XLis the inductive reactance of the cable core.
8. The method of claim 7 for determining the impedance spectrum of a distribution cable based on the improved infinitesimal equivalent model, further comprising:
using the DC resistance R of the cable core according toDCCorrection of the AC resistance R of the core wire of a cableAC：
RAC＝RDC(1+ys+yp)；
Wherein, ysIs the skin effect factor;
ypis the proximity effect factor.
9. Method for improved infinitesimal equivalent model based distribution cable impedance spectrum determination according to any one of claims 1 to 8,
the target distribution cable is suitable for 10-35kV lines;
the target distribution cable is a coaxial cable and sequentially comprises a conductor layer, an inner semi-conducting layer, an insulating layer, an outer semi-conducting layer and a metal shielding layer from inside to outside.
10. An apparatus for determining an impedance spectrum of a distribution cable based on an improved infinitesimal equivalent model, comprising:
the improved infinitesimal equivalent model determining unit is used for enabling the target distribution cable to be equivalent to a plurality of improved infinitesimal equivalent models and respectively determining the electrical parameters of the cable infinitesimal equivalent models;
the equivalent circuit model determining unit is used for cascading the plurality of cable infinitesimal equivalent models to form an equivalent circuit model corresponding to the target distribution cable and determining the upper frequency limit of the equivalent circuit model;
the electrical parameter adjusting unit is used for acquiring the information of the predetermined local defect and adjusting the electrical parameters of the corresponding improved infinitesimal equivalent model according to the information of the local defect;
and the impedance calculation unit is used for respectively determining amplitude-frequency data and phase-frequency data corresponding to the target distribution cable under the constraint of the upper frequency limit by using a predetermined head end impedance calculation method, and drawing an impedance spectrum.
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