CN115420998A - Cable characteristic extraction method and device based on broadband impedance spectrum - Google Patents
Cable characteristic extraction method and device based on broadband impedance spectrum Download PDFInfo
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
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Abstract
The invention relates to a cable characteristic extraction method and a device based on broadband impedance spectrum, comprising the following steps: measuring input impedance spectrum Z in case of open cable op (ii) a Measuring input impedance spectrum Z in case of short circuit of cable sc (ii) a From the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant; obtaining an attenuation constant and a phase shift constant according to the cable propagation constant; and calculating the wave speed propagated by the cable according to the phase shift constant. According to the invention, the influence of a measurement system on the extraction of the high-frequency characteristics of the cable can be reduced, and the accuracy of cable defects and fault location diagnosis is improved.
Description
Technical Field
The invention relates to the technical field of device loss measurement, in particular to a cable characteristic extraction method and device based on broadband impedance spectrum.
Background
With the development of urban construction, cable lines occupy greater and greater proportion in power transmission and distribution systems, and the normal operation of the cables is directly related to the development of social economy and the safety of power utilization. The power cable has a complex insulation structure, including conductors, insulation, semi-conductive shields, external shields and sheaths. Cross-linked polyethylene insulated (XLPE) cables are currently the most widely used type of cable in domestic use. The design service life of the cross-linked polyethylene cable for the distribution network is 30 years, but due to the factors of imperfect manufacturing process, damage of cable laying, bad cable operation environment and the like, the reliability of the long-term use of the cable is greatly influenced, the actual service life of a plurality of cables often cannot meet the design requirement, the cable is further led to enter the aging period in advance, and great hidden danger is brought down for the operation safety of a power system. Therefore, the timely diagnosis and positioning of the cable defects are a big problem to be solved in the operation and maintenance process of the cable at present.
In recent years, some domestic and foreign scholars begin to reflect changes of characteristic parameters of cables when local defects exist in cable insulation by using a Broadband Impedance Spectroscopy (BIS) technology of the cables based on a Frequency Domain Reflectometry (FDR). The method comprises the steps of injecting a sweep frequency signal into a cable, converting an echo signal (frequency domain impedance spectrum) into a space domain function by utilizing integral transformation, obtaining a diagnosis function of cable characteristic parameters changing along with the position, realizing the positioning of local defects, monitoring the change trend of cable insulation performance, and realizing the detection and diagnosis of cable aging and faults.
Since attenuation effects and dispersion of high-frequency signals affect the positioning accuracy, in order to accurately analyze the defect position of the cable, the transmission characteristics of the cable at high frequency must be considered. The propagation constant and the characteristic impedance are the basis for analyzing attenuation and dispersion, so how to accurately measure the propagation constant and the characteristic impedance of the cable under high frequency to further obtain the attenuation constant is very important for improving the positioning accuracy of the cable defect and detecting the latent defect position of the cable in time.
At present, the existing methods for calculating the transmission characteristics of a high-frequency power cable mainly include the following three methods:
one method is a time domain short pulse propagation method. The method is only used for extracting the propagation constant and the phase velocity, and noise interference and signal dispersion are obvious.
The second method is that firstly, according to the geometric structure of the cable, the dielectric properties of the core material and the insulating material and other parameters, the distribution parameter RLGC of the cable is calculated by using a formula, and then the characteristic impedance, the propagation constant and the phase velocity are calculated according to the RLGC parameters. The precondition of this method is to extract the complex dielectric constant of the insulating medium accurately. The complex dielectric constant of composite insulating media is often difficult to measure by "slicing" due to the need to exfoliate and slice the insulating material. In addition, due to the edge effect of the sample and the inductance effect of the connector, additional capacitance and inductance are generated at high frequency, which is difficult to correct, and further affects the measurement accuracy of the distributed inductance L and the capacitance C. Meanwhile, the additional capacitance and inductance at high frequency limit the frequency range of the test method.
The third method is a scattering matrix method; the method uses a short cable to form a two-port structure, and obtains characteristic impedance and propagation constant according to port voltage and current. Although this method can be used over a wide frequency range, it is still subject to the additional capacitance and inductance of the connection port, and little research has been done on how to eliminate this effect in particular.
Disclosure of Invention
The invention aims to provide a cable characteristic extraction method and device based on a broadband impedance spectrum, so as to reduce the influence of a measurement system on the extraction of the high-frequency characteristic of a cable and improve the accuracy of cable defects and fault location diagnosis.
In order to achieve the above object, the present invention provides a cable characteristic extraction method based on broadband impedance spectrum, which includes the following steps:
measuring input impedance spectrum Z in case of open cable op ；
Measuring input impedance spectrum Z in case of short circuit of cable sc ；
According to the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant;
obtaining an attenuation constant and a phase shift constant according to the cable propagation constant;
and calculating the wave speed propagated by the cable according to the phase shift constant.
Preferably, said method is based on said input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant, comprising:
according to
The cable propagation constant γ is calculated.
Preferably, the obtaining of the attenuation constant and the phase shift constant according to the cable propagation constant includes:
respectively taking a real part and an imaginary part according to the propagation constant gamma to obtain an attenuation constant alpha and a phase shift constant beta;
α＝Re[γ]，β＝Im[γ]；
where Re represents the real part of the propagation constant and Im represents the imaginary part of the propagation constant.
Preferably, said calculating the wave speed propagated by the cable according to said phase shift constant β comprises:
according to the phase shift constant beta and formula
The wave velocity v propagated by the cable is calculated.
The invention also provides a cable characteristic extraction device based on the broadband impedance spectrum, which comprises:
open-circuit impedance spectrum measuring module for measuring input impedance spectrum Z under open-circuit condition of cable op ；
Short circuit impedance spectrum measuring module for measuring input impedance spectrum Z under cable short circuit condition sc ；
A propagation constant calculation module for calculating a propagation constant based on the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant;
the attenuation and phase shift constant calculation module is used for obtaining an attenuation constant and a phase shift constant according to the cable propagation constant;
and the wave velocity calculation module is used for calculating the wave velocity propagated by the cable according to the phase shift constant.
Preferably, the propagation constant calculation module is configured to:
according to
The cable propagation constant γ is calculated.
Preferably, the attenuation and phase shift constant calculation module is configured to:
respectively taking a real part and an imaginary part according to the propagation constant gamma to obtain an attenuation constant alpha and a phase shift constant beta;
α＝Re[γ]，β＝Im[γ]；
where Re represents the real part of the propagation constant and Im represents the imaginary part of the propagation constant.
Preferably, the wave velocity calculating module is configured to:
according to the phase shift constant beta and formula
The wave velocity v propagated by the cable is calculated.
The invention has at least the following beneficial effects:
(1) The method is favorable for solving the inherent defect that the traditional method is greatly influenced by high-frequency noise, and contributes to defect diagnosis of cables in a distribution network system;
(2) Compared with the traditional method I, the method has the advantages that the test platform is simple and easy to use, and interference is small; compared with the traditional method II, the attenuation constant can be extracted without predicting the geometric structure of the cable, and the complex dielectric constant of the composite insulating medium can be measured without using a 'slicing method'; compared with the traditional method III, the calculation process is greatly simplified, and the measurement is convenient and quick;
(3) The method improves the positioning precision of the cable defects, relatively accurately extracts the high-frequency cable transmission characteristics of the cable, is beneficial to describing the propagation process of signals along the cable, and improves the accuracy of positioning and diagnosing the cable defects and faults.
(4) Compared with the singleness of the traditional method, the method is not only suitable for the distribution cable, but also suitable for other types of cables with different structures, such as high-voltage cables, communication cables and the like; as long as the transmission line equation (telegraph equation) is suitable, the cable can be regarded as a distributed parameter circuit in the process of transmitting power/signals, the attenuation constant of the cable can be measured, and the cable has high universality.
Additional features and advantages of the invention will be set forth in the description which follows.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a flowchart of a cable characteristic extraction method based on broadband impedance spectrum according to an embodiment of the present invention.
Fig. 2 is an equivalent circuit diagram of a cable according to an embodiment of the present invention.
Fig. 3 is a schematic diagram of a cable characteristic test using a network analyzer in an embodiment of the present invention.
Fig. 4 is a schematic diagram showing the comparison result between the method of the embodiment of the present invention and the conventional method for extracting the cable characteristics.
Fig. 5 is a schematic diagram of the analysis of the error of the cable characteristic extraction method according to the embodiment of the present invention.
Fig. 6 is a block diagram of a cable characteristic extraction apparatus based on broadband impedance spectrum according to an embodiment of the present invention.
Detailed Description
Various exemplary embodiments, features and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. In addition, in the following detailed description, numerous specific details are set forth in order to provide a better understanding of the present invention. It will be understood by those skilled in the art that the present invention may be practiced without some of these specific details. In some instances, well known means have not been described in detail so as not to obscure the present invention.
Referring to fig. 1, an embodiment of the present invention provides a method for extracting cable characteristics based on broadband impedance spectrum, including the following steps:
step S1, measuring input impedance spectrum Z under the condition of cable open circuit op ；
S2, measuring an input impedance spectrum Z under the condition of short circuit of the cable sc ；
S3, according to the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant;
specifically, in order to reduce the influence of a measurement system on the extraction of the high-frequency characteristics of the cable and simplify the extraction process, the embodiment of the invention provides a method for extracting the characteristic impedance Z of the cable from an impedance spectrum 0 Propagation constant γ, phase velocity v;
it is known from the transmission line theory that when the physical size of the electrical network is much larger than the wavelength of the electromagnetic wave, i.e. the cable length is longer or the signal frequency is higher, the electrical network needs to be expressed by using the distribution parameters. A typical equivalent circuit diagram of the cable is shown in fig. 2, wherein R, L, C, and G in fig. 2 represent resistance, inductance, conductance, and capacitance of the cable per unit length, respectively;
specifically, testing the impedance spectrum of the cable, namely adding a sweep frequency signal at the head end of the cable, and measuring the impedance spectrum of the head end in a form of measuring a reflection coefficient;
the known reflection coefficient, characteristic impedance, and propagation constant are:
wherein, Z L Is the load impedance, Z 0 R, L, G and C are distributed resistance, distributed inductance, distributed capacitance and distributed conductance, alpha is an attenuation constant, and the amplitude attenuation characteristics of voltage and current waveforms are represented;
for the phase constant, the phase change characteristics of the voltage wave and the current wave are represented, and the head end impedance spectrum obtained by measurement can be written as:
wherein l is the length of the cable;
when open circuit, the reflection coefficient of the cable is gamma op ＝1；
Reflection coefficient gamma of cable in short circuit sc ＝-1。
The input impedance spectrum Z under the conditions of open circuit and short circuit can be calculated by substituting formula (4) op And Z sc Respectively as follows:
thus, as shown in FIG. 3, if the input impedance spectrum Z is measured with the network analyzer for the case of open and short circuit of the cable op And Z sc Then the propagation constant can just be calculated:
therefore, the embodiment of the present invention provides step S3, the open-circuit impedance spectrum Z op And short circuit impedance spectrum Z sc Multiplying and then cutting a root number to obtain the characteristic impedance of the cable; open circuit impedance spectrum Z op And short circuit impedance spectrum Z sc Dividing the root number, taking the inverse hyperbolic tangent value, and finally dividing the inverse hyperbolic tangent value by the length l of the cable to obtain a propagation constant gamma;
s4, obtaining an attenuation constant and a phase shift constant according to the cable propagation constant;
as can be seen from equation (3), the attenuation constant and the phase shift constant can be obtained by taking the real part and the imaginary part of the propagation constant γ, respectively:
α＝Re[γ] (8)
β＝Im[γ] (9)
wherein Re represents the real part of the propagation constant, and Im represents the imaginary part of the propagation constant;
s5, calculating the wave velocity of cable propagation according to the phase shift constant;
specifically, the wave velocity can be obtained from the phase shift coefficient:
fig. 5 shows a comparison result of the extraction process between the method of the embodiment of the present invention and the conventional method for extracting the cable characteristics, which shows that the embodiment of the present invention has the advantages that the attenuation constant can be extracted without predicting the geometric structure of the cable, and the complex dielectric constant of the composite insulating medium does not need to be measured, so that the measurement process is greatly simplified, and the measurement is fast and accurate.
By using the test flow shown in the above diagram, comparing the cable characteristic extraction method based on broadband impedance spectrum provided by the embodiment of the present invention with the conventional cable characteristic extraction method, an error analysis diagram of the attenuation constants measured by the conventional method and the embodiment of the present invention as shown in fig. 6 can be obtained; it can be seen that the error deviation rate of the attenuation constant measured under high frequency and the attenuation constant measured by the conventional method in the embodiment of the invention is within 10%; the embodiment of the invention has the advantages of convenient and quick measurement, test accuracy and high field application value.
Another embodiment of the present invention further provides a broadband impedance spectrum-based cable characteristic extraction apparatus, which is used to implement the broadband impedance spectrum-based cable characteristic extraction method described in the foregoing embodiment, as shown in fig. 6, the apparatus includes:
open-circuit impedance spectrum measuring module 1 for measuring input impedance spectrum Z under open-circuit condition of cable op ；
Short circuit impedance spectrum measuring module 2 for measuring input impedance spectrum Z under short circuit condition of cable sc ；
A propagation constant calculation module 3 for calculating a propagation constant based on the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant;
the attenuation and phase shift constant calculation module 4 is used for obtaining an attenuation constant and a phase shift constant according to the cable propagation constant;
and the wave velocity calculating module 5 is used for calculating the wave velocity propagated by the cable according to the phase shift constant.
Preferably, the propagation constant calculation module 3 is specifically configured to:
according to
The cable propagation constant gamma is calculated.
Preferably, the attenuation and phase shift constant calculation module 4 is specifically configured to:
respectively taking a real part and an imaginary part according to the propagation constant gamma to obtain an attenuation constant alpha and a phase shift constant beta;
α＝Re[γ]，β＝Im[γ]；
where Re represents the real part of the propagation constant and Im represents the imaginary part of the propagation constant.
Preferably, the wave velocity calculation module 5 is specifically configured to:
according to the phase shift constant beta and formula
The wave velocity v propagated by the cable is calculated.
The above-described embodiments of the apparatus are merely illustrative, wherein the modules described as separate parts may or may not be physically separate, and the parts displayed as modules may or may not be physical modules, may be located in one place, or may be distributed on a plurality of network modules. Some or all of the modules may be selected according to actual needs to achieve the purpose of the solution of the apparatus of the embodiment.
It should be noted that the apparatus in the foregoing embodiment corresponds to the method in the foregoing embodiment, and therefore, details of the apparatus in the foregoing embodiment may be obtained by referring to the contents of the method in the foregoing embodiment, that is, specific step contents described in the method in the foregoing embodiment may be understood as functions that can be implemented by the apparatus in the foregoing embodiment, and are not described herein again.
As can be seen from the above description of the embodiments, the embodiments of the present invention have the following advantages:
(1) The embodiment of the invention is beneficial to solving the inherent defect that the traditional method is greatly influenced by high-frequency noise, and contributes to the defect diagnosis of the cable in the distribution network system;
(2) Compared with the traditional method I, the embodiment of the invention has the advantages that the test platform is simple and easy to use and has small interference; compared with the traditional method II, the attenuation constant can be extracted without predicting the geometric structure of the cable, and the complex dielectric constant of the composite insulating medium can be measured without using a 'slicing method'; compared with the traditional method III, the calculation process is greatly simplified, and the measurement is convenient and quick;
(3) The embodiment of the invention improves the positioning precision of the cable defect, relatively accurately extracts the high-frequency cable transmission characteristic of the cable, is beneficial to describing the propagation process of signals along the cable, and improves the accuracy of positioning and diagnosing the cable defect and the fault.
(4) Compared with the singleness of the traditional method, the method is suitable for not only the distribution cable, but also other types of cables with different structures, such as a high-voltage cable, a communication cable and the like; as long as the transmission line equation (telegraph equation) is suitable, the cable can be regarded as a distributed parameter circuit in the process of transmitting power/signals, the attenuation constant of the cable can be measured, and the cable has high universality.
While embodiments of the present invention have been described above, the above description is illustrative, not exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen in order to best explain the principles of the embodiments, the practical application, or technical improvements to the market, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
Claims (8)
1. A cable characteristic extraction method based on broadband impedance spectrum is characterized by comprising the following steps:
measuring input impedance spectrum Z in the case of open cable op ；
Measuring input impedance spectrum Z in case of short circuit of cable sc ；
According to the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant;
obtaining an attenuation constant and a phase shift constant according to the cable propagation constant;
and calculating the wave speed propagated by the cable according to the phase shift constant.
2. The cable characteristic extraction method according to claim 1, wherein the input resistance is based on the input resistanceResistance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant, comprising:
according to
The cable propagation constant γ is calculated.
3. The cable characteristic extraction method according to claim 1, wherein the obtaining an attenuation constant and a phase shift constant from a cable propagation constant comprises:
respectively taking a real part and an imaginary part according to the propagation constant gamma to obtain an attenuation constant alpha and a phase shift constant beta;
α＝Re[γ]，β＝Im[γ]；
where Re represents the real part of the propagation constant and Im represents the imaginary part of the propagation constant.
4. The cable characteristic extraction method according to claim 1, wherein the calculating a wave velocity propagated through the cable based on the phase shift constant β includes:
according to the phase shift constant beta and formula
The wave velocity v propagated by the cable is calculated.
5. A cable characteristic extraction device based on broadband impedance spectrum is characterized by comprising:
open circuit impedance spectrum measuring module for measuring input impedance spectrum Z under open circuit condition of cable op ；
Short circuit impedance spectrum measuring module for measuring input impedance spectrum Z under short circuit condition of cable sc ；
A propagation constant calculation module for calculating a propagation constant based on the input impedance spectrum Z op And the input impedance spectrum Z sc Calculating a cable propagation constant;
the attenuation and phase shift constant calculation module is used for obtaining an attenuation constant and a phase shift constant according to the cable propagation constant;
and the wave velocity calculation module is used for calculating the wave velocity of the cable propagation according to the phase shift constant.
6. The cable characteristic extraction apparatus according to claim 5, wherein the propagation constant calculation module is configured to:
according to
The cable propagation constant gamma is calculated.
7. The cable characteristic extraction apparatus of claim 5, wherein the attenuation and phase shift constant calculation module is configured to:
respectively taking a real part and an imaginary part according to the propagation constant gamma to obtain an attenuation constant alpha and a phase shift constant beta;
α＝Re[γ]，β＝Im[γ]；
where Re represents the real part of the propagation constant and Im represents the imaginary part of the propagation constant.
8. The cable characteristic extraction device according to claim 5, wherein the wave velocity calculation module is configured to:
according to the phase shift constant beta and formula
The wave velocity v of the cable propagation is calculated.
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