阅读说明：本技术 基于改进微元等效模型的配电电缆阻抗谱确定方法及装置 (Distribution cable impedance spectrum determination method and device based on improved infinitesimal equivalent model ) 是由 王昱力 欧阳本红 夏荣 李文杰 王格 刘松华 张振鹏 邓显波 赵鹏 刘宗喜 陈铮 于 2020-04-02 设计创作，主要内容包括：本发明公开基于改进微元等效模型的配电电缆阻抗谱确定方法及装置该方法包括：将目标配电电缆等效为多个改进微元等效模型,并分别确定各电缆微元等效模型的电气参数；将所述多个电缆微元等效模型级联,形成与所述目标配电电缆对应的等效电路模型,并确定所述等效电路模型的频率上限；获取预先确定的局部缺陷的信息,并根据所述局部缺陷的信息调整对应的改进微元等效模型的电气参数；利用预先确定的首端阻抗计算方法,在所述频率上限的约束下,分别确定与所述目标配电电缆对应的幅频数据和相频数据,并绘制阻抗谱。该方法利用分布式模型快速计算配电电缆的阻抗谱,准确度高,速度快,对计算资源的需求低,可应用在投运现场快速计算10-35kV配电电缆不同老化阶段的阻抗谱,服务于电力电缆在线性能检验测试。(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.)

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 core_cInductance L of cable core_cElectricity, electricityCable metal shielding layer resistance R_sCable metal shielding layer inductance L_sCable insulation capacitor C_I；

Wherein, the cable core wire resistance R_cAnd a cable core inductance L_cAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor C_IParallel connection; resistance R of cable metal shielding layer_sAnd cable metal shielding layer inductance L_sAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor C_IAnd (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 formula_max：

Wherein n is the number of the infinitesimal equivalent models;

l is the length of a single cable element;

v(f_max) As a function of the propagation speed of the signal in the cable;

L_(fmax)and C_(fmax)Are respectively the frequency f_maxThe 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 cable_IThe following formula:

wherein r is_cIs the outer radius of the cable core;

r_sthe 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, X_CCapacitive reactance for cable insulation;

R_ACan alternating current resistance of a cable core;

X_Lis 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 to_DCCorrection of the AC resistance R of the core wire of a cable_AC：

R_AC＝R_DC(1+y_s+y_p)；

Wherein, y_sIs the skin effect factor;

y_pis 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.

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 core_cInductance L of cable core_cResistance R of metal shielding layer of cable_sCable metal shielding layer inductance L_sCable insulation capacitor C_I；

Wherein, the cable core wire resistance R_cAnd a cable core inductance L_cAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor C_IParallel connection; resistance R of cable metal shielding layer_sAnd cable metal shielding layer inductance L_sAfter being connected in series, the two ends of the capacitor are respectively connected with the cable insulation capacitor C_IAnd (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 formula_max：

Wherein n is the number of the improved infinitesimal equivalent models;

l is the length of the cable element;

as a function of the propagation speed of the signal in the cable;

andare respectively the frequency f_maxThe core wire inductance and the insulation capacitance of the cable;

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 cable_IThe following formula:

wherein r is_cIs the outer radius of the cable core;

r_sthe 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, X_CCapacitive reactance for cable insulation;

R_ACan alternating current resistance of a cable core;

X_Lis 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 to_DCCorrection of the AC resistance R of the core wire of a cable_AC：

R_AC＝R_DC(1+y_s+y_p)；

Wherein, y_sIs the skin effect factor;

y_pis 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 core_cInductance L of cable core_cResistance R of metal shielding layer of cable_sCable metal shielding layer inductance L_sCable insulation capacitor C_I. Wherein the core resistance (R) of the cable_c) And cable core inductance (L)_c) Connected in series as a whole and respectively connected with cable insulation capacitors (C) at two ends thereof_I) 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 thereof_I) 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), X_CCapacitive reactance for cable insulation;

wherein, C_IFor cable insulation capacitance:

X_L＝2πfL；

wherein, L is the inductance of the cable core and the inductance of the metal shielding layer;

L＝L_c+L_s。

part 1 of the calculation of the rated current of the cable according to IEC 60287-1-1-2014: rated current equation (load factor 100%) and loss, ac resistance R of cable core_ACAnd a direct current resistance R_DCHas the following relationship:

R_AC＝R_DC(1+y_s+y_p)； (2)

in the formula (2), y_sIs the skin effect factor.

In the formula (2), R_DCCan 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 to_s：

Wherein, x is more than 0_s≤2.8；

y_s＝-0.136-0.0177x_s+0.0563x_s ²，

Wherein, x is more than 2.8_s≤3.8；

y_s＝0.354x_s+0.733；

Wherein x is more than 3.8_s。

Calculating the variable x according to_s：

In the above formula, k_sTo calculate the dimensionless number of skin effect, the value is obtained by engineering experience and/or experiment.

In the formula (2), y_pIs a proximity effect factor;

the single-core cable does not have the proximity effect factor y_pI.e. y_pIs 0.

For a three-core cable, its proximity effect factor y_pDetermined by equation (3):

in the formula (3), x_pIs determined by the following formula:

in the formula (4), k_pThe 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, x_pNot exceeding 2.8.

In formula (3), d is shown in FIG. 4_cThe outer diameter of the cable core;

s is the distance between two adjacent cores.