RU2795272C1 - Method for evaluating the ability of materials included in the design of cable fittings to equalize the electric field strength - Google Patents

Abstract

FIELD: cable fittings.

SUBSTANCE: invention relates to a method for assessing the ability of materials included in the design of cable fittings to equalize the electric field strength. An electrical voltage of industrial frequency is applied to a cable sample with prepared ends, the voltage value is increased, the value of partial discharges in the said sample is recorded. When partial discharges reach the threshold value, the first voltage value U1 is fixed. The test material is then applied to said prepared ends of said cable sample. Power frequency electrical voltage is applied again, the voltage value is increased, the value of partial discharges in said sample is recorded. When partial discharges reach the threshold value, the second voltage value U2 is fixed. A conclusion is made about the levelling ability of the tested material in relation to the second value of the voltage U2 to the first value of the voltage U1.

EFFECT: increasing the reliability of the results.

5 cl, 2 dwg, 1 tbl

Description

The field of technology to which the invention belongs

The invention relates to electrical cables, in particular to means for evaluating the properties of electrical polymeric materials.

State of the art

A known method for evaluating a material is to regulate the electric field at the edge of the screen for cable insulation (patents EP 0035271 B1, EP 0 522 760 A1) according to the magnitude of the overlap voltage of the prepared end of the cable 330 mm long (i.e., the distance between the bare cable core and the edge of the screen for insulation cable is 330 mm, patent EP 0035271 B1) [1]. However, such an assessment does not take into account the fact that partial discharges may occur in the coupling at operating voltage, which over time can lead to its breakdown.

The standard IEEE 48-1990 [2] is known, which provides for “Determination of the partial discharge (corona) extinction voltage”, in determining which partial discharges in the termination are measured at certain voltage values and must be below a certain value. Then the voltage at which the discharges go out is measured, and it must be above a certain value. Partial discharge and power frequency voltage characteristics shall comply with the requirements set out in the IEEE STD 48-1990 standard test procedures. However, these tests refer to mounted terminations, i.e. ready-to-use products and do not consider the possibility of testing individual elements of the end couplings for the ability to equalize the electric field strength.

The above well-known test tools during the installation of samples do not provide for a special, standardized preparation of cable insulation and screen for cable insulation, which can affect the final result, which does not allow a correct comparison of materials intended for equalizing the field strength at the edge of the screen for cable insulation, included into the end sleeve.

The essence of the invention

The invention solves the problem of creating a criterion and methodology for assessing the ability of materials included in the design of cable fittings to equalize the electric field strength.

The invention makes it possible to achieve the following technical results: increasing the reliability of a comparative assessment of the ability of materials included in the design of cable fittings to equalize the electric field strength; reduction of time and simplification of tests.

These technical results are achieved by the fact that the method for assessing the ability of materials included in the design of cable fittings to equalize the electric field strength consists in applying an electric voltage U of industrial frequency to a cable sample with prepared ends, increasing the voltage value, registering the value of partial discharges in mentioned sample, when partial discharges of the threshold value are reached, the first voltage value U1 is fixed, then the test material is applied to the said prepared ends of the cable sample, the electric voltage of industrial frequency is again applied, the voltage value is increased, the value of partial discharges in the mentioned sample is recorded, when partial discharges reach the threshold the values fix the second value of the voltage U2, in relation to the second value of the voltage U2 to the first value of the voltage U1, a conclusion is made about the leveling ability of the material under test.

The indicated technical results are also achieved by the fact that the device for implementing the method consists of a piece of polymer-insulated cable with a length of at least 3000 mm with prepared cable ends 500-1000 mm long, in the form of sections with the removal of all layers to the screen along the cable insulation and sections, with removal of the screen for cable insulation in a section 30 - 50 mm less than sections with the removal of all layers up to the screen for cable insulation.

The specified technical results are also achieved by the fact that the edge of the screen over the cable insulation is processed into a cone, with a taper of not more than 1 degree.

These technical results are also achieved by the fact that the surface of the cable insulation, at the edge of the screen along the cable insulation, over a length of 10 - 40 mm, in order to eliminate unevenness, is processed with an emery cloth, with a final grain size according to ISO 4344 (GOST 52381-2005) is not less than R400.

The specified technical results are also achieved by the fact that a test termination is mounted at the second end of the cable, in which, in the range of test voltages used, partial discharges do not obviously occur.

A distinctive feature of the invention is that a piece of cable with ends prepared for applying the test material is used as a test sample, i.e. tests are carried out under conditions as close as possible to the operating conditions of the material under test.

List of drawing figures

Figure 1 shows the application of the field adjustment tape to the edge of the screen along the cable insulation.

Figure 2 shows the application of a heat-shrinkable field control tube to the edge of the screen along the cable insulation

Implementation of the invention

From the operation of electric cables, it is known that in cable boxes at the edge of the screen along the cable insulation, a high electric field strength occurs, which leads to the appearance of partial discharges on the surface of the cable insulation or on the surface of the material applied over the cable insulation and, as a result, to electrical breakdown.

In medium voltage cable joints, to equalize the electric field strength at the edge of the screen along the cable insulation, polymer compositions with increased dielectric constant and/or dependence of conductivity and/or dielectric constant on tension are used, which are applied in the form of tapes, cuffs or heat-shrinkable tubes to the edge of the screen along cable insulation.

Existing test methods for individual materials used to reduce field strength, i.e. not included in the electrical cable, provide insufficient reliability of the results, since they do not take into account all operating conditions in real structures.

The variety of polymer compositions from different manufacturers with different mechanisms of influence on the distribution of field strength and technological features of their use in couplings complicates the choice of materials. Often, for these purposes, it is necessary to assemble and test a full-scale sample of the coupling, in which the tension distribution is influenced not only by the polymer composition itself, intended for field control, but also by other elements of the coupling design.

In addition, cable joints usually use at least two materials used to regulate the field at the edge of the screen along the cable insulation, superimposed one on top of the other, so it is important to evaluate not only the ability to equalize the electric field of each material, but also their combinations.

The proposed method for assessing the ability of materials to equalize the electric field intensity is based on measuring the magnitude of the electric voltage directly on the test sample of an electric cable.

When evaluating the ability of materials to equalize tension, one should take into account the dependence of this ability on the cable design (core diameter and cable insulation diameter). Therefore, in the proposed method, the partial discharge start voltage is measured twice, the first time without applying the evaluated material to the edge of the screen along the cable insulation and the second time with applying the evaluated material. As an evaluation criterion, the ratio of the voltage of the occurrence of partial discharges with the imposition of the evaluated material U2 to the magnitude of the voltage of the occurrence of partial discharges without the imposition of the evaluated material U1 is taken.

To evaluate materials, equalize the field strength at the edge of the screen according to the cable insulation, a design of the device is also proposed, which provides for the preparation of insulation and the screen according to the insulation of the end of the cable, which allows you to correctly evaluate the materials (or their combination) used to equalize the field strength at the edge of the screen according to the insulation cable.

The present invention makes it possible to take into account all the mechanisms for reducing the maximum electric field strength, which makes it possible to objectively evaluate and select a material for its use in cable fittings.

The evaluation method consists in that the device, which is a piece of cable with prepared ends, is supplied with a power frequency voltage, which gradually increases, with a constant registration of the value of partial discharges in the device, and the voltage U1 is recorded, at which partial discharges occur that exceed a certain value (for example, 10 pK). Then, on the prepared sections of the insulation and the electrically conductive screen, the test material is applied along the insulation of the device in the form of a tape, cuff or heat-shrinkable tube, and a power frequency voltage is applied to the cable core, which gradually increases, with a constant registration of the magnitude of partial discharges in the device, and the voltage U2 is recorded, at which partial discharges occur, exceeding a certain value (for example, 10 pK). As a quantitative measure of the leveling ability of the evaluated material, the ratio of the magnitude of the voltage U2 to the voltage U1 is taken.

In Fig.1 and Fig.2 shows embodiments of the device.

In the variant with the application of the field control tape on the edge of the screen for insulation, the cable contains a screen 1 for cable insulation, a conical part 2 of screen 1 for cable insulation (taper is not more than 1 degree), a tape 3 for field control, cable insulation 4 and a surface 5 treated with emery cloth. cable insulation.

In the heat shrink tube overlay variant shown in Figure 2, a heat shrink tube 6 is used instead of tape.

At the second end of the cable, a test termination can be mounted, in which, in the range of test voltages used, partial discharges certainly do not occur.

As examples of evaluation results, Table 1 shows the results of measuring the leveling properties of materials obtained on a cable with XLPE insulation for a voltage of 35 kV with a screen diameter of 12 mm along the core and a diameter of 30 mm along the insulation.

Table 1

No. Description of the material used U2 (kV) U1 (kV) K=U2/U1 1 Tape 1 applied to the insulation at a length of 30 mm 25 7 3.57 2 Tape 2 applied to the insulation at a length of 30 mm 27 7 3.85 3 heat-shrinkable tube 1 250 mm long 45 7 6.43 4 t/u tube 2, length 250 mm 13 7 1.86 5 t/u tube 3 250 mm long 20 7 2.86 6 t/u tube 4, length 250 mm 9 7 1.28 7 t/u tube 5, length 250 mm 40 7 5.71 8 t/u tube 6 250 mm long 36 7 5.14

Samples of tapes and tubes indicated in table 1 are made of materials from different manufacturers.

From table 1 it can be seen that the material - heat-shrinkable tube 1 250 mm long, as applied to a cable for a voltage of 35 kV, has the best electric field equalization properties of all the materials listed in the table.

The proposed method can be used for comparative testing of materials for regulating the electric field strength, input control of these materials, or in the design of couplings. Possible applications of the method and device are not limited to these examples.

Literature

1. EP 0035271 B1 "Elastomeric composition for providing electrical stress control", H01B1/20.

2. IEEE Standard Test Procedures and requirements for Alternating-Current

Cable Terminations 2.5kV Through 765 kV, IEEE Std48-1990, 31 pp.

Claims (5)

1. A method for assessing the ability of materials included in the design of cable fittings to equalize the electric field strength, consisting in the fact that an electric voltage of industrial frequency is applied to a cable sample with prepared ends, the voltage value is increased, the value of partial discharges in the said sample is recorded, upon reaching partial discharges of the threshold value, the first value of the voltage U1 is fixed, then the test material is applied to the said prepared ends of the mentioned cable sample, the electric voltage of industrial frequency is again applied, the voltage value is increased, the value of partial discharges in the said sample is recorded, when the partial discharges of the threshold value are reached, the second value is fixed voltage U2, make a conclusion about the leveling ability of the tested material in relation to the second value of the voltage U2 to the first value of the voltage U1. 2. A device for implementing the method according to claim 1, consisting of a piece of polymer insulated cable with a length of at least 3000 mm with prepared cable ends 500-1000 mm long, in the form of sections with the removal of all layers to the screen for cable insulation and sections, with the removal screen for cable insulation in a section 30-50 mm shorter than the sections with the removal of all layers up to the screen for cable insulation. 3. The device according to claim 2, characterized in that the edge of the screen on the cable insulation is processed into a cone, with a taper of not more than 1 degree. 4. The device according to claim 2, characterized in that the surface of the cable insulation, at the edge of the screen along the cable insulation, over a length of 10-40 mm, in order to eliminate unevenness, is processed with an emery cloth, with a final grain size of at least P400. 5. The device according to claim 2, characterized in that a test termination is mounted on the second end of the cable, in which, in the range of test voltages used, partial discharges certainly do not occur.

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