SU760196A1 - INSULATING COMPOSITION1 - Google Patents

Description

The invention relates to compositions for equalizing the voltage distribution along the length of electrical products and primarily to compositions for end joints of high-voltage cables.

The voltage distribution along the cable ^termination joint is extremely uneven. The section of the termination joint immediately adjacent to the end of the insulation shield bears 80% of the voltage applied to the cable, resulting in surface discharges in this section, which cause breakdown of the cable at the termination joint.

There are several ways to reduce the electric field strength at the end ^of the insulation screen: using a leveling cone; placing the cable end in a liquid with high permittivity and specific volume electrical resistance (10 ⁷ ~ -10* ⁰ Ohm cm); covering the end with a nonlinear dielectric with high permittivity.

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For working terminations, the first method is primarily used. For cables with plastic insulation, the equalizing cone is either wound with insulating tapes or made of a monolithic material, pre-pressed and placed on the insulation, or pressed onto the insulation. In these cases, sufficient equalization of the electric field is not achieved, and the terminations must either be placed in an oil environment or the termination length must be increased beyond the cone to prevent discharges across the surface of the cable termination.

The second method is only applicable in a test laboratory and requires complex equipment for deionization, circulation and cooling of water.

The third method uses various compositions containing silicon carbide. The volt-ampere characteristic (VAC) of silicon carbide can be described by the equation 1 = ki ^y where η is the nonlinearity index of the VAC. Typically, for the specified compositions

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Silicon carbide with a nonlinearity index of 2 to 7 is used in these compositions. A polymer solution is used as a binder in these compositions.

To equalize the stress distribution along the length of the end cutting, a mixture based on dissolved rubber, silicon carbide and a cross-linking agent is used, applied to the end cutting of the cable in a layer 0.5 mm thick [1].

Tests of this composition have not yielded satisfactory results in terms of the initial discharge voltage at the end joint. Furthermore, applying a 0.5 mm thick layer of mortar to the end joint is extremely labor-intensive.

The closest to the proposed composition is a composition containing a polymer binder, silicon carbide and a solvent [2].

The composition is applied to the cable end in a 0.25 mm thick layer and covered with an additional layer of insulating material, which significantly complicates the joint manufacturing process. Furthermore, the composition is prepared using special, scarce and expensive grades of silicon carbide produced for the radio engineering industry.

As tests have shown, the voltage at which discharges begin for termination joints with a coating based on the described composition is only 20 kV, which does not ensure reliable operation of cables at voltages of 35 kV and higher.

The purpose of the invention is to increase the voltage at the start of discharges and to reduce the dimensions of the end sleeve.

To achieve this goal, the proposed composition additionally contains acetylene black, and as silicon carbide, a mixture of silicon carbides with a dispersion of 1217 μm and 0.7-1.5 μm with the following ratio of components, wt.%:

Polymer binder 10-20

Silicon carbide with a particle size of 0.7-1.5 μm 10-25 Silicon carbide with a particle size of 12-17 μm 10-25 Acetylene black 0.3-1.0

Solvent Rest.

The composition recipes are prepared as follows.

The polymer binder (butyl rubber and polyisobutylene in a ratio of 3:1) is dissolved in a solvent (a mixture of toluene and trichloroethylene in a ratio of 9:1) at room temperature. The mixture is used as a solvent

toluene and trichloroethylene is caused by the fact that the addition of trichloroethylene increases the adhesion of the coating to the insulation. After the polymer binder has completely dissolved, the remaining components are added to the solution with constant stirring. A mixture of silicon carbides with particle sizes of 1 and 15 µm is used as silicon carbide. The content ^of components in the composition formulations varies

within the following limits, wt.%: Polymer binder 9-21 Silicon carbide with dimensions • 1 µm particles 8-27. Silicon carbide with dimensions particles 15 microns 8-27 Acetylene black 0.1-1.2 Solvent The rest. The composition is applied with a brush. to the ends Cable termination for voltage 35 kV with insulation

9 mm thick high-density polyethylene insulation. The end joints are 30 cm long sections of insulation coated with the composite over a length of 20 cm.

The toluene to trichloroethylene ratio for the proposed composition is not critical. The applied coating is air-dried at room temperature, and then the partial discharge onset voltage is measured with a gradual increase in the applied voltage to the cable.

The composition recipes and test results are presented in the table.

As follows from the table, the highest voltage at the onset of discharges is found in samples of end couplings with coatings based on formulations No. 2-7.

The proposed composition can be used in the manufacture of test and working termination joints for high-voltage cables with monolithic polymer insulation.

The composition is used in the manufacture of end joints for testing cable samples at a voltage of 35 kV for partial discharges."

• The discharge level of the termination joint itself, up to 35 kV, is below the sensitivity of the measuring equipment (KT ^-14 K). Tests confirm the stability of the obtained results. The process of manufacturing termination joints using the proposed composition is simple, labor-intensive, and does not require highly qualified performers.

The proposed composition can be used in the manufacture of end joints for power cables with plastic insulation for voltages of 6–220 kV.

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Composition Recipe, No. Component content, wt.% Acetylene black Silicon carbide with a particle size of 1 µm Silicon carbide with a particle size of 15 µm Polymer binder Solvent Discharge initiation voltage, kV I 0.1 17 17 15 50.9 18 2 03 , 10 .10 10 69.7 .31 3 0.3 10 yu 20 59.7 25 4 0.4 14 14. 11 60.6 40 5 ‘ 0.6 17 17 15 50.4 35 6 1.0 25 25 10 39.0 38 7 1.0 25 25 20 29.0 28 8 1.2 17 ¹⁷ 15 49.8 21 9 0.6 8 ' '8 ,15 68.4 17 yu 0.6 27 27 15 30.4 18 11 0.6 8 25 15 51.2 18 12 0.6 25 8 15 51.2. 17 13 0.4 14 14 9 62.6 ¹⁶ 14 0.4 14 14 —· ..... , , 21 50.6 21

Claims (1)

Claim Insulating composition for electrical products, mainly for terminations of high-voltage cables, containing polymer binder, silicon carbide and solvent, characterized in that, in order to increase the voltage of the start of discharges and reduce the dimensions of the terminal coupling, it additionally contains acetylene black, silicon contains a mixture of silicon carbides with a dispersity of 12-17 μm and 0.7-1.5 Mkm with the following ratio of components, wt.%: TsNIIPI Order 6027/18 Polymer binder 10-20 Silicon carbide with a particle size of 0.7-1.5 microns 10-25 Silicon carbide with a particle size of 12-17 microns 10-25 Acetylene black 0.3-1.0 Solvent Rest.