DE3516971A1 - Electrical insulations - Google Patents

Abstract

The invention relates to electrical insulations based on polyolefin for medium- and high-voltage from about 10kV. In insulations of this type, the object is effectively and durably to suppress the formation of water trees by means of suitable additives. To this end, it is proposed that the insulations contain, as additive, at least one compound of the following structure: <IMAGE> where R is H or alkyl, R<1> and R<2> are H, alkyl, cycloalkyl or aryl and X, Y and Z are O, S or NH, with the proviso that X is not O or S when Y and Z are each O, and that X, Y and Z are not simultaneously NH. The electrical insulations according to the invention are particularly suitable for use in cables and lines.

Description

Siemens share ^ & esel-ls-c.haf t ^: ,, ^: ,., "Our reference 3516971 Berlin and Munich VPA 85 P 3 1 6 7 OE

Electrical insulation

The invention relates to electrical insulation based on polyolefin, in particular for cables and wires, for medium and high voltage from approx. 10 kV with an additive to retard water trees.

In electrically stressed polyolefin insulation processes can take place which are referred to as "electrochemical treeing" (ECT) or "water treeing". These processes, which are particularly important with regard to the operational safety of plastic-insulated medium and High-voltage cables are important, lead to the emergence of tree-like structures, the so-called ECT structures.

The visual appearance of ECT structures that are particularly rich in contrast and visible in detail after suitable coloring is very diverse. Basically A distinction is made between two forms: _ »vented trees", from the surface of the insulation go out and extend into the insulation, and

- "bow-tie trees" that arise inside the insulation.
30th

The mechanism of ECT formation has not yet been clarified. In general, however, it is assumed that an electric field and the Presence of a polar liquid, especially water, is required; the ECT structures are therefore

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half also known as water trees. The initiation sites of the water trees always seem to be imperfections to be, such as impurities, aggregated admixtures, voids, gaps, cracks or disturbed interfaces, but only some of them lead to the formation of water trees. From the flaws which cannot be completely avoided in the case of insulation produced on an industrial scale the tree-like structures move in the direction of the electric field.

Because ECT structures local changes in the insulating material represent, they can cause insulation damage, especially with regard to electrical engineering Dielectric strength. Numerous attempts have therefore already been made to increase the growth of water trees to be prevented or at least delayed, especially in the case of plastic-insulated central and high voltage cables.

One of the measures to prevent the formation of water trees consists in the insulating layer with a metallic cladding or sheathing, for example made of lead or aluminum. But cables with such a water-shielding layer are not only more expensive, but also heavier and therefore more difficult to handle than cables without a metal sheath.

For this reason, attempts have already been made to the formation of ECT structures by adding additives to the insulating layer (or adjacent layers) to prevent. From the large number of compounds used, the following may be mentioned by way of example: lead stearate (DE-OS 24 25 760 and GB-PS 1 473 867), sodium chloride and sulfate or other strong electrolytes (DE-AS 25 37 283 and US Pat. No. 4,042,776), stable hydrates

ORIGINAL INSPECTED

85P3167DE

Forming salts, such as calcium and magnesium chloride, and basic anhydrides (DE-OS 28 1? 804 or GB-PS 1 584 501), lead oxides and basic lead compounds (DE-OS 25 23 844 and DE-OS 28 06 752), silica gel and Phosphorus pentoxide (DE-OS 2? 54 336), organic isocyanates (U.S. Patent 4,282,333), organosilanes (U.S. Patent Nos. 4,144,202, 4,212,756, and 4,263,158, as well as DE-OS 28 05 875), grafted onto the polymer material Silane compounds (DE-OS 29 35 224) and metal complexes of diketones, salicylic acid or Schiff's bases (EP-A1-0 027 300).

On the other hand and contrary to the proposals to add salt-like compounds or electrolytes to the insulating layer, but it has also already been proposed to increase the content of the insulation in finely divided water-soluble and / or hygroscopic salts below a value of

-1 -4

To keep 10 ppm and preferably below 10 ppm (DE-OS 29 11 756).

Obviously, however, all of these measures have to be taken in addition, partly also contradict, not yet brought the desired success. Because so is found in one Report on the Cigre, i.e. the International High Voltage Conference, from I98O the reference (see: "Plastics" 71, 1981, page 448 ff.) That it was recommended to apply a waterproof metal screen to (high-voltage) cables, to prevent moisture ingress.

The object of the invention is, in the case of electrical insulation of the type mentioned at the outset, i.e. in the case of insulation on the basis of crosslinked or uncrosslinked polyolefins, the growth of water trees through suitable To suppress additives effectively and permanently or to prevent their formation as much as possible, so that on costly measures, such as metal jacket, can be dispensed with

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can.

This is achieved according to the invention in that the electrical insulation is at least one additional Compound of the following structure included:

X H

R Il H

AA ^{(1) or}

/ \ Λ

Z / \ Y HZ | YH

R. R ^ H

where the following applies:
R = H or alkyl,

R ¹ , R ² = H, alkyl, cycloalkyl or aryl, and X, Y, Z = 0, S or NH,
with the proviso,

that X is not 0 or S when Y and Z are each 0,
and that X, Y and Z are not NH at the same time.

The formulas (1) and (2) each give only one of the possible tautomeric structures again. Otherwise are the compounds according to formula (1) derivatives of hexahydropyrimidine, the compounds according to formula (2) are based on the pyrimidine itself.

By means of the additives according to the invention, on the one hand prevent the formation of ECT in electrical insulating materials, i.e. the polymeric insulating material is improved with regard to a change in material under electrical stress by the formation of ECT structures in the isolation are delayed or completely suppressed; on the other hand, this connection

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fertilize the range of effective additives expanded. This So far, barbituric acid or 2-thiobarbituric acid and derivatives thereof have been offered (DE-OS 32 02 or US Pat. No. 4,458,044), in water-soluble alkali or alkaline earth phosphates and hydrolyzable phosphoric acid esters (DE-OS 32 02 896), in substances with a certain particle size, which are adsorptively active for heavy metal ions or bind heavy metals in ion exchange, such as pyrogenic and / or precipitated silicas as well as aluminum oxides or oxide hydrates and aluminum silicates (DE-OS 33 18 988), in alcoholates of magnesium, Calcium and aluminum (DE-OS 33 21 268) and in potassium stannate, sodium stannate and titanium oxide sulfate (German patent application Akt.Z. P 35 03 998.1).

From DE-OS 32 02 828 it is, as already mentioned, known in electrical insulation as additives for Retarding water trees to use barbituric acid or 2-thiobarbituric acid and derivatives thereof, i.e. connections of the following structure:

where X = O or S,

R = H or alkyl, and

R ¹ , R ² = H, alkyl, cycloalkyl, or aryl. The compounds according to formula (1) of the present patent application differ from these compounds in the following essential points: In addition to 0 and S -, the radical X can also mean NH, and the two radicals 0 in the 4- and 6-positions can each be replaced by S. or NH

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be replaced.

Except in cables and lines, the invention electrical insulation can also be found in sleeves and fittings. Serve as a basis these insulating materials are polyolefins, namely crosslinked or uncrosslinked materials. In particular, find Polyethylene (PE) and cross-linked polyethylene (VPE) are used in the insulation according to the invention. Besides but can also ethylene copolymers, such as ethylene-propylene copolymers (EPR), ethylene-vinyl acetate copolymers (EVA) and ethylene-alkyl acrylate copolymers (for example Ethylene-ethyl acrylate and butyl acrylate copolymers), or ethylene-propylene-diene terpolymers and Mixtures (blends) of these ethylene copolymers and terpolymers with polyolefins, especially polyethylene and polypropylene, are used. The polymers mentioned As already mentioned, or polymer mixtures can be used both crosslinked and uncrosslinked. the Crosslinking is preferably carried out peroxidically or by high-energy rays. The insulating materials can optionally also be provided with oxidation stabilizers.

The proportion of additives is approximately between 0.5 and 5% by weight , based on the total weight of the electrical insulation. The additives are added to the insulating material. In the case of cables and lines, the additives can be added to the actual insulating layer as well as to the field-limiting layers, ie the inner and / or outer conductive layer. The additives can be present in one layer or in all layers. The proportion of additives is preferably about 1 to 3% by weight.

The invention is also intended to be based on exemplary embodiments

are explained in more detail.

To demonstrate the reduction in the formation of water trees, high-purity, unstabilized thermoplastic low-density polyethylene (LDPE) 3 were now made of thick sheets - with and without additives. To produce insulation according to the invention, the ECT-retarding additives were mixed homogeneously in various concentrations into part of the sheet material before pressing. The plate-shaped test specimens were then subjected to an electrical load of 10 kV / 50 Hz, both surfaces being in direct contact with a 3% sodium chloride solution heated ^{to 70 ° C.} The exposure time was 130 hours.

The test results show that the plate test specimens containing ECT-retarding additives - among the same Test conditions - compared to the test items without such additives, fewer or none at all Have ECT structures, for comparison, test specimens containing barbituric acid were also used (see on this: DE-OS 32 02 828). There is also a considerable difference in the size of the ECT structures. While the linear expansion of the ECT structures in the direction of the electric field in the Test items without additives up to 1500 μm, have the test pieces containing additives only have ECT structures with a length of up to 50 μm, if at all such structures exist.

In detail - with additions of 1 % - the following results, for example:

-S-

VPA 85 P 3 1

additive

maximum length of the ECT structures

relative number of ECT structures (based on comparative experiment)

- (comparative
attempt) 1500 μπι - Barbituric acid 50 μηι less 4,6-dihydroxy
pyrimidine 50 μπι less 6-aminouracil - no 4-amino-2-mercapto-
6-hydroxypyrimidine - no

It can thus be seen that the ECT formation is already at low levels Concentrations of retarding additives can be reduced considerably. This has the further Advantage that the electrical properties of the insulation are not or only insignificantly affected. The compounds isobarbituric acid, i.e. 5-hydroxyuracil, and isomeric to barbituric acid or 6-aminouracil In addition, 5-aminouracil show no ECT-retarding effect, i.e. they give the same results as the Comparative experiment.

2 claims

Claims (2)

I - VPA 85 P 31 6 claims "· \ 1. Electrical insulation based on polyolefin, in particular for cables and wires, for medium and high voltage from approx. 10 kV with an addition for retarding characterized by water trees that they contain at least one compound of the following structure as an additive: where the following applies:
R = H or alkyl, R, R = H, alkyl, cycloalkyl or aryl, and X, Y, Z = 0, S or NH,
with the proviso, that X is not 0 or S when Y and Z are each 0, and that X, Y and Z are not NH at the same time. 2. Electrical insulations according to claim 1, characterized in that the proportion of the additive 0.5 to 5 wt .-%, in particular 1 to 3 wt -.%, By weight based on the total weight. ORIGINAL INSPECTED