DE2436413A1 - HIGH VOLTAGE CABLE - Google Patents

Description

The invention relates to high-voltage cables which, due to minimal dielectric losses, have an insulation of optimal dielectric strength and with simplified terminations and
Splice points can be provided. In order to reduce dielectric losses in high-voltage cables, it is desirable to make their insulation very thick. However, it is known that the dielectric strength of the insulation of such cables decreases with increasing
Insulation thickness. Two particular reasons are known for this drop in dielectric strength, namely contamination at the interfaces, if multilayered insulation is used. and the presence of holes that generally appear in the insulation. The impurities lead to electrical field strength concentrations and the holes to ionizations. Accordingly, both lead to electrical defects "" which have a negative effect on the dielectric strength of the insulation. '

In the manufacture of most high voltage solid dielectric cables the insulation is applied in an extrusion process. It is known that bubbles and air holes are formed in such a process are inevitable and lead to holes in the insulation. Many techniques have been tried to increase the number of holes to reduce. One of these techniques is to extrude the insulation in multiple layers, making most of the bubbles or air holes from each individual layer before or during it Drive out solidification. From a theoretical point of view, this technique leads to smaller and fewer holes than a construction, which consists of a single layer. Furthermore, this technique has no problems with irregularities in the structure resulting from sagging in the single layer construction. However, most of them have to Insulation materials are vulcanized in order to create a firm bond between the individual layers. The heat of the vulcanization process creates gaseous by-products, which become holes as inclusions, and thus the original effect destroy this technique. Furthermore, the treatment of insulating material, with or without vulcanization, creates surface contamination. Therefore, the previously known, multi-layer cable insulation leaves a lot to be desired.

Electrical breakdown problems occur at the terminations and splices on high voltage cables whose conductors are coaxial from a metallic shield is surrounded, and the insulation is located between the conductor and the shield. The breakthroughs occur because of an air gap or other aids with a lower dielectric strength than the insulation exist between the outer shield and the inner conductor at the terminations and splices. To solve the

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For breakthrough problems, stress relief cones are commonly used to reduce the stress gradient distribute between the conductor and the shield. This field relief funnel however, add up installation costs and have limited effect.

Therefore, the general object of the invention is at High-voltage cables by applying an insulation 'which reduces or eliminates the disadvantages of the known techniques, the dielectric strength increase, and the dielectric losses increase

to decrease.

A particular object of the invention is in high-voltage cables to increase the dielectric strength by applying insulation of optimal thickness and dielectric strength and to reduce dielectric losses.

In particular, it is the object of the invention in the case of coaxial high-voltage cables to increase the dielectric strength both in the radial and in the longitudinal direction in that a Insulation is attached, which reduces the voltage gradient between the Conductor and the shield at the terminations and splices more evenly distributed than could previously be achieved.

These objects are achieved in that, according to the invention, the cable insulation consists of at least three layers, with an insulator with a low specific capacitance (hereinafter abbreviated as SIC) in two or more layers and an insulator with high SIC as a breakage barrier for the electric field between the insulator layers with low SIC is used .. To achieve optimal dielectric strength To achieve isolation due to minimal dielectric losses, each layer will have high SIC over each layer kept thin with low SIC.

The invention will now be based on exemplary embodiments and using before. Schematic representations explained in more detailί

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In the figures show:

Figure 1 shows a cross section through a high voltage cable with a multilayer insulator structure according to the invention;

FIG. 2 shows a longitudinal section through a high-voltage cable according to the invention with simplified terminations attached thereto;

Figure 3 is a side view of a high voltage cable with comparative curves for the voltage gradients emerging from the cable and the improved electrical conditions illustrate that due to the multilayer insulator construction of the present invention at the terminations and splices to rule;

FIG. K shows a cross section through another high-voltage cable with a multilayer insulator structure according to the invention;

Figure 5 is a side view, partially in section, of one High voltage cable with a splice insert which is surrounded by a shield at a distance; and

FIG. 6 shows a cross section through a further high-voltage cable with a multilayer insulator structure according to the invention.

FIG. 1 illustrates a high-voltage cable do) with a multilayer insulator structure according to the invention. As with most high-voltage cables, the high-voltage cable (ίο) comprises an electrical conductor (12) and a shielding line (1 ¹ O) arranged coaxially therewith; however, the essence of the invention can also be used with high-voltage cables, which in addition to the insulation

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consist only of a conductor. An insulation (17), which is made up of several layers, is between the conductor (12) and the shielding line (i ^) arranged. The insulation (16) consists largely of an insulator with low SIC and is arranged in two layers (18). Between the layers (i8) however, a layer (2o) is arranged, which consists of an insulator with a high SIC. .

Of course, insulators have special electrical properties that clearly distinguish them from semiconductors, which are so often used in high-voltage cables. An isolator can pass through; a very high resistance at tree temperature (above 1o Ohra-cm), good dielectric strength (above 100 V / 2.5 ^ 1o ~ ^ cm (loo volts per rail)) and measurable values of the SIC; - on the other hand, a semiconductor is characterized by it characterized in that at room temperature it has a resistance below von.io ° ohm-cm, no dielectric strength and immeasurably large values of the SIC. Typical insulator materials for high-voltage cables consist of mixtures containing natural or synthetic rubber and thermoplastic polyolefins, for example polyethylene and mixtures containing polyethylene, or mixtures thereof. -

It is known to increase the SIC of insulators by adding appropriate amounts of titanium dioxide, carbon, or other materials in an appropriate manner. In the practical application of the invention , such modifying agents and their amounts must be carefully selected to ensure that the resistance at tree temperature does not drop so low that transition to semiconductor occurs. Titanium dioxide has been found to be a particularly useful modifying agent because significant amounts of it can be used to increase the SIC of the insulator without major adverse effects on its resistance at room temperature.

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Considerable dielectric losses in electrical transmission lines are due to their enormous length, especially when a conductor with a shield arranged coaxially to it in a high-voltage cable is used. Of course they are dielectric Losses directly proportional to the capacitance between the coaxial conductors. Because of this, such losses can either by reducing the relative dielectric constant or dielectric constant of that located between the coaxial conductors Isolator or by increasing their radial distance from each other. Because the dielectric strength across the insulation must be large enough to allow electrical breakthroughs To prevent between the conductors, the dielectric losses can then be kept to a minimum if the insulator has an optimal dielectric constant and radial thickness.

Although minimum dielectric losses appear to be just a matter of calculation, the radial thickness of an insulator is which are achieved in a conventional extrusion process may be limited for several reasons. Since the insulator is semi-fluid when using such a method, it only solidifies then if you let it harden for a while. Therefore, as the radial thickness of an insulator increases, the in one single extrusion step 'is produced, its tendency to sag or to flow into an irregular structure. Furthermore, in a semi-fluid isolator, which is created by an extrusion process produced, inevitably bubbles or air holes, and therefore by inclusion of such bubbles after solidification Holes are created in the insulation. The larger the radial thickness is made in a single extrusion stage, the larger is of course the number of holes included in it. There in

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Every hole ionization takes place, thereby the dielectric strength decreases of the isolator to a value that is below the calculated value. If the, insulator made of several layers which are manufactured in several extrusion stages, the number of holes in it decreases; -However, impurities inevitably appear on the outer surface of everyone Layer on, and there are electrical field strength concentrations on these impurities, which is why the dielectric strength the insulation decrease.

Even with vulcanized insulation, by-products are produced by the Curing originate and then become trapped present. These by-products must gradually be expelled to the outside will. The greater the wall thickness, the more difficult it is to drive off these by-products.

In general, the cable (iö) is used to transmit electrical Benefits used. It is connected in such a way that the current is led through the electrical conductor (.12) under high voltages and the shielding line (ΐΌ is grounded. The layers (18) and (2o) de insulation (16) are designed and arranged so that both the dielectric strength and the dielectric losses in the cable do) are optimal. This is because of it achieved since fewer holes exist in the layers (18) with lower SIC than would be the case if the optimal one Thickness would be achieved with the help of a single layer. Farther the layer (2o) with "the high SIC" forms a refraction barrier, the electrical field strength concentrations that occur due to surface contamination derive from the innermost layer (18), reduced. From the theory of the electric field it is known that the electric field strength and the dielectric displacement are rectified Vectors are which are broken at the interfaces of materials of different dielectric constant in such a way that

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the ratio of the tangent function of the angles between the normal to the interface and the lines of force equal to the ratio of the relative permittivities the materials is. It is also known that at the jumps, i.e. the discontinuous changes in the material properties, the normal of the dielectric displacement is continuous, i.e. has the same value on both sides.

The ratio between the dielectric displacement and the electric field strength corresponds to the SIC, ie D = L ·. E, where D is the dielectric displacement, £. the relative dielectric constant or dielectric constant and E is the electric field strength. Because a layer with a large SIC (capital t.) Is applied between the layers with a low SIC, the electric field strength in this layer is reduced in accordance with the proportions of the SICs. Because of this, the electric field strength concentration, which usually results from the surface impurities of the innermost layer (18), is decreased in proportion to the SIC ratio of the insulating materials; in order to achieve the best results, this ratio should be chosen as large as possible,

Due to the breakage barrier, the terminations and splice points of the cable do) are also simplified. According to FIG. 2, an end closure is produced in that the shielding line at the end of the cable do) and the insulation (i6) are removed from part of the electrical conductor (12). An approach (22) is firmly attached to the electrical conductor (12) and moisture seals (2 * +) and (26) are made ^{between the insulation (i6) and the shielding line (1 1 O or the approach (22), as} this is shown with dashed lines.

When using a Spleißeinsatzes, the two ends of the electrical conductor (12) conductively connects, as shown in Figure 5 at reference numeral 21, in the shield (1 ¹ O a fitting (15) is inserted, which surrounds at a distance the splice insert and a conductive connection in the shielding line (1 ¹ O establishes.

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The moisture seal (2 ^) is simplified compared to other moisture seals used, since no field strength relief funnel is used in it. Such funnels are usually required to distribute the voltage gradient between the electrical conductor (i2) and the shielding line (i4) at the end of the cable. They must be precisely shaped and placed within the moisture seal (24) so that they are complicated and expensive to use. The voltage gradient is distributed by the cable (1o) itself without the aid of stress relief funnels due to the effect of the refraction barrier for the vectors of the electric field strength and the dielectric displacement, as explained above. Of course, the resistance of a cable to breakthrough at its terminations and splices increases as the distribution of the voltage gradient improves. A typical distribution of the voltage gradient at an end closure of a high-voltage cable according to the invention is shown in FIG. 3, the distribution of the voltage gradient in conventional high-voltage cables being shown with dashed lines for comparison. Only two equipotential curves are shown, namely those with 5 % and 75 % of the line voltage. In the high-voltage cable according to the invention, however, the voltage gradient is also better distributed in every percentage range compared to conventional cables. If one extends this comparison to the use of field strength relief funnels at terminations and splice points, it follows that conventional high-voltage cables require such funnels at voltages above 5 KV, while the high-voltage cables according to the invention require such funnels only from 35 KV. Although the cable according to the invention prevents breakthroughs at the terminations and splice points even at higher conductor voltages, stress relief funnels can also be used to distribute the voltage gradient even more, and thereby reduce the susceptibility to breakouts even more for special purposes. - .. ".-;. .- \ ..

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The refraction barrier can also be inside the insulation (16) without taking into account the thickness, the layer (2o) or the thickness ratio the layers (18) and (2o) can be achieved. From the theory of the electric field it follows, however, that an insulation where the high and low SIC materials are equally thick, which spread over each material The voltage gradient is inversely proportional to the ratio of the STC values. That is why it spreads out as the difference increases in the SIC values the voltage gradient is less over the high SIC insulator material and more about the low SIC one. As already explained, the dielectric Losses in cables do) are kept to a minimum if the insulation (16) has an optimal thickness and dielectric strength. Therefore, the layers (18) should be much thicker than that Layer (2o) to distribute most of the stress gradient over the material with low SIC.

The practical feasibility of the manufacturing process must also always be considered and the limitations due to the material are considered. In known isolators, SIC values are that between 2 - 4.5, as low, and those between 1o - 5o are to be regarded as high. With these isolators and With the known extrusion process, thicknesses of up to 0.889 cm can be achieved (350 rails) with very few holes or structural irregularities can be achieved, while a thickness of about 0.116 cm Λο mils) is the thinnest producible thickness. Considering of the practical possibilities, the insulation (16) of a typical high-voltage cable do) according to the invention has an overall thickness about 1, ^ 224 cm (560 mils), with the layer (18) with the low SIC is about seven times as thick as the layer (2o) with a high SIC. Since the SIC values are relative values the ratio of the values of a high SIC to the values of a low SIC can be between about 2.5 and 25.

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Another advantage of the invention is electron capture or to see the energy absorption that takes place in the transition between, the thin layer (2o) with a high SIC and the thick layers (18) with a low SIC take place. This advantage further improves the dielectric strength of the invention Cables compared to conventional cables of the same overall dimensions.

Depending on the intended use, more than two layers made of a material with a low SIC can be used in a high-voltage cable according to the invention. Additional layers of a high SIC material can also be placed in a cable incorporating the conventional refractive barrier of the present invention. A cable (ίο ¹ ) illustrates these possibilities on the basis of FIG. K , whereby, because of the similarity to the cable do) according to FIG. 1 ', the same parts are provided with the same reference numbers plus a prime ('). The material with the low SIC is arranged in three layers (18 ¹ ), the layers (2ο ¹ ) made of materials with a high SIC each lying between adjacent layers (18 '). Additional layers (28) of materials with a high SIC are applied between the insulation (16 ¹ ) and each of the coaxial conductors (12 ') and (1 ⁴ P) used for common purposes. Furthermore, semiconductor materials can be used in high-voltage cables for common purposes and a high voltage cable with a refraction barrier according to the invention can also comprise semiconducting materials. In particular, according to FIG. 6, in which the same parts are provided with the same reference numbers as in the previous figures plus two lines (″), the layers that correspond to the layers (28) according to FIG. K consist of a semiconductor (30).

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The minimum dielectric is another particular advantage Losses in the high-voltage cable according to the invention, due to the refraction barrier for the electrical Field within the insulation and the resulting optimal thickness of the cable can be kept to a minimum, mentioned. Furthermore, the refractive barrier effectively distributes the stress gradient across the terminations and splices of the cable provided with coaxial conductors, so that field relief funnels are omitted in many uses can be.

PATENT APPEAL:

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Claims (1)

PATENT CLAIMS 1. J high-voltage cable with a metallic core of high electrical conductivity and an electrical insulation arranged circumferentially around the core, thereby geke η η-z eich η et that the insulation is made up of several layers, with several layers (i8; l8 '; 18 '') consist of 'a material with a low specific inductive capacitance, in each case between two adjacent layers (18; 18 ·; 18 ·') a layer (2ο; 2ό ';2o'') made of a material · with a high specific inductive Capacitance is arranged, the layers (18; 1-8 ';18'') have a sufficient total thickness to prevent a breakdown through them at an estimated voltage, each layer (2o; 2o ·; 2o' ·) a refraction barrier for the electric field emanating from the wire (12; 12 '; 12 *') forms every refraction barrier effectively keeps electric field strength concentrations between the layers (18; 18 '; 18 *') to a minimum and thereby leads to an optimal dielectric strength, and besides because the multi-layer structure keeps the dielectric losses in the high-voltage cable do; 1ο ¹ ; 1ο ¹ ') to a minimum » 2. High-voltage cable according to claim 1, characterized in that a metallic shielding line (i * f; 1V; / \ h ' ' ) of high electrical conductivity coaxially around the wire (12; 12 '; 12 *'), the insulation (i6; 16 '; 16 *') is arranged, and each refraction barrier effectively reduces the voltage gradient between the wire (12; 12 '; 12 · ·> and the shielding line (i4; 1 ^ f ·; 1Λ ") at the end caps and the splice points of the high-voltage cable (ίο; 1O ¹ JIo ¹¹ ), ' - tk. - 3- high voltage cable according to claim 2, using a End closure approach, characterized in that the shielding line (1 ^; 1V; 1V ') and the insulation (16; 16 '; 16' ') removed from part of the core (12; 12 *; 12' ') and the end closure approach (22) rait the wire is firmly connected, the shielding line (14; 1 V; 1V '), from the end closure approach (22) seen from a piece of the insulation (16; 16 '; 16 ·') is removed. k. High-voltage cable according to Claim 2, using a splice insert which electrically connects the ends of the wire, characterized in that in the shielding line (i * f; iV; 1V '). an electrically conductive fitting (15) is inserted which circumferentially surrounds the splice insert at a distance. 5. High-voltage cable according to claim 1, characterized in that the value of the specific inductive capacitance of the layers (i8; i8 ';18'') between 2 - - k, 5 and the value of the specific inductive capacitance of the layers (2o; 2o'; 2o '') is between To - 5o. 6. High-voltage cable according to one of claims 1-5, characterized in that the ratio of the layer thicknesses of the layer (i8; i8 '; 18 ¹ ') to the layer (2o; 2o ';2o'')' is at least equal to 5. 7 «high-voltage cable according to one of claims 1-6, characterized in that the layers (28) consist of a material with a high specific inductive capacitance and are located between the core (12 ¹ ) and the insulation (16) or the shielding line (IV) are arranged. - 15 - 409886/1107 8. High-voltage cable according to one of claims 1 - 6 characterized gekenn.zeichne t that layers (30) of semiconducting material between the core (i2 ^fl ) and the insulation (16 ¹¹ ) or the shielding line (1V ¹ ) are arranged. . ' PATENTANWXLTt OR.HMÖ.H.FINCKE, DIPL-ING.R · ΟΗ * O. B. STAESiR A09886 / 1107 Blank page