Most of the newly constructed high-voltage cable projects in China use prefabricated cable accessories. There are many types of prefabricated cable terminals, and the following is an analysis of the main types and structural characteristics popular at home and abroad. The inner insulation of conventional prefabricated terminals uses a prefabricated stress cone to control the electric field, and the outer insulation is a porcelain sleeve (or epoxy casing). The casing and the stress cone are generally filled with silicone oil or insulating oil such as polybutylene and polyisobutylene. When leaving the factory, the manufacturer provides rubber prefabricated stress cones, porcelain sleeves, insulating oils and other parts, which are then assembled into terminals during on-site installation. Modern prefabricated terminals have three basic structures.
(1) Put the rubber precast stress cone on the cable insulation after mechanical expansion:
This structure is characterized by the stress cone directly sheathed on the cable insulation, relying on the elasticity of the stress cone material itself to maintain the stress and electrical strength at the interface between the stress cone and the cable insulation.
Schematic diagram of the prefabricated termination structure of 110kV cross-linked cable (stress cone is mechanically expanded and sheathed on the insulation of the cable) .jpg
Schematic diagram of the prefabricated terminal structure of 110kV cross-linked cable
The domestic outdoor end product shown in Figure 1 is typical of this structure. Its outer insulation is a porcelain sleeve (GIS terminals generally use epoxy casing). The inner insulation is a synthetic rubber (silicone rubber or ethylene propylene rubber) pre-molded stress cone with a porcelain sleeve (or epoxy casing) injected with synthetic insulating oil.
Obviously, this structure is simple. However, there are two technical concerns about it: 1) the compatibility of synthetic rubber stress cone with impregnating oil; 2) Under the action of high electric and thermal fields, the aging of the pre-molded rubber stress cone will cause changes in interfacial pressure (relaxation), thereby reducing the electrical strength. The above two problems are actually a material problem. The right material can make synthetic rubber compatible with impregnated oils and ensure good aging properties. This can be proved by the long-term safe operation experience of a large number of products of cable manufacturers in the above-mentioned European and American countries.
(2) Spring pressing device is adopted:
The structural feature of this structure is to add a mechanical spring device to the stress cone to keep the stress at the interface between the stress cone and the cable constant (as shown in Figure 2), supplemented by the change in interface pressure (relaxation) that may be caused by the aging of the rubber stress cone under the action of high electric and thermal fields. Another important feature of this structure is that the rubber stress cone is basically isolated from the impregnating oil from Figure 2, thus eliminating the possibility of the stress cone material melting.
110kV cross-linked cable prefabricated outdoor terminal structure schematic (spring compression device) .jpg
Schematic diagram of 110kV cross-linked cable prefabricated outdoor terminal structure
The structure shown in Figure 2 with a spring device on the stress cone appears to be more thoughtful in design. However, the structure is complex, the requirements for both manufacturing and on-site installation have increased, and the time for on-site installation has also increased.
(3) A non-rubber stress cone is adopted, which can provide reliable stress control and avoid direct contact between the stress cone and cable insulation in design:
The typical structure is a product designed by the American G&W company, which has many users in China.
High voltage cross-linked cable accessories outdoor terminal, GIS terminal .jpg
Schematic diagram of outdoor terminal structure of 138kV cross-linked cable
Figure 3 shows a schematic representation of the structure of a 138kV crosslinked cable outdoor terminal and GIS terminal of this structure. It has assembled the main components in the factory: porcelain sleeve, stress cone (molded aluminum alloy sputtered epoxy), top cover, chassis and oil pressure adjustment device, etc., and filled with insulating oil. For installation, when the cable end is ready, the prefabricated terminal can be put into the cable. From a usage point of view, this construction allows for large diameter and eccentricity manufacturing tolerances for mateing cables.
Figure 3 Schematic diagram of the outdoor terminal structure of 138kV cross-linked cable (stress cone cable insulation does not directly contact) 1 - conductor lead rod 2 - shielding case 3 - sealing ring 4 - insulating oil compensation device 5 - cable insulation 6 - insulating oil 7 - porcelain sleeve 8 - stress cone 9 - sealing ring 10 - support insulator 11 - tailpipe 12 - epoxy casing 13 - aluminum shell 14 - valve 15 - grounding ring.
The above three structures have their own advantages, all have reached the practical level, and they have relatively mature use experience:
1. The basic structure of GIS terminal and transformer terminal is similar to the outdoor terminal of each company. Since GIS operates in a fully enclosed environment, it can be protected from atmospheric conditions and pollution, coupled with the good insulation characteristics of SF6 gas, the outer insulation of GIS terminal adopts epoxy resin sleeve, which is much smaller in size than the outdoor terminal porcelain sleeve. Its internal insulation with stress cone and insulating oil are similar to outdoor terminals;
2. The epoxy resin sleeve of the GIS cable termination in Figs. 1 and Figure 3 is filled with insulating oil, which is called wet (or oil-filled) GIS cable termination. In the GIS terminal in Figure 2, no insulating oil is filled, which is called dry GIS cable termination;
3. In order to standardize the specific matching size of GIS cable terminals and GIS switchgear and clarify the respective supply scope of cable manufacturers and switch manufacturers, the International Electrotechnical Commission has formulated IEC859 standards. Therefore, GIS cable terminals designed and manufactured in accordance with IEC859 standards can be installed on standard GIS equipment manufactured by any manufacturer;
4. The earliest version of the IEC859 standard was issued in 1986, when it was stipulated that the GIS cable termination, regardless of wet or dry, was the same height Ls in the cable compartment. For example, Ls=757±1mm for 110kVGIS cable termination. Since then, the standard has been revised several times, and the latest version of IEC60859-1999 issued in 1999 clarifies that GIS cable terminations are divided into wet and dry types, and the Ls of 110kV wet GIS cable termination is still 757±1mm. The Ls of the 110kV dry GIS cable termination is 470±1mm;
5. The basic structure of the transformer terminal is very similar to the basic structure of the GIS terminal, but the permittivity (permittivity) of transformer oil and SF6 gas is different, so the electric field distribution of the entire terminal is not exactly the same. In addition, the breakdown strength of transformer oil is also lower than that of SF6 gas. In fact, most manufacturers use to change the shape of the transformer terminal bushing high-voltage shielding cover to adjust the electric field distribution to achieve the same structure as possible between the transformer terminal and the GIS terminal;
6. With the successful use of silicone rubber in the field of electrical insulation, people began to expand the application of silicone rubber to the field of external insulation of cable terminals. First of all, people use silicone rubber composite sleeve instead of porcelain sleeve as the outer insulation of outdoor terminals. The composite casing is light in weight and has excellent explosion-proof properties, which ensures the safety of surrounding personnel and equipment. Therefore, its emergence has received general attention, especially in densely populated or equipped locations;
At the end of the 90s, a new type of fully prefabricated dry-type synthetic insulated outdoor cable terminal came out. The new outdoor terminal combines a stress cone, an umbrella skirt and an insulating layer into a monolithic prefabricated part. This structure greatly simplifies the installation process of the terminal, that is, after the cable is usually processed and the terminal rod is crimped, the entire terminal precast is put into the insulation of the cable.
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