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Most of the newly constructed high-voltage cable projects in China use prefabricated cable accessories. There are many types of prefabricated cable terminals. The main types and their structural characteristics popular at home and abroad are reviewed and analyzed below. The internal insulation of the traditional prefabricated terminal uses a prefabricated stress cone to control the electric field, and the external insulation is a porcelain bushing (or epoxy resin bushing). Silicone oil or insulating oil such as polybutene and polyisobutylene is generally filled between the casing and the stress cone. When leaving the factory, the manufacturer provides rubber prefabricated stress cones, porcelain sleeves, insulating oil and other parts, which are assembled into terminals when installed on site.
Modern prefabricated terminals have three basic structures.
(1) The rubber prefabricated stress cone is mechanically expanded and then sleeved on the cable insulation:
The characteristic of this structure is that the stress cone is directly sheathed on the cable insulation, and the stress and electric strength on the interface between the stress cone and the cable insulation are maintained by the elasticity of the stress cone material itself.
110kV cross-linked cable prefabricated terminal structure diagram
The domestic outdoor terminal products shown in Figure 1 are typical of this structure. Its outer insulation is porcelain sleeve (generally epoxy resin sleeve is used for GIS terminal). The internal insulation is a synthetic rubber (silicone rubber or ethylene propylene rubber) pre-molded stress cone, and synthetic insulating oil is injected into the porcelain sleeve (or epoxy resin sleeve).
Obviously, this structure is simple. However, there are two technical issues of concern: 1) the compatibility of the synthetic rubber stress cone with the immersion oil; 2) under the action of high electric and thermal fields, the aging of the pre-molded rubber stress cone will cause the change of the interface pressure (Relaxation), thereby reducing electrical strength. The above two issues are actually a material issue. Appropriate materials can not only make the synthetic rubber compatible with the impregnating oil, but also ensure good aging properties. The long-term safe operation experience of a large number of products of the above-mentioned European and American cable manufacturers can prove this point.
(2) Using spring compression device:
The structural feature of this structure is to add a set of mechanical spring devices to the stress cone to keep the stress on the interface between the stress cone and the cable constant (as shown in Figure 2), and to cope with the action of high electric and thermal fields. The change (relaxation) of the interface pressure that may be caused by the aging of the rubber stress cone. This structure also has a very important feature. It can be seen from Figure 2 that its rubber stress cone is basically isolated from the immersion oil, thereby eliminating the possibility of swelling of the stress cone material.
110kV cross-linked cable prefabricated outdoor terminal structure diagram
The structure of adding a spring device to the stress cone shown in Figure 2 seems to be more comprehensive in design. However, the structure is more complicated, the requirements for manufacturing and on-site installation are increased, and the time for on-site installation is also increased.
(3) A non-rubber stress cone is used, which can not only provide reliable stress control but also avoid direct contact between the stress cone and the cable insulation in design:
The typical structure is a product designed by American G&W Company, and there are already many users in my country.
Schematic diagram of 138kV cross-linked cable outdoor terminal structure
Figure 3 shows the structure of the 138kV cross-linked cable outdoor terminal and GIS terminal of this structure. It has assembled the main components in the factory: porcelain bushing, stress cone (formed aluminum alloy sprayed with epoxy resin), top cover, chassis and oil pressure adjustment device, etc., and are filled with insulating oil. When installing, when the cable ends are ready, put the prefabricated terminals into the cable. From the point of view of use, this structure can allow the supporting cable to have larger diameter and eccentricity manufacturing tolerances.
Figure 3 Schematic diagram of outdoor terminal structure of 138kV cross-linked cable (stress cone cable insulation does not directly contact) 1—conductor lead rod 2—shielding cover 3—sealing ring 4—insulating oil compensation device 5—cable insulation 6—insulating oil 7—ceramic Sleeve 8—stress cone 9—sealing ring 10—support insulator 11—tail tube 12—epoxy resin sleeve 13—aluminum shell 14—valve 15—grounding ring.
The above three structures have their own strengths, have reached the level of practicality, and have relatively mature use experience:
1. The basic structure of GIS terminal and transformer terminal is similar to outdoor terminals of various companies. Because GIS operates in a fully enclosed environment, it can be protected from atmospheric conditions and pollution, and the good insulation characteristics of SF6 gas, so the external insulation of the GIS terminal uses epoxy resin casing, which is smaller in size than the outdoor terminal porcelain sleeve Much. The stress cone and insulating oil for internal insulation are similar to those of outdoor terminals;
2. The epoxy resin sleeve of the GIS cable terminal in Figure 1 and Figure 3 is filled with insulating oil, which is called wet (or oil-filled) GIS cable terminal. The GIS terminal in Figure 2 is not filled with insulating oil and is called a dry GIS cable terminal;
3. In order to standardize the specific size of the GIS cable terminal and the GIS switchgear and clarify the respective scope of supply of the cable manufacturer and the switch manufacturer, the International Electrotechnical Commission has formulated the IEC859 standard. Therefore, GIS cable terminals designed and manufactured in accordance with the IEC859 standard can be installed on standard GIS equipment manufactured by any manufacturer;
4. The earliest version of the IEC859 standard was promulgated in 1986. At that time, it was stipulated that the height Ls of the GIS cable terminal is the same in the cable warehouse regardless of the wet or dry type. For example, the Ls of 110kVGIS cable terminal is 757±1mm. After that, the standard was revised several times. The latest version IEC60859-1999 issued in 1999 clarified that GIS cable terminals are divided into wet and dry types. The Ls of 110kV wet GIS cable terminals is still 757±1mm. The Ls of the 110kV dry GIS cable terminal is 470±1mm;
5. The basic structure of the transformer terminal is very similar to that of the GIS terminal, but the permittivity (dielectric constant) of transformer oil and SF6 gas are 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 changing the shape of the high-voltage shielding cover of the transformer terminal bushing to adjust the electric field distribution to achieve the same structure as possible for the transformer terminal and 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 sleeves instead of porcelain sleeves as the external insulation of outdoor terminals. The composite casing is light in weight and has excellent explosion-proof properties, ensuring the safety of surrounding personnel and equipment. Therefore, its appearance has received widespread attention, especially when used in densely populated or equipment-intensive locations;
In the late 1990s, a new type of fully prefabricated dry synthetic insulated outdoor cable terminal came out. The new outdoor terminal integrates the stress cone, umbrella skirt and insulation layer into a whole 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 preform is sheathed on the cable insulation.
