Development of Design and Manufacture of Cable Accessories (Part 2)

Material requirements and properties

Elasticity under electrical stress, corrosion resistance, and long-term stability are the most important properties required for materials used in the manufacture of cable accessories.

In the case of joints, due to the flexibility of silica gel and its inherent ability to generate and maintain permanent mechanical pressure, it can ensure a reliable electrical interface with epoxy resin accessories.

In this regard, the excellent electrical insulation properties of silicone elastomers, as well as their anti-aging and stable long-term elasticity, are rapidly making them the material of choice for many cable fittings. For example, in the case of joints, due to the flexibility of silicone resin and its inherent ability to maintain permanent mechanical pressure, a reliable electrical interface with epoxy resin fittings is ensured.

The latest generation of dry-type cable joints and terminals have a common feature that is, the insulating parts that include field control components. The "heart" of each dry fitting is usually made of silicone, the parts are pre-manufactured, and are usually tested in the manufacturer's facilities.

Professional grade silicone elastomers are increasingly meeting these types of requirements, and it is important to use the right grade for each application in terms of service performance and optimized manufacturing. For example, silicone with a relative dielectric constant of about 15 can be used to achieve prior art planar grading in the MV range. The conductivity grade is then used for field grading in HV applications.

Similarly, corrosion-resistant liquid silicone rubber (LSR) or high consistency rubber (HCR) with a hardness in the range of 25 to 40 Shore A has been found to be the ideal choice for cable terminations. In addition, their high elasticity makes it easy to assemble without the need for additional tools.

The most noteworthy benefit of silicone elastomers in outdoor cable applications (such as terminations (especially under contaminated conditions)) is their inherent hydrophobicity. The power industry has a good understanding of this characteristic, and compared with materials such as porcelain, compared with materials such as porcelain, allows more economical designs with shorter creepage distances and less maintenance Claim.

Manufacturing demand

Silicone elastomer joints are usually manufactured through various molding methods or through multilayer extrusion techniques.

At present, cable accessories suitable for all voltage levels and applications are increasingly based on silicone, with stress cone inner insulation with non-conductive silicone, and then conductive silicone. When it comes to cable termination, you can directly cover the external insulating material, or use a hollow insulator shell. In the past, these types of accessories were mainly made of RTV silicone. However, due to pressure to reduce costs and processing time, the use of LSR is now increasing. From a cost perspective and electrical and mechanical design, this material has shown good results. For example, the bonding between different silicones-conductive and non-conductive-can be achieved through the inherent properties of the material.

Different manufacturing techniques are used, usually depending on the voltage level of the accessory. For example, a horizontal mold parting line on a silicone stress cone at the MV level is generally accepted. In this case, typical manufacturing equipment includes a batching and mixing unit for two-component LSR, with a ratio of 1:1. The standard clamping machine (also used for MV epoxy parts) is nearby. The casing to be manufactured has a special manufacturing process, and the mold also has a special design. A special exhaust system and a closed-loop manufacturing and testing process can then be used to set up the production area.

The cable systems required for power facilities are moving towards higher voltages, and with this trend, the requirements have only increased from the point of view of material electrical properties and automated production processes. For example, a specific design of a cable termination with a stress cone and an external insulator requires a more specialized manufacturing setup, because the thickness of the silicone resin usually requires a long curing time. But in this case, a patented system called AVT was used, in which the two components of the silicone resin were preheated in a heat exchanger, which resulted in a 30% to 50% reduction in the curing cycle. Any unnecessary back grinding is also minimized, which also reduces quality control costs.

Electrical Cross Arm

Generally speaking, as the cable industry receives more and more cable joints and terminal orders at high voltages (eg, up to 500kV), optimizing the production of stress cones that may require up to 80 liters of silicone will become a growing demand. The use of RTV silica gel or even LSR in non-optimal processes can take hours, so the implementation of automated processes becomes increasingly important. Their common basic goal is to reduce the total cycle time by integrating each individual process step of the silicone body into a controlled manufacturing process, including mold preparation; preheating; filling the mold with LSR; controlling curing; cooling the mold To room temperature; open the mold; automatically extract the core from the silicone body; and clean the mold to prepare for the next cycle.

In order to achieve high product quality in high-volume HV cable accessories, optimizing process parameters will be crucial, which means controllable internal mold pressure (no back grinding) and very good material flow. In this way, the final accessory will provide the best mechanical and electrical performance as well as a perfect bond between the field control components and the insulating silicone.

Future outlook

At present, the application of overhead line fittings is becoming more and more demanding for the mechanical properties and the compression deformation of the materials used. At the same time, the voltage level is getting higher and higher, and the overall life cycle costs (including raw materials, production, on-site assembly, and maintenance costs) are under the lowest possible pressure. In this type of environment, ideally, one type of accessory should be suitable for a range of different cable cross-sections.

For the future, as the cable voltage level rises, the demand for further customized solutions for manufacturing large silicon-based accessories in the best way will continue to grow.