Self-supporting, fluid-filled terminations have been used since the advent of HV cables and continue to be the first choice among most network operators due to their excellent long-term performance. Recently, composite housings have increasingly replaced porcelain in such applications and also come to represent the state-of-the-art for these components. The basic design of the latest generation of fluid-filled terminations consists of a hollow core composite housing and a silicone rubber stress cone installed onto the cable insulation. The remaining volume is usually filled with a silicone oil compound, one of the most environmentally friendly choice among alternative insulation media. The main advantages of this design are its ease of application across a broad range of different cable sizes and types as well as its adaptability to the service environment and mechanical requirements of any installation. Although these types of terminations are usually reliable and safe, isolated cases of electrical breakdown can never be totally excluded. The worst-case failure mode results from a high temperature internal power arc that causes rapid vaporization and thermal expansion of the insulating medium. Due to sudden internal over-pressurization, the termination could easily explode, scattering debris and threatening collateral damage to nearby network equipment while also presenting a safety threat to the public. Given this, cable accessory manufacturers and users have tried to reduce risks by minimizing the consequences of such an event. The result has been development of explosion resistant terminations that provide a cost-effective and reliable solution in applications where terminations are sited in sensitive locations. Design and testing activities linked to this development were reported in this INMR article from 2016, contributed by Brugg Kabel in Switzerland.
Internal Power Arcs on Outdoor Terminations
During a power arc inside a termination, the electrical discharge channel in the form of plasma leads to rapid transition of the insulating medium from liquid to vapor. The result is a sudden increase in pressure of the vapor phase, generating shock waves that stress the entire construction. If these dynamic stresses surpass the maximum mechanical strength of any of the termination’s components (i.e. its hollow insulator housing, top and bottom flanges or the bolts that fasten it to the support structure), mechanical breakdown can occur. Debris can then be ejected at high velocity all around the installation. There are two possible options to address this problem. The first is to design a termination able to withstand even the high pressures developed during an internal electrical breakdown. This solution, however, would require costly over-engineering that would greatly restrict its economics and field of application. A more practical option is achieved through controlled release of the increased internal pressure during the early stages of a power arc. This would limit the maximum overpressure and resulting projection of any debris. To achieve this result, however, the termination would have to be equipped with embedded pressure release devices able to operate within only a few milliseconds of ignition of the internal arc.
Design of Pressure Release Devices
The main challenge during the development phase of this second option was designing a reliable pressure release device. For example, one of the limiting conditions was that the device would have to be capable of releasing the pressurized fluid within only 2 milliseconds after arc ignition, while also offering the mechanical strength needed for prolonged, safe service at normal operating pressures. A way of best integrating such pressure release devices into existing designs of composite-housed terminations also had to be found. In the end, this was achieved by placing the devices at the top (i.e. integrated into the corona shield) as well as at the bottom of the termination (i.e. integrated into the base plate).
Although having different shapes, both devices are designed to act similarly upon a burst disc but with different dynamics. In order to minimize work devoted to experimentation with different concepts, which then took place only during final product development, the design phase relied heavily on finite element analysis. In order to also function as a corona shield, the pressure device at the top of the termination took the shape of a cap with 8 grooves that break at a predefined mechanical stress, allowing the cap to open like a tulip (see Fig. 2).
The pressure device placed at the bottom of the termination was integrated into the termination’s basic design and supported by pins fastened to its base plate, as shown in Fig. 3. Should a predefined pressure level be reached, the disc is expelled in an axial direction towards the bottom of the termination.