Seismic evaluation and protection of high voltage disconnect switches

A disconnect switch used in sub-stations of large electrical grids is an important part that supports the functionality of the electrical network. Its failure may have consequences to the local network and to the entire grid. Such a switch is a combination of a support structure and the switching mechanism, which in turn is made of fixed and moving metal parts and of ceramic, or composite, electrical insulators. The construction includes therefore a combination of highly nonlinear components some with low and some with high stiffness, some extremely brittle and some ductile. In order to find the static and dynamic characteristics of these high voltage three-phases disconnect switches and their capacity to withstand earthquakes, testing was performed on different types of switching systems. The testing was performed using the standard IEEE693 procedures (with some modifications) for (i) a single post insulator; (ii) a sub-assembly of single insulator connected to the switch system; (iii) a sub-assembly of a single phase of the whole switch and (iv) a full assembly of the disconnect switch system including the support structure. It was found that the switch base, i.e. the structure which supports the mechanism and post insulators for each phase, has a large influence on the fundamental frequencies of post insulators assemblies. Changing the stiffness of the "switch base" could be a constructive way to alter the fundamental frequency of the system to avoid the frequency range in the input motion which has damage potential. Moreover, the switch base connection was found to be much more flexible than the post insulators and their connections, such that it caused smaller forces at base and larger displacements at the top of these insulators. In order to check the possibility to protect the post insulators mounted in the switch assembly, the entire systems was isolated with self-centering sliding isolators of the type of (i) Friction Pendulum System and (ii) wire rope isolators. Base isolation reduced the induced stresses in the post insulators by 50 to 90. Therefore, such isolation could be a good retrofit solution for existing overstresses disconnect-switches. However, this recommendation must be further checked for connector compatibility due to increased lateral movements of the entire switch structure.