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The paper examines the phenomenon of steady-state motion for a string traveling with constant velocity along an invariant curve under gravity in a viscous medium. This technically important phenomenon has been known in the literature for about 120 years and may be applied in high-speed turbines, the textile industry, etc. The conditions for the phenomenon’s existence are found. Concepts of two critical string velocities as well as sub, super, and hypercritical domains are introduced. The analytical solutions for the nonlinear differential equations and arbitrary constants for the general boundary conditions are found. The theoretical results are very close to the experimental ones.

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Short-circuit current sets the design loads for substation structure with flexible buses for voltage up to 115 kV and sometimes for higher voltage. Nonrecoverable force safety device (FSD) being included in flexible buses can restrict the transmission of great unwanted forces to substation portals. The device operates similar to a safety fuse of any electrical device while being replaced when tripped.The paper deals with theoretical investigation of FSD usage efficiency. Two mathematical models (one is to verify another) are built: The first model is based on the finite difference equations and the second one uses ANSYS finite elements program. The finite difference equations are derived by linearization of nonlinear equations of buses motion. Both models include bus conductor, insulators, FSD, portals masses, and portals stiffness. Results of actual calculations that show the efficiency of FSD are presented.