In this work we consider the overhead power transmission lines (OHL). Their specifics are related to the presence of cables (conductors) whose length between supporting towers may extend to dozens of thousand meters. The OHL components are exposed to a combination of natural actions – wind, rain, ice / snow / frost deposits. Compared to other structural parts, conductors have the highest flexibility and very low structural self-damping (of the order of 0.1 % of critical damping or lower, depending on frequencies). They are among structural elements the most sensitive to these actions.
Since early fifties the increased energy demand gave a rise to large construction of high-voltage and extra-high-voltage overhead lines equipped with bundled electrical conductors. For such arrangements there was noticed a kind of wind-induced oscillations originated by a zone of disturbed and retarded air flow (wake), that the cables located upwind(windward) cast onto the downwind (leeward) ones. The effect of this phenomenon called Wake-Induced Oscillations (WIO) resulted in fatigue damages of conductors, failures of insulator strings and cable suspension hardware and fatigue failures of spacers.
There have been identified analogues to transmission lines’ WIO in other regular structures subjectto the cross-flow of viscous fluids (air, gas, water etc.): heat exchanger tubes, clusters of fuel rods of nnuclear reactors, groups of chimneys, buildings. Early works in this field relate to the aerodynamics of tandem and staggered twin struts to support the wings of biplanes and published by Pannell, Griffits and Coales in 1915. Other cable structures like suspenders in suspension bridges or stays in cable-stayed bridges may be also subject to wake-induced oscillations. In each of these cases, conditions of oscillations’ occurrence and structural response depend on cable’s specific mass and stiffness, kind of fixation, dimension scale versus fluid viscosity and velocity (Reynolds number) etc. The cables’ separation plays important role, as there are different kinds of wake interference especially when the cables are closely spaced.
A number of research projects were entertained to study the wake-induced oscillations of different structures, which brought to development of analytical and experimental models and methods of protection against this phenomenon. A particular solution to overhead lines was found by unevenly distributing the spacers along the line span. To achieve that, no unique approach exists; virtually each grid company, or manufacturer of spacers proceeds with its own method. It may rely on different basis, either field experience or analytical study or a mixture of them. And, despite advances in numerical modelling of latest decades, few publications uncover phenomenological side of WIO.
The issues of modelling WIO in a view of helping to develop methods for protection of line against WIO are a main subject of this work.
Original advances studied in this thesis include:
- Current state-of-the art for analytical calculation of WIO, including the loads in the wake
- Overview of classic theory of wake-induced flutter and its evaluation from the standpoint of modern numerical tools for analytical applications (e.g., Matlab)
- Nonlinear Finite-Element Modelling of WIO using classic theory of wake-induced flutter,
study of its domains of application, advantages and limitations, including validation upon field experiments
- Foundation of basic methodology for optimal placement of spacers over the bundle conductor span