ON THE EFFICACY OF MECHANICAL DAMPING AND FREQUENCY DETUNING IN ALLEVIATING WAKE-INDUCED FLUTTER OF OVERHEAD POWER CONDUCTORS

A nonlinear, quasi-steady analysis of the wake-induced instability of a leeward conductor in the wake of a second identical windward conductor has been developed. In addition to the nonlinear analysis, the aerodynamic forces acting on the conductors are linearized, and a linear form of the equations of motion formulated. Both the linear and nonlinear analyses account for motion of the windward conductor. Although the quasi-steady assumption is employed, the time delay between flow leaving the windward conductor and influencing the aerodynamic forces on the leeward conductor is accounted for, as is the retardation of wake flow approaching the leeward conductor. Accounting for the above "unsteady effects" results in considerable differences in the equations of motion vis-à-vis the fixed windward conductor analysis; in particular, they change significantly the stability of the two conductors and induce a new form of instability-a negative damping instability. This new instability mechanism results in some configurations of the two conductors, which the fixed windward conductor analysis predicts to be stable, being unstable. Using both the linear and nonlinear analyses, the individual effects of mechanical damping and frequency detuning are investigated, the particular areas of interest being the threshold wind speed at which instability will occur and the limit cycle amplitudes once instability has occurred. In general, increasing mechanical damping, in both the in-plane and out-of-plane modes, is the most effective way of enhancing the stability of the system; it can, in some circumstances, raise the threshold velocity necessary to cause instability, and reduce the limit cycle amplitude of an unstable system. For the frequency coalescence instability mechanism, detuning the modes can be beneficial, but there are specific combinations of modes for which frequency detuning has little effect on either the minimum wind speed for instability to occur, or on the limit cycle amplitudes. Thus, for transmission line bundles, in which a multitude of modes may coalesce, it is unlikely that frequency detuning will be particularly beneficial. Also, not surprisingly, frequency detuning has almost no effect on the negative damping instabilities. © 1990 Academic Press Limited.