Dynamic analysis of lightly iced conductor galloping in two degrees of freedom

Abstract

Field observations have been repeatedly reported on the galloping of transmission lines having only a small acretion of ice of the order of 10% of the diameter on the windward side. Aerodynamic force measurements in wind tunnels reveal that significant lift forces are present, having a steep gradient with respect to angle of attack, and which satisfy the Den Hartog negative damping criteria. Drag forces are essentially constant, and aerodynamic moments are zero over the 3600 range of angle of attack. Similarly, no unbalanced inertia forces exist for such light-ice deposits, which precludes the dynamic inertia coupling between galloping and torsion modes. Blow back of the conductor is shown to explain such galloping owing to the angle of attack reaching the critical range, Within a finite range of wind speed. The addition of control devices, identified as the windamper, and the detuner, is shown to modify the distribution of angle of attack along the span, thus altering the angle of attack ‘exposure’ compared with that of an untreated span. Stability analyses in two degrees of freedom (galloping and dynamic twist) are performed for a number of representative cases. Stability is expressed as the root locus of a fourth-degree polynomial (quartic) equation, as a function of wind speed. Perforated cylinder dampers are also considered, and they are found to supply inadequate damping for control, unless the span coverage is unreasonably large. The twisting effect of the Windamper is shown to be its primary stabilising influence. The detuner appears to increase the range of instability in two out of three cases.