Transmission lines are extremely flexible structures, which can suffer from galloping under extreme environmental conditions. A better understanding of the phenomena is therefore necessary to predict occurrence and extent of this phenomenon. In this study, an aero-elastic experiment has been performed on sectional model of four-conductor transmission line considering different structural configurations.
The experiment is reproduced numerically using a nonlinear FEM code, in which aerodynamic force is estimated using quasi-steady and unsteady force model. Dependence of galloping on structural configuration and efficiency of aerodynamic force prediction models is investigated in light of experiment and simulation results.
Transmission lines that vary from supply to local feeders to countrywide supply to remote areas are essential part of every country’s development. These are extremely flexible structures and suffer from various types of structural instabilities. The basic concern of this research is “Galloping”, which refers to large amplitude oscillation in direction perpendicular to the applied load.
During winter, ice accretion takes place on transmission lines, changing their shape, as shown in Figure 1. The modified shape develops aerodynamic lift and rotational moment, which can result in negative aerodynamic damping and lead to galloping. To investigate the galloping phenomena, wind tunnel tests are usually carried out to determine the aerodynamic coefficients as a function of angle of attack.
The well-known Den-Hartog criterion is employed considering aerodynamic coefficients to evaluate the possibility of galloping occurrence and critical velocity of galloping for a given cable shape.
In most of the previous works galloping of transmission lines has been regarded as a quasisteady problem. In the last decade, some innovative studies have been carried out to determine influence of conductor motion and conductor wake on the aerodynamic characteristics of transmission lines considering single and 4-conductor bundle ice-accreted cable model (Kimura et al. 1999, Shimizu et al. 2004 and Phuc et. al. 2004).
These references put forth a model for representation of unsteady aerodynamic forces considering angle of attack and rotational velocity of cable to take into account the effect of cable motion. The results show that quasi-steady model provide much different results as compared to unsteady model especially for aerodynamic moment (Phuc et al. 2004) as is evident in Figure 2. The figure shows time history of aerodynamic moment on a cable rotating at amplitude of ±55° with respect to steady wind.
The unsteady model show very close agreement, whereas the quasi-steady model shows much different results, especially close to 0°.Shimizu and Sato (2001) have used simulation considering quasi-steady model and compared with field observations, some underestimations were observed in the results.
The reason of this difference may be because of the difference between the quasi-steady and unsteady aerodynamic coefficients (from Phuc et al. and Shimizu et al.), which is more distinguished for themoment coefficient.
This paper aims to develop a better understanding of galloping phenomena through an aeroelastic experiment. It investigates performance of the aerodynamic force models (quasi-steady and unsteady model) through a fully nonlinear FEM code considering the model for the aeroelastic experiment. Simulation results are used to explain the galloping phenomena.