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 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
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 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.
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