Overhead transmission lines play an important role in the operation of a
reliable electrical power system. Transmission Line (TL) towers are vital components of
the lines, and accurate prediction of their failure is very important for the reliability
and safety of the transmission system. When failure occurs, direct and indirect losses
are high, leaving aside other costs associated with power disruption and litigation.
The tower can fail due to the failure of any part or as a consequence of
foundation failure. In a number of cases, the tower failure is due to cyclonic storms.
Tower failure during construction is one of the common phenomena. The tower body
can also fail due to excess tension either during the stringing operation or due to
increase in the tension of conductor or earth wire. Stringing accidents also lead to
tower failure. There are ways and means to minimize the damage of the tower and
bring down the period of nonavailability of transmission line to a greater extent.
The question that arises is whether the TL tower can be made failure-proof or
not. The answer is definitely `No'. This is because of the fact that every utility has
to strike a balance between the economy and reliability. Thus, within a given circle
of reliability, it is possible to optimize the design. Stringent tests on the towers
increase the reliability and also help in optimizing, but there should be a strong will on
the part of the contracting agencies.
A TL tower is a highly indeterminate space structure. In the current analysis, a
tower is modeled as a space truss, all its members are assumed to be axially loaded
and pin connected at joints. In practice, such assumption can hardly be met. The
joints in transmission tower are not hinge joints, and the main members, such as
legs, usually retain their continuities at joint which may cause bending moment,
torque and shear in member, thus producing additional stresses not accounted for in
the space truss analysis. EPRI (1986) compared data from full-scale tests with
predicted results using current techniques and concluded that the behavior of
transmission towers, under complex loading condition, cannot be consistently predicted
using the present techniques. They found that almost 25% of the towers tested
failed below the design loads and often at unexpected locations. Furthermore,
available test data showed considerable discrepancies between member forces computed
from linear elastic truss analysis and the measured values from full-scale tests. It
becomes important to predict the actual strength and failure mechanism of such towers
with reasonable accuracy for failure scenarios in both static and dynamic regimes.
Post-elastic static and nonlinear static analysis of lattice structures may serve to
simulate failure scenarios in the existing overhead lines
or develop new designs of towers. |
