Conference,
A NEW METHOD FOR DETERMINING THE STRUCTURAL RELIABILITY OF TRANSMISSION LINES
(1982)
Abstract
To increase the power transfer capacity of various
transmission lines operated by Ontario Hydro, an
extensive program of transmission line uprating has
been undertaken. In general, this work involves
replacing the existing conductors with new higher
strength conductors of larger diameter ahd greater
current carrying capacity. Towers are strengthened,
as required, and in some cases structures may be
raised and placed on new foundations. In other
cases, new bottom sections are attached to the
towers, with new foundations.
In view of the substantial changes to the lines
resulting from the uprating work, questions arose
with regard to the reliability, or security level,
of the reconductored lines relative to the original
lines. In addition, the question of reliability of
the reconductored lines took on additional significance
because of extensive damage which a line had
suffered due to abnormally high winds in 1978.
Conductor tensions had been increased on this line
just prior to the damage.
A review of available techniques indicated that
various methods for assessing the reliability of
buildings and bridges were available, but that these
methods needed significant modification to be applied
to a line-like structural system such as a transmission
line. Thus, Ontario Hydro decided to develop
an appropriate methodology which could be used to
assess the relative security, from a structural
point of view, of transmission lines before and
after reconductoring.
This paper outlines the steps in the methodology.
The methodology is based on probabilistic concepts
and includes two alternate methods for assessing
line reliability. The first method is called the
Probability-of-Failure Approach. In this approach
the mean values and variance of the structure member
strengths and member loads are determined from
available structural test data and meteorological
records. These statistical parameters are used to
compute the annual probability of failure for an
existing tower line having a specified length. The
calculations are then repeated for the uprated line and the results compared with those of the existing
line to assess the impact of uprating on the
reliability of the line.
The security level of the line, both before and after uprating, can be
expressed in terms of an annual or lifetime probability-
of-failure or, alternately, in terms of
a return period of failures.
The concept of "Safety Index" is introduced as
an alternate measure of structural reliability.
This approach is gaining acceptance in North
America for use in designing buildings, bridges,
and other significant structures. The method is
gaining acceptance because the accuracy of the
calculations in the Probability-of-Failure Approach
is highly dependent upon the shape of the
distributions assumed for member strength and
loads. Thus, it is suggested that for the shortterm
the "Safety Index" Approach provides a more
substantive measure of reliability than the
Probability-of-Failure Approach. However, as
additional years of meteorological data are added
to existing data bases and, as additional data is
added to existing strength data bases, the accuracy
of the Probability-of-Failure Approach should
improve significantly.
