Q1 - What are the benefits experienced from the practical applications of dynamic
ratings of overhead transmission lines?
Dynamically rating equipment is important, since usually more load can be transferred.
This is important considering open access of transmission lines, uncertain load growth,
economic energy transfers, permitting new lines, expense of new lines, and maintaining high
reliability. Without measuring actual weather, equipment must be severely under-rated to
account for the occurrence of worst-case weather. By dynamically rating, the operator
knows the true circuit rating and can therefore make smart decisions when contingencies
arise or when there are pending economic energy transfers
Q2 - What is the difference between knowing the thermal rating of a line and knowing the
temperature of it's energized conductors?
The temperature/sag/tension of an energized conductor can be measured. The line rating cannot
be measured. It is calculated based on assumed or measured weather and line parameter data.
Comparing the conductor temperature to the temperature limit tells the operator whether the
line is operate the line but gives no guidance concerning what maximum current is allowed.
The line's thermal rating tells the operator directly what the maximum allowable current is.
Certain sophisticated procedures can also yield predictions of thermal ratings to warn the
operator of future problems.
Q3 - If I know the conductor temperature can I calculate tension and sag?
Yes and no. A classic "sag-tension" calculation such as the Alcoa SAG10 program allows the
calculation of "initial" and "final" sag-tensions as a function of conductor temperature for
a "ruling" span. There are several sources of uncertainty, however:
The calculated difference in initial and final sag-tension for a certain temperature
is result of certain assumptions about the conductor's plastic elongation as a function
of time since installation and previous occurrences of heavy ice and wind loading. This
error can be eliminated if the line's sag or tension is measured to establish the
tension-temperature at several temperatures with the line out of service.
The "ruling span" approximation depends on tension equalization at support points. In
cases where the suspension span lengths vary widely, tension equalization may not occur
and the predicted variation in sag-tension with temperature may be in error. This error
is negligible for lines in reasonably level terrain with reasonably equal suspension span
The thermal elongation of ACSR conductors with high steel content may differ from that
assumed in the program. This error can only be eliminated by measurement of sag-tension
under high current loads. This is primarily a problem with ACSR conductors having a high
Q4 - How should wind speeds be measured for line rating purposes?
For static or dynamic rating purposes, wind speeds need to be measured in the vicinity of the
line being rated and the anemometers need to be accurate at wind speeds below 2 m/sec. Bearing
friction needs to be considered in cup type and propeller type anemometers. Averaging periods
of 5 to 15 minutes are appropriate in most cases. The measurement height for wind should be
approximately the same as the low point of the line conductors. Placement of anemometers should
be considered relative to sheltered areas along the line, perhaps by reducing measured wind speeds
to account for sheltering.
Q5 - How many monitors should be used in order to calculate the dynamic thermal rating of an overhead
Weather conditions vary with time and distance. Air temperature and solar heating are reasonably
consistent spatially and according to time of day. Wind is highly variable with both distance
and time. For sag-based ratings, it is sufficient to monitor at one location per ruling span. For
several tandem ruling spans in the same terrain and having the same direction, monitoring of
one span may be sufficient.
Q6 - Several methods for determining dynamic line ratings are available, which is the best?
Weather-based, temperature-based and sag or tension-based rating methods are all based on similar
heat balance concepts. Since the rating of most lines is determined by sag clearance, the monitoring
of sag or tension is the most accurate method. Measurement of conductor temperature or of wind speeds
along the line requires an assumed relationship between conductor temperature and sag and is thus
inherently less accurate than the direct measurement of sag or tension.
Q7 - Is there a critical span where one should place real-time monitors?
Generally, the answer is no if thermal limits are intended to limit sag and yes if intended to limit
cumulative loss-of-strength. If sag in a certain section of line consistently determines the thermal
limit, then the sag clearance in that section should be increased by conventional mechanical upgrade
methods. If the conductor runs consistently hotter in a section of line, then monitors should be placed
in that section to avoid excessive loss of strength, there is little that can be done to expose the
critical section to higher winds.
Q8 - Why are tension and temperature monitors ineffective in determining the dynamic rating for lightly
The rise in conductor temperature due to current is proportional to the square of current. When the
current is less than 35% of the static rating, the temperature rise is less than 10% of that associated
with full load. Since the line rating is calculated based on the measured equivalent temperature rise,
and since errors in estimating equivalent conductor temperature rise from tension and solar temperature
measurements are typically several degrees C, large errors in rating are likely to result from lightly
Q9 - How can line tension be related to equivalent conductor temperature?
This is an artistic process, which requires several different simultaneous processes to reduce errors
to a minimum. For the CAT-1 monitoring system, these processes are:
Measure tension and solar temperature with the line out of service for an extended period of
time (at least 24 hours) during the coolest and hottest times of the year.
Measure tension and solar temperature with the line in service over an extended period of time
(at least one month preferably at the coolest and hottest times of the year).
Calculate sag-tension-temperature values using a program like SAG10 for the calculated ruling
span -or- use a program like SAG-SEC to estimate the temperature-tension-sag relationships,
which account for suspension point movement.