Q1 - Why do utilities place thermal limits (ratings) on transmission lines?
The power conductors of overhead lines are self-supporting and energized at
high voltage. As the current flow through these conductors increases, their
temperature increases, and they elongate. This elongation increases the sag
of the conductors between support points decreasing the clearance to people,
ground, other conductors, buildings, and vehicles under the line. Beyond a
certain "maximum allowable" sag, the lines may flashover, resulting in either
a power supply outage or injury to the public. Also, if the conductor temperature
remains high for an extended period of time, the strength of the conductors
and tensioned connectors may decrease, resulting in mechanical failure during
the next occurrence of ice or high wind loading. To avoid such excessive sag or
loss of strength, limits are usually placed on the power flow (MVA or amperes).
If such limits are based on worst-case weather conditions they are called static
ratings and, if based on actual weather conditions, dynamic ratings.
Q2 - What are the experienced benefits 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
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
Q3 - How is the temperature limit of a line determined?
Transmission conductors are typically stranded from aluminum wires with a steel core
added where increased strength is required. The temperature limit on all aluminum or
ACSR conductors is specified on the basis of maximum sag or maximum loss of strength
in the aluminum.
Temperature limits in use today range from 50C to 150C. The temperature limit,
corresponding to maximum sag, is normally selected at the time the line is designed.
The higher this temperature, the higher the thermal capacity of the line, the maximum
conductor sag, and the higher the structures required to maintain ground clearance.
Temperature limits above 95C may result in significant annealing of aluminum and a
total time duration at high temperature over the life of the line is normally specified
to limit the resulting loss of strength.
Q4 - How are thermal limits (ratings) calculated?
Thermal ratings are normally specified in amperes or MVA and can be directly compared
to the circuit electrical load in like units. To find the thermal rating, a heat balance
is performed, balancing the heat into the conductor due to Ohmic losses and solar heating
against the heat lost from the conductor by convection and radiation. The rating is
that electrical current for which the conductor temperature is equal to the maximum
temperature limit. When "worst case" weather conditions are used, the thermal limit
is called "static". When actual weather conditions are used, the thermal limit is called
Q5 - 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
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.
Q6 - 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. If 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 lengths.
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 steel content.
Q7 - How should I select "worst-case" weather conditions for a static thermal rating calculation?
Summer worst-case conditions with air temperature of 35 to 45C (depending on latitude), full solar
heating (approximately 1000 watts/m2) and a wind speed of 2 ft/sec perpendicular to the conductor,
are generally recognized as conservative. The use of less conservative weather conditions than these
based on field measurements should be approached with great caution. Adjustments to the assumed
ambient temperature by season are relatively easy to prove and are widely used to generate higher
ratings for winter.
Q8 - 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.
Q9 - How many monitors should be used in order to calculate the dynamic thermal rating
of an overhead line?
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.
Q10 - How much difference is there between the various heat balance methods for overhead
There are three well known methods - IEEE, CIGRE, and EPRI's DYNAMP. The difference in
radiation is little to none, but the difference in convection is up to 5%. Generally,
the three methods calculate conductor temperature and ratings that are quite close.
The maximum difference in calculated temperature rise above ambient is 10% and the maximum
difference in thermal rating is 5% given the same input parameters.
Q11 - Several methods for determining dynamic line ratings are available, which is the best?
Weather-based, Temperature-based and Tension-based methods are all based on performance
of a heat balance. Regression methods rely on extensive field measurements.
The most common dynamic rating method is using one of the heat balance methods with real-time
weather and line current data to calculate weather-based ratings. This method is best used
when the normal peak current density on the energized conductors does not exceed 1 amp/mm2,
when the line section being monitored is less than 2 km, and when the conductor has little
or no steel reinforcement.
Tension-based ratings are extremely accurate in establishing rating for current densities
in excess of 1 amp/mm2, with line ruling span sections longer than 2 km, and with conductors
having high steel content where thermal limit is determined by sag clearance.
Temperature-based ratings are accurate for current densities in excess of 1 amp/mm2, with line
ruling span sections less than 2 km, with conductors having little or no steel content, for lines
whose thermal limit is determined by loss-of-strength concerns.
Regression-based ratings are not in common use. A regression equation is found relating the
calculated rating (however it is calculated) to the measured parameters (weather, tension, sag,
Q12 - How are thermal line ratings dependent on line length, terrain, and orientation?
Traditionally, line length is not considered in specifying "static" thermal ratings. Field data
for dynamic rating studies indicates that the thermal rating decreases with increasing line length,
with the range of line directions, and with foliage or terrain shielding.
Q13 - 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 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.
Q14 - Why are tension and temperature monitors ineffective in determining the dynamic rating
for lightly loaded lines?
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 loaded lines.
Q15 - How should weather-based ratings be adjusted to account for line length and changes
Wind speed varies with distance. This is particularly true for low wind speeds. The
persistence of wind direction decreases with wind speed at any location. The solar
heating of the conductor and the air temperature vary much less than the wind except
between very sheltered and unsheltered areas. The variation in wind speed, solar
heating, and air temperature along the line can be incorporated by using multiple
monitoring locations, perhaps using an interval of 1 to 2 miles in critical line
sections. The variation in wind direction occurs at each monitoring location and
can only be incorporated by assuming a conservative near-parallel wind angle,
especially at low wind speeds.
Q16 - 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. 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.
In the approaches that involve measured tension under load, the current density in the
energized conductor must be at least 0.7 amp/mm2.
Q17 - How should risk be measured for overhead lines at high temperature?
Risk in lines limited by cumulative loss of strength is small. The use of typical weather
patterns is reasonable since momentary differences between different physical locations
average out. Risk in lines limited by sag clearance is a serious concern since momentarily
unfavorable conditions at a location translate into a problem (possibly involving public safety)
immediately. The risk is zero in lightly loaded lines except post-contingency.
Q18- What is the "ruling span" temperature?
Transmission lines are built in line sections, terminated by at each end by strain
structures. Within the line section the conductor is supported by flexible suspension
points. This method of construction yields tension equalization between "suspension"
spans under all loading conditions and the tension of all the spans varies with
temperature and load changes as though it were a single fictitious "ruling" span.
Since the conductor at all points along the line section experiences the same current
but the wind varies in direction and speed (particularly for longer sections), the
ruling span tension varies as a function of the temperature in each of the suspension
spans. The effective "ruling span" temperature is approximately equal to the average
temperature of conductor in all the spans.