Overhead Lines - FAQ

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.

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.

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.

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.

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.

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.

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, temperature, load).

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.

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.

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.

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.

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.

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.

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. 

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