Dynamic Ratings of Overhead Lines - FAQ

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

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. 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 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.

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.

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.

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.

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.

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. 

For comments or questions on this web site, please contact the PDC webmaster.
Copyright 2004 - Power Delivery Consultants, Inc.