MANUAL TOWER SPOTTING OF TRANSMISSION LINES DESIGN BASIC AND TUTORIALS

A celluloid template, shaped to the form of the suspended conductor, is used to scale the distance from the conductor to the ground and to adjust structure locations and heights to (1) provide proper clearance to the ground; (2) equalize spans; and (3) grade the line.

The template is cut as a parabola on the maximum sag (usually at 49#C) of the ruling span and should be extended by computing the sag as proportional to the square of the span for spans both shorter and longer than the ruling span.

By extending the template to a span of several thousand feet, clearances may be scaled on steep hillsides. The form of the template is based on the fact that, at the time when the conductor is erected, the horizontal tensions must be equal in all spans of every length, both level and inclined, if the insulators hang plumb.

This is still very nearly true at the maximum temperature. The template, therefore, must be cut to a catenary or, approximately, a parabola. The parabola is accurate to within about one-half of 1% for sags up to 5% of the span, which is well within the necessary refinement.

Since vertical ground clearances are being established, the 49 deg C no-wind curve is used in the template. Special conditions may call for clearance checks. For example, if it is known that a line will have high temperature rise because of load current, conductor clearance should be checked for the estimated maximum conductor temperature.

One crossing over a navigable stream was designed for 88 deg C at high water. Ice and wet snow many times cause weights several times that of the 1/2-in radial ice loading, and conductors have been known to sag to within reach of the ground.

Such occurrences are not normally considered in line design, and when they occur, the line is taken out of service until the ice or snow drops. Checks made afterward have nearly always shown no permanent deformation.

The template must be used subject to a “creep” correction for aluminum conductors. Creep is a nonelastic conductor stretch which continues for the life of the line, with the rate of elongation decreasing with time.

For example, the creep elongation during the first 6 months is equal to that of the next 91/2 years. All conductors of all materials are subject to creep, but to date only aluminum conductors have had intensive study. Creep is not substantial in other conductors, but the conductor manufacturers should be consulted.

The IEEE Committee Report, “Limitations on Stringing and Sagging Conductors,” in the December 1964 Transactions of the IEEE Power Group discusses creep, and the reader should examine that report.

Creep causes a continuous slow increase in the sag of the line which must be estimated and allowed for. The aluminum-conductor manufacturers will furnish creep-estimating curves, and most sag-tension computer programs now available are capable of calculating sags with and without creep.

These curves are at approximately constant temperatures, around 15.5 to 21 deg C, and plot stress against elongation, one curve for each period of time, 1 h, 1 day, 1 month, 1 year, 10 years, etc. The values are integrated values for the period and are considered to be reasonable estimates.

The temperature used is a reasonable average of the year’s temperature across the center of the United States. Precise values for creep are impossible to determine, since they vary with both temperature and tension, which are continuously varying during the life of the line.

From Fig. 3 of the committee report in Ref. 53, it is found that a 1000-ft span of 954,000-cmil 48/7 ACSR when subjected to a constant tension of approximately 18% of its ultimate strength at a temperature of 15.5 deg C will have a sag increase in 1 day of approximately 5.5 in; in 10 days, 13 in; in 1 year, 27 in; in 10 years, 44 in; and in 30 years, 52 in.

Unless it is known that the line will have a life of less than 10 years, not less than 10 years’ creep should be allowed for. Creep has come into consideration in transmission-line design only during the past 35 years, and to date no standards have been established for handling it.

Probably the simplest approach is to check all close clearance points on the profile with a template made with no creep allowance and to specify higher structures at these points if the addition of liberal creep sag infringes on the required clearances.

It is possible to prestress the creep out of small conductors, but for large conductors this requires time and special tensioning facilities not normally available. Also the time lost in constructing an EHV line will more than pay for the extra structure height required to compensate for the creep. Prestressing changes the modulus of elasticity, and this new modulus should be used in the design.

The vertical weight supported at any structure is the weight of the length of conductor between low points of the sag in the two adjacent spans. For bare-conductor weights, this distance between low points can be scaled by using a template of the sag at any desired temperature.

The maximum weight under loaded conditions should be scaled from a template made for the loaded sags. For most problems, the horizontal distance may be taken as equal to the conductor length. Distances to the low point of the sag may be computed.

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