Design manual for high voltage transmission lines

Purpose of design manual

The primary purpose of this design manual is to furnish engineering information for use in designing transmission lines. Good line design should result in high continuity of service, long life of physical equipment, low maintenance costs, and safe operation. This guide publication is a reference containing fundamental engineering guidelines and basic recommendations on structural and electrical aspects of transmission line design, as well as explanations and illustrations.

The many cross-references and examples should be of great benefit to engineers performing design. The guide should be particularly helpful to relatively inexperienced engineers beginning their careers in transmission line design.

Design of transmission lines < 230 kV

The engineering information in this bulletin is for use in design of transmission lines for voltages 230 kV and below. Much of this document makes use of standard Rural Utilities Service (referred to as the agency) structures and assemblies in conjunction with data provided in this bulletin.

Where non-standard construction is used, factors not covered in this bulletin may have to be considered and modification to the design criteria given inthis bulletin may be appropriate.

Since the agency program is national in scope, it is necessary that designs be adaptable to various conditions and local requirements. Engineers should investigate local weather information, soil conditions, operation of existing lines, local regulations, and environmental requirements and evaluate known pertinent factors in arriving at design recommendations.

Insulator swing and clearances of conductors from supporting structures

Suspension insulator strings supporting transmission conductors, either at tangent or angle structures, are usually free to swing about their points of support. Therefore, it is necessary to ensure that when the insulators do swing, clearances are maintained to structures and guy wires. The amount of swing varies with such factors as: conductor tension, temperature, wind velocity, insulator weight, ratio of weight span to wind span, and line angle.

The force due to line angle will cause suspension strings to swing in the direction of the line angle of the structure. Wind blowing on the conductor span will exert a force in the direction of the wind. These two forces may act either in the same direction or in opposite, the algebraic sum thereby determining the net swing direction. Line angle forces and wind forces also interact with the vertical forces of the conductor weight and insulator string weight.

The purpose of this chapter is to explain how insulator swing application guides called swing charts are prepared. Chapter 10 explains how these charts are used in laying out a line.

Clearances and Their Application

Table 1 provides information on three sets of clearances that can ensure proper separation between conductors and structures or guys under various weather conditions. Figure 1 illustrates the various situations in which the clearances are to be applied.

Table 1 – Recommended minimum clearances in inches at conductor to surface of structure or guy wires

No-Wind Clearance:

The no wind clearance provides a balanced insulation system in which the insulating value of the air gap is approximately the same as that of the insulator string for a tangent structure. Note that tangent structures do not include the extra insulators used with angle structures).

Conditions at which no-wind clearances are to be maintained follow:

• Wind: Assume no wind.
• Temperature: Assume a temperature of 60°F. See Figure 1 for conductor condition.

The engineer may also want to evaluate clearances at cold conditions (such as -20°F initial sag) and hot conditions (such as 167°F final sag).

Moderate Wind Clearance

This clearance is the minimum clearance that should be maintained under conditions that are expected to occur occasionally. A typical condition may be the wind that reoccurs no less than once every two years (probability of occurrence no more than 50 percent). Clearance values for moderate wind clearance conditions will have a lower flashover value than clearance values for the no-wind condition.

These lower clearance values are acceptable because under moderate wind conditions, the specified clearance will be sufficient to withstand most of the severe voltage stress situations for wind conditions that are not expected to occur often.

Conditions at which moderate wind clearances are to be maintained follow:

Wind:

Assume a wind of at least 6 psf blowing in the direction shown in Figure 1. Higher wind pressures can be used if judgment and experience deem them to be necessary. However, the use of excessively high wind values could result in a design that is overly restrictive and costly.

It is recommended that wind pressure values of no higher than 9 psf (60 mph) be used for the moderate wind clearance design unless special circumstances exist.

Figure 1 – Illustration of structure insulator swing angle limits and conditions under which they apply (excludes backswing)

Temperature:

Temperature conditions under which the clearances are to be maintained depend upon the type of structure. A temperature of no more than 32°F should be used for tangent and small angle structures where the insulator string is suspended from a crossarm. A lower temperature value should be used where such a temperature can be reasonably expected to occur in conjunction with the wind value assumed.

It should be borne in mind, however, the insulator swing will increase at lower temperatures because conductor tensions increase.

Even if the maximum conductor temperature is significantly greater than 60°F, a higher temperature need not be used as an assumed wind value of 40 mph (6 psf)) has quite a cooling effect.

Assume final sag conditions for 60°F temperature and initial sag conditions for 32°F.

High Wind Clearance:

This is the minimum clearance that should be maintained under high wind conditions that are expected to occur very rarely. The clearances provide enough of an air gap to withstand a 60 Hz flashover but not much more.

Choice of such values is based on the philosophy that under very rare high wind conditions, the line should not flashover due to the 60 Hz voltage.