ABSTRACT
An overhead transmission line (OHTL) is a very complex, continuous, electrical/mechanical system. Its function is to transport power safely from the circuit breaker on one end to the circuit breaker on the other. It is physically composed of many individual components made up of di¢erent materials having a wide variety of mechanical properties, such as
• Flexible vs. rigid • Ductile vs. brittle • Variant dispersions of strength • Wear and deterioration occurring at di¢erent rates when applied in di¢erent applications within
one micro-environment or in the same application within di¢erent micro-environments
is discussion will address the nature of the structures which are required to provide the clearances between the current-carrying conductors, as well as their safe support above the earth. During this discussion, reference will be made to the following deŸnitions:
Capability: Capacity ( ) availability×
Reliability level: Ability of a line (or component) to perform its expected capability
Security level: Ability of a line to restrict progressive damage after the failure of the first component
Safety level: Ability of a line to perform its function safely
Present line design practice views the support structure as an isolated element supporting half span of conductors and overhead ground wires (OHGWs) on either side of the structure. Based on the voltage level of the line, the conductors and OHGWs are conŸgured to provide, at least, the minimum clearances
10.1 Traditional Line Design Practice ................................................. 10-1 Structure Types in Use • Factors A¢ecting Structure Type Selection
10.2 Current Deterministic Design Practice ......................................10-5 Reliability Level • Security Level
10.3 Improved Design Approaches ......................................................10-9 10.A Appendix A: General Design Criteria-Methodology .............10-9 References .................................................................................................10-10
mandated by the National Electrical Safety Code (NESC) (IEEE, 1990), as well as other applicable codes. is conŸguration is designed to control the separation of
• Energized parts from other energized parts • Energized parts from the support structure of other objects located along the r-o-w • Energized parts above ground
e NESC divides the United States into three large global loading zones: heavy, medium, and light and speciŸes radial ice thickness/wind pressure/temperature relationships to deŸne the minimum load levels that must be used within each loading zone. In addition, the Code introduces the concept of an Overload Capacity Factor (OCF) to cover uncertainties stemming from the
• Likelihood of occurrence of the speciŸed load • Dispersion of structure strength • Grade of construction • Deterioration of strength during service life • Structure function (suspension, dead-end, angle) • Other line support components (guys, foundations, etc.)
Present line design practice normally consists of the following steps:
1. e owning utility prepares an agenda of loading events consisting of a. Mandatory regulations from the NESC and other codes b. Climatic events believed to be representative of the line’s speciŸc location c. Contingency loading events of interest; i.e., broken conductor d. Special requirements and expectations Each of these loading events is multiplied by its own OCF to cover uncertainties associated with it to produce an agenda of Ÿnal ultimate design loads (see Figure 10.1).
