Tampa Electric Company, an investor-owned electric utility serving approximately 2,000 square miles in west central Florida, was faced with providing electricity to a newly developed industrial park complex. The distribution engineering department needed to develop guidelines for the design of the distribution circuit. The "main feeder," which is the backbone of each 13-kV distribution circuit, represents a substantial investment by the company.5
Tampa Electric has four approved main feeder construction configurations: (1) cross arm, (2) vertical (horizontal line post), (3) vertical (standoff pin), and (4) triangular, as illustrated in Table ST5.3. The width of the easement sought depends on the planned construction configuration. If cross arm construction is planned, a 15-foot easement is sought. A 10-foot wide easement is sought for vertical and triangular configurations.
Once the required easements are obtained, the line clearance department clears any foliage that would impede the construction of the line. The clearance cost is dictated by the typical tree densities along road rights-of-way. The average cost to trim one tree is estimated at $20, and the average tree density in the service area is estimated to be 75 trees per mile. The costs of each type of construction are as given in Table ST5.3a
Additional factors to consider in selecting the best main feeder configuration are as follows. In certain sections of Tampa Electric's service territory, ospreys often nest on transmission and distribution poles. The nests reduce the structural and electrical integrity of the poles. Cross arm construction is most vulnerable to osprey nesting, since the cross arm and braces provide a secure area for construction of the nest. Vertical and triangular construction do not provide such spaces.
The insulation strength of a construction configuration may favorably or adversely affect the reliability of the line for which the configuration is used. A common measure of line insulation strength is the critical flashover (CFO) voltage. The greater the CFO, the less susceptible the line is to nuisance flashovers from lightning and other electrical phenomena.
The utility's existing inventory of cross arms is used primarily for main feeder construction and maintenance. The use of another configuration for main feeder construction would result in a substantial reduction in the inventory of cross arms. The line crews complain that line spacing on vertical and triangular construction is too restrictive for safe live line work. Each accident would cost $65,000 in lost work and other medical expenses. The average cost of each flashover repair would be $3,000. Table ST5.3b lists the values of the factors involved in the four design configurations.
All configurations would last about 20 years with no salvage value. It appears that non-cross arm designs are better, but engineers need to consider other design factors, such as safety, rather than just monetary factors when implementing the project. It is true that the line spacing on triangular construction is restrictive. However, with a better clearance design between phases for vertical construction, the hazard would be minimized. In the utility industry, the typical opposition to new types of construction is caused by the confidence acquired from constructing lines in the cross arm configuration for many years. As more vertical and triangular lines are built, opposition to these configurations should decrease. Which of the four designs described in the preceding table would you recommend to the management? Assume Tampa Electric's interest rate to be 12%.

Transcribed Image Text:

Design Configurations Factors Easements Line clearance Line construction Crossarm Triangular Horizontal Line Standoff $388,000 S1.188 $8,812 $487.000 $388,000 $1.188 $7,625 $388,000 $1,188 $12.828 $613 $7,630 Nesting Insulation strength, CFO (V) Annual flashover occurrence n) Annual inventory savings Safety Severe None None 476 None 474 462 54,521 OK Problem $4.521 $4,521 Problem Problem