Optimized Line Design in a Deregulated World
Line design and structure spotting are historically laborious, complicated tasks. Designers have not had time to thoroughly consider what-if scenarios. Intuition and experience help, but this has been a trial-and-error process. Only with the availability of new automated optimization tools can designers effectively work through the complicated interrelationship of variables that affect the spot, identifying and tracking the cost of each design decision. Automated optimization enables the designer to determine the factors that limit the design, manipulate those factors and then re-spot the line. By modeling designs and varying the factors described below, the designer is able to determine the optimum combination of price, performance, and reliability.
Overhead Optimization Process Factors
No two power line installations are the same. Differences in terrain, wire size, environmental loading, routing constraints, and other factors require that the designer consider each line as a new challenge. There are a number of factors the designer must consider in optimizing the cost and reliability of any line:
Maximum span length
The maximum allowable span length is the single biggest factor in potential savings during optimization. Rather than take the time to analyze each structure and location, designers have typically opted to stouten up the line. For overhead designs, this means shortening the spans. For underground, it means adding equipment (e.g., reducing the number of services per transformer).
- Excessive galloping motion under ice and wind conditions can bring conductors in contact and cause outages. Instead of the costly approach of shortening span length, the designer might consider a longer crossarm to increase phase wire separation or drop the neutral attachment to limit contact between phase and neutral lines.
- Designers might address iced-to-bare concerns by increasing vertical separation between conductors or using crossarm (instead of vertical) construction with the neutral on an offset bracket or on the crossarm.
- Horizontal and vertical separation required by NESC can limit a utility’s standard framing configurations. Designers can gain significantly longer spans by increasing phase and neutral separation at the structure, by increasing phase spacing with a longer crossarm, or by framing the neutral further down the pole.
- Right-of-way limitations often result in vertical construction to avoid overhanging private property. Sometimes, a few feet of overhang easement can allow the designer to use crossarm construction, increasing span length and reducing the total cost of the line, even with the added cost of the easement.
Conductor Tension
Conductor tension can have an enormous effect on line cost. Design tensions set low can result in large sags, excessive conductor motion, and shorter spans. Alternatively, design tensions set high (particularly in areas where short spans are required for reasons other than ground clearance) can result in expensive guying, oversized dead-end crossarms, or aeolian vibration with no appreciable cost benefit. Optimum design tension is particularly important during reconductoring when the line route contains fixed or existing structures. A loose conductor often results in a more flexible spot of new structures without putting existing structures into uplift. In hilly terrain, a slight reduction in design tension can eliminate dozens of hold-down (dead-end) structures in low areas, with considerable savings in material and labor. Designers often use standard sag and tension charts to calculate conductor loads and ground clearances. Without automation, they seldom considered alternative design tensions because
they lacked the tools to analyze multiple options in a reasonable time.
Strength of Structural Components
Structural component strength often restricts spots. Savings come from balancing structural components and the requirements of the line. Clearly, using insulators rated for 25,000 pounds is a poor material choice on a dead-end crossarm rated for 5,000 pounds. The designer can choose between a strong arm or a weak insulator only by investigating the line in question. All that can be seen at a glance is that the two are mismatched. The alternatives below should always be considered when preparing for optimization.
- Provide a Range of Weight Spans Without Skimping on the High End: An allowable
weight span, or vertical span, is the amount of conductor a structure can support vertically. Inadequate weight span for a structure family can significantly affect reliability and line cost and result in an awkward-looking spot. Weight span should rarely be the limiting factor in line spotting because it is usually inexpensive. The cost of doubling a distribution crossarm is insignificant when compared with the cost of installing two or three structures on every hill along the route. Alternatively, it is not practical to design all structures for a large weight span because few structures along the route will require it. The solution is to provide a range of weight spans with associated wind spans for each type of structure. A wide range of weight spans is obviously critical in hilly terrain, but it is often overlooked in flat areas. Limited weight spans can significantly impact line spotting for something as simple as raising the line for a railroad crossing. We recommend minimum weight spans of at least
one-half the maximum allowable span, and maximum weight spans of at least 1.5 times the maximum allowable span. In hilly terrain, a maximum weight span of two to three times the maximum allowable span is often appropriate.
- Cover All Line Angle Ranges with Appropriate Framing Types: The design should accommodate all ranges of line angles throughout the route with optimal structure types. For example, don’t use only buckarm corner structures for angles over 30º if a double-angle- pin or back-to-back, dead-end structure will suffice. Utilities often set angle ranges for framing configurations regardless of wire size or design tensions. This is an easy design, but a costly oversimplification. The only time this is appropriate is when there is a wire separation constraint. Allowable line angles should be determined based on the allowable transverse load of the structure, which is a combination of wire tension, wind loading, and line angle. Allowable line angles are intimately tied to conductor design tensions. A proper balance between the two is the recommended course of action.
- Guying: The design software should provide appropriate guy and anchor configurations for the full range of line angles on a project. Designers can make generalizations for spotting, but afterwards, the designer should study each guyed structure to see if guys or anchors could be eliminated. Consider the relationship between guying costs and conductor design tension: Do the benefits of stringing the conductor tighter justify the increased cost of guying?
