Optimized line design in a deregulated world
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. Cost and reliability of any line are affected by basic factors that can be evaluated
and modified. Some concepts that should be considered during every overhead line design
include:
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Maximum span length
- Conductor galloping
- Iced-to-bare separation
- Horizontal and vertical separation required by National Electric Safety Code
- Right-of-way limitations
- Conductor tension
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Strength of structural components
- Provide a range of weight spans without economizing on the high end
- Cover all line angle ranges with appropriate framing types
- Guying -
Routing
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Consider new materials and construction practices
The practices above directly impact the cost and reliability of a line. The optimal combination of
these factors isn't obvious without considering each individually and determining its importance
to the balance of the system. For example, increasing vertical conductor separation invariably
requires a taller pole to maintain the same ground clearance. However, the cost/benefit of longer
spans makes the cost of the extra pole height insignificant.
Viewing the line as a system increases the potential for the greatest cost savings. All variables
within the designer's control (from construction materials to conductor tension to allowable span
lengths) should be synchronized with the terrain and constraints outside the designer's control.
This is often overlooked in day-to-day line design. If span lengths are limited to 200 feet for
other reasons, ten-foot crossarms that allow 400-foot spans under galloping conditions are
unnecessary, costly, and of no benefit to the system.
Common reasons for not optimizing distribution lines
Distribution lines are notorious for exhibiting constraints not seen in transmission line design.
Some feel these constraints are so great no benefit can be gained by optimizing. This is not the
case. There is always something the designer can do to reduce the cost below standard "rule-ofthumb"
designs.
The standard objections to the value of optimization and our response to these objections follow:
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All the structure locations are fixed
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Are they all really fixed? Can I move some even 20 or 30 feet?
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Are the tap locations really fixed? Can I use slack spans or some other method to
allow the tap location to move?
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A designer can always optimize pole heights and classes, framing configurations,
guying, transformer size, and service size
- We only set the poles on property lines
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Do I really need to in all cases?
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Which property lines? Optimize to find out.
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What height and class poles are optimal? Optimize to find out.
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How should I model it to optimize?
- We only use one height and class of pole
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How much does it cost to stock a wider range of heights and classes?
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How much does it cost to not stock a wider range of heights and classes?
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Are there different optimal sizes for small wire, large wire, or double circuit?
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Should we pay extra to support joint-use wires?
