Electric Power Distribution Handbook, T. A. Short

3. Overhead Line Performance

Questions

  1. Mark which of the following are true during major storms.

    1. Pole damage increases as a portion of damage.
    2. Downed conductors increase as a portion of damage.
    3. Outside crews are often needed.
    4. Tree damage is more acute.

  2. Thermal inspections can find which problems? Mark all that apply.

    1. Cutout problems
    2. Bad arresters
    3. Tracking insulators
    4. Cracked insulators
    5. Automatics with poor connections

  3. Mark which of the following are true related to tree faults.

    1. Trees regularly cause momentary interruptions.
    2. Growth often accounts for 50% of vegetation outages.
    3. 35-kV lines are considerable more susceptible to vegetation outages than 15-kV lines.
    4. Wet branches are more likely to fault than dry branches.

  4. More vegetation clearance is better… (pick the best answer)

    1. If it removes branches that can fall into the line.
    2. So vegetation takes longer to grow into lines.

  5. True/false: doubling vegetation management will cut in half tree-caused outages.

    1. True
    2. False

  6. Animal faults are most common in which season?

    1. Winter
    2. Spring
    3. Summer

  7. Animal faults happen most frequently where?

    1. Transformers
    2. Phase to phase
    3. Reclosers

  8. True/false: animal guards should be placed as low on a bushing as possible.

    1. True
    2. False

  9. Pole deterioration happens mainly where? Mark all that apply.

    1. In the outer 2 in of a pole
    2. Near ground line
    3. Near third party attachments

  10. Vegetation faults are most common in which season?

    1. Winter
    2. Spring
    3. Summer

  11. True/false: doubling vegetation management will cut in half tree-caused outages.

    1. True
    2. False

  12. Relative to bare conductors, covered wire helps for which of the following? Mark all that apply.

    1. Ice loading
    2. Tree growth into conductors
    3. Downed conductors
    4. Small branches into conductors
    5. Trees falling into conductors

Problems

  1. Calculate the time it takes for a tree limb to cause a flashover on a 12.5-kV line with 4-ft phase spacings. Will it flash over?

    • Repeat for a 25-kV line.
    • Repeat for a 4.1-kV line.

  2. Assume it costs $4800 / mile to prune at a four-year cycle, and costs rise an additional $700 / mile for every year added to that cycle. What’s the optimal cycle just based on cost based on that linear relationship over the range of 4 to 12 years?

  3. Assume it costs $4800 / mile to prune at a four-year cycle, and costs rise by a factor of 1.2 for every year added to that cycle. What’s the optimal cycle just based on cost over the range of 4 to 12 years?

  4. Find the optimal maintenance cycle w (1 to 10 years) and the optimal hazard-tree removal program cycle h (1 to 10 years) with the following assumptions:

    • Tree SAIFI = 0.3 + w^2 / 100
    • A hazard tree inspection reduces tree SAIFI by 0.05
    • Pruning = $4800 / mile
    • Hazard tree removal = $800 / mile
    • Annual budget = $1300 / mile

Projects

  1. Vegetation management

    Find an area that has just been pruned. Photograph these areas. Document what you think the pruning spec is based on what you saw. Include any advice for improvements.

  2. Hazard trees

    Take a driving inspection (preferably with someone else driving) in an area with trees. Count how many hazard trees you identify in two miles of line. Take pictures and document.

  3. Animal issues

    Visit 5 poles on three-phase lines with equipment. Take pictures. Identify any animal-related design and/or protection defects. Document how it could have been avoided when built or upgraded and how it might be fixed now.

  4. Overhead line issues

    Ride and/or walk distribution lines in your area. Photograph and document at least 10 deficiencies that could affect performance. Document the main problem, how it could have been avoided when built or upgraded, and how it might be fixed now.



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To the extent possible under law, Tom Short has waived all copyright and related or neighboring rights to these study questions. This work is published from the United States. Please use this material however you want.