STEPS IN LAYOUTING DISTRIBUTION LINE IN OVERHEAD LINE DESIGN TUTORIALS

STEPS IN LAYOUTING DISTRIBUTION LINE IN OVERHEAD LINE DESIGN TUTORIALS
How to design overhead line especially in lay-outing distribution line?

The following steps are suggested as the approach to be followed in designing a line from scratch. With experience or by reference to the tables of common applications in the Design manual section “Pole Structures” many of these steps will not be required for jobs of a standard nature.

1. Determine conductor size and type based on planning requirements and application.

2. Determine the proposed stringing tension based on the situation eg. Urban, semi urban or rural. Consideration in this decision should be given to the difficulty of staying and frequency of angles required by route restrictions.

3. Determine the Limit state design wind pressure on conductors appropriate to the location (eg 900 or 1200 pa).

4. Determine strain/angle pole locations taking into account the deviation angle limits on pin insulators as per the table in the Design Manual. If ratios of adjacent span lengths exceed 2:1 in full tension rural situations, consider the use of a strain pole.

5. Determine expected span length on level ground from experience or by using suggested span and pole height / strength in the pole layout tables or the program Maximum span – ground clearance limitation. If poor soil foundations are anticipated, allowance should be made for additional pole setting depth at this stage. Consideration should also be given to any future requirement for subsidiary circuits.

6. If the terrain is not substantially flat, profile the line and determine pole locations and heights necessary to achieve ground clearances and likely strain/ angle positions.

7. Determine the ruling span using the Ruling span program for each section of line between strain structures.

8. Check any long spans to ensure that mid span phase to phase clearance requirements are met using the Maximum span - mid span clearance limitation program.

9. Use the Allowable pole tip load program to determine allowable (limit state) pole tip loads based on expected pole strengths and foundation conditions. These pole tip loads are after allowance has been made to take into account wind on the pole element.

10. Use the pole top loads from step 9 to input into the Allowable wind span program to determine the allowable wind span on unstayed intermediate poles. If these allowable spans are unrealistically low, return to step 9 using a greater pole or foundation strength. Consider the need for future subsidiary circuits in the selection of pole /foundation design. Use of bisect stays on small angles is an alternative option to increasing pole strengths.

11. Determine the weight span in particular on poles with a height which is significantly greater or less than their neighbours. This can be determined using the Weight span program which will output the weight span under the sustained load, maintenance and limit state conditions. If the weight span is negative, a strain structure should be selected.

12. Using the Crossarm design program, check that the proposed crossarm sizes are sufficient. Allowable weight spans for the selected crossarm sizes under the sustained load, maintenance and limit state conditions should exceed the weight spans determined from step 11.

13. Check that allowable horizontal stay loads from the Design Manual section “stays” exceed the limit state conductor wind and tension loads. Limit state conductor tensions can be determined using the Conductor tension change program.

14. For structures with multiple circuits or the stay attachment position away from the conductor attachment locations, use the Resultant stay load program to check that the stay horizontal load is not exceeded and that the bending moment in the pole at the stay attachment is not exceeded.

15. For any spans with different or unusual conductor configuration at one end and where mid span clearance may be an issue, use the Phase separation program to check clearances.

16. For any span where clearance to an adjacent structure may be an issue under conductor blowout, use the Conductor tension change program to calculate the horizontal swing under the 500 pa and 30 deg C condition. Add to this the relevant statutory clearance to check if clearance to the object from the line is sufficient. If not reduce span length or reposition poles and recalculate.

17. Conductor sagging information for listing on the construction plan for use by field staff in sagging the conductors can be determined using the Conductor sagging program.

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