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TitleGuidelines for Overhead Line Design
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Page 1

NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




Check this is the latest version before use. Page 1 of 20 Reference P56M02R09 Ver 1
Reference Approved by: Jim Brooks Network Lines Standards Manager

Ergon Energy Corporation Limited ABN 50 087 646 062
Ergon Energy Queensland Pty Ltd ABN 11 121 177 802



INDEX

Introduction

1. Selection of Insulators


1.1 Introduction
1.2 Pin Insulators
1.3 Post Insulators
1.4 Stay Insulators
1.5 Cap and Pin Disc Insulators
1.6 Insulator testing


2. Conductors


2.1 Introduction
2.2 Phase Conductors
2.3 Corrosion Performance


3. Conductor Sag Tension Theory


3.1 The Conductor profile Parabola vs Catenary
3.2 Sag
3.3 Slack
3.4 Factors that affect conductor tension
3.5 Multiple Span tension calculations – ruling Span
3.6 Sag tension calculations
3.7 Span ratios
3.8 Wind Span
3.9 Weight Span
3.10 Examples


4 Crossarms


4.1 Introduction
4.2 Design loads
4.3 Conductor spacing


5. Poles


5.1 Introduction
5.2 Wood pole Strength
5.3 Pole Design loads


6. Pole Foundations


6.1 Introduction
6.2 Foundation strength


7. Ground Stays


7.1 Introduction
7.2 Stay Application
7.3 Pole bending moment


APPENDIX 1 Conductor Loads
APPENDIX 2. Distribution Line Layout Steps

Page 2

NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




Check this is the latest version before use. Page 2 of 20 Reference P56M02R09 Ver 1
Reference Approved by: Jim Brooks Network Lines Standards Manager

Ergon Energy Corporation Limited ABN 50 087 646 062
Ergon Energy Queensland Pty Ltd ABN 11 121 177 802



GENERAL INTRODUCTION

What are we designing for?

Compliance with Statutory Regulations


Safety of both our employees and general public


Economic utilisation of materials


To best meet the needs of customers with minimum environmental impact


To obtain a standard acceptable both from an engineering view and aesthetically (ie.

have regard for the look of our construction from the public’s point of view).

What physical loadings do we have to allow for our design?

Weight of conductor and fittings


Conductor tension:- Terminal load

Deviation load
Differential conductor loads in adjacent spans
Vertical loads
Stay loads

Environmental Loads (eg. Wind) On Structures

On Conductors

Construction and maintenance loads



LIMIT STATE DESIGN

Current practice for the design of Overhead Line Structural Components is to use a Limit State
design approach as set out in C (b) 1-1999 Guidelines for Design and Maintenance of
Overhead Distribution and Transmission Lines.

The Limit State design approach uses a reliability based (risk of failure) approach to match
component strengths (modified by a factor to reflect strength variability) to the effect of loads
calculated on the basis of an acceptably low probability of occurrence. This approach allows
component strengths to be more readily matched and optimised by economic comparison.

The corresponding Limit State wind pressures which correspond to the previously used working
stress values of 500pa and 660 pa and which result in equivalent failure rates based on typical
component strengths factored by strength factors which incorporate appropriate component
reliability factors are approx 900pa and 1200pa respectively. Limit State wind load pressures
are therefore greater than permissible stress loads by a factor of 1.8.

Conductor tension loads will increase in response to the higher design wind pressures by a
factor of depending on conductor everyday tension and conductor characteristics and generally
in the range 1.3 to 1.6.

Conductor weight loads will increase due to the effect of increased tension on structures with a
height profile above the average of neighbouring structures, however in general this factor is
fairly minimal in relatively flat terrain.

Page 10

NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




Check this is the latest version before use. Page 10 of 20 Reference P56M02R09 Ver 1
Reference Approved by: Jim Brooks Network Lines Standards Manager

Ergon Energy Corporation Limited ABN 50 087 646 062
Ergon Energy Queensland Pty Ltd ABN 11 121 177 802



3.8 Wind Span
The wind span at a particular structure is the length of span that determines the transverse
load on the structure due to wind action on the conductor and is defined as:

Lw = one half the sum of the adjacent spans.


3.9 Weight Span
The weight span at a structure is the length of span between the catenary low points on
either side of the particular structure and determines the vertical load due to the weight of
conductor at that structure.


3.10 Examples

Example 1.
Consider a span of Raisin (3/4/2.5 ACSR) conductor strung to a tension of 22% NBL at 15
deg C. with the following properties:

Tension T = 5368 N (22 % NBL)
Weight w = 1.893 N/m
Span Length S = 250 m
Ruling Span Length is also 250 m

The sag under this condition is 1.893x250 2 /(8x 5368) = 2.76 metres
This sag can also be determined from the Conductor tension change program.

Example 2.
Now calculate the tension and sag under the maximum wind condition of 900 Pa
Using the conductor tension change program, the tension under this condition is 9895
newtons with vertical sag of 1.49 m and horizontal sag of 5.33 m.

Now calculate the tension and sag under the maximum operating temperature of 60 deg C
and no wind

Using the conductor tension change program, the tension under this condition is 3868 N with
vertical sag of 3.82 m.

Example 3 .
Now consider what happens if the conductor is over tensioned by pulling an additional 100
mm out of the span during stringing.

This will cause the tension to increase however the resulting increase in elastic stretch will
partly reduce the effect.

We can treat the removal of this conductor length as being similar to a reduction in
temperature, which can be calculated using the formulae for thermal expansion - L = T S.

Therefore T = L/ S
= 0.1 / 13.9x10-6 x 250
= 28.8 deg C

By going to the Conductor tension Change Program enter option and using a final condition of
15-28.8 ie –13.8 deg C we can calculate the resulting tension as 6682 N and sag as 2.21 m.
This means that the conductor is over tensioned by a factor of 25%

Example 4.

Page 11

NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




Check this is the latest version before use. Page 11 of 20 Reference P56M02R09 Ver 1
Reference Approved by: Jim Brooks Network Lines Standards Manager

Ergon Energy Corporation Limited ABN 50 087 646 062
Ergon Energy Queensland Pty Ltd ABN 11 121 177 802



Now consider what happens if the conductor is under tensioned because a stay foundation
relaxed to allow the pole head to move by 200 mm which, effectively puts an additional 200
mm of cable into the span.

This will cause the tension to decrease however the resulting decrease in elastic stretch will
partly reduce the effect.
We can treat the addition of this additional conductor length as being similar to an increase in
temperature which can be calculated using the formulae for thermal expansion - L = T S.

Therefore T = L/ S
= 0.2 / 13.9x10-6 x 250
= 57.6 deg C

By going to the Conductor tension Change Program enter option and using a final condition of
15 + 57.6 ie 72.6 deg C we can calculate the resulting tension as 3567 N and sag as 4.15 m.
This means that there is additional sag of 1.39 m, which will most likely to result in statutory
clearances not being maintained.

Of course if this span were one of a section, the effect of tension equalisation provided by
adjacent spans would tend to reduce these effects.

Example 5.
Now consider what happens if we raise one pole by 3 metres in a section with 250 m spans
either side on reasonably even ground.

The increase in chord length can be calculated by L = L- Sqrt( L2 + h2),

L = span length
H = increase in pole Height.

Therefore L = 250 – Sqrt(250 2+ 3 2) = 0.018 m

This will cause the tension to increase however the resulting increase in elastic stretch will
partly reduce the effect.

We can treat the reduction of this additional conductor length as being similar to a decrease in
temperature, which can be calculated using the formulae for thermal expansion - L = T S.

Therefore T = L/ S
= 0.018 /13.9x10-6 x 250
= 5.2 deg C

By going to the Conductor tension Change Program enter option and using a final condition of
15 – 5.2 ie 9.8 deg C we can calculate the resulting tension as 5586 N and sag as 2.65 m.
This means that there is an increase in tension of 4% which should be OK.

If we repeated the same exercise with a 100 m span (and 100 m ruling span), the tension
would increase to 6143 N which would be around 15 % overtension and may need correction
but then only if there are termination structures at each of the adjacent structures.


4 Crossarms

4.1 Introduction

Crossarms may be either wood or steel construction but the general design procedure is the
same. Wood crossarms do however have significant benefits with regard to electrical
performance associated with lightning outage performance. The mechanical loads to which

Page 19

NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




Check this is the latest version before use. Page 19 of 20 Reference P56M02R09 Ver 1
Reference Approved by: Jim Brooks Network Lines Standards Manager

Ergon Energy Corporation Limited ABN 50 087 646 062
Ergon Energy Queensland Pty Ltd ABN 11 121 177 802



APPENDIX 2


DISTRIBUTION LINE LAYOUT STEPS

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

Page 20

NETWORK LINES STANDARD
GUIDELINES FOR OVERHEAD LINE DESIGN




Check this is the latest version before use. Page 20 of 20 Reference P56M02R09 Ver 1
Reference Approved by: Jim Brooks Network Lines Standards Manager

Ergon Energy Corporation Limited ABN 50 087 646 062
Ergon Energy Queensland Pty Ltd ABN 11 121 177 802



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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