Guidelines for Overhead Line Design

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN

INDEX Introduction 1. Selection of Insulators 1.1 1.2 1.3 1.4 1.5 1.6

Introduction Pin Insulators Post Insulators Stay Insulators Cap and Pin Disc Insulators 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 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

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN 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:-

•

Environmental Loads (eg. Wind) On Structures On Conductors

•

Construction and maintenance loads

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

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

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN Design Component stresses is based on the ultimate stress at failure modified by a strength factor, which takes into account the material strength variability. Design component stresses are listed in the relevant sections of the Design Manual. What physical conditions do we have to allow for in our design? •

Conductor clearance:-

To ground, roads etc To railway lines Over flood country To buildings etc Other lines

•

Topography:-

Terrain Roads Railway lines Telecom Stays Special exclusion areas

•

Avoidance of obstructions:-

Airfields Roads Railway lines Telecom Stays Special exclusion areas

How do we allow for all these variable factors in our design? In order to minimise the risk of failure of an overhead line it is necessary to ensure that each component of an overhead line has been designed to meet all the electrical and mechanical loads likely to be experienced in service as far as reasonably practical. In order to achieve this, every line and every structure in that line could be individually designed to meet the project requirements. This would be extremely time consuming and is probably only justified for high value transmission lines. Another approach is to utilise a range of standard structures with pre-designed electrical and mechanical capabilities and apply them to a particular project. The advantage of this approach is that detailed structure design is not required, and as long as the structures are used within specification, a line can be constructed safely using standard building blocks. This approach allows for economics of scale on material purchases and achieved a measure of uniformity of construction. It is also worth noting that Ergon Energy has determined that, lines designed using the standard structure drawings do require such approval. Ergon Energy uses the standard structure approach for the majority of its lines and layout staff need only select the appropriate structure to suit the specific application. Where standard structures do not satisfy a line requirement, this should be referred to the Lines Standards Department for a detailed design and approval. To achieve a minimum cost line layout staff needs to apply the standard structures in the most cost efficient way. This requires: • • • •

Clear understanding of structure capabilities Methodical approach using all available tools “feel” and “insight” which come with experience detailed checking of work undertaken



While there are a range of standard construction, layout and design staffs who apply these standards to an overhead line need to be aware of some of the basic design principles so as to apply the most appropriate structure to a defined requirement. These notes provide an overview of the following factors in line design: • • • • • • •

Selection of Insulators Conductors Sag and tension theory Crossarms Poles Pole Foundations Ground Stays

1

SELECTION OF INSULATORS

1.1

Introduction

One of the most important and yet one of the most vulnerable links in transmission and distribution is insulators. Porcelain and toughened glass are the materials principally used for supporting conductors on overhead lines, and although these materials are relatively brittle and inelastic, they have proven service experience and are still widely used. The design of synthetic type insulators has improved both electrically and mechanically in recent times and they are being used in urban areas to minimise radio interference and in areas where gunshot or stone throwing is a problem. Insulator damage may occur due to such widely varying causes as lighting (puncture), power arcs, stone throwing, corrosion, gunshot and pollution. The following points must be considered in the selection of the appropriate insulation of an overhead line: • • • 1.2

50Hz performance (usually a pollution requirement) Impulse capabilities Switching capabilities

Pin Insulators

This type was amongst the earliest designs, and although it has improved both electrically and mechanically, it has altered little in appearance. It provides the most economic, simple and efficient method of conductor support for voltage up to and including 33kV. Pin type insulators for the lower voltages are designed so that the puncture voltage is higher than the flashover voltage, however if the insulator glazing under the conductor is damaged (usually caused by vibration) the insulator may puncture. 1.3

Post Insulators

These insulators are of one piece porcelain construction and have a cemented on a galvanised malleable cast iron base provided with a taped hole for fixing stud. It will be apparent that this type of construction renders it almost non-puncturable and a further advantage is that if any expansion of the cemented base joint does occur the porcelain is put into compression. If this occurs with the cemented joint of the screwed lead thimble of the pin type insulator as discussed above, the porcelain is placed in tension, a type of load, which it has little ability to withstand, and the porcelain will fail.


NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN Where adequate additional insulation is not provided by the support (eg. Timber, fibreglass etc.) then the insulators should be of two piece or non-puncturable construction to minimise the risk of an electric hazard due to insulator failure. 1.4

Stay Insulators

The stay insulator inserted in the stay wire is usually of porcelain and is so designed that in the event of failure of the stay insulator the stay wires will not fall to ground. All stays wires attached to wooden poles supporting active conductors should be fitted with stay insulators. The insulators should be mounted not less than 2.7 metres vertically above ground and have a wet power frequency flashover voltage not less than one and a half times the highest voltage conductor supported by the pole. The selection and placement of stay insulators should be in accordance with ESAA C (b) 1. 1.5

Cap and Pin Type Disc Insulators

These insulators are used at tension positions (ie. Termination and suspension) in high voltage lines and are available in 70kN and 160kV strengths to suit the various conductor loadings. The cap and pin design ensures that the porcelain or glass of the high insulator is always in compression. In areas of high pollution, particularly costal areas the pin of the insulator should be fitted with zinc collar. 1.6

Insulator Testing

All porcelain insulators taken out of service must be tested before re-erection. Toughened glass insulators however, need not be tested, since the smallest fault will cause disintegration of the insulator. Information on selection of insulators is contained in the Design manual section on “Insulators”

2

CONDUCTORS

2.1 Introduction Economically, conductors represent between 20 to 40% of the total cost of a line; consequently their selection is of prime importance. In earlier days of electrical power transmission, copper was mainly used as the material of overhead line conductors, however with the expansion of electricity networks, several factors, such as price, weight, availability and conductivity, have virtually compelled Overhead Line Design Engineers to concentrate on aluminium based conductors, eg. AAC = All Aluminium Conductor ACRS = All Aluminium Conductor Steel Reinforcement AAAC = All Aluminium Alloy Conductor Steel conductors are still widely used as overhead earth wires and also as phase conductors on rural distribution lines, eg. SC/GZ = Galvanised Steel Conductor SC/AC = Aluminium Clad Steel Conductor 2.2

Phase Conductors

The conductors fulfil an electromechanical function; hence both the electrical and mechanical aspects are to be considered. Check this is the latest version before use. Page 5 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

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN Electrical parameters: The most important parameter affecting the choice of conductor is its resistance, because it influences voltage regulation, power loss and current rating. For AC lines, the diameter of a conductor affects the inductance and the capacities. Up to a voltage of 132kV, the above considerations are generally adequate, however at higher voltages, the above gradient on the conductor surface may require the selection of a conductor on the basis of its diameter, thus leading to the use of bundled conductor (ie. 2, 3 or 4 phase). Mechanical parameters: As already indicated, Aluminium based conductors represent the highest proportion of conductor usage. The advantageous mechanical properties of aluminium alloys have also been recognised for long time, but AAAC has always been more expensive than ACSR, for equivalent conductivities. However there are cases where initial cost is not the governing factor. One of these is the corrosion performance, since being monometallic, the risk of bimetallic corrosion between the aluminium and the zinc on the steel core are nonexistent. Consequently AAAC conductors are used on lines in coastal areas. 2.3

Corrosion Performance

Table 3.3.1 provides an indication of the relative corrosion performance of various conductor types. The recommendations should be modified by local experience, for example, for salt spray pollution the relative distances from the source depend upon the prevailing winds and the terrain. Special circumstances such as crop dusting, which has been known to have severe effects, should also be taken into account. Table 2.1 Indication of relative corrosion performance of conductors SALT SPRAY POLLUTION CONDUCTOR TYPE

OPEN OCEAN

AAC

1

AAC/6201

BAYS INLETS SALT LAKES

INDUSTRIAL POLLUTION ACIDIC

ALKALINE

1

1

3

1

1

2

3

AAAC/1120

1

1

1

3

ACSR/GZ

3

2

2

3

ACSR/AZ

2

1

2

3

ACSR/AC

1

1

2

3

SC/GZ

3

2

3

2

SC/AC

1

1

2

3

HDCu

1

1

2

1

1 = Good performance 2 = Average performance 3 = Poor performance



3. CONDUCTOR SAG – TENSION THEORY 3.1

The Conductor Profile - Parabola versus Catenary

A parabola is the shape of a cable that supports a uniform horizontal load. An example of a parabola is the cable of a suspension bridge that supports the deck below. Whereas a catenary is the shape that is formed by a hanging cable whose weight is a constant per unit of arc length. The word catenary comes from the Latin word catena, meaning chain. Provided that the sag is less than 9% of the span length, there is less than 1% difference in their shapes. So for most practical distribution applications the parabola will suffice and is the assumption generally used for distribution design. The mathematical formulae, which are derived for the parabola, are much simpler than the catenary formulae. 3.2 Sag The following formula for the sag in a parabola can be used for level and non-level spans. A level span is a span where the conductor supports are at the same elevation.

S=

w L2 8T

S = mid-span sag (m) w = conductor weight (N/m) L = horizontal span length (m) T = conductor tension (N) The conductor tension T is the tension at the low point of the cable, however the tension does increase with conductor elevation. The tension at the supports will be no greater than an additional 7% of the tension at the low point for a level span where the sag is less than 9% of the span length. Normally the conductor weight is given in kg/km, which must be converted into N/m to use in the above equation.

w=

Wc × 9.81 1000

Wc = conductor weight (kg/km) 3.3 Slack The difference in distance between the straight line between the supports and the distance along the parabola arc (the stretched conductor length) is called the slack. For a level span the slack is given by

K=

8 S2 3L

K = slack (m) S = mid-span sag (m) L = span length (m)



3.4

Factors that Affect Conductor Tension

Temperature As the temperature increases, the unstretched conductor length will increase by an amount equal to: ∆ L = α T S

∆L = αTS α = the coefficient of thermal expansion T = the temperature increase in deg C S = the span length in metres This will result in a decrease in conductor tension and an increase in sag. Wind A wind load on the conductor will increase the apparent weight of the conductor resulting in an in increase in tension. The increase in tension will increase the cable length due to elastic stretch by an amount given by given by:

∆L = (To − T ) / EA To = the initial tension in newtons T = the final tension E = the coefficient of elasticity A = the cross section of the conductor in metres. This increase in resultant load will result in an effective sag in an inclined direction with both horizontal and vertical components. Ice Ice build up on the conductor will increase the apparent diameter and weight of the conductor. This is not an issue in Queensland however the same approach can be used for calculating loads and sags if bird darverters are installed along a span. Age Conductor sag over time may increase due to the effects of strand settling in and metallurgical creep. A higher tension may be used when the conductor is first erected to allow for “settling in of conductor strands and for subsequent metallurgical creep of the conductor material Pole movement Any movement of pole tops due to stay relaxation etc will have the effect of introducing additional length into the span.

3.5

Multiple Span Tension Calculations - Ruling Span

The ruling span (or equivalent span) is defined as that span which behaves identically to the tension in every span of a series of suspension spans under the same loading condition. In general the flexibility of a wood pole is sufficient to ensure that an intermediate pin structure can be considered as a suspension for the purposes of calculation of the ruling span provided that the ratio of adjacent span lengths is not too extreme (eg less than 1:2). Check this is the latest version before use. Page 8 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


The ruling span can be calculated using, n

Lr =

Σ L3i

i =1 n

Σ Li

i =1

Lr = ruling span Li = horizontal span length of span i n = number of spans between strain structures. This equation applies for lines in flat to undulating terrain. In very mountainous terrain with large differences in elevation between structures, use of Equation (4) in Appendix E of C(b)1-1999 Guidelines for Design and Maintenance of Overhead Distribution and Transmission Lines may be required. 3.6

Sag and tension Calculations

These calculations are primarily used to calculate the conductor tension under one set of conditions based on known tension under some other condition. Conductor Tension Limitations Conductor tension limitations are determined by the most onerous of the following conditions: • • • •

Serviceability Condition or everyday condition (relates to vibration, construction and anchoring practicalities)- as specified in the table of “Standard Conductor Applications” following in this section at a temperature of 15°C. Conductor Strength Limit State - Bare conductors – 70% of Conductor nominal breaking load at a temperature of 15°C. Serviceability Condition – low temperature condition – 50% of conductor nominal breaking load. This relates to structural loadings at a temperature of 0°C. (This condition will generally never govern for the range of conditions proposed.) Conductor Strength Limit State LV ABC conductors – 40% of Conductor nominal breaking load at a temperature of 15°C. (relates to insulation adhesion considerations).

In general the everyday or serviceability condition will govern and a tension change calculation is used to calculate tensions and sags under other conditions. In some cases however the maximum wind condition may govern at increased span lengths. The span at which the change occurs is called the transition span. Conductor stringing charts from which conductor tensions can be determined for differing temperature and wind loading conditions are located in the “Stringing Charts” section of the Design manual. 3.7 Span Ratios Large differences in the lengths of adjacent spans can result in significant tension differences across intermediate structures, which may not be able to be equalised by movement of the pole top and may cause ties or pins to fail. In rural situations practice is therefore to limit adjacent span ratios to 1:2. In short slack span urban situations, this practice is generally not necessary.


NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN 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 = α T S. Therefore T

= ∆ L/α 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.


NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN 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 = α T S. Therefore T

= ∆ L/α 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 = α T S. Therefore T

= ∆ L/α 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 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

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN crossarms may be subjected should take into account the conditions likely to be experiences in service so as to minimise the probability of failure, as far as reasonably practicable, these mechanical loads should be determined in accordance with ESAA C(b)1.

4.2

Design Loads

In designing crossarms for single supports the crossarms can generally be treated as two cantilevers fixed at the support. The crossarm at the support is therefore subjected to the following bending moments: • • • • • • •

Bending moment due to weight span of conductor, this may be either positive or negative, depending upon whether the profile imposes a down pull or an uplift of the conductors. Refer to Appendix 1 for illustration of weight span. Bending moment due to transverse conductor loads - wind and deviation loads acting at top of insulator pin (intermediate structures only). Refer to Appendix 1 for details on wind span.(These loads are fairly minimal) Bending moment due to direct horizontal pull of conductors (termination or strain structure only). Maintenance loads resulting from additional conductor lowering or anchoring activities and loads due to pole top rescue. The self weight of the crossarm (This load is minimal) Kingbolts must also be checked for allowable bearing loads perpendicular and parallel to the timber grain. Crossarm brace bolts must be checked for allowable bearing loads at an angle to the timber grain.

Allowable stresses for timber are dependant on the duration of the application of the load hence different allowable stresses are used for long duration, maintenance and short duration or wind loads. Table 4.1 gives the allowable long and short duration crossarm loads, for some of the more commonly used crossarms the table makes no allowance for vertical load. Table 4.1 Allowable Horizontal Crossarm Loads CROSSARM LENGTH CROSSARM

TYPE

Single Arm Double Arm

150x100 175x125 150x100 175x125

2400

2700

S.D.L. (kN)

Maint (kN)

L.D.L (kN)

S.D.L. (kN)

Maint (kN)

S.D.L. (kN)

9.6 17.5 19.2 35.0

7.7 14.2 15.4 28.4

4.5 8.2 9.0 16.2

8.3 15.4 16.6 30.8

6.7 12.4 13.4 24.8

4.0 7.4 8.0 14.8

S.D.L. = Short Duration limit state Loads eg. Wind Loads L.D.L. = Long Duration Loads eg. Conductor Weight Maint = Maintenance loads


NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN 4.3

Conductor Spacing

Crossarms must also be selected to give the required separation at the support and at midspan. For information on conductor separation refer to the Design Manual section “Layout Clearances”.

5

Poles

5.1

Introduction

Pole structures, and particularly single member pole supports are used to carry both high and low voltage conductors. Good pole supports, properly chosen with regards to local conditions and requirements, are a decisive factor in ensuring high continuity of service, long life of equipment and low maintenance costs. 5.2

Wood Pole Strength Rating

Previously wood poles were classified into light, medium and heavy class, regardless of strength group, and the Design Engineer had to determine the loading which each class and strength group would carry. All poles (wood or concrete) are now supplied with tip strength rating. The strength rating or short duration loads (S.D.L.) is the strength corresponding to the maximum allowable working pole tip load and must be multiplied by 1.8 to equate to limit state wind pressure loads on the project areas of both the pole and the conductor. Typical wind pressures:

Conductor 900 Pa or 1200 Pa in cyclonic areas Pole 1300 Pa or 1700 Pa in cyclonic areas

The pole long duration load (L.D.L.) is the continuous load that the pole has to withstand day after day. It is assessed as equivalent to the load applied by conductor tension at 15° C no wind and is half of the specified tip load (or approx 28% of the limit state load) 5.3

Pole design Loads

Unstayed poles may be subjected during service to the following horizontal loads:• • •

Horizontal load due to wind acting on pole Horizontal load due to conductor wind span Horizontal load due to conductor tension on angle, unstayed termination and unbalanced strain poles due to differential conductor tension in adjacent spans.

A vertical load is only imposed on unstayed poles by the conductor weight span and weight of fittings and very seldom becomes an important consideration, however for stayed poles a vertical load is imposed by the stay as well as the weight and fittings and this becomes a major consideration. Stayed poles are also subject to a bending moment, which is generally greatest at the stay foundation.

6

Pole Foundations

6.1

Introduction

The design of support foundations is rather more difficult than the design of other overhead line components, as the properties of soil are not as definite as those for other materials such Check this is the latest version before use. Page 13 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

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN as steel, aluminium, copper etc., and consequently for design purposes soil properties are selected within very widely varying limits. 6.2

Foundation Strength

The allowable pole tip Pο (kN) due to foundation strength is given by the following equation: Tip Load Pο = KmaxD J3 12 (h + ¾ J) where Kmax = Passive Soil Reaction (kPa/m) D = Average below ground pole diameter (m) J = Pole setting depth (m) H = Height of pole above ground (m)

7

Ground Stays

7.1

Introduction

It is necessary to stay overhead line supports at locations where the loads exceed the capacity of the pole/foundations so that the stay wire, rod, bed log/screw anchor etc. take the pull due to the conductors. Too much attention cannot be directed to the design, making off and setting of stays, as the future safety of the line, particularly under adverse weather conditions, depends equally as much on being correctly stayed as it does on the proper erection of the conductors. In the economic design of stays it is essential to match the strength of the component parts, ie. The stay wire, rod, bedlog etc. The maximum working strength of a particular stay type is determined by the least value of the strength of: • • • • • • 7.2

Stayrod Eyebolt Staywire Preformed Guy Grips Stay Insulators Foundations

Stay Application

It is important to note that when the stay attachment is at the load centre the horizontal component of the stay load is equal to the horizontal load occurring at the load centre. This occurs in the majority of cases as most stays on standard constructions are placed as close as possible to the crossarm. Where the stay attachment is not to close to the load centre, the horizontal load acting on the stay, P, due to the conductor termination or deviation, must be calculated.


NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN Select the appropriate stay whose allowable horizontal load, H, is greater than the calculated load, P. L x P y

P = L (1+3x/2y)

The calculation of the equivalent horizontal load, P, assumes that the bending moment occurring at a point one third the height of the stay attachment above ground level is zero. Table 7.2.1 lists the stay horizontal loads for a number of stay attachments for a conductor load L1 = 30kN and a stay height above ground y = 9.0 m. Table 7.2 Stay Horizontal Loads for 30 kN conductor load.

7.3

STAY ATTACHMENT (X)

CONDUCTOR LOAD (L1)

STAY HORIZONTAL LOAD (P)

At load centre

30kN

30.0kN

0.5m

30kN

32.5kN

1.0m

30kN

35.0kN

1.5m

30kN

37.5kN

2.0m

30kN

40.0kN

Pole Bending Moment

The pole must be designed to resist the maximum bending moment that will occur at the point of stay attachment. The allowable bending moment on wood a pole at the stay attachment points is given by the following equation: B.M. = fZ ..... .... .........................where

B.M. = bending moment F = design stress Z = section modules

fZ > L1 . x + wind on pole...........where

L1 x

= conductor = height of L1 above stay


NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN Table 7.3 lists the maximum allowable conductor short duration load (S.D.L.) and long duration load (L.D.L.) for a 14.0m x 8kN wood pole. Table 7.3 Pole Bending Moment – Allowable Conductor Tension 14.0m 8.0 kN Wood Pole

STAY ATTACHMENT

MAX. ALLOW S.D.L. CONDUCTOR TENSION

MAX ALLOW L.D.L. CONDUCTOR TENSION

(kN)

(kN)

At load centre

HORIZONTAL STAY TENSION (kN)

Governed by allowable stay load

0.5m

101.4

28.4

108.3

1.0m

50.7

14.4

57.6

1.5m

33.8

9.5

41.0

2.0m

25.3

7.2

32.8



APPENDIX 1 CONDUCTOR LOADS

Transverse Wind Load(N)

= Cond. Dia (m) x Wind Pressure (Pa) x Wind Span (m)

Vertical Load (N

= Cond. Weight (N/m) x Weight Span (m)

Wind Pressure

Wind Speed

500Pa

100 km/hr

900Pa

160 km/hr

1200Pa

184 km/hr

2 x Wind Span Weight Span 0°C 50°C

Weight Span @ 0°C Weight Span @ 50°C

0°C 50°C



Weight Span @ 50°C 0°C

Weight Span @ 0°C (negative) 50°C



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 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

NETWORK LINES STANDARD GUIDELINES FOR OVERHEAD LINE DESIGN 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.


Guidelines for Overhead Line Design

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