EHV TRANShlISSION LINE DESIGN Oppurtunities of Cost Reduction

8/16/2019 EHV TRANShlISSION LINE DESIGN Oppurtunities of Cost Reduction

1/8

IEEETransactions

on

Power Delivery,

Vol.

5 N O . 2 April 1990

EHV TRANShlISSION

L I N E

DESIGN

OPPORTUNITIES FOR COST REDUCTION

Richard

E.

Kennon, Senior Member

Electric Power Research Institute

Palo Alto, CA

KEYWORDS

EHV, Transmission, Line, Design, Economics, Optimization,

Standardization, Conductor, Terrain

ABSTRACT

The design of transmission lines is often limited to

a

few

standard conductors and structures in order to minimize the

costs of engineering, construction, inventory, and speed of

damage restoration. Certain design factors, such as unloaded

conductor tensions, maximum allowable conductor temperatures,

and phase spacing to avoid ice galloping induced flashovers,

are also fixed. Limiting the designer's choices can be economic

in certain situations but not in others. What makes economic

sense in flat terrain

o r

with a lightly loaded line may not be

economic in hilly terrain or with a line whose electric load

is consistently high.

This paper considers a range of line optimization

techniques which can be applied to decide whether standard

o r

optimized line designs

are

appropriate. It is found that even

simple methods of optimization can help the designer keep his

costs to a minimum.

The effects of electrical losses, structure family and

heights, conductor design limits on temperature and tension

are determined in flat and hilly terrain for a 500 kV example

case. In the example, in flat terrain, the selection of an

optimum design in place of the standard, results in savings

of from 8 to 15% in the total present worth of revenue

required for construction and losses over the life of the line.

In a specific section of hilly terrain, the example shows that

the use of optimization methods results in savings of from 15

to 19%. Th e costs

of

redesigning structures, using non-standard

conductors and investigating the proposed changes in design

limits must be measured against such potential savings.

1.0 BACKGR OUND INTRODUCTION

Many utilities in the United States and in Europe, utilize

standard transmission line structures in order to minimize the

costs of engineering, simplify construction, minimize inventory

and ease restora tion problems in case of damage. Since

structure loads are primarily determined by conductor loadings,

a

standard single

o r

bundled conductor corresponds to each

standard set

of

structures.

39 TD 435 2 PWRD 4 paper recommended and approved

by the IEEE Transro iss ion and 3 i s t r i bu t io n Commi tt ee

of

t h e

I E E E

Power Engineer ing 3ociety for p r e s e n t a t i o n

a t t he I EEE /PES

1989

Transmiss ion

an3

D i s t r i b u t i o n

Conference, New O r l e a n s , L o u i s i a n a , 4 p r i l

2 -

7 , 1989.

Yanuscr ip t submi t t ed October

7 ,

1983; made avai l a5 l e

f o r p r i n t i n g Y ar ch

8 ,

1989.

Dale A. Dou glas, Senior Member

Power Technologies, Inc.

New

York N Y

1145

Those who use standardized structure/conductor designs

do so for all but the largest line projects contending that the

engineering and test costs exceed the potential savings in line

losses and capital costs resulting from a full engineering line

design study. If a single structure/conductor combination is

used

for

a

particular voltage class, the only variables

considered in the line design process are route selection,

tangent structure heights, and structure spotting

of

the standard

structure family along the route.

Even in those cases where one attempts to optimize the

design of a particular line, the range of certain design factors

may be limited.

For

example, the minimum size of conductor

may be strongly influenced by radio noise limits, such limits

being fixed by regulation. Similarly, the cost of right-of-way

is a major factor in line cost but the cost of purchasing and

clearing the land is beyond the control of the designer.

Other design factors, however, that

within the control

of

the designer may be kept at traditional levels because of

a lack of technical data

-

for example, it may be

m o r e

economic

to install conductors to a higher unloaded tension than is done

in normal practice but the lack of field results on the

use

of

higher tensions prevents it

- or

simply because using traditional

values avoids th e need for eng ineering analysis.

A previous paper

[ I ]

described how line optimization

studies allow one to evaluate the potential savings offered by

conductor material and design innovations. Cost reductions

resulting from changes in conductor properties were determined

'for single and double circuit

345

kV lines in flat terrain.

Changes in conductor design leading to reduced aeolian

vibration and ice galloping and changes in conductor thermal

elongation coefficient, conductivity and the use of trapezoidal

rather than round strands were evaluated. The conclusions

of that paper were that the greatest opportunities for cost

reduction lay in increased everyday conductor tension, conductor

compaction, and reduced thermal elongation.

Using the optimization methods described herein, the utility

line designer may estimate the potential savings involved in

various changes in his standard design without needing to

develop detailed structure designs. This calculated savings can

then be compared to the costs of the designing and testing

structures, the additional cost of inventory/restoration and the

detailed engineering costs that would be required to build the

optimized line.

2.0 LINE DESIGN CHOICES

There is no unique process by which all transmission lines

are designed. There

are,

however, certain elements to any

line design that are common to all, The largest difference

between the design process at one utility and another is

determined by the degree to which the line design is

standardized or optimized. That is, the extent to which the

engineer is free to select the structures and conductor, and

to alter certain design parameters to obtain the optimum line

design solution in a particular design situation.

2.1 ODtimization Versus Standa rdizatio n

In a fully standardized design, the engineer essentially

fits a limited family of standard structures supporting the

corresponding standard conductor to the selected route. He

attempts, while doing this, to minimize both the number of

structures per mile and the use of expensive anglehtrain

structures whenever possible.

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2/8

1146

In flat terrain, structures are spotted at or near their

maximum allowable wind span and angle/strain structures are

only required for failure containment and at points where the

line changes direction. In hilly

or

mountainous terrain,

anglehtrain structures may be required to handle sharp changes

in elevation and the full utilization of tangent structure

capabilities is less likely.

In a fully optimized design, the use of any available

structure and/or conductor is possible

so

long

as

it can be

shown to be economic and reliable. Design limits are

questioned and studied. In flat terrain the wind/weight spans

are varied searching for the lowest cost for each potential size

and type of phase conductor. in hilly or mountainous terrain,

unloaded conductor tensions, structure wind/weight spans, H/V

ratios, and the available range of structure heights and types

are varied seeking to find the minimum cost support structure

for each potential phase conductor size and type.

2.2 St ructure Choices

Some of the major structure design parameters that may

be varied in a process of line optimization are:

o Structure type (steel lattice, guyed V , etc.)

o Structure mechanical characteristics (wind span,

weight span, H/V ratio)

o

Available structure family (tangent, Oo 15O

suspension, etc.)

o Available structure heights (range and

interval)

Admittedly, the designer may not be free to vary

ll

these

design parameters because of regulatory or aesthetic restraints,

but, unless completely restricted as to structure, available design

possibilities should be studied in a systematic manner.

2.3 Conductor Choices

Conductor design parameters (not all of which are

independent) that may be varied in a process of line

optimization are:

o

o

subconductor diameter

o

conductor strength weight

o

conductor resistance

o

coefficient of thermal elongation

o conductor weight per unit length

number of conductors per phase

2.4 D esign Limits

Design limits can result from safety, reliability or economic

considerations. They can be based on detailed engineering

studies or on simple tradition. Safety considerations determine

minimum ground clearance, structural safety factors and

maximum ice and wind loadings that the line must survive.

Reliability determines that the mid-span phase-to-phase spacing

must be adequate to avoid flashovers during occurrences of

ice galloping. Environmental limits may dictate the minimum

diameter subconductor in a particular bundle configuration.

3.0 ALTERN ATIVE DESIGN OPTIMIZATION APPROACHES

If the designer is limited to the use of a few standard

structu res (single tangent struc ture, 15O strain and a dead-end

structure) all designed for a particular conductor, then the

optimization of this line is limited to the minimum cost

placement of these structures on the chosen right-of-way.

This may be done by an experienced hand or by numerical

methods. This particular form of optimization is well understood

and is a standard part of every line design. The greatest

opportunities for minimizing cost in the process of spotting

towers is in uneven terrain[2].

This study assumes that the process of line design using

standard structures and conductor includes the optimum or near

optimum cost placement of available structures on the right-

of-way either by means of an experienced tower spotting

engineer or by means of a numerical program.

3.1 Con ductor ODtimization

The type and size of phase conductor are varied.

Standard

structures are not varied. The minimum allowable conductor

diameter is set by the minimum required thermal capacity for

low voltage lines and by radio noise, audible noise and corona

requirements for higher voltage bundled lines. The maximum

practical conductor diameter is determined by the load limits

of the standard structures (the larger the conductor diameter,

the shorter the wind and weight spans - the stronger the

conductor, the smaller the maximum line angle of angle

structures).

Within these constraints on conductor size, the

subconductor diameter is selected such that the sum of total

present worth of revenue required (PWRR) for electrical losses

over the life of the line and the levelized costs of construction

and maintenance, are a minimum [3],[4].

3.2 Conductor and Structure Ootimization

Variations in both the conductor and the structure are

considered. The lower limit on conductor diameter is still set

by either thermal or environmental design limits but larger

conductors can be accommodated by strengthening structures.

Conductors and structures having certain novel characteristics

are considered. Since the range of choices is large, the task

of design becomes both more complex and, potentially at least,

more rewarding.

For each type of structure investigated, the structure cost

varies with crossarm height, phase spacing, conductor diameter,

tension and span length.

A mathematical relationship between

structure cost and these variables must be determined before

this optimization approach can be undertaken. The resulting

optimal conductor and structures will involve the costs of

detailed structure design and perhaps some test work.

Design limits

-

unloaded conductor tension limits,

maximum conductor temperature, phase spacing, etc.

-

are

unchanged.

3.3 Condu ctor. Structur e and Design Limit ODtimization

The evaluation of certain innovations in conductor and

structure materials and/or design may require a re-evaluation

of conductor, structures and even design limits, limited only

by the need for safety and reliability at minimum cost. For

example, a new conductor design such as SDC[S], can be

installed at higher than standard unloaded tensions and offers

reduced structure wind loading due to the use of trapezoidal

rather than round strands. The resulting savings in structure

height and/or number of tangent structures per mile must be

compared to the increased costs from more heavily loaded angle

structures and possibly higher construction loads. The

advantages and disadvantages of such innovations require a

rather thorough rethinking of all the standard design

components and assumptions. Even with the use of standard

conductors, the consideration of non-standard design limits such

as reduced or increased tension[6] and reduced o r increased

maximum temperature[7] may offer significant savings in certain

types of terrain or may provide the justification for necessary

research concerning the use of nonstandard design limits.

4.0 OPTIMIZATION

OF

A STANDARD

500

KV DESIGN

In order to demonstrate the preceding optimization

alternatives, the design of a 500kV transmission line using a

standard conductor and limited family of standard structure

designs will be compared to various optimized alternatives.

The example designs are kept relatively simple but are sufficient

to illustrate the concepts clearly.


3/8

1147

'ooor------

6

I \

4 0 0 -

l

Certain major line costs (such as right-of-way purchase

and clearing and shield wire material and labor) and certain

minor line costs (such as insulators, spacers and hardware) are

insensitive to changes in the design of the standard steel lattice

structure . These costs essentially remain constant regardless

of optimization efforts regarding structures and conductor.

4.1 Standardized Design

It is assumed that the example line is to be built in a

National Electric Safety Code (NESC) Heavy loading area;

that the standard design consists of

a

3-conductor bundle

of

Rail ACSR; and that only single circuit, self-supporting steel

lattice structures are used either for standard or non-standard

structures. Th e standard structure family consists of a tangent

tower that can be used at line angles of up to 2 degrees, and

a 60° dead-end strain tower. The standard tangent structures

are available in three heights and the strain structure in only

one height. Minimum ground clearance at the maximum

conductor temperature of IOOC is 27 feet.

The standardized line design process as defined here,

allows one to minimize the line cost by selecting the least

expensive combination of structure heights and types from

those available but does not allow variation of the mechanical

characteristics of the various standard structure types nor the

available range of heights. The standard structure family is

described in Tables 1 and

2.

The mechanical capabilities

(maximum line angle, wind spans, weight spans, maximum wind

to weight span ratio) are listed for the suspension and the

strain structure. Structure costs include structure and

foundation material, labor and hauling, insulators, conductor

hardware, and conductor clipping costs.

MA X

MAX WIND SPAN

MA X I MU M MA X

ANGLE ANG.

ANG. LOADED BARE

RATIO

TY PE LI N E @ M A X

@

Oo WEIGHT SPAN H/V

P E G ) FT) n)

FT)

IW

T A N G E N T

SUSPENSION 2 1000

1000

1245 2000 1.4

A N G LE

STRAIN 60 1000

5000

2000 3 ~

TABLE la.

-

Mechanical Capabilities

of

Standard Structure

Family with Standard 3-Conductor Bundle

Rail ACSR

Structure AlTACHM ENT COST OF

Type X A R MH T H EIG H T STR U CTU R E

m

FT)

W 1s K)

TA N G EN T 73 60

21.9

SUSPENSION

83

70 24.0

9 3

80 26.1

A N G LE

STRAIN

60

60

63.1

TABLE Ib - Costs

for

Standard Structure Family.

Note

that costs include m aterial, erection. hardware insulators

and clipping.

Note that the maximum allowable wind/weight spans and

H/V

ratios

of

the standard structure family members, apply

to the standard phase conductor (a bundle of 3 Rail ACSR

conductors) installed to the standard design conditions

(15

final unloaded tension at 60F,

70

maximum loaded tension)

with NESC Heavy loading.

In flat terrain, a single tangent structure (crossarm) height

In hilly terrain, the standard tangent tower

f 93 feet is used.

is available in crossarm heights of 73, 83 and 93 feet.

The standard conductor and structure design meets the

environmental requirements -


4/8

1148

4 .2 Ootimized Conductor Selection

In this simplest optimization study, the standard

transmission line structures remain unchanged both in cost and

mechanical capability. Th e search for alternate conductors

is restricted to those that meet the environmental

requirements, and can be installed using the standard structure

family with the same design restrictions on unloaded tension

and maximum conductor temperature.

Consider the two possible electrical loading situations as

described in section

4.1

-

259/0

LF

and 75%

LF.

Given the

restrictions on changing the standard structures and the

environmental limits, the conductor selection study is limited

to other sizes of 45/7, low steel ACSR or mechanically

equivalent conductors in 2 and 3-conductor bundles. Only ACSR

conductors are considered here.

The use of larger 45/7 ACSR conductors in a 3-conductor

bundle meets the environmental restrictions but would require

redesign of the strain structures because of increased tension

loads. Two-cond uctor bundles with conduc tor diameters of less

than 1.7 inches do not meet the environmental requirements.

A two-conductor bundle of 2156 kcmil, 84/19 Bluebird, however,

presents approximately the same mechanical ice and wind

structure loads, only slightly higher RI and AN levels, and

nearly the same maximum sags as the standard 3-conductor

bundle of Rail ACSR.

Table 3 compares RI/AN levels, direct and present worth

material and labor costs, and present worth of electrical losses

over the

35

year life of the line for

Loss

Factors of

25

and

75%, for both the standard 3-conductor Rail and the alternate

2-conductor Bluebird bundle. Since the conductors have

approximately the same sag and utilize the standard structures,

the structure costs are approximately the same both in flat

and hilly terrain. For either

Loss

Factor, the Bluebird alternate

yields a lower net PW cost, however, the PW savings at the

higher loss factor is approximately three times as large.

From Table 3 , it can be se en that in creasing th e PW of

construction costs by $38K/mile to install the Bluebird

conductor, reduces the PW of electrical losses by $66K/mile

and $118K/mile for loss factors of 25 and 75%, respectively.

The corresponding net PW savings are $28K/mile and $SOK/mile.

3 RAIL 2. BLUEBIRD

ENVIRONMENTAL

EFFECTS

RI @ Edge of ROW (dB) 44 45

AN

@

Edge

of

ROW (dB(A) 47 51

CONDUCTOR MATERIAL LABOR

DIRECT COST ($K/MILE)

PW COST (SK/MILE)

PW OF BLEC LOSSES (SK/MILE )

LF

25%

LF 75%

61

100

84

138

236 I70

42 303

PW NE T SAVlN GS ($K/MILE)

LF 25% 28

LF 75%

80

TABLE 3 - Comparison of Present Worth

of

Conductor

Material & Labor and Electrical Losses

or

Standard

3*RAIL ACSR Bundle and Alternate 2.Bluebird Bundle

at Loss Factors (LF)

of

25% and 75%. Table data applies

to both flat and hilly terrain.

4.3 Optimized Conductor and Structure

In this example, it is assumed that either the standard

conductor and/or the standard structure family can be altered

if

R

lower cost design solution is found but that the standard

design limits (maximum conductor unloaded tension,

environmental effects, etc.) remain the same. The savings

indicated by this study of conductor and structure alternatives

must be compared to any design and testing costs that would

be required to implement the new design.

In order to evaluate possible cost reductions through

changes in the standard structure family, one must be able to

estimate structure cost as a function of conductor loadings

and tower dimensions. While it is possible to recalculate the

structure cost by means of a detailed analysis for each

combination of wind/weight span, conductor, tower height and

phase arrangement, section 4.3.1 discusses a simpler and quicker

method of estimating structure cost by means of a linear

regression equation.

4.3.1 Regression Cost Eauation s

-

Structure and foundation

cost depends upon conductor loading (maximum wind/weight

spans), phaseishield wire attachment points (phase-phase spacing

and height to bottom phase) and, for angle structures, on the

conductor tension and line angle. For the purposes of

optimization studies, structure and foundation costs under

different loading conditions may be estimated based upon a

limited number of structure designs through the development

of a linear regression structure cost equation.

While it is understood by the authors that structure and

foundation costs are not linearly dependent on loading and

geometry, experience has shown that a linear regression model

is adequate for optimization studies performed early in the line

optimization process. Once a combina tion of conduct or and

structure family is tentatively selected, the designer may want

to replace the approximate structure costs with more precise

values based upon detailed design studies. These improved

models could then be used in the final tower spotting process

in fine tuning the conductor tension and tower locations.

As an example of how such regression equations are

developed, steel lattice structure weights were calculated for

sixteen different combinations of conductor, crossarm height,

and with conductor loadings typical of an NESC area for single

circuit steel lattice structures, using a finite element design

program called SCTDES written by Dr. Alain Peyrot of the

University of Wisconsin although one could equally well obtain

such data from other design methods.

The resulting structure

weights for the eight cases are listed in column 1 of Table

4. The transverse (T), vertical (V),longitudinal

(L)

broken wire

loads and the crossarm height above earth are also shown in

the Table.

ACTUAL

ESTIMATED

T L V H t

WEIGHT

WEIGHT LOAD LOAD LOAD HT

TWR TWR ACT-EST TRNS LONG VERT XARM

9901.52 9910.56

-0.1

19.40 12.39 33.71 66.50

19098.86 19200.28 -0.5 33.95 9.18 59.00 109.20

9824.24 9560.86 2.7 15.55 11.92 30.48 67.10

18046.21 18195.65

-0.8

27.22 10.49 53.35 107.80

10172.73 10183.58

-0.1

19.77 12.51 35.47

66.90

19443.56 19255.46 1.0 34.60 10.76 62.08 107.10

1011S.15 9750.80 3.6 16.32 13.07 32.07 67.20

18648.11 1 8584.47 0.3 28.57 11.43 56.12 108.20

9865.15 9933.53 -0.7 19.21 30.50 32.05 71.40

24078.41 23800.07 1.2 33.62 6.66 56.09 144.50

9434.85 9415.91 0.2 15.07 38.28 28.59 72.50

20106.44 19973.30 0.7 26.38 19.84 50.03 125.30

9965.15 10070.33 1 . 1 19.57 35.10 33.61 71.80

23097.35 23262.15 -0.7 34.25 9.15 58.82 138.60

10056.06 10513.09 -4.5 16.53 8.60 31.36 72.50

20655.30 20899.06 -1.2 28.93 7.84 54.88 125.30

TABLE 4

-

Actual Structure Weights Compared

to

Estimated Weights

Based on the Following Regression.

Equation: SW - 3386 + 12.6.T -24.9 L + 122.6'V

+

138.8. Ht

Based on the design weights and the conductor loadings

and crossarm height, a linear regression equation, which is also

shown in Table 4, was determined. The primary independent

regression variables are transverse load and crossarm height,

but the fit is improved by including vertical and longitudinal

loading as well. The second and third columns of the Table

show the structure weights calculated on the basis of the

regression equation and the percent difference from the design


5/8

1149

weights, respectively. Clearly, the linear regression equation

yields excellent estimates of structure weight over the range

of loading and structure heights indicated in the Table.

A similar analysis of foundation costs and strain tower

weights lead to corresponding linear regression equations for

foundation cost and strain structure weight. The optimization

studies in the balance of this paper utilize such equations in

order to estimate the cost of structure and foundation for

various conductors, heights and spans.

4.3.2 ODtimum Conductor

&

Structure in Flat Terrain-

In section 4.2, a 2-conductor bundle of Bluebird ACSR was

shown to be less expensive than th e standa rd 3- conductor

bundle of Rail ACSR yet gave roughly the same RI and Audible

noise and the same structural loads. In this section, the

possibility of using a wider range of conductor types and sizes,

while adjusting the structure cost to reflect the conductor load

in each case, is considered. Also, the wind/weight span of

the tangent structure is varied for each candidate conductor

in searching for the minimum cost design. Design limits-

conductor unloaded tension in %RBS, maximum conductor

temperature, broken wire load assumptions, RI limits, etc.-

remain constant for each conductor and structure.

In flat terrain, angle towers are used wherever there is

a change in line direction. Thus the number of such towers

is constant no matter how the conductor and structure

wind/weight sp ans vary. The cost of each angle structure,

however, changes with the conductor tension and the

transverse load. Tangent towers are largely insensitive to

conductor tension, but increase in cost primarily with

transverse load.

Starting with the standard win d span of 1000 feet, the

total present worth of revenue required (PWRR) for electrical

losses and construction cost is plotted against conductor

diameter. Figures 2a and 2b are for loss factors of 25% and

75%, respectively. 3-conductor bundles of all-aluminum

conductor (AAC); 45/7 ACSR; and 54/19 ACSR were studied.

The subconductor diameter yielding minimum total present worth

of revenue required (PWRR) for construction and losses is not

sensitive to the conductor type, however, it sensitive to loss

factor. The largest diameter subconductors of each type - 1590

kcmil Coreopsis AAC, 1590 kcmil Lapwing, a nd 1590 kcmil

Falcon - offer savings at either

loss

factor over the standard

design.

TOTAL

PWR R (SKIMILEI

i

I

20

1

CONDUCTOR

DIAMETER

(IN)

Figure 2a - Total PWRR versus Subconductor

Diameter for 3 Conductor Bundles. Wind Span

is 1000 ft. Loss Factor is 25%. All Tangent

Structures in Flat Terrain.

TOTAL PWR R ( K/MILE)

I\

75

-

7

65 1

CONDUCTOR DIAMETER (IN)

Figure 2b

-

Total PWRR versus Subconductor

Diameter fo r 2 Conductor Bundles. Wind Span

1000 ft.

Loss

Factor is 75%. All Tangent

Structures in Flat Terrain.

Total PWRR depends slightly on the tangent tower wind

span for the three 1590 kcmil conductors. In this case, the

minimum total PWRR occurs for the standard wind span of 1000

feet.

With a 3-conductor bundle of Lapwing ACSR, with loss

factors (L F) of 25% and 75%, and a peak normal load of 1000

MVA, the net savings in comparison to the standard 3-conductor

Rail bundle is $39K/mile and $108K/mile, respectively.

4.3.3 ODtimum Conductor Structure in Hillv Terrain-

The effect of altering the standard range of tower heights

and the wind span of standard tangent structures will be studied

for the Lapwing conductor selected in section 4.3.2. The

standard structure/conduc tor family has tangent tower

attachment heights of 60 to 80 feet available for hilly terrain.

Table shows the effect of varying the wind span and the

range of structure heights.

RANGE OF MAX NO.

OF

COST OF

AlT AC H WIND STRUCTURES STRUCTURES

CASE HEIGHTS SPAN TANGENTtDE-TOTAL TANGENTtDE-TOTAL

FILE (FT)

FT)

(SUMILE)

3D

70 800 16 +

I 1

- 27 I l l + 224 335

121

+

162

=

2820

1000

I6

8 -

24

139 +

101

-

240

200

17

+

5 22

0

IO8

+ 183 = 292

E 60-80 800 17 + 9 = 26

60-80

1000

20

+ 5

25 142 +

101 -

243

1 3 9 + 8 0 - 2 1 9

0-80

I200 18 + 4

22

3F 60-120

1200

17 +

3 -

20 I44 + 60

-

204

60-120 I400 I S

+

2

=

17 145 + 41

=

I86

TABLE

5 -

Variation in Direct Structure Cost (for 3 L apwing B undle )

with Maximum Wind Span and Structure

Heights

in Hilly Terrain.

In this hilly terrain, the

use

of stronger tangent

structures (i.e. a 1200 foot wind span instead of the standard

1000 feet ) increases the cost of tangent structure but

reduces the QQ structurc cost from $243K/mile to $219K/mile

by reducing the numbei of tangent structures from 20 to 18

and the strain structures from to 4.

Also, referring again to Table 5 increasing the range

of

structure attachment heights from 60-80 feet up to 60-120 feet

further reduces the total structure cost for a 1200 foot tangent

wind span fro m $219K/mile t o $204K/mile. If the tangent wind

span is further increased to 1400 ft., the total structure cost

is reduced to $186K/mile.

By increasing the range of available tower heights and

using stronger tangent strudtures, the structure cost associated

with the 3-conductor Lapwing ACSR conductor can be reduced

by $57K/mile.


6/8

1150

When the optimized 3-conductor Lapwing design is

compared to the standard 3-conductor Rail design, net savings

of $86K and $155K/mile in total PWRR for losses and

construction are found for LF's of 25% and 75 , respectively.

4.4 Ootimization Studv of Design Limits

In this section, the value of changes in two standard

design limits will be considered - maximum allowable conductor

temperature and maximum final unloaded design tension

expressed as a percent of rated strength. First the sensitivity

to these design limits will be investigated for the standard

conductor and structures, then the broader impact on the use

of alternative conductors and structures will be considered.

4.4.1 - Changing Maximum Conductor TemDerature - The

maximum allowable conductor temperature determines the

maximum sag in EHV lines. Thus it seems apparent that any

decrease in this design limit will result in shorter structures

and less construction cost. Yet the maximum allowable

temperature also determines the thermal rating of the line.

The higher the maximum allowable temperature of the conductor,

the higher the thermal rating of the line. In low voltage lines,

thermal ratings can be of primary concern and insufficient

thermal capacity under emergency conditions may force

expensive reconductoring of lines.

In EHV lines, the environmental limits usually require the

use of relatively large bundled conductors whose thermal

capacity, even for moderate temperature limits is more than

sufficient over the life of the line. The line cost data in Table

6, shows the sensitivity to maximum allowable conductor

temperature both for flat and hilly terrain. If the maximum

conductor design temperature is reduced to 5OoC from 100°C,

with the standard conductor/structure in flat terrain, the

difference in direct construction cost of tangent structures

is $5K/mile. The savings is virtually the same whether the

terrain is flat or hilly. If it can be determined that the thermal

capacity of the line is sufficient for a 5OoC maximum conductor

temperature, then the savings is easily obtained.

SAG AT TOTAL

MAXIMUM MAXIMUM TANGENT

ALLOWABLE TEMP FOR TOWER

TERRAIN CONDUCTOR 1200

FT.

COST

__ TEMP (CC1 RULING

SPAN

@./MILE1

F L AT 100

60.0

151

F L AT 50 55.0 146

HILLY'

100 60.0 124

HILLY' so

55.0

120

*Structure Heights of 60 to 120

f t .

Wind Span 1200 ft.

TABLE

6

- Effect of Maximum Allowable Conductor Temperature

Upon Line Cost for

3

x Lapwing Conductor Bundle.

4.4.2 Effe cts of Maximum Unloaded Conductor Tension

-

If

the standard design limit on maximum unloaded conductor

tension is increased, the level of aeolian vibration will also

increase. This may require the use of vibration dampers.

Increasing conductor tension also affects structure cost in three

ways - the number of tangent structures per mile tends to go

down since the conductor sag is less for a given span; the

tension load on angle structures goes up requiring them to be

stronger and therefore more expensive; and strain structures,

the use of weights, or the redesign of suspension structures

may be required in order to deal with uplift problems in uneven

terrain.

Figure 3 shows curves of structure cost versus final

unloaded tension (expressed as a pelcent of rated breaking

strength) for a line in flat ierrain where there are only

tangent structures, and a line where there are assumed to be

4 angle structures within the 3.6 mile section. In the latter

case, the decrease in the number of tangent towers required

is offset by the increased cost of the four angle structures.

In the former case, the total PWRR of the line decreases by

$22K/mile as the tension is increased from

15

to 21%.

TOTAL PWRR (8KIMI)

I

7

WIT H AN GLES

66 k I

I

21

6

15

18

UNLOADED FINAL TENSION @60F

RES)

Figure 3 - Total PWRR versus Unloaded Final

Conductor Tension @ 60F for Flat Terrain

with and without Angle/Strain Towers.

Figure 4 shows the impact of conductor tension limit

changes on the line costs for the standard structure/conductor

combination in the hilly terrain shown in Figure 1. It is

assumed that there are no line angles between the strain towers

at each end of the section. Note that the tension (in YoRBS)

which yields the minimum total PWRR is different for each

of the three conductors considered. Above a certain value

of unloaded conductor design tension, the number of strain

structures required increases as does the total structure cost.

Clearly, in hilly terrain, even without line angles, the use of

higher conductor tensions is not always an advantage.

TOTAL

PWRR ( KIMI)

8

LAPWINO

COREOPSIS

7 8 0 .

FALCON

780

WIND SPAN

7

FINAL UNLOADED TENSION C60F ( RBS)

Figure 4 - Total PWRR versus Unloaded Final

Conductor Tension

@

60F for Hilly Terrain. Wind

Span of Tangent Towers is 1200 ft. and 1400 ft.

4.5 Comoarison of Standard and Ootimized Line Designs

In the preceding sections of this paper, the possibility

of reducing the costs of construction and electrical losses by

varying different line design factors has been evaluated for

a particular

500

kV line design situation. Clearly, the savings

due to each variable while not precisely additive, may

accumulate. Thus it is of interest to consider the possible

savings that could accrue if the standard line design is

optimized with respect to conductor size, type, maximum

allowable temperature, an d unloaded tension, and where the

structures are optimized with respect to wind span and, in hilly

terrain, with the range of available heights.


7/8

1151

Table 7a compares the standard line design (1000 foot

wind span, 3-conductor Rail ACSR bundle, 100°C maximum

allowable conductor temperature, 15% final unloaded tension,

etc.) with an optimized design in flat terrain with all tangent

structures. The net total present worth savings on the cost

of structure, conductor and electrical losses are $44K/mile and

$1 13K/mile f or loss factors of 25% and 75%, respectively. These

savings must be compared with the added costs of tower

redesign and the need for additional inventory

of

conductor

hardware, etc.

STANDARD OPTIMUM

Conductor

3

Rail 3 Lapwing

Max Wind Span

1000 1000

Tension (%RBS),T

15 .

IOOOC

21 . SOT

Direct Construction

Cost (SK/Mile) 191 219

Total PWRR (SK/Mile)

75

LF 734

621

25 LF 549 505

TABLE 7a

-

Comparison of Standard and Optimized Standard and

Optimized 500

kV

Conductor and Structure in Flat Terrain.

Table 7b compares the standard line design to that

of

the optimum in hilly terrain. As in the preceding, the net total

PW savings are $102K/mile and $168K/mile for 25% and 75%

LF's. These potential savings must be compared with the added

costs of tower redesign and the need for additional inventory

of conductor hardware, etc.

STANDARD OPTIMUM

Conductor 3. Rail

3.

Coreopsis

Max. Wind Span 1000 1400

Tension

(%R BS). OC

15,

100 21, so

Direct Cost of

Conductor & Structures

(SK/Mile) 275 265

Total PWRR (%/Mile)

75 871 703

25 686 584

TABLE 7b - Comparison of Standard and Optimized 500 kV

Conductor and Structure in Hilly Terrain

(3.6

miles

wi t hou t line angles).

5.0

CONCLUSIONS

Clearly, the indicated savings through line optimization

is not a net savings at all. The cost of implementing these

various changes in structure and conductor design and use will

clearly cost money as well as save it. The point is, that the

methods outlined in this paper point in the direction of

potential cost savings in EHV lines. The degree to which they

are actually implemented by the utility designer depends upon

the costs associated with the changes as well as upon the

savings indicated here.

Nevertheless, one may draw several conclusions from the

preceding examples of optimization:

(1)

The use of standardized conductor and structure designs

is not necessarily uneconomic. In many cases, the size

of conductor at EHV voltages, as determined by

environmental needs, requires a conductor larger than

is economic. In such cases, the use of a standard

conductor and corresponding structure designs minimizes

the engineering costs and eases inventory/restoration repair

problems.

(2) If the standard conductor design, which meets the

environmental and thermal needs of the line, is smaller

than the economic conductor size, then the savings in

present worth of electrical losses over the life of the

line can be significant. This is particularly true on long

lines where the use

of

a non-standard conductor is not

an inventory or restoration problem.

The potential savings from a line optimization study

increase with the number of factors considered. For

example, a study that considers redesigning the structures

as well as the conductor allows potentially higher savings

than a study of conductor alone.

Line design factors considered in this paper were conductor

size, conductor type, structure wind/weight span, structure

height range, unloaded conductor tension, and maximum

allowable conductor temperature. All appear to be

potential sources of significant savings.

The potential savings from optimization studies is difficult

to predict without actually performing the studies. With

the availability of increasingly sophisticated numerical

line design methods, the engineering time required for

such studies (and thus the cost) has decreased dramatically.

Even if standard conductor and structure designs are

retained after concluding an optimization study, the

designer has a solid basis for retaining standard designs.

ACKNOWLEDGMENT

The authors acknowledge the contribution of Messrs. H.

Bryan White and John Bates to the sections of this study

concerning structure optimization with regard to terrain. The

algorithm used in studying tower placement in hilly terrain

was developed by them.

In

addition, the authors wish to thank

the Electric Power Research Institute which sponsored the

studies from which this paper was excerpted and who also

sponsored the development of the Transmission Line Optimization

Program with Terrain (TLOPWT) which was used to perform

all the optimization calculations.

1.

2.

3

4.

5

6.

7.

6.0 REFERENCES

Dou gla s, D.A., Economic Measures of Bare Overhead

Conductor Characteristics, IEEE Transactions on

Power Delivery, Vol. 3, No. 2, April, 1988, pp 754-

761.

Bates, J. and White, H.B., Micro-based Program

Refines Transmission Design, Transmission

Distribution Magazine, April; 1987, pp 40-49.

Grant,

IS.

and Clayton, R.E., Transmission Line

Optimization, IEEE Transactions on Power Delivery,

Vol. PWRD-2, No. 2, April, 1987, pp 520-526.

Grant, I.S. and Longo, V.J. Economic Incentives for

Larger Transmission Conductors, IEEE PES Paper

81 WM 208-8 presented at the IEEE PES Winter

Meeting 1980.

Kirkpatrick, L.A., McCulloch, A.R. and Pue-Gilchrist,

A.C., Ten Years of Progress with Self-Damping

Conductor, Paper No. F79 736-0, presented at the

IEEE PES Summer Meeting, 1979.

Fritz, E., The Effe ct of Tighter Conductor Tensions

on Transmission Line Costs, IEEE Transactions

Paper, Vol. PAS-79, 1960, pp 513-527.

Day, P., Gaylard, B., et al, Influence of Conductor

Designs and Operating Temperature on the Economics

of Overhead Lines,

Proc.

IEEE, Vol. 118, No. 3/4,

March/April, 1971.

,


8/8

1152

APPENDIX

Economic Data

-

Base Case Values

Cost of conductor steel

Cost of 1350 aluminum

Cost of tower erection

Cost

of

tower material

Period of analysis

Discount Rate

Line fixed charge rate

Demand charge

Demand fixed charge rate

Demand reserve

Demand charge escalation rate

Energy charge

Energy charge escalation rate

0.40

$/lb

1.00 $/lb

1.00 $/lb

1.00 $/lb

35 years

12.00%

20.00

550.00 $/kW

20.00%

20.00%

3.50%

0.020 $/kWh

7.00%

Dale A. Douglass is a Senior Member of the IEEE. Born in

Cleveland, Ohio in 1941, he received a BSME in 1963, and an

MSEE and PhDEE in 1964 and 1967, respectively, from Carnegie

Mellon University.

After working as a member of technical staff with Bell

Laboratories and as a product design engineer with Kaiser

Aluminum, Dr. Douglass joined the staff of Power Technologies,

Inc. in 1978 as a Senior Enginee r. He is presently Manager

of the Overhea d Transmission Systems Unit.

He

has been

involved in many aspects of overhead transmission design

including wind-induced motions of conductor, optimal economic

transmission design, and dynamic thermal rating techniques.

He is a member of Tau Beta Pi and Pi Tau Sigma and is a

member of the working groups on Overhead Conductor

Temperature and Vibration and Galloping under the Towers,

Poles, and Conductors subcommittee of the IEEE.

He

has

chaired task forces on transient thermal ratings, AC resistance

of

bare overhead conductors, and safe design tensions for

conductor.

Mr. Richard E. Kennon is a graduate of the California Institute

of Technology with a B.S. degree in Electrical Engineering

obtained in 1952.

He

received a Masters degree in Business

Administration from Indiana University in 1972. A senior

member of the Institute of Electrical and Electronic Engineers,

Kennon holds six

U.S.

patents and has written several articles

and papers on the subject of overhead transmission lines.

He

is also an individual member of CIGRE.

Before moving to EPRI, Kennon was Manager, Capacitor

Equipment Engineering with the Westinghouse Electric

Corporation in Bloomington, Indiana. In that position, he had

responsibility for design of series capacitor protection, capacitor

banks and capacitor fuses.

He

also held the positions of

Supervising Engineer and Senior Engineer where he was

responsible for design of station class surge arresters and

development of ceramic bonded silicon carbide arrester blocks.

He was formerly a Sales Engineer with Westinghouse in Los

Angeles.

Kennon is the Manager of the Overhead Transmission Lines

Program, Electrical Systems Division, at the Electric Power

Research Institute (EPRI) in Palo Alto, CA. Kennon joined

the Institute in 1975 as a Project Manager in the Substations

Program. Moving to his presen t position in 1978, he became

responsible for research at two transmission research facilities

and for development of EPRI’s TLWorkstation.

Documents

EHV TRANShlISSION LINE DESIGN Oppurtunities of Cost Reduction