ehv transhlission line design oppurtunities of cost reduction
TRANSCRIPT
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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.
O55j-S950/9O/0jOO-l145 OlOO
990
EEE
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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.
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'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 -
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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
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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.
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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.
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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.
,
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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.