Variable 2: Rendimiento académico de arte
III. Resultados
3.1 Resultados descriptivos
Consider a 10-mile (16-km) long, 115-kV transmission line with 336.4 kcmil (172 mm2) Linnet ACSR conduc-tor installed to a final unloaded tension of 16% UTS at 60°F. The wood pole H-frame structures are spaced quite uniformly at 600 ft (180 m). According to the util-ity operating this line, it is assumed that the conductor has a summertime thermal capacity of 430 A (75oC con-ductor, 40°C air, 2 ft/sec (0.6 m/sec) cross-wind, with sun) that corresponds to a thermal line capacity of 85 MVA. The sag-tension data for the original conductor is shown below in Table 2.8-12.
The existing structures are in good condition, and it is possible to reinforce strain structures to allow an increase in the present maximum conductor tension (6410 lbs or 28.6 kN) of up to 50% for a cost equal to less than 10% of rebuilding all structures. The transverse load capability of the existing tangent structures is such that the diameter of the replacement conductor can be up to 10% higher than that of the existing conductor (0.72 in., or 1.8 cm) without reinforcing or replacing tangent structures.
The suspension structure conductor attachment height may not easily be increased, and the existing line’s ground clearances with a sag of 13.2 ft (4 m) at 75°C are barely adequate. Therefore this maximum high-temper-ature sag may not be exceeded in any of the uprating alternatives.
Table 2.8-12 ALCOA Sag and Tension Data
ALUMINUM COMPANY OF AMERICA SAG AND TENSION DATA 600 ft spans, 16% final, 75C max, w comp
Conductor LINNET 336.4 Kcmil 26/ 7 Stranding ACSR Area = 0.3070 sq in. Dia = 0.720 in. Wt = 0.463 lb/°F RTS = 14100 lb
Span = 600.0 ft NESC Heavy Load Zone Creep is a Factor Rolled Rod
Design Points Final Initial
Temp Ice Wind K Weight Sag Tension Sag Tension
(°F) (in.) (psf) (lb/°F) (lb/°F) (Ft) (lb) (Ft) (lb)
0. .50 4.00 .30 1.650 12.58 5915. 11.61 6409.
-20. .00 .00 .00 .463 5.85 3564. 4.13 5047.
0. .00 .00 .00 .463 6.67 3124. 4.44 4695.
60. .00 .00 .00 .463 9.55 2186. 5.75 3626.
120. .00 .00 .00 .463 12.08 1729. 7.78 2679.
167. .00 .00 .00 .463 13.23 1579. 9.78 2133.
212. .00 .00 .00 .463 14.33 1458. 11.78 1772.
The existing Linnet conductor is in reasonably good condition with an expected life of at least 20 years. It is capable of operation at a temperature above the present limit of 75oC, but only if it can be operated at a higher temperature without exceeding the present sag of 13.2 ft (4 m) at 75oC.
Because the line is short, stability problems are not of concern. The voltage drop per mile is considerable, how-ever, with a current of 430 A, a conductor temperature of 75°C, and a 90% power factor is:
2.8-2
Setting a 10% voltage drop as a limit during emergency loadings, all lines of this construction, having a thermal limit of 85 MVA are voltage constrained at lengths greater than 35 miles (56 km). Note that as the thermal rating of the line is increased, the line length at which the line is voltage drop limited decreases. Thus increas-ing the thermal ratincreas-ing of a 25-mile (40-km) line of this design to 125 MVA would make it voltage drop rather than thermally limited. In the present example, however, the 10-mile-long line’s rating would have to be increased to 300 MVA before it became voltage-drop limited.
The projected growth of the peak emergency line load-ing is shown in Figure 2.8-5. Note that the thermal capacity of the line will be exceeded by the peak contin-gency loading in 2 to 5 years for the pessimistic (1%) and the optimistic (3%) projections, respectively.
Preliminary Uprating Analysis
A preliminary uprating assessment of the line has been performed, as shown in this Uprating Analysis Table (Table 2.8-13). Certain uprating methods seem inappro-priate. For example, since the line is presently clearance limited with Linnet at 75oC and the tangent structures will not allow an increase in conductor attachment height, it is not possible to lift the conductor attachment points in order to increase the line design temperature.
Alternatively, the relatively modest increase in thermal rating and its uncertainty indicates that dynamic rating methods may work well.
After considering the capabilities and conventions of the transmission owner, the following three alternative uprating methods are identified as possible:
A. Reconductor the line with a lower resistance, trape-zoidal wire, Hawk/TW ACSR conductor, reinforcing the strain structures. The 10% larger diameter of Hawk/TW can be accommodated by the existing structures.
B. Install a dynamic thermal rating system based upon the use of conductor sag-tension monitors along the line. After some period of time depending upon the line load growth rate, remove the dynamic rating sys-tem, and increase the tension of the existing Linnet conductors to allow operation at higher temperature for the same maximum sag.
( )
( )
100 cos sin
%
3
100 430 0.328 0.90 0.336 0.436 115
3 0.286%
LL
I Rac X
VoltDrop
kV
per mile
φ φ
⎡ • • + ⎤
⎣ ⎦
= ⎡ ⎤
⎢ ⎥
⎣ ⎦
⎡ • • • + • ⎤
⎣ ⎦
= ⎡ ⎤
⎢ ⎥
⎣ ⎦
=
Figure 2.8-5 Example of projected growth of peak emergency line loading.
Table 2.8-13 Uprating Analysis Table
C. Reconductor the line with Linnet ACSS without modifying either tangent or strain structures.
Reconductoring with ACSR/TW Conductor
Reconductoring with 477 kcmil (243 mm2), 26/7 Hawk ACSR/TW conductor will yield a thermal rating of 525 A (105 MVA) at 75oC and 700 A (139 MVA) at 100oC due to the reduced resistance of Hawk/TW. The increased thermal capacity with Hawk/TW will be ade-quate for at least 10 years under the least-conservative assumption of 3% annual load growth.
In order to meet the maximum sag limit of 13.2 ft (4 m) at 75oC , Hawk/TW must be installed to an initial unloaded tension of 29% UTS at 0°F (-18°C) and a cor-responding maximum tension under NESC Heavy load-ing conditions of 7750 lbs (35 kN) (21% above the existing Linnet ACSR). If the initial unloaded tension at 0°F (-18°C) is increased to 33%, the sag limit is met at 100oC, but the maximum tension increases to 8310 lbs (37 kN) (30% above the present line). In either case, the maximum tension is well within the 50% increase limit in maximum tension load.
Hawk/TW has a diameter of 0.781 in. (2 cm) (8.5%
greater than Linnet), so it is likely that the existing tan-gent structures will not require reinforcement.
Having established the increase in thermal capacity pos-sible by reconductoring with Hawk/TW, other substa-tion equipment limits and replacement costs need to be reviewed.
The total line construction cost (i.e., strain structure modifications, replacement conductor, vibration damp-ers, labor cost, etc.) is a weak function of maximum ten-sion, since only strain structures need to be modified.
It’s assumed that the reinforcement of strain structures costs 5% of the structures for a new 115-kV line. It is further assumed that this amounts to $5,000 per mile ($3k per km).
In addition to the cost of upgrading structures, the most significant cost is that of the new conductor minus the scrap value of the old. Typically, one may obtain con-ductor costs and scrap value from a manufacturer. We will assume that the new Hawk/TW ACSR conductor costs $2.00 per ft ($6.70 per m) and that the scrap value of the old conductor is $0.50 per ft (1.50 per m). Recon-ductoring the line with Hawk/TW involves a total mate-rial cost of $24,000 per mile ($14,400 per km).
Other costs include stringing, sagging, and clipping the new conductor, new hardware, and engineering design costs. We will assume that this cost equals that of the Hawk/TW material.
Finally, the present worth of electrical losses over the life of the reconductored line should be calculated. It is assumed that the normal annual peak line load that is presently 50 MVA will increase to 65 MVA over an esti-mated 20-year useful life of the reconductored line. It is also assumed that the peak contingency load is 1.6 times the peak normal annual load of the line, and that the loss factor is 40%.
Economic Parameters for Loss Calculation
• Years of Analysis: 20 years
• Interest Rate: 8%
• Energy charge: $0.020/kW-hour
• Energy charge Escalation Rate: 7%
Using the economic data in the preceding paragraph and in Table 2.8-14, the present worth of electrical losses with the existing Linnet conductor over the 20-year period is $610k. The resistance of Hawk/TW is approxi-mately 70% (336.4/477 = 0.71) that of Linnet. Therefore, the savings in present worth of electrical losses for Hawk/TW is $18k/mile ($11k/km).
During the reconductoring of the line, the system opera-tor cannot use the circuit. Construction could take months and higher cost generation may have to be pur-chased during this period. The cost of this loss of the circuit could be determined by a system load flow analy-sis. The cost is primarily due to higher costs of genera-tion due to non-optimum generagenera-tion dispatch during construction and increased losses on other lines whose loads increase
Table 2.8-14 summarizes the costs and savings associ-ated with reconductoring the line with Hawk/TW con-ductor.
Table 2.8-14 Summary of Cost Savings Associated with Reconductoring
Conductor Name Hawk/TW
OD (in.) 0.782
Structures $5,000/mi
Conductor $24,000/mi
Conductor labor $24,000/mi
Total construction $53,000/mi
Cost of increased losses during
construc-tion ?
Savings in PW losses over 20-year life $18,000/mi Net PW cost of line operation over 20
years (ignore losses) $53,000/mi
Net PW cost of line operation over 20
years (include losses) $35,000/mi
Dynamic Uprating Method
If the existing Linnet conductor is in good condition, the line rating can be increased through the installation of sag-tension monitors along the line. Many papers have dealt with these techniques.
Before doing any economic calculations, it is essential that one determines just how much the use of a dynamic rating method increases the thermal rating of the line.
As was discussed previously, the dynamic thermal rating of the line is both random and chronological—that is, there is a certain amount of uncertainty combined with a certain degree of predictability based on season and time of day. Based on reference (Hall and Deb 1987), for a line in upstate New York, one may expect that the dynamic thermal rating of the 115-kV line with Linnet conductor will be:
• 10 to 20% above the static rating 50% of the time.
• Above the static rating 90% of the time.
• Below the static rating 10% of the time (usually at night).
One must decide how to interpret these numbers in terms of traditional planning criteria. One must recog-nize that the operator will have to intervene occasionally to reduce the loading of this line during times of peak loading and minimal rating. During these times when the load is high and the dynamic rating is low, the sys-tem will be operated in an uneconomic mode or load may need to be shed. It’s assumed that operating per-sonnel feel that they can intervene during 10% of the peak loading events without incurring significant costs.
Then one may credit the dynamic rating system with increasing the thermal rating of the line by 10%, or from 85 to 94 MVA.
For a 2% annual growth rate, the peak contingency load will reach 94 MVA in 8 years. Therefore, one may assume that the useful life of this dynamic rating approach is 8 years, after which the line must be modi-fied in some other manner to increase its thermal rating (e.g., raise structures, reconductor, etc). The installation of the dynamic rating system does nothing to reduce losses since the conductor resistance is unchanged.
It may be assumed that the dynamic rating equipment costs about $100,000 for a 10-mile (6.2 km) line. The monitoring system is reusable on other lines after 8 years, or earlier if the load increases more rapidly than predicted.
Line monitors are usually installed by bucket truck if terrain permits. Installation expenses vary widely since
they depend on terrain and accessibility along the line.
This is also true for maintenance. The monitors pres-ently available require periodic recalibration and proba-bly should be checked annually. A guess for initial installation cost of the line monitors might be $10,000 with an annual maintenance cost of the order of $5,000.
Prior to beginning the dynamic rating of the line, one must allow for a complete inspection of the structures and conductor to spot bad splices and impaired clear-ances. An inspection of the 10-mile (6.2-km) line is required for any of the three alternatives. The use of line monitors offers the unique advantage of establishing an experimental basis for high-temperature clearance.
Any line outage required to install the line monitors is brief, typically less than 24 hours. For EPRI’s video sag-ometer, no outage is needed.
Assume that at the end of 8 years the dynamic rating monitor system is worth 50% of its initial purchase price. After 8 years, the dynamic rating system will be removed and reused elsewhere in the system. The line would then be surveyed, certain critical spans selected for increase in clearance, the allowable conductor tem-perature increased, and 50 MVA of power transformer capacity added. The use of dynamic ratings would be discontinued at this point.
It’s estimated that the cost of retensioning the existing Linnet ACSR to 20% UTS at 60°F will be equal to half that of installing new conductor or approximately
$12,000/mile ($7,400/km). The increased everyday ten-sion will require the use of dampers costing about
$2,000/mile ($1240/km). The retensioned line will then have a line design temperature of 100oC and a rating of 575 A (115 MVA). If load growth is faster than antici-pated, then this may not be adequate, and the line will have to be either reconductored or rebuilt.
The present worth of $14,000/mile 8 years in the future is $10,000/mile, so the total present worth cost of this uprating approach is $21,000/mile.
Reconductor with Linnet/ACSS
Linnet/ACSS can be applied to the line without the need to rebuild or reinforce any of the structures. If the the Linnet/ACSS is installed to maximum NESC Code lim-its at 60oF, it reaches the sag limit of 13.2 ft (4 m) at about 150oC. The line rating is, therefore, 770 A (153 MVA).
The cost of reconductoring is limited to the conductor and its installation. Given the premium typical of ACSS of $2.00 per ft ($6/m) for the conductor and an equal
amount for stringing, sagging, and clipping it in place of the original Linnet, the total cost is $63,000 per mile ($39k/km).
Dampers will also be required because of the high initial tension levels. Therefore, the total estimated cost is
$65,000 per mile ($40k/km).
Economic Comparison of Uprating Alternatives
The total present worth of construction and losses to meet the increased thermal rating requirements of this 10-mile line in the three different uprating methods are summarized in Table 2.8-15.
Clearly, the use of dynamic ratings followed by a reten-sioning of the existing conductor (if required by actual load growth) is the most flexible approach, and requires the least initial and total capital investment. The major drawbacks involve the need to modify standard operat-ing procedures to utilize real-time ratoperat-ings and the mod-est rating increase that results.
The use of ACSS requires an absolute minimum of structure reinforcement since Linnet/ACSS has the same diameter as the original Linnet ACSR and yields reduced maximum tension because of its reduced modu-lus. There is no reduction in electrical losses since the Linnet/ACSS has nearly the same resistance as the orig-inal Linnet. In pursuing other alternatives, it is likely that an ACSS/TW conductor with a slightly larger diameter than Linnet would be a better choice.
The uprating option requiring the largest capital invest-ment is the reinforceinvest-ment of strain structures and the aggressive use of vibration dampers in order to recon-ductor the line with Hawk/TW ACSR. This option is unique in that it reduces electrical losses as well as increasing the line rating.
Review of other line uprating options and refinement of these three is clearly worthwhile. The means for identify-ing other possible upratidentify-ing options and selectidentify-ing the most appropriate has been presented in the preceding notes.
2.8.7 Conclusions
The impetus for line uprating comes as a result power system analysis. Present electrical loads are projected into the future, and the impact of various component outages (i.e., contingencies) on the electrical loading of the existing line is determined. Specific probabilities are seldom associated with post-contingency loadings, and even the prediction of normal loads is often uncertain, particularly with the advent of “open access” to com-mercial power generators.
For uprating to be possible, the existing line must be in good condition. Having established this, the identifica-tion of possible uprating methods depends upon the physical, electrical, and thermal characteristics of the existing line. An “Uprating Analysis Table” is developed here that simplifies the analysis of the existing line and provides a basis for identifying promising uprating methods in each specific line. Once the most promising uprating methods have been identified, a detailed analy-sis comparing the costs and capabilities of each method is required.
The final selection of an uprating method and its suc-cess in providing the nesuc-cessary increase in line capacity while maintaining system reliability and minimizing capital cost involves a good deal of engineering judg-ment, as well as the application of suitable numerical tools.
Table 2.8-15 Present Worth of Three Uprating Options Option A1 – Reinforcing strain
structures, adding dampers, and reconductoring with Hawk/TW for a line design temperature of 100oC.
$53,000/mile (ignoring losses)
Option A2 – Same as A1 but include loss savings
$35,000/mile (allowing for loss savings) Option B – Apply dynamic r
ating monitors and retension existing Linnet ACSR if pre-dicted load growth requires it.
$20,900/mile
Option C – Reconductor with Linnet/ACSS and go to a line design temperature of 125oC.
$65,000/mile
REFERENCES
Adams, H. W. 1974. “Steel Supported Aluminum Con-ductors (SSAC) for Overhead Transmission Lines.”
IEEE Paper T 74 054-3. Presented at the IEEE PES Winter Power Meeting.
Alcoa. 2003. “Alcoa Sag10 User’s Manual.”
Aluminum Association. 1974. “Stress-Strain-Creep Curves for Aluminum Overhead Electrical Conductors.”
Aluminum Association. 1989. “Aluminum Electrical Conductor Handbook.” Third Edition.
Aluminum Company of America. 1961. “Graphic Method for Sag Tension Calculations for ACSR and Other Conductors.”
Aronstein, J. 1990. “Conduction in Failing Aluminum Connections.” Proceedings of the Thirty-Sixth IEEE Holm Conference on Electrical Contacts. Montreal, Quebec. August.
Barrett, J. S., P. Ralston, and O. Nigol. 1982. “Mechani-cal Behaviour of ACSR Conductors.” CIGRE Interna-tional Conference on Large High Voltage Electric Systems. September 1-9.
Beers, G. M., et al. 1963 “Transmission Conductor Rat-ings.” AIEE Transactions. Paper 63-86.
Bennett, E. H. 1992. “Designing Compression Fittings for Long-Term Survival.” Bonneville Power Engineer-ing Symposium. April.
Black, W. Z., W. R. Byrd, R. A. Bush, and T. C. Cham-pion III. 1983. “Experimental Verification of a Real-Time Program for the Determination of Temperature and Sag of Overhead Lines.” Paper 83 WM 144-3.
January.
Black, W. Z. and W. R. Byrd. 1983. “Real Time Ampac-ity Model for Overhead Lines.” IEEE Transactions.
Vol. PAS-102. No. 7. July. pp. 2289-2293.
Black, W. Z. and R. L. Rehberg. 1985. “Simplified Model for Steady State and Real-time Ampacity of Overhead Conductors.” IEEE Transactions on Power Apparatus and Systems. Vol. 104. October. Pp. 29-42.
Boteler, D. H. 1994. “Geomagnetically Induced Cur-rents: Present Knowledge and Future Research.” IEEE Transactions on Power Delivery. Volume 9. Number 1.
January. pp. 50-58.
Braunovic, M. 1985. “Effect of Contact Aid Com-pounds on the Performance of Bolted Aluminum-to-Aluminum Joints Under Current Cycling Conditions.”
31st Annual Holm Conference. Chicago, IL. September.
Cahill, T. 1973. “Development of Low-Creep ACSR Conductor,” Wire Journal. July.
Campbell, H. E. and J. J. Burke. 1985. “Power Distribu-tion Systems Course.” PTI Course. September.
Chisholm, W. A. 1986. “Ampacity Field Studies on Line with Low Operating Temperature.” EPRI DTR Semi-nar. May.
CIGRE. 1992. WG 05 – Conductors. “The Thermal Behaviour of Overhead Conductors.” 22-81 (WG05) – 06. December.
CIGRE. 1997. WG 12-22. “Thermal State of Overhead Line Conductors.” Electra. No. 121. pp. 51-67.
CIGRE. 2000. WG 22-12. “Description of State of the Art Methods to Determine Thermal Rating of Lines in Real-Time and Their Application in Optimising Power Flow.” Paper 22-304.
Clapp, A. L. 1985. “Relationships of National Electrical Safety Code Vertical Clearances and Potentially Con-flicting Activity.” IEEE Transactions on Power Appara-tus and Systems. Vol. PAS-104. No. 11. November. pp.
3306-3312.
Dalle, B. 1982. Size and Aging of Joints for Bare Con-ductors of Overhead Line. Electricite de France.
Dalle, B. 1982. Size and Aging of Joints for Bare Con-ductors of Overhead Line. Electricite de France.