(*) jhmf@eln.gov.br

Eletronorte and the Challenge of Long-distance Transmission in Brazil

José H.M. Fernandes (*) R.P.Guimarães J.F. Nolasco P.R.R.L. da Silva M.C. Araújo SAE Towers JFN Cons. Engetower V.G. Machado M.N. Takai Eletronorte

Brazil

SUMMARY The Amazon region is characterized by its exuberant jungle, by the abundance of its hydroelectric resources, so far not well exploited and by the low amount and big sparsity of electric energy consumption in its vast territory. In such region, around 50% of the available Brazilian hydroelectric resources are situated in a distance of roughly 2000 km far from the big consumption centres, most of which at South-Eastern and Southern areas. The transmission of large blocks of power requires therefore the development of overhead lines having high power-carrying capabilities. Besides for developing such systems, huge financial amounts are necessary, so that the transmission systems need to be economically and reliably designed so as to provide the electric energy to the consumers at a competitive cost. During the last three decades, Eletronorte, the Brazilian state-owned Utility responsible for generation and transmission in Northern region of Brazil, has studied and implemented several overhead lines aimed at improving the cost/benefit ratio of its electric system, this meaning systems able to carry higher amount of electric energy at the lowest possible cost. Inside this context, the present paper describes the evolution of overhead line conception, especially their steel supports, as well as the basic line designs for 230 kV and 500 kV levels, developed in the last years by Eletronorte and engineering companies for carrying higher and higher power per circuit. At first, it will be shown how the Utility has faced and overcome the challenge of developing 230 kV and 500 kV overhead lines providing high Surge Impedance Levels, as required for transmitting large power blocks to distant regions. The evolution of the mechanical design of the towers will be presented, considering the options of compact towers and/or bundle expansion, starting with the self-supporting “racket” tower, continuing with cross armless towers, either of the Chaînette type or cross-rope version, ending with a new introduced type of tower recently under evaluation. Therefore, it will be shown that this new conception is gaining technical and economical attractiveness, namely single-mast guyed towers provided with expanded-bundle configurations. Concluding, the compaction techniques associated with the expanded-bundle conception using the last mentioned single-mast guyed structures have made possible to implement overhead lines with a transmission capacity 20% higher and 25 % less costly than the corresponding figures obtained with the conventional designs. These new “optimized” conceptions are available to transmission system planners and transmission line designers, opening therefore new possibilities for the optimization of electric systems.

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KEYWORDS Compact Line – Racket Tower – High SIL Line – Expanded Bundle – Cross-Rope Tower – Single-mast Guyed Tower – Live Line Maintenance – Insulation Coordination

1. INTRODUCTION Eletronorte is a state-owned Power Utility acting in Northern Brazil, which is an area worldwide known as Amazon Region. Since its foundation in 1973, Eletronorte has implanted hydro and thermoelectric power plants, and their respective transmission systems in a region, which is highly characterized by its exuberant jungle, the wealth of its hydroelectric resources and by the sparsity of loads and low electric energy consumption. This region, as shown in Figure 1, encompasses the highest portion of a not yet exploited Brazilian hydroelectric potential, which is characterized by a clean, renewable and low cost energy, despite being situated about 2000 to 2500 km far from South-Eastern and Southern regions, where the cities of Sao Paulo, Rio de Janeiro and Belo Horizonte are the biggest ones.

Additionally, considering the large area occupied by the Brazilian territory especially in North, its hydrologic basins are characterized by time-shifted hydrologic cycles. By interconnecting these basins through their electric systems it results in a considerable increase of the available electrical energy of the whole Brazilian system. In view of that, it turned out evident the need to implement long distance transmission systems linking the various systems of the country and for carrying the generated energy to the big consumption centres. However, the simultaneous supply of electric energy to the Amazon Region itself and to the intermediate loads of the regions crossed by the long transmission lines could not be neglected. Therefore, the development of efficient AC transmission systems was a challenge to be overcome by Eletronorte, aimed at achieving a better cost-benefit ratio (MW/ US$) as well as a better efficiency in the transportation of energy in each right-of-way (MW/m2), complying with the environmental constraints. The main objective consisted therefore in providing a reliable and low

costly energy to the consumers. It should be stressed that the DC transmission alternative turned out economically unfeasible, in view of several intermediate consumers to be supplied along the line route. 2. INCREASING THE TRANSMISSION CAPACITY 2.1 Compact Lines

The first 500 kV and 230 kV lines constructed by Eletronorte have adopted the conventional self-supporting flat-configuration towers (delta type). In the beginning of the eighties, the at the time recent technology of compact lines was introduced in Eletronorte [1] [2], and their engineering team considered that, together with series compensation, it could represent a valuable new opportunity for optimizing the new planned transmission systems for the region; moreover, the power to be generated by Tucurui Power

112 GW

43% 26 GW

10%

35 GW

14% 45 GW

17%

43 GW

16%

Total Power: 261 GW Operating / In Construction: 30% Prospected / Estimated: 70%

Figure 1 - Brazilian Hydroelectric Potential

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Plant (8370 MW) initiated at the time, could economically and technically be conveyed to the distant consumption centres through the use of such new technology. As a first real gain, the use of the compaction technology, associated with the installation of series capacitor banks, could preclude for the transmission of 5000 MW the construction of two additional 500 kV – 800 km long each one – transmission circuits; the adequate use of this technology could simultaneously increase the energy transmission rate through the same corridor (MW/m2) and improve the effectiveness of the costs associated thereof (MW / US$). Compaction of a transmission line consists in an adequate design of the tower-top geometry so as to place the phases as close as possible among each other. Herein the concept of Surge Impedance Loading or Natural Power is utilized as: the active power flow circulating over a line, for which the inductive power consumption along the conductors are supplied by the capacitive reactive power generated by the line itself. Defined by equations 1 and 2, the SIL reflects the interaction between line parameters, as follows: SIL = V2 / Z1 [1] Z1 = ZP – Zm [2] where: SIL – Surge Impedance Loading (MW) V – TL Operating voltage (kV) Z1 – Positive sequence impedance (Ω) Zp – Self impedance (Ω) Zm – Mutual impedance (Ω) Consequently, the compaction results in an increase of the coupling between phase conductors, so increasing the mutual impedance (Zm) and reducing the positive sequence impedance (Z1), causing a net increase in the SIL of the line. Such technology can provide a maximum increase of around 20 to 25% in SIL, as a function of some limiting factors as: minimum viable phase spacing able to guarantee adequate insulation coordination, asynchronous swing angles between phase conductors, appropriate limitation of conductor surface gradient. Based on that technology, Eletronorte developed the racket type compact towers for 230 kV and 500 kV [3][4], as per Figures 2 and 3, and the cross-rope type compact towers for 500 kV according to Figure 4.

Figure 2 – Compact Racket Tower 230 kV Figure 3 – Compact Racket Tower 500 kV

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2.2 High SIL Lines – HSILL Approximately in the middle of the nineties, Eletronorte started the studies of the so-called HSILL concept [5], whose the first applications in Brazil occurred in the beginning of 2000’s years [6]. Such technology consists in equalizing and maximizing the surface electric fields of the phase subconductors, so resulting in a tower-top geometry with optimized configurations, which present expansion of subconductors and an approach among the phases. This technology provides a simultaneous decrease in the self impedance (Zp) and in the mutual impedance (Zm) causing a great reduction in the positive sequence impedance (Z1), and consequently a great increase in the SIL of the line. Table I presents the possibly range of SIL for various voltage classes.

Table I – Typical Values of SIL

Thus, by utilizing expanded and symmetrical bundles, Eletronorte developed the single-mast towers for 230 and 500 kV, as shown in Figures 5 and 6. 3. ELECTRICAL ASPECTS 3.1 Insulation Coordination The following methodology has been used for defining the electrical parameters of compact racket, cross-rope and single-mast towers, and then carrying out the respective insulation coordination: − Maximum operating voltage: it is associated with high wind speed with a return period T=50 years

and with an integration period of 30 seconds. The withstand voltage (Vmed + 3σ) having σ = 2,0% is combined with the corresponding climate factor (RIS), and the electric clearance is obtained from Figure 10.1.6, page 494, of [7];

− Switching (slow-front) overvoltages: associated with reduced wind speed, it is equal to 60% of the high-wind speed. The required electrical clearances are obtained as a function of the probability of flashover (PFO) for the statistical distributions of switching overvoltages due to energization and reclosing operations, having maximum acceptable values of 10–3 or 10–2 for phase-to-ground and 10–4

or 10–3 for phase-to-phase, respectively; − Lightning (fast-front) overvoltages: they are taken into account without the simultaneous

occurrence of winds. The minimum clearance is equal to the length of the insulator string. The clearance distances required for withstanding the electrical stresses have been determined according to gap factor method [8], using curves of [7] and with the methodology for phase-to-phase as per [9].

Voltage (kV)

SIL-Traditional TL (MW)

SIL – HSIL TL (MW)

69 9-12 10-40

138 40-50 50-120 230 120-130 130-440 500 950-1000 1000-2000

Figure 4 – Compact Cross-rope Tower 500 kV

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The swing angles have been calculated by using wind effectiveness factors extracted from Hornisgrinde tests [7] [10], which fit theoretical equations proportional to the squared wind speeds to experimental field measured values. For switching surges, the average values of measured swing angles are used in the calculations, while for operating voltages, the envelop of maximum measured swing angles are considered. Consideration of asynchronous swing angles between conductors were taken into account according to [11], in which it is shown that the maximum measured angle was 3º for spans up to 800 m. Reference [12] presents graphs of the minimum required phase spacings as a function of the span length, considering the maximum allowable limits according to operational experience, so as not to cause phase-to-phase flashovers for the highest operating voltage.

3.2 Electric Characteristics

The electric characteristics of lines provided with compact racket and cross-rope towers, as well as with towers with expanded bundles have been defined taking the following criteria as reference: − Number of insulators per string: required creepage distance of at least 14 mm/kVrms phase-to-

phase; − Electric field: Maximum of 4,16 kV/m at the height of 1 m, at the edge of the right-of-way and of

15 kV/m under the conductors; − Magnetic field: Maximum of 83 µT at the height of 1 m, at the edge of the right-of-way; − No visible corona for maximum operating voltage during 90% of the year; − Radio-interference: Signal to noise ratio of at least 24 dB during 50% of the year at the edge of the

right-of-way; − Audible noise: Maximum at the edge of the right-of-way for wet conductors: 58 dB(A); − Lightning performance: For 500 kV: 1 outage / 100 km.year For 230 kV: 2 outages / 100 km.year Tables II and III show the main characteristics of the resulting 500 kV and 230 kV lines

Figures 5 and 6 – Single-mast towers with expanded bundles - 230 kV and 500 KV

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Table II – Characteristics of 500 kV lines Compact lines

Characteristics Conventional line Racket Cross-rope

HSIL Line Single-mast

Conductors/phase (ACSR) 4 x 636 – 26/7 4 x 954 – 45/7 4 x 954 – 45/7 4 x 954 – 45/7

Reactance [Ώ/Km] 0,326 0,267 0,267 0,265

Capacitance [nF/Km] 13,467 16,300 16,378 16,508

Subconductor spacing [m] 0,40 0,457 0,457 0,950

SIL- [MW] – 500 kV 986,5 1197,7 1201,7 1211,4

Nº of insulators/string - glass 26 – 12 Ton 22 – 16 Ton 22-16 Ton 22-16 Ton

Right-of-way [m] 60 60 60 60

Table III – Characteristics of 230 kV lines

Characteristics Conventional line Compact Racket

Tower HSIL Line Single-mast

Conductors/phase (ACSR) 2 x 795 – 45/7 2 x 795 – 45/7 2 x 795 – 45/7

Reactance [Ώ/Km] 0,353 0,303 0,304

Capacitance [nF/Km] 12,421 14,359 14,422

Subconductor spacing [m] 0,457 0,457 1,23

SIL- [MW] – 230 kV 192,49 223,67 223,73

Nº of insulators/string - glass 16 – 12 Ton 14 – 12 Ton 14 – 12 Ton

Right-of-way [m] 30 30 30

4. MECHANICAL ASPECTS 4.1 Mechanical Reliability

The mechanical design reliability was established based on probabilistic criteria, taking into account IEC 60826 recommendation [13] and specific reliability criteria required by ANEEL, the Brazilian Energy Regulatory Agency, which establishes minimum reliability level 2 , wind return period T = 150 years for 230 kV TL and wind return period of 250 years for 500 kV TL. In the final design however the wind return period of 250 years was adopted for both projects. Additionally, loadings due to high intensive winds (TS Thunderstorms), typical winds of tropical regions, characterized by its turbulence, high intensity and narrow front, were introduced in the transmission line designs. Although data base for those winds is not available in Brazil, the project adopted a wind velocity 20 % higher than the one determined for stationary winds in the Amazon region, adjusted to an integration period of 3 seconds. Its value was considered constant with height, acting through all the structures and on 25 % of the wind span for the conductors. Statistical wind studies for that region had shown values of 23,5 m/s for Reference Wind Velocity (VR) with return period T = 50 years. For a return period T = 250 years the reference wind velocity was 26,5 m/s. For high intensive wind the adopted value for wind velocity was 48,5 m/s (gust of 3 seconds). Table IV presents the final wind pressure values considered in the structure designs.

Table IV – Design Wind Pressures Wind Element Final Wind Pressure (kgf/m²)

Conductor 82,1 Shield-wire 87,8 Extreme Wind

Structure 81,4(H/10)0,149 Conductor Shield-wire TS Thunderstorms

Structure

124,7

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4.2 Mechanical Criteria for Conductors and Shield-Wires The following electromechanical design criteria were established for the design of the conductors and shield wires: • EDS Condition (no wind, final): maximum axial tension of 20% of RTS, associated with the

temperature of 25°C , limited to 22% of ultimate strength of the conductor, in the initial condition; • Minimum Temperature Condition: maximum axial tension associated with the temperature of 0°C,

limited to 33% of ultimate strength of the conductor; • Nominal Wind Condition (T = 50 years): maximum axial tension associated with the temperature of

10°C, limited to 50% of ultimate strength of the conductor; • Maximum Wind Condition (T = 250 years): maximum axial tension associated with the temperature of

10°C, limited to 70% of ultimate strength of the conductor; Additionally to those conditions, the shield wires have fulfilled the requirement that their sags should be smaller than 90% of the conductor sag in the same condition. 4.3 Structural Aspects The cross-rope suspension tower is composed of two guyed lattice masts, interconnected at their tops by an auxiliary assembly cable and by the cross-rope cable which sustains the three phases, as shown on figure 4. The auxiliary cable, which is used as a template during the tower assembly to keep the right distance between the two masts, is also used in live-line maintenance works to allow the lineman movement on tower. The cross-rope cable is composed of a galvanized high-strength steel strand where the suspension strings are connected by armour grip suspension clamps, as shown on Figure 7.

As this type of structure is very flexible, the cross-rope tower final geometry and configuration was only determined after the clearance verification of the deformed structure due to loadings corresponding to EDS Condition and Nominal Wind (return period T = 50 years). The cross-rope structure was designed as an angle suspension tower up to 2°. The final configurations for 230 kV and 500 kV single-mast towers were the result of iterative studies on which many

combinations of phase arrangement and sub-conductor distances within the bundle were considered. For each arrangement, the most adequate structural geometry was studied considering the mechanical and electrical aspects until achieving the most economical configuration as shown on Figures 5 and 6. Additionally, for the 500 kV single-mast tower, economical studies comparing the solutions of using III and IVI suspension isolator strings, were also carried out. The solution using III suspension isolator string proved to be more economical and that was the configuration used in 95 % of the single-mast suspension towers. The other alternative with IVI suspension strings was used in the remaining single-mast structures with angles up to 3°. The structural tower design including the member dimensioning was done according to ANSI / ASCE 10-90 – [14] requirements and the corresponding Brazilian standard. For the foundation design, the loads transmitted by the structures were increased by a coefficient of 1,10 to take into account the failure coordination criterion. 5. MAINTENANCE ASPECTS AND TESTS Every compact and expanded-bundle tower was submitted to electrical insulation tests, so that its top geometry could accept live-line maintenance work. Due consideration was given to maximum transient overvoltages possible to occur as resulting from load rejection during maintenance work. Reclosing is blocked in this case, and the clearance distances required to withstand such voltage stresses were determined according to [15], considering also constraints stated in [16] and [17]. Additional tests were carried out in Brazilian research centre CEPEL aiming at defining methods and tools for live-line

Figure 7 – Suspension Clamp

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maintenance. Particularly for the cross-rope towers, designed with the shortest phase spacings (5,5 m), the main results are shown in Tables V and VI.

Table V – Critical voltage (U50% - kV) in phase to ground tests Phase to Ground Tests U 50% (kV)

Insulator string without lineman 1457

Lineman on the conductors 1321

Lineman at a distance of 1,0 m from the conductors 1304

Lineman at a distance of 2,0 m from the conductors 1430

Lineman at a distance of 3,0 m from the conductors 1411

Table VI – Critical voltage (U50% - kV) in phase to phase tests

Phase to Phase Tests U 50% (kV) Lineman on the central phase 2005

Lineman at a distance of 1,0 m from the central phase 1983

Lineman at a distance of 1,5 m from the central phase 1985

Lineman at a distance of 2,5 m from the central phase 2160

In view of that the available clearance distances provided by the designed towers have provided enough safe conditions for live-line work. 6. CONSTRUCTION ASPECTS

From the construction point of view, the compact cross-rope tower presented a great advantage when compared with the compact racket tower. Due to its simplicity, the cross-rope tower can be assembled with an extraordinary productivity. Their masts, which have a simple geometric configuration, can be easily assembled on site. Furthermore, the auxiliary assembly cable and the cross-rope cable are furnished cut in their specified length with all fittings already connected to them. That procedure reduces dramatically the assembling work on site. As soon as the masts are assembled on the ground, the tower can be lifted by crane as shown on Figures 8 and 9. After the masts are fixed at their respective foundations, the pre-tensioning of the guys and the alignment of the structure may be easily carried out. The average assembly rate achieved during the construction of the cross-rope towers was eight structures per day by mechanized crew. Also the 230 kV and 500 kV single-mast suspension towers presented a great advantage from the construction point of view, when compared with the compact racket towers. They can be easily assembled by manual or mechanized

Figures 8 and 9 – 500 kV Cross-rope Tower Assembly

Figures 10 and 11 – 230 kV and 500 kV Single-mast Tower Assembly

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process, and they can be entirely lifted by crane in just one operation after having been horizontally assembled on the ground. One peculiarity of that type of structure with only one mast is that provisional guys are not necessary during the assembly operation. Their own guys can be used as auxiliary elements and may be definitively installed, even before the tower head is assembled, as shown on Figures 10 and 11. The average assembly rate achieved during the construction of the 500 kV single-mast suspension tower was six structures per day by mechanized crew.

7. OPERATIONAL EXPERIENCE The first lines using compact racket towers in Eletronorte were implemented in 1986 and 1994, at 500 kV and 230 kV voltages, respectively. So far, a total of 3761 km of such compact lines have been built in Brazil. On the other hand, the first 500 kV compact line using cross-rope towers was implemented in Brazil in 2002, while presently a total of 1665 km of such line type were already constructed and additional 681 km are in construction stage. In 2005 Eletronorte implemented its first 230 kV HSILL line, 171 km long, and presently other similar lines – 230 kV, 279 km long, and 500 kV, 623 km long – using the same concept, are in construction stage. All those lines have so far presented an excellent operational performance, in spite of the reduced distances adopted and of the optimized measures carried out.

8. SAVINGS OBTAINED In the first stage the line compaction technology provided an increase of around 20 to 25% in the natural power (SIL) of the lines, therefore resulting, for the same power to be transmitted, in a lower number of parallel circuits or in reduction of series capacitor requirements. However, the racket towers had approximately the same average weight as the conventional structures, i.e. about 23,0 ton/km for 500 kV lines and 13,0 ton/km for 230 kV lines. With the development of the compact cross-rope towers, the compaction concept was combined with the mechanical optimization of the supports, thus reaching, for the same SIL at 500 kV, an average weight of 15,60 ton/km, meaning a weight reduction of 32% as compared with the racket tower lines. The total cost of the cross-rope towers, including guy wires, auxiliary erection cables, upper cross-rope cables and foundations, turned out 20% lower than the equivalent racket tower and its foundations. The new generation of 500 kV lines with single-mast and expanded bundles, having the same SIL as the racket and cross-rope tower lines, presented savings of around 8% toward the latter one, taking into account only the tower and foundation costs. It should be emphasized that the cross-rope tower has one foundation more than the single-mast tower and that the lines with single-mast towers presented an average weight of 15,0 ton/km. Similarly, the 230 kV lines with single-mast towers and expanded bundles, with the same SIL of the racket tower line, presented an average weight of 9,20 ton/km, or a reduction of 30% as compared with the equivalent racket tower lines (13,0 ton/km). Therefore, when we combine the capacity increase of 20 to 25% provided by the compaction and bundle expansion techniques, with the cost decrease of around 30% due to the support optimization, a net cost reduction of about 25% in the line implementation cost was reached toward the conventional lines, beside an increase of about 60% in the efficiency of the transmission system (MW/US$). This higher efficiency resulted in savings of millions of US$ dollars for Eletronorte and other transmission companies using the same tower types, thus further amplifying the gains already obtained and the perspectives for the future. 9. CONCLUSIONS For overcoming the challenge of long-distance transmission, Eletronorte has along the years conceived, improved and implemented new towers for overhead lines; this was aimed at making the AC electric systems more competitive and efficient, and therefore able to carry more and more power at lower and lower cost per circuit, so increasing the energy density at the same line corridor.

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The study and introduction of the guyed single-mast towers represented so far the optimal economical choice, as a new gain of about 20 to 25% could be achieved in the line capacity and even a further savings of 5% toward the already economical cross-rope towers. This represented therefore the expressive amount of 25% of net savings as compared to the conventional lines. It should be emphasized that the operational performance of the new lines in the last 20 years has been kept equal, if not better, than the one of the traditional lines. The engineering efforts to optimize and mechanically improve the line and other components proved to be worthwhile. This shows clearly that transmission line engineering is an open horizon with plenty of opportunities, as the potential for new gains are always a challenge to be faced and overcome. All the phase compaction and bundle expansion techniques developed by Eletronorte and their associates or consultants have been put to the disposition of electric system planners and line designers, providing a great diversity of alternatives for fitting the transmission line capacity to the real necessities of the electric systems and bringing to them the opportunity to apply such technologies and even improve them. The high SIL concept was introduced in Brazil some years ago and found here a fertile field for further development, adding those alternatives to the many others already created and still being generated inside CIGRÉ and other technical transmission line bodies in the World. 10. BIBLIOGRAPHY

[1] EPRI – Electric Power Research Institute – “Transmission Line Reference Book 115-138 kV

Compact Line Design” – Palo Alto, 1978 [2] Zobel, E.S.; Rohlfs, A.F.; Flugum, R.W. – “Narrower Transmission Corridors Made Possible with New

Compacted Conductor Systems for EHV and UHV Lines”, (CIGRÉ Session Paper 22-06, 1980). [3] Fernandes, J.H.M.; Sganzela, F.; Tannuri, J.G.; Galiano, D.B.; Sato, W.; Takai M.N.; Massuda,

M.; – “500 kV Compact Line of Eletronorte Brazil – Conception, Electrical and Mechanical Design”, (CIGRÉ Session Paper 22-304, Paris 1990).

[4] Fernandes, J.H.M.; Tannuri, J.G.; Sato, W.; Filho, N. Kaba ; Arimori J.M.; “Eletronorte – Brazil – 500 kV and 230 kV Compact Lines: Design and Electrical Aspects”, (Paper CIGRÉ - 533-91 – Symposium Leningrad 1991).

[5] Alexandrov, G.N.; Nosov, I. M. – “The Increase of Effectiveness of Transmission Lines and Their Corridor Utilization”, (CIGRÉ Session Paper 38-104, Paris 1996).

[6] O.Régis Jr.; Cavalcanti S.J.G.; Neto, A.P.; Domingues L. A. de M.C.; Dart F.; Maia, M.J. A.; –“Expanded Bundle Technique: The Application of HSIL TL Concept to Increase the Capacity of Overhead Lines”, (CIGRÉ Session Paper 22-207, Paris, 1998).

[7] EPRI – “Transmission Line Reference Book 345 kV and Above” – Second Edition – Palo Alto – 1987. [8] Paris, L.; Cortina, R.; – “Switching and Lightning Impulse Discharge Characteristics of Large Air

Gaps and Long Insulator Strings” – IEEE Trans. Power Apparatus and Systems, Volume PAS – 87, No 4 – April 1968

[9] CIGRE – SC 33 – “Phase-to-Phase Insulation Coordination” – (Electra No 64 – May – 1979). [10] Leibfried, W.; Mors, H.; Versuchsanlage, Hornisgrinde; Schlussbericht; Badenwerk AG,

Karlsruhe, 1964. [11] Paris L. – “Azione Meccanica del Ghiaccio e Del Vento Sugli Elettrodotti – Indagini

Sperimentale” – (Confereza Tenuta all´ Instituto di Scienza delle Construzioni, della Facolta di Architettura del Politecnico de Milano”, il 19 dicembre 1963).

[12] United States Department of Energy – “EHV and UHV Transmission Line: Electrical, Structural and Environmental Design Concepts and Evaluations” – (DOE/ET/29236-2-1982).

[13] IEC 60826 – “Loading and Strength of Overhead Transmission Line’, Second edition, 1991-04; [14] ANSI/ASCE - Manual 10-90 – “Design of Latticed Steel Transmission Structure”, Edition

December 1991; [15] ANSI C2 – “National Electrical Safety Code”, Edition 1981. [16] U.S. Department of Labor – “Occupational Safety & Health Administration – OSHA – Working

on Exposed Energized Parts”, Appendix B to § 1910.269. [17] IEEE “Guide for Maintenance Methods on Energized Power Lines”, IEEE – std 516 – 2003

(revision of IEEE std 516-1995).