TRANSMISSION AND DISTRIBUTION A practical consideration of transmission line engineering by Dr. Ing W Bückner, Cigré

Europe-wide design projects for new generating plants, in particular new nuclear generating stations, as well as the power failures in in March 2006 caused by extreme weather conditions and in November 2006 caused by electrical overloads on power lines are making the construction of new transmission lines urgently necessary, and thereby attention is being drawn to the current state of electrical transmission line technology in the world.

Contemporary transmission line systems have been standardised for decades. Technical innovations which have been recommended and tried out have seldom been put into actual practice. Moreover, in many countries contracts are awarded separately for material and erection on the basis of unit prices, while in other countries fixed price lump-sum contracts are more common. In the latter case the bidder has the chance to make a winning bid by working out an economic solution for the tendered project. Optimisation of the project design by the bidder may be something which will gain importance in the future. The degree of standardisation as practised so far no longer makes sense. Fig. 1: 110/380 kV line – Costs per km as function In future, attention needs to be paid to of the Al-cross-section of the line. the degrees of freedom the electric utility allows the bidder in system design. Should personnel, not only in the construction technology is not taught at universities, the tower design and conductors as well as planning departments of the utilities, transmission line engineers must first the tower emplacement for a given route but also in the design and construction complete basic studies in civil, mechanical be specified, or should just the conductors departments of the construction and or electrical engineering, which then need and regulations be set and freedom left service companies in the sector. These to be augmented by several years of for the mechanical design? resources have partly been sent into practical on-the-job experience before the early retirement or transferred to other The realisation of the new investments training can be regarded as completed. departments. is facing a personnel problem. The low It will not be easy to find experienced investment level in transmission line On top of that, whole departments personnel and this is especially relevant construction that has prevailed for at have been out-sourced as independent for construction personnel, an aspect least ten years has led to a great drop companies. In this process the fact which became very evident during in staff levels of design and construction was overlooked that as overhead line the repair work in northern Germany in

No. of Conductors Al conductor – Conductor tension Thermal transmission Natural Cost1) Specific cost of the thermal systems per phase Al/St cross section of the line capacity capacity) transmission capacity mm2 % UTS MVA approx MW approx €/km approx €/MVA approx 110 kV 1 1x265/35 795 23,3 130 35 175 000 1347 2 2x265/35 3180 23,3 520 70 214 000 416 2 2x560/50 6720 28,1 800 72 387 000 459 2 4x560/50 1 3440 28,1 1600 74 594 000 371 1 1X560/50 1680 28.1 800 40 352 000 440 22) 2X265/35 3180 15,8 520 70 242 000 466 380 kV 1 3x265/35 2385 23,4 1340 420 332 000 248 2 3x265/35 4770 23,4 2680 900 476 000 178 2 4x435/55 1 0440 28,1 4700 930 628 000 134 2 4x680/85 1 6320 29,1 6000 1000 731 000 122 1 4x680.85 8160 29,1 3000 500 465 000 155 22) 3x34o/30 6120 17 4160 850 644 000 155 UTS = ultimate tensile stress of the conductor 1) At 85% suspension towers and 15% tension towers, 22) standard line according to German regulations

Table 1: Performance and costs as well as specific costs per km line for 110 kV and 380 kV lines with different conductor configurations.

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Fig. 2: Influence of the conductor temperature Fig. 3: 110/380 kV line – Costs per km on the thermal transmission capacity in MVA. as function of conductor tension.

2006. Forward looking innovations in overhead lines is a network planning lines as shown in Fig. 1. For integrated overhead line technology have evidently responsibility. For transmission over greater grids special investigations in a network not gained sufficient recognition by distances the natural capacity Na and the laboratory are necessary. Detailed the electrical power industry. Technical thermal ultimate capacity in MVA need to information on that can be found in [1, 2]. conferences such as that held by the ETG be considered. Table 1 gives indications The determination of the conductor type in Hamburg with the promising title “Energy of the values of capacities and costs per includes the prediction of the economic Technology for the Future” with the theme km as well as the specific costs in €/MVA current density of the line AW which is “Sustainability of Electrical Systems and per km line for 110 kV and 380 kV lines dependent on the line investments, Grids” make this attitude very evident. with different conductor configurations. on the energy rate per kWh and on There was no presentation of technical The costs cover the annual costs for the the load level of the line. As shown in innovations in overhead line technology. financing of the line investments, for power Fig. 2, the transmission capacity can be losses as well as for line maintenance. Choice of conductors – Transmission markedly increased with higher permissible With increasing conductor cross-section capacity conductor temperatures [3]. A transmission the specific transmission costs drop line engineer should be involved in the The desired transmission capacity of considerably, most notably for 110 kV selection of the conductor type in order to determine the line configuration with the best trade-off between optimum economy and operational reliability.

The configuration of a transmission line includes an electro-technical and a comprehensive civil engineering analysis [4, 5, 6, 7] which nowadays can be performed quickly using specialised computer tools.

A technical and economic investigation of a line section using Aluminium Mg-Si alloy (Aldrey) conductors instead of Al-St conductors demonstrates the advantages of the Aldrey conductor which enables the transmission costs to be reduced by about 4% [4]. Electricité de (EDF), the French state electricity provider uses Aldrey overhead line conductors exclusively [8]. The Aldrey conductors were developed in Germany around 1926 and used there for a short time.

Choice of support structure The optimisation a new transmission line including foundations with given operating voltage, conductor configuration and right-of-way starts with the determination of the conductor tension. Fig. 3 shows the cost minimisation for higher conductor tensions as a function of the span length for current common 110 kV and 380 kV transmission lines. The optimum span lengths are around 500 m for 380 kV lines due to the high costs for insulators and

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Fig. 4: Vibration recorder installed on a 110/380 kV overhead transmission line of the former Neckarwerke, now EnBW- Energie Baden- Wuerttemberg. fittings per tower and around 350 m for 110 kV lines. The higher conductor tensions Fig. 6: two-level lead to lower costs. The optimum span configuration, three-leg Siemens type lengths between suspension tower and on the Emden – Wiesmoor line. tension tower are lower [5;9].

Meanwhile, in Germany conductor tensions below 18% EDS (Every Day Stress conductor breaking stress) are preferred for fear of wind-induced vibration failures although the regulations allow 25% higher values if vibration dampers are used. But the higher values are seldom utilised in spite of the fact that operational experience shows that this would be possible without Fig. 5: Buckling loads of lattice any danger, as reports demonstrate tower member profiles. [10, 11, 12]. Modern vibration recorders (Fig. 4) allow the determination of members. Fig. 5 shows the permissible operational safety. These measurements compression/buckling load of steel angles make it possible to determine the safety each having a cross sectional area of level of the conductors with respect to 192 cm²: a common L-angle with wind-induced vibrations. At the same time, B = 100 mm and H = 10 mm, a channel the effectiveness of vibration dampers profile of B = 125 mm and H = 7,5 mm, fitted to the line can be measured, a round tube of 206 mm diameter and Fig. 7: Transmission tower single level configuration, three-leg Siemens something which cannot be done reliably 3 mm thickness as well as the same tube filled with concrete and a further one with type on the Emden – Wiesmoor line. in the laboratory. a partial (35%) filling of spun concrete. To summarise, it can be stated that higher All profiles in Fig. 5 are in use on operational conductor tensions with bundle conductors lines. The high moment of inertia of and spacer dampers can be safely the round tube is favourable for the operated up to an EDS of 30% and for dimensioning of lattice towers due to the single conductors with matched vibration key buckling load, as shown in Fig. 5. In dampers an EDS of 27% is possible without the 1930s tubular towers were used for a danger. A check by means of vibration 220 kV line Nuremberg- Berlin. Corrosion measurements over a period of two to on the inside wall made continued use three months, starting two months after difficult. Later, around 1950, tubular completion of the first test section, will towers were erected in Switzerland. During furnish a safety guarantee. There is no risk installation the tubes were filled with associated with this as a possible danger to concrete. the line would not arise until after a certain number of vibration cycles and is therefore The values for the Siemens type lattice time-dependent, i.e. it would be months towers recommended in 1952 – tubular before the line suffered any damage. corner legs filled with spun concrete – in Fig. 5 were determined by tests at the As the towers are the largest cost item of Bavarian State Trade Institute in Nuremberg an overhead transmission line, their design and at the Munich Technical University in Fig. 8: Transmission tower single-level therefore is an interesting task for the configuration, three-leg Siemens type 1953. Siemens type transmission towers on the Mainz – Bingen line. transmission line engineer. The key stress [13] use tubular steel legs, ST 35,29, with for the members of the tower body is the a minimum wall thickness of 3,5 mm, For economic reasons these towers were tension, and above all the compression with spun concrete filling, in lengths of constructed preferably with three legs, caused by the varying directions of stress, 3 to 5 m, crossed diagonal bracing of ST such as in 1956 for the Emden – Wiesmoor also referred to due to the slenderness 52 iron rods and ST 37 angle iron for the line in Northern Germany (Figs. 6 and 7) of the members as the buckling force. horizontal braces at the nodes as well as and the 110 kV railway grid line Mainz – Different profiles can be used for the tower for the bottom chords of the cross arms. Bingen in the Rhine valley (Fig. 8). Siemens

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Fig. 9: Cross-section of steel tubes with spun Fig. 10: Transmission line lattice tower with four Fig. 11: 110 kV Lattice tower concrete after one year in service. Legs and three foundations on the Vattenfall using 60° steel angles near (former Bewag) 110 kV line Reuter power station Kiruna in . – Spandau in Berlin. towers with three legs are environmentally advantages which pay off, especially of approx. 6% is always more economical friendly and due to only three foundations in poor ground conditions. than thick-walled angle sections, such are particularly economical with regard to Fears of internal corrosion, which led as with B = 60 mm and H = 6 mm due the total costs. In bad ground conditions unfortunately to the precautionary removal to the favourable moment of inertia in these towers are indispensable. The of the Siemens towers from the 110 kV line relation to the cross section. The 60° technical and economical results of the Tiefstack – Neuhof built in 1954, proved to angle sections used are more economic Mainz – Bingen 110 kV line are described be unfounded. The towers were replaced than the normal 90° angle sections due in detail in a publication by Deutsche by tubular towers filled with bitumen. The to their higher moment of inertia under Bahn AG (German Railways) [16]. In it two Siemens type tubular towers with compression, as shown in Fig. 5. In Austria, spun concrete mentioned above, which the advantages of the Siemens towers Voestalpine AG has constructed lattice have been in service for 50 years exhibit towers with diagonal struts of folded 60° compared with steel angle lattice towers no internal corrosion. This is shown in on the same line are described. The Mainz steel angles for 110 kV and 220 kV lines. towers that have been taken down due Transmission towers with insulated cross – Bingen line as constructed required the to reconstruction measures, and had arms were suggested as early as 1971 following quantities in comparison with already been ascertained in the long- [6], but were never used until 20 years normal four-leg towers that were planned term tests carried out in 1952 at Munich later in a re-design where composite for the same line: Technical University and the Bavarian State Trade Institute in Nuremberg. As could insulators were used as cross arms (Fig. 12).  Weight of steel 305 t instead of be demonstrated on tubes which were A 380 kV compact line in Switzerland built 421 t (71%) removed, the spun concrete exhibited on a former 110kV right-of-way (Fig. 13)  Concrete for foundations (78,2%) strengths between 479 and 512 kg/cm², made possible a construction which was still adhered to the tubing and even after space-saving as well as economical [18].  Area of painted surface (71,7%) 50 years of varying loading showed no  Total weight of tower 421 t instead damage to the reliable bond with the steel Conclusion of 435 t (97%) tubes (Fig. 9.) [17]. Transmission line engineering offers an  Lower construction costs of the towers So far, unfortunately no further use has interesting and viable proposition. To follow due to smaller number of towers been made of this type of tower, partly new, more economic paths and at the (84 instead of 98) and of parts (only due to the effort to continue the use of same time to pay attention to the costs three legs); existing standard towers, and partly due to of transmission as well as to environmental  For 110 kV lines the Siemens towers the fact that no support was available from protection is a worthwhile endeavour. have a larger economically optimal the original manufacturers Brüninghaus The necessary combination of electrical span length of 365 m compared to and Hilgers. It would be economically and structural problems needs to be 330 m; thus the quantities and costs advisable to use them again, especially considered. Old suggestions have been for insulators and fittings as well as the in view of today's higher steel prices. The recalled above, such as: costs for compensation and restoration economic advantages of fourleg lattice of landscape damage are reduced; towers with two legs of the lower tower  Choice of conductors  The visual profile and wind crosssection section drawn to a single point, that is with  Aldrey conductors three instead of four foundations, are also of the towers are definitely lower and  Higher conductor tensions due to their circular form are also less evident, but this design has unfortunately  Optimal span lengths sensitive to higher wind loads and less not been used very often. One exception prominent in the landscape; is two towers of the 110 kV line Spandau –  Tower configuration with three Reuter Power Station in Berlin, as illustrated foundations  The erection of the towers is just as in Fig. 10. Fig. 11 shows one of the steel simple as for normal lattice towers;  Siemens type towers use of com - lattice towers with three legs using 60° posite insulators for a compact and  The total cost reduction amounts to angle members, a design which was used economical overhead line design 9,5% Due to the use of tensioned by Siemens over 90 years ago on a 110 kV diagonal struts, the width of the line in Kiruna, Sweden. A further possibility Technical innovations do not only make lower part of the Siemens towers in lattice tower construction is the use of sense for new lines, but also for re- can be increased without additional thin-walled angle members. construction and changes to existing cost. That allows the tension load lines, such as raising the operating on the foundations, and hence the For example, in towers a steel angle with temperature of the conductors to foundation costs to be reduced – B = 60 mm and H = 6 mm with a thickness increase the transmission capacity,

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[3] W F Bueckner: Ein Beitrag zur Erhöhung der Übertragung- sleistung und zur Senkung der Übertragungskosten. Lecture –Fachtagung der Regioplan und WLK, Potsdam, 2002. [4] W F Bueckner:Elektrizitätswirtschaft und Freileitungstechnik. Electrizitätswirtschaft Jg.100 (2001), H.10,S.54-61. The Electrical Supply Industry and its Impact on Transmission Line Technology- Econo –mical Aspects. Cigré Report 2002 No.22 – 204. [5] W F Bueckner: Wirtschaftliche Gestaltung von Hochspannungs- Freileitungen. Siemens- Zeitschroft 1961 S. 705 – 714. [6] H Mors: Entwicklungsrichtungen im Freileitungsbau- Isoliertraversen. Elektrizizätswirtschaft Jg.70, (1971) H. 10, S. Fig. 12: Crossarms on a 380 kV line wi. 253-257. [7] W F Bueckner: Designing Overhead-Line replacement of the conductors or new Structures Economically. Siemens Review 1962, S. 305 – 311. environmental regulations. Moreover, the [8] F J Quey: Generalisation de l`'emploi de consolidation and extension of the lifetime l'almelec par Electricite de France Revue de of all components of a transmission L'aluminium December 1972. line is an economically effective goal. [9] K Herzig,H Mors: Kostenermittlung von optimalen Fig. 13: 380 kV Compact transmission line in The regulations should recognise new Freileitungen. Elektrizitätswirtschaft Jg.90 Switzerland with composite insulators. (1991), H. 21/22, S. 1151 – 1161. experience and allow verification by [10] Cigré 22 WG 04: Recommendations for Bingen. Elektrische Bahnen 29 (1958), S. 204- means of tests and measurements. Evaluating the Lifetime of Conductors. Electra 213. The supply of energy needs the help of No 63 (1979) S. 103 - 145. [17] Bayerische Landesgewerbeanstalt Nürnberg: experienced transmission line engineers [11] W F Bueckner,: Safety Determination of Versuche an Stahlrohren mit Schleuderbeton. in the development and construction of transmission line vibrations. T&D International, Bericht Nr. 26896 v. 16.10.1953. June 1992 p.37. future more economical overhead lines. [18] M Amman, P Dalleves,K O Papailiou, M Leva, S [12] W Phillips, W Carlheim, W F Bueckner, Endurance Villa: A new 400 kV Line with Compact Towers capability of transmission line conductors and and Composite Insulated Crossarms. Cigré References its evaluation. Cigré-Report 1972 No. 22- 05. Report 22/33/36-06 1998. [1] Wienken; Dorsch; Bueckner: Wirtschaftliche [13] H Rieger: Freileitungsmaste aus Stahlrohren mit Betrachtung zur Wahl der Spannung in Stromver Schleuderbeton. ETZ A 21 (1954), S. 725. Acknowledgement sorgungsnetzen, Siemens-Zeitschrift 1965. [14] W F Bueckner,: Freileitungsmaste mit [2] H J Koglin, R Zewe, W F Bueckner, F Hirsch, 3-Punktgründung. Cigré Meeting 1960. This article was published in Electra No 259 K H Weck: Technical economical and [15] W F Bueckner, H Rieger, Freileitungsmaste mit 3 Dec 2011 and is reprinted with permission. environmental aspects of compact lines. Eckstielen. Siemens-Zeitschrift 1956, S. 334-341. Cigré Report Leningrad No. 700-07, 1991. [16] R Fritsche,: Die 110 kVBahnstromleitung Mainz- Contact Cigre, [email protected] 

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