A Practical Consideration of Transmission Line Engineering by Dr
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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 Germany 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. energize - June 2012 - Page 34 TRANSMISSION AND DISTRIBUTION 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 France (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 energize - June 2012 - Page 36 TRANSMISSION AND DISTRIBUTION 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: Transmission tower 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.