User:Chanderpt/Trefoil cabling

EXAMINATION OF ALTERNATIVE FORMATIONS FOR 150 kV CABLES - POSSIBILITIES AND ADVANTAGES FROM THE USE OF TREFOIL FORMATION N. Drossos G. Kyritsis D. Tsanakas S. Papathanassiou Public Power Corporation (PPC) University of Patras National Technical University of Athens Abstract: The installation of single-core 150 kV cables with solid sheath bonding is examined, as an alternative to the current practice of flat formation with a distance between cables and continuous sheath cross-bonding. Results are provided for the current-carrying capacity of the cables for different formations and cable cross-sections, under various thermal conditions. Calculations are performed both for continuous and for cycling loading of the line. The examined alternatives are evaluated technically and economically. The advantages of the trefoil versus the flat formation of cables are analyzed, as well as the possibilities for its application in new cable lines for the alimentation of new substations in Attica. An important reduction in the installation cost of the HV lines is achieved by the trefoil formation. Keywords: HV cable lines, XLPE cables, Flat formation, Trefoil formation 1. INTRODUCTION 1.1 Scope of the paper The flat formation of single-core cables has been applied at the underground 150 kV grid of Attica (broader Athens region) in the last 25 years, with 250 mm axial distance of cables and cross-bonding of the sheaths. The flat formation of cables requires an excavation width of 80 cm and special equipment for the cross-bonding of the sheaths. In the paper, the trefoil formation is evaluated, i.e. the triangular formation of single-core cables in contact to each other, with no sheath cross-bonding. Application of this formation reduces the required excavations (60 cm excavation width versus 80 cm) and results in important savings in the installation and maintenance cost of the cross-bonding systems. To evaluate comparatively the two alternative formations, the acceptable line loadings for different soil thermal characteristics (thermal resistivity and temperature) are compared, as well as the relevant installation costs. The paper is an extended summary of study [1]. 1.2 Background The first 150 kV lines were installed in Greece as parts of the old Attica sub-transmission system, during the decade of 1960, for the supply of 150/22 kV outdoor substations of 3x66 MVA installed capacity. These first lines had a 120 MVA transportation capacity and consisted of single-core paper-oil insulated cables, with 240mm2 Cu conductors. The trefoil formation of the 3 single-core cables in contact was used in the above lines, Figure 1a. The development of the 20 kV system in Attica and the installation of 150/20kV GIS substations with power transformers of 100 MVA (up to 3x100 MVA) led to the adoption of cables of 200 MVA nominal current carrying capacity in the decade of 1970. The 200 MVA cable lines laid between 1973-1980 consisted mainly of single-core paper-oil insulated cables, with A1 conductors of 800 mm2 cross-section (in a few cases 700 mm2). The flat formation was used for these lines, figure 1b. Since then, newer cable lines have been installed in the system of Attica during the late 90’s. Synthetic (XLPE) insulation cables with A1 conductors of 800 mm2 cross-section and flat formation have been used for these lines. The current carrying capacity of a cable line, given its type and the cross-section of the conductors, varies according to the installation conditions (depth, cable formation, axial conductor distance), as well as with the temperature and thermal resistivity of the soil (seasonal variations, type of soil along the route). The determination of the current carrying capacity of the Attica 150 kV cable lines feeding 150/20 kV substations with 100 MVA transformers has been performed during the system design phase assuming a cyclic loading with a daily load factor of 0,75. 1.3 Methods of connection of the metallic sheaths When the metallic sheaths (Pb sheaths for the cables of Attica) of single core cables are interconnected, the minimization of the circulating currents in them is crucial for achieving a high transportation capacity for the 150 kV line. The currents in the phase conductors induce longitudinal voltages to the metallic sheaths. The magnitude of voltages depends on the conductor currents, the length of the parallel route and the mutual inductance between conductors and sheaths, which is related to the axial distance of single core cables and the sheath radius. Currents flow in the sheaths if they are connected to form a closed circuit. The solid connection of sheaths practically eliminates the sheath-to-earth voltages, but results in significant losses due to the sheath currents, which may reach 40% of the rated conductor current, when Pb sheaths are used (even greater for Al sheaths), with a significant impact on the current carrying capacity of the line. Reduction of these losses can be achieved if the axial distance of the cables is minimized by laying them in triangular formation, in contact to each other (symmetrical formation). Despite the fact that the dissipation of heat losses deteriorates, there is an overall increase in the current carrying capacity due to reduced sheath losses. 96.3 mm 250 mm250 mm Fig. 1. Formations of three single-core cables: (a) Trefoil formation in contact, (b) Flat formation. External diameter of 96.3 mm for XLPE 800 mm2 Al cable. Installation depth 1.4 m. The elimination of the sheath currents, which is basically the objective in the case of flat cable formation, is achieved through the application of the sheath cross-bonding system, Figure 2. At the cable joints, the sheath of each phase is interrupted by an insulator. The number of line segments between the positions of the joints is arranged to be a multiple of 3. The sheath segments at each side of a joint are connected to the segments of the other two phases, so that the induced sheath voltages cancel each other along three line segments. However, due to the lack of symmetry in the flat cable formation, as well as to non-equal lengths between joints in certain cases, there still exists a small residual current of the order of 1% of the rated line current. (a) The method that has been applied by PPC is the continuous cross-bonding of sheaths, as it appears in Figure 2, with sheaths grounded at both ends. For safety reasons, the maximum sheath voltage during normal operation is limited to 50 V, which in turn results in a limitation for the maximum length of cable between successive joints, for a given pole distance (250 mm for the cable lines 200 MVA of Attica). (b) 2. LOADING CALCULATIONS AND COST COMPARISON 2.1 Current-carrying capacity of old and new 150 kV cables The calculation of the current-carrying capacity for the paper-oil cables of the 150/20 kV system of Attica took place in the decade of 1970 [2]. For these calculations a 15 οC soil temperature was assumed (corresponding to winter weather conditions, since the peak power demand of Attica at that time occurred in the winter) Vkm333222444555666111RγSheatConductorΙλλλλλλΙRγΑ00.41.20.81.62.02.4Rγ ΙRTSIndicative variation of sheath-to-earth voltage as a function of the distance (λ=constant)L=n λλ ≅ 400 m: Earthing resistance(n=3,6,9,...)Earthleakagecurrent Fig. 2. Continuous cross-bonding of sheaths. and a soil thermal resistivity of 1,2 K.m/W. The maximum allowed loadings for paper-oil cables of 800 mm² nominal cross-section laid in the ground had been determined for continuous loading (100% load factor), as well as for cyclic loading with a load factor of 75%, corresponding to the following load variation pattern: • 6 hours at 100% load • 2 hours at 50% load • 6 hours at 100% load • 10 hours at 50% load The permissible maximum currents were also calculated for 5 additional cases, applying suitable reduction factors [1,2], corresponding to the following conditions: Soil temperature: 15 οC, Soil thermal resistivity: 0,85 K.m/W “ 15 οC, “ 1 K.m/W “ 25 οC, “ 1 K.m/W “ 25 οC, “ 1,2 K.m/W “ 25 οC, “ 1,5 K.m/W The last three cases occur in summer. The above reference temperatures were based on the statistical evaluation of long-term measurements, over several years. For the determination of the thermal resistivity of the soil, which is also a basic parameter for the calculations, research of the subsoil along the selected routes had been performed before the installation of the first 200 MVA underground lines. After extensive measurements, a thermal resistivity lower than 1,2 K.m/W was found for the winter period, with a high probability of exceeding this value in summer. For this reason, it was decided to use fine-grain sand, instead of the products of excavation, to fill the trenches after laying the cables, in order to achieve maximum compressibility and thus enhance heat exchange (practice that is followed today). In this way, even under the most unfavourable conditions of particularly dry subsoil, it is estimated that the soil thermal resistivity will not exceed 1,5 K.m/W. According to IEC 60287 ([3]), the current carrying capacity of 800 mm² Al paper-oil cables varies under continuous loading conditions from 162 MVA to 238 MVA, depending on the thermal characteristics of the soil. However, the current carrying capacity under constant load has a rather theoretical value when dealing with cables supplying distribution loads. Indeed, the exponential nature of cable heating, with very long time constants for usual Al cross-sections, results in the conductors reaching their maximum permissible temperature in time intervals exceeding 24 hours (3-5 days). Therefore, cable lines serving distribution loads with a strong daily variation can undertake maximum loads during each 24-hour interval well in excess of their continuous loading capacity. For this reason, the basic calculation of the current-carrying capacity of old paper cables has been carried out by the Neher-McGrath method [7], considering a daily load factor of 75%. This method is differentiated in the calculation of the external thermal resistance of the line, where the assumption is made that the largest part of the thermal resistance of the ground affecting the dissipation of the Joule losses is influenced by the average losses over the 24 hour interval. Using the Neher-McGrath method, the current-carrying capacity of 800 mm² Al paper-oil cables is calculated between 187 and 274 MVA, depending on the soil characteristics. The lowest values for winter and summer conditions are found equal to 228 and 187 MVA, respectively. For the 150 kV cable with synthetic insulation (XLPE) and the same 800 mm2 Al conductors, calculations have been carried out for both constant and cyclic loading, applying the IEC 60287 and the Neher-McGrath method respectively. As shown in Table 1, the current-carrying capacity of XLPE cables always exceeds the capacity of paper-insulated cables of the same cross-section. The capacity of XLPE cables varies between 181 and 241 MVA, for continuous loading. Under cyclic loading, it is found between 212 and 275 MVA, the lowest values for winter and summer conditions being 247 and 212 MVA, respectively. 2.2 Cross-section for 150 kV cables feeding new substations with 50 MVA transformers The rapid increase of the load in Attica, in conjunction with the anticipated additional needs for the 2004 Olympic Games held in Athens, led to the construction of 7 new 150/20 kV substations, including 4 GIS substations fed by cable lines. Three of the new GIS substations are equipped with 2x50 MVA transformers, with provision for the future installation of a third 50 MVA transformer. Thus, in the medium term, each of these substations will serve a load up to 80 MVA, under normal operation conditions, according to the system design principle for Attica (transformer loading up to 80% of the installed capacity, to ensure sufficient reserve margin for emergencies). In the long term, after installation of the third transformer, the maximum substation load under normal operation conditions will reach 120 MVA. Each of these substations is fed by two cable lines. Assuming a nominal capacity of 200 MVA for each line (corresponding to Al conductors of 800 mm² cross-section), a high reserve margin will exist, even under abnormal operation conditions, when the substation load might reach MVA. Consequently, it was worth exploring the possibility of using cables with conductor cross-section lower than 800 mm² Al for feeding the new substations with installed capacity 100-150 MVA. Calculations and comparisons of the current-carrying capacity were performed for 800 and 500 mm² nominal cross-sections (as standardized according to IEC 60228, [6]), assuming flat formation and continuous cross-bonding of sheaths, either for continuous or for cyclic loading (daily load factor 75%). It was found that the capacity of 500 mm² Al cables, although lower than 200 MVA under continuous loading, it exceeds 120 MVA (varying from 140 to 186 MVA). For cyclic loading, the capacity of the 500 mm² Al cable exceeds 200 MVA in winter conditions, whereas in the summer it is limited to 165 MVA, under the most unfavorable conditions. Therefore, this cable might undertake the full substation load of 120 MVA, while an additional capacity margin would still be provided. In comparison to the 800mm² cable, the resulting reduction of the transportation capacity is about 24%. Adopting the 500 mm² cross-section, the estimated reduction in the cable weight is of the order of 2,5 t per km of single-core cable (roughly 14%), taking into consideration only the basic components (conductor, insulation, Pb sheath, external PVC covering). However, this reduction does not result in a similar saving in the production cost of the cable itself, since raw materials roughly amount to 48% of the cable cost. More specifically, the reduction in the cost of the cable is estimated at 3.000 € per km of single-core cable, considering that manufacturing, packing and transportation costs remain practically undifferentiated. Consequently, a saving of 9.000 € per km of three phase line is achieved, corresponding roughly to 5% of the total cost of the cable. The cable cost amounts to 35% of the total installed line cost, while the remaining 65% (parts and installation costs) is very slightly influenced by the cable cross-section. Hence, the use of 500 mm² cross-section leads to a total cost reduction of about 1,75%. Notably, this saving diminishes over the long term, due to the increased Joule losses in the cables of smaller cross-section. Based on these considerations, the option of using cables with 500 mm² Al conductors for new cable lines in Attica was eventually abandoned. 2.3 Evaluation of alternative formations for 150 kV cable lines From the above investigation, it appeared that the possibility of installing 800 mm² cables in trefoil formation, with solid connection of sheaths, might be interesting to examine for the new cable lines feeding 3x50 MVA substations. This option might yield much more important savings, due to a significant reduction in the cost of excavations, parts (special joints and bonding materials) and cable installation, along with an simplification of line route studies. An additional benefit would be the cut down of costs and personnel involvement in cross-bonding system maintenance (periodical controls and interventions over the cables life span). Calculation results for the transportation capacity of 800 mm² Al XLPE cables in trefoil formation (solid connection of sheaths), under various loading conditions, are given in Table 1. It is observed that the maximum permissible cable loading for the trefoil formation varies between 159 MVA and 214 MVA, for unity load factor, while for cyclic loading at 0,75 load factor, it ranges from 188 MVA to 246 MVA. Lowest values for the winter and summer periods are 219 MVA and 188 MVA, respectively. These capacities are lower compared to the flat formation with sheath cross-bonding. Nevertheless, the resulting reduction is about 12% and under any circumstances it is not prohibitive. In Table 1, the 800 mm² Al XLPE cable loadings (for flat and trefoil cable formation, with cross-bonding and solid bonding of sheaths, respectively) are also compared with those of the old 800 mm² Al paper cables, all laid in flat formation with cross-bonded sheaths. The comparison is summarized as follows: For the same laying configuration of the lines (flat formation), the transition from paper-oil to XLPE insulated cables raised the transportation capacity of the lines. The gain is increased for unfavorable loading conditions, approaching 14% at maximum. Compared to the paper cables in flat formation (and cross-bonded sheaths), the XLPE cables in trefoil formation (and solidly bonded sheaths) present a capacity deficit, varying from -11% to + 1% over the considered range of soil conditions. The deficit is decreasing under unfavorable loading conditions, leading to practically equal capacities under the most unfavorable conditions (soil temperature 25 oC, thermal resistivity 1,2 or 1,5 K.m/W). It is also remarkable that, even when a deficit exists, the XLPE cables maintain a capacity in excess of the planned level of 200 MVA (for cyclic loading). The trefoil formation with solid connection of sheaths will increase losses, compared to the flat formation, due to the existence of currents circulating along the sheaths (sheath losses are roughly increased by 16%). For different reasonable loading hypotheses which were examined, the net present value of the long-term differential cost of losses per km was evaluated below 3.000 € and obviously it is compensated by the elimination of maintenance expenses. The evolution of Attica’s load presents an explicit upward trend for the daily load factor, leading to values higher than 75%, which was considered 30 years ago, when the design of the underground 150 kV network of Attica took place. This trend appears to be more constant during the winter period, being recorded for many years (from the beginning of the decade of 1990), with daily load factor values that very often approach 80%. For the summer period this increase is recorded over the very last years, in a less systematic way. The calculations were repeated indicatively for a load factor of 80%. The resulting reduction in cable line capacity is of the order of 4%. Since this reduction is uniform for the various types of cables and formations, the main observations of this Section remain valid. Loading calculations were also performed for a trefoil formation with a gap of 10 mm between the cables, in order to simulate a possible thin layer of sand among the cables. The resulting effect on the cable capacity was found insignificant. Table 1 Loading capacity of 150 kV, 800 mm²Al, XLPE insulated cables: (1) Flat formation with cross bonding of sheaths and (2) Trefoil formation with solid connection of sheaths, without cross-bonding. Correlation with paper cables. Winter Summer Thermal resistivity of soil (K.m/W) 0,85 1 1,2 1 1,2 1,5 Temperature of soil (°C) 15 25 Permissible rise of conductors’ temperature (°C) 75 65 Formation of cables (1) (2) (1)(2) (1)(2) (1)(2) (1) (2) (1) (2) Continuous Loading (load factor=1) Maximum intensity (Α) 929 822 877773 819720 816719 762 670 697 611 Thermal limit of cable capacity (MVA) 241 214 228201 213187 212187 198 174 181 159 Reduction of current carrying capacity -12% -12% -12% -12% -12% -12% Comparison with the capacity of paper cables 2% -11%5%-8% 7%-6% 7%-6% 10%-4% 12% -2% Cyclic loading (load factor=0,75) Maximum intensity (Α) 1059 946 1008 898 950843 938835 884 784 818 723 Thermal limit of cable capacity (MVA) 275 246 262233 247219 244217 230 204 212 188 Reduction of carrying capacity -11% -11% -11% -11% -11% -12% Comparison with the capacity of paper cables 1% -11%4%-8% 8%-4% 7%-5% 11%-2% 14% 1% 2.4 Economic comparison of alternative 150 kV cable formations The savings achieved by the trefoil formation comprise on one hand the reduction of the required installation space and excavations (trench width of 0,6 m, versus 0,8 m for the flat formation) and on the other hand the lack of special cross-bonding equipment, which represents an important purchase and installation cost. The components of cost being influenced are: ∗ civil works (excavation and relevant works) ∗ cable laying ∗ materials (joints, cross-bonding boxes) ∗ assembly labour for the cross-bonding systems The resulting reduction to the to the direct cost (supply and installation of cable and parts) of a cable line by using the trefoil formation is about 30.000 € per km of line, corresponding to about 8% of the total cable line cost. This reduction is quite significant and in fact more substantial than the one achieved by using smaller cross-section cables. Analyzing the various cost components, the largest reduction is due to the savings in material costs and to the considerably lower amount of civil works required (by approximately 11.000 € and 10.000 € per km, respectively). 2.5 Additional advantages from the introduction of trefoil formation of HV cables As already mentioned, the adoption of trefoil formation of cables with solid bonding of sheaths, apart from the reduction of the initial installation cost, eliminates the need for regular inspection of cross-bonding systems. It is also expected to enhance the reliability of operation, since sheath cross-bonding systems often constitute a source of problems, due either to assembly faults during initial construction or to the actual operating conditions. The repair of such faults and the restoration of service may require extensive and time consuming interventions along the whole length of the line. The resulting reduction of exploitation cost over the life span of a 150 kV line is difficult to quantify reliably but it will nevertheless be quite significant, particularly if is taken into account that qualified technical personnel will be released to perform other tasks on the cable network. 3. CONCLUSIONS The basic conclusions of this paper are summarized as follows: 􀂙 First of all, the application of the trefoil cable formation without cross-bonding of sheaths in 150 kV cable lines is perfectly acceptable. 􀂙 The trefoil formation leads to an estimated reduction in the direct cost of the cable line of the order of 30.000 € per km, that is 8% of the total line cost. In addition, the maintenance requirements are eliminated, due to the solid connection of sheaths. 􀂙 The application of this formation was adopted for the feeding lines of 3 new substations in Attica, with a total length of 30 km. The total savings in the investment expenses are estimated to be of order of 1.000.000 €. 4. REFERENCES [1] Public Power Corporation/Network Construction and Maintenance Department & Network Asset Management Department: Examination of alternative formations of 150 kV cables–Possibilities from the use of new cable lines of Attica, December 2002. [2] Ζ. Emiris: Underground oil-filled 150 kV cables, Greek Committee of CIGRE. [3] IEC 60287: Electric cables-Calculation of the current rating. [4] CIGRE-SC 21: The design of special bonded cable systems, Electra No 28 (1973), pp. 55-81. [5] CIGRE-SC 21: The design of special bonded cable systems (Part II), Electra No 47 (1976), pp. 61-86. [6] IEC 60228 (1978): Conductors of insulated cables. [7] G. J. Anders: Rating of electric power cables, IEEE Press, 1997.