In Recent Years, the Evolution and Enhanced Performance of Underground Cables with extruded insulations such as cross-linked polyethylene (XLPE) has refocused attention on the installation of underground extra-high-voltage (EHV) and high-voltage (HV) transmission circuits. The liberalization of the energy market and the need to connect new power plants to the grid has stimulated growing requirements to extend existing transmission systems.
The choice of whether to use overhead line (OHL) or underground cable (UGC) must be consistent with safety, reliability and operational constraints to ensure that the capacity of the transmission grid efficiently matches the supply and demand of electrical energy. The choice between OHL and UGC is driven by technical, environmental and economic considerations.
The operating costs over the life of an asset can be converted into an equivalent capital sum at the start of the project life and added to the capital cost of the investment. The environmental costs of a transmission facility can be quantified in terms of the burden on the built/developed/occupied land or territory. The presence of an electromagnetic field exceeding the value set by national laws (or rules or standards) may create a quarantined area of land unavailable for human activities or development. The economic impact to the land crossed by a transmission line can be estimated taking into account the loss of value of the rights-of-way.
For an OHL, the number of conductors per phase, the type and cross-sectional area of conductor, the tower design and height are dependent on technical requirements and geographical factors that may vary from country to country. These economical comparisons have been computed for the three EHV and HV Italian voltage levels.
Figure 1 shows the typical standard Italian towers used for 380-kV, 220-kV and 132-kV to 150-kV circuits, the conductor spacing and clearances, and the minimum clearance above ground level as specified in the Italian Standard CEI 11-4 used in the computation of rights-of-way. Traditional towers have been used in this comparison, even though more innovative designs such as compact towers are used on OHL.
Figure 1 also includes installation details for underground XLPE-insulated cables for the same circuit voltages and the same single-circuit ampacity. For example, the 380-kV cable system shows the typical underground installation of a double-circuit UGC with 2500-mm2 copper conductors, necessary to transmit the same ampacity of a single-circuit Italian OHL using 3 × 585 mm2 conductors.
Table 1 shows the positive-sequence parameters per unit length used to compute the steady-state regime and power losses, the losses being the sum of I2R and shunt conductance losses. OHL conductance is usually neglected, but in this comparison, the corona losses and the insulator leakage currents are considered. These losses depend on the prevailing weather conditions (dry or rainy). In Italy, for instance, 90 rainy days per year have been considered. The per-unit length resistance of OHL has been computed at 75°C (167°F), the conductor temperature when operating at the thermal limit in prevailing wind conditions.
The parameters for cables have been computed using IEC 287, assuming perfect cross bonding at a given spacing, which determines the inductance, apparent resistance values and all transmission constants.
Note that the cable ampacity is a variable dependent on cable spacing and the soil resistivity that needs to be evaluated for each planned cable route.
The UGC capital cost includes the excavation and installation costs, and the capital cost for the OHL includes the rights-of-way. The investment costs of an UGC circuit are not proportional to the route length due to the fixed costs of the terminations. The route lengths used in this comparison are longer than 5 km (3 miles) and, hence, the circuit capital cost can be considered a length linear function. Table 2 shows the initial capital cost ratios between cables and overhead lines for the three voltage levels.
The economic assessment of the power losses plays a significant role in the overall cost evaluation during the operational life of a transmission line. The load profile of a line is subject to the local system demand and there also could be load attributable to cross-border interconnections, connections between a power plant and the grid, or load distribution in the meshed transmission network.
With regard to the latter, the load diagram presents great fluctuations on both a daily and monthly basis. By analyzing some Italian EHV line-load diagrams, an equivalent operation at the maximum power with a 0.98 power factor for 350 hours per annum has been considered. If the line is directly linked to a power plant, the load profile depends on the power generation profile (base load and peak load power plants). In these cases, the power loss economic evaluation is more significant. Therefore, it is possible to compute the energy losses for two different transmission technologies of the same length (L) and the corresponding actual costs (VOHL and VUGC) based on the following assumptions:
Line lifetime = 35 years
Real rate of interest (discount rate) = 2.5%
Loss energy cost = 40 euros/MWh.
Figure 2 shows the power losses for three different Italian EHV/HV voltage levels for a 10-km (6-mile) circuit. The dielectric UGC losses vary from 1% for 132 kV to 4.4% for 380 kV of the total; whereas for OHL, they are noticeably lower: 0.26%, 0.56% and 0.44% for 380 kV, 220 kV and 132 kV, respectively. With the same current, VOHL is always greater than VUGC at all voltage levels owing to the major cable cross sections and consequent apparent resistances being much lower (Table 1), even if and when bundle conductors are used, which in Italy is only for 380-kV OHL.
Moreover, the kilometric costs per unit length for OHL and UGC are nearly independent for route lengths 1 km to 25 km (0.6 mile to 15.5 miles), the power losses being almost directly proportional to the circuit length.
CALCULATING THE COSTS
The standards adopted in each country for mitigating exposure to electromagnetic fields differ widely. In Italy, for example, the framework law of February 2001 has imposed a general discipline devoted to protection from electromagnetic field exposure. Conservatively, it is possible to determine a right-of-way where any building activity is restricted in close proximity to the ac transmission line route that has a width depending upon current, voltage limit and line arrangement. The width of the rights-of-way varies as a function of the magnetic induction limit of exposure and, hence, of the maximum current.
In Italy, for existing lines, there is a value of attention of 10 µT, whereas for new lines, the limit is 3 µT. Figure 3 shows the rights-of-way with reference to 3 µT for a new OHL (FOHL) and UGC (FUGC), depending upon the voltage levels and zone. It is noted that the magnetic field as well as the right-of-way for UGC may be reduced by the phase cable arrangement and/or screening.
To quantify the cost burden of land or territory due to the installation of a new circuit, UGC and OHL are considered to be erected on the same route on undeveloped land, but with a “buildability value” similar to that of the area adjacent to the rights-of way. It is necessary to evaluate the loss of value of the land based on difficulty or prohibition of future development and the presence of the transmission line. Therefore, it is a variable function of geographic location, building volume and surface area that gives parameters of 3 m3/m2 to 4 m3/m2 in urban areas and 0.8 m3/m2 to 1 m3/m2 for a suburban area. To evaluate the variation of land value located in the rights-of-way, it is necessary to have knowledge of the location and land values.
The economic evaluation of visual impact is extremely complex owing to its strongly subjective nature, as the value of the landscape is something very specific and a function of local views and preferences. Notwithstanding the ambiguity, when a new line has to be erected, this aspect could be evaluated, and therefore the advantage of an UGC circuit is understandable. Economic theory offers many possible approaches that, in general, are based upon contingency evaluation.
A comprehensive analysis of a transmission line must take into account the end of life (that is, the dismantling phase of the line). This operation foresees some costs to restore the place at the end of line life, with a considerable delay with respect to the investment and a subsequent lower burden.
The operation and maintenance (O&M) of a line, during its life, implies some costs that must be considered in the overall cost analysis. The evaluation of O&M refers to investment cost per kilometer, and the following values represent the annual cost to pay per kilometer of line as a percentage of investment cost.
For an OHL, the maintenance costs are between 0.7% and 1% a year with respect to investment costs and depend upon the weather conditions. The OHL operation ranges between 0.8% and 1%. The OHL O&M (flat installation with low salt pollution) can range between 1.5% and 2%. These values (Table 3) must be considered as an average indication that could increase in situations subjected to extraordinary environmental conditions.
For UGC, once installed, they do not need particular maintenance because there are no atmospheric external situations to contend with. The UGC maintenance can be evaluated as 0.1% of the capital cost (Table 3). The procedure does not take into account the different fault repair times of OHL and UGC and their influences on the system costs. These burdens are considered negligible because research indicates a nonavailability of 0.126 hours per year per circuit of OHL and 3.4 hours per year per circuit of UGC.
Studies indicate that 380-kV UGC circuits require reactive compensation at each end of the circuit when the length exceeds 10 km. Hence, the total cost of this compensation (equipment and land occupation) must be included in the overall life-cycle costs.
The life-cycle break-even costs for EHV and HV circuits, based on the use of OHL and UGC, have been evaluated and compared for each voltage for variable UGC capital costs and the average economic value of the rights-of way for the complete circuit. Table 4 details the break-even results for circuits installed in accordance with Italian engineering design standards and land values for circuit lengths not exceeding 10 km.
The life-cycle break-even cost for 380-kV circuits based on the use of OHL and UGC for circuit lengths between 10 km and 25 km has been determined for variable UGC capital costs. The average economic value of the rights-of way for the complete circuit is shown in Table 5.
It will be noted that the inclusion of the cost of reactive compensation has a negligible effect on the break-even point.
To ensure that reliable and economic transmission-system circuits respect the environment is a crucial task that often requires innovative solutions. Whereas the results of this life-cycle cost confirm that overhead lines offer the economic solution, the introduction of XLPE insulation in the design of EHV and HV cables has dramatically reduced circuit losses while maintaining an excellent level of cable-system performance.
In spite of the low investment cost of overhead lines, underground cables have other important tangible benefits, as well as some advantages that are less tangible. It is apparent that there is a need for a technical and economic assessment of these two different solutions that consider all the social issues for every system development project. Although the general procedure and approach presented is based on the Italian situation, this method of evaluation could be applied to any country with its own specific transmission standards, rules and laws.
In general, it is apparent that where land has already been developed for residential use or where development potential is very high, underground cables are the preferred option having less environmental impact even if there are higher capital costs. Conversely, in areas of low value, overhead line transmission technologies having a greater environmental impact and lower capital costs remain the preferred option.
The authors wish to acknowledge the support and assistance they received from M. Del Brenna of Prysmian Cables & Systems in Pikkala, Finland, and E. Zaccone of Prysmian Cables & Systems in Milan, Italy, in preparing this article.
Claudio Di Mario received his Dr.Ing. degree in electrical engineering from the University of Rome and his MBA in 2003 and his Ph.D. in 2005 in management engineering in power system economics from Rome University. From 1998 to 2000, he worked for Siemens AG in Germany, and in 2001, he joined GRTN (Italian system operator) — Grid Division before joining TERNA, the new Italian TSO. He is responsible for the National and European Regulation unit. Di Mario's field of interest includes HV hybrid switchgear and HV power cables. He is a member of CIGRÉ Study Committee C1 and Working Groups B3-01.
Roberto Benato received his Dr.Ing. degree in electrical engineering from the University of Padova in 1995 and his Ph.D. in power systems analysis in 1999. In 2002, he was appointed assistant professor in the Power System Group at Padova University. Among Benato's main fields of research are multiconductor analyses and EHV/HV transmission lines. He is a member of CIGRÉ WG B1-08, the secretary of CIGRÉ JWG B3-B1.09 and a member of the IEEE/PES Substations Committee. [email protected]
Arturo Lorenzoni received his laureate in electrical engineering in 1991 and his Ph.D. in energy economics in 1995 at the University of Padua. He obtained his master's in energy and environmental economics at Scuola Superiore ENI Enrico Mattei in Milan, Italy. Currently, Lorenzoni is a professor of energy economics at the University of Padua and research director at IEFE, the Centre for Research on Energy and Environmental Economics and Policy in Bocconi University, Milan. He has directed various research projects for distributed generation. He is a member of IAEE and CIGRÉ national committees C1 and C5.
|Cable geometrical data|
|Voltage level||380 kV||220 kV||132 kV|
|Cross-sectional area||mm2||2500 Cu||1600 A1||1000 A1|
|Diameter on conductor (Milliken)||mm||63.4||47.9||38.5|
|Diameter on XLPE insulation||mm||119.9||88.5||70|
|Diameter on metallic screen||mm||130.1 Al||95 Al||77 Al|
|Cross section of screen||mm2||≈ 500||≈ 237||≈ 237|
|Diameter on PE coating||mm||141.7||113.2||95|
|Voltage level||380 kV||220 kV||132 kV||Voltage level||380 kV||220 kV||132 kV|
|Conductor diameter||mm||3-ACSR 31.5||1-ACSR 31.5||1-ACSR 31.5||Cross section||mm2||2500 Cu||1600 Al||1000 Al|
|Resistance at 75°C (50 Hz)||mΩ/km||23.10||69.3||69.3||Apparent resistance at 90°C (50Hz)||mΩ/km||13.3||32.6||42.5|
|Series inductance||mH/km||0.858||1.282||1.213||Series inductance||mH/km||0.576||0.480||0.500|
|Shunt leakage (50 Hz)||nS/km||10||20||40||Shunt leakage (50Hz) with tan δ= 0.0007||nS/km||51.5||53.0||55.4|
|Capacitance||µF/km||0.0133||0.00894||0.00947||Capacitance With εr =2.3||µF/km||0.234||0.241||0.252|
|380 kV||c1/a1||10||c1 UGC is a double circuit|
|220 kV||c2/a2||4.5||See Fig. 1 and Table 1|
|132 kV to 150 kV||c3/a3||4||See Fig. 1 and Table 1|
|Type of Circuit||OHL||UGC|
|Operation||0.8% to 1%||0.1% to 0.3%|
|Maintenance||0.7% to 1%||0.1%|
|Operation and maintenance (O&M)||1.5% to 2%||0.2% to 0.4%|
|Circuit voltage||Width of rights-of-way||Average land value||Break-even cost|
|380 kV||100 m (327 ft)||14 m (46 ft)||34 euro/m2 (US$472/ft2)||4.0 million euro/km (US$3.2 million/mile)|
|220 kV||48 m (157 ft)||4 m (13 ft)||7.5 euro/m2 (US$104/ft2)||0.60 million euro/km (US$0.48 million/mile)|
|132 kV||39 m (128 ft)||3 m (10 ft)||10 euro/m2 (US$139/ft2)||0.50 million euro/km (US$0.40 million/mile)|
|Notes: For OHL at 380 kV, bundle conductors are used, while at 220 kV and 132 kV, single conductors are used. For UGC at 380 kV, cross-bonded double circuits are used, and at 220 kV and 132 kV, cross-bonded single circuits are used.|
|Circuit voltage||Width of rights-of-way||Average land value||Break-even cost|
|380 kV||100 m (327 ft)||14 m (46 ft)||38 euro/m2 (US$528/ft2)||4.2 million euro/km (US$3.36/mile)|
|Notes: For OHL at 380 kV, bundle conductors are used. For UGC at 380 kV, cross-bonded double-circuits are used.|