Planning the Interconnection of Islands to the Mainland Grid via Submarine Cables

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1 Planning the Interconnection of Islands to the Mainland Grid via Submarine Cables E. Κaramanou S. Papathanassiou M. Papadopoulos ( * ) Electric Power Division - National Technical University of Athens (NTUA) 9, Iroon Polytechniou St., Athens, Greece Tel , Fax , mppapad@power.ece.ntua.gr ABSTRACT: The paper includes a brief presentation and economic evaluation of alternative solutions for interconnecting autonomous island systems to the mainland grid, the main driver being the comparatively high generation cost and the need to maximize wind energy exploitation opportunities. Alternative interconnection schemes, including AC and DC technologies, are compared to each other and to the default autonomous operation scenario. A typical medium to large-size island is selected as a study case and the evaluation extends over a 25-year period. Results are presented on energy contributions and costs for the examined schemes, which demonstrate that interconnection of the island is economically attractive and permits full exploitation of the significant wind potential available on islands. Keywords: Islands, autonomous systems, interconnection, submarine cables, wind penetration. I. INTRODUCTION Generation of electricity in islands is usually made by autonomous power stations using as fuel expensive oil products. Consequently, interconnection of the islands to the mainland power system via submarine cables is today of primary economic interest, since electric energy produced by conventional local power stations can be reduced or entirely eliminated. Further, in the case of isolated island operation, the exploitation of the favorable wind conditions is severely restricted to about 10-15% of the island total energy demand. Hence, interconnection to the mainland power system offers the opportunity for much larger exploitation of the local wind potential. Since the 60s, more than 70 Greek islands of all sizes have been interconnected to the mainland grid or to each other, via HV (150 kv) or MV (15 or 20 kv) submarine cables, following the evolution of the related technology. Today, 56 autonomous power stations operate on Greek islands. Nevertheless, recent progress in submarine cable technology offers the possibility of realizing connections of higher capacity and longer distance, with more than one technology and design schemes being available. This is mainly due to the use of power electronics, such as voltagesource converter-based DC transmission, or reactive power compensators in AC transmission technology. A first application of such possibilities was explored in Greece, in the Cycladic islands study [1,2], but the interconnections may be extended to many other Greek islands. Besides the technical feasibility, the main criterion for interconnecting an island is the viability of the investment. In practice, the solution is selected that achieves minimum cost for the electrification of an island over a year period. The potential for development of the local Renewable Energy Sources (RES) has also to be taken into account. Evidently, interconnections are economically attractive when the generation cost in the mainland system is much lower than on the island, compensating the investment cost of the interconnection. The technical advantages offered by each solution (e.g. reliability and quality of supply) need also to be taken into account. In this paper a short presentation is first made of the state of the art on interconnection technologies. Next, alternative possibilities for the electrification of island systems are examined and discussed. An economic evaluation is the performed of the selected alternatives, taking into account the possibilities for RES (especially wind power) development on the island. II. TECHNOLOGICAL PROGRESS A. AC technologies The use of XLPE insulation in HV submarine cables was up to now restricted, compared to HV land cables, eventually due to the difficulty of constructing long onepiece cable lengths and the ensuing need for multiple flexible factory joints, [3]. Single-core submarine XLPE cables are usually used, but it appears that the three-core design with water impervious barriers (sheaths) on each core is nowadays preferred, [4]. SVC progress considerably extends the reactive power and voltage control possibilities offered by switched shunt reactors and capacitors. Further, due to the lower dielectric constant, XLPE insulated submarine cables present lower charging currents per unit length than the earlier used paper-oil insulation materials. In [4], it is stated that the length of a properly compensated 150 kv, 3x1x1000 mm 2 XLPE submarine cable line can exceed 200 km at 200 MW transmission capacity, but such installations have not yet been realized. A limit of 100 km seems to be more realistic. B. DC technologies HVDC transmission technology has been applied for many decades now for the interconnection of strong AC power systems, using Line Commutated Converters

2 (LCCs). Recent progress in power converters has permitted the extension of the DC transmission for feeding isolated AC networks, without the necessity for installing sources of commutating capacity in the remote AC network [3]. Extruded HVDC cables with polymeric insulation are now used in VSC-based schemes, instead of mass impregnated cables. These technologies are provided mainly for low transmission capacities (less than 1000 MW), being suitable for the interconnection of relatively small power systems, like those of islands or large off-shore wind parks. With DC technology, cable lengths and transmitted powers may be much higher. In addition, superior power and voltage control capabilities are offered, which are very important when considerable wind power is to be installed on the island. It is also remarkable that single-core DC cables are much simpler and lighter than corresponding AC ones. Consequently, taking into account that a large proportion of the total interconnection cost is due to the AC/DC converters, DC transmission becomes more cost effective for relatively long distances. III. ELECTRIFICATION SYSTEMS FOR ISLANDS The following three main cases need to be considered and comparatively evaluated: Electrification of the island is realized via development of the local conventional power stations (practically oilpower units) and the local network systems. Interconnection to a neighbouring power system (to the mainland grid or to the system of a larger island), in such a way that the security of supply, usually expressed by the N-1 criterion, is ensured by the design of the interconnection. In this case the local power station can be totally eliminated. Interconnection to another power system is made, but local generation remains in operation in parallel to the interconnection, for peak shaving and reliability purposes. In this case a simple interconnection scheme can be selected. In the following, the main points to be considered for each one of the above-mentioned schemes are briefly presented and discussed, including the possibility for exploitation of local RES potential. A. Autonomous power system development Isolated island grids are fed by oil-fired plants. Diesel engines, usually burning heavy fuel oil, are installed in the small islands (up to 10 MW). For medium size islands (up to 100 MW) gas turbines of aero-derivative or industrial type, using light fuel oil, may also be installed for peak service (up to about 1500 h/a). In large islands, steam turbines, using heavy oil products, as well as combined cycle plants, are also installed. Regardless of the composition of the autonomous generation system and the specific types of plants, in practice the existing RES potential (mainly wind) is restricted to about 20-30% of the peak power demand and 10-15% of the annual energy demand of the island. This is due to operating constraints imposed by the conventional units, related to their minimum loading level and dynamic regulation capabilities, which impose restrictions to the wind power penetration levels. The possibility to increase wind penetration by storage (e.g. batteries and dump loads in small islands or pumped storage systems in larger islands) have been tested in the past and are a promising solution for the future (especially hydro-storage), but their development has been so far restricted. B. Interconnection and elimination of local power stations All existing interconnections of Greek islands, via medium or high voltage cables, are of this type. The N-1 criterion for the security of supply is generally satisfied by installing independent cable circuits in such a way, that the simultaneous loss of all supply paths because of faults or other failures is practically excluded. With AC technology, this is practically achieved via installation of two three-phase or four single-phase submarine cable systems, with sufficient separation from each other along the sea route. Appropriate measures are also taken at the cable landing locations and at the on-coast switching stations. In the past, four single-phase submarine cables where usually installed, but now, with the use of extruded cables, the installation of two three-phase cables has approximately the same cost and it is considered as the preferable solution, because of its technical advantages. For DC interconnections, alternative configurations are proposed, employing different return circuit concepts (separate or common conductor) and possible use of the ground (not preferable). In any case, schemes that satisfy the N-1 criterion may present considerably increased costs. Selection of the appropriate technology and configuration scheme depends on characteristics of the interconnection such as the following: (a) Cable length, transmission capacity and sea depth. (b) Required reliability of supply. (c) Anticipated non-conventional on-island generation (usually RES, including wind and photovoltaic power, but also geothermal, biomass, solar thermal stations). C. Interconnection and operation of local power stations This case is of interest for large islands, where the generation cost (especially in peak hours) is comparable to that of the mainland system. In addition, the investment cost for meeting the peak demand may be excessive, while the additional investment for configurations providing the required reliability level may be higher than maintaining conventional local generation in cold reserve. Further, although the failure probability of the interconnection may be minimized by the selection of a proper configuration, there still remains a possibility for a combined failure events (e.g. due to landslide at the sea bottom), which cannot be dealt with by special measures, like transportable power units, as in the case of smaller islands. In this case, the interconnection offers the possibility of supplying the base load from the low-cost units of the mainland system, while the peak demand is covered by local (on-island) units, such as gas turbines. Notably, the interconnection capacity may be fully utilizable for transferring considerable amounts of RES energy from the island to the mainland, without the strict requirements imposed for the continuity of supply to the island.

3 The technology and configuration schemes for the interconnection are determined by the factors mentioned in the previous section, but also by the type and size of local conventional generation to be retained. The evaluation of this case is more complicated, since more alternative solutions need to be examined. IV. STUDY CASE The three alternative solutions for the electrification of an island, described in the previous section, are applied and compared in the following for a typical island system, over a period of 25 years. The distance of the island from the mainland is 80 km and the maximum sea depth is 500 m. The peak load demand at the first year of the evaluation period is 100 MW, with a mean annual rate of increase of 5% for the next 25 years. The existing power station comprises diesel units, utilizing heavy oil products, with a total installed capacity of 115 MW. The wind potential and local conditions on the island permit installation of up to 400 MW of wind power. The possibilities for development of other local power generation types are negligible. Following the categorization of the previous section, the three solutions described below are further examined. Input data used in the analysis are presented in Table 1. Case (A): The island system remains autonomous. The existing local power station is gradually expanded, following the increase of the load, maintaining an additional margin of 15% over the annual maximum demand. Diesel units, similar to the existing ones, are considered. The wind potential of the island is developed at its maximum permissible limit for autonomous island systems. This limit is assumed to correspond to installed wind capacity equal to 25% of the maximum load demand of each year. Table 1: Input data for the alternative solutions evaluated in the paper Case A Case B1 Case B2 Case C Load data Peak load demand (t=0) MW 100 Αnnual rate of increase 5% Load factor 54% Conventional unit data LPS LPS IPS LPS IPS LPS IPS Unit type ICE GT GT CC GT CC GT CC Fuel type OIL OIL OIL NG OIL NG OIL NG Capital investment /kw Fuel and O&M cost /MWh Variable cost annual rate of increase 5% 5% 5% 3% 5% 3% 5% 3% Fixed cost /kw Fixed cost annual rate of increase 0% CO 2 emissions tnco 2 /MWh Technical minimum 45% Dynamic penetration limit (for wind power) 28% Installed capacity (% peak load) 115% 100% Removed generation residual value 0% 10% 10% 0% Year of Emissions cost /tnco 2 25 Emissions cost annual rate of increase 5% Interconnection data Total interconnection capacity and additional investment 1st 5-year period MW / M - / / / / 200 2nd 5-year period MW / M - / / / / 0 3rd 5-year period MW / M - / / / / 0 4th 5-year period MW / M - / / / / 0 5th 5-year period MW / M - / / / / 0 Interconnection losses - 7% Wind data Installed wind capacity 1st 5-year period MW nd 5-year period MW rd 5-year period MW 25% P Lmax 4th 5-year period MW 5th 5-year period MW Mean wind speed m/s 9 Wind energy losses 10% Wind energy tariff /MWh Wind tariff annual rate of increase 3% LPS: Local Power Station, IPS: Interconnected Power System, GT: Gas Turbine, CC: Combined Cycle plant, ICE: Internal Combustion Engine

4 Case (B): Two interconnection technologies are examined: (B1) AC interconnection: One cable, with a rated capacity of 200 MW, is installed in year t=0. The local power station is maintained in cold reserve for reliability purposes. A second identical cable is installed in year 5, at which time the power station of the island is eliminated. To cater for the increase of the load, while ensuring the required reserve, a third cable system is installed in year 15. Increased exploitation of the wind potential of the island is now possible, by the installation of 200 MW of wind power in year 1 and another 200 MW in year 5. (B2) DC interconnection: One DC circuit and the associated converter stations, rated 350 MW, is installed in year t=0. The local power station is maintained in cold reserve. A second interconnection link of the same capacity is installed in year 5 and the local power station is then eliminated. Maximum exploitation of the wind potential is possible, with 350 MW of wind power in the first year and another 50 MW in year 5. Case (C): Interconnection of the island is performed via one AC cable circuit of 200 MW capacity in year t=0, with no future reinforcement of the interconnection. The mainland power system, via the interconnection, is the preferential source of supply for the load of the island (base and medium load), while the local units provide the required capacity for medium and peak load service and ensure the reserve in case of interconnection failures. For this reason, the capacity of the local power station is continuously expanded, to be able to meet 100% of the peak load demand. In this case, installation of 200 MW of wind power is assumed in year t=0. V. RESULTS Evaluation results over the whole 25-year period are presented in Tables 2 and 3. Table 2 summarizes the economic evaluation results, including investment costs for the interconnection and the development of conventional generation, fixed and variable operating costs and wind energy payments. The Net Present Values-NPV of the costs over the 25-year period are presented, as related to the local power station (LPS) and the interconnected power system (IPS). In Table 3, the contribution of the LPS, the IPS and the RES stations are presented (in percent of the annual load energy, to facilitate comparisons). Each line corresponds to the average year of one 5-year sub-period. In all cases interconnection of the island is more economic than the autonomous development of its system. This is mainly due to the fact that the variable generation cost in the IPS is lower from the corresponding LPS cost, compensating the interconnection investment. Comparing the three interconnection alternatives (Cases B1, B2, C), the last one appears as the most cost-effective. On the other hand, the results on energy contributions show that Case C achieves lower wind energy penetrations, since a comparatively lower wind capacity is assumed on the island. Currently, this does not entail any direct economic implications, but it is probable that this will change in the future, under the pressure of meeting national RES targets and possibly increased CO 2 emission costs. Another disadvantage for Case C is that it involves full development of on-island conventional generation, which in many cases is not possible, for environmental and practical reasons. From Tables 2 and 3 it is evident that the development of large amounts of wind power on the island is associated with high payments to wind energy producers, while a surplus of wind energy is returned to the IPS, reducing thus its variable generation cost. Although the wind energy related costs (incurred or avoided) dominate the overall balance, repeating the evaluation with a drastically reduced wind power deployment leads to identical conclusions for the relative economic merit of the various alternatives. The results of the economic evaluation depend on the fundamental cost assumptions, presented in Table 1, most important being the fuel cost (actually the oil prices), which affects the variable operating cost of all conventional generation, as well as the wind energy tariffs. The latter are related to the MV consumer tariffs, which are in turn affected by oil price changes. For this reason a sensitivity analysis has been performed, to evaluate the impact of wind energy tariffs and fuel prices on the conclusions drawn. Results are presented in Figs. 2 to 4, for the four cases analyzed. In Fig. 2 it is observed that substantial increases in the fixed wind energy tariffs make the interconnection less attractive or even more expensive than the autonomous development of the island system. This is more pronounced for Cases B1 and B2, where the installed wind power is the largest, while Case A is the least affected, as the assumed wind power levels are very low. Such a conclusion is reasonable, since wind energy now substitutes conventional energy of the IPS, whose generation is substantially lower. The effect of conventional fuel prices is illustrated in Fig. 3, all other parameters of the evaluation maintaining their base case values. In this case, changes in the oil prices affect the variable operating cost of the autonomous (oilfired) power stations to a greater extent, since the mainland system utilizes a diverse mix of primary fuels, including hydro, lignite and natural gas. To quantify this effect, it has been assumed that changes in the variable cost for the IPS are half of those for the LPS. Hence for a variation of e.g. 30% in the diagram, the respective change in the IPS cost is 15%. It is observed in Fig. 3 that a decrease of 30% in fuel costs would make the interconnection of the island system as economic as its autonomous development, while a further increase in oil prices (which is the most probable trend in the future) enhances the appeal of all interconnection alternatives. It is noted that the slightly declining trend of the curves for Cases B1 and B2 is due to the increasing avoided generation costs, as a result of the high amounts of wind energy in these cases. As already mentioned, changes in oil prices will affect both the conventional generation costs and the wind energy tariffs. The combined effect of these two factors is presented in Fig. 4, which is the most important for drawing realistic conclusions. From this diagram it is confirmed that increasing oil prices will make island interconnections even more attractive. Interconnections still remain the preferable solution for reductions of up to 30%. Case C is always the most economic solution.

5 Table 2: Evaluation results for the case studies analyzed: Net Present Value (NPV) of costs over the 25-year evaluation period, as related to the local generation (LPS) and the interconnected system (IPS) Cost categories (*): Negative costs denote savings Case A Case B1 Case B2 Case C Autonomous Development DC Interconnection & LPS development LPS IPS TOTAL LPS IPS TOTAL LPS IPS TOTAL LPS IPS TOTAL Interconnection investment (Μ ) Conventional generation investment (Μ ) Removed generation residual value (Μ ) Conventional generation, fixed (Μ ) Conventional generation, variable (Μ ) Fuel and O&M Emission rights Wind farm energy payments (Μ ) 0, Total (Μ ) Average generation cost ( /MWh) Table 3: Evaluation results for the case studies analyzed: Energy contribution of local generation (LPS), interconnected system (IPS) and wind farms (RES) (average annual values per 5-year sub-period) Case A Autonomous Development Case B1 Case B2 DC Interconnection & Case C LPS development 5-year period Annual load demand (MWh) LPS IPS RES LPS IPS RES LPS IPS RES LPS IPS RES 1 st % 0% 14% 0% -21% 121% 0% -113% 213% 0% -21% 121% 2 nd % 0% 14% 0% -94% 194% 0% -91% 191% 0% 4% 96% 3 rd % 0% 14% 0% -48% 148% 0% -48% 148% 0% 26% 74% 4 th % 0% 14% 0% -17% 117% 0% -16% 116% 0% 41% 59% 5 th % 0% 14% 0% 9% 91% 0% 9% 91% 3% 52% 46% (*): Negative energy signs denote energy returned to the interconnected power system

6 Figure 2: NPV sensitivity with respect to variations in the wind energy tariff. by assuming suitable wind power capacities and taking into account the energy payments to such stations. As a study case, a typical medium-to-large size island system has been selected, with a 25 year evaluation period. From the analysis presented it is deduced that the interconnection of the island to the mainland power system is always preferable to the autonomous development and operation, reducing the net present worth of the associated costs by up to 27%. This conclusion is valid whether the exploitation of the local wind potential is maximized or not. The evaluation has been based on current oil prices. In the most probable event of further increase in the price of oil in the future, the difference will become even more pronounced in favor of the interconnection solution. The same applies if the very increased potential for RES energy exploitation is somehow accounted for, either via the obligation for meeting national targets or via increased CO 2 emission costs. VII. ACKNOWLEGMENT This work has been financially supported by the Regulatory Authority for Energy (RAE) of Greece, to which the authors express their gratitude. VIII. REFERENCES Figure 3: NPV sensitivity with respect to variations in the conventional generation variable cost. [1] M. Papadopoulos et. al., Interconnection of Cycladic Islands to the mainland Grid. 5 th Intern. WSEAS Conf. 2005, Corfu, Greece. [2] J. Kabouris et. al., Supply of islands through long distance submarine cables: Problems and prospects (Case study of Cycladic interconnection). CIGRE Session 2006, Paris. [3] G.E. Balog et. al., Power transmission over long distances with cables. CIGRE Session 2004, Paris. [4] F. Rudolfsen et. al., Energy transmission on long three core/trefoil XLPE power cables. JICABLE 03. [5] S. Papathanasiou, N. Boulaxis, Power limitations and energy yield calculation for wind farms operating in island systems. Renewable Energy 31 (2006). [6] Regulatory Authority of Energy. Methodology for the assessment of wind penetration in non interconnected islands. 21st February BIOGRAPHIES Eleni Karamanou received her Diploma in Electrical Engineering in 2006 from NTUA, Greece. Currently she is pursuing postgraduate studies in NTUA, working also as a researcher in the field of renewable energy. Figure 4: NPV sensitivity with respect to simultaneous variations in conventional generation variable cost and wind energy tariffs. VI. CONCLUSIONS In this paper alternative island interconnection policies are presented and evaluated, in comparison to each other and to the autonomous operation and development of the island system. The evaluation encompasses all relevant costs, including investments for the interconnection and the deployment of the required generating capacity on the island or in the interconnected power system, fixed and variable operating power station costs and CO 2 emission rights. Further, the possibility for exploitation of the existing wind potential on the island is also accounted for, Stavros Papathanassiou received his Diploma in Electrical Engineering in 1991 and the Ph.D. degree in 1997 from NTUA, Greece. He worked for the Distribution Division of the Public Power Corporation, Greece, in distributed generation and distribution network design projects. In 2002 he joined the Electric Power Division of NTUA, where he is currently an assistant professor. His research deals with distributed generation and renewable energy technologies. Michael Papadopoulos received his Diploma in Electrical Engineering in 1956 and the Ph.D. degree in 1974 from NTUA, Greece. He worked for the Public Power Corporation, Greece, from 1956 to In 1985 he joined the Electric Power Division of NTUA. Currently is a professor emeritus of NTUA. His research interests lay in the fields of renewable energy generation and distribution networks.