APPLICATION OF PUMPED STORAGE TO INCREASE WIND PENETRATION IN ISOLATED ISLAND GRIDS. K. Protopapas, S. Papathanassiou (*)

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1 APPLICATION OF PUMPED STORAGE TO INCREASE WIND PENETRATION IN ISOLATED ISLAND GRIDS K. Protopapas, S. Papathanassiou (*) (*) National Technical University of Athens (NTUA), School of Electrical and Computer Engineering, Electric Power Division 9, Iroon Polytechniou st., 773 Zografou, Athens, GREECE Tel.: , Fax: , Abstract: Pumped storage is generally viewed as the most promising technology to increase wind penetration levels in autonomous island grids, where severe limitations are imposed by the constraints introduced by the conventional generating units. In addition a guaranteed generating capacity may be ensured, providing thus a degree of dispatchability to the hybrid (wind-hydro) system. In the paper, a hybrid windpumped storage system is considered, which operates in an isolated island grid powered by a diesel power station. Using a dedicated logistic model, the hybrid system performance is studied for various configurations of the wind park, pump and turbine station, reservoir sizes and operating strategies. It is shown that a significant increase in wind energy penetration can be achieved, while substitution of conventional units by hydro turbines is also possible, if the hybrid station is operated to provide dispatchable power. Keywords: Wind energy, island grid, hybrid system, pumped storage, logistic model, wind-hydro system 1. INTRODUCTION The objective of increasing the wind penetration levels in electric power systems, particularly in case of isolated island grids, calls for the implementation of suitable storage methods. This necessity is usually imposed by technical constraints of the conventional generating units (typically diesel generators, whose lowest operating capacity is of the order of 3-% of their rated power), as well as by dynamic response considerations, when dealing with randomly varying sources such as wind power, [1]. When the power system size increases beyond a few hundred kws, battery storage, flywheels and other similar means become technically and economically unappealing, leaving pumped storage as the only viable solution. Such systems require in concept a pump/turbine station and two water reservoirs at sufficient altitude separation (typically a few hundred meters). With pumped storage, wind energy which would be discarded due to penetration limits imposed by the conventional units (e.g. during periods of low load and high wind), can be stored by pumping water to the upper reservoir. This energy is subsequently recovered when the penetration constraints are relaxed or the available wind power is below the allowed penetration limits. Further, if a multiple tariff zone system is applied, with a higher energy price during the peak load hours, an additional financial interest for the hybrid system owner/operator is created, as proposed in [2]. The energy stored in the upper water reservoir may also be utilized to provide a guaranteed generating capacity, as in [3], providing thus a degree of dispatchability to the hybrid (wind-hydro) system and a corresponding capacity credit (guaranteed power remuneration), to enhance its viability prospect. In this paper, the operation of a wind farm and a pumped storage hydro station in a medium-sized diesel powered island grid is investigated. A dedicated logistic model is used, with wind and load time series as inputs, which simulates the steady state operation of the system over long time periods (typically one year), using a 1-minute time step. Taking into account the diesel engine dispatch strategy and wind power penetration restrictions imposed by the operators, different operating strategies are explored and a parametric investigation is carried out to evaluate alternative configurations of the hybrid system. An economic evaluation is also conducted, to investigate the feasibility of a hybrid station operating in the specific island grid. 2. SYSTEM MODELING 2.1 System description A schematic representation of the simulated system is presented in Fig.1. It comprises a wind farm, a pumping station with several units, for pumping water from the lower to the higher reservoir, and a hydro turbine station for generating power. Turbine and wind farm output power is supplied to the grid, which is considered to be a small autonomous power system supplied by a conventional diesel power station. A fictitious dump load is also shown, corresponding to wind power/energy, which cannot be absorbed by the system or the pumps and it is therefore discarded, i.e. not produced by the wind turbines, by decreasing their output power setpoint. 2.2 Wind park model Individual wind turbines are represented by their power curves. Input to the model are the wind speed time series (1-min average values), transformed to equivalent output power series via the WT power curves. The wind park output is the sum of the individual turbine powers, ignoring possible smoothing effects. As already mentioned, the dump load shown in Fig. 1 does not actually exist and when the available wind power exceeds the sum of the allowable penetration limit and of the pumping capacity, either the wind turbine output will be curtailed or individual WTs will be disconnected. 1

2 2.4 Diesel generator model Diesel engines are described by their operating restrictions (maximum and minimum output power) and the polynomial equation representing their specific fuel consumption as a function of their output power. 2. Power system load demand Load demand is represented by time series, usually expressed as hourly average values. To avoid step load variations when using smaller simulation time steps (1- min), interpolation is used. Fig. 1 Hybrid system and isolated grid structure. 2.3 Hydro station model Each pump and turbine unit is characterized by its efficiency curve. Given the electrical power at the terminals of the unit, the required water flow rate is calculated using the following equations, for pump and turbines units: ρ g Q ( H o + h f ) Ppump = 1 η pump ηel (1) ( H h ) ρ g Q o f Pturb = ηturb ηel 1 (2) P pump power consumed by the pump, kw P turb power produced by the turbine, kw ρ water density (x 1), kg/m 3 g gravity acceleration (9.81 m/s 2 ) Q water flow rate, m 3 /s H h f k η pump η turb η el head, m 2 = k Q, the head loss in the pipes, m friction factor pump efficiency, p.u. turbine efficiency, p.u. electrical efficiency of motor/generator, p.u. The head loss is given by the Darcy-Weisbach formula: f L D V 2 L V h f = f D 2g a numerical friction factor length of pipe, m penstock diameter, m velocity of flow in pipe, m/sec (3) A value of f=.2 can be set in the Darcy-Weisbach equation to get a rough estimate of the head loss. Equation (3) may be rewritten in terms of the water flow rate: fl 2 h f = Q 2 2g( π / 4) D (4) Equation (4) has been incorporated in (1) and (2) for the hydropower calculation. 3. OPERATION ALGORITHM Operation of non-dispatchable units in autonomous power systems is characterized by penetration restrictions related to the absorption capability of the system and more specifically the conventional generating units, [1]. The resulting constraints are time varying and depend on the load level, the dispatching policy for the conventional generators, as well as on stability considerations, related with the capability of the conventional units to compensate for fluctuations and the possible sudden loss of wind power. The nature and determination of these restrictions, detailed in [1], are only briefly outlined in this section, for the sake of completeness of the presentation. Given the system load demand and the dispatch of the conventional power station diesel generators (DGs), the maximum allowable penetration for a renewable power station is given by the stricter of the following two constraints: P HS P L Nop P i= 1 Nop D min, i PHS pmax P Dn, i i= 1 (6) P HS renewable (hybrid here) station output power P L total load demand P Dn,i rated power of the i th DG P Dmin,i lower technical limit (minimum) of the i th DG N op number of operating DGs p max dynamic penetration limit for the hybrid station, as a percentage of the rated power of the operating DGs The first constraint ensures that none of the operating DGs is loaded below its technical minimum. The second constraint limits the penetration to a predefined maximum of the DG operating capacity, related to dynamic response considerations. Experience on existing island grids with wind power has shown that the dynamic stability limit p max should not exceed 3-%, depending on the number, type and geographic dispersion of wind turbines, as well as on the diesel engines and their regulator characteristics. () The capacity of the operating DGs at any time interval is determined by P Don ( 1+ κ) PL + ( λ ) PHS 1 (7) 2

3 where λ(%) is the percentage of the available wind or hybrid power which may suddenly be lost (i.e. in times shorter than necessary for starting a new DG) and κ (%) represents the load forecasting error. A wind margin of λ=1% is adopted in this paper, corresponding to maintaining spinning reserve for the total available wind or hybrid power. A load margin κ=1% is used to compensate for relatively fast load fluctuations and forecasting errors. This conservative operating policy is typically adopted in small island systems, to minimize the probability of loss-of-load events, since a fault in the MV network may result in a sudden disconnection of all available renewable power. The dynamic penetration limit, p max, is set to 3%. Although substitution of diesel units by hydro turbines has been proposed (e.g. [4,]), a more conservative approach is adopted in the paper, the diesel units always maintaining the required spinning reserve. Simulation results ([6]) indicate that an improvement of the dynamic stability of an autonomous system may be achieved by the operation of pumped storage units, thus allowing for increased wind penetration levels. In any case, the issue of the spinning reserve policy depends also on several case-specific factors, including the probability of hydro generation loss e.g. due to system faults. Given the limit imposed by the system on the output power of the hybrid station, various alternative strategies can be thought of, in order to maximize the energy yield, reduce the variability of the output power and optimize the use of the hybrid station. Two such strategies are considered in this paper, which are definitely not unique and rather conservative, in the sense that the pumping/hydro station is treated as a storage/generation means internal to wind farm, and not as an independent unit of the isolated power system. Other operating policies are also possible, which would treat the pump/hydro station independently from a specific wind farm. Such strategies might prove more flexible and attractive from the point of view both of the system and the owner/operator of the hybrid station. However, the conservative approach adopted here provides a lower threshold for the potential benefits from the operation of the hybrid station and imposes no change to the operating regime of the autonomous grid. Strategy 1: The constraint expressed by equations () and (6) is considered to apply to the hybrid system output power in total. Hence, the storage unit is utilized to store excess wind power (exceeding the system penetration limit) by pumping water, whereas the hydro turbines operate when the available wind power is lower than the penetration limit. The diesel units always maintain full spinning reserve for the hybrid system output power. Strategy 2. Hydro turbines provide guaranteed power to the system at peak load hours, substituting thus peaking conventional units ( dispatchable hydro operation). The turbines operate at a predetermined power level (depending on the forecasted load demand, but not on wind power) based on a dispatch order similar to the diesel unit dispatch plan. Diesel units maintain full spinning reserve only for wind power and the penetration limit is calculated from eqs. () and (6), taking into account diesel power only. Any wind power surplus during dispatchable hydro operation is dumped, as it cannot be used for pumping (a single penstock is assumed for the hydro station). In this strategy, only wind energy is used for pumping (diesel power is never absorbed for filling the upper reservoir). The water level in the upper reservoir should be always sufficient to permit dispatchable hydro operation at a specified power and duration (e.g. during the peak load hours of the next days). If the energy stored in the reservoir is higher than required, the turbines may also operate as in Strategy 1 during off-peak hours, to complement the available wind power ( complementary hydro operation). 4. APPLICATION DATA A typical medium-sized isolated island grid with a maximum load demand of 17 MW and an annual load profile as shown in Fig. 2 is considered in this paper. Minimum load demand is 4.1 MW, mean load equals 9.33 MW and the annual energy demand 81,696 MWh. The total installed capacity of conventional units in the diesel power station is 23.3 MW, consisting of 6 DGs, rated from 1. to 6.3 MW, as shown in Table 1. The technical minimum of each unit is % of its rated capacity. The two alternative unit dispatch orders of Table 1 will be examined. Load Demand (ΚW) Load Demand (ΚW) Hours Fig. 2. Autonomous system load demand. Table 1. Diesel engine dispatch order. DG Rated power Dispatch Order (kw) Standard Modified A wind park consisting of several 8 kw pitch regulated turbines is considered. The annual wind speed time series used in the simulation is shown in Fig. 3 (average 9. m/s at hub height). 3

4 W indspeed (m/s) W indspeed (m/s) Hours Fig. 3. Annual wind speed time series. The pumping station consists of 2-1 fixed speed (FS) or variable speed (VS) pumps, each rated 1% of the wind park power. The hydro station comprises 1-6 Pelton turbines, with a rated power of 1 kw each. A 3 m head has been assumed, whereas the penstock length is taken equal to m. Head losses due to friction in the pipes are calculated as described in Section 2.3. Electrical efficiency is considered fixed at 9%, whereas typical pump and turbine efficiency curves are used. The water reservoir capacity is selected to ensure operation of the turbines at rated power for an interval of hours. A minimum charging level is assumed for the water reservoir, equal to 1% of its total capacity.. RESULTS.1 Wind power penetration limit of the system The wind penetration limit, calculated by eqs. () and (6), is illustrated in Fig. 4 and, as a function of the system load level. For the load statistics of Fig. 2 and the standard DG dispatch order of Table 1, the maximum wind energy which can be absorbed by the isolated system is GWh/year (26.9% of the load demand). Using the modified DG dispatch order, the energy increases to 2.4 GWh (31.1% of the load demand). The drawback of the modified order is the more frequent utilization of the peak load units..2 Wind-diesel operation (no storage) The operation of the autonomous system without any storage, simulated over a one year interval, is used as the evaluation basis for the effectiveness of the hybrid system. Simulation results are illustrated in Fig. 6 and 7 for two characteristic intervals. In Fig. 8, the variation of the produced and dumped wind energy over the whole year period is shown as a function of the installed wind power, for the two dispatch sequences of Table 1. It is clear that no power curtailments occur when the wind park size is relatively small. The wind energy absorbed by the grid tends to saturate for higher wind power penetration levels, where the energy dumped eventually exceeds the total production. Notably, wind energy fed to the grid is much lower than the limit calculated in Section.1, which represents the absorption capability of the system, regardless of prevailing wind conditions, wind park size and efficiency. Penetration Limit (kw) Due to DG Technical Minimum Due to Dynamic Limit Penetration Limit Load Demand (kw) x 1 4 Fig. 4. Wind penetration limit for the standard DG dispatch order of Table 1. Penetration Limit (kw ) Due to DG Technical Minimum Due to Dynamic Limit Penetration Limit Load Demand (kw ) x 1 4 Fig.. Wind penetration limit for the modified DG dispatch order of Table W indspeed (m/sec) Avail. W ind Wind DG W indspeed Fig. 6 Wind-Diesel operation, days of the year. Power (kw ) W indspeed (m/sec) 1 Fig. 7. Wind-Diesel operation, peak load days. Avail. W ind Wind DG W indspeed 4

5 2 Produced, Initial D.O. ed, Initial D.O. Produced, Modified D.O. ed, Modified D.O. 3 Wind Park, Initial D.O. Hybrid, Initial D.O. Wind Park, Modified D.O. Hybrid, Modified D.O. 2 2 Energy (GWh) 1 Produced Energy (GWh) 2 1,8 1,6 2,4 3,2 4 4,8,6 6,4 7,2 8 Wind Park Nominal Power (ΜW) Fig. 8 Annually produced and dumped wind energy Power (kw ) Tank State of Charge (%) Fig. 9. Operation with Strategy 1, days Operating Strategy 1 Avail. Wind Turbines Pumps Wind Hydro DG Tank SOC Operation of the hybrid system according to operating strategy 1 for the same interval as in Fig. 6 is illustrated in Fig. 9. The hybrid system consists of a 6x8 kw wind farm, 6x48 kw VS pumps, 2x1 kw turbines and a 16, m 3 reservoir. The annual energy yield of a hybrid system without limitations due to component size (pump or turbine minimum/maximum power, reservoir capacity) illustrated in Fig. 1 and represents the maximum potential benefit from installing a hybrid station in the island system. The impact of the hybrid system component size is illustrated in Fig Even a relatively small hybrid station can lead to a significant increase of the wind energy absorbed by the grid. However, to achieve a further increase a much larger system is required, the reservoir capacity being the most important factor.. Operating Strategy 2 Simulation results for a hybrid system consisting of a 6x8 kw wind park, 8 x 48 kw VS pumps, 3x1 kw turbines and a reservoir of 21, m 3 are illustrated in Fig. 14. The standard diesel dispatch order has been adopted. Comparing with Fig. 7, it is clear that the dispatch of the hydro turbines during peak load hours reduces substantially the utilization of peak load diesel units (No and 6).,8 1,6 2,4 3,2 4 4,8,6 6,4 7,2 8 Wind Park Nominal Power (ΜW) Fig. 1 Maximum benefit evaluation: Annual energy output of the wind farm and the hybrid station, without any size restrictions in the pumping/hydro station. Hybrid System Production (GWh) Wind 2 FS 4 FS 6 FS 8 VS 6 VS 8 VS Limit 3,2 4 4,8,6 6,4 7,2 8 Wind Park Size (ΜW) Fig. 11. Impact of number and type of pumps (FS: fixed speed, VS: variable speed). Hybrid Production (GWh) Hybrid Production (GWh) Wind Park 3.2MW 4.8 MW 6.4 MW 8. MW Limit Reservoir Capacity (x1 3 m 3 ) Fig. 12. Impact of reservoir capacity Wind 1x1 2x1 3x1 4x1 x1 6x1 Ideal Limit 3,2 4 4,8,6 6,4 7,2 8 Wind Park Size (MW) Fig. 13. Impact of the number of turbines

6 Tank State of Charge (%) Avail. Wind Turbines Pumps Wind Comp.Hydro DG Disp. Hydro Tank SOC Fig. 14. System operation during peak load days with operating strategy 2 and the standard DG dispatch order. Power (kw ) Power (kw ) Tank State of Charge (%) Avail. W ind Turbines Pumps W ind Comp.Hydro DG Disp. Hydro Tank SOC Fig.. System operation during peak load days with operating strategy 2 and the modified DG dispatch order. Fig. illustrates the operation of the system when the modified diesel dispatch order is adopted. The configuration of the hybrid system is the same as before, but the wind park size is increased to 6.4 MW, because the initial 4.8 MW size does not suffice to support dispatchable hydro operation. Diesel units No 4 and 6 are not dispatched at all, but an amount of wind energy has to be dumped, because the dynamic stability limit, based on the rating of the operating diesel units, is now reduced. The annual energy production of the hybrid station is almost the same as for Strategy 1 (Fig. 1), because it operates in dispatchable hydro mode only for a few hours per year, whereas the remaining time it operates in the complementary hydro mode (Strategy 1). 6. ECONOMIC EVALUATION 6.1 Methodology The investment cost of pumped storage stations, particularly regarding the reservoir cost, strongly depends on the specific topography of the selected site. For this reason, instead of assuming an indicative capital cost for the hybrid station and evaluating its feasibility, it is preferred to calculate the maximum investment cost for which the station would be viable. For this purpose, a tarriffication scheme has to be adopted, both for the produced energy and for the guaranteed power provided to the system. In general there exists no standard practice for the tarriffication of energy and power produced by hybrid stations. Under the assumption that no diesel power is ever used for pumping, as in this paper, the output of the hybrid station is generated solely from wind energy, while it never imports power from the grid. For this reason, the tariff applicable in Greece for electricity generated from wind energy in isolated grids is adopted. For the guaranteed power no specific tariff is either available. Here, an avoided diesel unit cost of 1 /kw-year is assumed. A subsidy of up to 4% of the total investment cost may also be available for the station. Maximum investment cost Supposing that the hydro station is compensated for energy E and guaranteed power P g supplied to the grid, while the wind energy surplus used for pumping is not billed (as long as the wind park and the pumped storage/hydro station are treated as a single generating station), then the station will be financially viable if its specific construction cost I s ( /kw) is less than: ( E p + P p ) f (8) a E g P max I s = CRF TAX Pn f a foc + 1 s Ν p E energy tarif p P guaranteed power tarif CRF capital recovery factor TAX income tax rate foc annual fixed operating cost P n total rated power of hydro turbines N investment lifetime s subsidy percentage of the investment cost 1 TAX f a = CRF The value of maxi s corresponds to zero NPV over the investment lifetime. If the construction cost C is given, then the actual NPV of the investment will be Electricity production cost NPV = P max I C (9) n In order to evaluate the electricity generation cost, the cost data for each production unit are separately evaluated. Costs related to electricity generation, such as fuel, maintenance and capital cost of production units are included, while other costs such as labour, power station building depreciation or grid maintenance are not included. Only the financial (internal) cost is taken into account, as long as the environmental (external) costs cannot be easily evaluated. The annual electricity generation cost of a unit may be expressed as: TCt = I CRF + FOC t + voct qt + fct qt (1) I the total debted investment cost of the unit CRF the capital recovery factor FOC t the annual fixed operating cost at year t voc t the variable cost per MWh produced q t the energy produced at year t (MWh) fc t fuel and lubricants cost per MWh produced s 6

7 The average generation cost is then expressed in /MWh as: TCt tc = (11) q Application data The annual fixed cost (power availability cost) for diesel units already installed on medium size island grids is estimated at 1 /kw-year and the fuel cost at 4 /MWh, plus a 7 /MWh cost for lubricants etc. [7]. The wind park investment cost is taken 1 /kw and the annual O&M cost equal to 2.% of the investment cost. A 3% subsidy of the initial cost is considered for the wind farm. The energy generated by the pumped storage unit is assumed to be compensated at the fixed wind energy tariff for isolated island grids, /MWh when this study was conducted. The guaranteed power is compensated at a rate equal to the avoided cost of the diesel units, 1 /kw-year. A subsidy equal to 4% of the investment cost is also assumed. As a crude estimate, the pumped storage unit cost is of the order of 3, 4, /kw of turbine power, depending on the system configuration, [8] Production cost The estimated electricity generation cost for the overall system under consideration is presented in Fig. 16, as a function of the installed wind power. It is observed that a reduction of % is achieved by the operation of a 3.2 MW wind park, whereas increasing the size of the wind park offers no additional benefit. A pumped storage system utilizing the discarded wind energy and operating by Strategy1 would not lead to any financial benefit, as the additional investment cost exceeds the fuel savings achieved. On the other hand, a pumped storage system supplying guaranteed power, which could replace conventional peaking units, leads to a system cost comparable to that of a simple wind-diesel system without storage. It is important to note that the sensitivity analysis conducted indicates that hybrid wind-diesel-pumped storage systems are less sensitive to fuel price fluctuations. A 1% rise in the price of oil would result in a 6.% rise of the total production cost for a diesel only system,.% for a system with a.6 MW wind park and 4.8-.% for a hybrid system based on the same wind park. t /kw) Max. Specific Investment cost ( ax. Specific Investment cost ( /MWh) Max specific cost change (%) Str. 1, Init. D.O. Str. 1, Mod. D.O. Str. 2, Init. D.O. Str. 2, Mod. D.O. 3,2 4 4,8,6 6,4 7,2 8 Wind Park Power (MW) Fig. 17. Maximum specific cost of pumped storage station with 1 MW turbine Str. 1, Init. D.O. Str.1, Mod. D.O. Str. 2, Init. D.O. Str 2, Mod. D.O. 3,2 4 4,8,6 6,4 7,2 8 Ισχύς Αιολικού Πάρκου (MW) Fig. 18. Maximum specific cost of a pumped storage station with 3x1 MW turbines Interest rate Energy price Subsidy Parameter Change (%) Fig. 19. Sensitivity of the maximum specific cost for operating strategy 1. 1 Interest Rate Energy price Guaranteed power price Subsidy 82 Wind-Diesel Hybrid, Str.1 DO1 Hybrid, Str.1,DO2 Hybrid, Str.2 DO1 Hybrid, Str.2 DO2 System Cost ( / MWH ) Max Investment cost change (%) ,,8 1,6 2,4 3,2 4, 4,8,6 6,4 7,2 8, Wind Park Power (MW) Fig. 16. Electricity production cost of the overall system as a function of the installed wind power Parameter Change (%) Fig. 2. Sensitivity of the maximum specific cost for operating strategy 2 7

8 6.3 Pumped Storage maximum investment cost The maximum acceptable specific cost for the pumped storage systems under consideration is presented in Fig. 17 and 18, for 1x1 MW and 3x1 MW hydro turbines, respectively. A pumped storage unit with a 1x1 MW turbine can be viable if it costs less than 3, /kw, if compensated for produced energy only (operating strategy 1). If it is also compensated for guaranteed power (operating strategy 2), the viability margin increases significantly. For larger pumped storage systems the maximum acceptable specific cost decreases, as the produced energy does not increase in proportion to the size of the hybrid station. However, applying the operating strategy 2, even large systems may be viable. Sensitivity analysis results for the maximum investment cost are illustrated in Fig. 19 and CONCLUSIONS In the paper, the operation of a hybrid wind-hydro station in a typical medium-sized autonomous island grid is analyzed. It is shown that even with a conservative RES penetration policy, a remarkable increase in wind energy utilization may be achieved, while the hydro station may provide guaranteed power to the system. A further increase in wind energy exploitation is possible, if the output power restrictions applied to the hybrid station are relaxed. However, this would require an investigation of the dynamic response of the overall system and it would inevitably depend on the dynamic characteristics of the hydro units and their capability to participate in the frequency regulation of the system, as well as to remain in operation in vase of network disturbances. In spite of the potential increase in wind energy penetration levels, the high investment cost of pumped storage poses questions for the economic feasibility of such systems, when evaluated with the standard economic assessment methodologies. A clear outcome of this investigation is that the construction of hybrid windhydro systems calls for special tariffication policies for both energy and guaranteed power, along with suitable subsidization schemes, possibly taking into account the environmental ( external ) costs avoided by their operation. 8. REFERENCES [1]. S. Papathanasiou, N. Boulaxis, Power limitations and energy yield calculation for wind farms operating in island systems. Renewable Energy 31 (26). [2]. E. Castronuovo A. Lopes, On the daily optimization of a wind-hydro power plant. IEEE Trans. Power Systems, Vol. 19, No.3, Aug. 24. [3]. J. Anagnostopoulos, D. Papantonis, Optimum Sizing of a Pumped-Storage Plant for the Recovery of Power Rejected by Wind Farms. Design Optimization International Conference 24, Athens, Greece. [4]. P. Theodoropoulos, A. Zervos, G. Betzios Hybrid systems using pump-storage implementation in Ikaria island, Proc. OPET Island International Conference 21, Chania, Greece. []. Z. Mantas, P Theodoropoulos, G. Betzios, A. Zervos Hybrid system using pump-storage for maximum wind energy penetration in Serifos island, Bulletin of the Hellenic Association of Mechanical & Electrical Engineers, Vol. 6/23 (in Greek). [6]. N. Hatziargyriou, J. Stefanakis, S. Alexandridis et al. Safe increase of wind energy penetration in island systems by pumped storage units - The Crete case. Proc. RENES 21, Athens, Greece. [7]. M. Papadopoulos, S. Papathanassiou, Preliminary study for the interconnection of the Cyclades islands to the Hellenic Interconnected Grid. Report prepared for the Regulatory Authority for Energy, 24. Available at (in Greek). [8]. J. Kaldellis, K. Kavvadias, D. Vlachou, Electricity load management of APS using Wind-Hydro solution. Proc. MedPower 22, Athens, Greece. 8