Wind Parks – Hydro Pumping Coordinated Operation: Application to the Portuguese System

Pedro Mendes

IST – Technical University of Lisbon

Lisbon, Portugal

pedr0_4@hotmail.com

Rui Castro and J.M. Ferreira de Jesus

IST – Technical University of Lisbon

Centre for Innovation in Electrical and Energy Engineering

Lisbon, Portugal

rcastro@ist.utl.pt ; fjesus@mail.ist.utl.pt

Abstract –Due to their ability to quickly vary their opera-tion point, hydro power plants emerge as the most viable solution to promote the secure integration of wind power production, as they are able to compensate the short term variations of wind production, maintaining the balance between global generation and demand. The hydro power plants equipped with pumping systems, in addition to the flexible operation, allows the storage of excess wind power in the form of water, pumping it to their upper reservoirs. In this paper, the technical feasibility of coordinating two hydro pumping plants with the wind power installed capac-ity in Portugal is assessed through a dedicated model. The objective is to maintain constant, throughout the year, the power output of this aggregate at a target value. Historical records of the power production and demand as seen from different points of the transmission grid constitute the background for the simulations. The results obtained show that the continuous supply of the target power throughout the year depends largely on the established target value itself, on the values of the installed capacity of each tech-nology (wind and hydro), as well as on the storage capacity of both reservoirs of the hydro pumping plants. Keywords: wind power, hydro pumping plant, coordinated operation, energy storage.

I. INTRODUCTION

Wind power is the technology that has been experienc-ing the fastest growth worldwide. In 2011, the global installed capacity of wind power reached almost 240 GW; in Portugal the wind capacity reached over 4 GW in that same year. The positive environmental effects associated with the integration of renewable energy production are well-known; however, the increasing incorporation of this type of generating units has some negative impacts on the operation of the electric system, due to the inherent irregularity of renewable resources. The wind power production depends on the availability of the wind re-source and on the values of wind speed. While wind speed experiences long term variations that follow sea-sonal patterns, it also experiences short term variations

that are hardly predicted. This last type of wind varia-tions, together with the increasing wind power installed capacity, may impair the stability of the existing electric system, since the balance between global demand and generation could be compromised. In order to compensate these sudden variations of wind production, the controllable power plants must possess load following capabilities and adjust their output power accordingly. However, due to the inherent characteristics of each technology, not all the power plants are suitable for this type of flexible operation, hydropower plants being recognized as the more capable solution of provid-ing power compensation, as they can change their output power approximately 100% per minute [1]. This ability to quickly change the output is the key feature to follow the short term variations on wind power. Furthermore, a few hydro facilities may also provide some storage ca-pacity to the electric system, which may be used to store the excess of wind production in periods of high wind speed and low demand. Therefore, the hydro pumping plants (HPPs) use the surplus of wind production to pump water to their upper reservoirs, keeping it avail-able to be used later when more power is needed in the electric system to maintain the generation-demand bal-ance. Recently, the subject of the wind–hydro coordinated operation has been addressed in several papers. In [2] the authors propose the maximization of the daily opera-tional profit of a wind–hydro pumping/generation plant. In [3] a multistage stochastic model for the optimal op-eration of wind farm, pumped storage and thermal power plants is presented. The unit commitment and dispatch of a power system with pumped storage is examined for increasing levels of installed wind power in [4]. In [5] a methodology for hydro plant and wind farm coordina-tion is developed using a Monte Carlo simulation tech-nique considering the chronological variation in the wind, water and the energy demand. The work reported in [6] is dedicated to study the impacts of the increasing integration of renewable energy in the power system, namely those related to the possible power unbalance

between generation and demand. In [7] the authors dis-cuss the operation policy of a Hybrid Power Station (HPS), installed in the island of Ikaria. The HPS inte-grates two small hydro power plants, one pumping hydro power station and a wind farm. Most of these works address the wind–hydro coordina-tion as an optimisation problem and hydro pumping storage is only considered in valley hours and when excess of wind power occurs. In this paper wind–hydro coordination is performed by defining a target value for the power to be delivered by the aggregate of both the wind and the hydro pumping power plants. Coordination of the operation of these power plants is performed in order to try to ensure that during the 8760 h of the year the aggregated injected power is equal to the target value established. Is it possi-ble to perform this coordination in order to guarantee a constant injected power during the 8760 h of the year? This is the research question addressed by this work. To answer this question, a study of the possibility of the coordinated operation between the Portuguese wind power installed capacity and two selected hydro plants with pumping capabilities installed, was performed. For that purpose, an algorithm to simulate the proposed coordinated operation was developed. As the aggregate of wind power and HPPs is intended to supply continuously, during the whole year, a certain target power, the coordinated operation could provide an overall integrated renewable energy generating facility, which is free of all the irregularity and intermittence inherent to the behaviour of renewable energy sources.

II. METHODOLOGY

A. General Aspects The simulations performed in this study use as a starting point the 2007 and 2009 generation power profiles re-corded, every two hours, in 8 thermal power plants, 6 hydro pumping stations and 21 other hydro plants with large installed capacity and the demand power profiles registered, with the same rate, in 48 substations. This enables the demand power profiles to be taken into ac-count. For the same period, the power flow in the interconnec-tion transmission lines between the Portuguese and the Spanish grids is also available. It is important to point out that this study has been carried out considering the power flow in the interconnection lines is fixed and equal to the values of the records. This means that if the dispatching actions undertaken result in a mismatch between the production and demand, the power flow in the interconnection lines is maintained and alternative measures carried to impose the balance between produc-tion and demand. In the following the modelling aspects of the wind parks, hydro pumping stations and thermal plants are described. The technical characteristics of other generating facili-ties, such as large and small hydro, are not relevant to this study, since for those facilities the historical records of production profiles were used.

B. Wind Parks In order to model the wind parks, a generic standard 2 MW wind turbine was considered. The wind turbine power curve is described by equation (1), in which u is the wind speed in m/s and Pe(u) is the electric power output in MW. This equation provides a good approxi-mation of standard wind turbine manufacturers’ curves.

0 4

7cos 1 4 15

( ) 11 11

2 15 25

0 25

<

π π≤ ≤

=

< ≤

>

e

u

u + + uP u

u

u

(1)

The wind speed profiles are randomly generated, follow-ing a Weibull probability distribution function. Further-more, to consider the geographical variations of wind speed over the national territory, a production profile is generated for each one of the 18 Portuguese districts, rather than a single wind production profile to the whole country. The Weibull parameters for each district were selected in two steps: the first step was to choose an approximate value for each district as made available in [8]; the sec-ond step was to make a fine adjustment of the selected values, aiming to approximately achieve the annual energy production yield in each district. As the Weibull parameters that characterize each area were determined for a 60 m height, the wind speed pro-files generated had to be corrected to the hub height of the generic 2 MW wind turbine, which is usually around 80 m. To do so, Prandtl logarithmic law was used. The temporal variations of wind over a year were con-sidered by simulating 10000 uncorrelated wind speed profiles for each district, all of them consistent with the Weibull probability function characteristic of that region. This was accomplished using Monte-Carlo method. As the historical data is recorded with a sampling rate of two hours, the wind speed was kept constant during this period. This limitation in the methodology was imposed by the available data for this study. Using equation (1), the wind speed profiles can be con-verted into wind power production profiles for a 2 MW standard wind turbine. Multiplying the power profile for this generic wind turbine by the installed capacity of the corresponding district, a wind power production profile for each district is obtained.

C. Hydro Pumping Plants In the present study, the HPPs working in coordination with wind parks are represented by two reservoirs that are characterized by the useful capacity and by a vari-able that indicates the quantity of water stored in the reservoir. If there is a surplus of wind production, rela-tively to the aggregated pre-established value, it is nec-essary to store that wind production by pumping water to the upper reservoir. On the opposite, when there is a deficit of wind power, the HPPs use the water stored in the upper reservoir and are operated as generators sup-plying the power required to attain the pre-established aggregate target power. The simultaneous operation of pumping and generating has not been considered.

Apart from the technical limitations of the equipments, the capacity of the reservoirs is also an important limita-tion to the operation of the HPPs. The pumping process is only possible when there is available capacity in the upper reservoir and water present in the lower reservoir. On the other hand, the generating process is only possi-ble if there is enough water in the upper reservoir, since it is considered that the lower reservoirs are not a limita-tion to this process. These considerations impose that it is required to con-sider the river inflow into the upper reservoirs, as it influences the availability of water in them, crucial both to the generating process, and to the pumping process. Historical water flow rates recorded by monitoring sta-tions installed in the two selected hydro pumping sta-tions are available in [9]. This data enables to obtain the incoming amount of water into the reservoir at each time step of the simulation. If the upper reservoirs reach the maximum level due to the inflow, the dischargers are opened and the excess water delivered to the lower res-ervoirs. The same is applicable to the lower reservoirs, which releases water to the river when its capacity is reached. In addition to the coordination between the wind parks and the two selected HPPs, the coordinated operation of the HPPs must be taken into account. This later was implemented by scheduling to operate first the hydro plant that has more water stored in the upper reservoir if a generating operation is needed or in the lower reservoir if pumping is the required operation. The second HPP only operates when the first reaches its limits. As the HPP are operated in coordination with the wind power plants, the water level in the reservoirs depends on this coordination and therefore is not controlled.

D. Thermal Plants The coordinated operation of the wind parks with the two HPP implies a modification in the historical data of the two HPP and of the wind parks. In order to reestab-lish the balance between generation and demand, dis-patch of the thermal power plants is carried. Therefore, the technical characteristics of the conventional thermal plants that are relevant for the coordinated hydro–wind operation are the technically admissible minimum values of the generating thermal units. In Portugal, the experi-ence gained so far by operating the electric system, showed that, at least, two thermal generating units should be always working, at its technical minimum, in this case, 210 MW.

III. METHODOLOGY DETAILS

The coordinated operation between wind and hydro power aims to maintain constant the output power of the aggregate of all wind power and two selected HPPs. For this purpose, the HPPs must be operated either in the pumping or generating mode of operation, accordingly to the wind power output in each time step (excess or deficit, respectively, in relation to the aggregate target power).

A. Pumping Operation Mode The operation plan for the primary HPP (1stHPP) fol-lows equation (2).

1 = − targetstHPP wind aggregateP P P (2)

If P1stHPP is a negative value, it means that the HPP has to consume that amount of power by pumping water to the upper reservoir. If the absolute value of P1stHPP is higher than the installed pumping capacity, it will not be possible to pump the desired volume of water, but only the share of that volume that corresponds to the rated power of the pumps. Given P1stHPP [MW], the volume of water to be pumped, V1stHPP [m3], is calculated by multiplying the volume flow rate, Q [m3/s] by 7200 (number of seconds corre-sponding to a 2 hour period) through:

1 1

1000 7200

9,81

η × ×= ×

×

pumpingstHPP stHPPV P

H (3)

where, pumping is the efficiency of the pumping process and H is the gross head of the installation. If the upper reservoir of the HPP has not enough avail-able capacity to store the volume of water to be pumped, this volume must be reduced so that the reservoir does not exceed its full capacity. The volume of water to be pumped must also decrease, if there is not enough water available in the lower reservoir. In this case, the HPP will pump up all the water left in the lower reservoir. The level indicators of both reservoirs are to be updated according to the volume of water, Vfinal, actually pumped. The actual power consumed by the hydro plant is then computed, once observed all technical limita-tions, through:

1

9,81

1000 7200

×= ×

η × ×stHPP final

pumping

HP V (4)

Using equation (5) the power output for the secondary hydro plant (2ndHPP) is calculated.

2 1= − −targetndHPP aggregate wind stHPPP P P P (5)

The computation of P2ndHPP follows the same procedure as the computation of P1stHPP. If P2ndHPP equals zero, it means that the primary HPP is able to accommodate all wind production. On the other hand, a negative value indicates the need to operate the second HPP. All the restrictions to its operation must be checked to define the new operation point as it was done with respect to the primary hydro plant. The power consumed by both hydro pumping plants may be insufficient to accommodate the surplus of wind production. In this case, the aggregate of wind and hydro pumping power does not supply the target power, but a larger value, which indicates that more pumping capac-ity or storage capacity is needed.

B. Generating Operation Mode The HPPs are assigned to operate in the generating mode in periods when wind power cannot supply by itself the aggregate target power. Equation (2) is used to compute the output power that the primary HPP must provide to compensate the low production of wind power. How-ever, due to technical limitations of the hydro turbines it may not be able to supply the required power. Other

restrictions might come from the availability of water in the upper reservoir. Once P1stHPP [MW] is known, the volume of water to be turbined [m3] can be computed through:

1 1

1000 7200

9,81

×= ×

× × ηstHPP stHPP

generating

V PH (6)

where generating is the efficiency of the power generation process and H is the gross head of the installation. If there is not enough water it will not be possible to generate the desired power, the available water in the reservoir, Vturbined, being used to generate part of the targeted power, which can be computed by:

1

9,81

1000 7200

× × η= ×

×

generatingstHPP turbined

HP V (7)

On the other hand, if the value of P1stHPP is lower than the technical minimum of the hydro turbines, the pri-mary HPP will not work. When the primary HPP cannot maintain the output power of the aggregate, the secondary one is assigned to supply the remaining amount of power, which is deter-mined by equation (5). The operation of the secondary HPP is limited by its own constraints, so it might be incapable of supplying the required power, but only a partial value. If that value is even lower than the techni-cal minimum of the turbines, two options are possible: either this HPP will not operate at all or it can operate at its minimum level, providing that the primary HPP is able to reduce its production to a level which is higher than its own technical minimum. The second option is preferable since it allows the aggregate to supply the target power, while the first option does not. If it is not possible to reduce the production of the primary HPP, the secondary one will not operate. Once the operating points of both HPPs are defined, their power output and the volumes of water to be used are known. It is then necessary to update the indicators of the water level in every reservoir, thus computing the gross head, H, of each reservoir. The lower reservoirs are not a constraint when the hydro plant is generating. If the quantity of water to be turbined is larger than the empty capacity of the lower reservoir, this reservoir will be filled up and the surplus of water is released to the water course.

C. Matching Power Generation and Demand As mentioned, the coordinated operation of the Portu-guese wind power plants with the two HPP modifies the historical data of the two HPP and the wind power gen-eration, thus leading to a violation of the balance be-tween generation and demand. In order to re-establish the balance, the generation of thermal power plants was adjusted through:

Re

1 2

+= − −

− − −

− − −

thermal load losses Hydro

newable Hydropumping

TotalWind stHPP ndHPP

P P P

P P

P P P

(8)

The value obtained from (8) can be lower than the minimum limit defined for the thermal production in the Portuguese electric system. In those cases, the thermal production is increased to that technical minimum. As a result of the increase in thermal production, the HPPs have to adjust their output power in order to keep the balance between generation and demand. The adjust-ments to be made on the output power of the hydro plants depend on several factors: the amount of power that was increased in thermal generation, the current operation point of the HPPs or the volumes of water stored in the reservoirs. Five distinct situations may occur. 1) One HPP running as a generator When one of the HPPs is working as a generator and the other is stopped, the adopted strategy was to first de-crease the production of the working HPP. However, this may lead to a value that is lower than the technical minimum of the turbines. In this case, the HPP is stopped and the thermal power plants increase further their output power. If reducing the hydro production is not enough, it is necessary to assign the HPPs to pump up water. The first HPP to start pumping is the one with the higher volume of water available in the lower reservoir. If it cannot absorb the necessary amount of power to compensate the increase in thermal production, then the second HPP is assigned to pump water too in order to increase the load as much as needed. Due to the referred restrictions in the pumping process, the HPPs might not be able to increase the load enough and, therefore, in this situation, the balance between load and generation is not achieved. 2) Both HPPs running as generators When both HPPs are operating as generators, the chosen strategy was to primarily decrease the production of the HPP that has the lower volume of water stored in the upper reservoir. If the new output power of that HPP is lower than the technical minimum, the HPP is stopped and thermal generators increase their production accord-ingly. On the other hand, if it is not enough to decrease the production of one of the HPPs, the other one also decreases its output power. In some situations, shutting down both HPPs might also be insufficient to compensate the increase of thermal production. Therefore, it could be required to reverse the operation of the HPPs, setting them to pump up water. In case of impossibility to consume the necessary amount of power by the primary HPP, due to its restrictions, the secondary one is also set to run as a pumping station, consuming the remaining power. Once again, due to the limitations in the operation of the HPPs, they might be incapable of consuming all the needed power and conse-quently the balance between load and generation is com-promised, in this situation. 3) Both HPPs stopped When both HPPs are stopped, the solution to balance load and generation is to increase the power consump-tion by setting the HPP with more water stored in the lower reservoir to pump up water. It might be required to set also the other HPP to operate as a pumping station. The balance between global power demand and genera-tion is achieved if the HPPs successfully pump up the

volume of water correspondent to the amount of power increased in thermal production. Otherwise, a mismatch between generation and demand occurs. 4) One HPP running as a pump The strategy adopted here is to increase the consumption of the HPP that is being run as a pump, and if necessary, to operate the second HPP as a pump. 5) Both HPPs running as pumps In this situation, the only HPP capable of increasing its power consumption is the second one, because the first HPP has already reached one of its technical constraints. If the second HPP cannot satisfy the mandatory increase of consumption, the balance between generation and demand will not be possible.

D. Remarks Whenever the balance between load and generation was compromised, no measures were undertaken to promote the matching, so that the necessity of more pumping or more storage capacity in the HPPs could be highlighted.

IV. SIMULATION RESULTS AND DISCUSSION

A. Case-Studies Description The wind–hydro coordinated operation was simulated for two wind power installed capacities, 2446 MW for the year 2007 and 3566 MW for 2009, and for two dif-ferent values of the target power, 500 MW and 750 MW. The selected HPPs were Aguieira and Alqueva with a total installed capacity in turbine operation mode of 596.4 MW and in pumping operation mode of 486.8 MW. It was found that the coverage of the whole year with the aggregate production equal to the target power is impos-sible due to constraints of the equipments of the HPPs. Therefore, a goal of at least 80% coverage of the year was established for all the 10000 wind production pro-files. The simulations performed in this study addressed the following scenarios: • sim0: real values of the installed capacity of tur-

bines and pumping systems, as well as of the stor-age capacity of the reservoirs; this is the base case.

• sim4: the installed capacity of the pumping systems to five times the real capacity.

• sim9: the installed capacity of the pumping systems to five times the real capacity, while the storage ca-pacity of the lower reservoirs is doubled.

• sim13: the installed capacity of the turbines and pumping systems, the storage capacity of the upper and lower reservoirs are increased until the aggre-gate of wind power and both HPPs successfully supplies the target power for at least 80% of the year and for all the 10000 different wind profiles.

B. Results Although the simulations were carried out for three different scenarios of the initial level of storage in the upper reservoirs (50%, 75% and 100%) [10], only the results for the reservoirs initially at 75% of their capacity are presented. The tables presented indicate the number of wind profiles in which it was possible to maintain the production of the aggregate equal to the target power, for

the indicated percentage of the year. This analysis has been carried out for the case studies (CS) stated above, but only a selection of the most relevant results is pre-sented. The results for the target power of 500 MW considering the wind capacity of the year 2007 (2446 MW) are pre-sented in Table I, whereas Table II depicts the results obtained when considering the installed wind power as it was in 2009 (3566 MW). Table III (wind power as in the year 2007) and Table IV (wind power as in the year 2009) show the simulation results obtained in the same conditions as above, except that the target power has now been set to 750 MW.

TABLE I: NUMBER OF WIND PROFILES IN WHICH TARGET POWER = 500 MW; WIND CAPACITY = 2446 MW, INITIAL

UPPER RESERVOIRS = 75%.

CS 70% 80% 90%

sim0 10000 9024 0

sim4 10000 10000 9137

sim9 10000 10000 9250

TABLE II: NUMBER OF WIND PROFILES IN WHICH TARGET POWER = 500 MW; WIND CAPACITY = 3566 MW, INITIAL

UPPER RESERVOIRS = 75%.

CS 30% 40% 80% 90%

sim0 10000 3941 0 0

sim4 10000 9471 0 0

sim9 10000 9629 0 0

sim13 10000 10000 10000 19 TABLE III: NUMBER OF WIND PROFILES IN WHICH TARGET

POWER = 750 MW; WIND CAPACITY = 2446 MW, INITIAL UPPER RESERVOIRS = 75%.

CS 20% 30% 80% 90%

sim0 10000 0 0 0

sim4 10000 0 0 0

sim9 10000 0 0 0

sim13 10000 10000 10000 2929 TABLE IV: NUMBER OF WIND PROFILES IN WHICH TARGET

POWER = 750 MW; WIND CAPACITY = 3566 MW, INITIAL UPPER RESERVOIRS = 75%.

CS 70% 80% 90%

sim0 9894 0 0

sim4 10000 5621 0

sim9 10000 4789 0

sim13 10000 10000 0

C. Discussion 1) Target Power: 500MW The results obtained using the 2007 wind installed ca-pacity and the real characteristics of the HPPs (Table I – sim0) are already satisfactory, since the aggregate is capable of maintaining the target power value during at least 80% of the year, for almost all the 10000 wind profiles. Furthermore, one can notice that increasing the

pumping capacity five times (sim4) improves the results obtained, since the aggregate is now capable of supply-ing the target power during 90% of the year for almost all the wind profiles (9137). On the other hand, doubling the storage capacity of the lower reservoirs (sim9) has no significant influence on the results. In Table II the results of the simulations for the wind capacity equal to 3566 MW are presented. The first observation to be made is that the results are much worse than the ones previously obtained for the same target power (Table I). In the base case (sim0), the aggregate can only supply the target power for 30% of the year. As the wind capacity increases, the HPPs are more often requested to pump and the amount of water to be pumped also increases; therefore, the HPPs do not have the required pumping and storage capacities. Only in the conditions of sim13 it was possible to achieve the goal of 80% of the year at the target power, but these condi-tions are unrealistic: the pumping capacity was five times the real value, the capacity of the lower reservoirs was raised to 200 times the real value and the capacity of the upper ones to 40 times. 2) Target Power: 750MW The results presented in Table III for the base case (sim0) refer to the wind capacity as in the year 2007. These results are not satisfactory, because the aggregate could never reach 30% of coverage at the target power. It can also be noticed that for none of the other case-studies, in which the pumping capacity and storage ca-pacity of the lower reservoirs are raised, the results are improved. This suggests that the major constraint here is related to the generating process. In sim13 it was possi-ble to attain the proposed objective. This simulation was however run considering the capacity of the turbines doubled and storage capacities of the upper reservoirs raised to six times the real capacities. This last require-ment is never an option due to the environmental effects on the flooding area. In Table IV (wind capacity = 3566 MW), it can be con-cluded that it is possible to guarantee 70% of the year for all wind profiles in most of the simulations, which is close to the established goal of 80%. In sim13 the stor-age capacity of the upper reservoirs was raised to three times the real values, keeping all the other characteristics of the HPPs in their real values, and in those conditions it was possible to accomplish the goal of 80% of the year at the target power. This can be explained by the fact that with the increase of wind capacity to 3566 MW, the number of periods with the wind production lower than 750 MW is reduced; consequently, an increase in the nominal power of the hydro turbines will not be re-quired.

V. CONCLUSIONS

The possibilities and the major limitations of using the coordinated operation between wind and hydro power facilities with the aim of maintaining their output power production at a fixed set point was dealt with in this paper. The results achieved by the coordination algorithm de-pended widely on the target power set to be continuously supplied and on the wind installed capacity. For the wind

capacity of 2446 MW and using the real characteristics of the hydro facilities, a target value of 500 MW could be satisfied during 80% of the year for almost all the wind profiles. However, when the target power was raised to 750 MW, the results dropped to around 30% of the year. For the simulation in the same conditions, but with the wind capacity of 3566 MW, better results were obtained for the target power of 750 MW, in which it was possible to satisfy that target power for around 80% of the year, while for a target power of 500 MW the results were around 30%. Based on the results obtained, one can conclude that as the wind installed capacity increases, the target power to be continuously satisfied by the aggregate must be read-justed. However, the definition of the proper target power is not a simple task. For higher values of wind capacity, the range of variation of the power production between windy periods and periods with no wind is also higher. Therefore, if the target value is low compared with the installed capacity, the pumping process is prevalent over the generating process, and so a higher pumping capacity is required, as well as a higher storage capacity in the reservoirs. On the other hand, if the target power is closer to the wind capacity, the generating process gains relevance due to the number of periods in which the wind production is lower than the target value. This situation would require a higher hydro turbine-generator capacity and a higher storage capacity in the upper reservoirs. This study suggests that setting the output power of the aggregate of wind parks and HPPs to a particular value could be feasible if local coordination is considered. For instance, the coordination operation of a single wind park and a small HPP may result in the supply of a fixed target power during almost 100% of the year.

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