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Barriers (and Solutions...) to Very High Wind Penetration in Power Systems A. I. Estanqueiro, Member, IEEE, J.M. Ferreira de Jesus, J. Ricardo, Amarante dos Santos and J. A. Peças Lopes, Senior Member, IEEE power system are presented. Abstract—In this paper the existing technical barriers that prevent the accomplishment of a very high wind generation penetration in a power system are presented. Since several countries and regions in Europe are already experiencing such high wind penetration or, as a minimum, planning their grids and operation strategies to cope with wind penetration from a high to a very high level, the solutions already identified for the most common constraints are also identified. Index Terms— high penetration, power system studies, wind energy, wind turbines.
I. INTRODUCTION
T
he current challenge for the wind energy research
area is motivated by the wind turbine manufacturers success. It is a recognized fact that the high capacity installed in the latest years, mostly in European countries, but recently also in the U.S., is introducing a new set of technological challenges for both grid planners [1]-[2]-[3] and Transmission System Operators (TSOs). These recent concerns are a real TSO challenge: are the conventional power systems capable of coping with the wind power generation in large quantities, without requiring new system operation tools, increased performance of the wind turbines or even a change in the power system conventional mode of planning and operation strategy? This is a legitimate TSO concern as it is their responsibility to manage the system within safety boundaries and respond to official regulatory bodies for the occurrence of serious events or even “blackouts”. The fact that large 100th megawatt wind parks start to be seen as “conventional power plants” and start to behave almost as any other generating unit is not only a positive issue, but also a clear sign of maturity of this technology. In this paper some actual methodologies to overcome the usual barriers to integrate wind power in the grid thus enabling to embed a large amount of wind generation in the Ana I. Estanqueiro is with INETI – National Institute for Engineering, Technology and Innovation, Estrada do Paco do Lumiar, 22, Lisbon, Portugal. (Ph: 351210924773; fax: 351217127195; e-mail:
[email protected]). J.M. Ferreira de Jesus is with TUL - Technical University of Lisbon. Instituto Superior Técnico. J. Ricardo and Amarante dos Santos are with the Portuguese TSO, REN– Rede Eléctrica Nacional, S. A. J. Peças Lopes, is with INESC-Porto and Faculdade de Engenharia da Universidade do Porto.
II. TECHNICAL BARRIERS TO HIGH WIND PENETRATION A. Transmission Limited Capacity The first historical reason normally invoked to limit the amount of wind generation embedded in the grid is the “exlibris” of limited grid capacity. That limitation of capacity usually refers only to the transmission capacity, once in most countries and power systems the developers of a new wind park are already asked to invest themselves on the distribution grid reinforcement and even pay the totality of the cost to build the interconnection lines to the already existing network. In European countries this limitation is being addressed in different ways, but the vast majority of countries are dealing with this classic barrier and nowadays include renewable energy in general and wind energy in particular in their transmission system development plans [4]-[5]. B. Security of Supply. Power Unit Scheduling 1) Balancing Power. Being a time depended and highly variable energy source, wind power gives no guarantee of firm power generation at all or, in the limit, gives a very reduced one at a very short production forecasting time scale. It is a commonly accepted fact that there is a threshold, above which, increasing the wind power penetration also increases the power reserve requirements of a system. It is a commonly accepted fact that there is a threshold, after which increasing the wind power penetration also increases the power reserve requirements of a system. This has been addressed in detail for some power systems or control areas, e.g. Nordpool [6] and the results are quite encouraging: the associated costs are much lower than expected up to a certain upper limit and are only representative for very high penetrations, the increase level strongly depending, as expected, on the system generation mix. For example, an hydro reservoir generation component will reduce these costs. 2) Wind Power Time and Space Variability It was back in the early 1990’ that some visionary scientists started to address the problematic issue of the excessive “wind variability” and, at that time, the almost impossible task of forecasting the wind production within time intervals useful
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for power system operation [7]. Another issue strongly related to the wind generation used to be the high frequency content of the power delivered to the system, mainly in the range of flicker emission (from 0.1 to 20 Hz). Those fluctuations could degrade the quality of the service in the surroundings of wind parks [8] and limits were successfully defined through international standards in order to guarantee an acceptable level of quality [9]. 3) Wind Generation Technical Reliability The main concern of every TSO with a large wind capacity in the grid is the sudden disconnection from the grid of all or most of the wind generation as a response to a fast grid perturbation, normally referred as a “voltage dips”. Low voltages or dips are usually originated by short circuits and may lead to the islanding of some parts of the network including some conventional generating units. . For the wind generation capacity to remain connected to the grid, under such circumstances, it is necessary that the wind turbine generators can withstand these voltage dips, a characteristic known as the “ride through fault -RTF” capability (or LVRTF – low voltage ride through fault) which is nowadays requested by most grid codes and national or local regulations. C. Operational Energy Congestion Surplus Management In power systems where the energy mix is flexible in terms of regulation (e.g. high penetration of hydro plants with storage capacity) another issue is commonly raised when the integration of large amounts of wind power is addressed: what if the situation of excess of renewable penetration (wind + hydro) occurs? Should the wind parks be disconnected, would the hydro be reduced: what is the most important value to preserve, the volatile energy that, if nor extracted from wind will be lost, or the sensible “business as usual” approach “if the hydro is historically in the system, it is a reliable and unexpensive renewable source”, therefore should never be disconnected?
III. THE FUTURE OF POWER SYSTEMS A.. Dealing with Grid Capacity For very high wind penetration, and assuming that the wind parks are connected to both the distribution and the transmission grids, capacity is an issue that has to be dealt with by both the TSO and DSO in close cooperation. The knowledge of the geographic location of the wind resource must be put together with the connection demands from the wind park developers to allow DSO and TSO to define, if necessary: 1) Where to extend the transmission grid, should there be important wind resources in remote areas with very small loads that would not justify, only by themselves, the need for
the transmission grid; 2) The other necessary internal distribution and transmission internal grid reinforcements, necessary to allow for the connection of the wind generation making sure that both grids cope with the planning and operation reliability criteria, The grid planning analysis have to take into consideration the special characteristics of wind generation, i.e., its variability in time and, in particular, the fact that most of the time the wind generation is quite lower than the installed nominal power in the wind parks. Probabilistic approaches are the most appropriate, with the wind modeled, if possible, with known correlation factors resulting from the wind resource geographic assessment studies. Should a deterministic approach be used, maximum, intermediate and low wind generation scenarios can be used, but the choice of maximum wind factor to be used must be wisely chosen to avoid unnecessary and costly asset investment. The possibility of control of maximum (but not frequent) generation in peak wind generation during periods of high transmission grid stress must also be considered. Of course, it is possible to introduce a reasonable amount of wind power in the grids, as planned in most parts of any grid; but if we want to go beyond a certain degree of wind penetration, the grids must be reinforced. The art is to take the most possible advantage of the characteristics and possible controllability of wind together other generation vectors, to minimize the necessary grid extensions. B. Integrated “HighTech” Design and Operation of Power Systems The beginning of power systems planning and development in the end of 19th century had a marked empirical approach: each innovative step was strongly dependent on recent experiences, not all of those positive, therefore this industry has evolved in an extremely cautious way. It is curious to conclude that, except from all the computing capacity - developed in the second half of the last century and promptly used in this sector - in what concerns the power system design and operation principles, the current approach is very similar to the original one: to take small steps, very conservative test first any innovation to the system, and trust that “what worked well in the past will work perfectly in the future”… It is astounding the fact that the power system evolved from the initial small dimension to cover the entire planet without much change on its basilar concepts. But it is also clear that the energy abundance period is about to change and that in addition to that scarceness fact, the severe environmental restrictions, climate change identification among others are new challenging phenomena that will require a different approach from this industry in the next years and decades. Recognizing the urgent need to have common scientific and technical approaches to the design and operation of power systems with high wind penetration, the International Energy
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Agency Implementing Agreement on Research, Development and Deployment of Wind Energy Systems started a new international cooperation (Task 25) entitled “Design and Operation of Power Systems with Large Amounts of Wind Power”, in the beginning of 2006. [10]-[11]. Within the program of this new R&D Task, the wind impacts on the system were mainly related to their time/range of scale and are globally identified as follows. 1) Regulation and load following [1 to 30 min.] How the uncertainty introduced by wind power affects the allocation and use of reserves in the system. 2) Efficiency and unit commitment [hours to days] Impact of wind power time variability and forecasting errors on unit commitment. 3) Adequacy of power [several years] Avoided investment in conventional units due to existing wind capacity. Total secure supply available during peak load situations. 4) Transmission adequacy and efficiency [hours to years] Depends on the location of wind farms relative to the load consumption Correlation between wind production and load. C. The Power System of the Future: Power Stations for Regulation and “Energy Stations” for Generation? The recent occurrence of severe blackouts both in US and in Central Europe was a clear warning that the electric energy design and operation principles have to change. While the warning seemed to have been very effective in US where new transmission lines are being planned and Federal regulations for the Power sector are being defined and implemented, in Europe, the plans to have a common European electricity regulator remain just as plans. It is most awkward that, in one age that sees the whole world connected to share non-vital information through internet – the European TSOs still do not share fundamental information for the operation of their power systems and have not implemented the available warning tools to help each other during events such as the one that took place on the 4th November 2006. Taking into consideration the increasing difficulties felt by most, if nor all, TSOs to have environmental approval for the construction of new transmission lines it becomes “mandatory” to improve the existing network efficiency. The increase of power line utilization by the use of online monitoring (temperature, wind, loads, etc), the increase of maximum transmission capacity by introducing new components such as FACTS or the upgrading of existing degraded components such as conductors, protections and transformers, are examples of measures taken by both TSO and DSO to increase network efficiency.
D. Storage of Renewable Energy The concept of wind energy storage - and other highly variable time-dependent renewable primary sources - is already in use. In a recent public call for wind parks access to the transmission grid held in Portugal [12], the possibility to contribute to the power system operation using added storage in conjunction with wind production gave the opportunity to the presentation of very innovative solutions, such as the use of vanadium batteries, and the more classical and large capacity-oriented possibility of energy storage in hydro power stations dams. When hydro pumping storage is available, the functionalities able to identify the best combined wind - hydro pumping storage strategies should be used. For that purpose wind power forecasts for the hours ahead are requested together with the electricity price forecasts. The identification of the optimised daily operation strategy can be determined by solving a linear hourly-discretized optimisation problem where the objective function is the economic benefits of such strategy [13]. If and when available in the generation mix, hydro pumping storage is a possibility for any power system to provide compensation energy, whenever necessary. An identification of the operational rules to be adopted is part of the research plan of IEA Task 24 – Wind Hydro Annex, whose scope also covers the compatibility and the correlation assessment of these two renewable sources. There is also a clear link between large wind penetration and the economic interest in increasing hydro capacity, for those countries that posses already some (but not much) hydro installed capacity. That is why ‘more wind’ can mean ‘more hydro’, in special more pumping capacity. Moreover, in systems with a large presence of hydro generation, and assuming that some water inflow regulation capability always exists in the hydro cascades, it is possible to implement optimal managing procedures, exploiting wind power forecasting tools, such that a common generation modulation can be achieved leading to the maximization of the integration of wind – hydro renewable resources in given area. IV. TECHNICAL SOLUTIONS ALREADY IN USE: WIND POWER INNOVATIVE CONCEPTS. A. Innovative Characteristics of the Wind Systems 1) Low Voltage Ride Through Fault Nowadays, recognizing the large potential of wind energy, but also revealing an extreme concern towards its growth and future development, most TSOs have issued grid codes requiring the wind turbines and power plants to contribute with some basic - although “anti-natural” for the wind technology”- power system operation functionalities. The more publicized one is the RTF – ride through fault capability one, whose characteristics for the E-ON and Spanish grid
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codes, as well as Portuguese technical requirements for new concessions are depicted in Fig. 1. 120
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total power the wind park can inject into the grid. However, in wind generation most of the time wind turbines are operated far from their nominal ratings. So, some over capacity installation in wind parks will be allowed provided that a control of production is performed to avoid an injection of power larger than the initially defined by grid technical constraints (see Fig. 3). Since monitoring and control of this generation can be performed using the wind power dispatch centres, this limit can be adapted to the network operating conditions without compromising network security operational levels.
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Fig. 1 – LVRTF ((t=0) characteristic response curves required to the wind systems.
2) Participation in the primary frequency control. Low Frequency Ride Through Fault
Fig. 2 – Frequency dip in the European network on the 4th November 2006.
Power purchase agreements that safeguard the possibility to interrupt the wind generation in cases technically documented and justified are already being used. This is a legal innovative approach in Europe where the permanent access of renewable sources to the grid is widely accepted.
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Duration Curve Single wind farm (WF) Transmission connected WF's
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Large scale recent events (4th November 2006) that were propagated to almost all the European Network (1st UCTE synchronous area) and affected even some North-African countries raised the issue of wind turbine response to extreme low frequency occurrences as the one depicted in Fig.2 If wind generators with primary frequency regulation capabilities are used, which means adopting a specific primary frequency control and a deload operation strategy - below the maximum extraction power curve, a considerable contribution can be obtained from these units to reduce the impact of this frequency dip. [14]-[15]. Such control strategy may provide a considerable contribution for the frequency regulation, specially in windy regions and power systems with reduced hydro power regulation capability. Isolated very windy power systems with traditional frequency control are a typical example for the application of this recent functionality of the wind turbines. The use of “frequency flexible” power electronics will definitely provide a relevant contribution for the power system robustness, by avoiding grid electronic interfaced wind generators disconnection from the grid when these system disturbances take place: the recent 4th November event was extremely useful to show that different wind turbine manufacturers show completely different capabilities.
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B. Wind Power Control and Curtailment The replacement of large conventional power plants by hundreds of wind generation units spread over the transmission and distribution system requires the development of new concepts for monitoring, controlling and managing these generation resources having in mind network operational restrictions and also market procedures. Some innovative strategies and equipments are already in operation in some European countries. The capacity of a wind park is usually limited by the capacity of the grid to accept the
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Fig. 3 – Comparison of wind power duration curves for a single wind park and the all the wind farms connected to the transmission network.
It may be economically interesting and very relevant for low wind regions where the wind park nameplate power is never or very seldom achieved (areas with a wind Weibull distribution with “no tail”), but should be handled with care in windy areas as the one illustrated by the single wind farm duration curve of Fig. 3.
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C. Wind Generation Aggregation Dispatch Centers
Fig. 4 – Architecture for the management of the power system.
C. Additional Remote Reactive Power Control; In order to assure the wind power plants the capacity to deliver reactive power during voltage dips, thus providing support for the network voltage, some TSOs are requiring reactive voltage support similar to the one presented in Fig. 5 (Portuguese example, [12]). 120
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Wind power has developed in varied forms in different countries: while in some remains an essentially distributed electrical energy source as in Denmark and Netherlands connected to the medium voltage distribution grid, and sometimes even to the low voltage; in others as Spain, Portugal and also US this topology is being overcome by the installation of extremely large wind parks (with several hundreds of MW) connected to high (or even very high) voltage transmission system. This recent and innovative tendency of the wind industry required the operation of these power plants to adapt to the new configuration. In Spain the generation of large transmission connected wind parks is already being aggregated and centrally managed by clusters that constituted a “local wind power dispatch center” and adopt an hierarchical control architecture as depicted in Fig. 4. Similar approach is already defined for the Portuguese power system for the “next generation of wind parks” and will be the technical basis for the future development of the remaining sustainable wind energy potential. This aggregation of the wind generation has several positive side effects as it enables to take advantage of one of the most basic characteristics of the wind resource: its spatial lack of correlation in what concerns the fast wind fluctuations (typically above 0.1 Hz) [16]. Other studies [17], [18] have shown that a part of this smoothing effect may extend to the spatial scale of one control area, but deep knowledge of the frequency of the fluctuations involved in the cancellation effect is still not available. Nevertheless, what could be, at “a first glance”, a negative characteristic may turn, in fact, to be extremely beneficial for the power system operation, since the most hazardous wind oscillations naturally tend to cancel themselves. Of course, in order to profit from that effect, it is required the share of common grid interconnection, otherwise large power fluctuation may not be felt by central dispatches, while they are affecting local or regional parts of the transmission network. The smoothing effect is also not present when a whole country (or power system) is immersed in high (or low) pressure atmospheric circulations or passed by large frontal areas. The need to monitor remotely the state and level of generation of wind power plants was recognized both by the manufacturers of wind turbines and the IEC - International Electrotechnical Commission [19] whose Technical Committee 88 – Wind Turbines started the development of a new international standard on communications (IEC 6140025-XX). This standard is already in its final phase and the publication is expected within a year. Having in mind wind generation can inject power either in the transmission network or in the distribution grids, a dialogue with the TSO and the DSO operators is required, as well as with the market operator, assuming therefore that in this case the wind generation can participate also in the market.
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Fig. 5 – Characteristic curve of reactive power delivery by wind power plants during/after voltage dips (at t=0) [12].
This capability is also required to enable the adjustment, by request of the TSO, of the reactive power injected in the network for tangent (phi) in the predefined ranges, that in Portugal assume values within [0; + 0,2].
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V. CONCLUSION The wind industry has experienced a remarkable increase in performance in what concerns the power system interface. Although the design studies needed in order to assess high wind penetration are still being accomplished in many countries, it is already clear that the wind industry has moved in the right direction with the integration of functionalities as RTF-ride through fault, remote monitoring and added power production control. The close cooperation of TSOs, DSO’s and the wind industry indicates it is possible to have, at a much larger scale, what some pioneering countries already did in terms of wind penetration. The barriers usually indicated in the end of last century that prevented the achievement of large wind penetration are nowadays turning into questions as: “what part of the wind capacity needs to be equipped with RTF capability; and “when, where and under what circumstances do the wind power stations need to be deloaded in order to provide primary frequency control”. REFERENCES [1]
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DENA, “Planning of the grid integration of wind energy in Germany onshore and offshore up to the year 2020”, DENA Grid study. Deutsche Energie-Agentur, 2005. A.I. Estanqueiro, A., R. Castro, J. Ricardo, M. Pinto, R. Rodrigues, and J. P. Lopes, “How to Prepare a Power System for 12% Wind Energy Penetration: The Portuguese Case Study” presented at the 2006 Nordic Wind Power Conference, Espoo, Finland. R. Doherty, H. Outhred, and Mark O’Malley, ”Establishing the Role That Wind Generation May Have in Future Generation Portfolios”, IEEE Trans. Power Syst., vol. 21, no. 3, pp. 1415-1422, Aug. 2006. Plano de Investimentos da Rede Nacional de Transporte 2006-2011, REN, S.A., Lisbon, November 2005 (in Portuguese). Available: http://www.ren.pt (URL). Sucena Paiva, J.P.; J.M. Ferreira de Jesus; Rui Castro; Pedro Correia; João Ricardo; A. Reis Rodrigues; João Moreira and Bruno Nunes, “Transient stability study of the Portuguese transmission network with a high share of wind power”, XI ERIAC CIGRÉ – Undécimo Encuentro Regional Iberoamericano de Cigré, Paraguay, May 2005. Holttinen, H, 2004. The impact of large scale wind power production on the Nordic electricity system. VTT Publications 554. Espoo, VTT Processes, 2004. 82 p. + app. 111 p. Available: http://www.vtt.fi/inf/pdf/publications/2004/P554.pdf (URL). I. Troen and L. Landberg, “Short Term Prediction of Local Wind Condition”, W. Palz, CEC (Ed.), Proceeding of 1990 ECWEC, H. S. Stephens and Associates, Bedford, pp. 76-78, 1990 IEA: Variability of wind power and other renewables. Management options and strategies. 2005. Available: http://www.iea.org/Textbase/publications/ free_new_Desc.asp?PUBS_ID=1572(URL). IEC 61400-21:2001 “Wind turbine generator systems - Part 21: Measurement and assessment of power quality characteristics of grid connected wind turbines”, IEC Standard, 2001. Hannele Holtingen et al. available at: http://www.ieawind.org/ AnnexXXV/ PDF/25_Workplan.pdf (URL) Hannele Holtingen et al, Design and Operation of Power Systems with Large Amounts of Wind Power Production, IEA collaboration, EWEC’06 online proceedings. Available: www.ewea.org (URL). Concurso para Atribuição de Pontos de Recepção de Energia Eléctrica para Ligação à Rede do SEP de Parques Eólicos - Programa de Concurso, DGGE – Direcção Geral de Geologia e Energia, Lisbon, July 2005 (in Portuguese). E. D. Castronuovo, and J. A. Peças Lopes, “On the Optimization of the Daily Operation of a Wind-Hydro Power Plant”, IEEE Trans. Power Syst., vol. 19, no. 3, pp. 1599-1606, Aug. 2004.
[14] R. G. de Almeida, and J. A. Peças Lopes, “Primary Frequency Control Participation provided by Doubly Fed Induction Wind Generators”, in Proc. 15thPower System Computation Conference, Liège, Belgium, Aug. 2005. [15] R. G. de Almeida, E. D. Castronuovo, and J. A. Peças Lopes, “Optimum Generation Control in Wind Parks When Carrying Out System Operator Requests”, IEEE Trans. Power Systems, vol. 21, No. 2, pp. 718-725, May. 2006. [16] N. H. Lipman, E. A. Bossanyi, P. D. Dunn, P. J. Musgrove, G. E. Whittle, and C. Maclean; “Fluctuations in the output from wind turbine clusters”, Wind Engineering, vol. 4, nº 1, pp.1-7, 1980. [17] H. G. Beyer, J. Luther and R. Steinberger-Willms. “Fluctuations in the combined power output from geographically distributed grid coupled WECS - An Analysis in the frequency domain”, Wind Engineering, 14, Nº 3, 179-192, 1990. [18] H. G. Beyer, J. Luther, W. Pahlke, R. Steinberger-Willms, H. P. Waldl e U. Witt. “Characteristics of the power output of wind turbines and large scale dispersed wind energy systems”. W. Palz, CEC (Ed.), Proceeding of 1990 ECWEC, H. S. Stephens and Associates, Bedford, pp. 606-610, 1990. [19] http//www.iec.ch: (International Electrotechnical Comission site) (URL)
Ana Estanqueiro (M’07) was born in Coimbra in 1963. She received her electrical engineer degree from the Technical University of Lisbon (TUL) in 1986 where she also did her M.Sc and PhD. in mechanical engineering, respectively in 1991 and 1997. She works as a research scientist at INETI, Lisbon, Portugal since 1987, being currently Director of the Wind and Ocean Energy Research Unit as well as associate professor at Universidade Lusiada. Her research interests are broad within wind energy with a focus on grid integration and dynamic behavior wind turbines benefiting from her electrical and mechanical background. Ana Estanqueiro is currently chair of the IEA International Energy Agency Wind Agreement and President of the PT IEP/IEC CTE 88 – Wind Turbines.
J.M. Ferreira de Jesus received the electrical engineering degree in 1975 and the Ph.D. degree in electrical engineering in 1982 from Instituto Superior Técnico and the University of London, respectively. He is currently an Associate Professor at Instituto Superior Técnico of the Technical University of Lisboa. His main interest research areas are in the fields of renewable energy sources, transient stability of electrical power systems and power conditioning. J. Ricardo was born in Lisbon, Portugal, in 1952. He received an electrical engineer degree in 1975 from the Instituto Superior Técnico, Technical University of Lisbon, where he also taught courses in Electrical Machines and Transients in Electrical Machines. He completed the Master in Buiseness Administration in 1989, in the Universidade Nova de Lisboa. He joined EDP, the Portuguese TSO, in 1976, working in electric system dynamics, network planning and asset management control. Later, he was appointed Deputy Manager of the National Dispatch and, from 1996 on, as Manager of the Transmission Network Planning Division. From 1992 to 1996 he served as CEO of the “Centro para a Conservação de Energia” , a national energy agency. Amarante dos Santos, received the electrical engineering degree in 1971 from Instituto Superior Técnico and has joined the Portuguese utility EDP – Electricidade de Portugal S.A.. He is currently the System Operator of the Portuguese TSO, REN, S.A.. J. A. Peças Lopes (M’80–SM’94) received the electrical engineering degree in 1981 and the Ph.D. degree, also in electrical engineering, in 1988, both from the University of Porto, Porto, Portugal. He received the Aggregation degree in 1996. He is an Associate Professor in the Department of Electrical Engineering, University of Porto. In 1989, he joined the staff of INESC-Porto as a Senior Researcher, and is now Co-Coordinator of the Power Systems Unit. Additionally, he has been leading several research and consultancy projects related with the integration of renewable generation and DG in the power system.
