Offshore wind farm connections to the grid

Transcription

1 Offshore wind farm connections to the grid Energy to Quality, S.L. Pº Castellana 114, 9, Madrid, Spain amolina@energytoquality.com Abstract In most countries and power systems, wind power generation covers only a small part of total power system load. However, there is a tendency to increase the amount of electricity generated from wind turbines especially in the use of offshore wind farms. The reason for offshore wind farm project is that wind speed offshore is potentially higher than onshore, which leads to a much higher power production. Until now, practical experience from offshore wind energy projects is limited. The installation and supporting structure of offshore wind turbine is much more expensive that of onshore turbines. Future projects include wind turbines with a higher rated capacity but also will be significantly large in total size and further away from the shore. This will require a new concept for the overall transmission system. Like HVAC connections and HVDC connections that have two technical options: line commutated converter () based on HVDC and voltage source converter () based on HVDC technology. This paper presents an overview of offshore wind farm (in Europe) and transmission system. Index Terms Offshore, wind farm, grid connection, HVAC, HVDC, E2Q. I. INTRODUCTION From the point of view of connection to the grid, offshore wind farms have specific characteristics with regard to onshore wind farms: - More powerful wind turbines, to compensate for the greater costs of foundation, - More powerful wind farms, - Greater length of the electric lines in the wind farm, due to the greater separation between wind turbines, - Greater distances to the connection to the grid, - New connection systems to the grid (alternating current AC or direct DC) - New problematic with regard to the fulfillment of the new requirements for the transmission in AC or DC and the connection to the grid. All of that entails greater costs of the electric system of the wind, compared to wind farms onshore. In the following sections prospects of the sector are described as for European as well as in Spain. Different alternatives of offshore wind farms connection systems to the grid, as well as their advantages and inconvenients, are analyzed later. II. FUTURE OF OFFSHORE WIND ENERGY IN EUROPE The EU has stated as an objective for the year 2020, coverage of renewable energy of 20% of the total of primary energy. Conclusions of the last international congress Offshore Wind that took place in Berlin, point that only with the development of offshore wind energy, these goals will be achieved. As for European, the following objectives have been previewed: Germany: offshore wind power until 2030, by contributing a 15% of the total. E.On Netz will build the first connection of direct current that will let to connect the offshore wind farms Alpha Ventus (1040), Bard Offshore I (400 ), Hochsee Windpark Nordsee (400 ) and Global Tech I (360 ) to shore. The German regulation of infrastructures of 2006 force the nearest system operator to provide connection to the grid, to those wind farms situated further than 3 nautical miles (5.556 km) from shore. United Kingdom: offshore wind power are previewed, and some have obtained the initial authorization during last months: - London Array (341 wind turbines of 3, in total), situated in the estuary of the Thames, 20 km from shore. This offshore wind farm is promoted by a consortium formed by the company DONG Energy and the electric E.On. - Walney (42 turbines), situated 15 km from the coast of Cumbria. This farm is promoted by the Danish DONG Energy. The future revision of the Law of Energy (Energy Bill) will include measures to develop the Renewable Obligation and finance offshore wind farms. Besides, it will obligate the operator of the electric transport system to contribute to the investments in infrastructures to evacuate energy, contributing to improve the economic imbalance that has blocked the development in the United Kingdom (current costs of development of offshore wind energy against previewed remunerations in current legislation). Sweden: offshore wind power compared with current 135, with the objective of increase their wind power production by a factor of 25 in Denmark is the country leading offshore sector in the world. 30% of the national energetic consumption supplied from renewable energies, thanks to a probable contribution of offshore wind energy before Danish companies

2 optimize projects for a 50 years life (foundation planning, towers, wrapper of the gondola, and main axis of the turbines). This entails a reduction of the costs of electricity produced, competitive against onshore installations. The European Wind Energy Association has presented a Joint Technology Initiative between the States members, to join forces. Three key components emphasize in this strategy, based on the Danish model as an example to follow, Copenhagen Strategy 2005 : - Channel of public and private funds towards the international R+D, for the development of the sector. - Creation of the necessary infrastructures in the sea and inland to access to the grid. - Elaboration and coordination of environmental studies. The Ministries of Industry, Tourism and Trade (MITYC) and Environment (MMA) have published the Environmental Strategic Study of the Spanish coast, carried out with the objective of determining the public knowledge zones that satisfy favorable conditions for the installation of offshore wind farms. As a result of the approval of the Law 9/2006, 28 of April, of evaluation of the effects of certain plans and programs in the environment, must be submit this Environmental Strategic Study of the Spanish coast for the installation of offshore wind farms to a process of Strategic Environmental Evaluation (EAE) which follows the procedure established in the aforementioned law. The Environmental sustainability Report is the result of the firsts phases of the process of EAE. The study concludes with an Environmental Report. The results of the Environmental Strategic Study of the Spanish coast, have been presented publicly, and are summarized graphically in Figure 1. The offshore wind zones are classified in: - Suitable zones (green) or the most adequate because of its reduced environmental impact. Once the study is approved, applications can only be presented for suitable zones. - Exclusion zones (red) because of conflict with other uses or because of environmental impact. - Zones with environmental conditionings (yellow), in which the development of offshore wind farms is determined, and its effects or conflicts must be analyzed in the later phase of design of the project. III. OFFSHORE WIND ENERGY IN SPAIN Nowadays, in Spain there is not any offshore installation in operation, in spite of high wind energy resources are available in the Spanish coastline. So, the government has the objective of install offshore wind farms for a power up to by the year Spain is in phase of demonstration of the viability and profitability of offshore installations, against the existing problematic as for administrative, environmental and of evaluation of wind energy resource. From the electric point of view, which are the points of difficulty of offshore wind farms against the transport grid is unknown. A. Administrative authorizations The Ministry of Industry, Tourism y Trade announced in December 2007, that in Spain will not be offshore wind farms before 2014, alter the publication of the Spanish Royal Order 1028/2007, in force since August 2007, which establishes the administrative procedure to obtain the authorization of offshore wind farms. B. Ecological implications Figure 1: Offshore wind zones suitable (green), excluded (red) and with environment conditionings (yellow) In summary, the document establishes that a 43% of the coastline is available for offshore installations further than 8 km from the coast. At present there are around thirty preapplication, which depend on the environmental impact and on how the regulation develops. These preapplications add up more than 7000, being most of them considered in the Atlantic coast. C. Costs increase regarding onshore wind energy Disadvantages of offshore wind farms are well known; in spite of remuneration of offshore wind energy considered (0,146 euros/kwh) is greater than current for onshore installations. Investments costs for offshore European projects are generally between 0.06 and 0.11 euros/kwh and are referred mainly to: - Foundation and structure costs: These concepts represent approximately 30% of the total cost of the project in an offshore installation, against the 5% in an onshore one. - Connection system to the grid and evacuation costs: Approximately 30% in the total budget of the offshore project, against the 5% for the same onshore project.

3 The risk and uncertainties over the real wind energy resource, contribute to practically duplicate costs regarding an onshore installation. Not everything is disadvantages. Studies executed in Denmark conclude that, in spite of needing an initial investment much greater, up to two times than onshore, electricity production is more stable and a 20% greater. The lifespan of the farm, by means of a good maintenance, may duplicate. IV. ALTERNATIVES TO THE TRANSMISSION SYS- TEM TO THE GRID In an offshore wind farm, the electric transmission system to the coast represents approximately the 30% of the total budget of the project [1]. That is why the importance of the previous design stage of the electric transmission system. Main design criteria for the electric transmission system in offshore wind farm are: - Equipments must be the most compact as possible, with great intervals for the maintenance or even designed for not having maintenance. - High reliability of operation and in critical zones redundancy must be included. - Installations must be resistant to corrosion caused by the air of the sea, which is highly humid and contains salt, so equipments placed inside or even in hermetically closed places must be used. All existing offshore wind farms have chosen the solution HVAC in its transmission system. Mainly, this is because they are located km from the coast and with powers oscillating between 2 (Lely in Holland) to 158 (Nysted in Denmark). In zones km from the coast and more powerful farms, it could be not so feasible this technology HVAC, mainly due to the production of important quantities of reactive power, offering the transmission technology HVDC more advantages. So, it is expected that for future developments of further from coast offshore wind farms and more powerful HVDC technology will be used. Next, a description of the existing technical solutions for transmission to the coast of power generated in an offshore wind faro is presented. A. High Voltage Alternative Current Transmission System (HVAC) As it has been discussed, offshore wind farms in operation and projected for a near future have chosen this technology HVAC for the transmission system. The main advantage is its significantly minor initial investment cost that HVDC technology HVDC and the greater experience. Furthermore, power as well as the distance to the coast is not too high in current farms, so problems of reactive power production in cables of HVAC technology are not so important and this would be a good option. However, for greater power and distances to the coast, AC cables produce great quantity of reactive power, which not reduces the capacity of transport of the cable but implies an additional system to absorb that reactive. This system would be a STATCOM or SVC connected at the beginning and the end of the AC network. Even wind turbines with converter at full power capable to regulate the reactive power produced may be used. Compensation at both extremes for long distances is not efficient in the intermediate point of the cable, where there would be a high capacity charge limiting the transmission capacity. As an example, a XLPE cable of 33 produces AR/km and of AR/km [2]. Figure 2: HVAC technology general schema B. High Voltage Direct Current transmission Systems (HVDC) based in a Line Commutated Converter It is a system based in high voltage with a converter made up of thiristors and commutated by line (). Its principal advantages are: - Possibility of transporting more power to higher distance than HVAC, because DC wires have not the problem of generating reactive power. - It is possible to control both active and reactive power, but not independently. - Optional to operate in a variable frequency state in the wind farm internal grid. - If there is a problem in the grid, the short-circuit current is smaller than the case with HVAC. The principal disadvantages are: - There is a higher initial investment. - Little experience using this technology in marine installations. - It is necessary to compensate reactive power in both AC sides of the circuit, due to reactive power consumption by converters. - It is necessary to use filters to eliminate the harmonic distortion due to converters. - It uses a lot of space to install the marine and land substations, converters, external equipment, transformers - Auxiliary services are needed in marine substation to operate with low wind or without generation. - The global efficiency of the conversion, considering both conversion stations AC-DC-AC, is about 97 and 98 %.

4 Figure 3: HVDC technology general schema [3] C. High Voltage Direct Current transmission Systems (HVDC) based in a Voltage Source Converter () It is a system based in high voltage with a voltage source converter made up of IGBTs that enables a decoupled control of both active and reactive power using a Pulse Width Modulation (PWM) in the converters control. Its principal advantages are: - Better stability than. - Control of reactive power on both grid extremes. - It is possible to operate in weak grids. - Independent control of active and reactive power. - It is not necessary to compensate reactive power in both AC sides of the circuit because it is implemented in converters- - It is not necessary to use filters to reduce the harmonic distortion. The principal disadvantages are: - There is a higher initial investment due to the price of converters. - There is a higher limitation of maximum power to be transported (<400 ) compared with classical HVDC thiristors technology. - There is not experience in offshore wind farms. - High commutation frequency that provides high losses (about 2% in each converter station). Global conversion efficiency is about 94%. Figure 4: HVDC_ Technology general schema [3] V. COMPARISON BETWEEN DIFFERENT TECH- NOLOGIES TO TRANSMIT THE ENERGY TO THE SHORE As in AC or DC, cables have different transmission capacity, depending on distance and the voltage level of the cable. System 3 phase cables of Cu AC Isolation XLPE Oil Oil Max voltage Max power Max longitude (km) MVA * MVA * phase cables DC Impregnated paper No limit Table 1: maximum characteristics to have a correct operation * Too much loses at this power XLPE No limit HVAC cables have a maximum of 200 nominal power for voltages about 150 and 170 and longitudes about 100 km. This capability can be increased if the longitude of the line is decreased or if there are equipments to compensate the reactive power in both sides of the line. DC cables used in HVDC solutions have not the problem of generating reactive power, so the maximum power to be transported is higher than HVAC solutions. Even though, the limit of maximum power to be transported with HVDC_ solution is determined by the converter station (about ). So, for more power, it will be necessary to use 2 or more converter stations. Both cables and stations of solution do not limit the transport of power to the grid, and can be reached up to 1000 in offshore wind farms. Load losses Besides of the capacity of transmission, load losses take a lot of importance. In HVAC technology it depends on the cable longitude, the voltage level and its characteristics. Even though, with HVDC systems, load losses depends less on the type of cable used and more on efficiency of the converter stations. As it is known, solutions have better efficiency (97-98%) than solution (96%) because of the fact that thiristors have lower conduction losses than IGBTs [4]. In figure 5 a comparative graph of losses in function to longitude, for HVAC and HVDC solutions, is presented.

5 Figure 4: Losses in function of longitude, for HVAC and HVDC Solutions. Size of offshore stations Offshore stations in case of HVDC are a third smaller than HVAC, because in HVDC stations, in addition to the transformer, it has to incorporate converters, filters and capacitors groups. With the solution it needs more room that using the one, because the first is made of auxiliary systems to compensate reactive power and filter the harmonics. Characteristics of the connection to the external grid It is important to take into account existing characteristics and requirements in offshore farms connection to the land distribution grid. HVAC and solutions are not indicated to weak external grids, even though one can be used with this kind of grids. In addition, some requirements must be fulfilled as for power range, voltage control, frequency control and voltage recovery capacity after a fail [5]. In this sense, HVDC solution has the advantage to contribute less than HVAC one to fail current. The principal advantage of is its capacity of voltage control, independent control of active and reactive power injection, and its capacity to remain connected in voltages dips [6]. Installations costs In this point a technique-economic comparison between different solutions in function of power and marine farm longitude to the shore (table 2) will be done. Power Until Longitude l<100 km 250<l>100 km l>250 km Best solution HVAC (Vmax= 170) l<100 km HVDC Justification No conversion stations in HVAC Excessive losses in AC AC solution more expensive Power > 900 Longitude l>100 km Best solution because of voltage level ( ) and reactive compensation systems. Transmission capacity reduction with HVAC. HVAC is competitive only in short longitudes. In case of it is necessary to use double converters and cables both in offshore and onshore stations. is more competitive, but there is more risk due to the absence of redundant systems. is more competitive, but there is more risk due to the absence of redundant systems. Not important Not important Not important Justification Table 1: Estimation of the best technique-economic solution for different cases VI. CONCLUSIONS Grid connection of offshore wind farms presents specific problems respect to onshore farms: - Calculating the transmission systems to the shore. - Calculating reactive power compensation systems and necessary filters. - It is necessary to have specific grid codes and verify procedures. Analyzing considered transmission systems to the shore (HVAC,, ) it is drawn that the experience acquired in AC systems is an important point for this technology, against the advantages of any of DC sys-

6 tems presented. The principal advantage of a DC system, against an AC one, is its capacity of control and the possibility to contribute to the primary and secondary control. Extra cost associated to these advantages is not justified. Another associated inconvenient to the capacity of control in a DC system is how this system establish priorities in active and reactive power control. The 20 DC system installed in 1954 in Gotland, and actualized in the end of the 80 s to 260, gives priority to active power generation with the objective of maintaining the voltage level in DC bus. This characteristic calls into question the system capacity to react in the reactive power control in the same way. E2Q is a pioneer company in Spain doing voltage dip tests in wind generators to verify that fulfill all requirements in PO 12.3 of Red Electrica de España (REE), and another international grid codes. Wind generators technologies tested by E2Q came from classical machines adapted to fulfill requirements of operation, to cutting-edge technology of megawatts. Energy to Quality, S.L. has received in January 2008 the accreditation by ENAC (National Entity of Accreditation and Certification) according to UNE-EN ISO/IEC 17025:2005, to realize test to validate simulation models according to the conditions established in Procedimiento de Verificación, Validación y Certificación de los requisitos del Procedimiento de Operación (P.O.) 12.3 de Red Eléctrica de España. The laboratory is one of the firsts that test wind generators that receives the accreditation for validation of a simulation model according to ISO/IEC in Spain, and all over the world. This accreditation, obtained for a test procedure based in simulation, confirms the technological advance of Spanish companies in wind sector. Actually, there are not any studies of the capacity of ridethrough of a HVDC system. The obligation of this requirement in a lot of grid codes, must get manufacturers to adequate their systems to not be disconnected before a voltage dip. This ride-through capacity during a voltage dip can not be accredited doing a test because of the high power, so accredited simulation models are required to realize simulation studies that permit accredit the capacity of ridethrough of HVDC solutions without interruptions during voltage dips. REFERENCES [1] Raul Manzanas, Parques Marinos, II Asamblea General de REOLTEC. 25 Septiembre [2] Sally D. Wright, Anthony L. Rogers, James F. Manwell, Anthony Ellis, Transmission Options for Offshore Wind Farms in the United States, American Wind Energy Association [3] Miguel Eduardo Montilla de Jesús, Estado del Arte y Tendencia Técnicas de los Parques Eólicos Offshore, Trabajo de Investigación Tutelado, Universidad Carlos III de Madrid, julio [4] S. Meier, S. Norrga, H. P. Nee, New Voltage Source Converter Topology for HVDC Grid Connection of Offshore Wind Farms, in proceedings of the 11 th International Power Electronics and Motion Control Conference, EPE-PEMC 04, Riga, Latvia, September [5] Stephan Meier, Novel Voltage Source Converter based HVDC Transmission System for Offshore Wind Farms, Royal Institute of Technology Department of Electrical Engineering Electrical Machines and Power Electronics, [6] Ervin Spahié, Gerd Balzer, Control Possibility for Offshore Wind Farm, 15 th PSCC, Liege, August 2005 Marta Hernández Alarcón, Ana Morales Martínez, Xabier Robe ACKNOWLEDGEMENTS and Miguel Eduardo Montilla D Jesus