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technical description lillgrund 11336934

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Technical Description Lillgrund Wind Power Plant Lillgrund Pilot Project September 2008 Type of document Document identification REPORT 2_1 LG Pilot Report Rev No 1.0 Report date Project No September 29, 2008 21858-1 Author Project name Joakim Jeppsson Poul Erik Larsen Åke Larsson Lillgrund Pilot Project Customer Reviewed by Vattenfall Vindkraft AB Approved by The Reference Group Distribution No of pages 78 The Swedish Energy Agency No of appendices PREFACE Vattenfall’s Lillgrund project has been granted financial support from the Swedish Energy Agency and Vattenfall will therefore report and publish experiences and lessons learned from the project This report is compiled in a series of open reports describing the experiences gained from the different aspects of the Lillgrund Wind Farm project, for example construction, installation, operation as well as environmental, public acceptance and legal issues The majority of the report authors have been directly involved in the Lillgrund project implementation The reports have been reviewed and commented by a reference group consisting of the Vattenfall representatives Sven-Erik Thor (chairman), Ingegerd Bills, Jan Norling, Göran Loman, Jimmy Hansson and Thomas Davy The experiences from the Lillgrund project have been presented at two seminars held in Malmö (4th of June 2008 and 3rd of June 2009) In addition to those, Vattenfall has presented various topics from the Lillgrund project at different wind energy conferences in Sweden and throughout Europe All reports are available on www.vattenfall.se/lillgrund In addition to these background reports, a summary book has been published in Swedish in June 2009 An English version of the book is foreseen and is due late 2009 The Lillgrund book can be obtained by contacting Sven-Erik Thor at sven-erik.thor@vattenfall.com Although the Lillgrund reports may tend to focus on problems and challenges, one should bear in mind that, as a whole, the planning and execution of the Lillgrund project has been a great success The project was delivered on time and within budget and has, since December 2007, been providing 60 000 households with their yearly electricity demand Sven-Erik Thor, Project Sponsor, Vattenfall Vindkraft AB September 2009 DISCLAIMER Information in this report may be used under the conditions that the following reference is used: "This information was obtained from the Lillgrund Wind Farm, owned and operated by Vattenfall." The views and judgment expressed in this report are those of the author(s) and not necessarily reflect those of the Swedish Energy Agency or of Vattenfall (78) Technical Description Lillgrund Wind Power Plant SUMMARY Lillgrund offshore wind power plant comprises 48 wind turbines, each rated at 2.3 MW, bringing the total wind farm capacity to 110 MW The Lillgrund offshore wind power plant is located in a shallow area of Öresund, km off the coast of Sweden and km south from the Öresund Bridge connecting Sweden and Denmark An average wind speed of around 8,5 m/s at hub height, combined with a relatively low water depth of to meters makes it economically feasible to build here Vattenfall Vindkraft AB is the owner and operator of Lillgrund offshore wind power plant Lillgrund is a Swedish pilot project supported by the Swedish Energy Agency (STEM) The bidding process was completed during 2005 and the offshore power plant was constructed in the period 2006 to 2007 Vattenfall awarded the contract for foundation and seabed preparation work to the DanishGerman joint venture of Pihl & Sohn A/S and Hochtief Construction AG, and the contract for wind turbines and electrical systems to Siemens Wind Power A/S The Lillgrund project is considered a success story not only from a technical point of view but also from a social point of view The wind farm was constructed on time and has now been successfully operational since December 2007 The project team, composed by specialists from different parts of Sweden and Denmark, have truly lived up to the vision “One Vattenfall” There is, however, always potential for improvement and the aim of this report has been to determine and highlight these areas It is worth pointing out that only the electrical system and the foundations are tailor made at offshore wind power plants The wind turbines are more or less standard products with none or very limited possibilities for project specific design changes Geotechnical investigations are expensive and it can be difficult to balance the risks as well as the benefits of this expense in the early phases of a large infrastructure project As a whole, the geotechnical surveys at Lillgrund proved to be useful They identified potential issues, such as the fact that extra excavation was required for two of the foundations It also revealed the location of a small number of boulders that would have to be removed Vattenfall requested a complete study of the electrical system for Lillgrund to be delivered with the bids That request was not met Instead Siemens Wind Power began a complete electrical system study after being awarded the Contract The electrical system study was completed during the construction period and revealed that (i) the insulation level in the main transformer was too low, (ii) surge arresters needed to be installed in all 48 wind turbines and (iii) some large transients occurring when the 130 kV main circuit breaker was switched on This caused extra costs and the experience shows that it is vital to perform an electrical systems study in good time before the construction period begins In general, the working conditions at the Lillgrund site have been good However, late autumn and winter 2006 the combination of harsh winds and inconsistent current directions made it impossible to perform the offshore work Situations like these need to be taken into consideration when writing the contract to ensure that the appointment of risk between owner and contractor is clearly defined (78) Many minor problems and disputes with the contractors can be avoided if the owner has a site representative present on-site during the whole project This must be required both for production sites for the foundations, concrete or steel, as well as for the offshore work The foundation contractor and designer underestimated the reinforcement needed to fulfil the requirements from the agreed design code Experience from earlier projects designed after other codes were not valid Different kinds of cement can be used for the foundations If a long lifetime is required the choice of cement can be of importance A Portland cement with a higher amount of alkali can make cracks self heal, which is beneficial The characteristic is not present in cement with micro silica, which was the cement chosen for the Lillgrund project It is recommended that anodes be used as cathode protection system on all foundations, including the transformer station foundation The influence of the cable armouring should also be taken into consideration in the design Due to corrosion problems, hand railings are preferably made of aluminium as opposed to painted or galvanised carbon steel Boat landings should be as simple as possible, if ice is a problem, consider a solution where you accept that some of them disconnect during hard winters This might be the overall cheapest solution Cable laying should be avoided during wintertime At Lillgrund a propeller breakdown on the vessel resulted in the cable being placed on the seabed ±15 meter within the trench line After repair of the vessel and waiting for proper weather conditions the cable was picked up from the seabed and re-laid in the trench During this delay of almost months the pre-excavated trench was partly backfilled by natural causes After re-laying the cable in the trench, water jetting had to be used to bring the cable to the bottom of the preexcavated trench (78) TABLE OF CONTENTS INTRODUCTION 1.1 Purpose 1.2 Background and limitations GENERAL DESCRIPTION 2.1 General 2.2 Location 2.3 Park layout 2.4 Site conditions 2.4.1 General 2.4.2 Wind resources 2.4.3 Water depth 2.4.4 Wave conditions 2.4.5 Current 10 2.4.6 Ice 10 2.4.7 Other 10 2.5 Discussion 11 FOUNDATIONS 12 3.1 Technical specification 12 3.1.1 Geometry 12 3.2 Geotechnical investigations 14 3.2.1 Year 2001; Phase I Geotechnical investigation 14 3.2.2 Year 2002, Hydrographical survey 14 3.2.3 Year 2003, Phase II Geotechnical investigation 14 3.2.4 Year 2005, Geophysical investigation 15 3.3 Certification of design 15 3.4 Design requirements 16 3.5 Construction method of the main structure 16 3.5.1 Onshore 16 3.5.2 Offshore 18 3.6 Foundation tower interface 22 3.7 Discussions 24 ELECTRICAL SYSTEM 25 4.1 Electrical system study 25 4.2 130 kV system 27 4.2.1 General 27 4.2.2 Onshore substation “Bunkeflo” 27 4.2.3 Techniques to reduce switching transients 30 4.2.4 130 kV Onshore cable 31 4.2.5 130 kV Sea cable 33 4.3 Offshore substation 36 4.3.1 General 36 4.3.2 Electrical system 41 4.4 Main transformer 42 4.4.1 General 42 4.4.2 Technical data 43 4.4.3 Gas-in-oil transmitter 43 4.4.4 Oil collector 44 4.4.5 FAT 44 (78) 4.5 4.6 4.7 4.8 Internal grid 45 4.5.1 General 45 4.5.2 33 kV sea-cables 45 4.5.3 J-tubes 51 4.5.4 33 kV Switchgear 51 System grounding 53 4.6.1 General 53 4.6.2 130 kV system 54 4.6.3 33 kV system 54 4.6.4 Wind Turbines 55 4.6.5 Energization 55 Relay protection 56 4.7.1 General 56 4.7.2 130 kV system 58 4.7.3 33 kV system 59 4.7.4 33 kV wind turbine feeders 59 4.7.5 Wind turbines 59 Discussion 60 WIND TURBINES 62 5.1 General information and technical data 62 5.2 Power Curve and Energy Production 64 5.3 Noise 65 5.4 Electrical Layout 65 5.4.1 Wind Turbine Transformer 66 5.4.2 Wind Turbine Generator 67 5.4.3 Wind Turbine Converter 67 5.5 Mechanical Layout 68 5.5.1 Rotor 68 5.5.2 Transmission system 69 5.5.3 Tower 71 5.6 Lightning protection system 72 5.7 Coating 73 5.8 SCADA 73 COMMENTS AND CONCLUSIONS 76 REFERENCES 78 (78) 1.1 INTRODUCTION Purpose This report summarises the design and gives a technical description of Lillgrund Offshore Wind Power Plant The report was written on behalf of the Swedish Energy Agency It aims at providing valuable practical information based on experience gained during the construction of the Lillgrund offshore wind power plant This experience cab be used to ensure that the construction of future offshore wind power plants are more cost efficient 1.2 Background and limitations Vattenfall AB has received governmental support for the construction of Lillgrund offshore wind power plant A requirement for the financial support was that experiences gained during the project development and installation phase are outlined in a report to the Swedish Energy Agency The full report will include areas such as economy, design and technical solutions, installation and commissioning, environmental impact, operation and maintenance, production analysis and communication This report is limited to design and technical solutions (78) GENERAL DESCRIPTION 2.1 General The Lillgrund offshore wind power plant is comprised of 48 wind turbines, each rated at 2,3 MW, resulting in a total wind power plant capacity of 110 MW The wind power plant system also includes an offshore substation, an onshore substation and a 130 kV sea and land cable for connection to shore 2.2 Location The Lillgrund offshore wind power plant is located in a shallow area of Öresund, km off the coast of Sweden and km off the coast of Denmark The wind power plant is situated km south of the Öresund bridge, which connects Copenhagen and Malmö Köpenhamn Saltholm Dragör Malmư Flintrännan Pepparholm Ưresundsbron km km Lernacken Bunkeflostrand km Klagshamn 14 km 10 km Skanör Figure 2.1 Höllviken Location of the Lillgrund offshore wind power plant (78) 2.3 Park layout The wind power plant incorporates 49 foundations in total, of which 48 are turbine foundations and one is for the offshore substation The substation is placed at position W01 in Figure 2.2 The turbines are connected to each other and to the substation through five radials as shown below Figure 2.2 2.4 2.4.1 Park layout showing the radials Note the “hole” in the park; this is due to the shallow water, which prevents vessels from being able to manoeuvre in this area Site conditions General Site conditions, with respect to wind, waves, water depths, water levels, ice and current have been studied and numerically modelled in order to establish expected values along with design values [4] The data is used for the design of the foundations and the combined foundation and wind turbine structure The information is also used as a basis on which contractors can develop their bids during the bidding process Site conditions such as expected wind speed, waves and currents are of great importance when contractors decide on suitable equipment and methods 2.4.2 Wind resources The wind resources for the site were estimated in different ways The expected wind resource was presented in a report from Risø [1] Extreme winds were analysed in [2] and observed results from the onsite wind measurement mast was presented in [3] Mean wind speed for the site is estimated to be 8,5 m/s at 65 meters height and a prevailing wind direction of 225 to 255 degrees (Figure 2.3) (78) Figure 2.3 2.4.3 Wind frequency for Lillgrund offshore wind power plant Water depth The water depth at the site is studied for more than one reason It is important to know the depth to the seabed in order to design the foundations It is also important to know the variation of the mean sea level since there is a minimum depth in which the sea vessels can operate Table 2.1 from [4] is an example of the results from the study of the water level Table 2.1 Return period (year) 10 50 100 Estimated water levels (m) from mean sea level [4] Low-water level Skanör -1,25 -1,40 -1,45 Drogden -1,30 -1,60 -1,70 High-water level Skanör 1,30 1,55 1,65 Drogden Köpenhamn Drogden Klagshamn 1,25 1,26 1,25 1,35 1,40 1,46 1,45 1,59 1,45 1,54 1,54 1,68 The sea bottom is not all flat and for this reasons five types of foundations with different shaft heights were used Table 2.2 shows the difference between design seabed level, caisson bottom level and excavation level Table 2.2 Design seabed level, caisson bottom level and excavation level for the Lillgrund site, all depths in m Type 2.4.4 Design seabed level -4,7 -5,7 -6,7 -7,7 -8,7 Caisson bottom level -6,8 -7,8 -8,8 -9,8 -10,8 Excavation level -7,1 -8,1 -9,1 -10,1 -11,1 Wave conditions Numerical modelling of the wave conditions at the site was done in [4] in order to establish the expected wave conditions for the site This information is useful for contractors when choosing suitable equipment for the project The wave height is dependent on wind direction, direction of the current and wind speed, and in order to establish the expected wave conditions a number of data sources were studied and numerical simulations performed for more than 240 different scenarios (78) 5.2 Power Curve and Energy Production The wind turbine starts producing power at a wind speed of approximately m/s and reaches nominal power at approximately 12-13 m/s At wind speeds above 25 m/s the wind turbine shuts down for safety reasons, and connect automatically to the grid again, when the wind speed has dropped below a pre-set value for a definite time Figure 5.2 Siemens 2,3 MW MkII Power Curve The calculated annual energy production for the wind turbine, based on this power curve and assuming a Rayleigh wind speed distribution, can be seen for below figure indicated for various mean wind speeds (assuming no array losses and air density 1.225 kg/m3) Figure 5.3 Calculated annual energy production (Siemens 2,3 MW Mk II) 64 (78) 5.3 Noise Noise measurements on the Siemens 2,3 MW Mk II wind turbine has been made on the basis of Technical Guideline (IEC 61400-11) Below, the summary of measured values can be seen for different wind speeds Wind speed in 10 m height (m/s) 10 Electrical power output calculated from the power curve 8kW) 1049 1651 2106 2260 2295 Measured pitch angle (degrees) -0,8 -0,8 -0,8 >1 Measured rotor speed (min-1) 15,1 15,3 15,4 15,8 16,0 Sound power level (dB) 103,4 104,9 105,1 105,0 105,0 Combined uncertainty in the sound power level, UC (dB) 1,2 1,1 1,2 1,3 1,3 Tonality, ΔLK -5,58 -4,68 -6,36 -5,43 -5,91 Tonal audibility, ΔLa, K (dB) -2,58 -1,69 -3,36 -2,43 -3,58 Frequency of the most prevalent tone (Hz) 1200 1200 1200 1200 530 5.4 Electrical Layout The main electrical layout of the wind turbine comprises a 2,3 MW induction generator feeding power through a full size quadrant frequency converter to a 2,6 MVA machine transformer (see figure 7.1) The generator is located in the wind turbine nacelle and the frequency converter and 0,69/33 kV transformer is located in the bottom of the turbine tower The 33 kV terminals of the transformer are connected to the marine cable system through a 33 KV switchgear also located in the bottom of the tower Gear box C G Frequency converter Figure 5.4 Electrical layout in Siemens 2,3 MW, MkII 65 (78) 5.4.1 Wind Turbine Transformer The turbine transformer is a two-winding, liquid-filled transformer manufactured by Pauwels in Belgium The transformer has a very compact design where the overall dimensions are kept to a minimum The design of the transformer allows it to be exchanged through the door in the turbine tower, see figure 5.5 The transformer is filled with silicon liquid with a fire point above 360 ºC, and the materials used are self-extinguishing Figure 5.5 The wind turbine transformer from Pauwels being lifted through the door Technical specifications transformer Manufacturer Pauwels, Belgium Type SLIM transformer Rated power 600 kVA Rated voltage (at no load) 33 / 0,69 kV Connections Delta / star Impedance voltage 6% No load loss 2,6 kW Full load loss (at 120ºC) 22,5 kW Cooling KNAN Dielectric / cooling medium Silicone liquid 66 (78) 5.4.2 Wind Turbine Generator The generator is a standard asynchronous machine with a squirrel cage rotor from ABB The generator is 4-pole and has a rated nominal power of 300 kW The generator is an IP 55, which is air-to-air cooled using a heat exchanger At one side the air is circulating through the generator and through heat exchanger where the heat is exchanged At the other side air is taken from outside underneath the nacelle through the heat exchanger and out through the rear end The air-to-air cooling means that the generator is not subjected to polluted air The airflow in the two systems is driven directly by the generator shaft in the non-drive-end, which makes it a reliable construction Technical specifications generator 5.4.3 Manufacturer ABB, Finland Type Asynchronous, squirrel cage Number of poles Rated power 300 kW Rated speed 1550 rpm Rated voltage 750 V Speed range 600 – 1800 rpm Frequency 16,5 – 60 Hz Enclosure IP 55 Generator weight 580 kg Wind Turbine Converter A 4-quadrant frequency converter from Alstom is used The design of the frequency converter is based on a parallel connection of IGBT-modules cooled by water Both the grid and the generator inverter have modules connected in parallel In order to improve the power quality, a filter is installed between the low-voltage circuit breaker and the grid inverter The filter is constructed by use of a main reactor and some additional capacitors and resistors The power factor is controlled by use of the frequency converter The 2,3 MW Mk II turbine will, according to standard, deliver a power factor equal to one At Lillgrund the power factor is kept equal to one at the onshore substation Bunkeflo This is achieved using a park pilot The park pilot continuously measures the power factor at Bunkeflo and control the power factor at each single turbine 67 (78) Technical specifications converter 5.5 Manufacturer Alstom Type 4Q full scale converter Switching PWM Parallel modules (grid inverter), (gen Inverter) Cooling Water PWM filter Installed Rated voltage 690 V (grid), 750 V (generator) Switching frequency (grid inv.) 500 Hz Switching frequency (gen inv.) 250 Hz Mechanical Layout The mechanical layout of the wind turbine nacelle is based on the state of the art layout used since the early days of modern wind turbine design 5.5.1 Rotor The rotor is a 3-bladed up-wind rotor A double row 4-point ball bearing is mounted on each blade root connecting the blade with the cast iron hub The blade is made of fibreglass-reinforced epoxy and is cast in one piece Each individual blade can be pitched in 80 degrees for shutdown purposes, allowing the rotor to idle at low speed, thereby avoiding stand-still marks in the main gearbox Figure 5.6 B45 blade for Siemens 2,3 MW Mk II 68 (78) Technical specifications blade 5.5.2 Manufacturer Siemens Wind Power Type Self-supporting Primary material Fibreglass-reinforced epoxy cast in one piece Weight Approximately 10,6 t Length 45 m Colour Leight grey RAL 7035 Profile FFA Transmission system The rotor hub is bolted to the main shaft flange The main shaft is forged in alloy steel and is hollow for transfer of power and signals to the blade pitching system The main shaft is supported by a self-aligning double spherical roller bearing at the rotor end and connected to the gearbox in the other end in a so-called 3-point suspension The coupling between the main shaft and the gearbox is a shrink disk design The gearbox is a three-stage planetary-helical design The first high torque stage is of helical planetary design and the medium and high-speed stages is of normal helical design A mechanical disk brake is mounted on the high-speed shaft of the gearbox between the gearbox and the generator The brake is used as a parking brake and emergency brake in certain situations The transmission system is mounted on a strong steel bedplate The bedplate position is controlled by electrical yawing motors/gears, securing that the nacelle is positioned correctly according to the actual wind direction 69 (78) Figure 5.7 Transmission system for Siemens 2,3 MW Mk II Technical specifications gearbox Manufacturer Winergy Type Combined planetary/helical, 3-stage P mech 2525 kW Oil volume Approx 400 l Ratio 1:91 Weight Approximately 23 t 70 (78) (PEAB 3356.2) 5.5.3 Tower The tower is a tapered tubular steel tower in two sections, bolted together with a flange connection The tower is fitted with a personnel hoist, capable of hoisting people at a time from the tower base to the nacelle Additionally, a ladder is mounted from tower entrance to top level just beneath the nacelle Interior layout of tower bottom section Figure 5.8 Interior layout of tower top section Tower sections interior layout MV transformer and switchgear are located on the first platform level Full-scale converter is located on second and personnel hoist is located on third In the top section (picture right) a connection box for power cables are located Hoisting of smaller components from ground to nacelle is done using the nacelle internal crane This can carry up to 250 kg If it is necessary to exchange larger components, a larger add-on crane can be mounted in the nacelle Otherwise a floating crane must be used The tower and nacelle is equipped with various health and safety equipment such as fire extinguishers, fire blanket, first aid equipment, eye flushing and emergency rescue equipment for lowering personnel from the nacelle on the outside or inside of the tower The tower, as well as the nacelle and rotor, are painted in a light grey colour (RAL 7035) 71 (78) 5.6 Lightning protection system Large wind turbines with total height of more than 100 m are highly susceptible to lightning strikes The lightning protection system of the Siemens 2,3 MW Mk.II wind turbine is designed according to IEC 61400-24 The fundamental design principle is to create a current path that can lead the lightning current to ground with minimum risk of damage to the structure and the electrical system The blades are provided with sets of lightning receptors One located near the blade tip and the other two further approximately and 16 meters towards the root The receptors are electrically connected to the hub, mainly through the blade bearing, and through carbon brushes to the nacelle steel structure From there, the current path follows the bronze brushes to the steel tower, down to the foundation armouring to be then grounded Figure 5.9 Lightning receptor in blade and location on a 40 m blade Using lightning rods etc also protects other exposed components on top of the nacelle, such as aviation lights and wind sensors 72 (78) 5.7 Coating The corrosion protection system includes two systems • External surface treatment • Internal surface treatment All external surfaces directly exposed to the environment, are protected according to EN ISO 12944-2 Class C5-M and internal surfaces, such as internal tower and components in the nacelle, are protected according to Class C4 Additionally, the tower and nacelle are equipped with dehumidifiers, keeping the relative humidity in these areas at approximately 40-50% This is considered to be sufficient to keep the tower and components protected against corrosion 5.8 SCADA The Lillgrund SCADA System is split in two separate systems Siemens SICAM PASS Supervision and control of the offshore substation are managed by the Siemens SICAM PASS system Predefined alarms from SICAM PASS, is forwarded to the WPS SCADA system, which is the primary system for operation of the wind power plant Operation of the SICAM PASS can only be done by Vattenfall, in contrary to the WPS system that can be operated by the WTG supplier as well Siemens WEB-WPS SCADA The supervision and control of the wind turbines and of the offshore meteorological station is managed by the Siemens WEB-WPS SCADA system, which is a standard system used by Siemens Wind Power at various other wind power plants The WPS SCADA system is an Internet-based control system and the main features comprise the following functions: • On-line supervision and control • Storage of almost unlimited amounts of historical data in database • Local storage at wind turbines and transfer to database if communication is interrupted • Remote system access from anywhere using a standard web browser • Assigned individual user names and passwords • E-mail function for fast alarm response • Grid measurement with designated Grid Code functions • Park pilot functions for enhanced control of the wind power plant and remote power regulation 73 (78) • Condition monitoring integrated with the turbine controller • Power curve plots and efficiency calculations • Utility interface • MW control, Voltage control, Frequency control, Ramp rate control etc Figure 5.10 WPS Single Line Diagram for Lillgrund Wind Power Plant Figure 5.11 WPS Park View for Lillgrund Wind Power Plant 74 (78) The communication network in the wind power plant is established with optical fibres included in the array and export marine power cables Figure 5.12 Lillgrund Ring Network 75 (78) COMMENTS AND CONCLUSIONS When writing this kind of report there is naturally a considerable focus on the project challenges and not enough on the overall success of the project The Lillgrund project is, most definitely, considered a success story, not only from a technical point of view but also from a social point of view The wind farm was constructed on time and has now been successfully operational since December 2007 The project team, composed by specialists from different parts of Sweden and Denmark, have truly lived up to the vision “One Vattenfall” There is however always potential for improvements and the aim of this report has been to determine and highlight these areas It is worth to pointing out that only the electrical system and the foundations are tailor made at an offshore wind power plant The wind turbines are more or less standard products with none or very limited possibilities for project specific design changes Geotechnical investigations are expensive and it can be difficult to balance the risks as well as the benefits of this expense in the early phases of a large infrastructure project As a whole, the geotechnical surveys at Lillgrund proved to be useful They identified potential issues, such as the fact that extra excavation was required for two of the foundations It also gave the location of a small number of boulders to be removed Vattenfall requested a complete study of the electrical system for Lillgrund to be delivered with the bids That request was not met Instead Siemens Wind Power began a complete electrical system study after being awarded the contract Consequently, the electrical system study was completed during the construction period causing the following difficulties: • The insulation coordination study showed that an increased insulation level and extra surge arresters were required in the main transformer Fortunately the insulation coordination was completed during the manufacturing of the main transformer and the changes could be performed at the factory • The switching transient study showed large transients occurring at the 130 kV busbar in Bunkeflo when the main circuit breaker for Lillgrund was switched on E.ON does not accept the occurrence of large transients Unfortunately the study had been completed after the ordering of the components for the new 130 kV bay At this stage it was not possible to change the circuit breaker design in order to avoid the transients and still keep the time schedule for the commissioning of the wind power plant This experience shows that it is vital to perform an electrical systems study in good time before the construction period begins In general, the working conditions at the Lillgrund site have been good However, the late autumn and winter 2006 combination of harsh winds and turbulent currents had made it impossible to perform the offshore work Situations like these need to be taken into consideration when writing the contract to ensure that the appointment of risk between owner and contractor is clearly defined Many minor problems and disputes with the contractors can be avoided if the owner has a site representative present on-site during the whole project This should be required for both the foundation, concrete or steel production site, as well as for the offshore work 76 (78) The foundation contractor and designer underestimated the reinforcement needed to fulfil the requirements from the agreed design code Experience from earlier projects designed after other codes were not valid It could be argued that the design requirements used are to rigorous, however since the criteria is a service life of 50 years the requirements are reasonable from a durability point of view Different kinds of cement can be used for the foundations If a long lifetime is required the choice of cement can be of importance An offshore wind power turbine is exposed to a high ratio of dynamic loads This means that fatigue in the reinforcement bars is the main design factor when determining the required amount of reinforcement Fatigue loads also indicate that there will be cracks on the concrete surface that open and close The cement type chosen influences how these cracks behave A Portland cement with a higher amount of alkali can make the cracks self heal, which is beneficial This characteristic is not present in cement with micro silica, which was the cement chosen for the Lillgrund project It is recommended that anodes are used as cathode protection system on all foundations, including the transformer station foundation The influence of the cable armouring should also be taken into consideration in the design Hand railings are preferably made of aluminium, as opposed to painted or galvanised carbon steel The need for Davit cranes should be carefully investigated for each project If the operation and maintenance crew does not require their use, they can be omitted If needed, it should be ensured that they have a locking device for the boom Boat landings should be as simple as possible, if ice is a problem, consider a solution where you accept that some of them disconnect during hard winters This might be the overall cheapest solution Cable laying should be avoided during wintertime At Lillgrund a propeller breakdown on the vessel resulted in the cable being placed on the seabed within 15 meters of the trench line After repair of the vessel and waiting for proper weather conditions, the cable was picked up from the seabed and re-laid in the trench During this delay of almost months the pre-excavated trench was partly backfilled by natural causes After re-laying the cable in the trench, water jetting had to be used to bring the cable to the bottom of the preexcavated trench 77 (78) REFERENCES [1] Barthelmie, R Wind resource at Lillgrund Risø-I-2339(EN) 18 April 2005 [2] Mann, J Extreme winds at Lillgrund Department of Wind Energy Research Center Risø DK-4000 Roskilde Denmark, 31 August 2001 [3] Törnkvist M Observed wind climate at Lillgrund Wind data statistics summary for the period: September 2003- January 2005 Vattenfall AB 2005-02-11 [4] Sloth P Hydrographic Conditions fưr Ưrestads Vindkraftpark, Sweden Final Report October 2001 DHI Water and Environment [5] Design of offshore wind turbine structures OS-J101 June 2004 Det Norske Veritas [6] A review of the Sacrificial Cathodic Protection System Design for an Offshore Wind Farm IACS Corrosion Engineering Ltd July 2007 07-130 Report 01 rev Attachement to communication form PH-0792 dated 2007-08-01 [7] Lillgrund wind farm, 145 kV Onshore Feeder Cable 145 Submarine Feeder Cable 36 kV Submarine Collection grid Cables, ABB, Ref 05-1115, Rev 5, Sept 2006 [8] Lillgrund Wind Power Plant, Detailed Design Specification 138/33 kV transformer, Siemens AG, Document G81050-U3801-R014-A, Dec 2006 [9] Affärsverket Svenska Kraftnäts föreskrifter och allmänna råd om driftsäkerhetsteknisk utformning av produktionsanläggningar, Svenska Kraftnät Regulation, SvKFS 2005:2, Dec 2005 (in Swedish) [10] Technical Specifications 2.3 MW MkII, Siemens Wind Power A/S, Document PG-R03-10-0000-0002-04, Aug 2005 78 (78)

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