technical description lillgrund wind power plant

Technical Description Lillgrund Wind Power Plant

Lillgrund Pilot Project

September 2008

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Type of document Document identification Rev. No. Report date Project No.

REPORT 2_1 LG Pilot Report 1.0 September 29, 2008 21858-1 Author Project name

Joakim Jeppsson Poul Erik Larsen Åke Larsson

Lillgrund Pilot Project

Customer Reviewed by

Approved by

Vattenfall Vindkraft AB

The Reference Group Distribution No. of pages No. of appendices

The Swedish Energy Agency 78 0

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 [email protected]. 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 do not necessarily reflect those of the Swedish Energy Agency or of Vattenfall.

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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, 7 km off the coast of Sweden and 7 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 4 to 8 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 Danish-German 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.

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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 2 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 pre-excavated trench.

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TABLE OF CONTENTS

1 INTRODUCTION................................................................................................... 6 1.1 Purpose ....................................................................................................... 6 1.2 Background and limitations.......................................................................... 6

2 GENERAL DESCRIPTION ................................................................................... 7 2.1 General ........................................................................................................ 7 2.2 Location ....................................................................................................... 7 2.3 Park layout................................................................................................... 8 2.4 Site conditions ............................................................................................. 8

2.4.1 General .......................................................................................... 8 2.4.2 Wind resources.............................................................................. 8 2.4.3 Water depth ................................................................................... 9 2.4.4 Wave conditions............................................................................. 9 2.4.5 Current ......................................................................................... 10 2.4.6 Ice ................................................................................................ 10 2.4.7 Other ............................................................................................ 10

2.5 Discussion ................................................................................................. 11 3 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

4 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

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4.5 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

4.6 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

4.7 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

4.8 Discussion ................................................................................................. 60 5 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

6 COMMENTS AND CONCLUSIONS ................................................................... 76 7 REFERENCES ................................................................................................... 78

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1 INTRODUCTION

1.1 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.

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2 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, 7 km off the coast of Sweden and 9 km off the coast of Denmark. The wind power plant is situated 7 km south of the Öresund bridge, which connects Copenhagen and Malmö.

Figure 2.1 Location of the Lillgrund offshore wind power plant.

9 km

10 km

14 km

7 km

7 km

Flin

trän

nan

Köpenhamn Saltholm

Malmö

Lernacken

Bunkeflostrand

Höllviken Skanör

Öresundsbron

Dragör

Klagshamn

Pepparholm

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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 W-01 in Figure 2.2. The turbines are connected to each other and to the substation through five radials as shown below.

Figure 2.2 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.

2.4 Site conditions

2.4.1 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).

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Figure 2.3 Wind frequency for Lillgrund offshore wind power plant.

2.4.3 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 Estimated water levels (m) from mean sea level [4]. Low-water level High-water level Return

period (year)

Skanör

Drogden

Skanör

Drogden

Köpenhamn

Drogden

Klagshamn

10 -1,25 -1,30 1,30 1,25 1,26 1,25 1,35 50 -1,40 -1,60 1,55 1,40 1,46 1,45 1,59

100 -1,45 -1,70 1,65 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 Design seabed level

Caisson bottom level

Excavation level

1 -4,7 -6,8 -7,1 2 -5,7 -7,8 -8,1 3 -6,7 -8,8 -9,1 4 -7,7 -9,8 -10,1 5 -8,7 -10,8 -11,1

2.4.4 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.

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2.4.5 Current

The speed of the current is very important for many offshore activities and must be specified correctly. At high currents, diving can be prohibited causing delays for the project. Diving work is often a critical part of the foundation work, the installation of secondary structures and cable laying.

2.4.6 Ice

Estimation of ice conditions is of importance for the design of the foundations. Ice conditions are also of importance from a maintenance point of view.

2.4.7 Other

During the construction work special consideration was taken with regards to the existing gas pipe located to the north of the site. Transport was prohibited over the gas pipe prolonging the transportation time for staff and supplies from Limhamn harbour. Positioning of the most northern foundations along with dredging in vicinity of the gas pipe was done with great care.

Figure 2.4 Marked area is the working area; the drawing also shows the existing gas pipe just north of the site and the existing optic cable passing through the working area.

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2.5 Discussion

In general, the working conditions at the Lillgrund site have been good. However, late autumn and winter 2006 and 2007 the combination of harsh winds and turbulent current directions made it impossible to perform the offshore work. Situations like these needed to be taken into consideration when writing the contract to ensure that the apportionment of risk between owner and contractor is clearly defined.

It has yet to be seen if the design of the wind power plant layout is optimal. There were a number of constraints, including the gas pipe, the proximity to the fairway for the ships through Öresund and proximity to the Danish border. The above-mentioned constraints combined with the constraints from the environmental permit with regards to total height of the turbines, have given rise to a layout where the wind turbines are situated in close proximity to each other. If turbines are placed too close to each other, park efficiency will be reduced.

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3 FOUNDATIONS

3.1 Technical specification

The foundations for the Lillgrund wind turbines are gravity based. They are made of reinforced concrete and filled with ballast to ensure they become heavy enough to withstand the overturning moment created from the turbine.

3.1.1 Geometry

Figure 3.1 shows the overall geometry and dimensions of the bottom slab. In Figure 3.2 a section of the foundation is shown along with the heights of the five different foundation types that were used. Finally Figure 3.3 shows how the foundation is placed in a pit excavated in the seabed, and how the erosion protection is solved.

Figure 3.1 The six-sided bottom slab of the foundations. The six pockets and the circular section in the shaft are filled with ballast. Units are in millimetres.

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Figure 3.2 Section of the foundation. Five different shaft heights were used to adjust the foundations for different water depths. Units are in millimetres.

Figure 3.3 Section showing dredging depth, ballast in the ballast pockets, ballast fill in the shaft

and scour protection.

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The weights of the foundations are shown in Table 3.1.

Table 3.1 Weight of the foundation with and without ballast. Type Weight without

ballast [tons] Weight Ballast [tons]

Total weight [tons]

1 1299 803 2102 2 1318 822 2140 3 1337 841 2178 4 1356 860 2216 5 1375 879 2254

3.2 Geotechnical investigations

The Danish Geotechnical Institute (GEO) performed the geotechnical investigations for Lillgrund in two phases. The first phase was carried out in autumn 2001 and the second phase was carried out in autumn 2003.

In 2002 a hydrographical survey and sub bottom seabed profile investigations were carried out and in the autumn 2005 a geophysical survey was carried out for the wind power plant area as well as the internal cable routes between the wind turbines.

3.2.1 Year 2001; Phase I Geotechnical investigation

In this phase the sea floor was inspected and seabed sediment samples were taken at ten locations. In seven of the locations soil/rock coring was done to 40 m below the sea floor and a geophysical profile was logged from the boreholes before they were grouted.

Static triaxial compression and oedometer tests were used to determine the strength and deformation properties of the core samples.

3.2.2 Year 2002, Hydrographical survey

Results from the hydrographical survey are presented in section 2.4, detailing information about water depth, wave conditions, currents and ice occurrence.

3.2.3 Year 2003, Phase II Geotechnical investigation

For this phase CPT’s (Cone Penetration Test) were carried out at three points at each of the 49 locations for the foundations. At five of the windmill locations, geotechnical boring/rock coring was carried out to a depth of 20 m below the seabed.

Laboratory testing of samples from the five locations was carried out in order to establish the material characteristics. In addition, strength and deformation tests, primarily static triaxial compression and shear box tests, were performed on the samples.

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3.2.4 Year 2005, Geophysical investigation

Seismic mapping was carried out in order to identify the depth to the bedrock and the thickness of the overlaying sediment. This was done for the all foundation locations as well as for the cabling routes between the foundations. An example of the results from these investigations is presented in Figure 3.4.

Figure 3.4 Depth to bedrock colour chart.

3.3 Certification of design

A consultant subcontracted by the foundation contractor carried out the design of the foundations. The foundation contractor was also responsible for the certification of the design and had therefore a contract with a certification body. It is standard procedure to use a certification body but the contract can be setup in different ways.

The above setup was not ideal. The certification body and the consultant doing the design were unable to prepare certified drawings prior to the start of the construction of the foundations in Poland. More than 40 of the 49 foundations were placed at Lillgrund before the design of the primary structure was certified. The certification of the secondary structures was delayed in a similar manner.

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The reason for the delay is not fully clear to Vattenfall. Vattenfall did not have any contract with the certifying body and was, because of the setup, not included in the correspondence between the designer and the certifying body. Two meetings have been held in order to discover what the reason for the delay was and why this situation developed. One issue identified was that a prolonged iteration took place; design proposals were sent back and forth between the designer and the certifying body. This indicates that the designer was not as skilled as required. It also became clear that the certifying body was not sufficiently staffed to handle this prolonged iteration phase.

One lesson learned is that the organisational setup, with one contract between the designer and the foundation contractor and another contract between the foundation contractor and the certifying body, needs to be revised. This set up meant that Vattenfall, as the owner, could not influence the process.

3.4 Design requirements

The design was carried out according to [5], “Design of offshore wind turbine structures, OS-J101, June 2004” from Det Norske Veritas. This project is one of the first to be designed according to this standard. The crack width requirements in combination with the fatigue criteria, proved to the hardest to fulfil.

3.5 Construction method of the main structure

3.5.1 Onshore

The foundations were cast directly on barges in Poland, see figure 3.5. Tower bolts (Figure 3.6), bolts for bollards and other secondary structures were cast into the foundation. After completion of four foundations, the maximum number per barge, the foundations were then towed to the Lillgrund site, see figure 3.7.

Figure 3.5 Form and reinforcement work, Swinoujscie Poland.

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Figure 3.6 Tower bolts to be cast in the concrete. Total height of the bolts is 1600 mm.

Figure 3.7 Barge with foundations leaving Poland.

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3.5.2 Offshore

During the casting in Poland, the dredging work had started at Lillgrund (Figure 3.8).

Figure 3.8 Dredging work at Lillgrund.

In the first phase the dredging was only carried out for the foundations. After dredging, a crushed bed of rock was placed to level the floor of the excavation. This was done using a centrepiece standing on the seabed. The centrepiece was long enough to reach above the water in order to facilitate the positioning. A steel frame was placed around the centrepiece, and this was carefully levelled horizontally. A vessel then delivered crushed bedrock into the excavated pit, where a diver levelled the crushed material using a beam connected to the centrepiece and dragged on top of the frame (Figure 3.9). No compacting of the material was carried out.

Centrepiece

Crushed bedrock

FrameBeam

Vessel with crushedbedrock

Figure 3.9 Outline of the work with the crushed bedrock.

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After finalising the dredging and placing the crushed bedrock in the excavated pit, the foundations were positioned using a crane barge see figure 3.10.

Figure 3.10 Positioning of foundations at the Lillgrund site.

After placing the foundations, dredging was started for the J-tube extensions. J-tube extensions are pipes mounted to the side of the foundations to provide access for the inter array cables. Figure 3.11 shows how the J-tubes are placed in the foundations and Figure 3.12 shows a J-tube extension lying on the quay.

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Figure 3.11 Drawing of a typical foundation showing the J-tube position as dashed line.

Figure 3.12 J-tube extension on the quay.

Next step was to place filter rock (see Figure 3.3 for the placement of filter rock). When the placement filter rock was finalised, J-tube extensions were fitted using divers. The next step was to place ballast rock, and armour rock I and II together with filter rock around the J-tube extensions. Thereafter, ballast fill was placed in the shaft of the foundation and a concrete slab was cast on the top of each foundation as shown in figure 3.2.

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Figure 3.13 shows the connection between the concrete slab cast after the ballast fill offshore and the shaft wall. Figure 3.13 also shows a cast in hang off flange. At the lower part of the slab, between the wall and the slab, there is an expanding water stop. On some foundations the water stop was placed close to the top surface of the concrete slab. When the water stop started to expand, as it should when subjected to water, the concrete cracked (Figure 3.14). The repair work had to be executed in hurry to avoid problems with the installation of the wind towers. It is suggested that more site inspections are carried out to verify the correct installation if this solution is used on future projects.

Figure 3.13 Connection between slab cast offshore and foundation shaft.

Figure 3.14 Damages due to expansion of the water stop.

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3.6 Foundation tower interface

The physical interface between the tower and the foundation is of course of outmost importance. If a mismatch occurs during the installation, with a fully loaded vessel, the costs for the delay that follows will be significant. The installation vessel can only unload in a specified sequence. If this is interrupted, the ship needs to seek harbour, and then unload and reload on quay.

To avoid this potential problem, narrow tolerances was set up for the tower bolts. This tolerance was however only defined and followed locally, i.e. for each bolt. A problem was discovered when it became clear that it was not sufficient even if the tolerance requirements for one singe bolt was fulfilled.

Before placing the tower on the foundation, shims are used to make sure that a horizontal surface is available, making the tower stand vertical, see Figure 3.15. When this was done it was discovered that the top surface of the foundation was so uneven that the bolts on the lower parts proved too short (Figure 3.16). A certain distance was required for the jacket used to pretension the bolts before fastening the washer. The situation was resolved by removing the concrete locally at the affected parts, see figure 3.17.

Figure 3.15 Shims placed on the foundations to achieve a horizontal surface for the tower.

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X ± y

Local tolerance

Global tolerance

To short

Figure 3.16 Sketch showing the problem with the tower bolt tolerances.

Figure 3.17 Solution of the problem: the careful observer can see that the concrete has been lowered locally give enough height above the shims.

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3.7 Discussions

Different kinds of cement can be used for the foundations. If a long lifetime is required the choice of cement can be of important. An offshore wind power turbine is exposed a high ratio of dynamic loads. This means that fatigue in the reinforcement bars is the main design factor when determining the appropriate 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. The characteristic is not present in cement with micro silica, which was the cement chosen for the Lillgrund project.

The contractor and designer underestimated the reinforcement needed to fulfil the requirements from the agreed design code. Experience from earlier projects designed after other codes could not be taken into consideration. It could be argued that the design requirements used are too rigorous, however the criteria is designed for an endurance of 50 years, so the requirements are reasonable from a durability point of view.

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 proved to be useful. They identified potential issues, such as the fact that extra excavation was required for two of the foundations. They also revealed the location of a small number of inconveniently positioned boulders that would have to be removed. Adjustments regarding the excavation levels had to be made on more than one foundation at Lillgrund. If unfortunate, the soil conditions could have been so bad that the turbines could not have been put in the chosen positions. The relocation of the turbines could be difficult since it may influence the efficiency of the surrounding turbines in a negative way. The economy of the project could then be in jeopardy.

Many minor problems and disputes with the contractor can be avoided if the owner has a site representative during the whole project. This is required both at the site for the production of the foundations, concrete or steel, and for the offshore work.

It is recommended that anodes be used as cathode protection system on all foundations, including the transformer station foundation, and that the influence of the cable armouring is 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 shall be carefully investigated for each project, if not needed by the operation and maintenance crew they can be omitted. If used, ensure that they have a locking device for the boom.


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4 ELECTRICAL SYSTEM

Lillgrund offshore wind power plant is installed in five internal 33 kV feeders. The feeders are connected to a 33 kV switchgear on the offshore substation. The offshore substation is connected through an offshore three-phase 130 kV cable and three single phase onshore 130 kV cables to E.ONs 130 kV station Bunkeflo close to Malmö.

4.1 Electrical system study

A complete review of the electrical system at Lillgrund was performed. This comprised 13 different studies listed below. These 13 studies settled the final design of the electrical system at Lillgrund.

• Cable sizing study. The cable sizing study was performed by the cable manufacturer ABB High Voltage Cables in Karlskrona, Sweden. The purpose of the study was to find an optimal design of cables with respect to current rating, heating and losses.

• Load flow study. The load flow study is comprised of steady state calculations based on cable data from the cable sizing study from ABB. The purpose of the study was to identify maximum currents, voltage variations and reactive power flow in the electrical system.

• Short circuit study. The objective of this study was to determine the maximum short circuit current stresses for all individual 138/33 kV components.

• Harmonics study. The harmonic model of Bunkeflo and the harmonic model for the wind turbines were used to calculate the harmonic distortion at the point of common connection (PCC) in Bunkeflo.

• Grounding study. A grounding study was provided for the Lillgrund offshore wind power plant. High voltage installations require a grounding system to protect human life against excessive touch voltages and to keep transferred potential to a minimum. The increase of fault currents to earth underlines the importance of grounding systems and the need for low resistance of the grid.

• Voltage fluctuation study. The purpose of the study was to analyse voltage fluctuations caused by switching operations and power variations occurring during normal operation.

• Reliability study. A reliability study of the offshore substation was performed in order to identify the availability and requirement for the availability of spare parts.

• Arc fault study. The objective of this study was to determine the maximum short circuit current stresses for all individual 138/33 kV components.

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• Dynamic simulation study. Each wind turbine is fitted with fault ride-through (FRT) capabilities and the wind park is fitted with a Pilot park controller (PPC) that controls the reactive power from the wind park. A dynamic study was performed by Siemens Wind Power A/S in Brande, Denmark, to verify the dynamic performance of the wind power plant

• Protection coordination study. The protection coordination study covers the protective relay settings and the protection concept for the 33 kV transmission system, the electrical equipment located at the offshore platform including the main transformer, all 33 kV cables and all 33 kV switchgear / auxiliary power components.

• Insulation coordination study. The objectives of this study are to identify maximum over voltages for all individual 138/33 kV components and if required modify the insulation and / or reduce excess voltages. The insulation coordination study showed that an increased insulation level and extra surge arresters were required in the main transformer. The insulation coordination study also showed that surge arresters on the 33 kV switch gear in all 48 wind turbines were required.

• Electromagnetic interference study. The study was produced in order to evaluate electromagnetic fields in and around the offshore substation with respect to human exposure. The report from the study provides a quantitative description of the levels of electromagnetic fields associated with the operation of the transformer station.

• Lightning protection study. A lightning protection study for the offshore substation was performed. Buildings and structures require a lightning protection system to protect people and structures against the negative effects of a lightning strike. The lightning protection system cannot prevent the lightning strike itself. It cannot guarantee the absolute protection to a structure or person. However the lightning protection system will significantly reduce the risk of damage caused by lightning.

Siemens AG, Power Transmission and Distribution, High Voltage Division in Erlangen, Germany performed all studies, except for the cable sizing study and the dynamic simulation study.

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4.2 130 kV system

4.2.1 General

Lillgrund offshore wind power plant is connected to E.ON’s 130 kV station Bunkeflo, near Malmö.

The 130 kV system is illustrated below in figure 4.1. The 130 kV system consists of a 130 kV bay at the onshore substation Bunkeflo, 2 km land cable and 7 km sea cable. At the offshore substation the sea cable is connected directly to the main transformer, i.e. there is no 130 kV circuit breaker or any other switchgear on the offshore substation.

Bunkeflo 130 kV

2 km land cable

7 km sea cable

Offshore substation

~

33 kV

0,4 kV

WT WT

WT WT

. .

.

Figure 4.1 Single line layout diagram of the electrical system at Lillgrund.

The 130 kV cable system generates approximately 10 MVAr reactive power. One requirement from E.ON is that the wind power plant keeps unity power factor at the point of common connection at Bunkeflo, e.g. the reactive power exchange shall be equal to zero. As can be seen in figure 3.1, Lillgrund does not have any reactor to absorb the reactive power. Instead, the reactive power at Lillgrund is controlled through the frequency converters in the wind turbines.

4.2.2 Onshore substation “Bunkeflo”

The main circuit breaker for Lillgrund is located at the onshore substation in Bunkeflo.

4.2.2.1 130 kV Circuit breaker

Figure 4.2 shows the new 130 kV bay for Lillgrund at Bunkeflo. Closest to the camera is the surge arresters followed by cable termination, voltage transformer, current transformer, grounding switch and finally the circuit breaker.

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Figure 4.2 The new 130 kV bay for Lillgrund.

4.2.2.2 Switching transients

During the construction period, a complete systems study (i.e. load-flow calculations, dynamic simulations and transient studies) were performed by Siemens AG in Erlangen, Germany. The switching transient study revealed some large transients occurring at the 130 kV busbar in Bunkeflo when the main circuit breaker for Lillgrund was switched on, see figure 4.3.

The measured voltage at the 130 kV busbar in Bunkeflo during direct switch of the circuit breaker is shown in figure 4.4. The measured voltage in figure 3.4 corresponds well to the simulated one in figure 4.3.

The switching transient causes an oscillation, which is composed of a high frequency voltage (approximately 650 Hz) overlapping the fundamental voltage (50 Hz). The oscillation emanates from the capacitance in the 130 kV cable and the inductance in the main transformer.

The grid operator, E.ON, could not accept this since the multiple zero crossings caused by the transients are likely to disturb other customers. This forced E.ON to take special precautions when connecting Lillgrund to the grid. E.ON created a separate 130 kV line to the 400 kV transmission line for Lillgrund by switching all other loads to other 130 kV lines.

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SIEMENS AG, PTD H

Produced with PSS/NETOMAC(R) PSS/NETOMAC is a registered trade-mark of Siemens AGLILLGRUND NON-SYNCHRONOUS SWITCHING, WORST CASE SWITCHING TIME;

BASIC CASE 1: 138 kV CABLE & MAIN TRANSFORMER ENERGIZATION.VOLTAGE VARIATION STUDY DUE TO ENERGIZATIONLILLGRUND WIND POWER PLANT TRANSIENT STUDY Nov.2006 BY Y. LU, PTD H 111

1

RMS Voltage and Variation at Bunkeflo 138 kV

0 25 50 75 100 [ms]

-0.1

0

0.1VARIATION Urms of BUNKEFLO (pu)Phase R

-200

0

200Voltage of BUNKEFLO (kV) Phase R

-2

0

2FLUX OF THE MAIN TRANFORME PHASE R (p.u.)

-3

0

3TRAFO INRUSH CURRENT R (kA)

-200

0

200Voltage of MAINTRANSFORMER HV R (kV)

-0.1

0

0.1

VARIATION Urms of BUNKEFLO (pu)Phase S

-200

0

200

Voltage of BUNKEFLO (kV) Phase S

-2

0

2

FLUX OF THE MAIN TRANFORME PHASE S (p.u.)

-3

0

3

TRAFO INRUSH CURRENT S (kA)

-200

0

200

Voltage of MAINTRANSFORMER HV S (kV)

-0.1

0

0.1VARIATION U4ms of BUNKEFLO (pu)Phase T

-200

0

200Voltage of BUNKEFLO (kV) Phase T

-2

0

2FLUX OF THE MAIN TRANFORME PHASE T (p.u.)

-3

0

3TRAFO INRUSH CURRENT T (kA)

-200

0

200Voltage of MAINTRANSFORMER HV T (kV)

Figure 4.3 Simulation of direct switch of the 130 kV circuit breaker. The second graph from the top shows the voltage at the busbars at Bunkeflo.

Figure 4.4 Measured voltage at the 130 kV busbar in

Bunkeflo during direct switch of the circuit breaker.

This specific operational mode causes limitations for Vattenfall. Lillgrund can not be switched on during day time, only during low load periods (typically late evenings and nights) when E.ON can switch all other customers to other lines.

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4.2.3 Techniques to reduce switching transients

Vattenfall and Siemens have investigated point-of-way switching and the use of a pre-insertion resistor to mitigate the transients.

Point-of-way switching is a method used to reduce transients during switching capacitive or inductive loads by using a synchronized connection. The technique is based on controlling each of the three poles in the circuit breaker individually and to close each pole with a certain time delay. Different time delays are used depending on whether the load is capacitive or inductive.

In the specific case of Lillgrund, the load is both capacitive (the 130 kV cable) and inductive (the main transformer). Simulations showed that it was most difficult to derive suitable synchronization parameters and this method was abandoned.

An alternative solution to reduce transients is to use pre-insertion resistors. Simulations showed that use of pre-insertion resistors is a successful method to reduced transients. Figure 4.5 show a single line diagram of a switchgear equipped with a pre-insertion resistor.

130 kV busbar Bunkeflo

130 kV cable to Lillgrund

R

B A

Figure 4.5 Single line diagram of a switchgear equipped with a pre-insertion resistor.

As can be seen in figure 4.5 the switchgear with pre-insertion resistor consists of two circuit breakers and a resistor. At the moment when the switchgear is switched ON, the circuit breaker “A” is closed and the inrush current is limited by the resistor “R”. After a few line periods circuit breaker “B” is closed, completing the switching by short-circuiting the resistor.

At the time when the switching transients were identified, the components for the new 130 kV bay was already ordered. Due to long delivery time for components like 130 kV circuit breakers it was at that stage in the project not possible to re-design the 130 kV bay and still keep the project main time schedule. Hence the 130 kV bay was build with one conventional 130 kV circuit breaker.

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4.2.3.1 Transient Fault Recorder

An ION 7650 is installed at the onshore substation “Bunkeflo”. The instrument fulfils three purposes; (i) active and reactive power measurements at the grid connection point, (ii) Power Quality measurements and (iii) TFR (Transient Fault Recorder). The instrument is fed with voltage and current signals from CT:s and VT:s in the 130 kV bay for Lillgrund.

The reactive power measurements performed by the instrument is used for generating feedback signals to the regulating loop in the Park Pilot, which in turn controls the reactive power from the 48 wind turbines. The reactive power is kept equal to zero (unity power factor) at the “Bunkeflo” grid connection point.

The instrument provides instantaneous 3-phase voltage, current and frequency measures. It measures energy in all four quadrants with an accuracy exceeding Class 0,2. The instrument measures individual harmonics up to the 63rd and a transient detection up to 20 μs. Power Quality is measured in accordance with IEC 50 160. The TFR is event driven by set points and can sample up to 512 samples/cycle and maximum 96 cycles, i.e. 512 samples per cycle for 4 cycles, 32 samples per cycle for 54 cycles or 16 samples per cycle for 96 cycles.

Manufacturer Power Measurements

Type ION 7650

Voltage inputs 4

Current inputs 5

Digital inputs 8

Voltage accuracy 0,1 %

Current accuracy 0,1 %

kW, kVAr, kVA accurracy Class 0,2

kWh, kVArh, kVAh accurracy Class 0,2

4.2.4 130 kV Onshore cable

The 130 kV onshore cable, consisting of three single-phase cables and one fibre optical cable, is buried approximately one meter below ground. The 130 kV onshore cable was manufactured by ABB in Karlskrona. The cable route is located close to existing roads.

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4.2.4.1 Technical data

The structure and the technical specifications for the 130 kV land cable [7] can be found below.

Manufacturer ABB HVC in Karlskrona, Sweden

Type AXLJ 1x630 mm2

Rated voltage 145 kV (Umax)

Conductor material Aluminium

Conductor cross-section 630 mm2

Shield 95 mm2 Cu (wires)

Cable diameter 74 mm

Weight 5,9 kg/m

Length of route Approximately 2 km

Construction period February to April 2007

4.2.4.2 Cable laying

One of the environmental requirements from the authorities was that the magnetic field from the cable should be less than 0,2 μT. In order to limit the magnetic field the three single-phase cables are placed in a triangular formation, see figure 4.6.

Figure 4.6 The magnetic field is limited to less than 0,2 μT by placing the three single-phase cables in a triangular formation.

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4.2.5 130 kV Sea cable

The 130 kV sea cable from the offshore substation to shore was manufactured by ABB in Karlskrona, Sweden. The cable is a three-core-copper-conductor with a lead screen around each conductor to prevent water penetration. A fibre optical cable is integrated into the cable as shown in the figure below.

4.2.5.1 Technical specification

The structure and the technical specifications of the 130 kV sea cable [7] can be found below.


Type FXBTV 3x400 mm2


Conductor material Copper


Fibre optical cable 48 fibres


Weight 56 kg/m

Length of route Approximately 7 km

Embedding depth 1 m

Laying vessel Nautilus Maxi

Construction period December 2006 to July 2007

4.2.5.2 Cable excavation

The 130 kV sea cable was placed in a pre-excavated trench. The dredger GRÄVLINGEN, which is equipped with a stationary backhoe excavator, performed the excavation, see figure 4.7.

GRÄVLINGEN has three spuds but no propulsion. It is towed to the trench location by a tugboat. When in position, the dredger is anchored using the spuds. The dredger is moved around the site by lifting the two spuds in the stern. It moves forward with the bucket while the spud in the bow is inclined. The two spuds in the stern are lowered into the new position and the spud in the bow is changed to vertical mode.

The depth of the pre-excavated trench was approximately one meter below the existing seabed level and had a width of approximately 1,5 meters at the bottom. In order to ease the backfill operation the excavated material was placed to one side of the trench only.

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Figure 4.7 Dredger GRÄVLINGEN used for pre-excavation of export cable trench.


The 130 kV sea cable laying took place on December 18, 2006, and was performed with the cable laying vessel NAUTILUS MAXI. According to the plan, the laying was due to start with the landfall of the export cable, immediately followed by the placing of the cable into the pre-excavated trench all the way to the offshore substation.

The 130 kV sea cable was loaded from the turntable at ABBs cable factory in Karlskrona onto the turntable onboard NAUTILUS MAXI. After arrival in Öresund the laying started by bringing the cable to shore. Close to shore cable floaters (air bags) were used to float the cable, see figure 4.8.

Figure 4.8 NAUTILUS MAXI with the first end of the cable on floaters.

The floating cable was pulled to shore using a small boat and two excavators, see figure 4.9.

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Figure 4.9 Onshore landing of the 130 kV sea cable.

The landfall operation was a success but the cable laying operation initially failed due to propeller breakdown on NAUTILUS MAXI. The vessel hit boulders on the seabed and got stuck a number of times during the first 500m. The stern’s starboard thruster did not work properly as its output was too weak. It is believed that stones had been sucked into the tunnel, damaging the screws.

The laying had to be continued with a tugboat to control the forward movement whilst controlling the lateral movement with the vessel thrusters. After a short distance the stern’s starboard thruster broke down completely, losing the ability to manoeuvre the vessel in line with the trench. This resulted in the cable being placed on the seabed within 15 meters of the trench line using only the tugboat.

After repair in a dry dock at the shipyard in Landskrona (approximately 4 hours from site) NAUTILUS MAXI was ready to pick up the cable from the seabed and re-lay it in the trench. Unfortunately bad weather forced NAUTILUS MAXI to remain in the harbour, resulting in the export cable being re-laid on February 10th, 2007, a delay of almost 2 months, which allowed the pre-excavated trench to partly backfill by natural causes.

After re-laying the cable, water jetting was used to bring the cable to the bottom of the pre-excavated trench.

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4.3 Offshore substation

4.3.1 General

The offshore substation at Lillgrund is designed to visually harmonize with the marine environment. Its cylindrical shape with a glass façade is reminiscent of a lighthouse, see figure 4.10. The cylindrical substation has a diameter of 22 meters and reaches approximately 25 meters above sea level.

Figure 4.10 Offshore substation at Lillgrund.

Figure 4.11 shows installation of the substation. The total weight is approximately 520 tons.

Figure 4.11 Offshore substation at Lillgrund during installation.

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The offshore substation is designed by ISC A/S in Copenhagen. It is built on three floors with the main transformer placed in the middle, see figure 4.12.

Figure 4.12 Layout of the offshore substation.

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The first floor of the offshore substation is an open cable deck. Second floor, denoted the lower deck, contains the transformer in the middle, a room for the 33 kV switchgear and a room containing the transformer for the local power supply and an emergency diesel generator, see figure 4.13.

Figure 4.13 Layout of the lower deck of the offshore substation. Main transformer in the middle,

33 kV switchgear to the right, transformer for local power supply and emergency diesel generator to the left.

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Third floor, denoted upper deck, has rooms for UPS (the battery back-up system), and the control and monitoring equipment in addition to room for the main transformer, see figure 4.14.

Figure 4.14 Layout of the upper deck at the offshore substation. Main transformer in the middle,

control equipment to the left and the battery backup system and batteries to the right.

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Bladth Industries A/S in Aalborg, Denmark, was contracted by Siemens Wind Power to construct the offshore substation. Bladth, in turn, subcontracted Mostostal-Chojnice in Poland for steelwork and painting. Figure 4.15 shows welding during manufacturing at the factory in Poland and the painted steel construction loaded on a barge ready for transportation to Aalborg.

Figure 4.15 The supporting steel structure during welding at the factory in Poland to the left and ready for transportation to Aalborg to the right.

The offshore substation was transported on a barge from the steelworks in Poland to Bladth Industries A/S Aalborg.

The offshore substation was completed in Aalborg where all electrical equipment was installed and tested. Several subcontractors to Siemens Wind Power were involved in the installation of electrical equipment. Siemens AG in Dresden installed the main transformer. Siemens Ballerup delivered and assembled all MV equipment and Semco Maritime A/S did all the installation work.

Figure 4.16 shows installation work at the Bladth Industries A/S factory in Aalborg. The left picture shows installation of the 120 MVA main transformer and the right picture shows installation of the glass façade.

Figure 4.16 Installation of equipment at Bladth Industries factory in Aalborg.

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4.3.2 Electrical system

The electrical layout of the offshore substation is shown in figure 4.17.

Figure 4.17 Electrical layout of the offshore substation.

The offshore substation basically comprises the following electrical systems:

• 138/33 kV main transformer, 120 MVA, with tap changer

• 33 kV switchgear for each feeder and the local power supply

• 33 kV/0.4 kV transformer for local power supply, 150 kVA

• 0.4 kV switchgear system for local power supply

• Emergency diesel for back-up, 110 kVA

• Control/monitoring system

• Mechanical vibration protective device (which trips all electrical equipment in case of a ship collision)

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4.4 Main transformer

4.4.1 General

The transformer is a two winding, three phase, oil immersed, 120 MVA, with natural oil and natural air-cooling.

The transformer is manufactured by Siemens AG in Dresden. Figure 4.18 shows the transformer during FAT (Factory Acceptance Test) at the factory.

The main transformer is placed indoors at the offshore substation and is cooled by ambient air. The transformer is protected against corrosion by special paint.

The main transformer at Lillgrund is a key-component since it is the only transformer and it has no back up. Having only one main transformer represents a project risk since the wind power plant will be disconnected for months in case of a transformer break down. In order to reduce the project risk, emphasis has been put on extended factory acceptance tests and an intelligent fault monitor providing early warning in case of fault conditions that could lead to transformer failure.

Figure 4.18 Main transformer during FAT at Siemens factory in Dresden.

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4.4.2 Technical data

Some technical specifications for the main transformer [8] can be found below.

4.4.2.1 General

Manufacturer Siemens Dresden, Germany

Transformer type 2 winding, three phase, oil immersed

Type of cooling ONAN

Vector group YNd5

On load tap changer Yes

No load voltage ratio 138kV –8/+16% in 19 steps / 33 kV

4.4.2.2 Nominal Data

Rated nominal voltage, HV 138 kV

Rated nominal voltage, LV 33 kV

Rated frequency 50 Hz

Rated power 120 MVA

4.4.3 Gas-in-oil transmitter

Except from standard devices for protection, supervision and control the transformer is also equipped with a gas-in-oil transmitter, se figure 4.19.

The gas-in-oil transmitter, Hydran M2 from GE, is an intelligent fault monitor that reads composite values of gases, in parts per million (ppm). The gases are generated by faults and the composition of the gas will indicate the type of fault. It is an early warning device that will alert personnel to developing fault conditions that could lead to transformer failure and unscheduled outages.

The gas-in-oil transmitter measures the following components in the transformer oil:

• relative humidity (%)

• hydrogene

• carbone monoxide

• acetylene

• ethylene

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Figure 4.19 Gas-in-oil transmitter.

4.4.4 Oil collector

An oil collector is placed below the main transformer in accordance with the same principles, which govern onshore transformers. In case of oil leakage the oil will be conveyed to a 40 m3 storage tank inside the gravity foundation.

4.4.5 FAT

The following factory acceptance tests were performed on the transformer:

• Lightning impulse test

• "Visual inspection" (very carefully done with an instrument to measure the thickness of the corrosion protection)

• Measuring of zero-sequence impedance

• Measure at no load operation and with the transformer short circuit on the low voltage windings

• FRA (Frequency Response Analysis), power factor and tangents delta (extremely valuable to have a "finger print" of the transformer in case of malfunction)

• PD-tests (if the transformer for some reason has PD from the beginning it will probably end up with real problems after some years of operation)

• Heat test (verification that the transformer is able to handle full load)

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4.5 Internal grid

4.5.1 General

The internal grid at Lillgrund consists of approximately 22 km of 33 kV sea cables divided in five feeders. Each feeder connects 9 or 10 wind turbines to the offshore substation, see figure 4.20. As can be seen in the figure, feeder 2, 4 and 5 connect 10 wind turbines each to the substation while feeder 1 and 3 connect 9 wind turbines each.

W-01

A-01

A-02

A-03

A-04

A-05

A-06

A-07B-07

B-08

B-01C-01D-01E-01

B-02C-02D-02E-02F-02G-02H-02

B-03C-03D-03E-03F-03G-03H-03

B-04C-04D-04E-04F-04G-04H-04

B-05F-05G-05

F-06

C-05

D-06E-06

E-07 D-07

B-06C-06

C-07

C-08D-08

Feeder 1

Feeder 2

Feeder 3

Feeder 4

Feeder 5

W-01

A-01

A-02

A-03

A-04

A-05

A-06

A-07B-07

B-08

B-01C-01D-01E-01

B-02C-02D-02E-02F-02G-02H-02

B-03C-03D-03E-03F-03G-03H-03

B-04C-04D-04E-04F-04G-04H-04

B-05F-05G-05

F-06

C-05

D-06E-06

E-07 D-07

B-06C-06

C-07

C-08D-08

Feeder 1

Feeder 2

Feeder 3

Feeder 4

Feeder 5

Figure 4.20 33 kV internal grid at Lillgrund.

4.5.2 33 kV sea-cables

The 33 kV sea cable inside the wind power plant (the inter array cable), was manufactured by ABB in Karlskrona, Sweden. The sea cable is a three-core-copper-conductor with a lead screen around each conductor to prevent water penetration. Fibre optic cables for

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communication are integrated in the cable. Three different conductor cross-sections are used; 95 mm2, 185 mm2 and 240 mm2, see figure 4.21.

As can be seen in figure 4.23, the sea cables between the first 6 wind turbines at the end of a feeder have a cross-section of 95 mm2. The sea cables between wind turbine number 7 to 9 has a cross-section of 185 mm2. Finally, the sea cable between wind turbine number 10 and the offshore substation in feeder 2, 4 and 5 has a cross-section of 240 mm2.

Figure 4.21 Different cross-sections in the 33 kV sea cables in the internal grid.

The use of a cross-section of 240 mm2 between wind turbine number 10 and the offshore substation is due to the mutual heating of the cables inside the gravity foundation. All five feeders and the export cable pass through the gravity foundation for the offshore substation. The mutual heating from the cables limit their capacity to transmit power.

4.5.2.1 Technical data

The structure and the technical specifications of the 240 mm2 33 kV sea cable [7] can be found below. The technical specifications of the sea cables with cross-sections of 95 mm2 and 185 mm2 are similar.


Type FXBTV 3x240 mm2


Conductor material Copper


Fibre optical cable 48 fibres


Weight 22 kg/m

Length Approximately 22 km

Embedding depth 1 m

Laying vessel Pleijel

Construction period May 2007 to July 2007

G G G

33/138 kV

95 mm2 185 mm2 240 mm2

1-6 WT 7-9 WT 10 WT

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The inter array cables are, just like the export cable, buried approximately one meter below the seabed. The cable laying works for the inter array cables are similar to the works for the export cable and cover the following working operations:

• Survey of the cable route (also denoted in-survey)

• Pre-excavation of cable tray

• Pre-lay survey (survey of the pre-excavated cable trench)

• Cable laying (i.e. pull-in of the cable into one foundation, cable laying in the pre-excavated trench between two foundations and pull-in into the second foundation)

• Post-lay inspection

• Backfilling

• Survey as-backfilled (also denoted out-survey)

Survey of the cable route Before trench work started, a survey of the presumed cable route between the wind turbines (or foundations) was carried out. The purpose of the survey was to document any excessive amounts of boulders or other obstacles e.g. wrecks, cables, pipes etc. existing along the seabed of the cable route which might interfere with the dredging or the cable laying. The survey also constitutes a reference for the backfill works.

The survey was performed by the marine survey vessel M/V SOUND SEEKER, equipped with multibeam echo sounding equipment. The vessel is positioned by the use of DGPS working in RTK mode. The area covered by the survey was approximately 30 meters. Figure 4.22 shows the in-survey between foundations C04 and C05.

Figure 4.22 In-survey of the cable route between foundations C04 and C05.

Pre-excavation of cable tray The pre-excavation for the inter array cables was performed by the dredger GRÄVLINGEN in the same manner as for the export cable, see figure 4.9. The depth of the pre-excavated trench was approximately one meter below the existing seabed level with a width of approximately 1,5 meters at the bottom. In order to ease the backfill operation the excavated material was placed to one side of the trench only.

Pre-lay survey Before cable laying was allowed to start the minimum depth of the trench had to be verified. It is vital to ensure that no foreign objects are located at the bottom of the trench. If any obstacles are found during the survey they are removed prior to cable laying.

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The cable laying was allowed to commence as soon Vattenfall had accepted the pre-excavated trench.

The pre-lay survey was performed using the same multibeam equipment in the same manner as described in 6.2.2.2 Survey of cable route. Figure 4.23 shows the pre-lay survey between foundations C04 and C05. The upper graph in the figure shows the location (route) of the trench between the two foundations. The lower graph in the figure shows the trench depth, where the thin grey line is the seabed, the thick black line is the target depth one meter below seabed and the red line is the excavated trench depth.

Figure 4.23 Pre-lay survey of the cable route between foundations C04 and C05. The upper graph shows the trench route between the two foundations and the lower graph shows the trench depth (seabed = thin grey line, target depth one meter below seabed = thick black line and actual excavated trench depth = red line)

Cable laying The inter array cable laying took place between May and July 2007, and was performed with the cable laying vessel C/S PLEIJEL, see figure 4.24.

Figure 4.24 Cable laying vessel C/S PLEIJEL.

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Inter array cable installation between two foundations start with the pull-in of the cable at the first foundation. This was performed as follows:

• An hydraulic winch fixed to a three legged structure was mounted on the first foundation

• The winch wire was attached to the guideline wire pre-installed through the J-tube

• C/S PLEIJEL was positioned approximately 50 meters from the first foundation

• With assistance from a diver the winch wire from the foundation was pulled onboard C/S PLEIJEL and connected to the cable end

• The cable end was pulled into the J-tube and through the foundation by use of the winch

• Cable installation with C/S PLEIJEL at low speed along the pre-excavated trench under inspection of a diver following at a secure distance and reporting the progress of the cable laying

• C/S PLEIJEL anchored approximately 30-40 meters from the second foundation and the other cable end was pulled-in in a similar way as for the first foundation. Figure 4.25 shows an inter array cable installed in a foundation.

Figure 4.25 Inter array cable installed in a foundation.

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Post-lay inspection Post-lay inspection was executed directly after the cable installation to ensure that the cable was located at the bottom of the trench. The inspection was performed by a diver measuring the difference in the level between the seabed and the top of the cable, by use of a digital pressure gauge (tolerance less than ± 2 cm). The positions of the measurements were documented by reading the DGPS-position of the diver vessel, corrected by the length of the umbilical used by the diver.

The measurements were taken at least once every 100 meters of the trench.

Backfilling Backfilling was performed with M/V GRÄVLING. The previously excavated material, piled beside the trench, was backfilled into the trench. It was essential that the seabed was restored, as closely as possible, to its original shape.

Survey as-backfilled On completion of the backfilling a final inspection was carried out using the same vessel and multibeam equipment as in the survey prior to trenching.

This survey (out-survey) was compared to the survey performed prior to trenching (in-survey) to ensure that the seabed is re-established. Figure 4.26 shows the out-survey between foundations C04 and C05.

The seabed in figure 4.28 is coloured in order to visualize the shape of the seabed. The green fields indicate that the seabed surface is within a tolerance of ±0,3 metre. At grey fields the seabed surface is within a tolerance of -0,6 metre and yellow fields within a tolerance of +0,6 metre.

On the left hand side in the figure it can be seen that the backfilling was not sufficient and that backfill material is missing. The grey field in the last part of the cable trench near the foundation clearly shows where that backfill material is missing.

Figure 4.26 Out-survey of the cable route between foundations C04 and C05.

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4.5.3 J-tubes

J-tubes for the cables, made of 400 mm HDPE (High Density PolyEthylene) pipes, are cast into the concrete in the gravity foundations, see figure 4.27.

J-tube Bellmouth

Figure 4.27 Outline of a J-tube in a gravity foundation.

The J-tube emerges from the foundation approximately one meter below the seabed and has an extension with a “bell-mouth” at the end. The purpose of the extension is to protect the cable from getting damaged by the rocks around the foundation (the scour protection). The purpose of the “bell-mouth” is to avoid the cable from chafing against the pipe end during pull-in.

4.5.4 33 kV Switchgear

The 33 kV switchgear for the internal grid is located on the offshore substation. The 33 kV switchgear is a Siemens SF6 insulated switchgear type NXPLUS, see figure 4.28.

The 33 kV switchgear has 9 panels of which:

• 5 panels type NXPLUS for the cable feeder with line protection relay 7SJ63

• 1 panel type NXPLUS for 33/138 kV transformer with protection relay 7SJ63

• 1 panel type NXPLUS for 33/0,4 kV transformer with protection relay 7UT61

• 1 panel type NXPLUS for busbar grounding

• 1 panel type NXPLUS for busbar voltage measuring

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Each switchgear panel consists of the following main components:

• Busbar module with three-position switch

• Busbar module with disconnector

• Circuit-breaker module with vacuum circuit-breaker

• Module couplings

• Cable connection compartment

• Low-voltage compartment

• Panel enclosure

Circuit-breaker modules and busbar modules have hermetically welded stainless-steel enclosures.

Figure 4.28 33 kV switchgear at the offshore substation.

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4.5.4.1 Technical data NXPLUS

Technical data for the NXPLUS switchgear can be found below.

Rated voltage 36 kV

Operating voltage 33 kV

Rated short-duration withstand voltage 70 kV

Rated short circuit breaker current 20 kA

Rated short circuit duration 3 s.

Rated normal current at busbar 2 500 A

Degree of protection, primary parts IP 65

4.6 System grounding

4.6.1 General

Each of the gravity foundations constitutes its own grounding plate and all foundations are electrically connected to each other by means of cable armouring. The 49 gravity foundations combine to form the grounding system for the Lillgrund wind power plant, see figure 4.29.

Figure 4.29 All foundations at Lillgrund are electrically connected to each other by means of cable armour.

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4.6.2 130 kV system

The 130 kV system has a low impedance zero-point grounding in the main transformer, see figure 4.30.

Figure 4.30 Single line diagram of the 130 kV grounding.

The offshore substation is located approximately 200 metres from a gas-pipe, which runs from Denmark to Sweden in the Öresund sea. It is vital that any grounding faults on the 130 kV system are prevented from causing voltage rise, which could affect the gas pipe. Since the short circuit power on the 130 kV system is high, and this can cause high earth fault currents through the cabling, the grounding resistance must be kept low.

4.6.3 33 kV system

The 33 kV system at Lillgrund is grounded through a resistor connected to the 33 kV neutral star terminal of the 150 kVA auxiliary power transformer, see figure 4.31. The 138/33 kV main transformer has a delta winding on the 33 kV side. Therefore the neutral resistor cannot be connected to this transformer.

The 67 ohms neutral grounding resistors limit the maximum earth fault current to 300 A rms. The maximum permissible protective time is 1.5 seconds based on 100 % energy safety margin of 3 seconds and with a total impulse rating of 18.1 MJ.

Figure 4.31 Single line diagram of the 33 kV grounding resistor.

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4.6.4 Wind Turbines

The wind turbines at Lillgrund are equipped with a DYn 33/0,69 kV transformer. The low voltage system of 0,69 kV has a low impedance zero-point grounding in the transformer, see figure 4.32.

~ ~

IM

0.69/33 kV

Figure 4.32 Single line diagram of wind turbine.

4.6.5 Energization

At the time when the offshore substation was going to be energized, work on the 33 kV feeders was still in progress. None of the five feeders at the offshore substation had been installed and there was no electrical connection between the offshore substation foundation and the turbine foundations. This meant that the grounding system was limited to one foundation instead of 49, which increased the grounding resistance at the offshore substation.

In order to allow energization of the 130 kV cable to the offshore substation, Elsäkerhetsverket (Swedish authority for electrical safety) requested that Vattenfall perform measurements of the earth resistance in the grounding system.

The earth resistance was measured using a 40 Hz alternating current of approximately 40 A, that was injected into the 130 kV cable at the onshore substation at Bunkeflo. The 130 kV cable was, at the same time, connected to ground at the offshore substation, forcing the current through the foundation down to ground. Measurements were then carried out to determine the voltage potential on the gas-pipe as well as on the water surrounding the offshore substation foundation.

The measurements showed that the grounding resistance at Lillgrund is very low and that the offshore substation foundation alone provides sufficient grounding.

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4.7 Relay protection

4.7.1 General

Figure 4.33 shows a block diagram of the relay protection of the 130 kV and 33 kV systems.

The general protection philosophy for Lillgrund is as follows:

• The export cable from Bunkeflo to the main transformer at the offshore substation is protected with line differential protection and an over current protection as back up.

• The transformer differential protection relay will clear all faults inside the differential protection area. The additionally installed over current protection relays operate as back-up protection.

• Offshore substation 33 kV busbar faults will be cleared by the 33 kV transformer feeder over-current protection.

• 33 kV cable faults will be cleared by feeder protection (feeder 1 to feeder 5).

• 33 kV internal wind turbine faults such as cable faults or transformer faults will be cleared by the internal wind turbine protection scheme and the internal 33 kV circuit breaker.

• 0.69 kV internal wind turbine faults will be cleared by the internal wind turbine protection and the internal 690 V circuit breaker.

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Figure 4.33 Block diagram of the relay protection of the 130 kV and 33 kV systems.

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4.7.2 130 kV system

4.7.2.1 Main protection export cable

The 130 kV export cable from Bunkeflo to the offshore substation is equipped with:

• line differential protection

• over current protection.

The line differential protection relay uses the fibre optics in the export cable for communication purposes. The line differential protection relay will trip the 130 kV circuit breaker at Bunkeflo and the 33 kV busbar circuit breaker.

The over current protection relay will trip the 130 kV circuit breaker at Bunkeflo.

4.7.2.2 Back-up protection for the export cable

E.ONs relay protection for the out-going 130 kV bay for Lillgrund act as a back-up for Lillgrund’s protection relays. E.ONs 130 kV bay protection consists of:

• zero sequence voltage protection

• over current protection

• distance protection

All three protection relays will trip the 130 kV circuit breaker at Bunkeflo.

4.7.2.3 Main transformer protection

The 138/33 kV, 120 MVA, main transformer is equipped with:

• transformer differential protection

• restricted ground fault protection

• Buchholz relay

• oil temperature protection

The protection relays will trip the 130 kV circuit breaker at Bunkeflo and the 33 kV busbar circuit breaker.

In addition, the main transformer is also equipped with:

• ground fault protection

• pressure rise protection

These two protection relays are being fed from another source than the rest of the protection relays (sub 2). The pressure rise protection relay will trip the 130 kV circuit breaker at Bunkeflo and the 33 kV busbar circuit breaker. The ground fault protection will trip the 33 kV busbar circuit breaker.

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4.7.3 33 kV system

4.7.3.1 33 kV busbar

The 33 kV busbar is equipped with an over current protection relay. The over current relay will trip the 33 kV busbar circuit breaker.

4.7.4 33 kV wind turbine feeders

The 33 kV wind turbine feeders are equipped with:



Both the over current protection relay and the ground fault protection relay will trip the 33 kV wind turbine feeder.

4.7.4.1 33 kV auxiliary power feeder

The 33 kV auxiliary power feeder is equipped with:



Both the over current protection relay and the ground fault protection relay will trip the 33 kV auxiliary power feeder.

4.7.5 Wind turbines

Each wind turbine comprises a 690 V and a 33 kV circuit breaker to protect the 33 kV feeders in case of an internal wind turbine fault, see figure 4.34.

Figure 4.34 33 kV and 0.69 kV main circuits in a wind turbine.

Offshore substation

33 kV

WT

~ ~

IM

WT

~ ~

IM

0.69 kV 33 kV

0.69 kV 33 kV

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The setting for the internal 690 V busbar breaker is mandatory and critical for the wind turbines. Therefore, it cannot be modified for other or additional system aspects.

4.7.5.1 33 kV

The 33 kV transformer in a wind turbine is equipped with:


• oil level protection

• pressure protection

• oil temperature protection

The transformer protection relays will trip the 33 kV circuit breaker inside the wind turbine.

4.7.5.2 0.69 kV

The wind turbine is equipped with:

• over / under voltage protection

• over / under frequency protection

• asymmetrical voltage protection


The wind turbine protection relays will trip the 0.69 kV circuit breaker inside the wind turbine.

4.8 Discussion

Vattenfall requested a complete study of the electrical system for Lillgrund to be delivered with the bidding. 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 insulation coordination study also showed that surge arresters on the 33 kV switch gear in all 48 wind turbines were required.

• 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

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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 a system study in good time before the construction period begins.

The delivery of the substation was delayed which in turn caused a three-month delay of energization. The reasons for the delay were:

• The substation had a lot of welding and painting defects when it arrived from Poland. Repair work on these defects took longer than planned.

• Several subcontractors were involved with the installation of electrical equipment. They were obstructed by ongoing repair work by a number of people working in a limited area with tasks that needed to be carried out in a specific order.

• The offshore substation for the Dutch wind farm Q7 was under construction at Bladth Industries at the same time. The heavy workload at Bladth Industries also contributed to the delay by occupying required equipment.

The main transformer is the heaviest component of the offshore substation. It is also one of the most difficult ones to repair. It is therefore very important to perform FAT onshore before it is installed and shipped offshore. Having only one main transformer represents a project risk, since the wind power plant will be unproductive for months in case of a transformer break down. A gas-in-oil transmitter will be useful in tracking changes in the transformer oil and to get an early warning of fault conditions that could lead to transformer failure.

Cable laying should not be performed during wintertime. At Lillgrund, 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 2 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 pre-excavated trench.

The inter array cable laying was a success due to the fine weather conditions during the laying period from March to July and the highly skilled subcontractors Baltic Offshore and SSE proved to be.

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5 WIND TURBINES

Since Lillgrund was the first large scale offshore wind power plant constructed in Sweden, there was substantial attention from the authorities. Many environmental protection measures regarding fish, birds and other marine life were required to be implemented, as well as controls for noise levels and technical control of the wind power plant. The grid connection and the wind power plant properties were required to comply with the recent National Grid Code concerning generation plants [9], formulated by the Swedish National Grid Company (Svenska Kraftnät).

The requirements regarding noise levels and National Grid Code played a crucial part in the choice of wind turbine technology. Each of the wind turbines at Lillgrund operate at variable speeds and are equipped with a full-size converter. Variable speed turbines emit less noise than fixed speed turbines and adjusting the rotational speed of the turbines can control the noise levels. The full-size converter makes it possible to control both the active- and the reactive power from the wind turbine.

5.1 General information and technical data

The 48 wind turbines at Lillgrund are of the type Siemens 2,3 MW Mk II. The Siemens 2,3 MW Mk II wind turbine is a variable speed version of the well-known 2,3 MW fixed speed type, used at, for example, the Nysted offshore wind farm in Denmark.

Technical specification for the Siemens 2,3 Mk II wind turbine [10]

Rotor type 3-bladed, horizontal axis

Rotor position Upwind

Rotor diameter 93 m

Swept area 6800 m2

Rotor speed 6 – 16 rpm

Aerodynamic regulation Pitch regulation

Yaw system Active

Controller type Microprocessor

SCADA system WPS

Tower Cylindrical

Rotor weight 60 ton

Nacelle weight 82 ton

Tower weight (70 m) 134 ton

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The wind turbines are CE marked and designed in accordance with the IEC 61300 series.

The basic design of the 2,3 MW Mk II is similar to the design of the 2,3 MW type. However, due to the increased requirements regarding noise and grid control the Mk II version has the following new features:

• Larger rotor with variable speed and pitch regulation.

• The generator is connected to the MV transformer with a 4-quadrant frequency converter.

Variable speed makes it possible to adjust the rotor speed to the actual wind speed, thereby maximising the aerodynamic efficiency, reducing the dynamic loads on the transmission system and at the same time reducing noise from the rotor at various wind speeds.

Pitch regulation makes it possible to control the active power, and the frequency converter makes it possible to control the reactive power of the wind turbine, and as such for the complete wind power plant. The frequency converter also provides maximum flexibility in the wind turbine response to voltage and frequency control, fault conditions, etc. and meets the requirements of all applicable grid codes.

1. Spinner 2. Spinner bracket 3. Blade 4. Pitch bearing 5. Rotor hub

6. Main bearing 7. Main shaft 8. Gearbox 9. Brake disk 10. Coupling

11. Service crane 12. Generator 13. MET sensors 14. Yaw bearing 15. Yaw gear

16. Yaw ring 17. Tower 18 Nacelle bedplate 19. Canopy 20. Oil filter

21. Oil filter 22. Generator fan 23. Oil cooler 25. Rotor lock 26 Hub controller

Figure 5.1 Nacelle overview (Siemens 2,3 MW Mk. II).

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5.2 Power Curve and Energy Production

The wind turbine starts producing power at a wind speed of approximately 4 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)

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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) 6 7 8 9 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 0 >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 4 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.

Figure 5.4 Electrical layout in Siemens 2,3 MW, MkII.

GCGear

box

Frequency converter

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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 2 600 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

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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 2 300 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

Manufacturer ABB, Finland

Type Asynchronous, squirrel cage

Number of poles 4

Rated power 2 300 kW

Rated speed 1550 rpm

Rated voltage 750 V

Speed range 600 – 1800 rpm

Frequency 16,5 – 60 Hz

Enclosure IP 55

Generator weight 6 580 kg

5.4.3 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 3 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.

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Technical specifications converter

Manufacturer Alstom

Type 4Q full scale converter

Switching PWM

Parallel modules 3 (grid inverter), 3 (gen. Inverter)

Cooling Water

PWM filter Installed

Rated voltage 690 V (grid), 750 V (generator)

Switching frequency (grid inv.) 2 500 Hz

Switching frequency (gen. inv.) 1 250 Hz

5.5 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

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Technical specifications blade

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

5.5.2 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 8 electrical yawing motors/gears, securing that the nacelle is positioned correctly according to the actual wind direction.

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Figure 5.7 Transmission system for Siemens 2,3 MW Mk. II

Technical specifications gearbox

Manufacturer Winergy

Type Combined planetary/helical, 3-stage (PEAB 3356.2)

P mech. 2525 kW

Oil volume Approx. 400 l

Ratio 1:91

Weight Approximately 23 t

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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 2 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 Interior layout of tower top section

Figure 5.8 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).

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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 3 sets of lightning receptors. One located near the blade tip and the other two further approximately 8 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.

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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

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• 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.

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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.

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6 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.

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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.


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 2 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 pre-excavated trench.

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7 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 för Örestads 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 1. 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-R-

03-10-0000-0002-04, Aug. 2005.

technical description lillgrund wind power plant

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