Offshore_technology

1. Introduction
2. Size and configuration
3. Support structure
4. Standards
5. Project Experience
6. Operation and maintenance
7. Electrical
8. general REFERENCES

CA-OWEE 1.7 Offshore Technology | Electrical

7 Electrical

The aim of this section is to establish the state of the art, in the wind industry and in research, in offshore wind electrical technology. In particular, it summarises important technology developments that are in place, foreseen, or considered necessary or beneficial. Network connection is excluded from this document, as it is covered in Work Package 2.2. Transmission to shore is included in this document.

7.1 Electrical Systems within the Wind Turbine

7.1.1 Variable or fixed speed

Recent developments in operational strategy, variable or fixed speed, show a tendency towards variable-speed designs as can be seen in [1]. Despite this, some big manufacturers, such as Bonus or NEG Micon, still make use of fixed speed (often two-speed) technology in their large designs ( ³2 MW) for future offshore applications.

A list of the operating philosophies is given in [1]. Some principal manufacturers of variable-speed machines and the technology used are outlined below:

Wide range variable speed operation – conventional

Several manufacturers have followed this route. It appears that Vestas are moving to this option in place of Optislip (see below) as converter costs reduce.

Wide range variable speed operation - direct drive

· ENERCON - direct-driven synchronous generator with wound rotor.

· LAGERWEY – direct-driven synchronous generator with wound rotor.

· JEUMONT – direct-driven synchronous generator with a permanent magnet rotor.

· SCANWIND - direct-driven synchronous generator with a permanent magnet rotor and high-voltage winding stator. (see Section 7.1.3)

Limited range variable speed

· NORDEX - ‘doubly-fed’ induction machine.

· ENRON - ‘doubly-fed’ induction machine plus optionally a dynamic VAR control system (DVAR).

Narrow band variable speed operation

· VESTAS – Induction generator with variable slip of as much as 10% by an electronically controlled resistance in series with the rotor resistance (OPTISLIP).

Wide range variable speed has well known benefits [1]. A further advantage offshore is the ability to avoid damaging resonances. This is important for offshore turbine structures, where the resonant frequencies have proved difficult to predict accurately, particularly for monopile structures, and also due to different seabed conditions. As a result such frequencies may change over the lifetime of the structure [4].

However, looking at operating statistics from wind turbines using power electronics according to the German ISET Institute [3], it also seems that availability rates for these machines tend to be somewhat lower than conventional machines, probably due to failures in the power electronics.

Therefore, special attention must be paid to the electronic converter required to interface the synchronous or induction generator to the utility grid. At the moment, wind turbine manufacturers are pushing the wind energy market with larger and larger turbine rotor diameters, which are specially suited for offshore developments. Wind turbines up to 2 MW are currently being sold as commercial products on the market. There is competition between Insulated Gate Bipolar Transistor (IGBT), Gate Turn-Off Thyristor (GTO) and integrated gate-commutated thyristor (IGCT) in the market for powers around 1 MW. However, IGBT may be favoured because of their use in motor drives of this size. For offshore applications, technologies which have demonstrated reliability with many units in industrial locations onshore will be attractive.

All the options used onshore will probably be used offshore, with the possible exception of Optislip. The only important factor in this area that is different offshore than onshore is availability, which would appear to favour fixed-speed machines, and direct-drive (because of the omission of the gearbox). It is not clear whether power electronic converters can be made reliable enough at suitable cost.

Future developments in this area are therefore expected to be:

Reliability

Work on converter design and remote monitoring to reduce downtime.

Benefits of variable speed

Work to establish whether the different conditions offshore (particularly turbulence) affect the pros and cons of variable speed.

Progress with device characteristics

Power electronic devices will get larger, cheaper and more efficient, and these may change the balance in favour of variable-speed.

Voltage and power factor

Research to optimise the converter in terms of control of power factor and voltage is likely to be useful [2].

Housing of equipment onshore

An ideal situation is to employ simple turbines offshore generating unregulated electric power as ‘raw-material’ in terms of voltage, frequency etc. Cables are laid to shore where the electricity is refined prior to grid connection. However, poor 'quality' of the generated electricity, in other words, a wide voltage and frequency range, will add cost to the electrical system within the wind farm and to shore. It is also possible to reduce the equipment required offshore (i.e. offshore transformer station) by accepting increased electrical losses in the connection to shore. However, any decision to locate complex items offshore rather than onshore must be supported by detailed analysis of the failure mechanisms and expected downtime.

There has to be a compromise between the simplicity of the electrical equipment offshore and the cost and efficiency of the transmission system to shore. It is not clear where the best compromise lies. The Scanwind/ABB Windformer concept assumes that for large distances to shore, an offshore converter station may be required to step up the DC voltage to a more economic level.

7.1.2 Direct drive

Direct-drive generators are considered above. There is scope for incremental improvement, particularly to suit the offshore environment. The principal aims are to make direct-drive cheaper, and with smaller diameters. Other types of machines may also be considered, like axial-flux and transverse-flux generators [2].

7.1.3 Scanwind: Windformer concept

The Windformer uses advanced cable technology developed by ABB’s Powerformer high-voltage generator. Powerformer is capable of generating electricity at up to 400 kV, allowing it to be connected directly to the transmission system.

This has been achieved by changing the conventional stator windings consisting of mica-epoxy insulated rectangular conductor-bars to windings with circular conductors insulated with conventional solid dielectric high-voltage cable insulation materials. As a result of this, the conventional generator, the generator surge arresters, the medium-voltage generator breaker and busbars, and the step-up transformer are all replaced by one single component, as can be shown in Figure 7.1.3.1. However, this new design will also have the relatively high top mass and large torque levels typically of large direct drive systems, which can be a potential problem for future 4-5 MW concepts.

The Windformer generator operates at voltages ranging from 18 to 25 kV depending on the rotor speed. A directly connected diode rectifier is used to rectify the AC voltage from the generator. This option is taken to maximise the reliability and minimise the losses. The high voltage characteristic of the generator rectifier system facilitates the connection within the cluster of wind turbines with minimum losses. The wind turbines are all connected to a common DC node from which the energy is transmitted to a converter station.

Figure 7.1.3.1 Diagram comparing conventional and Scanwind concepts

(Source http://www.newscientist.com/news/news_224335.html)

The principal claims for this concept are:

Higher energy production (see below)

Control of reactive power in order to control steady-state voltage and voltage fluctuations (flicker): this is also possible with most variable-speed concepts in principle, and with all turbine concepts if HVDC is used for transmission to shore.

Simple integration with HVDC transmission to shore, saving cost and losses

Low maintenance / high availability, due to the omission of the gearbox and power electronics (except for the diodes, which are very reliable).

High energy production

There are no published figures so this claim cannot be quantified. However, there are some positive factors which are likely to lead to higher energy production:

· Losses in the DC-transmission cable vary with the DC-level, which varies with the rotational speed of the turbine.

· Mechanical losses associated with the gearbox are avoided.

· The generator is likely to have high efficiency due to the permanent magnet rotor and its design.

· Losses related to the step-up transformer are avoided (typically 1% of annual production).

· The diode rectifier has lower losses than the active rectifiers habitually used in variable wind turbines.

GH estimate that the most that can be saved from gearbox, generator and transformer losses is probably about 10%.

7.1.4 Voltage level for output

The Scanwind concept has a benefit in avoiding the turbine transformer. This benefit is available to all design options if the generator is designed for a voltage sufficiently high (probably above 10 kV) to be suitable for interconnection of the turbines within an offshore wind farm. The technology exists to do this, but the effect on generator cost is significant. No commercial turbine manufacturer uses high-voltage generators, onshore or offshore. There would be advantages in studying the technology and the costs of high-voltage generators (up to 35 kV) in volume production.

7.1.5 Control system and SCADA

Turbine control systems are not expected to be different in principle offshore. However there is likely to be considerable effort to improve reliability, as control systems are a significant source of downtime. This effort will cover:

· formal techniques for estimation of reliability;

· redundancy of components (principally sensors) and complete subsystems;

· condition monitoring:

· remotely via the SCADA system;

· locally within the turbine controller;

· increased numbers of sensors to allow improved remote diagnosis, either manually or automatically by the SCADA system (perhaps by an expert system).

7.1.6 Robustness

This is a vague term, but it is intended to cover the need offshore for items of equipment to cope with a wider range of conditions. Principally these are environmental conditions, although temperature range is expected to be more benign offshore than onshore. In particular, it is likely that in the life of any offshore wind turbine, there will be periods when, due to cable failures, there is no power on the turbine for heaters and dehumidifiers for periods of several weeks or months. Is it cheaper to accept an extended recommissioning phase after such an event, or to design the turbines to allow generation to recommence after restoration of supplies without maintenance? This question can only be answered by studying the likelihood of cable failures, the restrictions on access to the turbines, and the effect of extended outages on individual components.

Electrical conditions, such as voltage range and voltage steps, could also be allowed to become more extreme if it resulted in an overall system (wind turbine to network connection point) which produces lower cost-of-energy. It is no longer necessary or perhaps even desirable to design turbines as though they will be connected directly to the distribution system.

7.1.7 Earthing and lightning protection

Earthing and lighting protection is an issue that should be addressed as offshore structures may be more exposed to positive polarity lighting strokes. Positive downward lightning is more destructive than the more common negative strikes, due to higher peak currents and charge transfers. This should be further investigated in order to establish and improve protection arrangements for offshore structures. It would be useful to have the same understanding of lightning phenomena offshore as is now available onshore.

7.2 Electrical Systems within the Wind Farm

7.2.1 Voltage level

This issue has been partly addressed above. In the Middelgrunden offshore wind farm, 30 kV XLPE cables dug into the ground are used within the wind farm. The idea of using oil-insulated cables was also carefully considered, but the tenders showed that the XLPE cable solution was by far the cheapest. Eventually authorities decided due to environmental concern not to allow oil-cables anyway. On the other hand, for the Horns Rev offshore wind farm to be built in Denmark [6] with an initial capacity of 150 MW, the cables within the wind farm will be operated at 22 kV nominal voltage and then a transformer station will increase the voltage up to 150 kV for transmission to shore.

A voltage of 36 kV within the wind farm is thought to be the highest which is acceptable, due to the cost of switchgear for higher voltages.

There may be a benefit in development of switchgear at these voltage levels specifically for offshore wind turbines. Such switchgear would ideally be highly reliable, able to withstand humidity and salt, and require no maintenance.

7.2.2 Cable laying techniques

Conventional cable laying vessels are expensive and may have too large a draught to operate in relatively shallow waters. There is a need to develop new techniques for installing the relatively short cables within the wind farm (~ 1000 m lengths). Hauling the cables within the wind farm could be relatively straightforward and could be handled by winches temporarily mounted on the foundations, or on simple barges.

There is also a need to consider new techniques for cable recovery and repair, which can be carried out in most sea states.

7.3 Transmission to Shore

7.3.1 Voltage level

Three possible options could be used for connecting an offshore wind farm:

(a) multiple medium voltage links (up to 35 kV)

(b) single high-voltage link (100 to 200 kV)

According to [13]:

· the first option appears to be the cheapest for distances offshore of a few kilometres and relatively small wind farm size (say up to 200 MW);

· the second option is appropriate for longer distances offshore and larger wind farms;

· the final option is appropriate for distances to shore above 25 km and for power levels of more than 200 MW.

In the Middelgrunden wind farm, (40 MW and 3 km to shore), the first option has been selected. Each turbine contains a 690 V/30 kV transformer in the bottom of the tower. From the central turbine of the wind farm two 3 kilometres long parallel 30 kV XLPE cables connect the wind farm to the national grid at the nearest point on shore. At this point 500 MW coal-fired power plants are situated, and provide an excellent point of connection for the wind farm. The tenders showed that two parallel cables, equal to the cable used between the turbines, are the cheapest solution.

However, higher installed capacity is expected for future offshore developments. Possible technical solutions will range from 150 kV or 400 kV for multiple wind farms to one 150 kV cable for a wind farm alone. HVDC is discussed below. In the Horns Rev Wind Farm [6], the solution finally chosen is one 150 kV cable for this wind farm alone. Later expansion of the site may result in a ring system. Three single-conductor cables or one three-conductor cable will be used to connect the wind farm to shore. Both types can be made with XLPE insulation and the three-conductor with fluid filled (oil/paper) insulation as well, although as seen before, environmentally-speaking oil insulation presents disadvantages.

7.3.2 Offshore substations

If voltages greater than 33 kV are used for the links to shore, then an offshore substation will be required, containing a step-up transformer. Unfortunately, there is no precedent for a small substation located at sea. It is likely that offshore transformer stations would be a three-legged steel structure with all the equipment necessary and supplied as a “turnkey” solution. Packaged substations are available, but these are usually used as emergency replacements or for quick installation in remote areas. The manufacturers are cautious about offering these for offshore installation. The reticence may disappear if a sizeable market appears.

For any site, there is some optimisation required to decide the number and size of offshore substations. A single large substation is likely to be cheaper due to the structure costs, but a failure results in the loss of the output from the entire wind farm. The same argument applies to the cable link to shore. It is likely that offshore wind farm design will include formal assessment of these risks, in order to select the optimum configuration.

The main item in the offshore substation will be the transformer, but there will also be medium-voltage switchgear and possibly high-voltage switchgear.

An emergency diesel generator may be included in the equipment. Due to the rough weather conditions and difficulties with access, electricity supply cuts for prolonged periods are possible. It may be justified to equip the station with a diesel generator in order to keep all essential equipment, such as climate conditioning, control and safety systems operating during these periods. The diesel generator could also supply the auxiliary loads in the wind turbines.

For large onshore wind farms, it is likely that on-load tap changers on the transformer would be required for voltage control. There is the same need for offshore wind farms, but maintenance requirements would be excessive. Table 7.3.2.1 summarises failures in substation transformers, where it can be seen that mechanical failures, and in particular on-load tap changer failures, are the most common cause of outage [11].

Origin	Less than 1 day	1 to 30 days	More than 30 days	Total
Mechanical	24.3	20.5	8.3	53.1
Dielectric	7.1	7.9	15.8	30.8
Thermal	2.3	4.6	2.3	9.2
Chemical	1.1	-	-	1.1
Unknown	5.8	1.4	1.6	2.8
Total	36.2	34.6	29.2	100

Table 7.3.2.1 Substation transformers.

Failures with forced and scheduled outage, as a percentage of total number of failures.

Solid-state load tap changers for medium power transformers (15 kV to 34 kV) with conditioning monitoring are being investigated, and it is claimed that they could reduce maintenance costs by 50-80% while increasing safety, reliability and power quality. This could be a line of research for higher voltage applications in conjunction with capacitor and reactor compensation [7].

The alternatives to on-load tap-changers are:

· specifying the turbines to be able to operate with a wide voltage range, so that voltage control is unnecessary;

· fitting off-load tap-changers, which are cheaper and smaller, and accepting that occasionally it will be necessary to shut down the wind farm for a few minutes in order to adjust the tap position.

The conclusion is that there is a need for detailed consideration of offshore substation design. It is likely that there will be a substantial market for such products, and there is substantial scope for detailed design to produce high availability and low cost.

7.3.3 HVDC

Since the establishment of the HVDC industry over 40 years ago, the technology and its application has undergone dramatic transformation. Nowadays, fast progress in the field of power electronics devices with turn off capabilities such as IGBT and GTO, makes Voltage Source Converters (VSC) more attractive for HVDC applications. To date, there are three manufacturers that have developed the state-of-the-art HVDC technology suitable for offshore wind farms; ABB, Alstom and Siemens.

As an example case, Siemens Power Transmission and Distribution Division has outlined a preliminary version of a possible 675 MW offshore DC/AC-Converter Station as can be seen in Figure 7.3.3.1 [10]. The dimensions of this station would be approximately 50 m in length, 50 m deep and 28 m in height. As shown, it would be designed with a platform for helicopter access for maintenance operations.

Figure 7.3.3.1 675 MW Siemens Offshore DC/AC-Converter Station

HVDC by ALSTOM [8]

Alstom makes use of conventional technology based on thyristor devices. Thyristor converters in conventional HVDC always require reactive power. Additional power components such as switched capacitor banks or Static Var Compensators (SVC) must be used in order to supply the reactive power demand of the converter station.

HVDC-Light by ABB [9]

The technology uses IGBTs as opposed to the thyristors used in traditional HVDC systems. The IGBTs are characterised by switching very fast between two fixed voltages. PWM and low pass filtering are used to achieve the desired AC waveform. Active and reactive power can be controlled by the PWM switching technique. As less components are required than conventional designs, the area required for a converter station is 20% lower.

HVDC^PLUS by SIEMENS [10]

The HVDC^PLUS converter is also equipped with IGBTs, and the important characteristics are similar to HVDC-Light. The technology can deal nowadays with up to 200 MW offshore capacity through a single sea cable. Future developments, with Light Triggered Thyristors (LTT), will be able to cope with up to 600 MW capacity. Recently, SIEMENS has been awarded the contract for the HVDC converter stations of a 500 MW submarine cable link between Northern Ireland and Scotland. For the first time in a commercial HVDC system, direct-light-triggered thyristors with integrated overvoltage protection will be used for the AC/DC converter stations.

Published cost information is not available to allow a comparison of the technologies, but it can be concluded that for the distances and power levels being considered for offshore wind farms, HVDC is more expensive than a conventional AC solution. Nevertheless, HVDC may well be used for offshore wind, because:

· Restrictions in building new overhead power lines onshore may require underground cables onshore, which narrows the gap between AC and HVDC.

· HVDC allows the entire offshore wind farm to operate at a variable frequency, which can give some benefit in energy capture.

· HVDC provides independent control of reactive power at the shore converter station, which could be of great benefit to the network operator, and could allow the network connection point to be on a weaker section of network, closer to the landfall.

· HVDC provides almost no contribution to fault currents, which in many areas are a major limitation on the connection of new generation of any type.

7.3.4 Cable installation

Submarine cables are vulnerable to damage by shipping, unless buried or otherwise protected. Burial is often the preferred method, although in some conditions other techniques are appropriate. Available information on actual likelihood of this sort of damage in the likely sites for offshore wind farms is sparse [12].

The major risk of damage is from ships’ anchors and trawl equipment. The risk therefore varies greatly with location. It is also affected by seabed conditions. In areas with a hard bottom, anchors and trawl gear will not penetrate: therefore, the cable could be buried to a shallower depth than in areas with soft soils. Consequently, in a softer sea bottom, the cable would need deeper burial to have adequate protection, though the cost of burial would be lower.

To date, there are no developments on minimum standards for cable route surveys. There are several industry standard techniques for subsea cable route surveys:

· Multibeam bathymetry is for developing seafloor topography along a proposed route and enables large swaths to be surveyed with a single pass of the survey vessel. Various systems are available on the market. Basically the higher the system frequency, the greater the resolution and data density, but the shorter the system range.

· Side scan sonar is for seabed imaging. Side scan provides excellent target detection and seabed classification capabilities.

· Sub-bottom profiling is for the collection of data concerning shallow geological and sedimentary conditions. The technique is an essential component in pre-installation surveys for buried marine cables.

There may be scope for development of new techniques and equipment suitable for route selection and installation of cables for offshore wind farms, particularly as the water depths will generally be shallower than for cables for other applications.

7.3.5 Energy storage

The connection to shore forms a greater fraction of the project cost than for the equivalent grid connection for onshore wind farms. This connection to shore will have a capacity factor of 0.3 to 0.4, depending on the site wind conditions. In other words, it is approximately three times larger than it needs to be, in terms of the energy it transmits per year. There is therefore some scope for examining techniques for storage of energy offshore, one benefit of which would be to reduce the size and cost of the connection to shore. Recent developments in fuel cells may possibly lead to energy storage which is cheap, reliable and small enough to be located offshore. This is considered a ‘long shot’, but worth investigation [14]. There may also be benefits in electricity trading, and in reducing the adverse effects of large wind penetrations on national electricity systems. The planned Laesø offshore wind farm in Denmark will include a small installation onshore, to investigate these latter benefits [15].

7.4 Summary

In conclusion, it can be said that there are many areas where technical developments are expected which will improve the economics and reliability of offshore wind farms. Some of these will arrive because of developments in other industries and in onshore wind, but others are specific to offshore wind and are therefore more risky.

There are also several areas where the risk is too high for commercial wind farm developers or turbine manufacturers, and which are therefore suitable for pre-competitive or collaborative investigation.

7.5 References

[1] Gardner P, Generators and Drive Trains, Wind Directions, Jan. 2000

[2] Dubois M, Review of Electromechanical Conversion in Wind Turbines, TUDELFT, April 2000.

[3] ISET, http://www.iset.uni-kassel.de/index_eng.html

[4] Smith G, Design for Improving the Reliability and Accessibility of Offshore Wind Plant, September 200, MSc project, CREST.

[5] Middelgrunden wind farm, http://www.middelgrunden.dk/summary/40MWoffshore.htm

[6] Christiansen P, Jorgensen K, Grid Connection and Remote Control for the Horns Rev 150 MW Offshore Wind Farm in Denmark.

[7] Substation Operation and Maintenance, EPRI , http://www.epriweb.com/pf99/trgt054.html

[8] Alstom, http://www.tde.alstom.com/systems/en/pes/products/hvdc.htm

[9] ABB, http://www.abb.com

[10] Siemens, http://ww.ev.siemens.de/en/pages/lighttri.htm

[11] Heathcote M.J., J & P Transformer Book, 12^th edition, Newnes, 1998. ISBN 07506 1158 8.

[12] Lyall G, Minimum Standards for Subsea Cable Route Surveys, UnderWater Magazine, Nov/Dec2000, http://www.diveweb.com/telecom/features/novdec2000.01.htm

[13] Rogers N, Border Wind Ltd, Offshore Wind Energy, MSc course, Loughborough University

[14] IIR Conferences, Commercially Viable Electricity Storage. Conference, London 30 & 31 January 2001. www.iir-conferences.com

[15] Windpower Monthly News Magazine, September 2000. British storage for Danish offshore wind.

[16] Variable Speed Drives

VALLIADIS SA ( ΒΑΛΛΙΑΔΗΣ AE): Manufacturer of electrical generators for wind turbines; Contact: Mr. G. Koulepis; tel: +1‑2817217, 2832602; valiadis@hol.gr; www.valiadis.gr; Research conducted at the National Technical University of Athens focuses on permanent magnet generator design, gearless generator design, artificial intelligence techniques, a.o.

[17] Flexible Cables

FULGOR – GREEK ELECTRIC CABLES SA; Production & deployment of submarine power cables; Contact: Mr. N. Boutopoulos; tel: 6852100; nboutopoulos@fulgor.gr; www.fulgor.gr

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Updated September 2008