OFFSHORE WIND FARM ELECTRICAL CONNECTION OPTIONS W.Grainger 1 and N.Jenkins 2 1 Border Wind Ltd, Hexham 2 Dept of Electrical Engineering and Electronics, UMIST SYNOPSIS The development of large scale offshore wind farms raises new issues in the electrical connection of the wind farm to the electricity grid. Wind farms may well be located 10 to 30 km offshore. Three options for connection to shore are explored in the paper 1 multiple 33 kv submarine cables 2 one 132 kv submarine cable with a 33 kv to 132 kv substation located offshore 3 one High Voltage DC link The connection issue has been addressed in the past, but advances in electrical and wind technologies in the last few years require it to be reassessed. Previous studies and the latest developments are reviewed. The results of the study indicate that the first wind farms are likely to be connected to shore at 33kV and that, in the medium term, High Voltage DC links look promising. INTRODUCTION Large wind farms offshore have not been built before although there are small wind farms off the coast of Denmark and Sweden, and one in the Ijsselmeer in Holland. Offshore wind farms are likely to be larger than those on shore as the economies of scale in offshore projects are more significant. These wind farms are likely to be located some distance from the shore. There are no comparable offshore electrical installations to give guidance on the design of the electrical infrastructure. PREVIOUS IDEAS The three offshore wind farms constructed to-date have been small and relatively simple to connect electrically. Practice has broadly followed that of onshore wind farms although, of course, with submarine cables. A UK study completed in 1993 considered only small wind farms (1) although there was some work done in the eighties (and revised in 1991) on the design of a very large (2000 MW) wind farm in the Wash (2). Both of these studies are interesting and reflect the state of knowledge at that time. However, wind turbine technology and onshore wind farm practice has advanced so rapidly since the studies were completed that they are not directly relevant to the commercial offshore wind farms now being considered. The 2000 MW study (2) considered a range of turbine ratings up to 8.6 MW. The proposed generator voltage was 11 kv and 70 MVA 11/132 kv substations were to be located in specially enlarged towers of some turbines. The main wind farm connection voltage was then 132 kv and a large 132/400 kv substation
was to be constructed on a platform at the centre of the wind farm. This substation was a conventional double busbar design although with reactive power compensation. Some 200 MVAr of mechanically switched reactors was required to compensate the charging current of the 400 kv submarine cables while some 600 MVAr of mechanically switched capacitors were connected to the 132 kv busbars to compensate the reactive power demand of the induction generators. Local power factor correction capacitors at each turbine were not proposed. It is interesting to note that paper insulated cables were considered for both the 132 kv and 400 kv circuits, Ethylene Propylene Rubber (EPR) was considered for the 11 kv circuits and there was considerable concern over the high fault level rating required of the 11 kv switchgear due to the fault level contribution of the generators. The later study (1) was a detailed feasibility study of a single 400 kw offshore turbine although the electrical study was extended to consider 10 turbines (4 MW) and 54 turbines (21.6 MW). The generator voltage was assumed to be 690 V and the wind farm connection voltage was 11 kv. For the largest wind farm (21.6 MW) both 11 kv connection to shore, a distance of 7 km, and 33 kv were considered. In addition to EPR cable, Cross Linked Poly-Ethylene (XLPE) submarine cables were also considered. The advantage of EPR insulation is that no metal sheath is required and the cables can be of "wet" construction. However, XLPE is now commonly used on land and, although an impermeable moisture barrier is required over the insulation, there may be economies because of its widespread use. The study found that synchronous generators can offer advantages in terms of power factor control, reduced losses and increased stability. Although conventional synchronous generators are very difficult to use in wind turbines because of their lack of compliance, there may be advantages in using variable speed wind turbines with a voltage source network converter as this operates in a similar manner to a synchronous generator. The study also proposed a rule of thumb that "To ensure stable operation the fault level of the main supply should be approximately 9 times the installed MW rating of the array". A conservative rule of thumb of this type is very convenient but the operation of an induction machine in its generating mode, rather than as a motor, can allow this rule to be relaxed in certain circumstances and a number of onshore wind farms are operating satisfactorily with this ratio reduced to between 5 and 6. CURRENT THINKING OF WIND FARM ELECTRICAL DESIGN ONSHORE As wind farms have increased in size and under the intense pressure to improve reliability and reduce cost, various new concepts have been introduced. Removal of local turbine switchgear At one time, each turbine transformer had a separate switch-fuse on the HV side. Now turbines are generally switched on and off in banks and individual turbine transformers isolated off line in the rare event this is necessary. Although the impact on operation of the lack of isolating switches can be predicted, care is necessary to ensure faults on the LV winding and terminal area of the transformer can be detected and cleared rapidly. This was an important function of the local switch-fuse, which included a mechanical switch, operated by a striker pin in the fuses, to interrupt low fault currents. Single phase to ground faults are notoriously problematic to detect and clear with low fault levels and "full-range" fuses at 33 kv are difficult to source. One response to this problem using a new type of source protection relay is described in Ref (3). Increase in distribution voltage In many onshore wind farms, the wind farm collection voltage has been increased from 11 kv to 33 kv. This reduces electrical losses and there is a potential capital cost saving by eliminating the main wind farm site transformer. The cost of the 33 kv collection circuits has been contained by using single core polymeric insulated 33 kv cables and new cable termination techniques. The 33/0.69 kv turbine transformers appear to be performing well and, on some onshore wind farms, they are located within the turbine towers. For the foreseeable future, on cost and
technical grounds, the transformers inside towers will be limited to 33 kv and this maximum collection voltage may prove to be an important limitation on the distance offshore of wind farms. Elimination of the site transformer If at all possible, the wind farms are connected directly to the local REC primary distribution voltage, often at 33 kv, to avoid the need for a site transformer. In the unlikely event of a site transformer failing, the whole wind farm could be out of action for several months. There is also a cost saving in switchgear and protection relays. There are some technical difficulties caused by dispensing with the main wind farm transformer as voltage control is more difficult without an on-load tap-changer and earth fault currents are not restricted to the wind farm circuits. However, for offshore wind farms the advantage of not having to locate a large transformer substation at sea are clear. ELECTRICAL COMPONENTS FOR OFFSHORE CONNECTIONS Submarine cables These cables are more expensive than land based ones to withstand their location. If XLPE insulation is used then particular care is required to prevent the ingress of moisture (4). The design voltage stress may also be lower for submarine cables than for those intended for use on land. Typically cables are available with a single wire or double wire armour layer. (There are more complicated designs for laying in deeper water than likely for this application.) Several fibre optic cables can be easily incorporated in the cable for use in protection circuits and for communications. Submarine cables are vulnerable to damage by shipping, unless buried. There is little information available on the actual likelihood of this sort of damage in the likely sites for offshore wind farms. These sites are likely to be in shallow water, 5 to 10 m, and away from shipping lanes. The failure rate in 1981 for submarine cables around Scotland is given as 1.3*10-2 per annum per km (6), which is high. The total length of submarine cable around Scotland at that time was 150 km and some links are in very vulnerable locations such as the Pentland Firth and Sullom Voe Oil terminal. XLPE insulated cables have a lower capacitance than paper insulated cables of a similar rating. Although the capacitance values vary with cable designs, most credible sizes of 33 kv cable circuit will generate less than 100-150 kvar/km. Thus the reactive power generation at this voltage level can easily be accommodated although its impact on induction generator self-excitation must be recognised. At 132 kv the reactive power generation will be in the region of 1 MVAr/km and so may have a more significant impact on the wind farm electrical design. Once onshore the connection to the network is more cheaply handled by overhead lines if the planning authorities allow their use. Wood pole designs are available up to 132 kv with a rating of up to approximately 100 MVA, although some unearthed 132 kv constructions can lead to difficulties with a high ground potential rise caused by earth fault currents. 132 kv wood pole lines use stronger supports and longer insulators than 33 kv designs but are of a similar concept and there is only a proportional increase in costs. For submarine cable, by contrast, different construction techniques have to be used as the voltage increases and the price increases out of step with the voltage. Current costs for 132kV links are double that of 33kV for similar power levels. HVDC Links Advances in power electronic technology have led to the development of HVDC systems at lower ratings than were previously cost effective. For instance, one manufacturer has a system in the range 2 to 200 MW based on IGBT voltage source converters (5). Typically a 110 MW link including cable, but not laying the cable, might cost 13M. One advantage of voltage source forced commutated converters, over traditional HVDC current source converters based on thyristors, is that synchronous rotating machines are not required at each end of the link. The AC connection voltage at the ends does not have to be the same, possibly saving a site transformer at the shore. To date only a demonstration
system has been installed and there has yet to be an offshore installation. The technology is in its infancy and further advances are likely. In the longer term, there is the possibility that offshore turbines will be connected at DC offshore, making HVDC links more attractive. This was the concept used at the Sexbierum wind farm in Holland many years ago. Switchgear and transformers If voltages greater than 33 kv are used for the link to shore, then a substation will be required at sea. There is no precedent for a small substation located at sea. Oil-rig switch-rooms and generators are located on much larger structures and have staff monitoring them continuously. Onshore switchgear and transformers for the higher transmission voltages are generally outdoor to take advantage of large air gaps for insulation. Offshore substations will require more expensive indoor equipment and additional environmental protection. Switchgear will be required to isolate parts of the wind farm on occasion for maintenance, if one main feed is used as well as to provide adequate protection. 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. Cable laying This is a significant cost for an offshore wind farm. The projects will be too small for the specialist cable laying boats, even if they could manoeuvre in the shallow waters. Modified general purpose boats are most likely to be used. Hauling the 33 kv cable between turbines is relatively straightforward and could be handled by winches temporarily mounted on the foundations. The link to shore is going to be several three core 33 kv cables or several separate cables comprising a higher voltage AC or an HVDC link. Cable laying companies contacted to date indicate that the single cores will need to be laid separately, ie the cost of laying three 33 kv cables would be the same as laying one 132 kv AC link. POSSIBLE CONNECTIONS Three options for connecting an offshore wind farm have been examined to establish the electrical characteristics, feasibility and costs. Option 1 - Multiple 33 kv links This option has no wind farm transformers offshore, only the individual turbine transformers. The wind farm is divided into blocks. Each block is fed by its own, 3 core, cable from shore. The maximum practical conductor size for operation at 33 kv appears to be 300 mm 2, giving a block rating in the range 25 to 30 MW. If necessary, two blocks could be connected by one cable, while the faulty cable is repaired. Option 2 - Single 132 kv link and transformer This is the simplest system with a higher transmission voltage for the link to the shore. From discussion with cable manufacturers, 132 kv is their preferred voltage rather than 66 kv. However, an offshore substation is required, on a separate platform. Also, if the link fails, the whole wind farm is disconnected. Option 3 - HVDC link The HVDC link uses the technology available at present on the assumption that a marinised version is available. Again a platform is required to house the offshore converter and switchgear, and the whole wind farm is lost if the cable fails. RESULTS Obtaining cost information on the different options for different wind farm sizes proved quite difficult as none of the equipment is off-the-shelf. Budget costs, see Figure 1, and some simple assumptions on scaling have led to the following conclusions.
Option 1 - Multiple 33 kv links This appears to be the cheapest option for distances offshore up to 20 km and power levels up to 200 MW. Outside these ranges the cable laying costs and electrical losses are the limiting factors. Also it must be recognised that 200 MW is a large injection of power for any distribution circuit to accept and so there may well be limitations imposed by the utility for large wind farms. Option 2 - single 132 kv link This option is expensive for all distances out to 30 km and power levels less than 200 MW. Putting a substation offshore for this type of link is novel and the first few would be expensive and require careful monitoring. Thus, it is unlikely to be used for the first offshore wind farms around the UK. Option 3 - HVDC link This option is too expensive for distances closer to shore than 25 km and for power levels less than 200 MW. There is also a risk, which will decrease with time, associated with applying this new technology offshore. OTHER ISSUES Earthing, particularly for lightning protection, will need to be addressed as offshore structures may be more exposed to positive polarity lightning strokes. Positive downward lightning has higher peak currents and charge transfer, and is likely to be more destructive than the more common negative downward strike. Coupled with the difficulties of offshore access, this may lead to a much higher economic benefit of improved lightning protection. Also for directly connected wind farms with 33 kv collection circuits, some form of reactive power compensation/voltage control may be required. It will, of course, be cheaper to locate this on land. CONCLUSIONS The equipment required for the electrical infrastructure of offshore wind farms is available. The early offshore wind farms are likely to use electrical designs quite similar to those adopted for recent on shore developments. A maximum voltage of 33 kv both for the wind farm collection circuits and for the connection to land is likely in the first instance. However, as large offshore installations are developed (>100 MW) then HVDC transmission to shore may be more cost effective. Considerable development work is still required for the large offshore substations or converter stations which will be required for large offshore wind farms. REFERENCES 1) Renewable Energy Systems Ltd, Feasibility Study for a Prototype Offshore Wind Turbine, ETSU W/35/00251/REP/C Vol I, Vol II, Vol III, 1993. 2)Burton, A.L. (ed) Offshore Wind Energy Assessment Phase IIB Study ETSU -WN-5009 Vol I, Vol II, Vol III, 1985, revised 1991. 3)Haslam SJ, Crossley PA, Jenkins N, Burt M and Borrill A, "Source based protection of wind farms using multi-function protection relay - its design and site experience" CIRED 97, 2-5 June, IEE Conference Publication No 438, paper 4.22 4) Moore, G.F.(ed), 1998, Electric Cables Handbook, 3rd Edition, Blackwell Science. 5) Asplund G. et al, "DC Transmission based on voltage source converters", CIGRE Session 1998 paper No 14-302. 6) Wallace, A.R.S & Whittington H.R., 1981, Routing of submarine cables for wave power transmission. Third International Conference on Future Energy Concepts, IEE Conference Publications No 192, p 123-126. ACKNOWLEDGEMENTS The authors would like to thank the following companies for information used in this paper, ABB Power Systems, AEI Cables, Alstom Transmission and Distribution, BICC Cables and Paulwels International.
Offshore Wind Farm HV Link Costs 25 30 distance 100 200 MW from Option shore 1 / km 100 200 MW Option 23 C a Figure 1 - Shore to Wind Farm Connection Costs