Offshore wind farm

Plugging in offshore wind power

With wind farms being built at greater distances to shore, the challenge is to channel the energy they generate to shore.

The transmission of power from large, offshore wind farms is a considerable technical challenge. It is similar to that faced by oil exploration companies in the 1960s when they first began to exploit offshore oil and gas reserves in the North Sea.

A reliable, modern and efficient grid, both onshore and offshore, is required. Offshore, the challenge is to more efficiently connect power harvested at sea with the onshore transmission system, while at the same time building a system that can actively contribute to stability and security of supply by enabling further integration of the European power market.

This was the dilemma mulled over by global renewable energy consultancy 3E when it analysed the situation in a report for the OffshoreGrid project, co-financed by the European Commission under the EU's Intelligent Energy Europe programme. The consultancy published its final report late last year.

Not surprisingly, the technical solution deemed most viable was a meshed grid consisting of linked hub connections. This would entail connecting projects located in close proximity so that they could share a single transmission line to shore. The report predicted that this would shave £11.5bn from the original £67bn.

The Hub

Hub connections generally become economically viable for distances above 50km from shore, when the sum of installed capacity in a small area (about 20 km around the hub) is relatively large, and standard available HVDC Voltage Source Converter (VSC) systems can be used.

Wind farms situated closer than 50km to an onshore connection point are virtually always connected individually to shore. "Apart from the costs savings, offshore hubs can also help to mitigate the environmental and social impact of laying multiple cables through sensitive coastal areas and allow for more efficient logistics during installations," Geert Palmers, chief executive officer at 3E, explains.

If these hub connections were combined with an even more interconnected meshed grid, the necessary additional costs of £3.5bn-£4.5bn would be compensated by £13-£17bn of additional benefits over 25 years of grid operation.

The project outlined two potential cost-efficient grid designs, that they called the Direct Design and the Split Design. In the Direct Design, interconnectors are built to promote unconstrained trade between countries and electricity markets as average price difference levels are high. Once additional direct interconnectors become non-beneficial, tee-in, hub-to-hub and meshed grid concepts are added to arrive at an overall grid design.

Tee-in connections are when a wind farm or hub is connected to a pre-existing or planned transmission line or interconnector between countries, rather than directly to shore, while hub-to-hub involves the interconnection of several wind farm hubs to form transmission corridors.

The Split Design is essentially designing an offshore grid around the planned offshore wind farms. As a starting point interconnections are built by splitting the connection of some of the larger offshore wind farms between countries. These split connections establish a path for trade, and the offshore wind farm nodes are further interconnected to establish an overall meshed design where beneficial.

Which connection concept is best depends on several factors, such as the distribution of the offshore wind farms, the distance to shore, and in the case of interconnecting several wind farms and/or countries, the distance of the farms to each other.

Technical solution

One solution to connecting offshore wind farms is a concept developed by ABB - self-installing gravity-based structure (GBS) platforms. The company is currently working on a contract worth around £600m for TenneT, the Dutch-German transmission grid operator, to create the DolWin2 power link that will connect offshore wind farms in the North Sea to the German mainland grid.

DolWin2 is the largest power transmission project in ABB's history. It will feature the world's largest offshore high-voltage direct current (HVDC) system with a rating of over 900MW, keeping electrical losses to less than 1 per cent per converter station. On completion in 2015, the link will be capable of supplying more than one and a half million houses with clean wind-generated electricity.

DolWin2 is ABB's third offshore wind connection order for TenneT, following the 800MW Dolwin1 link awarded in 2010 and the BorWin1 project. Both the BorWin1 and Dolwin1 projects feature conventional fixed platforms to house the offshore converter stations. However, the DolWin2 platform will be based on a new GBS design concept, building on experience gained from semi-submersible floating platforms for the oil and gas sector.

Wind farms in the DolWin cluster will be connected by 155kV AC cables to the HVDC converter station platform situated in the North Sea. This will then transmit the electricity at +320 kV DC via 45km of subsea cable and 90km of land cable to the HVDC onshore station at Dörpen-West on the German mainland grid, where it will be converted to 380kV AC.

"ABB pioneered HVDC technology in the early 1950s and is a world leader in the field," Peter Jones, engineering manager grid systems ABB UK, explains. "In total around 140GW of HVDC transmission capacity is installed in some 145 projects worldwide."

HVDC Light transmission technology deploys a new Insulated Gate Bipolar Transistor converter in combination with XLPE (cross-linked polyethylene) DC cable systems. This innovation complements the traditional bipolar semiconductor-based converter technology to provide a state-of-the-art power system with increased controllability.

"It is easy to install and offers a number of environmental benefits, such as neutral electromagnetic fields, oil-free cables and compact converter stations," Jones adds. "It is ideal for connecting remote offshore wind farms to mainland networks and overcoming distance limitations and grid constraints, while ensuring minimal electrical losses and efficient performance. The continuous development of HVDC Light has increased the power range from tens of megawatts up to 1,200MW at 320kV."

Initially, HVDC Light had losses of around 3 per cent per converter station. In the latest generation the losses have been reduced to about 1 per cent. Improvements in reliability have also been driven by dynamic performance, harmonic generation and the flexibility to accommodate changing grid conditions.

The latest generation of converters is based on cascaded two-level converter topology, eliminating the need for AC filters and enabling a more compact converter design with low harmonics and audible noise. With a footprint of 220m x 150m, the new converter stations are substantially smaller than previous generations.

The first HVDC link to connect an offshore wind farm with an AC grid is the 400MW BorWin1. Based on HVDC Light technology, this 200km link connects the Bard Offshore 1 wind farm off Germany's North Sea coast to the HVAC grid on the German mainland.

Once complete, the BARD Offshore 1 wind farm will consist of 80 wind generators, each with a capacity of 5MW. These will feed their power into a 36kV AC cable system. This voltage will then be transformed to 155kV AC before reaching the HVDC Light converter station on a dedicated platform. Here the AC is converted to '150kV DC and fed into two 125km sea cables, which then continue into two 75km land cables to the land-based converter station at Diele in Germany.

The conventional fixed platform featured on BorWin1 is a well proven concept which is reliable and fully certified. There is also the added advantage that there are a number of shipyards around the world with experience in fabricating this type of platform.

The drawback of the fixed platform is that installation/lifting is only possible in better sea conditions during May to September. Furthermore, with a 1,000MW HVDC station weighing in at around 10,000t, it requires the world's largest crane vessel, which has implications for both costs and availability, and multiple offshore lifts.

An alternative is the jack-up self-installing (floating) platform. This does not require a large crane vessel to install and there are many yards with the capability to fabricate platforms with no design risk. However, experience is limited with large platforms. A complex design of jack-up system and platform is also required to handle offshore jack-up operation for this weight.

The GBS platform is mainly intended for use with wind farms in sea depths of between 15m and 45m. It is constructed onshore and all the platform systems fully commissioned in dock. This minimises offshore hook up works. Offshore commissioning is limited to energisation and trial runs after installation of the HV cables.

The platform is towed into position by tugs and secured on the seabed by its own weight and ballasting. This approach significantly reduces the weather dependence of the installation operation.

The GBS platform is designed to reduce environmental impact. There are minimal marine operations required for installation, with only limited seabed preparation required, while the elimination of noisy piling operations ensures there is no impact on wildlife. The platform is also easy to remove and decommission at the end of its service life.

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