Hydropower, Offshore, Wind Power

Offshore transmission: Offshore options for grid connection

Issue 6 and Volume 10.

Offshore wind is a step ahead of marine renewable deployment, but how can the industries pull together to overcome shared challenges? Steve McDonald suggests some openings.

While engineers and inventors have long aspired to harness the sea as a vast power resource, it is only relatively recently that we have seen many years of engineering endeavour bring the dream very much closer to reality. Though the technical, commercial, social and political concerns need to be addressed, the global potential for offshore renewables is an opportunity that cannot be missed. Once the renewables industry breaks through the remaining technical milestones, it seems assured that marine renewables will follow where the wind industry is now advancing into the sea.

Nonetheless, the industry must work collaboratively to overcome the many technical and economic challenges of offshore renewables. The technical challenges are multidisciplinary, but developers often focus on the power capture element, rather than take a system-wide view. In particular, the power conversion and grid compliance issues tend to be disregarded until quite late in the day.

Moving offshore

Offshore wind has developed from land-based solutions offshore by mounting most of the complex equipment out of the surf zone, including any sub-stations. The majority of the sensitive electrical equipment is usually located high up in the tower or in the nacelle, making it more robust in face of the harsher sea environment. Thus, the technology developed for onshore wind can be re-located out at sea.

The turbine and its power conversion technology is a complex package, but having been subjected to mass production, improvements in quality, reliability and cost-effectiveness will be carried forward by the industry as it moves offshore. Marine renewables don’t have this history and, therefore, cannot easily make the gradual evolution to offshore deployment. Developing the concept and associated power conversion technology with the requirement to produce competitive generation costs and assure operational reliability is a tall order for such an embryonic industry.

Burbo Bank Offshore Wind Farm in Liverpool Bay – the 25 wind turbines supply power to more than 80,000 homes siemens power generation

The variable nature of wind and marine energy resources, and the extreme conditions, mean that both technologies face significant engineering challenges in integrating products into larger and more cost-effective farms. For example, offshore wind farms need to be larger in order to offset the higher installation and maintenance costs of the project with higher returns, but this in turn can expose the technology to more onerous connection conditions, incurring greater technical and financial risk should the power connection fail.

Some aspects of offshore wind technology have already transferred over to marine renewable developers. Some examples include subsea piling solutions, cable laying, blade technology and offshore access. The oil industry has also been a source of relevant transferable skills.

Common ground

There are a number of common issues for power quality and electrical transmission for offshore wind and marine renewables. Indeed, tidal generation probably has more in common with wind than wave technology due to the inherent nature of the power source, with some companies already moving across from wind to marine, as they attempt to move their technology below the surface. As with wind, the fluctuating power characteristics of wave and tidal power make power conversion and grid connection issues a real challenge for marine renewables. Wave power has a massive dynamic range, is subject to constant change and the output from power capture devices is usually oscillatory in nature, a function of the wave period. Tidal is more consistent and predictable, but still has a large dynamic range and operates from zero to maximum twice a day.

Marine technologies need to be robust and engineered to cope extremely well with the extreme conditions out at sea. Two reliability issues requiring special consideration are energy conversion and power transmission. To operate efficiently, traditional standard rotational electrical generators need a high frequency power input. To be able to use these standard generators, the low frequency, high torque input generated from the marine resource requires an energy conversion intermediary, such as a gearbox. While this is fairly straightforward, the intermediary conversion must be interfaced with the mechanical power take-off technology in order to allow effective conversion of marine power into electricity. This power conversion process has intrinsic challenges associated with power loss, reliability and system complexity, all of which are multiplied by submersion metres below the sea and many miles from the nearest sub-station.

Marine technologies with fewer subsea systems will inevitably reduce the likelihood of maintenance and hence mitigate the high costs of working at sea or retrieving a device. Likewise, reducing the number of interconnections to a device is important in minimizing the risk of cable damage. Straightforward solutions such as removing heat losses by using passive cooling to the hull can avert some of the risks involved in using complicated heat exchangers and associated pumping systems.

The challenges of grid connection

A key challenge for both wind and marine renewables, with their intrinsically fluctuating power generation, are the Grid Codes and Distribution Codes for electrical transmission and distribution, which underpin the entire electrical network operation. These rules require electricity suppliers to match their device to the point of common coupling. Issues such as frequency stability, voltage, power factor, harmonics and fault level all need to be taken into account.

Managing the national grid requires accurate forecasting of both the consumer demand and the electricity generation, every half hour in the UK. At present, small scale generators are exempt, but in the coming years, as the renewables industry moves well beyond the 100 MW benchmark, transmission and trading will grow more and more complex.

Ideally, for grid connection of any generation technology, a predictable, consistent power flow is needed. As a result, the stable and predictable qualities of the power curve generated from tidal and wind turbines potentially make them more grid-friendly. However, wave farms may use a range of methods to level the peaky power flow seen from an individual device.

A fundamental consideration, for both wind and marine technologies, is that the site of offshore electricity generation is dictated by the best locations for energy resource. However, the majority of these are far from the main load centres and often have only a weak distribution network available. Linking electricity generation in these remote areas to the local network can result in network problems and requires costly reinforcement and hence project costs may be prohibitive if deep reinforcement is deemed necessary. The electrical network systems in Pentland Firth, Shetland and Orkney, for example, have previously had electrical network infrastructure in place to suit a rural location, coping with relatively small capacities for supply and demand. They are as yet unprepared to face the greater system demands that will be made once significant quantities of offshore renewables come on-line.

Once marine renewables are ready to progress to full scale farms, identifying appropriate locations should be quite straightforward, given the fact that much research has already been done in terms of resource. And, it is likely that very large marine renewable farms will be built to take advantage of economy of scale and justify the construction of a common shore-based grid connection.

The wind turbines at Blyth, NE England, were an important part of the learning curve as wind power took to the sea

Transmitting the power generated from offshore renewables, over the long distances involved to reach demand, is a significant challenge for any grid network, particularly in the UK where the system is relatively isolated from interaction with other networks on the continent.

In the future, the industry will need to consider whether investing in greater interconnection of the UK electricity system with other energy grids and the European system could be a practical way of expanding the penetration of renewable sources, including the contribution of wind and marine. Even so, it is likely that the increase of renewable sources will require concerted action in Europe to develop a more robust universal mode of operation and a common grid code. Transmission and distribution systems of the future will be transformed from passive infrastructures into actively managed, flexible networks.

As the sizes of offshore farms increase, so does the need for higher voltage transmission. Presently, marine prototypes are connecting at 11 kV and in most cases less than 6.6 kV, since lower voltage levels greatly reduce the issues of insulation and subsea connection. Transmission of offshore power could be achieved using High Voltage Direct Current (HVDC), but this is only economic for very large-scale farms transmitting over long distances. However, this could become more viable in the future when new silicon devices become more readily available.

The UK currently has one HVDC link to Europe and another to Ireland. An increase in interconnections may mitigate the intermittent effects of wind and marine resources by enabling larger power flows between countries. HVDC could also be used to collect the energy from larger farms, which are anticipated to be located very far offshore. To ensure a more effective relationship between offshore renewables and the grid network, it is likely that subsea or offshore substations will be needed for the larger farms to marshal and collect the generator’s power before transmission to the shore and the wider electrical network. In the case of multiple synchronous generators, these sub-stations would collect the AC power at various frequencies, convert it to a common grid-frequency, raise the voltage and transmit it to the shore.

Electrical cable connection is a key issue particularly for wave devices where the power take off is subject to tidal lift and fall, or where the device needs to re-orient itself to capture the tidal flow or the waves’ energy. In these cases, flexible cables are required. These issues have, to some degree, been solved for oil and gas applications. However, ensuring cable reliability remains an area of concern. It is not yet clear if generic or standardized electrical connection techniques can be developed for all marine renewable technologies.

Wave and tidal technologies each face very different challenges in converting the kinetic power of waves and tides into transmittable electricity. Wave power is oscillatory in nature and typically goes through a mechanical conversion or storing process before being converted to electricity. Wavegen’s LIMPET, for example, converts the wave movement to compressed air which drives a Wells turbine. Wave Dragon uses a stored head of water to drive a turbine, whereas OPD’s Pelamis uses multiple hydraulic cylinders to charge accumulators and then drive a hydraulic motor. Tidal devices on the other hand typically convert the flow directly into rotational motion, thus being more compatible with conventional generator technology.

Another great conundrum which poses a challenge for all offshore renewables, is the observation that if technology developers create a simple device, then the power interfaces required to connect them to the grid network tend to be more complicated. For example, if a marine device uses an asynchronous generator, the output is AC at a frequency proportional to the shaft speed. However, for many generators in a tidal farm, the frequency will be different for each machine – depending on the flow characteristics at each device. To connect them together, they must all operate at the same frequency and hence speed. This will result in some devices operating outside their optimal range. Furthermore, both large synchronous and asynchronous machines cannot self-start – meaning that some way of starting and synchronizing the generators is needed.

Learning from the oil and gas industry

The UK’s exploitation of North Sea oil and gas resources gives the country’s renewable industry ready access to experience in solving problems related to installing and operating offshore installations. However, while many of the lessons, technologies and techniques learnt in the oil and gas industry may at first appear to be directly transferable to the marine renewable sector, the major factor which prevents the direct transfer of knowledge, and in some cases technology, is the difference in economic margin.

The approach adopted by both the oil and gas sector and offshore wind is reliance upon fixed structures with limited dependence on remote unmanned operations. However, offshore renewables installations will almost certainly be unmanned, remote systems.

The vastly different economic margins currently seen between the oil and gas sector and the renewable sector are posing their own unique challenges to marine deployment. The same vessels needed to install renewable technologies are used by the oil and gas industry, which can afford to pay much higher rental prices than marine renewables developers. It is reported that a device developer has experienced significant project delays during the installation of technology when the price of the vessel that had been contracted increased by over 90% overnight because of changes in circumstances and requirements in the oil and gas sector.

Oil rig platforms also last a long time and are generally reliable, with maintenance carried out by personnel who operate the rig on a daily basis. However, with smaller profit margins, downtime – as a result of damage or excessive maintenance requirements – can bankrupt a renewables project. Furthermore, the risk of errors is greater in the offshore renewable industry, not least because the technology is designed to interact as much as possible with the harsh environment, rather than simply protect itself from it.

Nonetheless, some subsea companies are already creating a stake for themselves in the fledgling marine renewable industry. The partnership between Rotech, a subsea excavation and dredging company, and Lunar Energy Limited, for example, has created the Rotech Tidal Current Turbine, which is currently at the 1 MW prototype stage.


Within the wind industry several codes, methodologies and procedures have developed gradually over many years to place the industry in a position of being able to raise large sums of project finance for both on- and offshore projects. It is only through the same controls of verification and certification that the marine industry will be able to raise the same levels of investor confidence needed to develop the industry.

The marine industry has not been able to afford such a gradual harmonization of standards over time enjoyed by the wind industry, since the development of standards and methodologies at an early stage has been essential in creating a uniform approach in this fledgling sector. The broad range of power conversion technologies has not helped the standardization process.

Scaled development of prototypes towards full-scale is the key to mitigating the high technical and financial risks of marine technology and more quickly establishing commercial operations. To resolve the technical and economic challenges it is ultimately necessary to demonstrate the capability and performance of production technology. In the wind industry, this has already been proved many times over onshore, but for marine renewables, various incremental stepping stone stages of design and testing are essential in bringing about full-scale offshore farms.

In order to advance significantly the large-scale manufacture of marine renewables, the industry must follow the example of the wind industry by identifying the most viable concept, which in the case of wind has been the horizontal axis, three-bladed turbine, and work to push this agreed concept through to full-scale success.

This will give renewed focus and pace to marine renewable commercialization and pave the way for the wider range of technologies which will follow. Supporting the most feasible technologies will work to drive down the costs of operation and maintenance – which so significantly challenge the financials of the marine renewable industry – and help to overcome some of the fundamental power issues of grid connection and conversion.

In the long-standing battle that has raged between technology and the sea, it now seems that the stakes are moving in our favour as more and more offshore wind farms are connected. With perseverance, it is certain that marine renewables will soon follow suit.

Dr Steve McDonald is the Director of Electrical Technology at NaREC, the New and Renewable Energy Centre.
web: www.narec.co.uk