Stepping up to serve the future grid

Cable carousel with 33 kV offshore wind cable in Drammen

Future grid capacity is currently a hot topic in the Netherlands. With the wind projects becoming larger, especially at sea, what are the requirements in terms of cable designs?

Wind Energy Magazine spoke to Mario Loi and Hans Schwirtz at the Delft production facility and head office for the Nordics of international energy and telecom cable and system provider Prysmian Group.

Hans Schwirtz, Commercial Manager Projects of Prysmian PowerLink S.r.l. is responsible for the company’s subsea activities in the Nordics. “After the telecom boom, following the introduction of the internet, we now see a huge hype in power, both onshore and offshore, in interconnecting countries, connecting wind farms but also in optimising the grid,” he tells. While the Netherlands is moving away from gas and towards a more electricity based energy system, with large influx from renewable energy in the future, this will have a big impact on the current national distribution network, for both the national Transmission System Operator and distribution grid operators. Mario Loi, as Business Manager Utilities Netherlands, responsible for the onshore market, explains: “There are already difficulties in keeping up with the current renewable energy influx, especially with solar energy.

We are currently working on several high-voltage networks to keep up but it seems that in the near future it’s already not enough, he stresses. All parties involved are still debating on this topic. “Nobody wants wind farms in their backyard but they neither want these huge high-voltage pylons. Something has to happen.” Prysmian is taking this very serious, he stresses, regular meetings have been set up with all parties involved to discuss this topic. Where are we headed in the electricity market, what is the desired capacity and how does this affect our company? Should we and when do we adapt our production process is one continuous question we ask ourselves.

Bigger and longer
One thing is sure, cables will become bigger in size and capacity and larger. For one, clients opt for cables with better efficiency. This generally means cables will be larger in diameter. Another reason is the growth in wind turbine size. Mr Schwirtz elaborates: “20 years ago, we talked about kilowatts and now we are talking about megawatts. Old turbines are being replaced by modern, more powerful turbines. As voltages and currents increase, these require higher transmission powers. This has an impact on the size of cables and the installation components.” Offshore, he sees not only an increase in turbine size but also in wind farm size and wind farms being built further offshore. “We are now already building 700 MW wind farms. After 2025 we will see 2 to 4 GW wind sites. These are no longer wind farms but proper power plants!”

With regards to the inter-array cables, 66 kV cables are becoming the standard. “The 33 kV inter-array cables will still be used for smaller wind farms or nearshore, such as Fryslăn Wind Farm. The turbines in the range of 9.5 to 12 MW that are currently being developed already require 66 kV cables. Those in the range of 15 to 20 MW will even need 132 kV cables. We are already designing these.” After that, he warns, it will become a challenge. When you step up to 220 kV, the present cable designs need a fundamental change. Because of the continuous force on the cables, caused by the movement of the wind turbines, the cables need to be light and very flexible. Lead sheaths, or aluminium sheaths cannot stand much movement although they are still the best solutions to keep the cable dry. The company is already talking to a number of parties in the offshore wind to look into this. Also, the bigger the offshore turbines, the bigger the distance required between the turbines. This means longer inter-array cables are needed. Mr Schwirtz already sees examples of wind farms where they require inter-array cables of 15 km.

The size and the far away location also affects the length and size of export cables. Mr Schwirtz: “We already produce export cables with a diameter of 30 cm that weigh around 170 kilos per metre. That’s almost a redoubling from the first 33 kV cables used at one of the early Dutch offshore wind farms.” The current standard is 220kV AC but the market is moving towards 275 kV AC and 320 to 500 kV DC. AC cables loose efficiency on longer distances. DC cables are cheaper but require expensive converters. However these pay themselves back with each km of cable. There is a grey area around 700 MW and 70 km in which, on a project-by-project base, the optimum AC-DC solution has to be evaluated, he explains. As to the capacity of the export cables, Prysmian recently showed with the contract for the 600 kV Western HVDC link, connecting the Scottish and the English/Wales power grids, that it is able to provide high capacity cables.

Onshore, Mr Loi also sees an increase in size and length of the cables. “Take Fryslăn Wind Farm, we are talking about one complete connection of at least 55 km! This has an impact on the grid but also on the permitting process.” The latter, he explains, is much more complicated than offshore as it involves many more players, both governmental and private, especially if a cable route is intersecting multiple municipality borders. In the case of Fryslăn Wind Farm, the situation is even more complicated as a large section of the cable will be installed in the Afsluitdijk dike, at the same time as the Ministry of Waterways and Public Works is working on a dike reinforcement project.

He also sees a shift towards the use of DC connections. When DC is used for connecting offshore wind farms to the land station it means that parts of the onshore track will also consist of DC cables. Due to the high capacity, less cables (2 instead of 3), and thus less space, are required.

Growing focus on sustainability
Sustainability is a focus area for Prysmian. Until 30, 40 years ago, Prysmian still produced paper insulated, lead covered (PILC) cables. Pressure was placed on the cable manufacturers to look into more cleaner solutions. This lead to the introduction of XLPE (crosslinked polyethylene) and EPR (ethylene propylene rubber) cables. However, these materials are not easy to recycle. Around ten years ago Prysmian developed their P-Laser insulation material. P-Laser is an eco-friendly cable, manufactured using thermoplastic materials. Its production process utilises ‘zero-gas’ technology, reducing CO2 emissions. It is also capable to perform at higher temperatures, allowing for higher voltages. Prysmian currently only produces P-Laser cables for onshore use but is already in the process of developing them for subsea use also.

“We were pioneers in this field. Unfortunately the use of sustainable materials currently only takes up around a small percentage of the market, Mr Loi says, in fact, European tender regulations even prevented us back then to include the product in a tender on grounds of preventing a level playing field!” Nowadays, as the interest and sense of necessity for sustainable solutions is growing amongst clients, there is more competition in the market. For most clients, cost is still a deciding factor.

Prysmian’s cable laying vessel Cable Enterprise

QA, monitoring and prediction
Mr Schwirtz: “Cables are a critical product. They take up a relatively small percentage of the total value of a wind farm but if they are not functioning, there is no revenue. This becomes even more so important with subsidy-free wind farms.” Extreme QA is applied to the cable production, especially to subsea cables. “Half of the time is spent on testing. With each new production step the entire cable is tested again”, he adds. Prysmian has test labs in the UK, Italy and the Netherlands. Prysmian decided to invest in their own mobile diagnostic and testing facilities from the Delft facility. With mobile road-transportable equipment, high voltage cables can be tested under extreme voltages. It is currently the only mobile diagnostic and testing facility were very long export cables can be tested before being put in operation. The export cable for the Gemini offshore wind farm was tested here for this reason, even though Prysmian did not provide the cable itself.

In general, however, he explains, an offshore cable generally does not get damaged because of production errors but due to external elements, by trawling and sometimes by errors made during installation. Most of the time, a cable breaks down when it is operating at its highest capacity, when there are strong winds. This is also the worst weather situation to pick up the broken cable, repair it and install it again. It also depends on the availability of the required equipment at that time. “When an onshore cable is damaged it will take between 2 to 7 days to repair the cable. Offshore, this could take a few months.”

It is for this reason that monitoring of subsea cables is extremely important. The quality of a cable can determined by measuring the temperature. This is done by making use of the optical fibres, by sending a thin light-pulse and wait for the reflection. Mr Schwirtz: “We can measure the temperature and location with a precision of up to a few metres. By correlating current and temperature, we can see if a cable is heating up disproportionally or is not heating up as expected. These can be caused by a cable part being buried too deep or a cable being exposed. Both scenarios can put a cable at a risk.” This method is now also used for inter-array cables.

The most important indicator of the quality of the cable, however, is the insulation. Prysmian has developed its own system based on partial discharge sensoring. Mr Schwirtz: “Damage on the  insulation leads to small electrostatic discharge. We can, say with some restrictions, measure these discharges. By integrating this add-on system with temperature measurement we can not only determine the actual quality of a cable but also predict future quality. This way we can plan repair activities at a more convenient time, for example during planned maintenance activities, better weather windows, or when the required equipment is available. This leads to better cost-efficiency.”

Future outlook Prysmian
In the offshore wind market the two Prysmian men see a trend towards project developers using less contracts. Mr Schwirtz elaborates: “For the UK Gwynt y Môr offshore wind project, our client managed around 120 contracts. For the Westermeerwind Wind Farm, only one contractor was assigned for the complete wind farm installation.” Prysmian can perform both supply only, as is the case for Borssele 3 & 4, or turn-key projects, as required with for instance TenneT’s tenders for the 220 kV offshore connections. Playing a different role can at times create tension. Mr Schwirtz: “Sometimes you are a supplier and sometimes a competitor. You have to carefully consider each project because there can easily be a conflict of interest.” Fortunately he sees the market becoming more mature.

Even though the power market is booming, it does not mean it necessarily is a growth market. Mr Schwirtz: “Due to cables becoming more efficient, the cable market itself in general is not growing substantially, not in volume nor profit. We use less cables and therefore need less volume per megawatt. To survive in this market, it is necessary to scale up your business. Prysmian’s strategy is to acquire other healthy cable companies that complement our existing activities. With the acquisition of Draka and General Cables we showed that we have been able to do so successfully.”

This article appeared in Wind Energy Magazine, 2019 02. Text: Sabine Lankhorst Photo credits: Prysmian

 

 

 

 

 

 

 

 

 

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