TenneT is building two transformer platforms in the IJmuiden Ver wind energy area that convert the alternating current from offshore wind energy into direct current. A first in the Netherlands. High Voltage Direct Current (HVDC): direct current with high voltage is used to transport power. Over long distances, HVDC has relatively fewer losses than alternating current (AC) systems. Yet the choice for HVDC is not just an obvious one.
Origin
Sea cables for electricity have been around for a long time. A number of the Wadden Islands and many other islands and wind farms worldwide are connected to the electricity grid by sea cables. Cables that use direct current are also not new.
Heavy HVDC cables are a fairly new phenomenon compared to the traditionally used AC high voltage. They are often used for submarine connections to transport electricity internationally and to connect supply and demand of countries, where this was previously not possible because there was a sea in between. They have become indispensable in the net.
Marine cables capable of transporting the total power of a large power plant are a relatively new phenomenon. This is because it is technically complicated to convert power from alternating current to direct current and then changing it back to alternating current at the other end of the cable.
The first of the new generation of high-capacity HVDC interconnectors is the Baltic Cable (Germany-Sweden), delivered in 1991.
Developments
Since that year, developments in cable technology and conversion stations have been rapid on both sides. About thirty HVDC cables are currently running through the European seabed (and partly above ground).
In the Netherlands, the NorNed cable , between the Netherlands and Norway, was completed in 2008. This connection has a capacity of 700 MW and is currently the longest submarine power cable in the world.
The BritNed cable was also completed in 2010, with a slightly higher capacity (1000 MVA). A third connection is currently under construction, the COBRA Cable (between Eemshaven and Endrup).
Comparison
Alternating Current (AC), or alternating voltage, differs from Direct Current (DC) direct current because with alternating voltage the current constantly changes direction.
Overall, alternating current has higher costs per kilometer and lower costs for the stations on either side. With a high-voltage DC grid it is the other way around: there are higher costs for more complex on- and offshore converter stations and lower costs per kilometer. How exactly does that work?
- Costs per kilometer The energy production from offshore wind farms that are further away from the coast can be transported to the mainland more cost-efficiently via an HVDC grid because the costs per kilometer are lower than those of AC. This is partly because the HVDC cables are on average smaller in size than those of AC. The underlying reason is that HVDC cables have relatively less heat loss, so less material is required.
- More efficient transport The transport of electricity over long distances is generally more efficient via HVDC systems than via traditional alternating current (AC) systems. This is mainly because no reactive power is transported with DC connections, as is the case with AC connections. Reactive power is power that shuttles back and forth through the cable and therefore does not contribute to the transport of energy through the grid, but does contribute to energy losses in the grid.
- Interconnection of asynchronous zones Almost the entire mainland of Europe forms one synchronously linked zone. The United Kingdom, Ireland, Scandinavia and the Baltic States each form their own synchronous zone, in which the grid frequency is not synchronized with the rest of mainland Europe. AC interconnections are not an option in these cases. Electricity connections with these countries are therefore only possible via HVDC.
- Space differences The cross-section of AC cables is determined by the amperage and the cable length. For alternating current there are additional requirements according to NEN10102. The cross-section of HVDC marine cables is often larger than that of AC cables due to the higher powers and lower losses. Converter stations for HVDC also require much more space and technology than transformer stations for AC.
- Higher costs In contrast to the relatively lower costs per kilometer, there are many times more complex and therefore higher costs for the on- and offshore converter stations than those required for the transformer stations of alternating current systems. This is mainly due to the power electronics. The technology behind this involves many high-frequency components that switch approximately 10,000 times per second. In total, HVDC has higher costs for:
- Maintenance and logistics;
- The converter stations at the beginning and at the end at landfall;
- Higher initial and investment costs.
Into practice
Although the choice for HVDC is especially obvious for longer distances, it requires quite a bit of investment from grid operators. Also in other areas, such as knowledge development and the much more complex logistics processes.
Work is now underway on the construction of HVDC systems in various places, such as for IJmuiden Ver.
To bring the ever-increasing yield of wind energy onshore, TenneT and its partners will build at least 14 high-voltage direct current (HVDC) offshore grid connection systems by 2032, each with a transport capacity of 2 gigawatts (GW) in the Dutch and German North Sea. Once implemented, they will supply up to 35 million households with green wind energy from the North Sea.
The future also seems to point in the direction of HVDC in a European context. The European grid can be connected into a much more effective network in which different geographical areas can make much more efficient use of existing and renewable production and shifting use.
New planning
On April 28, 2024, outgoing Minister Jetten sent the additions to the offshore wind roadmap to Parliament. This includes an adjusted schedule for the completion of a number of lots and tenders. As a result of the longer period for connecting the wind farms to the TenneT platforms and as a result of delays in the delivery of some platforms, the dates for commissioning the wind farms are shifting.
