A multi-terminal HVDC -connection is needed
A HVDC-connection (High Voltage Direct Current) is normally a point-to-point connection, typically between two countries. This means that a failure in one end results in a forced shut-down at the other end. However, with more than two end-terminals to a HVDC-connection, any failing terminal would result in shut-downs at all terminals.
This is unacceptable in a meshed grid. Thus the need to invent a key component for the future: A switch, that will isolate the failing terminal while keeping the rest of the HVDC-network alive.
The next step in offshore power generation depends on the successful development of the multi-terminal HVDC network. The pilot roject will be the power connection to the Kriegers Flak wind farm.
By Kent Krøyer
The world’s first multi-terminal transnational High Voltage DC connection will be built in the shallow waters of the Baltic Sea, at the meeting of the borders of Denmark, Germany, and Sweden. In this area, called Kriegers Flak, a 400 MW German wind farm is now under construction and will later be followed by a 600 MW Danish wind farm. A proposed Swedish wind farm in the same area is now postponed.
Technical innovations are required
This project has gained international interest because of its innovative nature, particularly regarding power transmission. The new and technically challenging aspect is the connection of more than two terminals in a single HVDC -connection. For instance, a connection between three countries or between two countries and a wind farm.
The technical solution chosen here will represent a model for much larger international power transmission structures in the future, for example for the great wind farms of the North Sea. This is why the European Union has granted substantial economic support to the Kriegers Flak project, up to 150 million euros.
New thinking: Nationally independent power islands
“Future wind farms do not have to belong to a certain country. This is quite a new way of thinking. Gigantic wind farms can be nationally independent power units out at the sea, directing their power to the countries that need it. This demands a whole new network of high-capacity power cables at sea. But before we begin building that, we need to have thoroughly tested components that are reliable in the harsh environment at sea because service at sea is expensive. This is where the Kriegers Flak project comes in. This project will be thoroughly studied and reported,” says Kim Behnke, head of Research & Development at the Danish TSO (Transmission System Operator) Energinet.dk.
The wind farm is part of a larger strategy
The HVDC project is a consequence of the Danish government’s commitment to increasing its use of green electrical energy from wind farms. It is about more than just providing the necessary electrical connection to the Danish 600 MW wind farm at Kriegers Flak.
In periods with little wind, this HVDC connection will also serve as a transit connection for Northern European energy suppliers. It will ease eastern Denmark’s dependency on a handful of energy suppliers and will strengthen competition. It will also stabilize transmission systems in both countries against power fluctuations from an increasing number of wind farms and help deliver more energy to the load centers of Europe, particularly the eastern part of Germany.
A DC-star in the middle of three AC-grids
The technical challenge is to connect the two countries’ out-of-phase AC-grids with the out-of-phase, AC-producing wind farm – all through a star-connected DC network. Although the DC-cables and the AC /DC converters of the VSC -type (Voltage Source Converter) are new but recognized technologies, this is not the case for the necessary DC-breakers. Switching off an electrical current of close to 2000 Amperes is no easy task. It is nevertheless necessary to have the option of taking out a single DC segment in case of failures.
The Swedish contribution to the Kriegers Flak wind farms has not been planned yet, but the HVDC-connection Denmark – Kriegers Flak – Germany will nonetheless be the world’s first star-connection of this type with more than two terminals, and that poses a technical challenge. Graphics: Energinet.dk
One hurdle down, one to go
One solution could be to send a fast break signal to the AC -sides in both countries, precisely at the time when the local AC -cycle passes through zero. But this would mean an abrupt switching off of the entire wind farm segment, which is not desirable. As a result, it is vital to develop a tough DC -breaker.
“These breakers do not exist in the market today. Large companies like Siemens, ABB, and Alstom are developing them right now. Another challenge is a control system with an open interface, so that components from different suppliers can be interconnected,” says Peter Jørgensen, vice president of Energinet.dk.
Solutions are not far off:
“We have a DC-breaker ready, and it works. It is tested in our laboratory but still not in a transmission network”, says senior engineer Erik Koldby, ABB. The necessary control system is not yet ready however:
“Software is fundamental to this system, and it is still too early in the defining process to determine its exact properties,” he says.
ABB’s presentation of a durable breaker for HVDC power lines