The transition towards renewable energy sources poses new challenges for many transmission networks. This applies to the German portion of TenneT’s transmission network. Large amounts of wind energy have to be transported from northern Germany to the demand centers situated in the south and west of the country. In addition to new high-voltage direct current transmission systems, so-called HVDC links, high-voltage alternating current networks, are being expanded. In order to accelerate this process, underground cables are laid out in sections (see figure below; source: TenneT publication).

These expansions are considered to be pilot projects from which to evaluate practical experience with underground cables in the high-voltage AC networks regarding the stability and reliability of the electrical transmission system. If implemented successfully, more underground transmission projects will be considered.

From a power system’s point of view, underground cables mainly represent capacitances and consume reactive power. To maximize the active power transport and minimize reactive power flows, inductive reactive power must be added depending on the length of the underground cable. For that reason, high-voltage shunt reactors are installed at the beginning and / or end of the underground cable section, in so-called transition stations.

For the first time in history, TenneT requested 420kV dry-type air-core shunt reactors for its pilot project. As the leader in air-core, dry-type reactor technology, Coil Innovation’s proposal emerged as the winner.

The following figure shows the Coil Innovation reactor design for a single phase. Each phase consists of two, two-coil reactor stacks connected in series.

In each transition station, two three-phase coil systems are installed (see title image; source: TenneT publication). Each three-phase coil system provides the reactive power compensation of 120 MVAr.