For the Skagerrak 4 subsea interconnector between Norway and Denmark, Statnett has utilised the voltage source converter (VSC) technology in order to enable the security of the electricity supply while combining wind and hydro energy. Among others, the new technology provides better stability in the AC grid, a lower need for filters, and the possibility to black start a blacked-out grid.
The fourth interconnector between Denmark and Norway, Skagerrak 4, increased the total capacity between the two countries from 1,000 to 1,700 MW. Combining these two power systems is highly beneficial: the intermittent wind power and relatively slow thermal power plants of Denmark work perfectly together with Norwegian hydro power plants, which are fast and easily controllable. By utilising the new VSC technology, Statnett aimed at adding value to the power system, such as supporting the grid during failure (voltage control), reducing the risk for commution failures (stable operations) and making it possible to use the link during a blackout situation (black-start capability).
The multilevel VSC is built up of transistors and diodes in parallel to capacitors. The capacitors can be either connected or bypassed by switching off the transistors. Hence, the VSC can use the capacitors to build up a controlled AC source with small harmonic distortion and a DC voltage on the DC side. Compared to classic HVDC, where the reactive consumption increases with the transmitted power, the new converter can control reactive and active power independently. Capacitor banks for compensation are no longer needed.