Skagerrak (power transmission system)

Skagerrak went in service in 1977 as a bipolar HVDC scheme. This facility was built with thyristor valves. When installed this underwater cable was the world's longest and deepest underwater HVDC power cable. The cable, manufactured by Alcatel, is laid in a maximum water depth of 530 metres (1,740 ft).[8]

Both cables have a capacity of 250 MW at 250 kV.[1]

In 1993 the scheme was extended by HVDC Skagerrak 3. Skagerrak 3 is a monopolar line for a voltage of 350 kV with a capacity of 440 MW at 350 kV.[1] In installing Skagerrak 3, the old poles Skagerrak 1 and Skagerrak 2 were converted to monopolar HVDC schemes, which run with opposite polarity to Skagerrak 3.[3] In 1999, the full 1,000 MW capacity was reserved by large power companies.[9]

In November 2009, Statnett and Energinet.dk signed the agreement to construct Skagerrak 4.[10][11] The capacity of Skagerrak 4 is approximately 700 MW with a DC voltage of 500 kV.[12] It had been in a test phase since 1 October 2014 and became operational at the end of 2014, when strong winds created negative prices despite the new connection.[13] It was officially inaugurated by Frederik, Crown Prince of Denmark and Haakon, Crown Prince of Norway on 12 March 2015.[14] As for the existing Skagerrak 1-3, the grid connection points are Kristiansand and Tjele. Differently from Skagerrak 1–3, for Skagerrak 4 a cable solution is chosen for the complete route length.[1] The 300 million DKK Prysmian land cable on the Danish side is approximately 90 kilometres (56 mi), while the 137 kilometres (85 mi) subsea cable and the 12 kilometres (7.5 mi) Norwegian land cable is to be made by Nexans for 638 million DKK.[15] Converter stations were built by ABB, as for Skagerrak 1–3.[16] The combined budget is 2.8-3.2 billion DKK.[15][17]

The technology used was VSC, capable of black start. Although thyristor-based converters have a loss of only 0.7%, the IGBTs of VSC get close with a loss of 0.8 to 1%. Skagerrak 1 and 2 previously used Skagerrak 3 as a return cable, but 1 and 2 were again coupled so that Skagerrak 4 can use number 3 for returns.[18] The cable also supplies 110 MW of reserve power.[19] Some of the 24 fiber pairs may be rented for business by telecommunications companies.[20]