132kV Inter Array System for Floating Wind Projects

Hans Petter Øvrevik, CCO, Deep Wind Offshore
The current state of the art inter array voltage is 66 kV. The voltage level was previously raised from 33 kV to 66 kV, which facilitated connection of more turbines per string, thereby reducing inter array cable length, and reducing inter array losses. The transition to 66 kV also made alternative inter array designs that resulted in higher cable availability more feasible. The benefits of increasing the voltage to 66 kV were shown to be larger than the cost increase for the cable (per meter cost) and other components (e.g. circuit breakers). As turbine sizes increase from the current size of ca. 10 MW, there may be beneficial to raise the inter array voltage further to 132 kV.

The Carbon Trust participated in making the transition to 66 kV by launching a project to qualify 66 kV wet cable design, where Nexans, Prysmian and JDR qualified designs. The Carbon Trust have since launched a new project, the High Voltage Array Systems, to look at the next inter array voltage increase, perhaps to 132 kV or 150 kV. Cable manufacturers such as Prysmian and Unitech has also stated they are looking at inter array cables at 132 kV.

Raising the voltage from 66 kV to 132 kV enables similar advantages as was identified when raising the voltage from 33 kV to 66 kV. This presentation examines the previous studies made when transitioning to 66 kV and evaluates these studies with regards to findings from wind farms that have been put into operation with 66 kV inter arrays. The findings from these studies are then updated to assess a similar transition from 66 kV to 132 kV, showing that a transition to 132 kV will be beneficial for next generation commercial scale offshore wind farms with wind turbines from 15 MW and higher.

For floating offshore wind turbines, dynamic cables are required. The current state of the art for dynamic cables are also 66 kV, used for example in Windfloat Atlantic and planned to be used in Hywind Tampen to be installed in 2022 and the EU supported flagship project to be installed in 2022/23. An assessment of challenges with regards to raising the voltage for dynamic cable designs to 132 kV is also performed.

Deep Wind Offshore is contemplating utilizing a 132kV dynamic array system when developing the Utsira Nord project offshore Haugesund, Norway, a planned 500MW floating offshore wind project in 180-280m water depth. The project is evaluating utilizing a cost efficient onshore substation on the Utsira Island, which even through longer step-out beyond 15-20km is anticipated to be commercially feasible through having higher voltage (132kV) in the array system from the wind farm back to the onshore transformer station which will reduce losses to similar levels as having a floating substation on the field. Some findings from the initial studies of this solution will be presented.