Abstract
The objective of this thesis is to show, analyze and compare the options for connecting offshore wind farms (OWF) to shore, the infield power collection and the physical installation procedures of the turbine connection in order to provide recommendations for future developments. For the offshore wind farm near Egmond (Netherlands) specific solutions for the infield power collection were developed.
The power connection for the entire OWF was divided in three sections, the shore connection transporting the electricity from the farm collection point to shore, the infield power collection which deals with the connection of the turbines to the shore connection point and the turbine connection describing the required physical installation procedures to connect the turbines to the infield power collection. The first step was to give an overview of the options to connect an OWF to the onshore grid connection point where the produced electricity is handed over to the integrated power grid. The options for the shore connection are alternating current at infield power collection voltage level, which is usually in the medium voltage range, alternating current at high voltage and high voltage direct current. At 100 MW transfer power the coverable distances with medium voltage are 15 km, 100 km for high voltage and high voltage direct current for distances larger than 100 km. For the Egmond OWF a shore connection at infield power collection voltage level was a starting guideline.
For the infield power collection at Egmond OWF several connection schemes were developed and applied for a given geometrical layout. Their implications on the electrical infrastructure were investigated and compared in respect to cable dimensions, required switchgear and redundancy aspects. The layout schemes can be divided in string and loop layouts. String layouts are simpler, require less switchgear than loops, but have low redundancy possibilities. Loop layouts require extensive switchgear equipment, but have high redundancy capabilities. The number of turbines per string or loop determines the required cable dimensions. A medium voltage cable can transport about 50 MW of load, limiting the maximum number of turbines per string or loop. To optimize the cable dimensions for specific loads per string or loop a thermic model of the cable was developed. The model showed that the installation parameters like laying depth, soil type and surrounding temperature have a high influence on the actual power transport capacity of the cable and therefore close investigations of the actual conditions on site are required to optimize the cable dimensions. The number of strings or loops at a specific layout determines the required switchgear at the shore connection point. With the developed connection scheme it is possible to house the shore connection switchgear in an additional standard container at one turbine; an additional platform offshore is not required. This is valid for shore connections at infield power collection voltage. The cable failure rates determine the required redundancy and selectivity of the cable layout. Redundancy can be obtained with multi string or loop layouts with extensive switchgear (3 power switches per turbine). But with currently used failure rates (1 cable fault in 25 years) the extensive switchgear is not justified by the gain in energy production. With the before stated parameters a 3 string layout with simple switchgear (1 power switch per turbine) is the optimal layout for the OWF near Egmond.
The physical turbine connection for OWFs was investigated for monopile foundations. The basic installations steps are laying the cable close to the turbine with a cable installation vessel, inserting the cable into a cable riser (J-Tube) at the seafloor and pulling the power cable to the turbine connection point above sea level. In standard installation procedures used by the offshore oil industry this process is supervised by divers or remote operated vehicles (ROV). Scour protection is also an issue that has to be considered, as ether scour protection has to be app lied around the monopile or additional precautions have to be undertaken to cover the scour hole, for example with an horizontal extended J-Tube. To bypass the need to either apply scour protection or cover the scour hole directional drilling can be used to drill a well inside the monopile and resurface it at a scour save distance from the monopile. This installation approach has promising aspects, but needs further development to become a standard procedure.