Reliability-based Reserve Connections in the Sheringham Shoal Offshore Wind Farm - A Technical-Economical Evaluation of Increasing the Reliability in an Existing Offshore Wind Farm
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- Institutt for elkraftteknikk 
Offshore wind has become one of the fastest growing renewable energy sources over the last twenty years. It has the advantage of having great energy generating capabilities, which may be of good value for the energy companies. Due to the constantly increasing power demand, the trend today is towards larger components, more complex control systems and locations further away from shore. As the wind farms are supplying more customers, the need for reliable electrical systems becomes critical. Studies conducted in the area of offshore wind and reliability may therefore be very interesting. In this master thesis, five layouts with reserve connections have been compared with an existing offshore wind farm, Sheringham Shoal, located on the east coast of England. The wind farm is installed with 88 turbines and has a total capacity of 316.8 MW. It has two export cables connected to different offshore substation platforms, each at 132 kV, and several inter array cables at 33 kV. No reserve connections between the radials are installed, and the benefits of investing in such connections are therefore investigated. A reliability analysis is conducted to find the expected power delivery for each layout. The Relrad methodology is introduced and discussed, and used to find the energy not delivered to shore and the annual system availability index for the layouts. In reliability analyses done for regular distribution systems, the energy not supplied (ENS) to the load points is found. This index would have to be modified, due to the consideration of the load points in this master thesis, looking at each turbine as a negative load point consuming negative power. To fit the wind system analyzed, the previous index is modified and set to energy not delivered (END) from the load points. In addition to the reliability analysis, a power flow analysis is carried out to investigate if the power flows occurring during failures are manageable. If the power is exceeding the cable capacity, actions have to be taken if the reserve connections are to be installed. To find the most profitable reserve connection investments, an economic analysis is also conducted. The potential income from the saved energy is compared with the additional investment cost, and a profit for each layout is calculated. The result shows that for the three first layouts analyzed (Layout 1-3), the expected energy delivered to shore has increased compared with the radial system, and the profits are positive. The two remaining layouts (Layout 4 and 5) have a significant increase in the expected energy delivered to shore compared with all previous layouts, and are much more reliable. Layout 4 however, is discarded as a result of the power flow analysis and Layout 5 has, as for now, a negative profit. The sensitivity analysis shows that the results are very much depending on the failure rates and repair times given to the reliability analysis, in addition to the energy price used in the economic analysis. Layout 2, with four reserve connections installed, is found to be the best layout of the ones analyzed in this master thesis. In this layout, the reliability has increased and the profits are high, and is proven to be a good alternative for the existing Sheringham Shoal. In addition to investing in reserve connections, a control system should be installed to prevent the power flow from exceeding the cable capacity. The reliability analysis do also show that in future systems, a connection between the two offshore platforms should be considered because of the significant reduction of energy not delivered to shore. If the energy price increases both Layout 1 and 5 becomes more profitable, and are therefore good alternatives to the one suggested in the Sheringham Shoal offshore wind farm.