New offshore wind farms are being proposed to be located at distances of 50–100 km from the shore. These large distances make HVDC connection to grid a feasible option. DC collection grids have been proposed to eliminate offshore platforms in wind farms. Two main topologies previously proposed for DC offshore wind farms with series connection of wind turbines to eliminate offshore platforms have been studied. The aspects of reliability of wind farm collection system are important in order to attract investors and make the designs physically realizable. Wind farms are large economic investments and in order to make good economic decisions a very important factor is the amount of additional income that can be generated over the life time of the wind farm. Thus the reliability calculations have to be included as a part of design process. The collection system reliability for DC Series and Series-Parallel topologies is mainly addressed in this paper. Various reliability indices like the interruption frequency, interruption duration and expected energy not supplied are calculated using an analytical method. The largest contributors to unreliability are identified. Alternative designs with added redundancy are proposed and evaluated. The important factor of possible additional income that can be generated is calculated to select and improve designs to be implemented. The evaluation results indicate that the DC Series-Parallel wind farms offer a great promise in terms of feasibility and reliability. It is observed that the DC Series design can be used as base system for calculations and understanding of the important factors in reliability.
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