For the future, a large-scale expansion of offshore wind energy is expected in Europe. To collect this wind energy and to enable electricity trading between countries, an offshore network will be implemented in the North Sea. Maintaining a high level of security of supply at affordable costs is one of the key objectives in the design and operation of power systems and therefore, the reliability of offshore grids is an important topic of discussion. Whereas onshore, the security of supply is assured by reliability criteria like n-1 redundancy, the same n-1 redundancy might not be an economical solution for offshore networks. For todays (small) offshore networks, n-1 redundancy is hardly economically justifiable, seen from a wind farm owner's point of view. The question then arises how the reliability of large-scale offshore networks should be evaluated and what measures can be taken to maintain a high security of supply onshore. This paper aims at discussing this topic by reviewing the results of recent research work. It is found that whereas for smaller offshore networks reliability evaluation is mainly an economic analysis seen from a wind farm owner's point of view, for large-scale offshore networks, it is necessary to consider the interaction of offshore–onshore networks in reliability analysis. It is proposed to analyze the reliability of combined offshore–onshore power systems in an integrated approach, such that various (offshore and onshore) measures can be considered to find the most economical solution. This article is categorized under: Wind Power > Systems and Infrastructure Energy Infrastructure > Systems and Infrastructure Energy Systems Economics > Systems and Infrastructure.

Western Europe is heavily investing in offshore wind power, but the costs of these wind farms remain high. For that reason, current transmission grids for offshore wind energy are predominately radial to reduce investment cost. To date, selection approaches of offshore grid configurations are mainly based on the level of reliability. This paper introduces an alternative selection approach to comprehensively assess the net present value (NPV) of near-shore power grids. The assessment encompasses, among others, a load flow analysis, as well as the sensitivities to parameters like failure rates, repair times and energy prices, for different grid configurations (i.e. radial/ meshed). A case study of the Dutch offshore transmission system shows that this approach helps in finding a suitable tradeoff between lowest feasible NPV, while also improving the reliability (in terms of reduced lost energy) of the offshore grid.