Investigation on reliability-driven network expansions of offshore transmission systems

As a consequence of government objectives in terms of CO2 reduction and with the aim of increasing the share of renewable energy sources within the power system of Europe, offshore sites for wind power gain more and more importance. Within the last years the installed capacity of offshore wind reached 3 GW in the German Bight of the North Sea, most of them being connected through high voltage direct current (hvdc) transmission systems. Since the capacity of installed offshore transmission systems is increasing, redundancy is of great interest to guarantee system security in case of contingencies. This contribution investigates the possibility of offshore ac interconnections to reduce the risk of wind power being curtailed due to network outages and evaluates the economic feasibility of such grid expansion projects. The interconnection of BorWin gamma and DolWin beta is chosen as study case. For this purpose, reliability analyses are performed together with load flow studies based on wind power time series. In addition, economic considerations are carried out by means of investment evaluation. Reliability analyses are performed utilizing analytical Markov method in order to clearly distinguish system states in the latter process. The results show that ac interconnection can be beneficial for distances less than 40 km between converter stations assuming moderate reliability of offshore components. Furthermore, strong sensitivities are pointed out between the economic benefit and the reliability.