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Incorporating DC Grid Protection, Frequency Stability and Reliability Into Offshore DC Grid Planning

Various options for dc grid protection strategies are currently investigated which differ in terms of speed and selectivity. In order to compare these options fairly, the impact on the connected ac grid needs to be studied. This paper analyzes the ac grid impact in terms of frequency stability and calculates the reserve capacity necessary to retain the stability. Assuming fast frequency reserves as a possible mitigation, a probabilistic method is provided to determine a trade-off between under frequency load shedding and additional reserve requirement. In general, non-selective strategies require higher and faster reserves compared to their fully selective counterparts for both cases analyzed in this paper. Both the selectivity and the speed of protection have an influence on the reserve requirement. The dc grid topology also plays an important role in the reserve sizing by determining the impact of dc contingencies on the ac grid. The concept of reserve sizing based on temporary loss of supply shows value in terms of lower capacity requirement. A grid specific CBA study shows that it is possible to compensate the higher investment cost associated with dc breakers by the benefit of lower reserve procurement cost.