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Selective and decentralized underfrequency protection schemes in the distribution grid

The increasing feed-in of renewable energy sources leads to fundamental changes in the European energy system. Due to the integration of converter technologies the electrical frequency experiences a higher sensitivity towards severe disturbances leading to new challenges for underfrequency protection. Hence, if an imbalance in a power system threatens its stability, automatic load shedding is the ultimate countermeasure to prevent large-scale blackouts. In conventional under frequency load shedding schemes relays are located in primary substations and trip when a critical frequency is detected. The tripping process disconnects a part of the grid from the system to reduce the overall load. In future scenarios the risk of isolating renewable energy sources due to conventional load shedding schemes rises because of a high penetration of photovoltaic and wind farms in the medium and low voltage level. Therefore, it is necessary to develop new load shedding strategies to ensure a selective underfrequency protection process differentiating power consumers and producers. In this paper, the localization of underfrequency relays in lower voltage levels is investigated making use of already existing information and communication infrastructures of smart grids. In addition, novel parametrization algorithms are presented that allow for an optimal allocation of loads that needs to be shed in every step based on real-time measurements or forecasts, therewith increasing the accuracy of the load shedding scheme.