Under Frequency Load Shedding Plan in Active Power Systems: Analysis and Innovative Proposals

The Under Frequency Load Shedding Plan (UFLSP), also called Low Frequency Demand Disconnection Plan in EU Regulation 2017/2196 (LFDD Plan), is a fundamental element of the Defense Plan of the Electrical Power System. Its purpose is to avoid that the active power imbalances due to network separations lead to the disconnection of the generation groups and therefore to the blackout. The increase in production by the plants connected to the distribution networks in medium and low voltage makes the management of the UFLSP increasingly complex, whose optimization process started at TERNA in 2014 to ensure compliance with the required performances. In this paper an Adaptive UFLSP (A-UFLSP) is presented, based on the dynamic selection of the medium voltage lines to be disconnected. After defining a general algorithm, the main characteristics of the devices necessary for its implementation are established. The application to a real case demonstrates the capability of the A-UFLSP to remain equally effective as the load and generation conditions change in the distribution networks, even in the face of further foreseeable increases in production from Renewable Energy Sources (RES) through Distributed Generation (DG).