Optimal and predictive under-frequency load shedding against critical islanding contingencies

Grid separation is a dangerous and often severe contingency. For this reason, ensuring a secure N-1 operation with regard to the potential electric islands is of great importance. Nevertheless, situations appear where operating the system closer to its limits is more effective, and consequently, grid operators are challenged to perform a difficult risk assessment. This paper proposes a procedure that could assist the operator in deciding whether the system can be operated in the so-called N-1/2 secure state (with respect to grid islanding) or not. The proposed method is a model-based procedure that pre-calculates the optimal amount of under-frequency load shedding for a known potential grid separation. It is based on a simplified dynamic model, and a sensitivity-based optimal power flow as presented in [1]. The functionality is evaluated with two scenarios in the IEEE 39 bus system and has shown satisfactory results.