Semi-Adaptive Setting of Under Frequency Load Shedding Relays Considering Credible Generation Outage Scenarios

Severe generation outages may affect the power system frequency stability and cause abnormal frequency declines especially under the high penetration of low inertia generation technologies. Under frequency load shedding (UFLS), as a special protection scheme, is utilized to restore the power balance and maintain system frequency in an acceptable range. This paper develops a semi-adaptive UFLS plan in which the multistage load shedding is triggered based on the rate of change of frequency value. The proposed semi-adaptive UFLS plan activates predetermined later stage as early stage under the severe generation deficiencies. Most credible power imbalance scenarios including conventional generation outages and islanding scenarios are considered in semi-adaptive setting of the proposed UFLS plan. In order to achieve the optimal settings, the proposed UFLS plan is formulated as a mixed integer linear programming model. The dynamic of governors and load damping are considered in the discretized system frequency response. A scoring method is developed to select the UFLS settings with the most desired system frequency response. The performance of the proposed UFLS plan is verified over the IEEE 39-bus test system under credible and most likely scenarios of generation outages.