Feasible region of under-frequency load shedding and its application

Under-frequency load shedding (UFLS) is an effective way of reducing the imbalance between generation and load before the system frequency falls under the minimum allowable value. However, the load shed during the UFLS procedure may sometimes be insufficient or excess for system frequency to recover. This paper introduces the concept of the feasible region of UFLS (FROUFLS) which could force the perturbed system to a new equilibrium state with acceptable frequency. The proposed FROUFLS is composed of upper boundary and lower boundary for each stage of UFLS. The upper boundary of stage (UBOS) is the maximum percentage of load, which guarantees system frequency will not exceed the maximum allowable value. The lower boundary of stage (LBOS) is the minimum percentage of load, which guarantees the system frequency will not be under the minimum allowable value. The FROUFLS varies with the load on different feeders. It can be employed in monitoring the feasibility of an UFLS program. Evaluations for the FROUFLS in a power system in north China are described in detail to demonstrate the effectiveness of the FROUFLS.