Influences of the time delay on the control effect of under-frequency load shedding in power systems

Recently, the worldwide blackouts have shown that disadvantages of under-frequency load shedding (UFLS) devices such as settled threshold, constant time delay and determined load curtailments may result in frequency instability by executing insufficient or excessive load curtailments. Based on the analysis of frequency dynamics, this paper analyzes the influences of the time delay on the effect of UFLS. Simulations illustrate that whether the time delay has influence on the effect of UFLS depends on the magnitude of the active mismatch power, while the detailed influence of the time delay on the effect of UFLS depends on the disturbance location. The change of time delay may have obvious influence on the effect of UFLS, therefore, to guarantee the frequency stability, we could adjust the time delay of UFLS devices in real time according to the practical operating condition of the power system.