Rotational Inertia Analysis and Underfrequency Load Shedding Strategy of a Microgrid System

This paper takes the transient response of the energy storage system (ESS) into account for the study of the rate of change of underfrequency and load shedding strategy of a microgrid system. The rotational inertia of the existing power system can be contributed by tradition power generation sets. The strategy of Underfrequency Load Shedding (UFLS) is based on the rotational inertia of synchronous generation sets and transient response of the system frequency. Since a microgrid system consists of more distributed energy resources, due to its lower rotational inertia, the traditional strategy of UFLS may not be applied to a microgrid. Hence, the formula for the curve of rotational inertia of a voltage source based of an ESS has to be analyzed, to plan for the strategy of UFLS for a microgrid system so that when a failure occurs, the renewable energy keeps generating power and the feeding of important loads. An UFLS strategy is proposed in this paper, and it is verified in the microgrid in Institute of Nuclear Energy Research (INER) in Taiwan. It shows the proposed UFLS strategy can recover the microgrid to be stable in voltage and frequency within 800ms after the disconnection from power grid, while PV and some loads are kept connected. Hence, the proposed UFLS strategy can be applied to keep the important loads in stable operation when the demand from loads is larger than power feeding in a microgrid under the islanded mode operation.