Multi-stage Under Frequency Load Shedding Relay in Islanded Distribution System

Disconnecting microgrid from main grid, may lead to severe frequency changes. Under frequency load shedding method provides a proper protection scheme by shedding certain amount of loads. Under frequency load shedding relay is used for frequency control of islanded microgrid. Each synchronous distributed generator (DG) of microgrid is equipped with a protective islanding detection device such as rate of change of frequency (ROCOF) relay. When microgrid disconnects from main grid DGs may trip by activation of their anti-islanding device. Therefore this paper utilizes ROCOF constraint in order to avoid DGs tripping in an islanding condition. A discretized swing equation is used for modeling the system frequency response. Genetic algorithm is employed to minimize the amount of load to be shed. The proposed method is applied to IEEE-34 bus distribution test system.