Method for identification of grid operating conditions for adaptive overcurrent protection during intentional islanding operation

Currently, it is considered that distributed synchronous generators may operate in intentional-islanding operation mode, in which a portion of the power grid, which consists of load and distributed generation, is isolated from the remainder of the utility system. However, the fault current level of the feeders drastically decreases when the utility grid is disconnected and the protection will not properly operate if the overcurrent settings are not changed. In this paper it is proposed a method for identification of the grid operating conditions, utility connected or islanded. The method is based on the use of a local scheme installed close to the protection device. The scheme uses a thyristor which is fired during a short waveform interval of voltage. The thyristor voltage and current are monitored and used to calculate the grid equivalent impedance. According to the grid equivalent impedance it is possible to determine whether the utility is connected or not. Simulations considering a distribution system and a distributed synchronous generator are used to validate the proposed method. The results show that the method can be used to update the protection settings when the operating condition of the system is changed from utility-connected to islanded operation mode or vice-versa. As a result, the method allows implementing an adaptive protection without the use of communication system or can be used as a backup to increase the resilience in case the communication is lost.