Changing the operation mode of the sources leads to change in the fault current passing through overcurrent relays, their operating time and relay pairs coordination. In this paper, a new adaptive method is proposed to maintain the coordination of overcurrent relays under uncertainty of the presence of DGs in a distribution network. In this method, the relay settings are first calculated in the base structure (in the absence of DGs) and coordination is established. The fault currents of the new structure are then calculated from the Thevenin equivalent circuit. The Thevenin equivalent circuit is determined from the relay point of view using voltage and current measured at the relay location before a fault occurrence. Adaptive coefficients are then defined as ratios of the base structure fault currents to the new structure fault currents. Using these adaptive coefficients, the fault currents in the new structure are converted into the equivalent values in the base structure, and the coordination of relay pairs is maintained. The proposed adaptive relays coordination is compared with the conventional relays coordination in networks with radial, ring and mesh configurations. The simulation results show the superiority of the proposed method in maintaining coordination.

Keywords