Considering transient state in interconnected networks during fault for coordination of directional overcurrent relays

•Considering transient state in coordination prevents miscoordination in practice.•The network must be simulated in the time domain to obtain the exact operating time.•The complete model of the network elements should be used to avoid miscoordination.•A new way to show the operating time of relays due to fault current redistribution.•Using relay exact formula and instantaneous fault currents improve the coordination. The object of optimal coordination in networks is to obtain the minimum operating time (OT) of directional overcurrent relays and to maintain the coordination time interval. Coordination of relays in the steady state causes miscoordination in practice. Conventional methods have some drawbacks such as ignoring the exact formula of the relay, using the simplified model of the network elements (ignoring complete models and nonlinear elements), and disregarding backup relays chain and fault current redistribution during topology transient change. In this study, a coordination method is proposed that considers the transient state by simulating the network using a complete model of the elements and precise formulas. Network simulation was performed in three stages in Simulink during the coordination. A new method is also proposed to obtain and correctly indicate the backup relays OTs incorporating the topology transient change. Initially, using one conventional method, the IEEE 8-bus and IEEE 30-bus networks were coordinated. The results show 23 and 94 miscoordinations, respectively. Moreover, since relays’ OTs show some errors, they were then coordinated using the proposed method. Consequently, all miscoordinations were eliminated and error-free OTs of relays were obtained. This study confirms that the proposed method has more practical results compared to conventional methods.