Impact of Critical Clearing Time on Over-current Relays Coordination in a Multi-Machine Power System in Presence of Resistive SFCL

Introduction of distributed generations (DG) to electrical networks has many virtues. Nevertheless, by connecting DGs to a network, there might happen some defects in coordination of overcurrent (O/C) relays. In order to eliminate these likely defects and restoration of O/C relays’ coordination, a resistive superconducting fault current limiter (SFCL) is used in series with DG. The resistive SFCL also affects the transient stability of a multi-machine power system, and as a result, change the critical clearing time (tc) of the network. In this paper, the effect of consideration of tc on the determination of optimal size of resistive SFCL in order to simultaneously satisfy both the coordination of backup and main (B/M) relays and the operation of relays before reaching tc is studied. The results show that the determination of SFCL size, without consideration of tc, is not appropriate since after installation of SFCL in the network, the network tc changes, and the determined size of SFCL causes the operation time of some relays to be more than tc, although it removes the miscoordination between B/M relays. Ignoring this point makes the network unstable at fault occurrence.

Introduction of distributed generations (DG) to electrical networks has many virtues. Nevertheless, by connecting DGs to a network, there might happen some defects in coordination of overcurrent (O/C) relays. In order to eliminate these likely defects and restoration of O/C relays’ coordination, a resistive superconducting fault current limiter (SFCL) is used in series with DG. The resistive SFCL also affects the transient stability of a multi-machine power system, and as a result, change the critical clearing time (tc) of the network. In this paper, the effect of consideration of tc on the determination of optimal size of resistive SFCL in order to simultaneously satisfy both the coordination of backup and main (B/M) relays and the operation of relays before reaching tc is studied. The results show that the determination of SFCL size, without consideration of tc, is not appropriate since after installation of SFCL in the network, the network tc changes, and the determined size of SFCL causes the operation time of some relays to be more than tc, although it removes the miscoordination between B/M relays. Ignoring this point makes the network unstable at fault occurrence.