Study on Protection Coordination of a Flux-Lock Type SFCL With Over-Current Relay

In this paper, the method for the protection coordination of a flux-lock type superconducting fault current limiter (SFCL) with an over-current relay (OCR) was proposed. Immediately after the fault happens, the fault current can be limited by the flux-lock type SFCL and, if the fault continues, can be interrupted by the circuit breaker (CB) after several cycles. However, as the fault current is decreased by the impedance occurrence of the flux-lock type SFCL, the trip signal of the OCR for the interruption of the CB can be delayed and the delayed operation of the OCR can result in the failure of the protection coordination with the backup protective device as well. As the method to keep the protection coordination of the flux-lock type SFCL with the OCR, the current in a coil 3 of the flux-lock type SFCL, which only flew during the fault period, was used as the input current of the OCR. Therefore, through the adjustment of the turn number's ratio between the coil 1 and the coil 3 of the flux-lock type SFCL, the operating time of the OCR could be kept to be the same as the operating one of the OCR before the flux-lock type SFCL was applied. The suggested method could be confirmed to be available through the simulation using PSCAD/EMTDC for the protection coordination between the flux-lock type SFCL and the OCR.