Comment: It appears from the posting that the swing relay is at swing bus. The swing bus is supposed to be on, therefore, a transmission line connected to the swing bus may have each end tripped or to isolate it, both ends tripped. The swing bus is applicable in multi-bus network only.  The swing bus or slack bus is a special type of generator bus that is needed by the computational solution process. There is normally one swing bus. In utility studies, a large generator is picked as the swing bus. In industrial studies, the utility supply is usually represented as the swing bus. The swing generator adjusts its scheduled power to supply the system MW and Mvar losses that are not otherwise accounted for.