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This paper discusses the arrangement of differential relays at the Langsa substation. The ratio of CT mounted on the transformer to the primary side is 150/1 A and on the secondary side is 1000/1 A. This result is taken with consideration of the current value calculated from 127.02 A on the primary and 952.62 A on the secondary side. For the sensitivity of the mismatch error is still good because it is still under 5% on the primary side which is 0.89% and the secondary side is 1.13%. The differential current also shows the normal condition of 0.1 A and the restraint current 1.205 A. Using the ETAP 12.6.0 simulation, relay can work well when interference occurs at the current setting indication 1.29 A is greater than the operating current 1.65 A and the relay does not work during an external fault with a setting current of 1.29 A equals to an operating current of 1.29 A.