This article presents a novel per-phase differential admittance (DA) concept-based relaying scheme applied in transmission system with fixed series capacitors. For a given two-bus system, DA is defined as the difference in the sending-end and receiving-end bus admittances at a given instance. This per-phase DA relaying scheme detects, classifies and trips the faulty phase accurately with fast response. The system is modeled and validated with PSCAD/EMTDC software with real-time 400 kV system parameters, and the algorithm has been programmed in MATLAB. The global positioning system time-stamped synchronized measurement is used. Also, the proposed algorithm has been verified successfully with two-machine system hardware with real-time data. The proposed scheme uses only one-cycle moving window, sample-by-sample checking information and has a low computational burden. The benefit of this scheme is that there is no need to change relay threshold settings for uncompensated and series-compensated line. The proposed scheme is tested on a variety of fault parameters. Simulation and hardware results demonstrate the reliability, accuracy and the fast approach of the proposed scheme over other conventional schemes.