Transmission lines are typically protected using a combination of different protective relaying schemes, including time-graded and current-graded overcurrent protection, as well as distance protection.

Time-graded overcurrent protection is a protective relaying scheme that uses a series of time-delayed overcurrent relays to provide graded protection for a transmission line. The relays are set to operate in a sequence based on the distance from the relay to the fault, with the relay closest to the fault operating first and subsequent relays operating with increasing time delays.

Current-graded overcurrent protection is another protective relaying scheme that uses overcurrent relays to provide graded protection for a transmission line. In this scheme, the relays are set to operate based on the magnitude of the fault current, with relays closer to the fault operating at a lower current threshold than relays further from the fault.

Distance protection is a protective relaying scheme that uses the measurement of voltage and current at the relay location to calculate the distance to a fault on a transmission line. This scheme is typically used as the primary protection scheme for transmission lines, as it provides accurate and reliable fault detection and location.