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This paper describes several commonly applied line protection schemes, including distance schemes, directional comparison schemes using distance and directional elements, and line current differential schemes. Using analysis tools like fault trees, power system studies, and event analysis, we evaluate and compare these protection schemes in terms of speed, sensitivity, dependability, security, and selectivity. The paper considers the use of various communications channels, including direct relay-to-relay fiber-optic channels and multiplexed digital fiber-optic networks. The paper also discusses some practical considerations for evaluating line protection schemes when faced with complications like series compensation, mutual coupling, single-pole tripping and reclosing, three-terminal lines, and short lines.

The last three decades have seen significant improvements in power system protection. With the rise of digital relays, recalibration during maintenance tests became obsolete. The model of one IEEE element per electromechanical relay is not the case anymore. Digital relays build a system of elements freely combinable with programmable logic conditions. Additionally, to be able to protect the increasingly complex power system, these relays communicate with other relays, building an even bigger system. This being said, with few exceptions, like the use of RTDS (real-time-digital simulator) to simulate real world test cases, protection testing has neither changed significantly since the days of one element per electromechanical relay, nor adapted to the new challenges.