IEEE Std C37.243-2015 pdf download – IEEE Guide for Application of Digital Line Current Differential Relays Using Digital Communication .
3. Definitions, special terms, acronyms, and abbreviations 3.1 Definitions For the purposes of this document, the following terms and definitions apply. The IEEE Standards Dictionary Online should be consulted for terms not defined in this clause. 3 current ratio plane: A geometrical representation in the complex plane of the ratio of the two phase or sequence current phasors measured at all terminals of the transmission line. ping-pong method: A method when relays are exchanging timestamps over communication channel in order to calculate channel delay and align current measurement values from all terminals of the transmission line. 3.2 Special terms line current differential scheme: A protection scheme designed to detect faults by measuring the differences between the currents at the terminals of a transmission line. Data from the relay at the remote terminal(s) is provided to the local relay by using a communications channel between the terminals. This data is used by the local relay to reconstruct the phasor of the currents measured by remote relay(s). Phasors of currents measured by the local relay and remote relay(s) are executed by the differential algorithm to determine if a fault internal to the line exists.
4. Current differential line protection applications 4.1 General The most common pilot line protection today is directional comparison by use of distance relays. However, the increased availability of digital communications channels has renewed the interest in current differential line relaying. 4.2 Current differential versus other forms of line protection relaying In comparison of line current differential protection schemes to other forms of protection, there are advantages and disadvantages. Primarily the selectivity of the system has significant advantages as compared to inherently overreaching schemes such as overcurrent and distance relaying. Differential protection reacts to faults in a specific zone on the power system and when properly set and applied does not react for faults outside of that zone. This advantage does present a shortcoming of the protective scheme itself in that while confined to a specific zone, it offers no backup protection for faults outside of its zone of protection.