Transmission line differential protection with an enhanced characteristic

This paper describes a new digital line differential relaying system with an enhanced characteristic. In the paper, we first present the concept of the complex current-ratio plane for the analysis of current-only pilot unit-type relay characteristics. Next, we briefly discuss different unit-type protection relaying systems including their advantages and disadvantages. We then propose a new line differential relay characteristic in the current-ratio plane, describe the relay design, and finally present and discuss the relay performance using digital simulation of the algorithms. The new relaying system design, presented in this paper, provides sensitive protection for transmission line and cable faults, and high security and stability for external faults. The relay system is tolerant of the unequal communications channel delays that are typical of modern networked digital communications channels. The operating time of this new differential relaying system is less than one cycle, and it is applicable for the protection of HV and EHV lines, including series-compensated lines.