Modern line current differential protection solutions

Line current differential protection creates challenges for relay design and application. From a design perspective, the distributed nature of the line current differential system imposes limits on the amount of data that can be exchanged between the system terminals and calls for data alignment schemes to enable the differential protection principle. From the application perspective, line current differential schemes are concerned with CT saturation, particularly in dual-breaker applications; in-zone reactors and line-charging current; in-line and tapped transformers; sensitivity to high-resistive faults; single-pole tripping; security on channel impairments; application to lines with more than three terminals; and so on. This paper reviews technical solutions to the line current differential design and application, addressing the common design constraints and utility-driven application needs. The paper is a tutorial in this challenging area where protection principles and applications mix with communications and signal processing.