Tutorial on operating characteristics of microprocessor-based multiterminal line current differential relays

Line current differential (87L) protection schemes face extra challenges compared with other forms of differential protection, in addition to the traditional requirements of sensitivity, speed, and immunity to current transformer saturation. Some of these challenges include data communication, alignment, and security; line charging current; and limited communications bandwidth. To address these challenges, microprocessor-based 87L relays apply elaborate operating characteristics, which are often different than a traditional percentage differential characteristic used for bus or transformer protection. These sophisticated elements may include adaptive restraining terms, apply an Alpha Plane, use external fault detection logic for extra security, and so on. While these operating characteristics provide for better performance, they create the following challenges for users: • Understanding how the 87L elements make the trip decision. • Understanding the impact of 87L settings on sensitivity and security, as well as grasping the relationship between the traditional percentage differential characteristic and the various 87L operating characteristics. • Having the ability to transfer settings between different 87L operating characteristics while keeping a similar balance between security and dependability. • Testing the 87L operating characteristics. These issues become particularly significant in applications involving more than two currents in the line protection zone (multiterminal lines) and lines terminated on dual-breaker buses. This paper is a tutorial on this relatively new protection topic and offers answers to the outlined challenges.