Upgrading High-Impedance Bus‑differential Protection

High-impedance (Hi-Z) bus-differential protection is economical and clears a faulted bus very quickly (in less than one power-system cycle). Hi-Z bus-differential protection schemes have been employed for more than a century. These schemes are very popular in industrial environments. This paper presents the fundamentals of high-impedance bus protection, CT requirements, and settings methods. Also discussed are newer methods of CT supervision, and MOV health monitoring. This paper presents lessons-learned from a recent Hi-Z bus differential upgrade.