The greatest challenge of bus differential protection is dealing with false differential current due to current transformer (CT) saturation. Percentage-restrained differential relays are often applied for bus differential protection applications. The percentage restraint principle provides security by requiring that the differential current exceed some percentage of the current flowing through the zone. There are few application guidelines for choosing a secure slope setting when applying this

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... ype of relay for bus protection. This paper reviews basic concepts of differential relays and CTs and then describes a study that was performed to determine secure settings for a relay in a specific bus protection application. Results of the study and recommendations are included. Additional simulations were run to develop a general application guideline for selecting a secure dual-slope setting. Michael J. Thompson received his B.S., Magna Cum Laude from Bradley University in 1981 and an MBA from Eastern Illinois University in 1991. He has broad experience in the field of power system operations and protection. Upon graduating, he served nearly 15 years at Central Illinois Public Service (now AMEREN), where he worked in distribution and substation field engineering before taking over responsibility for system protection engineering. Prior to joining Schweitzer Engineering Laboratories in 2001, he was involved in the development of a number of numerical protective relays. He is a senior member of the IEEE and has authored and presented several papers on power system protection topics.