Phase angle comparison and differential rate of change methods used for differential protection of busbars and transformers

Modern microprocessor based relays used for busbar and transformer protection, typically implement a low impedance percentage differential principle using dual slope characteristics. These relays may sometimes misoperate in response to the CT saturation during close-in external faults, and the subsidence current present after clearing external faults. A primary reason for such misoperations is the fact that the traditional percentage differential principle relies exclusively on current magnitudes rather than directionality for tripping decisions. This paper describes two novel principles that distinguish between external and internal faults so as to enhance the security of differential protection; directional phase angle comparison and rate-of-change of differential current. The paper also discusses mathematical models used for each principle and their interaction during operation as well as their interaction with the traditional dual slope method. In addition, the paper compares the performance of a protective device using the traditional dual slope method vs. one with the new principles. Test cases are presented to illustrate the effectiveness of the new principles in enhancing the security for external faults.