Application guidelines for microprocessor-based, high-impedance bus differential relays

High-impedance bus differential relays have been applied successfully for decades. These relays are preferred for many reasons, including their speed, security, and ease of setting and wiring, as well as the high number of breakers that can be included in the zone of protection. However, these relays present challenging application considerations. The relationship between the CT knee-point voltage and the relay setting voltage remains mysterious to many. This paper provides a review of the high-impedance differential principle. It details the performance of CTs used in high-impedance applications and shares the CT and relay current waveforms experienced during primary high-current laboratory testing. The paper explains the resulting digitally filtered signal used by the relay. It also investigates dependability for internal faults, including sensitivity for low-magnitude faults, and security for external faults. Throughout the paper, practical application guidelines are provided for common and challenging applications.