Negative-sequence differential protection - principles, sensitivity, and security

This paper explains the principles of negative-sequence differential (87Q) protection, its basis for excellent sensitivity and speed, and the need for securing it with external fault detectors to deal with the saturation of current transformers. The paper reviews applications of 87Q elements to lines and transformers. It explains why 87Q elements cannot be used for turn-to-turn fault protection in shunt reactors and stators of generators and motors. The paper presents applications of negative-sequence directional elements for turn-to-turn fault protection in reactors and stators. Finally, it derives two new protection principles based on negative sequence for generator turn-to-turn fault protection.