FOR GROUND FAULT PROTECTION
Joe Mooney, P.E., Jackie Peer
Schweitzer Engineering Laboratories, Inc.
Modern digital relays provide several outstanding methods for detecting ground faults. New
directional elements and distance polarization methods make ground fault detection more
sensitive, secure, and precise than ever. Advances in communications-aided protection further
advance sensitivity, dependability, speed, and fault resistance coverage.
The ground fault detection methods and the attributes of each method discussed in this paper are:
• Directional Zero-Sequence Overcurrent
• Directional Negative-Sequence Overcurrent
• Quadrilateral Ground Distance
• Mho Ground Distance
Comparison of the ground fault detection methods is on the basis of sensitivity and security. The
advantages and disadvantages for each method are presented and compared.
Some problem areas of ground fault detection are discussed, including system nonhomogeneity,
zero-sequence mutual coupling, remote infeed into high-resistance faults, and system unbalances
due to in-line switching. Design and application considerations for each problem area are given
to aid in setting the relay elements correctly.
This paper offers a selection and setting guide for ground fault detection on noncompensated
overhead power lines. The setting guide offers support in selecting the proper ground fault
detection element based upon security, dependability, and sensitivity (high-resistance fault
Following the body of this paper is an annotated bibliography of publications related to topics
discussed in this paper. The list of references allows the reader to further research and build
upon the ideas and theory presented in this paper.
Finally, Appendix A discusses how to model in-line switching and open-phase conductors. This
appendix presents the effects of in-line switching on sensitive set ground fault detecting
elements, as well as corrective measures to ensure secure operatio