Understanding the dynamic mho distance characteristic

In order to retain dependability and security in cases of close-in faults when the loop voltage is zero, mho distance elements use cross-phase and/or memory polarization. Polarization techniques in distance elements cause the mho distance characteristic to dynamically expand and then contract as the polarization memory decays during a fault. The dynamic behavior of this mho element continues to be a confusing topic to many engineers. Many papers and text books written on the subject are mired in long derivations and produce equations that can seem abstract. This paper bridges the theory with familiar phasor diagrams and visual aids to promote an intuitive understanding of the dynamic mho expansion during balanced and unbalanced faults in the forward and reverse direction. We examine the dynamic mho expansion for common fault types, load flow conditions, and under typical relay test scenarios. We emphasize visualizing the expansion for positive-sequence memory polarization while demonstrating its practical benefits in terms of security, directionality, and increased resistive coverage.