Hi mull, In the field, I have seen negative sequence relays trip at a generating station step up transformer due to current imbalance. In looking through the "GE Art and Science of Protective Relaying," I found:
"When there is no zero-phase-sequence-current or voltage source for polarizing the directional unit of a ground relay, it is often possible to use a negative-phase-sequence directional unit if separate ground relaying is required. However, one must be sure that sufficient negative-phase-sequence current and voltage will be available to assure reliable operation of the directional unit for all conditions for which it must operate. In some systems that are grounded through impedance, the negative-phase sequence quantities may be too small. A negative-phase-sequence directional unit may be either a simple directional unit supplied with negative-phase-sequence current and voltage from filter circuits or it may consist of two polyphase directional units with opposing torques, as described in Chap. 9. Another advantage of negative-phase-sequence directional units is that they are not
affected by mutual induction between paralleled circuits when ground faults occur. Chapter 15 shows that directional relays with zero-phase-sequence polarization may operate undesirably under such circumstances. In spite of any advantages the negative-phase-sequence relay may have, it is used only as a last resort, because the zero-phase-sequence relay is simpler and easier to test, and because it produces more reliable torque under all conditions where it is applicable."
Hope this helps.
