What is positive to negative sequence coupling and negative to positive sequence coupling ?

Mutual coupling...

Mike

See:
EPRI AC Transmission Line Reference Book—200 kV and Above, Third Edition
Chapter 2: Electrical Characteristics of Conductor Configurations and Circuits

I’m not clear what you are trying to do, but by your terms I am assuming you’re connecting to a distribution circuit. The utility assumes their lines are perfectly transposed, which they are not, but the voltage imbalanced caused by balanced load on a poorly transposed distribution circuit is generally not a problem, I’ve never heard of it coming up. Unbalanced load, which is likely if your utility isn’t diligent about balancing circuits, is a much bigger problem that has nothing to do with mutually coupled sequence impedances. (My utility doesn’t do a great job balancing circuits, and it periodically causes significant power quality problems)

Hopefully the OP can clarify their question.

Casey-Even when I have perfectly balanced loads, I still end up with voltage unbalance at the end of the feeder. I have some balanced circuits where the lack of distribution transposition causes at least a few percent voltage imbalance at the end of the feeder. We have a couple of long circuits where the voltage imbalance from lack of transposition is the most limiting constraint.

In my region the phase B voltage on the transmission system is roughly 1% higher than phases A and C. My understanding is that this is due to phase B commonly being the middle phase. While this 1% is not directly a problem, it does contribute to other limitations. If the substation voltage regulator uses B phase, it must control to -4/+5% rather than -5/+5%. Additionally, the transmission voltage imbalance reduce the allowable imbalance within the distribution system.

For exact modeling in my region, just having the impedance matrix (e.g. Z12) is insufficient due to the transmission level imbalanced. Since the source voltages have differing magnitudes and are not precisely 120 degrees apart, one also needs the zero sequence and negative sequence voltages to precisely model things.

Well, I stand corrected - I should more accurately have stated it’s never come up in my utility that I know of.
Load imbalance, now that comes up all the time.