Trust me, I'm a Professional
I have this funny and annoying habit of correcting even the "experts". Because I think things through, rather than just parroting what I was taught in school. The common knowledge is often correct only in special cases.
Yes, if you have 240V source, auto-transformer, two 120V 100A loads distributed across the two phases, their neutral currents flow from one to the other and auto-transformer neutral carries zero. Windings of auto-transformer carry zero.
But I've figured out when neutral carries 200A. This turns out to be a reason for the "120% rule", although the reasonings of NEC committee that I read would indicate they were unaware. Maybe one of them understood, but it wasn't mentioned.
Consider a utility pole transformer sending 120/240VAC to a house over wires appropriate for 100A (e.g 4 awg, rated 95A in a bundle of 3 current carrying conductors but I think round up to 100A breaker is allowed.)
In the house, a 100A 2-pole main breaker and 100A busbars in the panel.
The 120% rule would let me put a 20A PV breaker at far end of panel, but I'm going to outsmart it and put 100A PV breaker there. After all, driving other end of busbar means busbar doesn't get 100A + 100A = 200A anywhere because current is flowing in opposite directions to feed loads. All points in the busbar carry 100A or less, dropping to zero somewhere in between.
Stick breakers on busbar between 100A main and 100A PV breaker, with a total of 200A worth of 120V loads. That's 24kW on one phase "L1".
Let's say that's ten, 20A single-pole breakers.
Note that without PV, if more than 100A loads are active, the 100A main breaker will heat up and trip.
Feed 100A worth of PV (24kW of PV) into the 2-pole PV breaker.
That's 100A into L1, 100A into L2. Each branch circuit breaker draws off 20A from L1, So busbar carries 100A, 80A, 60A, 40A, 20A, 0A as you move along it.
Neutral bar connected to white wires returning from branch circuits carries 20A, 40A, 60A, 80A, 100A.
The PV current on L1 flows into loads, supplying 100A and 12kW, which is enough for half the loads.
With no more current available from GT PV inverter, the remaining five loads draw their current from main breaker on L1. That point on busbar between 20A load breaker #5 (counting from PV end) and breaker #6 is carrying 0A. Closer to main breaker, it carries 20A, 40A, 60A, 80A, 100A. That's 100A coming from main breaker. In addition to 100A from PV breaker.
Neutral bar has ten wires from ten branch circuits, and is fed with one 4 awg wire from utility.
We've counted up to 100A so far, at connection from circuit fed by breaker #5. Keep counting: 120A, 140A, 160A, 180A, 200A.
You now have 200A in the neutral busbar.
200A in the 4 awg utility drop cable, which causes 4x the heating that 100A would.
PV breaker is also feeding 100A into L2. No loads connected, so 100A through 4 awg L2 wire back to utility transformer.
In the utility transformer, power delivered to L2 couples by magic, I mean by magnetic, to the other winding.
This is how 24kW 100A 240V from GT PV inverter interacts with utility pole transformer (could have been your own auto-transformer) to become 24kW 200A 120V in the breaker panel busbar and certain wires. Zero power drawn from grid, from primary side of that transformer.
L1 and L2 each carry 100A at 120V (from ground), 240V from each other. Neutral from transformer carries 200A.
This does not occur with an isolation transformer getting say 24kW 100A 240V on primary. It can't deliver more than 100A to "neutral" center-tapped 120/240V secondary. A 100A 2-pole breaker protecting L1 and L2 is sufficient to protect N as well.
When an auto-transformer is used (or an isolation transformer backfed as I described above), Neutral can carry 2x the current of either L1 or L2, basically the sum rather than the difference between them.
That is OK if wires are heavy enough. Either run 4 wires from the two 120V windings of the auto-transformer to breaker panel, or join the to wires together and have a single neutral wire with ampacity twice what a 2-pole breaker is rated for. Breaker panel busbar must also be rated for that 2x current. If other sources are present (e.g. grid operating in parallel), available current from those sources must be included in the analysis, for an even higher ampacity neutral requirement.
Alternatively, you could use a 3-pole breaker to connect the autotransformer, limiting current from neutral. This could apply in your off-grid setup with auto-transformer, but with limited current from inverter you probably just want to use heavy enough wires.
In the case where utility transformer acts as auto-transformer for your GT PV (or backfeed from battery inverter), 3-pole breaker including neutral probably isn't an option. So obey the 120% rule and be less stressed, knowing neutral wire would only be moderately overloaded.