Wow - interesting problem. I have a SW5500-plus... and a TX6A and am curious!
Has this always happened or is it just being seen now with no other changes? Does the problem persist if the master inverter is swapped to the other of the pair? Has anything changed on the battery-generator leads to unbalance them? Have you heard of that happening to others using stacked SW5500's? Did Xantrex Support have anything to say? Have you pitched the problem-questions at the
Outback Forums?Just a note, when stating transformer or other inductive power its deigned Volt-Amperes (VA) for the label ratings, so that's a better search term on your hunt... Trace/Xantres had a larger auto-transformer the TX6K that was rated for 6000VA, versus the 2900VA allowable on the T240.
I'm not sure the autotransformer design being inherently non-isolated is what you need - more like a 1:1 ratio isolation transformer capable of 12,000VA constant duty which is a manly hunk of iron! I know toroidal transformers can be stacked in parallel to increase the through-put amperage, not sure about autotransformers.
Long ago on the Web, and now lost to time, was a project where someone wound their own high-power toroid transformer using nearly a roll of narrow steel pallet banding wound up opposite from what it was on the roll it came on - several thousand continuous strand tiny doughnuts using teflon anf kapton tape as inter-layer insulation on a form about two feet in diameter... It'd be unwind the coil onto a long board to act as the bobbin of a winding loom to pass through the eye of the toroidial circle and cinch it down tightly as its being wound. Not something to attempt lightly - but something I always wanted to try someday..
There are many listed on ebay -
http://www.ebay.com/sch/?_nkw=Toroidal%20Transformer , today I'm seeing a 5kVA 220:220 direct from India for a mere $300 with free shipping.