Hi romanboy,
Unfortunately, captains are not automatically promoted to the rank of General after winning a Heroic Victory, although gaining one helps. Captains are usually promoted and added to your family tree if a faction has a small general to province ratio and the captain has won a high quality victory - either heroic or clear.
Heroic victories are usually triggered by the odds, specified on the pre-battle screen, being heavily against the concluding winner. It also helps if the winner vanquishes most of the enemy while taking as few losses as possible himself.
For Clear Victories, although the odds being against the concluding winner helps earn this sort of victory, the victor has to have vanquished a large proportion of the enemy forces while taking a small number of losses himself.
Sorry for the lack of clarity on this matter, but, sadly, as far as I know, no research has been performed on the matter of victories and captain promotion.
Hope this helps you, my appologies again for the lack of details, good luck, cheers!
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