Dear sky_123but beyond about 1MHz
This is true, but what I meant was for the same diameter (i.e. compare 1mm of Litz to 1mm of single wire), then at the lowest frequencies (e.g. 50Hz)
then the single core can be a better choice, because less space is wasted in insulation compared to Litz, and therefore the wire can have less resistance compared to
the same-sized Litz). Then, as you say, Litz becomes the better performer (100kHz etc), but at the much higher frequencies, then the skin effect per strand also gets compounded by each strand affecting the other strands, and causing currents to be induced in the other strands, which makes it worse than a single wire. I'm not sure on the overall detail (I'm only familiar with low-power RF, not power transformers) but certainly for the smaller RF transformers, a point is reached (about 1MHz) where the single wire becomes more preferable again, if we're comparing an equivalent-sized litz wire.
But i'm disagree with this , because i saw many power transformers for frequencies , more than 50MHZ and the powers about 12KW .to experience >1MHz for a power transformer! :