Although I cannot find the Welsh "Powell" tartan that you speak of in the Scottish Register of Tartans, there are a number of tartans that are "Surname" of Wales, all with the same entry, to wit:

Despite there being no known tradition of tartan in Wales, this is one of a growing series of commercial Welsh 'name' tartans marketed by the Wales Tartan Centre in Cardiff. Their inclusion in this Index does not confer upon them any historical or genealogical credibility and the use of the words 'of Wales' is not of conventional territorial significance but is purely to identify the source and thus avoid confusion with surnames which have a genuine tartan connection. Designed for The Wales Tartan Centre in Cardiff by Sheila Daniel of Cambrian Woollen Mill, Powys. They're unusual in that almost all of them incorporate odd numbered threads and have quite different warp & weft, both in thread numbers and sometimes colours. This places the classification of some of them as tartans in some doubt. Welsh Tartan Centre Cardiff.

They are dated 2002 onwards.

Your may well be one of these, perhaps some research at the Wales Tartan Centre maybe bear fruit.