Does Skin Effect Happen during Alternating current? Or just changing current?

i've read it happens during AC-- current that reverses direction every cycle.



But i've been told it happens even with a positive-only wave.

It's the same thing, the only difference is the DC offset.

The wave below has many pieces added together. Any DC on the wave will pass down the wire with out skin effect problems.
The main frequency will have to fight skin effect. There are high frequency harmonics that will cause even larger skin effects.

Each piece of the square wave will cause different skin effects. You almost need to think of the square as many different pieces.

If your picture was a sign wave (0.0.5,1) you will have 0.5 DC and the one AC part, with no harmonics.