Can I use an electric plane to to smoothen large boards/planks?
I have done what you want to do with a electric hand plane. It will leave ridges where the cuts lap. If you want it flat, one side needs to be ran over a jointer of the proper width for the piece then a thickness planer to get a uniform thickness across the board.

As I mentioned, I have done this, but as a prelude to running the material through a planer. Instead of using a jointer, I would run the electric hand planer over the surface here and there to flatten it with care, then run the other side through the thickness planer. once that side is flat enough, I flip it over and run the side I used the hand plane on to remove the ridges left by that method.