We actually have all of the expansions. They add a small but non-trivial element to the game. The additional powerplant deck also helps; otherwise, you begin to memorize the plants and know to wait until 25/26 hit the board and whatnot.

If I have a complaint with Powergrid, it's that our group has collectively opted into a strategy of delaying the start of phase 2. Sometimes this grinds on interminably such that we skip it entirely, and then enter phase 3 with giant cash hordes and bizarrely high priced powerplant auctions. It's still pretty fun, but sometimes feels a bit broken.