The Power Plant is a part of the map with only electric pokemon and is a little 'maze'. The power plant is abandoned and full of pokemon and it's a good place for train rock and steel pokemon. For example, Aron/Lairon/Aggron can be trained there as they have an immunity to electric type attacks.
The Power Plant can be found near the end of Waterfall Pond. You need 3 Gym badges to enter this area. The construction site has lvl 50 Pikachu's so if you beat David's gym, go there to catch them.