Kalos Power Plant can be accessed from Kalos Route 13. There are 5 facilities on Route 13 but only one of them can be visited.
Ash and his friends came across the Kalos Power Plant when they were on their way to Lumiose City. They decided to visit it when Clemont told his friends some facts about it. Nearing the Power Plant, Ash's Pikachu, Clemont's Dedenne and Luxio began to spark and clutched their heads, seemingly getting headaches. Their eyes turned red and they moved towards the Power Plant. The heroes followed them and when they were inside searching for their Pokémon, they came across three men, saved them, and they told the heroes that they were captured by a man, a woman and a talking Meowth. After a while, the heroes found the controlled Pokémon and the heroes tried to save them. Having enough from everything, Clemont intervened and he got hit by his Luxio's Thunder Fang. Using Luxio's electricity, Clemont used his Aipom Arm to overload the Power Plant and the Power Plant shut down, turning the controlled Pokémon back to normal. Luxio evolved into Luxray and used Electric Terrain to power up electric-type moves. The former controlled Pokémon used a combined electric attack, blasting Team Rocket off and saving the Power Plant.
Apparently, if you haven't done the raid on the Power Plant, the game will still let NPCs past the workers stopping you from getting past them due to the blackout. However, if you play during nighttime, then the buildings will still light up at night even if you haven't restored the power to Northern Lumiose.