The idea here on the forum is that you write the letter as well as you can, post it here, and the members will help you finish it off nicely.
So, start by introducing yourself and your company, tell the company why your group would like to visit the generating station, say something nice about their company, tell them what benefit you will have by being able to learn more about their company.
Go on, it's not that difficult.
I am not a teacher.
Student or Learner