Please give the exact steps you are using exactly to change and save these power options, and where are you doing them? I am not certain now we are even talking about the same thing.
Later,after 15 min of inactivity, my screen will go black, and when I check the settings, they are back to 15,30,20 on power options
Please explain exactly WHAT is back to 15, 30, 20

Also believe you must be logged in as Adminstrator to change the settings.