So the thermal power plant is not owned by your company. OK ?
You have been given the facility and infrastructure to operate and maintain. OK ?
You have competency to do this and when you do this the output is the electricity. OK ?
Here your business scope and QMS scope is simply...
"Operation and Maintenance of thermal power plant"
Your output is a service and you have a huge customer property that are in your control and being used. Your QMS has nothing to do with 220V / 440V / 50Hz / 60Hz /3 Phase ...etc etc.
Your operation and maintenance are as per the thermal plant owner's specifications and recommendation and you do not do anything outside this domine. There is a clear design and development exclusion here.
Wow

You opened Pandora's box, by the part I marked in the quoted text!
Before I start, I'll say that I agree with all you observations so far. They are very simple and logical.
Believe or not, till last year we have excluded 7.5.4 from our QMS. Then, recently someone noted that we at least have employees diplomas and their work cards kept in the company - and that's considered as "customer property". So we included 7.5.4, but not because of the bigger reason, you mentioned!
We are public organization, owned by the state, and we have our "mother" company which directs every power plant what to do. We do not sell our electricity. We just "Operate and Maintain the thermal power plant"
Huh. Complete disaster and mess...
Best Regards,
Vladimir Stefanovic