Although I have worked in (many) fewer coal-fired stations than Una, I have still worked in a few, and have no experience of seeing such large low buildings on site.
I notice the two examples are pretty much identical in design.
Here’s what I don’t think it is: coal storage; ash storage; any part of the power generation cycle. I don’t think it’s switching equipment either.
I could go along with the idea that it is some kind of storage facility (for example, for storing cable drums for the network business), that exceeds the needs of the power station itself. Possibly also containing workshops, and there is certainly an admin block adjacent.
But it’s also very reminiscent of some kind of factory, with what looks like a high-voltage power supply at the front, and boilers at the rear.