The whole point of the missing context is that it it going to be crucial in determining the sense here.

For the sake of illustration, I'm going to mkae some sweeping assumptions. MLD says this is "something to do with power generation", so my assumption is that this is the generating equipment at a power utility factory intended to feed the national or local power network.

Now since the essais are mentioned as being 'en absence secteur' and 'en charge', I am assuming these are 2 contrary conditions; the output network or 'secteur' would represent the 'load' of the system.

So I suspect this is really off-load / on-load testing.

HOWEVER, it could also mean 'tests in isolation' with no incoming mains supply available; most generating equipment is designed to be stand-alone (otherwise not much use in a power-cut), but of course complex systems may well rely on the availability of external power to get them started, or whatever.

And finally, without knowing more about it, it could be referring to the functioning of some other piece of equipment that normally relies on a mains supply input, in which case the 'essai absence secteur' might be quite unrelaetd to the 'essai en charge'.