While doing some reading on the power system resource adequacy studies I have noticed that the standard (baseline?) model for generation availability is the two state model - assumes two possible capacities, zero and maximum. It also requires that the capacities from different units are independent. While I think I get the independence reasoning for conventional and wind/hydro generation and why sometimes it cannot be achieved (e.g. wind capacities at different locations might be driven by the same weather systems - it can be windy or calm in the whole power system at some time $t$). I have a problem understanding the logic behind using two state model.
The papers are quite advanced and they talk about more complicated scenarios and thus, authors do not explain the two state model assumptions in detail. From what I have seen there's usually just something along the lines: "generating units are usually represented by a two state model" or "two-state model is well suited for modeling conventional generators". But I haven't been able to find a proper answer to WHY the two state model is justified here.
My question is whether the assumption of operating under two states, zero and maximum, is justified for conventional and unconventional generation. It might be super obvious but I just can't see why this is true.