After selling 500 MW of non-firm ATC from a total of 1000 MW, what amount remains as firm ATC?

The scenario presented discusses available transfer capacity (ATC), specifically differentiating between firm and non-firm capacity. ATC is a measure of the capability of a transmission system to accommodate additional electricity flows without violating reliability or operational constraints.

Initially, there was a total of 1000 MW of ATC. When 500 MW of non-firm ATC is sold, it does not impact the firm ATC directly since non-firm ATC refers to capacity that can be curtailed under certain conditions (like during peak demand or transmission constraints).

Therefore, after the sale of 500 MW of non-firm ATC, the remaining ATC still consists of 500 MW of non-firm capacity and the 1000 MW original firm ATC. The firm ATC is unaffected by the sale of non-firm capacity, which means the total firm ATC remains intact at 1000 MW.

This indicates that the firm ATC remains at 1000 MW, which is crucial for maintaining reliability and ensuring that critical services have guaranteed transmission capacity. Thus, the accurate understanding leads to the conclusion that the amount of firm ATC remaining is indeed 1000 MW.