In the real-time operating window, some generators may have must-run characteristics. For instance, hydro with downstream flow requirements for fish habitat or

In the real-time operating window, some generators may have must-run characteristics. For instance, hydro with downstream flow requirements for fish habitat or agricultural use or nuclear without thermal bypass. It's easy to say 'let the operators deal with the operational complexity', but if you're going to need to that nuclear unit in 6 hours you can't tell them to pound sand and poison their reactor now. Furthermore, even if the energy from some conventional generators may not be needed, they may be required to run to provide grid ancillary services such as operating reserve (the ability to ramp up or down rapidly) or to avoid violating system operating limits due to physical constraints on the transmission system. Since wind turbine generators will not experience or cause physical damage due to rapid curtailment, only commercial harm, in a scenario where the spot price is going to zero or negative (i.e. the marginal energy being produced is not valued by anyone), it can make operational sense to curtail them before higher emitting generators under certain conditions. Keep in mind I'm speaking only to the real-time operating considerations (think a time horizon of 24 hours) not the larger generation planning perspective. The system should be planned in a way that minimizes the need for renewable curtailment, but that's a long-term goal that isn't accomplished yet. Operating on a strict zero-curtailment basis for wind turbine generators reduces system flexibility and maneuverability and results in costs for all ratepayers.