[eigenvector]

Base-load nuclear is the opposite of dispatchable. Dispatchable means you can turn it on and off at will. It takes a long time to shut down a nuclear plant and start it back up again (on the order of days not minutes). By contrast, natural gas plants can be highly dispatchable hence their use as operating reserve in many ISOs.

[breischl]

Hm. I was about to tell you that you're wrong, but then noticed you're an electrical engineer... so I guess you probably know more about this than I do.

But, my understanding is that dispatchable is the opposite of intermittent generation (solar, wind), because you can choose when it's running or not. Also, baseload is the opposite of peaking, because baseload generators take a relatively long time to turn on/off.

Thus nuclear would be dispatchable, baseload power since you can decide when it runs, but it takes a long time to get there.

Or am I totally off base here?

[eigenvector]

Well yes, it's dispatchable in that sense. If you frame dispatchable as a binary property, it's certainly dispatchable in the way that wind is not.

But if you think of it from a power system planning perspective of economic dispatch (not just, can I dispatch - but can I afford it), nuclear isn't really there. Yes, you can E-stop a nuclear plant in seconds, but the cost is astronomical. Some nuclear plants like Bruce NGS in Ontario have thermal bypass - this increases their dispatchability by allowing them to dump steam and reduce electrical output quickly without touching the thermal output.

Nuclear's dispatchability doesn't really counter-act wind or solar's lack thereof since the time scale you're looking at is much different. Being able to start and stop my nuclear plant in 72 hours doesn't really help me if the wind stops blowing for a couple of hours. So in practice, you'd build other, more easily and economically dispatchable assets to meet your needs.

http://en.wikipedia.org/wiki/Economic_dispatch

[ghshephard]

My company does work in the UK, and we're putting together communications system that will allow the wind generations systems to stop loading the network when the transmission lines get too hot (apparently it's less expensive to shut down the wind power x% of the time, than it is to build new transmission lines).

I'm wondering if you have any insight into what happens when our comms system sends a signal to those wind turbines and tells them to "Stop" - it has to happen fairly quickly, we have working factors of 15, 30, and 60 seconds, at which point we start escalating and instructing groups of wind turbines, and then eventually the whole farm to cease production. The wind turbines also have a keep-alive that has them auto-shutdown if they lose comms, as obviously it's far more important that the transmission line isn't damaged, than it is to stop producing energy for awhile. (From the perspective of the Distribution Utility, obviously the private turbine owners take slightly different perspective).

I see you have familiarity with wind farms, and I'm wondering if you know what the turbines do - do they free spin? Send load to ground? Come to a halt?

[eigenvector]

The primary way that a wind turbine "stops" is by pitching its blades (in some vendors' terminology they call this "pause" because the machine is still connected to the grid just not producing any energy). On modern machines this can reliably be achieved in less than 3 seconds. There is also a mechanical brake that can be applied for emergency stop in 1 second or less. However, mechanical braking is not to be used in normal operations.

However, going to the maximum ramp-down rate (which for a large wind park can approach hundreds of MW/s) is usually not desirable because it will impact grid voltage. At most wind parks voltage control is accomplished using on-load tap changes of the main transformers as well as switched capacitors. Both of these need a time delay of at least 10 seconds to avoid wearing out quickly so if you ramp your whole wind farm from max output to zero in 1 or 2 seconds, you will see a big impact on grid voltage. So if we need to ramp down for a transmission system thermal constraint, we will do it at the slowest possible rate that still meets the transmission utility's needs.

[ghshephard]

Ah, that makes perfect sense. It also explains why we have an expectation of a minimum of 15 seconds before we see any response to a command to stop loading the transmission lines, and why the emergency command to the entire wind-farm doesn't occur until all other options have been exhausted.