Back to the OT.
Scale down a bit. What happens when your portable generator puts out more power than is being produced?
For one thing, it can't. It's either being made into juice or used up as heat. Same for a big steam turbine.
As the load increases on your home generator, the fuel consumption increases.
The same is true for steam turbines and gas turbines. It takes less fuel to free wheel any generator than it uses under a full load.
Agreed, when it comes to a steam turbine it is hard to imagine a throttle and a governer being employed but that is just how it works. There are valves that regulate the steam pressure.
Now here is where the big difference is. You can't just back down the steam supply like letting off the throttle. If the amount of steam being rejected by the regulation valve exceeds the amount that the liquid pumps are able to remove from the system, the steam is explosively discharged into the atmosphere via pop off valves. This is known as a 'power pop' and if you are anywhere near the exhaust of one of these when they pop off you will never forget it. I was working on the top of a 6 turbine steam plant when they popped off at 1250 psi. I thought the world was coming to an end. Some systems can hold up to 2500 psi before they need to be popped. That is like a large bomb going off. Following the explosion there is a deafening rush of steam that sounds like a jet fighter engine that lasts from a few seconds to a minute or two. It's really, really scary to be around. Especially when you aren't expecting it. Trust me, I know from first hand experience.
Gas turbines, as you can imagine, just require less natural gas which can be turned off instantly via throttle valves.
It may interest some of you to know that in some of the gas turbine peak plants there are steam turbines that run off the heat from the exhaust of the gas turbines. The process is called Heat Recovery Steam Generation, or Hertzig, as we referred to it. The pic on this Wiki page looks just like the one I worked on in Zeeland, MI.
http://en.wikipedia.org/wiki/Heat_recovery_steam_generator