The article mentions that the turbine is French made. Presumably the French have tested it, so why do the Americans have to build such an expensive test rig? Mention is made of the possibility of overloading the grid. Surely the output capacity is known?
I do hope they are making use of the massive amounts of very hot gas, such as for supplying hot water for domestic use, if the powerplant is actually situated near any towns or cities.
Just curious, but does this type of powerplant actually produce a thrust, similar to a jet engine?

Why? Why build a machine that is destined to be a logistics nightmare purely because of its size, when a dozen or so smaller proven turbines could do the job. Also, if this thing breaks down, large populations are inconvenienced, whereas if there were multiple smaller turbines it is unlikely they would all break down, so risk is reduced to a minimum. They would be better off putting their funds into developing tidal power instead of this fossil-fueled monstrosity.

"Presumably the French have tested it, so why do the Americans have to build such an expensive test rig? "
Google says it was built at GE’s Manufacturing Facility in Belfort, France. It might have been operationally tested before shipping, but it's the largest ever built so I doubt it could be fully tested without building a new test rig somewhere in the world. It's hard to scale, but this is a very large object. In the first picture of the image gallery, notice there are 4 people standing in and around the housing.
"Mention is made of the possibility of overloading the grid. Surely the output capacity is known? "
The output is certainly know, but the amount of grid load can not be. It would be hard to test full load and fault conditions unless you know for certain that the grid can take all of that energy. If I understand correctly, the speed of the turbine is fixed to synchronize with the grid, the amount of torque is the variable?
"I do hope they are making use of the massive amounts of very hot gas, such as for supplying hot water for domestic use"
Even better than hot water, that's the difference between simple cycle and combined cycle. The high quality waste heat is used to make steam for a second turbine.
It would be difficult and costly to provide hot water for domestic use. The plant output varies, therefore the amount of waste heat also varies. Meaning the domestic load would need to vary along with plant output, and always be able to have the capacity to cool the plant. A vary unlikely situation in the real world.
"Just curious, but does this type of powerplant actually produce a thrust, similar to a jet engine? "
The article says the volume of air coming out is enormous. I suspect the exhaust housing is designed to minimize thrust, notice how the output flairs open? A jet's exhaust constricts down to increase flow velocity, providing greater thrust.
This is a very impressive feat by the GE teams, both design and testing.

What is the big deal ? Single steam Turbines of 500MW capacity are passe, a decade back. For a gas turbine, 600MW is big. But the technical challenges of a Gas Turbine are the high temperatures, which are the same for smaller Turbines, and are passe, a decade back. Once you marry the size of 500MW steam generators (sealing, bearing, lubricating, exciting, vaning) with the technology developed for higher temperatures in smaller gas turbines, it is merely routine technical design and development. I also dont see why GE had to make a new Turbine Test bed, when they have been making 500MW and more Steam Turbines. Surely the testing load etc would be the same for Steam Turbine and Gas Turbine ? N. M. Havewala havewala@gmail.com

" It might have been operationally tested before shipping, but it's the largest ever built so I doubt it could be fully tested without building a new test rig somewhere in the world." It was not tested in France (although production units will be Full Speed tested before leaving the factory). The Greenville facility is unique in the world to be able to test these units across a huge range of conditions and represents a $180M + investment by GE. Industrial, power generation Gas Turbines convert the chemical energy of the fuel into rotational energy which then drives a generator to make electricity. Modern Aircraft engines do the same but turn a huge fan to create thrust. Generators are connected to power grids and are therefore fixed in speed (60 Hz in the US) and are part of a system so rapid changes in power from a large generator (either up or down) will impact the rest of the grid. To really put Gas Turbines through their paces, the Greenville test Stand doesn't use a generator to absorb the energy. This means GE can test at variable speeds and loads (very important to demonstrate performance at various locations, rapid starts, and load rejections).
"Why build a machine that is destined to be a logistics nightmare purely because of its size, when a dozen or so smaller proven turbines could do the job" In Gas Turbines, size is power and efficiency. Double the size of a turbine and you increase the flow by 4 times and the power increases with flow. Other things like clearances and other loses don't go up as much so you get higher efficiency. The power plant actually gets much more complicated with more units so fewer is better.

What an amazing amount of cogent information provided by the readers. Thanks guys!

Let us pray that it does not alter the big spin.
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If it's spewing a Category 5 hurricane out the back end I presume they'll have wind generators positioned behind it to capture that power too.

Quite a few questions and some very good answers here. A few additional points about how such a turbine could be meant to fit within our grid structure. It comes in two formats; one at 41% peak efficiency, which equates well with current steam plants, but can come up to speed from cold much, much quicker, so its function is, like most stationary gas turbines in grid service, as a 'peaker' to add to the base load of steam plants a utility already has. Two; the combined cycle version which can bring up the efficiency to 61%, but probably has to be used for base load itself, as it is limited by the steam-side's need to keep its temperature within thermal efficiency constraints. The variable stators widen the power envelope while allowing the speed to remain constant within emissions constraints. It is interesting to note that the first attempt at a gas turbine electric plant preceded by decades the development of aircraft turbines, but could not come at all close to the peak efficiency of the steam plants of that era. This idea has finally arrived to surpass them.

LOOK OUT NUCLEAR PLANTS ===> Just imagine what will be "online" in 10 to 30 years from now at much less cost than using Nuclear...
I believe that turbines this size and even larger ones which will follow will be used to maintain Large Capacity Grid Storage batteries in various locations across the USA and make sharing Energy an even better deal, once the Grid gets upgraded.