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# What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/08/2011 4:37 PM

I need to know the 3 phase short circuit torque(lb-ft) on a 450 MW generator turning 3600 rpm at the turbine generator coupling should protective relays fail to function.

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#1

### Re: What is the 3 phase short circuit torque on a 450 mw generator

05/08/2011 6:55 PM

Follow the manufacturer's guidelines. I think that you'll find them a good place to start.

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#2

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/08/2011 11:10 PM

Cool Question, Professor. It will be quite a bit more than the textbook answer for torque loading at nameplate KW. 'Scuze, me, that would be MW. Lots of torque. The literature for the equipment would assume proper protection, and thus spec out mechanical stresses in line with nameplate values.

A question comes to mind: The length of conductors in the short-circuit loop would come into play. If there is some, albeit very small, resistance in the loop, that would result in less instantaneous braking torque than a theoretical big contactor across the generator output terminals.

We use a "Dynamic Braking Resistor" in motor circuits to limit armature current/resultant torque. Infinitely low resistance results in things coming to a stop rather abruptly.

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#4
In reply to #2

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 6:30 AM

Not my field, but just a thought - won't the prime mover stall? I estimate running torque 106 ft.lb and generator input power clearly a bit higher than 450MW. For reasons of economic design, I wouldn't expect the prime mover to have much in hand over that, and short-circuit condition will be a lot higher.

If that's rubbish, somebody tell me.

Cheers..........Codey

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#6
In reply to #4

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 9:11 AM

Yes the short circuit torque would be much higher but I'm still in need of SCT(short circuit torque) for the coupling design on driver end.

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#5
In reply to #2

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 9:08 AM

The conductors are 25 feet long. In the power business we call the stationary winding the stator windings and the rotating the "field".

Thanks, Turbine professor

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#3

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 3:55 AM

Find out the short circuit power, you got the rpm. that's all you need as input data, see this http://www.engineeringtoolbox.com/angular-velocity-acceleration-power-torque-d_1397.html

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#7

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 9:43 AM

If I were you, I would design the coupling to be able to handle the 450MW at 3600 rpm and as a measure of security, take a margin (that is multiply the value) of X such that The result is below what the Turbine shaft is capable of handling. (Also what margin ??).

Otherwise, if the short circuit torque is high enough(and it will be great!), the Turbine shaft will be damaged! Therefore, it is better that the coupling fails in a controlled way to protect the Turbine.

The Alternator should be protected by it's devices(electrical) and failing that, it is better that the Turbine escapes the eventual damage (The Alternator only will be damaged in this case ...)

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#8
In reply to #7

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 9:59 AM

Agreed, why not make the coupling release torque about 1/2 way between generator full-load torque and turbine max running torque?

Presumably there are controls to stop the turbine running away when the clutch operates.

Cheers.........Codey

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#9
In reply to #8

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 12:37 PM

Sorry but there is no clutch! Rigid solid coupling bolted together with 16ea 3.0" diameter bolts 12" long.

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#10
In reply to #9

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 1:52 PM

Yes, I can't imagine a clutch for a 500 kW turbine coupling.

But this is more reason to make sure that the short circuit torque will rather shear the coupling bolts rather than twist the turbine shaft (or shear it).

For the shourt circuit torque, I am afraid that I cannot give you a figure. The OEM for the alternator should be able to enlighten you on the maximum possible short circuit current (depending on where it is taking place...?).

The Turbine maker will also be the one to give you the limits not to exceed in an emergency like that.

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#11
In reply to #9

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 3:36 PM

OK I misunderstood, I thought you meant an overtorque protection clutch. Can you design the coupling (and everything else) for the stalling torque of the turbine (plus a safety margin)? Thinking about it again, the turbine stalling torque could be a fair bit higher than the maximum running torque. The turbine supplier should be able to give you a figure.

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#12

### Re: What is the 3 Phase Short Circuit Torque on a 450 MW Generator

05/09/2011 6:22 PM

Don't know, but I bet it would be exciting!

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