runaway speed of hydrogenerator

[rotman]

Hi,
I have a question regarding meaning of "runaway speed" requirements specified for hydrogenerator sets. My doubt refers to the duration usually specified together with runaway speed value (eg. 200% of rated speed during 10 min.). I've never seen additional information about limitation of 'runaways' number in certain period of time (or generator life). Does it mean, that all elements of generator have to be designed (with regard to their machanical endurance) in fact for continuous operation at runaway speed? Are there any normative references to this aspect of runaway speed rating of hydro machines?

 

[dpc]

In most cases, the unit will be subject to overspeed every time it trips, so it is not really an unusual occurence. The generator should be able to deal with it. The time limit may be more related to bearings and bearing cooling, etc.

 

[whycliffrussell]

1. bearings are designed for a certian speed mechanically

2. motor is built with a structural integrity such that id does not 'fly apart' due to the centrifugal forces acting on the motor b/c it's spinning

*i'd be more worred about 1! the motor will most likely have to rotate at a phenominal speed to 'fly apart'...

 

[waross]

hello whycliffrussell
In the case of hydro-generators, we may be dealing with a rotating mass of several tons and a diameter of 10, 20 or more feet. The rotation may be around 100 RPM (depending on the number of poles and the freqwuency). The designers must be concerned with centrifugal forces. The self destruction speed may be less than "phenominal".
respectfully

 

[davidbeach]

A hydro plant that suffers a total (or near total) load rejection will go into a severe overspeed condition, speeds approaching 2 per unit. It just isn't possible to shut off that column of water fast enough to avoid the overspeed.

 

[wbd]

That is true davidbeach. The wicket gates on the hydro units at my facilities are timed to fully close in 15 - 30 secs on a full load rejection. If fast close the forces on the penstock are tremendous. The overspeed seen is about 2 times normal running speed and I know for sure that one unit is rated to run at overspeed for an hour.

 

[edison123]

rotman

To answer your question, yes, the hydro generators are designed to withstand the stress to runaway speed which varies from 1.7 to 2.1 times depending on the turbine type.

I have seen remnants one 50 MW hydro generator completely wrecked and it was proved the design was inadequate.

The wreckage is still kept outside the power house as a museum piece to show how things fail.