When evaluating your machine for a generator you have to assume that a load rejection happens and the wicket gates do not close, hence the unit goes to the runaway speed. Your generator must survive this speed the specific amount time necessary to shut the unit by additional means. Example close the headgates.
In the event of a load rejection and correct operation of wicket gates (they close) a balance needs to be reached between maximium overspeed and pressure rise in the penstock.
Using the calculated maximium pressure (+ safety margin)the penstock can handle and the maximium rotor speed a series of unit trips are performed.
Install a high speed pressure transducer into the penstock, and quick responding speed transducer on the unit shaft and if available a turbine flow transduser (ie winter-kennedy). The pressure needs to elevation adjusted.
Perform a series of trips starting at full speed breaker open than full speed breaker closed. Continue the trips at higher loads. Trips are initiated as a breaker opening.
Perform a balancing act, adjust the gate trip closing time to get the lowest over speed and lowest penstock pressure rise. Adjusting the cushion stroke should have little effect on pressure rise. After completing the testing adjust the numbers for maximium head.
This is general outline, you will need to develop specific test plans.
Overspped limits per NEMA MG-1 1993
Syn Speed % Overspeed (<1 minute)
1801 and over 20
1800 and below 25
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