How can I simulate transients from an instant load rejection, if the wicket gates are less than 100% open? If I try to set up the operating rule starting less than 100%, the following error occurs:
"For the Operating Case selected, the gate opening values must start at 100% and decrease to 0%."
This can be done with the Load Rejection operating case:
1) Select the "Load Rejection" operating case for your turbine
2) Set up the wicket gate pattern to start at 100% (still a requirement), ramp down to your desired starting position (for example 75%) over a period of time that does not cause a damaging transient, then hold it steady until any transient waves dampen out. Some trial and error may be needed to determine the length of time needed for the system to settle down.
3) Set up the electrical torque curve to start at the initial conditions torque, wait until the wicket gate drops to the desired partially-open position and things settle down, then "reject" the load after that point by dropping it to zero within one timestep. The article seen at the bottom can help determine the initial electrical torque to enter in the table.
Note: if your wicket gate is closing in order to keep the turbine at synchronous speed (wicket gate varies to match the varying electrical load, keeping the speed near-constant), you will also need to account for that in the electrical torque curve. Meaning, the electrical torque will need to change accordingly as the wicket gate closes, so as to keep the speed constant. You can observe the turbine speed in the "Extended Node Date" tab of the Transient Results Viewer or use the spreadsheet in this article to estimate the electrical torque corresponding to the changed flow/head through the turbine.
4) Finish the wicket gate operating rule pattern by dropping it down to zero just as you would with instant load rejection (just with the time shifted forward)
5) Use the “Report History After” calculation option if you want the results to appear as if they start at the time when the system has settled down to the desired part-open position. For example if the "real" rejection occurs 90 seconds after the start of the simulation, set this option to 90 seconds and the graphs and profiles will start at the 90 second timestep.
Using Turbines in Bentley HAMMER
Calculating initial electrical torque for use with the Load Rejection turbine Operating Case