Modeling turbine instant load rejection with wicket gate starting less than 100% open

Product(s): HAMMER
Version(s): 08.11.XX.XX and higher
Area: Modeling

Problem

How can I simulate transients from an instant load rejection, if the wicket gates are starting less than 100% open? (partially closed) 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%." 

Solution

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% open (still a requirement), gradually 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 maintain synchronous speed until the time of rejection. You will need to first determine the electrical torque at 100% open to match the initial conditions, then the electrical torque at your desired partially-open position. As the gate closes partially, the electrical torque will need to drop as well, to maintain a constant rotational speed. Set up the electrical torque curve to start at the 100% open torque position, then have it drop to the electrical torque for the partially open position at the same timing, then hold it constant for the same duration as the gate opening pattern (to allow for transients to settle down).

Note: 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). The pattern should have two drops - one at the time when the gate pattern drops from its initial 100% open position, and the another drop at the time of load rejection.

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.

See Also

Using Turbines in Bentley HAMMER

Calculating initial electrical torque for use with the Load Rejection turbine Operating Case

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