Problem

When using the Load Rejection turbine Operating Case, a table of time vs electrical torque is required. What does this electrical torque represent exactly, and how can I calculate what it should be set to for the time zero row in this table, so that the electrical torque in the first transient timestep matches the electrical torque?

Background

With the Load Rejection operating case, the user must specify the time at which the load is rejected and over what duration of time that occurs. This is done by way of the Electrical Torque Curve, where a table of time vs. electrical torque is entered. This allows you to control exactly when the rejection (drop in electrical torque) occurs, and over how long. Contrast this to the Instant Load Rejection operating case, where the electrical torque is assumed to drop to zero instantly at the beginning of the simulation.

Note: if you simply want the load to be rejected instantly at time zero of the transient simulation, set the Operating Case to Instant Load Rejection. In this case you will not need to enter the electrical torque curve.

In a Load Rejection scenario, the turbine is initially in balance with the electrical load. Meaning, the hydraulic torque at the turbine, minus efficiency losses, is equal to the electrical torque load on the generator, so the speed is constant. When the load rejection occurs, there is no external load on the turbine-generator unit and the speed of the runner increases rapidly until the wicket gates are closed.

So in summary, you basically need to enter the shaft torque of the turbine in the initial conditions, for the time zero entry in the electrical torque field, because the shaft torque accounts for the turbine's efficiency and is equal to the electrical load demanded by the generator.

Solution

If you need to use the Load Rejection operating case and are not sure how to calculate the initial electrical torque to enter for the time zero row in the electrical torque curve, it can be calculated with the below spreadsheet (be sure to log in first before using the link). Use the "Turbine Power and Torque calculator" section, entering the flow, head, efficiency, fluid weight (keep that the same if using the default fluid properties in HAMMER) and rotational speed, and use the computed Shaft Torque as the time zero electrical torque entry in HAMMER.

Pump Power and Torque.xlsx

Pump Power and Torque (from the OpenFlows | Hydraulics and Hydrology File Gallery)

Here are the steps of the calculation:

1) First, calculate the Hydraulic Power of the turbine based on the initial head (drop) and flow
2) Calculate the shaft power based on the hydraulic power and efficiency.
3) Calculate the Shaft Torque based on shaft power and rotational speed

References:

Fluid Transients in Systems, Wylie, 1993, pg. 146
http://www.engineeringtoolbox.com/pumps-power-d_505.html
http://www.engineeringtoolbox.com/pumps-speed-torque-d_1114.html

See Also

Using Turbines in Bentley HAMMER

Change in results before turbine load rejection occurs

Turbine Efficiency

Calculating Nominal Torque for a Transient Pump Startup or Variable speed