Problem

After computing the transient simulation for a model with a turbine, the following user notification appears and the results are generally very unstable or extremely high hydraulic grades are seen in the transient results viewer.

"Turbine calculations do not converge. Check electrical torque or four quadrant curve values."

Background

This can occur in some situations where the transient numerical solver is not able to converge on a balanced solution using the turbine's quadrant curve (based on the selection of Specific Speed). This could happen as the turbine shifts around a particularly challenging part of the characteristic curve, such as a steep area of inflection point.

In some cases the impact of the non-convergence may be minor enough to ignore - animate some profile paths in the Transient Results Viewer, look at a time history of head and flow on either side of the turbine and the rotational speed in the Extended Node Data tab. You may find that the non-convergence does not significantly effect the transient envelope (minimum and maximum head over the profile) or the rotational speed.

If the impact of the non-convergence is significant ...

Solution

1) If you are using version 10.00.00.40 (initial CONNECT Edition release), either apply the latest available cumulative patch set for this version, or upgrade to a newer version when available. This includes improved turbine convergence (reference ID # 592237). It may not eliminate the non-convergence and related user notification, but the effect on the results should be much less.

2) Check that the appropriate Specific Speed has been selected for your turbine. See below article for how to estimate from one of the available default specific speeds:

Estimating the Specific Speed of a Pump or Turbine

3) Check that the initial conditions and other turbine parameters are correct. If using the Load Rejection Operating Case, check the electrical torque curve. If the curve needs to start at the initial conditions electrical torque, see below article:

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

See Also

Flow choking effect with turbines as speed increases

Using Turbines in Bentley HAMMER