I'm encountering something with my diesel generators, and I am wondering if there is a setting I can adjust.  In my model, there are many times when the wind resources is large enough that the wind turbines can meet the complete electrical load.  In this situation, the optimization shuts the diesel generators off completely. 

However, for stability, many utilities would continue to run their diesel generators at a safe level in this situation so that the generators would be prepared to meet the load when the wind stops.  Is there a good way to ensure that the diesel generators are not turning off in the optimization?  I tried the "Forced On" scheduling, and this runs the diesel generator at what appears to be an arbitrary 57% when the power is not needed, where I think it could be running closer to the minimum that I set (30%).
 

Yes, you can prevent the system from turning all the diesels off during times of high wind power output.  The operating reserve inputs, which appear on the Constraints window, allow you to do this.  If you set the operating reserve as a function of wind power output to 100%, then the system will always run at least one diesel whenever the electric load exceeds zero.

You may want to read up on operating reserve in the HOMER help file.  For wind diesel systems, the punchline is that HOMER calculates the required operating reserve in each time step as a summation of some fraction of the load plus some fraction of the wind power output.  If you set that fraction of wind power output to 100%, you are saying you want the system to provide enough operating reserve to cover the load even if the wind power output suddenly dropped by 100%.  (In other words, you don’t trust the wind turbines at all.)  So the diesels have to operate even in the presence of surplus wind power.

Scheduling a generator to run all the time should work though, and I think you are right that the generator should run at its minimum power level in times of surplus wind power.