Dear Pierre,
You should only change the simulation time step if you have weather data in a time step other than 60 minutes. If you change the simulation time step to 30 minutes and then run simulations with 60 minute (hourly) data, SAM will misinterpret the data in the weather file.
Only the physical trough model has been tested to run with weather data with time steps other than 60 minutes. We do not recommend changing the simulation time step unless you are using the physical trough model, and have weather data to match the simulation time step.
Best regards,
Paul.