Hi Sarah,
Without seeing the model it is difficult to know exactly what the issue is, but in general changing the unit from seconds to minutes should not introduce instabilities like you are seeing. The problem may have to do with how you converted the model. What you probably want to do is go through the model and convert all the block parameters that have time units into minutes (for example, add a '*60' in the mask if it was /s initially). That should do it. You don't need to put gains in front of the integrators and controllers in that case.
