Answering old question to clean up unanswered questions:
Common mode voltage requires that an accurate model where discrepancies in "ground" voltages can be simulated. Simscape motors can be simulated with their wye connections external, so that you can explicity model any ground path that you want. This is not available in specialized power systesm. It is important to never explicitly ground anything in the simulation model, but instead to create an accurate model fo the non-ideal path to ground. Some of these paths may be high impedance, and some of them may be low impedance, and accurately modelling them is quite challenging as it is very dependent on the physical setup of your device and not just the circuit topology that you are using. Dialectrics, shielding, geometry, can all play a role in this and are not included in your schematic. This is why, in general, common mode issues are still mostly handled with prototypes or the real system, as it is not necessarily cost effective to create such a detailed model even though it is possible.
