in a standard voltage 3 phase inverter, I have seen the ground at the negative terminal of the DC source, or there could be two capacitor in series then parallel with the DC source. The ground is defined at the center of the the capacitor. Could anyone comment on the difference between these two configuration.

Shaohua (or Eric?),
if I understand correctly your question, the second configuration you are considering is the one used in three-level inverters (NPC, for example).

In the theoretical study of two-level inverters (H-bridge), defining a neutral point at half the DC bus is just for simplifying the analysis (since you can define any point as null potential, you choose one that makes analysis simpler), but this doesn't necessarily correspond to an actual circuit.

Thank you very much for your reply Sandro, I really appreciate it. I think you are right, it is for analysis purpose only, does not alternate the performance of the inverter. take care

Eric

You're welcome.