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I think I can see what you are getting at. The power supply for your inverter cannot share any common reference with the load - if you want to make one side of your load grounded, then the power supply must be floating with respect to ground. That way, you can connect one output of your full bridge to ground (call that "neutral") and the other will be "live".
If you measure the power supplies of the inverter with respect to ground, you will see ripple equal to half the output voltage swing superimposed on the DC rail voltage.
I hope this makes sense - I don't have the facility to draw a picture at the moment, otherwise I would do a diagram to explain it better.
I'm not sure I fully understand your question, but here is my best guess:
Use a pair of tight-tolerance resistors connected between the two outputs.
The point where the two resistors connect, will be equipotential from both outputs. And could be considered a virtual ground point.
As we can connect one side of the load of a half-bridge inverter to any ground in a device (as it can be earthed for protection), I was asked if can have such a point in full bridge inverter and I was wondering if we can create such a point and I was a little bit ambiguous (and still I am ) in describing what I was asked to do. I think what schmitt trigger suggested, can be a useful way to create such a point. And also I searched to find the attached figure to make sure that we can use the point in the DC link too (it is found in Mohan book).
Thank you all for your answers( tomizett, Nigel Goodwin and schmitt trigger). I think my problem is now solved.