I don’t know of ANY brand of VFD that would REQUIRE an ungrounded source, in fact in my experience it is exactly the opposite. Most require a solidly grounded source, a few can be modified to ALLOW for an ungrounded source, but recommend against it. So if you are seeing someone purposely eliminate a ground on the source to a VFD, they are looking for trouble. There is ZERO value in doing so.
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
There are so many VFD units working on HRG & also on IT systems. Therefore, for such a VFD the input transformer
may be either delta or ungrounded star. The secondary side of the VFD transformer may be also DELTA or ungrounded star
provided the GF are detected by the VFD by somne other means.
A standard VFD can't detect ground fault if the system is not grounded. I've seen more than one modulate an upstream ungrounded power system at the drive switching frequency because of a motor ground.
You need special circuitry that actually measures the system to ground impedance or that introduces a high resistance ground to measure leakage current across.
The normal way to wire an isolation transformer is to follow the rules on "separately derived neutral" for the secondary side which is the side that powers the VFD. The ground bond and derived neutral can be connected to a earth, the building structure, a bonding system, the electrical bonding (green) wire, but not to the primary side neutral.
