The term "open delta" is used for two very different VT configurations. In the US, it SHOULD mean two voltage transformers connected in delta fashion, but with one phase missing or "open".
However, I have seen the term "open delta" used in this forum by European and UK members to mean three voltage transformers connect in delta but with one corner of the delta left unconnected and used for measurement (typically) of zero sequence voltage. In the US, this configuration is referred to as "broken delta".
As with any other voltage transformer configuration, grounding is done for safety and to establish a ground reference point.
