According to standard each secondary circuit of instrument transformers should be a completely self-contained metallic circuit insulated from ground, except at a point in the secondary circuit, to prevent of dangerous touch voltage due to insulation damage or secondary current transformers opening. The grounding should be done at one point, to prevent differences of potential; because of differences of voltage between different points in the station ground grid. Also to facilitate the temporary removal and reestablishment of the ground connection, when desired.
The best point in three phase connection is the star point of CTs and relay elements. In 2CTs scheme, this point can not be the star point of relay elements; because in this case one of three elements of relay is bypassed by grounding connections.