The standard insists for the linear characteristics between the said range. However, you may check the factory test reports of the actual VT of the manufacturer where you bought.
The sensitivity majorly affected in bus bar protection where the no. of CT's are more. If the actual values of Imag are not considered, the relay tends to operate on the inappropriate fault current than the setting current due to multiplying factor of Imag currents of all CT's.
Delta tertiary generally not grounded. Power utilities uses separate BCPU which can sense the OC & EF faults. For this you may need to add ISO, CB, CT, VT in the tertiary before connecting station trafo.
