Isolated phase busbar connection between generator and generator transformer bank for large units - issues & novel solution

The Generator Step up Transformers (GST) for large rating units are formed by a bank of three single phase units. A common vector group chosen by utilities is YNd11 (Sometimes kept different to match the existing generators and/or MV systems). The LV side of a GST is generally Delta connected and the HV side is Star connected The Generator to GST connections and also the Delta formation is carried out by Isolated Phase busduct (IPBD). It has been experienced several times in past, that the phase sequence are different on high voltage line side and the Generator. The actual phase sequence mismatch of the units has been experienced by utilities during Generator Protection System testing prior to synchronization. This mismatch is despite the fact that the delta formations are carefully designed during engineering stage. The solution carried out in past involved cumbersome modifications in Isolated Phase busduct.