Vector grouping indicates the relative phase displacement and winding connections of a three phase transformer, for example Dyn11 or YNd1. This information is vital when paralleling transformers, designing protection, and analyzing fault currents. Different vector groups produce different phase shifts between primary and secondary, which can cause circulating currents or misoperation of relays if mismatched. Engineers use vector groups to ensure compatibility with existing network phasing, motor connections and grounding schemes. The designation also helps interpret test results and diagnose issues like unbalance or unexpected zero sequence behavior.