Vector group classification tells you how the windings of a three phase power transformer are connected (delta, star, zigzag) and what phase shift exists between primary and secondary. This is critical for paralleling transformers, avoiding circulating currents, coordinating protection, and ensuring correct phase relationships in the network. Grid operators use vector groups to check compatibility when connecting transformers into existing systems and to select proper CT and relay settings. Using an incorrect vector group can cause unbalanced voltages, unwanted trips or even equipment damage.