T-T Tranformer Analysis and Application

Equations are presented for calculating the self and mutual sequence impedances of T-T transformers with either a zero degree or 30 degree phase shift and either with or without a grounded neutral on the secondary teaser winding. It is shown that the voltage unbalance caused by the unsymmetrical characteristics of the T-T unit is less than 0.2 percent and 0.4 percent for units with 30 degree and zero degree phase shift respectively. Maximum values of secondary fault current can be determined by neglecting mutual sequence impedances and assuming the sequence self impedances are a fixed percentage of the nameplate impedance of the transformer.