The delta-star connection has the same characteristics as the Y-_ bond, being different only in its transformation relation. In the transformer, the line voltage of the primary winding connected to – is converted to the phase voltage of the Y-connected secondary winding. One of the conveniences of the transformer is that it can be kept in operation even when one phase is lost, or when it is one of the units for maintenance. This type of connection is called _ open (open delta) or V-V connection.
Thus, as in the Y-_ connection, the Y-transformer causes a 30 ° gap between the primary and secondary voltages. In the direct sequence, the secondary voltages will be 30 ° ahead of the primary voltages, that is, the lag will be around 30 °.
Applying the opposite phase sequence in the primary, a -30 ° phase-shift in the secondary voltages and the corresponding phasor diagram will be observed in the phase phasor diagram of the three-phase transformer interconnected in Δ-Y: primary phase and line voltages; phase and secondary line voltages.
As in the Y-_ connection, other lags between primary and secondary are plausible depending on the connection of the windings and the primary phase sequence.