DELTA-STAR and Distribution Transformer Unbalanced Loades

 DELTA-STAR and Distribution Transformer Unbalanced Loades

There are two types of unbalanced load. One type can be connected line-to-neutral and the other can be connected line-to-line. Although both unbalances can occur simultaneously during transformer loading, let us consider each separately.

Given a Dyn1 transformer, if a single phase load is connected line-to-neutral, the currents can be shown as follows:

When the load is connected to phase-a and neutral, the resulting load current $I$ will flow in only one of the three transformer secondary windings. On the primary side, the current flow will appear as a line-to-line connected load. Current $I_A$ will have the same phase angle as the load current and $I_C=-I_A$. Thus, a single phase load connected between any phase and neutral on the secondary side will appear as a line-to-line connected load on the primary.

If a single phase load is connected line-to-line, the currents can be shown as follows:

This type of unbalance is a bit more complicated. The load current flows in two of the secondary windings and each will have equal magnitude but opposite phase angle. On the primary side, current will appear in each of the three lines. Line current $I_A$ will have twice the magnitude as $I_B$ and $I_C$ but opposite phase angle. Thus, a single phase load connected line-to-line on the secondary will produce imbalanced line currents on the primary side. Two of the line currents will have equal magnitude and phase angle. One of the line currents will have twice the magnitude and opposite phase angle.

The unbalanced loading of a transformer is typically analyzed using the method of symmetrical components, but the approach shown above might be more intuitive for those not familiar with that method.