What is Delta-Delta Connection of Transformer?

The delta-delta connection of the primary and secondary windings of a three-phase transformer is shown in the figure. Here, the secondary winding r₁r₂ corresponds to the primary winding R₁R₂, and the terminals R₁ and r₁ have same polarity. Also, the polarity of terminal 𝐫 connecting the r₁ and b₂ is the same as that of R connecting R₁ and B₂.

The phasor diagram is drawn for the usual case of lagging power factor load. Here, the magnetising current and voltage drops in the impedances have been neglected. Under balanced condition, the line currents are √3 times of the phase currents and lags behind the phases currents. In case of the delta-delta connection, the corresponding line voltages and phase voltages are same in magnitude on both primary and secondary windings.

The secondary line voltages V_ry, V_yb and V_br are in phase with the primary line voltages V_RY, V_YB and V_BR and their voltage ratios are given as follows −

$$\mathrm{\frac{𝑉_{𝑅𝑌}}{𝑉_{𝑟𝑦}}=\frac{𝑉_{𝑌𝐵}}{𝑉_{𝑦𝑏}}=\frac{𝑉_{𝐵𝑅}}{𝑉_{𝑏𝑟}}=𝐾}$$

And when the magnetising currents are neglected, the current ratios are

$$\mathrm{\frac{𝑉_{𝑅𝑌}}{𝑉_{𝑟𝑦}}=\frac{𝑉_{𝑌𝐵}}{𝑉_{𝑦𝑏}}=\frac{𝑉_{𝐵𝑅}}{𝑉_{𝑏𝑟}}=\frac{1}{𝐾}}$$

There is no phase shift in case of a delta-delta transformer and no problems with unbalanced loads or harmonics.

Advantages of Delta-Delta Transformer

The delta-delta connected transformer has the following advantages −

• The delta-delta connection can be used for both balanced and unbalanced loads.

• If the third harmonic is present, it circulates in the closed path of the delta loop and does not appear in the output voltage.

Disadvantages of Delta-Delta Transformer

The main disadvantage of the delta-delta transformer is that there is no star-point or neutral terminal available. Therefore, the delta-delta connected transformer is used when neither primary nor secondary requires neutral terminal and the voltages are low and moderate.