Star-Delta Transformer | Electrical Devices | Electrical Engineering

In a star-star or delta-delta 3-phase transformer there is no phase shift between the corresponding voltages of any phase to neutral on either side. However, a delta-connected winding is desirable in many power transformers for reasons of harmonic elimination. As such most of the power transformers are either star-delta or delta-star connected.

In such transformers, even under normal operating condition, the phase-to-phase voltages and phase-to-neutral voltages of hv side are displaced from the corresponding voltages of Iv side. The phase shift of 30° comes in group 4 and that of – 30° comes in group 3. Similarly the currents on two sides are displaced. Generally in short circuit calculations the phase shift needs not to be considered.

Now let us consider 3-phase star-delta transformer with primary side Y-connected and secondary side delta-connected, as shown in Fig. 3.6. Windings shown parallel to each other, being wound on the same core, are magnetically coupled. The polarity markings are indicated on each phase.

The dots at the windings indicate the terminals which are positive at the same time with respect to the un-dotted terminals. With phase marked as ABC on the star side, there are number of ways of labelling the phases a b c on the delta side.

The labelling indicated on the diagram corresponds to + 90° connection in which the positive sequence phase a to neutral voltage (delta side) leads phase A to neutral voltage (star side) by 90° and so the line currents in a and A.

This labelling is computationally convenient – The alternative way is to label delta as b → a, c → b and a → c, and thus we get standard Yd₁ – 30° connection. If the polarities on the delta side are also reversed, we have standard Yd₁₁, 30° connection.

Double suffixes are used for line-to-line voltages and delta currents and single suffix are used for line currents and phase (line-to- neutral) voltages. Line-to-line transformation ratio is being taken to be unity.

The positive and negative sequence voltages on primary (star) and secondary (delta) sides of the transformer are shown in Fig. 3.7 while the positive and negative sequence currents on the two sides of the transformer are shown in Fig. 3.8.

From these figures it is observed that –

V_a1 = jV _A1; I_a1 = jI_A1; V _a2 = – jV _A2 and I _a2 = – jI _A2                           …(3.5)

In case of reversal of power flow i.e. when delta acts as primary and star as secondary, the voltage phasors do not change but all the current phasors reverse. The phasor relationship between star and delta voltages and currents therefore remain the same.

Thus we see that magnitude of phase shift is same for positive sequence components and negative sequence components. However, the direction of phase shift in case of negative phase sequence compo­nents is reverse of that applicable to the positive sequence compo­nents (due to reverse phase se­quence).

The magnitude and direc­tion of phase shift depends on the transformer group and allocation of phase references. Phase shift of zero sequence quantities need not be considered in star-delta transformers because the zero sequence currents do not flow in lines on delta-connected side.