Choice of Transformer Connection Diagram

Choice of Transformer Connection Diagram:

The different choice of Transformer Connection Diagram are namely,

Star/star:

This is economical for small HV transformers as it minimizes the turns/phase and winding insulation. A neutral connection is possible. However, the Y/Y connection is rarely used because of difficulties associated with the exciting current.

Delta/delta:

This suits large LV transformers as it needs more turns/phase of smaller section. A large load unbalance can be tolerated. The absence of a star point may be a disadvantage. This connection can operate at 58% normal rating as open-delta when one of the transformers of the bank is removed for repairs or maintenance.

Star/delta:

This is the most commonly used connection for power systems. At transmission levels star connection is on the HV side, i.e. Δ/Y for step-up and Y/Δ for step-down. The neutral thus available is used for grounding on the HV side. At the distribution level the Δ/Y transformer is used with star on the LV side which allows mixed 3-phase and 1-phase loads, while delta allows the flow of circulating current to compensate for neutral current on the star side (Fig. 3.50).

The Y/Δ connection has an associated phase shift off ±30° which must be accounted for in power system interconnections.

Harmonics:

It was seen already that when the third-harmonic current is permitted to flow, by circuit conditions, along with the sinusoidal magnetizing current in a transformer, the core flux is sinusoidal and so is the induced emf. On the other hand, when the circuit does not permit the flow of the third-harmonic current, i.e. the magnetizing current is sinusoidal, the flux is flat-topped containing “depressing” third-harmonic and as a consequence third-harmonic voltages are present in the induced emfs. This problem in 3-phase transformers will now be examined.

It is to be observed here that the phase difference in third-harmonic currents and voltages on a 3-phase system is 3 x 120° = 360° or 0° which means that these are cophasal. Therefore, third-harmonic (in general harmonics of order 3n called triplens) currents and voltages cannot be present on the lines of a 3-phase system as these do not add up to zero.