Choice of Transformer Connections

Choice of Transformer Connections:

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. (This has already been explained.)

Star Delta

This is the most commonly used connection for power systems. At transmission levels star connection is on the HV side, i.e. z /Y for step-up and Y/A for step-down. The neutral thus available is used for grounding on the HV side. At the distribution level the A/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/zt connection has an associated phase shift off 30° which must be accounted for in power system interconnections.

Harmonics

It was seen in Sec. 3.10 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.

IL 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.

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