Transformer Primary Connections
Transformer primaries are usually connected in delta. The wye connection is occasionally used and may or may not have its neutral grounded. If the transformer secondary is connected wye, as in a four-wire 415Y/240-volt system, the best connection for the primary is delta. Unbalanced loads do not create abnormal regulation problems. Third-harmonic magnetizing currents are contained within the closed delta, and do not produce objectionable telephone interference in the primary system.
Wye-Wye Connection Problems
Wyg-wye connected transformers should not be used because of problems they frequently introduce, Regardless of the combination of grounding or non-grounding of the system neutral or primary neutral, troubles can occur, which do not exist with the delta-wye connection. These troubles include telephone interference, neutral potential, instability, inability to serve a sizcable load unbalance without neutral shift, and dangerous overvoltage from third-harmonic resonance between system capacitance and transformer reactance.
The most suitable wye-wye connection includes a delta tertiary winding to provide a path in the transformer for third-harmonic and other zero-phase sequence currents. This practically eliminates the problems that are special to the wye-wye connection but requires a special transformer larger than the conventional delta-wye transformer, since the ampere-turn capacity of the delta tertiary must be about 35 percent of the other windings.
Delta-delta Connection is Past
The delta-delta connected transformer has had extensive use in the past. One reason for its past popularity was because it is practical to continue operation with only two or three single-phase units, in case maintenance was required on one unit of a three-phase bank. This is not usually an important advantage because of the extremely high reliability of modern transformers, and also because of the growing popularity of three-phase units. The delta-delta connection has the disadvantage, pointed out previously, that single-phase loads at a lower voltage cannot be as easily served as with a delta-wye connection. Also, the fact that the neutral is not tied to ground on the secondary is a severe disadvantage. Corner of the delta grounding is not as satisfactory as neutral grounding and is rarely applied in modern systems. The wye-delta connection is not used for the same reasons as apply to the delta-delta.
Conclusion
In conclusion, the delta-wye transformer connection is the favored connection in most applications. It permits flexibility, reliability, positive automatic protection, safety and low-cost system design. The low-voltage auxiliary generator is also usually wye-connected for similar reasons. Neutral grounding of generators and transformers provides greatest reliability and safety.