Power and Distribution Transformers
Power transformers make efficient AC distribution practical. Power can be transmitted at high voltage and lower current, reducing conductor loss, then stepped down near the load.
Single phase
A common North American residential distribution transformer has a center-tapped 120/240 V secondary: each outer conductor is about 120 V from the center tap and 240 V from the other outer conductor.
Three phase
Three-phase transformers may be one unit or a bank of single-phase units. Windings are commonly connected wye or delta.
| Connection | Characteristic |
|---|---|
| Wye | Provides a neutral point; phase-to-neutral voltage is lower than line-to-line voltage. |
| Delta | No inherent neutral; closed delta path can carry some harmonic and unbalanced currents. |
| Delta-wye / wye-delta | Changes voltage, grounding arrangement, and phase relationship. |
Phase shift
Some winding connections shift secondary phase angles relative to the primary. This is critical when paralleling transformers and coordinating protection.
Dry and liquid-filled
Dry-type transformers rely on air and solid insulation and are common indoors. Large utility units often use insulating liquid for both insulation and cooling.
Taps
Fixed taps let installers compensate for supply voltage. Utility units may use load tap changers that regulate voltage while energized.
Inrush
At energization, residual flux and the exact switching instant can drive the core temporarily into saturation, producing magnetizing current many times normal full-load current.
Fault current
Transformer percent impedance is a major limit on downstream short-circuit current.