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Transformer connections delta wye

The delta-wye connection (Δ‑Y) is the predominant configuration in three-phase distribution transformers worldwide, characterized by a primary winding connected in delta and a secondary in wye, which allows a typical line voltage of 400 V / 400 V on the secondary from 11 kV / 11 kV on the primary. Its main advantage is the ability to simultaneously supply three-phase and single-phase loads thanks to the accessible neutral on the wye side, in addition to blocking third-order harmonic currents in the primary.

Delta-wye configuration (Δ-Y)

Section titled “Delta-wye configuration (Δ-Y)”

The delta-wye connection is constructed by closing the three primary windings in a triangle (delta) and joining one end of each secondary winding at a common point that constitutes the neutral (wye). The neutral terminal is grounded and run as the fourth distribution wire. The most common IEC standardized designation is Dyn11: “D” indicates delta primary, “y” wye secondary, “n” accessible neutral, and “11” the hour index representing a phase shift of 30° between primary and secondary line voltages. A variant without an accessible neutral is designated Dy11.

Angular phase shift

Section titled “Angular phase shift”

The angular phase shift introduced by the delta-wye connection has a fundamental value of 30° / 30° (hour index 11), although depending on the marking and orientation of the windings, phase shifts of 150° / 150°, 210° / 210° or 330° / 330° may appear. This phase rotation prevents direct paralleling with transformers that do not have the same phase shift, such as wye-wye or delta-delta configurations, unless additional phase shifters are used.

Connection Hour index Primary line voltage Secondary line voltage Angular phase shift
Dyn11 11 11 kV / 11 kV 400 V / 400 V 30° / 30°
Dyn5 5 13.8 kV / 13.8 kV 480 V / 480 V 150° / 150°
Dyn1 1 33 kV / 33 kV 1000 V / 1000 V 330° / 330°
Dyn7 7 6.6 kV / 6.6 kV 380 V / 380 V 210° / 210°

Transformation ratio

Section titled “Transformation ratio”

The relationship between the line-to-line voltages of the primary (Δ) and secondary (Y) depends on the turns ratio per phase and the connection factor. For an ideal transformer the following holds:

VLLp / VLLs = a / √3

where a = N1/N2 is the turns ratio between primary and secondary. The division by √3 appears because on the wye side the phase voltage is the line voltage divided by √3, while in delta they coincide. For example, with a turns ratio a = 47,6, a primary voltage of 11 kV / 11 kV produces on the secondary 400 V / 400 V line and 230 V / 230 V phase.

Variable Symbol Unit
Primary line voltage VLLp V / V
Secondary line voltage VLLs V / V
Turns ratio per phase a = N1/N2 dimensionless
Connection factor √3 ≈ 1.732 dimensionless
Secondary phase‑neutral voltage VLN = VLLs / √3 V / V

Advantages

Section titled “Advantages”
  • Availability of accessible neutral for single-phase loads and grounding.
  • Circulation of third-order harmonic currents within the primary delta, preventing their propagation to the supply network.
  • Flexibility to simultaneously supply three-phase and single-phase loads in the same system.
  • Better utilization of insulation on the wye side, since the phase voltage is lower than the line voltage.
  • Reduction of voltage unbalance on the secondary thanks to the solidly grounded neutral.

Limitations

Section titled “Limitations”
  • Fixed phase shift of 30° / 30° (or multiples) that prevents direct coupling with transformers of other connection groups without adaptation.
  • Requires specific protection against neutral overloads when large unbalanced currents flow.
  • Paralleling with other transformers is only possible if they share identical hour index, transformation ratio, and short-circuit voltage.
  • On the wye side, the phase-to-ground voltage may be higher than in isolated delta-delta configurations, which may require a higher insulation level.
  • Voltage dips and inrush currents may be higher due to neutral inductance and core saturation.

Typical applications

Section titled “Typical applications”
  • Distribution transformers in medium-to-low voltage networks (e.g., 11 kV / 11 kV to 400/230 V / 400/230 V in IEC systems; 13.8 kV / 13.8 kV to 480/277 V / 480/277 V in ANSI systems).
  • Power supply for commercial buildings, industries, and high-density residential areas that combine three-phase motors with lighting and single-phase equipment.
  • Substations in wind farms and photovoltaic plants where a solid neutral-to-ground is required to connect inverters and power transformers.
  • Transformation centers on oil platforms and mining, where neutral availability is critical for installation safety.
  • Secondary distribution systems in underground urban networks, especially in Europe and North America.

Comparison with other connections

Section titled “Comparison with other connections”
Connection Accessible neutral 3rd harmonic suppression Typical phase shift Main application
Delta‑Delta (Δ‑Δ) No No (circulates in Δ) 0° / 0° Industrial motors, long-distance transmission
Wye‑Wye (Y‑Y) Yes No (requires tertiary) 0° / 0° or 180° / 180° Large power transformers with δ tertiary
Delta‑Wye (Δ‑Y) Yes Yes (circulates in Δ primary) 30° / 30° Low-voltage distribution with mixed loads
Wye‑Delta (Y‑Δ) No (primary Y with optional neutral) Yes (circulates in Δ secondary) 30° / 30° Voltage step-up in generators

Frequently Asked Questions (FAQ)

Section titled “Frequently Asked Questions (FAQ)”

What secondary voltage is obtained from a delta-wye transformer with turns ratio a = 47,6 and supply at 11 kV / 11 kV?

Section titled “What secondary voltage is obtained from a delta-wye transformer with turns ratio a = 47,6 and supply at 11 kV / 11 kV?”

Applying the formula VLLs = VLLp × √3 / a = 11 kV × 1,732 / 47,6 gives 400 V / 400 V line and 230 V / 230 V phase.

Why is the angular phase shift in a delta-wye connection always 30° or multiples?

Section titled “Why is the angular phase shift in a delta-wye connection always 30° or multiples?”

The difference of 30° / 30° is due to the product of the phase shifts introduced by the windings and the topology itself; depending on the winding arrangement, 150° / 150°, 210° / 210° or 330° / 330° may appear, but all are submultiples of 180°.

How are third-order harmonics suppressed in a delta-wye connection?

Section titled “How are third-order harmonics suppressed in a delta-wye connection?”

The third harmonic currents, all in phase, find a closed path within the primary delta, circulating exclusively through the delta winding, which eliminates their propagation to the supply network and avoids distortions in the secondary voltage waveform.

Can two delta-wye transformers with different hour indices be connected in parallel?

Section titled “Can two delta-wye transformers with different hour indices be connected in parallel?”

No, unless phase-shifting transformers are used or the connection is made with a corrected phase shift; the difference of 30° / 30° or more causes internal circulating currents that can damage the equipment.

What happens if the neutral is lost on the wye secondary of a delta-wye transformer?

Section titled “What happens if the neutral is lost on the wye secondary of a delta-wye transformer?”

When the neutral disappears, single-phase loads lose a stable reference, and if the system is unbalanced, the phase voltages can reach dangerous values that exceed the design 480 V / 480 V in a 277 V / 277 V system.

What is the main advantage of a delta-wye connection in industrial distribution systems?

Section titled “What is the main advantage of a delta-wye connection in industrial distribution systems?”

It allows supplying three-phase motors at 480 V / 480 V and simultaneously offering lighting circuits and outlets at 277 V / 277 V with the same transformer, optimizing investment and substation space.

References

Section titled “References”