Transformer Delta Wye Connection: Delta vs Wye Explained

A transformer delta wye connection is a three-phase winding arrangement in which the primary windings are connected in delta and the secondary windings are connected in wye, so the secondary delivers both a line-to-line voltage and a neutral point that can be earthed. A wye winding gives line voltage = √3 × winding voltage with line current equal to winding current; a delta winding gives line voltage = winding voltage with line current = √3 × winding current. Neutral availability, harmonics, zero-sequence impedance, phase shift and protection all follow from those two relationships.

This guide covers what the two windings do, why the four standard combinations of three phase transformer connections exist, and what a supplier needs before the unit can be built.

What is a transformer delta wye connection?

A transformer delta wye connection joins the three primary windings end to end in a closed loop — a delta — and the three secondary windings at a common star point — a wye. The delta side has no neutral, the wye side has one, and the two sides sit 30 electrical degrees apart.

Three properties follow. The wye supplies a neutral for four-wire systems. The closed delta gives a low zero-sequence impedance, because zero-sequence current on the wye side is balanced by circulating current in the delta. And the delta gives triplen harmonics a closed path, so third-harmonic magnetising current circulates inside it instead of flattening the core flux and distorting the secondary voltage. One transformer can therefore serve a mixed load from a three-wire medium-voltage feeder — which is why the distribution transformer with a delta primary and an earthed-wye secondary, connection symbol Dyn11, is the standard arrangement.

Where does the 30° phase shift come from?

The winding voltages on the two sides are in phase; the shift appears only when line quantities are compared: on the delta side a line-to-line voltage is a winding voltage, while on the wye side it is the phasor difference of two winding voltages 120° apart, which places it 30° from the winding phasors. The IEC clock system puts the high-voltage phasor at 12 o'clock and the low-voltage phasor at the clock hour, each hour being 30°. Delta-wye and wye-delta units always land on an odd clock number — 1, 5, 7 or 11; delta-delta and wye-wye units land on 0 or 6.

Which voltage should you use when checking the ratio?

The winding voltage, not the line voltage. In a 10 kV / 0.4 kV delta wye connection transformer, each high-voltage winding sits across the full 10 kV line voltage while each low-voltage winding carries 400 / √3 = 231 V — a winding ratio of about 43.3 : 1 against a line-voltage ratio of 25 : 1. In general: delta primary to wye secondary, winding ratio = √3 × line-voltage ratio; wye primary to delta secondary, winding ratio = line-voltage ratio / √3; delta-delta or wye-wye, the two ratios are equal. A specification quoting only line voltages leaves the manufacturer to derive the rest, so state both.

What is a 3 phase delta connection?

A 3 phase delta connection joins the three windings in a closed ring and takes the line conductors from the junctions. There is no neutral point.

• Voltage: line voltage = winding (phase) voltage; each winding sits across the full line-to-line voltage.
• Current: line current = √3 × winding current, so each winding carries 57.7% (1 / √3) of the line current it supplies.
• Harmonics: triplen harmonic currents circulate inside the delta and never appear in the line currents.
• Zero sequence: no external path, so zero-sequence current cannot transfer through the winding.

The trade-off: a delta secondary cannot supply line-to-neutral loads, and in an unearthed delta system the phase-to-earth voltages are set by capacitance, so one earth fault produces very little current.

What is a 3 phase wye connection?

A 3 phase wye connection, or star connection, joins one end of each winding at a common star point and takes the lines from the free ends. That point is the neutral; it may be brought out, earthed or left isolated.

• Voltage: line voltage = √3 × winding voltage, so winding voltage = line voltage / √3 — 231 V on a 400 V system, 6.35 kV on an 11 kV system.
• Current: line current = winding current, so each winding carries the full line current and needs more conductor cross-section than a delta winding.
• Neutral: a fourth conductor from the star point supplies line-to-neutral loads and gives a defined reference for phase-to-neutral measurement and protection.
• Harmonics: a wye winding set offers no closed path, so triplen harmonic current has to be absorbed by a delta winding elsewhere in the transformer.

The wye secondary makes a three-phase four-wire network possible: line-to-neutral loads connect between any phase and the neutral, and on a solidly earthed system the neutral carries earth-fault current. Where the star point is unearthed on a three-wire source it is free to move, and unbalance shifts the phase voltages.

How is the neutral point of a wye winding earthed?

The earthing method is a system design decision, but it changes what the transformer has to withstand.

• Solidly (effectively) earthed: high earth-fault current, straightforward residual protection, limited healthy-phase overvoltage during a fault.
• Resistance earthed: a neutral resistor limits fault current and damage while leaving a measurable current for protection, and is rated for the fault duration.
• Resonant or high-impedance earthed: an arc-suppression coil or high resistance holds fault current low so transient faults self-extinguish; detection then relies on residual voltage.
• Isolated: fault current is small and capacitive, but the healthy phases rise to full line-to-line voltage to earth during a sustained fault, so insulation coordination becomes critical.

Tell the supplier which applies and on which side: it decides whether a neutral bushing is needed and how the winding is insulated.

Delta delta, wye wye, and transformer delta wye arrangements: which should you use?

Four standard combinations exist. The choice comes down to four questions: does the load need a neutral, how is each system earthed, what happens to triplen harmonics and zero-sequence current, and does the phase displacement matter.

• Does the load need a neutral? Line-to-neutral loads, or an earthed neutral for a four-wire network, need a wye winding with the neutral brought out, which rules out a delta delta transformer for that side.
• What clears an earth fault? A delta winding cannot supply zero-sequence current, so an earth fault on the delta side must be cleared by source-side protection or a core-balance scheme.
• Where do triplen and zero-sequence currents go? If the secondary neutral is earthed, or the load is unbalanced, a delta winding somewhere in the transformer is what makes the arrangement work — the primary of a delta wye connection, or a delta tertiary on a wye-wye unit.
• Does the phase displacement matter? Delta-delta and wye-wye sit at clock 0 or 6; delta-wye and wye-delta at clock 1, 5, 7 or 11. If the unit may run in parallel, the clock number is a hard requirement.

Two transformers may run in parallel only if their ratios agree, their impedances are compatible, their phase sequences match and their vector groups are the same or equivalent. A Dyn11 unit paralleled with a Yy0 unit circulates a large current before any load is applied, because the secondaries are 30° apart.

What are the phase and line voltage and current relationships in delta and wye?

The √3 factor is the phasor sum of two equal quantities 120° apart. In a balanced wye the line-to-line voltage is the phasor difference of two winding voltages; in a balanced delta the line current at a node is the phasor difference of two winding currents. Stated explicitly, with the side and the quantity named:

• Wye: line voltage = √3 × phase (winding) voltage; line current = phase (winding) current.
• Delta: line voltage = phase (winding) voltage; line current = √3 × phase (winding) current.

Each relationship applies to whichever side is connected that way, and the two sides are independent — which is why the winding ratio and the line-voltage ratio differ by √3, and why a phase current on one side does not equal the phase current on the other even at nominally equal line voltages. Nahu Group's current transformers for metering and protection are specified against these line currents, so CT ratio and connection are settled with the transformer connection rather than after it.

What does a 3 phase voltage diagram show?

A phasor diagram shows the three voltages as vectors of defined length with their phase displacement. In a wye winding the three phase-to-neutral phasors are drawn from the star point 120° apart, and the line-to-line phasors follow by subtracting one phase phasor from the next — each is √3 times longer and 30° from the phase phasors. A balanced three-wire load sums to zero and the star point stays put. In a delta winding the three winding phasors form a closed triangle, each line-to-line voltage equals a winding voltage, and nothing in the diagram represents a neutral. The clock system is that diagram turned into a number: high-voltage phasor at 12 o'clock, read the hour the low-voltage phasor points to, multiply by 30°. That number is the displacement to match when paralleling and the compensation differential protection must apply — and a transformer built to the wrong clock number cannot be corrected on site.

How do transformer connections affect metering and protection?

Once the winding arrangement is fixed, the instrument transformers on each side must be connected to match it. Three voltage transformers in wye, with the secondary neutral earthed at one point only, provide the phase-to-neutral voltages for four-wire metering and phase-to-earth protection. Two in open delta give the three line-to-line voltages with no neutral reference, which suits three-wire circuits. A broken-delta connection — three secondaries in series with one corner left open — produces the residual (zero-sequence) voltage across that corner, the usual way of detecting an earth fault on an isolated or impedance-earthed system where fault current is too small to measure reliably.

The 30° displacement of a delta wye transformer also appears across the protected zone. Electromechanical schemes compensated physically, connecting CTs in wye on the delta side and in delta on the wye side, which also removed zero-sequence current; numerical relays do the same correction mathematically, so CTs are now commonly wye-connected on both sides with the vector group entered as a setting. The relay setting and the physical CT connection must describe the same transformer. Never short-circuit a voltage-transformer secondary while its primary is energised, and never leave a current-transformer secondary open while its primary carries current.

How do you specify a transformer delta wye connection to a supplier?

A manufacturer can build almost any standard combination, but only if the requirement is stated:

• Rated voltage of each side as line-to-line, plus the phase-to-neutral value where a neutral is involved.
• The required connection symbol (vector group), such as Dyn11 or YNd11, and whether a neutral must be brought out and on which side.
• The earthing method of each system — solidly earthed, resistance earthed, resonant earthed or isolated — and whether residual voltage or residual current is needed for earth-fault detection.
• The load description, including single-phase line-to-neutral load and how unbalanced it may become.
• Anything the unit will run in parallel with: existing vector group, ratio, impedance and tap position. This is the item most often omitted and the one that most often forces a redesign.
• Frequency, rated power or current per phase, prospective short-circuit level, tap range and tapping winding, insulation levels, installation environment, and cooling arrangement.
• The standard to be built and tested to — IEC 60076-1 is the general power-transformer standard and the usual starting point.
• Metering and protection requirements, plus the documentation expected: outline and connection drawings, rating-plate data including the connection symbol, and routine test certificates.

If some values are not yet fixed, send the single-line diagram, the existing nameplate and the load schedule, and ask which items remain open. A manufacturer such as Nahu Group can confirm the winding and neutral arrangements from that starting point.

Frequently asked questions about transformer delta wye connections

What is the difference between a delta and a wye connection in a transformer?

In a delta connection the windings form a closed loop: line voltage equals winding voltage, line current is √3 times winding current, and there is no neutral. In a wye connection the windings share a star point: line voltage is √3 times winding voltage, line current equals winding current, and the star point can be brought out as a neutral.

What is the line-to-line voltage on the secondary of a delta wye transformer?

It is the rated secondary line voltage, not the winding voltage. Each wye secondary winding carries the phase-to-neutral voltage, so the secondary line-to-line voltage is √3 times the individual winding voltage: 231 V windings give 400 V line to line, and 240 V windings give 415 V.

What is the difference between a delta delta transformer and a delta wye transformer?

A delta delta transformer has no neutral on either side, so it serves only three-wire loads, but it has no phase displacement and can be run open delta. A delta wye transformer provides a secondary neutral for four-wire supply, gives a low zero-sequence impedance and a triplen harmonic path, and introduces a 30° displacement.

What does a vector group such as Dyn11 mean?

The capital D means the high-voltage winding is delta connected; the lower-case y means the low-voltage winding is wye connected; the n means the low-voltage neutral is brought out; and 11 is the clock number, so the low-voltage phasor is displaced from the high-voltage phasor by 11 × 30°, that is 330°.

Does a delta wye transformer provide a neutral?

Yes, on the wye side. The delta primary has no neutral point at all; the wye secondary has a star point that can be brought out and earthed. Whether it is earthed solidly, through a resistor or through an arc-suppression coil is a system design decision to state in the specification, because it affects the neutral bushing and the winding insulation.

Why does a delta winding reduce third-harmonic distortion?

Triplen magnetising currents need a closed path in which to circulate, and a delta winding provides it. They flow inside the delta instead of distorting the core flux, so the flux stays close to sinusoidal and the induced voltages carry no third-harmonic component, unlike a wye-wye transformer without a delta path.

Which transformer connections can be operated in parallel?

Only units whose ratios match, whose impedances are compatible, whose phase sequences agree and whose vector groups are the same or equivalent. A Dyn11 unit and a Yy0 unit are 30° apart and will circulate a large current between them even with no load connected, so the connection symbol is as important as the voltage rating.

Can a delta delta transformer be operated with only two transformers?

Yes. Two transformers of the same rating in open delta, also called V-V, supply a balanced three-phase three-wire load at about 57.7% of the capacity of the equivalent three-unit closed-delta bank, using the two units at about 86.6% of their combined rating.

Sources and further reading

• IEC 60076-1:2011 — Power transformers, Part 1: General
• IEEE C57.105-2019 — IEEE Guide for Application of Transformer Connections in Three-Phase Electrical Systems
• Distribution Transformer Handbook, 5th edition (Alexander Publications)

For the wider context, the guide to the three-phase transformer and the guide to electrical transformers cover the surrounding terms and ratings.