GuideTransformers9 min read
Delta-wye transformers
A delta-wye transformer has its primary windings connected in a triangle (delta) and its secondary windings in a star (wye) with a neutral at the center. It’s the standard distribution connection: the delta primary needs no neutral from the supply and traps third-harmonic currents, and the wye secondary provides a neutral for 277 V or 120 V loads and a solid ground reference. The connection shifts the secondary voltages 30 degrees from the primary, which matters only when transformers are paralleled.
The short version
- Wye: three windings joined at a neutral point. Line-to-line voltage = 1.732 × winding voltage; the neutral gives line-to-neutral loads (480Y/277, 208Y/120).
- Delta: three windings joined end to end in a loop. Line-to-line voltage = winding voltage; no neutral.
- Delta-wye (Δ-Y) is the standard for a plant’s distribution transformer; grounded wye-grounded wye is common on utility pad-mounts; delta-delta survives in older plants.
- A delta-wye or wye-delta connection shifts the low-voltage side 30° from the high-voltage side (ANSI: low side lags). Only transformers with the same shift, ratio and similar impedance can be paralleled.
- On a nameplate, “12470GrdY/7200 – 480Y/277” means a grounded-wye primary and a wye secondary; “12470 Δ – 480Y/277” is delta-wye.
Delta and wye, defined
A three-phase transformer has three primary windings and three secondary windings, and each set of three can be connected two ways.
- Wye (Y, or star)
- One end of each winding joins the other two at a common point, the neutral; the other ends are the three line terminals. The line-to-line voltage is √3 (1.732) times the winding voltage, and the line-to-neutral voltage is the winding voltage: 480 V line-to-line gives 277 V to neutral, 208 V gives 120 V. Each line carries the winding’s current.
- Delta (Δ, or mesh)
- The windings join end to end in a closed triangle, and the line terminals are the three corners. The line-to-line voltage is the winding voltage, and each line carries √3 times the winding current. There’s no neutral, and no line-to-neutral voltage unless one corner is grounded.
The windings themselves are the same; the connection is made at the terminals, and on many units can be changed there. On a nameplate the secondary is written with its connection: 480Y/277 V (wye, 277 V to neutral), 208Y/120 V, or 480 V delta.
The four connections
| Connection | Secondary neutral | Third harmonics | Where it’s used |
|---|---|---|---|
| Delta-wye (Δ-Y) | Yes, from the wye, solidly grounded | Circulate in the delta; kept off the supply | The standard plant and building distribution transformer: 12.47 kV or 4.16 kV delta to 480Y/277 V, and 480 V delta to 208Y/120 V |
| Wye-wye (Y-Y) | Yes | No path unless the core or a tertiary provides one | Utility pad-mounts and pole-mount banks on multi-grounded-neutral systems (12470GrdY/7200 to 208Y/120 or 480Y/277), with a five-leg core |
| Delta-delta (Δ-Δ) | No (ungrounded, or one corner grounded) | Circulate in both deltas | Older plants with 480 V or 240 V delta distribution; keeps running at 58% capacity in open delta if one unit of a bank fails |
| Wye-delta (Y-Δ) | No | Circulate in the delta | Generator step-up (delta on the high side), some industrial loads; a grounded-wye primary acts as a ground source for the supply, so it’s usually ungrounded |
Three-wire delta secondaries, which once fed every machine shop, have mostly given way to grounded wye, because a grounded wye makes ground faults easy to detect and clear and gives every building its 277 V lighting and 120 V receptacles from the same transformer.
Why delta-wye is the standard
Four things, each worth something on its own, come together in the delta-wye connection.
- A neutral where the supply has none. A 12.47 kV delta feeder has three conductors. The wye secondary creates the neutral, grounded at the transformer, so the plant gets 480Y/277 V with line-to-neutral lighting and a ground reference. In the NEC’s terms the secondary is a separately derived system (250.30).
- Ground faults stay on their own side. A ground fault on the secondary sees the transformer’s neutral, not the utility’s; a fault on the primary doesn’t drive current through the secondary neutral. Protection on each side is simpler.
- Third harmonics are trapped. Fluorescent ballasts, drives and switch-mode supplies draw third-harmonic currents that add up in the neutral rather than cancelling. On the secondary they flow in the wye neutral; reflected into the primary they circulate around the closed delta and never reach the supply.
- Unbalanced loads are handled. A single-phase load on one secondary phase is spread across two primary windings by the delta, so the primary current stays nearer balanced than it would through a wye-wye.
The 30-degree phase shift
In a delta-wye transformer the primary line-to-line voltage is applied to each delta winding, and each wye winding produces a line-to-neutral voltage on the secondary. In any three-phase system the line-to-neutral voltages sit 30° from the line-to-line voltages, so the secondary line voltages end up 30° from the primary line voltages. The same happens, the other way, in a wye-delta. Delta-delta and wye-wye units have no shift.
The ANSI standard (IEEE C57.12.00) fixes the direction: on a delta-wye or wye-delta transformer the low-voltage side lags the high-voltage side by 30°. IEC writes the same thing as a vector group, Dyn1 (delta high side, wye low side with neutral, low side at 1 o’clock, lagging 30°); Dyn11, common outside North America, leads by 30° instead.
Grounding and the neutral
The wye secondary’s neutral (X0) is normally bonded to ground at the transformer, and from there the system is a solidly grounded wye: line-to-neutral voltages are fixed, ground faults draw large currents that breakers clear quickly, and the neutral is carried to the loads that need it. That’s the arrangement in nearly every building.
Plants that can’t afford a trip on the first ground fault, such as continuous processes, often ground a 480 V wye through a high resistance instead (high-resistance grounding). The neutral still exists, and the first fault only raises an alarm. A 480 V delta secondary, with no neutral, can be left ungrounded for the same reason, though hunting for a fault on an ungrounded system is harder and the second fault is a line-to-line short.
On the primary side, a delta needs no connection to ground. A wye primary can be grounded (GrdY on the nameplate) or not, and it matters: a grounded-wye primary with a delta secondary turns the transformer into a grounding bank that supplies ground-fault current to the system feeding it, which the utility or the plant’s relaying may not expect. Grounded wye-grounded wye units are built for multi-grounded-neutral utility systems and carry the system neutral straight through.
Reading the connection on a nameplate
Nameplates and listings write the two windings and their connections in a compact form. The common ones:
- 12470 Δ – 480Y/277
- Delta primary at 12,470 V; wye secondary, 480 V line-to-line, 277 V to the neutral. The standard plant transformer.
- 12470GrdY/7200 – 480Y/277
- Grounded-wye primary, 12,470 V line-to-line and 7,200 V line-to-neutral, with the neutral grounded; wye secondary. The standard utility pad-mount; the primary needs the system neutral.
- 12470GrdY/7200 – 240/120
- A single-phase unit on a 7,200 V line-to-neutral primary, with a 240 V secondary center-tapped for 120 V.
- 4160 – 480
- Both windings delta (a connection written without a Y is delta). No secondary neutral: 480 V three-wire only.
- 13800 Δ – 4160Y/2400
- Delta primary, wye secondary at 4,160 V with a 2,400 V line-to-neutral: a medium-voltage distribution transformer feeding motors.
- 480 Δ – 208Y/120
- The dry-type transformer in every building’s electrical room.
The vector diagram on the nameplate shows the two triangles or stars and the 30° displacement, and the terminal markings follow IEEE C57.12.70: H1, H2, H3 (and H0 for a wye neutral) on the high side, X1, X2, X3 and X0 on the low side.
Which connection to buy
- A plant or building fed from a three-wire medium-voltage supply, needing 480Y/277 or 208Y/120: delta-wye. This covers most of what buyers search for.
- A utility-style service on a multi-grounded-neutral line: grounded wye-grounded wye, as the utility specifies; the primary neutral must be available.
- A replacement in an existing system: whatever the old unit was. The connection, the shift and (if anything parallels it) the impedance must match; the old nameplate is the specification.
- A generator step-up: usually wye on the generator side, delta on the high side, with the generator’s neutral grounding arrangement decided by the engineer.
When you search, write the connection the way the nameplate would: “1,500 kVA, 12.47 kV delta to 480Y/277 V”. Magic Search reads the nameplate details on each seller’s page and tells you which units are delta-wye, which are grounded wye-wye, and what that means for your supply.
Questions people ask
What is the difference between delta and wye?
In a wye connection the three windings are joined at one end to form a neutral, and the line voltage is 1.732 times the winding voltage, with a line-to-neutral voltage available. In a delta connection the three windings are joined end to end in a triangle, the line voltage equals the winding voltage, and there is no neutral. Wye gives you a neutral and a ground reference; delta gives you a closed path that absorbs third-harmonic currents and needs only three conductors.
Why is the secondary of a delta-wye transformer shifted 30 degrees?
Because the primary line-to-line voltage is applied across each delta winding, while each wye winding produces a line-to-neutral voltage, and in a three-phase system the line-to-neutral voltages sit 30 degrees from the line-to-line voltages. The windings are in phase with each other; it’s the line quantities on the two sides that differ by 30°. By the ANSI standard the low-voltage side lags the high-voltage side by 30° on both delta-wye and wye-delta units.
Can you parallel a delta-wye transformer with a wye-wye transformer?
No. The delta-wye secondary is shifted 30° and the wye-wye secondary isn’t, so their outputs would differ by 30°, and connecting them would be a short circuit through both transformers. Transformers paralleled on a common bus must have the same angular displacement (vector group), the same voltage ratio and taps, and impedances within about 7.5% of each other.
What does Dyn11 mean?
It’s the IEC way of writing a connection: D for a delta high-voltage winding, y for a wye low-voltage winding, n for a neutral brought out, and 11 for the clock position of the low-voltage phasor, so the low side leads the high side by 30° (11 o’clock). Dyn1 has the low side lagging by 30°, which is the ANSI standard for North American transformers. A Dyn11 unit can’t be paralleled with a Dyn1 unit.
Does a delta-wye transformer need a neutral from the supply?
No. A delta primary connects to the three line conductors only. The wye secondary makes its own neutral, which is grounded at the transformer and becomes the neutral of a separately derived system (NEC 250.30). That’s one of the main reasons for the connection: it provides a neutral where the supply has none.
Why do utilities use grounded wye-grounded wye pad-mounts?
On a multi-grounded-neutral distribution system the neutral is already there, a grounded-wye primary lets single-phase and three-phase units be built the same way, it avoids the ferroresonance that a delta primary can suffer when one primary phase opens (as it does when single-phase fuses or elbows are pulled), and it costs less. The price is that the wye-wye needs a five-leg core or a tertiary to handle unbalance and third harmonics, and it passes some of the primary’s unbalance through to the secondary.
Sources: IEEE C57.12.00 (angular displacement and terminal markings); IEEE C57.12.70; IEEE 142 (grounding of industrial systems); NEC 250.30 (separately derived systems).