3 Phase Transformer: Working Principle, Connections and Selection

A 3 phase transformer is a transformer with three primary and three secondary windings on one magnetic circuit, used to change voltage in a three-phase AC power system, and it can be built as a single three-phase unit or as a 3 phase transformer bank of three single-phase transformers. Because electricity is generated, transmitted and distributed in three phases, almost every voltage change on a network — from a transmission intertie to the 400 V supply at the end of a feeder — is made by three-phase transformation.

This guide covers how a three phase transformer works, why its core is one magnetic circuit, how star, delta and zig-zag connections set the phase and line values, what a 3 phase transformer wiring diagram shows, the unit-versus-bank trade-off and the nameplate items to verify.

What is a 3 phase transformer?

A 3 phase transformer is a static device with three primary and three secondary windings, one pair per phase, wound on a common core in a single enclosure. Its rated power is a total three-phase value in kilovolt-amperes (kVA), not the sum of three separate ratings.

The same job can be done by three single-phase transformers connected together — a transformer bank. Both are legitimate; the choice depends on cost, transport, redundancy, spare parts and the neutral the load needs.

Liquid-immersed units place the windings and core in insulating liquid inside a tank, with radiators and bushings as the rating requires. Dry-type units are air-cooled, cast-resin encapsulated or impregnated, which suits indoor switchrooms where an insulating liquid is not wanted.

A three-phase power transformer is not the same product as a three-phase instrument transformer — covered in our guide to electrical transformer types — which scales voltage or current down to a signal for meters and relays.

How does a three phase transformer work?

It works by electromagnetic induction, with three winding sets energised by voltages 120 electrical degrees apart. Current in each primary winding drives flux through the core limb that carries it, and that flux links the corresponding secondary winding and induces a voltage. Per phase, the voltage ratio equals the turns ratio:

V_p / V_s = N_p / N_s

Only a winding's phase voltage is fixed by its turns ratio. The line quantities at the terminals depend on how the windings are connected, which is why the connection symbol belongs in the rating. For a balanced transformer, power follows from the line quantities:

S = √3 × V_L × I_L

A 1000 kVA transformer rated 11 kV / 400 V therefore carries roughly 52 A per primary line and 1443 A per secondary line, whichever connection produces those voltages.

Why is a three-phase core built as one magnetic circuit?

In a balanced system the three fluxes are equal and 120 degrees apart, so their instantaneous sum is zero. Three limbs joined by top and bottom yokes exploit that: each limb uses the yokes as a shared return path, so no fourth return limb and no three separate cores are needed. The result is less core steel, a shorter mean turn length and a smaller, lighter, generally cheaper transformer than three single-phase units of equal total rating.

Two consequences follow. Unbalanced loading does not cancel completely, so residual flux still needs a return path — tolerable within design limits on a three-limb core, and handled more gracefully by a bank, where each phase has its own magnetic circuit. Triplen harmonics (third, ninth and so on) are in phase in all three limbs and also need a path: a closed delta winding provides it, letting harmonic current circulate without appearing in the line current. Without a delta winding, a five-limb or shell-type core can supply the return path. IEC 60076-8, the application guide to the power transformer family standard, covers these connection and magnetic-circuit characteristics in detail.

How do phase and line values differ?

A star-connected secondary can therefore feed a four-wire network at 1/√3 of its line voltage, while a delta winding carries √3 times its winding current in each line.

One three-phase unit or a 3 phase transformer bank?

A 3 phase transformer bank is three single-phase transformers interconnected at their terminals to serve a three-phase load. Electrically it achieves what a single unit achieves; commercially and operationally the two differ.

For a new substation within transport limits and with a reasonably balanced load, a single three-phase unit is normally the better economic choice. A bank earns its place where the largest lift is limited by transport, where a utility keeps a common single-phase spare, or where heavily unbalanced loading matters more than the lower cost of one unit.

What do the common three phase transformer connections mean?

Each winding is connected in star (wye, Y or y) or delta (D or d). The choice fixes phase displacement, neutral availability, harmonic paths and insulation distribution.

• Star-star (Yy). A neutral on either side, and insulation can be graded towards the star point, which cuts cost at high voltage. Without a delta winding, third-harmonic currents have no path.
• Delta-delta (Dd). No neutral, but the closed delta gives third-harmonic currents somewhere to circulate. Suits delta-connected loads.
• Delta-star (Dy). The classic distribution configuration: the delta primary provides the harmonic path and a balanced load upstream, the star secondary supplies a four-wire low-voltage system.
• Star-delta (Yd). Used at transmission interties and generator connections where the delta side stabilises the system.
• Zig-zag (Z). Each winding is split so its halves oppose for balanced currents and reinforce for zero-sequence, giving a low-impedance path for earth-fault and unbalanced current. Used for four-wire distribution with heavy single-phase loading, and to create a neutral where none exists.
• Open delta (V-V). Two units instead of three for a reduced-capacity supply, typically temporary or after a unit failure.

Our companion post on delta and wye transformer configurations compares these arrangements in more detail.

What does a vector group such as Dyn11 tell you?

The vector group codes the connections and the phase displacement: upper case for the high-voltage winding, lower case for the low-voltage winding (D/d delta, Y/y star, Z/z zig-zag, N/n for a brought-out neutral), plus a trailing number in clock hours of 30 degrees. Dyn11 is therefore a delta primary feeding a star secondary with neutral, lagging 30 degrees. Units run in parallel need a compatible vector group and the same ratio, or the difference between their secondary voltages circulates current before any load is connected.

What does a 3 phase transformer wiring diagram show?

A 3 phase transformer wiring diagram shows how the windings and terminals are interconnected, and it is the document used to check that the unit as built matches the rating ordered. It shows:

• Terminal designations — primary H1 to H3, secondary X1 to X3, any neutral N, and the tap terminals.
• Winding connections — which ends form the star point or closed delta. On a single unit these are factory-made; on a bank the diagram also shows the site-installed inter-unit jumpers.
• Star point treatment — whether the neutral is brought out and where it is earthed.
• Vector group — the connection symbol the wiring must implement, with the polarity convention used.
• Tapping arrangement and the voltage at each tap, plus auxiliary terminals for embedded current transformers, temperature devices and space heaters.

Treat it as a verification document, not a source of improvisation: its connections are the ones tested at the works. If a site drawing and the nameplate disagree, resolve the difference before energising — a reversed phase or an unexpected 30-degree shift is cheaper to find on paper than after paralleling.

How do you size and select a 3 phase transformer?

Selection starts with the load and works back to the rating:

1. Load in kVA. Sum the connected load, apply the allowed diversity factor and add an allowance for growth. Assess motors and power-electronic loads on kVA and harmonic current, not rated kW alone.
2. Rated voltages. Primary and secondary line-to-line values, plus the tapping range needed for supply variation.
3. Connection and vector group. Whether a neutral is needed on either side, and what phase displacement the downstream equipment expects.
4. Impedance. Higher values limit fault current and ease switchgear duty; lower values reduce voltage drop. Paralleled units need impedances close enough to share load predictably.
5. Cooling class. Sets the cooling method and how much of the rating is available in each mode.
6. Insulation and environment. Indoor or outdoor, altitude, ambient temperature and pollution all change the design; a temperate indoor rating is not automatically suitable for a coastal or high-altitude site.
7. Physical and protection interfaces. Footprint, mass, cable or busbar entry, earthing terminals and the test and documentation package.

Then verify these nameplate items against the enquiry and drawings:

For three-phase power transformers with a winding above 1.1 kV, general requirements and the rating and test framework come from the IEC 60076 series — Part 1 for general requirements, Part 11 for dry-type transformers. Ask the supplier to name the exact edition and part, and to issue drawings and routine test documentation with the order.

Which medium-voltage and low-voltage applications use three phase transformers?

On the medium-voltage side: distribution substations stepping 11 kV, 22 kV or 33 kV feeders down to a low-voltage supply, industrial main and unit substations, packaged substations, renewable generation collection, generator step-up interties, and dedicated transformation stages in traction, marine and mining installations.

On the low-voltage side: four-wire distribution boards, motor control centres and control panels, machine and process sub-distribution, uninterruptible power supplies and rectifier front ends, and commercial buildings taken from a medium-voltage supply and distributed at 400 V or 415 V. Vector group, impedance, cooling class and neutral treatment come from the application, not from the kVA alone.

Which installation and safety mistakes should you avoid?

Most three-phase transformer problems are specification or installation errors that appear under load, not manufacturing defects. The recurring ones are:

• Paralleling incompatible units. Different vector groups, phase displacements or ratios produce circulating current before any load is applied.
• Neutral errors. A floating star point displaces the phase voltages; earthing the neutral at two separated points invites circulating current in the earthing system.
• Changing taps while energised. An off-circuit tap changer may only be operated with the transformer isolated and the winding discharged.
• Wrong phase sequence. Motors downstream rotate in reverse and metering and protection read incorrectly. Verify the sequence at commissioning.
• Ignoring phase displacement in protection. A 30-degree shift changes the phasor relationships differential and directional relays expect.
• Under-sizing the neutral. A harmonic-rich four-wire load can carry more current in the neutral than in a phase.
• Blocking cooling and clearances. Cable trays tight against dry-type windings raise the operating temperature and shorten service life without overload.
• Skipping pre-commissioning checks. Confirm insulation resistance, winding resistance, ratio, vector group, terminal torque, earthing and protection settings first.

Frequently asked questions about 3 phase transformers

What is a 3 phase transformer used for?

It changes voltage between two levels on a three-phase system: stepping distribution feeders down to a low-voltage supply, stepping generation up to a transmission or collection voltage, and providing a separate voltage level for an industrial process or a large building.

What is the difference between a three phase transformer and a 3 phase transformer bank?

Both make the same three-phase transformation. A three phase transformer is one unit with three windings on a shared magnetic circuit, usually cheaper and more compact per kVA. A 3 phase transformer bank is three single-phase transformers wired together: easier to transport, redundant through a common spare, better with unbalanced loads, but more expensive and with more site connections.

How do you calculate the current of a 3 phase transformer?

Divide the kVA rating by 1.732 times the rated line voltage. A 1000 kVA transformer rated 11 kV carries about 52 A per primary line. Use the same formula on the secondary side, remembering that line current differs from winding current in a delta connection.

What does a 3 phase transformer wiring diagram show?

The terminal designations for each phase and any neutral, which winding ends form the star point or the delta, where the star point is earthed, the tap voltages, the vector group the wiring must implement, and the auxiliary terminals for embedded current transformers and temperature devices.

Why is a delta winding often used on the primary of a distribution transformer?

It gives third-harmonic magnetising currents a closed path, so the secondary voltage waveform stays close to sinusoidal. It also keeps zero-sequence current from an unbalanced or earth-faulted secondary out of the supply network, and presents a balanced three-wire load upstream.

What is the vector group of a three phase transformer?

A code for the winding connections and the phase displacement between primary and secondary, such as Dyn11. Capital letters indicate the high-voltage winding, lower case the low-voltage winding, and the trailing number gives the displacement in clock hours of 30 degrees. Parallel units need compatible vector groups.

Can a three phase transformer supply single-phase loads?

Yes, when a star-connected secondary brings out a neutral, giving four-wire distribution with single-phase loads connected between a phase and neutral. Distribute that loading across phases, because a three-limb core tolerates severe imbalance less well than three independent single-phase units.

What should I check on a three phase transformer nameplate before ordering?

The kVA rating, primary and secondary line-to-line voltages, tapping range and tap changer type, frequency, vector group, short-circuit impedance, cooling class, insulation level and the standard the unit is tested to, plus mass and dimensions if transport or civil work depends on them.

Sources and further reading

• IEC 60076-1:2011 — Power transformers, Part 1: General — general requirements for three-phase power transformers
• IEC 60076-8:1997 — Power transformers, Part 8: Application guide — guidance on transformer connections and magnetic circuit designs
• Montana State University, EELE 354 — Lecture 17: Three-Phase Systems — lecture notes on three-phase systems and the star and delta phase-to-line relationships