Three Phase Power Transformers
Every network above a few hundred kilowatts is three-phase, and almost every transformer serving it is too. A three phase power transformer transforms all three phases on a single magnetic circuit — cheaper, lighter and more efficient than three separate single-phase units for the same duty, though occasionally less flexible. We manufacture three-phase power transformers across the ratings and voltage classes we build to, for utilities, industrial plants, renewable projects and EPC contractors.
What a Three Phase Transformer Is
Physically it is one core with three limbs, each carrying the windings for one phase, assembled in a single tank with three-phase terminations. Because the magnetic flux generated by each phase returns through the other two limbs, the core does the job that six return limbs would otherwise do — hence the saving in material and loss.
Compare this with three single-phase units: greater flexibility (you can keep running on two phases in an emergency, and a spare unit costs less than a spare three-phase transformer), but more steel, more oil, more footprint and typically a few percent more loss.
Winding Connections and Vector Groups
Each winding can be connected star (Y), delta (D) or, less commonly, zig-zag (Z). The combination of HV and LV connections plus the resulting phase displacement is the vector group, expressed in standard notation.
| Vector group | HV / LV connection | Common application |
|---|---|---|
| Dyn11 | Delta primary, star secondary with neutral | Distribution substations supplying four-wire LV networks |
| YNd11 | Star primary with neutral, delta secondary | Generator step-up units where the generator neutral is earthed |
| Yyn0 | Star / star, no phase displacement | Smaller distribution units where harmonic loading is light |
| Yd11 | Star primary, delta secondary | Step-down to three-wire loads, traction and industrial drives |
| Dd0 | Delta / delta | Heavy unbalanced loads, harmonic-rich industrial supplies |
Vector group selection is not arbitrary. It has to suit how the network is earthed, whether the transformer will run in parallel with existing units, and whether triplen harmonics need a circulating path. Getting it wrong produces circulating currents that persist whether or not load is connected — and no amount of commissioning adjustment will fix it.
The Third Harmonic Question
This is the reason delta windings exist on most power transformers. In a star-connected winding with an isolated neutral, third harmonic currents have nowhere to circulate, so the flux becomes distorted, inducing harmonic voltages that distort the output waveform.
Adding a delta winding, even a small tertiary one carrying no external load, gives those harmonic currents a closed path and restores a sinusoidal flux. It is why Yd and Dyn configurations dominate in practice, and why a purely star-star connection with an unearthed neutral is rarely specified.
Advantages of Three Phase Construction
- Lower cost — one tank, one core assembly, one set of bushings instead of three.
- Smaller footprint — critical where land is expensive or the substation is enclosed.
- Higher efficiency — less core material means lower no-load loss.
- Simpler installation — one lift, one oil fill, one termination set instead of three.
The counter-argument is availability: a single-phase spare is cheaper and easier to store than a whole three-phase unit, which is why some utilities specify banks of single-phase units for their largest and least replaceable positions.
Three-Limb Versus Five-Limb Cores
Standard construction uses three limbs. A five-limb design adds two unwound outer limbs that give zero-sequence flux a return path, which reduces tank heating and allows transport height to be lowered — genuinely useful where route restrictions limit the height of the load.
Five-limb cores cost slightly more and are heavier to build, so they are specified when height matters, or where zero-sequence impedance matters for earth-fault protection, rather than by default.
Applications
- Utility distribution and sub-transmission substations — see grid applications.
- Industrial intake and plant distribution substations — see industrial project applications.
- Renewable collector substations and inverter stations — see renewable energy applications.
- Power plant auxiliary and station service duty — see power plant applications.
- Compact and prefabricated substations where footprint matters — see our compact substation range.
Selection Checklist
- Rating in kVA or MVA against the firm load case, including future growth.
- Voltage ratio on both sides, plus tapping range and whether on-load or off-circuit.
- Vector group, confirmed against existing units if parallel running is possible.
- Impedance, from the system study rather than a catalogue default.
- Cooling class and whether additional stages (ONAF, OFAF) are needed.
- Site conditions: altitude, ambient temperature, pollution level, seismic zone, noise limits.
- Applicable standard, plus any utility-specific specification clauses.
Our guides to calculating transformer capacity, cooling methods and testing standards each cover one step in more depth.
Frequently Asked Questions
Can you supply three single-phase units instead? Yes, for projects where the spare-unit or transport argument justifies it.
Which vector group should we specify? It depends on the earthing arrangement and what else runs on the network. Send us details of existing units and we will confirm what will parallel correctly.
Do you build to both 50 Hz and 60 Hz? Yes — state the frequency with your enquiry, since it affects core loss and dimensions.
What standards apply? IEC 60076 as standard, IEEE C57 available where the specification requires it.