A three-winding transformer is a single-core magnetic device featuring three electrically isolated coils—typically one primary and two secondaries—that transfer power across three distinct voltage levels simultaneously. Instead of installing two separate two-winding transformers to step down 115 kV to both 13.8 kV and 4.16 kV, a single three-winding unit handles both loads in one tank. This changes the physical footprint of an installation, reduces core losses, cuts mineral oil volume, and provides a dedicated tertiary path for zero-sequence fault currents and harmonic suppression. People commonly confuse three-winding transformers with three-phase transformers (which refer to the number of AC phases, not the physical coils per phase) or autotransformers (which share a physical electrical connection between windings rather than providing isolated circuits).
Anatomy and Nameplate Ratings of a Three Winding Transformer
In a standard core-type three-winding transformer, the physical arrangement of the coils dictates the leakage reactance and the short-circuit forces. The primary (high-voltage) winding is typically placed on the outside, the secondary (medium-voltage) on the inside, and the tertiary (low-voltage) sandwiched between them or placed closest to the core. This specific stacking order minimizes the leakage flux between the primary and secondary, which are usually the main power-transfer pair.
The tertiary winding is almost always connected in a closed delta configuration, even if it supplies no external load. This delta loop acts as a trap for third-harmonic currents generated by the non-linear magnetization of the transformer core, preventing those harmonics from propagating back into the power grid. When the tertiary does supply a load, it is typically sized for a fraction of the total transformer MVA capacity, often reserved for station service, shunt capacitor banks, or synchronous condensers.
Below is a representative nameplate spec sheet for a substation-grade three-winding unit. Notice that the MVA ratings are not identical across all three windings; the transformer is constrained by the maximum thermal limit of any single winding.
| Parameter | Winding 1 (HV Primary) | Winding 2 (LV Secondary) | Winding 3 (TV Tertiary) |
|---|---|---|---|
| Nominal Voltage | 115 kV | 13.8 kV | 4.16 kV |
| Maximum MVA Rating (ONAN/ONAF) | 15 MVA / 20 MVA | 10 MVA / 13.3 MVA | 5 MVA / 6.6 MVA |
| Connection Type | Wye (Solidly Grounded) | Wye (Resistance Grounded) | Delta (Closed Loop) |
| BIL (Basic Impulse Level) | 550 kV | 110 kV | 60 kV |
| Pairwise Impedance (at 15 MVA base) | Z_HL = 10.0% | Z_LT = 8.0% | Z_HT = 14.0% |
Worked Numeric Example: Calculating Equivalent Circuit Impedances
When modeling a three-winding transformer in power systems software like ETAP or SKM PowerTools, you cannot simply input the pairwise impedances listed on the nameplate. The software requires the individual star-equivalent impedances for each winding (Z_H, Z_L, and Z_T) to build the positive-sequence network.
The nameplate provides the pairwise impedances, which are measured in the factory by short-circuiting one winding, leaving the third open, and applying voltage to the remaining winding. Using the values from Table 1 (on a common 15 MVA base):
- Z_HL (High to Low) = 10.0%
- Z_HT (High to Tertiary) = 14.0%
- Z_LT (Low to Tertiary) = 8.0%
To find the individual equivalent impedances, we use the standard star-delta transformation equations:
Z_H (Primary) = 0.5 × (Z_HL + Z_HT - Z_LT)
Z_H = 0.5 × (10.0 + 14.0 - 8.0) = 0.5 × 16.0 = 8.0%
Z_L (Secondary) = 0.5 × (Z_HL + Z_LT - Z_HT)
Z_L = 0.5 × (10.0 + 8.0 - 14.0) = 0.5 × 4.0 = 2.0%
Z_T (Tertiary) = 0.5 × (Z_HT + Z_LT - Z_HL)
Z_T = 0.5 × (14.0 + 8.0 - 10.0) = 0.5 × 12.0 = 6.0%
Verification: If we add Z_H and Z_L (8.0% + 2.0%), we get 10.0%, which perfectly matches the nameplate Z_HL. This mathematical model allows protective relaying engineers to accurately calculate fault currents on any of the three buses and set the differential relay (87T) slopes accordingly.
Where You Meet This in Practice
You will rarely see a three-winding transformer on a commercial rooftop or in a residential padmount. These are heavy-duty assets deployed in specific high-power scenarios:
- Utility Transmission Substations: A 115kV-to-13.8kV substation might use the tertiary 4.16kV winding exclusively to feed station service transformers, power the SCADA systems, and run the cooling fan motors. If the grid drops the 13.8kV distribution feeders, the tertiary winding can still pull auxiliary power from the 115kV transmission line to keep the substation controls alive.
- HVDC Converter Stations: High-voltage direct current systems require massive harmonic filtering. The tertiary winding of the converter transformers is often dedicated to connecting large banks of passive harmonic filters and reactive power compensation capacitors without interfering with the main AC/DC power transfer.
- Large Industrial Facilities: A steel mill or water treatment plant might receive power at 69 kV. The three-winding transformer steps this down to 13.8 kV for large 5000 HP induction motors, while simultaneously providing a 4.16 kV tertiary bus for medium-sized pumps and plant-wide distribution.
For deeper insights into how these units integrate into grid resilience and infrastructure planning, the US Department of Energy's Office of Electricity maintains extensive documentation on large power transformer siting and vulnerability.
Common Confusions and Protection Pitfalls
The most frequent mistake junior engineers make is confusing a three-phase transformer with a three-winding transformer. A standard three-phase distribution transformer has three primary coils and three secondary coils (two windings per phase). A three-winding transformer has three coils per phase, resulting in nine total coils inside a three-phase tank.
Another major pitfall occurs in protective relaying. According to standard practices outlined in resources like All About Circuits' transformer guides and IEEE C37.91, a transformer differential relay (ANSI 87T) compares the current entering the primary with the current leaving the secondary. But what about the tertiary?
Frequently Asked Questions
Can I use a three-winding transformer as a standard two-winding transformer?
Yes. If you leave the tertiary winding open-circuited, the unit functions exactly like a two-winding transformer. However, if the tertiary is designed to provide a path for third-harmonic flux (which is common in wye-wye configurations), leaving it open can result in severe voltage distortion and overheating of the transformer tank due to stray flux. Always check the manufacturer's nameplate and connection diagram before leaving a tertiary winding unloaded.
Why is the tertiary winding usually rated for a lower MVA than the primary?
Physical space inside the transformer tank is limited. The tertiary winding is often made with smaller-gauge copper because it is only intended to carry the station service load or harmonic circulating currents, not the full through-fault power of the transmission line. Sizing it for the full MVA would require a massively larger core and tank, destroying the economic advantage of using a three-winding design in the first place.
