What Is a Power Transformer?

A power transformer is a device that transfers electrical energy between two or more circuits through electromagnetic induction, changing voltage in the process. When electricity travels long distances from a generation plant to a city, it travels at very high voltage — typically 115 kV to 765 kV — because high voltage means lower current, and lower current means much less energy lost to resistance in the transmission line. Before that power enters a distribution network and eventually reaches homes and businesses, it must be stepped down through a series of transformers. Those transformers — typically rated from a few MVA up to several hundred MVA — are power transformers.

Understanding what a power transformer is, how it is constructed, and how it stays cool is foundational knowledge for anyone involved in substation operations, maintenance, or equipment procurement.

How electromagnetic induction produces voltage transformation

The operating principle is Faraday's law: a changing magnetic field induces a voltage in any conductor passing through it. A transformer has two or more coils of wire wound around a common magnetic core. When alternating current flows through the primary (input) winding, it creates a constantly changing magnetic flux in the core. That changing flux induces a voltage in the secondary (output) winding. The ratio of the output voltage to the input voltage equals the ratio of the number of turns in the secondary winding to the number of turns in the primary winding.

A transformer with a 10:1 turns ratio steps voltage down by a factor of 10 and simultaneously steps current up by a factor of 10 (minus losses). A 115 kV primary winding with 10 times as many turns as the 11.5 kV secondary delivers about 10 times the current at the output that it draws at the input. Power is approximately conserved — losses in a well-maintained transformer are typically less than 0.5% of rated power.

Core construction

The magnetic core is made of thin laminations of grain-oriented silicon steel, typically 0.23 mm or 0.27 mm thick, stacked together and bound into a rigid structure. The laminations are electrically insulated from each other by a surface coating; this breaks up the path for eddy currents (induced currents in the core material that would otherwise cause resistive heating and energy loss). The grain orientation of the silicon steel aligns the material's magnetic properties with the direction of flux flow, reducing core losses compared to non-oriented steel. Core construction is either core-form (windings surround the core limbs) or shell-form (the core surrounds the windings); core-form is more common in utility power transformers.

Winding construction

The coils are wound from copper or aluminum conductor insulated with thermally upgraded kraft paper. The paper insulation — soaked with and surrounded by the transformer oil — forms the primary solid insulation system. The high-voltage (HV) and low-voltage (LV) windings are wound concentrically on the same core limb, with the LV winding typically on the inside (closer to the core) and the HV winding on the outside. Insulating barriers of pressboard and oil ducts separate the windings from each other and from the core, providing both electrical insulation and cooling channels for oil circulation.

Insulating oil

The core-and-coil assembly is immersed in mineral oil inside a steel tank. The oil serves two purposes: it insulates, filling all the spaces between conductors and between the windings and the tank; and it cools, carrying heat away from the core and windings by natural convection or forced circulation. The condition of the oil — its moisture content, acidity, dielectric strength, and dissolved gas content — is one of the most important indicators of transformer health. Degraded oil loses its insulating and cooling ability and accelerates aging of the paper insulation it contacts.

Cooling classes

IEEE C57.12.00 defines standard cooling classes for power transformers using a four-letter code that describes the internal cooling medium, the internal circulation method, the external cooling medium, and the external circulation method. The most common classes are:

ONAN(Oil Natural, Air Natural) — oil circulates by natural convection inside the tank; heat is radiated to air through the tank walls and radiator fins by natural convection. The base cooling class. No pumps or fans required; the transformer is fully self-cooled.

ONAF(Oil Natural, Air Forced) — same as ONAN but with fans added to the external radiators. Forced air cooling increases heat dissipation significantly, allowing a higher MVA rating from the same core-and-coil. Fans are typically switched on in stages as load increases.

OFAF(Oil Forced, Air Forced) — both oil pumps and fans are used. Oil is actively circulated through external coolers by pumps, improving heat transfer from the windings and allowing still higher ratings.

ODAF(Oil Directed, Air Forced) — oil is directed through specific cooling channels in the winding assembly by pumps, providing the most efficient cooling of the winding hot spot. Used in the largest and most heavily loaded transformers.

A transformer nameplate will show multiple MVA ratings corresponding to different cooling stages — for example, 40/50/60 MVA ONAN/ONAF/OFAF means the transformer is rated 40 MVA with natural cooling, 50 MVA with fans on, and 60 MVA with fans and pumps on. Operating above the rating for the active cooling stage risks accelerating insulation aging.

Reading the nameplate

The nameplate is the permanent record of the transformer's design specifications and must be referenced for any testing, maintenance, or operational decision. Key nameplate items include: kVA or MVA rating, voltage ratings of all windings (primary and secondary, and any tertiary), the vector group (winding connection arrangement such as delta-wye or wye-wye), the cooling class and corresponding MVA ratings at each stage, the temperature rise rating (typically 65°C for newer transformers), the impedance percentage at rated MVA, the weight (core and coil weight, oil weight, total weight for shipping and installation planning), the serial number and manufacturer, and the year of manufacture.

The impedance listed on the nameplate — typically 5% to 8% for distribution substation transformers — determines how much fault current the transformer can deliver for a bolted fault on the secondary terminals. This figure is critical input for protective relay coordination and fault analysis.

The load tap changer (LTC)

Most transmission and large distribution transformers include a load tap changer (LTC) — a mechanism that adjusts the effective turns ratio while the transformer is energized and loaded, allowing output voltage to be held within a narrow band as load and generation vary. The LTC moves through a range of tap positions (typically ±10% of rated voltage in 16 or 32 steps) by switching in or out sections of the HV winding. Each tap change requires switching a current-carrying conductor; this is accomplished by a reversing switch and a transition resistor arrangement that prevents interrupting the load current during the transition. LTCs contain moving parts in oil and are among the highest-maintenance components on a power transformer — they accumulate contact wear proportional to the number of operations, and their oil becomes contaminated with arcing products much faster than the main tank oil.