Transformer Types and Construction: Core, Shell and Windings

From the outside, most transformers look like a steel box with bushings on top. Inside, they are precise machines whose construction decides their behaviour: how they handle faults, how they are cooled, how they are tested, and how long they last. This guide opens the box in words — core and windings, insulation and oil, tanks and conservators, tap changers and bushings — and explains the main types you will meet on site, so that when you read a nameplate, a test report or a protection setting, you understand what kind of machine it describes. It is written for engineers, technicians and students who want the physical picture behind the electrical theory.

As with everything on this site, always work to your project specification, the manufacturer's instructions and your site safety rules; the general guidance here never overrides those documents. Internal inspection of a transformer is outage work for competent persons under permit, with the unit isolated, earthed and drained as the procedure requires. If you are new to transformer testing as a whole, start with our power transformer testing and commissioning guide, which explains how each part of the construction is proved by testing.

The active part: core and windings

At the heart of every transformer is its active part: the magnetic core and the windings that sit on it. The core is built from thin laminations of grain-oriented silicon steel, stacked and clamped together. The laminations matter: thin sheets separated by insulation keep eddy currents small, and grain orientation aligns the steel's magnetic properties with the direction of the flux, keeping core losses low. The core is assembled into limbs, on which the windings sit, joined by yokes at top and bottom to complete the magnetic circuit. Joints between limbs and yokes are carefully made, because a poor joint increases losses, noise and vibration.

The windings are coils of copper or aluminium conductor, insulated turn by turn and layer by layer, arranged concentrically around each core limb in most large transformers: typically the low-voltage winding innermost, closest to the core, and the high-voltage winding outside it, with insulation and oil ducts between them. Winding construction varies with voltage and current: layer windings for lower voltages, disc windings with interleaved sections for high voltages, where the arrangement controls voltage distribution under surge conditions. The windings are clamped firmly, because short-circuit forces try to throw them apart and crush them together in turn; a winding that can move under fault forces will not survive many faults. This is why mechanical integrity — clamping, blocking and bracing — is as much a part of transformer quality as its electrical design, and why tests such as frequency response analysis exist to detect winding movement after a fault or transport.

Core form versus shell form

The two classic construction families are core form and shell form, and the difference is which part surrounds which. In a core-form transformer, the windings surround the core limbs: the magnetic circuit is essentially a single loop, the windings are cylindrical, and the core is visible as a frame around which the coils sit. This is the dominant construction for large power transformers worldwide, because it suits high voltages, large ratings and the disc windings that handle surge voltages well. In a shell-form transformer, the core surrounds the windings: the magnetic circuit has multiple return paths around the coils, the windings are often rectangular or pancake coils, and the core effectively shells the active part.

Shell form has advantages in some applications: the multiple flux paths can reduce the height of the unit, and the construction can be mechanically very robust, which is why it appears in certain large, special and furnace transformers. But it is less common for general power transformers, and its winding and core arrangement makes some tests and repairs different. For most engineers the practical point is simpler: when you see a core-form unit, expect cylindrical windings, a tank that roughly follows the core frame, and the standard test set; when you meet a shell-form unit, check the manufacturer's documentation before assuming the standard arrangements apply. The nameplate and the general arrangement drawing, not habit, tell you which construction you are dealing with.

Distribution transformers versus power transformers

The most useful everyday distinction is between distribution transformers and power transformers, and it is about duty, not just size. Distribution transformers perform the final step down to consumers — from medium voltage to low voltage — and they are everywhere: pole-mounted, pad-mounted, in substations and buildings. They are usually modest in rating, they run at widely varying load, often light for much of the day, and they are designed for good efficiency at partial load, with minimal attention over a long life. Many are sealed or hermetically protected, with no conservator, and their protection is often limited to fuses and, on larger units, basic overcurrent and temperature devices.

Power transformers serve transmission networks, generating stations and large industrial loads. They are much larger, they run near full load for long periods, and they are designed for maximum efficiency at full load, with full monitoring and protection: differential and restricted earth fault protection, Buchholz relays, temperature and pressure devices, on-load tap changers, and forced cooling systems with fans and pumps. The boundary between the two classes is a matter of convention and varies by utility, but the principle holds: the bigger and more critical the transformer, the more of its life is actively managed, monitored and protected. A distribution transformer is expected to be forgotten; a power transformer is expected to be watched.

Oil-filled versus dry-type transformers

Most large transformers are oil-filled: mineral oil insulates and cools, the tank contains it, and a conservator accommodates its expansion. Oil-filled construction scales to the largest ratings and the highest voltages, and it is the construction this site's testing and maintenance guides mainly describe. Its disadvantages are the ones you would expect: oil is flammable, it can leak, and it needs containment, fire protection and environmental care. Those disadvantages are managed by design — bunded foundations, fire walls, oil containment pits — and by maintenance, but they never go away entirely.

Dry-type transformers use air, and sometimes cast resin, instead of oil. The windings are insulated with materials rated for higher temperatures and cooled by air, natural or forced. They are chosen where oil is unacceptable: inside buildings, hospitals, tunnels, ships, offshore platforms and water-sensitive sites, where fire safety or environmental rules forbid an oil-filled unit. They cost more per unit of capacity, they are usually smaller in rating, and they run hotter and noisier than an equivalent oil unit, but they remove the oil fire and leakage risks entirely. Their testing differs too — insulation resistance and temperature rise behave differently without oil — so the test procedures for an oil-filled unit should never be applied blindly to a dry-type one.

Insulation, tanks, bushings and tap changers

The insulation system is what makes the whole machine possible, and it is mostly paper and oil. Cellulose paper insulates turn from turn and layer from layer, oil fills the spaces and carries heat away, and the two age together: moisture, heat and acids degrade both, which is why oil testing is also, indirectly, paper testing. Solid insulation also includes pressboard barriers and supports that shape the electric field and hold the windings in place. In dry-type units the equivalent role is played by resin and high-temperature insulation materials. Whatever the system, its enemy is the same trio — heat, moisture and contamination — and its health is judged by the same evidence: insulation tests, oil or temperature records, and the transformer's history.

Around the active part sits the tank: a welded steel vessel that holds the oil, supports the core and windings, and carries the fittings. The conservator, mounted above, accommodates oil expansion and keeps the main tank full; a breather with drying agent keeps the air above the conservator oil dry. Bushings carry the winding connections through the tank wall and out to the external conductors, and they are themselves precision insulators — oil-filled or capacitive-graded — whose condition is tested separately. Tap changers adjust the turns ratio to regulate voltage: off-circuit tap changers, moved only with the transformer de-energised, on smaller units, and on-load tap changers, covered in detail in our OLTC guide, on larger ones. Each of these parts has its own maintenance and testing regime, and a transformer is only as reliable as the weakest of them — which is why a thorough inspection looks at all of them, not just the windings.

Choosing and recognising a transformer on site

When you arrive at a transformer you have not met before, the nameplate is your first document. It tells you the rating, voltages, vector group, impedance, cooling designation, insulation class, oil type and quantity, weight, and year of manufacture, and it is the reference for every setting, test limit and loading decision that follows. The general arrangement drawing tells you the construction: core or shell, conservator or sealed, tap changer type, cooling equipment and bushing arrangement. Together they answer the questions that matter before any work: what kind of machine is this, what can it carry, how is it cooled, how is it protected, and what does its manufacturer require?

That habit — nameplate first, drawing second, habit last — is what connects construction knowledge to everything else on this site. It tells you which tests apply and which do not, which protection schemes to expect, what cooling to check, and what maintenance the unit needs. A transformer is not a generic box; it is a specific machine with a specific construction, and the engineers who last longest in this field are the ones who always find out which one they are standing in front of before they touch it.

Frequently asked questions

What is the difference between core-form and shell-form transformers?

In a core-form transformer the windings surround the core limbs, with the magnetic circuit essentially a single loop. In a shell-form transformer the core surrounds the windings, providing multiple return paths for the flux. Core form dominates large power transformers; shell form is common in some large and special designs.

What is the difference between a distribution transformer and a power transformer?

Distribution transformers serve the final step down to consumers, are usually smaller, run at varying and often light load, and are designed for efficiency at partial load with minimal attention. Power transformers serve transmission and large industrial loads, are much larger, run near full load, and are designed for maximum efficiency at full load with full monitoring and protection.

When is a dry-type transformer chosen over an oil-filled one?

Dry-type transformers are chosen where fire safety, environmental sensitivity or indoor location rules out oil: buildings, hospitals, tunnels, offshore platforms and water-sensitive sites. They cost more per unit of capacity and are usually smaller, but they remove the oil fire and leakage risks entirely.