Transformers Internal Components: Core, Windings & Insulation

Quick answer: The primary transformers internal components consist of the active assembly (magnetic core, primary and secondary windings, structural clamping frames), dielectric insulation systems (pressboard barriers, Kraft paper, and liquid or solid resin dielectrics), voltage-regulating tap changers, and internal lead connections routing power to high-voltage bushings.

Understanding what sits beneath the steel tank of an electrical power transformer is critical for electrical engineers, utility planners, and procurement teams. A power transformer functions as a vital transformer of energy, converting alternating current (AC) electricity between voltage levels across modern transmission and distribution grids. The mechanical integrity, thermal endurance, and electromagnetic performance of a transformer are directly governed by how these internal sub-assemblies are engineered and fabricated. Whether specifying substations or assessing lifecycle costs, examining the structural interior reveals how modern equipment achieves operating efficiencies exceeding 99% while enduring severe electrical and physical stresses.

What is inside a transformer: The core and active part

Inside an electrical transformer, the active part serves as the electromagnetic engine, comprising the laminated magnetic core clamped rigidly to withstand mechanical forces. The core provides a low-reluctance magnetic path to channel the mutual flux linking the electrical circuits. Engineers construct transformer cores from high-permeability, cold-rolled grain-oriented (CRGO) silicon steel laminations, typically graded M4, 0.23 mm, or 0.20 mm in thickness. Stacking these ultra-thin sheets with laser-scribed domain refinement drastically reduces eddy current circulation and hysteresis losses, adhering to strict no-load loss criteria in standards such as IEC 60076-1.

Modern designs employ step-lap mitered core joints cut at 45-degree angles. This continuous step-lap geometry eliminates air gaps at the corner joints, which minimises magnetising inrush current, lowers operational noise levels by 3 to 6 dB(A), and mitigates stray flux. As detailed in our comprehensive 3-phase transformer guide, high-grade core clamping frames made from structural steel channels apply uniform surface pressure (often between 1.0 and 2.0 MPa) across the core limbs and yokes. This mechanical rigidity prevents lamination vibration during energisation, ensuring that the transformer maintains stable magnetisation over a 30-to-40-year operational design life.

Transformer coil and winding architecture

A transformer coil assembly converts electric current into magnetic flux and back into voltage through conductive wire or strip arranged coaxially around the core limbs. The primary side of transformer equipment connects to the supply source, while the secondary side delivers the transformed voltage to the downstream load. Depending on the current capacity and voltage class, engineers specify electrolytic copper (with conductivity ≥ 101% IACS) or electrical-grade aluminium conductors.

Different winding configurations are deployed according to application parameters:

• Continuous disc windings: Utilised extensively for high-voltage (HV) windings above 33 kV. These discs provide superior mechanical withstand against axial short-circuit forces and optimise dielectric stress distribution under transient lightning impulses.
• Helical and spiral windings: Ideal for low-voltage (LV), high-current applications, such as the secondary side of distribution units, providing optimal cooling duct placement to dissipate resistive heat.
• Foil windings: Constructed from continuous copper or aluminium sheets across the full axial height of the coil, commonly used in cast resin dry-type transformers to eliminate layer-to-layer dielectric stresses and minimise axial short-circuit displacement forces.

The operational foundation relates directly to the fundamental transformer equation:

V_p / V_s = N_p / N_s = I_s / I_p

Here, V represents voltage, I represents current, and N represents the physical number of turns in the primary and secondary coils. When calculating losses and system reliability, understanding conductor cross-sections and winding arrangements helps engineers evaluate operational economics, as explored in our guide to transformer losses and total cost of ownership.

What is a transformer made of: Dielectric and insulation systems

A transformer is made of conductive metals, magnetic alloys, structural steel, and a sophisticated combination of solid and liquid dielectric materials. Electrical insulation prevents phase-to-phase flashovers, turn-to-turn short circuits, and coil-to-ground dielectric breakdowns under high electrical fields. In high-voltage transformers, insulation must endure continuous electrical stress alongside elevated operating temperatures defined by thermal classes (such as Class A 105°C, Class F 155°C, or Class H 180°C).

Solid insulation materials include high-density cellulose pressboard, thermally upgraded Kraft paper wrapped tightly around individual conductors, crepe paper tubes for flexible lead routing, and laminated wood blocks for winding clamping. In dry-type transformers, solid insulation relies on epoxy resin loaded with silica fillers, cast under strict vacuum conditions to eliminate partial discharge below 10 pC at 1.3 times rated phase-to-ground voltage. For liquid-immersed units, uninhibited or inhibited naphthenic mineral oil (meeting IEC 60296) or synthetic/natural esters (meeting IEC 61099) serve a dual purpose: providing dielectric withstand (breakdown voltage ≥ 30 kV across a standard 2.5 mm test gap) and circulating via natural or forced convection to extract winding heat. The mechanical and chemical differences between these insulation methodologies are detailed in our analysis of oil-immersed vs dry-type transformers.

Key transformer parts: Internal regulation and protection

Beyond the core and coils, auxiliary transformer parts installed inside the main tank manage voltage stability, terminal transitions, and fault detection. Voltage control is handled by tap changers, which mechanically alter the effective turns ratio on the high-voltage winding without disturbing the core flux density. These mechanisms fall into two distinct engineering classes:

1. De-Energised Tap Changers (DETC): Operated manually only when the unit is disconnected from the electrical network, commonly providing a ±2 × 2.5% regulation range via rotary internal contact bridges.
2. On-Load Tap Changers (OLTC): Driven by automated motor drives to shift taps under full load conditions without interrupting power, employing transition resistors or vacuum interrupters to bridge circulating currents during the contact transfer sequence.

Internal copper busbars and flexible copper braids link the winding extremities to the lower terminals of the porcelain or resin-impregnated paper (RIP) bushings. Inside conservator-equipped transformers, internal piping routes evolving fault gases upward toward the cover-mounted Buchholz relay. Furthermore, magnetic shielding shunts and laminated flux collectors line the interior tank walls to prevent stray leakage flux from inducing localised eddy-current overheating in structural steel plates during heavy loading.

Technical specifications of core internal components

Evaluating the internal build quality of power and distribution transformers requires precise alignment with international manufacturing standards. The table below outlines the core internal components, their primary engineering materials, functionality, and governing industry specifications:

How does an electrical transformer work: Electromagnetic sequence

An electrical transformer operates on the principle of electromagnetic induction, specifically Faraday's law and Lenz's law, allowing alternating electrical energy to transfer between circuits at identical frequency without moving mechanical components. When looking at the internal interaction between the core, primary, and secondary systems, the functional sequence follows a defined physical progression:

1. Primary energisation: An alternating voltage applied to the primary terminals forces an alternating excitation current through the primary winding coils.
2. Core magnetisation: This primary current establishes an alternating magnetomotive force (MMF) that drives dynamic magnetic flux through the closed loop of the CRGO steel core.
3. Flux linkage: The laminated core directs virtually all magnetic flux lines through its low-reluctance path, linking the turns of both the primary and secondary coils with negligible leakage.
4. Electromotive force induction: The continuous time-variation of the magnetic flux induces an electromotive force (EMF) across every turn of both windings in proportion to the rate of change of flux (dΦ/dt).
5. Secondary power delivery: When a closed electrical circuit is connected across the secondary terminals, the induced voltage drives a load current, supplying power transformed to the desired voltage level.

To guarantee that these internal mechanisms function flawlessly before site delivery, equipment undergoes rigorous factory verification, as outlined in our step-by-step transformer factory acceptance testing checklist.

Next steps: specifying and sourcing

When specifying internal components for substation transformers, project engineers must define exact electrical parameters in their procurement tenders. Essential data includes rated capacity (kVA or MVA), primary and secondary nominal voltages, vector group configuration, impedance (%Z), winding conductor material (copper or aluminium), and short-circuit withstand capabilities according to IEC 60076 or IEEE C57.12. Explore our engineered power transformers and rugged oil-immersed transformers built to international quality benchmarks. For project pricing, custom internal design requirements, or factory documentation, submit your specification to our engineering team via our transformer quotation portal.

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