Transformer Windings: Types, Insulation Classes and Short-Circuit Withstand
May 19, 2025
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How Windings Transfer Energy Between Circuits
A transformer has no moving parts and no direct electrical connection between its circuits. Energy crosses from the primary to the secondary through a shared magnetic flux in the core, and the windings are the elements that link that flux to the external circuit. The electromotive force induced in a winding follows the rate of change of flux linkage, so the ratio of the voltages is set by the ratio of the turns: with N1 turns on the primary and N2 turns on the secondary, the no-load voltage ratio is V1 divided by V2 equals N1 divided by N2.
That relationship is only exact under no-load conditions. Under load, winding resistance and leakage reactance cause a voltage drop, which is why an applied voltage ratio is verified as part of the routine tests defined in IEC 60076-1 rather than assumed from the nameplate.
Conductor Materials and Insulation Systems
Copper is the default conductor because of its conductivity and its ability to withstand short-circuit forces when wound into a compact structure. Aluminium windings are used where weight and cost dominate and where the larger cross-section required can be accommodated in the window. Winding wire for enamelled conductors is specified under the IEC 60317 series, with IEC 60317-0-1 covering general requirements for enamelled round copper wire and defining dimensions, elongation and electrical breakdown of the enamel film.
The insulation system as a whole is classified thermally under IEC 60085, where class 105, 120, 130, 155 and 180 correspond to the traditional A, E, B, F and H designations. The class defines the temperature the insulation system can tolerate continuously, and it is the reference point for evaluating ageing during overload.
Winding Types and Where Each Is Used
| Winding type | Typical application | Practical consideration |
|---|---|---|
| Layer winding | Small distribution transformers | Simple to manufacture, limited axial cooling path |
| Helical winding | Low-voltage windings at higher current | Good cooling, needs careful clamping against short-circuit forces |
| Disc winding | Medium and large power transformers | Excellent voltage distribution and cooling, more labour |
| Foil winding | Low-voltage winding of distribution units | Very high space factor, low axial short-circuit stress |
| Continuous disc | High-voltage windings | Uniform insulation, suitable for impulse stress control |
| Interleaved winding | Windings exposed to steep impulse fronts | Improved initial voltage distribution, higher cost |
In most oil-immersed designs the low-voltage winding is placed nearest the core and the high-voltage winding outside it, with a duct between them. The arrangement equalises insulation distances and gives the high-voltage winding a lower capacitance to the core, which improves impulse voltage distribution along the winding.
Losses, Leakage Reactance and Thermal Design
Two loss mechanisms originate in the windings. The first is resistive loss, calculated from the current squared multiplied by the resistance at the reference temperature of 75 degrees C. The second is eddy and stray loss produced by leakage flux acting on the conductors and on the surrounding steel structure. Short-circuit impedance measurement during routine testing captures the sum of these effects, and impedance is therefore stated together with the load loss and the reference temperature.
Cooling is designed by controlling duct width, conductor aspect ratio and oil flow paths. A winding with a tall, thin conductor has a larger cooling surface but higher eddy loss than a wide, flat conductor of the same area, so thermal and loss performance trade against each other and the optimum depends on the rating and the cooling mode, whether ONAN or ONAF.
Withstanding Short-Circuit Forces
Under a short circuit the current can reach many times rated value, and the mechanical force between conductors rises with the square of that current. IEC 60076-5 defines the ability to withstand the thermal and dynamic effects of an external short circuit, with the reference duration taken as 2 s. Compliance depends less on conductor material than on how the winding is supported: interleaved insulating cylinders, spacer blocks and a rigid clamping structure determine whether the winding survives without permanent deformation.
Deformation is not always visible after the event and may not show up in a ratio measurement. Frequency response analysis, defined in IEC 60076-18, is the established method for detecting mechanical displacement of windings and core movement by comparing the measured response against a baseline fingerprint.
Inspection and Quality Control Points
The critical control points during winding are turn count, conductor alignment, insulation lap and clearance, tightness of the winding after pressing, and free height before assembly into the core. Turn count errors of a single turn are measurable as a ratio deviation and are one of the most frequent causes of rejection at the routine test stage.
After drying and oil impregnation, the winding dimension and clamping pressure should be re-recorded, because insulation shrinks during the drying cycle. Records of these dimensions provide the manufacturing baseline against which later condition assessment is compared.
Frequently Asked Questions
Q: Why is the secondary winding usually closer to the core?
A: Placing the low-voltage winding next to the core reduces the insulation gap needed to the grounded core, and it lowers the stray loss in the core clamping structure. It also reduces the mean length of the high-voltage winding, which reduces conductor cost.
Q: What limits the current a winding can carry continuously?
A: The allowable temperature rise of the insulation system, generally taken as 65 K average winding rise for liquid-immersed transformers under IEC 60076-2. Current, conductor cross-section, cooling duct arrangement and ambient temperature all feed into that limit.
Q: Can aluminium windings replace copper?
A: Yes, where the larger conductor cross-section needed for the same current fits in the window and where the additional space is available. The design must be reworked, because resistance, short-circuit force and thermal behaviour all change with the conductor material.
Q: How is a winding deformation detected?
A: By frequency response analysis under IEC 60076-18, comparing the response of each winding with a baseline recorded earlier. Ratio and impedance measurements can both remain within tolerance while the internal geometry has already moved.
Q: What does class 155 insulation mean on a winding?
A: Under IEC 60085, class 155 corresponds to the traditional class F and denotes the temperature the insulation system can tolerate continuously. It defines the ageing reference for the winding rather than a guaranteed hot-spot limit under overload.
Q: Why is load loss quoted at 75 degrees C?
A: Because conductor resistance depends on temperature, and a loss figure is only comparable when the reference temperature is defined. Reporting load loss at 75 degrees C is the conventional basis, and the same temperature is used when stating short-circuit impedance.