The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported, and cooled to withstand operational and test conditions. The terms winding and coil are used in same meaning.
Copper and aluminum are the primary materials used as conductors in power-transformer windings. While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and is used almost exclusively in all, but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength. The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors. Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends.
Individual strands may be subjected to differences in the flux field due to their respective positions within the winding, which create differences in voltages between the strands and drive circulating currents through the conductor loops. Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents. In core-form transformers, the windings are usually arranged concentrically around the core leg, a winding being lowered over another winding already on the core leg of a three-phase transformer for minimizing leakage flux.
Shell-form transformers use a similar concentric arrangement or an interleaved arrangement. With an interleaved arrangement, individual coils are stacked, separated by insulating barriers and cooling ducts. The coils are typically connected with the inside of one coil connected to the inside of an adjacent coil and, similarly, the outside of one coil connected to the outside of an adjacent coil. Sets of coils are assembled into groups, which then form the primary or secondary winding.
In concentric windings, circular windings have inherently higher mechanical strength than rectangular windings, whereas rectangular coils can have lower associated material and labor costs. Rectangular windings permit a more efficient use of space, but their use is limited to small power transformers and the lower range of medium-power transformers, where the internal forces are not extremely high. As the rating increases, circular coils, or shell-form construction, are used. In some special cases, elliptically shaped windings are used.


A variety of different types of windings have been used in power transformers through the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils; or they can be wound horizontally and placed upright upon completion.
Pancake Windings:
Several types of windings are commonly referred to as “pancake” windings due to the arrangement of conductors into discs. However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers. The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically.
Layer (Barrel) Windings:
Layer (barrel) windings are among the simplest of windings in that the insulated conductors are wound directly next to each other around the cylinder and spacers. Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination. Variations of this winding are often used for applications such as tap windings used in load-tap-changing (LTC) transformers and for tertiary windings used for, among other things, third-harmonic suppression.
Helical Windings:
Helical windings are also referred to as screw or spiral windings, with each term accurately characterizing the coil’s construction. A helical winding consists of a few to more than 100 insulated strands wound in parallel continuously along the length of the cylinder, with spacers inserted between adjacent turns or discs and suitable transpositions included to minimize circulating currents between parallel strands. The manner of construction is such that the coil resembles a corkscrew. Helical windings are used for the higher-current applications frequently encountered in the lower-voltage classes.
Disc Windings:
A disc winding can involve a single strand or several strands of insulated conductors wound in a series of parallel discs of horizontal orientation, with the discs connected at either the inside or outside as a crossover point. Each disc comprises multiple turns wound over other turns, with the crossovers alternating between inside and outside. Most windings of 25-kV class and above used in core form transformers are disc type. Given the high voltages involved in test and operation, particular attention is required to avoid high stresses between discs and turns near the end of the winding when subjected to transient voltage surges.
Spiral Winding:
Spiral winding are mainly used for high current. So it is used at low voltage side, as low voltage side carries high current. But in case of high voltage winding with high current, it is also applicable. Here the conductors are positioned one by one without leaving a gap to make a spiral shape.
Sandwich Winding:
In shell type transformer, sandwich winding are used. We can reduce leakage flux by this type winding. Like layer winding, both high and low voltage winding are split into number of sections and H.V section is sandwiched between two low voltage sections.

Factors to be consider for transformer winding:

  • The winding should have proper insulation.
  • Should have mechanical toughness to withstand the force occurred in sudden short circuit.
  • Electrical strength of winding should be high to withstand the over voltage.
  • Should be economical.
  • Temperature rising of winding should be in considerable limit.
  • In transformer, the same magnetic fluxes are linked by both the primary and secondary winding. In actual 3 phase transformer both windings are interleaved to link with maximum flux as well as reduce leakage flux. In single phase, where two separate winding on two separate leg required, half of the both primary and secondary winding wound on two leg one after another, again for same reason, to reduce leakage flux.
    Low voltage windings are placed nearer to the core, because less insulation required between L.V. winding and core than high voltage winding and insulating core.

    Uniform and graded insulation in case of transformer winding:

    In case of star connected winding, there is a neutral point and the voltage near the star or neutral point is almost zero. And voltage gradually increases from neutral point to terminal. So, in case of star connection, we made the insulation also lower at star point and increased step by step to the terminal. This type of stepping insulation is known as graded insulation. These types of insulations are normally imposed in case of extra high voltage.
    In delta connection, as there is no neutral point, it is uniformly insulated, same as low voltage star connection too.


    Transformer windings and leads must operate at high voltages relative to the core, tank, and structural elements. In addition, different windings and even parts of the same winding operate at different voltages. This requires that some form of insulation between these various parts be provided to prevent voltage breakdown or corona discharges.


    The surrounding oil or air which provides cooling has some insulating value. The oil is of a special composition and must be purified to remove small particles and moisture. The type of oil most commonly used is called transformer oil. Further insulation is provided by paper covering over the wire or cables. When saturated with oil, this paper has a high insulation value.
    Other insulating structures which are generally present in sheet form, often wrapped into a cylindrical shape, are made of pressboard. This is a material made of cellulose fibers which are compacted together into a fairly dense and rigid matrix. Key spacers, blocking material, and lead support structures are also commonly made of pressboard. When power is suddenly switched on or off in some part of the system combination of oil and pressboard barriers can withstand higher voltages for shorter periods of time. In other words, a short duration high voltage pulse is no more likely to cause breakdown than a long duration low voltage pulse. This means that the same insulation that can withstand normal operating voltages which are continuously present can also withstand the high voltages arising from lightning strikes or switching operations which are present only briefly. In the case of oil, it has been found that subdividing the oil gaps by means of thin insulating barriers, usually made of pressboard, can raise the breakdown stress in the oil. Thus large oil gaps between the windings are usually subdivided by multiple pressboard barriers. The leads which connect the windings to the bushings or tap changers or to other windings must also be properly insulated since they are at high voltage and pass close to tank walls or structural supports, which are grounded.

    Parameters that affect degradation of cellulose:

    The rate of degradation reactions greatly depends on temperature. With temperature rise, chemical reaction increases. The useful life of cellulose and oil is markedly reduced at higher temperature.
    All cellulose reacts with oxygen. Due to oxygen, oxidation occurs. Carbon di oxide, water, carbon mono oxide are produced as the product. The level of these products in the oil continues to increase as oxidation continues.
    Cellulose has a great affinity for holding water. The moisture content of cellulose is recommended to not more than 0.5%. Water distributes between the oil and paper in a constant ratio, depending on the temperature of the system. As temperature increases, water moves from paper into oil and with temperature decreasing, water moves in opposite direction.
    Water is a product of the oxidation of cellulose, and therefore increasing in concentration with time.
    Cellulose can degrade by a chemical process referred to as hydrolysis. This process is catalyzed by acids. Acids are present in the oil that is in contact with the cellulose. Carboxylic acids are produced from the oil as a result of oxidation. The acid content of the oil increases as the oil oxidizes. With an increase in acidity, the degradation of the cellulose increases.


    Mineral insulating oil is used for transformer and switchgear. It is used mainly for two purposes,

  • For insulation, where oil used as a liquid insulator for core and winding.
  • For cooling, where oil acts as a coolant by dissipates heat of the transformer.
  • All mineral oils are mixtures of hydrocarbon compounds with about 25 carbon atoms per molecule. Crude oils from different geographical areas will have different chemical structure which means arrangements of carbon atoms within the molecules are different. Crude oils from some sources are higher in paraffinic compounds whereas, others are higher in naphthenic compounds. This oil also contains significant amounts of aromatic and poly aromatic compounds.
    The refining of crude oil for the production of dielectric fluids reduces the aromatic and poly-aromatic content to enhance the dielectric properties and stability of the oil.
    The terms paraffinic and naphthenic refer to the arrangement of carbon atoms in the oil molecule. Carbon atoms which bonded with one another in straight or branched line are referred to as being paraffinic. And when bonded as rings with five, six or seven carbons referred as being naphthenic. When bonded as ring of benzene referred as aromatic.
    The differences in the chemical composition will result in differences in physical properties. For electrical equipment, the main concerns are at low temperature, paraffinic oils tend to form waxes and have higher viscosity. It has lower thermal stability than naphthenic oil.

    Oxidation rate is slower Oxidation rate is high.
    Consist of wax content. Does not consist much wax content.
    Sludge stored in the bottom of the tank. Sludge is soluble.
    Easily available. Not easily available.
    Lower thermal stability. Higher thermal stability.
    Low cost. High cost.
    Used for warm climate. Can be use in any climate.

    Though paraffin based oil has some disadvantages, it is still used in many countries due to low cost and easy availability.


    In case of choosing transformer oil we should follow some parameters specified by the country such as
    1) Dielectric strength: It stands for the break down voltage of the transformer oil. Normally 145 KV systems, dielectric strength of the oil should be minimum 50 KV and up to 145 KV, dielectric strength should be minimum 30 KV.
    2) Dielectric dissipation factor: It stands for tan-delta of a transformer. And by the tan-delta test, we come to know about the leakage current through insulating oil. Less the value of tan-delta, better the insulating oil.
    3) Specific resistance: The resistivity of the oil decreases with increasing temperature. At full load, the temperature of the insulating oil increases near about 90˚C.
    The standard value of specific resistance at 90˚C is $$ 30X{ 10 }^{ 12 }\quad ohm-cm.$$
    4) Water content in transformer oil: As the mineral oil is hygroscopic, increasing water content decreases the breakdown voltage rapidly.
    5) Acidity: Acidity in the transformer oil decreases the break-down voltage as well as deteriorates the insulation mainly paper insulation. The value of acidity is 0.03 mg-KOH per gm. This is also called the neutralization value.
    6) Appearance: The oil should be neat and clean. There should not be any visible sludge.
    7) Sludge content: Maximum sludge content inside oil should be 0.1% of its weight.
    8) Density: The density of transformer oil at 27˚C should be 0.89 gm/cm3 (maximum).
    9) Flash point: It indicates the temperature to produce a flame. As very high flash point is desirable, minimum temperature should be 125˚C.
    10) Pour point: Pour point indicates the wax content of the transformer oil. It is the temperature under which, oil flows stop.


    Power transformers are greater than 99% efficient, the input and output power are nearly the same. However because of the small inefficiency, there are losses inside the transformer. The sources of these losses are I2R losses in the conductors, losses in the electrical steel due to the changing flux which it carries, and losses in metallic tank walls and other metallic structures caused by the stray time varying flux. These losses lead to temperature rises which must be controlled by cooling. The primary cooling media for transformers are oil and air. In oil cooled transformers, the coils and core are immersed in an oil filled tank. The oil is then circulated through radiators or other types of heat exchanger so that the ultimate cooling medium is the surrounding air or possibly water for some types of heat exchangers.
    In small distribution transformers, the tank surface in contact with the air provides enough cooling surface so that radiators are not needed. Sometimes in these units the tank surface area is augmented by means of fins or corrugations. Transformers come in various cooling classes, as defined by the industry standards. In small oil-filled distribution transformers, the surface of the tank is sufficient for transferring heat from the oil to the air. Ribs are added to the tanks of some distribution transformers to increase the surface area of the tank and to improve heat transfer. Large distribution transformers and small power transformers generally require radiator banks to provide cooling.
    Different cooling classes are:

    ONAN – OIL NATURAL AIR NATURAL; – In this process, produced heat is minimized by natural air and oil flow.
    ONAF – OIL NATURAL AIR FORCED; – Here produced heat comes on radiator through oil, where external fan fitted outside the radiator cools the transformer oil.
    OFAN – OIL FORCED AIR NATURAL; – Oil is forcefully flow by pump and naturally cooled.
    OFAF – OIL FORCED AIR FORCED; – Here pump needed for oil circulation and also fan used outside the radiator.
    OFWF – OIL FORCED WATER FORCED; – Forced oil is cooled by water flow.
    ODAN – OIL DIRECTED AIR NATURAL; – In this process, oil is forced by pump near to the transformer winding, cooled by natural air.
    ODAF – OIL DIRECTED AIR FORCED; – Forced oil nearer from winding, cooled by fan.
    ODWF – OIL DIRECTED WATER FORCED; – Directed forced oil cooled by water flow.

    Keep in pocket:→

    ✓Insulated copper or alluminium conductors are used for transformer winding.
    ✓Mainly copper conductors are used. In smaller sizes aluminium is preferable.
    ✓Paper insulation is used for winding insulation.
    ✓Cross section of the conductors is circular or rectangular types.
    ✓Low voltage and high voltage windings are interleave, where low voltage winding placed nearer to the core & high voltage above the insulated low voltage winding.
    ✓In core form, normally disc windings are used and pancake windings for shell form.
    ✓Paraffinic and naphthenic compounded petroleum oil are used for transformer oil.
    ✓Different cooling classes are used for transformer cooling.

    Short questions related to this topic:

    Q. Transformer windings are generally made by?

    A. Copper.

    Q. Disc winding used in which form?

    A.Core form.

    Q. Why low voltage winding are placed nearer the core?

    A. Reducing insulation cost.

    Q. Why L.V and H.V. winding placed in same leg?

    A. Reduce leakage flux.

    Q. Which type of insulation require on winding?

    A. Paper insulation.

    Q.Does graded insulation require in delta type winding?

    A. No, only star type winding require it..

    Q. What moisture affect on insulation?

    A. It degrades paper as well as oil insulation.

    Q. Paraffin or naphtha which one has better quality ?

    A. Naphtha based oil.

    Q. Paraffin or naphtha which one has greater use and why?

    A. Paraffin based. Because low cost and easy availability.

    Q. How much sludge content is admissible in transformer oil?

    A.0.1% of its weight.

    Q. Break-down voltage of transformer oil should be?

    A.30 KV to 50 KV.

    Q. What does ONAF means?

    A.Oil natural air forced.

    Q. ODWF cooling used for?

    A.Large power transformer.

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