Core type transformer:

Primary Winding: Winding connected to the voltage source and creates a flux.

Secondary Winding: The winding where voltage is induced by induction.

Step down transformer: Secondary voltage is less than that of the primary

Step up transformer: Secondary voltage is more than that of the primary

A step down transformer can be made a step up transformer by making the low voltage winding its primary.

Hence it may be more appropriate to designate the windings as High Voltage (HV) and Low Voltage (LV) windings.

The winding with more number of turns will be a HV winding.

The current on the HV side will be lower as V-I product is a constant and given as the VA rating of the machines.

Also the HV winding needs to be insulated more to withstand the higher voltage across it.

HV also needs more clearance to the core, yoke or the body.

These aspects influence the type of the winding used for the HV or LV windings.

Transformer coils can be broadly classified in to concentric coils and sandwiched coils

The former are very common with core type transformers while the latter one are common with shell type transformers.

In concentric arrangement, in view of the lower insulation and clearance requirements, the LV winding is placed close to the core which is at ground potential.

The HV winding is placed around the LV winding.

Also taps are provided on HV winding when voltage change is required.

Three most common types of coils viz. helical, cross over and disc coils are shown in Fig.

Disc coils:

• Consist of flat conductors wound in a spiral form at the same place spiralling outwards.

• Alternate discs are made to spiral from outside towards the center.

• Sectional discs or continuous discs may be used.

• Excellent thermal properties and the behavior of the winding is highly predictable.

• Winding of a continuous disc winding needs specialized skills.

Cross over coils:

• Made of circular conductors not exceeding 5 to 6 sq mm in cross section.

• Used for HV windings of relatively small transformers.

• Turns are wound in several layers.

• The length and thickness of each block is made in line with cooling requirements.

• A number of such blocks can be connected in series, leaving cooling ducts in between the blocks, as required by total voltage requirement.

Helical Winding:

• Made of large cross section rectangular conductor wound on its flat side.

• The coil progresses as a helix and commonly used for LV windings

• The insulation requirement also is not too high.

• Between layers no insulation (other than conductor insulation) is needed as the voltage between layers is low.

• The complexity of the winding rapidly increases as the current to be handled becomes more.

• The conductor cross section becomes too large and difficult to handle.

• The eddy current losses in the conductor rapidly increases.

• Hence two or more conductors have to be wound and connected in parallel.

• The parallel circuits bring in problems of current sharing between the circuits.

• Transpositions of the parallel paths reduce unequal current distribution.

• The modern practice is to use continuously transposed and bunched conductors.

Sandwich coils:

• They permit easy control over the short circuit impedance of the transformer.

• By bringing HV and LV coils close on the same magnetic axis the leakage is reduced and the mutual flux is increased.

• By increasing the number of sandwiched coils the reactance can be substantially reduced.

The insulation used in the case of electrical conductors in a transformer is varnish or enamel in dry type of transformers.

In larger transformers to improve the heat transfer characteristics the conductors are insulated using un-impregnated paper or cloth and the whole core-winding assembly is immersed in a tank containing transformer oil.

The transformer oil thus has dual role : insulator and coolant.

The porous insulation around the conductor helps the oil to reach the conductor surface and extract the heat.

The conductor insulation may be called the minor insulation as the voltage required to be with- stood is not high.

The major insulation is between the windings.

Annular bakelite cylinders serve this purpose.

Oil ducts are also used as part of insulation between windings.

The oil used in the transformer tank should be free from moisture or other contamination to be of any use as an insulator.

Why larger unit sizes of transformers economically attractive ?

• Consider a transformer of certain rating designed with certain flux density and current density.

• If now the linear dimensions are made larger by a factor of \(K\) keeping the current and flux densities the same; the core and conductor areas increase by a factor of \(K^2\).

• The losses in the machine, which are proportional to the volume of the materials used, increase by a factor of \(K^3\).

• The rating of the machine increases by a factor of \(K^4\)

• The surface area however increases by a factor of \(K^2\) only.

• Thus the ratio of loss per surface area goes on increasing by a factor of \(K\).

• The substantial increase in the output is the major attraction in going in for larger units.

• However cooling of the transformer becomes more and more difficult.

• As the rating increases better cooling techniques are needed.

Simple air cooling of the transformers is adopted in dry type transformers for a rating of few kVA.

Hence air cooling is used in low voltage machines.

This method of cooling is termed as AN (Air Natural).

Air Blast(AB) method improves on the above by directing the blast of air at the core and windings.

Substantial improvement is obtained when the transformer is immersed in an oil tank.

The oil reaches the conductor surface and extracts the heat and transports the same to the surface of the tank by convection.

This is termed as ON (Oil Natural) type of cooling.

This method permits the increase in the surface available for the cooling further by the use of ducts, radiators etc.

OB (Oil Blast) method is an improvement over the ON-type and it directs a blast of air on the cooling surface.

In the above two cases the flow of oil is by natural convective forces.

The rate of circulation of oil can be increased with the help of a pump, with the cooling at the surface remaining natural cooling to air.

This is termed as OFN (Oil Forced Natural).

If now a forced blast of air is also employed, the cooling method become OFB (Oil Forced Blast).

A forced circulation of oil through a radiator is done with a blast of air over the radiator surface.

Substantial amount of heat can be removed by employing a water cooling.

Here the hot oil going into the radiator is cooled by a water circuit.

Due to the high specific heat of water, heat can be evacuated effectively.

Next in hierarchy comes OFW which is similar to OFB except that instead of blast of air a forced circulation of cool water in the radiator is used in this.

Basic properties: (a) Insulation (b) Cooling

There are many other properties which make a particular oil eminently suitable

Organic oils of vegetative or animal origin are good insulators but tend to decompose giving rise to acidic by-products which attack the paper or cloth insulation around the conductors

Mineral oils are suitable from the point of electrical properties but tend to form sludge.

The properties that are required to be looked into before selecting an oil for transformer application are as follows:

• Insulating property : Most of the oils naturally fulfil this. Therefore deterioration in insulating property due to moisture or contamination may be more relevant.

• Viscosity : Determines the rate of flow of the fluid. Highly viscous fluids need much bigger clearances for adequate heat removal.

• Purity : The oil must not contain impurities which are corrosive. Sulphur or its compounds as impurities cause formation of sludge and also attack metal parts.

• Sledge formation : Thickening of oil into a semisolid form is called a sludge. Sludge cause the oil to slowly forms semi-solid hydrocarbons. These impede flows and due to the acidic nature, corrode metal parts. Heat in the presence of oxygen is seen to accelerate sludge formation. If the hot oil is prevented from coming into contact with atmospheric air sludge formation can be greatly reduced.

• Acidity : Oxidized oil normally produces \(CO_2\) and acids. The cellulose which is in the paper insulation contains good amount of moisture. These form corrosive vapors. A good breather can reduce the problems due to the formation of acids.

• Flash point and fire point : Flash point of an oil is the temperature at which the oil ignites spontaneously. This must be as high as possible (not less than \(160^0~C\) from the point of safety). Fire point is the temperature at which the oil flashes and continuously burns. This must be very high for the chosen oil (not less than \(200^0~C\)).

Synthetic transformer oil like chlorinated diphenyl has excellent properties like chemical stability, non-oxidizing, good dielectric strength, moisture repellant, reduced risk due fire and explosion.