Construction of Transformer ~ Electrical & Electronics Knowledge

Construction of Transformer

Basically a transformer consists of two inductive windings and a laminated steel core. The coils are insulated from each other as well as from the steel core. A transformer may also consist of a container for winding and core assembly (called as tank), suitable bushings to take our the terminals, oil conservator to provide oil in the transformer tank for cooling purposes etc. The figure at left illustrates the basic construction of a transformer. In all types of transformers, core is constructed by assembling (stacking) laminated sheets of steel, with minimum air-gap between them (to achieve continuous magnetic path). The steel used is having high silicon content and sometimes heat treated, to provide high permeability and low hysteresis loss. Laminated sheets of steel are used to reduce eddy current loss. The sheets are cut in the shape as E,I and L. To avoid high reluctance at joints, laminations are stacked by alternating the sides of joint. That is, if joints of first sheet assembly are at front face, the joints of following assemble are kept at back face.

Magnetic circuit :-

The magnetic circuit of a transformer consists of core and yoke. The circuit provides the path to the flow of magnetic flux. The transformer consists of laminated steel core and the two coils. The two coils are insulated from each other and also from the core. The core of the transformer is constructed from laminations of sheet steel or silicon steel assembled to provide a continuous magnetic path. At usual flux densities, the silicon steel material has low hysteresis losses.

The vertical position on which the coil is wound is called the limb while the horizontal position is known as the yoke.

Electric circuit :-

Construction of electric circuit of transformer consists of primary and secondary windings usually made of copper. The Conductors of rectangular cross section are generally used for low voltages winding and also for the high voltage winding for large transformers. Conductors of the circular cross-sectional area are used for high voltage winding in the small transformer.

According to the core construction and the manner in which the primary and secondary windings are placed around it, the transformer is named as core type and shell type.

Types of Transformer :-

Transformers can be classified on different basis, like types of construction, types of cooling etc.

(A) On the basis of construction, transformers can be classified into two types as; (i) Core type transformer and (ii) Shell type transformer, which are described below.

i) Core type Transformers :-

In core type transformer, windings are cylindrical former wound, mounted on the core limbs as shown in the figure above. The cylindrical coils have different layers and each layer is insulated from each other. Materials like paper, cloth or mica can be used for insulation. Low voltage windings are placed nearer to the core, as they are easier to insulate.

Shell type construction

ii) Shell type Transformer :-

The coils are former wound and mounted in layers stacked with insulation between them. A shell type transformer may have simple rectangular form (as shown in above fig), or it may have a distributed form.

B) On the basis of their purpose

Step up transformer: Voltage increases (with subsequent decrease in current) at secondary.

Step down transformer: Voltage decreases (with subsequent increase in current) at secondary.

Single phase transformer

Three phase transformer

(D) On the basis of their use

Power transformer: Used in transmission network, high rating

Distribution transformer: Used in distribution network, comparatively lower rating than that of power transformers.

Instrument transformer: Used in relay and protection purpose in different instruments in industries

Current transformer (CT)

Potential transformer (PT)

(E) On the basis of cooling employed

Oil-filled self cooled type

Oil-filled water cooled type

Air blast type (air cooled)

Dielectric Circuit:-

The dielectric circuit consists of insulations used in different places in the transformer to insulate the conducting parts. The core is laminated to minimize the eddy current losses. The laminations are insulated from each other by a light coating of varnish or by an oxide layer. The thickness of laminations varies from 0.35mm to 0.5mm for a frequency of 50 Hz.

Tanks and Accessories :-

Other different parts and accessories are also fitted on the transformer for its efficient work as well as for longer life and better services of the transformer. They are as follows

Conservator:-

The Conservator is a cylindrical tank placed on the top or on the roof of the transformer main tank. A large cover is provided which can be opened from time to time for the proper maintenance and cleaning of the transformer. It acts as a reservoir for the transformer insulating oil.

When the transformer is fully loaded and the temperature of the transformer rises high, an increase in the volume of the air inside the transformer takes place. As the level of the oil increases and decreases simultaneously, thus, a conservator provides adequate space for this expanded oil inside the transformer.

Breather:-

As in the human body, there is a heart similarly a breather acts as a heart for the transformer. When the temperature of the transformer rises, the insulating oil in the transformer gets heated up. This oil expands and contracts. When the oil heats up and expands the transformer breaths air in and thus the oil gets cooled and the level of oil goes down and the air is absorbed in it. This process of taking air in and out is called breathing of the transformer.

The level of oil in the chamber increases and decreases when the breather takes the air in and out for cooling of the oil. This air carries moisture, which contaminates the oil and thus the quality of oil gets deteriorate. For eliminating this moisture content , the breather is filled with Silica Gel. The main function of the silica gel is to separate moisture from the oil, maintaining the quality of the insulating oil. Initially, the colour of the silica gel is blue and as it absorbs the moisture from the oil its turns into pink colour.Fresh Silica gel drys down the air to a dew point below -40 degrees Celsius.

Explosion vent:-

The explosion vent is a thin aluminium pipe placed at both the ends of the transformer to prevent the transformer from the damage. When the temperature increases in the transformer drastically and the excessive pressure is created inside the transformer, the explosive vent helps in releasing the pressure.

Radiator:-

The main function of the radiator is to cool the oil in the transformer. Radiator are the detachable device whose upper and lower portion is connected by a valve to the transformer tank. When the transformer cleaning and maintenance is done the valve prevent the draining of the oil when the radiator is detached from the transformer.

When the transformer is in the working conditions, the oil of the transformer gets heated and moves up in the main tank and enters into the radiator through the upper valve, where it gets cooled and from the lower valve of the radiating unit the oil again enters the transformer tank and this process continues.

Bushings:-

The Bushings in the transformer are the insulting device which allows an electric conductor to pass electrical energy safely through it. It provides electrical field strength to the insulation of the conductors to withstand if a large amount of electric energy passes through it. Solid porcelain type bushing is used in smaller transformer and oil filled condenser type bushing is used in large transformer.

The most common cause of the failure of the bushing resulting in damage of the transformer is the entrance of the moisture. The power factor of the bushing will always be in stable condition, but if the variation is seen in the power factor that means there is deterioration in the insulation. This can be identified by the tests known as acceptance or routine test and Doble Power Factor Test.

B(I

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