Transformer- Construction & Types

The transformer is very simple in construction and consists of magnetic circuit linking with two windings known as primary and secondary windings.

Besides magnetic circuit and windings it consists of a suitable container for the assembled core and windings, such as a tank, a suitable medium for insulating the core and windings from its container such as transformer oil, suitable bushings (either of porcelain, oil-filled or condenser type) for insulating and bringing out terminals of the windings from the container, temperature gauge for measurement of temperature of hot oil or hottest spot temperature, and oil gauge to indicate the oil level inside the tank.

Some transformers are also provided with conservator tank in order to slow down deterioration of oil and keep the main tank full of oil, emergency vent to relieve the pressure inside the tank in case the pressure inside the transformer rises to a danger point and gas operated relay Buchholz relay) in order to give alarm to indicate the presence of gas in case of some minor fault and take the transformer out of circuit in case of serious fault.

Magnetic circuit consists of an iron core. Since core is magnetic link between the two systems in a transformer and it itself contains a lot a energy, it is not by any means the passive component it would first appear.

Construction Of Transformer

Ideally the steel, of which the transformer core is made, is simple in chemical composition, the hot rolled steel has 4-5% silicon and the cold rolled has 3%. Nowadays cold rolled steel is used almost exclusively inspite of its high price, because fax density 30% higher can be used. This is because of high permeability and lower losses at given flux density.

Since flux is an alternating one, in order to reduce eddy current losses the are is laminated. The thickness of laminations or stampings varies from 0.35 mm to 0.5 mm. The laminations are insulated from each other by coating them with a thin coat of varnish. The iron core is rectangular in x-section for small size transformers and circular for large size ones to reduce the quantity of copper required.

Types of Transformers

Depending upon the type of construction used, the transformers are classified into two categories viz.

(1) core type
(2) shell type.

(1) In core type construction

As shown in Fig. 1 the coils are wound around the two limbs of a rectangular magnetic core. Each limb carries one half of the primary winding and one half of the secondary winding so as to reduce the leakage reactance to the minimum possible.

The lv winding is wound on the inside nearer to the core while the hv winding is wound over the lv winding away from the core in order to reduce the amount of insulation materials required.

Construction Of Core Type Transformer

(2) In shell type construction

As shown in Fig. 2 the coils are wound on the central limb of a three limb core. The entire flux passes through the central limb and divides into two parts going to side limbs. Consequently, the x-sectional area (and hence width) of the central limb is twice of that of each of the side limbs. Sandwich type winding is used in such a construction.

Construction Of shell Type Transformer

The present practice is to use core type transformers in large high voltage installations.

The shell type construction is commonly used for small transformers where a square or rectangular core x-section is suitable for economic considerations.

Power Transformers

The term is used to include all transformers of large size (250 kVA and above) used in generating stations and substations for transforming the voltage at each end of a power transmission line. They may be single phase or 3-phase; 3-wire delta/delta or delta/star connected: and of voltage rating 220/11 kV or in high voltage range.They are put in operation during load hours and disconnected during light load hours i.e. they are usually operated on approximately full load.

This is possible because they are arranged in banks and can be thrown in parallel with other units or disconnected at will. They may be self oil cooled, forced air cooled or forced water cooled.

Distribution Transformers

Transformers of rating up to 200 kVA, used to step down the distribution voltage to a standard service voltage are known as distribution transformers. They are kept in operation all the 24 hours a day whether they are carrying any load or not. They are of the self cooling type and are almost invariably oil-immersed.